Extended Abstract
 
Less than 1% of the Earth's water resources are suitable for human consumption, and clean water is essential for sustaining life and human activities. However, factors such as population growth, agriculture, industrialization, urbanization, and poor management have degraded water quality, rendering it unsuitable for consumption. Heavy metal pollution in aquatic environments is particularly concerning due to its direct impact on living organisms and indirect effects on human health. Toxic heavy metals, like cadmium (Cd), chromium (Cr), nickel (Ni), arsenic (As), lead (Pb), and iron (Fe), accumulate in fat and liver tissues of humans and animals, adversely affecting the nervous, blood circulatory, and immune systems. Some of these metals, such as Cd and Cr, are also carcinogenic. Therefore, accurate determination of the heavy metals concentration and monitoring water quality is crucial. The Karun-Dez river basin, located in southern Iran, is a critical water resource. The Dez River, a major tributary of the Karun River, supplies water to numerous cities and villages, thousands of acres of farmland, several fish farming projects, and industrial factories. The Karun River is vital for industries such as metal, petrochemical, and oil, which are situated along its banks. This river provides drinking water for cities like Ahvaz, Abadan, and Khorramshahr, and fish from the river are a primary source of protein for the local population, making heavy metal contamination a significant threat to food safety. As a result, monitoring heavy metal levels in the Karun and Dez rivers has become a focus for researchers. This study measured the concentrations of heavy metals in the Karun and Dez rivers, which supply water to treatment plants 1 to 5 in Ahvaz. Additionally, the outlet flows of these treatment plants and six distribution areas were examined. The metal levels were compared with World Health Organization standards, and water quality was assessed for heavy metal pollution using recognized global indices.
The study area encompasses the Karun and Dez rivers in Khuzestan Province. Given the vast extent of this region and the critical importance of river water quality for various uses, the section of the river serving as the water intake for the drinking water treatment plants of Ahvaz has been designated as the focus area. This study examined treatment plants No. 1, to 5 in Ahvaz, as well as six points from the city's distribution network. Sampling was conducted in two seasons: the wet season (winter 2022) and the dry season (summer 2023), from specified stations with three repetitions each. Samples were collected in acid-washed polyethylene containers, and the concentration of metals in each sample was determined using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, model 730-ES, Varian). The results were plotted using GraphPad Prism software version 9 and compared with standard levels of metals in drinking water according to Iranian National Standards No. ISIRI1053, the World Health Organization (WHO, 2008) guidelines, the US Environmental Protection Agency, and Health Canada standards. The difference in heavy metal concentrations between the wet and dry seasons was analyzed using the T-test with SPSS software version 17. The Heavy Metal Pollution Index (HPI) and the Heavy Metal Evaluation Index (HEI), were determined for all inlet and outlet samples of the treatment plants and distribution points. The Metal Index (MI) was used to evaluate the potential impact of overall heavy metal pollution on public health and to aid in the rapid estimation of drinking water quality. Additionally, the degree of contamination (Cd), calculated as the sum of the contamination factors of individual components exceeding the permissible limit, was determined for all samples in both the wet and dry seasons.
The measurement results showed that the levels of metals Ag, As, Cd, Co, Hg, Pb, Sb, and Sn were zero in all inlet and outlet samples, and at six distribution points during both wet and dry seasons. Metals Ba, Cr, Cu, Mn, Mo, Ni, and Zn were detected below permissible standard limits in all samples from treatment plants and distribution points, with concentrations higher in the wet season than in the dry season (p<0.05). Concentrations of Cr, Cu, Mn, and Ni were significantly lower in the outlet samples from treatment plants compared to the inlet samples (p<0.05), indicated effective removal by the treatment plants. However, none of the treatment plants effectively removed Ba and Mo. The concentrations of Ti and V in all samples were below 0.19 ppm and 0.009 ppm, respectively. Their reduction in the outlet samples (p<0.05) suggested effective removal. During the wet season, Al levels exceeded permissible limits (p<0.05), primarily due to mineral waste released during thunderstorms and non-point sources. The presence of Al in outlet samples was linked to the use of polyaluminum chloride as a coagulant during treatment. Fe concentrations significantly decreased from inlet to outlet samples (p<0.05) and remained below permissible limits. The Heavy Metal Pollution Index (HPI) was below 100 in both winter and summer, indicated suitability for drinking regarding heavy metals. The Heavy Metal Evaluation Index (HEI) was below 10, also suggested the water's suitability for drinking. However in winter, the inlet water to treatment plants 3, 4, and 5, sourced from the Karun River, had an HEI value above 20 due to mineral waste, revealed unsuitability for drinking before treatment. Except for treatment plant 2 and distribution points 1, 2, 4, and 6, the Metal Index (MI) values generally exceeded 1that was in warning threshold. The high MI in the outlet samples from the treatment plants was due to the Al and Fe released from the coagulant materials used during the treatment process. Additionally, the elevated MI in the inlet streams during the wet season can be attributed to increased rainfall and the release of mineral wastes from thunderstorms and non-point sources. Inlet streams of treatment plants 3, 4, and 5 had Cd levels significantly higher than 3, classified them as highly polluted. A Cd index above 1 indicates metals exceeding permissible limits, with Al and Fe being the main contributors. Therefore, reducing the coagulant dose in the treatment process is recommended to lower MI and Cd indices.