Understanding Mg isotope systematics of variably saline water along the west coast of southern Africa

[摘要] ENGLISH ABSTRACT: Isotope techniques are a highly effective tool for fulfilling critical hydrologic information and aretherefore crucial for supporting effective water management. The most frequently used isotopes inhydrology are isotopes of the elements of the water molecule, hydrogen and oxygen, and of the elementcarbon. These stable environmental isotopes have been used for over four decades in the study ofhydrology. With the almost exponential increase in analytical capabilities in recent years, much focus hasbeen on more novel isotope systems to provide further insights into hydrological processes. Magnesiumisotopes are a potentially underutilised tool in the study of groundwater hydrology. The principle reasonsfor this are the technically challenging nature of magnesium isotope analysis and the relatively smallfractionation range observed in nature. However, as magnesium is a major component in both water androck, magnesium isotopes are a theoretically useful tracer. Sixty-six groundwater and surface watersamples were collected from the catchments of nine major west-draining river systems along the westcoast of southern Africa. Catchments were grouped into three regions based on their proximity to oneanother. The southern region catchments occur in the Western Cape province of South Africa. Thesecatchments have NaCl-type waters, show intermediate δ 18O and δD values relative to the sample set, with characteristically low δ13C-DIC compositionsconsistent with soil 13CO2 in C3-vegetated regions. δ26Mg values show enrichment in southerncatchment waters, and Mg isotopes are fractionated by means of secondary clay formation incatchment soils. The central region catchments occur in the Northern Cape province, and show similarcharacteristics to the southern region catchments, however with a strong 87Sr/86Sr component andless negative δ13C-DIC values, which are influenced to a greater extent by atmospheric 13CO2 in thisregion. The northern region catchments drain carbonate lithologies, and waters show a dominantHCO3- anion chemistry, with mixed cations. δ13C-DIC compositions in the northern region are theleast negative for the sample set, resulting from carbonate dissolution. δ18O and δD values are moredepleted relative to the southern and central region catchments, ranging between -5‰ and -7‰ forδ18O and -30‰ to -50‰ for δD. δ26Mg values are also low, and reflect a dominant contributionfrom carbonate lithologies. The use of Mg isotopes in west coast aquifers has provided insight intothe processes controlling fractionation of their isotope ratios, and, when used in conjunction withother isotopic and hydrochemical parameters, provide a better understanding of the hydrologicalsystems, which is essential for the formulation of sustainable resource development andmanagement strategies.KEYWORDS: δ26Mg variation, groundwater and surface water chemistry, baseline, watermanagement strategies.

[发布机构] Stellenbosch University