Environmental Isotope Tracers Constrain River—Groundwater Interactions in a Stream Vulnerable to Mining Impacts

Protection of river flows and riparian ecosystems is reliant on a clear understanding of river–groundwater interactions. Uncertainty regarding these exchanges has hindered understanding of how Australia's largest coal mine will impact baseflow in the Carmichael River and discharge to the Doongmabulla Springs. Environmental isotope tracers ( 2 H, 18 O, 87 Sr/ 86 Sr, 3 H and 36 Cl), major ions and groundwater levels helped to identify the sources of water and the extent of gaining and losing reaches in this poorly‐gauged river system. Elevated δ 18 O values (−3.4‰–0.9‰) and 3 H activities (1.8–1.9 TU) in the river indicate that surface runoff and shallow groundwater are the main water sources upstream of the Doongmabulla springs. Adjacent to the springs, lower 3 H (0.33–0.91 TU) and depleted R 36 Cl values (35.8 × 10 −15 –72.1 × 10 −15 ) indicate that very old (residence time > 500 ka) and intermediate (residence time > 50 ka) groundwater sustains baseflow during the dry season. This same stretch of river periodically recharges underlying groundwater during major flooding events, as indicated by fluctuations in groundwater levels, Cl concentrations and measurable 3 H (0.81 TU) in near‐river groundwater. Downstream, within 1 km of the projected open‐cut mine pit, increased 3 H (0.64–0.70) and R 36 Cl (75.69 × 10 −15 –78.07 × 10 −15 ) in the river suggest there is no additional influx of older regional groundwater into the stream. Groundwater levels and Cl concentrations also suggest that river leakage is less significant near the mine than upstream. This conclusion contradicts the previous conceptualisation of the river, which assumed that river leakage would buffer mining‐related groundwater drawdown along the river valley. Impacts to the Carmichael River and Doongmabulla springs have most likely been under‐represented in numerical modelling and require re‐evaluation. This study highlights the importance of river‐groundwater exchanges in sustainable water management and presents a widely applicable methodology for characterising these exchanges in poorly‐gauged rivers.