Browsing by Author "Hattingh, Magdaleen"

Now showing 1 - 1 of 1
  • Thumbnail Image
    Item
    Structure and chemistry of two heuweltjies in areas with contrasting aridity in the Olifants/Doorn catchment: evidence for downward salt movement
    (Stellenbosch : Stellenbosch University, 2022-04) Hattingh, Magdaleen; Clarke, Catherine E.; Francis, Michele Louise; Miller, Jodie A.; Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science.
    ENGLISH ABSTRACT: 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. This sequence of precipitation of increasing soluble salts suggested that the net direction of water movement occurs downward in mounds of both high rainfall and lower rainfall areas. The clay mineralogy did not support the parent material as a provenance. Additionally, increased coarse soil texture with depth and large macropores of heuweltjies indicated that groundwater is an unlikely cause of mound salinity. The chemical signature of salts indicated a marine origin. Concentrated hotspots of ions and minerals within biogenic features of mound soils suggested that burrowing fauna are responsible for accumulating marine-derived salts in heuweltjie soils. Preferential flow paths that aid solute movement were observed in gravelly dorbank, fractured platy dorbank and as termite channels and rodent burrows in the mound centres of both Piketberg and Klawer. This study reinforced the hypothesis that heuweltjie salts are generated within mounds, possibly through biological activity, and that salts are potentially translocated to the groundwater through preferential flow paths in mounds of the Olifants/Doorn catchment.