Groundwater seepage into Great South Bay, New York

Great South Bay (New York) is a large lagoon on the northeast coast of the United States. The flow of groundwater across the floor of Great South Bay has been reported to account for as much as 2/3 of the total freshwater inflow. In situ measurements of this seepage flow have been made along four offshore transects in the Bay. These measurements show that the flow rate decreases rapidly offshore; within 30 m of the shoreline, the submarine outflow rates were typically 40 l (day m²)⁻¹ and decreased to less than 10 l (day m²)⁻¹ at a distance of 100 m from shore. The Bay floor at the study locations was sand or silty sand with vertical intrinsic permeabilities ranging from 14 to 78 darcys.

The flow rate across the Bay floor may be described by an exponentially decreasing function. The flow distribution may, therefore, be specified with two parameters—the flow value at the shoreline, A, and a ‘decay’ constant, c, that governs the rate of decrease of the flow offshore. The calculated total flows along the four transects were 2·1 × 10³, 1·1 × 10³, 8·5 × 10³ and 3·9 × 10³ l (day m)⁻¹. Between 40% and 98% of this flow enters the Bay within 100 m from shore. The total flow of groundwater across the Bay floor was calculated to be about 2 × 10⁸ l day⁻¹ or 10–20% of the total freshwater inflow.