DISTINGUISHING REGIONAL-SCALE GROUNDWATER DISCHARGE TO A PERENNIAL RIVER USING A SUITE OF ENVIRONMENTAL TRACERS

A mix of older regional groundwater and modern local groundwater was identified as the sources of baseflow for the Daly River in the Northern Territory of Australia. This study used a suite of environmental tracers to identify the location of groundwater discharge (²²²Rn) and to differentiate water of different residence times (CFCs, SF₆, ⁴He). Tracer concentrations were measured in the river, groundwater, and springs along a 60 km reach of the Daly River. At the location where a group of springs intersected the river, groundwater discharge contained elevated ⁴He and very low concentrations of CFCs and SF₆. The group of springs is located at the terminus of an extensive dolostone aquifer, and the measured ⁴He concentrations indicate residence times on the order of 10,000 yrs. Elevated ⁴He in waters discharging from the springs and from a well completed deep in the geologic basin clearly indicates a regional-scale groundwater source. Upstream and downstream of the springs, the source of baseflow was composed of waters containing SF₆ and CFCs from local groundwater sources adjacent to the river. Close to the river a redox fence was detected in groundwater, and reducing conditions are hypothesized to have degraded CFCs. CFC degradation could have masked the contribution of local groundwater sources without comparison with SF₆. Using the suite of environmental tracers, we conclude that 45% of baseflow originated from a regional groundwater for the 60 km river reach. This study demonstrates that in order to identify multiple sources of baseflow and waters having different residence times, multiple tracers are needed. The knowledge gained from identifying sources of baseflow and mixing between local and regional groundwater advances the conceptual understanding of water cycling in catchments.