Groundwater Discharge to Long Island Marshes: Theory, Inferences, and Data

Groundwater discharges in an unconfined aquifer in locations where the head elevation for the aquifer is greater than its corresponding sediment surface elevation (Freeze and Cherry, 1979). On Long Island, where the Upper Glacial aquifer is intersected by the ground surface, streams and ponds occur. USGS modeling suggests 28 percent of annual recharge discharges to stream systems in Suffolk County (Buxton and Smolensky, 1999). This discharge to streams impacts the water table, causing lower head pressures in the near vicinity of streams. The measurable effect, especially for smaller streams, is often extremely local. At Connetquot Brook, the difference in heads was detectable only 30 vertical feet below the creek and approximately the same distance from each bank (Prince et al., 1989). However, other modeling has suggested that even modest streams can drain large portions of individual watersheds (the freshwater portion of Meetinghouse Creek collected 25 percent of the recharge of that area) (Schubert, 1999). Research on coastal plain streams fed by groundwater indicates that the upper stretches are often receiving recently recharged groundwater (which thus is from the immediate vicinity of the stream). This changes fo