Hydrologic feasibility of water-supply-development alternatives in Cape May County, New Jersey

Increasing public-supply withdrawals in Cape May County, New Jersey associated with increasing residential and seasonal tourist populations have led to regionally lowered ground-water levels, a reversal of ground-water flow directions toward onshore, and landward encroachment of saltwater in the shallow aquifer system. The three aquifers composing the shallow system are, in order of increasing depth, the unconfined Holly Beach water-bearing zone and the confined estuarine sand and Cohansey aquifers. The changes to the ground-water system have been greatest in the confined aquifers near the three major well fields on the Cape May peninsula. Formerly productive water-supply wells have been abandoned because of saltwater contamination. Concern about anthropogenic contamination has prevented shifting of withdrawals to the unconfined aquifer. Surface- water sources have also been little used. Further development on the peninsula involving increased water demand will exacerbate the current saltwater-encroachment problems. The purpose of this study was to test the feasibility of possible water-supply-development alternatives by use of predictive ground-water flow simulations. The alternatives involve (1) injection of tertiary- treated wastewater to replenish aquifer storage and create a hydraulic barrier to saltwater encroachment, (2) withdrawal of brackish water in order to create a hydraulic barrier, (3) conjunctive use of ground water and surface water, enabling the reduction of ground-water withdrawals, and (4) redistribution of withdrawals inland to the unconfined aquifer. Results of these simulations can potentially be used in the design of a water-supply-development strategy that preserves supply and a monitoring program that ensures early warning of saltwater encroachment, thereby allowing sufficient time for development of an alternative supply. The water-supply- development alternatives were evaluated by comparison of results of predictive simulations made with a previously calibrated ground-water flow model of the shallow aquifer system. The quasi-three-dimensional sharp-interface model was calibrated to 1988 annual average hydrologic conditions. The planning period for the predictive simulations is 1989-2049. For the planning period, total public-supply withdrawals were increased 100 percent over average 1983-88 withdrawals. Results of a baseline simulation involving only the increased withdrawals were compared to each of the simulated alternatives, which also include the withdrawals. Hydraulic heads, saltwater- freshwater interface movement, and ground-water flows were compared. Simulation results indicate that the barrier-injection or barrier-withdrawal scheme could be useful in managing the water supply for a specific location. The conjunctive- use scheme would provide a marginal regional hydrologic benefit. Redistribution of withdrawals appears to be the only regional alternative that would result in recovery of ground-water levels and would substantially slow saltwater encroachment; however, anthropogenic land-surface contamination of the unconfined aquifer would have to be considered if the redistribution alternative is acted upon.