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

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 SO₄ ^2-. The CH₄ and SO₄ ^2-concentrations were best predicted by the oxidation/reduction potential of the studied groundwater. CH₄ was abundant in more reducing waters, and SO₄ ^2- 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.