Sr Isotope Quantification of Deep Brine and Shallow Acidic Coal Mine Drainage Inputs to High TDS Gas Well Discharges in Western Pennsylvania

Chapman, E. C., Capo. R. C., Stewart, B. W. Dept. of Geology & Planetary Science, University of Pittsburgh, Pittsburgh, Pennsylvania, 15260 Hedin, R. S. and Weaver, T. R., Hedin Environmental, 195 Castle Shannon Blvd., Pittsburgh, Pennsylvania, 15228 In western Pennsylvania, numerous abandoned oil and gas wells discharge contaminated water to the surface. Many of these discharges have circumneutral pH as well as high concentrations of iron and sulfate. Total dissolved solid (TDS) content is also high relative to local groundwater. Possible sources of this water include deep brines, which have circumneutral pH and very high TDS, or shallow acidic coal mine drainage (AMD). Hundreds of hilltop strip mines are found in this area, and mine seeps have low pH (3.5-4) and high TDS. Geochemical data alone have not been definitive in determining the source of the gas well discharges. Strontium isotopic compositions of deep brine, gas well discharges drilled 150-240 m deep into Upper Devonian strata, and local AMD associated with the Leeper anticline in Clarion County strongly suggest that the water chemistry in the gas wells is dominated (>99%) by mine drainage. Because of its high Sr concentrations, even small contributions of brine (<1%) can significantly change the 87Sr/86Sr of the groundwater. With the ability to determine the source of these discharges, other questions about subsurface geochemical reactions can be addressed. For example, iron concentration in the gas well discharges is much higher than either the deep brines or shallow AMD. This could be due to siderite (FeCO3) dissolution by AMD; previous work identified the presence of siderite in sedimentary strata within the subsurface path of the AMD flows. Carbonate mineral dissolution could also explain the circumneutral pH of flows from the gas wells. Sr isotopes can be used as a sensitive tracer for the interaction of shallow and deep fossil fuel byproducts with natural waters, including produced waters from hydrologic fracturing and coalbed methane.