Applicant proposes wet pond + bioretention to address water quality; experts say infiltration would better protect fish habitat

Project engineers told the Kitsap County hearing that the Meadowview development’s water-quality system is built around two wet ponds with bioretention features downstream of the ponds. The design team said the combined pond-plus-bioretention approach addresses both coarse and fine pollutant loads raised by the public.

Miss Heffern, the project’s civil engineer, said the project includes two wet ponds sized for water-quality and flow-control, and bio-retention swales through which pond outflows will pass. She said the team selected a “list 2” compliance path under Kitsap County’s stormwater manual, implemented full-dispersion where feasible at the north lakefront, and used bioretention at outfalls downstream of the ponds. “The stormwater pond, the wet pond helps, capture the larger particles that are dense and helps them settle out. ... then as the water releases from the pond and goes through the bioretention, this helps with the microscopic,” she testified.

Where experts differ

- 6PPD-quinone (6PPD-q): Several participants flagged 6PPD-q, a tire-derived contaminant that research has linked to fish toxicity in some settings. Project engineers said they reviewed Department of Ecology guidance and responded in the drainage memo and by adding bio-retention features. Heffern said the county manual does not mandate treatment specifically for 6PPD-q; nonetheless the project added bioretention downstream of the ponds as an additional treatment layer.

- Infiltration vs. underdrained bioretention: Independent expert Dr. Rosin told the hearing that infiltration is the preferred approach for reducing certain contaminants and for sustaining cold groundwater-fed baseflow important to salmonids. He warned that underdrained bioretention (the design the project is using in places) provides less protection than infiltration. “Under drain systems are worse,” he testified, and argued the site’s soils could accommodate more infiltration than the applicant concluded.

Regulatory context and staff review

Kitsap County stormwater rules require water-quality treatment that fits the site. The county reviewer present said the applicant’s chosen system meets the county manual at the preliminary stage and that additional testing or final design can refine the approach. Engineers said the project does not claim enhanced water-quality credits or reduced pond sizing based on infiltration at this stage; doing so would require final-engineering testing and county approval.

What remains unresolved

- Effectiveness of specific BMPs against 6PPD-q: engineers noted that Department of Ecology and national guidance are evolving; the county manual does not currently require 6PPD-q-specific treatment.

- Whether additional infiltration can be implemented to reduce pond reliance and increase baseflow: independent experts urged more in-situ testing at final engineering elevations before the county approves any infiltration credits.

Ending: The applicant proposes a wet-pond-plus-bioretention treatment train that the county reviewer finds compliant at the preliminary stage; outside experts and appellants proposed more aggressive infiltration testing and design to reduce pond dependence and to better protect baseflow and fish habitat.