Following the 2019 Karun River's flood, this study was conducted to evaluate the impact on the physicochemical characteristics and potentially toxic elements (PTEs) concentrations in the sediments as compared to 2015 pre-flood study. Surficial and core samples were collected from the river bed stretching through Ahvaz city for analysis and evaluation. The physicochemical properties of the sediments and the concentrations of PTEs (Mn, Fe, Cr, Ni, As, Zn, Pb, and Cu) in samples were determined by standard methodology. The PTEs contamination in sediments was assessed by calculating the contamination factor and the contamination degree. The potential ecological risk index (RI) was used to assess the contamination degree and ecological effects of PTEs in sediments. The comparative results showed that the flood reduced the concentration of PTEs and organic carbon in the sediments. The flood also decreased the silt and clay content and increased the acidity and sand content compared to pre-flood samples. The post-flood ecological risk assessment revealed that the sediments remained as low risk as compared to pre-flood state. Reduction of post flood PTEs concentration in the sediments as compared to pre-flood concentrations is probably due to long-term exposure to pollutants in the pre-flood sediments. It appears the reduction in the amount of clay particles and organic carbon (as important adsorbents for pollutants) has resulted in reduction of pollutants in the sediments. Statistical analysis of PTEs in the post-flood sediments showed that the major source of metals is geogenic. It seems that despite being destructive for the inhabitants, the flood, has reduced the amount of pollutants and the ecological risk, in the study area, at least for a while.
Keywords: Ecological risk; Flood; Iran; Karun River; PTEs; Sediment.