In 1988 a deposit with some 500,000 m³ of coarse sulphidic mine waste at Bersbo, Sweden, was covered with compacted illitic clay or cement stabilised coal fly-ash (CeFill) to prevent eathering and the transport of metals. Theoretically, oxidation of pyrite would give a sulphate/iron ratio in solution exceeding 2, if oxygen is the electron acceptor, but below 2 if Fe(III) is the oxidizing agent. The effluents had ratios of 35—50 and 2—0.8 before and after covering, respectively, why it is concluded that weathering continued. The altered hydrological regime contributes to changes in metal concentrations in the surrounding surface water.