Metals in combined conventional and vegetated road runoff control systems

Combined conventional and vegetated runoff treatment systems were installed on the rural Newbury Bypass in Southern England, which was estimated to carry an average daily traffic of 30,000 vehicles. The system components were arranged in series to give progressive pollutant removal, comprising an oil separator, sediment trap, grassy slope and constructed wetland. In the absence of specific design criteria for road runoff control, the system design was based on available guidance notes. Since the road opened, an 18-month monitoring programme has examined the performance of these systems. Early indications are that the systems, and especially the wetlands, promote removal of suspended solids (>1.2 μm) and effectively trap fine (<63 μm) sediments. The waters were generally oxidising with neutral to basic pH which may have favoured metal partitioning to the solid phase. The distribution pattern for Zn and Cu in fine sediments contrasted in two of the systems studied, indicating the complex behaviour of metals in wetlands. Further work will increase the data set and examine the adsorption behaviour of metals. These results will contribute to the development of future design criteria for the application of wetlands to road runoff treatment.