Chemical and isotope compositions of shallow groundwater in areas impacted by hydraulic fracturing and surface mining in the Central Appalachian Basin, Eastern United States

•CH4 concentrations were analyzed in groundwater from Central Appalachia.•CH4 was of immature thermogenic and mixed biogenic/thermogenic origin.•CH4 content was not correlated with proximity to hydraulically fractured natural gas wells.•Groundwater in greater proximity to surface mining was more likely to be oxidized.•Oxygenated shallow groundwater increased the likelihood of CH4 oxidation.

Hydraulic fracturing of shale deposits has greatly increased the productivity of the natural gas industry by allowing it to exploit previously inaccessible reservoirs. Previous research has demonstrated that this practice has the potential to contaminate shallow aquifers with methane (CH4) from deeper formations. This study compares concentrations and isotopic compositions of CH4 sampled from domestic groundwater wells in Letcher County, Eastern Kentucky in order to characterize its occurrence and origins in relation to both neighboring hydraulically fractured natural gas wells and surface coal mines. The studied groundwater showed concentrations of CH4 ranging from 0.05 mg/L to 10 mg/L, thus, no immediate remediation is required. The δ13C values of CH4 ranged from −66‰ to −16‰, and δ2H values ranged from −286‰ to −86‰, suggesting an immature thermogenic and mixed biogenic/thermogenic origin. The occurrence of CH4 was not correlated with proximity to hydraulically fractured natural gas wells. Generally, CH4 occurrence corresponded with groundwater abundant in Na+, Cl−, and HCO3−, and with low concentrations of SO42−. The CH4 and SO42−concentrations were best predicted by the oxidation/reduction potential of the studied groundwater. CH4 was abundant in more reducing waters, and SO42− was abundant in more oxidizing waters. Additionally, groundwater in greater proximity to surface mining was more likely to be oxidized. This, in turn, might have increased the likelihood of CH4 oxidation in shallow groundwater.