The Malmesbury Group - an aquifer of surprising significance

Abstract

Abstract The Malmesbury Group has often been overlooked as a useable source of groundwater. This is due to its previously generalised low yields and relatively higher salinities of groundwater found in these weathered to fresh sedimentary and metasedimentary rocks. However, during a period of severe drought in the Western Cape of South Africa from 2016 to 2018, the Malmesbury Group was investigated for groundwater more intensively, mainly out of necessity by those who had little other choice geologically. This study presents two case studies from distinctly different geological and hydrogeological zones within the Malmesbury Group. A selection of 57 boreholes that have been drilled into the Malmesbury Group were used for this study. Of these, 30 boreholes are in close proximity to Paarl while the remaining 27 boreholes are located in the Cape Flats. Although these areas are noticeably different, both sites revealed that the Malmesbury Group could serve as a host to a high yielding aquifer, under certain geological conditions. Borehole yields of up to 9 l/s and 21 l/s were recorded at the Cape Flats and Paarl sites respectively. However in all cases the high yields were due to high yielding fractures in the bedrock, rather than the inherent bedrock properties. At the Paarl site, a correlation is seen between aquifer parameters and their proximity to an inferred extension of the Wellington – Piketberg Fault and Shear Zone from published geological maps. Boreholes in both the Cape Flats and Paarl sites that did not intersect these fractures were found to have similar bedrock yields to previous Malmesbury Group generalisations (<0.5 l/s).