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

Abstract

The following work was undertaken in this study to provide insight about the role of tidal fluctuations in influencing rates of SGD: (1) Improvements were made to the installation method and accuracy of a new tool developed for measuring the transient changes in vertical differential fluid pressures, referred to as the differential pressure system (DPS-II), (2) a field experiment was carried out to obtain measurements of SGD rates by a continuous heat type seepage meter, transient changes in vertical differential fluid pressures heads in shallow sediment by the DPS-II and tidal fluctuations in the near-shore environment, (3) the interpretation of the field data sets were constrained based on calculations using Darcy’s Law and a 1D uniform density flow model. The field experiment was carried out between October 26-28 2005, which was the last of a sequence of nine shorter tests completed to refine field procedures and gain experience using the instruments. Spanish Banks West beach in Vancouver was chosen as the field site because it is close proximity to The University of British Columbia, reasonable rates of SGD were measured during preliminary tests and the location fulfilled many of the logistical demands of testing. Results of the experiment showed that the highest differential fluid pressure heads (measured between about 0.3 and 0.6 m below the seabed) at two piezometers of the DPS-II and highest SGD rates occurred at low tide. Using ii Darcy’s Law, SGD rates were calculated based on the differential fluid pressure heads and a hydraulic conductivity value within the constraints of the hydraulic conductivity data set derived from falling head tests completed on core samples from the site. The calculated SGD rates provided a good match with SGD rates by the seepage meter. Results of the 1D hydrogeological modelling suggest that the field based measurements of differential fluid pressure heads and SGD rates can be explained reasonably well by a 1D uniform density dependent flow model.

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