Provided in cooperation with New Jersey Department of Environmental Protection
Program to Maintain and Update Groundwater Models
Upper Rancocas River Basin
Simulated effects of alternative withdrawal strategies on groundwater flow patterns, New Jersey Pinelands
By Edward Modica
MODEL VERSION/TYPE: MODFLOW-96, steady state
AREA STUDIED: Rancocas Creek and Wading River Basins, Burlington and Ocean Counties
AQUIFERS SIMULATED: Kirkwood-Cohansey aquifer system
MOST RECENT WITHDRAWALS SIMULATED: pre-pumping conditions
MODEL SIZE: 2 layers, 65 rows, 70 columns
MINIMUM GRID SPACING:1,500 feet x 1,500 feet
REPORT: USGS Water-Resources Investigations Report 95-4133
Archived model files are available.
A steady-state, three-dimensional groundwater-flow model of the unconfined part of the Kirkwood-Cohansey aquifer system beneath the upper parts of the Rancocas Creek and Wading River Basins in the New Jersey Pinelands was developed to (1) define groundwater-flow patterns and residence times in an aquifer system typical of the New Jersey Coastal Plain and (2) demonstrate the effects of alternative withdrawal strategies on groundwater-flow patterns and streams. Groundwater flow near the McDonalds-Middle Branch area was analyzed by using a particle tracker to demonstrate the effects of three hypothetical withdrawal scenarios on the configurations of source areas of groundwater flow to withdrawal wells, streams, and other discharge outlets in the Kirkwood-Cohansey aquifer system. Under natural conditions, more than 98 percent of the groundwater in the part of the Kirkwood-Cohansey aquifer system underlying the upper Rancocas Creek and Wading River Basins enters the system as recharge. More than 87 percent of the groundwater discharges to streams and wetlands. Groundwater seepage to the underlying Piney Point aquifer accounts for about 8 percent of discharge from the system. Areas near major drainage divides are the source of flow to distant parts of the system. Consequently, source areas of flow to small basins located near major drainage divides do not necessarily coincide with the physiographic boundaries of the basins. Groundwater residence times ranged from slightly greater than zero to about 200 years. Much of the groundwater remained in the system for less than 20 years because it discharged to streams. Residence times of groundwater were reduced significantly by persistent withdrawals. The sizes and shapes of source areas of flow to local stream systems and to the Piney Point aquifer are affected by the location of a withdrawal well. The source area of flow to the withdrawal well includes areas of the water table that would, under natural conditions, be incorporated into source areas of flow to streams or to the Piney Point aquifer. Simulated withdrawals of 1.85 million gallons per day (Mgal/d) at the divide have negligible effects on source areas of groundwater flow to adjacent streams, but capture recharge that under natural conditions would flow deep into the aquifer. Simulated withdrawals of 1.85 Mgal/d located away from the divide, however, capture the most recharge from source areas of flow to adjacent streams. Large-scale regional withdrawals that cause a 10- to 15-foot decrease in heads in the Piney Point aquifer and divert 6.2 Mgal/d of groundwater from the Kirkwood-Cohansey aquifer system increase the source area of flow to the Piney Point aquifer by diverting groundwater from streams. Results of withdrawal simulations indicate that well-location strategies applied in the Kirkwood-Cohansey aquifer system can mitigate the adverse effects of withdrawals on streams and that large-scale regional withdrawals in confined aquifers can adversely affect streams although the effects are dispersed over numerous streams.