Understanding Mg isotope systematics of variably saline water along the west coast of southern Africa
Thesis (MSc)--Stellenbosch University, 2018.
ENGLISH ABSTRACT: Isotope techniques are a highly effective tool for fulfilling critical hydrologic information and are therefore crucial for supporting effective water management. The most frequently used isotopes in hydrology are isotopes of the elements of the water molecule, hydrogen and oxygen, and of the element carbon. These stable environmental isotopes have been used for over four decades in the study of hydrology. With the almost exponential increase in analytical capabilities in recent years, much focus has been on more novel isotope systems to provide further insights into hydrological processes. Magnesium isotopes are a potentially underutilised tool in the study of groundwater hydrology. The principle reasons for this are the technically challenging nature of magnesium isotope analysis and the relatively small fractionation range observed in nature. However, as magnesium is a major component in both water and rock, magnesium isotopes are a theoretically useful tracer. Sixty-six groundwater and surface water samples were collected from the catchments of nine major west-draining river systems along the west coast of southern Africa. Catchments were grouped into three regions based on their proximity to one another. The southern region catchments occur in the Western Cape province of South Africa. These catchments have NaCl-type waters, show intermediate δ 18O and δD values relative to the sample set, with characteristically low δ13C-DIC compositions consistent with soil 13CO2 in C3-vegetated regions. δ26Mg values show enrichment in southern catchment waters, and Mg isotopes are fractionated by means of secondary clay formation in catchment soils. The central region catchments occur in the Northern Cape province, and show similar characteristics to the southern region catchments, however with a strong 87Sr/86Sr component and less negative δ13C-DIC values, which are influenced to a greater extent by atmospheric 13CO2 in this region. The northern region catchments drain carbonate lithologies, and waters show a dominant HCO3 - anion chemistry, with mixed cations. δ13C-DIC compositions in the northern region are the least negative for the sample set, resulting from carbonate dissolution. δ18O and δD values are more depleted relative to the southern and central region catchments, ranging between -5‰ and -7‰ for δ18O and -30‰ to -50‰ for δD. δ26Mg values are also low, and reflect a dominant contribution from carbonate lithologies. The use of Mg isotopes in west coast aquifers has provided insight into the processes controlling fractionation of their isotope ratios, and, when used in conjunction with other isotopic and hydrochemical parameters, provide a better understanding of the hydrological systems, which is essential for the formulation of sustainable resource development and management strategies. KEYWORDS: δ26Mg variation, groundwater and surface water chemistry, baseline, water management strategies.
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