Salt-freshwater dynamics in a subterranean estuary over a spring-neap tidal cycle

This paper presents field measurements and numerical simulations of pore water salinities and groundwater flow in the intertidal zone of an unconfined coastal aquifer over a spring-neap tidal cycle. The study provides insight into the extent and time-scales of mixing between fresh groundwater and recirculating seawater in a tidally influenced subterranean estuary. Salt-freshwater dynamics in subterranean estuaries are currently not well understood despite their potentially important implications for fluxes of chemicals to coastal waters via submarine groundwater discharge. The data and simulation results show that changes in the tidal shoreline excursion over the spring-neap cycle induce significant variations in the intertidal salinity structure. Observed higher frequency salinity fluctuations demonstrate further the intensity and complexity of the salt-freshwater mixing process. In contrast with the salinity variations, fresh groundwater was found to discharge around a distinct intertidal beach slope break throughout the spring-neap period. This suggests that the slope break may affect significantly groundwater flow and salt transport near the shore. Measurements of pH and dissolved oxygen distributions revealed important biogeochemical zonations in the system. These zonations are expected to strongly influence the fate of many reactive chemicals in the nearshore aquifer and their subsequent discharge to coastal waters.