Interpretation of geophysical logs, aquifer tests, and water levels in wells in and near the North Penn Area 7 Superfund site, Upper Gwynedd Township, Montgomery County, Pennsylvania, 2000-02

Abstract

Ground water in the vicinity of various industrial facilities in Upper Gwynedd Township and Lansdale Borough, Montgomery County, Pa., is contaminated with various volatile organic compounds (VOCs). The 2-square-mile area was placed on the National Priorities List as the North Penn Area 7 Superfund site by the U.S. Environmental Protection Agency (USEPA) in 1989. The U.S. Geological Survey (USGS) conducted geophysical logging, aquifer testing, water-level monitoring, and streamflow measurements in the vicinity of North Penn Area 7 beginning autumn 2000 to assist the USEPA in developing an understanding of the hydrogeologic framework in the area as part of the USEPA Remedial Investigation. The study area is underlain by Triassic and Jurassic-age sandstones, siltstones, and shales of the Lockatong Formation and the Brunswick Group. Regionally, these rocks strike northeast and dip to the northwest. The sequence of rocks form a fractured-sedimentary-rock aquifer that acts as a set of confined to partially confined layered aquifers of differing permeabilities. The aquifers are recharged by precipitation and discharge to streams and wells. The Wissahickon Creek headwaters are less than 1 mile northeast of the study area, and this stream flows southwest to bisect North Penn Area 7. Ground water is pumped in the vicinity of North Penn Area 7 for industrial use and public supply. The USGS collected geophysical logs for 16 wells that ranged in depth from 50 to 623 feet. Aquifer-interval-isolation testing was done in 9 of the 16 wells, for a total of 30 zones tested. A multiple-well aquifer test was conducted by monitoring the response of 14 wells to pumping a 600-ft deep production well in February and March 2002. In addition, water levels were monitored continuously in three wells in the area and streamflow was measured quarterly at two sites on Wissahickon Creek from December 2000 through September 2002. Geophysical logging identified water-bearing zones associated with high-angle fractures and bedding-plane openings throughout the depth of the boreholes. Heatpulse-flowmeter measurements under nonpumping, ambient conditions indicated that borehole flow, where detected, was in the upward direction in three of the eight wells and in the downward direction in three wells. In two wells, both upward and downward flow were measured. Heatpulse-flowmeter measurements under pumping conditions were used to identify the most productive intervals in wells. Correlation of natural-gamma-ray and single-point-resistance logs indicated that bedding in the area probably strikes about 40 degrees northeast and dips from 6 to 7 degrees northwest. Aquifer intervals isolated by inflatable packers in wells were pumped to test productivity and to collect samples to determine chemical quality of water produced from the interval. Interval-isolation testing confirmed the presence of vertical hydraulic gradients indicated by heatpulse-flowmeter measurements. The specific capacities of isolated intervals ranged over two orders of magnitude, from 0.02 to more than 3.6 gallons per minute per foot. Intervals adjacent to isolated pumped intervals showed little response to pumping the isolated zone. The presence of vertical hydraulic gradients and lack of adjacent-interval response to pumping in isolated intervals indicate a limited degree of vertical hydraulic connection between the aquifer intervals tested. Concentrations of most VOC contaminants generally were highest in well-water samples from the shallowest isolated intervals, with some exceptions. Trichloroethylene, cis-1,2-dichloroethylene, and toluene were the most frequently detected VOCs, with maximum concentrations of greater than 340, 680, and greater than 590 micrograms per liter, respectively. Results of the aquifer test with multiple observation wells showed that water levels in 4 of the 14 wells declined in response to pumping. The four wells that responded to pumping are either along str