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.