BASELINE DISSOLVED-METHANE CONCENTRATIONS IN WATER-SUPPLY WELLS IN THE APPALACHIAN PLATEAU PHYSIOGRAPHIC PROVINCE OF MARYLAND

The extensive development of the natural gas resources of the Marcellus Shale in the northeastern Appalachian basin has led to concerns that methane and other produced fluids may be contaminating shallow aquifers that are the primary water supplies in rural areas. However, because methane is not usually tested for in private water wells, there is relatively little data regarding methane concentrations prior to natural-gas-extraction activities. The Marcellus Shale is considered prospective for gas production in the Appalachian Plateau province of western Maryland, but it has not yet (2015) been developed due to a moratorium on drilling in Maryland.

In order to collect baseline data on methane in wellwater, Maryland Geological Survey personnel collected water samples from 87 water-supply wells (mostly private wells) throughout the Appalachian Plateau region of western Maryland in 2012-2014. The region is underlain by gently folded Paleozoic strata, and slightly more than half of the study area is underlain by Pennsylvanian-age rocks that include significant coal seams. Water samples were analyzed for dissolved methane, ethane, propane, ethene, and field-measured parameters (pH, specific conductance, dissolved oxygen, and chloride). Dissolved-methane concentrations ranged from less than 0.0015 milligrams per liter (mg/L) to 8.55 mg/L, and methane was detected in all nine geologic formations in which sampled wells were located. Samples from seven wells exceeded 1 mg/L, and more than half the wells had less than 0.0015 mg/L methane (the reporting level). Ethane was detected in six wells. Methane was detected in 63 percent of wells that were located in valleys, compared with 30 percent of wells located in upland settings. Methane was also detected in 54 percent of wells in areas underlain by coal, compared with 40 percent of wells in non-coal areas. Methane samples collected at monthly intervals from three wells showed a 20 to 30 percent average variation from the median monthly methane concentration in each well, although individual variations were frequently much larger. The data indicate that low-level concentrations of methane in wellwater are common throughout the region.