Delineation of areas contributing recharge to wells in central Long Island, New York, by particle tracking

Abstract

Particle tracking was applied to a three-dimensional, seven-layer groundwater flow model of a 270-square-mile area in central Long Island to delineate the recharge-contributing areas to five hypothetical well sites that represent a variety of hydrologic settings. These sites are (1) on the northern shore, (2) near the regional groundwater divide where a confining layer is present, (3) near the regional groundwater divide where no confining layer is present, (4) at the southern shore near the Patchogue River where a confining unit is present, and (5) at the southern shore near a tidal wetland where no confining unit is present. Groundwater flow at each site was simulated (1) under nonpumping conditions, and (2) at two pumping rates 36,000 and 72,000 cubic feet per day. The model was calibrated to long-term, steady-state conditions and coupled to a previously developed Long Island regional model to obtain boundary flows and to define regional-scale system geometry. The study-area model has finer discretization than the regional model and can provide detailed flow-path resolution in the well-site areas. Particletracking analyses showed that the size and shape of the contributing areas to wells and the directions of groundwater flow can be significantly affected by (1) the presence or absence of a confining unit; (2) proximity to flow boundaries, such as streams, the shore, the saltwater-freshwater interface, and public-supply wells; and (3) pumping rates.

Hydraulic Properties of the Magothy and Upper Glacial Aquifers at Centereach, Suffolk County, New York

“…Hydraulic conductivity of the 600-ft thick aquifer system centered at S125632 and including TW1-5 is estimated to be about 50 ft/d horizontal and 0.5 ft/d vertical, typical values for the Magothy aquifer. The K h estimate of 50 ft/d horizontal and 0.5 ft/d vertical agrees with previous hydrogeologic framework and flow modeling studies of the Magothy aquifer (McClymonds and Franke, 1972;Koszalka, 1980;Misut and Feldman, 1996;and Buxton and Shernoff, 1999). However, some highly permeable glacial sands likely were included within the upper part of the modeled aquifer system, indicating that the aquifer system directly affected by pumping of S125632, as conceptualized to include parts of the both the TW1 TW5 TW4 TW2 TW1 TW2 TW3 TW4 TW5 PRELIMINARY RESTING PUMPING RECOVERY PRELIMINARY RESTING PUMPING RECOVERY -1 0 glacial and Magothy aquifers, is less hydraulically conductive than previously estimated.…”

“…For example, estimates of aquifer properties are needed for delineation of groundwater flow paths, evaluation of optimal pumping rates and other aspects of well field design and maintenance, and prediction of local-scale water-level changes in response to pumping stresses. Previously published estimates of the hydraulic properties of aquifers on Long Island, N.Y., (Koszalka, 1980, Misut and Feldman, 1996, and Buxton and Shernoff, 1999 provide a general range of values and a regionalized view but lack detail.…”

Simulation of zones of contribution to wells at site GM–38, Naval Weapons Industrial Reserve Plant, Bethpage, New York

“…Head values for the boundary conditions are estimated from USGS 2006 regional contour maps, which are representative of present steady state conditions (Monti and Busciolano, 2009). Justification for the use of constant-head boundaries in Long Island groundwaterflow models is given in Misut and Feldman (1996). Recharge is a specified flux boundary with several factors affecting the rate, as described by the model stresses section below.…”

Simulation of groundwater flow pathlines and freshwater/saltwater transition zone movement, Manhasset Neck, Nassau County, New York

“…Particles that remain active indicate that pumping captures groundwater which recharged the system prior to 1920; the maximum flowpath duration analyzed in this study is 100 years. Some groundwater on Long Island may be expected to exceed several thousands of years in age (Misut and Feldman, 1996). Groundwater flowpath travel times are directly related to specified effective porosity values, which were set to 20 percent for all aquifers in the SEAWAT and MODPATH6 models (within the range given by Freeze and Cherry, 1979).When particles travel near a pumped well or other point of model discharge, deflection of the particle path toward that well typically occurs.…”