Addressing risks to bioretention performance: The potential for pollutant leaching and groundwater interactions to overshadow design features

Bioretention cells are a popular approach to manage urban runoff, yet few studies have compared the role of plant selection and design features on performance in systems treating similar catchments. Four bioretention cells with varied planting palettes and loading ratios (i.e., ratio of drainage area to cell surface area) treating runoff from a 1.3 ha parking lot in Cleveland, Ohio, USA were monitored for 11 months following construction. While mean concentrations of total suspended solids (TSS) were effectively reduced (by 50–83 %), elevated nutrient (i.e., N and P) concentrations were often observed in the effluent, indicative of leaching of organics from compost in the bioretention media. While concentrations of most metals were reduced or unchanged from relatively low concentrations in influent runoff, higher concentrations of metals known to associate with organic material (e.g., Cu) in the effluent from all cells supported this hypothesis. No significant seasonal differences in runoff reduction performance for any of the bioretention cells were observed, suggesting that volume reduction via evapotranspiration was far exceeded by exfiltration from the oversized cells. While runoff volumes were substantially reduced by three bioretention cells, effluent volumes exceeded influent runoff at the fourth, lowest-lying cell (Tree cell). Suspected interactions with the local groundwater table were confirmed through field assessment of the soil profile after the completion of the study, where redoximorphic features were observed immediately below the underdrain. Findings highlight the trends in bioretention performance after construction until readily leachable material has been washed from the media layer, the critical importance of material sourcing for bioretention media blends, and the implications of groundwater interactions on bioretention function and ultimately receiving waters. • Four bioretention cells installed in a parking lot for 11 months post-construction. • Planting plan, loading ratio varied for each bioretention cell. • Nutrient, Cu leaching from bioretention cells observed, linked to compost in media. • Substantial runoff volume reduction at three cells; overshadowed evapotranspiration. • Outflow from lowest-lying cell exceeded inflow due to groundwater intrusion.