THE USE OF GEOCHEMICAL TRACING METHODS TO ASSESS SEDIMENT TRANSPORT AND PROVENANCE WITHIN THE MKABELA CATCHMENT, SOUTH AFRICA

Geochemical fingerprinting and provenance studies were carried out for the Mkabela basin to attain an understanding of pollutant source(s), transport, and storage processes within a representative agricultural catchment in the KwaZulu-Natal Midlands of South Africa. The provenance studies, based on a refined, multivariate mixing model, suggest that silt- and clay-rich layers found within wetland and reservoir deposits are derived from the erosion of fine-grained, valley bottom soils frequently utilized as vegetable fields. Coarser-grained deposits within the wetlands and reservoirs were presumably derived from the erosion of sandier hillslope soils extensively utilized for sugar cane. Erosion of these upland cane fields occurs during relatively high magnitude runoff events that are capable of transporting sand-sized sediment off the slopes, and which create reservoir deposits lacking significant fine particles. Thus, the source of sediment to the axial valley varies as a function of runoff magnitude. Cu and Zn were also used as a sediment tracer to assess downvalley transport processes as their concentrations were elevated within fertilizers used on headwater vegetable plots. Comparison of down core variations in both elements at selected location along the valley, combined with changes in sediment source through time, indicate that sediment transport through the basin was significantly altered in the 1990s. The alteration was associated with the construction of a drainage ditch through a previously unchanneled mid-basin wetland that altered the hydrologic connectivity of the catchment. Prior to ditch construction, sediment was largely deposited in wetlands that encompassed a majority of the valley floor within upper catchment areas. Following construction of the ditch, sediments were transported from the headwaters through downstream wetlands and dams (reservoirs) to a low-gradient alluvial channel bordered by an extensive riparian zone. The axial drainage system is now geomorphically and hydrologically connected during most flood events. The study indicates that increased valley connectivity partly negated the positive benefits of controlling sediment/nutrient exports from the catchment by means of upland based, best management practices.