Articles | Volume 14, issue 1
Research article
17 Jan 2017
Research article |  | 17 Jan 2017

Biogeochemical constraints on the origin of methane in an alluvial aquifer: evidence for the upward migration of methane from underlying coal measures

Charlotte P. Iverach, Sabrina Beckmann, Dioni I. Cendón, Mike Manefield, and Bryce F. J. Kelly

Abstract. Geochemical and microbiological indicators of methane (CH4) production, oxidation and migration processes in groundwater are important to understand when attributing sources of gas. The processes controlling the natural occurrence of CH4 in groundwater must be understood, especially when considering the potential impacts of the global expansion of coal seam gas (CSG) production on groundwater quality and quantity. We use geochemical and microbiological data, along with measurements of CH4 isotopic composition (δ13C-CH4), to determine the processes acting upon CH4 in a freshwater alluvial aquifer that directly overlies coal measures targeted for CSG production in Australia. Measurements of CH4 indicate that there is biogenic CH4 in the aquifer; however, microbial data indicate that there are no methanogenic archaea in the groundwater. In addition, geochemical data, particularly the isotopes of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC), as well as the concentration of SO42−, indicate limited potential for methanogenesis in situ. Microbial community analysis also shows that aerobic oxidation of CH4 occurs in the alluvial aquifer. The combination of microbiological and geochemical indicators suggests that the most likely source of CH4, where it was present in the freshwater aquifer, is the upward migration of CH4 from the underlying coal measures.

Short summary
This research characterised the biogeochemical constraints on the origin of methane in an alluvial aquifer, concluding that the most likely source was the upward migration from a directly underlying coal seam. This research was undertaken due to concerns about the effect of coal seam gas production on groundwater quality in the study area. The implications include the fact that no methane is being produced in the aquifer (in situ) and that there is local natural connectivity in the study area.
Final-revised paper