EXTREME ACIDIFICATION OF A LAKE IN SOUTHERN NORWAY CAUSED BY WEATHERING OF SULFIDE-CONTAINING BEDROCK

In 1986 Lake Langedalstjenn in southern Norway was a weakly acidified lake with a pH of 5.2-5.6, and an average concentration of SO4 of 330 mueq L-1. The total Al concentration varied between 10 and 20 mueq L-1 (expressed as Al3+). The lake supported populations of brown trout an d perch and had supplied about 100 people with drinking water until th e late 1980's. During 1986-1989, a dramatic change in the water chemis try occurred because of blasting of and weathering of sulphidic gneiss es in the watershed. The oxidation of sulphide to sulphate (sulphuric acid) caused an increase in the SO4 concentration of the draining stre am of up to almost-equal-to 4800 mueq L-1. Weathering and/or cation ex change of Ca and Mg neutralized approximately 52% of the protons from the sulphuric acid production, while about 46% were consumed by mobili zation of aluminium and iron. Nevertheless, about 2% of the hydrogen i ons from the sulfuric acid were still present, which resulted in a str eam pH of 4.0. In the lake, the pH was 4.4, and the concentrations of all major cations and anions were significantly lower than in the heav ily affected stream. Mixing of the stream water with lake water, forma tion of aluminium-sulphate complexes and coprecipitation of Ca may exp lain the resulting concentrations of major ions in the lake.