Acidic, metal-rich water that had accumulated within two abandoned, adjacent copper mines in north Wales was removed to prevent its possible catastrophic release. About 274 000 m3 of acidic (pH ∼2.4) mine water was pumped out of the mines over a 14-week period. Concentrations of dissolved species (iron, sulfate, aluminium, copper, manganese and zinc) increased as water at lower depths within the mines was accessed. The discharged water flowed through a small wetland area, reaching the sea about 3 km north of the mine site. Analysis of the water at three sampling stations revealed that there was very little removal of aluminium and most of the heavy metals present except (ferrous) iron, which was partially removed as a result of oxidation and hydrolysis of the resulting ferric iron. The dominant bacterium in the subterranean mine water, which was essentially devoid of oxygen, was the iron- and sulfur-oxidizer Acidithiobacillus ferrooxidans. The microbial populations in the pumped mine water were monitored using combined cultivation-dependent (isolation on solid media) and cultivation-independent (terminal restriction fragment length polymorphism and clone library) techniques. Other bacteria detected in the mine water included other iron-oxidizers (Leptospirillum spp., ‘Ferrimicrobium acidiphilum’ and Gallionella-like organisms) and heterotrophic acidophiles (Acidiphilium, Acidisphaera and Acidobacterium). Archaeal clones were also detected; most of these were related to methanogens. Owing to the absence of an effective remediation strategy, an estimated 7.5 tonnes of copper, 3.1 tonnes of manganese, 14.8 tonnes of zinc and 15.3 tonnes of aluminium was discharged into the Irish Sea as a consequence of the dewatering of the mines.