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Highway runoffs introduce numerous traffic-related pollutants – such as heavy metals, suspended solids and different kinds of hydrocarbons – into surface waters, thus endangering environmental health. In Germany, sedimentation basins with oil retention baffles are mandatory for the treatment of highway runoff in water protection areas. If the distance to the groundwater table is too low, further treatment with event-loaded vertical-flow treatment wetlands (VFWs) is required. In this study, we evaluated if retrofitting of stormwater retention basins to floating treatment wetlands (FTWs) can serve as an alternative to post-treatment VFWs and if native plant species could adapt to the oxygen-deprived salt-enriched environment. Two such basins with two hydraulically separated parallel chambers each near the municipality of Cologne were equipped with floating mats on one chamber, while the other one served as control. One location additionally had hydraulically separated post-treatment VFWs, allowing for a comparison with the treatment efficiency of the FTW. The removal of chloride, sulfate, suspended solids (total and < 63 μm), nutrients, metals, petroleum derived hydrocarbons and polycyclic aromatic hydrocarbons (PDH and PAH) was examined in 54 events over a period of two years. As a result, it was found that plants can create an intense root network despite hostile conditions in the basins. For the target parameters (TSS 63 , PAH and PDH), the removal efficiency of the stormwater retention basin was already sufficient. For metals and ammonium nitrogen, the VFWs proved to be more efficient, whereas for nitrate nitrogen, this was the case for FTWs. None of the treatment systems proved effective against reducing chloride concentration from deicing salts. [Display omitted] [ABSTRACT FROM AUTHOR]