Geological and geophysical investigations of groundwater resource in the area of Ndlambe Municipality, Eastern Cape Province, South Africa.

Groundwater is an essential resource contributing extensively to overall total annual supply. However, overexploitation has depleted groundwater availability significantly and also brought about land subsidence at some places. The evaluation of groundwater potential zones is essential in protecting the quality of water and managing of groundwater structures. The Eastern Cape Province is one of the provinces faced with the challenge of supplying clean fresh water to the communities residing in it. Ndlambe municipality is a case in point where people rely on groundwater for agriculture and domestic activities. Few studies have been undertaken to evaluate groundwater resource in the study area. This study aims to assess groundwater resources as a way of achieving the goal of water sustainability. In the southeast of the Cape Fold Belt (CBF), is the Ndlambe municipality underneath folded Cape and Karoo Supergroup rocks. The rock sequence in the region reflects the geological history over a considerable period. In general, the area of study comprises of the Bokkeveld and Witteberg groups. The Bokkeveld comprises of shale along with sandstone whilst the Witteberg Group consists predominantly of quartzites with minor interbedded shales. The aims of the study were to target potential zones for groundwater exploration, analyse zones of lineaments, investigate the networks of fractures and the mineralogy of the area of study. In line with studies for groundwater exploration, a number of methods were undertaken for obtaining the results. These included literature review, remote sensing and geographic information system, thin section analysis, scanning electron microscope (SEM), X-ray fluorescence (XRF), structural analysis and geophysical investigations (magnetic and vertical electrical sounding). Potential zones for groundwater were identified using remote sensing techniques and geographical information systems (GIS). Maps were generated using GIS tools. Geology, slope, lineaments and Normalised Difference Vegetation Index (NDVI) were considered as parameters for determining groundwater potential zones. A geological and geophysical study was conducted in the Ndlambe municipality. Twelve samples for thin sections, 12 samples for XRF analysis and 4 samples for SEM analysis were collected at different locations in the area of study and were analysed. Thin section analysis revealed that quartz arenite has been metamorphosed to quartzite due to high pressure and temperatures and the cracks in the quartz grains are due to pressure and brittle deformation that has in turn enhanced secondary porosity resulting in joints and fractures which are good for groundwater movement. Scanning electron microscopy complemented the findings from optical microscopy by revealing splatter pores which are due to intense fracturing and these contribute to increasing permeability of the rock structures. XRF revealed the chemical composition of various rock types in the area of study. The analysed samples were dominated by silica reflecting their source from weathering of silicate rocks. The Chemical Index of Alteration (CIA) and Al2O3 values of the analysed samples reflected moderate to intensive weathering. In the study area, three main hydrogeologically important structures were taken into account. These included primary structures (bedding), contacts (geological) and secondary structures like fractures and folds. The study area comprises of several faults that are an important factor for porosity and permeability. The study area is dominated by blocky quartz veins that have been crushed due to compression hence higher permeability of groundwater. The Syscal Pro multichannel resistivity equipment was used to conduct the survey using the Schlumberger- Dipole-dipole array on three lines of varying length (350 m, 530 m and 340 m). Data collected were processed using RES2DINV software, producing a subsurface 2-D geoelectrical model from the pseudosections of apparent resistivity. Low resistivities in the study area were inferred to be weathered moist sandstone while low resistivities with depth indicated an overlying zone of fractured sandstone and quartzite which may be hosting some groundwater. An area of low resistivity zones between two large bodies suggests that the massive rock bodies are being weathered and fractured and fractures act as conduits for groundwater. High resistivity values indicate hard rock bodies that are unweathered hence low potential zone for groundwater localisation. The ground magnetic survey was used to delineate the subsurface structures as these control groundwater potential