Analysis of ground-water flow and saltwater encroachment in the shallow aquifer system of Cape May County, New Jersey

Abstract

Cape May County, the southernmost county in New Jersey, is on a natural peninsula that is virtually surrounded by saltwater, including the Atlantic Ocean and Delaware Bay. Nearly all of the county's water supply comes from ground water, half of which comes from the shallow aquifer system. Because of its proximity to saltwater bodies, the county's freshwater supply is very limited. This report describes the results of a conceptual and numerical analysis of the shallow-ground-water resources of the county, with emphasis on the effects of saltwater encroachment on water supply. The conceptual analysis was conducted by investigating the hydrogeologic framework, water use, flow system, and water quality. The shallow aquifer system consists of one unconfined aquifer and two confined aquifers. Recharge to the shallow aquifer system is derived mainly from precipitation. Although water-supply is greatest in the unconfined part of the system, the introduction of contaminants from the land surface has precluded extensive use of this aquifer. Withdrawals from the confined aquifers have increased through time in response to the summer influx of tourists, and the water used is ultimately discharged offshore to the Atlantic Ocean. Extensive cones of depression have resulted in these aquifers. The net freshwater loss from the system has led to saltwater encroachment and chloride contamination of the water withdrawn. Chloride contamination is even more severe in the deep aquifer system. The numerical analysis was conducted by using a quasi-three-dimensional finite-difference model of the ground-water system and the sharp-interface approach. Limitations and assumptions inherent in the model involve data quality, computer code, and model application. The model is calibrated to predevelopment and to current hydrologic conditions. The calibrated model was used to simulate ground-water flow under two water-supply-development alternatives for a 30-year planning period. The alternatives involve only modest increases in withdrawals in combination with desalination of brackish ground water or relocation of wells toward inland areas. Simulation results indicate that projected withdrawals for the two alternatives can be sustained without significant additional saltwater encroachment over the planning period. However, saltwater will affect some wells if the current withdrawal scheme is maintained. Finally, information is provided on the use of ground-water monitoring to detect saltwater encroachment.