Effects of sanitary sewers on ground-water levels and streams in Nassau and Suffolk Counties, New York; part 2: Development and application of southwest Suffolk County model

Abstract

By 1990, sanitary sewers in Nassau County Sewage Disposal Districts 2 and 3 and Suffolk County Southwest Sewer District are expected to divert to ocean outfall 140 cubic feet of water per second that would otherwise be returned to the groundwater system through septic tanks and similar waste-disposal systems. To evaluate the effects that this loss of ground water will have on groundwater levels and base flow, the U.S. Geological Survey developed a groundwater flow model that couples a fine-scale subregional model to a regional model of larger scale. The regional model includes the natural hydrologic boundaries and-was used to generate flux-boundary conditions for the subregional model. The subregional model was then used to study in detail the area of primary concern, southwest Suffolk County. Model results indicate that the water table will decline as much as 8 feet along the Suffolk-Nassau county line, with effects decreasing eastward. Base flow is predicted to decrease by as much as 73 percent along the county line, but this effect will decrease to zero just east of the sewered area. This report is one in a three-part series describing the predicted hydrologic effects of sewers in southern Nassau and southwest Suffolk Counties. Part 1 is an introduction that describes the hydrogeologic system and groundwater -modeling principles; part 3 describes the development and results of a subregional model of southern Nassau County, adjacent to the area described herein. potentiometric head throughout the groundwater system, which in turn has caused a decrease in streamflow and in subsea outflow to the surrounding saltwater bodies (