Assessment of the hazard posed by metal forms in water and sediments

This study aimed to describe the prevalence heavy metals (Zn, Cu, Pb, and Cd) forms in the ecosystem of the Utrata river in order to determine the mobile forms and bioavailability of metals. To extract the dissolved forms of metals in the water of the Utrata PHREEQC2 geochemical speciation model was used. The river waters show a high percentage of mobile and eco-toxic forms of Zn, Cu and Pb. The percentage of carbonate forms for all the studied metals was low (<1%). The content of carbonates in the water and the prevailing physical and chemical conditions (pH, hardness, alkalinity) reduce the share of toxic metal forms, which precipitate as hardly soluble carbonate salts of Zn, Cu, Cd and Pb. Cu in the water in 90% of cases appeared in the form of hydroxyl compounds. To identify the forms of metal occurrence in the sediments Tessier's sequential extraction was used, allowing to assay bound metals in five fractions (ion exchange, carbonate, adsorption, organic, residual), whose nature and bioavailability varies in aquatic environments. The study has shown a large share of metals in labile and bioavailable forms. The speciation analysis revealed an absolute dominance of the organic fraction in the binding of Cu and Pb. Potent affinity for this fraction was also exhibited by Cd. The rations of exchangeable Zn and Cu forms in the sediments were similar. Both these metals had the lowest share in the most mobile ion exchange fraction.