Investigating regional recharge dynamics through the use of tritium and radiocarbon isotopes to assess the hydrological resilience of groundwater in southern Africa

Thesis (PhD)--Stellenbosch University, 2021.ENGLISH ABSTRACT: Water resources are integral in the global pursuit of equitable growth and sustainability. Although groundwater is the most abundant freshwater resource accessible to humans and the environment, it is often underrepresented in water management strategies and policy. Furthermore, groundwater reservoirs are not only vulnerable to contamination, but to depletion through climate change, land use change and increased water demand. As groundwater monitoring in southern Africa is sparse and long-term records are rarely available, it is difficult to constrain the groundwater system on a regional scale. Radioactive isotope tracers have proven effective in estimating recharge and residence times of groundwater over a local to global scale and could provide insight to regional groundwater vulnerability in an otherwise data poor region. The use of tritium (3H) and radiocarbon (14C) isotopes requires a comprehensive understanding of initial tracer abundance in recharge as well as subsurface physiochemical processes that affect the accuracy of predicted recharge rates and residence times. The release of 3H and 14C from thermo-nuclear bomb tests during the 1950’s and 1960’s changed the applications of these tracers in hydrology. However, the ‘bomb peak’ has been mostly attenuated in modern times and the return of atmospheric tracers to background levels presents new challenges and applications in hydrological studies. To constrain the natural variability of 3H in South African rainfall, backward trajectories of ‘water parcels’ that produced sampled rainfall events were modelled to assess the controls of water mass origin on 3H activity. It was found that ~90% of 3H variability could be completely or partially explained by the origin of the water mass. By understanding the variability of 3H in South African rainfall, the theoretical activity of 3H in groundwater can be predicted through lumped parameter models. When combined with 14C theoretical abundances, measured tracer abundance can be assessed as the proportion of modern groundwater and age of the fossil component in major aquifers across southern Africa. It was found that aquifers across southern Africa have diverse proportions of mixed groundwater (0.6 - 100%) that are mixed within a wide range of older groundwater, that range in calculated ages (105 – 59400 years) and mean residence times for each aquifer system (4698 - 27841 years). The distribution of 3H activity in groundwater across South Africa was modelled using geostatistics that evaluated the effects of 3H variability in precipitation and unsaturated zone travel time. The predicted 3H surface agrees well to expected controls, with proximal (