Restoration of lake Borrevannet - Self-purification of nutrients and suspended matter through natural reed-belts

A three-year large-scale study for estimating the retention of nutrients and suspended matter has been carried out in a wetland (littoral reed-belt) in the southern part of Lake Borrevannet, in the south-east of Norway. The reed-belt adjacent to the lake was isolated with a 250 m long reinforced PVC-membrane equipped with a tunnel for transportation of water out of the wetland. Inputs, mainly from agricultural runoff, were measured at seven inlet stations. Calculations of retention show a substantial retention of suspended matter and residue on ignition, respectively 94 and 97% for the whole period 1994-96. Total phosphorus shows a mean value of 65%. The retention of orthophosphate was 85%. Nitrogen shows a lower retention rate, 44% for total nitrogen, somewhat lower for ammonia and somewhat higher for nitrate. For suspended matter/residue on ignition and for the phosphorus fractions, there was a high retention rate also during the winter period. For nitrogen, however, there was a significant reduction in the retention rare in the winter period. While in the summer period, 50-85% of the total nitrogen was held back this was down to 30% in the winter period. (C) 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved.