Biofilters to improve road run-off quality : interactions between water, soil and the biosphere

International audienceBiofilters have been widely implemented in the past few years due to combined ecological and economic advantages, such as improved road runoff quality, impervious surface reduction and low cost. However, poor is known about the role of microorganisms and plants on pollutants remediation in such infiltration systems. Thereby, we studied during one year trace elements (TE), total hydrocarbons (HC) and polycyclic aromatic hydrocarbons (PAH) in water, soil and plant in two experimental road side biofilters, one covered with grass, the other one being planted. We also faced the challenge to build four representative large-scale outdoor mesocosms, spiked with Cd, Pb, Zn, phenanthrene, pyrene and benzo[a]pyrene,, to better understand the biofilters functioning. Four plant species were tested for remediation in the mesocosm experiments: Juncus effusus, Iris pseudacorus, Phalaris arundinacea and grass (50% Lolium perenne and 50% Festuca L.). PAHs were extracted from soils by microwave assisted extractions and analyzed by gas chromatography while microwave-acid digestion was used to extract metals, which were then analysed by ICP. Total, fungal and bacterial biomass were investigated using Q-PCR, 16S rDNA and 18S rDNA. Fluorescein diacetate (FDA) enzymatic activity allowed us to measure the total microbial activity. The passage of runoff through the experimental swale showed a significant removal of organic and inorganic pollutants in water: from 27 to 79% for Cu, 21 to 37% for Pb, 27 to 62% for Zn, and 56 to 70% for total HC. Hg, As and Cd were under their detection limits.. Phenanthrene and pyrene concentrations in the soils of mesocosms were divided by 100 in 6 months and by 5 to 10 for benzo[a]pyrene, depending on the plant species. The mesocosm experiments showed that Juncus effusus facilitated PAH degradation and that Phalaris arundinacea had the highest shoot concentration of TE. Both in the experimental swale and in the mesocosm, grass appeared to be the best filter for suspended solids (5 to 77% removal) because of its dense rhizosphere. Swales design could be improved using a mix of plants composed of grass, Juncus effusus and Phalaris arundinaceae that respectively, trap suspended solids, facilitate PAH degradation and absorb TE. These results show that biofilters could be an alternative to hydrocarbon separators to treat road runoff