The beginning of polymetallic sulphide ore exploitation at Almagrera, Iberian Pyrite Belt, SW Spain, has been dated back to pre-Roman times; mining activity lasted until 2001 when it was definitely halted. The Almagrera sulphide tailings dam was built at the Sotiel–Coronada mine, where Cu, Zn and Pb concentrates were obtained by flotation processes, and sulphide (mainly pyrite) wastes formed. The dam was built as a water-reservoir, on a tectonic fault; its hazard is currently documented by the released wastes after a strong storm in the region, then, its stability is not monitored by the owner.The present study focuses on the pollutant potential of the Almagrera sulphides tailings, and estimates the environmental impact (volume affected) that would be caused by a hypothetical dam collapse. The chemical speciation of potentially toxic elements (As, Cd, Cr, Cu, Fe, Mn, Pb, S, Sb and Zn) in the sulphide tailings was performed by modified BCR sequential extraction tests. Results show that the most pollutant fraction in these wastes consists of metals bounded to sulphides and released under oxidizing conditions in acid mine drainage (AMD) production processes. Part of this fraction is temporally retained in solid phases by precipitation of soluble secondary minerals in warm periods, but such minerals are easily dissolved in rainy periods.Taking into account the amount of pollutants in the bio-available fraction (capability of these polluting elements to be absorbed or become available for the human body), and GIS modelling, the mining wastes from Almagrera could release to the environment up to e.g. 6960 tons (t) of Fe, 5857 t of S, 2756 t of Zn, 2463 t of Pb, 741 t of Cu, 633 t of Mn, and 589 t of As. Although these estimates would be affected by seasonal variations, high amounts should be expected every year by the continuous oxidation of the sulphide fraction. The supposed toxic spill would have a dramatic impact, like the one that occurred in 1998 at Aznalcóllar, an abandoned mining site showing strong similarities with Almagrera.