Ecological risk assessment of trace elements in sediment: A case study from Limpopo, South Africa

This study aimed to determine the ecological risk posed by metals, in sediments from the Nyl River system in Limpopo, South Africa. Metals were extracted from sediment samples by aqua regia microwave digestion and were analysed using standard ICP-OES techniques. The ecological risk indices applied to the data included Contamination Factor, Pollution Load Index, Geo-accumulation Index and Enrichment Factor. The results showed that the levels of Ni at STW and NYL in the HF period exceeded the Canadian Sediment Quality Guidelines by a factor of 1.36 and 1.83 respectively whereas NYL and MDD had 2.57 and 1.32 times the allowed limit of Ni in the LF period. During the HF period, the GC site exceeded the allowed limit of Zn by a factor of 1.04 and NYL had 1.21 times the allowed Zn in the LF period. The levels of metals are generally low near the origin of the river and increase moving downstream. The levels of metals in the Nyl River floodplain, a Ramsar accredited wetland, were high with CF scores ranging between 0.905 and 5.82, Igeo values with a range of -0.541 to 2.441 and EF scores ranging from 0.959 to 6.171. and posed a greater risk than the other sites. This indicated that the wetland is performing its ecological function by trapping and removing toxins from the water body. The Pollution Load Index determined that the Golf Course (PLI=4.586) and STW (PLI=2.617) sites were polluted only in the low flow period whereas the Nyl River floodplain (HF PLI=79.845; LF PLI=30378.768) and Moorddrift Dam (HF PLI=1.903; LF PLI=9.256) sites were polluted in high flow and low flow periods.