Injection of fresh river water into a saline floodplain aquifer as a salt interception measure in a semi-arid environment

•Floodplains in arid and semi-arid environments are ecologically essential.•Salinization has been highlighted as a significant risk for riparian tree health.•River water injection into floodplain aquifer leads to environmental improvement.•A physically-based, fully integrated numerical model was developed and calibrated.•River water injection is only able to maintain a temporary freshwater lens.•For long-term strategy, river water injection needs to be periodically repeated. Floodplains in arid and semi-arid environments are hydrologically and ecologically essential components of the landscape. However, floodplain salinization has been highlighted as a significant risk for riparian tree health and river water quality. While various salt management measures have been developed, some of these have limited application in arid and semi-arid regions because of insufficient infrastructure and limited availability of supplemental environmental flows. Fresh river water injection into a saline floodplain aquifer can lead to environmental improvement using a relatively small amount of water and without the need for water disposal infrastructure. To explore the impacts of fresh river water injection on floodplain salinity, a physically-based, fully integrated numerical model was developed and calibrated against the observed data from a trial conducted in September 2006 at Clark’s Floodplain in the Lower Murray in South Australia. It is shown that injection of an increased volume of river water leads to a larger extent of the subsequent freshwater lens. In addition, it is shown that for a given injection volume, it is more efficient to inject at a lower injection rate and for a longer duration. Also, the interface of the saturated/unsaturated zone appears to be the most effective injection screen depth. Moreover, in this case, a linear configuration of the injection wells was more effective compared to a rectangular configuration. Overall, the fresh river water injection is only able to maintain a temporary and spatially limited local freshwater lens. For long-term salt management, the river water injection needs to be periodically repeated in the absence of overbank flooding in the meantime.