Aqueous geochemistry of pit lakes : two cases studies at Rävlidmyran and Udden, Sweden

Abstract: Two abandoned pit lakes in northern Sweden were studied in order to improve the understanding of the geochemistry of pit lakes, Rävlidmyran Pit Lake and Udden Pit Lake. Both lakes show strong similarity with other mining lakes in terms of low pH and high metal and sulphate contents. Still, the two lakes have distinctly limnological and geochemical characteristics. Rävlidmyran Pit Lake is oligotrophic and meromictic. The lake is permanently stratified and three layers, the mixolimnion at the top, the chemocline below and the monimolimnion further below, are identifiable. Groundwater filling and stratification strongly control the elemental distributions in the lake. Most elements such as Ca, Mg, Na, K, S, Mn, Fe and Zn have lowest and constant concentration in the mixolimnion. Concentrations increase in the chemocline and keep rising in the monimolimnion. These elements show quite similar behaviour in the water with strong correlations. Groundwater is supposed to be the major source. Fe is removed from the mixolimnion due to oxidation and precipitation, but it is redissolved in the monimolimnion. Udden Pit Lake is a dimictic lake. Distinct overturning in spring and autumn occurs in the lake, during which the whole lake circulates and most of the parameters and chemical components become homogeneously distributed. The lake is thermally stratified in summer and winter. Three layers, the epilimnion at the top, the metalimnion below and the hypolimnion at depth could be identified. The thermal strata do not control the elemental distribution. During stratification, the concentrations of dissolved elements appear to be constant at most depths. Variations mainly occur in the top and bottom waters. Most elements such as Ca, Mg, Na, K, S, Mn, Cu Al and Zn have lowest concentration in the epilimnion. Concentrations increase in the metalimnion and kept constant in the hypolimnion. Fe concentration distribution reflects the changes of redox potential. High amounts of Fe2+ reaching the surface water by drainage waters from closely situated waste rock dumps cause higher dissolved Fe concentration and lower redox potential in the top water. Precipitation of dissolved Fe into suspended Fe occurs in deeper water where the redox potential is higher. In both lakes, a strong relationship between Cu and Al is found. Gibbsite is important since it controls both dissolved Al and Cu concentrations in the lake through processes of adsorption and precipitation. Otherwise scavenging processes by particles are not a significant factor controlling trace metal distributions due to the low suspended particle concentration in both lakes. Sulphate reduction was neither observed in Rävlidmyran nor in Udden, even though the oxygen concentration is poor in the deep waters. The high redox potential limits the formation of sulphides, thus precluding an important precipitation of trace metals as metal sulphides.