It is hard to tell just by looking at a tree-lined street, but soil and its microbes — in combination with trees — can form a powerful system for improving stormwater quality. Some pollutants are filtered or retained by soil, while others are taken up or transformed by plants and microorganisms.

The U.S. EPA’s 2023 Bioretention Design Handbook explains that bioretention uses physical, chemical, and biological processes to treat runoff. Suspended solids can be removed through sedimentation and filtration, while dissolved pollutants may be treated through sorption, precipitation, plant uptake, and microbial activity. The amount of time runoff remains in contact with the soil can also influence treatment performance.

The table above summarizes some of the primary ways bioretention treats common stormwater pollutants. Research has consistently found strong treatment potential for suspended solids, metals, polycyclic aromatic hydrocarbons, and other organic compounds. Nutrient removal — particularly dissolved nitrogen and phosphorus — has been more variable.

Vegetation can improve bioretention performance by reducing flow velocities, stabilizing soil, encouraging infiltration, taking up nutrients, and supporting biological activity around the roots. Studies comparing vegetated and unvegetated bioretention media have found that plants can improve nitrogen and phosphorus retention. However, performance depends on the plants, soil media, drainage conditions, and pollutants being targeted.

The soil itself must also be selected carefully. Particulate phosphorus is primarily removed through sedimentation and filtration, while dissolved phosphorus may be treated through plant uptake, adsorption, and precipitation. The EPA cautions that unsuitable existing soils or bioretention media can sometimes release nutrients instead of retaining them — particularly in nutrient-sensitive watersheds. There is no single soil mixture that will provide optimal treatment for every pollutant or project.

In a Silva Cell stormwater application, runoff is directed into the lightly compacted soil beneath the pavement. As water moves through that soil, pollutants can be filtered, retained, or biologically transformed before the water infiltrates into the ground or leaves through an underdrain. At the same time, the soil supports tree growth, allowing the same underground space to provide both bioretention and long-term urban-forestry benefits.

Conventional pipes are effective at conveying stormwater, but soil-based bioretention adds treatment at the source. Trees growing in generous soil volumes can therefore become an active component of green infrastructure — helping cities improve stormwater quality while creating healthier, greener streets above.

Sources

• U.S. Environmental Protection Agency. Bioretention Design Handbook: Designing Holistic Bioretention for Performance and Longevity. EPA 841-B-23-002, 2023.
• Davis, A.P., Hunt, W.F., Traver, R.G., and Clar, M. “Bioretention Technology: Overview of Current Practice and Future Needs.” Journal of Environmental Engineering, 135(3), 109–117, 2009.
• Lucas, W.C., and Greenway, M. “Nutrient Retention in Vegetated and Nonvegetated Bioretention Mesocosms.” Journal of Irrigation and Drainage Engineering, 134(5), 613–623, 2008.
• LeFevre, G.H., Hozalski, R.M., and Novak, P.J. “The Role of Biodegradation in Limiting the Accumulation of Petroleum Hydrocarbons in Raingarden Soils.” Water Research, 46(20), 6753–6762, 2012.