The distribution and partitioning of silver and other heavy metals in sediments associated with an acid mine drainage stream

Abstract

This study reports an investigation into fluvial contamination by the Cwm Rheidol ZnPb mine near Aberystwyth, Wales. This mine was last worked at the end of the last century, but acidic drainage waters are still a major source of water-borne contamination to the River Rheidol, despite the installation of a static treatment plant below the mine over twenty years ago. Acidity in the main discharge stream often approaches pH 3, causing the precipitation of hydrated ferric oxides and subsequent sorption of metals along its length. The concentrations and movement of Ag and other metals (Cd, Cu, Pb and Zn) before and after treatment are reported. The fate of these metals after discharge into the River Rheidol was also studied, using a sequential extraction procedure performed on the sediments. Much of the discharged Cd remained ‘easily or freely leachable and exchangeable’ (Badri & Aston, 1983), non-lithogenic forms (i.e. not bound in the mineral lattice) of Ag and Cu were released from sediments by oxidation, while Cd and Zn were solubilised predominantly by an acid-reduction extraction. Studies of the distribution and partitioning of metals in sediments at and near the mine suggest similarities in the behaviour of Zn and Cd and of Ag, Cu and Pb. However, the data indicate that caution should be exercised when interpreting the chemical partitioning of elements solubilised by so-called ‘selective’ extractants.