Geochemical data analysis and evaluation of ecological risk index of potentially toxic elements in surface sediments of Karun River, Vays to Ahvaz

Rivers are highly variable systems and one of the most important water ecosystems that play an important role in the life of humans and other living beings and are easily affected by pollution. (Maanan et al., 2015). The concentration of heavy metals in sediments is generally several times that of water bodies, and heavy metals are not destroyed and decomposed during natural processes, therefore, heavy metals can be stored in sediments and remain there for a long time. For this reason, surface sediments are one of the main reservoirs of heavy metals and other pollutants (Islam et al., 2016). The Karoun River is not immune from these pollutions and many wastes from metal, petrochemical and oil, cellulose and food industries, domestic and hospital sewage and agriculture are discharged into this water environment. On the other hand, this river supplies water for these industries and is a source of drinking water for cities such as Ahvaz, Khorramshahr and Abadan. The fishes of this great river are one of the main sources of nutrition for the people of the region, as a result of the presence of pollutants, especially potentially fertilizing elements in Karoun, it can cause pollution of sediment, water and food and affect the life cycle of this great river. damage (Rastmanesh et al. 2015). Considering the quantitative extent and diversity of human activities in the Khuzestan plain and the entry of all kinds of effluents and sewage into the Karoun River, investigating the pollution, pollutants and risk levels is an inevitable necessity. Therefore, this research aims to determine the level of enrichment of elements, to determine the ecological risk and dangerousness of pollution in comparison with the quality standards of sediments in the Karoun river in the Weis basin to the beginning of Ahvaz city using geochemical data and statistical analysis. It was done by XLSTAT2018 software. 

In order to evaluate the pollution of the sediments of the Karoun River bed, 22 samples were taken with a boat from a depth of 0-10 cm and based on common methods in sedimentary geology Tucker (1988) and Arzani (1997) of sediments was harvested. Then, the samples were prepared in the water and soil laboratory of the Soil Conservation and Watershed Research Institute for the granulation test and determination of the concentration of toxic metal elements. In order to investigate the state of sediment pollution, the obtained concentrations have been compared with the standard values of ISQGs, severe effect level (SEL), probable effect level (PEL) and global base values. 

According to the results obtained from the grading, the sediments of the river bed in most of the stations have a granular texture and are of the type of mud, silty sand and sandy mud. These sediments have an average of 0.62% organic matter in terms of organic matter. The results of the pollution level also showed that the concentration of toxic elements As, Cr and Ni have values beyond the standard values of ISQGs and PEL. Based on the comparison between sediment quality criteria and standards, Karun River in the studied area has clean sediments from the point of view of Cd and Pb elements. The results of calculation of enrichment factor showed that Zn element has moderate to significant enrichment. The enrichment of Cu, Ni and Cr was in the range of moderate pollution. The semi-metal As is in the range of low to medium enrichment and the two elements Cd and Pb have low enrichment. The average enrichment of elements is Zn>Ni>Cu>Cr>As>Pb>Cd. Finally, the results obtained from the risk index showed that the values of the risk index for all samples are in the low risk range (RI<150). The statistical analysis of the samples showed that there is a significant positive relationship between Cr, Ni, Cu and AS with clay. So clay particles are the main carriers of Cr, Ni, Cu and AS elements. A high correlation coefficient between elements indicates a common source, mutual dependence, and the same behavior during transportation. 

The current research leads to sufficient understanding of the geochemical situation and any change from natural conditions and revealing the local and thematic enrichment of pollution levels for aquatic animals and users, especially in the field of agriculture and food cycle in the Karun River in the Vays basin to the city of Ahvaz. has been The results of the enrichment factor showed low to high pollution for selected elements. The potential ecological risk values of all selected elements except As in sample 21 are in the low risk range, and AS in sample number 21  is in the medium risk range . The risk index values for all samples are in the low risk range . Based on the comparison of sediment quality criteria with the standards, Karoun River has clean sediments from the point of view of Cd and Pb elements in the studied period. So that 100 percent of the samples have a concentration lower than ISQG. Cr with 27% and Ni with 100% has a concentration beyond the PEL pollution level and all the values obtained for Cr and Ni elements have a concentration beyond the minimum concentration of the ISQG pollution level. In this way, there is a possibility of poisoning for aquatic animals and water exploitation by the toxic elements Ni and Cr. Comparing the concentration of elements with the standards showed that in stations number seven (subordinate to Mahi Shiban), 21 (east coast of KianPars) and 10 (Kouresh sewage), Cr and Ni elements have concentrations beyond the PEL pollution level and more As. It is from the minimum level of ISQG and the most polluted stations are in the study period. So, the maximum enrichment of Cr and As has also happened in station number seven. Element clustering analysis showed that organic materials are the main carriers of Cu and Zn elements and clay particles are the main carriers of Ni and Cr elements. Also, in the case of Cu and As elements, clay particles play the main role. But the toxic metal Pb has not shown any significant relationship with other elements as well as organic materials and clay particles. So Pb element has a different origin than Cu, Zn, Cr, Ni and As elements. The results of principal component analysis, while confirming the correlation coefficient and cluster analysis, showed that Cu, Zn, Cr, Ni, and As are of anthropogenic origin, and Pb and Cd are of terrestrial origin. This study has an important contribution in determining the origin, pollution and ecological risk of potential fertilizing elements and can help in identifying pollutant sources and pollutant control.