Noble gas and radiocarbon constraints on the residence times of groundwater in and around the City of Cape Town

Thesis (MSc)--Stellenbosch University, 2020.ENGLISH ABSTRACT: The TMG Aquifer is one of the largest aquifer systems in the country and is currently targeted as a potential source of potable water for the City of Cape Town (CoCT) in times of water stress. This study has aimed to provide proper constraints on turnover time of groundwater in the aquifer system before large-scale abstraction takes place, in order to evaluate the sustainability of the resource. This study has used the 3H/3He system to date modern water (500 years). Groundwater residence times were determined for six aquifers in the Western Cape, namely the alluvial, Witteberg, Bokkeveld, TMG, Cape Granite Suite (CGS) and Malmesbury aquifers. Noble gas and radiocarbon dating of the TMG Aquifer range from 0.3 – 44.6 and <70 – 1 626 years, respectively. Residence times calculated for the alluvial aquifer are also relatively young, ranging from 0.0 – 1 783 years. Good correlation between 3H/3He and 14C ages indicate relatively short residence times for the alluvial and TMG aquifers whereas groundwater from the Witteberg (6 699 years), Bokkeveld (58.5 – 6 038 years), CGS (3.7 – 8 648 years) and Malmesbury (45.5 – 11 146 years) aquifers indicate mixing of older water bodies with modern recharge resulting in distinctly different ages derived from the two dating systems. Noble gas recharge temperatures (9.4 – 28.6°C), 3H and 14C activities suggest that groundwater in the region is primarily recharged at high elevations, but also that rainfall events further down the flow path at lower elevations, make substantial contributions to groundwater recharge. Evaluation of isotopic tracers and basic chemistry have led to the identification of potential proxies for residence time, namely pH, HCO3- concentrations and Ca/Mg ratios. These components can be used to make assumptions on groundwater residence times without the costly process of analysis. Residence times indicate short turnover times for groundwater in the TMG Aquifer (typically