The performance and perception of wetland systems for the treatment of highway runoff

Constructed wetlands are increasingly being seen as a viable solution to the treatment of highway runoff although currently there are no established design and performance criteria for highway runoff treatment systems. In order to assess the heavy metal removal performance by different wetland systems receiving urban runoff, a natural and constructed wetland site were monitored for metal removal performance and metal accumulation by the sediment and four macrophyte species. Although there is active metal uptake by the sediment and macrophytes, aqueous metal concentrations remain unchanged in the natural wetland. The initial results from the constructed wetland system show variable metal removal performance although an improvement in performance is expected as the system becomes more established.
An experimental laboratory-based wetland system showed efficient treatment performance of water dosed with heavy metals. The distribution of metals in the substrate highlight the importance of the hydraulic design, and the need to reduce the possibility of shortcircuiting, in constructed wetlands for runoff treatment.
A questionnaire survey was used to assess the public's perception of the aesthetic and wildlife value of wetlands and their attitudes towards the use of wetland treatment systems for water pollution control. A predictive model for the visual preference of wetland landscapes was developed from the results of the survey. The results show that the public has a clear preference for clean natural looking wetland environments with landscape complexity, diverse vegetation and the presence of wildlife. The public have reservations about the use of wetlands for wastewater treatment indicating the need for background information to allay these concerns.
Design criteria are proposed for the engineering of the wetland that will be constructed by the Environmental Agency (EA) at the natural wetland site to treat highway runoff. The results of the survey provide recommendations for the landscaping design of treatment wetland systems.