The contribution of physical and chemical sediment characteristics to environmental risk from an irrigation scheme in South Africa

Abstract

The physical and chemical characteristics of sediment from a South African irrigation scheme were used to determine the sediment stressors within the system and to determine whether or not stressors have the potential to contribute to pollutant risk within the system. The information generated also strengthens the database of sediment chemical concentrations in South Africa. The sediment condition at various sites along the Vaalharts Irrigation Scheme (VHIS) was assessed based on physical and chemical characteristics. Sediment samples were collected during three surveys from 2007 to 2009. Physical analysis included organic content and grain size determination. The chemical characteristics were analysed using sequential metal extractions and pesticide analysis. Sediment samples from the Harts River showed increasing organic content along the VHIS while some sediment deposition was evident. Chemical analysis indicated that no pesticide residues were present in the sediment. In general, the sediment metal concentrations indicated lower concentrations than measured at other sites in South Africa. The sequential extractions indicated that cadmium and manganese were contained mostly in the soluble phase resulting in a high to very high risk of metal pollution effects, as measured with the Risk Assessment Code (RAC). A medium risk was identified for cobalt and zinc concentrations. The high fractions of cadmium present in the soluble fraction indicated anthropogenic enrichment, potentially due to phosphate fertiliser which is used in agriculture. Sediment assessments are not routinely applied in South Africa resulting in very little reference or background data available for the area. The metal concentrations for the study area were generally lower than those for other studies in more polluted regions. A low risk for the majority of metals was identified using the RAC methodology with the exception of manganese and cadmium. The study also contributes to the available knowledge surrounding metal pollution in riverine sediments in South Africa.