AbstractCoal 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 (CH₄) concentration and isotopic composition (δ¹³C-CH₄), groundwater tritium (³H) and dissolved organic carbon (DOC) concentration. A continuous mobile CH₄ survey adjacent to CSG developments was used to determine the source signature of CH₄ derived from the WCM. Trends in groundwater δ¹³C-CH₄ versus CH₄ concentration, in association with DOC concentration and ³H analysis, identify locations where CH₄ 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.