EFFECT OF TIDAL SALINITY VARIATIONS ON HYPORHEIC EXCHANGE IN ESTUARINE BEDFORMS

Hyporheic exchange is important for stream ecosystems and biological processes. Hyporheic exchange driven by currents over bedforms has been studied extensively in freshwater environments where stream chemistry is relatively steady over time. However, in estuaries the salinity of the water column changes as the saltwater front moves upstream and downstream due to the competing influences of tides and river discharge. When combined with current-driven hyporheic exchange, oscillations in salinity along the estuary bed may provide a mechanism for both fresh and saltwater to enter the hyporheic zone. In this study, we built numerical variable-density groundwater flow and salt transport models of hyporheic exchange driven by currents over bedforms in an estuary and explored the role of tidally-varying surface water salinity on salinity distributions in the estuary bed. Model simulations showed that tidal salinity variations in surface water flowing over bedforms led to alternating pulses of fresh and saline water infiltrating into the hyporheic zone. The variation in salinity of infiltrating water resulted in alternating layers of freshwater and saltwater under bedforms. When tidal fluctuations were added to the model, the combined effects of tides and currents created a more complex salinity distribution than when currents were considered alone. The results of this study are a first step toward understanding the role of tides, currents, and changes in surface water salinity on affecting advective flow and transport processes in estuarine sediments. Ultimately, a better understanding of groundwater-surface water interactions in estuaries can be used to improve coastal water quality forecasts and help predict the fate of pollutants along the aquatic-ocean gradient.