Assessment of PAH dissipation processes in large-scale outdoor mesocosms simulating vegetated road-side swales

Biofilters have been implemented in urban areas due to their ability to improve road runoff quality. However, little is known about the role of soil microorganisms and plants on pollutant remediation in planted swales. Therefore, four large-scale outdoor mesocosms were built and co-contaminated with metals and model polycyclic aromatic hydrocarbons (PAHs) (phenanthrene (Phen), pyrene (Pyr) and benzo[a]pyrene (BaP)), to better understand the complex functioning of swale-like environments. Three macrophyte plant species were tested for enhanced remediation of PAHs: Juncus effusus, Iris pseudacorus, Phalaris arundinacea and a grass mix. Long-term dynamics of PAHs in water outflow and soil was studied. Results showed that only 0.07 to 0.22% of total PAHs were released in water outflow after one year. Two years after contamination, soil sample analyses showed a dissipation of 99.6% for Phen and 99.4% for Pyr whatever the mesocosm considered and ranging from 75.5 to 91% for BaP, depending on plant species. Furthermore, dissipation time-courses may be described by a biphasic process. Experiments showed that the grass mix facilitated BaP long-term biodegradation. Grass appeared also to be the best filter for suspended solids because of its dense rhizosphere, which prevents the transfer of BaP to groundwater.