Conceptual hydrogeological characterisation of the Cedarville Aquifer System, Eastern Cape Province, South Africa

Gqiba, Dumisani Sanele
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The research study focuses on the characterisation of the aquifer, which entails a comprehensive hydrogeological investigation that involves field geological mapping, pumping test data analysis, hydrogeochemical analysis, environmental stable isotope analysis and interpretations of the Cedarville Aquifer. The primary objective of the study is to develop a conceptual hydrogeological model of the Cedarville Aquifer System. Cedarville is situated in the Eastern Cape Province of South Africa and forms a basin-shaped topographical profile filled with Quaternary alluvial deposits referred to as the Cedarville Flats. Management of groundwater resources is of paramount importance in ensuring a sustainable water supply to groundwater-dependent communities such as the Cedarville Community. The sustainable management of groundwater resources requires an in-depth understanding of the aquifer system; hence, the conceptual characterisation of the Cedarville Aquifer System is indispensable. The Cedarville Area is underlain by the rocks of the Tarkastad Subgroup, which belong to the Beaufort Group of the Karoo Supergroup. The underlying rocks consist of fractured alternating sandstone and mudstones intruded by dolerite. The aquifer system constitutes alluvium, which forms an unconfined to semi-confined water-bearing strata and is underlain by the fractured semi-confined to confined aquifer of the Karoo Sedimentary Sequence .Assessment of the dominant water-bearing units and reliable hydrogeological features is essential in the characterisation of the Cedarville Aquifer System. Analyses of joints in the area show that the NE-SW trending joint sets are open and were formed due to tensional stresses. These joints were later intruded by magma, which resulted in the NE-SW striking linear dolerite dykes. Pumping test data were analysed using various analytical models to determine the hydraulic parameters, which exhibit that the transmissivity ranges from 23.9 to 105.3 m2/day with an average storage co-efficient of 0.01. Groundwater recharge was evaluated based on the Chloride Mass Balance (CMB) to the value of 98mm/yr. The hydrochemical analysis demonstrated that ion exchange (Na+and Ca2+ replacement) is the most dominant hydrogeochemical process in the Cedarville Aquifer System. The groundwater in the Cedarville Aquifer System is characterised to be recently recharged groundwater from rainfall based on hydrogeochemical analysis. This is further collaborated by stable environmental isotopes, which characterise groundwater in the Cedarville Aquifer System to be of meteoric origin. The Cedarville Aquifer System is conceptually modelled as a two-layered system consisting of an unconfined to semi-confined alluvium top layer and semi-confined to confined, fractured hard-rock bottom layer. Rainfall recharges the system, which percolates and reaches the contact zone between the two layers of the model resulting in obvious water strikes when intercepted by drilling. The fractured bottom layer is recharged from the top alluvium layer with horizontal fractures transmitting groundwater laterally, resulting in significant water strikes especially in places where fracturing is enhanced by dolerite intrusion. High yielding productive boreholes are attained by drilling deeper and high yields are attributable to more cumulative water strikes being intercepted at depths
A research report submitted to the School of Geosciences, Faculty of Science, University of the Witwatersrand, in partial fulfilment of the requirement for the degree of Master of Science in Hydrogeology, 2021