Sediment geochemistry, mineral precipitation and clay neoformation on the Mkuze River floodplain, South Africa

The Mkuze Wetland System, situated in northern KwaZulu-Natal, is South Africaʹs largest freshwater wetland area, and acts as a major sink for naturally occurring solutes within the landscape. The mineralogy and geochemistry of 135 sediment samples from two transect sites on the Mkuze River floodplain were investigated to identify processes involved in the control of solute concentrations. Results show that clastic sediment is dominated by silt and fine sand, with clay mineralogy comprising primarily Fe-rich smectite. Smectite is concentrated within the lower zone of sediment profiles where it is believed to be neogenetic in origin. Smectite neoformation is typically associated with the precipitation of CaCO3 and SiO2 which results from the evapotranspirative concentration of groundwater solutes. Evapotranspiration locally lowers water table elevation and plays an important role in controlling the geochemical behaviour of Fe and Mn. Seasonal water level fluctuations result in the accumulation of Fe and Mn oxyhydroxides, which likely favours smectite neoformation. The precipitation of minerals and immobilisation of solutes within the Mkuze River floodplain sediments has important implications for the long-term development of the geomorphology and ecology of the wetland system, as it has potential to alter sediment properties, groundwater composition, salinity, vegetation distribution, and topographic relief.