EXTENDED ABSTRACT
Introduction
Reservoir sediments are key components of aquatic ecosystems due to their capacity to accumulate, store, and potentially remobilize pollutants. Among inorganic contaminants, heavy metals are of particular concern because of their persistence, non-biodegradability, toxicity, and tendency to bioaccumulate through aquatic food webs. After entering aquatic systems, heavy metals bind to fine particles and organic matter, eventually settling into bottom sediments where they may remain for extended periods. However, environmental fluctuations such as changes in pH, redox conditions, and hydrodynamics can trigger their release into the overlying water, posing ecological and human health risks. Large dam reservoirs, especially those influenced by both natural geological inputs and human activities, are highly vulnerable to heavy metal accumulation. Therefore, assessing metal concentrations, contamination intensity, and distinguishing lithogenic from anthropogenic sources is essential for environmental monitoring and sustainable management.

Materials and Methods
This study evaluated the level, intensity, and origin of heavy metal contamination in sediments of the Khersan 3 Dam reservoir in southwestern Iran using geochemical indices. Seasonal sediment sampling was conducted during autumn and winter 2023 and spring and summer 2024. Surface sediments (0–5 cm) were collected using a grab sampler and transported to the laboratory under cooled conditions. Samples were freeze-dried, homogenized, and sieved (<63 µm). Sequential extraction based on Tessier’s procedure was applied to separate anthropogenic (labile) fractions from total (labile + residual) concentrations. Total digestion was carried out using nitric acid. Iron (Fe), zinc (Zn), copper (Cu), cadmium (Cd), and lead (Pb) were measured using flame atomic absorption spectrometry, while mercury (Hg) was analyzed by cold vapor atomic absorption. To evaluate contamination, the Müller geoaccumulation index (Igeo), contamination factor (CF), and pollution load index (PLI) were calculated using accepted background values. These indices were applied to both total and labile concentrations to better assess contamination intensity and identify dominant pollution sources.

Results and Discussion
Iron, zinc, and copper showed low concentrations in the labile fraction, and their Igeo and CF values classified them as unpolluted, indicating predominantly lithogenic origins governed mainly by geological processes and natural weathering. Seasonal variations were minor and mostly hydrological. Lead showed an unpolluted to moderately polluted status, suggesting mixed origins; although natural background inputs dominate, limited anthropogenic contributions, likely from agricultural runoff or atmospheric deposition, may exist. Cadmium was identified as the most critical contaminant. Elevated Igeo values indicated moderate to heavy contamination, while CF values greater than unity in all seasons confirmed anthropogenic enrichment. A considerable fraction of cadmium occurred in the labile form, highlighting its bioavailability and potential ecological risk. Agricultural activities, particularly phosphate fertilizer application, appear to be the primary source. Mercury, although present in relatively low concentrations, was mainly detected in the labile fraction, indicating dominant anthropogenic influence, possibly linked to combustion processes, waste disposal, or diffuse environmental sources. Despite these element-specific concerns, PLI values remained below 1 in all seasons, suggesting that sediments are not critically polluted from a cumulative perspective. However, reliance solely on PLI may underestimate localized or element-specific risks, particularly for cadmium.

Conclusion
Overall, sediments of the Khersan 3 Dam reservoir are classified as unpolluted when evaluated cumulatively. However, metal-specific assessments reveal important concerns. Iron, zinc, and copper are mainly lithogenic and environmentally non-critical, whereas cadmium represents a significant anthropogenic contaminant with high ecological risk potential, and mercury also reflects human influence despite its lower levels. These findings highlight the necessity of using multiple geochemical indices simultaneously rather than relying solely on cumulative indicators. Continuous monitoring, improved agricultural management, and control of anthropogenic discharges are essential to prevent future deterioration of sediment quality and to ensure sustainable environmental management of the Khersan 3 Dam reservoir.


EXTENDED ABSTRACT
Introduction
Reservoir sediments are key components of aquatic ecosystems due to their capacity to accumulate, store, and potentially remobilize pollutants. Among inorganic contaminants, heavy metals are of particular concern because of their persistence, non-biodegradability, toxicity, and tendency to bioaccumulate through aquatic food webs. After entering aquatic systems, heavy metals bind to fine particles and organic matter, eventually settling into bottom sediments where they may remain for extended periods. However, environmental fluctuations such as changes in pH, redox conditions, and hydrodynamics can trigger their release into the overlying water, posing ecological and human health risks. Large dam reservoirs, especially those influenced by both natural geological inputs and human activities, are highly vulnerable to heavy metal accumulation. Therefore, assessing metal concentrations, contamination intensity, and distinguishing lithogenic from anthropogenic sources is essential for environmental monitoring and sustainable management.

Materials and Methods
This study evaluated the level, intensity, and origin of heavy metal contamination in sediments of the Khersan 3 Dam reservoir in southwestern Iran using geochemical indices. Seasonal sediment sampling was conducted during autumn and winter 2023 and spring and summer 2024. Surface sediments (0–5 cm) were collected using a grab sampler and transported to the laboratory under cooled conditions. Samples were freeze-dried, homogenized, and sieved (<63 µm). Sequential extraction based on Tessier’s procedure was applied to separate anthropogenic (labile) fractions from total (labile + residual) concentrations. Total digestion was carried out using nitric acid. Iron (Fe), zinc (Zn), copper (Cu), cadmium (Cd), and lead (Pb) were measured using flame atomic absorption spectrometry, while mercury (Hg) was analyzed by cold vapor atomic absorption. To evaluate contamination, the Müller geoaccumulation index (Igeo), contamination factor (CF), and pollution load index (PLI) were calculated using accepted background values. These indices were applied to both total and labile concentrations to better assess contamination intensity and identify dominant pollution sources.

Results and Discussion
Iron, zinc, and copper showed low concentrations in the labile fraction, and their Igeo and CF values classified them as unpolluted, indicating predominantly lithogenic origins governed mainly by geological processes and natural weathering. Seasonal variations were minor and mostly hydrological. Lead showed an unpolluted to moderately polluted status, suggesting mixed origins; although natural background inputs dominate, limited anthropogenic contributions, likely from agricultural runoff or atmospheric deposition, may exist. Cadmium was identified as the most critical contaminant. Elevated Igeo values indicated moderate to heavy contamination, while CF values greater than unity in all seasons confirmed anthropogenic enrichment. A considerable fraction of cadmium occurred in the labile form, highlighting its bioavailability and potential ecological risk. Agricultural activities, particularly phosphate fertilizer application, appear to be the primary source. Mercury, although present in relatively low concentrations, was mainly detected in the labile fraction, indicating dominant anthropogenic influence, possibly linked to combustion processes, waste disposal, or diffuse environmental sources. Despite these element-specific concerns, PLI values remained below 1 in all seasons, suggesting that sediments are not critically polluted from a cumulative perspective. However, reliance solely on PLI may underestimate localized or element-specific risks, particularly for cadmium.

Conclusion
Overall, sediments of the Khersan 3 Dam reservoir are classified as unpolluted when evaluated cumulatively. However, metal-specific assessments reveal important concerns. Iron, zinc, and copper are mainly lithogenic and environmentally non-critical, whereas cadmium represents a significant anthropogenic contaminant with high ecological risk potential, and mercury also reflects human influence despite its lower levels. These findings highlight the necessity of using multiple geochemical indices simultaneously rather than relying solely on cumulative indicators. Continuous monitoring, improved agricultural management, and control of anthropogenic discharges are essential to prevent future deterioration of sediment quality and to ensure sustainable environmental management of the Khersan 3 Dam reservoir.