Groundwater methane in a potential coal seam gas extraction region

• This study provides baseline groundwater chemistry prior to coal seam gas extraction.
• Methane may trace coal seam gas water mixing with shallow aquifers.
• Methane distribution was best explained by geology in the Richmond River Catchment.
• Stable carbon isotopes revealed variable methane sources in the catchment.

Study regionThis study investigates dissolved methane distribution in groundwater from the Richmond River Catchment (New South Wales, Australia) before proposed coal seam gas (CSG, or coal bed methane) development.Study focusUnconventional gas exploration has rapidly expanded in recent years. However, the impact of these operations on groundwater systems is poorly understood. A total of 91 groundwater samples were analyzed from 6 geological units. Our observations act as regional baseline research prior to CSG extraction and may assist with long term impact assessment.New hydrological insights for the regionMethane was found in all geological units ranging between 0.26 and 4427 μg L−1 (median 10.68 μg L−1). Median methane concentrations were highest in chloride-type groundwater (13.26 μg L−1, n = 58) while bicarbonate-type groundwater had lower concentrations (3.71 μg L−1). Groundwater from alluvial sediments had significantly higher median methane concentrations (91.46 μg L−1) than groundwater from both the basalt aquifers (0.7 μg L−1) and bedrock aquifers (4.63 μg L−1); indicating geology was a major driver of methane distribution. Methane carbon stable isotope ratios ranged from –90.9‰ to –29.5‰, suggesting a biogenic origin with some methane oxidation. No significant correlations were observed between methane concentrations and redox indicators (nitrate, manganese, iron and sulphate) except between iron and methane in the Lismore Basalt (r2 = 0.66, p < 0.001), implying redox conditions were not the main predictor of methane distribution.