IDENTIFYING THE CAUSES OF SHALLOW GROUNDWATER SALINIZATION IN SE AUSTRALIA USING PALEO-CLIMATICALLY DRIVEN HYDROGEOLOGIC MODELING OF GROUNDWATER AGE

This study reconstructs the salinization history of the southwestern Murray Basin, in southeastern Australia, using a hydrogeologic model driven by paleo-climatic reconstructions of ground-water recharge for the last 50,000 years. In large (100's of km), shallow groundwater settings the pattern of salinization is often a primary function of the paleo-climatic history of the basin. In these settings, high salinity fluids are generated in groundwater fed lakes under arid and semi-arid conditions. Yet the location of groundwater supported lakes depends on the geometric relationship between the water-table and the land surface which can produce counter-intuitive results. For example, high groundwater recharge rates under wetter climatic conditions would likely result in higher water-table levels which may support saline lake basins (and their concomitant saline reflux) that are dry under drier climates.

We force the finite volume hydrogeologic model SECOFL_3D with a simplified reconstruction of the drying of the climate that that has occurred over the last 50,000 years. SECOFL_3D was specifically designed to accurately describe the fluctuations of water-tables in large sedimentary basins by using a moving boundary. Groundwater age dates calculated using the modified method of characteristics were compared to the model results.

We tested two hypotheses regarding the origin of groundwater salinity. One assumes large-scale fluid migration from the basin margin, while the other is based on enhanced freshwater recharge in sandy regions along the flanks of the basin. Results support the latter hypothesis but suggest that flow geometry resulting from the interaction of the water table and the land surface is responsible for the location of enhanced freshwater recharge.