Investigating groundwater–river interactions using environmental tracers

Groundwater and surface water are hydraulically connected in many landscapes, and a better understanding of their connectivity is critical for effective management of water resources. Environmental tracers are a useful preliminary tool to study the interaction between groundwater and surface water and provide independent means for corroborating or refuting information based on more traditional investigations. This paper discusses the results of using major ions, stable isotopes (deuterium and oxygen-18) and a radioactive isotope (radon-222) as environmental tracers to better understand groundwater–surface water interactions in the Border Rivers catchment, Australia. In the upstream reaches of the catchment, shallow groundwater close to the river has a similar major-ion and stable-isotope chemistry to that of the river water, and is different to the groundwater distant from the river. The near-stream groundwater has an enriched isotopic signature (less negative) whereas groundwater far from the river has a depleted isotopic signature. Overall, the comparison of chloride concentrations with deuterium suggests that three types of groundwater occur in the Border Rivers catchment: (i) the near-stream groundwaters influenced by direct recharge from the river; (ii) the groundwaters marginal to the river that are more influenced by diffuse rainfall recharge; and (iii) saline groundwaters in the downstream part of the catchment which never (or rarely) receive recharge from surface water. River water samples obtained during the high-flow season show a very low variation in radon concentrations (0.11–0.39 Bq/L). The longitudinal transect of radon concentration measurements in river water during the high-flow season indicates that there is no groundwater contribution to stream flow. Radon concentrations are lower in groundwater close to the rivers and increase with distance from the river, in general coincidence with the salinity and chloride concentration. This indicates river water infiltration into nearby alluvial aquifers, rather than groundwater discharge to the river. The results of hydrochemical and environmental isotope sampling indicate that in the upper catchment area (upstream of Keetah) the river is connected to and actively recharges the near-stream shallow alluvial aquifer. Using the environmental isotope data, we have also demonstrated that recharge of the alluvial aquifers by surface water occurs by bank infiltration, with diffuse recharge during high-rainfall events more dominant further away from the river. This information would be useful for a better understanding of the nature and extent of hydrogeological processes at the river–aquifer interface and their links with biogeochemical processes and ultimately water allocation policies.