Analysis of geophysical logs, at North Penn Area 6 Superfund Site, Lansdale, Montgomery County, Pennsylvania

Abstract

The U.S. Geological Survey (USGS), as part of technical assistance to the U.S. Environmental Protection Agency (USEPA), collected borehole geophysical log data in 34 industrial, commercial, and public supply wells and 28 monitor wells at the North Penn Area 6 Superfund Site, in Lansdale, Pa., from August 22, 1995, through August 29, 1997. The wells range in depth from 50 to 1,027 feet below land surface and are drilled in Triassic-age shales and siltstones of the Brunswick Group and Lockatong Formation. The geophysical log data were collected to help describe the hydrogeologic framework in the area and to provide guidance in the reconstruction of the 28 monitor wells drilled during summer 1997. At the time of logging, all wells had open-hole construction. The geophysical logs, caliper, fluid-resistivity and fluid-temperature, and borehole video logs were used to determine the vertical distribution of water-bearing fractures. Heatpulse-flowmeter measurements were used to determine vertical borehole flow under pumping and nonpumping conditions. The most productive fractures generally could be determined from heatpulse-flowmeter measurements under pumping conditions. Vertical borehole flow was measured under nonpumping conditions in most wells that had more than one water-bearing fracture. Upward flow was measured in 35 wells and probably is a result of natural head differences between fractures in the local ground-water-flow system. Downward flow was measured in 11 wells and commonly indicated differences in hydraulic heads of the fractures caused by nearby pumping. Both upward and downward flow was measured in three wells. No flow was detected in eight wells. Natural-gamma-ray logs were used to estimate the attitude of bedding. Thin shale marker beds, shown as spikes of elevated radioactivity in the natural-gamma logs of some wells throughout the area, enable the determination of bedding-plane orientation from three-point correlations. Generally, the marker beds in and near Lansdale strike about N. 48°-60° E. and dip about 11° NW. Acoustic televiewer logs run in selected boreholes indicate that the attitude of many water-bearing fractures commonly is similar to that of bedding. INTRODUCTION Ground water at the North Penn Area 6 (NP6) Superfund Site is contaminated with organic solvents. The site covers about 3 mi2 in and near Lansdale, Montgomery County, Pa., and includes at least six properties identified as potential sources of contamination (Black and Veatch, 1994). The location of the 62 production-type and monitor wells logged at the site are shown in figure 1. Some industries in the area have been operating since the 1940's, whereas others began operations as late as the 1980's (Black and Veatch, 1994). Various solvents, degreasers, and other types of organic compounds such as trichloroethylene (TCE), 1,1,1-trichloroethane (1,1,1-TCA), and tetrachloroethylene (PCE) and metals were used by these industries (CH2MHill, 1991). In 1979, TCE was measured in concentrations greater than 4.5 Jig/L in eight North Penn Water Authority (NPWA) public supply wells in the Lansdale area (Black and Veatch, 1994). This concentration nearly exceeds the U.S. Environmental Protection Agency's (USEPA) maximum contaminant level in drinking water of 5 Jig/L (U.S. Environmental Protection Agency 1994). In 1986, the USEPA Region III requested water-use information from 17 industries in the area, and numerous residential wells were sampled. The NP6 was placed on the National Priority List (NPL) in March 1989. Results of environmental investigations by property owners, the Pennsylvania Department of Environmental Protection (PADEP), and by USEPA and its contractors confirmed that the ground water beneath the Borough of Lansdale is contaminated with volatile organic compounds (VOC's). Concentrations of TCE as great as 9,240 jig/L were measured in water samples from municipal public supply wells, and VOC's also were detected in