Investigation of factors influencing borehole yields in the Nzhelele-Makhado Area in Limpopo Province, South Africa

Abstract

This dissertation focused on the assessment of borehole yields within the Nzhelele- Makhado area, which is located in the northern part of South Africa within the Vhembe District Municipality of Limpopo Province. The aim of the study was to identify factors that influence the yields of water supply boreholes within the study area. This information will be used to improve the groundwater resource knowledge required in assessing the potential of groundwater resources in augmenting the Nzhelele Regional Water Supply Scheme. The study area is mostly underlain by the ‘hard rock’ formations of the Soutpansberg Group, which practically has no primary porosity. The groundwater is residing mainly within the weathered and fractured or discontinuities, considered being secondary porosities. Due to the complexity of the underlying fractured and hard rock aquifer systems and the fact that most of the boreholes drilled in the area were not scientifically sited, the study area is dominated by very low yielding boreholes. Majority (48%) of the boreholes were drilled into the Nzhelele formation due to the fact that it occupies the central, relatively flat and low lying sections of the study area. The variations in average yields in boreholes drilled in different formations within the study area is relatively low suggesting that the difference in lithology of different formations do not to have any major influence in the yields of boreholes. The topographical settings of the area do not have any influence in the borehole drilling depths and yields. The high borehole yields in shallow boreholes located in mountainous areas is due to local groundwater systems, which recharges and discharges locally. Mapped lineaments are slightly low yielding (average yield of 0.32 l/s) compared to the faults (average yield of 0.43 l/s) within the study area. Boreholes drilled along the NE-SW trending lineaments support double the yields (0.41 l/s) on average of those along the SE-NW (0.28 l/s) and W-E (0.20 l/s) trending lineaments. The high yields in boreholes closer to non-perennial streams compared to perennial rivers is due to the fact that non-perennial streams are comprised of thick layer of overburden capable of supporting high yielding boreholes, whereas the overburden along the perennial rivers are washed away during rainy season leaving bedrock exposed or covered with thin layer of sediments. The proximity to the young faults trending SE-NW and dry non-perennial streams has proved to be the most the favourable areas for development of high yielding boreholes in the study area, compared to lithological difference and topographical settings of the area. However, it should be noted that there are no simple relationship between various factors that control the yield of the boreholes in the area. Despite the similarities in some factors that influence borehole productivity on a regional scale such as faults and drainage systems, the complexity of the weathered-fractured aquifer system suggests an over-riding influence of local features, which results in significant variations in yield and response to abstraction.