The treatment of metals in urban runoff by constructed wetlands

The use of constructed wetlands for the treatment of domestic wastewater is now well established in the UK and their ability to treat a range 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 operational benefits, in both industrial and developing countries. In response to this, the Environment Agency have developed constructed wetland treatment systems at two selected sites in south-east England, both of which receive large volumes of urban runoff. The sites are located at Brentwood and Dagenham and were completed in April 1995. Water and sediment samples have been collected at bi-monthly intervals at each site since October 1995 and analysed for a range of parameters including the total concentrations of six trace metals — cadmium, copper, nickel, chromium, lead and zinc. Similar analysis has been carried out on plants 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 maximum removal efficiencies being recorded at the Dagenham wetland during a storm event. Analysis of plant tissues indicates that the reeds bioaccumulate trace metals and that metal uptake is greatest in the roots. Sediment metal concentrations are typical of a site receiving urban runoff. At both sites the highest sediment concentrations are consistently recorded in samples collected from the settlement tanks.