The Wood Cadillac mining site, in north-western Quebec, is an important source of arsenic contamination. The arsenic-rich effluent is treated through a reductive biofilter, which allows arsenic to precipitate as a sulphide (likely As2S3). The top-fed biofilter, built at the end of 1999, is 50 m x 57 m by 1 m thick. The reducing conditions are provided by the decomposition of yellow birch bark chips. At the outlet, dissolved arsenic was maintained below 57 μg/L during the entire periods investigated, which is May 2000 to October 2002, except June 20th 2001 (93 μg/L). Relative to mean input values of 103, 227, and 168 μg/L in 2000, 2001, and 2002, arsenic concentrations were removed on average by 92, 88, and 76 %, respectively. Dissolved sulphates were removed in the summer up to 99%, 95%, and 67% in 2000, 2001, and 2002. Alkalinity is a direct product of sulphate reduction, and its variation was found to be inversely proportional to that of sulphate removal. Unfortunately, water is also enriched with organic matter after its journey through the biofilter, with high COD, BOD, and, to some extent, phenols. The physical properties of the biofilter have been modified over time due to its compaction since its construction (6 to 10 %). Thus, the heterogeneity of the porous media has increased and preferential flow paths have been developed (shorter residence time), which could explained part of the decrease in arsenic removal in 2002. Furthermore, the geotextile near the surface of the biofilter got partly plugged by organic material limiting the infiltration. As a consequence, the maximum discharge decreased from 7 L/s in 2000 to 5 L/s in 2002. To sum up, the biofilter has improved the water quality of Wood Cadillac effluents with respect to arsenic. However, its performance has somewhat decreased in 2002 in comparison to the two previous years. This decrease is due to preferential flow paths, which shorten the residence time, and/or to less available easily degradable organic matter available to reducing sulphate bacteria. The quality of the biofilter effluent has somewhat degraded when one's consider organic matter. However, chemical and biological demands have significantly decreased in 2002 in comparison to the two previous years.