PAH Removal, Fate and Transport in Stormwater Bioretention Systems Amended with Biochar and Fungi

Bioretention is a widely implemented form of Green Stormwater Infrastructure used to prevent the pollution of receiving waters by stormwater runoff. Polycyclic Aromatic Hydrocarbons (PAHs) are a family of organic contaminants which are ubiquitous in stormwater. Several PAHs are toxic to aquatic species. Little is known about the transport and fate of individual PAH compounds in bioretention systems. Even less is known about how amendments to standard bioretention formulations might optimize PAH treatment. We conducted a mesocosm-scale experiment to evaluate the removal of PAHs using four types of bioretention media. We used a standard, planted bioretention media mixture (60:40 sand:compost by volume) and three other formulations containing the standard mixture and two amendments - biochar and fungi. This mixture was amended with biochar (50% replacement of compost) or inoculated with Stropharia rugosoannulata. Twelve bioretention columns (4 treatments in triplicate) were dosed with highway runoff during 8 storm events. Influent and effluent samples from each mesocosm were analyzed for 24 parent PAHs. Samples of bioretention media were taken horizontally from ports in the columns four times throughout the study and analyzed for 20 parent PAH compounds. Our results showed that all media types provided efficient PAH removal (>97% removal for 54 of 56 samples). Fluoranthene, pyrene, and phenanthrene were the predominant PAHs in runoff and media samples. Initial Total PAH concentrations were twice as high in columns that did not contain the 50% compost replacement with biochar. We observed a net loss of PAHs in all mesocosms despite repeated PAH inputs from stormwater and almost no breakthrough in the effluent. Our results suggest that bioretention media containing sand and compost provides excellent PAH treatment at the influent concentrations used in this study (0.089-4.62 ug/L). We hypothesize that our bioretention systems support internal PAH loss mechanisms through microbial bioremediation and plant uptake.