Temporal variability of methane in domestic groundwater wells, northeastern Pennsylvania

We herein document temporal changes in dissolved and gaseous methane concentrationsin12domestic water wells completed in Late Devonian clastic aquifers in northeastern Pennsylvania over time periods from 0.2 to 2 yr. Wells with initial methane concentrations regarded as low (<5 mg/L), moderate (5-15 mg/L), and high (>15 mg/L) were all used in our study for comparative purposes. The temporal variability of dissolved methane concentrations in groundwater from water wells ranged from tens to hundreds of percentage points in all categories of methane concentration, from low to high. Dissolved methane concentrations from most water wells with initial dissolved methane greater than 6 mg/L correlated (p ≤ 0.05) to natural changes in regional groundwater levels. Dissolved methane concentrations in groundwater are highest when regional groundwater levels are at seasonal lows, and they vary in response to groundwater fluctuation. Water-level drawdown during water well pumping lowers methane solubility and induces it to exsolve from the groundwater into the well headspace. We observed that methane exsolution occurred when concentrations in groundwater exceeded 6 mg/L. In some cases with large drawdown or sustained water use, methane headspace in water wells can rapidly increase, reach ignitable concentrations, and remain elevated for days later.