Assessing groundwater-surface water connectivity using radon and major ions prior to coal seam gas development (Richmond River Catchment, Australia)

•Radon indicated that 28 out of 77 surface water segments were gaining groundwater.•Major ions revealed that groundwater discharge accounted for 8–55% of river flows.•Groundwater discharge was more significant following floods than during baseflow.•Geochemical tracers allow quick qualitative assessments in poorly gauged rivers.•Baseline assessments are required prior to unconventional gas development.

Coal seam gas (CSG, or coal bed methane) mining is rapidly growing, with poorly understood impacts on groundwater and surface water systems. Here, we use chemical tracers to investigate groundwater-surface water connectivity in an Australian river system (Richmond River Catchment, New South Wales) prior to CSG extraction but after ∼ 50 exploratory CSG wells were drilled. We performed four surveys of 29 interconnected creek and river sites, over contrasting hydrological conditions. Radon was used to determine if a surface water segment was gaining groundwater. Radon observations over four seasons revealed that 28 out of 77 surface water segments were clearly gaining groundwater, 5 were possibly gaining groundwater and 44 were undetermined. This is equivalent to gaining segments in 333 km (39%) of surface water from the 864 km being investigated. High spatial and temporal variability in groundwater gaining segments was found. Na/Cl ratios were used to determine the fraction of groundwater in surface water. Overall, the groundwater contribution in surface waters was 14–24% higher in post flood conditions than during the other three surveys of baseflow and moderate flow conditions. The results serve as a regional baseline assessment of river water chemistry and groundwater-surface water connectivity prior to the planned development of CSG fields. Our geochemical tracer approach allows for a quick qualitative assessment of groundwater-surface water connectivity in poorly gauged river systems and can define priority locations where groundwater extraction for CSG mining should be carefully managed.