Effects of Dolerite Intrusions on the geohydrology of Karoo sediments in the Mount Frere area, South Africa

Abstract

The Karoo Supergroup has a geohydrological regime which is largely controlled by Jurassic dolerite dyke and sill complexes. The study area is located in the central interior of the Eastern Cape Province, in the former Transkei. The sedimentary rocks of the middle Karoo constitute fractured and intergranular aquifers, due to relatively hydro-conductive lithologies. The main groundwater production targets within the upper Karoo are related to dolerite intrusions that have a number of characteristics that influence groundwater storage and dynamics. Magnetic and electrical resistivity geophysical techniques are used in this study to determine the different physical characteristics of the dolerite intrusions, such as size, orientation and the level of weathering. Trends in the data collected from a large-scale development programme, can provide evidence that intrusion characteristics also play a role in determining the geohydrological characteristics of the area. Interpreted geophysical, borehole drilling, aquifer testing and water chemistry data can be used to indicate geohydrological differences between dolerite intrusions, undisturbed sedimentary formations and structurally controlled lineaments. Using the Flow Characteristics (FC) software, pump testing data is analysed for the presence of no-flow boundaries and to determine sustainable duty cycle yields per successful borehole. The chemical content of each successful borehole is also determined by laboratory analysis. This is done to aid in determining the character of the water as well as determining correlations between basic chemical indicators such as pH, electrical conductivity (EC) and total dissolved solids (TDS) and problematic elements such as fluoride and iron. Observed trends in the data collected could be used to identify more accurately future well field target areas and the development thereof.