A steady-state three-dimensional groundwater-flow model that simulates present conditions was coupled with the particle-tracking program MODPATH to delineate zones of contribution to wells pumping from the Magothy aquifer near a chlorinated volatile organic compound (VOC) plume. This modeling was part of a study by the U.S. Geological Survey in cooperation with the Naval Facilities Engineering Command to delineate groundwater near the Naval Weapons Industrial Reserve Plant in Bethpage, New York. Because rates of advection within the coarse-grained sediments typically exceed 0.1 foot per day, transport by dispersion and (or) diffusion was assumed to be negligible. Resulting zones of contribution are complex shapes, influenced by hydrogeologic features including confining beds and a basal gravel zone, and the interplay of nearby hydrologic stresses. The use of two particle tracking techniques identified zones of contribution to wells. Particles are backtracked from pumping well screens, and particles are forward tracked from the location of a VOC plume, as defined by surfaces of equal total VOC concentration. During any period of 5 years or less, about 1 to 3 percent of particles backtracked from pumping wells within a focus area intersected the 5-part per billion (ppb) VOC plume shell, indicating that the vast majority of particles were not sourced from the plume. During 5 years or less, none of the particles backtracked from pumping wells intersected the 50-ppb VOC plume shell. Forward-tracking techniques identified the fate of water within the VOC plume after 5 years as it moves toward ultimate well capture and (or) discharge to model constant head and drain boundaries. Out of 4,813 forward tracked particles started within the 50-ppb VOC plume shell, 1 forward-tracked particle was captured by well ANY8480. Out of 22,958 forward tracked particles started within the 5-ppb VOC plume shell, 100 were captured by production wells (less than 1 percent). The subset of forward pathlines that represent well plume capture are similar in number and shape to those of backtracked pathlines.

Model simulations were conducted to assess uncertainties and improve understanding of how variability in hydraulic properties, pumpage rates, and maximum particle traveltime affect delineation of zones of contribution. By use of driller’s’ logs, a transitional probability approach generated nine alternative realizations of heterogeneity within the Magothy aquifer to assess uncertainty in model representation. Fine-grained sediments with low hydraulic conductivity were realized as laterally discontinuous, thickening towards the south, and comprising about 27 percent of the total aquifer volume within the transitional probability subgrid. Model simulations with alternative pumpage rates, porosity terms, and alternative maximum particle traveltime were also used to demonstrate how the size and shape of zones of contribution may vary.