Water framework directive mixing zone guidelines : applied on a smelter and mine scenario at two Boliden sites

Abstract: The Water Framework Directive (WFD) aims to protect the aquatic environment and human health by reducing pollutants at the source. In order to achieve this, the concentration of priority substances should be lower than Environmental Quality Standards (EQS) values at the point sources. There are cases when the priority substance exceeds the EQS at the effluent discharge, however, they should be met at a distance close to the discharge point. The zones in the vicinity of discharge points where the priority substances exceed the relevant EQS values are called mixing zones.Article 4 of Directive 2008/105/EC allows member states to designate mixing zones when the pollutant concentrations exceed the EQS values at effluent discharges. But mixing zones should not affect the compliance of the rest of the water body with EQS standards. Therefore, mixing zone design should meet some defined criteria. Moreover for ensuring that design criteria are met and for facilitating the mixing zone application, member states are provided with a Mixing Zone Guideline.This project applies the Mixing Zone Guidelines at the Rönnskär Smelter located in the Northern Sweden with effluent discharges to the Baltic Sea. A second assessment of mixing zones is done in a mine area with discharge to Brubäcken stream. A “Tiered Approach” used in the guidelines is followed in assessment of mixing zones. The priority substances considered in the effluents are Hg, Cd, Ni and Pb. In the Rönnskär Smelter at Tier 2 determination of mixing zone length is done by the Discharge Test provided with Mixing Zone Guideline. CORMIX model is used for prediction of mixing in Tier 3.In the mine area the concentration of metals at the measured points showed no exceedence of EQSs, therefore there was no need for the mixing zone design. In Rönnskär, for effluents where more than one priority substance exceeded the EQS values, the mixing zones determination is done separately for each metal. Mixing zone AA-EQS criteria were met for each substance within a predefine distance of 500 m from each of the discharge points. However, much more investigation is needed related to the use of total and dissolved metals concentrations, consideration of background concentration and other input data which are site specific. Further research is also needed for mixing zone assessment where the effluent discharges contain multiple priority substances.We also concluded that climate effects, seasonal changes and other factors related to the receiving water characteristics need further investigation for a proper implementation of mixing zones for protection of aquatic and human life in Sweden.