Monitoring concentration and isotopic composition of methane in groundwater in the Utica Shale hydraulic fracturing region of Ohio

Degradation of groundwater quality is a primary public concern in rural hydraulic fracturing areas. Previous studies have shown that natural gas methane (CH₄) is present in groundwater near shale gas wells in the Marcellus Shale of Pennsylvania, but did not have pre-drilling baseline measurements. Here, we present the results of a free public water testing program in the Utica Shale of Ohio, where we measured CH₄ concentration, CH₄ stable isotopic composition, and pH and conductivity along temporal and spatial gradients of hydraulic fracturing activity. Dissolved CH₄ ranged from 0.2 μg/L to 25 mg/L, and stable isotopic measurements indicated a predominantly biogenic carbonate reduction CH₄ source. Radiocarbon dating of CH₄ in combination with stable isotopic analysis of CH₄ in three samples indicated that fossil C substrates are the source of CH₄ in groundwater, with one ¹⁴C date indicative of modern biogenic carbonate reduction. We found no relationship between CH₄ concentration or source in groundwater and proximity to active gas well sites. No significant changes in CH₄ concentration, CH₄ isotopic composition, pH, or conductivity in water wells were observed during the study period. These data indicate that high levels of biogenic CH₄ can be present in groundwater wells independent of hydraulic fracturing activity and affirm the need for isotopic or other fingerprinting techniques for CH₄ source identification. Continued monitoring of private drinking water wells is critical to ensure that groundwater quality is not altered as hydraulic fracturing activity continues in the region. Graphical abstractA shale gas well in rural Appalachian Ohio. Photo credit: Claire Botner. A shale gas well in rural Appalachian Ohio. Photo credit: Claire Botner.