Characterizing groundwater CH4 and 222Rn in relation to hydraulic fracturing and other environmental processes in Letcher County, KY

Hydraulic fracturing of shale deposits has greatly increased the productivity of the natural gas industry by allowing it to exploit previously inaccessible reservoirs. However, previous research has demonstrated that this practice can contaminate shallow aquifers with CH₄ [methane] from deeper formations. This study compares concentrations and isotope compositions of CH₄ sampled from domestic groundwater wells in Letcher County, Kentucky in order to characterize its occurrence and origins in relation to neighboring hydraulically fractured natural gas wells. Additionally, this study tests the reliability of ²²²Rn [radon] as an alternative tracer to CH₄ in identifying processes of gas migration from Devonian shale. Other chemical and isotopic tracers – including isotope compositions of H₂O [water] and dissolved SO₄ [sulfate], as well as concentrations of major dissolved ions – were also compared in order to characterize groundwater in relation to other environmental processes.

Approximately half of the 59 households sampled in Letcher County showed elevated CH₄ concentrations (> 1 mg/L). CH₄ concentrations measured in groundwater ranged from < 0.05 mg/L to 10 mg/L (mean: 4.92 mg/L). δ¹³C [delta-13 of carbon] values of CH₄ ranged from -66 ‰ [per mil] to -16 ‰ (mean: -46 ‰), and δ²H [deuterium] values ranged from -286 ‰ to -86 ‰ (mean -204 ‰). The isotope composition of observed CH₄ was characteristic of an immature thermogenic or mixed biogenic/thermogenic origin, similar to that of coalbed CH₄ sampled from shallower, Pennsylvanian deposits. The occurrence of ²²²Rn was rare, and determined not to be linked to the occurrence of CH₄. CH₄ and ²²²Rn occurrences were not correlated with proximity to hydraulically fractured natural gas wells. Instead, CH₄ occurrence corresponded with groundwater abundant in Na⁺ [sodium], Cl^- [chloride], and HCO₃^- [bicarbonate], and CH₄ concentrations were best predicted by the oxidation/reduction potential of the aquifer sampled. These results suggest that hydraulic fracturing has had a negligible impact on processes of stray gas migration in Letcher County. Furthermore, CH₄ found in shallow groundwater likely originated from shallower depositional and/or microbial processes unrelated to gas migration from Devonian shale.