ABSTRACT

Withdrawals from the Potomac-Raritan-Magothy aquifer system in New Jersey, which includes the Upper, Middle, and Lower Potomac-Raritan-Magothy aquifers, are the principal source of groundwater supply in northern Gloucester and northeastern Salem Counties in the New Jersey Coastal Plain. Water levels in these aquifers have declined in response to pumping. With increased population growth and development expected in Gloucester County and parts of Salem County over the next 2 decades (2005–2025), withdrawals from these aquifers also are expected to increase.

A steady-state groundwater-flow model, developed to simulate flow in the Potomac-Raritan-Magothy aquifer system in northern Gloucester and northeastern Salem Counties, was calibrated to withdrawal conditions in 1998, when groundwater withdrawals from the Potomac-Raritan-Magothy aquifer system in the model area were more than 10,100 Mgal/yr (million gallons per year). Withdrawals from water-purveyor wells accounted for about 63 percent of these withdrawals, and withdrawals from industrial self-supply wells accounted for about 32 percent. Withdrawals from agricultural-irrigation, commercial self-supply, and domestic self-supply wells accounted for the remaining 5 percent. Results of the 2000 baseline groundwater-flow simulation, incorporating average annual 1999–2001 groundwater withdrawals, indicate that the average simulated water levels in the Upper, Middle, and Lower Potomac-Raritan-Magothy aquifers are 31, 27, and 30 feet below the National Geodetic Vertical Datum of 1929 (NGVD 29), respectively, and the lowest simulated water levels are 77, 65, and 59 feet below NGVD 29, respectively.

In the full-allocation scenario, the maximum State-permitted (allocated) groundwater withdrawals totaled 16,567 Mgal/yr, an increase of 72 percent from the 2000 baseline simulation. Results of the full-allocation simulation indicate that the average simulated water levels in the Upper, Middle, and Lower Potomac-Raritan-Magothy aquifers are 49, 43, and 48 feet below NGVD 29, respectively, which are 18, 16, and 18 feet lower, respectively, than in the 2000 baseline simulation. The lowest simulated water levels are 156, 95, and 69 feet below NGVD 29, respectively, which are 79, 30, and 10 feet lower, respectively, than in the 2000 baseline simulation. Simulated net flow from the Potomac-Raritan-Magothy aquifer system to streams is 8,441 Mgal/yr in the 2000 baseline simulation but is 6,018 Mgal/yr in the full-allocation scenario, a decrease of 29 percent from the 2000 baseline simulation. Simulated net flow in the 2000 baseline simulation is 1,183 Mgal/yr from the aquifer system to the Delaware River but in the full-allocation scenario is 1,816 Mgal/yr from the river to the aquifer system.

Four other simulations were conducted that incorporated full-allocation conditions at water-purveyor wells in Critical Area 2 but increased or decreased withdrawals at selected water-purveyor wells outside Critical Area 2 and agricultural-irrigation and industrial-self-supply wells in the study area. The results of the four simulations also indicate net flow from the Delaware River to the Potomac-Raritan-Magothy aquifer system.

A growth scenario was developed to simulate future withdrawals in 2025 estimated from population projections for municipalities in the Salem-Gloucester study area. Simulated withdrawals for this scenario totaled 10,261 Mgal/yr, an increase of 6 percent from the 2000 baseline simulation. This total includes about 25 Mgal/yr withdrawn from the Englishtown aquifer system for domestic self-supply. This scenario incorporated full-allocation withdrawals at water-purveyor wells in Critical Area 2, and increased withdrawals at water-purveyor wells outside Critical Area 2. Results of this simulation indicate that the average simulated water levels in the Upper, Middle, and Lower Potomac-Raritan-Magothy aquifers are 32, 29, and 32 feet below NGVD 29, respectively, which are 1, 2, and 2 feet lower, respectively, than in the 2000 baseline simulation. Simulated net flow from the Potomac-Raritan-Magothy aquifer system to streams in the 2025 scenario is 8,189 Mgal/yr, a decrease of 3 percent from the 2000 baseline simulation. Simulated net flow from the Potomac-Raritan-Magothy aquifer system to the Delaware River in this scenario is 1,010 Mgal/yr, a decrease of 15 percent from the 2000 baseline simulation.

An analysis of the sensitivity of water levels at key boundaries in the study area to withdrawals from a hypothetical well was completed for the Upper and Middle Potomac-Raritan-Magothy aquifers. These boundaries include the 250-mg/L (milligrams per liter) isochlor, the western boundary of Critical Area 2, the aquifer outcrop, and the southwestern boundary of the model area. This analysis indicated that water levels are affected less by withdrawals from the northern part of the study area than by withdrawals from the southern part.

Saline water has threatened the potability of groundwater supplies derived from the Potomac-Raritan-Magothy aquifer system in the study area. Four areas in the study area have experienced acute problems with saline water. Example pumped wells were simulated in each of the four areas, particle tracking was used to define groundwater flow paths, and a budget analysis of the withdrawal zones was conducted to assess the movement of saline water and the likelihood of continued saltwater intrusion. Water withdrawn from wells screened in the Upper Potomac-Raritan-Magothy aquifer in the Glassboro Borough area at average rates for 1999 to 2001 of the 2000 baseline simulation or the withdrawal rates of the adjusted full-allocation scenario will likely remain potable with respect to chloride for at least several hundred years. However, chloride concentrations in the wells closest to the 250-mg/L isochlor probably will continue to rise slowly. The elevated chloride concentrations observed in water from wells screened in the Upper Potomac-Raritan-Magothy aquifer in Harrison Township and the Middle Potomac-Raritan-Magothy aquifer in Woodstown Borough likely result from proximity of the wells to the 250-mg/L isochlor in each aquifer, rather than from substantial lateral updip movement of the saline water. The elevated chloride concentrations found in wells screened in the Lower Potomac-Raritan-Magothy aquifer in Oldmans Township are likely to persist because of the proximity of these wells to the 250-mg/L isochlor in this aquifer and the orientation of the contributing flow path that directs recharge water through saline areas.

First posted March 25, 2011