Geogenic Controls on the Distribution of Potentially Toxic Elements in the Saridareh River (Northwestern Iran)

This study investigates the geochemical characteristics and elemental enrichment patterns of stream sediments in the Saridareh River, a mineralized catchment associated with Au–Cu hydrothermal systems in northwestern Iran. Geochemical indices, including Igeo, EF, CF, Cd, mCd, PLI, PCI, and NIPI, were applied to characterize sediment geochemical status and to identify the controls governing elemental distribution. The sediments are characterized by elevated concentrations of Al (54,829–80382 mg/kg), Fe (21,843–51454 mg/kg), Mg2+ (4,205–10599 mg/kg), Cu2+ (27–158 mg/kg), Pb (9–26 mg/kg), Zn2+ (17–72 mg/kg), and As (8.3–47.3 mg/kg), reflecting the influence of the heterogeneous geological setting, weathering of local lithologies, sediment transport, and mineralization-related geogenic inputs. Most lithogenic elements exhibited negative Igeo values, indicating unpolluted conditions, whereas Cu2+ displayed notable enrichment with a maximum Igeo value of 2.23 at station St14, corresponding to moderate-to-strong enrichment. The highest EF value was also recorded for Cu2+ (EF = 6.65), indicating severe enrichment associated with downstream dispersion of mineralized materials. The calculated Cd values ranged from 6.45 to 15.52, with a mean of 9.73, suggesting moderate contamination levels across the watershed. Modified contamination degree values remained below 1.5 at most stations, while St14 exhibited a mCd value of 2.01, indicating moderate contamination. PCI values revealed substantial enrichment for Sn (48.0), Mg2+ (23.45), Cu2+ (21.63), W (9.60), and Te (8.40), highlighting localized geochemical anomalies. NIPI values varied between 0.72 and 2.12, identifying St14 as the principal enrichment hotspot. Similarly, PLI values ranged from 0.77 to 1.27, with the highest values observed at stations influenced by the Au-Cu mineralization zone. Importantly, the elevated concentrations and index values are interpreted exclusively as indicators of geochemical enrichment related to the underlying Au-Cu mineralization and local lithology, and not as evidence of demonstrated ecological effects. No site-specific ecotoxicological or biological data were collected in this study; therefore, the conclusions are deliberately restricted to geochemical enrichment patterns, and the identified anomalous zones are presented as locations warranting further environmental assessment rather than as confirmed cases of ecological deterioration.