Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Structure and chemistry of two heuweltjies in areas with contrasting aridity in the Olifants/Doorn catchment : evidence for downward salt movement

Auteur : Hattingh, Magdaleen

Université de soutenance : Stellenbosch University

Grade : Master of Science (MScAgric) 2022

Résumé partiel
The Olifants/Doorn catchment in the West Coast region of South Africa is variably affected by saline groundwater. Other areas along the coast of southern Africa experience similar mean annual precipitation rates but do not display the same variability in saline groundwater. This suggests additional contributions to groundwater salinisation in the West Coast. Regularly spaced earthen mounds, termed heuweltjies (up to 2 m high and about 32 m wide), occur in abundance in the Olifants/Doorn catchment area. These mounds are characterised by significantly higher salinity levels compared to surrounding soils. Variable saline groundwater seems to spatially correspond with salt affected heuweltjies, suggesting that mound salts might be contributing to groundwater salinisation in the region. Two heuweltjies, one in a semi-arid climate (Klawer) and the another in a more Mediterranean climate (Piketberg), were excavated to determine the morphological properties and distribution of soluble salts and ions within the mounds. The study was conducted to determine and compare if salts in heuweltjies with different mean annual precipitation rates could be contributing to groundwater salinisation of the Olifants/Doorn catchment. The mineralogy, soil texture, electrical conductivity, pH, anion and cation profiles and dissolved silica was analysed to determine if these heuweltjies are potentially contributing to the groundwater chemistry. Dominant salts exclusively present in heuweltjie soils were identified in both the mineralogy and modelling of ion concentrations. Calcite (in both heuweltjies) and gypsum (in the mound in Klawer) were identified to be enriched in mound soils. The less soluble calcite was saturated at closer to the surface compared to more soluble gypsum at greater depths in both mounds.

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