Constructed Wetlands and Buffer Zones as Measures for Agricultural Phosphorus Leakage on a Sub-catchment Scale : The Söderköping River Project

Abstract: The Baltic Sea has a major problem with eutrophication where acts have been taken by the EU commission to sign a common action plan, the Baltic Sea Action Plan (BSAP). The overall goal is to reach a good environmental status by the year 2021, where one of the sub-goals is that the Baltic Sea should be unaffected by eutrophication. For Sweden, the goal for phosphorus (P) is to reduce the annual load with 290 tonnes by the year 2021. Since phosphorus is the main limiting nutrient, it is targeted for reduction when it comes to addressing problems with eutrophication. The objective of this thesis is to estimate the effect of constructed wetlands (CW) and buffer zones (BZ), as measures for reducing agricultural phosphorus, in a specific sub-catchment area of the Söderköping river. The waters in this sub-catchment area have the status unsatisfying and poor. If these measures are suitable for this area and where, and if the effect implementation would fulfill a good status for these waters, are questions that this thesis will aim at. An assigned P reduction has been calculated for the sub-catchments where the waters with unsatisfying and poor statuses are present. Areas for CWs have been calculated with the help of the assigned reduction and retentions found in the literature, and location for them has been suggested. With the help of calculated areas for potential BZs for this area, the effect of them have been calculated by retentions found in the literature with 9 and 10 m widths. The results show higher results for assigned P reduction than the actual P load that is present in the sub-catchments. This gives odd results for the effect of the CWs where they show a P reduction of 59-234 %. The method for assigned P reduction is therefore questioned, where the method for the reference value that is used is not without flaws. For BZs, a reduction of 5-14 % is shown, where the reduction is larger with larger areas for potential BZs. Whether the implementation of the measures will fulfill a good status for the waters is difficult to say, due to the inadequate methods and the odd results given in this thesis.