About Bioretention Basins

Bioretention basins are shallow depressions that use a special soil mixture and specific plants to collect, absorb, and filter stormwater pollutants flowing off hard surfaces. The soil mixture uses soil, compost, sand, and specific minerals to allow water to soak in rapidly, treat runoff, and support plant growth. Hardy, low-maintenance plants adapted to the local climate and soil moisture conditions are chosen including trees, shrubs, and grasses. These plants can have the added benefit of attracting pollinators, providing habitat for beneficial insects and birds, adding beauty, and improving neighboring property values.

Engineered rain gardens are one type of bioretention basin. Some other bioretention basins include proprietary devices such as the Filterra tree box media filter on Ohio and Ellis Street, and the Filterra BioScape on 8^th Street.

Water Quality Benefit: Bioretention basins can capture sediment and bacteria, capture oil, pesticides, fertilizers, and dissolved metals. Plant roots also are host to diverse microbial and fungal populations that filter pollution using biological processes. Bioretention basins that have under-drains usually only filter out large sediment and can allow dissolved pollution to pass through.

Water Quantity Benefit: Bioretention basins reduce flooding by either holding rainwater and releasing it slowly, or by soaking it into the ground. Special soil mixtures are chosen to allow water to soak into the ground. Roots break up the soil so water can soak into the ground more easily. If they are built in an area where the existing soils drains well, then water will soak in and recharge the groundwater below. If the existing soil does not drain well, the bioretention basin will be built with an underdrain that lets water out slowly. In a large storm, if there is too much rainwater to fit in that bioretention basin, it will drain out an overflow pipe or bypass the basin.