METHANE OCCURRENCE, ORIGIN, AND RELATION TO PH AND ARSENIC AND OTHER TRACE CONSTITUENTS IN GROUNDWATER FROM SULLIVAN AND WAYNE COUNTIES, PENNSYLVANIA

Water samples were collected from 54 domestic wells in Sullivan and Wayne Counties, Pennsylvania, by the U.S. Geological Survey and the Pennsylvania Geological Survey during 2012-13 to provide baseline groundwater-quality data prior to shale-gas development in areas underlain by the Marcellus Shale. The water samples were collected from wells completed in Pennsylvanian, Mississippian, and Devonian age sedimentary rocks and analyzed for nutrients, major ions, metals and trace elements, radioactivity, and dissolved gases, including methane and radon-222. Fifty-four percent of the sampled wells had a detectable [greater than 0.001 mg/L (milligrams per liter)] methane concentration. Concentrations of dissolved methane ranged over five orders of magnitude (nondetect to 51.1 mg/L); the median concentration was 0.001 mg/L. Five water samples (9 percent) had a dissolved methane concentration greater than 0.9 mg/L and were analyzed for carbon and hydrogen isotopes of methane. The δ¹³C values ranged from -42.37 to -60.32 per mil, and the δD values ranged from -170.2 to -226.6 per mil, which is generally indicative of a thermogenic origin. Six water samples with methane concentrations greater than 0.08 mg/L also had pH values greater than 8 and elevated concentrations of arsenic, barium, boron, bromide, lithium, and molybdenum. The groundwater samples with the five highest methane concentrations also had the highest boron concentrations [91 to 165 micrograms per liter (µg/L)] and the highest bromide concentrations (0.26 to 2.7 µg/L). Arsenic concentrations ranged from less than 0.03 to 21.8 µg/L and exceeded the 10 µg/L drinking water standard in three water samples (6 percent). The four water samples with arsenic concentrations greater than 8 µg/L had a pH of 8.4 to 9.3 and methane concentrations from 0.08 to 4.1 mg/L.