Characterisation of a Coastal Aquifer System in the Eyre Peninsula, South Australia, Using Nuclear Magnetic Resonance Methods

Abstract

The coastal aquifers of the Uley Basin, which are the most important source of potable groundwater for the Eyre Peninsula, consist of unconfined Quaternary limestone overlying Tertiary clays and sandstones. Despite its importance, elements of the connectivity and total water resource basin remain relatively poorly understood. To address this, hydrogeophysical methods have been employed to better characterise the aquifer systems present. Interpretation of airborne electromagnetic data provided evidence for the delineation of the base of the Quaternary (limestone) aquifer and a basement low in the southwest corner of the South Uley Groundwater Lens, where there is a limited number of lithological bores or groundwater wells. The basement low, adjacent to the coast, suggests a preferential groundwater flow path and a possible connection between the Basin aquifers and the Southern Ocean.

Geophysical methods are routinely employed for groundwater exploration, assessment, and aquifer characterisation, particularly where access to land is limited and where other investigation techniques such as drilling may be limited or prohibited. In areas of environmental significance, or where access is generally difficult, non-invasive hydrogeophysical methods offer an alternative to exploratory drilling, by targeting areas of interest and better defining groundwater and aquifer characteristics in advance. We discuss the application of the hydrogeophysical technique of surface nuclear magnetic resonance (sNMR) for groundwater assessment. Presently, sNMR is the only hydrogeophysical technique that allows for direct detection of groundwater in the subsurface.

To better understand the possible inter-connectivity between the Uley Basin and the Southern Ocean; and as a precursor to considering whether new groundwater resources could be tapped, we investigate a series of (sNMR) soundings along a transect and also at locations where information about the aquifer is better known. We confirm the presence of a Quaternary limestone aquifer containing potable water, extending 1–2 km across the south western corner of the Uley South Basin. The aquifer is defined to be about 15–20 m in thickness and possesses an effective porosity of around 20 %; it overlies clay layers that separate the limestone from a brackish Tertiary sandstone aquifer beneath. In conjunction with conductivity-depth sections derived from AEM data, our sNMR results deliver completely new knowledge and extend the hydrogeological understanding of this corner of the Uley Basin at a relatively low cost and minimal environmental impact.