Improved Predictions of Groundwater Salinisation in the SE of South Australia – A Case Study On the Combined Role of Geology and Geophysics.

In many areas of South Eastern Australia, saline soil-water in the unsaturated zone overlies fresher groundwater. in areas where remnant vegetation has been removed and has been replaced with pasture or cropping, enhanced drainage has resulted, which after a lag time, flushes the saline soil-water down into deeper unconfined aquifer systems that contain fresh groundwater. for an area near Tintinara in south-eastern South Australia, spatial simulations of groundwater salinisation under dryland and irrigation scenarios were developed for a 200 year timeframe using a distributed parameter recharge model. inputs to the model include spatial patterns of near surface clay materials determined from the constrained Inversion of helicopter frequency domain EM data. the AEM data were used to define the spatial distribution and thickness of a near surface clay-rich sedimentary unit. the presence of clay coupled with it’s thickness significantly affects the lag time for the downward movement of saline soil water and the salt flux to the aquifer. Under dryland agriculture leaching of the salt to the aquifer takes between 50 and 200 years, whereas under irrigation leaching takes between 20 and 50 years. Under irrigation the salinity of the groundwater is expected to increase from 1000 – 6000mg/L over the next 25-40 years. While there are no obvious economic measures available to prevent salinisation, the modelling suggests that efficient irrigation on appropriate soils could slow the process.