Assessing the Natural Variability of Dissolved Methane in Groundwater over the Marcellus Shale in New York: Results from a Year of Monthly Sampling of Domestic Groundwater Wells

Despite the proliferation of high volume hydraulic fracturing (HVHF) in recent years, natural variability of dissolved methane in shallow groundwater remains relatively unpredictable, especially in areas overlying the Marcellus Shale. For this reason, baseline water quality data collected pre-drilling is critical for differentiating between naturally occurring methane and stray gas contamination. Often, only one or two samples of domestic well water are taken to determine baseline conditions before gas well drilling commences. Therefore, it is critical to know if point-in-time observations of baseline methane concentrations are representative of long-term conditions. Few areas remain that have not been impacted by HVHF and that can be used for investigation of how baseline conditions vary through time. New York, where there is a ban on HVHF, is an ideal study area where methane concentrations cannot be attributed to HVHF and has similar geology and landforms as adjacent areas in Pennsylvania significantly developed using HVHF. We worked with homeowners spread across three different counties in southern New York to sample their groundwater wells (n=11 wells) monthly for a year (n=13 months). Methane concentrations in these wells ranged from barely detectable ( 0.0002 mg/L) to above saturation (>28 mg/L). The objective of our study is to determine whether point-in-time observations of dissolved methane in domestic wells are representative of average dissolved methane concentrations in groundwater over time. Most wells with average methane concentrations >1 mg/L (n=7) had coefficients of variation (CV) less than 0.25, and several wells had CV < 0.15 (n=5). All wells with <1 mg/L methane (n=3) had high CV (>1) but low absolute variability. Some factors increase the variability of methane concentrations in wells over the year. Microbial sources of methane can show seasonal patterns with highest concentrations in summer and lowest in winter months. Methane concentrations can also vary through time if the water source for the well varies. Signs of a changing water source include seasonal changes in water use and/or changing water type. Overall, point-in-time measurements of naturally occurring methane in domestic groundwater wells are representative of long-term conditions.