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An integrated geophysical survey of groundwater resources, Mamre Area, western Cape Province

CSIR, PO Box 320, Stellenbosch 7600, e-mail: lsoltau{at}csir.co.za

N.L. Anderson

Department of Geology and Geophysics, University of Missouri-Rolla, 125 McNutt Hall, Rolla, Mo. 65409-0410, e-mail: nanders{at}umr.edu

The town of Mamre is located on the water-scarce western coast of South Africa, and has been completely dependent on treated (softened and chlorinated) groundwater from the Atlantis wellfield, located about 10 km to the south. Atlantis wellfield water has also been used for livestock watering and irrigation. In an effort to conserve potable water reserves within the Atlantis wellfield and reduce the cost of irrigation water, locally occurring groundwater was investigated as an alternative source of irrigation water. The fractured zone within the Malmesbury shale in a study area proximal to Mamre was identified as the target for groundwater exploration, based on previously acquired borehole and geophysical data. To assess the potential for development of an aquifer in the Malmesbury shale within the study site and with a view to identifying prospective borehole locations, an integrated geophysical data set was acquired. The non-invasive techniques used, were shallow reflection seismics, electromagnetics, and resistivity. The interpretation of the geophysical data was constrained by previously acquired resistivity data and borehole control. The integrated geophysical survey was successful. The seismic data were used to quantitatively determine depth to bedrock and the electromagnetic and resistivity data were used to qualitatively delineate zones of deeper weathering. Areas prospective for the presence of fractured Malmesbury shale (with associated enhanced porosity and permeability) were identified. Priority locations for exploratory boreholes were also delineated.