Performance assessment of a street-drainage bioretention system.

Event-based, flow-paced composite sampling was carried out at the inlet and outlet of a street-side bioretention facility in Seattle, Washington, to assess its ability to reduce street runoff quantity and pollutants. Over 2.5 years, 48 to 74% of the incoming runoff was lost to infiltration and evaporation. Outlet pollutant concentrations were significantly lower than those at the inlet for nearly all monitored constituents. In terms of mass, the system retained most of the incoming pollutants. Besides soluble reactive phosphorus (the mass of which possibly increased), dissolved copper was the least effectively retained; at least 58% of dissolved copper (and potentially as much as 79%) was captured by the system. Motor oil was removed most effectively, with 92 to 96% of the incoming motor oil not leaving the system. The results indicate that bioretention systems can achieve a high level of runoff retention and treatment in real-weather conditions.