SHALLOW STRATIGRAPHIC CONTROLS ON SURFACE WATER-GROUNDWATER MIXING AND GEOCHEMICAL FATE IN THE BENTHIC ZONE OF AN ESTUARY

Paleovalleys act as conduits that influence the discharge of fresh groundwater to coastal waters and the recirculation of saline surface water across the sediment-water interface. Here, we quantify the spatial distribution of submarine groundwater discharge (SGD) rates, shallow surface water-groundwater mixing, and associated geochemistry near a paleovalley in Indian River Bay, Delaware using a hand resistivity probe, seepage meters, and pore water samples. In the interfluve far from the paleovalley, fresh groundwater discharges rapidly near the coast. The discharging groundwater is relatively oxygenated and high in nitrate. Salinity and redox gradients are shallow along fast upwelling flow paths, providing little opportunity for geochemical transformation of discharging groundwater. In contrast, near the peat-covered paleovalley, fresh groundwater discharges farther offshore along the paleovalley sides at a moderate rate, and recirculation of saline pore water occurs at greater depths. Fresh groundwater near the paleovalley is relatively reducing, lower in nitrate, and higher in iron(II) and sulfide. Groundwater discharges through decimeter-scale salinity and redox gradients that may provide opportunities for transformation of groundwater chemistry. By modifying patterns of groundwater discharge along coasts, shallow stratigraphic features such as paleovalleys can influence the geochemistry of fresh SGD and redox reactions within the benthic layer.