It’s a very small Island with more than 140 distributed bores, so why would geophysics make a difference to our conceptualization of the groundwater resource?

Abstract:

Groundwater resources supporting small indigenous communities in tropical north of Australia are often poorly defined and understood. In part this is linked to their remoteness, the practicalities of undertaking ground investigations in isolated areas. One small community, on South Goulburn Island in Australia’s Northern Territory is wholly reliant on groundwater for its water supply. Recent drought conditions stressed the available supply, particularly at the end of the dry season. Although the Island is peppered with >140 bores, installed through the 80’s and 90’s, options for supplementing the existing supply are required to secure the wellbeing of the community and provide a basis for its limited growth.

To better conceptualise the groundwater resource, aquifer systems present, and provide spatial context for existing bore data, an airborne EM survey was undertaken over the whole Island. This was supported by a ground geophysical investigation involving surface NMR. The inverted AEM data identified a relatively thin (~20m) unconfined “lateritic” aquifer present, developed primarily in weathered Cretaceous mudstones. This freshwater “lens” system overlies a largely unweathered mudstone containing saline groundwater. Most of the island, extending over ~58km², is underlain by this saline aquifer. The AEM indicated that the extent of the freshwater system was ~ 29km², considerably smaller than originally defined. A review of existing bore, pump test and climate data coupled with some limited groundwater tracer work, showed that while the aquifer recharged rapidly in the wet season, it then discharged quickly into the surrounding sea. At the end of the dry, mean standing water levels were often below or at mean sea level, indicating that water levels dropped by ~10m from the end of the wet. The lower part of the freshwater aquifer was determined to be low yielding, supported by the NMR, thereby limiting options for extended development.

Geophysical investigations extended the hydrogeological conceptualisation of the island, helping identify management options for supplementing the existing production bores. Specifically, the AEM helped define locations for the siting of a distributed suite of additional, low yield, production bores that would tap parts of the freshwater system that was not being used.