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 (\u3e 1 mg/L). CH4 concentrations measured in groundwater ranged from \u3c 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
