Présentations Colloques

Session 5.05: Groundwater and Ecosystems – Hydrological role of forests and wetlands
Sigidi (orateur)
Geochemical and isotopic tracing of salinity loads into the RAMSAR listed Verlorenvlei freshwater coastal estuarine lake, Western Cape, South Africa.
This study characterises the nature of salinity within surface water and shallow groundwater in the Verlorenvlei catchment to develop better water management strategies for the RAMSAR listed Verlorenvlei estuarine system. It is assumed that all salts in the Verlorenvlei River are transported downstream to the Verlorenvlei estuarine wetland, and the increase in salt load has led to the degradation of this water resource. Samples were collected on a monthly basis to account for seasonal variation, and major ions, stable isotopes of O and H, strontium isotope ratios, tritium activities, EC, T, pH and selected trace elements were analysed for, to delineate salt load flow paths. The amount of salt contributed from each tributary into the Verlorenvlei River varies considerably- the Krom Antonies River has the lowest salt loads reflected in relatively low EC < 300 mS m+ the Kruismans River is considerably higher, 500 mS m < EC < 2000 mS m+ the Hol River is intermediate with 400 mS m < EC < 1500 mS m+ and the Bergvallei River 500mS m < EC < 800 mS m. The Verlorenvlei River showed lower salt loads, with values between 80 ms m and 300 mS m. Stable isotopes indicate that the Kruismans River is strongly influenced by evaporation, but the Hol River, also with high EC values, does not show the same evaporation trend and suggests a different source of salinity. Major ion data indicate high concentrations of Na+ and Cl- associated with elevated Mg2+. The Krom Antonies and the Begvallei Rivers reflect mean Na Cl molar ratio of 0.56, the Kruismans a mean Na Cl molar ratio of 0.45 while in the Hol and Verlorenvlei the mean Na Cl molar ratio is 0.61.The low Na Cl ratios result in a decrease of Na2+, due to ion exchange with clay minerals associated with the geology of the catchement The high Cl- values are derived from rain and marine aerosols.**
South Africa