NO DIRECT EVIDENCE OF METHANE IN SHALLOW GROUNDWATER FROM HYDRAULIC FRACTURING ACTIVITIES IN EASTERN KENTUCKY

Previous research has demonstrated that hydraulic fracturing 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 relation to neighboring hydraulically fractured natural gas wells. The studied groundwater shows elevated CH₄ concentrations ranging from 0.05 mg/L to 10 mg/L (mean: 4.92 mg/L). However, this is considerably lower compared to the northern Appalachian Basin (e.g., Marcellus Shale) where the CH₄ concentrations reach as high as 70 mg/L. The measured δ¹³C values of CH₄ range from -66 ‰ to -16 ‰ (mean: -46 ‰), and δ²H values range from -286 ‰ to -86 ‰ (mean -204 ‰). The occurrence of CH₄ is not correlated with proximity to hydraulically fractured natural gas wells. Generally, CH₄ occurrence corresponds with Na-Cl-HCO₃-rich and more reducing groundwater, and with low concentrations of SO₄. Additionally, groundwater in greater proximity to surface mining is more likely to be oxidized and contains less CH₄. We infer that mining activities increase the probability of CH₄ oxidation in shallow groundwater of Letcher County. Our results suggest that hydraulic fracturing does not increase the likelihood of stray gas migration into the groundwater of eastern Kentucky, although CH₄ concentrations can be elevated in this region. Local residents have reported occasional accidents of domestic well combustion as a result of excessive CH₄ accumulation. The isotopic values of CH₄ suggest an immature thermogenic and mixed biogenic/thermogenic origin rather than thermogenically mature shale gas. More research is needed to explain the distribution of CH₄ concentrations. Future studies should include more intensive sampling within a smaller area as the region's mountainous terrain creates relatively isolated valleys, making the chemical and isotopic results spatially heterogeneous.