An assessment of the extent of mercury pollution of the Mngceweni stream, the Umgeni River and the Inanda Dam in Kwa Zulu-Natal

Abstract

The debate surrounding mercury pollution of the river system below Thor Chemicals in Kwa Zulu Natal, South Africa has been of concern to the people of the region for several years. No scientific assessment of Mercury pollution has been carried out to date, other then a study conducted by Johnston et al. (1991) in 1990 in an area restricted to within 5 kilometres of the plant. Due to elevated levels being detected directly below the plant, Johnston et al. (1991) have expressed concern that this could result in the mobilisation of Mercury into the ecosystem and possibly threaten the Ecology of the Inanda Dam which is located approximately 20 kilometres south of Thor Chemicals which is a major reservoir for Durban. Added to this, the community residing in the area has expressed a great deal of concern and the matter was taken up in parliament in 1998. These factors provide impetus for a follow up study to determine the extent of mercury transformation into the higher trophic levels in the area surrounding Thor Chemicals. In order to quantify the extent to which mercury has become mobilised in the ecosystem and to identify possible pathways of exposure to the community, samples of sediment, algae, cattle hair and fish were taken at ten sample sites. These sample sites were selected at intervals along the Mngceweni and Umgeni River. The first sampling site was situated below Thor and the final sample site was located at the head of the Inanda Dam. Sediment, algae, cattle hair and fish were specifically chosen as they are considered to be reliable indicators of mercury pollution and mobilization. Fish,form an important part of the diet of the community residing in the study area. Mercury concentrations for the above parameters, were compared to mercury concentrations found in the control area, upstream from Thor, as well as to international and local standards. Composite sediment samples taken at the first sample site, within 500 metres of Thor Chemicals, revealed a mercury level of 54j.lg/gram. Mercury concentrations detected in the remainder of the composite sediment samples were significantly lower and revealed similar levels of magnitude throughout the rest of the study area. The higher concentration of mercury in the sediment at the site directly below Thor Chemicals may be attributed to a high