Integrating major ion geochemistry, stable isotopes (¹⁸O, ²H) and radioactive isotopes (²²²Rn, ¹⁴C, ³⁶Cl, ³H) to understand the interaction between catchment waters and an intermittent river

Determining the locations and sources of baseflow and the transit times of water is important for understanding catchment behaviour and functioning. Major ion geochemistry, stable isotopes (O-18 and H-2), and radioisotopes (Rn-222, H-3, C-14, and Cl-36) were used to investigate the sources and transit times of water in the upper catchment of the intermittent Avoca River in southeast Australia. Rn-222 activities and Cl concentrations implied the presence of baseflow inputs and the distribution was mainly controlled by local topography. Fluctuation of Cl concentrations implied that low-salinity near-river water was an important component of baseflow. The H-3 activities of laterally disconnected pool waters during the summer months were 1.64 to 5.11 TU. The higher of these values exceed those of average annual rainfall (2.8-3.2 TU), probably due to the input of later winter to spring rainfall. The stream water had H-3 activities ranging from 2.21 to 2.40 TU in July and 2.39 to 2.77 TU in August, which yield mean transit times of 4.0 to 7.0 years and 1.4 to 4.8 years respectively. These H-3 activities were significantly higher than those of regional groundwater (H-3 activities <0.1 TU), implying that the river is largely sustained by young near-river stores at all flow conditions. Regional groundwater had C-14 activities of 34.0 to 98.1 pMC, which yield mean residence times of up to 12,900 years. (RCl)-Cl-36 values of regional groundwater (50.9-61.9 x 10(-15)) were higher than those of modern rainfall, probably reflecting the (RCl)-Cl-36 values of recharge. Similar (RCl)-Cl-36 values of the pool and stream water (33.3-58.7 x 10(-15)) implied that some Cl is derived from the regional groundwater influx. As with other intermittent streams in southeast Australia, the upper Avoca River was mainly sustained by relatively small water stores, and it will be vulnerable to short-term changes in climate and land use.