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Heavy metals are considered as one of the most important environmental threats, especially for aquatic ecosystems, due to their toxicity, stability, widespread distribution and bioaccumulation in the food chains. Urban runoff is the main non-point source of heavy metal emissions to receiving environments, which its quality and evaluation is particular importance. The major part of Tehran’s surface runoff, especially the northern and eastern catchment, is transferred to the southern areas by drainage network and affects receiving environment including surface water, groundwater and sensitive ecosystems such as Band- e- Alikhan wetland. The aim of this study was to investigate the content of heavy metals in urban runoff and compare the efficiency of the index of Iran’s surface water resources quality toxic parameters with the Contamination Index, heavy metal evaluation index and Heavy metal pollution index. For this purpose, sampling was performed from the outlet of Tehran’s Sorkheh – Hesar catchment during three flood events in 2018- 19 and the concentration of Arsenic, Iron, Lead, Zinc, Cadmium, Chromium, Copper, Manganese, Molybdenum and Nickel were measured by ICP-MS. Iron, Manganese, Zinc, Lead, Copper, Chromium, Nickel, Arsenic, Molybdenum and Cadmium had the highest abundance in all samples, respectively The values of the index of Iran’s surface water resources quality toxic parameters, Contamination Index, heavy metal evaluation index and Heavy metal pollution index were in the range of 5.5 to 68.2, 9.6 to 74.4, 83.2 to 192.7 and 8.8 to 23.7, respectively. The contamination index indicated that all samples were in the highly contaminated class, while according to the and heavy metal evaluation index and Heavy metal pollution index indices, 67 and 86 percent of the samples were highly contaminated, respectively. Also, the index of Iran’s surface water resources quality toxic parameters values showed that 70 percent of the samples are in very bad quality class, and the other samples are in bad quality class. In general, a comparison of the results of the studied indices indicate that most of the samples were polluted. The results of the present study showed that the studied indices presented similar results in determining the contamination status of most samples. The results of Spearman's rank correlation coefficient showed strong correlation (0.775-0.999) between all of four indices in runoff, which confirmed the similar behavior of the indices in determining the contamination status of the samples. In more detail, comparing the values of the indices with each other showed that their response to a significant increase in the concentration of elements is different. So that, the changes in the value of the Iran’s surface water resources quality toxic parameters gradually decrease with increasing the concentration of elements and its value is fixed in very high levels of pollution, but the values of the other three indices increase linearly with increasing the concentration of elements. Comparison of the results of the indices for the runoff quality assessment, showed that index of Iran’s surface water resources quality toxic parameters had the highest affinity with the pollution index, Heavy metal pollution index and heavy metal evaluation index, respectively, which shows the appropriate efficiency for evaluating these streams. Thus, the index is suitable for evaluation of heavy metal contamination in urban storm-water runoff due to high separation of pollution degree and less susceptibility to very high concentrations. It is recommended that, more elements in a wide range of concentrations should be considered in order to performance evaluation of the indices in future researches.

Keywords: Contamination index, heavy metals, urban runoff, flood event, Tehran

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