Elevated dissolved methane (CH4) concentrations in groundwater are an environmental concern associated with hydraulic fracturing for shale gas. Therefore, determining dissolved CH4 baselines is important for detecting and understanding any potential environmental impacts. Such baselines should change in time and space to reflect ongoing environmental change and should be able to predict the probability that a change in dissolved CH4 concentration has occurred. We considered four dissolved CH4 concentration datasets of English groundwater using a Bayesian approach: two national datasets and two local datasets from shale gas exploration sites. The most sensitive national dataset (the previously published British Geological Survey CH4 baseline) was used as a strong prior for a larger (2153 measurements compared to 439) but less sensitive (detection limit 1000 times higher) Environment Agency dataset. The use of the strong prior over a weak prior improved the precision of the Environment Agency dataset by 75%. The expected mean dissolved CH4 concentration in English groundwater based on the Bayesian approach is 0.24 mg/l, with a 95% credible interval of 0.11 to 0.45 mg/l, and a Weibull distribution of W(0.35 ± 0.01, 0.34 ± 0.16). This result indicates the amount of CH4 degassing from English groundwater to the atmosphere equates to between 0.7 and 3.1 kt CH4/year, with an expected value of 1.65 kt CH4/year and a greenhouse gas warming potential of 40.3 kt CO2eq/year. The two local monitoring datasets from shale gas exploration sites, in combination with the national datasets, show that dissolved CH4 concentrations in English groundwater are generally low, but locations with concentrations greater than or equal to the widely used risk action level of 10.0 mg/l do exist. Statistical analyses of groundwater redox conditions at these locations suggest that it may be possible to identify other locations with dissolved CH4 concentrations ≥10.0 mg/l using redox parameters such as Fe concentration.
