Novel Combined Toxicity Indices (CTI) to assess the ecological risk of metals in sediments dependent on fractions and total metal content, application on Qarun Lake sediment, case study

Fractions and total contents of 21 heavy metals were evaluated in Qarun Lake sediment, a closed and shallow turbid saline lake. The metal contents in the sediments were in the following order: Fe > Al > Ti > Mn > Sr > Zn > Cr > Pb > Ba > Ni > B > Cu > Li > Co > As > Ga > Cd > Se > Bi > Ag > Hg. Several indices based on the total content (T-indices) or the fraction ratios (F-indices) assessed the pollution degree of the sediment. All the indices used revealed that the sediment of Qarun Lake suffers from different degrees of metal pollution. The T-indices evaluate the pollution or potential risk depending on the total content of metals, regardless of the species of these metals (hazardous mobile or non-mobile forms). Also, the F-indices evaluate pollution depending on the fraction ratios of metals, regardless of their metal content (even if at a deficient level). This study suggested a new combined toxicity index (CTI) and mean CTI (m-CTI) to combine T-indices and F-indices to assess the potential toxicity of metals and the pollution levels of sites, respectively. Based on integrated T-indices, all selected sites in Qarun Lake were extremely or highly polluted. Also, the GCF (as F-index) revealed that the investigated metals highly contaminated all sites. In contrast, according to the m-CTI, sites 1, 3, and 4 were highly polluted, while sites 2, 5, and 6 were moderately polluted. The present study's findings indicate that using CTI and m-CTI may be more favorable and accurate in assessing the metal pollution status of sediment than using T-indices or F-indices separately.