Natural methane occurrence in domestic wells is common in sodium-rich shallow groundwater in valley settings overlying the Marcellus Shale

Abstract:

Unconventional gas production from the Marcellus shale has proliferated in the past decade, raising concerns regarding impacts on fresh groundwater resources. Methane contamination of shallow groundwater can result from faulty seals on gas production wells or migration of methane along fractures during well development. Characterizing such contamination is of particular concern in the rural Marcellus shale region, where the water supply is primarily private, domestic wells and methane occurs naturally in shallow groundwater. High volume hydraulic fracturing (HVHF) is currently used to produce gas in all states overlying the Marcellus shale, with the exception of New York (NY), where HVHF is permanently banned. Given the similar regional geology, climate, and land use across areas underlain by the Marcellus, studies of methane occurrence in domestic wells in NY are representative of methane occurrence prior to HVHF in an area with conventional gas production. We measured methane concentrations in 137 domestic wells across 5 counties in southern NY covering an area of 10,230 km². For each well, we determined the topographic position (valley or upland), the geologic unit of water extraction, the chemical water type, and distances to the nearest fault, lineament, and active or other conventional gas well. Methane concentrations in domestic wells were not significantly different between geologic units of extraction, nor did they correlate with distances to faults, lineaments, or gas wells. Methane concentrations did differ between valleys and uplands, but the significance of such differences varied based on the method of classification. Methane concentrations were significantly different between water types; Na-HCO₃ waters had significantly higher methane concentrations than Ca-HCO₃ waters (median values of 0.78 and 0.002 mg/L, respectively). Combining methane and water quality data from this study and other prior studies in NY and Pennsylvania (n=724), we found that although only 7.7% of domestic wells had >1 mg/L dissolved methane, 52% of valley wells producing Na-rich water had >1 mg/L dissolved methane. Our results suggest high methane concentrations in valley wells producing Na-rich water are likely to be naturally occurring, rather than the result of gas production by HVHF.