New Jersey Water Science Center
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A numerical ground-water-flow model was constructed to simulate ground-water flow in the Pohatcong Valley, including the area within the U.S. Environmental Protection Agency Pohatcong Valley Ground Water Contamination Site. The area is underlain by glacial till, alluvial sediments, and weathered and competent carbonate bedrock. The northwestern and southeastern valley boundaries are regional-scale thrust faults and ridges underlain by crystalline rocks. The unconsolidated sediments and weathered bedrock form a minor surficial aquifer and the carbonate rocks form a highly transmissive fractured-rock aquifer. Ground-water flow in the carbonate rocks is primarily downvalley towards the Delaware River, but the water discharges through the surficial aquifer to Pohatcong Creek under typical conditions.
The hydraulic characteristics of the carbonate-rock aquifer are highly heterogeneous. Horizontal hydraulic conductivities span nearly five orders of magnitude, from 0.5 feet per day (ft/d) to 1,800 ft/d. The maximum transmissivity calculated is 37,000 feet squared per day. The horizontal hydraulic conductivities calculated from aquifer tests using public supply wells open to the Leithsville Formation and Allentown Dolomite are 34 ft/d (effective hydraulic conductivity) and 85 to 190 ft/d (minimum and maximum hydraulic conductivity, respectively, yielding a horizontal anisotropy ratio of 0.46). Stream base-flow data were used to estimate the net gain (or loss) for selected reaches on Brass Castle Creek, Shabbecong Creek, three smaller tributaries to Pohatcong Creek, and for five reaches on Pohatcong Creek. Estimated mean annual base flows for Brass Castle Creek, Pohatcong Creek at New Village, and Pohatcong Creek at Carpentersville (from correlations of partial- and continuous-record stations) are 2.4, 25, and 45 cubic feet per second (ft3/s) (10, 10, and 11 inches per year (in/yr)), respectively.
Ground-water ages estimated using sulfur hexafluoride (SF6), chlorofluorocarbon (CFC), and tritium-helium age-dating
techniques range from 0 to 27 years, with a median age of 6 years. Land-surface and ground-water water budgets were calculated, yielding an estimated rate of direct recharge tothe surficial aquifer of about 23 in/yr, and an estimated net recharge to the ground-water system within the area underlain by carbonate rock (11.4 mi2) of 29 in/yr (10 in/yr over the entire 33.3 mi2 basin).
A finite-difference, numerical model was developed to simulate ground-water flow in the Pohatcong Valley. The four-layer model encompasses the entire carbonate-rock part of the valley. The carbonate-rock aquifer was modeled as horizontally anisotropic, with the direction of maximum transmissivity aligned with the longitudinal axis of the valley. All lateral boundaries are no-flow boundaries. Recharge was applied uniformly to the topmost active layer with additional recharge added near the lateral boundaries to represent infiltration of runoff from adjacent crystalline-rock areas. The model was calibrated to June 2001 water levels in wells completed in the carbonate-rock aquifer, August 2000 stream base-flow measurements, and the approximate ground-water age.
The ground-water-flow model was constructed in part to test possible site contamination remediation alternatives. Four previously determined ground-water remediation alternatives (GW1, GW2, GW3, and GW4) were simulated. For GW1, the no-action alternative, simulated pathlines originating in the tetrachloroethene (PCE) and trichloroethene (TCE) source areas within the Ground-Water Contamination Site end at Pohatcong Creek near the confluence with Shabbecong Creek, although some particles went deeper in the aquifer system and ultimately discharge to Pohatcong Creek about 10 miles downvalley in Pohatcong Township. Remediation alternatives GW2, GW3, and GW4 include ground-water withdrawal, treatment, and reinjection. The design for GW2 includes wells in the TCE and PCE source areas that withdraw water at a total rate of 420 gallons per minutes (gal/min) and 100 gal/min, respectively. Flow-path analysis shows the system would capture all ground water within the 500-µg/L TCE isopleth and ground water from a small area in the PCE source area. The design for GW3 includes wells in the TCE and PCE source areas that withdraw and re-inject at a total rate of 1,400 gal/min and 420 gal/min, respectively. Flow-path analysis shows the system would capture all ground water within the 100-µg/L TCE isopleth in the TCE source area and all of the ground water in the estimated PCE source area. The design for GW4 includes 35 wells withdrawing a total of 10,820 gal/min. Most particles started in the flow-path analysis in the TCE source area and in an arbitrary area representing contamination farther downvalley were captured by the GW4 system, although a few particles traveled beneath the withdrawal wells and flowed down the valley, ultimately discharging to Pohatcong Creek.
Abstract
Introduction
Hydrogeology
Geology
Hydrology
Fracture and Permeability Analysis
Analysis of Fracture Data from Borehole Acoustic Televiewer Logs
Packer Test Analysis
Specific-Capacity Analysis
Aquifer-Test Analysis
Stream Base Flow
Stream-Hydrograph Separation
Estimated Mean Annual Base Flow at Partial-Record Stations
Direct Discharge to the Delaware River from the Carbonate-Rock Aquifer
Ground- and Surface-Water Withdrawals
Ground-Water Levels
Ground-Water Age
Water Budgets
Simulation of Ground-Water Flow
Conceptual Model of Ground-Water Flow
Model-Grid Design
Boundary Conditions
Lateral Model Boundaries
Surface Water
Ground-Water Withdrawal and Injection Wells
Recharge
Aquifer Hydraulic Conductivity
Model Calibration
Simulated Water Levels
Simulated Stream Base Flow
Simulated Ground-Water Age
Model Limitations
Simulated Ground-Water-Flow Paths
Simulation of Ground-Water Remediation Alternatives
Alternative GW2
Alternative GW3
Alternative GW4
Summary
Acknowledgments
References Cited
Download: PDF of SIR2006-5269 (9.2Mb)
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