| Summary: | Rävlidmyran pit lake situated in Northern Sweden had low pH, and a permanent stratification with anoxic water at depth. To improve the water quality, the pit lake was limed by using Ca(OH)2. Before liming, the lake was stratified with low pH (3.0 - 3.4) and high concentration of dissolved oxygen between 0 and 5 m depth, with a transition zone from 5 to 8 m depth, and a zone with pH 3.2 - 5.5 and very low dissolved oxygen from 8 m depth to the bottom. Metals such as Al, Cu and Pb had highest concentrations in the upper zone whereas the concentrations of most other elements including As, Ca, Cd, Fe, Mg, Mn, S and Zn increased with depth. The stratification remained also after the liming treatment. pH increased to about 8 in the whole water column although only one sixth of the Ca(OH)2 was rapidly dissolved, and the remaining part accumulated in the bottom sludge. Redox decreased, especially in the oxygen poor zone at depth. Concentrations of Fe, Mn, Zn and Mg decreased strongly in the upper oxic water, due to formation of oxyhydroxides. In the anoxic bottom water, the concentrations of these elements gradually increased towards pre-liming levels. Aluminium and Cu were almost completely depleted in the entire water column due to pH increase and precipitation of minerals such as gibbsite. The bottom sludge formed after the liming was dominated by Ca(OH)2, calcite and a group of Fe(II)-Fe(III) oxyhydroxides, so called green rust. Metals removed from the dissolved phase of the water column accumulated in the sludge. Although the bottom sludge will have a buffering effect for some time, the poor water mixing in the pit lake will rapidly result in acid water in the upper zones when liming ceases.Key Words: AMD, pit lake, liming, geochemistry, water column, bottom sludge Godkänd; 2007; 20071207 (ysko)
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