THE CONTROLS OVER THE NATURAL OCCURRENCE OF METHANE IN POTABLE GROUND WATER IN THE APPALACHIAN BASIN

Unconventional natural gas production in the Marcellus and Utica Formations of the Northeastern United States raises concerns about potential impacts to shallow groundwater. We examine the results of baseline (pre-drilling) sampling for methane in 13,040 potable ground water samples in Northeastern Pennsylvania and 8,004 samples from a “Western Area” (southwest Pennsylvania, eastern Ohio, and north-central West Virginia) that were collected on behalf of Chesapeake Energy Corporation as part of its pre-drill monitoring program, and the results of a year-long study on temporal variability of methane in ground water at 12 locations in NE Pa.

Methane naturally is common. In NE Pennsylvania, measureable dissolved methane occurred in 24% of samples with 3.4% naturally exceeding the PADEP methane notification level of 7 mg/L. In the western area, dissolved methane occurred naturally in 36% of groundwater sampled and in Ohio, 4.1% of samples exceeded the Ohio dissolved methane action level of 10 mg/L. More methane occurs where hydrogeochemical facies types trend towards Na-Cl and Na-HCO₃ type waters in valleys and along hill flanks. However, we found no relationship between the concentration of methane and proximity to pre-existing gas wells. Finally, concentrations of methane in domestic wells can vary by factors, depending on pumping regime and time of year.