Hydrologic and soil properties of mature bioretention cells in Ontario, Canada

Bioretention systems, which mimic natural hydrology and reduce volume of stormwater runoff, are a preferred solution for meeting water balance objectives, but lack of knowledge about the long-term performance of these systems hinders their wider adoption. This study was a field survey of mature (>3 years and up to 10 years post-construction) bioretention cells across Ontario, Canada. The survey involved visual inspections, determination of soil physical parameters and soil-water interaction parameters, infiltration capacity testing and synthetic drawdown testing. Results indicate that infiltration capacity remains above the recommended minimum of 25 mm/hr, likely due to high content soils and development of soil structure due to biological factors over time. The drawdown times for three sites ranged from 5 minutes to 6 hours, much less than the maximum allowed drawdown time of 24–48 hours. Ksat (saturated hydraulic conductivity) was only moderately negatively correlated with age, and where data existed on KSat at the beginning of operation, KSat improved for six out of nine sites. Soil-water interaction properties more closely resembled loam soils than sandy soils, which may be due to the development of a soil structure over time. We recommend conducting visual inspections regularly over infiltration capacity testing for quick determination of maintenance needs. HIGHLIGHTS Hydrology of bioretention continues to perform well over long-term (up to 10 years).; Media infiltration rates remain high and increased for 6/9 sites.; Drawdown times much faster than design.; Soil structure developed over time likely contributes to more water holding capacity available for plant uptake.;