ANALYSIS OF SUBSURFACE MOUND SPRING CONNECTIVITY USING ERI, WESTERN MARGIN GAB, SOUTH AUSTRALIA

Mound Springs in South Australia provide the primary discharge mechanism for waters of the Western Margin of the Great Artesian Basin (GAB). The GAB is the largest groundwater basin in Australia covering 22% of the continent. These springs are an important resource both for the environment and the Australian people, but their hydraulics are poorly defined. The springs have a range of morphologies and discharge rates and can include extensive tails (groundwater-dependent wetlands associated with the springs) and hundreds of springs in a given spring complex. Electrical Resistivity Imaging (ERI) was used to evaluate mound spring subsurface hydraulic connectivity characteristics at three spring complexes. The results demonstrate that GAB water appears as resistors in the subsurface at these sites with high salinity and electrically conductive surface conditions. Using an empirical method developed for this work, the ERI data indicate that the spring complexes have multiple subsurface connections that are not always easily observed at the surface. The connections are focused along structural deformation allowing fluids to migrate through the confining Bulldog shale. The data also suggest that extensive spring tail wetland systems may not be the result of a single discharge point flowing over the surface, but include secondary subsurface discharge points along the tail. The hydrogeologic conceptual model developed from the ERI and other data provides a basis for hydraulic modeling of the mound springs.