The environmental effects of the Finnish industrial loadings are in practice decided by water courts as they give permits. In decision-making they use an inspection procedure. Since 1970s in Finland within this procedure one method of environmental evaluation has been the application of numerical water quality models. At the Kokemäenjoki watercourse water quality effects of the paper and pulp mills in Mänttä, Lielahti and Valkeakoski were studied with two or three dimensional water quality models. On the basis of these results the costs and benefits of different loading options were considered. Now, after more than a decade, the response of the lake waters to the nutrient and BOD loading can be seen in the water quality monitoring results. We used these observations and studied the accuracy and limitations of these early water quality models.

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