36 Chlorine isotope systematics in saline groundwater in the Buffels River Valley

Van Gend, Jani (2018-03)

Thesis (MSc)--Stellenbosch University, 2018.

Thesis

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.

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