Assessing Connectivity Between an Overlying Aquifer and a Coal Seam Gas Resource Using Methane Isotopes, Dissolved Organic Carbon and Tritium

Coal seam gas (CSG) production can have an impact on groundwater quality and quantity in adjacent or overlying aquifers. To assess this impact we need to determine the background groundwater chemistry and to map geological pathways of hydraulic connectivity between aquifers. In south-east Queensland (Qld), Australia, a globally important CSG exploration and production province, we mapped hydraulic connectivity between the Walloon Coal Measures (WCM, the target formation for gas production) and the overlying Condamine River Alluvial Aquifer (CRAA), using groundwater methane (CH 4 ) concentration and isotopic composition (δ 13 C-CH 4 ), groundwater tritium ( 3 H) and dissolved organic carbon (DOC) concentration. A continuous mobile CH 4 survey adjacent to CSG developments was used to determine the source signature of CH 4 derived from the WCM. Trends in groundwater δ 13 C-CH 4 versus CH 4 concentration, in association with DOC concentration and 3 H analysis, identify locations where CH 4 in the groundwater of the CRAA most likely originates from the WCM. The methodology is widely applicable in unconventional gas development regions worldwide for providing an early indicator of geological pathways of hydraulic connectivity.