Geochemical and isotopic tracing of salinity loads into the Ramsar listed Verlorenvlei freshwater estuarine lake, Western Cape, South Africa

Sigidi, Nthabeliseni Thendo (2018-03)

Thesis (MSc)--Stellenbosch University, 2018.


ENGLISH ABSTRACT: The Verlorenvlei estuarine lake is a natural, semi-fresh lake along the West Coast of South Africa. The lake is situated in a semi-arid region, and is subject to high evaporation rates of up to 2400mm, and average rainfall of less than 300mm/per year. The lake is also an important host for diversified species of flora and fauna. Due to normal functions of the lake, variation in salinity of the lake is due to several factors. These include: (1) ingress and mixing with sea water; (2) floods, droughts and evaporation due to climate change and (3) salt transported into the lake from terrestrial sources. For the long term health and management of the lake, it is required to understand the role of terrestrial salinity and transport processes into the lake. Therefore, the study focuses on four main tributaries which are the Krom Antonies, Hol, Bergvallei, and the Kruismans. These four tributaries join at the Verloren confluence and thereafter form a single river system called the Verloren River. The Verloren River and its tributaries drain on predominantly fine- grained rocks of the Malmesbury group (contain high concentration of salts) and of the Table Mountain Group. The geology of this area has an important bearing on the composition of the water that enters the lake. To understand the origin and processes of salinity for surface water and shallow groundwater to the Verloren River, analysis of coupled major ion hydrochemistry and environmental isotopes (δ18O, δ2H, 87Sr/86Sr and 3H) has been undertaken. To fully evaluate the contribution from each tributary, the TDS values in both the surface and shallow groundwater are quantified, the daily overall discharge (surface runoff, interflow and baseflow) in each tributary has been established. Interaction with the aquifer matrix, evaporation, and precipitation-dissolution control the hydrochemistry of surface water and shallow groundwater. Due to the dominance of Na and Cl ions surface water and shallow groundwater can be characterised as Na-Cl water type. The presence of Mg and Ca ions in surface water and shallow groundwater corresponds to dissolution of dolomite limestones. The decrease in Ca2+ and increase in Na+, is a result of cation exchange processes and weathering of feldspars and micas. High sulphate concentrations correspond to dissolution of gypsum and nitrate concentrations can be related to the use of fertilisers. Evaporation for surface water and shallow groundwater in the Hol, Bergvallei and Kruismans is a dominant source for salt concentration, therefore increasing Na and Cl ions in water. The use of strontium isotopes indicates mixing between the Hol and the Bergvallei as source of observed salinity and water composition for surface and shallow groundwater in the Verloren confluence. The results show that the Hol contributes the least salt load (5.3%), followed by the Krom Antonies (6.2%), then the Bergvallei (35.0%) and the Kruismans contribute the most salt load (53.5%), into the Verloren confluence.

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