Dissolved methane in shallow groundwater of the Appalachian Basin: Results from the chesapeake energy predrilling geochemical database

We report the results of analysis and interpretation of 19,278 predrilling groundwater samples from water wells in the Appalachian Basin for dissolved methane collected from 2009 to 2012 (11,309 samples from northeastern Pennsylvania and 7969 samples from a western area that included north-central West Virginia, eastern Ohio, and southwestern Pennsylvania). We evaluate how concentrations of dissolved methane relate to geology, topographic position, well depth, groundwater circulation pathways, and hydrochemical facies. This data set, the most comprehensive to date for this part of the Appalachian Basin, shows 22.9% of predrilling samples naturally exceed the analytical detection limit for dissolved methaneof0.026 mg/Linnortheast Pennsylvania, and 24.8% exceed the analytical detection limit of 0.026 mg/L in the western area. The concentration of dissolved methane broadly relates to the waterbearing geological unit penetrated by the water well, the hydrochemical facies (e.g., Na-Cl, Na-HCO₃, or Ca-HCO₃ groundwater type), whether the water well is found in a valley or an upland location, whether the water well intersects restricted or confined saline zones, and where the well is completed within the groundwater circulation pathways. Dissolved methane in shallow groundwater commonly occurs in sodium-dominated groundwater types (Na-Cl or Na-HCO₃) and originates from the surrounding and underlying rocks or from deeper connate brines commonly found in valley settings at shallow depths in northeast Pennsylvania. A clear understanding of natural background conditions and natural variability must be considered when investigating water quality and stray gas complaints.