São Domingos is one of the most emblematic Portuguese mining districts in the Iberian Pyrite Belt (IPB). The
beginning of mining exploitation in the area has been dated back to pre-Roman times, remaining in activity
until 1966 when it was definitely halted. The intense mining labours are reflected in the presence of a huge
amount of sulphide-mining wastes and downstream production of acid mine drainage (AMD). The chemical
speciation of potentially toxic elements (As, Cd, Cr, Cu, Fe, Mn, Pb, S, Sb and Zn) in the highly heterogeneous
mine wastes from São Domingos was performed by modified BCR-sequential extraction. The oxidizable
fraction is the most pollutant fraction for the environment in this type of residues. Part of this fraction is
retained by precipitation of soluble secondary minerals in warm periods, moving to the bio-available fraction
that is dissolved in rainy periods. Just considering the bio-available fraction, all mine wastes from São Domingos
may leach up to 207031 tons of Fe, 14859 t of S, 6305 t of Pb, 3259 t of Zn, 1316 t of Mn, 1143 t of Cu, 219
t of Cr, 163 t of As, 40 t of Sb and 1.7 t of Cd. As a function of the seasonal variation, this fraction is self-fed
every year by oxidation of the oxidizable fraction. Thus, the present study shows the pollutant potential of
this IPB mining district and justifies the spoiled state of the fluvial courses in the region
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