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Accueil du site → Doctorat → Afrique du Sud → 2021 → Isotopic and elemental ratios to assess the relationship between heuweltjies and saline groundwater in the Northern Cape of South Africa.

Stellenbosch University (2021)

Isotopic and elemental ratios to assess the relationship between heuweltjies and saline groundwater in the Northern Cape of South Africa.

Van Gend, Jani

Titre : Isotopic and elemental ratios to assess the relationship between heuweltjies and saline groundwater in the Northern Cape of South Africa.

Auteur : Van Gend, Jani

Université de soutenance : Stellenbosch University

Grade : Doctoral Degree (Earth Sciences) 2021

Résumé partiel
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.

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