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University of Cape Town (2005)

Geochemical study of soil salinity in a toposequence near Riebeeck West, South Africa

Folefoc, Asongbecap Daniel

Titre : Geochemical study of soil salinity in a toposequence near Riebeeck West, South Africa

Auteur : Folefoc, Asongbecap Daniel

Université de soutenance : University of Cape Town.

Grade : Master of Science (MS) in Environmental Geochemistry 2005

Résumé
A geochemical investigation of salt distribution in soils of the semi-arid of Western Cape, South Africa, was carried out following a pilot study which revealed widespread soil salinity in the Berg River catchment. The present study looks at the distribution of the salts along a toposequence in a subcatchment of the Berg River underlain by Malmesbury Group shale. The objectives were : to investigate salt distribution in relation to landscape topography ; to identify potentially harmful trace elements associated with the salinity ; and to determine the processes responsible for salt distribution. Ten soil profiles were dug to a depth of 200 cm at points along a slope of angle 10.20 along and 367 m long. The profiles at the crest of the toposequence are underlain by silcrete and alluvium while Malmesbury shale parent material underlies the rest of the profiles. Contour drains disrupt the natural shape of the slope. The soil texture is loamy sand to clay loam at the crest and loamy to sandy-clay loam in the midslope. Mineralogy at the crest is quartz dominated and kaolinite occurs in the mid and lower slope. The pH(H20 & KCI) were measured in a 1 :2.5 soil:water/1 M KCI suspension and it was found that the soils were acidic at the crest ( pH(KCI) < 4.5) ; alkaline (pH( KCI) >8 ) in the midslope and moderately acidic to neutral (pH(KCI) 5.4-6.9) at the footslope and valley bottom. The saturated paste extract (SPE) results indicated highest ECe of 10.97dS/m at the midslope while the crest and valley bottom segments had highest ECe of 0.54 and 3.75 dS/m respectively. In general the order of dominance for soluble cations was Na +>Ca2+>Mg2+>K+ and anions cr>SO/->N03->F, although samples from the footslope and valley bottom were dominated by Ca2+ and SO/-. NH40Ac extractable cations had highest concentrations in order of dominance Ca2+>Mg2+>Na+>K+. The midslope had highest concentrations of soluble and exchangeable ions. Trace elements (Se, B & As) were determined by ICP-MS and were found to concentrate within the salts in the midslope but not to detrimental levels. Eluviation, leaching at the crest and footslope, and evaporation are responsible for the observed salt distribution in this study. Most soils in this study are salt-affected although those at the crest are categorized as normal (ECe < 4dS/m ; ESP < 15%). Soils in the mid and lower segments of the toposequence are either saline-sodic or sodic (ECe < 4dS/m, ESP > 15%). There is need for immediate reclamation and preventive measures to be put in place if agricultural activities are to be continued in a sustainable manner.

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