Thesis Abstract Lusanda Vinqi

MSc Student: Ms LUSANDA VINQI

Academic supervisors: Dr Thokozani Kanyerere, PhD & Dr Kevin Pietersen, PhD

Thesis Title: Assessing the role of groundwater recharge in semi-arid catchments, Hout River Catchment, Limpopo Province, South Africa

Abstract

Many countries in sub-Sahara African region are characterised by complex crystalline basement aquifers where groundwater explorations remain complex. This is due to poor understanding on groundwater process [Recharge-flow-discharge process] and its associated interaction in groundwater system. At a local scale, crystalline basement aquifers are heterogeneous and anisotropic due to fractures, geological structures and discontinuities including varying hydraulic characteristics which need to be understood. The lack of detailed investigations of site-specific conditions to assess influence on groundwater recharge process limits implementation of initiatives for groundwater abstraction that supports environment and socio-economic projects. Based such a gap the current study characterised and conceptualised site-specific conditions to demonstrate their influence on groundwater process. The Hout River catchment in Limpopo province of South Africa was used as a case study. In this study the argument is that site specific conditions are essential to improve understanding about their influence on groundwater processes. To validate such argument, the study (i) characterised the aquifer system i.e. recharge and flow, (ii) determined discharge sites i.e. sources of water along the river reach and (iii) conceptualised the influence of local geological characteristics on hydraulic parameters and river flows. Record review and hydrogeological field work were conducted to collect correct data. Analytical and laboratory-based methods were used for analysis and interpretation of geophysics, geological, groundwater level, pumping test, hydro-chemical and environmental stable isotopic data sets. Insights from geophysics results showed that groundwater occurrence is characterized by a multiple layer of varying depths inferred to be caused by different levels of weathering, geology and fractures. Furthermore, VES sections constructed from field measurements suggested a high groundwater potential zone to be mostly in range of 30m to 72m, and being mostly controlled by highly weathered pegmatite lineaments characterizing the study area.

Groundwater distribution pattern is constrained by geological structures as showed by weak correlation (R2=0.53) between depth to groundwater level and topography. Theoretically, this implies that groundwater occurrence was dominated by compartmentalization through control of structural features, and that flow was controlled by geological structures. Transmissivity values ranged from 37m²/day to 63.53m²/day. The diagnostic and derivative curves, shown on log-log plots indicated that there are several changes in the flow-controlling mechanism suggesting a well-connected fracture network. Linear flow regime and wellbore storage effect alternate during early times of pumping while late time data showed half unit slope associated with linear flow conditions to a well in a channel aquifer. All boreholes showed a similar pattern except HO4-3125 which suggested boundary conditions associated with closed no flow boundary. Using hydrochemical data, results showed that Na-HCO₃ was the dominant water composition for river water while Na-Cl was for groundwater. Environmental stable isotope data indicated river samples in areas with outcrops of geological structures having depleted δ¹⁸O (- 3.88 to - 5.12‰) and δ²H (-21.0 to -19.9‰) signatures similar to groundwater indicating a stable and continuous groundwater contribution to river flows. High