Articles | Volume 18, issue 12
Research article
12 Dec 2014
Research article |  | 12 Dec 2014

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

N. P. Unland, I. Cartwright, D. I. Cendón, and R. Chisari

Abstract. Bank exchange processes within 50 m of the Tambo River, southeast Australia, have been investigated through the combined use of 3H and 14C. Groundwater residence times increase towards the Tambo River, which suggests the absence of significant bank storage. Major ion concentrations and δ2H and δ18O 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 covariance between 3H and 14C indicates the leakage and mixing between old (~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.

Short summary
Periodic flooding of rivers should result in increased groundwater recharge near rivers and thus - younger and fresher groundwater near rivers. This study found the age and salinity of shallow groundwater to increase with proximity to the Tambo River in South East Australia. This appears to be due to the upwelling of older, regional groundwater closer the river. Other chemical parameters are consistent with this. This is a process that may be occurring in other similar river systems.