The Wood Cadillac mine site in Northwestern Quebec features sulfide-poor but As-rich tailings laid down between 1939 and 1949. A reducing biofilter was planned for the passive treatment of As-contaminated seep and surface waters. The As load is precipitated as As2S3 (orpiment). Various types of barks were tested in water-saturated columns at different flow rates to determine the efficiency and performance of wood wastes in the biofilter. Sulfate reduction and As precipitation occurred with a removal often higher than 80%. The precipitated solid was stable under acid leach down to pH 2. Barks from deciduous trees performed better than conifer barks. Thirty-year-old wastes were still reactive, but to a much smaller degree. As expected, performance increased with residence time. The data allowed for the design and implementation of a biofilter (50 m x 57 m x 1 m thick) with a vertical flow and a residence time of 25 hours. Available at http://www.ott.wrcc.osmre.gov/library/proceed/sudbury2003/sudbury03/148.pdf