NÄRINGSÄMNESRETENTION I EN ANLAGD VÅTMARK – EN INTENSIVSTUDIE AV ORMASTORP S / Nutrient retention in a constructed wetland – case study South Ormastorp
South Ormastorp is a constructed wetland in the Råå river catchment area located in the northwest of Skåne. The aim of the wetland is to decrease the diffuse leaching of nutrients from Råå river to Öresund. During three weeks South Ormastorp was intensively studied. Runoff was continuously measured and water samples were collected daily (Monday–Friday), both using composite sampling and spot sampling. The water samples were analysed for total nitrogen, total phosphorus, dissolved phosphorus, phosphate, nitrate+nitrite, ammonia and suspended matter. Different methods of measuring and calculating nutrient retention was also compared. It was not possible to prove a statistically significant difference between the concentration of total nitrogen and the concentration of nitrate. Therefore all nitrogen is supposed to exist as nitrate. The concentration of nitrate clearly followed the runoff. The relative retention of total phosphorus was high and it increased when the runoff increased. Different methods of calculating nutrient retention gave very different results depending on how water samples were collected and runoff was measured. Composite sampling is to be preferred instead of spot sampling and to continuously measure runoff is to be preferred instead of using recalculated values from SMHI’s runoff measuring station, Bröddebacken. The recalculated values from Bröddebacken showed a runoff that was just 10 % of the real runoff into the wetland during the first peak runoff.