Impact of Grassed Swales on the Fate of Metals Leached from Roads Built with Municipal Solid Waste Incineration Bottom Ashes

Abstract

Roads built with municipal solid waste incineration (MSWI) bottom ashes may possess a risk to the environment if contaminants (e.g. heavy metals) are leached out. In this study, roadside swales were constructed and studied as a trap for pollutants leached from an experimental road. Soil from the slopes and bottoms of the swales was sampled twice yearly (in spring and autumn) and analysed for metal content. Common plant species in Northern Sweden were planted and a grass mixture was sowed in the swales. The aerial plant parts were sampled in the autumn and analysed for Cu, Cr, Ph, and Zn. One season of observations did not reveal any significant effect of the road on the soil and plants. The metal concentrations in the soil of the swales varied insignificantly between the samples collected in the spring and autumn. Metal concentration in the soil did not exceed the generic guideline values for Swedish contaminated soils. ne dry summer of 2002 could have hindered the formation of ash leachate from the road. As the soil proper-ties (pH, metal content) of all studied swale sections remained unaltered, a lack of humidity might be the reason for the observed poor plant establishment in the experimental and reference swales. The impact of the road material on the soil quality started to show up during the second season (2003). Slopes of the experimental swale became contaminated with Cu. Yet the metal translocation from grassed swales through the trophic chain is unlikely; in soil containing higher concentrations of metals grew vegetation with lower metal uptake.