Referensvåtmarker för uppföljning av växtnäringsretention i anlagda våtmarker

One of the environmental problems today in seas, lakes and streams is eutrophication. This is often caused by nutrients such as phosphorus (P) and nitrogen (N) that leak from agricultural areas. A measure to partly prevent the nutrient discharge is to construct or restore wetlands. In order to control the efficiency of nutrient reduction of existing wetlands in the county, the administrative board in Västra Götaland wants to find different criterias for reference wetlands. These reference wetlands should represent other wetlands and be used in future evaluations of reduction of nutrients and design of constructed wetlands. According to the administrative board in Västra Götaland the reference wetlands should have a catchment area of about 50 hectare consisting of at least 70 % arable land, to represent wetlands created to remove nutrients. The surface area should exceed 0,5 hectare and the inlet-nitrogen concentration should be around 5 mg N-1. This report investigates nitrogen and phosphorus retention in two wetlands, Härstad and Åmot in the county of Västra Götaland, and if they fulfill the criteria of being a reference wetlands. In addition to this, a tracer study was performed in one of these wetlands with the purpose to study the hydraulic efficency. Neither the wetland in Åmot or Härstad achieves the guidelines of about 5 mg N l-1 in the incoming water. Therefore, they can be seen as inappropriate as reference wetlands according to their nitrogen retention. The wetland of Härstad, however, has significantly better N retention than the wetland of Åmot. Results from the report shows that N retention in the wetland of Härstad was relatively high not only due a higher N load, but also due to that N was largely in the form of NO3- facilitating efficient transformation of NO3- to N2 by denitrification bacteria. Nitrogen removal was much lower in the wetland in Åmot due to that N in incoming water was not in the form of NO3- and could therefore not be efficiently transformed to N2 by denitrification. Incoming total phosphorus to the Härstad and Åmot wetlands exceeded 100 µg P l-1, which means "extremely high" tot-P concentrations according to environmental quality criteria from the Swedish Environment Protection Agency. Phosphorus load per wetland area was slightly higher in the wetland in Åmot than in the Härstad wetland. In spite of this, P retention per wetland area as well as relative P retention was clearly higher in the Härstad wetland. This can be explained by that P in incoming water to the Härstad wetland was to a larger degree than in the Åmot wetland bound to particles, facilitating P retention through sedimentation. According to the tracer study, the Härstad wetland has a hydraulic efficency (λ) of about 0,13 which means it has a low hydraulic efficiency. The effective volume ratio (e) in Härstad was calculated to 18 % which means that the water has an inadequate spreading in the wetland, which is not good for the nutrient elimination in this wetland. This study illustrates the difficulties in finding representative reference wetlands due to potential differences between wetlands in nutrient concentrations in incoming water, the degree that incoming P is bound to particles, the degree that incoming N is in the form of NO3-, and hydraulic efficiency