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University of Cape Town (2000)

A geochemical investigation of the Darling and Ysterfontein saline pans, Western Cape, South Africa

Smith, Meris

Titre : A geochemical investigation of the Darling and Ysterfontein saline pans, Western Cape, South Africa

Auteur : Smith, Meris.

Université de soutenance : University of Cape Town

Grade : Master of Science (MS) 2000

Résumé
Saline pans are an component of the cycle in arid areas, and are common in South Africa. Natural saline pans on the coastal lowlands of the Western Province, South Africa, were studied to determine the origin and evolution of salts within the pans. h,,,,ri·,,,·,, stream and water as well as pan sediments were collected from two large coastal pans near Ysterfontein, and smaller pans 10- 20 km from the sea, on the inland side of the Darling hills. Pan waters have between 2 and 379 gIL of total dissolved salts (TDS) and are neutral to with a of6.5 to 9.2 buffered carbonate precipitation. Inland pans can be divided into (168 - 379 giL TDS) or brackish to saline types (2 64 gIL TDS). 0180 and 5D values indicate a meteoric of water, with moderate evaporation of pan water. Ionic dominance and ion ratios of elements in the brackish-saline, coastal and stream/spring samples are similar to seawater. The order of cation dominance is Na »Mg »Ca and anion dominance is With the of F, Si, alkalinity and U, all elements behave in streams and springs, brackish-saline pans and coastal pans. Calcium, alkalinity and per’hal)S F are depleted in all samples due to precipitation of calcite. Removal of elements from the brine-type pans is due to precipitation of """"-" .. ’" gypsum and fluorite HCO), immobilisation by redox S) and ion or clay mineral formation Si, K, Rb, Ba). Modelling using PHRQPITZ and PHREEQC indicates that pan waters are slightly undersaturated with to halite, but have reached eqUilibrium with gypsum, barite, quartz and are su )en ;atllTalted with respect to calcite, "V’V’U"’-, epsomite and kaolinite. Evaporite mineral zonation in the inland pans shows the following stratigraphic sequence with depth : low-Mg calcite high-Mg calcite - gypsum consistent with concentration of water capillary The sequence in coastal pans is low gypsum precipitated by evaporation of seawater. Precipitation of calcite and gypsum in the sediments permanently removes ions from solution whereas halite is not nTF’’’PTVP l in the sediments and dissolves on an annual basis. Salts in the catchment soils by ev tporatJlon of coastal rainfall are flushed out by heavy salinising the regional groundwater and contributing nmp’rTIIV pans. Seasonal influxes of this brackish ground- and runoff-water result in accumulation of salts in the pans. Major ions S04) in inflow waters are derived from coastal rainfall and marine aerosols. Chemical contributes additional Ca and S to the inflow water, and is the main source and trace elements U). Inland pans are estimated to be no more than a few thousand years old while coastal pans are relict Pleistocene marine pvotn"’r,,t<·< : currently cut off from the sea and being fed by terrestrial water. Most pan waters are unfit for any human related use because of the extreme concentrations of ions 4642 Concentrations of Se - 47 IJ.mol/kg) and B 7310 IJ.mol/kg) are sufficiently waters to be a hazard to wildlife and the livestock farmed in the area, a exacerbated by the from pans, and removal of natural from the hillsides. in pan of halite Salts from coastal rainfall modified by contributions from chemical "’’’11’’’’’’’5 of As the pans evolve, mineral precipitation, ion exchange and redox processes affect the chemical "’5’’’’’’"’’’’ ofthe pan.

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