Methane occurrence is associated with sodium-rich valley waters in domestic wells overlying the Marcellus shale in New York State

Prior work suggests spatial parameters (e.g., landscape position, distance to nearest gas well) can be used to estimate the amount of dissolved methane in domestic drinking water wells overlying the deep Marcellus Shale. New York (NY) provides an opportunity to investigate methane occurrence prior to expansion of high-volume hydraulic fracturing because unconventional gas production is currently banned in the state. We sampled domestic groundwater wells for methane in 2013 (n=137) across five counties of NY bordering Pennsylvania, and then resampled a subset of those wells in 2014 for methane concentrations and δ¹³C-CH₄ and δD-CH₄. The majority of waters from wells sampled (77%) had low concentrations of methane (10 mg/L). Dissolved methane concentrations did not change as a function of proximity to existing vertical gas wells, nor other parameters indicating subsurface planes of weakness (i.e., faults or lineaments). Methane levels were significantly higher in wells closer to hydrography flow lines, and most strongly correlated to Na-HCO₃ water type. The distribution of methane between Ca-HCO₃ (n=76) and Na-HCO₃ (n=23) water types significantly differed (p