Combined geophysical and analytical methods to estimate offshore freshwater extent
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Offshore fresh groundwater is increasingly suggested as a potential water resource for onshore human demands. In many cases, onshore pumping already draws significant fresh groundwater from offshore. However, offshore aquifers and the extent of offshore freshwater are usually poorly characterised due to data scarcity. This study combines geophysical data, hydraulic information and a first-order mathematical analysis to investigate offshore freshwater extent in the Gambier Embayment (Australia). A large seismic data set, combined with onshore and offshore bore-log geological profiles, are used to explore the regional offshore hydro-stratigraphy. Aquifer hydraulic parameters and onshore heads are obtained from onshore investigations. A novel application of Archie’s law, geophysical data and onshore hydrochemical data provide useful insights into the salinity profiles within four offshore wells. These are compared to steady-state, sharp-interface estimates of the freshwater extent obtained from a recently developed analytical solution, albeit using simplified conceptual models. Salinities derived from resistivity measurements indicate that in the south of the study area, pore water with total dissolved solids (TDS) of 2.2 g L-1 is found up to 13.2 km offshore. Offshore pore-water salinities are more saline in the northern areas, most likely due to thinning of the offshore confining unit. The analytical solution produced freshwater-saltwater interface locations that were approximately consistent with the freshwater-saltwater stratification in two of the offshore wells, although analytical uncertainty is high. This investigation provides a leading example of offshore freshwater evaluation applying multiple techniques, demonstrating both the benefit and uncertainty of geophysical interpretation and analytical solutions of freshwater extent.
© 2019 Published by Elsevier B.V. This manuscript version is made available under the CC-BY-NC-ND 4.0 license: http://creativecommons.org/licenses/by-nc-nd/4.0/ This author accepted manuscript is made available following 24 month embargo from date of publication (June 2019) in accordance with the publisher’s archiving policy