THE TREATMENT OF METALS IN URBAN RUNOFF BY CONSTRUCTED WETLANDS

The use of constructed wetlands for the treatment of domestic wastewat er is now well established in the UK and their ability to treat a rang e of industrial wastewaters is now being investigated. However, their ability to treat urban runoff is relatively untested despite the fact that this application could have important environmental and operation al benefits, in both industrial and developing countries. In response to this, the Environment Agency have developed constructed wetland tre atment systems at two selected sites in south-east England, both of wh ich receive large volumes of urban runoff. The sites are located at Br entwood and Dagenham and were completed in April 1995. Water and sedim ent samples have been collected at bi-monthly intervals at each site s ince October 1995 and analysed for a range of parameters including the total concentrations of six trace metals - cadmium, copper, nickel, c hromium, lead and zinc. Similar analysis has been carried out on plant s collected from both sites in the spring of 1997. Results show a wide variation in pollutant levels, reflecting the highly variable quality characteristics of urban runoff. Mean removal efficiencies of metals in the water vary between sites in dry weather conditions, with maximu m removal efficiencies being recorded at the Dagenham wetland during a storm event. Analysis of plant tissues indicates that the reeds bioac cumulate trace metals and that metal uptake is greatest in the roots. Sediment metal concentrations are typical of a site receiving urban ru noff. At both sites the highest sediment concentrations are consistent ly recorded in samples collected from the settlement tanks. (C) 1998 E lsevier Science B.V.