The rapid increase in shale gas production in recent years has led to increased attention to its potential negative environmental effects, including the risks of contaminating groundwater with methane and other substances. In this context, the uncontrolled gas migration that is triggered during well blowouts is an understudied environmental hazard. We show that the methane chemistry in shallow groundwater overlying the site of a catastrophic underground blowout continues to be impacted 50 y later. The occurrence of anaerobic methane oxidation limits the spatial extent to which the dissolved thermogenic methane plume could be observed and discerned from local biogenic methane sources. However, it also highlights the requirement to carry out monitoring in close proximity to potential gas leakage sources.
Blowouts present a small but genuine risk when drilling into the deep subsurface and can have an immediate and significant impact on the surrounding environment. Nevertheless, studies that document their long-term impact are scarce. In 1965, a catastrophic underground blowout occurred during the drilling of a gas well in The Netherlands, which led to the uncontrolled release of large amounts of natural gas from the reservoir to the surface. In this study, the remaining impact on methane chemistry in the overlying aquifers was investigated. Methane concentrations higher than 10 mg/L ( n = 12) were all found to have δ 13C-CH 4 values larger than −30‰, typical of a thermogenic origin. Both δ 13C-CH 4 and δD-CH 4 correspond to the isotopic composition of the gas reservoir. Based on analysis of local groundwater flow conditions, this methane is not a remnant but most likely the result of ongoing leakage from the reservoir as a result of the blowout. Progressive enrichment of both δ 13C-CH 4 and δD-CH 4 is observed with increasing distance and decreasing methane concentrations. The calculated isotopic fractionation factors of ε C = 3 and ε D = 54 suggest anaerobic methane oxidation is partly responsible for the observed decrease in concentrations. Elevated dissolved iron and manganese concentrations at the fringe of the methane plume show that oxidation is primarily mediated by the reduction of iron and manganese oxides. Combined, the data reveal the long-term impact that underground gas well blowouts may have on groundwater chemistry, as well as the important role of anaerobic oxidation in controlling the fate of dissolved methane.