METHANE AND BRINE-RELATED INORGANIC CONSTITUENTS IN GROUNDWATER – BASELINE RECONNAISSANCE SPATIAL AND TEMPORAL ASSESSMENT, PIKE COUNTY, NORTHEASTERN PENNSYLVANIA

In 2012, the U.S. Geological Survey sampled water from 20 domestic and supply wells completed in bedrock in Pike County, Pennsylvania to provide baseline reconnaissance data on concentrations of methane and brine-related constituents in shallow groundwater prior to shale-gas development as part of cooperative study with the Pike County Conservation District. Pike County is underlain by Devonian age shales, including the Marcellus Shale, but has no current unconventional shale-gas development. Sample analyses included major ions, trace constituents, nutrients, radon-222, stable isotopes of water, and methane. Methane was detected in water samples from 16 of 20 wells (80 percent), with most concentrations less than 0.10 milligrams per liter (mg/L) but two greater than 1 mg/L. Samples from these two wells had about 3.8 and 5.8 mg/L methane, respectively, and isotopic composition of methane in the samples indicated a predominantly microbial source, with values of –64.5 and –64.4 per mil for δ¹³C_CH4 and –217 and –202 per mil for δD_CH4, respectively. The two samples with elevated methane also differed in composition from samples with low to undetectable methane concentrations, having pH values greater than 8.2 and relatively elevated concentrations of sodium, lithium, boron, bromide, fluoride, and tungsten; one sample also had elevated arsenic and barium concentrations. Many of these constituents have been reported to occur in high concentrations in brines in deep shales in Pennsylvania. Four of the 20 wells were sampled monthly for one year ending June 2013 to assess temporal variability in groundwater quality over a range in methane and ion concentrations. Temporal concentrations of major ions generally varied less than 20 percent, although chloride and other constituents likely related to road salt spiked in samples collected during two winter months from one well. Temporal concentrations of methane varied by less than 0.01 mg/L for three wells with low methane and by about 1 mg/L for one well with high methane. The isotopic composition of methane in water from one high-methane well varied little in monthly samples, ranging from –64.5 to –64.8 per mil for d¹³C_CH4 and from –217 to –228 per mil for δD_CH4.