Oral Presentation Australasian Groundwater Conference 2017

Where do the methanogens live? New insights into the origin of methane in the Condamine River alluvial aquifer (#27)

Charlotte P Iverach 1 2 , Sabrina Beckmann 3 , Dioni I Cendón 1 2 , Mike Manefield 3 , Bryce FJ Kelly 1
  1. Connected Waters Initiative Research Centre, UNSW Australia, UNSW Sydney, NSW, Australia
  2. Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, Australia
  3. School of Biotechnology and Biomolecular Sciences, UNSW Australia, UNSW Sydney, NSW, Australia

There is considerable debate about the origin of methane (CH4) in aquifers surrounding coal seam and shale gas exploration and production. Numerous processes control the natural occurrence of CH4 in groundwater. To determine the origins of CH4 in an aquifer both the geochemical and microbial community need to be characterised, however this is rarely done. We use geochemical and microbiological data, along with measurements of CH4 isotopic composition (δ13C-CH4), to determine the processes acting upon CH4 in the Condamine River Alluvial Aquifer, a freshwater aquifer that directly overlies coal measures targeted for coal seam gas production in Australia.

Groundwater samples for geochemical and microbiological analyses were collected from private irrigation boreholes in the Condamine Catchment. The groundwater was analysed for the major ions, water stable isotopes (δ2H and δ18O), the isotopic composition of dissolved organic carbon (δ13CDOC) and dissolved inorganic carbon (δ13CDIC). Quantitative real-time PCR was used to determine abundances of bacterial and archaeal 16S rRNA gene targets and functional gene targets in the groundwater. 

Measurements of CH4 concentration and isotopic composition suggest that there is biogenic CH4 in the aquifer, however microbial community analysis indicates that there are no methanogenic archaea in the groundwater. In addition, geochemical data, particularly the isotopes of DIC and, as well as the concentration of SO42-, indicate limited potential for methanogenesis in situ. Microbial community analysis also showed that aerobic oxidation of CH4 is occurring in the alluvial aquifer despite the absence of a microbial pathway to produce the CH4.

The combination of microbiological and geochemical indicators suggests that the most likely source of CH4, where it was present in the deeper freshwater aquifer, is the upward migration of CH4 from the underlying coal measures.

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