The aim of this study was to investigate the feasibility of using a floating treatment wetland for removal of
dissolved metals from urban runoff under full-scale conditions. The study site is a retention pond located
in northeast Nantes (France) that receives the storm water runoff from a section of highway
(27,000 vehicles/day, basin surface 375 m2
). Three floating marshes were anchored in the retention pond,
each raft with a surface area of 1.5 m2 (125 cm 120 cm) supporting 16 plants (8 Juncus, 8 Carex). After
4 months, plant samples were harvested, mineralized and analyzed by atomic spectrometry absorption
to determine cadmium, nickel and zinc concentrations. Analysis showed Ni concentration in leaves
between 23 and 31mg/gdrymatter, and between 113 and 131mg/g in roots. Accumulation of Zn was 45–
80mg/g in leaves and 168–210mg/g in roots. The root/leaf ratio, between 2.6 and 5.7 for Ni and Zn
respectively, highlights the important function of roots in heavy metal accumulation. No cadmium
accumulation was detected in plants. All three metals were also present in the biofilm on roots. Moreover,
roots demonstrated a capacity to filter particulates. This study shows that floating marshes can effectively
remove some metals from stormwater ponds.