ABSTRACTThe East Sullivan mine was closed in 1966, leaving 15 Mt of acid-generating tailings free to contaminate theenvironment. An experimental wood waste cover was implemented in the early 80's in order to prevent sulphideoxidation. Contaminant production and dispersion were investigated in the 90's to validate that concept and guide lateractions. Ligneous wastes stop oxygen, and also favour infiltration, rising the watertable and allowing a rapid flush of precover,acid-prone, groundwater. These are replaced by reducing and alkaline waters, a geochemical environment thatfavours sulphate reduction and base metal precipitation. The initial restoration strategy was modified to take advantageof these properties.An innovative treatment of acid effluents was put forward, capitalizing on the sulphate reduction expected to occur in thewater collected around the impoundment and recirculated through the organic cover. The treatment system wasimplemented in 1998, and operated until 2005, when not required anymore. Despite acid feed water down to pH 2.5,groundwater’s pH next to the dispersal zone was maintained above 6. Alkalinity decreased during the recirculation, butwas back to 800 mg/L and more the following spring, thanks to sulphate reduction. Fe2+ concentrations were maintainedbelow 2 mg/L. Evolution of dissolved iron mass in the reservoirs surrounding the impoundment suggests that thecontaminated groundwater flush was completed in the north sector in 1999, in the west sector in 2001, and in the southsector in 2006. It is near completion in the east sector. Since 1999, the water quality at the final effluent, located southof the pumping station, meets the requirements of the Ministry du Développement durable, de l’Environnement et desParcs du Québec. The wood-waste cover, besides limiting sulphide oxidation, can fill the role of alkaline reducingbioreactor for the treatment of acidogenic waters, until the mass of alkalinity exceeds that of acidity in the effluents.Such situation occurred in 2006. These unconventional approaches have allowed to cut restoration cost by several M$in comparison to those required by standard techniques of the early 90's.