Rivers in arid and semiarid regions may traverse saline aquifers, creating buoyant freshwater lenses in the adjoining riparian and floodplain zones. The River Murray in South Australia is an example, as the groundwater within the floodplain is naturally highly saline, but lenses of lower salinity exist adjacent to the river and creeks. These lenses provide an additional water source to vegetation such as river red gum and black box. Changes to the management of the River Murray and its floodplain can alter the extent and stability of the lenses, potentially impacting the health of the floodplain ecosystem. A study is investigating lens dynamics through hydrogeochemistry, laboratory studies, and modelling; this presentation discusses the modelling results so far.
A series of analytical solutions were developed to describe the interface between fresh and saline waters within the floodplain, under static conditions with and without dispersion. The analysis was compared with laboratory results from sand tanks. A numerical model simulating variable-density groundwater flow and transport was developed to explore a wider range of conditions than is possible with analytical methods.
The analytical solutions and sand tank experiments demonstrate that it is possibly for a low-salinity lens to form even when the river is under gaining conditions. Deeper aquifers, more transmissive riverbeds, and larger freshwater-saltwater density differences produce more extensive lenses. The numerical model illustrates the importance of transient effects such as changing river levels, floodplain inundation, and evapotranspiration. The analytical solutions provide a screening method for the occurrence and long-term stability of low-salinity lenses while the numerical modelling is being used to assess the potential impacts of floodplain management in South Australia.