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Hyporheic exchange processes in a coastal headwater stream Scordo, Elisa Branson 2007

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HYPORHEIC E X C H A N G E PROCESSES IN A COASTAL HEADWATER  STREAM  by  ELISA BRANSON  SCORDO  B . N R S . , T h o m p s o n R i v e r s U n i v e r s i t y , 2003  A THESIS SUBMITTED IN P A R T I A L F U L F I L L M E N T OF THE REQUIREMENTS  FOR T H E DEGREE OF  M A S T E R OF SCIENCE  in  THE F A C U L T Y OF G R A D U A T E STUDIES (Geography)  UNIVERSITY OF BRITISH C O L U M B I A October 2007  © E l i s a Branson Scordo, 2007  ABSTRACT H y p o r h e i c exchange f l o w i n v o l v e s the t w o - w a y m o v e m e n t o f water between the stream c h a n n e l and the b e d a n d b a n k s . T h e s e exchange f l o w s create d i s t i n c t i v e habitats and i n f l u e n c e b i o g e o c h e m i c a l processes and water temperatures. T h i s study focused o n the c h a r a c t e r i z a t i o n o f the spatial d i s t r i b u t i o n o f subsurface f l o w p a t h w a y s and associated travel times t h r o u g h the h y p o r h e i c z o n e w i t h i n a l o w - o r d e r , high-gradient headwater stream located i n the U B C M a l c o l m K n a p p R e s e a r c h Forest, a p p r o x i m a t e l y 60 k m east o f V a n c o u v e r , B r i t i s h C o l u m b i a . H y p o r h e i c z o n e processes w e r e e x a m i n e d M a y to O c t o b e r 2 0 0 6 , at three spatial scales i n East C r e e k : point, channel-unit and reach. H y d r o m e t r i c data c o l l e c t e d from piezometers i n s t a l l e d w i t h i n the stream c h a n n e l , a l o n g w i t h solute i n j e c t i o n tracer experiments, w e r e used to characterize subsurface f l o w p a t h w a y s w i t h i n a 100 m stream section. S t r e a m tracer b r e a k t h r o u g h curves w e r e u s e d to m o d e l the processes o f a d v e c t i o n , d i s p e r s i o n , lateral i n f l o w and transient storage w i t h i n the h y p o r h e i c z o n e u s i n g the n u m e r i c a l m o d e l O T I S - P . T r a c e r injections at i n d i v i d u a l step-pool units w e r e used to identify l o c a t i o n s o f h y p o r h e i c discharge, as w e l l as to estimate separate travel times for h y p o r h e i c and surface-water transient storage zones. S o l u t e transport process v a r i e d w i t h discharge at the reach scale. T r a n s i e n t storage area ( A s ) increased w i t h discharge, w h i l e transient exchange coefficient (a) r e m a i n e d f a i r l y constant. A t the scale o f i n d i v i d u a l p o o l s , transient storage area and residence times w e r e h i g h e r than the r e a c h scale estimate, suggesting that p o o l s and b a c k eddies do contribute to transient storage i n headwater streams. W a t e r fluxes calculated w i t h D a r c y ' s L a w i n one channel-unit d i d not " s c a l e - u p " to the reach scale estimate o f h y p o r h e i c exchange (a), and w a s t w o orders s m a l l e r than the reach scale. D i r e c t measurements o f water fluxes into the streambed, i n c l u d i n g v e r t i c a l h y d r a u l i c gradients and i n f i l t r a t i o n rates, d i d not v a r y s y s t e m a t i c a l l y w i t h discharge. H y d r a u l i c gradients v a r i e d s i g n i f i c a n t l y w i t h s c a l e d l o c a t i o n w i t h i n the channel-unit, but not w i t h the d o w n s t r e a m step height. H y d r a u l i c c o n d u c t i v i t y v a r i e d w i t h site c o n d i t i o n s ( u p w e l l i n g , d o w n w e l l i n g and neutral sites), suggesting that c h a n n e l g e o m e t r y and h y d r a u l i c c o n d u c t i v i t y c o n t r o l exchange f l o w . T h i s m u l t i p l e scale a p p r o a c h h i g h l i g h t s the considerable spatial and t e m p o r a l v a r i a b i l i t y and c o m p l e x i t y o f h y p o r h e i c exchange processes w i t h i n step-pool streams.  T A B L E OF CONTENTS ABSTRACT  .  . - i i  T A B L E OF CONTENTS  iii  LIST OF T A B L E S  v  LIST OF FIGURES....  vii  ACKNOWLEDGMENTS DEDICATION  x '.  xi  CHAPTER ONE:INTRODUCTION  1  1.1.  H y p o r h e i c exchange f l o w i n s m a l l streams  1  1.2.  P h y s i c a l controls o n exchange f l o w  3  1.2.1.  B e d f o r m scale  4  1.2.2.  C h a n n e l - u n i t scale  1.2.3.  R e a c h scale - Transient storage processes  ,  1.2.4.  S t u d y objective and scale-dependent questions  5 8 12  CHAPTER TWO:METHODS 2.1.  Study location  14 .".  14  2.2.  Study design  19  2.3.  Stream discharge and geometry  21  2.3.1.  D i s c h a r g e measurements  21  2.3.2.  D i s c h a r g e calculations  22  2.3.3.  C h a r a c t e r i z i n g lateral 'exchanges  24  2.3.4.  C r o s s - s e c t i o n measurements  24  2.4.  Stream tracer experiments - R e a c h scale  24  2.4.1.  M e t h o d o f injection  2.4.2.  Q u a n t i f y i n g p o o l storage and residence t i m e  25  2.4.3.  H y d r a u l i c parameters ( O T I S - P )  25  2.4.4.  E v a l u a t i o n o f parameter uncertainty  27  2.5.  J  ....24  Stream tracer experiments - C h a n n e l - u n i t scale  28  2.5.1.  Q u a l i t a t i v e observations o f h y p o r h e i c d i s c h a r g e  28  2.5.2.  Q u a n t i f y i n g residence times  29  2.5.3.  M o d e l l i n g m e a n residence t i m e  2.6.  29  Subsurface f l o w measurements - P o i n t scale  30  2.6.1.  P i e z o m e t e r d e s i g n and installation  30  2.6.2.  H y d r a u l i c head  31  2.6.3.  Hydraulic conductivity  2.6.4.  R e l a t i n g discharge and recharge zones to stream geometry  2.6.5.  Stream b e d infiltrometers  35  2.6.6.  Subsurface relative c o n n e c t i v i t y  37  2.7.  Statistical A n a l y s i s  CHAPTER THREE: RESULTS  :  32 34  38 39  3.1.  Study period conditions  3.2.  Data quality  41  3.3.  S o l u t e transport m o d e l analysis - R e a c h scale  42  3.3.1.  S u m m a r y o f O T I S - P simulations  39  42  3.3.2.  V a r i a b i l i t y o f fitted parameters  44  3.3.3.  Parameter uncertainty  45  3.3.4.  D e r i v e d quantities  47  3.3.5.  L a t e r a l i n f l o w rates  3.3.6.  Q u a n t i f y i n g p o o l storage and residence t i m e s .  3.3.7.  Subsurface relative c o n n e c t i v i t y  3.4.  49 .....50  .'  :  53  S o l u t e injection experiments - C h a n n e l - u n i t scale  54  3.4.1.  Q u a l i t a t i v e observations o f h y p o r h e i c discharge  54  3.4.2.  Q u a n t i f y i n g residence times  56  3.5.  Subsurface f l o w - P o i n t scale  58  3.5.1.  H y d r a u l i c gradients  58  3.5.2.  V H G and scaled l o c a t i o n w i t h i n c h a n n e l units  63  3.5.3.  Hydraulic conductivity  65  3.5.4.  Streambed i n f i l t r a t i o n rates  67  3.5.5.  Streambed water fluxes c o m p u t e d f r o m D a r c y ' s l a w  68  CHAPTER FOUR: DISCUSSION 4.1.  R e a c h scale  70 70  ;  4.1.1.  M o d e l l e d parameter uncertainty  4.1.2.  S o l u t e transport parameters and discharge  ,  70 72  4.1.3.  R e s i d e n c e times and retention  74  4.1.4.  In-channel transient storage  75  4.2.  C h a n n e l - u n i t scale  75  4.2.1.  V a r i a b i l i t y i n exchange f l o w p a t h w a y s  75  4.2.2.  Transient storage m o d e l l i n g  77  4.3.  P o i n t scale  4.3.1.  '.  77  Interpretation o f f l o w p a t h w a y s  77  4.3.2.  Interactions b e t w e e n h y p o r h e i c f l o w and lateral i n f l o w  4.3.3.  W a t e r fluxes and discharge  4.3.4.  S c a l i n g streambed water fluxes  :  80 81  CHAPTER FIVE: CONCLUSIONS 5.1.  S u m m a r y o f m a i n results  5.2.  A r e a s for future research  REFERENCES APPENDIX A: M O D E L SIMULATIONS  79  83 83 85 •.  87 93  iv  LIST OF TABLES T a b l e 3.1: C o m p a r i s o n o f 2 0 0 6 m e a n d a i l y temperature and m o n t h l y p r e c i p i t a t i o n to 30 year c l i m a t e n o r m a l (1961-1990) as measured at the H a n e y - U B C R e s e a r c h Forest A d m i n c l i m a t e station ( E n v i r o n m e n t Canada)  39  T a b l e 3.2. S u m m a r y o f streamflow ( Q ) measurements c o n d u c t e d d u r i n g l o w e r reach ( L R ) , upper reach ( U R ) stream tracer injections o v e r the study p e r i o d M a y 31 to O c t o b e r 20, 2 0 0 6 . I n c l u d e d are the rates o f i n j e c t i o n (Rj), slope o f the c a l i b r a t i o n r e g r e s s i o n (k), and standard error o f k ( S E ) , e l e c t r i c a l c o n d u c t i v i t y at b a c k g r o u n d ( E C k , ) and plateau D  g  (ECp| t) and the p r o b a b l e error i n stream f l o w measurement  41  a  T a b l e 3.3: S u m m a r y o f best fit m o d e l parameters for solute releases i n c l u d i n g stream d i s c h a r g e ( Q ) , d i s p e r s i o n coefficient ( D ) , stream cross-sectional area ( A ) , cross-sectional area o f storage z o n e ( A ) , storage z o n e exchange coefficient (a), net lateral i n f l o w ( Q L ) , s  the D a m k o h l e r n u m b e r ( D a l )  :  42  T a b l e 3.4. S u m m a r y o f uncertainty ratios for the parameter estimates o f d i s p e r s i o n coefficient ( D ) , stream cross-sectional area ( A ) , cross-sectional area o f storage z o n e ( A s ) , a n d the storage z o n e exchange coefficient (a)  46  T a b l e 3.5. S u m m a r y o f d e r i v e d quantities i n c l u d i n g stream v e l o c i t y (w), h y d r a u l i c residence t i m e for the stream ( T ) and storage z o n e ( T s l r  s l o r  ) , h y d r a u l i c uptake l e n g t h ( S i d ) , iy  h y d r a u l i c retention factor ( R ) and the standardized storage z o n e coefficient ( A s / A )  47  n  T a b l e 3.6. S u m m a r y o f the s i m u l a t e d parameter estimates o f d i s p e r s i o n coefficient ( D ) , stream cross-sectional area ( A ) , cross-sectional area o f storage z o n e ( A s ) , and the storage z o n e exchange coefficient (a), stream v e l o c i t y (u), the standardized storage z o n e coefficient ( A s / A ) and the D a m k o h l e r n u m b e r ( D a l )  51  T a b l e 3.7. M e a n residence times for h y p o r h e i c z o n e (step) and p o o l storage zones i n P o o l 4. V e r t i c a l h y d r a u l i c gradients ( V H G ) measured u s i n g p i e z o m e t e r 61  .56  T a b l e 3.8. S p e a r m a n c o r r e l a t i o n coefficient (r ), associated p - v a l u e s , n u m b e r o f s  observations (n) a n d v e r t i c a l h y d r a u l i c gradients ( V H G ) for each p i e z o m e t e r i n d i c a t i n g a significant c o r r e l a t i o n w i t h discharge  62  T a b l e 3.9. A n a l y s i s o f v a r i a n c e table c o m p a r i n g three l i n e a r mixed-effects m o d e l s for v e r t i c a l h y d r a u l i c gradients i n c l u d i n g a base m o d e l w i t h o n l y channel-unit, a s e c o n d m o d e l w i t h channel-unit and step height ( S H ) , and a t h i r d m o d e l w i t h channel-unit, stepheight and c h a n n e l p o s i t i o n ( X / L ) . A chi-square (% ) statistic w a s used to test for 2  significance  63  v  T a b l e 3.10. A n a l y s i s o f v a r i a n c e table c o m p a r i n g t w o linear mixed-effects m o d e l s for h y d r a u l i c c o n d u c t i v i t y ( l o g transformed), i n c l u d i n g a base m o d e l w i t h o n l y reach as a factor and a s e c o n d m o d e l w i t h reach and site c o n d i t i o n ( u p w e l l i n g , neutral and downwelling)  65  T a b l e 3.11. S p e a r m a n con-elation coefficient (r ), associated p-values and n u m b e r o f s  observations (n) for infiltration rates versus discharge at each infiltrometer l o c a t i o n  68  T a b l e 3.12. W a t e r fluxes w i t h i n one step-pool unit ( P o o l 1) a l o n g w i t h scaled-up and reach-scale estimates o f h y p o r h e i c exchange (s" ) :  69  1  T a b l e 4 . 1 . R a n g e o f parameter values reported for high-gradient streams ( W a g n e r and H a r v e y 1997) c o m p a r e d to m o d e l l e d parameter values i n East C r e e k  71  vi  LIST OF FIGURES F i g u r e 1.0.1. D i a g r a m s h o w i n g a t y p i c a l subsurface f l o w p a t h w a y f r o m the stream c h a n n e l to the h y p o r h e i c z o n e  6  F i g u r e 2 . 1 . Site m a p s h o w i n g l o c a t i o n o f U B C R e s e a r c h Forest  15  F i g u r e 2 . 2 . Site m a p s h o w i n g East C r e e k drainage b a s i n  16  F i g u r e 2 . 3 . East C r e e k study reach w i t h upper and l o w e r sub-reaches and stream tracer experiment i n j e c t i o n and s a m p l i n g locations  17  F i g u r e 2 . 4 . E l e v a t i o n gradient for East C r e e k study reach d o w n s t r e a m f r o m a culvert c r o s s i n g at R o a d M  '.  19  F i g u r e 2 . 5 . D o w n s t r e a m v i e w o f l o g steps and piezometers i n upper reach o f East C r e e k 20  F i g u r e 2 . 6 . U p s t r e a m v i e w o f l o w e r reach o f East C r e e k s h o w i n g steps and p o o l complexes  20  F i g u r e 2 . 7 . M a p o f s a m p l i n g locations, m o r p h o l o g y and t h a l w e g p r o f i l e for b o t h reaches 33  F i g u r e 2 . 8 : Streambed infiltrometer. A d a p t e d from M a r t i n ( 1 9 9 6 )  35  F i g u r e 3 . 1 : D a i l y p r e c i p i t a t i o n , m a x i m u m and m i n i m u m d a i l y temperatures, measured discharge and net lateral i n f l o w f r o m tracer injections conducted d u r i n g the study p e r i o d o f M a y to O c t o b e r 2 0 0 6 . D i s c h a r g e values represent streamflow measured at the l o w e r reach b o u n d a r y . L a t e r a l i n f l o w measured as the difference b e t w e e n upstream and d o w n s t r e a m streamflow measurements. N o t e l o g scale for Q and Q L . C l i m a t e data recorded at the H a n e y - U B C R e s e a r c h Forest A d m i n c l i m a t e station ( E n v i r o n m e n t Canada)  40  F i g u r e 3 . 2 . M o d e l s i m u l a t i o n s u s i n g O T I S - P for June 1 9 for the upper reach (a) and the l o w e r r e a c h (b)  .....43  F i g u r e 3 . 3 . S i m u l a t e d m o d e l parameters for solute releases i n the upper and l o w e r stream reach. D i s p e r s i o n coefficient ( D ) , stream cross-sectional area ( A ) , cross-sectional area o f storage z o n e ( A s ) , storage z o n e exchange coefficient (a) versus stream discharge ( Q ) . E r r o r bars represent ± 1 standard d e v i a t i o n  44  F i g u r e 3 . 4 . E x p e r i m e n t a l D a m k o h l e r n u m b e r ( D a l ) versus stream discharge ( Q )  45  vn  F i g u r e 3 . 5 . U n c e r t a i n t y ratio ( U R ) for the s i m u l a t e d m o d e l parameters o f d i s p e r s i o n ( D ) , stream cross-sectional area ( A ) , cross-sectional area o f storage z o n e ( A s ) , storage z o n e exchange coefficient (a) versus stream discharge ( Q )  46  F i g u r e 3 . 6 . H y d r a u l i c residence t i m e o f solutes i n the stream ( A ) and storage z o n e ( B ) versus stream discharge ( Q ) for the u p p e r and l o w e r reach.  48  F i g u r e 3 . 7 . Stream v e l o c i t y ( A ) , h y d r a u l i c uptake length ( B ) , h y d r a u l i c retention factor ( C ) and the standardized storage z o n e coefficient ( D ) versus stream discharge ( Q ) for the upper and l o w e r reach 49 F i g u r e 3 . 8 . N e t lateral i n f l o w rates ( Q L ) versus discharge ( Q ) for a l l solute releases  50  F i g u r e 3 . 9 . M o d e l s i m u l a t i o n s u s i n g O T I S - P for September 3 0 . R e s u l t s are from one p o o l l o c a t i o n i n the l o w e r reach : 51 F i g u r e 3 . 1 0 . S i m u l a t e d m o d e l parameters for solute releases i n the upper and l o w e r stream reach d u r i n g September 2 9 and 3 0 . P o o l s 1 and 2 w e r e located i n the upper reach. P o o l 3 w a s located i n the l o w e r reach. D i s p e r s i o n coefficient ( D ) , stream cross-sectional area ( A ) , cross-sectional area o f storage z o n e ( A s ) , storage z o n e exchange coefficient (a) versus l o c a t i o n . E r r o r bars represent ± 1 standard d e v i a t i o n 52 F i g u r e 3.1 l . H y d r a u l i c residence t i m e o f solutes i n the stream and storage z o n e for solute releases i n the upper and l o w e r stream reach d u r i n g September 2 9 and 3 0 . P o o l s 1 and 2 w e r e located i n the upper reach. P o o l 3 w a s l o c a t e d i n the l o w e r reach  53  F i g u r e 3 . 1 2 . R e l a t i v e c o n n e c t i v i t y ( R C ) as m e a s u r e d u s i n g a n o n - d i m e n s i o n a l m i x i n g ratio (%) for piezometers s a m p l e d d u r i n g tracer i n j e c t i o n experiments i n the upper and l o w e r reach  54  F i g u r e 3 . 1 3 . C h a n n e l - u n i t observations o f f l o w p a t h w a y s i n P o o l 4 , i n c l u d i n g side v i e w and aerial v i e w  56  F i g u r e 3 . 1 4 . S t e p - p o o l residence t i m e e x p e r i m e n t c o n d u c t e d o n September 2 5 , 2 0 0 6  57  F i g u r e 3 . 1 5 . S t e p - p o o l residence t i m e e x p e r i m e n t c o n d u c t e d o n O c t o b e r 5, 2 0 0 6  .57  F i g u r e 3 . 1 6 . V e r t i c a l h y d r a u l i c gradients m e a s u r e d i n the upper ( A ) and l o w e r ( B ) reaches. S y m b o l s indicate study p e r i o d means  59  F i g u r e 3 . 1 7 . V e r t i c a l h y d r a u l i c gradients m e a s u r e d o v e r the study p e r i o d i n piezometers 1-6 i n the upper reach 60 F i g u r e 3 . 1 8 . V e r t i c a l h y d r a u l i c gradients m e a s u r e d o v e r the study p e r i o d i n piezometers 1 0 , 1 1 , 1 3 - 1 5 i n the upper reach  60  Vlll  F i g u r e 3.19. V e r t i c a l h y d r a u l i c gradients measured o v e r the study period^in the p i e z o m e t e r s 21-23 and p i e z o m e t e r s 24-26 i n the upper reach  61  F i g u r e 3.20. V e r t i c a l h y d r a u l i c gradients measured o v e r the study p e r i o d i n the p i e z o m e t e r s 60, 6 1 , 62 and 67 to 68 i n the l o w e r reach  62  F i g u r e 3.21. V e r t i c a l h y d r a u l i c gradient ( c m / c m ) versus scaled l o c a t i o n w i t h i n the channel-unit. H y d r a u l i c gradients are averaged o v e r the entire study p e r i o d .  64  F i g u r e 3.22. V e r t i c a l h y d r a u l i c gradient ( c m / c m ) versus step height (m) as a f u n c t i o n o f scaled l o c a t i o n w i t h i n the channel-unit ( X / L ) . H y d r a u l i c gradients are averaged o v e r the entire study p e r i o d  64  F i g u r e 3.23. H y d r a u l i c c o n d u c t i v i t y ( K ) for d o w n w e l l i n g ( D ) , neutral ( N ) and u p w e l l i n g ( U ) sites located i n the l o w e r (n = 24) and upper reach (n = 17). N o t e l o g scale  66  F i g u r e 3.24. H y d r a u l i c c o n d u c t i v i t y ( K ) w i t h depth o f p i e z o m e t e r i n s t a l l a t i o n for the upper and l o w e r reaches. N o t e l o g scale  66  F i g u r e 3.25. Infiltration rates o v e r the study p e r i o d . E r r o r bars represent p r o b a b l e errors based o n E q u a t i o n 2.25 67 F i g u r e 3.26. H y d r a u l i c c o n d u c t i v i t y calculated u s i n g infiltration rates and slug-tests for f i v e l o c a t i o n s . V a l u e s represent the geometric m e a n ± standard error  68  F i g u r e 3.27. W a t e r fluxes c a l c u l a t e d u s i n g D a r c y ' s L a w for each X / L category w i t h i n one step-pool channel-unit d u r i n g l o w f l o w ( Q = 1.1 L / s ) and h i g h f l o w ( Q = 15.4 L/s)........69 F i g u r e A . l . M o d e l s i m u l a t i o n s u s i n g O T I S - P for M a y 31 for the l o w e r reach  93  F i g u r e A . 2 . M o d e l s i m u l a t i o n s u s i n g O T I S - P f o r J u n e 27 for the l o w e r reach  93  F i g u r e A . 3 . M o d e l s i m u l a t i o n s u s i n g O T I S - P for Sept 21 for the upper r e a c h (a) and the l o w e r reach (b) '. 94 F i g u r e A . 4 . M o d e l s i m u l a t i o n s u s i n g O T I S - P for Sept 29 for the upper reach  95  F i g u r e A . 5 . M o d e l s i m u l a t i o n s u s i n g O T I S - P for Sept 3 0 for the l o w e r reach  .95  F i g u r e A . 6 . M o d e l s i m u l a t i o n s u s i n g O T I S - P for O c t o b e r 2 0 for the u p p e r reach (a) and the l o w e r reach (b)  96  F i g u r e A . 7 . M o d e l s i m u l a t i o n s u s i n g O T I S - P for September 29. R e s u l t s are f r o m t w o p o o l s located i n the upper reach  97  ix  ACKNOWLEDGMENTS  R.D.  Moore  M. Weiler H. Schreier E. Morgan T. Lagemaat S. Guenther J. Leach A. Bier A.  Zimmermann J. Caulkins J. Phillips  A. Home Perkins A. See ton UBC Geography/ Forestry  National Science and Engineering Research Council of Canada (NSERC-CGM) Forest Investment Account (FIA)  La mia famiglia  DEDICATION  For Claire and Kees, the Adventurers  CHAPTER ONE: INTRODUCTION 1.1.  Hyporheic exchange flow in small streams H y p o r h e i c exchange f l o w i n v o l v e s the t w o - w a y m o v e m e n t o f water b e t w e e n the  active stream channel and subsurface sediments i n the stream b e d and banks. T h e s e exchange f l o w s p l a y an important r o l e i n the f u n c t i o n i n g o f stream ecosystems. Frequent h y p o r h e i c exchange keeps stream water i n close contact w i t h c h e m i c a l l y o r b i o l o g i c a l l y active stream b e d sediments, w h i c h increases the opportunities for b i o g e o c h e m i c a l p r o c e s s i n g . Interactions between the stream c h a n n e l and subsurface influence water q u a l i t y b y generating gradients o f nutrients and d i s s o l v e d gases ( B o u l t o n et a l . 1998) a n d r e g u l a t i n g water temperatures ( M o o r e et a l . 2005a). T h e h y p o r h e i c z o n e p l a y s an important role i n ecosystem f u n c t i o n i n g i n c l u d i n g stream m e t a b o l i s m ( M u l h o l l a n d et a l . 1997), nutrient retention and c y c l i n g ( T r i s k a et a l . 1989, W o n d z e l l and S w a n s o n 1996b), habitat for benthic invertebrates (Stanford and W a r d 1988), and general e c o s y s t e m stability ( V a l e t t et a l . 1994). Spatial heterogeneity i n exchange f l o w p a t h w a y s results i n a h y d r o l o g i c a l l y l i n k e d r e g i o n beneath and adjacent to streams and r i v e r s w h e r e surface water and subsurface g r o u n d water m i x ( T r i s k a et a l . 1989, Stanford and W a r d 1993, F i n d l a y 1995). T h i s m i x i n g z o n e is defined as the " h y p o r h e i c z o n e , " i n w h i c h the water c h e m i s t r y reflects a m i x t u r e o f streamwater and groundwater. E x c h a n g e f l o w s t h r o u g h the h y p o r h e i c z o n e l i n k aquatic and terrestrial c o m p o n e n t s o f the riparian e c o s y s t e m ( W o n d z e l l and S w a n s o n 1996b). E a r l y attempts to define the p h y s i c a l boundaries o f the h y p o r h e i c z o n e w e r e based o n the distributions o f aquatic invertebrates, i n c l u d i n g h y p o g e o n (groundwater o r i g i n ) and e p i g e o n (channel o r i g i n ) . T r i s k a et a l . (1989) used solute patterns to o p e r a t i o n a l l y define the boundaries o f the h y p o r h e i c z o n e as the depth to w h i c h greater than 1 0 % advected channel water a n d less than 9 0 % g r o u n d w a t e r is present. B e n e a t h the h y p o r h e i c z o n e is the groundwater z o n e where the water c h e m i s t r y is not i n f l u e n c e d b y stream water. H y p o r h e i c zones are l i n k e d to a nested series o f f l o w paths that can travel b o t h  1  laterally and v e r t i c a l l y t h r o u g h subsurface f l o w paths, rather than entering the stream b e d i n one l o c a t i o n as does groundwater ( H a r v e y and B e n c a l a 1993, H a r v e y et a l . 1996, H a r v e y and W a g n e r 2 0 0 0 , K a s a h a r a and W o n d z e l l 2003). H y p o r h e i c exchange creates distinct zones o f aquifer discharge ( u p w e l l i n g o r o u t w e l l i n g ) f r o m the sediments into the stream c h a n n e l , and recharge ( d o w n w e l l i n g ) f r o m the stream channel into the saturated sediments. U p w e l l i n g water can s u p p l y stream o r g a n i s m s w i t h the nutrients w h i c h i n f l u e n c e p r i m a r y p r o d u c t i o n , w h i l e d o w n w e l l i n g water c a n p r o v i d e d i s s o l v e d o x y g e n and o r g a n i c matter to benthic invertebrates l i v i n g i n the sediments ( B o u l t o n et a l . 1998). D o w n w e l l i n g water also p r o v i d e s o x y g e n to fish eggs i n the subsurface ( B a x t e r and H a u e r 2 0 0 0 ) . S m a l l streams (<10 m w i d t h ) w i t h active exchange b e t w e e n surface and subsurface waters ( h y p o r h e i c exchange) are thought to facilitate n i t r o g e n - r e m o v a l and reduce the export o f nitrate ( N C v ) d o w n s t r e a m ( T r i s k a et a l . 1989, Jones and H o l m e s 1996, D u f f and T r i s k a 2 0 0 0 ) . N i t r a t e is c o n s i d e r e d a major pollutant o f aquatic systems i n m u c h o f the northern h e m i s p h e r e ( N R C 2 0 0 0 ) . T h e c o m p e t i n g processes o f nutrient retention and h y d r o l o g i c a l export are expressed i n the nutrient s p i r a l i n g concept ( W e b s t e r and Patten 1979, N e w b o l d et a l . 1981). V a l e t t et a l . (1996) p r o p o s e d a c o n c e p t u a l m o d e l suggesting that the solute retention is a product o f c h e m i c a l transformation rates and surface-subsurface interactions w h i c h increase residence times. A d d i t i o n a l studies support the theory that the extent o f subsurface interactions influences solute retention ( M u l h o l l a n d et a l . 1997, H i l l and L y m b u r n e r 1998). Solute retention and residence t i m e i n the subsurface depend p r i m a r i l y o n h y d r a u l i c gradients and h y d r a u l i c c o n d u c t i v i t y . T h e h y p o r h e i c z o n e increases the residence t i m e for water w i t h i n the stream ecosystem and enhances transient storage ( B e n c a l a 1984). T r a n s i e n t storage refers to the temporary detainment o f solutes i n s l o w m o v i n g areas such as side p o o l s or b a c k eddies relative to the faster f l o w i n g areas i n the m a i n channel. Solutes, s u c h as n i t r o g e n and phosphorus, m a y also enter the p e r m e a b l e substrate s u r r o u n d i n g the stream (i.e. h y p o r h e i c zone) and travel at a s l o w e r v e l o c i t y than that o f the m a i n c h a n n e l . Solutes detained w i t h i n transient storage zones are e v e n t u a l l y re-released b a c k into the m a i n channel, but at a s l o w e r rate relative to solutes t r a v e l i n g at the a d v e c t i o n rate i n the m a i n channel (Jones and M u l h o l l a n d 2 0 0 0 ) . In the m a i n channel, solutes are transported  2  through the h y d r o l o g i c a l processes o f a d v e c t i o n and d i s p e r s i o n (Stream S o l u t e W o r k s h o p 1990). A d v e c t i o n refers to the d o w n s t r e a m transport o f solute mass at the m e a n v e l o c i t y o f the streamwater. D i s p e r s i o n is the spreading o f the solute mass due to shear stress and m o l e c u l a r d i f f u s i o n i n the d o w n s t r e a m d i r e c t i o n . Interactions between the stream c h a n n e l and subsurface h a v e been a focus o f research for o v e r t w o decades (Jones and M u l h o l l a n d 2 0 0 0 ) . N a t u r a l r i f f l e - p o o l and stepp o o l units have been c o m m o n l y studied as c h a n n e l m o r p h o l o g y that exerts a strong c o n t r o l o n h y p o r h e i c exchange. C u r r e n t k n o w l e d g e o f h y p o r h e i c z o n e processes is based l a r g e l y o n studies conducted w i t h i n s m a l l (<10 m w i d t h ) , a l l u v i a l , l o w to m i d order g r a v e l - b e d , headwater streams w i t h r i f f l e - p o o l (e.g. T r i s k a et a l . 1989, H a r v e y et a l . 1996, H i l l et a l . 1998, W a g n e r and B r e t s c h k o 2 0 0 2 ) and step-pool m o r p h o l o g i e s (e.g., H a r v e y and B e n c a l a 1993, W o n d z e l l and S w a n s o n 1996a, V a l e t t et a l . 1996, H a g g e r t y et a l . 2 0 0 2 , K a s a h a r a and W o n d z e l l 2 0 0 3 , Storey et a l . 2 0 0 3 , A n d e r s o n et a l . 2 0 0 5 ) , as w e l l as w i t h i n high-order, l o w gradient, b r a i d e d to m e a n d e r i n g , r i v e r systems (e.g. Stanford and W a r d 1988, B o a n o et a l . 2 0 0 6 ) . T h i s thesis is concerned w i t h the characterization o f the spatial d i s t r i b u t i o n o f subsurface f l o w p a t h w a y s and associated residence times through the h y p o r h e i c z o n e w i t h i n a l o w - o r d e r , high-gradient headwater stream. S e c t i o n 1.2 r e v i e w s the current literature o n the p h y s i c a l controls o f h y p o r h e i c exchange to p r o v i d e the b a c k g r o u n d for the s p e c i f i c research objectives presented i n S e c t i o n 1.3.  1.2.  Physical controls on exchange flow Surface-subsurface interactions, or h y p o r h e i c exchange, are d r i v e n b y v a r i a t i o n s  i n h y d r a u l i c h e a d gradients as a result o f instream structural c o m p l e x i t y created f r o m large w o o d y debris (e.g. l o g j a m s ) and g e o m o r p h i c features s u c h as step-pool sequences, or breaks i n t o p o g r a p h y ( H a r v e y and B e n c a l a 1993). H y p o r h e i c f l o w varies c o n s i d e r a b l y over space scales (1 c m - 100 m ) and t i m e scales (10 s - 100 days) and at v a r i o u s rates through different types o f substrate ( H a r v e y et a l . 1996). D e p e n d i n g o n l o c a l g e o l o g y and channel m o r p h o l o g y , the extent o f the h y p o r h e i c z o n e c a n range i n l e n g t h f r o m centimeters to hundreds o f meters (Stanford and W a r d 1993). L o c a l c h a n n e l features  3  i n c l u d i n g sediment c o m p o s i t i o n , p e r m e a b i l i t y ( h y d r a u l i c c o n d u c t i v i t y ) and the b e d t o p o g r a p h y c o n t r o l the lateral extent o f the h y p o r h e i c z o n e b e l o w the saturated stream channel ( T r i s k a et a l . 1993). E x c h a n g e f l o w s o c c u r at different spatial scales i n c l u d i n g those o f (1) i n d i v i d u a l b e d f o r m s , (2) channel units and (3) reaches. T h e f o l l o w i n g sections w i l l further e x a m i n e the exchange f l o w processes o c c u r r i n g at the different spatial scales.  1.2.1.  Bedform scale  T o p o g r a p h i c features k n o w n as bedforms (riffles and dunes) d e v e l o p due to streamflow o v e r a l o o s e sediment b e d . O b s t r u c t i o n s or irregularities i n the streambed s u c h as sand riffles ( J o h n s o n 1980) or even fish redds ( T o n i n a and B u f f m g t o n 2 0 0 5 ) create a high-pressure z o n e upstream o f the o b s t r u c t i o n and a l o w pressure r e g i o n d o w n s t r e a m . F l u m e studies c o n d u c t e d at this scale h a v e s h o w n that f l o w is i n d u c e d b y pressure i m b a l a n c e s generated f r o m gradients o f temperature, density and hydrostatic head ( T h i b o d e a u x and B o y l e 1987, E l l i o t t and B r o o k s 1997). T h e process o f solutes and water f l o w i n g between high-pressure and l o w - p r e s s u r e zones i n the b e d is referred to as " a d v e c t i v e p u m p i n g e x c h a n g e " (Savant et a l . 1987, T h i b o d e a u x and B o y l e 1987). W a t e r also m o v e s through the sediments t h r o u g h the process o f "turnover". T u r n o v e r occurs as m o v i n g bedforms trap and release interstitial f l u i d . Studies i n laboratory f l u m e s h a v e i n d i c a t e d that h y p o r h e i c exchange rates increase w i t h discharge, s p e c i f i c a l l y w i t h stream f l o w v e l o c i t y , due to an increase i n the pressure difference b e t w e e n h i g h and l o w pressure regions ( T h i b o d e a u x and B o y l e 1987, E l l i o t t and B r o o k s 1997). W o n d z e l l (2005) speculated that interactions b e t w e e n s t r e a m f l o w and channel b e d f o r m s m u s t d r i v e exchange f l o w i n headwater streams; unfortunately  field  studies have not been able to incorporate finer scale effects (as studied i n flumes) into a c o h e s i v e f r a m e w o r k for exchange i n headwater streams. A s a result, the i n f l u e n c e o f i n d i v i d u a l b e d f o r m s o n h y p o r h e i c exchange f l o w has not been investigated w i t h i n a  field  setting.  4  1.2.2.  Channel-unit scale  A t the scale o f i n d i v i d u a l channel units, m o r p h o l o g i c a l features i n the stream channel, such as large w o o d y debris, create head gradients that d r i v e a d v e c t i o n o f stream water t h r o u g h the h y p o r h e i c zone. S e v e r a l authors have s h o w n that l o n g i t u d i n a l gradients i n step-pool and r i f f l e - p o o l sequences d r i v e s m a l l scale exchange f l o w b o t h v e r t i c a l l y and laterally ( H a r v e y and B e n c a l a 1993, H i l l et a l . 1998, Storey et a l . 2 0 0 3 , A n d e r s o n et a l . 2005, G o o s e f f e t a l . 2006). H y p o r h e i c exchange has been described c o n c e p t u a l l y as short p a t h w a y s that enter the subsurface and return to the stream channel at m u l t i p l e locations ( H a r v e y a n d B e n c a l a 1993). S t r e a m water f l o w i n g t h r o u g h w e l l - d e f i n e d f l o w p a t h w a y s i n the a l l u v i u m m a y enter a streambed at the top o f a riffle o r step and then return to the stream a short distance d o w n s t r e a m i n the b o t t o m o f p o o l ( F i g u r e 1.1). F l o w begins w h e n the total head i n the surface channel is greater than that i n the subsurface, r e s u l t i n g i n a negative vertical h y d r a u l i c gradient ( V H G ) , w h i c h d r i v e s stream water d o w n into the subsurface sediments. Surface water m a y m i x w i t h o r d i s p l a c e groundwater and e v e n t u a l l y return to the surface w h e r e the total head i n the subsurface is greater than the stream c h a n n e l . H y p o r h e i c exchange creates d i s t i n c t zones o f aquifer discharge ( u p w e l l i n g or o u t w e l l i n g ) f r o m the sediments into the stream c h a n n e l , and recharge ( d o w n w e l l i n g ) f r o m the stream channel into the saturated sediments. F l o w paths m a y travel v e r t i c a l l y between the channel to the subsurface o r laterally f r o m the adjacent r i p a r i a n z o n e ( W h i t e 1993, F i n d l a y 1995). W o n d z e l l and S w a n s o n (1996) a l o n g w i t h K a s a h a r a and W o n d z e l l (2003) further identified channel-unit features w h i c h d r i v e h y p o r h e i c exchange, s u c h as side channels, meander bends, g r a v e l bars and b o u l d e r or log-steps. K a s a h a r a and W o n d z e l l (2003) f o u n d that steps accounted for a p p r o x i m a t e l y 5 0 % o f the exchange f l o w s i n s e c o n d and fifth-order  stream reaches based o n the results o f a s e n s i t i v i t y analysis u s i n g g r o u n d w a t e r  f l o w m o d e l s . C h a n n e l m o r p h o l o g y has also been d o c u m e n t e d as a s i g n i f i c a n t c o n t r o l for lateral h y p o r h e i c exchanges ( V e r v i e r et a l . 1993, M o r r i c e et a l . 1997, S t o r e y et a l . 2 0 0 3 ) . T h e s e studies suggest that channel-unit f o r m and g e o m e t r y are s i g n i f i c a n t controls o n h y p o r h e i c exchange f l o w .  5  F i g u r e 1.0.1. D i a g r a m s h o w i n g a t y p i c a l subsurface f l o w p a t h w a y f r o m the stream channel to the h y p o r h e i c z o n e .  Studies e x a m i n i n g h y p o r h e i c exchange f l o w at the channel-unit scale t y p i c a l l y e m p l o y a h y d r o m e t r i c approach. T h i s approach requires an extensive n e t w o r k o f piezometers and/or w e l l s to measure h y d r a u l i c gradients and h y d r a u l i c c o n d u c t i v i t y i n order to characterize and m a p exchange f l o w s . Studies are therefore l i m i t e d to a s m a l l spatial area (e.g. H a r v e y and B e n c a l a 1993), and inter-reach c o m p a r i s o n s are c h a l l e n g i n g . A s a result, B e n c a l a (2000) expressed the need to identify the p h y s i c a l and h y d r o m e t r i c properties o f the stream system that contribute to solute transport w i t h i n the h y p o r h e i c zone, and that can be r o u t i n e l y m e a s u r e d or m a p across spatial scales. R e c e n t studies have b e g u n to e x a m i n e channel-unit s p a c i n g i n stream l o n g i t u d i n a l profiles to predict the s p a c i n g between zones o f u p w e l l i n g and d o w n w e l l i n g i n step-pool and p o o l riffle m o r p h o l o g i e s u s i n g a h y d r o m e t r i c ( A n d e r s o n et a l . 2005) or m o d e l l i n g approach ( G o o s e f f e t a l . 2006). In both these studies, channel-unit s p a c i n g , s i z e and sequence were c o n s i d e r e d important controls i n d e t e r m i n i n g exchange patterns o f u p w e l l i n g and d o w n w e l l i n g . T h e s e results suggest that a s c a l i n g relationship to identify zones o f u p w e l l i n g and d o w n w e l l i n g based o n channel-unit g e o m e t r y w o u l d be a useful t o o l for c h a r a c t e r i z i n g  6  and p r e d i c t i n g exchange f l o w i n step-pool streams. A n objective o f this thesis is to d e v e l o p a g e o m e t r i c s c a l i n g r e l a t i o n s h i p relating v e r t i c a l h y d r a u l i c gradients to c h a n n e l unit g e o m e t r y i n c l u d i n g p o o l length and d o w n s t r e a m step height, i n order to determine i f h y p o r h e i c discharge and recharge zones are a f u n c t i o n o f stream c h a n n e l l o c a t i o n . W h i l e it is k n o w n that structural c o m p l e x i t y f r o m large w o o d y debris and g e o m o r p h i c features such as step-pool sequences d r i v e exchange f l o w , there are still uncertainties r e g a r d i n g the spatial patterns o f h y p o r h e i c f l o w , and the l o c a t i o n s where h y p o r h e i c water discharges b a c k into the stream. R e c e n t studies o f h y p o r h e i c exchange i n steep, headwater streams i n the L o o k o u t C r e e k b a s i n ( O r e g o n , U S A ) h a v e g e n e r a l l y not observed coherent u p w e l l i n g o f h y p o r h e i c water b e l o w steps, as d e s c r i b e d b y the t y p i c a l f l o w p a t h w a y , despite p r e d i c t i o n s f r o m groundwater f l o w m o d e l s that u p w e l l i n g s h o u l d o c c u r ( A n d e r s o n et a l . 2 0 0 5 , G o o s e f f et al. 2 0 0 5 , W o n d z e l l 2 0 0 5 ) . T h e s e results suggest that h y p o r h e i c discharge occurs under different m e c h a n i s m s , i n c l u d i n g lateral i n f l o w or o u t w e l l i n g f r o m other l o c a t i o n s w i t h i n the stepp o o l . F o r e x a m p l e , M o o r e et a l . (2005b) observed u p w e l l i n g sites w i t h i n a concentrated z o n e o f lateral i n f l o w that w a s consistent w i t h the convergent t o p o g r a p h y and h i l l s l o p e o f a section o f headwater stream i n coastal B r i t i s h C o l u m b i a . S o l u t e tracer tests i n d i c a t e d that these u p w e l l i n g sites underwent little to no m i x i n g w i t h water f r o m the stream c h a n n e l , suggesting that u p w e l l i n g sites are a result o f lateral i n f l o w . In this scenario it is h y p o t h e s i z e d that f l o w p a t h w a y s c o u l d i n c l u d e an interaction b e t w e e n groundwater or lateral i n f l o w and h y p o r h e i c exchange pathways. A n objective o f this thesis is to determine w h i c h m o d e l best c o n c e p t u a l i z e s exchange f l o w w i t h i n steep, step-pool streams. Subsurface f l o w p a t h w a y s that create zones o f discharge (i.e. u p w e l l i n g or o u t w e l l i n g ) and recharge (i.e. d o w n w e l l i n g ) w i t h i n step-pool units can be d e s c r i b e d u s i n g three different conceptual m o d e l s : 1.  M o d e l l a - Represents a t y p i c a l f l o w p a t h w a y i n w h i c h d o w n w e l l i n g o f water occurs at the top o f a riffle or step and returns to the stream c h a n n e l a short distance d o w n s t r e a m i n the b o t t o m o f p o o l . F l o w is a l i g n e d w i t h the c h a n n e l creating u p w e l l i n g i n the p o o l . M o d e l s l b and 2 represent p o s s i b l e alternative hypotheses to the t y p i c a l f l o w p a t h w a y and c o u l d e x p l a i n the l a c k o f o b s e r v e d u p w e l l i n g i n p r e v i o u s studies.  7  2.  M o d e l l b - H y p o r h e i c f l o w is a l i g n e d w i t h the channel and d r i v e n b y v e r t i c a l h y d r a u l i c gradients as per M o d e l l a , but f l o w creates u p w e l l i n g or o u t w e l l i n g sites at another l o c a t i o n w i t h i n the step-pool, s u c h as d i r e c t l y b e l o w the step. M o d e l s l a and l b do not i n c l u d e a lateral i n f l o w component, w h i c h is represented b y M o d e l 2.  3.  M o d e l 2 - H y p o r h e i c exchange f l o w i n c l u d e s a lateral i n f l o w component i n w h i c h zones o f u p w e l l i n g are a result o f lateral i n f l o w from the r i p a r i a n zone and adjacent h i l l s l o p e due to convergent topography. A s w e l l , f l o w pathways c o u l d i n c l u d e an interaction between groundwater or lateral i n f l o w and h y p o r h e i c exchange pathways. F o r e x a m p l e , h y p o r h e i c water c o u l d f l o w l a t e r a l l y into the r i p a r i a n z o n e after i n f i l t r a t i n g i n a step, then f l o w laterally into the c h a n n e l .  1.2.3.  Reach scale - Transient storage processes  H y p o r h e i c exchange processes are t y p i c a l l y studied at the reach scale u s i n g a transient storage m o d e l ( T S M ) c o n s i s t i n g o f a o n e - d i m e n s i o n a l a d v e c t i o n - d i s p e r s i o n equation w i t h an a d d i t i o n a l term for transient storage ( B e n c a l a and W a l t e r s 1983, R u n k e l 1998). T h e T S M p r o v i d e s reach-scale estimates o f the solute transport processes o f a d v e c t i o n , d i s p e r s i o n , transient storage and lateral i n f l o w , b y s i m u l a t i n g the breakthrough curves ( B T C ) generated f r o m stream tracer injections (e.g. D ' A n g e l o et a l . 1993, H a r v e y and B e n c a l a 1993, M o r r i c e et a l . 1997, M u l h o l l a n d et a l . 1997, G o o s e f f et al. 2003). A d d i t i o n a l l y , T S M s h a v e been w i d e l y a p p l i e d to l i n k h y d r o l o g i c a l transport parameters to b i o l o g i c a l processes (e.g. V a l e t t et a l , 1996, M u l h o l l a n d et a l . 1997, G o o s e f f et a l . 2004). Transient storage i n c l u d e s (1) i n - c h a n n e l storage i n "dead z o n e s " (i.e. side p o o l s , back eddies) and (2) storage i n the h y p o r h e i c zone. Current T S M s , s u c h as the O n e D i m e n s i o n a l Transport w i t h I n f l o w and Storage ( O T I S - P ; R u n k e l 1998) m o d e l , l u m p b o t h transient storage zones together into a s i n g l e m o d e l - d e r i v e d estimate o f the crosssectional area o f the transient storage z o n e ( A s ) . E x c h a n g e between the m a i n channel and the transient storage z o n e is c o n t r o l l e d b y a transient exchange coefficient (a), w h i c h is c o n s i d e r e d an estimate o f h y p o r h e i c exchange. T y p i c a l l y , the m o d e l - d e r i v e d transient  storage area ( A s ) parameter is assumed to represent storage i n the streambed, or the h y p o r h e i c z o n e ( B e n c a l a et a l . 1993), due to the i n a b i l i t y to separate i n - c h a n n e l storage f r o m storage i n the h y p o r h e i c zone. H o w e v e r , p r e v i o u s studies have suggested that b o t h transient storage m e c h a n i s m s o c c u r at the reach scale ( H a r v e y and B e n c a l a 1993, G o o s e f f et a l . 2 0 0 3 , W o n d z e l l 2005). H a r v e y and B e n c a l a (1993) observed a difference i n the transient storage residence-times estimated f r o m stream tracer experiments and h y d r o m e t r i c w e l l breakthrough d y n a m i c s , and c o n c l u d e d that at the reach-scale solute transport processes are sensitive to b o t h transient storage m e c h a n i s m s . A t t e m p t s to d i s t i n g u i s h between in-channel storage and h y p o r h e i c transient storage h a v e resulted i n the development o f T S M s w i t h t w o storage or m u l t i p l e e x p o n e n t i a l residence t i m e distributions (Castro and H o r n b e r g e r 1991, C h o i et a l . 2 0 0 0 , G o o s e f f et a l . 2 0 0 4 ) . H o w e v e r , these m o d e l s are often difficult to c o r r e c t l y parameterize u s i n g current field techniques ( R u n k e l 2002). A s a result, there is a need to better understand processes at the p o i n t and channel-unit scale for interpretation o f reach scale tracer tests, p a r t i c u l a r l y i n terms o f the relative roles o f h y p o r h e i c exchange and transient storage i n p o o l s . Solutes detained w i t h i n dead zones are e v e n t u a l l y re-released b a c k into the m a i n c h a n n e l , but at a s l o w e r rate than the m a i n solute p u l s e (Jones and M u l h o l l a n d 2 0 0 0 ) . T h i s s l o w release is manifested b y a l o n g tail i n the breakthrough curves ( B e n c a l a and W a l t e r s 1983, Stream S o l u t e W o r k s h o p 1990, R u n k e l 1998, C h a p r a and R u n k e l 1999, C h o i et a l . 2 0 0 0 ) . T h e O T I S - P transient storage m o d e l assumes that the late-time residence times o f solutes w i t h i n these dead zones are e x p o n e n t i a l l y distributed ( R u n k e l 1998). H o w e v e r , recent w o r k e r s h a v e s h o w n residence t i m e distributions that are better characterized u s i n g a l o g n o r m a l d i s t r i b u t i o n ( W o r m a n et a l . 2 0 0 2 ) , or d i s p l a y e d scaleinvariant b e h a v i o r , w i t h the u n d e r l y i n g residence t i m e f o l l o w i n g a p o w e r l a w d i s t r i b u t i o n ( H a g g e r t y et a l . 2 0 0 2 ) . G o o s e f f et a l . (2003) characterized exchange processes i n three reaches at L o o k o u t C r e e k b a s i n u s i n g t w o solute transport m o d e l s w i t h different residence t i m e distributions (exponential and p o w e r - l a w ) . Transient storage w i t h i n a b e d r o c k reach f o l l o w e d an e x p o n e n t i a l d i s t r i b u t i o n w i t h a m e a n residence t i m e o f three hours. I n contrast, transient storage i n an a l l u v i a l reach f o l l o w e d a p o w e r l a w d i s t r i b u t i o n w i t h a  9  m e a n residence t i m e o f > 100 hours. A l t h o u g h this study m a k e s the a s s u m p t i o n that the a l l u v i a l reach represents both in-stream and transient storage processes, whereas the b e d r o c k reach represents o n l y i n - c h a n n e l transient storage due to the absence o f a h y p o r h e i c z o n e , these results s h o w that in-ehannel features s u c h as p o o l s and b a c k eddies do contribute to transient storage i n s m a l l , headwater streams. H a r v e y et a l . (1996) assumed that the residence-time for i n - c h a n n e l transient storage to be v e r y short and is therefore accounted i n the d i s p e r s i o n coefficient rather than the transient storage coefficient. H o w e v e r , G o o s e f f et a l . (2003) h i g h l i g h t s the v a r i a b i l i t y i n residence times and late-time distributions that can o c c u r d e p e n d i n g o n the d o m i n a n t transient storage m e c h a n i s m at the reach scale. A l t h o u g h it is sometimes p o s s i b l e to e l i m i n a t e one o f the transient storage m e c h a n i s m s based o n m o r p h o l o g y (i.e. a l l u v i a l versus b e d r o c k reach), studies h a v e not been able to isolate the relative c o n t r i b u t i o n from each transient storage z o n e i n stream reaches w h e r e b o t h processes o c c u r together. A s w e l l , the c o n t r i b u t i o n f r o m i n - c h a n n e l storage has o n l y been e x a m i n e d at the reach scale, and not at the channel-unit scale w i t h i n i n d i v i d u a l p o o l s u s i n g a stream tracer approach. In stream reaches w h e r e p o o l s are located, no p r e v i o u s study has s p e c i f i c a l l y treated an i n d i v i d u a l p o o l , as a "stream r e a c h " to test the assumptions o f the transient storage m o d e l . T h i s a p p l i c a t i o n o f the T S M m a y h e l p to characterize the spatial extent and residence times o f transient storage as measured f r o m stream tracer experiments. A k e y objective o f this thesis is to quantify transient storage and estimate separate travel times for h y p o r h e i c and surface-water transient storage zones. Several studies have sought to i d e n t i f y the factors that c o n t r o l transient storage and h y p o r h e i c exchange, i n c l u d i n g channel m o r p h o l o g y ( H a r v e y and B e n c a l a 1993, D ' A n g e l o et a l . 1993, K a s a h a r a and W o n d z e l l 2 0 0 3 , G o o s e f f et a l . 2 0 0 3 , W o n d z e l l 2 0 0 5 ) , discharge ( H a r v e y et a l . 1996, W o n d z e l l 2 0 0 5 , Z a r n e t s k e et a l . 2 0 0 7 ) , parent material ( M o r r i c e et a l . 1997), groundwater i n f l o w s ( H a r v e y and B e n c a l a 1993, W r o b l i c k y et a l . 1998) and stream c o m p l e x i t y (Patschke 1999, G o o s e f f et a l . 2 0 0 7 ) . D ' A n g e l o et a l . (1993) u s e d the stream tracer approach to investigate the influence o f g e o m o r p h o l o g i c a l constraint o n transient storage w i t h i n a fifth-order stream i n the L o o k o u t C r e e k b a s i n . Transient storage was l o w e r i n constrained stream reaches (n = 5) as c o m p a r e d to reaches that w e r e u n c o n s t r a i n e d (n = 2). T h i s study also reported a  10  decrease i n transient storage w i t h stream order (first-order to fifth-order), i n d i c a t i n g that transient storage is v a r i a b l e w i t h spatial scale. M o r r i c e et a l . (1997) c o m p a r e d three headwater streams w i t h different parent l i t h o l o g i e s and s h o w e d that the extent o f h y p o r h e i c exchange increased w i t h increased h y d r a u l i c c o n d u c t i v i t y . T h i s study also suggested that c o n d u c t i v i t y and infiltration c a p a c i t y influences the amount o f water that infiltrates the stream b e d and enters the h y p o r h e i c z o n e . W o n d z e l l (2005) conducted stream tracer experiments i n t w o s m a l l , steep-mountain streams i n the H . J . A n d r e w s E x p e r i m e n t a l Forest ( O r e g o n , U S A ) at l o w b a s e f l o w and h i g h b a s e f l o w c o n d i t i o n s . T h i s study found that transient storage v a r i e d w i t h channel m o r p h o l o g y and to a lesser extent w i t h discharge. A n increase i n stream discharge is h y p o t h e s i z e d to result i n a greater wetted area, w h i c h p r o v i d e s for enhanced h y p o r h e i c exchange. In headwater streams, the i n f l u e n c e o f discharge o n h y p o r h e i c exchange and spatial extent w i l l v a r y w i t h catchment wetness, h i l l s l o p e c o n n e c t i v i t y and the strength o f h y d r a u l i c gradients to the stream channel ( W o n d z e l l and S w a n s o n 1996a). W r o b l i c k y et a l . (1998) a p p l i e d a 2 - D groundwater f l o w m o d e l to study the surface-subsurface interactions w i t h i n t w o  first-order  headwater  streams i n N e w M e x i c o ( U S A ) , and found that the spatial extent o f the h y p o r h e i c z o n e decreased b y a p p r o x i m a t e l y 5 0 % d u r i n g h i g h e r f l o w s . A s e n s i t i v i t y a n a l y s i s i n d i c a t e d that exchange f l o w s w e r e g o v e r n e d b y the interplay o f e x t r i n s i c (e.g. v a r i a b i l i t y i n p r e c i p i t a t i o n and recharge) and i n t r i n s i c g e o m o r p h o l o g i c a l and h y d r o l o g i c a l factors (e.g. hydraulic conductivity). Stream tracer experiments have c o n f i r m e d that solute transport processes v a r y w i t h discharge at the reach scale ( L e g r a n d - M a r c q and L a u d e l o u t 1985, D ' A n g e l o et a l . 1993, H a r v e y et a l . 1996, M o r r i c e et a l . 1997, H a r t et a l . 1999, P a t s c h k e 1999, W o n d z e l l 2 0 0 5 ) ; although c o n f l i c t i n g responses h a v e been documented. M o r r i c e et a l . (1997) c o m p a r e d transport processes d u r i n g 4 tracer releases w i t h i n one headwater stream reach, and o b s e r v e d a decrease i n transient storage ( A s ) w h i l e transient exchange (a) increased w i t h i n c r e a s i n g discharge. S i m i l a r l y , D ' A n g e l o et a l . (1993) f o u n d that the transient exchange coefficient increased w i t h discharge and that the ratio o f storage z o n e to cross-sectional area ( A s / A ) decreased w i t h discharge. A d d i t i o n a l studies h a v e reported s i m i l a r trends ( H a r v e y et a l . 1996, H a r t et a l . 1999, W o n d z e l l  11  2 0 0 5 ) , h o w e v e r trends that are i n c o n t r a d i c t i o n to those studies have also been d o c u m e n t e d ( L e g r a n d - M a r c q and L a u d e l o u t 1985, H a r t et a l . 1999, P a t s c h k e 1999). P a t s c h k e (1999) o b s e r v e d an increase i n A w i t h discharge i n reaches w i t h a s  greater degree o f stream c o m p l e x i t y . H a r t et a l . (1999) reported that transient storage r e m a i n e d constant w i t h discharge, but transient exchange increased as discharge increased. T r a c e r tests b y L e g r a n d - M a r c q and L a u d e l o u t (1985) s h o w e d a r a p i d decrease i n A s as discharge increased to a threshold v a l u e o f 2 L / s , after w h i c h transient storage r e m a i n e d constant to a v a l u e o f 12 L / s . L e g r a n d - M a r c q and L a u d e l o u t (1985) and P a t s c h k e (1999) o b s e r v e d that the transient exchange coefficient r e m a i n e d constant w i t h discharge. T h e s e c o n f l i c t i n g results i n the literature h i g h l i g h t the uncertainty s u r r o u n d i n g the response o f transient storage area and exchange parameters to discharge i n s m a l l headwaters streams. Part o f this uncertainty is attributed to the T S M , w h i c h is sensitive to e x p e r i m e n t a l d e s i g n ( W a g n e r and H a r v e y 1997) and parameter fitting routines ( R u n k e l 1998). H a r v e y et a l . (1996) e x p l a i n e d that because stream v e l o c i t y increases w i t h discharge, a m a j o r i t y o f the o b s e r v e d differences i n parameters for intra-reach or interreach c o m p a r i s o n s w i l l result from a s w i t c h i n d o m i n a n c e o f a d v e c t i v e f l o w o v e r transient storage, and not from a change i n p h y s i c a l processes. F o r e x a m p l e , W o n d z e l l (2005) f o u n d that the results o f T S M s i m u l a t i o n s were at odds to h y d r o m e t r i c data, w h i c h s h o w e d little change i n the l o c a t i o n and extent o f the h y p o r h e i c z o n e despite a four-fold increase i n discharge (1.0 - 11.5 L / s ) . T h e s e results are s l i g h t l y c o n f o u n d e d b y h i g h uncertainty i n parameter values as indicated b y D a m k o h l e r values w h i c h w e r e >2 (ranged f r o m 2.4 to 21.1) for 5 o f the 9 s i m u l a t e d tracer experiments. H o w e v e r , W o n d z e l l (2005) cautioned that T S M c o m p a r i s o n s s h o u l d be restricted to stream tracer experiments p e r f o r m e d w i t h i n different reaches o f a s i n g l e stream under c o m p a r a b l e f l o w c o n d i t i o n s .  1.2.4.  Study objective and scale-dependent questions  T h e b r o a d objective o f this study is to e x a m i n e h o w h y p o r h e i c exchange processes v a r y s p a t i a l l y and t e m p o r a l l y w i t h i n a l o w - o r d e r , high-gradient headwater stream under a range o f f l o w c o n d i t i o n s . T h i s study f o c u s e d o n the characterization o f the  12  spatial d i s t r i b u t i o n o f subsurface f l o w p a t h w a y s a n d associated travel times through the h y p o r h e i c and.surface-water transient storage zones u s i n g b o t h a h y d r o m e t r i c and stream tracer approach. A s i g n i f i c a n t c h a l l e n g e o f h y p o r h e i c z o n e research has been to scale up s m a l l scale p h y s i c a l measurements to the results o f reach-scale stream tracer injections. A s a result, specific questions are a p p l i e d to the spatial scale o f interest i n c l u d i n g the reach scale, the channel-unit scale and the l o c a l or point scale. A m u l t i p l e scale approach to e x a m i n i n g h y p o r h e i c e x c h a n g e processes has not been e x p l i c i t l y a p p l i e d i n p r e v i o u s research. It is expected that the v a r i a b i l i t y i n solute transport processes observed at the reach scale, s p e c i f i c a l l y transient storage area ( A s ) and the transient exchange coefficient (a), c a n be e x p l a i n e d u s i n g observations m a d e at s m a l l e r spatial scales.  T h e f o l l o w i n g research questions w i l l be addressed i n this thesis:  1.  H o w do solute transport processes at the reach-scale, s p e c i f i c a l l y transient storage area a n d transient exchange, v a r y o v e r space and t i m e (i.e. w i t h discharge)? O f specific interest is the c o n t r i b u t i o n o f h y p o r h e i c and surfacewater transient storage zones to transient storage area ( A s ) .  2.  , C a n separate residence times for h y p o r h e i c and surface-water transient storage zones be quantified u s i n g a stream tracer approach at the c h a n n e l u n i t scale?  3.  W h e r e are zones o f h y p o r h e i c discharge and recharge located w i t h i n the channel-unit, and do exchange f l o w s v a r y w i t h p o s i t i o n i n the channel-unit and d o w n s t r e a m step height?  4.  H o w do water fluxes at the p o i n t scale v a r y , and can these be quantified a n d " s c a l e d u p " to reach scale estimates o f h y p o r h e i c exchange?  4' T h e f o l l o w i n g four chapters e x a m i n e the research q u e s t i o n i n detail. C h a p t e r 2 describes the study area, f i e l d and l a b o r a t o r y methods, and data analysis. R e s u l t s are presented i n C h a p t e r 3 and are d i s c u s s e d i n C h a p t e r 4. T h e m a i n c o n c l u s i o n s o f the study and r e c o m m e n d a t i o n s for future research are s u m m a r i z e d i n C h a p t e r 5.  13  CHAPTER TWO: METHODS T h i s chapter p r o v i d e s a detailed d e s c r i p t i o n o f the study l o c a t i o n and study reaches. A n o v e r v i e w o f the study design, a l o n g w i t h the methods used for field s a m p l i n g , laboratory analysis a n d data analysis are also described.  2.1.  Study location T h i s study w a s conducted i n the U n i v e r s i t y o f B r i t i s h C o l u m b i a ( U B C )  Malcolm  K n a p p R e s e a r c h Forest ( 4 9 ° 16' N , 1 2 2 ° 3 4 ' W ) , located i n the temperate Fraser V a l l e y foothills o f the C o a s t M o u n t a i n s , a p p r o x i m a t e l y 6 0 k m east o f V a n c o u v e r , B r i t i s h C o l u m b i a ( F i g u r e 2.1). T h e research forest is located w i t h i n the C o a s t a l W e s t e r n H e m l o c k (Tsuga heterophylla) b i o g e o c l i m a t i c zone ( C o u p e et al. 1991). T h i s z o n e is characteristic o f the m a r i t i m e c l i m a t e w i t h wet, m i l d winters and w a r m , d r y summers. M e a n annual p r e c i p i t a t i o n at the U B C R e s e a r c h Forest headquarters (147 m elevation) i s a p p r o x i m a t e l y 2 1 8 4 m m ( E n v i r o n m e n t C a n a d a 1993), o f w h i c h 7 0 % falls p r i m a r i l y as r a i n between O c t o b e r a n d A p r i l due to P a c i f i c frontal systems. S n o w f a l l c o m p r i s e s o n l y 5 % o f the total annual p r e c i p i t a t i o n at the headquarters. T h e h i g h e r e l e v a t i o n areas are s n o w c o v e r e d for a p p r o x i m a t e l y four months o f the year. S t r e a m f i o w is t y p i c a l l y v e r y " f l a s h y " i n response to r a i n f a l l . P e r i o d s o f l o w f l o w d o m i n a t e d u r i n g the s u m m e r m o n t h s (base f l o w <1 L / s ) . P e r i o d s o f peak f l o w correspond to fall a n d w i n t e r storms. R u n o f f generation processes are d o m i n a t e d b y subsurface f l o w a n d saturation o v e r l a n d f l o w ( H u t c h i n s o n a n d M o o r e 2000). S o i l s w i t h i n the watershed are h i g h l y permeable, a n d are d o m i n a n t l y s h a l l o w • p o d z o l s (averaging about 1 m deep) f o r m e d f r o m g l a c i a l t i l l ! S o i l parent material is g l a c i o f l u v i a l i n o r i g i n ( K l i n k a a n d K r a j i n a 1986). S o i l s are t y p i c a l l y u n d e r l a i n b y basal t i l l o v e r granitic b e d r o c k ( K l i n k a a n d K r a j i n a 1986). I n some locations, the soils d i r e c t l y o v e r l i e b e d r o c k . G e o l o g i c a l c o n d i t i o n s have contributed to the observed l o w b a c k g r o u n d concentrations o f nitrogen, phosphorus and l o w c o n d u c t i v i t y i n surface water.  14  C h e m i c a l l y the study stream i s nearly pristine w i t h l o w concentrations o f major ions ( F e l l e r and K i m m i n s 1984). V e g e t a t i v e c o v e r a l o n g East C r e e k is d o m i n a t e d b y coniferous forest c o n s i s t i n g o f Douglas-fir  (Pseudotsuga menziesii) and western cedar {Thuja plicata), that are  a p p r o x i m a t e l y 130 years o l d . R e d alder (Alnus rubra) occurs a l o n g the stream banks i n patches. S e l e c t i v e forest harvesting o c c u r r e d adjacent to the stream i n the 1 9 7 0 ' s , but instream large w o o d y debris was retained i n the stream. T h e study w a s c o n d u c t e d w i t h i n the East C r e e k watershed, a 4 k m l o n g s e c o n d order stream that drains a p p r o x i m a t e l y 100 h a at the study r e a c h ( F i g u r e 2.2). East C r e e k is located upstream f r o m the confluence o f S p r i n g C r e e k . R e s e a r c h w a s c o n d u c t e d f r o m M a y to O c t o b e r , 2 0 0 6 . H y p o r h e i c exchange processes were e x a m i n e d w i t h i n a 100 m l o n g section o f East C r e e k , w h i c h extends d o w n s t r e a m f r o m a culvert c r o s s i n g at road M to the stream's c o n f l u e n c e at S p r i n g C r e e k ( F i g u r e 2.3). T h i s stream was selected due to its h i g h structural c o m p l e x i t y and v a r i e t y o f g e o m o r p h i c features, w h i c h i n c l u d e a p p r o x i m a t e l y 7-10 p o o l - c a s c a d e sub-units w i t h i n the reach (Patschke 1999). Structural c o m p l e x i t y f r o m i n - s t r e a m large w o o d y debris and heterogeneous substrate d e p o s i t i o n has contributed to distinct pool-cascade sequences w i t h b o t h b o u l d e r and l o g steps.  F i g u r e 2 . 1 . S i t e m a p s h o w i n g l o c a t i o n o f U B C R e s e a r c h Forest  15  I I I I I I I 0 0.1250.25 0.5 Kilometers Projection: UTM Zone 10 Datum: NAD 1983  1:15,500  Legend ==== Roads Streams Lakes Cutblocks/Thinning Study Location  F i g u r e 2.2. Site m a p s h o w i n g East C r e e k drainage b a s i n  Legend 100 Meters  =  Roads Streams Contour Lines (5m)  Projection: UTM Zone 10 Datum: NAD 1983 1:2,500  it  Reach Upper Boundary (EC sample)  •  Reach Lower Boundary (EC sample)  A  Tracer Injection Site  Figure 2.3. East Creek study reach with upper and lower sub-reaches and stream tracer experiment injection and sampling locations  T h e reach has an e l e v a t i o n drop o f a p p r o x i m a t e l y 2 0 m ( F i g u r e 2.4), and varies i n m o r p h o l o g y and substrate a l o n g the study l e n g t h ( F i g u r e 2.5, F i g u r e 2.6). T h e c h a n n e l w i d t h for the reach is 2.5 - 3 m w i d e . A large sediment d a m exists at m i d reach, w h i c h d i v i d e s the reach into t w o sections. A b o v e the sediment d a m , the top 45 m o f the reach has a gradient o f a p p r o x i m a t e l y 4 % . Stream m o r p h o l o g y consists o f l o w gradient  riffle-  r u n and p o o l sequences w i t h i n natural l o g steps, and the substrate is c o m p o s e d o f fine to coarse gravel and m e d i u m s i z e cobbles. D u r i n g periods o f l o w f l o w , the stream goes subsurface t h r o u g h preferential f l o w pathways i n the sediment d a m (at a p p r o x i m a t e l y 4 2 m d o w n s t r e a m from the culvert at M road). D i r e c t l y b e l o w the l o g j a m , the stream is b r a i d e d for a p p r o x i m a t e l y 5-10 m . A t the b e g i n n i n g o f the study p e r i o d , seepage f r o m a cutbank was o b s e r v e d at this l o c a t i o n . T h e b o t t o m 45 m section has a gradient o f a p p r o x i m a t e l y 1 2 % and contains p o o l cascade sequences w i t h b o u l d e r and l o g steps. F i n e to m e d i u m gravels i n d e p o s i t i o n areas b e h i n d steps and large anchor cobbles and boulders contribute to a s p a t i a l l y heterogeneous substrate. D u r i n g periods o f h i g h f l o w (> 100 L / s ) , a secondary channel at a p p r o x i m a t e l y 55 m d o w n s t r e a m from the culvert becomes active. S e d i m e n t transport processes i n East C r e e k v a r y seasonally w i t h discharge. S i n c e 2 0 0 3 , a p p r o x i m a t e l y 10 to 15 sediment m o b i l i z i n g events per year have been observed, o f w h i c h 3 to 4 peak r a i n f a l l events d u r i n g the fall and w i n t e r months o f each year m o v e the m a j o r i t y o f sediment (J. C a u l k i n s , pers. c o m m . ) . T h e s e events result i n the greatest g e o m o r p h i c changes to the channel.  18  0  20  40  60  80  100  D i s t a n c e d o w n s t r e a m (m)  F i g u r e 2.4. E l e v a t i o n gradient for E a s t C r e e k study reach d o w n s t r e a m from a culvert c r o s s i n g at R o a d M  2.2.  Study design T h i s study e m p l o y e d a c o m b i n a t i o n o f h y d r o m e t r i c data and solute i n j e c t i o n  experiments to characterize subsurface f l o w p a t h w a y s . T r a c e r experiments were designed: (1) to e x a m i n e the extent o f stream-subsurface interactions at the reach scale (Stream Solute W o r k s h o p 1990), (2) to measure the relative c o n n e c t i v i t y o f the subsurface to the stream c h a n n e l , (3) to quantify storage i n i n d i v i d u a l p o o l s d u r i n g r e a c h scale tracer injections, and (4) to p r o v i d e reach scale estimates o f the processes o f a d v e c t i o n , d i s p e r s i o n , lateral exchange and transient storage. Stream tracer experiments w e r e c o n d u c t e d at a range o f f l o w c o n d i t i o n s to e x a m i n e h o w transport processes v a r y w i t h discharge. T r a c e r injections at i n d i v i d u a l step-pool units w e r e used to identify l o c a t i o n s o f h y p o r h e i c discharge, as w e l l as to estimate separate travel times for h y p o r h e i c and surface-water transient storage zones. H y d r o m e t r i c data i n c l u d e d streamflow, h y d r a u l i c head, h y d r a u l i c c o n d u c t i v i t y m e a s u r e d u s i n g piezometers, a l o n g w i t h direct measurements o f infiltration into the stream b e d .  19  F i g u r e 2.6. U p s t r e a m v i e w o f l o w e r reach o f East C r e e k s h o w i n g steps and p o o l complexes  2.3.  Stream discharge and geometry  2.3.1.  Discharge measurements  S t r e a m f l o w , or discharge, was measured u s i n g the constant-rate salt i n j e c t i o n m e t h o d ( M o o r e 2004a). T h i s m e t h o d i n v o l v e s injecting a conservative solute tracer s o l u t i o n o f k n o w n concentration into a stream at a constant rate and then m e a s u r i n g the stream water electrical c o n d u c t i v i t y ( E C ) as it b e c o m e s u n i f o r m l y m i x e d across the stream some distance b e l o w the injection point. D o w n s t r e a m m i x i n g was m o n i t o r e d u s i n g a c o n d u c t i v i t y probe to v e r i f y lateral and v e r t i c a l m i x i n g at a d o w n s t r e a m l o c a t i o n . T r a c e r injections w e r e p e r f o r m e d a c c o r d i n g to procedures o u t l i n e d b y M o o r e (2004a). S o d i u m c h l o r i d e ( N a C l ) or c o m m o n table salt w a s used as a tracer because it is i n e x p e n s i v e , r e a d i l y a v a i l a b l e and e n v i r o n m e n t a l l y b e n i g n for the concentrations and durations i n v o l v e d i n discharge measurement ( M o o r e 2004a). C h l o r i d e is a c o n s e r v a t i v e i o n that occurs i n l o w b a c k g r o u n d concentrations (0.6-1.2 m g / L ) i n East C r e e k ( F e l l e r and K i m m i n s 1984). S o d i u m c h l o r i d e was injected into E a s t C r e e k to raise the concentration above b a c k g r o u n d u n t i l the c o n c e n t r a t i o n reached plateau ( a p p r o x i m a t e l y 2-3 m g / L ) . S o l u t i o n s o f N a C l and water w e r e m i x e d i n the laboratory u s i n g p r e - w e i g h e d bags o f salt. In the f i e l d , solutions were v i g o r o u s l y shaken to ensure that the added salt was c o m p l e t e l y d i s s o l v e d . S o l u t i o n s were prepared at concentrations b e l o w the saturation p o i n t o f 238 g / L ( W e b s t e r and E h r m a n 1996). F o r injection trials p r i o r to A u g u s t 11, s o l u t i o n s were added at a constant rate u s i n g a M a r i o t t e bottle ( S t o r y et al. 2 0 0 3 , M o o r e 2 0 0 4 b ) . T h e M a r i o t t e bottle consisted o f a 30 L c a r b o y sealed at the top w i t h a rubber stopper. T h e release rate was c o n t r o l l e d u s i n g a pipette connected to a spigot at the base o f the M a r i o t t e bottle. R e l e a s e rates w e r e m e a s u r e d for each tracer experiment b o t h p r e - i n j e c t i o n and post-injection since changes i n b a r o m e t r i c pressure c a n affect the release rates ( W e b s t e r and E h r m a n 1996). B e g i n n i n g o n A u g u s t 11, a battery-operated S o l i n s t M o d e l 4 1 0 peristaltic p u m p connected to a 30 L d r u m was used to c o n d u c t l o n g e r d u r a t i o n injections d u r i n g base f l o w c o n d i t i o n s . A voltage regulator attached to the 12 v o l t battery was u s e d to prevent fluctuations i n the release rate due to changes i n the battery voltage.  21  E l e c t r i c a l c o n d u c t i v i t y w a s measured as a surrogate for tracer concentration d u r i n g stream tracer tests. E C increases l i n e a r l y w i t h salt concentration and is c o n s i d e r e d an e c o n o m i c a l alternative to direct measurements o f c h l o r i d e ( G o o s e f f and M c G l y n n 2 0 0 5 , W o n d z e l l 2005). E C w a s measured at the d o w n s t r e a m end o f the stream reach d u r i n g discharge measurements u s i n g either a W T W ™ L F 3 4 0 , 3 4 0 i or 3 5 0 i c o n d u c t i v i t y meter. A n o n - l i n e a r c a l i b r a t i o n b u i l t into the W T W ™ meter w a s used to correct a u t o m a t i c a l l y the E C values to a standard temperature o f 2 5 ° C . D o w n s t r e a m changes i n c o n d u c t i v i t y w e r e r e c o r d e d e v e r y 30 seconds u n t i l a steady-state plateau w a s reached (Stream Solute W o r k s h o p 1990). T h e L F 3 4 0 and 3 4 0 i W T W ™ c o n d u c t i v i t y meters w e r e attached to a C a m p b e l l S c i e n t i f i c C R 5 1 0 data logger to r e c o r d measurements. E C values measured u s i n g the W T W ™ 3 5 0 i meter w e r e recorded to the m e t e r ' s internal m e m o r y . O n c e steady-state w a s a c h i e v e d , as determined as a constant conductivity reading for more than 10 minutes at the farthest downstream location, the injection was stopped, and c o n d u c t i v i t y r e c o r d i n g s c o n t i n u e d u n t i l the stream water returned to b a c k g r o u n d l e v e l s to characterize the " f a l l i n g l i m b " o f the B T C .  2.3.2.  Discharge calculations  D i s c h a r g e w a s cal culated as:  Q=  (2.1) k • (ECss -  ECBG)  w h e r e Q is stream discharge ( L / s ) , q is the injection rate o f tracer s o l u t i o n ( L / s ) , k is the c a l i b r a t i o n coefficient, E C s s is steady state c o n d u c t i v i t y (u.S/cm), and E C B G is the b a c k g r o u n d or pre-release c o n d u c t i v i t y ( u S / c m ) . T h e c a l i b r a t i o n coefficient (k) represents the slope o f the r e l a t i o n b e t w e e n relative concentration ( L / L ) and electrical c o n d u c t i v i t y ( f i S / c m ) . E l e c t r i c a l c o n d u c t i v i t y is l i n e a r l y related to relative concentration ( R C ) for d i l u t e solutions. A s a result, the relative concentration at steady state ( R C s s ) c a n be d e t e r m i n e d f r o m E C measurements: RCss = (ECss - ECBG) -k  (2.2)  22  S e c o n d a r y and c a l i b r a t i o n solutions w e r e created i n the f i e l d for each tracer e x p e r i m e n t i n order to determine k. T o p e r f o r m the c a l i b r a t i o n , t w o 1-L v o l u m e s o f stream water w e r e first measured u s i n g a glass v o l u m e t r i c flask and p o u r e d into s a m p l e bottles. T h e secondary s o l u t i o n consisted o f 10 m L o f the p r i m a r y i n j e c t i o n s o l u t i o n , m e a s u r e d u s i n g a glass pipette, m i x e d w i t h 1 L o f stream water. T h e s e c o n d a r y s o l u t i o n w a s then added i n 10 m L increments into a separate 1 L o f stream water to create the c a l i b r a t i o n s o l u t i o n . E l e c t r i c a l c o n d u c t i v i t y w a s r e c o r d e d w i t h each a d d i t i o n o f s e c o n d a r y s o l u t i o n i n t o the c a l i b r a t i o n s o l u t i o n . R e l a t i v e concentration was c a l c u l a t e d as:  RC = S,  RC  (2.3)  VT  w h e r e S A is the v o l u m e o f secondary s o l u t i o n added ( m L ) , R C  s e c  is the r e l a t i v e  concentration o f the secondary s o l u t i o n (10 m L / 1 0 1 0 m L ) , and V T is the total v o l u m e o f the c a l i b r a t i o n s o l u t i o n . I f assumptions o f the constant rate i n j e c t i o n m e t h o d (i.e c o m p l e t e m i x i n g , steadystate plateau) are met, the m e t h o d can measure f l o w s w i t h i n ± 5 % uncertainty (Johnstone 1988). H o w e v e r , the potential error associated w i t h the discharge measurements w a s c a l c u l a t e d u s i n g an error analysis ( S t o r y 2 0 0 2 ) : ^Sk^  2  +  Q  where:  +  8(ECss - ECBG) = ^{5ECssf  SQi  a  Qi  K J K  w h e r e sources o f error i n c l u d e d i n j e c t i o n rates  +  5(ECss - ECBG)  (2.4)  ECss - ECBG  +(5ECBG)  2  a K J t  (Q;),.electrical  c o n d u c t i v i t y measurements  and the slope o f the regression (k) r e l a t i n g e l e c t r i c a l c o n d u c t i v i t y to the r e l a t i v e concentration. T h e error associated w i t h the i n j e c t i o n rates  (8Qj/Qj) w a s  estimated based  o n the error associated w i t h m e a s u r i n g the i n j e c t i o n rate (r) and t i m e (t).The uncertainty w i t h r e a d i n g the graduated c y l i n d e r (8r) w a s estimated as ± 0.5 m L ; the uncertainty w i t h m e a s u r i n g t i m e (8t) w a s ± 2 s (1 s o n either o f the t i m i n g ) . T h e average i n j e c t i o n rate  23  was 74 m L / m i n (n = 14). E r r o r associated w i t h the measurement o f the e l e c t r i c a l c o n d u c t i v i t y at plateau ( E C s s ) and b a c k g r o u n d ( E C B G ) w e r e 0.1 uS / c m for E C s s <200 LiS/cm and 1 LIS / c m for E C s s >200 LiS/cm. Standard error o f the slope (k) f r o m the regression w a s used to calculate the error i n k.  2.3.3.  Characterizing lateral exchanges  Constant-rate i n j e c t i o n tracer experiments also p r o v i d e d estimates o f the rate o f lateral i n f l o w . L a t e r a l i n f l o w rates w e r e estimated as the difference i n discharge measurements ( E q u a t i o n 2.1) as measured at upstream and d o w n s t r e a m E C s a m p l i n g locations w i t h i n the reach ( F i g u r e 2.3) d i v i d e d b y the reach length:  Q= ~ Qtb  L  Q,a  L  (2.5)  w h e r e Q L is the net lateral i n f l o w rate ( L s " ' m " ' ) , Qds an&Qus are streamflow ( L / s ) measured at the d o w n s t r e a m and upstream l o c a t i o n s w i t h i n the reach r e s p e c t i v e l y , a n d L is the reach l e n g t h (m).  2.3.4.  Cross-section measurements  C r o s s sections w e r e m e a s u r e d at 10 m intervals a l o n g the stream reach. A t each l o c a t i o n , the depth, measured at 2 0 c m intervals across the stream, and the wetted c h a n n e l w i d t h w e r e r e c o r d e d . M e a s u r e m e n t s o f depth and channel w i d t h w e r e used to calculate the stream cross-sectional area. T h e reach averaged cross-sectional area w a s used as an input v a r i a b l e for h y p o r h e i c z o n e m o d e l l i n g at the reach scale.  2.4.  Stream tracer experiments - Reach scale  2.4.1.  Method of injection  S t r e a m tracer tests w e r e c o n d u c t e d u s i n g the constant-rate salt i n j e c t i o n m e t h o d i n order to e x a m i n e the extent o f surface-subsurface interactions at the reach scale. W a g n e r  24  and H a r v e y (1997) d e t e r m i n e d that the constant i n j e c t i o n m e t h o d w i t h s a m p l i n g t h r o u g h the c o n c e n t r a t i o n rise, plateau and fall p r o v i d e s m o r e r e l i a b l e parameter estimates than other s a m p l i n g designs (e.g. s l u g injections). T r a c e r injection points and s a m p l e l o c a t i o n s w e r e the same for a l l f l o w c o n d i t i o n s ( F i g u r e 2.3). T r a c e r injections w e r e i n i t i a l l y c o n d u c t e d b y separating the study stream into t w o a p p r o x i m a t e l y 50 m sections (i.e. l o w e r a n d upper reach) and i n j e c t i n g a b o v e the upper b o u n d a r y for each reach. T r a c e r experiments w e r e c o n d u c t e d o n the same d a y or w i t h i n 1-2 days d e p e n d i n g o n c o n d i t i o n s . F u l l r e a c h tracer experiments w e r e also c o n d u c t e d for ease o f f i e l d w o r k . S t r e a m tracer w a s injected above the u p p e r b o u n d a r y o f the upper reach ( a p p r o x i m a t e l y 1 0 m d o w n s t r e a m f r o m the c u l v e r t at M road) and the b r e a k t h r o u g h curves w e r e measured at the l o w e r boundaries o f the reaches. D u r i n g base f l o w c o n d i t i o n s ( Q < 2 L / s ) , the reach w a s again separated into t w o sub-reaches to conduct l o n g e r duration tracer experiments.  2.4.2.  Quantifying pool storage and residence time  D u r i n g t w o reach-scale tracer injections, electrical c o n d u c t i v i t y meters w e r e p l a c e d at the i n f l o w and o u t f l o w l o c a t i o n o f three p o o l sub-units i n order to quantify p o o l storage and residence time. B r e a k t h r o u g h curves f r o m i n d i v i d u a l p o o l s w e r e s i m u l a t e d u s i n g O T I S - P . S i m u l a t i o n s w e r e c o n d u c t e d for t w o p o o l s located i n the upper reach (September 29) and one p o o l located i n the l o w e r reach (September 30). E a c h p o o l w a s s i m u l a t e d as a distinct reach.  2.4.3.  Hydraulic parameters (OTIS-P)  Transport and transient storage m e c h a n i s m s i n the stream reach w e r e a n a l y z e d b y fitting a n u m e r i c a l m o d e l to b r e a k t h r o u g h curves generated from stream tracer experiments. T h e processes o f a d v e c t i o n , d i s p e r s i o n , lateral i n f l o w and transient storage w i t h i n the h y p o r h e i c z o n e w e r e m o d e l e d u s i n g the O T I S - P code, w h i c h w a s d e v e l o p e d b y the U S G e o l o g i c a l S u r v e y ( R u n k e l 1998). O T I S - P , a m o d i f i e d v e r s i o n o f O T I S ( O n e d i m e n s i o n a l Transport w i t h I n f l o w and Storage), n u m e r i c a l l y solves finite difference  25  approximations to Equation 2.6 with the Crank-Nicolson Method, and uses a nonlinear least squares method to optimize parameters by minimizing the sum of squared errors. The model uses a modification of a one-dimensional advection-dispersion model with additional terms for groundwater inputs and transient storage (Bencala and Walters 1983, Stream Solute Workshop 1990). The following set of differential equations (Runkel and Broshears 1991) is used by OTIS-P to solve for solute mass balance between the main channel (Equation 2.6) and the transient storage zones (Equation 2.7): dC QdC I d , , „ 3 C . aim . • „ — = -—— + -—(AD—) + ^—(CL - C) + a(Cs - C) dt A ax A dx dx A  (2.6)  = -a — (Cs-Cs) As  (2.7)  — dt  „  where A is main channel cross-sectional area ( L ); As is storage zone cross-sectional 2  area (L ); C is main channel solute concentration (M/L ); C L is lateral inflow solute 2  3  concentration (M/L ); Cs is storage zone solute concentration (M/L ); D is dispersion 2  3  3  coefficient (L /T); Q is volumetric flow rate (L IT); quN is lateral inflow rate (L / T - L ) ; t is time (T) and x is distance (L); q is storage zone exchange coefficient (1/T). Crosssectional area (A, m ) as entered into the model is calculated as mean of the summed 2  products of mean wetted depth and channel width for each cross-section. The model estimates a reach-averaged A to match observed solute concentration data. Average stream velocity (u, m/s) is calculated as Q/A, using the model-estimated A value. 2  2  Dispersion (D, m Is), reach-averaged transient storage zone cross-sectional area ( A , m ), s  and transient storage exchange coefficient (a, s" ) are estimated using the model. 1  Transient storage refers to any zone within the channel, such as an eddy, pool, or hyporheic zone, where some water is temporarily detained relative to the faster moving water near the center of the channel. Model parameters solved using OTIS-P were used to derive the following quantities: (1) hydraulic uptake length (Mulholland et al. 1994), (2) hydraulic residence time (or contact time) in the storage zone and stream (Mulholland et al. 1994), (3) the hydraulic retention factor (Morrice et al. 1997), and (4) the standardized storage zone area (Stream Solute Workshop 1990, D'Angelo et al. 1993). Derived quantities are often 26  u s e d to c o m p a r e solute transport processes b e t w e e n stream reaches (Stream S o l u t e W o r k s h o p 1990). T h e h y d r a u l i c residence t i m e , T  s l o r  (s), i n the storage z o n e c a n be c a l c u l a t e d as:  Tstor = —  Aa  (2.8)  whereas residence t i m e i n the stream ( T y i s calculated as: s t r  Tsir = -  (2.9)  a  T h e h y d r a u l i c uptake length (Shyd) is the average distance a water m o l e c u l e travels d o w n s t r e a m before entering the storage zone:  SM = -Q-  Aa  (2.10)  T h e h y d r a u l i c retention factor ( R ) is a measure o f the storage z o n e residence t i m e H  per unit o f stream r e a c h traveled: R„ = — Shyd  (2.11)  T h e standardized storage z o n e area ( A s / A ) is the ratio o f storage cross-sectional area to stream cross-sectional area and is the m a t h e m a t i c a l equivalent o f storage z o n e residence t i m e to stream residence t i m e .  2.4.4.  Evaluation of parameter uncertainty  U n c e r t a i n t y s u r r o u n d i n g the parameter estimates was e x a m i n e d u s i n g t w o i n d i c e s : the e x p e r i m e n t a l D a m k o h l e r n u m b e r ( D a l ) and the uncertainty ratio. T h e D a l n u m b e r w a s c a l c u l a t e d as:  ^ a(l + AI Dal = ——= T  As)L —  (2.12)  27  where L is length o f the stream reach o v e r (m). W a g n e r and H a r v e y (1997) s h o w e d that w h e n the relationship b e t w e e n the exchange rate and a d v e c t i o n deviates f r o m 1.0, the uncertainty i n the m o d e l l e d parameters increases. Parameter uncertainty is m i n i m i z e d w h e n D a l is close to 1.0. H i g h values m a y o c c u r because exchange w i t h the streambed is r e l a t i v e l y fast c o m p a r e d to the water v e l o c i t y or the reach length m a y be too l o n g . A D a l less than 1.0 indicates that o n l y a s m a l l amount o f the stream tracer is e x c h a n g i n g w i t h the storage zone. S m a l l D a l numbers (<0.1) c o u l d result f r o m : (1) h i g h stream v e l o c i t y ,  f (2) l o n g exchange t i m e scale as i n d i c a t e d b y a l o w a and A s / A ratio, and/or (3) short reach length. T h e uncertainty ratio for each estimated parameter ( D , A , A s , a) is equal to the parameter estimate d i v i d e d b y its standard d e v i a t i o n . A l o w ratio indicates that the parameter estimate is h i g h l y uncertain.-  \  2.5.  Stream tracer experiments - Channel-unit scale >  2.5.1.  Qualitative observations of hyporheic discharge  T r a c e r injections o f the d y e tracers B r i l l i a n t B l u e F C F ( C . I . 4 2 0 9 0 ) and R h o d a m i n e W T ( R W T ) were c o n d u c t e d at three locations ( I I , 14 and 15; F i g u r e 2.8) u s i n g infiltrometers i n s t a l l e d w i t h i n the stream bed as described i n S e c t i o n 2.5.6. Injections were used to identify q u a l i t a t i v e l y the l o c a t i o n o f h y p o r h e i c discharge w i t h i n i n d i v i d u a l channel units. A p p r o x i m a t e l y 1-5 g o f R W T was injected, and less than 50 m L o f b r i l l i a n t b l u e was injected at one time. E x p e r i m e n t s u s i n g R W T were conducted o v e r s i x dates ( J u l y 11, A u g u s t 14, A u g u s t 2 1 , September 2 7 / 2 8 , O c t o b e r 6) i n the l o w e r r e a c h at t w o locations (1-4,1-5). T r a c e r i n j e c t i o n experiments were also conducted i n the upper reach at one l o c a t i o n (1-1) o n four dates ( J u l y 12, A u g u s t 2 3 , and September 2 8 , O c t o b e r 6).  28  2.5.2.  Quantifying residence times  S o d i u m c h l o r i d e w a s also used as a tracer to quantify solute residence t i m e w i t h i n one b o u l d e r step-pool unit at the l o w e r reach. A s o l u t i o n o f 1 L d e i o n i z e d water and 2 0 g o f salt w a s added d i r e c t l y to the infiltrometer 15 ( F i g u r e 2.8) i n s t a l l e d w i t h i n the . streambed. E l e c t r i c a l c o n d u c t i v i t y w a s measured at the base o f the step (or p o o l i n f l o w ) and at the p o o l o u t f l o w l o c a t i o n to quantify residence t i m e w i t h i n the h y p o r h e i c z o n e a n d p o o l s r e s p e c t i v e l y . C a l i b r a t i o n s w e r e c o n d u c t e d u s i n g the same p r o c e d u r e as d e s c r i b e d i n section 2.3.2. T w o channel-unit experiments were c o n d u c t e d o v e r the study p e r i o d (September 2 5 a n d O c t o b e r 5, 2 0 0 6 ) .  2.5.3.  Modelling mean residence time  A s i m p l e m o d e l i n g a p p r o a c h was used to m o d e l the m e a n residence t i m e o f water w i t h i n the transient storage zones. T h e system w a s m o d e l e d u s i n g l i n e a r r e s e r v o i r theory a s s u m i n g that the p o o l unit behaves as a  continuously stirred tank reactor ( C S T R )  ( C h a p r a 1997). C o m p l e t e m i x i n g o f the solute a n d steady-state water f l o w for the duration o f the tracer i n j e c t i o n experiment w a s assumed. A mass b a l a n c e for the p o o l c a n be expressed as:  ^  = «J0-^(0  (2-13)  dt where  M(t) i s the m a s s o f tracer i n p o o l (kg), q (t) i s the input o f tracer (kg/s) a n d qout(0 in  is the output (kg/s). M a s s c a n be related to tracer concentration u s i n g the equations:  M(t) = V-c(t)  (2.14)  q ,At) = Q-c {t) o  (2.15)  0HI  w h e r e Vis the v o l u m e o f water i n p o o l ( m ) , c(t) i s the concentration o f tracer i n the p o o l 3  3  at t i m e  3  t ( k g / m ), Q is the water discharge at the outlet o f the p o o l ( m /s), a n d c (t) is the out  tracer c o n c e n t r a t i o n at the outlet. U s i n g C S T R theory, the m e a n residence t i m e ( M R T ) o f solutes w i t h i n the system c a n b e expressed as: 29  MRT  V 1 =— =Q  (2.16)  k  w h e r e A: is a first order exchange coefficient ( s ) . T o w a r d the end o f the experiment, 1  tracer discharge to the p o o l w i l l b e c o m e s m a l l , and the r e l a t i o n s h i p b e t w e e n concentration and time c a n be m o d e l l e d as a first-order reaction:  ^ - = -k-c(t) dt  "  (2.17)  Integrating E q u a t i o n 2.17, subject to the i n i t i a l c o n d i t i o n c(t ) = c , y i e l d s : G  c(t) = c e * ' - ' ' _  (  o )  0  0  •  (2.18)  w h e r e Co is the c o n c e n t r a t i o n at t i m e = to, w h i c h is an arbitrarily selected t i m e . T h i s equation specifies an e x p o n e n t i a l d e p l e t i o n o f the tracer concentration o v e r t i m e . E q u a t i o n 2.18 can be transformed b y t a k i n g l o g a r i t h m s o f b o t h sides to y i e l d :  ln[c(0]  = ln[c ]-M<-O 0  •  (2-19)  I f a p l o t o f the l o g a r i t h m o f concentration against t i m e y i e l d s a straight l i n e , E q u a t i o n 2.15 h o l d s true, and k c a n be c a l c u l a t e d as the slope o f a straight l i n e fitted to the linear p o r t i o n o f the l o g - t r a n s f o r m e d b r e a k t h r o u g h c u r v e .  2.6.  Subsurface flow measurements - Point scale  2.6.1.  Piezometer design and installation  P i e z o m e t e r s w e r e i n s t a l l e d i n a dense n e t w o r k near the center o f the channel i n s i x step-pools to p r o v i d e a h i g h spatial r e s o l u t i o n o f subsurface f l o w p a t h w a y s ( F i g u r e 2.7). A total o f 68 p i e z o m e t e r s w e r e i n s t a l l e d i n the stream d u r i n g early s u m m e r ( M a y to June, 2006). T w o types o f p i e z o m e t e r designs w e r e used: (1) a l u m i n u m piezometers for t a k i n g water q u a l i t y samples (n = 2 8 ) , and (2) p l a s t i c p i e z o m e t e r s to measure h y d r a u l i c gradients and measure h y d r a u l i c c o n d u c t i v i t y (n = 40).  30  A l u m i n u m piezometers w e r e constructed u s i n g an a p p r o x i m a t e l y 6 0 c m length o f 1 c m internal diameter a l u m i n u m tube w i t h a slot z o n e o f 5 c m . A l u m i n u m p i e z o m e t e r s w e r e d r i v e n into the streambed u s i n g a sledgehammer. H y d r a u l i c gradients w e r e also measured for the a l u m i n u m piezometers. P l a s t i c p i e z o m e t e r s w e r e a p p r o x i m a t e l y 60 c m i n length and constructed u s i n g 0.7 c m internal diameter p o l y v i n y l c h l o r i d e ( P V C ) p i p e w i t h a 5 c m slot zone. T h e slot zones w e r e screened w i t h n y l o n m e s h to prevent c l o g g i n g w i t h fine sediments. P l a s t i c p i e z o m e t e r s w e r e i n s t a l l e d into the streambed b y first d r i v i n g i n a steel r o d , c o n t a i n e d w i t h i n a metal sleeve. T h e m e t a l r o d w a s then r e m o v e d from the sleeve and the p l a s t i c p i e z o m e t e r w a s p l a c e d i n s i d e . O n c e the metal sleeve and r o d w e r e r e m o v e d , the p i e z o m e t e r w a s left i m b e d d e d i n the subsurface. T h e piezometers w e r e i n s t a l l e d at m u l t i p l e depths (0-15, 15-30, 3 0 - 5 0 c m ) b o t h l o n g i t u d i n a l and p e r p e n d i c u l a r to the wetted stream c h a n n e l . T h e l o c a t i o n and e l e v a t i o n o f a l l piezometers w e r e s u r v e y e d u s i n g a L e i c a G e o s y s t e m s © total station.  2.6.2.  H y d r a u l i c head  V e r t i c a l h y d r a u l i c gradients ( V H G ) i n the streambed w e r e used to locate areas o f u p w e l l i n g and d o w n w e l l i n g w i t h i n the h y p o r h e i c zone. V H G w a s c a l c u l a t e d as:  Ah VHG = —  Al  (2.20)  w h e r e Ah is the e l e v a t i o n o f the water i n the p i e z o m e t e r m i n u s the e l e v a t i o n o f the stream water surface ( c m ) , and Al is the distance b e t w e e n the surface o f the stream b e d a n d the m i d d l e o f the slot z o n e ( c m ) . P o s i t i v e V H G indicates u p w e l l i n g h y p o r h e i c o r g r o u n d w a t e r f l o w ( f l o w potential f r o m the b e d towards the channel); negative V H G indicates d o w n w e l l i n g f l o w ( f l o w potential is directed from the c h a n n e l into the bed). R e v e r s a l o f exchange potential w a s defined as a shift b e t w e e n p o s i t i v e and negative V H G . N e u t r a l p i e z o m e t e r s w e r e defined as h a v i n g h y d r a u l i c gradients b e t w e e n -0.05 to 0.05 c m / c m , w h i c h l i e w i t h i n the uncertainty o f V H G measurements ( G u e n t h e r 2 0 0 7 ) . H y d r a u l i c heads w e r e m e a s u r e d u s i n g a water l e v e l sensor. T h e sensor consisted o f t w o electrical w i r e s attached to a r o d and connected to a battery and b u z z e r . T h e r o d w a s  31  lowered into the piezometer. When the wires contact water, the electrical circuit is closed and a buzzer sounds. The water depth was measured using a measuring tape, resulting in an accuracy of ± 0.05 cm.  2.6.3.  Hydraulic conductivity  Piezometers were also used to measure saturated hydraulic conductivity of the bed sediments using a Hvorslev test (also known as falling head slug test; Freeze and Cherry 1979). Water was added to the piezometer using a syringe connected to a tube and inserted to the top of the piezometer. The tube was disconnected, and the time required for the water to return to a specified level on the piezometer was measured using a stopwatch. Hydraulic conductivity (K) was computed based on the empirical equation of Hvorslev (1951) as modified by Baxter et al. (2003) for closed-bottom perforated piezometers: K^SfM"' ^  '•  1  (2.21)  (t2-t\)  where: d - In ( ) 2  SF =  L  +  \  f  2  +  where Hi and H2 are the head in the piezometer (cm) at time t/ and  (s), SF is the shape  factor for the piezometer intake (m), L is the length of the perforations (m), D is the diameter of the perforated intake (m) and d is the inside diameter of the piezometer (m). Hydraulic conductivity measurements of stream sediments are typically reported to be positively skewed or not normally distributed (Ryan and Boufadel 2007). As a result, the geometric mean (rather than arithmetic mean) was calculated. The mean was computed from of all falling head tests taken at each site (n =4).  32  24  Pool!  ^  Upper Reach  i i i i i i i  0 1.25 2.5  5 Meters  Projection: UTM Zone 10 Datum: NAD 1983  ^J2/ Infiltrometer (35)  Piezometer Thalweg  Channel Morphology Pool Step  F i g u r e 2.7. M a p o f s a m p l i n g locations, m o r p h o l o g y and t h a l w e g p r o f i l e for b o t h reaches  33  2.6.4.  Relating discharge and recharge zones to stream geometry  Calculated vertical hydraulic gradients (Equation 2.20) were used to map zones of hyporheic discharge (upwelling) and recharge (downwelling) along the stream profile in ArcGIS version 9.1. A geometric scaling relationship was used to describe vertical hydraulic gradients as a function of location in the stream channel based on mapping results. The general form of the relation is:  -^ = f£,SH) dl L  (2.22)  C  where X is the distance from the upstream end of the pool to the piezometer (m), L is the distance from the upstream end of the pool to the edge of the step downstream of the pool (m), and SH is the height of the downstream step (m), as defined by Zimmermann and Church (2001). It is hypothesized that dh/dl should be negatively related to X / L , with positive values for values near 0, and increasingly negative values as X / L approaches 1. Furthermore, hydraulic gradients in the downwelling zone should be negatively associated with SH. That is, higher steps should exhibit stronger downwelling gradients. A total of 7channel-units were used for the analysis. The average water flux for one step-pool unit (Pool 1) was calculated under baseflow (September 29) and high flow conditions (June 19) using Darcy's law: dh q = -K^A dl  .  (2.23)  dh where q is the volume of water (L/s), K is the average hydraulic conductivity.(m/s), — dl is the average hydraulic gradient (cm/cm), and A is the area of stream channel (m ) as determined by the X / L category. The area above and below the step was segmented into spatial units based on the X / L category. The average flux for each segment was computed and totaled for each channel sub-unit. The total channel-unit flux was divided by the wetted channel volume (L) of the reach in order to obtain a scaled-up estimate of hyporheic exchange (s" ). This estimate was compared to the reach-scale estimates of 1  34  transient exchange o b t a i n e d b y fitting the O T I S - P m o d e l to tracer i n j e c t i o n breakthrough curves.  2.6.5.  Stream bed infiltrometers  D i r e c t measurements o f stream bed infiltration rates were c o n d u c t e d u s i n g a constant-head stream b e d infiltrometer ( F i g u r e 2.8). Infiltrometers w e r e constructed u s i n g an a p p r o x i m a t e l y 2 0 - 3 0 c m l o n g c y l i n d r i c a l , open-ended, P V C pipe w i t h a 7.5 c m internal diameter. A h o l e d r i l l e d into the P V C pipe at m i d length was used to connect the p i p e to a M a r i o t t e c y l i n d e r u s i n g a p i e c e o f t y g o n tubing. T h e P V C p i p e w a s installed into the stream b e d at a depth o f 5-10 c m such that the m i d - l e n g t h o p e n i n g to the P V C p i p e w a s l e v e l w i t h the stream bed. T h e M a r i o t t e tube w a s i n s t a l l e d v e r t i c a l l y i n the stream u s i n g a p i e c e o f rebar h a m m e r e d into the stream bed. T h e M a r i o t t e tube was f i l l e d w i t h water p r i o r to c o n d u c t i n g measurements. T h e basic premise for the M a r i o t t e tube is that w h e n water w i t h i n the P V C pipe "infiltrates" into the bed, the M a r i o t t e tube supplies a d d i t i o n a l water to m a i n t a i n a constant water l e v e l . T h e M a r i o t t e tube consisted o f a 30 c m length o f plastic tube w i t h both a plastic stopper at both ends. T h e b o t t o m end o f the tube attached to the P V C p i p e installed i n the stream, w h i l e the stopper at the top w a s used to h o l d a s m a l l e r diameter tube i n place.  2 0 cm  F i g u r e 2.8: Streambed infiltrometer. A d a p t e d from M a r t i n (1996).  35  Infiltrometers w e r e installed into the stream b e d i n five d o w n w e l l i n g l o c a t i o n s w i t h i n the reach, s p e c i f i c a l l y above the step w i t h i n a step-pool unit ( F i g u r e 2.7) to measure the amount o f water d i r e c t l y infiltrating into the h y p o r h e i c zone. I n f i l t r a t i o n measurements w e r e taken p e r i o d i c a l l y d u r i n g the study p e r i o d at a range o f f l o w s . T h e infiltration rate ( I R ) w a s calculated as:  Ah n{r{ - r ) 2  IR  2  A'  (2.24)  K(r ) 2  o  w h e r e Ah is the change i n water l e v e l i n the M a r i o t t e tube (cm) o v e r an i n t e r v a l , At (s), ri, rj and r  are the i n s i d e radius o f the M a r i o t t e r e s e r v o i r ( c m ) , the outside radius o f the  p  b u b b l e r tube i n the M a r i o t t e reservoir (cm), and the i n s i d e radius o f the p a n ( c m ) , respectively. T h e uncertainty associated w i t h the infiltration measurements w a s c a l c u l a t e d u s i n g an error analysis:  S(Ah) Y , (S(At)) + Ah At  2  I  , (2(r, +  +  r  2  •* )Y f 2  2 r  +  J  ~  S r  ^  2 \1/2 (2.25)  V  w h e r e b(Ah) is the error associated w i t h m e a s u r i n g the change i n water l e v e l i n the M a r i o t t e tube, estimated at ± 0.2 c m ; 8(At) is the uncertainty w i t h m e a s u r i n g t i m e , estimated at ± 2 s ( l s o n either o f the t i m i n g ) ; & > / , or 2 and or  p  are the uncertainty i n  m e a s u r i n g the i n s i d e radius o f the M a r i o t t e reservoir, the outside radius o f the b u b b l e r tube i n the M a r i o t t e reservoir, and the inside radius o f the e v a p o r a t i o n pan, r e s p e c t i v e l y . Infiltrometers w e r e installed adjacent to piezometers ( a p p r o x i m a t e l y 10 c m ) i n order to b a c k - c a l c u l a t e h y d r a u l i c c o n d u c t i v i t y (Ki):  IR VGH  (2.26)  w h e r e I R is the measured infiltration rate (cm/s) a n d V G H is the vertical h y d r a u l i c gradient ( c m / c m ) measured u s i n g E q u a t i o n 2.20. Infiltrometer measurements o f h y d r a u l i c c o n d u c t i v i t y w e r e then c o m p a r e d to measurements o f c o n d u c t i v i t y as measured i n the p i e z o m e t e r s u s i n g the f a l l i n g - h e a d test.  36  2.6.6.  Subsurface relative connectivity  Subsurface water samples w e r e c o l l e c t e d f r o m the a l u m i n u m piezometers to measure the relative c o n n e c t i v i t y o f the subsurface to the stream channel d u r i n g reach scale tracer injections. P i e z o m e t e r s w e r e i n i t i a l l y purged a n d a l l o w e d to r e f i l l p r i o r to s a m p l i n g . A p o l y p r o p y l e n e s y r i n g e (60 m L ) attached to t y g o n t u b i n g ( 3 - m m i n s i d e diameter) w a s used to w i t h d r a w water samples f r o m the piezometers. S m a l l v o l u m e s o f water ( a p p r o x i m a t e l y 15-30 m L ) w e r e r e m o v e d for p u r g i n g . A v o l u m e o f a p p r o x i m a t e l y 2 5 - 3 0 m L w a s r e m o v e d to a n a l y z e for e l e c t r i c a l c o n d u c t i v i t y . S a m p l e s w e r e r e m o v e d i n s m a l l quantities to m i n i m i z e the i n f l u e n c e o n the subsurface f l o w system. T h e syringe, t u b i n g and s a m p l e bottles w e r e d e i o n i z e d . W a t e r samples w e r e taken p r i o r to tracer release and a g a i n once the stream concentration reached plateau. T h e electrical c o n d u c t i v i t y o f the water samples was measured to calculate relative c o n n e c t i v i t y . A t w o c o m p o n e n t m i x i n g equation u s i n g the e l e c t r i c a l c o n d u c t i v i t y o f stream water and p i e z o m e t e r porewater at stream tracer i n i t i a t i o n (t = 0) and steady state w e r e used to calculate the fraction o f stream water present i n the subsurface ( G o o s e f f and M c G l y n n 2005). T h e percent stream water i n the h y p o r h e i c z o n e w a s c a l c u l a t e d as:  ECp(D  — ECp(0)  X = ECs(t) —  • 100%  (2.27)  ECs(0)  where E C ( ) , EC (o), E C ( ) and E C o ) refer to the c o n d u c t i v i t y i n the piezometers and P  t  P  s  t  s(  stream at steady state (t) and b a c k g r o u n d (time 0) r e s p e c t i v e l y . T h e m i x i n g ratio w i t h i n the h y p o r h e i c z o n e w a s c a l c u l a t e d for a l l piezometers. It is assumed that no stream water has e x c h a n g e d w i t h the subsurface w h e n % is equal to 0; w h e n % is equal to 1, c o m p l e t e replacement o f the h y p o r h e i c z o n e water w i t h tracer l a b e l e d stream water has o c c u r r e d .  37  2.7.  Statistical Analysis V a r i a b i l i t y o f exchange f l o w was assessed b y c o m p a r i n g changes i n V H G and  infiltration rates o v e r the range o f f l o w c o n d i t i o n s observed. S p e a r m a n ' s rank c o r r e l a t i o n analysis w a s used to e x p l o r e the relationships between discharge, v e r t i c a l h y d r a u l i c gradient and i n f i l t r a t i o n at each p o i n t l o c a t i o n . In a d d i t i o n , V H G as a f u n c t i o n o f the d o w n s t r e a m step height was also tested u s i n g this approach. A v a l u e o f 1 or -1 indicates a strong p o s i t i v e (or strong negative) c o r r e l a t i o n between t w o n o n p a r a m e t r i c variables ( K u t n e r et a l 2 0 0 4 ) . A l i n e a r mixed-effects m o d e l ( M a i n d o n a l d and B r a u n 2 0 0 7 ) was u s e d to e x a m i n e the in-site v a r i a b i l i t y i n h y d r a u l i c gradients and h y d r a u l i c c o n d u c t i v i t y u s i n g the L M E R -function i n R 2.5.1 for u n b a l a n c e d e x p e r i m e n t a l designs ( R D e v e l o p m e n t C o r e T e a m 2 0 0 7 ) . T o m o d e l the v a r i a b i l i t y i n h y d r a u l i c gradients, channel-unit (n = 7) was c o n s i d e r e d a r a n d o m effect w h i l e d o w n s t r e a m step height (7 levels) and p o s i t i o n w i t h i n the c h a n n e l (as d e f i n e d b y X / L ; 5 levels) were f i x e d effects. T h r e e separate l i n e a r m o d e l s were created i n c l u d i n g (1) a base m o d e l w i t h o n l y channel-unit, (2) channel-unit w i t h step height and (3) channel-unit, step height and X L factors. A sequential analysis o f v a r i a n c e w a s u s e d to determine the s i g n i f i c a n c e o f the f i x e d effects o n h y d r a u l i c gradients b y c o m p a r i n g a l l three m o d e l s u s i n g the A N O V A function i n R 2 . 5 . 1 . D a t a were w e i g h t e d b y the inverse o f X / L to account for the heteroscedasticity o f the residuals ( K u t n e r et a l . 1994). T h i s statistical m e t h o d was also used to e x a m i n e the v a r i a b i l i t y i n h y d r a u l i c c o n d u c t i v i t y due to site c o n d i t i o n (i.e. u p w e l l i n g , d o w n w e l l i n g o r neutral) and reach l o c a t i o n (i.e upper or l o w e r reach). R e a c h (n = 2) was c o n s i d e r e d a r a n d o m effect w h i l e site c o n d i t i o n was a f i x e d effect. D a t a were l o g transformed to account for the heteroscedasticity o f the residuals ( K u t n e r et a l . 1994). A s i g n i f i c a n c e l e v e l o f 0.05 was used for a l l analyses.  38  CHAPTER T H R E E : RESULTS T h i s chapter presents the results o f field observations i n East C r e e k between M a y and O c t o b e r , 2 0 0 6 . T h e chapter starts w i t h an o v e r v i e w o f the study p e r i o d c o n d i t i o n s (section 3.1) and d i s c u s s i o n o f the data q u a l i t y (section 3.2). R e s u l t s are then presented for each scale o f i n v e s t i g a t i o n b e g i n n i n g w i t h the reach scale ( S e c t i o n 3.3), f o l l o w e d b y the channel-unit scale (section 3.4), and the i n d i v i d u a l p o i n t scale (section 3.5). T h e chapter concludes w i t h a s u m m a r y o f the k e y f i n d i n g s (section 3.6).  3.1.  Study period conditions B a s e d o n data f r o m the H a n e y - U B C R e s e a r c h Forest A d m i n climate station  ( E n v i r o n m e n t C a n a d a ) , located w i t h i n the U B C R e s e a r c h Forest ( 4 9 ° 1 6 . 2 ' N , 1 2 2 ° 3 4 . 2 ' W ) , s u m m e r temperature and p r e c i p i t a t i o n c o n d i t i o n s were w a r m e r and drier than the 30 year n o r m ( T a b l e 3.1). F i g u r e 3.1 p r o v i d e s an o v e r v i e w o f f l o w c o n d i t i o n s i n East C r e e k a l o n g w i t h d a i l y p r e c i p i t a t i o n and temperature i n the R e s e a r c h Forest. Stream tracer experiments were conducted f r o m M a y 31 to O c t o b e r 20, 2 0 0 6 , over a range o f f l o w c o n d i t i o n s to e x a m i n e h y p o r h e i c exchange processes over time. A majority o f the tracer injections were c o n d u c t e d d u r i n g b a s e f l o w c o n d i t i o n s ( Q <5 L s " ) 1  w h e n the c o n t r i b u t i o n o f the h y p o r h e i c z o n e is h y p o t h e t i c a l l y m a x i m i z e d . S t r e a m f l o w s ranged f r o m 0.21 L / s o n September 13 to 30.6 L / s o n M a y 31 o v e r the study p e r i o d . D i s c h a r g e l e v e l s were too h i g h b y early N o v e m b e r ( Q >200 L / s ) to continue the study.  T a b l e 3.1: C o m p a r i s o n o f 2 0 0 6 m e a n d a i l y temperature and m o n t h l y p r e c i p i t a t i o n to 30 year c l i m a t e n o r m a l ( 1 9 6 1 - 1 9 9 0 ) as measured at the H a n e y - U B C R e s e a r c h Forest A d m i n c l i m a t e station ( E n v i r o n m e n t C a n a d a ) . M e a n d a i l y temperature ( ° C ) Month  Normals  2006  Difference  May  11.8 14.6  12.7  +0.9  Jun  16.1  +1.5  Total precipitation (mm) 2006  % of Normal  114  121.8  106.8  93.1  55.2  59.3  Normals -  Jul  16.8  18.7  +1.9  80.9  27.2  33.6  Aug  17.0  17.8  +0.8  74.3  17.0  22.9  Sep  14.5  15.7  +1.2  119.7  90.8  75.9  Oct  9.9  11.6  +1.7  223.8  32.6  14.6  •  39  o  \  May 01  Jun 01  Jul 01  . Aug 01  Sep 01  Oct 01  Nov 01  F i g u r e 3 . 1 : D a i l y p r e c i p i t a t i o n , m a x i m u m and m i n i m u m d a i l y temperatures, measured discharge and net lateral i n f l o w f r o m tracer injections conducted d u r i n g the study p e r i o d o f M a y to O c t o b e r 2 0 0 6 . D i s c h a r g e values represent streamflow measured at the l o w e r reach b o u n d a r y . L a t e r a l i n f l o w measured as the difference b e t w e e n upstream and d o w n s t r e a m streamflow measurements. N o t e l o g scale for Q and Q l , C l i m a t e data recorded at the H a n e y - U B C R e s e a r c h Forest A d m i n c l i m a t e station ( E n v i r o n m e n t Canada).  40  3.2.  Data quality E r r o r rates v a r i e d between 3-7 % for a l l f l o w c o n d i t i o n s ( T a b l e 3.2). T h e s e error  rates are r o u g h l y consistent w i t h the cited uncertainty o f ± 5 % (Johnstone 1988). D u r i n g three tracer injections c o n d u c t e d d u r i n g l o w f l o w c o n d i t i o n s ( J u l y 20, A u g u s t 13 and 31), the l o w e r reach b o u n d a r y d i d not achieve plateau. S t r e a m f l o w at the l o w e r reach b o u n d a r y w a s then estimated as a percentage o f s t r e a m f l o w as measured at the upper reach b o u n d a r y . T h i s was based o n the relationship between the measured discharge at the upper and l o w e r reach boundaries for full reach tracer injections that reached plateau (n = 4). T h e increase i n discharge w a s estimated at 2 0 . 5 % o f the upstream discharge. M o d e l s i m u l a t i o n s w e r e o n l y c o n d u c t e d for tracer injections w h e r e the upper and l o w e r reach b o u n d a r y concentrations reached plateau (n = 10).  T a b l e 3.2. S u m m a r y o f streamflow ( Q ) measurements c o n d u c t e d d u r i n g l o w e r r e a c h ( L R ) , upper reach ( U R ) stream tracer injections o v e r the study p e r i o d M a y 31 to O c t o b e r 20, 2 0 0 6 . I n c l u d e d are the rates o f i n j e c t i o n (R;), s l o p e o f the c a l i b r a t i o n regression (k), and standard error o f k ( S E ) , electrical c o n d u c t i v i t y at b a c k g r o u n d (ECbkg,) and plateau (ECpi t) and the probable error i n stream flow measurement. a  k  Date  Q (L/s)  R (mm/mi)  (io- )  M a y 31* June 7 June 19* June 19* June 27* July 20 A u g 13 A u g 31 Sept 13 Sept 2 1 * Sept 29* Sept 30* Oct 2 0 *  30.6 24.9 15.4 17.2 5.5 2.1 0.53 0.33 0.21 9.9 1.1 1.9 11.9  71 34  2.8 5.4  33 23 59 87 40 116 70 109 92 92 104  1.7 3.4 3.3 3.3 9.2  6  130 52 6.6 21 21 4.5  SE (kx 10" )  ECbkg (jj,S/cm)  ECp| i (u.S/cm)  Probable E r r o r (%)  3.0 6.8 14 1.7 13 13 12 3.0 3300 8.3 60 60 140  20.0 20.4  34.0 24.6 43.2 27.8 79.0 270.0 174.0 84:0 143.0 58.9 102.0 71.9 74.6  3.6 5.0 3.8 4.8 3.5 3.4 3.6 4.0  9  22.6 22.3 23.8 29.0 29.6 38.8 35.4 31.1 33.0 32.4 30.2  a  7.1 3.4 3.7 3.4 4.5  Reach LR LR UR LR UR/LR UR/LR UR LR LR UR/LR UR LR UR/LR  T r a c e r injections s i m u l a t e d b y O T I S - P  41  3.3.  Solute transport model analysis - Reach scale  3.3.1.  Summary of OTIS-P simulations  B e s t fit m o d e l parameters for s i x tracer experiments c o n d u c t e d i n the l o w e r reach and four tracer experiments c o n d u c t e d i n the upper reach are s u m m a r i z e d i n T a b l e 3.3. A l l tracer experiments d o u b l e d or q u a d r u p l e d the b a c k g r o u n d electrical c o n d u c t i v i t y o f stream water to a plateau concentration. F i g u r e 3.2 s h o w s an e x a m p l e o f s i m u l a t e d and o b s e r v e d tracer concentrations for the June 19 tracer injection c o n d u c t e d i n the u p p e r and l o w e r reach. A d d i t i o n a l b r e a k t h r o u g h curves and m o d e l s i m u l a t i o n s are p r o v i d e d i n Appendix A .  T a b l e 3.3: S u m m a r y o f best fit m o d e l parameters for solute releases i n c l u d i n g stream discharge ( Q ) , d i s p e r s i o n coefficient ( D ) , stream cross-sectional area ( A ) , cross-sectional area o f storage z o n e ( A s ) , storage z o n e exchange coefficient (a), net lateral i n f l o w ( Q L ) , the D a m k o h l e r n u m b e r ( D a l ) .  Parameters Reach Upper June 19 Sept 21 Sept 29 Oct 20  D  Q (10"  3  mV)  (10"  2  mV ) 1  A  A  s  (10" m )  (10" m )  2  2  2  2  a (10V)  QL  (lO^mV'nr ) 1  Dal  15.9 9.8 1.0 9.7  31.6 18.3 2.9 4.0  11.6 6.73 2.82 9.28  5.83 1.45 0.6 1.46  2.1 2.7 3.0 3.5  26.8 8.5 3.3 10.5  0.12 0.22 0.50 0.38  30.6 17.2 5.5 9.9 1.9 11.9  48.6 29.5 1.9 8.4 4.6 5.0  21.7 19.2 13.7 14.3 6.5 19.2  13.5 5.9 4.1 2.8 1.3 2.0  1.4 1.9 2.7 2.1 0.8 1.6  31.8 20.4 5.6 6.9 6.9 9.4  0.08 0.14 0.44 0.36 0.17 0.28  Lower M a y 31 June 19 June 27 Sept 21 Sept 30 Oct 20  42  Upper boundary Lower boundary Simulated  A  °  A . A A A  AA"y A  A  A  A A •  -TL_  A  A AAA  •  •  •• •  •  o  c O  o CO  O O  o CN  CD  Time (hour)  LO CM  A  Upper boundary Lower boundary Simulated  D  A  13  AA  A A  r F * ^ A A ^' 1 3  O CM  A  l  A  7  a  A  ^  A  A  A  A  A  1  C  g  Ic  A  1  8 c  8  .1  A  1  LO A  O  A  (  B  )  ^H^-^St"-a—a...  -4 0.0  0.5  1.0  1.5  2.0  2.5  Time (hour)  F i g u r e 3.2. M o d e l s i m u l a t i o n s u s i n g O T I S - P for June 19 for the upper reach (a) and the l o w e r r e a c h (b)  43  3.3.2.  Variability of fitted parameters  S o l u t e transport processes v a r i e d b o t h t e m p o r a l l y ( w i t h variations i n discharge) and to a lesser extent, spatially (i.e. b e t w e e n reaches). D i s p e r s i o n rates, transient storage area and stream c r o s s - s e c t i o n a l area increased w i t h discharge ( F i g u r e 3.3). T h i s general trend is observed for b o t h the upper and l o w e r reaches; h o w e v e r , for a l l tracer s i m u l a t i o n s the stream cross-sectional area o f the l o w e r reach is greater than the u p p e r reach. Increased channel c o m p l e x i t y and p o o l storage i n the l o w e r reach m a y e x p l a i n this difference. T r a n s i e n t e x c h a n g e rates b e t w e e n the m a i n channel and the storage z o n e d i d  0.6  not v a r y s y s t e m a t i c a l l y w i t h discharge or b e t w e e n reaches ( F i g u r e 3.3).  5  LO -  o ^ ^  E  11  i  -  "  -  o  -  o '  m o  I  o o  a"  1 i i  T o  _  '•I  _  • o o  A  Upper Lower  <  •  <°  Q(Ls"')  A  Upper Lower  o o  Q(Ls"')  F i g u r e 3.3. S i m u l a t e d m o d e l parameters for solute releases i n the upper and l o w e r stream reach. D i s p e r s i o n coefficient ( D ) , stream cross-sectional area ( A ) , cross-sectional area o f storage z o n e ( A s ) , storage z o n e exchange coefficient (a) versus stream discharge ( Q ) . E r r o r bars represent ± 1 standard d e v i a t i o n .  44  3.3.3.  Parameter uncertainty  A general decreasing trend i n D a l numbers w a s o b s e r v e d as discharge i n c r e a s e d ( F i g u r e . 3 . 4 ) . D a m k o h l e r numbers ranged f r o m 0.08 o n M a y 31 (30.6 L / s ) to 0.5 o n September 2 9 (1.04 L / s ) . T h e tracer injection w i t h the best parameter estimates, as i n d i c a t e d b y a D a l n u m b e r closest to one, w a s o n September 2 9 .  A  A  A  D A A A  • ^  Upper Lower  1  I  I  I  I  I  I  0  5  10  15  20  25  30  Q(Ls~ ) 1  F i g u r e 3.4. E x p e r i m e n t a l D a m k o h l e r number ( D a l ) versus stream discharge (Q)  The uncertainty ratio for each estimated parameter ( D , A , A s , a), calculated as the  '  parameter estimate divided b y its standard deviation, varied over the range o f flow conditions ( T a b l e 3.4). N o trend between discharge and the uncertainty ratio was observed ( F i g u r e 3.5).  45  T a b l e 3.4. S u m m a r y o f uncertainty ratios for the parameter estimates o f d i s p e r s i o n coefficient ( D ) , stream cross-sectional area ( A ) , cross-sectional area o f storage z o n e ( A s ) , and the storage z o n e exchange coefficient (a). Reach Upper June 19 Sept 21 Sept 29 Oct 20 Lower M a y 31 June 19. June 27 Sept 21 Sept 30 Oct 20 Mean Upper Lower  D  A  1.9 5.1 5.5 1.2  211.1 69.5 56.5 35.2  8.1 2.7 14.2 6.2  9.8 7.6 4.2 7.9 13.7 5.6  .76.7 60.2 78.7 87.1 99.1 51.4  3.1 0.9 25.8 11.9 8.7 6.0  3.4 8.1  93.1 75.5  7.8 9.4  • A  A  A  s  20.7 8.2 5.2 2.1 4.8 .1.4 ' 8.6 5.2 9.3 2.5  •  9.1 5.3  Upper . Lower  • A  Upper Lower  D  CN  DC  "i  1  1  1  1  1  r  0  5  10  15  20  25  30  Q (Ls" )  i 10  i — n 15  20  r 25  30  1  Q (Ls~ ) 1  F i g u r e 3.5. U n c e r t a i n t y ratio ( U R ) for the s i m u l a t e d m o d e l parameters o f d i s p e r s i o n ( D ) , stream cross-sectional area ( A ) , cross-sectional area o f storage zone ( A s ) , storage z o n e exchange coefficient (a) versus stream discharge ( Q )  46  3.3.4.  D e r i v e d quantities  D e r i v e d quantities i n c l u d i n g h y d r a u l i c residence t i m e and retention and uptake lengths are s u m m a r i z e d i n T a b l e 3.5. H y d r a u l i c residence t i m e i n the stream w a s higher than i n the storage zone for a l l tracer injections and d i d not appear to v a r y w i t h discharge for either reach ( F i g u r e 3.6). T h e average stream residence t i m e was higher i n the l o w e r r e a c h (109 m i n ) c o m p a r e d to the upper reach (49.8 m i n , F i g u r e 3 . 6 A ) . Storage z o n e residence t i m e was also h i g h e r i n the l o w e r reach (31.0 m i n ) c o m p a r e d to the upper reach (10.2 m i n , F i g u r e 3 . 6 B ) . T a b l e 3.5. S u m m a r y o f d e r i v e d quantities i n c l u d i n g stream v e l o c i t y (u), h y d r a u l i c residence t i m e for the stream ( T ) and storage z o n e ( T ) , h y d r a u l i c uptake length (Shyd), h y d r a u l i c retention factor (Rh) and the standardized storage z o n e coefficient ( A s / A ) . s t l  Reach Upper June 19 Sept 21 Sept 29 O c t 20 Lower M a y 31 June 19 June 27 Sept 21 Sept 30 O c t 20  st01  u  Q (10" m V )  (ms" )  15.9 9.8 1.0 9.7  0.13 0.15 0.04 0.10  30.6 1.7 5.5 9.9 1.9 11.9  0.14 0.09 0.04 0.07 0.03 0.06  3  T tr  T  (min)  1  stor  Shyd (m)  80.7 61.8 • 55.2 48.3  40.4 13.3 11.6 7.6  648 542 122 302  3.7 1.5 • 5.7 1.5  0.50 0.22 0.21 0.16  121.5 86.2 61.0 80.2 196.5 106.3  75.7 26.7 18.1 16.0 38.9 11.0  1029 465 146 334 343 397  4.4 3.4 7.4 2.9 6.8 ' 1.7  0.62 0.31 0.30 0.20 0.20 0.10  s  1  (min)  R (sm" ) h  1  A /A s  Stream v e l o c i t y increased w i t h discharge ( F i g u r e 3 . 7 A ) . G e n e r a l l y , v e l o c i t y w a s higher i n the upper reach than the l o w e r reach. T h e h y d r a u l i c uptake length increased as discharge increased for b o t h the upper and l o w e r reach ( F i g u r e 3 . 7 B ) . V a l u e s ranged f r o m 122 m (1.04 L / s ) i n the upper reach to 1028 m (30.6 L / s ) i n the l o w e r reach. O n average the uptake length was higher i n the upper reach (318 m ) than the l o w e r reach (305 m ) . H o w e v e r , d u r i n g t w o tracer injections (June 19, September 21), uptake l e n g t h was greater i n the upper reach than the l o w e r reach, and less than the l o w e r reach d u r i n g t w o a d d i t i o n a l injections (September 2 9 , O c t o b e r 20).  47  T h e h y d r a u l i c retention factor, a measure o f the storage z o n e residence t i m e per unit o f stream reach traveled, s h o w e d no clear trend w i t h discharge ( F i g u r e 3 . 7 C ) . D u r i n g l o w f l o w c o n d i t i o n s , retention was highest, r a n g i n g from 5.7 s/m at 1.0 L / s to 7.4 s/m at 5.5 L / s . A t m i d to h i g h stream f l o w rates (10 L / s to 30 L / s ) a slight i n c r e a s i n g trend i n retention was o b s e r v e d ; h o w e v e r , retention factors d i d not reach l o w f l o w values. T h e standardized storage z o n e area also d i d not s h o w a clear trend w i t h discharge ( F i g u r e 3 . 7 D ) . A m a j o r i t y o f the values ranged f r o m a p p r o x i m a t e l y 0.10 to 0.30, w i t h the e x c e p t i o n o f the M a y 31 injection at a v a l u e o f 0.62.  o  -  O U")  -  CM  X—  A  A  A  001.  A A  •  o  o  -  o o  -  A  • • • A  Upper Lower B  o  A  co  o CD  o  •  A  A  •  CM  O  .  •  A  A  -  Upper Lower  1  1  1  I  I  I  i  0  5  10  15  20  25  30  Q(Ls" ) 1  F i g u r e 3.6. H y d r a u l i c residence t i m e o f solutes i n the stream ( A ) and storage z o n e ( B ) versus stream discharge ( Q ) for the u p p e r and l o w e r reach.  48  • Upper Lower  A  o ID  O  <  Q(Ls")  Q(Ls )  Figure 3.7. Stream velocity (A), hydraulic uptake length ( B ) , hydraulic retention factor (C) and the standardized storage zone coefficient (D) versus stream discharge (Q) for the upper and lower reach . 3.3.5.  Lateral inflow rates  Lateral inflow rates increased with discharge (Figure 3.8). Inflow rates ranged 6  3 1 1  5  3  1 1  from 3.26x10" m s" m" at low flow conditions to 3.18 x 10" m s" m" at high flow conditions. Net inflow represents less than 1% gain of streamflow. Lateral inflow rates did not appear to vary significantly between reaches.  49  • A  Upper Lower  H  0  5  10  15  20  25  30  Q(Ls" ) 1  F i g u r e 3.8. N e t lateral i n f l o w rates ( Q l ) versus discharge ( Q ) for a l l solute releases  3.3.6.  Quantifying pool storage and residence times  B r e a k t h r o u g h curves f r o m t w o p o o l s located i n the upper reach and one p o o l located i n the l o w e r reach w e r e s i m u l a t e d u s i n g O T I S - P . R e s u l t s are presented for one injection o n September 30 ( F i g u r e 3.9). A d d i t i o n a l m o d e l s i m u l a t i o n s are presented i n A p p e n d i x A . F o r a l l p o o l s i m u l a t i o n s , the transient storage area w a s greater than c h a n n e l area, r e s u l t i n g i n a larger A s / A ratio than the reach s i m u l a t i o n ( T a b l e 3.6). H i g h uncertainty values were o b s e r v e d for parameter estimates associated w i t h d i s p e r s i o n ( D ) and storage area ( A s ) for t w o p o o l s i m u l a t i o n s ( F i g u r e 3.10). T h e s i m u l a t i o n s for p o o l ' 2 and p o o l 3 h a d the greatest uncertainty based o n the D a m k o h l e r number. T h e D a l numbers w e r e 0.09 and 2.8, r e s p e c t i v e l y , c o m p a r e d to a D a l n u m b e r o f 0.8 i n p o o l 1. Storage zone residence times w e r e h i g h e r than for the entire reach for b o t h p o o l s located i n the upper reach ( F i g u r e 3.11). R e s i d e n c e times w e r e also greater than the reach scale estimated values. H o w e v e r , this pattern w a s not supported i n the p o o l located i n the l o w e r reach ( P o o l 3).  50  T a b l e 3.6. S u m m a r y o f the simulated parameter estimates o f clispersion coefficient ( D ) , stream cross-sectional area ( A ) , cross-sectional area o f storage z o n e ( A s ) , and the storage z o n e exchange coefficient (a), stream v e l o c i t y (ii), the standardized storage z o n e coefficient ( A s / A ) and the D a m k o h l e r n u m b e r ( D a l ) . Parameters Reach S e p t 29 Reach 1 Pool 1 Pool 2 S e p t 30 Reach 2 Pool 3  (l(r m )  a (lOV)  U (ms-)  As/A  Dal  (10- mV)  (10- mV)  A (loW)  1.0  2.9 2.5 1.8  2.8 6.3 0.8  0.6 60.7 6.1  3.0 3.1 3.4  0.04 0.02 0.13  0.21 9.61 7.40  0.50 0.80 0.09  6.5 2.5  1.3 5.9  0.85 145  0.03 0.07  0.20 2.33  0.17 2.60  Q 3  1.9  D 2  4.6 8.4  •  A  s  2  2  1  F i g u r e 3.9. M o d e l s i m u l a t i o n s u s i n g O T I S - P for September 30. Results are f r o m one p o o l l o c a t i o n i n the l o w e r reach.  51  E. Q  E,  E,  <  <  Pool 3  Upper  Pool 3  F i g u r e 3.10. S i m u l a t e d m o d e l parameters for solute releases i n the upper and l o w e r stream reach d u r i n g S e p t e m b e r 2 9 and 30. P o o l s 1 and 2 w e r e located i n the upper reach. P o o l 3 w a s located i n the l o w e r reach. D i s p e r s i o n coefficient ( D ) , stream cross-sectional area ( A ) , cross-sectional area o f storage z o n e ( A s ) , storage z o n e exchange coefficient (a) versus l o c a t i o n . E r r o r bars r e p r e s e n t ' ± 1 standard d e v i a t i o n .  52  o o -  •  CO  A  Stream Storage  A  o CO  o o A  o CO  •  o o CM  O O  -  o  -  •  •  •  A-  1 Pool 1  1 Pool 2  1 Lower  A  ^ Upper  *  r^ Pool 3 -  Location  F i g u r e 3.11. H y d r a u l i c residence t i m e o f solutes i n the stream and storage z o n e for solute releases i n the upper a n d l o w e r stream reach d u r i n g September 29 and 30. P o o l s 1 and 2 w e r e located i n the u p p e r reach. P o o l 3 w a s located i n the l o w e r reach.  3.3.7.  Subsurface relative connectivity  A total o f 5 to 8 observations o f m i x i n g ratios ( E q u a t i o n 2.27) w e r e m a d e for each piezometer, except for piezometers P 5 6 , P I 8, and P 4 8 , w h e r e o n l y three observations w e r e c o n d u c t e d due to the d r y i n g out o f the stream channel o v e r the study p e r i o d . T h e relative c o n n e c t i v i t y v a r i e d b e t w e e n reaches ( F i g u r e 3.12). T h e upper reach appeared to have a h i g h e r percentage o f piezometers w h e r e the tracer concentration i n the p i e z o m e t e r decreased d u r i n g the tracer injection, as i n d i c a t e d b y a negative v a l u e o f % (e.g. P I 6, P 2 0 , P 2 2 , and P 2 1 ) . A negative m i x i n g ratio c o u l d be a result o f stratification o f water c h e m i s t r y i n the streambed. In general, the tracer concentration for piezometers l o c a t e d w i t h i n the l o w e r reach increased d u r i n g tracer injections as i n d i c a t e d b y a p o s i t i v e v a l u e o f x- T h i s suggests that p i e z o m e t e r s located w i t h i n the l o w e r reach have a h i g h e r degree o f c o n n e c t i v i t y to water i n the stream c h a n n e l . T h e r e w a s no clear d i s t i n c t i o n b e t w e e n u p w e l l i n g and d o w n w e l l i n g zones i n terms o f relative c o n n e c t i v i t y . In the upper reach,  53  h y d r a u l i c gradients w e r e not measured i n piezometers 16, 18 and 19. A l l three sites located at the upstream end o f a riffle habitat.  o  Upper 0.5  o ,  o  O  1  i  0  o  d m d ~  i  0  o  o  p f P12  o  • •  0  o  I  i  I  1  1  I  i  1  P16  P18  P19  P20  P21  P22  P23  P27  Lower  1 P7  —j— 1  0.5  -T-  °  o  i  o  .0  nil \W*.....-±:....mmm  o  1  •  i  '  i  j  L  P8  1  —  --  1  Downwelling Neutral i  o  0 o  0  1  1 4  1  a  Q  -0.5  o  1  —•—  •1.0  • • I P31  I I P 3 3 P35  i P36  1  1  P38  P47  P48  I  i  I  1  1  1  1  P50  P51  P52  P53  P54  P56  P64  Downwelling Upwelling •! Neutral 1 i P67  P68  F i g u r e 3.12. R e l a t i v e c o n n e c t i v i t y ( R C ) as measured u s i n g a n o n - d i m e n s i o n a l m i x i n g ratio (x) for piezometers s a m p l e d d u r i n g tracer injection experiments i n the upper a n d l o w e r reach  3.4.  Solute injection experiments - Channel-unit scale  3.4.1.  Qualitative observations of hyporheic discharge  T w o different f l o w p a t h w a y s were observed w i t h i n one channel-unit i n the l o w e r reach ( F i g u r e  3.13). T h e f o l l o w i n g observations w e r e m a d e d u r i n g e x p e r i m e n t s at the  infiltrometer 1-5 l o c a t i o n :  •  July 11 - R W T tracer water infiltrated the bed, traveled l a t e r a l l y a r o u n d a large anchor b o u l d e r i n the riparian z o n e and returned to the stream at the base o f the step w i t h i n 5-10 m i n u t e s post-injection.  54  •  A u g u s t 14 - R W T f o l l o w e d the same f l o w p a t h w a y as J u l y 11. R W T was v i s i b l e at the start o f the p o o l w i t h i n four minutes post-injection and was v i s i b l e w i t h i n the p o o l u n t i l a p p r o x i m a t e l y 18 minutes post-injection.  •  S e p t e m b e r 27 - D i s c h a r g i n g h y p o r h e i c water w a s again observed at this l o c a t i o n ; h o w e v e r , d u r i n g this experiment R W T infiltrated the streambed, t r a v e l l e d v e r t i c a l l y t h r o u g h the step and returned to the stream at the start o f the p o o l . R W T w a s v i s i b l e i n the p o o l w i t h i n 9 minutes, and was still s l i g h t l y v i s i b l e w i t h i n the p o o l after one h o u r .  S i m i l a r observations w e r e m a d e at infiltrometer 1-4: •  A u g u s t 21 - N o R W T was v i s i b l e w i t h i n the p o o l after 4 hours o f o b s e r v a t i o n . H o w e v e r , an increase i n fluorescence (as m e a s u r e d u s i n g a fluorometer) was observed. F l u o r e s c e n c e returned to b a c k g r o u n d i n a p p r o x i m a t e l y 24 hours, suggesting the presence o f h y p o r h e i c f l o w at this l o c a t i o n .  •  S e p t e m b e r 28 - R W T infiltrated the streambed, traveled v e r t i c a l l y t h r o u g h the step and returned to the stream at the start o f the p o o l .  •  O c t o b e r 6 - D i s c h a r g i n g h y p o r h e i c water was observed at the base o f the p o o l ( s i m i l a r l o c a t i o n as the September 27 and 28 injections). R W T was o b s e r v e d 13 minutes after i n j e c t i o n and was still v i s i b l e i n the p o o l after 1 h r 24 m i n . N o tracer was o b s e r v e d at 1 h r 48 m i n after injection.  •  Injections o f B r i l l i a n t B l u e F C F were also c o n d u c t e d ; h o w e v e r , d i s c h a r g i n g d y e was not observed at any l o c a t i o n s w i t h i n the l o w e r reach.  O b s e r v a t i o n s w e r e also m a d e at infiltrometer 1-1 i n s t a l l e d i n a log-step i n the upper reach; h o w e v e r , d i s c h a r g i n g o f R W T or B r i l l i a n t B l u e F C F was not observed d u r i n g any tracer i n j e c t i o n experiments at this l o c a t i o n ( J u l y 12, A u g u s t 2 3 , and September 2 8 , O c t o b e r 6), except for an i n i t i a l trial o n June 2 7 . H y p o r h e i c discharge o f B r i l l i a n t B l u e F C F was o b s e r v e d w i t h i n the log-step channel-unit d u r i n g the trial o n June 2 7 . T r a c e r infiltrated the stream b e d and was observed u p w e l l i n g f r o m the sediments at the upstream end o f the p o o l .  55  F i g u r e 3.13. C h a n n e l - u n i t observations o f f l o w pathways i n P o o l 4, i n c l u d i n g side v i e w and aerial v i e w  3.4.2.  Quantifying residence times  T h e p o o l b e h a v e d l i k e a c o n t i n u o u s l y stirred tank reactor ( C S T R ) , as i n d i c a t e d b y a straightening o f the late t i m e p e r i o d o f the b r e a k t h r o u g h c u r v e w h e n c o n c e n t r a t i o n is plotted o n a l o g a r i t h m i c scale ( F i g u r e 3 . 1 4 B , F i g u r e 3 . 1 5 B ) . M R T v a r i e d o v e r the experiments p o s s i b l y due to v a r i a b i l i t y i n streamflow and h y d r a u l i c gradients ( T a b l e 3.7). E l e c t r i c a l c o n d u c t i v i t y increased r a p i d l y to a peak c o n c e n t r a t i o n w i t h i n the step and p o o l sub-units. D u r i n g both experiments, the m a x i m u m e l e c t r i c a l c o n d u c t i v i t y was greater i n the step than p o o l . A d d i t i o n a l m i n o r peaks i n E C are v i s i b l e o n the r i s i n g and r e c e s s i o n l i m b o f the breakthrough curves for each e x p e r i m e n t . T h e s e peaks c o u l d indicate separate flow p a t h w a y s w i t h different residence times w i t h i n the channel-unit.  T a b l e 3.7. M e a n r e s i d e n c e times for h y p o r h e i c z o n e (step) a n d p o o l storage z o n e s i n P o o l 4. V e r t i c a l h y d r a u l i c gradients ( V H G ) measured u s i n g p i e z o m e t e r 6 1 . M e a n Residence T i m e (min)  Date  Q (Ls )  VHG (cm/cm)  Step  Pool  Sept 25 Oct 5  2.4 1.4  -0.96 -0.89  23.8 68.8  26.4 71.2  1  56  o LU  o LU  "i 00:00  00:12  1 00:24  1 00:36  1 00:48  1 01:00  ~\  r 01:12  00:00  Time (hr:min)  00:12  00:24  00:36  00:48  01:00  r 01:12  Time (hr:min)  F i g u r e 3.14. S t e p - p o o l residence t i m e experiment c o n d u c t e d o n S e p t e m b e r 2 5 , 2 0 0 6  O  tu  o  CO  O  LU  01:00  03:00  05:00  Time (hnmin)  F i g u r e 3.15. S t e p - p o o l residence t i m e experiment c o n d u c t e d o n O c t o b e r 5, 2 0 0 6  57  3.5.  Subsurface flow - Point scale  3.5.1.  Hydraulic gradients  C o n s i d e r a b l e spatial and t e m p o r a l v a r i a b i l i t y i n h y d r a u l i c gradients w a s o b s e r v e d at a l l l o c a t i o n s . S t r o n g negative h y d r a u l i c gradients t y p i c a l l y o c c u r r e d at the start o f a step or riffle i n d i c a t i n g infiltration into the stream b e d ( F i g u r e 3.16). C o n s i s t e n t negative gradients w e r e o b s e r v e d i n piezometers 1 to 6 w i t h i n the log-step channel-unit, a l t h o u g h gradients v a r i e d t e m p o r a l l y ( F i g u r e 3.17). In general, gradients tended to get w e a k e r ' a s stream f l o w decreased d u r i n g l o w f l o w c o n d i t i o n s i n m i d to late s u m m e r . P o s i t i v e h y d r a u l i c gradients w e r e consistently observed d o w n s t r e a m f r o m obstructions i n the stream c h a n n e l , such as the l o g step located i n the upper reach 1 3 - 1 5 m d o w n s t r e a m . P i e z o m e t e r s 10, 11, 13, 14 and 15 s h o w e d consistent p o s i t i v e h y d r a u l i c gradients o v e r the study p e r i o d ( F i g u r e 3.18); h o w e v e r , h y d r a u l i c gradients v a r i e d o v e r short spatial scales. F o r e x a m p l e , p i e z o m e t e r s 10 and 13 w e r e located a p p r o x i m a t e l y 2 0 c m apart and w e r e \  i n s t a l l e d at s i m i l a r depths; h o w e v e r , the average gradient i n p i e z o m e t e r 13 w a s 1.06 c m / c m o v e r the study p e r i o d c o m p a r e d to 0.03 c m / c m i n p i e z o m e t e r 10. A s i m i l a r pattern o f h y d r a u l i c gradients w a s observed i n a boulder-step c h a n n e l unit located i n the upper reach 2 0 m d o w n s t r e a m f r o m the c u l v e r t c r o s s i n g . P i e z o m e t e r s 21 to 23 s h o w e d a consistent strong d o w n w e l l i n g response ( F i g u r e 3 . 1 9 A ) . T h e magnitude o f response increased as the distance to the step increased w i t h p i e z o m e t e r 23 s h o w i n g the strongest response (average = -0.6 c m / c m ) . P i e z o m e t e r s 24 to 26 a l l s h o w e d consistent u p w e l l i n g and were located near the head o f the p o o l ( F i g u r e 3 . 1 9 B ) .  N  58  o o  ..^A^... AA.  -A.  #  o  x > • p c\i  A  A —  'T-  r~ 13  14  15  16  17  IS  —  Step/Riffle Pool i r~ 19  20  o o o E O  x  > o c\i  Distance downstream(m)  F i g u r e 3.16. V e r t i c a l h y d r a u l i c gradients measured i n the upper ( A ) and l o w e r ( B ) reaches. S y m b o l s indicate study p e r i o d means.  o  P  o  P3 P4  A  •  o d  -K-.T.^T.  ,>  ...  A  1  p2  A  LO  •  P5  +  P6  A ' -  :-•  I  >  1  1  Jul 01  1 Sep 0 1  A u g 01  1 O c t 01  0  i  0.3  F i g u r e 3.17. V e r t i c a l h y d r a u l i c gradients measured o v e r the study p e r i o d i n piezometers 1 -6 i n the upper reach  i  0.2  ..A1  o ~  A A • 4  A  i  A  A.,  A.  . ,.o  O  .0.  A  ••••fl--\v;;::!i:;:;: A  ""  CN  d  0...  */  o d  P10 P11 P13 P14 P15  A •""  6'  9'' _  T  JinOI  JJ01  AteOI  SepOl  Oct 01  F i g u r e 3.18. V e r t i c a l h y d r a u l i c gradients measured o v e r the study p e r i o d i n piezometers 10,11,13-15 i n the upper reach  60  (A)  (B)  £  I  I  >  >  P21 P22 P23  Jul 01  Aug 01  S e p 01  Oct 01  + • x 1  1  1  Aut3 01  Sep 01  Oct 01  F i g u r e 3.19. V e r t i c a l h y d r a u l i c gradients measured o v e r the study p e r i o d i n the piezometers 21-23 and p i e z o m e t e r s 24-26 i n the u p p e r reach U p w e l l i n g sites w e r e also observed w i t h i n the l o w e r reach, a l t h o u g h h y d r a u l i c gradients w e r e not as strong as i n the u p p e r reach. V a l u e s for u p w e l l i n g sites tended to be w i t h i n measurement error ( ± 0.05 c m / c m ) . R e v e r s a l o f h y d r a u l i c head gradients w a s also m o r e c o m m o n w i t h i n the l o w e r reach, w i t h p i e z o m e t e r response s w i t c h i n g b e t w e e n neutral, d o w n w e l l i n g and u p w e l l i n g o v e r the course o f the study p e r i o d . F o r e x a m p l e , i n one b o u l d e r step channel-unit, piezometers i n s t a l l e d at the d o w n s t r e a m end o f a large step (60, 61 and 62) s h o w e d consistent d o w n w e l l i n g (average - 1 . 3 , -0.88, -1.1 c m / c m respectively), whereas the gradients measured i n p i e z o m e t e r s located w i t h i n the p o o l (67 and 68) fluctuated f r o m neutral to slight u p w e l l i n g and b a c k to d o w n w e l l i n g d u r i n g the late s u m m e r ( F i g u r e 3.20). P o s i t i v e V H G ' s o b s e r v e d i n p i e z o m e t e r s 67 and 68 d u r i n g m i d to late September c o r r e s p o n d to qualitative observations m a d e at the channel-unit, where R W T injected into the streambed traveled v e r t i c a l l y through the step and returned to the p o o l at the start o f the p o o l d u r i n g an i n j e c t i o n o n September 28. P r i o r to this, exchange f l o w was o n l y observed f l o w i n g l a t e r a l l y around a large anchor b o u l d e r located w i t h i n the channel-unit.  61  P25 P24 P26  *  A o • •  t  •---.-W  -"t— A,  ... A '•A  " " ^ < ' * "  P60 P61 P62 P67 P68  -vp........  .•  ...-O--.. . '"" S3  'o  ET'  Jul 01 '  Aug 01  Sep 01  Oct 01  F i g u r e 3.20. V e r t i c a l h y d r a u l i c gradients measured o v e r the study p e r i o d i n the p i e z o m e t e r s 60, 6 1 , 62 and 67 to 68 i n the l o w e r reach O f the 66 p i e z o m e t e r s tested, o n l y four locations had correlations b e t w e e n discharge and V H G u s i n g a non-parametric S p e a r m a n ' s rank c o r r e l a t i o n that are significant at a = 0.05 ( T a b l e 3.8). A n u p w e l l i n g ( P 2 5 ) and d o w n w e l l i n g site (P2) h a d negative correlations w i t h discharge, w h i l e t w o d o w n w e l l i n g sites had p o s i t i v e correlations ( P 2 7 , P 4 7 ) w i t h discharge. T h e s e results suggest that stream discharge does not c o n t r o l v e r t i c a l h y d r a u l i c gradients i n East C r e e k .  T a b l e 3.8. S p e a r m a n c o r r e l a t i o n coefficient ( r ) , associated p-values, n u m b e r o f observations (n) and v e r t i c a l h y d r a u l i c gradients ( V H G ) for each p i e z o m e t e r i n d i c a t i n g a significant c o r r e l a t i o n w i t h discharge. s  Piezometer 2 25 27 47  P-value -0.87 -0.72 0.84 0.62  0.002 0.02 0.02 0.04  N 9 10 7 11 '  VHG (cm/cm) -0.01 0.03 -0.12 -0.03  62  3.5.2.  V H G a n d scaled location w i t h i n channel units  Z o n e s o f h y p o r h e i c discharge and recharge appear to b e a f u n c t i o n o f the scaled l o c a t i o n i n the channel unit ( F i g u r e 3.21). Z o n e s o f h y p o r h e i c discharge, or u p w e l l i n g (as i n d i c a t e d b y a p o s i t i v e V H G ) , were g e n e r a l l y c o n f i n e d to the upper p o r t i o n o f the channel units ( X / L = 0.0 to 0.4). F o r X / L > 0.2, there is a trend to i n c r e a s i n g l y negative h y d r a u l i c gradients w i t h increasing distance f r o m the head o f the channel-unit. T h e height o f the d o w n s t r e a m step does not appear to c o n t r o l h y d r a u l i c gradients. A S p e a r m a n ' s rank c o r r e l a t i o n analysis i n d i c a t e d that h y d r a u l i c gradients w e r e not correlated w i t h step height ( r = -0.186, p = 0.17). In a d d i t i o n , a sequential analysis o f s  v a r i a n c e u s i n g three linear mixed-effects m o d e l s c o n f i r m e d that p o s i t i o n w i t h i n the stream channel (as defined b y a category o f X / L ) s i g n i f i c a n t l y contributed to the observed v a r i a b i l i t y i n h y d r a u l i c gradients (x = 40.9, p < 0.001, T a b l e 3.9). T h e a d d i t i o n 2  o f d o w n s t r e a m step-height to the base m o d e l ' (i.e. c h a n n e l unit as a r a n d o m effect) d i d not s i g n i f i c a n t l y contribute to the observed v a r i a b i l i t y i n h y d r a u l i c gradients % ( ~ = 4.6, p = 009, T a b l e 3.9).  T a b l e 3.9. A n a l y s i s o f variance table c o m p a r i n g three l i n e a r mixed-effects m o d e l s for vertical h y d r a u l i c gradients i n c l u d i n g a base m o d e l w i t h o n l y channel-unit, a second m o d e l w i t h channel-unit and step height ( S H ) , and a t h i r d m o d e l w i t h channel-unit, stepheight and channel p o s i t i o n ( X / L ) . A chi-square (% ) statistic was used to test for 2  significance. 2 X  DF  4  4.6  2  0.09  8  40.9  4  <0.001  Model  DF  Unit  2  Unit + S H Unit + S H + X L  2 X  p-value  63  °...<&s..°..... A..s>  a..  r  °  o E  -  o  o  o  o  °  0  o  O  x  >  0.0  1  1  1  0.2  0.4  0.6  1  0.8 •  r  1.0  X/L  Figure 3.21. Vertical hydraulic gradient (cm/cm) versus scaled location within the channel-unit. Hydraulic gradients are averaged over the entire study period.  & + 44-  x A/  CD X  >  4-  X/L<0.2 0.2<X/L<0.4 0.4<X/L<0.6 0.6<X/L<0.8 0.8<X/L< 1.0 1^ 0.0  0.2  0.4  0.6  0.8  1.0  Step height (m)  Figure 3.22. Vertical hydraulic gradient (cm/cm) versus step height (m) as a function of scaled location within the channel-unit (X/L). Hydraulic gradients are averaged over the entire study period. 64  3.5.3.  Hydraulic conductivity  T h e geometric means o f c o n d u c t i v i t i e s for the l o w e r and upper reaches w e r e 2.54 x 10" (n = 24) and 2.37 x 10" m/s (n = 16) respectively. C o n d u c t i v i t i e s appeared to be 4  4  h i g h e r at neutral and u p w e l l i n g sites than at d o w n w e l l i n g sites i n the l o w e r reach ( F i g u r e 3.23); h o w e v e r , o n l y three sites i n the analysis w e r e considered u p w e l l i n g sites, c o m p a r e d to d o w n w e l l i n g (n = 21) and neutral sites (n = 17). A sequential analysis o f v a r i a n c e u s i n g t w o linear mixed-effects m o d e l s c o n f i r m e d that site c o n d i t i o n s i g n i f i c a n t l y contributed to the observed v a r i a b i l i t y i n h y d r a u l i c c o n d u c t i v i t y (% = 6.7, p = 0.01, T a b l e 2  3.10). H y d r a u l i c c o n d u c t i v i t y also d i d not appear to v a r y s y s t e m a t i c a l l y w i t h i n s t a l l a t i o n depth ( F i g u r e 3.24).  T a b l e 3.10. A n a l y s i s o f variance table c o m p a r i n g t w o linear mixed-effects m o d e l s for h y d r a u l i c c o n d u c t i v i t y ( l o g transformed), i n c l u d i n g a base m o d e l w i t h o n l y reach as a factor and a second m o d e l w i t h reach and site c o n d i t i o n ( u p w e l l i n g , neutral and downwelling). Model  DF  Reach  2  R e a c h + Site C o n d i t i o n  3  "  I  DF  6.7  1  1  2 X  p-value  0.01  65  F i g u r e 3.23. H y d r a u l i c c o n d u c t i v i t y ( K ) for d o w n w e l l i n g ( D ) , neutral ( N ) and u p w e l l i n g ( U ) sites located i n the l o w e r (n = 24) and upper reach (n = 17). N o t e l o g scale.  o  •  CN  0  E  Upper Lower  N  10  15  "T  T  20  25  30  Depth (cm)  F i g u r e 3.24. H y d r a u l i c c o n d u c t i v i t y ( K ) w i t h depth o f p i e z o m e t e r installation for the upper and l o w e r reaches. N o t e l o g scale.  66  Streambed infiltration rates  3.5.4.  Temporal variation in infiltration rates was observed over the study period (Figure 3.25) . Rates were difficult to measure with the constant head infiltrometer during low flow conditions, resulting in a lack of observations during base flow conditions (mid to late August). The probable errors for infiltration measurements were almost ± 60% of the measured value. The relationship between discharge and infiltration rates was tested using a nonparametric Spearman's rank correlation. In two locations, infiltration rates were significantly correlated with discharge (Table 3.11), including the sediment-step (1-3) and boulder step (1-5). The boulder step (1-5) location also had the strongest mean V H G (-1.1 cm/cm). Hydraulic conductivity estimates based on streambed infiltrometers (Equation 2.26) were higher than estimates from falling head tests (Figure 3.26). This result suggests that bed infiltration computed from piezometer data alone may underestimate actual infiltration rates.  o LO  o  Cxi  or  LO  ci  o d Jul  Aug  Sep  Oct  Figure 3.25. Infiltration rates over the study period. Error bars represent probable errors based on Equation 2.25. 67  T a b l e 3.11. S p e a r m a n correlation coefficient (r ), associated p-values and n u m b e r o f s  observations (n) for i n f i l t r a t i o n rates versus discharge at each infiltrometer l o c a t i o n . Location  s  L o g step (1-1) Sediment step (1-3) B o u l d e r step (1-4) B o u l d e r step (1-5)  12  r 0.37 0.92 0.51 0.83  p-value  n  0.21 0.01 0.25 0.01  '  13 5 6 7  • Infiltrometer k El Slug k  10  11 Log step (1-1)  Boulder step (I-2)  Sediment step (I-3)  Boulder step (I-4)  i n  Boulder step (I-5)  F i g u r e 3.26. H y d r a u l i c c o n d u c t i v i t y c a l c u l a t e d u s i n g infiltration rates and slug-tests for f i v e locations. V a l u e s represent the geometric m e a n ± standard error.  3.5.5.  Streambed water fluxes computed from Darcy's law  Streambed water fluxes w i t h i n one step-pool unit i n the upper reach ( P o o l 1) v a r i e d w i t h f l o w c o n d i t i o n s . T h e total c o m p u t e d f l u x into the bed was larger d u r i n g h i g h f l o w c o n d i t i o n s o n June 19 ( Q = 15.4 L / s ) than d u r i n g l o w f l o w c o n d i t i o n s o n September 29 ( Q = 1.1 L / s ) , as calculated u s i n g E q u a t i o n 2.23. T h e fluxes into the bed were also larger than fluxes out o f the b e d as s u m m a r i z e d i n T a b l e 3.12. T h e average fluxes out o f  68  the b e d , as c a t e g o r i z e d w h e r e X / L < 0.4, d i d not change s i g n i f i c a n t l y w i t h f l o w ( F i g u r e 3.27). H o w e v e r , Jhe f l u x e s into the area a b o v e the step, w h e r e X / L > 0.6, d i d increase s l i g h t l y w i t h f l o w , s p e c i f i c a l l y w h e r e X / L ~ 0 . 8 . T h e reach scale estimate o f h y p o r h e i c e x c h a n g e w a s two-orders o f m a g n i t u d e greater than the scaled-up estimate o f e x c h a n g e for b o t h f l o w c o n d i t i o n s ( T a b l e 3.12).  L o w flow  o o  High flow  o o o  CO  o o  I  I  I  I  0.2  0.4  0.6  0.8  T~  r.O  X/L  F i g u r e 3.27. W a t e r fluxes calculated u s i n g D a r c y ' s L a w for each X / L category w i t h i n one step-pool channel-unit d u r i n g l o w f l o w ( Q = 1 . 1 L / s ) and h i g h flow ( Q = 15.4 L / s )  T a b l e 3.12. W a t e r fluxes w i t h i n one step-pool unit ( P o o l 1) a l o n g w i t h scaled-up and reach-scale estimates o f h y p o r h e i c exchange ( s ) . _l  Water Fluxes (L/s) Date  Q (Ls" )  Scaled-up  R e a c h scale  qin  qout  (10'V)  a(10" s )  June 19  15.4  0.78  -0.07  4.1  2.1  September 2 9  1.1  0.46  -0.02  2.6  3.0  1  4  _l  69  CHAPTER FOUR: DISCUSSION T h i s chapter discusses the results presented i n C h a p t e r 3. Sections 4.1 to 4.3 discuss the research objectives o u t l i n e d i n C h a p t e r 1. T h e last section integrates observations from the different scales o f interest.  4.1.  Reach scale  4.1.1.  Modelled parameter uncertainty  W a g n e r and H a r v e y (1997) e x p l a i n e d that parameter uncertainty is m i n i m i z e d w h e n D a l = 1.0. Parameter uncertainty, e s p e c i a l l y w i t h respect to the estimates o f transient storage and exchange, increases i n experiments w i t h v e r y h i g h or l o w D a l values. In East C r e e k , D a l values w e r e less than 1.0 for a l l experiments and ranged f r o m 0.1 to 0.5, w h i c h is consistent w i t h W a g n e r and H a r v e y ' s (1997) c o n c l u s i o n that " w e l l estimated" parameters are l i k e l y to be obtained w h e n the D a m k o h l e r n u m b e r is o n the order o f 0.1-1.0. M o d e l l e d parameter estimates i n East C r e e k w e r e also f a i r l y consistent w i t h the t y p i c a l range o f parameter values reported i n the literature for h i g h gradient streams ( 1 - 1 5 % ) before 1997 ( W a g n e r and H a r v e y 1997, T a b l e 4.1). U n d e r c o n d i t i o n s w h e n D a l < 1.0, parameter uncertainty is h i g h because o n l y a s m a l l amount o f tracer interacts w i t h the storage z o n e ( W a g n e r and H a r v e y 1997). T h i s m a y o c c u r because: (1) stream v e l o c i t y is h i g h , (2). exchange timescales are short, as indicated b y l o w values o f a or (3) the reach l e n g t h is short. Parameter uncertainty w a s greatest d u r i n g p e r i o d s o f h i g h e r f l o w s , p o s s i b l y due to the s l o w rates o f transient exchange relative to the stream water v e l o c i t y . T h e transient exchange coefficient (a) w a s estimated at 0.00014 s" under h i g h f l o w c o n d i t i o n s o n M a y 31 ( Q = 30.6 L / s , u = 0.14 1  m/s), w h i c h w a s h a l f the average a for a l l s i m u l a t i o n s (0.00022 s~'). H a r v e y et a l . (1996) c o n c l u d e d that the stream tracer m e t h o d does not r e l i a b l y characterize exchange at h i g h e r f l o w s , w h i c h c o m p l i c a t e s the efforts o f studies e x a m i n i n g the i n f l u e n c e o f discharge o n transient storage processes o v e r a range o f f l o w c o n d i t i o n s (e.g. H a r t et a l . 1999).  70  The Damkohler number was highest in pool 3 (Dal = 2.8) compared to values of 0.8 and 0.09 in pools 1 and 2 respectively, indicating higher uncertainty in the parameter estimates associated with transient storage. However, parameter uncertainty was lower in pool 1 than at the reach scale (Dal = 0.5). The low Dal numbers within pool 1 and 2 may be attributed to the length of the pool (4 m) compared to 50 m at the reach-scale. Wagner and Harvey (1997) indicated that the length of the study reach should be adjusted to maintain a balance between advective transport and transient storage. This suggests thatreducing the "reach-length" to the scale of individual pools may have contributed to parameter uncertainty. The high Dal number in pool 3 indicates that exchange rates were fast relative to the stream water velocity and that all the solute was exchanged with the storage zone over the reach length (Wagner and Harvey 1997). The uncertainty ratio was also used to quantify parameter uncertainty for reach scale and channel-unit scale simulations. Ratios were lowest for the reach-scale dispersion and transient exchange parameters (Table 3.4), which indicates a higher degree of uncertainty. Low uncertainty ratios were also associated with the dispersion and transient storage parameters for two of the three pool simulations. Uncertainty ratios were within the ranges reported by a previous study examining the broad heterogeneity of hyporheic zone processes across seven small streams in western Washington (Reidy 2004). However, that study encompassed a wider range of flow conditions (0.7 to 216 10 m /s) and channel morphologies than studied in East Creek. 3  3  Table 4.1. Range of parameter values reported for high-gradient streams (Wagner and Harvey 1997) compared to modelled parameter values in East Creek.  Parameter Q (m /s) Q (m /s/m) A(m ) D (m /s) A (m ) a(s-') 3  3  L  2  2  2  s  Range Wagner and Harvey East Creek (1997) 0.005 - 0.2 0.002 - 0.03 0-0.0001 0 - 0.00003 0.02 - 0.6 0.06-0.2 0.025 - 0.8 0.02-0.5 0.01 -2.0 0.006 - 0.1 5.0 x 10" -0.001 8.0 x 10" - 0.0001 6  5  71  4.1.2.  Solute transport parameters and discharge  Solute transport parameters v a r i e d w i t h discharge at the reach scale. T h e results o f the O T I S - P s i m u l a t i o n s were i n partial agreement w i t h p r e v i o u s studies w h i c h reported an increase i n d i s p e r s i o n ( D ) and the channel cross-sectional area ( A ) w i t h discharge ( L e g r a n d - M a r c q and L a u d e l o u t 1985, D ' A n g e l o et a l . 1993, H a r v e y et a l . 1996, M o r r i c e et a l . 1997, H a r t et a l . 1999, W o n d z e l l 2 0 0 5 ) . H o w e v e r , those studies reported that transient storage area ( A s ) decreased w i t h discharge w h i l e transient exchange (a) increased w i t h increasing discharge, w i t h the e x c e p t i o n o f L e g r a n d - M a r c q and L a u d e l o u t (1985) w h o reported that transient exchange r e m a i n e d constant w i t h discharge. In E a s t C r e e k , o n the other hand, exchange rates r e m a i n e d f a i r l y constant w i t h discharge w h i l e the storage area increased. A p r e v i o u s study i n the upper and l o w e r reaches o f East C r e e k observed s i m i l a r trends i n the response o f a and A s (Patschke 1999), as d i d H a r t et a l . (1999). A recent study i n a fourth-order stream i n central M i c h i g a n ( U S A ) f o u n d that the size o f the transient storage z o n e increased w i t h discharge f r o m 1.9 m m / s ) to 7.3 m 3  2  at b a s e f l o w (2.5  at a f l o w o f 19.1 m / s ( P h a n i k u m a r et a l . 2 0 0 7 ) . 3  Studies have attributed a c o n s t r i c t i n g o f the transient storage area d u r i n g p e r i o d s o f h i g h e r discharge to increased groundwater discharge and catchment wetness ( B o u l t o n et a l . 1998, W h i t e 1993). A s the catchment wetness increases, h y d r a u l i c gradients to the stream from the r i p a r i a n z o n e are stronger and c a n o v e r w h e l m the i n f l u e n c e o f channel m o r p h o l o g y ( H a r v e y and B e n c a l a 1993, W o n d z e l l and S w a n s o n 1996a), r e s u l t i n g i n a decrease i n the extent o f the h y p o r h e i c z o n e . C h a n n e l c o m p l e x i t y is cited as a p o s s i b l e e x p l a n a t i o n for the o b s e r v e d differences i n the response o f the transient storage area ( A ) to discharge i n East C r e e k (Patschke s  1999). Storage area increased w i t h discharge i n t w o reaches associated w i t h a r e l a t i v e l y h i g h degree o f stream c o m p l e x i t y (i.e. upper and l o w e r reach) and r e m a i n e d constant w i t h discharge i n t w o less c o m p l e x reaches l o c a t e d upstream f r o m the current study l o c a t i o n (Patschke 1999). T h i s study attributed the difference to v a r i a b i l i t y i n stream c o m p l e x i t y and suggested that as discharge increased, the c o n t r i b u t i o n f r o m storage i n p o o l s and b a c k eddies m a y h a v e contributed to the increase i n storage area. S t o r y (2002)  72  also attributed the v a r i a b i l i t y i n storage z o n e cross-sectional area w i t h i n three sub-reaches to differences i n channel c o m p l e x i t y . T h e ratio A s / A increased w i t h discharge i n both reaches. T h i s trend is i n c o n t r a d i c t i o n to results presented b y H a r v e y et a l . (1996) and M o r r i c e et a l . (1997) i n w h i c h A s / A decreased w i t h discharge. P a t s c h k e (1999) reported a s i m i l a r trend for the l o w e r reach; h o w e v e r , a d e c r e a s i n g trend i n A s / A was o b s e r v e d for the upper reach. D ' A n g e l o et a l . (1993) and M o r r i c e et a l . (1997) reported transient storage zones that were as large as or larger than the surface water area. M u l h o l l a n d et a l . (1997) d e t e r m i n e d that m e t a b o l i c rates and nutrient uptake were s i g n i f i c a n t l y greater i n streams w i t h a larger transient storage z o n e relative to the channel cross-sectional area ( A s > A ) . T h e transient storage area ( A s ) i n E a s t C r e e k was consistently s m a l l e r (on average 7 0 % ) than the surface water area ( A ) . Estimates o f A s / A ranged f r o m 0.1 to 0.6 i n b o t h reaches. A t h i g h e r discharges, l o w - l y i n g areas adjacent to the m a i n channel i n East C r e e k w e r e f i l l e d w i t h stagnant water w h i c h m a y have p r o v i d e d a d d i t i o n a l surface storage. A s w e l l , at h i g h e r f l o w s , a side channel at m i d - r e a c h ( a p p r o x i m a t e l y 45 - 50 m ) d o w n s t r e a m f r o m the culvert at M r o a d was activated. W o n d z e l l and S w a n s o n (1996a) o b s e r v e d significant exchange f l o w between p r i m a r y and secondary stream channels. A t h i g h e r discharges m o r e wetted channel area m a y also be a v a i l a b l e for h y p o r h e i c exchange to occur. T h e transient exchange coefficient r e m a i n e d f a i r l y constant w i t h discharge. S o m e e v i d e n c e o f a threshold response c o u l d be suggested, as transient exchange plateau to a v a l u e o f 2.7 x 10~ s"' at 5.5 L / s and then d e c l i n e d w i t h discharge i n the l o w e r reach. 4  M o r r i c e et a l . (1997) also reported a threshold response i n w h i c h a increased then decreased w i t h discharge i n a first-order stream. A d d i t i o n a l studies h a v e reported either a steady increase ( D ' A n g e l o et a l . 1993, H a r v e y . e t a l . 1996, H a r t et a l . 1999, W o n d z e l l 2005) or a decrease i n transient exchange w i t h discharge ( L e g r a n d - M a r c q and L a u d e l o u t 1985, H a r t et a l . 1999, P a t s c h k e 1999).  73  4.1.3. i.  Residence times and retention  R e s i d e n c e times w e r e h i g h e r i n the stream channel than i n the transient storage z o n e for a l l tracer injections. R e s i d e n c e times also v a r i e d s p a t i a l l y (i.e. b e t w e e n reaches), but d i d not v a r y t e m p o r a l l y w i t h discharge ( F i g u r e 3.6). B e t w e e n reaches, stream channel and transient storage residence times w e r e consistently h i g h e r i n the l o w e r reach. T h i s v a r i a t i o n m a y be attributed to stream c o m p l e x i t y . W o n d z e l l (2005) observed that the storage z o n e residence times and the h y d r a u l i c retention factor (R|,) w e r e greater i n reaches w i t h l o g j a m s than i n a c o m p a n i o n pool-step reach. T h e R h factor w a s also consistently greater i n u n c o n s t r a i n e d reaches than constrained reaches. R e t e n t i o n factors w e r e f a i r l y c o m p a r a b l e b e t w e e n the t w o reaches i n East C r e e k ( T a b l e 3.5), despite the upper reach h a v i n g a greater n u m b e r o f l o g steps and a h i g h e r degree o f i n c i s i o n . T h e l o w e r reach w a s generally less c o n f i n e d than the upper reach, w h i c h m a y e x p l a i n the longer residence times o b s e r v e d for transient storage. R e a c h - s c a l e estimates o f residence times and retention are not consistent w i t h estimates for i n d i v i d u a l p o o l s d u r i n g t w o tracer injections. F o r b o t h p o o l s l o c a t e d i n the upper reach ( P o o l s 1 and 2), storage z o n e residence times w e r e greater than i n the stream channel. Storage z o n e residence times w e r e also greater than reach scale estimated values. H a r v e y et a l . (1996) assumed that the residence t i m e for the i n - c h a n n e l transient storage is v e r y short and is therefore accounted i n the d i s p e r s i o n coefficient rather than the transient storage coefficient. R e s u l t s f r o m T S M s h o w that residence times w i t h i n the p o o l are h i g h e r than the m a i n c h a n n e l . T h i s suggests that it m a y b e i n v a l i d to assume that the residence times w i t h i n the i n - c h a n n e l storage zones (i.e. p o o l s or b a c k eddies) are n e g l i g i b l e . H o w e v e r , this pattern w a s not c o m p l e t e l y supported i n the p o o l located i n the l o w e r reach ( P o o l 3). R e s i d e n c e times i n b o t h the stream c h a n n e l and storage zones w e r e between 1 to 2 m i n and w e r e l o w e r than the reach-scale estimates o f 196 and 39 m i n , respectively.  74  4.1.4.  In-channel transient storage  A major c r i t i c i s m o f current transient storage m o d e l s is the i n a b i l i t y to separate in-channel transient storage f r o m storage w i t h i n the h y p o r h e i c z o n e ( H a r v e y et a l . 1996). B a s e d o n the a s s u m p t i o n that the d o m i n a n t transient storage processes w i t h i n the p o o l s are f r o m in-stream transient storage, T S M s i m u l a t i o n s conducted at the scale o f i n d i v i d u a l p o o l s w e r e used to separate i n - c h a n n e l storage from h y p o r h e i c exchange. Transient storage area ( A s ) w a s g e n e r a l l y higher w i t h i n the p o o l s , resulting i n a h i g h e r A / A ratio than at the reach scale ( T a b l e 3.6). T h e transient storage area parameter ( A s ) is s  assumed to incorporate b o t h storage processes despite the i n a b i l i t y to separate i n - c h a n n e l transient storage f r o m h y p o r h e i c z o n e storage ( H a r v e y et a l . 1996). T h e results f r o m the i n d i v i d u a l p o o l s i m u l a t i o n s suggest that this m a y be a v a l i d assumption.  4.2.  Channel-unit scale  4.2.1.  Variability in exchange flow pathways  Q u a l i t a t i v e and quantitative observations m a d e at the channel-unit scale h i g h l i g h t the t e m p o r a l v a r i a b i l i t y i n exchange f l o w s . W i t h i n one channel-unit ( P o o l 4), t w o separate f l o w p a t h w a y s w e r e o b s e r v e d ( F i g u r e 3.11). O n J u l y 11 and A u g u s t 14, R W T tracer water infiltrated the b e d , traveled laterally due to the d e f l e c t i o n from a large anchor b o u l d e r i n the r i p a r i a n z o n e and returned to the stream at the base o f the step. T h i s c o n f i r m s that exchange f l o w p a t h w a y s can i n c l u d e a lateral f l o w component, as d e s c r i b e d b y m o d e l 2 i n S e c t i o n 1.2.2. H o w e v e r , i n this channel-unit, lateral f l o w was a result o f the step m o r p h o l o g y , and therefore does not c o n f i r m whether zones o f u p w e l l i n g are the result o f lateral i n f l o w or return f l o w f r o m the r i p a r i a n z o n e and adjacent h i l l s l o p e . O n September 27, R W T infiltrated the streambed, traveled v e r t i c a l l y through the step and returned to the stream at the start o f the p o o l . T h i s is described as m o d e l 1 a, or the t y p i c a l f l o w p a t h w a y , i n S e c t i o n 1.2.2. A d d i t i o n a l injections i n a different l o c a t i o n w i t h i n the channel-unit ( l o c a t i o n 1-4) and i n the upper r e a c h ( l o c a t i o n 1-1) c o n f i r m e d M o d e l l a . D i s c h a r g i n g B r i l l i a n t B l u e F C F  75  tracer water was o n l y o b s e r v e d o n one o c c a s i o n w i t h i n the upper reach (June 27). E v e n t h o u g h R W T tracer was injected i n a c o m p a r a b l e quantity at this l o c a t i o n ( l o c a t i o n 1-1), d i s c h a r g i n g tracer water was not observed w i t h the log-step i n the upper reach. T h i s o b s e r v a t i o n m a y be due to R h o d a m i n e W T a b s o r b i n g to the sediments or organics i n the streambed. R h o d a m i n e has been observed to absorb to sediments i n laboratory experiments, e s p e c i a l l y finer sediments ( M u n n and M e y e r 1988). E x c h a n g e f l o w s are i n d u c e d b y the h y d r a u l i c s o f s t r e a m f l o w o v e r an i r r e g u l a r streambed i n the process k n o w n as " a d v e c t i v e p u m p i n g e x c h a n g e " as d e s c r i b e d b y Savant et a l . (1987) and T h i b o d e a u x and B o y l e (1987). T h e s e exchange f l o w s are c o n t r o l l e d b y the spatial and t e m p o r a l v a r i a b i l i t y i n h y d r a u l i c heads a l o n g the stream b o u n d a r y , r e s u l t i n g i n a d i s t r i b u t i o n o f travel times through the h y p o r h e i c zone. O b s e r v a t i o n s m a d e at the channel-unit scale h i g h l i g h t the v a r i a b i l i t y i n residence times associated w i t h transient storage. Solute residence times w i t h i n an i n d i v i d u a l p o o l w e r e consistently greater than those i n the step (or h y p o r h e i c zone) based o n m o d e l l i n g the m e a n residence time o f solutes w i t h i n both storage zones u s i n g l i n e a r r e s e r v o i r theory. M e a n residence times v a r i e d o v e r between experiments p o s s i b l y due to v a r i a b i l i t y i n streamflow and h y d r a u l i c gradients ( T a b l e 3.7). P r e v i o u s studies have reported a w i d e range o f residence times i n the h y p o r h e i c zone; for e x a m p l e , W o r m a n et a l . (2002) reported residence times i n the range o f 160 to 800 m i n u t e s E x a m i n i n g the b r e a k t h r o u g h curves f r o m these stream tracer experiments also h i g h l i g h t s the v a r i a b i l i t y i n exchange f l o w pathways o b s e r v e d w i t h i n a s m a l l spatial area ( F i g u r e 3.14, 3.15). E l e c t r i c a l c o n d u c t i v i t y increased r a p i d l y to a peak c o n c e n t r a t i o n w i t h i n the step and p o o l sub-units. T h e m a x i m u m E C was also greater i n the step than p o o l . T h i s c o u l d be an effect o f d i s p e r s i o n or p o s s i b l y indicate a d i l u t i o n i n concentration w i t h i n the p o o l . H o w e v e r , a d d i t i o n a l m i n o r peaks i n E C are v i s i b l e o n the rising and recessional l i m b o f the breakthrough curves for each experiment, w h i c h c o u l d indicate separate f l o w pathways w i t h different residence times w i t h i n the channel-unit.  76  4.2.2. *  Transient storage modelling  M o d e l l i n g residence times w i t h i n the p o o l u s i n g linear reservoir theory i n d i c a t e d that the p o o l b e h a v e d l i k e a c o n t i n u o u s l y stirred tank reactor ( C S T R ) . R e s i d e n c e times w i t h i n the p o o l w e r e h i g h e r than the h y p o r h e i c z o n e . T h i s suggests that it m a y be an i n v a l i d a s s u m p t i o n to assume that the residence times w i t h i n p o o l s are n e g l i g i b l e ( H a r v e y et a l . 1996). R e s u l t s f r o m O T I S P - P s i m u l a t i o n s i n P o o l 1 and 2 also support this hypothesis. R e s i d e n c e times w i t h i n b o t h transient storage zones (i.e. step and p o o l sub-units) f o l l o w e d an e x p o n e n t i a l d i s t r i b u t i o n . T h e s e results support the use o f an e x p o n e n t i a l residence t i m e d i s t r i b u t i o n to m o d e l transient storage. L a t e - t i m e solute residence times o r " t a i l i n g " o f solute c o n c e n t r a t i o n after the m a i n solute pulse, are represented as an exponential p r o b a b i l i t y d e n s i t y i n current T S M ' s ( B e n c a l a and W a l t e r s 1983). A d d i t i o n a l studies have suggested alternative approaches to represent the t i m e s c a l e o f transient exchange i n c l u d i n g T S M ' s w i t h m u l t i p l e storage zones ( C h o i et a l . 2 0 0 0 ) . H o w e v e r , the results from this study suggest that it m a y be v a l i d to use one transient storage z o n e to represent residences times u s i n g an e x p o n e n t i a l d i s t r i b u t i o n .  4.3.  Point scale  4.3.1.  Interpretation of flow pathways  Repeated observations o f vertical h y d r a u l i c gradients measured f r o m piezometers installed w i t h i n the stream channel c o n f i r m e d the c u r r e n t l y accepted c o n c e p t u a l m o d e l o f exchange f l o w w i t h i n step-pool streams, w h i c h i n v o l v e s infiltration into the b e d upstream o f a step and subsequent discharge a short distance d o w n s t r e a m i n the b o t t o m o f p o o l as described b y H a r v e y and B e n c a l a (1993). In East C r e e k , d o w n w e l l i n g f l o w w a s o b s e r v e d upstream o f obstructions i n the channel (i.e. steps and logs) w i t h u p w e l l i n g o c c u r r i n g at the start o f the p o o l b e l o w a step ( F i g u r e 3.5). P a t s c h k e (1999) s i m i l a r l y o b s e r v e d strong d o w n w e l l i n g i n p i e z o m e t e r s located upstream f r o m steps i n East C r e e k . In contrast to the observations o f u p w e l l i n g  77  observed i n East C r e e k and b y M o o r e et a l . (2005a) at another stream i n M a l c o l m K n a p p R e s e a r c h Forest, W o n d z e l l (2005) failed to locate areas o f u p w e l l i n g w i t h i n a step-pool reach, despite predictions f r o m groundwater f l o w m o d e l s that u p w e l l i n g s h o u l d o c c u r . T h i s m a y be p a r t i a l l y e x p l a i n e d b y that author's d i f f i c u l t y w i t h i n s t a l l i n g piezometers i n the streambed w i t h o u t s i g n i f i c a n t l y d i s t u r b i n g the bed sediments s u r r o u n d i n g the p i e z o m e t e r ( S . W o n d z e l l , pers. c o m m . ) . A d d i t i o n a l studies i n the L o o k o u t C r e e k b a s i n ( O r e g o n , U S A ) , h a v e also not observed coherent h y p o r h e i c discharge b e l o w steps, despite p r e d i c t i o n s f r o m groundwater f l o w m o d e l s that u p w e l l i n g s h o u l d o c c u r ( A n d e r s o n et al. 2 0 0 5 , G o o s e f f et a l . 2 0 0 5 ) . A l t h o u g h u p w e l l i n g was observed i n locations where it was expected i n E a s t C r e e k , the h y d r a u l i c gradients were substantially w e a k e r than i n the d o w n s t r e a m sections o f the step-pool units (i.e., X / L > 0.6). G i v e n that h y p o r h e i c exchange s h o u l d be a p p r o x i m a t e l y at a steady state d u r i n g extended periods o f b a s e f l o w , u p w e l l i n g and d o w n w e l l i n g fluxes s h o u l d balance. T h e fact that d o w n w e l l i n g gradients w e r e g e n e r a l l y stronger and m o r e s p a t i a l l y w i d e s p r e a d than u p w e l l i n g gradients c o u l d be e x p l a i n e d b y one or m o r e o f the f o l l o w i n g : (1) greater h y d r a u l i c c o n d u c t i v i t i e s i n u p w e l l i n g zones, (2). discharge b e i n g concentrated a l o n g preferred p a t h w a y s , and (3) the presence o f lateral f l o w paths. A l l three are p o s s i b i l i t i e s at East C r e e k . T h e presence o f h i g h e r c o n d u c t i v i t i e s i n u p w e l l i n g zones w o u l d be consistent w i t h the o b s e r v a t i o n that penetration o f fine sediments into the b e d (resulting i n c l o g g i n g o f larger pore spaces) is greater i n d o w n w e l l i n g areas ( S c h a l c h l i 1992, P a c k m a n and M a c K a y 2 0 0 3 ) . F u r t h e r m o r e , there is w e a k e m p i r i c a l e v i d e n c e supporting the contrast i n h y d r a u l i c c o n d u c t i v i t i e s b e t w e e n u p w e l l i n g and d o w n w e l l i n g zones ( F i g u r e 3.23). T h e r e is s o m e support for the presence o f preferred p a t h w a y s f r o m the unit-scale tracer experiments, although it is difficult to quantify h o w m u c h discharge occurs v i a these preferred p a t h w a y s (section 3.4). T h e presence o f a lateral c o m p o n e n t to h y p o r h e i c exchange is p o s s i b l e , p a r t i c u l a r l y at s o m e areas a l o n g the reach, but the study d e s i g n d i d not a l l o w the assessment o f lateral h y p o r h e i c exchange because h o r i z o n t a l gradients were not measured. Z o n e s o f h y p o r h e i c discharge and recharge v a r i e d w i t h p o s i t i o n i n the stream channel (as defined b y X / L , T a b l e 3.9). G e n e r a l l y , ' f o r X / L > 0.2, negative h y d r a u l i c gradients increased w i t h distance to the step; h o w e v e r , the actual step height does not  78  appear to c o n t r o l V G H ( T a b l e 3.9). W h e r e X / L > 0.8, there appears to be a slight • negative trend w i t h i n c r e a s i n g step height ( F i g u r e 3.22); h o w e v e r , this relationship was not significant. T h e s e results suggest that the r e l a t i o n b e t w e e n V H G and X / L c o u l d be a useful tool for c h a r a c t e r i z i n g and p r e d i c t i n g h y d r a u l i c gradients i n step-pool streams, a need expressed b y B e n c a l a (2000). C h a n n e l - u n i t s p a c i n g , size and sequence are also d o c u m e n t e d as s i g n i f i c a n t controls o n exchange f l o w i n p r e v i o u s studies ( A n d e r s o n et a l . 2 0 0 5 , G o o s e f f et a l . 2 0 0 6 ) . H o w e v e r , the influence o f step-height o n exchange f l o w does not appear to h a v e b e e n d i r e c t l y quantified i n p r e v i o u s studies.  4.3.2.  Interactions between hyporheic flow and lateral inflow  Increased lateral or g r o u n d w a t e r i n f l o w s f r o m adjacent .hillslopes can reverse head gradients a l o n g the stream m a r g i n , l e a d i n g to a r e d u c t i o n i n h y p o r h e i c z o n e extent and the degree o f h y p o r h e i c exchange ( H a r v e y and B e n c a l a 1993, W r o b l i c k y et a l . 1998, Storey et a l . 2 0 0 3 , W o n d z e l l 2 0 0 5 ) . H y d r a u l i c head reversals (fluctuations b e t w e e n u p w e l l i n g , d o w n w e l l i n g and neutral) w e r e c o m m o n i n the l o w e r reach ( F i g u r e 3.7). G r o u n d w a t e r d i s c h a r g e f r o m the stream banks c o u l d have contributed to the observed reversal o f h y d r a u l i c gradients. T h i s o b s e r v a t i o n appears to be consistent w i t h d o c u m e n t e d c l i m a t i c c o n d i t i o n s , as h y d r a u l i c h e a d reversals w e r e o b s e r v e d f o l l o w i n g p r e c i p i t a t i o n events d u r i n g September and O c t o b e r . T h i s m a y indicate that exchange f l o w s are c o n t r o l l e d b y seasonal v a r i a b i l i t y i n lateral i n f l o w from the h i l l s l o p e , further suggesting that areas o f u p w e l l i n g c o u l d be lateral i n f l o w f r o m the r i p a r i a n zone. L a t e r a l i n f l o w s t e a d i l y d e c l i n e d d u r i n g the early part o f the study p e r i o d ( M a y to A u g u s t ) due to the a t y p i c a l d r y s u m m e r c o n d i t i o n s and general increased w i t h i n c r e a s i n g , discharge. S t r e a m f l o w measurements also c o n f i r m e d that East C r e e k is a g a i n i n g reach; h o w e v e r , net lateral i n f l o w r e m a i n e d less than 1% o f the total streamflow. W o n d z e l l and S w a n s o n (1996a) f o u n d that the strength o f h y d r a u l i c gradients towards the stream v a r i e d w i t h season. D u r i n g the s u m m e r m o n t h s , h y d r a u l i c gradients towards the stream channel w e r e w e a k e r than d u r i n g the w i n t e r m o n t h s . A r e d u c t i o n i n groundwater discharge d u r i n g the s u m m e r m o n t h s due to less r a i n f a l l and drier s o i l c o n d i t i o n s m a y have contributed to the v a r i a b i l i t y i n gradients o b s e r v e d i n this study.  79  M o o r e et a l . (2005b) suggested that the u p w e l l i n g sites m a y be c o n n e c t e d to lateral i n f l o w s f r o m the h i l l s l o p e , p a r t i c u l a r l y those w i t h convergent t o p o g r a p h y based, o n observations that u p w e l l i n g sites underwent little to no m i x i n g (% = 0) w i t h stream water d u r i n g tracer injections i n one coastal headwater stream. In the l o w e r reach, u p w e l l i n g and d o w n w e l l i n g sites appeared to be h i g h l y connected to water i n the stream c h a n n e l , based o n consistently p o s i t i v e m i x i n g ratios d u r i n g reach scale tracer injections. T h e s e ratios i n d i c a t e a partial replacement o f h y p o r h e i c z o n e water w i t h tracer-labeled stream water. In the upper reach, negative m i x i n g ratios w e r e o b s e r v e d a n d the m e d i a n m i x i n g ratio for most sites w a s near 0. T h e s e findings c o u l d i n d i c a t e that water w a s b e i n g d r a w n f r o m different depths o f the h y p o r h e i c z o n e and thus represent water w i t h a different c h e m i c a l signature or residence t i m e .  4.3.3.  Water fluxes and discharge  O b s e r v a t i o n s o f vertical h y d r a u l i c gradients from piezometers i n s t a l l e d w i t h i n the streambed (n = 66) s h o w e d considerable spatial and t e m p o r a l v a r i a b i l i t y , suggesting that h y p o r h e i c exchange o r water fluxes into the stream also v a r y . T h e h i g h w i t h i n - s i t e v a r i a b i l i t y demonstrated is not u n c o m m o n ( T h i b o d e a u x and B o y l e 1987, B a x t e r and H a u e r 2 0 0 0 ) . P h y s i c a l processes, such as discharge or l o c a l b e d f o r m characteristics, are c o n s i d e r e d p o s s i b l e controls o n temporal and spatial patterns o f exchange f l o w . C o n s i d e r a b l e t e m p o r a l v a r i a t i o n i n water fluxes i n c l u d i n g i n f i l t r a t i o n rates and h y d r a u l i c gradients w e r e observed o v e r the study p e r i o d . H o w e v e r , stream discharge w a s not a s i g n i f i c a n t p h y s i c a l c o n t r o l o n water fluxes into the bed, based o n the S p e a r m a n ' s rank c o r r e l a t i o n analysis r e l a t i n g h y d r a u l i c gradients and infiltration rates to discharge (Tables 3.3, 3.4). T h e s e findings suggest that a d d i t i o n a l m e c h a n i s m s contribute to the observed t e m p o r a l v a r i a b i l i t y i n water fluxes, i n c l u d i n g lateral i n f l o w f r o m the r i p a r i a n . H y d r a u l i c c o n d u c t i v i t y was also h y p o t h e s i z e d to be a significant p h y s i c a l c o n t r o l o n the spatial v a r i a b i l i t y o f water fluxes into the streambed. T h e results o f a sequential analysis o f v a r i a n c e determined that h y d r a u l i c c o n d u c t i v i t y v a r i e d w i t h site c o n d i t i o n (i.e. u p w e l l i n g , d o w n w e l l i n g and neutral gradients), but not between reaches ( T a b l e 3.10). H y d r a u l i c c o n d u c t i v i t y d i d not appear to v a r y w i t h depth o f i n s t a l l a t i o n i n the subsurface  \  .  -  80  ( F i g u r e 3.24). H o w e v e r , h y d r a u l i c c o n d u c t i v i t y as calculated f r o m streambed infiltrometers (depth - 1 0 c m ) , was h i g h e r than estimates f r o m f a l l i n g head tests ( F i g u r e 3.10), suggesting that b e d infiltration c o m p u t e d f r o m p i e z o m e t e r data alone m a y underestimate actual infiltration rates. T h e s e results are i n c o n t r a d i c t i o n w i t h p r e v i o u s studies, w h i c h observed an increase i n c o n d u c t i v i t y w i t h depth ( L a r k i n and Sharp 1992, C o n r a d and B e l j i n 1996). T h o s e studies attributed the difference i n c o n d u c t i v i t y at the streambed to the settling o f silt, c l a y and o r g a n i c materials o n the surface i n the process referred to as c o l m a t i o n ( B r u n k e and G o n s e r 1997). T h i s process can also reduce the degree o f h y p o r h e i c exchange. In contrast, supplementary studies e x a m i n i n g the c o n d u c t i v i t y o f porous streambed sediments support the results o b s e r v e d at East C r e e k ( L a n d o n et a l . 2 0 0 1 , S o n g et a l . 2 0 0 7 ) . T h e streambed c o n d u c t i v i t y was t y p i c a l l y greater than the sediments d i r e c t l y b e l o w this layer (~ 3 0 c m ) for b o t h studies. S o n g et a l . (2007) h y p o t h e s i z e d that h y p o r h e i c exchange f l o w s f o r m e d l o c a l i z e d p a t h w a y s w h i c h increased the sediment pore size r e s u l t i n g i n an increase i n h y d r a u l i c c o n d u c t i v i t y . T h e observed spatial v a r i a t i o n i n c o n d u c t i v i t y c o u l d be caused b y the i m p r e c i s i o n o f each measurement m e t h o d . T h e p r o b a b l e error associated w i t h infiltration measurements was almost ± 6 0 % o f the measured v a l u e . In a d d i t i o n , the h i g h e r c o n d u c t i v i t y v a l u e at the streambed interface c o u l d be a result o f the larger s a m p l i n g area o f the infiltrometer (diameter ~ 6 c m ) c o m p a r e d to the p i e z o m e t e r (diameter ~ 1 c m ) , such that the infiltrometers are m o r e l i k e l y to capture the effects o f infiltration v i a preferred p a t h w a y s , consistent w i t h the results o f S o n g et a l . (2007). Saturated h y d r a u l i c c o n d u c t i v i t y has been w i d e l y d o c u m e n t e d to increase w i t h the v o l u m e o f p o r o u s m e d i u m under c o n s i d e r a t i o n (Freeze and C h e r r y 1979).  4.3.4.  Scaling streambed water fluxes  T h e streambed water f l u x e s c o m p u t e d f r o m D a r c y ' s L a w w i t h i n one channel-unit i n the upper reach ( P o o l 1) w e r e s i g n i f i c a n t l y greater for i n f i l t r a t i o n (where X / L > 0.6) than for discharge (where X / L < 0.4). T h i s indicates that h y p o r h e i c discharge was a l o w e r p r o p o r t i o n o f the total f l u x . F l u x e s into the b e d also increased w i t h discharge, as o b s e r v e d o n June 19 ( Q = 15.4 L / s ) , c o m p a r e d to l o w f l o w c o n d i t i o n s o n September 2 9 ( Q = 1.1.  81  L / s ) . T h e area o f the channel-unit that contributed to the highest p r o p o r t i o n to the total f l u x w a s w h e r e X / L > 0.8, w i t h a rate o f o v e r 0.5 m L / s o n June 19. P r e v i o u s studies have suggested that at h i g h e r discharges, the h y d r a u l i c potential for d o w n w e l l i n g into the b e d increases, thus i n c r e a s i n g the potential for h y p o r h e i c exchange f l o w (e.g. W o n d z e l l 2 0 0 5 ) . A l t h o u g h water fluxes w i t h i n this channel-unit appeared to v a r y w i t h discharge, h y d r a u l i c gradients and infiltration rates measured at the p o i n t scale d i d not v a r y s i g n i f i c a n t l y w i t h discharge. A s a result, it is difficult to c o n c l u d e whether discharge w a s a first order c o n t r o l o n water fluxes w i t h i n East C r e e k . A t t e m p t s to " s c a l e - u p " the total flux to the reach scale estimate o f h y p o r h e i c exchange (s"') i n d i c a t e d that the reach-scale exchange coefficient w a s t w o orders larger than the scaled-up estimate o f h y p o r h e i c exchange ( T a b l e 3.12). S e v e r a l processes m a y e x p l a i n this result. F i r s t l y , u s i n g D a r c y ' s L a w to calculate water fluxes m a y underestimate the amount o f exchange, due to the tremendous spatial v a r i a t i o n i n h y d r a u l i c c o n d u c t i v i t y . A t least part o f this bias m a y result f r o m u n d e r e s t i m a t i o n o f K b y the s l u g tests. I f the i n f i l t r o m e t e r measurements are accurate, t h e y suggest that the estimated values o f K m a y be an order o f m a g n i t u d e too l o w . S e c o n d l y , lateral fluxes o r h o r i z o n t a l exchange f l o w m a y h a v e contributed to a p o r t i o n o f the h y p o r h e i c exchange f l o w that w a s not quantified at the channel-unit scale. T h i r d l y , the T S M m a y overestimate the amount o f exchange, p o s s i b l y due to transient exchange i n p o o l s . H o w e v e r , this analysis w a s o n l y c o n d u c t e d w i t h i n one channel-unit, and s h o u l d be extended to a d d i t i o n a l channel-units.  82  CHAPTER FIVE: CONCLUSIONS T h e final chapter s u m m a r i z e s the m a i n results o f the thesis research and c o n c l u d e s w i t h areas o f future research.  5.1.  Summary of main results H y p o r h e i c z o n e processes were e x a m i n e d at three spatial scales d u r i n g the p e r i o d  o f M a y to O c t o b e r 2 0 0 6 i n East C r e e k : reach scale, channel-unit scale and p o i n t scale. A t the reach scale, the b r e a k t h r o u g h curves f r o m a total o f 10 stream tracer injection experiments were s i m u l a t e d u s i n g O T I S - P . S o l u t e transport processes v a r i e d both t e m p o r a l l y ( w i t h v a r i a t i o n s i n discharge) and to a lesser extent, s p a t i a l l y (i.e. b e t w e e n reaches). D i s p e r s i o n rates ( D ) , channel area ( A ) and transient storage area ( A s ) s h o w e d an i n c r e a s i n g trend w i t h discharge, w h i l e the transient exchange coefficient (a) r e m a i n e d f a i r l y constant w i t h discharge i n both reaches. T h e ratio A s / A increased w i t h discharge. S t r e a m and storage z o n e h y d r a u l i c residence times d i d not v a r y w i t h discharge at the reach scale. R e t e n t i o n was highest d u r i n g l o w f l o w c o n d i t i o n s . M o d e l parameter uncertainty was greatest d u r i n g periods o f h i g h f l o w s , p o s s i b l y c o n f o u n d i n g the a b i l i t y to e x a m i n e transient storage processes o v e r a range o f f l o w c o n d i t i o n s . D u r i n g t w o tracer injections (September 29 and 30), b r e a k t h r o u g h curves f r o m i n d i v i d u a l pools, were s i m u l a t e d i n order to quantify p o o l storage and residence times. R e s i d e n c e times w i t h i n the transient storage z o n e o f the p o o l (assumed to be i n - c h a n n e l storage) w e r e h i g h e r than the residence t i m e i n for the entire reach. T h e transient storage area ( A s ) was also g e n e r a l l y h i g h e r w i t h i n the p o o l s , r e s u l t i n g i n a h i g h e r A s / A ratio than at the reach scale. T h e s e results suggest that it m a y be v a l i d to assume that the transient storage area adequately incorporates b o t h storage z o n e processes at the reach scale. T w o different f l o w p a t h w a y s were o b s e r v e d d u r i n g stream tracer experiments c o n d u c t e d at the channel-unit scale. O n e f l o w p a t h w a y was a l i g n e d w i t h the stream c h a n n e l , w h i l e a second f l o w p a t h w a y i n c l u d e d a lateral c o m p o n e n t w i t h i n the r i p a r i a n z o n e . T h i s second p a t h w a y w a s associated w i t h f l o w around a large b o u l d e r .  83  O b s e r v a t i o n s m a d e at this spatial scale h i g h l i g h t the t e m p o r a l and spatial v a r i a b i l i t y i n exchange f l o w s . A t the channel-unit scale, stream tracer experiments were used to determine the m e a n residence t i m e o f solutes i n b o t h transient storage zones, s p e c i f i c a l l y the h y p o r h e i c z o n e versus i n - c h a n n e l storage i n p o o l s . U s i n g c o n t i n u o u s l y stirred tank reactor theory,  ,  m e a n residence times w e r e found to be greater w i t h i n the p o o l than the step (i.e. h y p o r h e i c z o n e ) d u r i n g t w o stream tracer experiments. T h e s e results, a l o n g w i t h the results f r o m O T I S - P s i m u l a t i o n s w i t h i n t w o i n d i v i d u a l p o o l s , suggest that it m a y be i n v a l i d to assume that residence times w i t h i n p o o l s are n e g l i g i b l e . M o d e l results also c o n f i r m that the residence t i m e distributions w i t h i n the step and p o o l sub-units f o l l o w an e x p o n e n t i a l d i s t r i b u t i o n , suggesting that current T S M (e.g. O T I S - P ) do accurately represent the late-time solute residence times u s i n g an e x p o n e n t i a l p r o b a b i l i t y d e n s i t y . w i t h one transient storage z o n e . A t the point scale, direct measurements o f water fluxes into the stream b e d , i n c l u d i n g v e r t i c a l h y d r a u l i c gradients and infiltration rates, s h o w e d considerable t e m p o r a l and spatial v a r i a b i l i t y . H o w e v e r , these water fluxes d i d not v a r y statistically w i t h discharge, suggesting that other processes contribute to the observed v a r i a b i l i t y . V e r t i c a l h y d r a u l i c gradients v a r i e d s y s t e m a t i c a l l y w i t h the scaled l o c a t i o n w i t h i n the channel-unit, i n d i c a t i n g that stream g e o m e t r y is a significant c o n t r o l o n water fluxes. Repeated observations o f V H G , as measured i n piezometers i n s t a l l e d w i t h i n the streambed, i n d i c a t e d a strong d o w n w e l l i n g o f water upstream f r o m obstructions i n the streambed such as boulders and l o g s , c o r r e s p o n d i n g to w h e r e X / L > 0.8. Z o n e s o f u p w e l l i n g o c c u r r e d d o w n s t r e a m at the base o f p o o l s , and corresponded to areas i n the stream channel w h e r e X / L < 0.2. Step height was not a s i g n i f i c a n t control o n h y d r a u l i c gradients. U p w e l l i n g sites w e r e located w i t h i n b o t h reaches, a l t h o u g h gradients w e r e stronger w i t h i n the upper reach. R e v e r s a l o f h y d r a u l i c gradients was also m o r e c o m m o n w i t h i n the l o w e r reach, p o s s i b l y due to h i l l s l o p e discharge. H y d r a u l i c c o n d u c t i v i t y measurements w e r e s p a t i a l l y heterogeneous, but w e r e w i t h i n the same order o f m a g n i t u d e ( 1 0 " m / s ) i n b o t h reaches. H o w e v e r , h y d r a u l i c 4  c o n d u c t i v i t y estimates based o n streambed infiltrometers w e r e higher than estimates f r o m f a l l i n g h e a d tests. T h i s result suggests that b e d i n f i l t r a t i o n c o m p u t e d f r o m p i e z o m e t e r  84  data alone m a y underestimate actual infiltration rates. A sequential analysis o f v a r i a n c e indicated that h y d r a u l i c c o n d u c t i v i t y v a r i e d w i t h site c o n d i t i o n (i.e. u p w e l l i n g , d o w n w e l l i n g and neutral sites), suggesting that c o n d u c t i v i t y is an a d d i t i o n a l c o n t r o l o n exchange f l o w s . T h i s f i n d i n g is also consistent w i t h the n o t i o n that d o w n w e l l i n g zones s h o u l d be m o r e i n f l u e n c e d b y the c l o g g i n g o f pore space b y infiltration o f fine sediment. C h a n n e l - u n i t water fluxes calculated w i t h D a r c y ' s L a w d i d not " s c a l e - u p " to the reach scale estimate o f h y p o r h e i c exchange (a), w h i c h w e r e t w o orders o f magnitude l o w e r than the reach. A d d i t i o n a l processes s u c h as lateral i n f l o w or transient storage i n p o o l s c o u l d have resulted i n the observed differences, i n a d d i t i o n to bias i n the h y d r a u l i c c o n d u c t i v i t y measurements.  5.2.  Areas for future research T h i s research contributes to a b o d y o f w o r k e x a m i n i n g the p h y s i c a l properties o f  the stream c h a n n e l that influence solute transport and retention i n s m a l l , headwater streams. A h y d r o m e t r i c and stream tracer a p p r o a c h w a s used to characterize the spatial d i s t r i b u t i o n and the associated residence times t h r o u g h the h y p o r h e i c and surface-water transient storage z o n e at three spatial scales o f interest i n c l u d i n g the reach, channel-unit and l o c a l or p o i n t scale. A m u l t i p l e scale approach to e x a m i n e h y p o r h e i c exchange has not been e x p l i c i t l y a p p l i e d i n p r e v i o u s research, a n d h i g h l i g h t s the considerable spatial and t e m p o r a l v a r i a b i l i t y and c o m p l e x i t y o f h y p o r h e i c exchange processes w i t h i n stepp o o l streams. T h i s study shows that channel-unit s p a c i n g is a d o m i n a n t c o n t r o l o n the m a g n i t u d e o f exchange f l o w and the extent o f the h y p o r h e i c z o n e . In a d d i t i o n , this research suggests that a s c a l i n g relationship based o n the channel-unit g e o m e t r y c o u l d be a useful and p r a c t i c a l t o o l for c h a r a c t e r i z i n g and p r e d i c t i n g exchange f l o w i n step-pool streams. C o n t i n u e d research s h o u l d focus o n e x a m i n i n g the l o n g i t u d i n a l patterns i n f l u v i a l g e o m o r p h o l o g y , s u c h as channel unit s p a c i n g , i n order to characterize and a p p l y the h y p o r h e i c exchange f l o w processes across a b r o a d range o f stream size and scales. Studies attempting to l i n k b i o l o g i c a l and g e o c h e m i c a l processes to p h y s i c a l characteristics o f the h y p o r h e i c z o n e have h i g h l i g h t e d the i m p o r t a n c e o f the h y p o r h e i c z o n e and i n - c h a n n e l " d e a d " zones for nutrient uptake and t e m p o r a r y retention o f surface  85  water nutrients. T h e results f r o m this study s h o w that i n - c h a n n e l features s u c h as p o o l s and b a c k eddies do contribute to transient storage i n headwater streams. O b s e r v a t i o n s m a d e at the channel-unit scale demonstrate that separate residence times for h y p o r h e i c and surface-water transient storage zones can be quantified u s i n g a stream tracer approach. T h e s e observations p r o v i d e d insight into the residence t i m e d i s t r i b u t i o n o f water i n the h y p o r h e i c z o n e . H o w e v e r , spatial and t e m p o r a l r e p l i c a t i o n was l i m i t e d i n this study, and future studies s h o u l d a p p l y the a p p r o a c h to m u l t i p l e channel-units w i t h i n a s i n g l e reach, o v e r a range o f f l o w c o n d i t i o n s . In a d d i t i o n , a m e t r i c r e l a t i n g i n - c h a n n e l residence times to p o o l g e o m e t r y w o u l d be a v a l u a b l e c o n t r i b u t i o n towards u n d e r s t a n d i n g the interplay b e t w e e n channel m o r p h o l o g y and solute residence times. 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M o d e l s i m u l a t i o n s u s i n g O T I S - P for M a y 31 for the l o w e r reach  F i g u r e A . 2 . M o d e l s i m u l a t i o n s u s i n g O T I S - P for June 27 for the l o w e r reach  F i g u r e A . 3 . M o d e l s i m u l a t i o n s u s i n g O T I S - P for Sept 21 for the upper r e a c h (a) and the l o w e r reach (b)  Time (hour)  F i g u r e A . 4 . M o d e l s i m u l a t i o n s u s i n g O T I S - P for Sept 29 for the upper reach  mAA  /£A*A^z&  &  A D  Upper boundary Lower boundary  Time (hour)  F i g u r e A . 5 . M o d e l s i m u l a t i o n s u s i n g O T I S - P for Sept 30 for the l o w e r reach  o o o  Upper boundary Lower boundary — Simulated A  A  A  ^ A ^  D  o o o  CO  o ro o c o O ro cu  o o o  CM  o o o  a: (A)  -a-  —r~ 0.0  2.5 Time (hour)  o o o ^r  o o o co  c  0)  o c o O  ro CD  Upper boundary Lower boundary — Simulated A  ZAA  A  A A  ^ A  A  D  A A  o o o  CM  o o . o  rr  A  A  " I —  0.0 Time(hour)  F i g u r e A . 6 . M o d e l s i m u l a t i o n s u s i n g O T I S - P for O c t o b e r 2 0 for the u p p e r r e a c h (a) and the l o w e r r e a c h (b)  96  o  8  A  A  A ^  A  A  A  A  °  Upper boundary Lower boundary Simulated  c o  1 <3 '16 (A) n  Time (hour)  Time (hour)  F i g u r e A . 7 . M o d e l s i m u l a t i o n s u s i n g O T I S - P for September 29. Results are f r o m t w o p o o l s located i n the upper reach  97  

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