DYNAMIC SURFACE WATER-GROUNDWATER INTERACTIONS AND NITROGEN CYCLING IN A TIDALLY INFLUENCED RIVER

Tides cause dynamic changes in river-groundwater exchange in costal riparian zones. The timing of maximum groundwater discharge with respect to river discharge can impact export of nutrients and contaminants from watersheds to oceans. We investigated the effect of tidal fluctuations on river-groundwater exchange in the tidal freshwater zone of White Clay Creek, located in the Cristina River Basin (Delaware, USA), one of ten NSF-funded critical zone observatories. River-groundwater exchange rates were determined using hydraulic head values from shallow water table wells and streambed piezometers in the riparian aquifer and hyporheic zone. River discharge and velocity were recorded with an acoustic doppler current profiler. We also sampled surface water and groundwater at high spatial resolution over a tidal cycle to constrain the influence of river-groundwater interactions on nitrogen cycling. Preliminary results show at that at high tide when river discharge is low, river water infiltrates into the riparian aquifer. During low tide, river discharge increases, and groundwater discharges to the river. Tidal bank storage and release may attenuate nutrient and contaminant export from the watershed by temporarily storing surface water in reactive sediments. A better understanding of surface water-groundwater exchange and nutrient cycling in tidally influenced rivers can inform management decisions for coastal watersheds.