Qualitative assessment of surface sediments of the Gavehroud and Qeshlaq tributaries of the Sirvanroud to predict the phenomenon of eutrophication at the construction site of the Zhaveh Dam (2023-2024)

Background and objectives: The Sirvan River catchment in western Iran— particularly, the Zhaveh Dam area near Sanandaj city—formed by the confluence of the Qeshlaq and Gavehrud rivers—has been subjected to increasing environmental stress over the past few years. The disposal of city and industrial wastewaters into river tributaries, particularly the Qeshlaq branch, has raised concerns about sediment contamination and the initiation of eutrophication in the Zhaveh Dam reservoir. Eutrophication, caused by the overloading of water with nutrients such as phosphorus, can lead to uncontrolled algal growth, poor water quality, and danger to aquatic organisms. Rivers' surface sediments serve a twofold purpose within aquatic ecosystems: on the one side, they are stores of nutrients, and on the other side, under certain conditions, they can release the nutrients into the water column. Phosphorus reserves within the sediments were several times higher than in water, particularly in areas influenced by untreated wastewater outflow.
This study investigates spatiotemporal variations in the sediment characteristics within the Zhaveh Dam, focusing on phosphorus speciation to assess the possibility of eutrophication. Results are further compared with national and international standards to ascertain sediment quality and environmental risk.
Materials and Methods: This research was conducted in 2023–2024 at six sites on the Qeshlaq and Gaveh Rivers, and in the reservoir of Zhaveh Dam. Sediment quality parameters under investigation were various forms of phosphorus, pH, redox potential (Eh), and total organic carbon (TOC).The fractions of sediment phosphorus were quantified according to sequential extraction procedure that assesses five forms of phosphorus: adsorbed, Fe-bound, Al-bound, Ca-bound, and residual P.
Phosphorus Pollution Index (PPI) was calculated to predict the ecological risk based on comparison of measured phosphorus levels with standards for contamination.
Parametric statistical tests like stepwise regression, Pearson correlation, and one-sample t-tests for comparison with standards were performed using SPSS version 18.
Results and Discussion: Available phosphorus in sediments from stations 3 and 4 (Qeshlaq branch) accounted for approximately 50% of total phosphorus, which could be due to infiltration of wastewater into the river. Among phosphorus forms present, loosely bound phosphorus was the lowest percentage because it is utilized directly by aquatic organisms. The major form of phosphorus in sediments was aluminum-bound phosphorus fractions (more than 60%). At all stations and under all seasons, the inorganic phosphorus percentage was more than 95% of total phosphorus, while organic phosphorus concentration was recorded as less than 5%.
The results of this research showed that the TP content in Sirvan River basin (2519 mg/kg.dw) was 3.44 times higher than China's national standard (733 mg/kg.dw), in a way that based on total phosphorus contamination concentration range in sediments, all those with TP content above 1000 mg/kg.dw were highly polluted.
Station 6, near the existing construction of the Zhaveh Dam, contained more phosphorus than stations 1 and 5. The Phosphorus Pollution Index (PPI) was above 1 at the majority of the sampling stations except for stations 1 and 5, and this shows that the majority of the research area was polluted. The mean PPI in the current study (2023–2024) was 3.4 ± 0.81, a bit higher than in the former study in 2020–2021. Stations 1 and 3 showed the lowest and highest PPI, respectively. The results of the phosphorus ecological risk assessment in sediments via the single pollution standard index method (PPI) showed that the standard index of TP ranged from 1.60 to 8.08. It indicated that phosphorus's ecological pollution risks in this area were relatively low to high.
It provides means of efficiently assessing the ecological risk of nutrients in watershed sediments, the dominant sources and factors of nutrients in watershed sediments, as well as eutrophication prevention and ecological restoration of the basin. Total organic carbon (TOC%) in sediment in the basin was generally within Canadian standard values except for the case of maximum values at stations 3 and 4, exceeding the standard. In comparison to the original study, the total phosphorus (TP) increased by 1.2 times with the percentages of TOC being generally consistent.
Total phosphorus (TP) and total organic matter (TOM) levels ranged between 0.17–0.39% and 4.65–19.23%, respectively, and seasonally and annually were higher than Canadian standard measurements. TP percentages in this research were higher in all seasons except summer, while annual values were similar to the previous study. In contrast, TOM percentages were lower in all seasons except autumn and annually compared to the previous study.
The balance of mineral phosphorus in the sediments and water in the current research (2023–2024) ranged between 3366 and 475, whereas the balance in the previous study (2020–2021) ranged between 267 and 796. It depicts higher diffusion of mineral phosphorus from sediment to the water column, according to the diffusion principle from high to low concentration environments. Comparative analysis indicated overall phosphorus and mineral phosphorus in this study were 1.5 and 0.3 times higher, respectively, than that in the previous study. Curiously, for spring season, river discharge in this study was 26.32 m³/s, while it was only 1.98 m³/s in 2020–2021. These changes in riverbed turbulence and discharge likely aided in transporting downstream the upstream sediment phosphorus, thereby enhancing its impact on water phosphorus concentration near dam location.
Conclusions: This study encountered very elevated levels of total phosphorus (TP: 2518 ± 231 mg/kg.dw) and bioavailable phosphorus (1126 ± 141 mg/kg.dw) in Sirvan river sediments discharged at the Zhaveh Dam location. They were several times above the Chinese national standard (733 mg/kg.dw), indicating a very high degree of ecological risk (High PPI). Significantly, TP levels in the reservoir sediments were over a thousand times higher than in the river water, indicating that sediments have the potential to act as a long-term phosphorus source and to amplify eutrophic conditions. Implications of this research indicate the importance of management of nutrient loads and regular sediment quality monitoring in the region.