State and local governments, academia, and the private sector worked together to develop and test several alternative bioretention soil mix (BSM) blend that does not export phosphorus, like Ecology's stormwater management manuals default 60/40 BSM, for use in areas near sensitive surface waterbodies.
- Are there alternative mixtures for the bioretention soil mix (BSM) that will meet basic, enhanced, and phosphorus treatment objectives?
- Do these alternative mixes prevent toxicity to aquatic organisms?
- Are these mixes affordable and available to permittees in Washington state?
Scope of Work
This bench-scale study successfully tested the performance of eight experimental BSM treatments over five storm events using highway runoff stormwater. Influent stormwater was compared to effluent from each BSM blend for total suspended solids (TSS), total and dissolved copper (Cu), lead (Pb) and zinc (Zn), total phosphorus (TP), ortho phosphorus (ortho-P), nitrate+nitrite, fecal coliform bacteria, polycyclic aromatic hydrocarbons (PAHs), and total petroleum hydrocarbons. Pollutant reduction targets were defined by Ecology's TAP-E treatment goals for TSS, metals, and phosphorus.
One experimental BSM (treatment #4) accomplished all the experimental pollution reduction goals including export of phosphorus, but was able to reduce phosphorus carried in the influent. Treatment 4 consisted of several layers (primary, polishing, and compost mulch) that when used in combination, could be anticipated to meet basic, enhanced, and phosphorus reduction goals. The study recommendations provide the specifications for Ecology to adopt the combination or component parts of the treatment 4 experimental blend as a new Washignton bioretention media. A bioretention media specification that meets Ecology’s basic, enhanced and phosphorus treatment objectives will greatly increase the surface water settings where designers and jurisdictions can confidently apply bioretention systems to manage stormwater runoff.