Surface water and groundwater interaction in the Vredefort Dome, South Africa

Welguś, Marcja Natalja
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The growing importance of groundwater as freshwater supply in semi-arid areas such as the Vredefort Dome World Heritage Site (VDHS) demands the judicious management of this vital resource. A comprehensive understanding of the mechanisms operating during hydrological connectivity is, therefore, necessary to achieve sustainable groundwater development. An integrated study based on hydrochemical multivariate statistical techniques, baseflow separation, microorganisms, stable water isotopes, and groundwater level mapping has been conducted to investigate the nature of surface water-groundwater (SW-GW) interaction under the influence of anthropogenic practices, coupled with a structurally complex geological setting. Surface water loss to the underlying fracture network was observed in the northern and south-western river reaches, underlain by metamorphosed sedimentary and lava units. These areas exhibit declining groundwater levels that are below the riverbed. The continuous wastewater discharge released into the Vaal River coupled with increased groundwater exploitation may be prompting induced recharge conditions. Analysis of seasonal baseflow from 2013 to 2016 indicates that subsurface contribution occurs primarily during the dry season (BFI ~ 7%) as opposed to the wet season (BFI 1%). The limited baseflow that was detected has an isotopic composition consistent with springs and shallow groundwater near the channel in the south-eastern river reach, underlain by the crystalline basement complex. Principal Component Analysis reveals the combined effect of natural hydrochemical processes and anthropogenic sources as controlling factors of groundwater composition. The microbiological quality of the water shows the prominence of groundwater contamination across the study area, most of which points to faecal sources. The cluster multivariate analysis highlights the multi-source control over the groundwater hydrochemical properties-certain sites proximal to the channel (0 -500 m) are evidently severely compromised from mixing with river water. In some instances, these sites were found to be additionally polluted by human-derived contaminants; in others, sites were solely contaminated by anthropogenic practices. Sites located further from the river channel (> 1 km) demonstrated improved water quality, however, are susceptible to contamination from localised land-use practices. Groundwater sites influenced by the river water are characterised by more enriched isotopic signatures, reflecting more recent, shallow groundwater. In contrast, sites located > 1 km from the river were found to have depleted isotopic compositions, representing deep circulating, colder climate groundwater. This may be attributed to recharge originating from high altitude and/or elevation, possibly under cooler conditions. Recharge occurs through direct and indirect modes, where the latter constitutes evaporation prior to infiltration and mixing of water resources. The results suggest compartmentalisation of the groundwater systems, where certain areas in the vicinity of the channel are not influenced by river-induced contamination, indicating that SW-GW interaction is governed by site-specific hydraulic properties. The interaction weakens gradually with distance and is limited to a maximum distance of 1 km from the channel. This is likely attributed to the topographic controls over groundwater flow direction and the presence of certain barriers governed by the riverbed sediments, as well as regional structural and geological configurations. Regional aquifer connectivity is highlighted by the depleted groundwater isotope signals at various distances from the river. Therefore, it can be deduced that the heterogeneous geological morphology and structure, represented as faults and tectonic zones, facilitate hydrological connection, and serve as significant groundwater drainage systems in the VDHS
A research report submitted to the School of Geosciences, Faculty of Science, University of the Witwatersrand in partial fulfilment of the requirements for the degree of Master of Science in Hydrogeology, 2021