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SEAWAT, MODFLOW-2000, and SHARP models used to simulate potential water-allocation changes, Cape May County, New Jersey

November 30, 2021

Three existing groundwater flow models, using MODFLOW-2000, SEAWAT, and SHARP model codes, were used by the U.S. Geological Survey (USGS) to determine the effects of increased withdrawals, and shifts of withdrawals between 2 aquifers, on the limited water resources in the Cape May County, New Jersey. Saltwater intrusion and declining water levels have been a water-supply problem in Cape May County for decades. Several communities in the county have only one aquifer from which freshwater withdrawals can be made, and that sole source is threatened by saltwater intrusion and (or) substantial declines in water levels caused by groundwater withdrawals. Growth of the year-around and summer
tourism populations have caused water demand for some purveyors to approach full-allocation withdrawal rates leading these purveyors to request increases in allocations. The three groundwater flow models were used to evaluate the shallow and deep aquifer systems of Cape May County. The groundwater flow in the shallow and deep aquifer systems were simulated separately. The lateral hydrologic boundaries of the shallow aquifer system generally coincide with the political boundary of Cape May County, whereas the boundaries for the deep aquifer system extend well beyond the county boundaries. Flow in the shallow aquifers was simulated with the saltwater-transport modeling code, SEAWAT. Flow in the deep aquifers was simulated using MODFLOW-2000 with a medium-cell-size numerical model (CMAC)
encompassing Cape May County (https://doi.org/10.3133/wri954280) that was revised to include the Rio Grande water-bearing zone and recalibrated with recent (2003) withdrawal data and water-level measurements for a previous study of the Cape May County water resources (https://doi.org/10.3133/sir20095187). Boundary flows to the CMAC model were provided from the New Jersey Coastal Plain regional model (NJCP SHARP) (https://doi.org/10.3133/wri984216). This coarse-cell-size Coastal Plain-wide model uses the SHARP model code and simulates saltwater movement by treating the transition from freshwater to saltwater as a sharp interface, and therefore, only predicts large-scale movements of the 10,000-mg/L isochlor. Future groundwater withdrawal scenarios for the shallow and deep system were compared to baseline scenarios in an effort to balance the need for additional water with protection of the limited water resources in the county. This USGS data release contains all the input and output files for the simulations described in the associated model documentation report (https://doi.org/10.3133/sir20205052)

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