Potentially problematic elements in sediment and fish from the Orange-Vaal River system

Abstract

Potentially problematic elements in sediment and fish from the Orange-Vaal River system Levels of Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Ag, Cd, Pt, Au, Hg, Tl, Pb and U were investigated in sediment and fillet tissue (Clarias gariepinus), from sites along the Orange-Vaal River system, South Africa. Sediment sampling sites were chosen up- and downstream of major pollution sources such as mining, industrial and farming areas. The two fish collection sites in the Vaal River were Standerton and Parys and in the Orange River, Boegoeberg and Rooipoort. ICP-MS analysis was done to determine the concentrations the elements. Indices were calculated to describe the quality of the sediment regarding a number of factors: The enrichment factor (Ef) of individual elements evaluated elevation in levels above natural geology whereas the geo-accumulation index (Igeo) determined the degree of pollution by the enrichment levels of the individual elements at each site. The pollution effect of the total mixture of elements at each site was investigated by the elemental contamination index (ECI) and pollution load index (PLI). The ECI is the geometric mean concentrations of elements, while the PLI uses the contamination factor (Cf), that is, how many times the measured concentration is higher than a background value. Ecological risk was determined by calculating the sediment quality guideline index (SQG-I) at each site. The SQG-I is the arithmetic mean of how many times the measured concentration of individual elements at a specific site were higher than a guideline level. In this case, threshold sediment quality guideline values for Australia, the Netherlands, and Canada were used, since South Africa does not have them. In addition, a sediment quality index (SQI) was calculated to incorporate the percentage of elements that did not meet guidelines and the magnitude of exceedance. The bio-accumulation factor (BF) was also calculated between the sediment and the fish. The Igeo and Ef of Se, Ag and Au were the highest at all the sites. The Parys site had the most elements that were enriched to different degrees, but the Rooipoort site had the highest ECI and PLI. The SQG-I indicated that three of the four sites had a moderate chance of posing an ecological risk to its biota. However, the Rooipoort site had a high probability to be toxic to biota. The SQI classified Rooipoort as 'fair' in terms of sediment quality and the rest of the sites as 'good'. The elemental concentrations present within the fish were below the allowed standards for human consumption. Se, Ag, Pt, Au and Hg were the elements that bio-accumulated most in the fish.