Baseline Methane Concentrations in Drinking-Water Wells in the Appalachian Plateau Province of Maryland
The potential development of natural gas from the Marcellus Shale in western Maryland has raised concerns about whether such development could result in methane contamination in drinking-water wells. Because methane is not routinely analyzed in drinking water in Maryland, virtually nothing was known of its occurrence and distribution. In 2012 and 2013, the Maryland Geological Survey collected water samples from 77 water-supply wells in Garrett and Allegany Counties in the Appalachian Plateau physiographic province of western Maryland. The near-surface geology of the region is characterized by gently folded Devonian, Mississippian, and Pennsylvanian rocks. Coal is present in the synclinal basins, and natural gas is produced from and stored in the (Devonian) Oriskany Formation in parts of the anticlinal structures. Well selection was based on geology (coal vs. non-coal areas) and topographic setting (valley vs. hillside or hilltop), resulting in wells being assigned to one of four categories: coal/valley (15 wells); coal/hilltop+hillside (20 wells); non-coal/valley (17 wells); and non-coal/hilltop+hillside (25) wells. Dissolved methane was detected in about 44% of all wells (reporting level: 1.5 micrograms per liter [µg/L]). Concentrations ranged from less than 1.5 to 8550 µg/L; four of the 77 wells exceeded 1000 µg/L. Methane was detected in wells from all geologic formations. Wells located in coal/valley settings had the highest percentage of methane detections (about 73%), followed by wells in coal/hilltop+hillside settings (45%), non-coal/valley settings (41%), and non-coal/hilltop+hillside settings (28%). Monthly methane concentrations analyzed in samples from three wells generally varied by 20% to 30% from the median monthly methane concentration in each well. Isotopic analyses for 13C-CH4 and 2H-CH4 suggest a thermogenic origin for the methane. Additional methane sampling is planned in the vicinity of a natural-gas storage facility in Garrett County.