Identifying the contribution of regional groundwater to the baseflow of a tropical river (Daly River, Australia)

A mixture of older regional-scale groundwater flow and relatively modern local-scale groundwater was identified as the source of baseflow to a perennial river in a tropical savanna. Multiple environmental tracers, including 222Rn, CFCs, SF6, and 4He were measured in the river, groundwater, and springs along a 60 km segment of the Daly River in the Northern Territory of Australia. At the location where a group of springs intersected the river, groundwater discharge contained elevated 4He and very low concentrations of CFCs and SF6. This influx of regional-scale groundwater could be detected at downstream locations in the river and was used to parameterize a one-dimensional model for estimating the groundwater discharge flux. Upstream and downstream of the springs, the source of baseflow is composed of waters containing SF6 and CFCs from local-scale groundwater sources adjacent to the river. Within 1 km of the river, a redox fence was detected, with reducing conditions leading to degradation of CFCs that could have masked detecting the contribution of local-scale sources. This study confirmed the applicability of a new technique using 4He to identify regional-scale groundwater flow contributions to rivers, and demonstrated that multi-tracer studies are needed to identify the locations, rates, and sources of baseflow.