Herrera’s Role in Developing New High Performance Bioretention Soil Media

Bioretention remains the foremost Best Management Practice (BMP) in the Green Infrastructure toolkit. Reducing toxic runoff, helping prevent combined sewer overflows, providing traffic calming, urban habitat and placemaking, the list of benefits runs long.

However, a 2015 study conducted with the City of Redmond discovered that bioretention built with underdrains and the standard Ecology media specification of 60 percent sand and 40 percent compost (60/40), exports phosphorus, nitrogen, and copper. These pollutants remain detrimental to sensitive downstream receiving waters. With this discovery, the region embarked upon a 6-year journey to develop an alternative bioretention soil media (BSM). From 2016 to 2020, Jenee Colton (King County) and Chris May (Kitsap County), with help from Herrera, lead the effort to obtain Ecology grant funding to study alternative BSMs that would cease the export of nutrients and copper.

This work culminated in 3 new specifications to be used as an alternative to the 60/40 BSM. The new BSM replaces the compost with components such as biochar, coconut coir, iron filings, and activated alumina. These new specifications are dubbed high performance bioretention soil media (HPBSM) and constitute a marked improvement in water quality performance, while retaining all previous benefits of the 60/40 BSM.  Herrera is honored to be involved with the process of making our best stormwater treatment BMP even better!

The new specification can be found on Ecology’s website. The studies leading up to the new HPBSM specifications can be found on our website here and here.

  • Project Manager Curtis Hinman, explains the test column array to colleagues at Seattle University.
  • Herrera staff assess biomass of plants grown in the new HPBSM.
  • Measuring infiltration rates at 185th in Redmond. The study that alerted the region
  • Western Washington University students helping Herrera assess new media options.
  • We love to work with students during our research efforts and frequently end up hiring them!
  • A new typical bioretention cross section showing the layered HPBSM.