Metal dispersion in sediments and waters of the River Conwy draining the Llanrwst Mining Field, North Wales

Abstract

The dispersion and sediment-water partitioning of Pb and Zn have been studied in the Conwy River, North Wales. Analysis included major and trace element water chemistry and concentrations of sediment-hosted Pb and Zn. In situ solution pH, Eh, temperature and conductivity were also measured. Sediments were characterized via SEM, XRD, nitric acid leaching and sequential chemical extraction to quantify metal distribution and sediment phase associations. Dissolved and sediment-bound Pb and Zn within river and estuary waters and sediments have been used to calculate whole sediment- and phase-specific apparent partition coefficients.Weathering of galena and sphalerite, associated with the Llanrwst Mining Field, provide point sources of elevated dissolved and sedimentary Pb and Zn in the upper catchment. Dissolved Pb is actively adsorbed onto sedimentary surface coatings of Fe-Mn (hydr)oxides and organics whilst Fe-Mn (hydr)oxides, organics and carbonates were the main hosts for Zn. Systematic changes in metal sorption are evident between the estuary and the upper catchment, with organic matter becoming progressively more important upstream. This change is ascribed to the sorptive properties of sedimentary organic material. Solution pH, [Cl] and aqueous metal speciation are the dominant solution controls on metal partitioning. Laboratory sorption experiments parallel Pb sorption behaviour in the natural system.