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Simulation of Ground-Water Flow in the Potomac-Raritan-Magothy Aquifer System, Pennsauken Township and Vicinity, New Jersey

By Daryll A. Pope and Martha K. Watt


U.S. Geological Survey Scientific Investigations Report 2004-5025

Prepared in cooperation with the U.S. Environmental Protection Agency

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Abstract

The Potomac-Raritan-Magothy aquifer system is one of the primary sources of potable water in the Coastal Plain of New Jersey, particularly in heavily developed areas along the Delaware River. In Pennsauken Township, Camden County, local drinking-water supplies from this aquifer system have been contaminated by hexavalent chromium at concentrations that exceed the New Jersey maximum contaminant level. In particular, ground water at the Puchack well field has been adversely affected to the point where, since 1984, water is no longer withdrawn from this well field for public supply. The area that contains the Puchack well field was added to the National Priorities List in 1998 as a Superfund site.

The U.S. Geological Survey (USGS) conducted a reconnaissance study from 1996 to 1998 during which hydrogeologic and water-quality data were collected and a ground-water-flow model was developed to describe the conditions in the aquifer system in the Pennsauken Township area. The current investigation by the USGS, in cooperation with the U.S. Environmental Protection Agency (USEPA), is an extension of the previous study. Results of the current study can be applied to a Remedial Investigation and Feasibility Study conducted at the Puchack well field Superfund site.

The USGS study collected additional data on the hydrogeology and water-quality in the area. These data were incorporated into a refined model of the ground-water-flow system in the Potomac-Raritan-Magothy aquifer system. A finite-difference model was developed to simulate ground-water flow and the advective transport of chromium-contaminated ground water in the aquifers of the Potomac-Raritan-Magothy aquifer system in the Pennsauken Township area. An 11-layer model was used to represent the complex hydrogeologic framework. The model was calibrated using steady-state water-level data from March 1998, April 1998, and April 2001. Water-level recovery during the shutdown of Puchack 1 during March to April 1998 was simulated to evaluate model performance in relation to changing stresses. The Delaware River contributes appreciable-flow to the ground-water system from areas where the Middle and Lower aquifers crop out beneath the river. A transient simulation of an aquifer test near the Delaware River was run to help characterize the hydraulic conductivity of the riverbed sediments represented in the model. Vertical flow across confining units between the aquifers is highly variable and is important in the movement of water and associated contaminants through the flow system. The model was imbedded within a regional model of the Potomac-Raritan-Magothy aquifer system in Camden County.

In general, a simulation of baseline conditions, which can provide a representation on which simulations of various alternatives can be based for the feasibility study, incorporated average conditions from 1998 to 2000. Ground-water withdrawals within the model area during this period averaged about 14 Mgal/d. Regional ground-water flow is from recharge areas and from the Delaware River to downgradient pumped wells located just east of the model area in central Camden County. Simulation results show an important connection between the Intermediate sand and the Lower aquifer of the Potomac-Raritan-Magothy aquifer system in the vicinity of the chromium-contaminated area. The Delaware River contributes nearly 10 Mgal/d to the flow system, whereas recharge contributes about 6 Mgal/d. Ground-water withdrawals within the model area account for nearly 14 Mgal/d (mostly from the Lower aquifer of the Potomac-Raritan-Magothy aquifer system).

Contents

Abstract
Introduction
  Purpose and scope
  Previous investigations
  Well-numbering system
Hydrogeology and stratigraphy
Simulation of ground-water flow
  Model development
    Aquifer-system geometry and model-grid design
    Boundary conditions
    Recharge
    Surface water
    Lateral model boundaries
    Ground-water withdrawal data
    Hydrogeologic properties
      Horizontal hydraulic conductivity of aquifers and confining units
      Vertical hydraulic conductivity of confining units
      Specific storage
  Model calibration
    Calibration criteria
    Simulation of March 1998 conditions
    Simulation of April 1998 conditions and Puchack 1 shutdown
    Simulation of April 2001 conditions
    Simulation of Delaware Gardens aquifer test
    Summary of calibration
  Simulation of baseline conditions
Summary and conclusions
Acknowledgments
References cited
Appendix A: Simulated and measured water levels and residuals in the
 Potomac-Raritan-Magothy aquifer system, Pennsauken Township and vicinity,
 Camden County, New Jersey, March 1998 and April 2001
Appendix B: Simulated and measured water levels and drawdowns in the
 Potomac-Raritan-Magothy aquifer system, Pennsauken Township and vicinity,
 Camden County, New Jersey, March and April 1998



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