It is important to find economic and effective methods to solve the problem with acid mine drainage. To cover tailings with water is thought to be a good method to prevent oxidation and weathering of sulphides by limiting the supply of oxygen. One method to cover tailings with water is flooding by raising the water level in tailings and clarification ponds. Flooding has the potential to be an important method for treatment of mine waste, but there is an urgent need for research of how flooding affects weathering and transport processes in the waste. The abandoned mine at Stekenjokk, northern Sweden, offers a good possibility to perform in situ, full-scale geochemical studies of flooding since the tailings were covered with water 1991. In accordance with this, Stekenjokk has been selected for a geochemical study. The intention is to study the weathering in the tailings, the amounts of metals and other elements released to the pore water, and the interactions between sediments (tailings), pore water and the water column in the dam.
Stekenjokk is a stratabound Zn-Cu deposit of Caledonian age, situated in northern Sweden close to the Norwegian border, and at an altitude of c. 800 m above sea level. Operations by Boliden Mineral AB lasted from 1976 to 1988, leaving waste rock dumps and some 4.4 Mtons of tailings containing c. 20 % sulphur, and containing on average 0.19 % Cu and 0.64 % Zn. A decommissioning program based on flooding was completed 1991 (for a detailed description, see Broman & Göransson, 1994). The high content of sulphur makes the tailings at Stekenjokk a potential source of acid mine drainage.
Sediments, pore water and the water column have been sampled in detail. Seasonal variations in the water column have been studied. Sampling started from the ice at the end of May, 1995. Three 15-20 cm long sediment cores, split into subsamples every cm, were taken. Pore water was pressed from the three cores. Two profiles of the water column in the dam were sampled. In the deepest part of the profiles, samples were taken every 5 cm, and in the upper part every 10 cm. The water was filtered through filters with a pore size of 0.45 µm. From one of the profiles, the suspended phase was sampled for analysis. All sample types were chemically analysed by multi-element techniques (ICP-AES and
ICP-MS, anions by ion chromatography). The ice-free period extends from mid June to early October. Additional sampling of the suspended and dissolved phase of the water column was performed in mid-July, late August and late September.
Results from all these samplings will be presented at the conference. Preliminary interpretations of the results from the sampling indicate that weathering of pyrite, buffered by carbonates, occurs in the uppermost sediment layer. pH in the water column is high, between 7.3 and 7.8 throughout the whole season. The contents of metals in the water are higher than background values, but not higher than expected from the decommissioning plan. Cu content is low, only 2-3 µg/l, and Zn varies between 180 and 360 µg/l. There is a concentration gradient in the water column in May, with the highest values at the deepest levels, indicating a diffusion of Zn from the pore water to the water column. Cd, Ca and sulphate have similar concentration gradients. The water column in July, August and September are well oxygenated and mixed and there are no concentration gradients visible in the dissolved phase. The content of dissolved elements are lower in the water column during the ice-free period due to dilution with surface-water from the drainage area. The chemical composition of the suspended phase indicates that there is some resuspension of the tailings.
Broman, P. G. & Göransson, T., International Land Reclamation and Mine Drainage Conference and the Third International Conference on the Abatement of Acidic Drainage, Pittsburgh, PA 2, 32-40 (1994).