In cooperation with the U.S. Environmental Protection Agency
Use of Borehole Geophysical Logging and Water-Level Data to Characterize the Ground-Water System in the Lockatong and Brunswick Formations
SENIOR, Lisa A., and CONGER, Randall W., U.S. Geological Survey
Borehole geophysical logging and ground-water level data were used to characterize the ground-water system at two sites underlain by siltstones and shales of the Lockatong and Brunswick Formations in Montgomery County, Pennsylvania. Water-bearing zones in the boreholes and stratigraphic marker beds were identified using caliper, natural gamma, fluid-resistivity and fluid-temperature, fluid-movement (brine-tracing or heat-pulse flowmeter), and borehole video logs. Nine wells ranging in depth from 110 to 900 feet (ft) were logged at the site underlain by the Lockatong Formation and 30 wells ranging in depth from 83 to 1,027 ft were logged at the site underlain by the Brunswick For mation. At both sites, the most productive water-bearing zones are within 350 ft of the land surface and commonly are associated with near vertical fractures close to the depth of the static water level in the well. Although large fractures were noted at depths of 600 ft or greater, ground-water circulation probably is most active at shallower depths. The depth to water in the wells was related to hydrologic setting. In recharge areas on topographic highs, depth to water generally ranges from 50 to 75 ft below land surface, but in discharge areas near streams, depth to water generally is within 20 ft of and sometimes higher than the land surface. Water levels monitored in open-hole bedrock wells ranging in depth from 110 to 900 ft consistently showed the effects of earth tides indicating confined or semi-confined conditions. Packer-test and the borehole-flow data show potentiometric head differences between water-bearing zones in some boreholes, indicating a multi-aquifer system. Seepage along borehole walls above the water level was noted in some boreholes, suggesting that there may be a shallow, perched system above the observed water level. Reported low estimates of recharge for these formations may be related to low permeability of the weathered bedrock near the surface.
Senior, L.A., and Conger, R.W., 1997, Use of borehole geophysical logging and water-level data to characterize the ground-water system in the Lockatong and Brunswick formations: (abs.), GSA Abstracts with Programs, vol. 29, no. 1, p. 78.