DISSOLVED GAS GEOCHEMISTRY OF SHALLOW GROUNDWATER SYSTEMS IN PENNSYLVANIA AND NEW YORK, ASSOCIATED WITH NATURAL GAS EXTRACTION

The Marcellus organic-rich shale has been developed extensively as one of the largest natural gas reservoirs in the U.S., in large part due to advances in drilling technologies and production strategies. Concerns over potential environmental impacts have accompanied natural gas extraction but lack sufficient data for a thorough scientific assessment. To address this, we sampled shallow groundwater systems of homeowners in NE Pennsylvania and New York for dissolved gases beginning in 2010. Samples were divided into active natural gas extraction areas (defined as having one or more gas wells within 1 km of a private water well) and non-active natural gas extraction areas (defined as no gas wells within 1 km of a private water well).

Methane was detected in approximately 85% of homeowner water wells regardless of gas industry operations. However, the dissolved methane concentrations were 17.4 times higher on average in active extraction areas (19.2 mg/L; n=26) than in non-active gas extraction areas (1.1 mg/L; n=34). The higher chain hydrocarbons (ethane, propane, and butane) were detected more often in active areas (21, 8, and 3, respectively) compared to non-active areas (3, 0, and 0, respectively), indicating a more thermogenic methane character. The carbon isotope values of methane (δ¹³C-CH4), in active extraction areas were on average less negative (-37±7 ‰) than non-active extraction areas (-54±11‰). We conclude from these and other results that there are important differences in methane concentration, natural gas composition, and δ¹³C-CH₄ values in shallow groundwater wells between active and non-active natural gas extraction areas. Furthermore, the dissolved gas geochemistry in active extraction areas is consistent with a more thermally mature source of organic matter, whereas the dissolved gas in shallow ground water from non-active extraction areas tends to be more mixed biogenic/thermogenic in character. Results from additional sampling in 2011 will also be presented.