The migration of methane into shallow groundwater is the most cited water quality issue associated with shale gas extraction in Pennsylvania, U.S., and methane contamination has been documented in other shale gas basins across the county. In aquifers impacted by methane migration from oil and gas activities, geochemical evidence suggests anaerobic oxidation of methane (AOM) couples with iron and/or sulfate reduction. While AOM has the potential to attenuate methane contamination, iron reduction coupled with AOM can mobilize harmful species associated with iron oxides (e.g., arsenic). Additionally, sulfide produced via sulfate reduction coupled with AOM may lower metal concentrations via metal sulfide precipitation. As such, the availability of iron oxides/sulfate and respective rates of these reactions are likely an important influence on the geochemical impacts of methane migration nearby oil and gas wells.<br/><br/>We assessed the impacts of AOM coupled with iron and sulfate reduction on aquifer (bio)geochemistry using field, lab, and modeling-based observations. Using the results of microcosm experiments, we fit reactive transport models to investigate the role of reaction kinetics and Fe:S ratios in shaping aquifer redox zonation, as well as how such zonation contributes to the concentrations of methane and iron- and sulfate-reduction related species. Our results indicate Fe:S ratios, modeled using the initial concentrations of goethite and sulfate, determine whether the simulated aquifer reaches a dissolved iron or sulfide-rich chemistry. However, AOM-coupled iron and sulfate reduction play an important role in shaping redox zonation within methane-contaminated aquifers. Our results provide a preliminary framework for predicting the magnitude of redox-related secondary contamination in aquifers contaminated by leaking oil and gas wells.

Methane oxidation and redox-active contaminant mobilization in aquifers impacted by leaking oil and gas wells

ACS Spring 2022

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