METAL FIXATION AND MOBILIZATION IN THE SEDIMENTS OF THE AFON-GOCH ESTUARY - DULAS BAY, ANGLESEY

The surface drainage waters of Parys Mountain, Anglesey (Wales), a sit e of former mining for base metals, are highly acidic and metal-rich d ue to the oxidation of sulphide minerals. These acid waters mix with m ore neutral waters in the Afon Goch, downstream of Parys Mountain, all owing the formation of ochre precipitates which are found throughout t he length of the Afon Goch and Dulas Bay. X-ray fluorescence (XRF) ana lysis of the ochres that settle where the Afon Goch enters Dulas Bay ( Fe2O3 - 18 wt%), reveals that they are heavily contaminated with Cu (1 3,000 mu g/g) and Zn (7700 mu g/g). These sediments are black immediat ely below the surface and porewater analysis confirms that sulphate re duction is taking place. Samples of both stream ochre and anoxic black mud have been analysed by EXAFS spectroscopy. Data from the EXAFS ana lysis of the ochre sample reveals that the Fe, Cu and Zn are bonded to oxygen in poorly ordered or amorphous solids. In the anoxic black mud , however, Fe, Cu and Zn are all present as sulphides. The Fe sulphide is either amorphous or poorly ordered whereas Zn forms a discrete sul phide phase similar to sphalerite. The Cu sulphide has short range ord er with a chalcopyrite-like structure. Sequential Extraction analysis of the same samples was also performed. For the ochre sample, Cu and Z n release is controlled by the Fe hydrated oxides, being recovered pri marily in the mildly acid 'Carbonate' and more strongly acid and reduc ing 'Reducible' fractions. Fe and Zn are also recovered in these fract ions from the black mud, indicating that these metals are present as a cid-soluble sulphides. Cu, however, is almost exclusively recovered in the 'Oxidisable' fraction, indicating that it is incorporated into a more stable sulphide such as chalcopyrite. Copyright (C) 1996 Elsevier Science Ltd