Occurrence and Concentration of 6 Metals and 28 Organic Micropollutants in the Forebays of Bioretention Facilities

Abstract

Pollutant loads in urban runoff from anthropogenic sources contribute to degradation of downstream waters. Cities are turning toward green infrastructure to manage urban stormwater. Bioretention is popular as green infrastructure and is commonly installed to remove runoff pollutants. A significant proportion of pollutants in urban runoff are particulates or particulate-bound and are effectively removed in bioretention cells. Pollutants accumulate in concentrated areas of the bioretention (e.g., forebays, inlets, surficial filter layers), which require maintenance to restore effective treatment and to increase the operational lifespan. Particles trapped in forebays risk diminished effectiveness of the pretreatment, which may eventually lead to filter clogging and leaching of toxic pollutants. Studies have examined pollutant accumulation and distribution in bioretention filter media, but less is known about processes in bioretention forebays. In this study, 28 bioretention forebays were examined in urban areas of Ohio and Michigan (United States) as well as Stockholm and Malmö (Sweden) to investigate the occurrence and accumulation of metals (i.e., Cd, Cr, Cu, Ni, Pb, and Zn) and 38 analytes of organic micropollutants [OMPs, i.e., alkylphenols, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and phthalates]. Investigated metals were present in all 28 samples, except Cd detected in 27 samples. Of 38 OMP analytes, 31 were detected in at least one sample. PAHs and PCBs were the most frequently detected pollutants found at all examined sites. In general, high concentrations of pollutants were detected in all forebay sediments. Cu, Ni, Zn, PAHs with high molecular weight, and PCBs were detected at concentrations above US and Swedish soil quality guidelines. It was concluded that forebays regularly need to be excavated to maintain their function, and excavated sediments must be handled safely during maintenance work and disposal. High pollutant concentrations in forebay sediments have critical implications for bioretention operators and regulators. Previous studies indicate clogging as a limiting factor for long-term function of bioretention systems, and sedimentation forebays are used to reduce the sediment load reaching the filter surface. However, operators should consider that removed sediment may be contaminated, exceeding guidelines and soil screening levels, and that removed material should be handled accordingly. Zn was the metal most commonly exceeding the Eco-SSL and Swedish EPA guidelines, followed by Cu and Ni. PAH-H exceeded the US EPA Eco-SSL and Swedish EPA standards, PAH-M and Σ7PCBs exceeded Swedish EPA. Concentrations exceeding Swedish EPA may be classified as hazardous waste, need certification for transportation, and must be disposed of at licensed landfill. The large variation between the different forebays (land use, catchment characteristics) underlines that sediment removal frequency and disposal are site-specific and difficult to generalize. However, the recommendation for bioretention operators is regular monitoring and sampling to inform forebay maintenance procedures.