Residence times and mixing of water in river banks: implications for recharge and groundwater-surface water exchange

Bank exchange processes within 50 m of the Tambo River, southeast Australia, have been investigated through the combined use of H-3 and C-14. Groundwater residence times increase towards the Tambo River, which suggests the absence of significant bank storage. Major ion concentrations and delta H-2 and delta O-18 values of bank water also indicate that bank infiltration does not significantly impact groundwater chemistry under baseflow and post-flood conditions, suggesting that the gaining nature of the river may be driving the return of bank storage water back into the Tambo River within days of peak flood conditions. The co-variance between H-3 and C-14 indicates the leakage and mixing between old (similar to 17 200 years) groundwater from a semi-confined aquifer and younger groundwater (< 100 years) near the river, where confining layers are less prevalent. It is likely that the upward infiltration of deeper groundwater from the semi-confined aquifer during flooding limits bank infiltration. Furthermore, the more saline deeper groundwater likely controls the geochemistry of water in the river bank, minimising the chemical impact that bank infiltration has in this setting. These processes, coupled with the strongly gaining nature of the Tambo River are likely to be the factors reducing the chemical impact of bank storage in this setting. This study illustrates the complex nature of river groundwater interactions and the potential downfall in assuming simple or idealised conditions when conducting hydrogeological studies.