Event-based sampling on a street-side bioretention facility in Seattle found that over a 2.5-year period, 48-74% of the incoming runoff was infiltrated or evaporated. Outlet pollutant concentrations were significantly lower than those at the inlet for total suspended solids (TSS), total nitrogen, total phosphorus, copper, zinc, and lead. Motor oil was removed most effectively, with 92-96% removal efficiency.
Chapman, Cameron, and Richard R. Horner. (2010). Performance assessment of a street-drainage bioretention system. Water Environment Research, 82(2),109-119.