HYDROGEOCHEMICAL ORIGIN OF IRON-CONTAMINATED WATERS FROM ABANDONED NATURAL GAS WELLS, CLARION COUNTY, PENNSYLVANIA

Iron-contaminated artesian flows pollute hundreds of miles of streams in northwestern Pennsylvania. A common assumption is that the polluted water is associated with the gas-producing sands emanating from decades-old abandoned natural gas wells. However, the geochemistry of a number of these artesian discharges is more characteristic of acid mine drainage (AMD) than deep subsurface brines. If the source of the pollution is shallow AMD from near surface coal mines rather than deep brines, plugging the wells (which is common practice) would simply redirect the water pollution to downstream wells, springs or fractures, which could be very difficult to remediate. The discharges have similar concentrations of manganese and sulfate as AMD from local coal mines, but higher concentrations of iron (>40 ppm and up to 215 ppm), very low concentrations of dissolved aluminum (<1 ppm), and generally higher pH (4.7-6.1). Chloride and sodium are below 100 ppm, and cations are dominated by up to 200 ppm of calcium. Hedin et al. (2005, Mine Water Environ. 24: 106) proposed that these discharges could be explained by the interaction of hilltop coal mine drainage with subsurface siderite (FeCO3), an iron carbonate mineral found in local sedimentary rocks. In this study, mineralogical (XRD) analysis of drill core cuttings from a well in Clarion County confirms the presence of siderite in sandstone and siltstone units at depths between 50 and 350', and minor siderite in calcite-containing limestone at a depth of 432'. In addition, a borehole camera investigation shows significant fracturing and water flow at 132' depth. Because the flows are not net alkaline, these results suggest that the bulk of water-rock interaction is occurring in the shallower siderite-dominated units. Similar discharges and lithologies have been identified not only in Pennsylvania, but throughout the Appalachian coal belt, and in Great Britain as well. If these artesian discharges do in fact originate from shallow AMD as our results suggest, manipulating the artesian discharges directly into a treatment system would be the most effective remediation strategy.