Isotopic and elemental ratios to assess the relationship between heuweltjies and saline groundwater in the Northern Cape of South Africa.
Date
2021-12
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
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.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (PhD)--Stellenbosch University, 2021.
Keywords
Natural groundwater salinity -- South Africa -- Northern Cape, Heuweltjies -- South Africa -- Northern Cape, Groundwater -- South Africa -- Northern Cape -- Management, Isotopes -- Environmental aspects, Hydrochemistry, Salinity -- South Africa -- Northern Cape, Arid regions -- Environmental aspects, UCTD