Fresh groundwater entering an estuary can be mixed with salt water in the upper few decimeters of the sediment. As a result, net measured discharge rates at the sediment-water interface are equal to the volume discharge of fresh groundwater although the salinity of the escaping water is high. Seepage meters were used to measure discharge rates over about 50 l/day/m² near the shoreline of a wide, shallow lagoon (Great South Bay, New York) situated at the surface of a coastal plain aquifer. These rates decreased to 15 l/day/m² at a distance of 100 m from shore. The tide range here is about 0.21 m. No consistent variation in discharge with tidal phase was found, but water collected at a particular location freshened over time from 30 ppt to 23 ppt in twelve hours. Piezometers recorded vertical hydraulic gradients (at ambient salinity, 28 ppt) between 0.08 and 0.02 in the upper meter of the sediment and the vertical hydraulic conductivity was measured by a falling head test to be between 1 and 20 m/day. The discharge rates, calculated by Darcy's law, decreased offshore, being several hundred l/day/m² within 10m of shore to 18 l/day/m² at a distance of 100 m. Conductivity measurements showed the pore water salinity decreasing from ambient bay values at the surface to near fresh water values at a depth of 0.6m. The vertical downward dispersion coefficient for salt was estimated to be 0.02 m²/day. This could be due to the superposition of water-wave induced dispersion in the sediment with gravitational convection (salt fingering).

This research was supported by NSF OCE-0097497 REU Site: Research Experience for Undergraduates in Estuarine Processes (to J.Y. Aller). We are grateful to have had the help of Daniel O'Rourke and Ronald Paulsen, Cornell Coopertive Extension, to make the conductivity probe measurements.