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Regulatory programs to evaluate and control potential sources of contamination of public supply wells require knowledge of recharge mechanisms, flow patterns, and areas contributing recharge to wells. Ground-water-flow patterns and areas contributing recharge to wells and streams in a complex valley-fill and carbonate-rock aquifer system in southwestern Morris County, New Jersey, were simulated by using a three-dimensional numerical ground-water-flow model. Areas contributing recharge to supply wells under recent, projected 2005, and fullallocation pumping conditions were delineated and areas contributing recharge that reaches supply wells in 5and 12-year time periods were identified. Limitations of the analysis include factors related to conceptual-model reliability, model discretization, and parameter error. Withdrawals from the aquifer system are increasing in large part to meet growing water demands of communities in southwestern and central Morris County. As withdrawals increase, areas contributing recharge to wells expand, displacing areas that previously contributed recharge to streams. The area contributing recharge to a shallow well typically is located near the well. The area contributing recharge to a deeper well typically consists of a highly irregular or fragmented area, parts of which can be located thousands of feet from the well. Ground-water travel time from distant contributing recharge areas to deep wells typically is many decades or hundreds of years, whereas travel time to shallow wells from local contributing recharge areas typically is shorter. The aquifer system in the study area consists of three aquifers~an upper valley-fill aquifer, a lower valley-fill aquifer, and an underlying carbonate-rock aquifer. Recharge occurs as direct infiltration of precipitation through the valley floor and as seepage from surface water. Irregular aquifer boundaries and complex hydraulic gradients result in ground-water-flow patterns that are tortuous in places. Flow patterns in the upper valley-fill aquifer are dominated by the effects of surface-water features and aquifer boundaries, and most water in the upper valley-fill aquifer discharges to streams. Flow patterns in the lower valley-fill and carbonate-rock aquifers in the study area are dominated by the effects of supply wells, aquifer boundaries, and zones of varying aquifer permeability. Withdrawals from the aquifer system in the study area averaged 4.2 Mgal/d during 1991-95 and are projected to increase by 38 percent, to about 5.8 Mgal/d, by 2005. If 1996 allocations were fully utilized, withdrawals would reach 6 Mgal/d, a 43-percent increase over recent (1991-95) withdrawals. The areas contributing recharge to wells under projected 2005 and fullallocation conditions were larger than those under recent conditions. Areas contributing recharge to wells under projected 2005 and full-allocation conditions were generally similar because ground-water withdrawals under these respective conditions were similar. Differences in contributing areas and travel times under different conditions are attributable to differences in pumping rates. INTRODUCTION In the New Jersey Highlands, the valley-fill 1 and carbonate-rock aquifer system extending from Hoffmans to Picatinny Arsenal (fig. 1) has become an increasingly important source of water supply for communities in southwestern and central Morris County. The hydrogeology of the aquifer system and surrounding area were described in a previous investigation by Nicholson and others (1996), which included the development and application of a numerical ground-water-flow model (fig. 1). Eighteen water-supply wells tap the aquifer system in the present study area which includes the headwaters of Drakes Brook and the Lamington River (fig. 1). These wells provide water to communities in and east of the study area. As these communities continue to experience residential, commercial, and industrial growth, withdrawals from the aquifer system in the study area are expected to increase. The study area includes several potential sources of ground-water contamination. Efforts to protect and monitor the quality of ground-water resources in the study area have been constrained by a limited understanding of recharge areas and flow patterns. This study was undertaken to improve this understanding. The U.S. Environmental Protection Agency (USEPA) instituted the Wellhead Protection Program to provide a regulatory framework for the evaluation and control of potential sources of contamination of public water-supply wells. A critical element of such evaluations is the determination of the areas contributing recharge to supply wells. In hydrogeologically simple settings and circumstances, simplified analytical-modeling methods or arbitrary approaches are sometimes used to estimate these areas (U.S. Environmental Protection Agency, 1987; New Jersey Department of Protection and Energy, 1991). In complex hydrologic-flow systems, such as the valley-fill and carbonate-rock aquifer system in southwestern Morris County, New Jersey (fig. 1), areas contributing recharge to wells can be estimated more reliably by the application of numerical-modeling techniques. The U.S. Geological Survey (USGS), in cooperation with Randolph Township, conducted a study of the aquifer system in which numerical-modeling techniques were used to estimate areas contributing recharge to all ground-water discharges from the study area. This study is part of the Alamatong Wellhead Protection Demonstration Project, which was administered by Randolph Township as part of the New Jersey Department of Environmental Protection (NJDEP) Wellhead Protection Program. The NJDEP Wellhead Protection Program was initiated under the auspices of the USEPA Wellhead Protection Program, which requires each state to implement programs to l Terms in bold are defined in the glossary. 74°52'30" 74°45' 74°37'30"

water-resources investigations report

Simulation of ground-water-flow patterns and areas contributing recharge to streams and water-supply wells in a valley-fill and carbonate-rock aquifer system, southwestern Morris County, New Jersey