Groundwater methane in a potential coal seam gas extraction region

Abstract

Study region: This 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 focus: Unconventional 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 region: Methane was found in all geological units ranging between 0.26 and 4427μgL -1 (median 10.68μgL -1). Median methane concentrations were highest in chloride-type groundwater (13.26μgL -1, n=58) while bicarbonate-type groundwater had lower concentrations (3.71μgL -1). Groundwater from alluvial sediments had significantly higher median methane concentrations (91.46μgL -1) than groundwater from both the basalt aquifers (0.7μgL -1) and bedrock aquifers (4.63μgL -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 (r 2 =0.66, p<0.001), implying redox conditions were not the main predictor of methane distribution.