Multi-isotope studies investigating recharge and inter-aquifer connectivity in coal seam gas areas (Qld, NSW) and shale gas areas (NT)

Large sedimentary basins with multiple aquifer systems like the Great Artesian Basin and the Beetaloo Sub-Basin are associated with large time and spatial scales for regional groundwater flow and mixing effects from inter-aquifer exchange. This makes them difficult to study using traditional hydrogeological investigation techniques. In continental onshore Australia, such sedimentary aquifer systems can also be important freshwater resources. These resources have become increasingly stressed because of growing demand and use of groundwater by multiple industries (e.g. stock, irrigation, mining, oil and gas). The social licence to operate for extractive oil and gas industries increasingly requires robust and reliable scientific evidence on the degree to which the target formations are vertically and laterally hydraulically separated from the aquifers supplying fresh water for stock and agricultural use. The complexity of such groundwater interactions can only be interpreted by applying multiple lines of evidence including environmental isotopes, hydrochemistry, hydrogeological and geophysical observations. We present an overview of multi-tracer studies from coal seam gas areas (Queensland and New South Wales) or areas targeted for shale gas development (Northern Territory). The focus was to investigate recharge to surficial karst and deep confined aquifer systems before industrial extraction on time scales of decades up to one million years and aquifer inter-connectivity at the formation scale. A systematic and consistent methodology is applied for the different case study areas aimed at building robust conceptual hydrogeological models that inform groundwater management and groundwater modelling. The tracer studies provided (i) in all areas increased confidence around recharge estimates, (ii) evidence for a dual-porosity flow system in the Hutton Sandstone (Queensland) and (iii) new insights into the connectivity, or lack thereof, of flow systems.