Geochemical fractionation of mercury and trace metals in river sediments of the Almadén mining district (Spain): implications for ecological risk

Stream sediments from the drainage basin of the Almadén mercury mining district, affected by long-term activity, were investigated to distinguish geogenic from mining-related sources of trace metals and to assess their geochemical forms. The main objective of this study was to perform a screening-level ecological risk assessment, based on total concentrations and operationally defined geochemical fractions, and aimed to support future management of the freshwater environment. A total of 65 samples were collected from ten sites along the main streams, including nine within the mining district and one control site. Samples were analysed for major and trace element, and their geochemical partitioning was determined using a sequential extraction procedure. Statistical analyses were also applied to identify geochemical relationships among elements. Potential ecological risk was assessed by combining sediment guideline values with the Risk Assessment Code, calculated from the proportion of each element in the operationally defined exchangeable fraction. Results show that most trace metals exhibited median concentrations above Spanish and European background values, with Hg displaying the strongest enrichment (Hg range: 1.6–277.4 μg g −1 , mean 106.3 μg g −1 ). Elevated concentrations were also observed for Mn, Zn and Pb with ranges of 282.5–1350.1, 53.7–314.1 and 20.0–140.7 μg g −1 , respectively. Copper, Ni and Pb are mainly associated with the residual fraction, indicating low mobility under current conditions, whereas As, Hg, Mn and Zn showed higher proportions in non-residual fractions, suggesting greater potential mobility. The screening-level risk assessment indicates potential concern mainly for Hg, Mn and Zn; however, RAC results should be interpreted as indicators of potential mobility and bioavailability rather than direct evidence of ecological effects. Overall, sediments in the Valdeazogues River basin act as sink for TMs but also represent a potential source of trace metal remobilisation and ecological risk, posing a persistent threat to the freshwater ecosystem despite mine closure.