Numerical modelling of surface water and groundwater flow and solute interactions between a river and a saline floodplain in a semi-arid region
The Murray River is one of Australia’s longest rivers but in South Australia it has become degraded through river regulation, water extraction and adjacent highland irrigation. These have decreased the natural flood frequency and increased rates of floodplain salinization. Concerns have been raised about the quality of water extracted from the Murray River for industrial, agricultural and potable uses, including for metropolitan Adelaide’s water supply. This has been highlighted as the most significant hydrological risk by the Murray Darling Basin Authority (MDBA) and therefore a comprehensive understanding of flow and solute dynamics within the river and floodplain environment is essential. Hence, this research is aimed at developing a better understanding of surface water (SW) and groundwater (GW) flow and solute interactions in a semi-arid river-floodplain system using a numerical modelling approach.
Collaborating closely with CSIRO, DEWNR, SA Water and NCGRT, this research initially involved a comprehensive review of the current understanding of numerical modelling of salt mobilization arising from SW-GW interactions. The review concluded that the level of understanding of arid and semi-arid environments is still relatively basic, particularly in relation to SW-GW interactions in floodplains. Following this, Clark’s Floodplain in the Lower Murray River was chosen as a study site where sufficient observation facilities were available. The Hydro-Geo-Sphere model was selected for this research because it is a 3D physically-based fully integrated surface-subsurface numerical model with variable saturation and solute transport simulation capabilities. A calibrated model was developed and a number of scenarios were designed to investigate the impacts of different drivers on the river-floodplain system processes such as groundwater-table dynamics, evapo-transpiration (ET), bank storage, regional groundwater recharge and floodplain salinization. The identified drivers include floodplain vegetation cover, groundwater lowering, river stage manipulation, artificial flooding and water injection to the saline floodplain aquifer.
The results show that vegetation cover type can have significant impacts on the flow and solute interaction dynamics due to the influence of ET as a dominant hydrological driver. It was also found that groundwater lowering mitigates the floodplain salinization risk via two mechanisms, namely extraction of the solute mass and creating a divide which stops saline water reaches the floodplain by lowering the groundwater-table. Also, it appears that groundwater extraction is able to remove some of the solute stored in the unsaturated zone. Furthermore, river stage manipulation is beneficial for floodplain health because it amplifies the freshwater lens during high-flow pulses through the mixing of fresh river water with saline groundwater. In addition, it was shown that artificial flooding can temporarily form less saline groundwater and soil profiles that improve water availability for vegetation. However, it was found that this effect is generally limited to the inundated zone. Finally, it was shown that injection of fresh river water to the saline floodplain aquifer may potentially improve soil water availability in the capillary fringe. However, application of this technique seems to be relatively costly and has associated potential problems such as aquifer clogging and well breaching.
To conclude, it appears that all of these salt interception measures are limited spatially and temporally. Indeed, none of these measures are able to permanently change the natural condition of the floodplain groundwater salinity and flow regime. Hence the interventions should be considered only as short term management techniques. However, if longer term strategies are required, it may be possible to implement these salt interception measures periodically. The outcomes of this research contribute to a better understanding of how to maintain a healthier floodplain in arid and semi-arid environments using different available management strategies. This research also provides knowledge regarding the ecological implications of SW-GW flow and solute interactions in a semi-arid river-floodplain system, with a particular focus on the Lower Murray River region.