Quantifying near-surface, diffuse groundwater discharge along the south-west margin of the Great Artesian Basin

Few field measurements are available to constrain the diffuse discharge (or vertical leakage) component of the water balance of the Great Artesian Basin (GAB), even though numerical modelling indicates that it accounts for over half of the outflow of the GAB in South Australia. High leakage rates are observed along the southwestern margin of the GAB where the aquifer is close to the surface with the groundwater under artesian pressure. These zones contain both sporadic springs and distinct areas of leakage into the surface environment as shown by salt deposition at surface and very shallow water tables. These leakage zones typically occur along stratigraphic and structural features. Quantifying the component of vertical leakage occurring along the margin of the GAB will better constrain the water balance of the GAB and increase confidence in sustainable yields from this vital water resource. This project uses evapotranspiration monitoring (eddy correlation, microlysimeters) and soil water chloride profiles to estimate groundwater discharge rates. Preliminary data have been collected from the western side of the Peake-Denison Ranges and the northern edge of the Gammon and Willouran Ranges of South Australia. In these areas, diffuse discharge commonly occurs along or near the outcropping aquifer-aquitard. The discharge rates were consistently higher for the areas showing substantial salt crusting at the surface, with flux rates in the order of 10’s mmyr. In contrast, modelled annual flux rates from soil profiles drilled in areas lacking widespread surface salt precipitation were <10 mmyr and similar to previously published rates in the Lake Eyre South region.