Hydrogeological characterisation and groundwater vulnerability to pollution mapping of the Thyspunt Nuclear Site, Eastern Cape
Date
2020
Authors
Mohuba, Seeke Carol
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Abstract
A thorough understanding of the hydrogeological system and its dynamics are crucial to achieve sustainable development in the area. A detailed hydrogeological characterisation and mapping the groundwater vulnerability to pollution of the Thyspunt Nuclear Site is presented based on desktop studies, field investigations and laboratory analyses. The geology of the area comprises of the Table Mountain Group (TMG) and the Bokkeveld Group of the Cape Supergroup, indurated coastal Cenozoic deposits of the Algoa Group and Quaternary to Recent sand deposits. Algoa Group sediments, dunes and fractured quartzite of the TMG comprise unconfined aquifer system in the area. Underlying the unconfined aquifers are confined aquifer composed of fracture TMG quartzites. Additionally, as a result of ductile deformation that resulted in the formation of regional anticline, artesian system has been developed, which is characterised by slightly acidic and oxidising water. The aquifers show high variability in physical properties, including variability within each aquifer type, as indicated by the wide range of values in hydraulic conductivity (0.0115 - 19.13 m/day), transmissivity (0.359-108.3 m2/day) and aquifer thickness (12.85-112.4 m).
The mean annual rainfall ranges between 302 and 922.6 mm (ave. 612.3 mm/year), while the estimated potential evapotranspiration (PET) and the actual evapotranspiration (AET) are 821.5 and 535.9 mm/year, respectively. Annual recharge estimated using the Water Table Fluctuation Method ranges between 0.588 and 72.183 mm/year (ave. 36.092 mm/year). The main hydrochemical facies across the study area is Ca-Mg-HCO3 type. The groundwater hydrochemical evolution appears to be from a fresh Ca-Mg-HCO3 type at shallow depth to saline Na-Cl type water in the direction of groundwater flow from west to east. Similarities in the stable isotope signatures indicate a strong hydraulic link between most aquifers and springs, and a strong link between some of the different aquifers. No evidence of seawater intrusion in the sampled aquifers has been noted. All groundwater samples have depleted isotope signatures, indicating that recharge occurred during colder season or from rainfall originating from a high latitude region. The baseline geochemical analyses indicate that the shales of the Ceres Subgroup and Baviaanskloof Formation contain elevated concentrations of metals, especially Cobalt, Thorium, Lead and Uranium. The highest uranium and thorium concentrations were encountered at the Thyspunt site in the quartzite of the Skurwerburg Formation, where most of the rocks contain highly elevated metals and radionuclides. Moreover, elevated concentrations of uranium were also noted in soils sampled from the footprint area of the site. A conceptual hydrogeological model for the Thyspunt area was proposed based on data from field, laboratory and desktop studies. Multiple hydrostratigraphic units based on lithological logs and aquifer properties exist, where unconfined aquifers with weathered quartzite/sandstone/shale and interstitial sand is the most dominant system underlain by fractured quartzite that is often generates artesian system. Groundwater flow direction is towards the Ocean and hence, helps to buffer the effect of seawater intrusion. The fact that most springs discharge into the sea, coupled with the fold orientation signifies less chance for seawater to penetrate into the aquifers. Based on the DRASTIC model, the foot print area is situated at the medium and high vulnerability zone comprised of Algoa Group sediments underlain by fractured quartzites
Description
A research report submitted in fulfilment of the requirements for the degree Master of Science in the School of Geosciences, Faculty of Science, University of the Witwatersrand, Johannesburg, 2020