Drainage waters affected by pyrite oxidation in a coal mine in Galicia (NW Spain): Composition and mineral stability

The quality of the drainage waters from the As Puentes lignite mine dump (Galicia, Spain) was evaluated along with the geochemical processes which determine its composition. Analysis of water from different areas of the dump was carried out at monthly intervals over a period of 2 years. During this period, pH values ranged between 2.1 and 8.0 and redox potential values between 150 and 750 mV, although most of the water samples were strongly acidic (pH 2.5–3.5) and oxidising (Eh 600–750 mV); conditions which are usually found in mine drainage systems where pyrite is present. In general, the water samples were characterised by the presence of elevated concentrations of Fe, SO42− and H+, liberated from the oxidation of pyrite, and of Si, Al, Ca and Mg derived from the accelerated mineral hydrolysis occurring under these conditions. At the same time, very high concentrations of elements, in particular Mn, Zn, Ni and Co, which were liberated from both processes, were recorded. The best water quality was found in the most recently constructed areas of the dump, comprising pre-selected material low in sulfides.