BioRetention

BioRetention, commonly referred to as bioretention, is a stormwater management practice that uses a planted, soil-based filtration system to capture and treat urban runoff. It aims to reduce runoff volume and improve water quality before discharge to receiving waters.

A typical bioretention facility consists of a shallow vegetated surface, an engineered soil media layer, sometimes a mulch or organic top layer, and an underdrain that conveys infiltrated water to a sewer system or groundwater. It may be integrated into street medians, parking lot islands, curb extensions, or dedicated bioswales along transportation corridors.

When runoff enters the facility, ponded water infiltrates through the media where physical filtration removes sediments; the media adsorbs nutrients and metals, and plant uptake plus microbial processes further reduce contaminants. In many designs, a portion of treated water percolates into the subsoil, while the underdrain provides a reliable outlet to prevent standing water.

Applications include new development and redevelopment in urban areas, as well as projects along roads, commercial sites, and public spaces. Benefits include reduced peak discharges, increased infiltration and groundwater recharge, pollutant removal, and enhanced aesthetics and habitat for urban flora and fauna.

Design and maintenance considerations cover appropriate media depth (typically 18–24 inches of engineered media), media composition, plant selection (often native species), and regular maintenance to remove sediment and prevent clogging. Limitations include variable pollutant removal performance, climate or soil constraints, potential groundwater or freeze-thaw issues, and the need for ongoing upkeep and monitoring.

Related concepts include rain gardens, bioswales, and bioretention cells.