Doctoral Degrees (Earth Sciences)

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Browsing Doctoral Degrees (Earth Sciences) by browse.metadata.advisor "Francis, Michele Louise"

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    Isotopic and elemental ratios to assess the relationship between heuweltjies and saline groundwater in the Northern Cape of South Africa.
    (Stellenbosch : Stellenbosch University, 2021-12) Van Gend, Jani; Miller, Jodie A.; Clarke, Catherine E.; Francis, Michele Louise; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: Globally, groundwater is becoming one of the most important resources. This is specifically the case in semi-arid to arid southern Africa where surface water resources are limited. In the Buffels River catchment, part of a coastal desert and global diversity hotspot in Namaqualand, South Africa, many communities and the local economy are largely dependent on groundwater as the only source of potable water in the region. However, the groundwater is variably saline. In this study, hydrochemistry and stable and radiogenic isotopes from groundwater in the Buffels River catchment is used to determine the origin of salts in the groundwater as well as the mechanism of salinisation. In order to do this, a better understanding of the aquifer systems was required. Basic cation and anion data together with δ¹⁸O, δ²H and ⁸⁷Sr/⁸⁶Sr data indicated that evaporative concentration of salts is not the main contributor to salinisation as previously thought, but that dry deposition of marine aerosols and water-rock interaction are the main sources of salts. Heuweltjies are prominent features in this region and it was known that they generally consist of nutrient rich soils, but geophysics data revealed that these soils are extremely saline with the salinity increasing with depth and towards the centre of the heuweltjie. Thus, heuweltjies are zones where salts accumulate and given that heuweltjies consist of aerated soils and contain tunnels which could act as preferential flow paths, their contribution to salinisation was further investigated. A new groundwater recharge model for was conceptualised which include recharge through heuweltjies, and total mean groundwater ages were calculated using a combination of ¹⁴C and ³H and a lumped parameter approach to understand when recharge has been taking place. The age of groundwater in the Buffels River catchment range between modern and ~18 000 years, with modern fraction of up to 80 %. The relationship between heuweltjie salts and saline groundwater was further investigated by determining the relative depths and ages of the different carbonate horizons. Heuweltjies are up to ~30 000 years old and three distinct wetting fronts, which is an indication of mean annual rainfall amounts, are seen. This proved that heuweltjies act as preferential flow paths and that salts are transported downwards through the centre of the heuweltjies. δ¹⁸O SO4 and δ S SO4 isotope signatures of heuweltjie soils indicated that the salts in 2- 34 2- heuweltjies is directly related to dry deposition of aerosols containing both marine and non-marine- salts. δ¹⁸O SO4 signatures of groundwater hosted in the granitic gneisses are similar to that of the 2- heuweltjies, suggesting that the mechanism of formation of these salts are the same, while the δ34S 2- SO4 signature indicate a “granitic gneiss”-influence. In contrast to this. In areas were the heuweltjie density is high, the δ³⁴S SO4 and δ O SO4 signatures of groundwater and heuweltjie soils are 2- 18 2- comparable indicating that salts stored in heuweltjies are flushed into the aquifer system and that heuweltjies play a role in salinisation of groundwater and have been doing so for thousands of years.