Performance of constructed wetland for highway runoff treatment.

Highways have a long-term impact on the environment, therefore a sustainable approach to their design is vital. In the spring of 2001 a pilot constructed wetland (CW) system was set up at a section of the northeast-southwest motorway in Slovenia. It was designed for a critical flow of 11.75 l/s for 0.75 ha of catchment area. It consisted of a sedimentation basin covering an area of 36 m2 and a CW of 85 m2. The CW was filled with sand media and planted with reeds. Performance efficiency of the system was evaluated from summer to autumn 2001. Some of the physical and chemical parameters monitored varied noticeably. Removal efficiency was 69% for suspended solids, 97% for settleable solids, 51% for COD, 11% for BOD5 and 80% for Fe. Heavy metals such as Cu, Zn, Cd, Ni and Pb were below limited level at the inflow with reduction efficiency in the system of over 90%. Concentrations of N and P showed a limited level of nutrients for biological processes. Results of the study suggest that CW could be an alternative for highway runoff treatment. Further long-term investigations are needed to provide more data on their proper design.