Load and concentration based performance outcomes for emerging organic contaminants in bioretention stormwater treatment systems

Summary

The performance capabilities of bioretention systems for a range of stormwater constituents remain lightly characterized, particularly for emerging organic contaminants. To address this gap, we evaluated concentration- and load-based treatment outcomes at four urban bioretention systems for tire wear particles (TWPs) and 21 chemical contaminants, including 6PPD-quinone (6PPDQ), 1,3-diphenylguanidine (DPG), hexa(methoxymethyl)melamine (HMMM), benzothiazoles, benzotriazoles, pharmaceuticals and personal care products (PPCPs), and pesticides. Inlet concentrations of vehicle-derived contaminants were generally at least twice the values reported for regional stormwater, consistent with these systems draining primarily roadway and parking lot runoff. Concentration-based removal varied widely across compounds, storms, and sites and was moderately associated with predicted organic carbon–water partitioning, although compound-specific deviations indicate additional influences beyond hydrophobicity alone. Median concentration-based removal efficiencies exceeded 76% for 9 of 19 contaminants, whereas the remaining compounds exhibited lower and more variable removal. Although bioretention reduced 6PPDQ concentrations by 95%, the median outlet concentration of 12 ng L−1 remained above the US EPA acute freshwater screening value. At one site with modeled flow data, load-based estimates provided complementary context for treatment performance by integrating measured concentrations with modeled event volumes, revealing high median mass reductions (>90%) for most contaminants over the monitoring window. Soil-core characterization of particulate contaminants showed retention of TWPs and other microplastics (MPs) within the media, the latter predominantly in near-surface layers. Overall, the results demonstrate that bioretention systems can substantially reduce contaminant mass through combined reductions in outlet concentrations per unit volume, stormwater volume reduction, and particle retention, though achieving protective concentrations for toxic contaminants like 6PPDQ may require complementary upstream source control or other treatment enhancements.