Investigating regional recharge dynamics through the use of tritium and radiocarbon isotopes to assess the hydrological resilience of groundwater in southern Africa
Date
2021-03
Authors
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Publisher
Stellenbosch : Stellenbosch University
Abstract
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 (<100km) coastal regions, winter rainfall zones and
deeper depth to groundwater predicted to have lower 3H activities. Conversely, inland localities with shallower depth to
groundwater and/or summer rainfall are predicted to have elevated 3H activities. The predictive surface was then included as
a recharge indicator in a novel groundwater vulnerability assessment, that incorporates both the risk of reducing groundwater
quantity but also a deterioration in quality through modern contamination and sustained salinization. The assessment found
that large portions of South Africa (22.4%) have a very high vulnerability to depletion in quantity and deterioration in quality,
adding that this will likely increase (+3-6%) in the next 50 years as a result of climate change. This thesis outlines the need for
an improved understanding of the vulnerability, sustainability and resilience of South Africa’s groundwater resources and offers
insight through an investigation to the efficacy of 3H and 14C as tracers of regional recharge dynamics.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (PhD)--Stellenbosch University, 2021.
Keywords
Groundwater -- Africa, Southern -- Management, Tritium dating, Carbon -- Isotopes, Groundwater recharge, Water resources development -- South Africa, UCTD