Mixing zones can be designed by EU member states when the pollutant concentration exceeds the EQS values at effluent discharge points. A Mixing Zone Guideline is applied at the Rönnskär Smelter located in the Northern Sweden with effluent discharges to the Baltic Sea. And a second assessment of mixing zones is done in a mine area with discharge to Brubäcken stream. Further at the last step of the project CORMIX model is used. Climate effects and season changes have resulted to be a matter of discussion for mixing zone design in the Swedish waters.