Geohydrology and Water Quality of Three Deep Test Holes in Fractured Bedrock, North-Central Pennsylvania
During 2012 and 2013, the U. S. Geological Survey, in cooperation with the Pennsylvania Geological Survey, characterized the geohydrology and water quality of three deep test holes in the fractured-bedrock uplands of Bradford, Tioga, and Sullivan Counties, north-central Pennsylvania. The 1,500-feet deep test holes were drilled as part of the State Survey’s geologic mapping program in this region of extensive Marcellus shale-gas development.
A suite of geophysical logs was collected from the test holes including nuclear, electric, electromagnetic, and sonic; acoustic and optical televiewer and video; and fluid-resistivity, temperature, and flow under ambient and pumped conditions. Based on interpretation of the geophysical logs, water-quality samples were collected at specific depths with a wireline sampler for analysis of specific conductance, total dissolved solids, major cations and anions, trace metals, and gas isotopes. The specific conductance and flow rate of the air-lifted discharge was monitored during drilling of the Sullivan County test hole. A straddle-packer system was also used in this test hole to isolate selected fractured zones for hydraulic measurement of transmissivity and head and collection of water-quality samples.
The test holes penetrated multiple water-flow zones at bedding-related fractures and, in a few shallower zones, steeply dipping fractures related to regional jointing. Minimal fracture transmissivity was penetrated below 700 feet as indicated by the depth at which the temperature-log gradients approached the geothermal gradient. Fractured zones above 300 feet produced very fresh calcium-bicarbonate type water with no detectable methane. Fractured zones below 900 feet produced very small inflows of saline sodium-chloride type water with thermogenic methane concentrations that ranged up to 120 milligrams per liter. Intermediate-depth fractured zones produced water that was transitional between that produced by the shallow and deep zones.