An assessment of metal removal from highway runoff by a natural wetland

Constructed wetlands are increasingly being seen as a viable solution to the treatment of highway runoff. This paper presents a preliminary assessment of the heavy metal removal performance of a natural wetland. The results are compared with the levels found in an adjacent stream which also receives runoff from the same major road. Both subsequently discharge effluent at proximal locations into the Brent Reservoir, a site of special scientific interest in NW London. The concentrations and temporal trends of Cd Cu, Pb and Zn in the water, sediment and emergent macrophytes of the wetland and stream are discussed with respect to the treatment efficiency of the wetland. Although there is evidence of active metal uptake by the sediment and macrophytes, aqueous metal concentrations generally remain unchanged. The results support the proposal to construct a wetland planted with selected species of macrophyte to treat highway runoff on approximately half the area of the current site.