Occurrence and origin of groundwater methane in the Stellarton Basin, Nova Scotia, Canada

Highlights: • Groundwater from 45 domestic wells in the coal-bearing Stellarton Basin, Nova Scotia, had methane levels of up to 73 mg/L. • Isotopic data indicate a primarily microbial origin of the methane, with trace thermogenic gases. • Thermogenic components of gas may originate from deeper coal and/or oil shales, suggesting possible gas migration scenarios. • Atmospheric measurements of methane indicated gas seepage pathways along building foundations. Groundwater methane (CH₄) in areas of fossil fuel development has been a recent focus of study as high CH₄ concentrations pose water quality concerns and potential explosive hazards. In 2013, a provincial study in Nova Scotia identified areas with elevated groundwater CH₄. However, due to limited data, the specific sources and local distribution of CH₄ in those areas remain unknown. In this study, we examined the Stellarton Basin in central Nova Scotia, Canada, a region with an abundance of coal formations, numerous abandoned coal mines, and an active open pit coal mine. Methane was detected in 94% of water samples that were sampled from 45 private water wells. Six water wells exceeded the 28 mg/L hazard mitigation threshold with CH₄ levels of up to 72.7 mg/L. The δ¹³C_CH4 (−85.5 to −48.5‰) and the δ²H_CH4 (−280 to −88‰) indicated that >95% of samples had CH₄ of microbial origin. However, the detection of ethane (C₂H₆) up to 2.97 mg/L and propane (C₃H₈) up to 0.008 mg/L, as well as the δ¹³C_C2H6 values (−30.1 to −15.6‰) suggested a mixture of microbial CH₄ with trace thermogenic gas, likely migrated from Stellarton coals (δ¹³C_C2H6 of −27.6 to −15.35‰). A mobile greenhouse gas analyzer survey was conducted within the perimeter of residences and off-gassing from taps had atmospheric CH₄ measurements as high as 66 ppmv. This study integrates multiple sampling and monitoring methods to investigate groundwater CH₄ in a coal-bearing region. The findings advance the understanding of the origin and occurrence of CH₄ in complex groundwater systems. The data acquired in this study may be used as a pre-drill baseline for groundwater CH₄ concentrations and origins should coal-bed methane operations in Nova Scotia proceed in the future.