New Jersey Water Science Center
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Since 1996, when the New Jersey Department of Environmental Protection (NJDEP) restricted ground-water withdrawals from the Potomac-Raritan-Magothy aquifer system in the southern New Jersey Coastal Plain as a result of excessive drawdown, Coastal Plain communities have been interested in developing alternate sources of water supply for their residents. The use of ground water from areas near the updip parts of the overlying confined aquifers where withdrawals are not restricted is being considered to meet the demand for drinking water. Concerns have arisen, however, regarding the potential effects of increased withdrawals from these areas on ground-water flow to streams and wetlands as well as to the deeper, confined parts of the aquifers. Therefore, the U.S. Geological Survey, in cooperation with the NJDEP, conducted a study to investigate the sources of water to currently inactive wells in the updip part of the Wenonah-Mount Laurel aquifer in Gloucester and Camden Counties, New Jersey. Of particular interest is whether the primary source of the increased withdrawals is likely to be the aquifer outcrop or the downdip, confined part of the aquifer.
The outcrop of the Wenonah-Mount Laurel aquifer covers nearly 8 mi2 (square miles), or about 46 percent of Deptford Township’s 17.56-mi2 area. The Deptford Township Municipal Utilities Authority owns six currently (2005) inactive wells in the Wenonah-Mount Laurel aquifer at the southeastern boundary of Deptford Township, 1.25 mi (miles) from the outcrop. For the purposes of this study, an existing ground-water-flow model of the New Jersey Coastal Plain aquifers was used to simulate ground-water-flow conditions in Gloucester and Camden Counties in 1998.
Two alternative withdrawal scenarios were superimposed on the results of the 1998 simulation. In the first (the “full-allocation” scenario), full-allocation withdrawal rates established by the NJDEP were applied to 45 existing wells in the Deptford Township area. In the second (the “additional-withdrawal” scenario), the full-allocation scenario was modified by adding an additional withdrawal of 1.62 million gallons per day from the six inactive Deptford Township withdrawal wells.
Simulated drawdown for the full-allocation scenario is zero to near zero in Deptford Township. Changes are greatest downdip from Deptford Township, where a broad area of 5- to 10-ft (feet) drawdowns is simulated; maximum drawdown at the center of the cone of depression is 20 ft. Water levels declined as much as 10 ft around individual wells whose current withdrawals are only a small percentage of their allotted allocation.
Simulated drawdown for the additional-withdrawal scenario exceeds 40 ft and is centered around the six inactive Deptford Township withdrawal wells. The area in which the simulated drawdown is 5 ft extends approximately 3.75 mi downdip from the wells and 2 mi updip, into the outcrop.
Water budgets based on the simulation results for the full-allocation and additional-withdrawal scenarios were calculated and compared, with particular focus on a 75-mi2 area in and around Deptford Township that includes the outcrop of the Wenonah-Mount Laurel aquifer and part of the area downdip from the outcrop (budget zone 2). The comparison of the two water budgets for zone 2 shows that 46 percent of the withdrawals from the six inactive Deptford Township wells would result from reduced stream base flow in the outcrop of the Wenonah-Mount Laurel aquifer and 35 percent would result from increased downward flow from the overlying Vincentown aquifer. Four percent would result from increased flow from the downdip areas of the Wenonah-Mount Laurel aquifer, 5 percent would result from decreased flow to the downdip areas of the Wenonah-Mount Laurel aquifer, and 5 percent would result from decreased flow to the underlying Englishtown aquifer system. The remaining 4 percent was attributed to decreased upward flow to the overlying Vincentown aquifer.
Records from three streamflow-gaging stations and four low-flow partial-record stations around the Deptford Township area were analyzed to determine base flow for comparison to the water-budget values. Statistics from only one station, Still Run near Mickleton, N.J. (01476600), were used in the estimation of base-flow reduction because the Wenonah-Mount Laurel outcrop covers 75 percent of the drainage basin’s area. The unit-area base flow of 1.05 cubic feet per second per square mile calculated for the Still Run station was assumed for all streams draining the outcrop. Using this base-flow value, the outcrop area of 22.61 mi2 within budget zone 2 would yield 23.7 ft3/s (cubic feet per second) of base flow. Simulation results for this budget zone include a 1.15-ft3/s decrease in ground-water flow to streams from the full-allocation scenario to the additional-withdrawal scenario. This decrease represents a 4.9-percent reduction in the average stream base flow of 23.7 ft3/s estimated for streams draining the Wenonah-Mount Laurel aquifer outcrop within the area of budget zone 2. Therefore, on the basis of the simulations, the primary sources of the water withdrawn from the Wenonah-Mount Laurel aquifer when the six inactive withdrawal wells in Deptford Township are pumped would be the Wenonah-Mount Laurel aquifer outcrop and the Vincentown aquifer rather than downdip parts of the Wenonah-Mount Laurel aquifer. The relatively low transmissivity of the Wenonah-Mount Laurel aquifer (500-1,000 feet squared per day) and the proximity of the wells to the outcrop area are the primary factors that control the source of water to these wells.
Purpose and Scope
Description of Study Area
Simulation of Ground-Water Flow
Description of Scenarios
Estimation of Withdrawals
Water Levels and Drawdown
Sources of Water to Wells
Estimation of Base Flow
Estimated Base-Flow Reduction
Limitations of the Model
Summary and Conclusions
Download: PDF of SIR2005-5250 (5.2Mb).
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