Browsing by Author "Van Gend, Jani"
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- Item36 Chlorine isotope systematics in saline groundwater in the Buffels River Valley(Stellenbosch : Stellenbosch University, 2018-03) Van Gend, Jani; Miller, Jodie A.; Clarke, Catherine E.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: It is typical for coastal aquifers in arid regions to be affected by salinisation, populations in these areas that rely upon groundwater resources are often directly afflicted by this phenomenon. Large parts of the western coast of South Africa are affected by variably saline groundwater as a result of varying degrees of salinisation, primarily driven by evaporative processes. Initial stable δ2H and δ18O isotopic investigations suggest rainfall and groundwaters carry evaporative signatures, further investigation has suggested that there are other significant salt contributors in the region, although these sources and pathways are poorly constrained. One such alternate salt source may arise from paleo-termite mounds, called heuweltjies, that are found in areas along the west coast. These structures typically consist of salt- and nutrient-rich sediments and the recorded sediment EC values for these structures are an order of magnitude higher than that of the adjacent interheuweltjies. Furthermore, sediment EC values of heuweltjies increase with depth and the difference between deep and shallow EC values in heuweltjies are between 1000 and 3000 μS/cm, with the deeper samples having the higher EC value. The possibility of heuweltjie salts entering the groundwater system and contributing to groundwater salinisation in this area has not been investigated as yet. The town of Buffels Rivier in the western region of the Northern Cape has limited surface water resources and is dependent on local groundwater for subsistence. For this reason, it is an ideal site to investigate these atypical salinisation drivers. In order to effectively isolate the additional salt sources, several geochemical, isotopic and geophysical methods were implemented. Highly variable soil and groundwater EC values were observed in the field area. Furthermore, 36Cl isotope ratios together with noble gas data provides some evidence that aquifer mixing occurs. This is further evidenced by the spatial discontinuity in groundwater 87Sr/86Sr ratios. The geology of this area consist of basement granite gneisses and the contribution of rock salt to the groundwater cannot be excluded. 87Sr/86Sr ratios in groundwater in the Buffels River Valley are elevated, ranging between 0.73030 and 0.78240, which is typically associated with the water-rock interaction granitic rocks. The extent of the salt contribution from the various systems in the Buffels River Valley is still not fully understood but it is evident that conventional, semi-arid salinisation through evaporation is not the only driver in this regional system.
- ItemIsotopic 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.