Simulation of groundwater flow and pathlines at NAWC NJ (2006) Completed

REPORT ABSTRACT:

The effectiveness of groundwater pumping by the use of recovery wells in preventing contaminated groundwater at the Naval Air Warfare Center (NAWC), West Trenton, N.J., from moving off the NAWC property was evaluated by use of a digital model of groundwater flow. The finite-difference, three-dimensional, porous-medium model uses the MODFLOW code to solve the groundwater-flow equation. Groundwater flowpaths from areas at and near contaminated well sites for a no-pumping and two-recovery-well-pumping scenarios were computed using the post-processor MODPATH.

The NAWC is underlain by the Lockatong and Stockton Formations, which consist of alternating layers of mudstone, siltstone, and sandstone. A fault transects the area and forms the contact between the Lockatong and Stockton Formations. Water flows through these rocks primarily in fractures that are parallel to the strike of the bedding—N. 65° E.—and, to a lesser degree, in fractures that are nearly vertical.

The median horizontal hydraulic conductivity measured at 32 shallow wells (less than 25 ft (feet) deep) at the NAWC is 2.6 ft/d (feet per day). At 17 wells, 26 to 50 ft deep, the median horizontal hydraulic conductivity is 11.3 ft/d. The hydraulic conductivity is lower in the shallow rocks than in the rocks from 26 to 50 ft below land surface because the shallow rocks are weathered, and clay and silt from the weathering process fills many of the fractures. Deeper than 50 ft, the horizontal hydraulic conductivity decreases, probably because the interconnection of fractures decreases with depth. The broad range in measured hydraulic conductivity (0.001 to 227 ft/d) is a result of the heterogeneity of the fractured-rock aquifer. Wells where high hydraulic conductivities were measured probably are completed where one or more vertical fractures intersect horizontal fractures, whereas wells with low hydraulic conductivities probably are between vertical fractures.

Under no-pumping conditions, simulated groundwater flowpaths from the NAWC are primarily in the direction of the strike of the bedding units, with most water from the NAWC flowing west-southwest to discharge at Gold Run on and adjacent to the NAWC. Under recovery-well-pumping conditions, most flowpaths are diverted toward the pumped wells.

Two different recovery-well networks, each comprising six wells pumping a total of 56.5 gallons per minute, have been used at the NAWC. Modeling results indicate that both networks capture water from the area around 51 of the 55 contaminated wells at the NAWC and that the second network captures more potentially contaminated groundwater than would otherwise discharge to Gold Run.