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The role of tidal fluctuations in influencing rates of submarine groundwater discharge Safadi, Catherine Helen 2006

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The role of tidal fluctuations in influencing rates of submarine groundwater discharge by  Catherine Helen Safadi B.E.S'c., The University of Western Ontario, 2003  A T H E S I S S U B M I T T E D IN PARTIAL F U L F I L L M E N T O F THE REQUIREMENTS FOR THE D E G R E E OF MASTER OF APPLIED SCIENCE in The Faculty of Graduate Studies (Geological Engineering)  T H E UNIVERSITY O F BRITISH C O L U M B I A July 2006 © Catherine Helen Safadi, 2006  Abstract The following work was undertaken in this study to provide insight about the role of tidal fluctuations in influencing rates of S G D : (1) Improvements were made to the installation method and accuracy of a new tool developed for measuring the transient changes in vertical differential fluid pressures, referred to as the differential pressure system (DPS-II), (2) a field experiment was carried out to obtain measurements of S G D rates by a continuous heat type seepage meter, transient changes in vertical differential fluid pressures heads in shallow sediment by the DPS-II and tidal fluctuations in the near-shore environment, (3) the interpretation of the field data sets were constrained based on calculations using Darcy's Law and a 1D uniform density flow model.  The field experiment was carried out between October 26-28 2005, which was the last of a sequence of nine shorter tests completed to refine field procedures and gain experience using the instruments. Spanish Banks West beach in Vancouver was chosen as the field site because it is close proximity to The University of British Columbia, reasonable rates of S G D were measured during preliminary tests and the location fulfilled many of the logistical demands of testing.  Results of the experiment showed that the highest differential fluid pressure heads (measured between about 0.3 and 0.6 m below the seabed) at two piezometers of the DPS-II and highest S G D rates occurred at low tide. Using  Darcy's Law, S G D rates were calculated based on the differential fluid pressure heads and a hydraulic conductivity value within the constraints of the hydraulic conductivity data set derived from falling head tests completed on core samples from the site. The calculated S G D rates provided a good match with S G D rates by the seepage meter. Results of the 1D hydrogeological modelling suggest that the field based measurements of differential fluid pressure heads and S G D rates can be explained reasonably well by a 1D uniform density dependent flow model  Table of Contents Abstract  ii  List of T a b l e s  vi  List of F i g u r e s  vii  Acknowledgements  xii  1. Introduction 1.1 1.2 1.3 1.4  1  Introduction to R e s e a r c h T o p i c and P u r p o s e B a c k g r o u n d a n d Motivation Key Questions Outline of T h e s i s  1 2 4 4  2. R e v i e w of S u b m a r i n e G r o u n d w a t e r D i s c h a r g e 2.1 2.2 2.3 2.4  6  Introduction a n d S i g n i f i c a n c e Measurement Approaches Local and Global Estimates Relationship between S G D and Tidal Levels  6 8 12 13  3. Instrumentation D e v e l o p m e n t for M e a s u r i n g Differential Fluid P r e s s u r e H e a d s 3.1 A p p l i c a t i o n of Fluid P r e s s u r e M e a s u r e m e n t s to S G D 3.2 Fluid P r e s s u r e M e a s u r e m e n t A p p r o a c h e s 3.3 D e s c r i p t i o n of D P S - 1 3.3.1 Field R e s u l t s using D P S - I 3.4 D e s c r i p t i o n of DPS-II 3.4.1 D e s i g n O v e r v i e w 3.4.2 D a t a A c q u i s i t i o n S y s t e m 3.4.3 Differential Fluid P r e s s u r e H e a d s from T r a n s d u c e r s 3.4.4 D e p l o y m e n t 3.5 Error A n a l y s i s 3.5.1 Error E s t i m a t e  19 19 20 24 25 29 30 33 34 37 41 56  :  4. Field E x p e r i m e n t  79  4.1 Field Site 4.2 E x p e r i m e n t a l M e t h o d 4.2.1 DPS-II 4.2.2 C o n t i n u o u s H e a t T y p e A u t o m a t e d S e e p a g e M e t e r 4.2.3 L e v e l o g g e r a n d B a r o l o g g e r 4.2.4 P o r e W a t e r S a m p l e r 4.2.5 C o r e S a m p l e r 4.3 R e s u l t s 5. A n a l y s e s of Field M e a s u r e m e n t s  •  79 79 82 82 83 83 84 85 107  5.1 Differential P r e s s u r e H e a d s a n d C a l c u l a t e d S G D F l u x e s  107  5.2 H y d r o g e o l o g i c M o d e l  112  6. D i s c u s s i o n a n d R e c o m m e n d a t i o n s  137  6.1 Differential P r e s s u r e H e a d s , S G D a n d Tidal L e v e l s  iv  137  6.2 M e a s u r e m e n t s by the DPS-II  141  7. C o n c l u s i o n s a n d S u m m a r y  148  References..  151  A p p e n d i x A : M e t h o d for Calibration of Differential P r e s s u r e T r a n s d u c e r s  160  A p p e n d i x B: W i r i n g C o n f i g u r a t i o n of the E l e c t r o n i c C o m p o n e n t s of D P S - I I  168  A p p e n d i x C : C a l i b r a t i o n C u r v e for C o n t i n u o u s Heat T y p e S e e p a g e M e t e r  171  v  List of Tables Table 2.1: Simplified relationships between components, driving forces and contributing factors of S G D (Burnett et al. 2003) 16 Table 3-1: Calibration curves completed several days to weeks apart typical show variation less than +/-1-2 mm. Transducer C was used in the DPS-II prior to the transducer being replaced by Transducer A  59  Table 3-2: Calibration curves developed at different temperatures to investigate the effect of temperature on the electronic components of the DPS-II. Calibration curves were not developed for transducer B at 7- 8 ° C because of significant noise in the output of the transducer 59 Table 4-1: Values of E C measurements (mS/cm) from pore water samples collected at low tides over the experiment. Two pore water samples from 10 cm below the seabed (each adjacent to a piezometer) were collected only on the last day of the experiment. 92  vi  List of Figures Figure 2 - 1 : S c h e m a t i c of a h y d r o g e o l o g i c a l c r o s s section s h o w i n g a c o n c e p t u a l representation of S G D . M o d i f i e d from S l o m p a n d V a n C a p p e l l e n 2 0 0 4  17  F i g u r e 2-2: S c h e m a t i c of a m a n u a l type s e e p a g e meter  17  Figure 2 - 3 : S c h e m a t i c of a c o n t i n u o u s heat type s e e p a g e meter  18  Figure 2-4: L o c a t i o n s of p u b l i s h e d investigations of S G D by s e e p a g e m e t e r s , p i e z o m e t e r s , or g e o c h e m i c a l / g e o p h y s i c a l tracers. T h e n u m b e r s refer to T a b l e I in T a n i g u c h i (2002)  ..18  Figure 3 - 1 : T h e P I S S P I - 3 d e v e l o p e d at the University of L e h i g h (photo provided by B.Carson)  60  Figure 3-2: T h e D O P P I , d e s i g n e d by G E O T E K Ltd for m e a s u r e m e n t of pore water p r e s s u r e in s h a l l o w m a r i n e s e d i m e n t , ( s c h e m a t i c provided by G E O T E K Ltd.)  60  F i g u r e 3-3: S c h e m a t i c of a newly d e v e l o p e d multi-depth pore p r e s s u r e m e a s u r e m e n t d e v i c e by U r a k o s h i et a l . (2005) 61 F i g u r e 3-4: T h e D P S - I d e p l o y e d at high tide n e a r the F S M U L in 2 0 0 2 (photo by J.Caulkins)  62  Figure 3-5: S c h e m a t i c of a DPS-II p i e z o m e t e r  63  F i g u r e 3-6: T h e d a t a acquisition c o m p o n e n t s of the DPS-II ( t r a n s d u c e r s , resistors, d a t a l o g g e r a n d p o w e r s o u r c e ) h o u s e d inside a s u b m e r s i b l e (i.e. w a t e r proof) box for deployments  64  Figure 3-7: T h e DPS-II after c o m p l e t i o n of installation at low tide at S p a n i s h B a n k s W e s t beach, Vancouver 65 Figure 3-8: T h e instrument, N o l d D e a e r a t o r , u s e d to p r e p a r e d e a i r e d w a t e r for the installation of the D P S - I I  65  Figure 3-9: C a l i b r a t i o n c u r v e s d e v e l o p e d for t r a n s d u c e r A of the D P S - I I in this study. (A) Pre-field d e p l o y m e n t calibration curve d e v e l o p e d O c t o b e r 2 1 , 2 0 0 5 . (B) Post-field d e p l o y m e n t calibration c u r v e d e v e l o p e d O c t o b e r 3 1 , 2 0 0 5 66 Figure 3-10: C a l i b r a t i o n c u r v e s d e v e l o p e d for t r a n s d u c e r B of the DPS-II in this study. (A) Pre-field d e p l o y m e n t calibration curve d e v e l o p e d O c t o b e r 2 1 , 2 0 0 5 . (B) Post-field d e p l o y m e n t calibration c u r v e d e v e l o p e d O c t o b e r 3 1 , 2 0 0 5 67 Figure 3 - 1 1 : C o m p a r i s o n b e t w e e n the loading of the t r a n s d u c e r s in the field configuration a n d calibration set up. N o t e the orientation/position of t r a n s d u c e r in the s u b m e r s i b l e box d o e s not c h a n g e in the configurations  vii  68  Fig. 3-12: Setup of the deaired.water, peristaltic pump and data acquisition components in a small inflatable boat in preparation for insertion of the DPS-II piezometers into the seabed 69 Fig. 3-13: A piezometer of the DPS being installed at low tide. This photo was taken during an earlier trial deployment of the DPS-II in May 2005  69  Figure 3-14: Schematic of set up inside the submersible box. During insertion of the piezometers of the DPS-II, deaired water is pumped through the manifold past the sides of the transducer and out the ports of the piezometer via low density polyethylene tubing. When a piezometer is inserted, only the pair of valves on the manifold that regulate flow to that piezometer are open 70 Figure 3-15: The sources of error identified within the main stages of the DPS-II development  71  Figure 3-16: Calibration of the DPS-II differential pressure transducers to assess nonlinearity, non-repeatability, and hysterisis. (A) Calibration of transducer A. (B) Calibration of transducer B 72 Figure 3-17: 1s measurements that show the variation in output when a constant zero differential pressure is applied to the transducers. (A) Transducer A. (B) Transducer B. 73 Figure 3-18: One minute averages of 1 s measurements that show the variation in output when a constant zero differential pressure is applied to the transducers. (A) Transducer A. (B) Transducer B 74 Figure 3-19: The discharge of water at the seabed from along the side of the DPS-II piezometers was observed during the insertion of the piezometers  75  Figure 3-20: Temperature measured inside the submersible box, housing the electronic components of the DPS-II, by the internal temperature of the CR10X. The decrease in temperature after about 20 hours from the start of the experiment corresponds to the timing of the low tide that uncovered the box 76 Figure 3-21: Variation in the output of the transducers at 1 second measurements is compared to 1 minute averages of the data, for a 10 minute interval occurring at the highest high tide (2.3 m) over the experiment. (A) Transducer A. (B) Transducer B 77 Figure 3-21: Variation in output of the transducers at 1 second measurements is compared to 1 minute averages of the data, for a 10 minute measurement interval occurring at the lowest low tide (0.76 m) over the experiment. (A) Transducer A. (B) Transducer B  78  Figure 4-1: Map of Vancouver, BC with location of field site, Spanish Banks beach  93  Figure 4-2: Tidal level measurements over the period of the field experiment at the site. The tide line receded to several 100s of meters away from where the field instruments were deployed during the low tide that occurred about 20 hours after the beginning of the experiment 94  vin  Figure 4-3: Schematic of installation configuration. (A) Top view. (B) View in the direction offshore. The A-ln, B-ln, and B-Out labels refer to the position of core samples relative to the piezometers collected at the end of the field experiment 95 Figure 4-4: The boat containing the equipment to install the piezometers of the DPS-II and the seepage meter is positioned about a meter in land of the location where the DPS-II and seepage meter were installed in this study. 96 Figure 4-5: A continuous heat type seepage meter was deployed between the piezometers of the DPS-II during the experiment at the Spanish Banks West site (photo taken looking west of the site) 97 Figure 4-6: The seepage meter and piezometers were uncovered by the low tide on the second day of testing for about 5 hours (photo taken in the direction of shore) 98 Figure 4-7: Water sampler used to collect pore water samples from within the seabed. The water level was typically 30 to 60 cm above the seabed during pore water sampling. (A) An example of a pore water sample being retrieved using the water sampler.. 99 Figure 4-8: Core sampler used to collect sediment samples from adjacent to the piezometers of the DPS-II  100  Figure 4-9: Comparison between 1 minute averages of the differential pressure head measurements by the DPS-II and 5 minute measurements of tidal levels collected in the near-shore at the Spanish Banks site. There is a gap in the field data because of the low tide occurring between 20-25 hours after the beginning of the experiment, which uncovered the instruments 101 Figure 4-10: Comparison between 10 minute instantaneous measurements of SGD by a continuous heat type seepage meter and tidal levels 101 Figure 4-11: Averaged plots of 10 minute SGD measurements over different time intervals. (A) 20 minute average. (B) 30 minute average. (3) 40 minute average  102  Figure 4-12: EC measurements of pore water samples collected during the low tides over the course of the experiment. (A) Seawater samples. (B) The average EC value of the water column, and 10 and 30 cm (top piezometer port) below the seabed derived from all pore water samples collected at these positions 103 Figure 4-13: Core samples (A-Out and A-ln) collected from adjacent to piezometer A. The locations of the cores relative to the piezometer in the field are shown in Figure 4-3. (A) A-Out. (B) A-ln. 104 Figure 4-14: Core sample (B-ln) collected from adjacent to piezometer B. The location of the core relative to the piezometer in the field is shown in Figure 4-3 105 Figure 4-15: Hydraulic conductivity data derived from falling head tests for the core samples collected adjacent to the piezometers of the DPS-II. The letter labels indicate the estimates of hydraulic conductivity for a position relative to piezometer A or B, see Figure 4-3 for the location of these positions. The two estimates for each top and bottom section of core were based on using tap water and seawater as the hydraulic fluid for the  ix  falling h e a d tests. B o t h e s t i m a t e s for the bottom c o r e s e c t i o n of A - I N w e r e 1 x i O " m/s. 106 5  Figure 5 - 1 : F r e s h w a t e r h e a d gradients calculated b a s e d o n the differential p r e s s u r e h e a d m e a s u r e m e n t s by the DPS-II at p i e z o m e t e r A a n d B 122 F i g u r e 5-2: R e l a t i o n s h i p u s e d to convert E C m e a s u r e m e n t s to salinity ( E O S C 4 8 2 / 5 3 2 : Field T e c h n i q u e s in G r o u n d w a t e r H y d r o g e o l o g y c o u r s e notes) 123 Figure 5-3: R e l a t i o n s h i p u s e d to convert salinity to fluid density ( E O S C 4 8 2 / 5 3 2 : Field T e c h n i q u e s in G r o u n d w a t e r H y d r o g e o l o g y c o u r s e notes) 123 F i g u r e 5-4: C o m p a r i s o n b e t w e e n S G D rates c a l c u l a t e d b a s e d o n the differential p r e s s u r e h e a d m e a s u r e m e n t s by the DPS-II using D a r c y ' s L a w a n d m e a s u r e d S G D rates by the s e e p a g e meter. C a l c u l a t e d S G D rates w e r e d e r i v e d u s i n g hydraulic conductivity v a l u e s of 1.5 x 10" m/s at p i e z o m e t e r A a n d 4 x 10" m/s at p i e z o m e t e r B, w h i c h w e r e b a s e d o n the v a l u e that provided the best m a t c h with field b a s e d S G D rates a n d w e r e within the r a n g e of the a v a i l a b l e hydraulic conductivity d a t a set 124 5  5  Figure 5-5: G e n e r a l i z e d s c h e m a t i c of set up of h y d r o g e o l o g i c m o d e l e x a m i n e d in this study 125 Figure 5-6: C o m p a r i s o n b e t w e e n field b a s e d m e a s u r e m e n t s a n d M o d e l 1 simulation: K = 1x 10" m/s, S s = 1 x 10" m" a n d H = 1.0 m. (A) H y d r a u l i c gradients b e t w e e n 0.3 a n d 0.6 m b e l o w the s e a b e d . (B) S G D rates. 126 5  4  1  b a s e  F i g u r e 5-7: C o m p a r i s o n b e t w e e n field b a s e d m e a s u r e m e n t s a n d M o d e l 1 simulation: K = 1 x 1 0 - 5 m/s, S s = 1 x 10-4 m-1 a n d H b a s e = 2.5 m. (A) H y d r a u l i c gradients b e t w e e n 0.3 a n d 0.6 m b e l o w the s e a b e d . (B) S G D rates 127 Figure 5-8: C o m p a r i s o n b e t w e e n field b a s e d m e a s u r e m e n t s a n d M o d e l 1 simulation: K = 4 x 10" m/s, S s = 1 x 10" m" a n d H e = 2.5 m. B e s t m a t c h of the s i m u l a t e d results for p i e z o m e t e r B d a t a (A) H y d r a u l i c gradients b e t w e e n 0.3 a n d 0.6 m b e l o w the s e a b e d . (B) S G D rates 128 5  4  1  b a S  Figure 5-9: Sensitivity a n a l y s i s of m o d e l 1 to v a l u e s of Ss. (A) S  = 5 x 10~ m" 5  s  129  1  F i g u r e 5-10: T h e H s e v a l u e for m o d e l 1 w a s i n c r e a s e d to c o m p a r e s i m u l a t e d gradients with higher hydraulic g r a d i e n t s at P i e z o m e t e r A . (A) H e = 3.0 m. (B) H = 5.0 m . . 130 b a  b a s e  b a S  Figure 5 - 1 1 : C o m p a r i s o n b e t w e e n field b a s e d m e a s u r e m e n t s a n d M o d e l 2 simulation: 1 m thick top  layer, H  b a s e  = 2.5  m; K p L a y e r = 2 X 10"  m/S,  5  To  S S j o p L a y e r =1  X 10""  m" ; 1  K ottomLayer B  = 4 x 10" m/s, SseottomLayer = 1 x 1 0 " * m" . (A) H y d r a u l i c gradients b e t w e e n 0.3 a n d 0.6 m b e l o w the s e a b e d . (B) S G D rates. „ 131 5  1  F i g u r e 5-12: C o m p a r i s o n b e t w e e n field b a s e d m e a s u r e m e n t s a n d M o d e l 2 simulation: 1 m thick top l a y e r , H = 2.5 m; K p L a y e r = 1.5 x 10" m/s, Ss _pLayer = 1 x 10" m ' ; K ottomLayer = 4 x 10" m/s, Ss ttomLayer = 1 x 10" m" . (A) H y d r a u l i c gradients b e t w e e n 0.3 a n d 0.6 m b e l o w the s e a b e d . (B) S G D rates 132 1  5  b a s e  T0  B  4  To  5  4  Bo  x  1  1  Figure 5-13: The H se value for model 2 was increased to try to match the hydraulic gradients in the latter portion of the experiment (greater than 24 hours): 1 m thick top layer, H e = 3.0 m; K p L a y e r = 1 .5 X 10" m/S, S S p L a y e r = 1 X 10" IT!" ; K ottomLayer = 4 X 10," m/s, SsBottomLayer =1 x 10" IT)" . (A) Hydraulic gradients between 0.3 and 0.6 m below the seabed. (B) SGD rates 133 ba  1  5  b a S  4  To  1  T o  5  4  B  1  Figure 5-14: Simulation using Model 1 to investigate if a lower value of hydraulic conductivity (K = 1 x 10" m/s) would influence the timing of the highest SGD rates relative to the tidal level. Other Model 1 parameters: S = 1 x 10" m" and H e = 2.5 m. (A) Hydraulic gradients between 0.3 and 0.6 m below the seabed. (B) SGD rates. Note the different y-axes 134 6  4  1  s  b a S  Figure 5-15: Simulation using Model 2 to investigate if values of hydraulic conductivity 1 x 10" m/s) and Ss (S opL yer = 1 x 10 -3 m" ) similar to those used by the model constructed by Caulkins (2003) would influence the timing of the highest SGD rates relative to the tidal level. Other Model 2 parameters: 1 m thick top layer, H = 2.5 m; K pLayer = 1.5 x 10" m/s; S ttomLayer = 1 x 10" in" . (A) Hydraulic gradients between 0.3 and 0.6 m below the seabed. (B) SGD rates. Note the different y-axes... 135 (K ottomLayer=  6  1  B  sT  a  b a s e  5  To  4  1  sBo  Figure 5-16: Simulation completed for comparison with modelled results by Caulkins (2003). Plots B and C show hydraulic head with depth for specified times over the tidal cycle. The number labels refer to position on the tidal cycle shown on plot A. K and Ss labels indicate the parameter values assigned for the upper 6 m layer and the bottom 4 m layer 136 Figure AA-1: Flow path of deaired water (blue arrows) during preparation for bleeding the pressure ports of the transducer 165 Figure AA-2: Bleeding of a pressure port of the Setra 230 differential pressure transducer  166  Figure AA-3: Set up for taking readings during calibration  167  Figure AB-1: Flow chart illustrating the interaction between the data acquisition components of the DPS-II  169  Figure AB-2: Wiring of the Electronic Components through the CR10X  170  xi  Acknowledgements I owe many thanks to my supervisor Dr. Leslie Smith for giving me the opportunity to work with him on this project. I have especially valued Leslie's many suggestions, patience and of course, humour. From this work I have gained a greater appreciation for the effort involved in creating and implementing a new design as well as an understanding of the driving mechanisms of submarine groundwater discharge (I will not look at a beach the same way!). I am also grateful for the assistance and guidance provided by Dr. Roger Beckie and Dr. Uli Mayer, who were both always available throughout my studies at U B C .  I would like to thank my fellow hydros who have helped with my field work, contributed to my learning by sharing there own experiences and have provided such enjoyable company.  I am grateful to have been a part of such a knowledgeable, supportive and caring group at U B C . I wish you always, all the best.  xii  1. Introduction 1.1 Introduction to R e s e a r c h Topic and P u r p o s e Submarine groundwater discharge is defined as the total mixture of seawater and fresh groundwater flowing out from the aquifer, into the coastal water, through the underlying sediments (Destouni et al. 2003). Unlike river flow, S G D is a less obvious influx to the ocean, typically occurring as diffuse and temporally variable discharge, in some cases augmented by focused seeps or springs. S G D in the near-shore has been measured and predicted by many researchers (e.g. Kohout 1964; Bokuniewicz 1992; Young 1996; Burnett et al. 2003; Taniguchi et al. 2006) but the exact magnitude of the phenomenon, as well as its impact on the nearshore zone is still not known with certainty. In some cases, the fresh water component of S G D , discharging from local surficial or deeper aquifers, has been shown to represent a potentially important pathway for the transport of anthropogenic substances such as fertilizers and sewage from land to sea (Valiela et al. 1990; Capone and Slater 1990; Corbett et al. 1999; Krupa et al. 2003; Slomp and Van Cappellen 2004; Boehm et al. 2004).  A major question that exists is to what extent S G D is controlled by hydraulic gradients driving groundwater of terrestrial origin into the ocean, and how much is driven by oceanic processes such as tidal forcing, wave action or saltwater fingering (e.g., Li et al. 1999; Rasmussen et al. in press) that could cause a higher quantity of re-circulating seawater to discharge across the seabed. Discharges influenced by terrestrial and marine forces are typically coincident in  1  time and space but may differ significantly in magnitude. Since such factors as the hydraulic gradient, tidal range, and position of the freshwater-seawater interface change over time; it is possible that the conditions in any one area could shift (e.g., seasonally) between terrestrially governed and marine dominated systems (Burnett 2006). In attempts to distinguish between the mechanisms driving S G D in the near-shore, S G D characteristics such as flux rate and salinity may be monitored in addition to measurements of the S G D driving forces which may include hydraulic gradients and tidal (sea) levels. In this study, transient changes in differential fluid pressure heads in shallow marine sediments, S G D flux rates, and tidal levels were measured at a coastal site at Spanish Banks West beach in Vancouver, British Columbia. This site was chosen because it is in close proximity to The University of British Columbia, reasonable rates of S G D were measured at the site during preliminary tests, and it fulfilled many of the logistical demands for testing. The objectives of this study are to improve the accuracy of a new instrument developed to measure transient changes in vertical differential pressure heads in near-shore shallow marine sediments, and to investigate the role of tidal fluctuations in influencing rates of S G D using the measurements of transient changes in vertical fluid pressure heads, with direct measurements of S G D flux rates, and tidal levels.  1.2 B a c k g r o u n d and Motivation Within the last decade there has emerged recognition that at least in some cases, S G D may be both volumetrically and chemically important (Johannes  2  1980). One of the major objectives of the recent interest in S G D has been to provide both the scientific and coastal zone management communities with the tools and skills necessary to evaluate the influence of S G D in the coastal zone. The three principal approaches used to assess S G D are: (1) direct measurements using manual or automated seepage meters (e.g., Cable 1997a and 1997b; Burnett et al. 2003); (2) geochemical tracer techniques (e.g., Moore 1996; Moore and Wilson 2005); and (3) hydrogeological modelling (e.g., Bokuniewicz 1992; Smith and Zawadski 2003). Many studies have been conducted to evaluate S G D , however up until recently rarely was more than one approach employed in any one study and estimates of measurement uncertainty were almost never provided (Burnett et al. 2001). To improve the confidence in S G D assessments, intercomparison experiments have been performed that directly compare several of the independent measurement approaches (e.g., Burnett et al. 2002). These experiments have shown that careful measurements can accurately estimate S G D , quantify some of the driving mechanisms, and provide insight to the spatial and temporal scales at which these mechanisms operate. In this study I attempt to improve the accuracy and better constrain the errors associated with a new tool, referred to as the differential piezometer system (DPS-II), developed for measuring transient changes in vertical differential fluid pressure heads in the seabed. I also use measurements of the transient changes in vertical differential pressure heads by the DPS-II, in conjunction with S G D rates by an automated seepage meter to investigate the relationship the role of tidal fluctuations in influencing rates of S G D .  3  1.3 Key Q u e s t i o n s This work focuses on better understanding the mechanisms controlling S G D in the near-shore environment, specifically assessing the role of tidal fluctuations. The key questions are: (1) What is the relationship observed at the field site between measurements of sea level fluctuations due to semi-diurnal tides, transient changes in vertical differential pressure heads in shallow marine sediment and S G D rates? (2) What are the roles of transient changes in vertical differential pressure heads in shallow marine sediment and tidal level fluctuations in modulating S G D rates? (3) What are the best approaches that can be implemented in all aspects of the development of the DPS-II (e.g., design, deployment) to obtain accurate and reliable measurements of transient changes in vertical differential pressure heads by the DPS-II? (4) What is the uncertainty associated with field measurements of vertical differential pressure heads by the DPS-II?  1.4 Outline of T h e s i s The research questions stated above in section 1.3 were addressed, in part, by analyzing field measurements of S G D rates by a continuous heat type seepage meter, transient changes in vertical pressure gradients by the DPS-II and tidal levels. The field measurements were obtained from an experiment completed between October 26 - 28, 2005 which was the last field experiment of a  4  sequence of nine shorter tests (hours to 1 day) completed to refine field procedures and assess the performance of the seepage meter and DPS-II. The analysis of the data involved calculations using Darcy's Law and a 1D uniform density numerical model to better constrain the interpretation of the field data sets. An error analysis of the DPS-II was also completed to evaluate the error associated with the measurements by the tool and provide recommendations to minimize the error of future DPS-II measurements.  5  2. Review of S u b m a r i n e Groundwater Discharge 2.1 Introduction and  Significance  In addition to the advective flow of fresh groundwater driven by a hydraulic gradient between the land and sea, there are several oceanic processes that drive the flow of re-circulated seawater across the seabed. The term S G D has been used in different ways over the years, typically to include only freshwater (Zektser et al. 1983) or both freshwater and re-circulating seawater (Church 1996). A clear definition of S G D is important for two key reasons: (1) the ratio of S G D to total water flux into the ocean has a different meaning depending on whether S G D includes re-circulated seawater or not, and (2) S G D values compared to other freshwater discharge estimates to the ocean may lead to misunderstandings if re-circulated seawater is included.  A definition of S G D that is compatible to both terrestrial and oceanic derived flows is important, since either flow could be significant from a biogeochemical perspective. Consistent with recent definitions of S G D (e.g., Kim and Hwang 2002; Burnett et al. 2003; Taniguchi et al. 2004), in this study S G D is defined as all discharge of subsurface fluids across the land-ocean interface without regard to its composition (e.g., salinity), origin, or phenomena driving S G D .  S G D in the near-shore is typically diffuse, patchy and temporally variable and may involve multiple aquifers (Slomp and Van Cappellen 2004; Burnett et al. 2006). The composition of S G D is usually a mixture of waters reflecting different  6  fluid histories (Figure 2-1). Freshwater is a component of S G D that may discharge from a surficial aquifer driven by a hydraulic gradient between the land and the sea. The oceanic processes that drive the re-circulated seawater component of S G D are not as well understood and it is unclear the amount these processes contribute to S G D . In addition to S G D in the region beyond the low tide line, water will also drain from the seepage face that forms along the shoreline during the tidal cycle. This water includes both freshwater and seawater components (Smith and Zawadzki 2003).  Burnett et al. (2003) summarized some of the more obvious or important components and driving forces of S G D , as shown in Table 2-1. This table is a simplification of the relationships between contributing factors and driving forces. For example, the tidal range likely influences the tidal forcing of re-circulated seawater and would also affect S G D rates by modulating the hydraulic gradient in a coastal setting.  An annual recharge cycle causing a seasonal inflow and outflow of seawater within an unconfined coastal aquifer is a new concept introduced by a team at MIT (Michael et al. 2005). The group had shown earlier in a seepage meter experiment carried out in Waquoit Bay, Massachusetts that the groundwater discharge was largely saline (Michael et al. 2003). A seasonal shift in the freshwater-seawater interface in response to the annual recharge cycle was  7  proposed to explain the source and timing of the high flux of saline water recorded by the seepage meters.  In addition to the ecological impact of S G D in the coastal zone by groundwater inputs such as nutrients, heavy metals, radionuclides and organic compounds (La Roche et al. 1997), groundwater discharge may also influence seawater intrusion into coastal aquifers. Seawater intrusion is an important process that leads to groundwater salinization to levels exceeding acceptable drinking and irrigation water levels (Van Dam 1999). Due to increasing population growth and the fact that about 70% of the world population occupies coastal regions, such groundwater contamination is clearly a significant issue ( S C O R / L O I C Z Working Group 112 2000). Seawater intrusion into terrestrial aquifer systems and S G D are closely linked processes that directly affect each other. The freshwater component of discharge to the sea controls seawater intrusion in some conditions. However, the over-pumping of coastal aquifers increases seawater intrusion and may significantly reduce or eliminate S G D , or at least the freshwater component of discharge.  2.2 Measurement Approaches When S G D assessments are performed, the rate of groundwater discharge can be described in several different ways. One way that is widely used is the specific volume flux across the sea floor, typically expressed as cm/d (cm *cm" «-d~ ). 3  2  1  S G D has also been expressed as the total volume discharge per unit length of  8  shoreline per unit time (e.g., Lm" -d" ). Such assessments are difficult because 1  1  more measurements are required but are useful for extrapolating to larger areas (Burnett et al. 2003).  Seepage meters are the only direct method for measuring S G D . The lower bound for reliable measurements using seepage meters is about 2 cm/d (Burnett et al. 2002). The manual seepage meter (Lee, 1977), is essentially a chamber inserted open end down into the sediment (Figure 2-2). A plastic bag covers a small hole drilled into the top of the chamber and is used to collect water that is displaced from the chamber by S G D . The change in the volume of water in the bag over a measured time interval provides the S G D flux measurement. Studies involving seepage meters have reached the following general conclusions: (1) many seepage meters are needed because of the natural spatial and temporal variability of S G D rates (Shaw et al. 1990), (2) the resistance of the tube (Fellows et al. 1980) and bag (Shaw et al. 1989; Belanger et al. 1992) should be minimized to the degree possible to prevent artefacts; (3) use of a cover for the collection bag may reduce the effects of surface water movements due to wave, current or stream flow activity (Libelo et al., 1994), (4) caution should be applied when operating near the seepage meter detection limit (Cable et al. 1997) and, (5) artefacts may exist from pressure gradients developed by unidirectional currents passing over the meter (Shinn et al. 2002).  9  Manual seepage meters are a simple and inexpensive tool for assessing S G D in a coastal setting but a significant disadvantage of the meters is that they are labour intensive. In order to obtain S G D rates automatically and continuously, various types of automated seepage meters have been developed (Taniguchi and Fukuo 1993; Krupa et al. 1998, Sholkovitz et al. 2003; Paulsen et al. 2001). One example of such an approach is the continuous heat type meter developed by Taniguchi and Iwakawa (2001). The meter uses two thermocouples sensors and a heating wire positioned in a flow tube above an inverted chamber covering a known area of sediment (Figure 2-3). The basis of the method is to measure the temperature gradient of the water flowing between the thermocouple sensors that are located at downstream and upstream positions in the flow tube. Heat is continuously generated at the upstream position. The temperature gradient between the downstream and upstream sensors is dependent on the S G D flux rate. When there is no S G D flux, the temperature gradient in the flow tube is the maximum, and it decreases with increasing S G D . Once the system has been calibrated, measurements of S G D can be made automatically on a near continuous basis. The continuous heat type seepage meter can not measure recharge (i.e. inflow into the seabed). The expected output of the meter if it is deployed in a recharge zone would be measurements outside the range of the calibration curve for the meter (T. Ishitobi personal communication). The continuous heat type meter has successfully measured S G D over several days to months at a typical rate of about one measurement every 10 minutes (Taniguchi 2002; Taniguchi et al. 2004; Taniguchi et al. 2006). Measurement  10  rates of one measurement every 1 minute have also been recorded using the continuous heat type meter (Taniguchi 2005). In this study, a continuous heat type seepage meter is used to measure S G D rates over a period of about 48 hours at a rate of one measurement every 10 minutes.  The use of geochemical tracers offers a promising approach for regional assessments of S G D . Small scale variability is a serious problem for the use of seepage meters, but small spatial scale variations tend to be smoothed out over time and space in the case of tracer methods. The smooth measurement output is a result of the tendency of the marine water column to integrate the groundwater tracers entering via the aquifers. The use of geochemical tracers requires that all other tracer sources and sinks except groundwater be evaluated and this task can be difficult. Several studies have employed the use of the natural U-decay series nuclides  2 2 6  R a and  2 2 2  R n to assess S G D (Moore, 1996;  Burnett et al. 1996; Corbett et al. 1999; Kim and Hwang 2002; Burnett and Dulaiova 2003). Natural geochemical tracers should be greatly enriched in groundwater relative to coastal waters, be conservative and easy to measure.  Modelling to predict S G D can be categorized into three basic groups: (1) flow equations, i.e. analytical or numerical solutions of Darcy's Law; (2) mass balance approaches that usually consist of water or salt budgets; and (3) hydrograph separation techniques that examine the baseflow from streams and extrapolate the interpreted groundwater flow to the coastal zone. These approaches are  11  carried out on a variety of spatial scales and levels of complexity but all have certain limitations. Estimates of S G D using Darcy's Law often assume homogeneity of the aquifer when this is not the case. Although numerical solutions are capable of accounting for heterogeneity, it is difficult to obtain representative values of the hydraulic conductivity within an aquifer. Another source of uncertainty may result from the assumption of steady state conditions in some modelling analyses that may not necessarily apply especially considering the effects of tidal and density driven forces in the near-shore zone.  2.3 L o c a l and G l o b a l Estimates Locations of S G D estimates compiled by Taniguchi (2002) show that many studies have been performed on the east coast of the U S A , Europe, and Japan (Figure 2-4). No quantitative S G D data was located from South America, Africa, India or China, although indications of S G D were reported for India (Moore, 1997) and Kenya (Kitheka, 1998).  Many studies have observed that S G D decreases exponentially away from the shoreline. The world wide compilation of S G D estimates by Taniguchi (2002) indicated that most S G D rates are below about 10 cm/d. Considering the length of coastal shoreline, this S G D estimate could represent a significant quantity of discharge into the ocean. Various estimates of the role of S G D in the global water balance range over about three orders of magnitude (approximately 0.01 to 10 % of total river flow). Comparing S G D assessments is complicated because of  12  the differences in spatial scales, assumptions and artefacts associated with each method. A s well, one must be cautious about drawing too many conclusions from these S G D results. Most S G D measurements have been performed where it is easily detected and large volumes of S G D are expected. Therefore, the estimates may be more representative of maximum S G D values.  2.4 Relationship between S G D and Tidal Levels Temporal trends of S G D recorded by seepage meters typically show variations that correspond in timing to the tidal period in that area. In general, the timing of the S G D spikes relative to the tidal level varies depending upon the hydrogeologic setting. For example, Lee (1977) observed that S G D rates were distinctly higher at low tide in a coastal site in Beaufort, North Carolina, whereas, Burnett et al. (2002) showed that the highest S G D rates occurred during the transition from highest to lowest tide during an intercomparison experiment carried out at a site along the northeastern coast of the Gulf of Mexico. Taniguchi et al. (2006) concluded that measurements of S G D in the near-shore at a site near Kyushu Island, Japan, that showed a direct inverse relationship with tidal levels could be explained mainly by terrestrial groundwater, while offshore S G D that showed a lag time of three hours between highest S G D rates and low tide was controlled mostly by oceanic processes that caused the discharge of recirculated seawater.  13  Studies have shown longer-term (weeks to months) tidally-modulated cycles in S G D based on continuous measurements of the groundwater tracers radon and methane (Kim and Hwang 2002) and measurements by automated seepage meters (Taniguchi 2002). Taniguchi (2002) continuously recorded S G D rates in Osaka Bay, Japan from May to August 2001 and using a Fast Fourier Transfer (FFT) method was able to identify a semi-diurnal to diurnal variation in S G D as well as a semi-monthly variation in discharge reflecting the neap spring lunar cycle.  It may be possible to explain the S G D spikes corresponding to changes in tidal levels by factors other than tides. It is recognized that benthic chambers deployed on the seabed can cause local pressure perturbations that drive pore water flow (Huettel and Gust, 1992). Tidal currents could also be at a maximum during tidal transition periods suggesting that pressure induced flows may be responsible for these spikes. However, groundwater tracers measured in the overlying water column also show similar variations between S G D and tidal levels (Burnett et al. 2002; Kim and Hwang, 2002). Pressure induced artefacts in seepage meter data would likely not be present in the same manner for tracer assessments.  Superimposed on the tidally driven behaviour of S G D rate, are variations in hydrogeological properties (e.g., water table) that have been shown to have an influence on S G D rates. In a tracer study completed in Korea by Kim and Hwang .  14  (2002), S G D rates were more limited in the dry season when the aquifer was not recharging. These results show that despite it being typical to find a correspondence between tides and S G D rates in the near-shore, the overlapping nature of terrestrial and marine S G D forcing components can also be important.  15  Components  Driving F o r c e s  Contributing F a c t o r s  M e t e o r i c w a t e r s (fresh)  H y d r a u l i c gradient  T o p o g r a p h y , Transmissivity, Precipitation, E v a p o r a t i o n  Re-circulated seawaters (salt)  H y d r a u l i c gradient, T i d a l forcing, W a v e set-up  T i d a l r a n g e , period, frequency, Wind Force, direction  C o n n a t e w a t e r s (very salty)  Density, T h e r m a l gradient  G e o l o g y , G e o t h e r m a l heating  Table 2.1: Simplified relationships between components, driving forces and contributing factors of SGD (Burnett et al. 2003).  16  Figure 2-1: Schematic of a hydrogeological cross section showing a conceptual representation of SGD. Modified from Slomp and Van Cappellen 2004.  4-1 plastic bag Two-way valve with 0.9 cm inner diameter Threaded port  Seepage Figure 2-2: Schematic of a manual type seepage meter.  17  End section of 55-gallon steel drum  Sediment  SL  sea level  converter & data logger  converter & data logger  flow direction <?  sea floor  + t t  heater & thermistor B thermistor A  t  groundwater seepage Figure 2-3: Schematic of a continuous heat type seepage meter  • 12  341  •  3 S  ?»  _83_j  J%i  23  22  • 19.20  32 14  Figure 2-4: Locations of published investigations of SGD by seepage meters, piezometers, or geochemical/geophysical tracers. The numbers refer to Table I in Taniguchi (2002).  18  3. Instrumentation Development for Measuring Differential Fluid Pressure Heads 3.1 Application of Fluid Pressure Measurements to SGD Direct in-situ measurements of fluid pressures are one method to detect and quantify S G D . Using measurements or estimates of the aquifer permeability and fluid density, the flux rates of S G D are related to fluid pressure gradients through Darcy's Law written in terms of fluid pressure: kfdP q =  -  pg  — +  (Equation 3.1)  u^dz  where q is the specific discharge, k is aquifer permeability, u is fluid viscosity, dP/dz is the pressure gradient, p is the fluid density at the calculation point (and time) that S G D is to be determined, and g is the gravitational constant.  Alternatively, for a uniform density flow field, fluid pressure can be converted to hydraulic head using the relationship: h =—+ z  (Equation 3.2)  pg  where z is the elevation above a datum, and the calculation of S G D can be simplified by using the conventional form of Darcy's Law: q =- K ^  (Equation 3.3)  where dh/dz is the hydraulic gradient and K is the hydraulic conductivity. In addition to directly assessing S G D rates, data sets of fluid pressures or hydraulic gradients are useful as a calibration constraint in the development of S G D hydrogeological models.  19  3.2 Fluid Pressure Measurement Approaches Typically, the hydraulic potential is assessed at several depths in the sediment using multi-level piezometer nests (Freeze and Cherry 1979). It is relatively easy to install piezometers in shallow-water sediments by direct pushing and hammering. Piezometers have been successfully used in conjunction with seepage meters to explain the periodic fluctuations of S G D from measured hydraulic gradients and S G D flux rates (Barwell et al.1981 ;Taniguchi 1995). However, conventional piezometers may not be suitable for resolving the small hydraulic head gradients at some S G D sites (Smith and Zawadski 2003).  A variety of instruments to measure fluid pressure are used in geotechnical applications (Hanna 1985; Dunnicliff 1988). For measuring fluid pressures in marine sediments, the differential piezometer design is likely the most relevant of the tools used in geotechnical applications and offers an advantage over conventional piezometers. To continuously monitor fluid pressures in the sediment using a conventional piezometer, a pressure transducer may be used to measure the water level above the screen of the piezometer. The accuracy of a pressure transducer is usually expressed as a percentage of the full scale (FS) of the instrument. The F S of the instrument refers to the range of signal input the transducer is intended to measure (i.e., a pressure transducer with a F S of 1 PSI is rated to measure a range of pressures from 0 to 1 PSI). Because of the head of seawater, using a conventional piezometer to measure fluid pressures in the seabed requires the F S of a pressure transducer to be larger and thus increases  20  the error of the instrument. At some S G D sites, an error associated with a measuring instrument on the order of centimetres may approach the magnitude of the small changes in fluid pressure gradients and/or the small hydraulic gradients that are resolved from the fluid pressure measurements. The differential piezometer design used in this study overcomes the limitations of resolving small changes in fluid pressure gradients from independent measurements of fluid pressures by making a direct differential fluid pressure head measurement between two ports on a piezometer that is driven into the seabed. Since the early 1970s, several designs of differential pressure piezometers have been developed as a means to analyze soil strength (Bennett etal.1979; Davis et al. 1991; Schultheiss 1989; Anderson e t a l . 1996). The majority of these instruments were used to assess fluid pressures in relatively deep-water sediments requiring specialized installation techniques.  Although less well documented and with fewer piezometers developed than for the deep sea, attempts have also been made to measure fluid pressures in relatively shallow waters using the differential piezometer approach. The lack of literature and limited success using shallow water differential piezometers likely reflects fewer applications in the past for highly accurate shallow marine fluid pressure measurements.  There were three generations of Portable In-situ Pore Pressure Instruments (PISPPIs) developed at the University of Lehigh in Pennsylvania, U S A . The  21  PISPPI-3, shown in Figure 3-1, was designed exclusively for shallow water. The intent of the PISSPI-3 was to determine S G D across the sediment-water interface, for which measurements were needed of both permeability and hydraulic head. The instrument utilizes a differential pressure transducer to monitor the pressure gradient between the sediment water interface and a depth in the sediment determined by the penetration of a probe. Insertion of the probe into the sediment induces a transient excess pressure field around the probe and upon dissipation of this pressure pulse, a small electric pump and a series of valves are activated to conduct a pump or slug test. The instrument also collects a small sample of pore water for subsequent chemical analysis. The probe is hydraulically connected to the pressure transducer through a carrying case left on the seabed and also a datalogger attached to a buoy. The only documentation for the PISSPI-3 is the User's Guide and Technical Manual. The project to develop the instrument was completed in 2001 (B. Carson personal communication).  Figure 3-2 shows the Diver Operated Pore Pressure Instrument (DOPPI) developed by G E O T E K Ltd in the United Kingdom. The only testing of the DOPPI was by G E O T E K Ltd in the Adriatic S e a where many problems were encountered as a result of nearby trawler activity. The instrument is no longer in operational condition (P. Schulthesis personal communication).  22  A differential pressure sensor instrument was developed for the Geological Survey of Canada to investigate slope failures in the Fraser River Delta. A small consulting company, Adara Consulting in Vancouver, designed the instrument. The differential pressure sensor consisted of two probes and a data acquisition system. The probes were inserted by divers into the seabed to a depth of about 3 to 5 m and each measured the fluid pressure at the seafloor and at a point in the seabed. Several data sets were obtained but information is not readily available about the quality of the measurements. Weather and funding problems prevented the recovery of either of the probes (G. Jolly personal communication; T. Lightfoot personal communication).  Pore pressures and seepage flux rates were measured and compared to tidal levels at a site near Kurobe, Japan (Urakoshi et al. 2003). Pore pressures were measured for 25 days using a newly developed multi-depth pore pressure instrument (Figure 3-3). This device measures pore pressures independently at the slit of two hollow lances that are imbedded into the sediment, using absolute pressure transducers positioned on the seabed. Pore pressure gradients are calculated from the differences in the pore pressure measurements. A disadvantage of this configuration is that errors in the installation depth of the lances will directly influence the interpretation of the data. Obtaining accurate measurements of the installation depth of the lances is a challenging aspect of the design (T. Urakoshi personal communication).  23  3.3 Description of DPS-I A new tool, referred to as the differential piezometer system (DPS-I), was developed within the Earth and Ocean Sciences Department at U B C for the measurement and short term monitoring of vertical hydraulic gradients in nearshore, shallow marine sediments (Caulkins, 2003).  The DPS-I consisted of two piezometers and a control box housing the electronics used for data acquisition (2 differential pressure transducers, 4 resistors, 1 datalogger, and 1 power supply). The piezometers were approximately 1 m (3.3 ft) in length, and 4.5 cm (1 % in) outside diameter (OD) and were designed to permit installation by sledge hammering into sandy sediments. Each piezometer was constructed of 0.3 m (1 ft) length steel drill rods and contained two measuring ports with screened diameters of about 6 mm spaced 31 cm apart. By the addition of short lengths of drill rods, this separation distance could be adjusted. The measuring ports of a piezometer were hydraulically connected through plastic tubes filled with deaired water to the pressure sides of a capacitance-type differential pressure transducer (manufactured by Setra, model 230, range +/- 0.5 PSI) mounted inside the control box. Deaired water was used as the hydraulic fluid in the tubes to minimize the potential for air bubbles within the tubes that could affect the accuracy of the measurements. Deaired water was obtained by boiling tap water for 15 minutes, and subsequently cooling and storing the water in 4L glass jars.  24  Each piezometer was connected to a separate transducer, providing independent measurements of the vertical hydraulic gradient.  In addition to short preliminary tests, the DPS-I was deployed four times over the course of a three week experiment in August 2002. In this experiment, installation of the DPS-I system began just after high tide and took five to six hours for completion. Measurements at the sites began immediately after installation of the system. Data were recorded with a Campbell Scientific C R 1 0 X datalogger that measured the voltage output (linearly related to differences in hydraulic head using laboratory calibrations) of the differential pressure transducer. The control box housing the datalogger and transducers did not have a water tight seal and the electronics were not submersible. To prevent submersion of the control box during deployment, it was positioned on a scaffold that was erected to a height above the maximum expected sea level (Figure 3-4).  3.3.1 Field Results using DPS-I Prior to the first field experiment that implemented the DPS-I to estimate vertical hydraulic gradients in shallow marine sediment (Caulkins, 2003), the performance of the tool was tested by Caulkins (2003) in convenient near-shore locations close to The University of British Columbia in Vancouver, British Columbia (e.g., Spanish Banks, beaches below Burrard Street bridge). Satisfactory hydraulic gradient data were obtained from preliminary tests (2-3 hours in duration) using the DPS-I.  25  The first field experiment implementing the DPS-I was completed between August 17 to September 7, 2002. The purpose of the experiment was to investigate the influence of tidal levels fluctuations on rates of S G D using measurements of vertical hydraulic gradient, S G D and tidal level. The experiment was conducted in an area adjacent to the Florida State University Marine Laboratory (FSUML) at Turkey Point, Florida, in the northeastern coast of the Gulf of Mexico. This site was chosen to enable data comparisons with an S G D intercomparison study previously completed at the site in August of 2000 by members of the Scientific Committee on Oceanic Research Working Group 112 (Burnett e t a l . 2002).  During the 2002 experiment the DPS-I was installed at two sites located roughly 25 and 80 m offshore of the low tide line. Two tests were completed at each site, with the tests lasting between two and nine days. The quality of the hydraulic gradient data collected using the DPS-I was too poor to be used for the purposes of the field experiment. The discrepancy between the performance of the DPS-I during preliminary tests and the field experiment was likely in part a result of the different testing duration. A s the field experiments were longer than a few hours, the DPS-I may have been affected by significant environmental conditions such as temperature fluctuations, wind and waves that were not present during the brief preliminary tests conducted in Vancouver.  26  From an analysis of the 2002 field experiment using the DPS-I, completed in this study, as well as considering the sources of errors identified by Caulkins (2003), the main issues identified with the field data collected using the DPS-I during the field experiment were:  1. The measurement error w a s the same magnitude as the measurements. A zero shift of -2 cm for transducer I and -1 cm for transducer II was found between the laboratory calibrations of the transducers undertaken prior to deployment and after completion of the field experiment. The time or reason was not known for the zero shifts and consequently, a regression analysis was performed to account for the shifts. The regression analysis yielded a single calibration curve that was used to interpret the field data and also an error that was associated with each field data point. At a 95% confidence interval, the regression analysis error for the field data for transducer I was +/2.1 cm and for transducer II was +/-1.4 cm. The magnitudes of these errors were significant in comparison to the small hydraulic head differences (averages of 0 to 1 cm using a measuring port separation of 31 cm) measured in the field by the DPS-I. The temperature fluctuations experienced by the electronic components of the DPS-I were suggested as the most likely cause of the zero shifts. The electronic components of the DPS-I (transducers, resistors, datalogger) that were positioned on the scaffold in a closed control box, experienced daily temperature fluctuations over the course of the experiment. A temperature increase from about 25°C at sunrise  27  to 45°C in the late afternoon was recorded by a temperature probe inside the control box. The performance of transducers and resistors can be affected by temperature changes but the extent of the influence is specific to the electronic device. The potential effects of temperature for the DPS-I were not quantified. 2. The significant noise in the field data prevented recognition of the true signal. The significant noise present in the data collected using the DPS-I was considered to be a result of wind and wave disturbances to the exposed portions of the fluid filled tubes. The fluid in these tubes transmits the formation pressures from the measuring ports of the piezometer to the differential pressure transducers housed in the control box. The exposed tubes were shielded using stilling tubes filled with sand and flexible plastic tubing but this may not have been adequate to satisfactorily dampen the winds and wave action at the site. 3. The small hydraulic gradients at the site were inherently difficult to resolve. The first two tests at the site used a measuring port separation distance of 31 cm, with the bottom port at a depth of roughly 0.7 m below the seafloor. This separation distance was predetermined prior to deployments and believed to be sufficient to resolve the hydraulic head gradients at the site. The separation distance was constrained by the steel drill rods that were available in 0.3 m (1 ft) lengths. Because small hydraulic head differences were being recorded at the site that were the same magnitude as the accuracy of the DPS-I, the 31 cm port separation distance was increased for  28  the last two tests to 62 cm. Although larger hydraulic head differences are produced using an increased measuring port separation, the drawbacks are that the assumption of a linear hydraulic gradient between the measuring ports is more uncertain, density variations may exist preventing the application of the conventional form of Darcy's Law and the complete insertion of the piezometer may be more difficult. 4. Early termination of testing. Higher than expected water levels as a result of strong waves forced the removal of the control box housing the electronics and consequently early termination of measurements during the last of the four tests. The control box positioned on the scaffold above the high tide level permitted periodic collection of the DPS-I data during testing, however the electronics were also susceptible to water and other environmental damage.  3.4 Description of DPS-II A key objective of this study was to determine the best approach that would enable accurate field measurements of the small transient changes in vertical differential pressure heads in near-shore shallow marine sediments. To achieve this goal, an attempt was made to improve the accuracy of the DPS-I by recognizing the strengths and weaknesses of the system identified during the F S M U L tests. The instrument used in this study to measure vertical differential fluid pressure heads in shallow marine sediments is a modified version of the DPS-I, referred to hereafter in this study as the DPS-II.  29  3.4.1 Design Overview The main components and general installation approach of the DPS-I design were utilized by the DPS-II. However, to improve the performance of the system, particularly with respect to accuracy, the limitations of the system identified in the 2002 field experiment were addressed as follows: 1. An error analysis of the D P S - II system was completed in an attempt to identify, quantify and minimize or eliminate all known factors adversely influencing the performance of the DPS-II. 2. Modifications to the calibration procedure were carried out to improve the accuracy and reliability of the calibration curves. 3. During testing, the electronic components of the DPS-II were housed in a submersible box located on the seabed rather than on a scaffold above the high tide level. The intent of this modification was to stabilize the temperature experienced by the electronics during DPS-II deployments as well as minimize the noise associated with the movement from wind and waves of the fluid filled tubes of the DPS-II. 4. A total of nine trial type tests of the DPS-II, lasting between one and nineteen hours, were completed at near-shore locations close to U B C between May 18 and September 14, 2005. The main goals of the tests were to improve the performance of the DPS-II by identifying problems with the installation method or field data collection and implementing suitable modifications.  30  The DPS-II consists of two piezometers (Figure 3-5) and a submersible box that houses two differential pressure transducers, four resistors, a datalogger and a power supply (Figure 3-6). The piezometers are roughly 1 m (3.3 ft) in length and 4.5 cm (1 YA in) O D and are designed to permit installation by sledge hammering into sandy sediments. The piezometers are constructed of 0.3 m (1 ft) A W steel drill rods with two screened measuring ports of about 6 mm diameter, spaced 31 cm apart. The top port of the piezometer is approximately 30 cm below the seafloor, when the piezometer is inserted into the seabed. The depth to the top port (30 cm) corresponded to the length of a drill rod section that is added above the top port of the piezometer. The 30 cm distance between the seabed and the top port of the piezometer was assumed to be a sufficient depth to allow for pressure perturbations caused by small waves to attenuate and not affect the fluid pressure measurements at the top measuring port (i.e., the top port of the piezometer is assumed to be affected by sea level fluctuations caused by tides but not small passing waves). Formation pressures are transmitted from the top and bottom ports of the piezometer, via fluid-filled, low density polyethylene tubes to a capacitance type differential pressure transducer (manufactured by Setra Systems, model 230, range +/- 0.5 PSI) housed inside a submersible box placed on the seabed (Figure 3-7). Valves located near the positions of the transducers in the submersible box, prevent the deaired water from draining out of the plastic tubes in the field set up of the DPS-II.  31  Deaired water is used as the hydraulic fluid to minimize the likelihood of air bubbles and thus discontinuities in the fluid filled tubes that can cause erroneous differential fluid pressure head measurements. The effect of air bubbles on measurements by the DPS-II is discussed in detail in the error analysis of the instrument (section 3.5). Deaired water is prepared by reducing the dissolved gas content (i.e. de-airing) of Vancouver tap water using an instrument called the Nold Deaerator, shown in Figure 3-8. The Nold Deaerator consists of a sealed tank, electric motor, impeller and water powered aspirator and operates by mechanical agitation with the application of a vacuum. The agitation/vacuum system is significantly more efficient at removing dissolved gases from water than conventional boiling methods. The dissolved oxygen (DO) level of six litres of water was reduced from air saturation (about 8 ppm) to 1-2 ppm in about 5 minutes using the Nold Deaerator. The deaired water is stored in completely filled 4L glass containers.  The differential fluid pressure values over which the differential pressure transducers of the DPS-II are intended to measure (i.e., range) is specified by the manufacturer specifications as +/- 0.5 PSID. An explanation of the terminology used by the manufacturer specification to express the range of the transducer is required. Differential transducers are designed to simultaneously measure two independent pressure sources; the output is proportional to the pressure difference between the two sources. The +/- part of the expression for the range of the transducer indicates that the greater input pressure can be applied to  32  either pressure port (i.e., a bidirectional transducer). The value of the highest differential pressure the transducer is designed to measure is 0.5 PSI, which converts to a differential fluid pressure head of 35 cm of water at 4°C. A standard temperature used to express pressure in terms of centimetres of water is 4°C. There are insignificant changes in the conversion between pressure in terms of PSI and centimetres of water when the density of water (e.g., fresh vs. seawater) and/or temperature of the water (e.g., 4°C vs. room temperature) are considered. From hereafter in this study, fluid pressure head expressed in terms of centimetres of water will omit the reference to 4°C. The letter D that follows the PSI term refers to differential for the type of pressure transducer.  3.4.2 Data A c q u i s i t i o n S y s t e m Data from the differential pressure transducers are recorded with a 16 bit channel datalogger (CR10X manufactured by Campbell Scientific). Four 10 K Ohm resistors (1/4 Watt, +/- 2% tolerance, manufactured by NTE) create a voltage divider that provides a common range over which the transducers can transmit and the datalogger can receive data. A 12 volt rechargeable lead acid battery provides the necessary electrical voltage for the operation of the transducers and datalogger. A differential pressure measurement is obtained by comparing the pressure at the high pressure side relative to the low pressure side of the differential pressure transducer. Therefore, if the high pressure side of the transducer is coupled to the bottom piezometer port, an upward pressure  33  gradient is indicated when positive differential pressure measurements are recorded.  3.4.3 Differential Fluid P r e s s u r e Heads from T r a n s d u c e r s The differential pressure transducers output a linear voltage that is proportional to the applied pressure. The voltage outputs of the transducer recorded in the field are related to differential fluid pressure heads (duj) using a calibration curve. The outputs of the transducer could have been related to another expression for pressure (e.g., PSI), but the choice of pressure head was a result of using the difference between the heights of columns of water to apply the differential pressures to the transducers to develop the calibration curves. The term calibration, when used in association with transducers, refers to a test during which a known value is applied to the input of the transducer for the purpose of observing the system output. By the application of a range of known values to the input and observation of the system output, a calibration curve for the measurement system is developed. Details of the calibration procedure are found in Appendix A. The general calibration curve is represented by the following form: v = Cxd P + v x  0  (Equation 3.4)  where V is the voltage output of the differential pressure transducer, C is the calibration factor (slope of the best fit line), dqj is the applied differential pressure head to the transducer, and V is the voltage output corresponding to an applied 0  differential pressure head equal to zero.  34  As a result of the measurement system including two differential pressure transducers (each corresponding to a piezometer) with independent inputs and outputs, a unique calibration curve was developed for each transducer. The calibration curves developed before and after deployments are used to interpret the voltage output recorded by the DPS-II in the field as differential fluid pressure head (Figures 3.9 and 3.10). There is typically no significant difference between the calibrations completed prior to and after deployment. The average of the calibration curves developed for each transducer (differentiated in this study as transducer A and B) before and after the field deployment in this study were y = 35.4x + 1220 for transducer A and y = 35.5x + 1478 for transducer B. The yintercepts of the calibration curves for the transducers are different due to the unique conditions for each transducer such as the manufacturing process, material components, previous working conditions and age.  During calibration of the DPS-II, known fluid pressure heads are applied directly to the pressure sides of the differential pressure transducers using columns of waters as opposed to the loading of the transducers in the field configuration, where pressures are applied to the sensing element of the transducer via fluid filled tubes (Figure 3.11). The method for calibration was chosen because of the ease of implementing in the laboratory and relatively high accuracy of the calibration method. However, in the field configuration, the fluid filled tubes exert a constant differential pressure on the pressure sides of the transducer that is not accounted for when the calibration curves are directly used to relate the voltage  35  output from the transducers to differential pressure head. The constant differential pressure head caused by the fluid filled tubes is a result of the vacuum conditions that are created by the valves preventing the deaired water in the tubes from draining under gravity and is equal in magnitude to the vertical distance between the measuring ports of the piezometer. The constant differential pressure head of -31 cm exerted on the transducers was observed during laboratory tests using the DPS-II. In the laboratory, with the bottom measuring port of the piezometers connected to the high pressure side of the transducer and both measuring ports open to the atmosphere, a constant differential pressure head of -31 cm was recorded as the output when an output of 0 would have been expected considering only the pressures acting on the measuring ports of the piezometer (based on a port separation of 31 cm and a vertical orientation of the piezometer). When the piezometer was placed in a bucket of saturated sand with both measuring ports of the piezometer within the sediment, the differential pressure head output recorded was zero (output of 31 cm expected).  After the DPS-II has been deployed in the field, the voltage outputs of the transducer are converted to differential fluid pressure heads using the calibration curves. To yield only the differential fluid pressure heads contributed by the formation pressures in the field, 31 cm is added to each differential fluid pressure head measurement to correct for the constant differential pressure exerted by the fluid filled tubes.  36  3.4.4 Deployment A small inflatable boat was used for the deployment of the DPS-II during this study. The equipment used for deployment was positioned inside the boat and the piezometers were driven into the seabed off the side of the boat while they were attached through the plastic tubes to the differential pressure transducers mounted inside the submersible box (Figures 3-12 and 3-13). The total time required for the complete deployment (not including preparations at the field site) of the DPS-II is on average 0.5 to 1 hour. To ensure the collection of valid data, the following sequence of steps for deployment is completed: 1. Piezometer 2. Piezometer  Assembly Insertion  3. Bleeding of the Pressure Ports of the Transducer 4. Submersion of Data Acquisition 5. Differential Pressure Head  Piezometer  Components  Response  Assembly  Both piezometers are constructed in the laboratory prior to field deployment as shown by Figure 3-5.  Piezometer Insertion For the purposes of data comparison and reliability, differential pressure heads are recorded between the measuring ports of two piezometers. If required, the DPS-II can be deployed using only one piezometer. Typically the piezometers  37  are inserted reasonably quickly (5-10 minutes) into fine grained to sandy sediments by sledge hammering. This installation technique may not be practical for some coarse grained sediments because small sized stones may make it difficult to insert a drive point piezometer.  In this study, insertion of the piezometers is at low tide, when the water level is at or near the sediment surface. The hydraulic tubes leading from the measuring ports of the piezometers are connected to the pressure sides of the differential pressure transducers through the submersible box using swagelok fittings. Deaired water is pumped using a peristaltic pump (Masterflex portable sampler manufactured by Cole Palmer) from a 4L glass container through the manifold in the submersible box past the pressure sides of the transducer and through the plastic tubes until the water exits out the measuring ports of the piezometer (Figure 3-14). De-aired water is pumped through the system to prevent the measuring ports from clogging with sediment, while the piezometer is held vertically and hammered into the seabed.  Deployment of the DPS-II at high tide was attempted in a preliminary test using the DPS-II, however several difficulties were encountered because of the higher water levels that were: (1) the distance from shore the DPS-II could be installed was restricted and consequently the piezometers was inserted closer to shore and became uncovered (preventing measurements) during the succeeding low tide, (2) the sediment closer to shoreline at high tide was coarser than a few  38  meters seaward and hammering the drive point piezometers into the sediment was difficult because of obstruction from small rocks, (3) a longer piezometer (greater than 1 m) would have been more appropriate to permit the top measuring port of the piezometer to be driven to a sufficient depth (top port 30 cm below the seabed) without hammering the piezometer below the water level, (4) longer lengths of tubing between the transducers and pressure measuring ports of the transducer were required during deployment which could cause the DPS-II system to be more prone to noise from the random motions of the tubing, however the test did not last long enough to investigate noise from tubing movement and, (5) submersing the box housing the data acquisition components of the DPS-II on the seabed following the insertion of the piezometers had be done underwater and was cumbersome.  Bleeding of the Pressure Ports of the Transducer Air bubbles in the plastic fluid filled tubes of the DPS-II that are used to hydraulically transmit the formation pressure to the transducers and/or the pressure cavities (the space that is exposed to fluid and is in contact with the diaphragm of the transducer) of the transducers can cause measurement artefacts. The bleeding of the pressure ports of the transducers allow air bubbles to escape from the fluid filled tubes or pressure cavities of the transducer. After a piezometer has been driven into the sediment, the pressure ports of the transducer hydraulically connected to the installed piezometer are immediately bled. Deaired water is continuously pumped through the system to pressurize the  39  differential pressure transducers, while the pressure ports of the transducer are bled. The procedure involves backing off the bleed screws until only bubble free liquid flows out. The Setra model 230 transducer installation guide provides information about bleeding the pressure ports of the transducer. After the pressure ports of the transducer have been bled, the two valves on the manifold leading to that transducer are closed and the pumping of deaired water through the lines of that transducer is stopped. The two valves regulating flow to the other transducer are then opened and the insertion of the second piezometer is started. Once both piezometers have been installed, the power source is connected to initiate differential pressure head measurements and the data acquisition components are sealed in the submersible box and submerged.  Submersion of Data Acquisition  Components  The box housing the electronic components of the DPS-II is constructed of 1.9 cm (0.75 in) thick P V C plastic. The lid of the box has an o-ring fitted into a groove within the bottom of the lid and is secured to the base of the box, using 24 bolts, to provide a water proof seal for shallow water deployments of the DPS-II. To submerse the box on the seabed the box is strapped in a steel crate, machined to fit the dimensions of the box. The bottom of the steel crate is weighted down using lead shots stored in sealed plastic containers. The steel crate holding the control box is positioned at approximately a meter away from the inserted piezometers to minimize any measurement artefacts caused by the possibility of flow perturbations resulting from the heavy lead weights and/or surface area of  40  the box and crate. The exposed portion of the two fluid filled tubes leading to each piezometer from a differential pressure transducer in the submersible box is bound together using zip ties to minimize random movements of the hydraulic tubes.  Differential Pressure  Response  Differential fluid pressure head data is collected during this stage of the deployment. The beginning portion of the field data sets by the DPS-II (up to 11.5 hours) may be omitted to account for disturbances caused by working in the water after installation. Often the deployment of other instruments (e.g., seepage meter) nearby the installed DPS-II is completed after DPS-II measurements have been initiated.  3.5 Error A n a l y s i s An important part of the development of the DPS-II included efforts to minimize the error associated with measurements by the DPS-II and evaluating a reliable estimate of the upper bound of the errors. Dunnicliff (1989) examines the various types of errors that can affect field measurements and provides suggestion of how they may be minimized. The fundamental performance characteristics of transducers and also design related considerations are addressed by Horton (1989). The measurements by the DPS-II are affected by three main types of errors: (1) systematic errors, that refer to an error that is present for every measurement and may be caused by an observation bias, calibration etc., (2)  41  (  random errors, which are a result of random fluctuations in the measured value that yield different results each time the measurement is repeated, and (3) conformance errors, caused by the instrument altering the parameter being measured (e.g., drainage paths along the piezometer that reduce measured differential fluid pressure heads below the "true" value).  Sources of errors have been identified in the development of the DPS-II. A s shown by Figure 3-15, sources of errors were categorized based on the main stages of DPS-II development that are: (1) choice of instrument, which considers the accuracy of the two Setra model 230 differential pressure transducers that were used in the design of the DPS-II, (2) calibration of the transducers, which is particularly important since the calibration of the earlier version of the DPS-II (DPS-I) was a significant issue pertaining to the poor quality of data obtained using the tool (section 3.3), (3) deployment of the DPS-II, which is one of the most difficult aspect of the error analysis because to identify and/or quantify the errors requires the most experience using the tool, and (4) site conditions, which examines the environmental conditions (e.g., temperature fluctuations, wind and waves) that could adversely affect the DPS-II measurements. The key factors regarding the performance of the DPS-II have been identified within each of the DPS-II stages of development and are discussed in more detail below.  42  Choice of electronic  instruments  The two Setra 230 differential pressure transducers were chosen for use in the design of the DPS-II because they met several constraints relating to the field measurement of differential fluid pressure heads such as a suitable measurement range, relatively high accuracy and reasonable cost. In addition, the Setra 230 differential pressure transducers were a convenient choice as two of these transducers were already available from the design of DPS-I. Two additional Setra 230 differential pressure transducers have since been purchased (one transducer from the DPS-I malfunctioned during laboratory tests in this study and the other is used as an extra) and consequently the transducers used in this experiment were not the ones from the DPS-I.  The accuracy of the differential pressure transducers used by the DPS-II is expressed by the manufacturer specifications as the combined error of nonlinearity, non-repeatability, and hysteresis with a value of +/- 0.25 % F S . The F S of the transducers is 1 PSI, which converts to a pressure head of 70 cm of water. The corresponding accuracy for the given F S of the differential pressure transducers is +/- 0.2 mm of water. Non-linearity, non-repeatability and hysterisis are defined as follows: (1) non-linearity is the maximum deviation of the calibration curve from a specified straight line, (2) non-repeatability refers to the ability of the transducer to reproduce output readings when the same measurement value is applied to it consecutively under the same conditions and, (3) hysterisis, is the maximum difference in output at any measurement value,  43  within a specified range, when the value is approached first with an increasing and then with decreasing measurement values. Since the accuracy and precision of the calibration curve used to interpret the field data is affected by non-linearity and non-repeatability and the field measurements of differential pressure head show that the transducer is loaded by an increasing and decreasing pattern, all three measures of the performance of the transducers are relevant. Manufacturer specifications of pressure transducers are a general guide for the performance characteristics of a given model of differential pressure transducer. For the basis of comparison with the manufacturing specifications, a laboratory test, which assessed the non-linearity, non-repeatability and hysterisis characteristics of the differential pressure transducers used by the DPS-II, was performed at room conditions. The test was carried out by loading the transducers with slow changes in differential pressure head over the full range of the transducer, unloading the transducer and than immediately re-loading the transducer (Figure 3-16). The loading, unloading and reloading curves, (which are fitted to a best fit line) were all linear and in close agreement, reflecting an accuracy in good correspondence or better than the manufacturer specifications of accuracy of +/2 mm.  Drift is a common characteristic of transducers and can be a problem for obtaining high accuracy measurements, however the extent of the drift experienced by a transducer is specific to the type and testing conditions of a particular transducer. Defined as the change in output under constant loading  44  conditions, drift may occur due to several factors such as a change in voltage supply. Drift was characterized by measuring an applied zero differential pressure head to the transducers every 1 s for 24 hrs at room conditions. Figure 3-17 shows the 1 s measurements over the first 8 hours of the experiment. The variation in the output of the transducers is about 0.3 cm. Figure 3-18 shows 1 minute averages of the 1 s measurements over the entire experiment. It should be noted that for each plot the measurement values are rounded to the nearest mm.  Calibration The errors of the calibration method completed at room conditions are: (1) an error of  mm that is associated with reading the applied differential pressure  head using the rulers attached beside the water columns during calibration of the transducers, (2) the accuracy of the transducers that are specified (and supported by lab tests) as about +/- 2 mm, and (3) variations in the output of the systems between +/-1 to 3 mm for a given applied input caused by slight changes in the shape of the meniscus of the water columns used to apply the differential pressures to the pressure sides of the transducers. To provide consistency between calibrations an effort was made to maintain the same shape of meniscus at each applied pressure using the water columns. Given that the shape of the meniscus can cause variations of +/- 3 mm, by maintaining a consistent meniscus, the calibration curves may have a bias of +/- 3 mm. The  45  accuracy of the calibration curves developed for each transducer was thus estimated as +/- 3 mm (the largest error associated with the calibration method).  The repeatability of the calibration method is about +/-1 mm. Calibrations completed within a few days and sometimes weeks (even after they have been deployed in the field and susceptible to stress) are within +/-1 mm (Table 3-1).  Deployment An air bubble of sufficient size in the vertical segment of the fluid filled tubes of the DPS-II can cause erroneous measurements. A few small pin sized bubbles about the diameter of 1 mm or less in the tubing of the DPS-II did not appear to be significant during testing in the laboratory, although numerous bubbles of this size could form a larger air bubble. At each air/water meniscus, surface tensions cause the air bubbles to be at a slightly higher pressure than the water and these air/water menisci oppose the transmission of the formation pressure causing a pressure different than the formation pressure to be sensed by the transducer. As well, the contraction or movement of air bubbles in the system can cause fluid motion that can produce false changes in formation pressure head measurements. Errors on the order of at least centimetres were recorded during laboratory tests when air bubbles were visible within the system. The primary methods used to reduce the likelihood of air bubbles in the fluid filled tubes are: (1) the use of deaired tap water as the fluid in the hydraulic tubes, (2) colouring the deaired water with a dye to easily observe air bubbles in the visible portions  46  of the fluid filled plastic tubes of the DPS-II (i.e., the tubing not enclosed by the piezometers), and (3) bleeding of the pressure ports of the transducers during deployment of the DPS-II to eliminate air in the hydraulic tubing and pressure cavities of the transducers. No air bubbles were observed in the visible portions of the fluid filled plastic tubes either after installation of the DPS-II or completion of the field deployment.  During the insertion of the piezometers in the seabed, deaired water is constantly pumped out the measuring ports of the piezometer to prevent sediment from clogging the ports. Although clogged ports, itself, may not interfere with the measurement of differential fluid pressure, if the sediment clogging the ports is of a significantly lower hydraulic conductivity than the surrounding sediment at the installation depth, false measurements of differential pressure may be recorded. There is no method to determine if clogged ports are a source of error during field tests. Increasing the rate of flow discharging from the ports during installation of the piezometers may minimize the potential for clogged ports. A higher flow rate, however, increases the likelihood of creating temporary drainage paths along the sides of the piezometers that would reduce the measured differential fluid pressures to below the "true" value. Water that is pumped out of the piezometer ports at a higher rate than can be easily transmitted through the formation sediment will likely flow along the side of the piezometer. The discharge of water at the surface of the seabed from along the piezometer has been observed during installation of the piezometers in the  47  preliminary field deployments and during the deployment in this study (Figure 319). It is assumed that if preferential flow paths are created during installation they will be eliminated over a short period of time due to the relatively permeable marine sediment and dynamic action of the waves that will likely cause the sediment to collapse against the piezometers.  An aspect of the installation method that has not been satisfactorily addressed is whether the act of inserting the piezometers causes a significant increase in fluid pressures in the sediment and if so the length of time required for dissipation of the fluid pressure spike. Because of the relatively slow rate of installation, small diameter of the piezometer and the permeable marine sediment, installing the piezometers likely does not cause a significant insertion pressure. No measurements of fluid pressures in the seabed have been made to monitor fluid pressures during piezometer insertion. It may be difficult to use the differential fluid pressure measurements by the DPS-II, even if a long data set was obtained (e.g. 7 days), to observe if there is a decline in fluid pressure in the seabed over time after the installation of the piezometers, because the measurements are differential.  Site Conditions A deployment configuration has been chosen that would minimize the extent of environmental sources of errors. The box housing the electronic components of the DPS-II was submerged on the seabed, thus providing fairly stable temperature conditions for the electronics. Because both the zero output and 48  sensitivity of the differential pressure transducers can change with temperature, a stable temperature environment ensures the most stable measurements. It should be stressed the resistors used in the configuration of the electronics of the DPS-II are also susceptible to changes in performance given a change in temperature. Because the calibration curves for the measurements by the DPS-II are developed at room conditions but the tool is used at temperatures away from room temperature in most cases, the calibration curves need to be corrected, if temperature has a significant effect on the output of the transducers.  The operating temperature range of the differential pressure transducers is from - 1 8 to 80°C, which indicates the limits within which the transducer will not be damaged. The compensated temperature range, -1 to 65°C, is the range in which the pressure transducer will meet the specifications for zero and sensitivity (slope of calibration curve) shift as given in the data sheets. The thermal effects of the transducers have been compensated by the manufacturer to reduce the thermal zero and sensitivity shift to a maximum of + 2 % F S (+1.4 cm of water) given a change in operating temperature between 0 - 50°C from room temperature. To restate in another way, the thermal specification indicate that the maximum shift that could be expected is + 1.4 cm (although the shift could be less) and could occur for a temperature change of 1°C or 50°C from room temperature, depending on the transducer. Because of variations such as material properties and manufacturing processes, the performance of a group of the Setra 230 differential pressure transducers will scatter about the stated  49  thermal specifications. The manufacturer of the differential pressure transducers does not characterize the thermal effects of individual transducers because of the time and labour involved in the calibration process of numerous transducers. A shift in the zero and/or sensitivity of the transducer output because of a temperature change is not permanent (i.e., the output of the transducer may shift at different temperatures but should output the same value at a given temperature if all other conditions are constant).  The temperature inside the submersed box housing the data acquisition components of the DPS-II during the field test in this study was recorded using the internal temperature of the datalogger. The temperature inside the submersed box stabilized at about 11 ° C after about 8 hours from the start of the test (Figure 3-20). The period between 22 to 32 hours during field test, showed lower temperatures that corresponded to the time the submersible box housing the electronics was uncovered by a low tide.  The performance of the transducers and resistors of the DPS-II at conditions away from room temperature was investigated by comparing the calibration curves of the transducers developed: (1) at room temperature (23°C), (2) in a cold room set to a temperature between 11 to 13°C, and (3) in a cold room set to 5 ° C . The calibration tests were completed over three days, with a test at a given temperature carried out on each day. Before the calibrations in each cold room were started, the transducers, resistors and datalogger were placed in the cold  50  room for 4 hours to permit the temperature of the electronics to reach the testing temperature. The reason for not placing the instrument in the room for 8 hours (the time required for the instruments to reach a stable temperature in the field experiment in this study) was because the key objective at the time of the tests was not to simulate the exact conditions during the field deployment but to gain insight about the effect of temperature on the output of the transducers. Also the cold rooms were located in a room accessible to all individuals in the building, requiring constant supervision of the electronics, and consequently the amount of time chosen to leave the electronics in the room was also a practical consideration. Four hours was sufficient time to permit the transducer to reach the approximate temperature of the cold rooms. For the experiments, the electronics were housed in the submersible box that is used for the field deployments but the box remained uncovered. After the 4 hours had elapsed, the internal temperature of the datalogger was measuring a temperature 2-3°C higher than the expected ambient temperatures in both the 11-13°C and 5 ° C cold rooms. The slightly higher temperatures measured by the C R 1 0 X may have been because the expected temperatures of the cold rooms were not exact. No independent measurement was obtained of the temperature in either of the cold rooms. Also there is an error associated with the internal temperature measurement by the C R 1 0 X of about +/- 0.5 °C (Campbell Scientific personal communication). The transducers were loaded, unloaded and reloaded over the entire range of the transducer for the calibrations tests at room temperature and in the 11-13 °C cold room. The purpose of the loading, unloading and reloading  51  was to complete several calibrations and thus improve the reliability of the calibrations curves at the specified temperature. Other than it is slightly more time efficient, as a result of not having to refill the water columns after each calibration, no additional benefit or information was gained by the approach of loading, unloading and reloading the transducer as opposed to loading the transducer three times.  The calibration test in the 5°C cold room was completed by twice loading the transducers. The different calibration method used in the 5°C cold room was a result of the significant variations (e.g., +/- 5 mm) in the output of the transducers that were observed when loading each of the transducers. Instead of unloading and reloading the transducers to produce additional calibration curves for comparison at that temperature, as in the calibration test in the 11-13°C cold room, the entire calibration process was begun again. Deaired water was pumped through the system and the pressure ports of the transducers rebled before the transducer was again loaded. The purpose for beginning the calibration process from the start for the second calibration was to ensure the noise in the output was not from air bubbles present in the pressure cavities of the transducer. Because both of the loading curves for each transducer were affected by variations in the output of the transducer, the noise was likely caused by vibrations of the water in the water columns of the calibration stand, caused by continuously operating fans in the room that prevented the applied pressure from remaining constant. Typically during a calibration test, the outputs of the  52  applied pressures to the transducer are recorded from real time data. Because of the noise in the output (+/- 5 mm) during the calibrations in the 5°C room, it was more difficult to read an exact output for the applied pressures (usually output variations are +/-1 mm or less). Consequently, the average observed output value over about a minute was used to develop the calibration curves. Although there was noise in the output for both the transducers in the 5°C cold room, the variations in output were higher and not as consistent for transducer B making it more difficult to assess an average output value for a given applied differential pressure. A s a result, a calibration curve could not be developed for transducer B. Transducer B was closer than transducer A to the closest two fans in the cold room (about 30 cm away), which may account for the more significant noise. An effort to shield the water columns of the calibration stand from the wind produced by the fans, did not reduce the noise in the output of Transducer B.  Table 3-2 shows the calibration curves developed at each of the testing temperatures. The discrepancies between the calibration curves at room y  conditions and in the 11-13°C cold room showed a shift of +1 mm in output for transducer A and no measurable change for transducer B. The discrepancy between the calibration at room temperature and in the 5°C cold room for transducer A was +3 mm. No change could be noted at transducer B because a calibration curve was not developed as discussed earlier. Although there may be a bias associated with the accuracy of the calibration curves of about +/- 3 mm, the accuracy of the transducers is +/- 2 mm or better. Also in general the  53  repeatability of calibration curves is within +/-1 mm as shown by Table 3-1. The discrepancy of +3 mm is larger than would be typically expected for calibrations completed within days apart. The accuracy of measurements that is required to measure the changes in differential pressure at some coastal sites, require all sources of error at this order of magnitude be identified. A potential cause for the calibration shift is because of errors reading the output of the transducer due to the significant variations in output of the transducer. Given that both calibration curves developed for the transducer in the 5°C cold room showed the same magnitude of shift, errors in the reading of the output are a less likely source for the shift. Another potential explanation for the shift in output is the decrease in temperature.  The calibration tests completed at room temperature, and in the 11-13°C and 5°C cold rooms aided with better constraining the magnitude of the shift that may be caused by temperature changes. The tests showed that the potential maximum shift of +1.4 cm from thermal effects that is specified by the manufacturer's specifications for the transducers does not occur for a decrease in temperature of almost 15°C from room temperature and that the zero shift may be closer to +3 mm (at least for transducer A). However, the + 3 mm shift experienced between the calibration tests completed at room temperature and 5°C tests has not been clearly identified as a result of temperature change. It should be noted that the effects of temperature on the output of the DPS-II could change as the transducers and resistors age.  54  In addition to stabilizing the temperature of the electronics, submersing the electronic components of the DPS-II on the seabed aids with minimizing the motion of the fluid filled tubes of the DPS-II from wind and wave effects. The random fluctuations in the output of the DPS-II caused by wind and waves were also reduced by averaging the 1s measurements of differential pressure head by the DPS-II over a minute. Figures 3-21 and 3-22 shows the variation in output of the DPS-II at 1 second measurements for 10 minute periods of testing taken at the highest high tide (2.3 m of water above the seabed) and the lowest low tide (75 cm of water above the seabed). The variation in the 1 s measurements of the DPS-II was about +/- 0.4 cm at both piezometer A and piezometer B at the highest water level and increased by a factor of two to +/- 0.8 cm at piezometer A and by an order of magnitude to +/- 2 cm at piezometer B during the lowest low tide. Because of the time of day the lowest low tide occurred (10 pm), it was difficult to observe the condition of the DPS-II. The observations of the DPS-II at the lowest low tide were taken over a 30 minute period at the time of predicted low tide, from on shore (about 20 m away from the instruments) because the darkness, rough waves and high water levels at the site prevented entering the water. From observations of the DPS-II during the low tide, a portion of the plastic fluid filled tubes of the DPS-II, likely the section of tubing that comes out from the top of the piezometers, could be seen to be periodically uncovered by passing waves. The uncovered tubes would have been susceptible to movement from the wind in addition to the waves and may explain the larger variations in  55  output at low tide than at high tide. The greater variation in the output at low tide at piezometer B than at piezometer A may have been a result of the greater length of exposed tubes at piezometer B (about 30-50 cm longer), that are used to hydraulically connect the measuring ports of the piezometer to the transducers in the submersible box (Figure 3-7). The piezometers are prepared in the lab prior to deployment and excess tubing is left leading from the measuring ports of the piezometers to permit the tubes to be connected to the transducers in the field prior to insertion of the piezometers. Typically, the excess tubing from the piezometer is shortened in the field before a connection is made to the transducers. More effort could be made in the field to ensure a closer agreement between the lengths of exposed tubing for each piezometer, which may provide more consistency between the measurements at each piezometer. The exposed portions of the lengths of tubes at each piezometer were not measured in this . study but were about 1-1.5 m. The length of tubing was more than sufficient (could have used 0.5 m less tubing at each piezometer) to position the submersible box with the transducers, on the seabed away from the piezometers.  3.5.1 Error Estimate The error associated with the differential pressure head measurements by the DPS-II was estimated as +/- 3 mm. The error estimate was calculated based on the largest error that was estimated for the calibration method. The error estimate of the differential fluid pressure head measurements is likely an underestimate, given the uncertainty associated with several aspects of the DPS-II design during  56  field deployments. The largest sources of uncertainties with the field deployment is the possibility of a temporary calibration shift due to temperature fluctuations during deployment, the potential for clogged ports (for which there is currently no method to detect) and the formation of drainage paths along the sides of the piezometers and/or a transient excess pressure during insertion. Given the 1D modelling that showed a fairly good agreement with the measurements by the DPS-II (section 5.2), and also considering the temperature tests which showed little shift due to temperature changes and relatively high permeability of the sediment combined with wave action at the site (making conditions less likely for drainage paths and/or insertion pressures to be maintained) the error estimate of +/- 3 mm, likely only slightly underestimates the error of the DPS-II measurements. More experience using the tool in the field would increase the confidence associated with the estimate of error. Several recommendations have been proposed in section 6.2, to better constrain the error estimate. If a uniform density flow field is assumed, the differential pressure head measurements by the DPS-II can be used to calculate the magnitude and direction of hydraulic head gradients in the shallow marine sediment. The difference in hydraulic head between the measuring ports of a piezometer of the DPS-II can be calculated based on the measured differential fluid pressure heads by the DPS-II and the distance between the measuring ports of the piezometer using the following relationship: dh = di|/ + dz  57  (Equation 3.5)  The first term on the right side of Equation 3.5 represents the measurements of differential pressure head by the DPS-II. The error associated with the calculated difference in hydraulic head values is equivalent to the error of the differential fluid pressure head measurements (+/- 3 mm).  Hydraulic gradients based on the differences in hydraulic head can be calculated according to the following equation: Vh = —  (Equation 3.6)  An error of 0.01 is estimated for the calculated hydraulic gradients values, derived using the rule of error propagation (Taylor 1997) that is expressed for the general relationship defined as z = y/x and written as: 2  A z = z •.  Ix  + )  I  (Equation 3.7) y)  where A z is the absolute error associated with quotient (i.e. hydraulic gradient), Ax is the absolute error of the dividend (i.e. distance between the measuring ports of the piezometer) and Ay is the absolute term, of the divisor (i.e. difference in hydraulic head).  58  Date of Calibration Test  Transducer B  Transducer C  August 15, 2005  y =35.7x+ 1470  y = 35.0x+ 1658  August 23, 2005  y =35.5x + 1469  y = 34.8x + 1660  September 1, 2005  y =35.6x+ 1472  y = 34.8x + 1664  September 14, 2005  y =35.5x+ 1471  y = 35.0x + 1667  Table 3-1: Calibration curves completed several days to weeks apart typical show variation less than +/-1-2 mm. Transducer C was used in the DPS-II prior to the transducer being replaced by Transducer A.  Date of Test  Calibration Temperature (°C)  Calibration Curve  Transducer A  Transducer B  5/9/05  23  I  y = 35.6x+ 1224  y = 35.5x+1478  II  y = 35.5x+ 1228  y = 35.4x+ 1479  III  y = 35.5x+ 1227  y = 35.3x+ 1477  I  y = 35.7x + 1229  y = 35.4x+ 1479  II  y = 35.8x+ 1229  y = 35.5x+ 1478  III  y = 35.8x + 1232  y = 35.5x+ 1479  I  y = 35.7x + 1238  N/A  II  y = 35.6x + 1236  N/A  Average  y = 35.6x + 1237  Average 5/10/05  14-15  Average 5/11/05  7-8  Table 3-2: Calibration curves developed at different temperatures to investigate the effect of temperature on the electronic components of the DPS-II. Calibration curves were not developed for transducer B at 7- 8°C because of significant noise in the output of the transducer.  59  Figure 3-1: The PISSPI-3 developed at the University of Lehigh (photo provided by B.Carson).  Valves  Absolute Pressure Transckicer  Probe push plate  Pressure Port 1 •  Qjfferenlial Pressure Transducers  Pressure Port 2  igure 3-2: The DOPPI, designed by GEOTEK Ltd for measurement of pore water pressure in shallow marine sediment, (schematic provided by GEOTEK Ltd.).  60  pressure gauge 21cm  pressure sensor  Figure 3-3: Schematic of a newly developed multi-depth pore pressure measurement device by Urakoshi et al. (2005).  61  Figure 3-4: The DPS-I deployed at high tide near the FSMUL in 2002 (photo by J.Caulkins).  62  1.75" (45mm) OD, _ 1 ft AW steel drill rod or hammer head device  1.75" (45mm) OD, AW steel coupling  ^  1.75" (45mm) OD, _ 1 ft AW steel drill rod  %" OD Swagelok fitting Steel line from top port  Screened Inlet (top port)  1.75" (45mm) OD, AW steel coupling  %" OD low density polyethylene tubing  1.75" (45mm) OD, _ 1 ft AW steel drill rod  Single line from bottom ports (1/4" OD swagelok fitting)  AW steel tip  5 screened inlets set circumferentially (bottom port)  Figure 3 - 5 : S c h e m a t i c of a DPS-II piezometer.  63  Figure 3-6: The data acquisition components of the DPS-II (transducers, resistors, datalogger and power source) housed inside a submersible (i.e. water proof) box for deployments.  64  Submersible box housing electronics  Piezometer  Fluid filled hydraulic tubes  •  Figure 3-7: The DPS-II after completion of installation at low tide at Spanish Banks West beach, Vancouver.  Figure 3-8: The instrument, Nold Deaerator, used to prepare deaired water for the installation of the DPS-II.  65  Figure 3-9: Calibration curves developed for transducer A of the DPS-II in this study. (A) Pre-field deployment calibration curve developed October 21, 2005. (B) Post-field deployment calibration curve developed October 31, 2005.  \  66  3000-  (A)  I  ,  -40  ,  -35  -30  ,  -25  ,  -20  ,  ,  -15  ,  -10  e-i  -5  0  ,  ,  5  10  ,  15  ,  20  ,  25  ,  30  1  35  Applied differential pressure head (cm)  .  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _  .  3000-,  •  1  (B)  I  ,  ,  ,  ,  ,  ,  -40  -35  -30  -25  -20  -15  -10  ,—e-i -5  0  , 5  ,  ,  ,  ,  ,  10  15  20  25  30  1 35  Applied differential pressure head (cm)  Figure 3-10: Calibration curves developed for transducer B of the DPS-II in this study. (A) Pre-field deployment calibration curve developed October 21, 2005. (B) Post-field deployment calibration curve developed October 31, 2005.  67  \  Submersible box  (A)  Fluid filled tubes  Transducer  Formation pressure applied to top port, P i Formation pressure applied to bottom port. P  Piezometer:  (B)  2  Water Column Pressures  p  2 Transducer  V  Front view  Figure 3-11: Comparison between the loading of the transducers in the field configuration and orientation/position of transducer in the submersible box does not change in the configurations  68  Fig. 3-12: Setup of the deaired water, peristaltic pump and data acquisition components in a small inflatable boat in preparation for insertion of the DPS-II piezometers into the seabed.  Fig. 3-13: A piezometer of the DPS being installed at low tide. This photo was taken during an earlier trial deployment of the DPS-II in May 2005.  69  12 volt power source  Campbell Scientific CR10X datalogger  4-1 OK Ohm resistors Flow pathways in manifold „., > -| —  Input for deaired water  Manifold  Valve  Valve  Valve %" OD polyethylene t—tubing—>  Setra model 230 differential pressure  Setra Mo del 230 differential pressure  Low pressure side  Low High pressure pressure side side  v To piezometer measuring port  V  v  To piezometer measuring port  To piezometer measuring port  Valve  High pressure side  To piezometer measuring port  Figure 3-14: Schematic of set up inside the submersible box. During insertion of the piezometers of the DPS-II, deaired water is pumped through the manifold past the sides of the transducer and out the ports of the piezometer via low density polyethylene tubing. When a piezometer is inserted, only the pair of valves on the manifold that regulate flow to that piezometer are open.  70  drift  linearity, hysterisis, non-repeatability  <  reading error  temperature  Calibration  wind/waves  configuration  /  r»  i  *  \  b-  air bubbles in fluid filled tubes  clogged ports and/or drainage paths during •* insertion of piezometers pressurizing transducers on seabed Figure 3-15: The sources of error identified within the main stages of the DPS-II development.  71  Figure 3-16: Calibration of the DPS-II differential pressure transducers to assess nonlinearity, non-repeatability, and hysterisis. (A) Calibration of transducer A. (B) Calibration of transducer B.  72  0.4  (A)  0.3  0.2 4-  0.1 H-  4  5  6  10  6  10  Time Elapsed (hours)  0.4  (B)  0.3  4  m 0.2  4  5 Time Elapsed (hours)  Figure 3-17:1s measurements that show the variation in output when a constant zero differential pressure is applied to the transducers. (A) Transducer A. (B) Transducer B.  73  0.4  (A)  O < 0.2  0.1  12  18  24  Time Elapsed (hours)  Time Elapsed (hours)  Figure 3-18: One minute averages of 1 s measurements that show the variation in output when a constant zero differential pressure is applied to the transducers. (A) Transducer A. (B) Transducer B.  74  Figure 3-19: The discharge of water at the seabed from along the side of the DPS-II piezometers was observed during the insertion of the piezometers.  75  12  6  -I  ,  ,  ,  ,  ,  ,  ,  0  6  12  18  24  30  36  42  1 48  Time Elapsed (hours)  Figure 3-20: Temperature measured inside the submersible box, housing the electronic components of the DPS-II, by the internal temperature of the CR10X. The decrease in temperature after about 20 hours from the start of the experiment corresponds to the timing of the low tide that uncovered the box.  76  5  (A) 4.8  3.4  —  — * —  3.2  3  -I 0  1  1  1  2  1  3  1  4  .  5  1  6  1  7  1  8  1  9  Time Elapsed (minutes)  Figure 3-21: Variation in the output of the transducers at 1 second measurements is compared to 1 minute averages of the data, for a 10 minute interval occurring at the highest high tide (2.3 m) over the experiment. (A) Transducer A. (B) Transducer B.  77  10  4  5  6  Time Elapsed (minutes)  Figure 3-22: Variation in output of the transducers at 1 second measurements is compared to 1 minute averages of the data, for a 10 minute measurement interval occurring at the lowest low tide (0.76 m) over the experiment. (A) Transducer A. (B) Transducer B.  78  4. Field Experiment 4.1 Field Site The field data was collected from an area at Spanish Banks West beach, within English Bay, located in the most western part of Vancouver, British Columbia (Figure 4-1). The Spanish Banks West beach is bordered to the south by steep slopes and cliffs that are approximately 60 m above sea level. The base of the slopes is roughly 100 m back from the experiment area. The cliffs are composed primarily of a deposit known as Quadra Sands. In general, the stratigraphy of the cliffs is comprised of a layer of medium to coarse sized sands, overlying a layer of relatively finer grained sand, silts, and organic materials inter-layered with sand (Piteau Associates 2002). Jericho and Kitsilano beaches are located to the east of the site. The foreshore of these beaches is characterized by mostly Quadra medium-grained sands and well exposed bedrock in some areas (P. Mustard personal communication). There are no borehole data at the Spanish Banks West beach that define the local stratigraphy of the near-shore. Clague (personal communication), roughly estimates that bedrock may be within a few tens of metres of sea level at the site. English Bay is a tidal salt-water body and extensive sand flats are present at the Spanish Banks West beach.  4.2 Experimental Method From October 26 - 28, 2005 a field experiment was carried out at the Spanish Banks West site to measure: (1) the transient changes in vertical differential fluid pressure heads in shallow marine sediment by the DPS-II, (2) S G D rates by a  79  continuous heat-type seepage meter, (3) tidal levels by a water level sensor and, (4) the E C of pore waters and hydraulic conductivity of marine sediment from pore water and sediment samples collected in the immediate area of testing.  : Figure 4.2 shows the tidal level measured over the course of the experiment. The period of time represented by the plot is from October 2 6 at 9:30 am to October th  2 8 at 7:30 am. The period of time chosen for the experiment was constrained by th  several factors relating to the tidal level at the site that were: (1) during deployment the water level had to be sufficiently low to make working conditions suitable, (2) the instruments should remain fully submerged over the course of the experiment, (3) at least one tidal cycle should occur over the course of the experiment (several tidal cycles would provide more data to compare the relationship between differential fluid pressure heads, S G D rates and tidal levels) and, (4) a sufficiently low water level to enable removal of the instruments after the experiment was completed. Tidal predictions were obtained from The National t i d e s , Currents and Water Levels website (Vancouver station # 7735) provided by Fisheries and Oceans Canada. The period chosen for the experiment met all the requirements except that the instruments were uncovered for about 5 hours by the low tide approximately 20 hours after the start of the experiment. The uncovering of the instruments for the 5 hours during testing was acceptable compared to the limitations of other periods considered for deployment at the site.  80  The DPS-II, seepage meter and water level sensor were deployed approximately 20 m seaward of the high tide line in a configuration that would best permit comparison between the S G D rates measured by the seepage meter and the differential fluid pressure heads measured by the DPS-II (Figure 4-3). The piezometers of the DPS-II were spaced approximately 0.5 m on either side of the seepage drum to minimize effects from spatial heterogeneities in hydraulic conductivity between the measurements by the DPS-II and the seepage meter. Because S G D has been observed to decrease exponentially from shore in some studies (e.g., Bokuniewicz 1980; Shaw and Prepas 1990), the piezometers were also aligned parallel to each other. The piezometers and seepage meter were positioned away from the boxes housing the data acquisition components of the instruments to minimize any measurement artefacts caused by potential flow perturbations from the surface area or weight of the submerged boxes! To differentiate between the measurements from the piezometers, the piezometers are referred to as A and B (piezometer A is connected to transducer A and piezometer B to transducer B). Piezometer A is positioned east of piezometer B at the site as shown in Figure 4-3.  On the morning of October 2 6 , at the predicted low-low tide time for the day, the th  equipment used for the installation of the field instruments was set up in a small inflatable boat positioned in the near-shore at the site (Figure 4-4). Preparation for installing the instruments, which in addition to setting up the gear in the boat also involved preparing the electronics of the seepage meter, took approximately  81  1 hour. Insertion of the piezometers of the DPS-II was begun roughly 15 minutes after the set up for installation was completed.  4.2.1 DPS-II The water level was a few centimetres above the sediment-water interface when the piezometers of theTJPS-II were hammered into the seabed. Insertion of each piezometer took approximately 5-10 minutes. Differential fluid pressure heads were measured at the bottom measuring port of the piezometer relative to the top measuring port, thus a positive differential fluid pressure head indicated a vertical pressure gradient in the direction toward the seabed. Differential fluid pressure head was measured simultaneously at both piezometers of the DPS-II, at a rate of one measurement every 1 second.  4.2.2 Continuous Heat Type Automated Seepage Meter Approximately 1 hr after installation of the DPS-II, water levels at the site were sufficiently high to deploy the seepage meter (i.e., water was deep enough to fully submerge the seepage drum and flow tube of seepage sensor unit). A continuous heat-type automated seepage meters was installed directly between the piezometers of the DPS-II (Figures 4-5 and 4-6). Air bubbles were purged from the seepage drum and flow tube of the seepage sensor unit during deployment of the seepage meter. An attempt was made to position the flow tube of the seepage sensor unit parallel to the direction of the waves. Wave motions can effect S G D measurements made by the seepage meter by interfering with  82  discharge from the flow tube of the seepage sensor unit. Two small rocks were placed on the drum to provide stability from waves. Measurements of S G D were made every 10 minutes and provided an estimate of S G D at that instant in time.  4.2.3 Levelogger and Barologger Tidal levels were recorded every 5 minutes by a pressure transducer (F15/M5 LT levelogger, Solinst), that was attached to piezometer B, about 5 cm above the sediment-water interface. Barometric pressures used for the tidal level measurements, were recorded every 5 minutes by a pressure transducer (F5/M1.5 Barologger, Solinst) located onshore.  4.2.4 Pore Water Sampler Pore water samples (30 ml) were collected daily over the course of the experiment during the morning low tide with a disposable syringe connected to a 0.6 cm diameter, 90 cm long stainless steel Push Point sampler (MHE product), shown in Figure 4-7. The electrical conductivity (EC) of the samples was measured in the laboratory.  A pore water sample was collected each day, adjacent to the top measuring ports of both piezometers. It was not possible to obtain pore water samples adjacent to the bottom measuring ports of the piezometers. This condition may reflect finer sediment at this depth that clogs the port of the sampler. The sediment cores collected from adjacent to the piezometers (section 4.2.5) did not  83  support the assumption of finer sediment with depth. However, a complete recovery of the core samples collected using the coring instrument that was driven to a depth of 60 cm below the seabed was not achieved (cores represented about a 40 cm length from within the 60 cm depth). Thus the samples may not have been comprised of the sediment present at the bottom port (60 cm below the seabed). A s a result, the lack of observation of finer sediment at the bottom portion of the cores does not exclude the possibility of a finer layer at depth. After sampling pore waters from the top measuring port of the piezometers, two seawater samples were also collected from the surface water.  On the last day of the experiment, in addition to collecting a porewater sample adjacent to the top port of each piezometer, a pore water sample was also taken at about 10 cm below the seabed from directly beside each of the piezometers. These extra pore water samples were collected for comparison with the samples at the top ports of the piezometers, to provide insight about whether the E C of the pore water changed significantly closer to the seabed.  4.2.5 Core Sampler Three core samples (3.5 cm diameter and 40 cm length) were collected at the end of the experiment by a core sampler that utilizes a core catcher technique (Figure 4-8). The core sampler consists of a removable transparent plastic core tube that fits into a 0.9 m (35 1/2 in) length core barrel and a core catcher that is  84  attached to the bottom end of the core barrel. During core collection, the fingers of the core catcher allow the sediment to pass up into the core tube, but spring back into place to prevent the core material from falling out during core handling.  The core sampler was hammered into the sediment surface to a depth of approximately 0.6 m (bottom piezometer port) but a full recovery of the core sample was not possible due to such factors as the saturation of the sediment and grain size which influence how well the core catcher can retain the sample during retrieval. The location of the core samples are shown in Figure 4-3. Two samples (referred to as A-in and A-out) were obtained from either side of piezometer A, parallel to the shoreline. A core sample (referred to as B-in) was also taken from adjacent to piezometer B, between the seepage drum and the piezometer. A second attempt for a core at piezometer B was unsuccessful. The core samples were transported to the laboratory to examine the sediment profile and estimate the hydraulic conductivity of the sediment using the falling head test.  4.3 Results Figure 4.9 compares the measurements by the DPS-II of differential fluid pressure head at both piezometer A and B with the measurements of tidal level at the site. The plot shows 1 minute averages of the differential fluid pressure head measurements for the entire duration of the experiment. There is a gap in the field measurements because the piezometers of the DPS-II were uncovered  85  for five hours by the low tide occurring between 20 - 2 5 hours after the beginning of the experiment.  The differential fluid pressure head measurements at both piezometers A and B showed temporal variations that corresponded to the semi-diurnal tides at the site, with the highest differential fluid pressure heads occurring at low tide. The magnitude of the differential fluid pressures measured at piezometer A, however, were on average 13 % higher than at piezometer B. The differential fluid pressure heads at piezometer A ranged from 34.4 cm to 46.7 cm with an arithmetic mean of 39.9 cm. The differential fluid pressure head at piezometer B was between 32.6 cm and 37.3 cm with an arithmetic mean of 34.6 cm.  For the first high/low tidal cycle between 0 and 18 hours of the experiment, the tidal level changed by 1.37 m and the corresponding change in differential fluid pressure head at piezometer A was 6.8 cm and at piezometer B was 2.7 cm. The tidal level changed by 1.56 m for the second tidal cycle between 24 and 37 hours into the experiment, and the corresponding change in differential fluid pressure head at piezometer A was 10.3 cm and at piezometer B was 3.7 cm.  Figure 4-10 compares the S G D measurements recorded every 10 minutes by the seepage meter with the measurements of tidal level at the site. A s was the case for the DPS-II field data, there is also a gap in the S G D measurements that corresponds to the complete exposure of the meter during the low tide. Because  86  the exact depth of water that resulted in the seepage meter starting to become uncovered is not known with certainty, the S G D measurements immediately before and after the gap in the field data (i.e., the low tide completely exposing the seepage meter) may be affected by measurement artefacts. The high S G D rates immediately preceding the gap in the S G D field data may reflect measurement artefacts rather than true discharge rates. The seepage drum and flow tube of the seepage meter must be completely filled with water to permit valid S G D measurements. A s a result the high values of S G D measured at around 20 hrs after the beginning of the experiment may not be accurate.  The reason for the fluctuations in S G D rates over short measurement intervals is not certain but may be the result of several causes that are: (1) environmental factors (e.g., wind, waves), (2) poor electrical connections, (3) instrument noise and/or, (4) the natural variability in S G D over time. In some cases, S G D data sets obtained using a continuous heat type seepage meter have also shown abrupt changes in discharge rates (e.g., Taniguchi 2006). The deployment of two seepage meters was preferred for this experiment, to increase the reliability of S G D measurements, but was not viable because of problems with a second seepage meter that was constructed. Figure 4-11 shows the plots of the S G D measurements averaged over time intervals of 20, 30 and 40 minutes.  The S G D rates showed temporal variations that corresponded to the semidiurnal tides at the site, with the highest S G D rates occurring at low tide. S G D rates  87  averaged every 30 minutes over the duration of the experiment have been used for a comparison with tidal level fluctuations because the S G D pattern is clearer than the plot of the 10 minute S G D measurements. The S G D rates at the site varied from 5 cm/d to 103 cm/d, with an arithmetic mean of 38 cm/d. Over the first high/low tidal cycle of the experiment, the change in S G D rates was about 42 cm/d. The S G D rate changed by about 35 cm/d over the high/low second tidal cycle.  The averages of the E C measurements of the seawater samples collected at low tides during the experiment, showed an increase from 15 mS/cm to 31 mS/cm (Figure 4-12a. These results may suggest the E C of the seawater at the experiment site is affected by circulation patterns in English Bay that transports fresh water from the Fraser River as it moves around from Point Grey. Daily changes in the E C of the water column could be important if the E C of the pore waters in the shallow seabed are affected because of mixing processes operating close to seabed. However, a higher frequency of sampling is needed to validate and more accurately represent the daily fluctuations of E C for the water column.  Table 4-1 shows the estimates of the E C measurements for the pore water samples collected during the low tides over the experiment, at 10 cm below the seabed and adjacent the top measuring port of piezometer A and B. Figure 4-12b illustrates the averages of these E C measurements at each depth below the  88  seabed. The E C pattern at the site suggests: slightly brackish water (average E C 8 mS/cm) at a depth of 10 cm below the seabed that overlies a component of fresh water (average E C of 0.3 mS/cm) at 30 cm below the seabed during low tide conditions. Improving the characterization of the E C profile by additional E C measurements at smaller vertical intervals within the seabed was unsuccessful because of difficulties collecting pore water samples with the water sampler.  Figure 4-13 shows the three core samples collected from adjacent to the piezometers. With the exception of the core sample identified as A-in, each of the core samples was divided into two equal sections for the purpose of identifying any discrepancies in hydraulic conductivity with depth. The core sample, A-in, was divided by separating coarser sediment occurring in the top portion of the sample (about top % of core) from a finer sediment that was similar to that comprising the other core samples.  Figure 4-14 shows the estimate for hydraulic conductivity found for both the top section (closer to the seabed) and bottom sections using the falling head test. The hydraulic conductivity test was completed twice for each sample, once using tap water and also using seawater as the hydraulic fluid. Other than discrepancies, which were likely a result of the accuracy of the permeameter test, no differences were observed between estimates of hydraulic conductivity using tap water and seawater. The estimates of hydraulic conductivity (including estimates from the top and bottom sections of the core) at piezometer A ranged  89  from 1 x 10" to 1 x 10 " m/s, with a geometric mean of 4 x 10 " m/s. The values 5  4  5  of hydraulic conductivity at piezometer B ranged from 9 x 10" to 1 x 10 " m/s 7  5  with a geometric mean of 4 x 10" m/s. The geometric mean of all the samples 6  was 2 x 10 " m/s. It should be recognized that the values are from disturbed 5  samples that may have experienced possible compaction during sampling and disruption of macropore structures when the cores were prepared for the permeameter tests.  The average of the hydraulic conductivity measurements at piezometer A were an order of magnitude higher than at piezometer B. The higher average hydraulic conductivity does not agree with the higher differential fluid pressure heads measured at piezometer A, if the conventional form of Darcy's Law is to be applied to calculate S G D rates from the DPS-II field data. Because the differential fluid pressure heads measured by the DPS-II are over a depth of roughly 0.3 to 0.6 m below the seabed (the depths corresponding to the measuring ports of the piezometer), the estimates of the hydraulic conductivity of the bottom core sections may be a more appropriate choice for comparison with the differential fluid pressure heads measured at the piezometers. The average of the hydraulic conductivity estimates for the bottom sections at piezometer A, were still higher but by a factor of 3.5, compared to the average hydraulic conductivity of the bottom core sections at piezometer B.  90  Analytical and model based methods have been used to provide a basis for comparison with field measurements and also to better constrain the interpretation of the field data sets. A prediction of S G D rates based on Darcy's law at the Spanish Banks site has been developed using the transient changes in vertical pressure gradients in the seabed measured by the DPS-II, the hydraulic conductivity estimates from falling head tests completed on core samples and the E C of water samples collected from near the piezometers and the water column. Data available to develop a hydrogeologic model of S G D rates and shallow vertical pressure gradients in the seabed at the site consisted primarily of the hydraulic conductivity data set and a rough estimation of the depth to bedrock.  91  Location of pore water sample  Sample date: October 26  Sample date: October 27  Sample Date October 28  10 cm below seabed  N/A  N/A  6 and 9  Adjacent top port of piezometer A (30 cm below seabed)  0.23  0.47  0.27  Adjacent top port of piezometer B (30 cm below seabed)  0.20  0.27  0.33  Table 4-1: Values of EC measurements (mS/cm) from pore water samples collected at low tides over the experiment. Two pore water samples from 10 cm below the seabed (each adjacent to a piezometer) were collected only on the last day of the experiment.  92  West Vancouver  North Vancouver  ButTard Intei  Spanish Banks beach  Jericho and Kitsilano beaches  Strati of Georqia  Figure 4-1: Map of Vancouver, BC with location of field site, Spanish Banks beach.  93  250  200  I  150  •5 100  50  0  -I  0  ,  ,  ,  ,  ,  ,  ,  I  6  12  18  24  30  36  42  48  Time Elapsed (hours)  Figure 4-2: Tidal level measurements over the period of the field experiment at the site. The tide line receded to several 100s of meters away from where the field instruments were deployed during the low tide that occurred about 20 hours after the beginning of the experiment.  94  (A) Piezometer B  S e e p a g e Meter Submersible Box (data acquisition instruments)  A  DPS-II Submersible Box (data acquisition instruments)  I | Onshore  v  not to scale  (B)  V  I  S2.3 m Tidal Level Sensor  S e e p a g e Meter Drum B-ln  ~ 0.3 m  Water A-Out  A-ln  •  •  •  •  i e z o m eter B  P i e z o m eter A  ~ 0.3 m  3  Sediment  Pressure Measuring Ports not to  scale  Figure 4-3: Schematic of installation configuration. (A) Top view. (B) View in the direction offshore. The A-ln, B-ln, and B-Out labels refer to the position of core samples relative to the piezometers collected at the end of the field experiment.  95  Figure 4-4: The boat containing the equipment to install the piezometers of the DPS-II and the seepage meter is positioned about a meter in land of the location where the DPS-II and seepage meter were installed in this study.  96  Figure 4-6: The seepage meter and piezometers were uncovered by the low tide on the second day of testing for about 5 hours (photo taken in the direction of shore).  98  S a m p l e r Port  Figure 4-7: Water sampler used to collect pore water samples from within the seabed. The water level was typically 30 to 60 cm above the seabed during pore water sampling. (A) An example of a pore water sample being retrieved using the water sampler.  99  ~1  m  Figure 4-8: Core sampler used to collect sediment samples from adjacent to the piezometers of the DPS-II.  100  250 y Tidal level  7Y  i Piezor l e t e r A  Piezo neter B  200 \  150  | Q. CD  Q  100  j  \  \ 12  18  /  \  iJ  \  50  / 24  30  36  42  48  Time Elapsed (hours)  Figure 4-9: Comparison between 1 minute averages of the differential pressure head measurements by the DPS-II and 5 minute measurements of tidal levels collected in the near-shore at the Spanish Banks site. There is a gap in the field data because of the low tide occurring between 20-25 hours after the beginning of the experiment, which uncovered the instruments.  101  fe  i  18  24 Time  Elapsed  30 (hours)  I  2 5 4-t-M-  18  24 Time Elapsed  30 (hours)  I  18  24 Time Elapsed  30 (hours)  Figure 4-11: Averaged plots of 10 minute SGD measurements over different time intervals. (A) 20 minute average. (B) 30 minute average. (3) 40 minute average.  102  (A)  38653  38652 A  38651  10  15  20  25  30  35  EC measurement (mS/cm)  0  5  10  15  Average Electrical Conductivity (mS/cm)  Figure 4-12: EC measurements of pore water samples collected during the low tides over the course of the experiment. (A) Seawater samples. (B) The average EC value of the water column, and 10 and 30 cm (top piezometer port) below the seabed derived from all pore water samples collected at these positions.  103  Figure 4-13: Core samples (A-Out and A-ln) collected from adjacent to piezometer A. The locations of the cores relative to the piezometer in the field are shown in Figure 4-3. (A) A-Out. (B) A-ln.  104  Figure 4-14: Core sample (B-ln) collected from adjacent to piezometer B. The location of the core relative to the piezometer in the field is shown in Figure 4-3.  105  Top Core Section  A-OUT B-IN  A-OUT  A-IN, A-IN, B-IN (same values)  1.0E-06  A-IN A-IN  A-OUT  B-IN  Bottom Core Section  1.0E-07  B-IN  1.0E-05 Hydraulic Conductivity (m/s)  A-OUT  1.0E-03  1 .OE-04  Figure 4-15: Hydraulic conductivity data derived from falling head tests for the core samples collected adjacent to the piezometers of the DPS-II. The letter labels indicate the estimates of hydraulic conductivity for a position relative to piezometer A or B, see Figure 4-3 for the location of these positions. The two estimates for each top and bottom section of core were based on using tap water and seawater as the hydraulic fluid for the falling head tests. Both estimates for the bottom core section of A-IN were 1 x 10" m/s. 5  106  5. A n a l y s e s of Field Measurements 5.1 Differential P r e s s u r e Heads and Calculated S G D F l u x e s An objective of this study is to investigate the relationship between differential fluid pressure heads and S G D flux rates, consequently it is necessary to examine the relative importance of the pressure gradient component of the S G D flux to the density-related gravity (i.e., buoyancy) component of the S G D flux. By expanding the gravity term in Darcy's Law, written in terms of fluid pressures (Equation 3.1), an equivalent freshwater head term, which represents the pressure driven component of flow, can be separated from a density related term, which represents the gravity-driven component of flow (Davies 1989).  Equivalent freshwater head at point A is expressed as: H = — -t-z  (Equation 5.1)  f  Pf9  where H is the equivalent freshwater head and p is the density of freshwater f  f  (1000 kg/m ). The seepage flux is calculated using the following derived form of 3  Darcy's law, expressed in terms of an equivalent freshwater head and a densityrelated gravity term: q =-K ' z  z  V  H  f  +  (Equation 5.2)  ^ V z ' pf  ;  where K is hydraulic conductivity, Ap is the difference between the actual fluid z  density at point A and a reference fluid density. Equation 5.2 indicates seepage flux cannot be determined directly from the vertical gradient in freshwater head  107  and that a gravity driven component of flow must also be considered if it is of similar magnitude to the gradient in freshwater head.  Figure 5-1 shows the equivalent freshwater head gradients (dHf/dz) that have been calculated based on the differential fluid pressure heads measured by the DPS-II at piezometers A and B, using the differential form of Equation 5.1 which is written as:  _ 5 +— 8z ~ 5z Pf9 8z  8H  f  (Equation 5.3)  Direct measurements of fluid density in the field are not required for the calculation of equivalent freshwater head gradients. The first term on the right side of Equation 5.3 represents a gradient in differential fluid pressure head as shown by the following relationship: _5_ P 8z Pf9 F  A  ^  5z  (Equation 5.4)  Since differential fluid pressure head measurements between the measuring ports of the piezometers are obtained by the DPS-II, only the distance between the measuring ports of the piezometer is also required, to evaluate freshwater head gradients at the piezometers using Equation 5-3.  108  For the first high/low tidal cycle between 0 and 18 hours of the experiment, freshwater head gradients at piezometer A varied from about 0.11 at high tide to 0.33 at low tide. At piezometer B, over the same tidal cycle, freshwater head gradients ranged from 0:05 to 0.14. Over the second high/low tidal cycle between 24 and 37 hours into the experiment, the freshwater head gradients at piezometer A varied from 0.13 to 0.45 and at piezometer B from 0.05 to 0.17.  Because fluid density measurements were not obtained between the pressure measuring ports of the piezometer (i.e., at the point of the flux calculation), an upper bound for the gravity term in Equation 5.2 was constrained by assuming a fluid density equal to seawater (1025 kg/m ) at the point of the flux calculation. It 3  is unlikely that the fluid density in the seabed at depths between the ports of the piezometers is equal to seawater since E C measurements obtained at the top measuring ports of the piezometer at low tide indicated a significant component of freshwater. However, the possibility of seawater at these depths cannot be excluded, especially during the high tide, when the seaward movement of freshwater is at a minimum. Based on a reference density of 1000 kg/m , the calculated upper bound for the density-gravity term that could be expected at the field site had a magnitude of 0.025. In this case, the density- gravity component of flow is 1 order of magnitude lower than the lowest estimate for freshwater head gradient at piezometer A. However, the gravity component of flow is significant in comparison to the freshwater gradients estimated at piezometer B at high tide.  109  Estimates that are likely more reflective of the gravity driven component of flow at the site were calculated using actual fluid densities at the point of the flux calculation that were equivalent to the E C measurement at a depth of 10 cm below the seabed and the average of the E C measurement obtained from the pore water samples adjacent to the top port of the piezometers (30 cm below the seabed). E C was converted to fluid density using the relationships shown in Figure 5-2 and Figure 5-3 ( E O S C 428/532: Field Techniques in Groundwater Hydrogeology course notes). The magnitude of the calculated density-gravity component using an actual fluid density of 1004 kg / m (equivalent to E C 3  measurement from 10 cm below seabed) was 0.004 and for an actual fluid density of 998 kg/m (equivalent E C top measuring ports of piezometer) was 3  0.002. In these cases, the gravity driven component of flow is 2 orders of magnitude less than the lowest estimate of freshwater head gradients at the piezometers A and an order of magnitude less than the lowest freshwater head gradient at B. These findings suggest that for the conditions at the site, the pressure driven gradient component of flow is likely significantly more dominant than density gravity effects.  S G D rates were calculated based on the freshwater head gradients at piezometer A and B, for the purpose of comparison with S G D rates measured by the seepage meter. Using Equation 5.2 and assuming that gravity density effects were negligible, S G D rates at each piezometer were calculated by varying the estimate of hydraulic conductivity until a reasonable match with the S G D rates by  110  the seepage meter was obtained (Figure 5-4). S G D rates calculated based on the freshwater gradients at piezometer A were found to best match the S G D rates measured by the seepage meter using a hydraulic conductivity of 1.5 x 10"  5  m/s. At piezometer B, a hydraulic conductivity of 4 x 10" m/s, best matched the 5  S G D rates measured by the seepage meter. The hydraulic conductivity values assumed at each piezometer to calculate the best match of S G D with the field based S G D rates are within the constraints of the available hydraulic conductivity data set obtained from the core samples.  Given the variability in the available hydraulic conductivity data set (9 x 10" to 6 x 7  10" m/s) it is also plausible, that discharge at each of the piezometers could 5  have been different than that recorded by the seepage meter. Michael (2003) showed that differences in S G D over spatial scales of 1 m can be similar in magnitude to differences in S G D over larger scales. The 1999 cluster experiment by Michael (2003) indicated that seepage meters located next to each other may differ greatly in discharge. For example, during the same two hour period, two seepage meters less than 2 m apart registered 5 cm/d and 37 cm/d.  Three key modifications to the experimental method that would improve the interpretation of calculated and field based S G D rates are: (1) the addition of a second seepage meter to improve the reliability of field based S G D measurements, (2) more measurements of the pore water at depths between the measuring ports of the piezometer in order to better characterize the fluid  111  density variation in the seabed and thus provide further insight regarding the significance of the pressure driven component of flow compared to gravity effects and, (3) a better characterization of the hydraulic conductivity at the site.  5.2 H y d r o g e o l o g i c M o d e l Model based predictions of S G D rates and the vertical hydraulic gradients in the seabed have been developed using the finite element model F R A C 3 D V S (Therrien and Sudicky 1996). F R A C 3 D V S is used to simulate a 1D uniform density flow model that examines how S G D rates are enhanced and diminished by tidal fluctuations. A 1D model excludes the simulation of density dependent flow because convection cells that normally form due to density variations cannot be simulated by only vertical flow.  The hydrogeologic model examined is shown in schematic form by Figure 5-5. The domain of the model has dimensions of 1 m x 10 m. A transient hydraulic head boundary at the top of the domain is applied to account for the sea level fluctuations due to the semi-diurnal tides at the site. The specified head at the top boundary varies from 0.1 m to a maximum of 2.26 m with an arithmetic mean value of 1.28 m. A constant hydraulic head is specified at the base of the domain.  Two models have been constructed to provide a basis for comparison with the transient changes in vertical hydraulic head gradients in the seabed calculated based on the differential pressure head measurements by the DPS-II (section  112  3.5.1) and the S G D rates by the seepage meter. In Model 1 it is assumed that the model domain is homogenous. In Model 2, a domain consisting of two layers is used to characterize heterogeneity with depth.  The boundary conditions assigned to Model 1 are:  Base- No measurements are available to constrain the conditions in the sediment with depth at the field site. A constant hydraulic head value is assigned to the boundary. For the initial simulation of Model 1, a value of 1.0 m is assumed as an arbitrary value and provides a hydraulic gradient of 0.1 between the base and the top of the model domain.  Top- Along the seabed, a transient hydraulic head boundary condition is applied to simulate the semi-diurnal tides measured during the experiment at the site.  No information is available about the porosity of the sediment. A typical value of porosity for fine sand of 0.3 (Domencio and Schwartz 1997) has been specified for all simulations. Specific storage (S ) is defined as the volume of water that an s  aquifer releases from or takes into storage per unit surface area of the aquifer per unit aquifer thickness per unit change in head. Specific storage is expressed as: S s = p g(Pp + ilPw) w  113  ( E q u a t i o n 5.4)  \  where p is density of water, (3 is the vertical compressibility of the porous w  P  matrix, q is the porosity of the porous matrix and B is the compressibility of w  water. Using Equation 5.4, a value of S s for the simulations was calculated as between 1 x 10" to 5 x 10 ~ m" based on the assumption that water is 3  5  1  incompressible and a typical range for the vertical compressibility of loose sand to dense sand gravel (Fetter 2001). A value near the middle of the calculated range of S (1 x 10" m" ) has been used for the simulations unless otherwise 4  1  s  noted. In some cases, the sensitivity of the results to S has been evaluated by s  varying the value of S from 1 x 10" m" to 5 x 10" and 5 x 10" m" . 4  1  5  4  1  s  Hydraulic heads are simulated at vertical increments of 20 cm within the model domain. Hydraulic gradients are calculated based on simulated hydraulic heads in the domain between points at 0.3 and 0.6 m, for a datum defined at 0.6 m (i.e. a positive hydraulic gradient indicated upward flow). Simulated S G D rates are monitored by a flux output node specified along the top boundary of the domain.  Results Model 1 Figure 5-6 illustrates the hydraulic head gradients and the S G D rates for the case where it is assumed the hydraulic conductivity of the domain is equal to the geometric mean of the harmonic averages of the core sections estimated from the falling head tests (1 x 10 " m/s). A comparison of the magnitude of the 5  hydraulic head gradients and S G D rates with that obtained from field measurements by the DPS-II and seepage meter indicates a poor  114  correspondence. The simulation predicts recharge is occurring at the site at most times during the tidal cycle, which is not observed from the field measurements.  A second simulation was completed in which the bottom boundary condition was assigned a higher value of h= 2.5 m (Figure 5-7). A better agreement was obtained between the simulated hydraulic gradients and the field based hydraulic gradients values at piezometer B but not at piezometer A. In this simulation, predicted S G D rates indicated discharge at the site but were lower than fieldbased S G D measurements.  A third simulation was completed in which the model domain was assigned a hydraulic conductivity four times higher (4 x 10" m/s) than in the first two cases 5  (Fig. 5-8). A comparison of the magnitude of the hydraulic head gradients at piezometer B and S G D rates with that obtained from field measurements by the DPS-II and seepage meter indicates a good correspondence but again not at piezometer A. Discrepancies between the simulated and field based S G D rates at approximately 20 hours after the beginning of the experiment are likely a result of seepage meter measurement artefacts caused by the falling tide, which during this time was beginning to uncover the seepage meter (section 4.2).  For the next two simulations, the S value was varied from 1 x 10" m" to 5 x 10" 4  1  s  and 5 x 10" m" (Figure 5-9). There was no significant change in the simulated 4  1  results of hydraulic gradient.  115  5  The hydraulic head at the base of the model was increased to values of 3.0 and 5.0 m in an attempt to simulate the higher hydraulic gradients at piezometer A (Figure 5-10). Although the relative magnitude of the simulated hydraulic gradients is closer to the field based measurements, it does not appear possible to explain the larger fluctuations in the transient changes in vertical hydraulic gradients at piezometer A over the tidal cycle using Model 1. Given the negligible effect of S on the model results, it is likely not a storage effect causing the s  difference between the simulated and field based results of hydraulic gradient at piezometer A using Model 1. The purpose of the second model is to provide insight to the discrepancy between the hydraulic gradients measured at piezometer A and piezometer B.  Results Model 2  To maintain consistency with Model 1, the boundary conditions are not modified. To obtain higher hydraulic gradients, a 1 m thick layer was created at the top of the domain, and assigned a lower hydraulic conductivity equal to the geometric mean of all the core sections of 2 x 10" m/s. There was no physical basis for the 5  addition of a 1 m thick layer. Although the magnitude of the S G D rates is in approximate agreement with field based S G D measurements, the correspondence between simulated and field based estimates of hydraulic head gradients at piezometer A decreases after 1 day has elapsed (Fig. 5-11).  116  A second simulation using Model 2 was completed, in which a lower hydraulic conductivity of 1.5 x 10~ m/s was assigned to the top layer (Figure 5-12). A 5  comparison of the hydraulic head gradients and S G D rates with that obtained from field measurements at piezometer A by the DPS-II and seepage meter indicates a better agreement, however the magnitude of the simulated hydraulic gradient during the latter portion of the simulation (after 24 hours) are lower than field based measurements.  The hydraulic head specified at the base of the domain was increased from 2.5 to 3.0 m to account for the lower hydraulic gradients simulated in the latter portions of the simulation (Figure 5-13). The simulated hydraulic gradient over the first 20 hours of the simulation were higher than the field based measurements at piezometer A. In the period after the gap in the field measurements, the simulated results were also higher, but to a lesser extent.  For the next two simulations, the S value of the top layer was varied from 1 x 1 0 s  4  m" to 5 x 10" and 5 x 10" m" . There was no significant change in the 1  4  5  1  simulated results of hydraulic gradient or S G D rates (results not shown).  Although a reasonable agreement was obtained between simulated hydraulic gradients using Model 2 and hydraulic gradients at Piezometer A, there was not a consistent match over the entire tidal cycle. A better characterization of the heterogeneity with depth at the field site may be useful for improving the  117  conceptual set up of the models and providing a better match with the field measurements at piezometer A.  The 1D flow models were also used to provide insight about the highest S G D rates observed at low tide by the field measurements at the Spanish Banks site (and also predicted by the models) compared to other coastal sites that show a time lag of a few hours between the highest S G D rates and low tide. Caulkins (2003), using a 1D model similar to the models constructed in this study, obtained results that showed the highest S G D rates occurred at the transition between high and low tide for a field site located along the northeastern Gulf of Mexico. A plot of simulated hydraulic head values with depth at different times over the tidal cycle showed that the hydraulic gradient was at a minimum at the high and low tides.  A key difference between the simulations by Caulkins (2003) and the simulations completed in this study are the values of hydraulic conductivity and specific storage specified for the domain. The domain of the model constructed by Caulkins was divided into two layers to represent a surficial aquifer and a limestone layer (Intracoastal Formation) that characterized the coastline at the field site. The upper 6 m layer of the domain was assigned a hydraulic conductivity of 1 x 10" m/s and S of 1 x 10" m" . The bottom 4 m layer was 5  3  1  s  assigned a lower hydraulic conductivity of 1 x 10" m/s and S of 1 x 10" m" . The 6  4  1  s  lowest hydraulic conductivity applied to the model domains in this study was 1.5  118  x 10" m/s. The values of hydraulic conductivity for the two layers in Model 2 used 5  in this study were also closer in magnitude, than in the simulations by Caulkins. The S used by Caulkins was also varied between the layers and a higher value s  for S was applied to the top layer than for model 1 and model 2 simulations in s  this study.  A simulation using Model 1 was completed with a lower value of hydraulic conductivity of 1 x 10" m/s (Figure 5-14). The simulation showed little shift 6  between the highest S G D rates and low tide. To also investigate the effect of storage on the time lag, the value of S was changed from 1 x 1 0 " m" used in 4  1  s  the previous simulations to higher and lower values of 5 x 10" and 5 x 10" m" . 5  4  1  No significant effect was observed (results not shown).  A simulation was completed using Model 2 with a lower value of hydraulic conductivity (1 x 10" m/s) for the bottom layer. A s well the S value of the top 6  s  layer was increased to 1 x 10" m" to also match the values used by Caulkins 3  1  (Figure 5-15). The results show the highest S G D rates have shifted and occur at the transition between the high and low tide.  A second simulation was completed with the upper layer thickness increased to 6 m to provide a better comparison with the simulations completed by Caulkins. The simulated S G D rates again showed a time lag between the highest S G D rates and low tide. A plot of the hydraulic head with depth in the domain at  119  different times over the tidal cycle showed the hydraulic gradient near the top of the domain was at a minimum at high tide (Figure 5-16).  A third simulation was completed to investigate whether it was the difference in hydraulic conductivity between the layers that caused the time lag or also the lower values of hydraulic conductivity. The upper 6 m layer was assigned a higher hydraulic conductivity of 1 x 10" m/s and the bottom 4 m layer was also 4  assigned a higher value of 1 x 10" m/s. The results obtained showed the highest 5  S G D rates occurred at low tide with no time lag (results not shown).  Despite some discrepancies between the simulated and field based measurements, it appears possible to satisfactorily explain the field based estimates of the transient changes in the vertical hydraulic gradient in the seabed at piezometer A and B as well as the S G D rates using a 1D uniform density flow model with boundary conditions that consider the sea level fluctuations due to semi-diurnal tides at the site. Although a reasonable agreement was obtained between simulated hydraulic gradients using Model 2 and hydraulic gradients at Piezometer A, there was not a consistent match over the entire tidal cycle. The possible discrepancy in the hydraulic gradients at piezometer A and the simulated results could be the result of heterogeneity that was not well characterized by the models. Also the error associated with the calculated hydraulic gradients (estimated as +/- 0.01) may have contributed to the difference between the simulated and field based values of hydraulic gradients. However if  120  measurement error was the only source for the discrepancy between the modelled and simulated results, the discrepancy should have been more consistent over the simulated period (unless the error associated with the DPS-II changed over time).  The models have also shown that the timing of the highest S G D rates relative to the low tide is sensitive to the value of hydraulic conductivity. Simulations using model 2 showed a greater time lag than model 1 simulations, suggesting heterogeneity with depth may be important in influencing the timing between the highest S G D rates and low tide.  To advance the framework for modelling S G D and hydraulic gradients at-the site, data sets that would be beneficial are: (1) salinity-depth profiles in the near-shore to provide more insight about the applicability of using a uniform flow model to simulate flow at the site, (2) a better characterization of the hydraulic conductivity in the area of the deployment to constrain the heterogeneity with depth, and (3) more information about the geology at depth.  121  0.600  0.000 J  ,  0  6  -, 12  ,  ,  ,  ,  18  24  30  36  I 42  48  Time Elapsed (hours)  Figure 5-1: Freshwater head gradients calculated based on the differential pressure head measurements by the DPS-II at piezometer A and B.  122  Figure 5-2: Relationship used to convert EC measurements to salinity (EOSC 482/532: Field Techniques in Groundwater Hydrogeology course notes). 1025  Figure 5-3: Relationship used to convert salinity to fluid density (EOSC 482/532: Field Techniques in Groundwater Hydrogeology course notes).  123  0  6  12  18  24  30  36  42  48  Time Elapsed (hours)  Figure 5-4: Comparison between SGD rates calculated based on the differential pressure head measurements by the DPS-II using Darcy's Law and measured SGD rates by the seepage meter. Calculated SGD rates were derived using hydraulic conductivity values of 1.5 x 10 m/s at piezometer A and 4 x 10 m/s at piezometer B, which were based on the value that provided the best match with field based SGD rates and were within the range of the available hydraulic conductivity data set. 5  5  124  H  T o p  = s e m i diurnal tides at the site  i  .  10 m  H se= varied from 1 - 5 m ba  1m Figure 5-5: Generalized schematic of set up of hydrogeologic model examined in this study.  125  Figure 5-6: Comparison between field based measurements and Model 1 simulation: K = 1x 10" m/s, Ss = 1 x 10" m" and H = 1.0 m. (A) Hydraulic gradients between 0.3 and 0.6 m below the seabed. (B) SGD rates. 5  4  1  b a s e  126  0.60  250  ()  A  A  Tide  0.50  j IPiezo neter A  j  \  200  /  Piezometer B  0.40  A/  o  \  V /  /s mulated  1  150 a)  o 0.30  Q  / \ 0.20  0.10  si \  0.00  V  \  f  s  \  100  \ \  A \ f  /  I  50  r 12  18  24  30  36  42  48  Time Elapsed (hours) 250  18  24  30  Time Elapsed (hours)  Figure 5-7: Comparison between field based measurements and Model 1 simulation: K = 1 x 10-5 m/s, Ss = 1 x 10-4 m-1 and Hbase = 2.5 m. (A) Hydraulic gradients between 0.3 and 0.6 m below the seabed. (B) SGD rates.  127  250  0.60  18  12  24  30  Time Elapsed (hours) 250  18  12  24  30  Time Elapsed (hours)  Figure 5-8: Comparison between field based measurements and Model 1 simulation: K = 4 x 10' m/s, Ss = 1 x 10" m" and H se = 2.5 m. Best match of the simulated results for piezometer B data (A) Hydraulic gradients between 0.3 and 0.6 m below the seabed. (B) SGD rates. 5  4  1  ba  128  0.00  -I 0  1  1  1  1  6  12  18  24  -i  30  1  1  36  42  1- 0 48  Time Elapsed (hours)  Figure 5-9: Sensitivity analysis of model 1 to values of Ss. (A) S = 5 x 10" m" . (B) S =5x10" m". 5  s  4  1  s  129  1  0.60  12  18  24  30  Time Elapsed (hours)  0.60  0.50  0.40  o 0.30  0.20  0.10  0.00 18  24  30  Time Elapsed (hours)  Figure 5-10: The H value for model 1 was increased to compare simulated gradients with higher hydraulic gradients at Piezometer A. (A) H = 3.0 m. (B) H = 5.0 m. b a s e  b a s e  130  b a s e  250  0.60  0.50  0.40  o 0.30  0.20  0.10  0.00 18  12  24  30  Time Elapsed (hours)  250  125  100  Q  O CO  18  12  24  30  Time Elapsed (hours)  Figure 5-11: Comparison between field based measurements and Model 2 simulation: 1 m thick top layer, H = 2.5 m; K p e r = 2 X 10' m/S, S S p L a y e r = 1 X 10" m"; K ttomLayer = 4 x 10" m/s, SsBottomLayer =1x10" m . (A) Hydraulic gradients between 0.3 and 0.6 m below the seabed. (B) SGD rates. 5  b a s e  5  T o  U a y  4  T o  4  131  1  Bo  Figure 5-12: Comparison between field based measurements and Model 2 simulation: 1 m thick top layer, H e = 2.5 m; K pLayer = 1.5 X 10" m/S, S s L a y e r = 1x10"* m"; K ttomLayer = 4 x 10' m/s, SsBottomLayer = 1 x 10' m . (A) Hydraulic gradients between 0.3 and 0.6 m below the seabed. (B) SGD rates. 1  5  baS  5  To  1  Top  4  132  Bo  250  0.70 j  Tide  f\  0.60  ~-\  /  .  \ P i e z o m e ter A  / \  Simulated  0.50  _ 0.40 CD o |  i '/  \  Vf  \ /  0.30  /^  -  V /  150 § Q.  Q  \ II V  \ /  iezometer B  i  0.20  \ / / \ v P\  \\ A  /"  200  v  100  \  r  50  0.10  0.00  12  18  24  36  30  42  48  Time Elapsed (hours)  Figure 5-13: The H value for model 2 was increased to try to match the hydraulic gradients in the latter portion of the experiment (greater than 24 hours): 1 m thick top layer, H = 3.0 m; K o p L a y e r = 1.5 X 10' m/S, Ss pLayer = 1 X 10' m"; K ttomLayer = 4 x 10' m/s, SsBottomLayer = 1 x 10" m . (A) Hydraulic gradients between 0.3 and 0.6 m below the seabed. (B) SGD rates. b a s e  1  5  b a s e  T  4  To  4  133  1  5  Bo  i  Figure 5-14: Simulation using Model 1 to investigate if a lower value of hydraulic conductivity (K = 1 x 10" m/s) would influence the timing of the highest SGD rates relative to the tidal level. Other Model 1 parameters: S = 1 x 10' m" and H e = 2.5 m. (A) Hydraulic gradients between 0.3 and 0.6 m below the seabed. (B) SGD rates. Note the different y-axes. 6  1  s  134  baS  0  6  12  18  24  30  36  42  48  Time Elapsed (hours)  Figure 5-15: Simulation using Model 2 to investigate if values of hydraulic conductivity 1 x 10" m/s) and Ss ( S L a y e r = 1x10-3 m' ) similar to those used by the model constructed by Caulkins (2003) would influence the timing of the highest SGD rates relative to the tidal level. Other Model 2 parameters: 1 m thick top layer, H = 2.5 m; K o Layer = 1.5 x 10' m/s; S ottomLayer = 1 x 10" m"\ (A) Hydraulic gradients between 0.3 and 0.6 m below the seabed. (B) SGD rates. Note the different y-axes. 6  (K ottom_ayor=  1  B  s T o P  b a s e  5  T  P  4  sB  135  1.5 hydraulic h e a d (m)  (C) K = 1.5E-5 m/s Ss=10 m" 3  1  11  10  K = 1E-6 m/s Ss=10" m" 4  10  1  0  0.5  —  1  1  —  —  1  1.5  1  1  2  2.5  3  hydraulic h e a d (m)  Figure 5-16: Simulation completed for comparison with modelled results by Caulkins (2003). Plots B and C show hydraulic head with depth for specified times over the tidal cycle. The number labels refer to position on the tidal cycle shown on plot A. K and Ss labels indicate the parameter values assigned for the upper 6 m layer and the bottom 4 m layer.  136  6. Discussion and Recommendations 6.1 Differential Pressure Heads, SGD and Tidal Levels The field measurements of differential pressure heads (measured between 0.3 and 0.6 m below the seabed) by the DPS-II and S G D rates by the seepage meter showed a semi-diurnal pattern with the highest differential pressure heads and S G D rates occurring at low tide. A significant component of freshwater at a depth of 0.3 m below the seabed was indicated by E C measurements of pore water samples collected at low tide, which suggests that the flow of terrestrial freshwater may be a significant source of S G D at the site.  It was possible to match the S G D rates measured by the seepage meter with calculated S G D rates using Darcy's Law based on the measured differential pressure heads at each of the DPS-II piezometers and a hydraulic conductivity value within the range of the available data set derived from falling head tests completed on core samples collected from adjacent to the piezometers. A reasonable match was also obtained between predicted and field based values of S G D rates and hydraulic gradients at piezometer A and B using a 1D uniform flow model. Although a reasonable agreement was obtained between simulated hydraulic gradients and field based hydraulic gradients at Piezometer A, there was not a consistent match over the entire tidal cycle.  The results of this research suggests that the transient changes in differential pressure heads in shallow marine sediment and changes in seepage rates, are  137  likely predominantly in response to tidal level fluctuations, and a significant driving mechanism controlling S G D rates in the near-shore at Spanish Banks West beach.  To better constrain the interpretation of the field data sets the following modifications to the field experiment procedure and additional modeling considerations are suggested:  1. Obtaining E C measurements of the pore water over the entire tidal cycle and at vertical incremental distances of 5-10 cm to a depth of 1 m below the seabed. The additional E C measurements would be useful for supporting the results of the 1D uniform density flow model in this study. Also E C measurements of the seawater over the entire tidal cycle should be obtained to better characterize the variation in E C that was observed from the water samples collected from the water column in this study. 2. Measurements of the salinity of the discharge water may help to constrain the origin of the waters and provide more insight about the significant component of freshwater that has been measured from pore waters collected 0.3 m below the seabed. Water samples collected from the outflow of the seepage drum or the seepage drum, itself, could be used to assess the salinity of the discharge water. A conductivity probe within the drum of the seepage meter could also be used to continuously monitor the  138  salinity of discharge, which has been done in other studies using a continuous heat type seepage meter (Taniguchi 2006). 3. A second seepage meter would be useful for increasing the reliability of the S G D measurements. An additional seepage meter would also permit other field configurations of the DPS-II and seepage meter to be tested. As well a method to calibrate the seepage meters should be developed (the calibration curve for the seepage meter was provided when the meter was purchased) to permit periodic performance checks and accuracy of the calibration curves, similar to the calibration tests that are completed for the transducers of the DPS-II before and after field installations. 4. A higher frequency of measurements of seepage (i.e. every 1 or 5 minutes instead of 10 minutes) could be attempted to investigate the significant fluctuations in the S G D rates. 5. Collection of complete core samples from the upper 1 m of sediment. Only about a 75% partial recovery of the cores driven to a depth 0.6 m (bottom measuring port of piezometers) was achieved from the coring attempts. The core catcher was likely not able to retain the sample as the corer was being pulled up from the sediment because of the saturation of the sediment. The coring technique had only been used once previous to this attempt to recover core samples, and more experience using the tool in the field may increase the core sample recovery. 6. Estimation of the hydraulic conductivity of the core samples in the original collection tubes to minimize further disruptions to the samples. In this  139  study the core samples were transferred to permeameters that were compatible with a falling head test apparatus that was available in the laboratory of the department. 7. The Spanish Banks West beach provided a site in close proximity to the University, relatively high rates of SGD and accessibility to a location for loading and unloading field equipment near the field site. However, a different location may need to be considered for longer deployments (e.g. greater than 1 or 2 days), because of the significant tidal flats at the site which uncover the instruments. 8. The significance of the hydraulic gradient between land and sea in comparison to other modeling parameters such as hydraulic conductivity and specific storage could be investigated in controlling the timing of the highest SGD rates relative to the low tide. A sensitivity analysis using the models constructed in this study was completed to evaluate the effect of the values of hydraulic gradient and specific storage on the timing of the highest simulated SGD rates relative to the tidal levels. The steep cliffs at the Spanish Banks west beach likely set up higher hydraulic gradients that drive SGD in the nean-shore and should also be investigated to aid with explaining the highest SGD rates observed at low tide at Spanish Banks. 9. The construction of a 2D model to investigate the effect of horizontal gradients at the site may be beneficiall in providing more insight about the usefulness of the 1D model.  140  6.2 Measurements by the DPS-II The DPS-II was developed to provide measurements of the transient changes in differential pressure head in shallow marine sediment, which could be used to investigate tidal forcing as a driving mechanism of S G D in near-shore environments. The DPS-II is a new tool that can be relatively easily installed and monitored.  Key issues regarding the accuracy of an earlier version of the DPS-II used in a previous field experiment (Caulkins 2003) were addressed in this study. The significant achievements of this study with regard to the DPS-II are: (1) refining and documenting the method used for collecting data in the field, (2) minimizing the noise in the measurements of differential fluid pressure head (a tidal signal could be observed from the field data), (3) providing an estimate of the error associated with the measurements by the DPS-II, (4) identifying areas in the development of the DPS-II that need to be improved and recommending strategies to advance the DPS-II.  Recommendations to further improve and establish the performance of the D P S II are:  1. The calibration of the differential pressure t r a n s d u c e r s in the field configuration. During calibration the known differential pressure heads are applied directly to the pressure sides of the transducers, as opposed  141  to the field tests where formation pressures in the sediment are transmitted via fluid filled tubes. Although it is recognized that a constant differential pressure head is exerted on the transducers from the fluid filled tubes, a test has not been completed to ensure that the transducer senses differential pressure changes in a similar manner in both configurations over the full range of the transducer. A calibration curve should be developed with the DPS-II in the field configuration and compared to calibration curves derived from the current calibration technique. There is not expected to be a variation between the calibrations but by completing the calibration in the field configuration, any uncertainties regarding effects from the tubing and/or the performance of the transducer are eliminated and more insight regarding the DPS-II is gained.  2. Evaluate the effect, if any, of the pressurization of the transducers while deployed in the submersible box on the seabed. It is assumed there is that there is no effect on the transducers and consequently calibration curve, from the pressurization (maximum water pressure 1.5 m of head) of the transducers while deployed on the seabed in the submersible box. This assumption could be investigated by deploying the transducers, while reading a constant applied zero differential pressure head, on the seabed for a tidal cycle.  142  3. Assess if insertion pressures, due to the displacement of water during the insertion of the piezometers, are significant. Insertion pressures are dependent on such factors as the rate of piezometer insertion, the dimensions of the piezometer and the hydraulic conductivity of the marine sediment. It is assumed that the measurements by the D P S II are not affected by insertion pressures that would alter the measured differential fluid pressure heads from "true" field values. An independent measurement of the fluid pressure in the seabed (i.e. an absolute pressure transducer embedded in the seabed) during insertion of the piezometers could provide more insight regarding the effect of piezometer insertion on the fluid pressures in the shallow sediment.  4.  Completion of a laboratory test that simulates the field conditions under controlled conditions and can monitor the output of the DPS-II over a time similar to field deployments (e.g., 1-2 days). A laboratory test of the DPS-II to simulate field conditions was attempted in this study but was not unsuccessful. The apparatus consisted of a 0.6 x 0.6 x 0.6 m box constructed of 2 cm thick wood sheets and lined with a plastic used for small backyard ponds. The piezometers of the DPS-II were positioned in the box, with two piezometers constructed of glass columns with screens at the base of the columns, attached to each of the piezometers of the DPS-II. The screens of the glass piezometers were positioned at the measuring ports of the DPS-II piezometers. The box was filled with fine  143  sand and saturated. A hose was used to vary the water level above the sand layer. An outlet at the base of the box permitted water to be discharged and the flow was regulated by a valve. A coarser layer composed of medium gravel was required at the base of the box to prevent the sand from clogging the outlet at the base. The piezometers of the DPS-II were prepared for testing in a similar manner as for a field deployment. Each piezometer of the DPS-II was connected to a transducer mounted in the submersible box through plastic tubing, and deaired water was pumped through the"system in order to bleed the pressure ports of the transducers. The plastic tubes were filled with deaired water to permit the pressures at the ports of the piezometers to be hydraulically transmitted to the transducers. An attempt was made to set up hydraulic gradients in the sediment by varying the water level above the sand layer and/or discharging water at the base of the box. By comparing the output at each of the piezometers and also the response of the glass piezometers, insight could be gained regarding how the DPS-II may respond to tidal level fluctuations. Leakage through the liner of the box prevented prolonged testing before the sand became partially or fully unsaturated, which occurred in approximately 10-15 minutes. Another test of this nature would be useful to gain insight about the performance of the DPS-II, however a container to hold the piezometers that does not require waterproofing should be considered. A 55 gallon drum that is composed of  144  a thick wall (to prevent deformation from the weight of the saturated sand) may be more suitable choice.  5. The effect of temperature on the transducers and resistors used by the DPS-II s h o u l d be further investigated. T h e calibration tests completed at room temperature ( 2 3 ° C ) , 1 4 - 1 5 ° C a n d 7 - 8 ° C were useful to better constrain the potential shift in the calibration c u r v e s due to temperature c h a n g e . However, the temperature tests did not evaluate the effect of a fluctuating temperature on m e a s u r e m e n t s by the DPS-II, which could c a u s e a greater error associated with the DPS-II m e a s u r e m e n t s . Although the transducers are deployed on the s e a b e d , it takes time for the instrument to reach a stable temperature (8 hours in this study) and if the electronics are u n c o v e r e d by a low tide (as in this study) the temperature experienced by the electronics may vary. A l s o future tests should make an effort to differentiate between temperature effects c a u s e d by the transducers a n d resistors. S i n c e the resistors are inexpensive and e a s y to replace, it would be a d v a n t a g e o u s to better constrain the potential temperature errors a s s o c i a t e d with the individual electronic c o m p o n e n t s .  6. The addition of a valve enabling a zero differential pressure head measurement to be applied to the s y s t e m during deployment and c h e c k e d against the expected zero output. A z e r o c h e c k valve was not implemented in the DPS-II design b e c a u s e the valve w a s not considered a  145  critical feature that would improve the performance of the tool compared to the other changes in the design that were being implemented and assessed in this study. However, an ability to check the calibration of the transducers during deployment provides significant information about the performance of the DPS-II. The implementation of a zero check valve should be considered for future applications using the DPS-II. A zero check valve has been used in other differential piezometer designs (e.g. DOPPI by G E O T E K Ltd). 7. Additional short deployments (e.g. 1-2 days) and longer deployments (e.g. 7 days) to establish the performance of the tool. Longer deployments may generate additional questions regarding the use of the DPS-II that have not been considered in this study. In general, for each deployment of the DPS-II that is carried out under different testing conditions (e.g. environment, time) the effect of the specific conditions for that test on the performance of the DPS-II should be considered. For example, longer deployments of the DPS-II may require more insight to the likelihood of gas bubbles forming in the fluid filled tubes that are used to transmit the formation pressures at the measuring ports of the piezometers to the transducers. Temperature effects on the fluid in the tubes could cause degassing to occur over time, which is likely not a key issue in shorter tests. No gas bubbles have been observed in the fluid filled tubes of the DPS-II after test lasting less than 1 day or after the deployment in this study (almost 48 hours). No tests have been completed  146  to specifically investigate the potential for gas bubbles forming in the fluid filled tubes of the DPS-II.  147  7. Conclusions and Summary This research focused on improving the accuracy and refining the installation method of a new tool (DPS-II) developed for measuring differential pressure heads in shallow marine sediment, and was achieved in part by resolving the difficulties in obtaining accurate differential pressure head measurements by an earlier version of the tool (DPS-I). Another key objective of this research was to use the measurements of differential pressure heads by the DPS-II to investigate the relationship between transient changes in differential pressure heads in shallow marine sediment, S G D rates and tidal level fluctuations at a field site located at Spanish Banks West beach. Spanish Banks West beach was chosen for the location of this study because the site met many of the logistical demands for field testing.  In this study, the highest differential pressure heads measured between 0.3 and 0.6 m below the seabed by the DPS-II and the highest S G D rates by a continuous heat type seepage meter occurred at low tide. The average measurements of the E C at depths of 10 and 30 cm below the seabed (8 mS/cm and 0.3 mS/cm respectively) were different by an order of magnitude, which suggested the fluid density within the shallow marine sediment at the site may not be uniform. To better understand the relationship between the transient changes in differential fluid pressure heads, S G D rates and tidal level fluctuations, the significance of density effects on flow had to also be considered. A comparison between the magnitude of calculated equivalent freshwater head  148  gradients and the density driven component of flow at the site indicated the relative importance of density on flow was likely negligible. S G D rates calculated using Darcy's Law based on the transient changes in differential pressure head measured at each of the DPS-II piezometers and a value of hydraulic conductivity within the constraints of the available hydraulic conductivity data set, provided a good match with S G D rates by the seepage meter. Two uniform density 1D flow models were used to predict the S G D rates and hydraulic gradients at 0.3 and 0.6 m below the seabed. Simulations using Model 1, that assumed a homogeneous domain, showed a reasonable agreement between the predicted and field based S G D rates and hydraulic gradients at piezometer B. It did not appear possible to obtain a good match between the simulated and field based hydraulic gradients at piezometer A using Model 1. A better agreement was obtained between the hydraulic gradients at piezometer A and simulated results using Model 2, which assigned a lower hydraulic conductivity layer to the upper 1 m of the domain. Despite the better agreement with hydraulic gradients at piezometer A using Model 2, the results were not consistently a good match over the entire tidal cycle. A better characterization of fluid density with depth and time (e.g., over the tidal cycle) as well as heterogeneity at depth at the site may provide insight to better explain the hydraulic gradients at piezometer A. Recommendations to improve the field procedure and further constrain the interpretation of the field results have been suggested in section 6.1.  149  Measurements of differential pressure heads in the seabed were successfully recorded by the DPS-II over a period of about 48 hours. The result of this work suggest that using a careful measurement approach, the DPS-II can measure the transient changes in shallow marine sediment to an accuracy of about +/- 3 mm. Several recommendations have been proposed in section 6.2 to further advance the DPS-II.  /  150  References A n d e r s e n , G . R., Bennett, R., B a r b e r , M . , T o d o r o v s k i , L. a n d M a y n a r d , G . (1996). " A M u l t i - s e n s o r P i e z o m e t e r for S h a l l o w M a r i n e S e d i m e n t s in C o a s t a l E n v i r o n m e n t s . " G e o t e c h n i c a l T e s t i n g J o u r n a l 19(4): 3 7 3 - 3 8 3 . A t a i e - A s h t i a n i , B., L o c k i n g t o n , D.A. a n d V o l k e r , R . E . 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A n introduction to error a n a l y s i s : T h e study of uncertainties in physical m e a s u r e m e n t s . S a u s a l i t o , Calif, University S c i e n c e B o o k s . T h e r r i e n , R., a n d S u d i c k y , E A (1996). " T h r e e - d i m e n s i o n a l a n a l y s i s of variably-saturated flow a n d solute transport in discretely-fractured p o r o u s m e d i a . " J o u r . C o n t a m . H y d r o l o g y 2 3 : 1-44. Turner, I. L., a n d M a s s e l i n k , G . (1998). " S w a s h infiltration-exfiltration a n d s e d i m e n t transport." J o u r n a l of G e o p h y s i c a l R e s e a r c h 1 0 3 : 3 0 , 8 1 3 - 3 0 , 8 2 4 . U B C H y d r o g e o l o g y p r o g r a m (2004). E O S C 4 2 8 / 5 3 2 F i e l d T e c h n i q u e s in G r o u n d w a t e r Hydrogeology course notes. U c h i y a m a , Y . , N a d a o k a , K., R o l k e , P., S a d a c h i , K., a n d H i r o s h i , Y . (2000). " S u b m a r i n e g r o u n d w a t e r d i s c h a r g e into the s e a a n d a s s o c i a t e d nutrient transport in a s a n d y b e a c h . " W a t e r R e s o u r c e s R e s e a r c h 36(6): 1 4 6 7 - 1 4 7 9 . United S t a t e s N a v y (2003). C o a s t a l c o n t a m i n a n t migration monitoring: T h e trident probe a n d ultraseep s y s t e m . S a n D i e g o , C a l i f o r n i a , S y s t e m s C e n t e r S P A W A R : 1-23. U r a k o s h i , T., T o k u n a g a , T . , a n d M o g i , K. (2003). Determination of p r e s s u r e field a n d hydraulic diffusivity of s u b s e a formations by c o n t i n u o u s m e a s u r m e n t s of pore p r e s s u r e s . A G U fall m e e t i n g .  U r a k o s h i , T . , T o k u n a g a , T . , M o g i , K . , 2 0 0 5 , Estimation of pore p r e s s u r e field a n d hydraulic properties of s u b - s e a formation at s u b m a r i n e g r o u n d w a t e r d i s c h a r g e a r e a . J o u r n a l of G r o u n d w a t e r H y d r o l o g y , 4 7 , no. 2, p p . 1 8 1 - 1 9 7 . (in J a p a n e s e , with E n g l i s h abstract)  158  U r g e l e s , R., C a n a l s , M . , R o b e r t s , J . , a n d S N V " L a s P a l m a s " S h i p b o a r d Party" (2000). "Fluid flow from p o r e p r e s s u r e m e a s u r e m e n t s off L a P a l m a C a n a r y Islands." J o u r n a l of Volcanolgy and Geothermal R e s e a r c h 101: 253-271. V a l i e l a , I., C o s t a , J . , F o r e m a n , K., T e a l , J . M . , H o w e s , B., a n d A u b r e y , D. (1990). "Transport of g r o u n d w a t e r - b o r n e nutrients from w a t e r - s h e d s a n d their effects o n c o a s t a l waters." B i o g e o c h e m i s t r y 10: 1 7 7 - 1 9 7 . V a n D a m , J . C . (1999). " G r o u n d w a t e r fluxes in the global h y d r o l o g i c a l c y c l e : past, present, future." J . of H y d r o l o g y 144: 4 0 5 - 4 2 7 . W i l s o n , A . M . (2005). " F r e s h a n d s a l i n e groundwater d i s c h a r g e to the o c e a n : A regional perspective." Water R e s o u r c e s R e s e a r c h 4 1 : 2016-2027. W o e s s n e r , W . W . , a n d S u l l i v a n , K . E . (1984). " R e s u l t s of S e e p a g e M e t e r a n d M i n i P i e z o m e t e r S t u d y , L a k e M e a d , N e v a d a . " G r o u n d w a t e r 22(5): 5 6 1 - 5 6 7 . Y i m , C . S . , a n d M o h s e n , M . F . N . (1992). "Simulation of T i d a l Effects o n C o n t a m i n a n t T r a n s p o r t in P o r o u s M e d i a . " G r o u n d w a t e r 30(1): 7 8 - 8 6 . Y o u n g , J . E . (1996). T r a c i n g g r o u n d w a t e r flow into the N o r t h w e s t e r n G u l f of M e x i c o using naturally o c c u r i n g R a d o n - 2 2 2 . Florida State University. Z e k s t e r , I. S . , D z h a m a l o v , V . A , a n d M e s k h e t e l i , A . V . (1983). " R o l e of s u b m a r i n e g r o u n d w a t e r d i s c h a r g e in the w a t e r b a l a n c e of A u s t r a l i a . " I A H S - A I S H P u b l i c a t i o n , N o 142 " G r o u n d w a t e r in R e s o u r c e s P l a n n i n g " : 2 0 9 - 2 1 9 . Z e k s t e r , I. S . (1996). G r o u n d w a t e r d i s c h a r g e in the c o a s t a l z o n e , L O I C Z / R u s s i a n A c a d e m y of S c i e n c e s : T e x e l , N e t h e r l a n d / M o s c o w : 1 2 2 - 1 2 3 .  159  A p p e n d i x A : Method for Calibration of Differential P r e s s u r e Transducers  160  Method for Calibration of Differential Pressure Transducers Background T h e relationship b e t w e e n the input a n d output of a m e a s u r i n g s y s t e m is e s t a b l i s h e d during the calibration of a s y s t e m . T h e calibration c u r v e e n a b l e s a m e a s u r e m e n t s y s t e m ' s direct output to be interpreted during a n actual m e a s u r e m e n t . B y the application of a r a n g e of k n o w n v a l u e s to the input a n d o b s e r v a t i o n of the s y s t e m output, a calibration c u r v e for the m e a s u r e m e n t s y s t e m c a n be d e v e l o p e d . C a l i b r a t i o n m e t h o d s m a y vary widely. T h e m a i n i s s u e in a calibration is that the v a l u e of the input a p p l i e d to the t r a n s d u c e r a n d the errors a s s o c i a t e d with that v a l u e are k n o w n . T h e e x p e c t e d error of the a p p l i e d inputs must be s m a l l e r than t h o s e a l l o w e d for the t r a n s d u c e r . M o s t t r a n s d u c e r s a r e subject to a static calibration, w h i c h is performed u n d e r r o o m conditions a n d i n c l u d e s letting the input stabilize at v a r i o u s v a l u e s before a n output r e a d i n g is r e c o r d e d . T h e s e v a l u e s are g e n e r a l l y s e l e c t e d at e q u a l i n c r e m e n t s , with the input first i n c r e a s i n g a n d then d e c r e a s i n g .  C a l i b r a t i o n s w e r e c o m p l e t e d before a n d after field d e p l o y m e n t s a n d following a n y modifications to the e l e c t r o n i c s of the DPS-II s u c h a s re-wiring or r e p l a c e m e n t of a t r a n s d u c e r . A s a result of the m e a s u r e m e n t s y s t e m including two differential p r e s s u r e t r a n s d u c e r s with i n d e p e n d e n t inputs a n d outputs, s e p a r a t e calibrations particular to e a c h transducer were required.  Calibration Method Bleeding the Pressure Ports of the Transducer Prior to t r a n s d u c e r s being calibrated the p r e s s u r e ports of the t r a n s d u c e r w e r e bled to eliminate a n y air in the lines or p r e s s u r e cavities of the t r a n s d u c e r . T h e following s t e p s  161  w e r e t a k e n to b l e e d the p r e s s u r e ports of the t r a n s d u c e r a n d begin the calibration of the transducers:  1.  T h e t r a n s d u c e r s w e r e positioned in the s u b m e r s i b l e box that is u s e d to h o u s e the t r a n s d u c e r s during D P S - I I d e p l o y m e n t s .  2.  T h e p r e s s u r e ports of the t r a n s d u c e r are c o n n e c t e d to w a t e r c o l u m n s that are a t t a c h e d to a s t a n d , v i a s w a g e l o k c o n n e c t i o n s on the s u b m e r s i b l e box.  3.  T o b e g i n calibration of the first t r a n s d u c e r (either t r a n s d u c e r A or B ) , the two v a l v e s regulating flow toward the t r a n s d u c e r a r e o p e n e d . T h e two v a l v e s regulating flow to the other t r a n s d u c e r remain c l o s e d . D e a i r e d tap water, w h i c h h a s b e e n d y e d with c o l o u r i n g , is p u m p e d using a peristaltic p u m p from a 4 L g l a s s container, through a n inlet o n the manifold a n d p a s t the s i d e s of the p r e s s u r e t r a n s d u c e r . T h e d e a i r e d w a t e r flows out f r o m the t r a n s d u c e r a n d d i s c h a r g e s from the top of the water c o l u m n s (Figure A A - 1 ) . D e a i r e d water is u s e d for the calibration to m i n i m i z e s o u r c e s of air b u b b l e s . A i r b u b b l e s in a n y part of the m e a s u r e m e n t s y s t e m s u c h a s the water c o l u m n s or p r e s s u r e cavities of the t r a n s d u c e r m a y c a u s e discontinuities w h i c h c a n affect fluid p r e s s u r e m e a s u r e m e n t s a n d thus the a c c u r a c y of the calibration. T h e d e a i r e d water is d y e d blue u s i n g f o o d colouring to e a s i l y identify a n y air b u b b l e s p r e s e n t in the water c o l u m n s .  4.  A s the t r a n s d u c e r p r e s s u r e ports are b e i n g p r e s s u r i z e d with the d e a i r e d water, the p r e s s u r e ports of the t r a n s d u c e r are b l e d . T h e b l e e d i n g of the p r e s s u r e ports allow air b u b b l e s to e s c a p e from the p r e s s u r e cavities of the t r a n s d u c e r . T h e p r o c e d u r e for b l e e d i n g the p r e s s u r e ports is d e s c r i b e d in the S e t r a m o d e l 2 3 0 t r a n s d u c e r installation g u i d e . T h e p r o c e d u r e i n v o l v e s b a c k i n g off the b l e e d s c r e w s until only b u b b l e free liquid flows out (Figure A A - 2 ) . After the p r e s s u r e  162  ports h a v e b e e n b l e d , the v a l v e s regulating flow toward the t r a n s d u c e r are c l o s e d a n d the p u m p i n g of d e a i r e d water s t o p p e d .  Applying Known Differential Pressures Heads and Transducer Observing Transducer Output A differential p r e s s u r e m e a s u r e m e n t is obtained from a p r e s s u r e m e a s u r e d relative to a r e f e r e n c e p r e s s u r e . In the c a s e of the differential p r e s s u r e t r a n s d u c e r s u s e d by the DPS-II ( S e t r a S y s t e m s m o d e l 2 3 0 , r a n g e +/- 0.5 P S I D ) , differential p r e s s u r e s are m e a s u r e d at the high p r e s s u r e s i d e relative to the low p r e s s u r e s i d e . K n o w n p r e s s u r e s are a p p l i e d to the high a n d low p r e s s u r e s i d e s the t r a n s d u c e r to yield a k n o w n difference in p r e s s u r e . T h e p r e s s u r e h e a d s are a p p l i e d by varying the water levels in the w a t e r c o l u m n s c o n n e c t e d to e a c h of the t r a n s d u c e r p r e s s u r e s i d e s . T h e c h a n g e in w a t e r l e v e l s are read from rulers affixed next to the w a t e r c o l u m n s . A syringe c o u p l e d to a plastic tube w a s u s e d to r e m o v e or a d d w a t e r from the water c o l u m n s . T h e operating r a n g e of the S e t r a S y s t e m s m o d e l 2 3 0 differential p r e s s u r e t r a n s d u c e r is +/- 3 5 c m of water. T h i s operating r a n g e w a s t a k e n a s the u p p e r a n d lower limits of the differential p r e s s u r e m e a s u r e m e n t s y s t e m for the calibrations. 1.  Initially 4 0 c m of w a t e r is a p p l i e d to both s i d e s of the t r a n s d u c e r to yield a n a p p l i e d differential p r e s s u r e h e a d of 0. T h e output of the t r a n s d u c e r at this a p p l i e d p r e s s u r e is r e c o r d e d . T h e output of the s y s t e m in r e s p o n s e to the k n o w n differential p r e s s u r e input is d i s p l a y e d u s i n g the C a m p b e l l Scientific d a t a l o g g e r support software, P C 2 0 8 W V e r s i o n 3.2 (Figure A A - 3 ) .  2.  T h e a p p l i e d p r e s s u r e to the low p r e s s u r e s i d e is d e c r e a s e d by 5 c m i n c r e m e n t s until a differential p r e s s u r e h e a d of + 3 5 c m is o b t a i n e d . At e a c h 5 c m interval of c h a n g e in p r e s s u r e h e a d the output of the t r a n s d u c e r is recorded.  163  3.  T h e w a t e r c o l u m n c o n n e c t e d to the low p r e s s u r e s i d e is refilled to a water level of 4 0 c m to a g a i n p r o d u c e a 0 differential p r e s s u r e . T h e output of the z e r o a p p l i e d differential p r e s s u r e h e a d is r e c o r d e d a n d c o m p a r e d to the initial v a l u e o b t a i n e d for a z e r o a p p l i e d differential p r e s s u r e .  4.  T h e w a t e r c o l u m n c o n n e c t e d to the high p r e s s u r e port is d e c r e a s e d by 5 c m i n c r e m e n t s until a differential p r e s s u r e h e a d of - 3 5 c m is r e a c h e d . At e a c h 5 c m interval of c h a n g e in p r e s s u r e h e a d the output of the t r a n s d u c e r is recorded.  5.  T h e k n o w n input a n d c o r r e s p o n d i n g s y s t e m output r e c o r d e d at e a c h 5 c m i n c r e m e n t is u s e d to yield a calibration c u r v e of s y s t e m output (mV) v s . k n o w n a p p l i e d differential p r e s s u r e h e a d (cm).  6.  T h e calibration p r o c e d u r e is repeated a g a i n for the t r a n s d u c e r to obtain a s e c o n d calibration c u r v e for the p u r p o s e of c o m p a r i s o n with the first calibration.  References Norton Harry N . (1989). H a n d b o o k of t r a n s d u c e r s . E n g l e w o o d Cliffs, N J , P r e n t i c e Hall.  S e t r a S y s t e m s M o d e l 2 3 0 Installation G u i d e .  164  165  Figure AA-2: Bleeding of a pressure port of the Setra 230 differential pressure transducer.  166  A p p e n d i x B : W i r i n g Configuration of the Electronic C o m p o n e n t s of DPS-II  168  Wiring Configuration of the Electronic Components of DPS-II Set Up of the Differential Pressure Measurement System F i g u r e A B - 1 s h o w s the o r g a n i z a t i o n of the electronic c o m p o n e n t s m a k i n g up the data acquisition part of the D P S - I I . T h e data acquisition c o m p o n e n t s c o n s i s t e d of: 1.  2.  3.  4.  T w o S e t r a S y s t e m m o d e l 2 3 0 differential p r e s s u r e t r a n s d u c e r s , w h i c h m e a s u r e d the differential p r e s s u r e b e t w e e n two points in the s e a b e d a n d than c o n v e r t e d this m e a s u r e m e n t into a u s a b l e electrical output. A C a m p b e l l S c i e n t i f i c C R 1 0 X datalogger, w h i c h controlled the f r e q u e n c y of the differential p r e s s u r e m e a s u r e m e n t s a s well a s the s t o r a g e a n d d i s p l a y of the measurements. T w o 1 0 K O h m resistors, w h i c h created a voltage divider to provide a n a c c e p t a b l e r a n g e o v e r w h i c h the C R 1 0 X d a t a l o g g e r c o u l d r e c e i v e the output from the transducers. A p o w e r s u p p l y to provide the required v o l t a g e s to the t r a n s d u c e r s a n d C R 1 0 X .  Differential Pressure  _  ^  Setra model 230 differential pressure transducer  10 K O h m resistors 2 % Tolerance  Campbell Scientific CR10X  Data Interpretation  datalogger  12 Volt Power Supply  )  Figure AB-1: Flow chart illustrating the interaction between the data acquisition components of the DPS-II.  169  CR1 OX W I R I N G D I A G R A M Company: Project:  OUT Connection Transducer B  Resistois B Resistois A EXC Connection Ti.insdiicei B  OUT Connection Tiansducei A  _ EXC Connection Tiansducei A  Positive of Battel y Negative of Battery  rial  Resistoi B Resistor A ——  11  1 i  Ground CRIOX _ Sifin.il fiom Resistoi B ~ Giound CRIOX Sicjnnl fioin Resistoi A Low foi Channel 2 Diff. CR10X Low foi Channel 1 Diff. CRIOX  COM connection Transducer B. COM connection Tiansducei A"  —1  NOTES;  Figure  AB-2:  W i r i n g of the E l e c t r o n i c C o m p o n e n t s t h r o u g h the  170  CR10X.  A p p e n d i x C : Calibration Curve for C o n t i n u o u s Heat Type Seepage Meter  171  0.014 0.012  0.000  1  0.3  1  1  0.35  0.4 Voltage (v)  172  1  0.45  1  0.5  

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