Field trial: new layered bioretention media removed 6PPD quinone to nondetect in initial full‑scale tests

King County and consultant teams presented the first full‑scale field performance data for a high‑performance bioretention soil media (HPBSM) designed to reduce nutrient export while capturing tire‑derived contaminants.

Dylan Ahern (Herrera Environmental Consultants) described the layered media — a surface mix of sand, coir and biochar over a polishing layer (sand, activated alumina and iron aggregate) — developed to avoid nutrient export associated with compost‑heavy mixes. Chelsea Mitchell (King County) presented 20 storm events sampled at a retrofit roadside swale in Whatcom County and reported influent 6PPD quinone concentrations ranging from 11 to 72 nanograms per liter (flow‑weighted event means).

"We did not detect any 6PPD quinone in the effluent of the system throughout all these storms," Mitchell said, and the team noted the system met state treatment goals for total suspended solids and total phosphorus while achieving zinc removal targets though copper removal objectives were not met in that dataset.

The presenters cautioned that these are early field results and that testing will continue; they also noted that earlier lab work, plant tests and pilot trials informed the media design. The retrofit included a liner and reconfigured plumbing to direct flow through multiple media cells and monitoring stations.

Why it matters: a field‑proven media that reduces 6PPD quinone without exporting nutrients could be incorporated into transportation and retrofit projects that intersect salmon‑bearing streams. Presenters emphasized the need for additional multi‑site and long‑term deployments to assess lifetime performance and maintenance needs.