Acid sulphate (AS) soils cover extensive areas of the coastal plains in western Finland. These soils constitute an environmental hazard due to their ability to release acidity when exposed to atmospheric oxygen. Acidification of soil and water can be reduced or prevented if the factors influencing the release and transport of acidity in AS soils are known. The objective of this study was to develop mathematical models describing the acidity load from AS soils, thus providing tools for accurate soil and water management in the affected areas. The work resulted in a static and a dynamic model for predicting the runoff acidity from catchments containing AS soils. The main features of the models are described in the paper. Both models require significant catchment characteristics and hydrological monitoring results as input data, but no ion balance information is needed. Model applications showed that the static model is able to give good estimates of the mean acidity level to be expected during specific flood periods in the studied river, while the dynamic model, after calibration, is capable of predicting the daily acidity with appreciable accuracy. The information provided by the models can be used to assess adequate mitigration strategies for the watercourses affected by AS soils.

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