A preliminary analysis of water chemistry of the Mkuze Wetland System, KwaZulu-Natal: a mass balance approach

Abstract

The Mkuze Wetland System in northern KwaZulu-Natal constitutes an important source of freshwater to Lake St Lucia. The St Lucia System, including both the Mkuze Wetland System and Lake St Lucia, is recognised as a Wetland of International Importance under the Ramsar Convention. The long-term survival of Lake St Lucia depends on an adequate supply of freshwater. Consequently, the role of the Mkuze Wetland System in water quality enhancement is essential. In order to investigate the water chemistry of this system, water samples were collected throughout the study area from surface water, groundwater, pan and reed swamp sites, as well as a rainwater sample. These were analysed for chloride, sodium, potassium, calcium, magnesium, iron and silicon. Four main water bodies were identified using the multivariate techniques, classification and ordination. Of these four groups, two represented the major water sources to the system, these being the Mkuze River and floodplain and the Mbazwane System. The third represented outflow into Lake St Lucia, while the fourth represented scattered ground and surface water samples some distance away from the main watercourses. The solute concentrations of the two water sources were compared using a mass-balance approach. Chloride was identified as a conserved solute and the increasing chloride concentration from the less concentrated inflows (especially the Mbazwane System) to the concentrated outflow into Lake St Lucia, is considered to reflect evapotranspiration. By considering chloride to be conserved, the degree of evaporative enrichment was estimated in the Mkuze Wetland System and used to determine the percentage of solutes retained in the swamp. The wetland was found to be an important sink for calcium (~50% retention), potassium (~70% retention) and silicon (~80% retention), with magnesium and sodium being retained to a lesser extent. The TDS value supports these findings as it is lower than expected in the outflow by a similar percentage to the magnesium and sodium retention. The removal of solutes by the Mkuze Wetland System, not only has far-reaching implications for Lake St Lucia in maintaining a freshwater supply, but could also have significant impacts on the ecology and geomorphology of the Mkuze System itself. The fate of solutes and the processes of solute retention are subjects of ongoing research.


WaterSA Vol.28(1) 2002: 1-12