Submarine Ground Water Discharge and Salt Penetration Across the Sea Floor

Abstract

AbstractFresh ground water 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 can be equal to the volume discharge of fresh ground water, although the salinity of the escaping water is high. Benthic chambers, vented to a collection bag, were used to measure specific volume discharge rates over ∼5 cm/day 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 1.5 cm/day at a distance of 100 m from shore. Although the flow rate is expected to be inversely correlated to the tidal water level elevation, and has been shown to be so modulated at other sites, no consistent variation in discharge with tidal phase was found; the tidal range (25 cm) may have been too small to have a measurable effect. Water collected at a particular location freshened over time from 30 to 23 ppt in 12 hours. Piezometers recorded vertical hydraulic gradients (at ambient salinity) 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 from dozens of cm/day within 10 m of shore to 1.8 cm/day at a distance of 100 m, consistent with measured seepage rates. Conductivity measurements imply that pore water salinity decreases from ambient bay values at the sea floor to near fresh water values at a depth of 0.6 m into the sediment. 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).