IMPACT OF HIGH ALUMINIUM LOADING ON A SMALL CATCHMENT AREA (THURINGIA SLATE MINING AREA) – GEOCHEMICAL TRANSFORMATIONS AND HYDROLOGICAL TRANSPORT STEFAN PEIFFER 1 , CARL BEIERKUHNLEIN 2 , ALEXANDRA SANDHAGE-HOFMANN 3 , MARTIN KAUPENJOHANN 3 and SILKE B ¨ AR 1 1 Limnologische Forschungsstation, 2 Lehrstuhl f¨ ur Biogeographie, 3 Lehrstuhl f ¨ ur Bodenkunde, all at University of Bayreuth, D–95440 Bayreuth (Received 14 December, 1994; accepted 30 January, 1996) Abstract. A field study was performed on the effects of acid mine leachate from slate mine tailings seeping into a small river passing through the tailings. Before entering the tailings the river water has high alkalinity which neutralizes acidity upon mixing with leachate within the tailings. Downstreams of the tailings the pH of the river water ranges about pH = 8, the water contains high concentrations of sulfate ( 1500 mol/l) and particulate bound aluminium ( 80 mol/l), but low concentrations of dissolved aluminium ( 3 mol/l). It is therefore assumed that Al(OH)3 colloids are precipitated during the neutralisation process and transported out of the tailings. The concentration of particulate bound aluminium along the river shows a strong correlation with the concentration of sulfate, which indicates that particulate bound aluminium is conservative. It therefore seems that under dry weather conditions (under most of the sampling was performed) no chemical retention mechanism exists which confines the distribution of aluminium to a restricted part of the catchment area. In contrast, the white river sediment is rich in both aluminium and sulfate, which suggests the temporary formation of aluminium hydroxosulfate minerals. Favorable (i.e. acidic) conditions may prevail at high discharges where the acidity accumulated in the tailings is flushed into the river with its subsequent acidification. Key words: acid mine drainage, aluminium, river sediments, mixing behaviour 1. Introduction Acid mine drainage is one of the most severe environmental problems associated with mining. Due to oxidation of iron sulphides such as pyrite and pyrrhotite acidity is produced which leads to a subsequent leaching of potentially toxic metal ions into the receiving waters. Therefore damages to the adjacent river ecosystems are frequently observed (Theobald et al., 1963; Parsons, 1977; McKnight and Feder, 1984). In the East Thuringian slate mining area problems arise from high aluminium loadings in the brooks and rivers receiving water seeping out of slate mine tailings (Herrmann et al., 1989). The East Thuringian syncline consists mainly of carboniferous sedimentary rocks. Slate mining has a long history in this area. Economically exploitable slate occurs within a strongly metamorphosed host block of the East Thuringian Syn- cline, the ‘Frankenw¨ alder Querzone’. The blue slate is associated with strata rich in pyrite (‘Rußschiefer’) and mineral concretions (geodes). The crusts of these Water, Air, and Soil Pollution 94: 401–416, 1997. c 1997 Kluwer Academic Publishers. Printed in the Netherlands.