Bioretention Stormwater and Soil Pollutant Monitoring Performance of PCBs and Mercury to Support Management Planning

Abstract

Bioretention performance data for polychlorinated biphenyls (PCBs) and mercury (Hg) remain limited, yet are critically important to guide stormwater managers in places with PCB and Hg regulations. This study monitored four bioretention systems in San Francisco, California, with varying ages ( < 1 to 11 + years) and influent concentrations over two wet seasons. To help interpret the San Francisco findings, the data were integrated with a larger regional data set including nine other systems (13 sites total) and investigated using concentration-dependent efficiency curves and reference–effluent–influent (REI) plots, along with a Best Management Practice (BMP) Performance Index (PI) tool for objective assessment of water quality goals. The results of the synthesis show that influent concentration and system maturity played key roles; more mature systems outperformed newly installed ones, and sites with higher influent concentrations had higher percentage reductions in concentrations. A clear, irreducible concentration was apparent for both PCBs and Hg, and the study found significant capture of PCB-11 (dichlorobiphenyl), a lighter congener often found in the dissolved phase, despite past assumptions regarding its mobility. Mercury concentration reductions were more variable, consistently exhibiting reductions only when influent concentrations were greater than 32    ng / L . Additionally, hydrologic modeling of an undersized system [2% green stormwater infrastructure to drainage management area (GIS:DMA) ratio] demonstrated that even systems that do not meet standard sizing conventions can achieve substantial annual mass capture (e.g., 59% for PCBs). Soil profile analysis confirmed that pollutants accumulate primarily in the upper 50–100 mm of media and near inlets, with concentrations generally well below EPA regional screening levels (RSLs). The study shows that integrating efficiency curves to support parameterizing load-reduction models with REI plots for regulatory compliance supplies a robust framework for stormwater managers. The study findings suggest prioritizing bioretention placement in highly contaminated areas to maximize mass capture and allow for a multiyear stabilization period before formal performance assessment.