Bioretention cells use vegetation and soil media for source control of urban stormwater runoff, alleviating waterway impairment. Environmental performance of two cells was investigated. First, a cell capturing road runoff was monitored for one year. At a second cell, media were sampled to measure lifetime metal accumulation and evaluate the environmental, health, and maintenance implications of metal sequestration.

Monitoring found high metal and suspended solids removal, generally poor nutrient performance, and chloride export. Runoff volume and peak flow rate reduction occurred for small storm events. For larger events, outflow volume consistently exceeded inflow because of unique site conditions.

Lead, copper, and zinc media concentrations in the second cell were elevated but well below cleanup thresholds. Metals were strongly bound to bioretention media and largely immobile; lead bioavailability was comparable to generic soil estimates. Most metal accumulation was near the inflow point in the top 3 to 12 cm of media.