TEMPORAL VARIABILITY OF NATURALLY OCCURRING METHANE IN SHALLOW GROUNDWATER WELLS IN THE MARCELLUS SHALE REGION

Although much has been learned about the impact of unconventional hydraulic fracturing in the Marcellus Shale region, little is known about the natural variation of dissolved methane prior to production, which is crucial when assessing the possible impacts of energy extraction. Few studies have collected baseline methane data over time and month-to-month natural variability is relatively unexplored in the Marcellus Shale Play. Limited opportunities remain for baseline data collection in northeastern Pennsylvania and other areas of the play, where shale gas production has proliferated, but opportunities remain in New York (NY), where hydraulic fracturing is currently banned. The objective of this study is to characterize temporal variability of naturally-occurring methane in shallow groundwater in the Marcellus region. We are working with homeowners to sample 11 domestic groundwater wells monthly in the Marcellus Shale region of NY for dissolved methane concentrations. Samples were initially collected using an open-system, overflow method for yearly sampling (n=4) and more recently using a closed-system, evacuated container (IsoFlask) for monthly sampling (n=10). Sampling methodology appears to have a greater impact on variability in measured methane concentrations relative to actual monthly variation for most wells, and this is particularly true at higher methane concentrations (>1 mg/L). We also evaluate whether “snapshot” point-in-time sampling is representative of average dissolved methane concentrations in groundwater through time. For wells with sufficiently high concentrations of methane to do isotopic analyses (≥0.90 mg/L), the predominant methane source is informed by δ¹³C-DIC, δ¹³C-CH₄, and δD-CH₄ isotopes, allowing us to evaluate thermogenic versus microbial sources of methane. These data provide information on inter- and intra-year variability of naturally-occurring methane, as well as spatial variability between wells. Results of this study provide context for interpreting the spatial and temporal variability of methane concentrations in domestic wells that may be impacted by proximal natural gas production from the Marcellus Shale.