Shahid Rajaee Dam is located in Tange-Soleiman, Mazandaran state, around 40 km southeast of Sari. It is built on the Tajan River in the region to offer vital irrigation water for lush agricultural fields in the dam’s downstream plains, as well as to provide hydroelectricity, flood control, and tourism attractions in Northern Iran’s gorgeous woodlands. The dam is a double-curvature concrete arch dam with a crest length of 430 m, a foundation width of 27 m, and a crest height of 7 m. The height above the foundation is 138 m, while the distance from the riverbank is 116 m. The spillway controls 800 cubic meters of water each second, and the lake’s size at normal water level is 158 cubic meters. Figure 1 depicts the location of the Shahid Rajaee Dam.

Surface sediments were gathered from six stations around the Rajayi reservoir by a stainless steel grab. To prevent contamination, samples were packed in sacks and stored at 4°C before being transported to the laboratory. The sediment loss on ignition (LOI) was measured using ASTM-D7348-08. The samples of sediment were sifted using 63 μm sifter after air-drying at room temperature. The Tessier sequential extraction approach was utilized to investigate the repartition of metals in five different fractions: exchangeable (F1), carbonate (F2), reducible (F3), oxidizable (F4), and residual (F5). Cu, Co, Cr, Pb, Mn, Zn, Fe, and Ni concentrations were determined by ICP-MS (ELAN DRC-e; PerkinElmer). In order to assess the analytical procedure’s validity and accuracy, duplicate tests and certified reference substances (MESS-3) from the National Research Council of Canada were examined. The sequential extraction procedure achieved a recovery rate of 95%–104% for all components.

The SPSS software (IBM SPSS Statistics 27) was used for the analysis of data, including the generation of descriptive statistics. OriginPro 2022 were used for visualizations and statistical analyses. Pearson correlation was utilized to determine the correlations between elements. According to Fowler et al. (2013) (Fowler et al., 2013), correlation intensity was defined by the following values: 0.0 ≤ r < 0.19 (a very weak correlations), 0.19 ≤ r < 0.39 (weak correlations), 0.39 ≤ r < 0.69 (moderate correlations), and r ≥ 0.69 (high correlations). Hierarchical cluster analysis (HCA) was used to assess element similarity and their potential sources. Variances were considered significant if p < 0.05.

Sediment samples from six stations in the Shahid Rajayee Reservoir were analyzed for eight elements (Cu, Cr, Co, Pb, Ni, Zn, Mn, and Fe). The findings showed that the mean concentrations of the examined elements were ranked as Fe > Mn > Zn > Ni > Pb > Cr > Co > Cu, with mean values of 8500, 242.40, 170.80, 32.37, 29.82, 26.49, 17.93, and 14.19 mg/kg. In sediment and soil investigations, Fe is often found at the greatest levels, followed by Mn (Table 1). Table 1 also compares the average concentration of studied elements with similar environments around the world. The mean Zn contents in the Shahid Rajayee Reservoir was the greatest when compared to comparable environments described in Table 1. Chromium levels were greater (26.49 mg/kg) than those found in dam reservoirs in Poland (7.74 mg/kg) (Sojka et al., 2022) and Aguamilpa, Mexico (9.26 mg/kg). The Aguamilpa Dam is part of the reservoir waterfall system, which consists of four reservoirs along the Santiago River’s middle and lower reaches. For decades, this structure has absorbed industrial and urban from Guadalajara’s urban region, as well as runoff from agricultural lands in the area surrounding the river (Rangel-Peraza et al., 2015). Furthermore, only the Manwan Reservoir in China has a bigger PHE load than the Shahid Rajayee Reservoir. The Manwan hydroelectric dam is positioned in the central reach of the Lancang River, and according to the findings, the origins of PTEs in this region were anthropogenic, that include tributary river industrial operations (smelting and heavy metal mining) and agricultural practices (pesticide usage and fertilization) (Wang et al., 2012). The research region had relatively higher Pb levels than the Wadi Al-Aqiq water reservoir dam in Saudi Arabia (Alghamdi et al., 2019), the Ziqlab reservoir in Jordan (Al-Taani et al., 2015), and Aguamilpa in Mexico (Rangel-Peraza et al., 2015). However, compared to the current research, Cr and Co concentrations were found to be greater in the sediments of the Hongfeng reservoir in China (Wang et al., 2015) and the Castilseras reservoir in Spain. The Castilseras reservoir is an artificial water body situated alongside the Valdeazogues River, which runs south from the Almadén mercury mining sector and gets major input from numerous nearby cinnabar mines (García-Ordiales et al., 2016).

Table 2 displays the carcinogenic risk (CR and TCR) and non-carcinogenic (HQ, HI, and THI) levels for PHEs. All PHEs’ HQ_dermal and HQ_ingestion values were below the permitted limit of one, showing that studied PHEs had no negative health effects on recreational users (children and adults). Likewise, THI (HQ_ingestion + HQ_dermal) and the HI_ingestion and HI_dermal values were beneath one, suggesting that the combined impacts of the seven PHEs in Rajayi reservoir sediments would cause no health danger to receptors. Furthermore, the HI_dermal and HQ_dermal levels for each of the receptors were below the HI_ingestion and HQ_ingestion levels. This shows that the ingesting route had a stronger negative effect on receptor health than the dermal contact path (Canpolat et al., 2022).

Carcinogenic risk (CR) values for Cr, Ni, and Pb ingestion pathways were 7.6 × 10⁻⁶ and 3.16 × 10⁻⁵, and 1.45 × 10⁻⁷ respectively, whereas their dermal pathways showed CR values of 1.67 × 10⁻⁶, 2.26 × 10⁻⁶, and 4.16 × 10⁻¹⁰. The CR and TCR values in the research region were within the permitted range of 1 × 10⁻⁴ and 1 × 10⁻⁶, indicating that exposure to Pb, Ni, and Cr in the sediments would not produce carcinogenic health consequences for recreationists.

Since the pollutants exposure technique exhibits spatiotemporal variation, the input variables for the exposure model might be changed across both space and time. As a result, further research is needed into the geographical and temporal modeling of various contaminants, sources, and pathways. Although the model may overestimate or underestimate, the findings may nevertheless give some useful insights and forecasts for local governments to deal with the present severe environmental concerns.

It is critical to assess if the concentrations of PHE in sediments constitute a hazard to aquatic species. In this investigation, the concentrations of the PHE in sediment samples were contrasted to consensus-based PEC and TEC levels (Supplementary Table S2). Cr and Cu levels were below the TEC in all samples. Ni, Zn, and Pb were found between the TEC and PEC in 100%, 66.67%, and 33.4%, respectively. The findings indicate that none of the abovementioned ingredients have the potential to harm sediment-dwelling organisms.

The speciation features of elements in sediment are crucial for researching their origins and bioavailability. PHE migration is intimately associated with speciation. PHE speciation influences their ecological toxicity and natural cycle (Lin et al., 2022). Figure 4 depicts the distribution of the exchangeable (F1), carbonate (F2), reducible (F3), oxidizable (F4), and residual (F5) of the PHE fractions in the Shahid Rajaee Reservoir sediments. Ni, Cu, Co, Pb, Mn, Cr, and Zn the average concentrations in F1 and F2 bound fractions (labile fractions) were 17.6%, 17.5%, 16.5%, 14.5%, 12.6%, 12%, and 7.1% of the total quantity, respectively. The exchangeable and carbonate-bound fractions are partly the result of human intrusions. Pollutants from these fractions are readily discharged into the water, posing a significant danger to aquatic organisms (Saleem et al., 2018; Kachoueiyan et al., 2023).

F3 is the percentage that accumulates in the environment by coprecipitation or adsorption with (Fe)/(Mn) oxides. When water redox potential reduces or the water becomes hypoxic, Fe/Mn oxides are reduced, releasing elements into the environment (Ahn et al., 2020), causing water pollution. The F3 of PHEs in sediments in the Shahid Rajaee Reservoir ranged from 3.6% to 13.8%, with the principal PHEs in the F3 fraction occurring in the following order of quantity: Co (13.82) > Cu (13.81%) > Ni (12.2%) > Cr (9.64%) > Pb (7.54%) > Mn (6.45%) > Zn (3.6%). F4, or the fraction associated with organic matter and sulfides, is made up of element ions that bind to sulfide ions or organic matter active groups (ligands) to generate water-insoluble compounds (Shen et al., 2022). When there is a significant concentration of organic matter in the sediment, F4 may be mineralized and degraded by microorganisms, releasing a part of the PHEs into the overlying water or being absorbed by aquatic life. Based on the results, the level of organic matter (LOI = 7%) was insignificant, hence, the risk of releasing PHEs in the water column from sediment is almost low. All of the elements investigated at all stations were mainly linked to F4 fractions. The average F4 percentage of Zn, Mn, Pb, Cr, Co, Cu, and Ni in sediment was 85%, 77%, 66%, 60.8%, 51%, 47.7%, and 46.6% of their total quantity, respectively.

F5 is the proportion of elements in the crystal structure of primary and secondary ores that are exceptionally stable (Uddin et al., 2022). F5 has minimal effect on the mobility and bioavailability of elements in sediments, hence it is typically regarded as environmentally acceptable. F5 represented 3.77%–23.62% of all PHEs in sediment samples, with the following abundances: Ni (23.63%) > Cu (21%), Co (18.57%), Cr (17.5%), Pb (11.94%), Zn (4.23%), and Mn (3.77%).

Multivariate analysis, i.e., hierarchical cluster analysis (HCA) and correlation analysis (CA), are excellent procedures for tracing element origins (Aydın et al., 2023; Esmaeilzadeh and Mehdinia, 2024). CA (Figure 5A) reveals two main clusters: cluster one (Cu, Co, Cr, Ni, and Fe) and cluster two (Pb, Mn, and Zn). The elements found in the two clusters formed match the factors revealed by the Pearson correlation. The Pearson correlation (Figure 5B) for the sediment samples showed that Cu had a high correlation with Co (r = 0.92), Cr (r = 0.87), and Ni (r = 0.61), and Fe is also highly linked with Ni (r = 0.63), suggesting that these elements originate from a same source. The mean I_geo values of Co, Cu, Cr, and Ni were less than 0 and between 0 and 1, indicating that all four PHEs are mostly derived from crustal components or natural erosion.

Moreover, Mn displays a significant correlation with Pb (r = 0.89). Previous research suggested that Mn is a lithogenic metal, originating mostly from natural sources via erosion and weathering of soil and rock parent materials (Li et al., 2021; Proshad et al., 2022). In this investigation, Mn showed low CF and I_geo values. Thus, it is presumed that Mn was mostly influenced by the parent materials. Pb had substantial positive relationships with Mn, indicating that Pb also has natural origins. However, according to CF and I_geo readings, Pb at stations 1 and 2 was “moderately polluted.” As a result, it is possible to infer that Pb concentrations have both natural and anthropogenic origins. Prim Road is near the research area. Vehicle fuel emissions may cause Pb concentration in sediments (Shi et al., 2022). Fuel emissions from automobiles on Prim Road are likely to contaminate sediments with Pb. The connection between Zn and Pb was somewhat positive (r = 0.62). Additionally, Zn concentrations may be linked to human activities, as it was found to have a positive connection with Pb. Zn is an element that plants require for growth, and when applied as fertilizer, it can gets taken up surface sediments (Jin et al., 2019). It needs to be noted that there are agricultural fields around the research location. In order to minimize total pollution from automobiles, vehicle emissions regulations should be enhanced. Moreover, by employing sustainable agriculture techniques, the inputs of PHE can be mitigated. This finding is consistent with the results of the contamination factor analysis. Overall, all PHEs had natural origins except for Pb and Zn, which were derived both from natural and human activities. These findings validated pollution indices results.