ARE POINT-IN-TIME OBSERVATIONS OF METHANE IN DOMESTIC WELL WATER REPRESENTATIVE OF YEAR-ROUND CONDITIONS? RESULTS OF YEAR-LONG MONTHLY WATER QUALITY SAMPLING IN THE MARCELLUS PLAY

Development of shale gas plays has accelerated over the past decade due to advancements made in horizontal drilling with high volume hydraulic fracturing (HVHF). To assess possible future water quality impacts of drilling operations, baseline water quality conditions at domestic wells are typically established through 1 or 2 pre-drill samples. Yet, little is known about temporal changes of naturally occurring methane, particularly over the Marcellus shale, and whether one sample is representative of year-round conditions. With the extensive development of much of the Marcellus and other gas plays, few areas remain to study baseline conditions over time. New York State, unlike its southern neighbor Pennsylvania, has banned HVHF for the foreseeable future and is an ideal location for evaluating the variability of naturally occurring methane in shallow groundwater above the Marcellus shale. Over 13 months, we worked with homeowners to collect monthly samples from domestic groundwater wells (n=11) in three counties in southern New York. The wells are spatially distributed and reflect a wide range of dissolved methane concentrations typical in the area, from below detection limit to above saturation (<0.0002 mg/L to >28 mg/L). The objective of our study is to determine whether one-time sampling schemes produce results that are representative of long-term baseline conditions. All wells with mean methane concentrations <1 mg/L had coefficients of variation (CV) greater than 1, but small absolute variability. Of those wells with >1 mg/L mean dissolved methane concentrations, most (n=7) had CV less than 0.25 and many had CV less than 0.15 (n=5). Variability in methane over the year was higher when the water source shifted through time. Seasonal patterns of methane, with the highest concentrations in summer and lowest in winter, are attributed to microbial sources of methane. On the whole, long-term conditions for naturally occurring dissolved methane in domestic groundwater wells over the Marcellus shale are well represented by point-in-time measurements.