The rapid expansion of Marcellus shale gas extraction in the Appalachian basin of the northeastern US has prompted significant public concern regarding the potential for groundwater impacts due to hydraulic fracturing operations. This concern has been heightened by media coverage suggesting that hydraulic fracturing operations and gas extraction activities in northeastern Pennsylvania (Susquehanna County, in particular) have resulted in widespread occurrences of methane in shallow groundwater.

However, the results of an extensive water well survey by GSI Environmental Inc. indicate that methane is commonly found in drinking water aquifers prior to proximate drilling activities in Susquehanna County. In addition, a review of relevant geologic and historic information , as well as isotopic analyses of dissolved gases, suggests that methane in Susquehanna County water wells is most likely associated with sources in the shallow subsurface or Upper Devonian gas-charged deposits that overlie the deeper Marcellus shale targeted for commercial development. This finding reinforces the need for evaluation of potential stray gas impacts based upon multiple lines of evidence in order to properly assess the source of methane impacts and the risks associated with gas well drilling and hydraulic fracturing operations. Shale gas drilling activities first began in Susquehanna County in 2006. However, historical publications and geologic records show that elevated methane concentrations have been observed in springs and water wells for more than 200 years in the area. In the 1950s, the morning ignition of the water fountain at Rush High School was a daily ritual. In addition, local water well drillers report frequent encounters with shallow gas-bearing deposits, particularly within the sandstone units of the Upper Devonian Catskill and Lock Haven formations. Indeed, predrill sampling and testing of more than 1,700 water wells by Cabot Oil and Gas Corp. in Susquehanna County from 2008 through 2011 found methane to be present in 78% of the water wells prior to nearby gas well drilling activities. These predrill samples were obtained from water wells located at least 300 m (1,000 ft) away from active drilling operations. In this dataset, methane concentrations in water wells within 1 km (0.6 miles) of the nearest active gas well were comparable to those in water wells greater than 1 km away, suggesting no relation to gas development activities on a regional scale. Rather, elevated methane levels in groundwater showed a clear relation to topography.

In addition to geologic and historic evidence for shallow natural gas sources, the isotopic signatures of dissolved gases in Susquehanna County also suggest that the observed methane levels are not likely associated with deep Marcellus shale gas. In 2009 and 2010, the Pennsylvania Department of Environmental Protection and Cabot collected samples of dissolved gases from 14 water wells in Susquehanna County for geochemical analyses to determine the origin of impact. The carbon and hydrogen isotopic signatures of the dissolved gases were found to be consistent with either thermogenic gases from Upper and Middle Devonian deposits overlying the Marcellus shale, microbial gases formed in shallow glacial drift and alluvium, or a mixture of the two. These data show that the presence of elevated methane in water wells throughout Susquehanna County could be explained without any contribution from Marcellus shale related to the hydraulic fracturing process.

In several cases, site-specific investigations may be required to determine the source of methane in individual water wells. However, based upon available information it is premature at best to conclude that gas well drilling and hydraulic fracturing activities are having any regional-scale impacts on shallow groundwater resources.