Metal fixation and mobilisation in the sediments of the Afon Goch estuary — Dulas Bay, Anglesey

The surface drainage waters of Parys Mountain, Anglesey (Wales), a site of former mining for base metals, are highly acidic and metal-rich due to the oxidation of sulphide minerals. These acid waters mix with more neutral waters in the Afon Goch, downstream of Parys Mountain, allowing the formation of ochre precipitates which are found throughout the length of the Afon Goch and Dulas Bay. X-ray fluorescence (XRF) analysis of the ochres that settle where the Afon Goch enters Dulas Bay (Fe 2O 3 - 18 wt%), reveals that they are heavily contaminated with Cu (13,000 μg/g) and Zn (7700 μg/g). These sediments are black immediately below the surface and porewater analysis confirms that sulphate reduction is taking place. Samples of both stream ochre and anoxic black mud have been analysed by EXAFS spectroscopy. Data from the EXAFS analysis 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 sulphide phase similar to sphalerite. The Cu sulphide has short range order with a chalcopyrite-like structure. Sequential Extraction analysis of the same samples was also performed. For the ochre sample, Cu and Zn release is controlled by the Fe hydrated oxides, being recovered primarily in the mildly acid ‘Carbonate’ and more strongly acid and reducing ‘Reducible’ fractions. Fe and Zn are also recovered in these fractions from the black mud, indicating that these metals are present as acid-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.