| Summary: | Streams in Norrbotten County, near the coast of the Gulf of Bothnia in northern Sweden, suffer from recurrent episodes of low pH and high metal concentrations, which have resulted in fish kills and cadmium levels exceeding environmental quality standards defined in the EU Water Framework Directive. The cause of these acid episodes is that sulfide sediments, which are quite common under the highest coastline of the Littorina Sea, form acid sulphate soils when drained for agricultural and forestry purposes. Sulphuric acid and metals are leached from the acid sulphate soils and transported with the drainage water to nearby streams. The study was commissioned by the County Administrative Board in Norrbotten and the aim was to identify potential areas of risk where streams can be affected by acid sulphate soils. In order to identify areas of risk a method based on GIS (geographical information systems) was developed and the method was supported by water measurements (pH and conductivity). The study was based on a case study of five catchment areas in the size range 200-600 km2; Aleån, Rosån, Alterälven, Altersundet and Holmsundet. Of these catchment areas, Rosån, Altersundet and Holmsundet have previously been affected by acid episodes, while Aleån and Alterälven have not. The results show that acid sulphate soils occur throughout the study area, although, Rosån, Altersundet and Holmsundet probably have the largest proportional distributions of acid sulphate soils, which may explain why they are affected by acid episodes. Another explanation may be that Aleån and Alterälven have greater water flows, resulting in greater dilution of sulfuric acid and metals. The water measurements confirm the existence of acid sulphate soils in the catchment areas, and the method can be advantageously implemented for other areas below the highest coastline of the Littorina Sea. A follow-up study is needed, and it should focus on the smaller, coastal catchment areas (such as Holmsundet) because they are more likely to be affected by ...
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