King County evaluated how effectively a large improvement project treats stormwater at a regional water detention facility in Federal Way.
Study question
- How well does each individual practice treat stormwater in this large retrofit?
- What about combinations of treatments?
- What is the overall flow control and pollutant reduction to the stream?
Scope of Work and
Amendment 1
The study evaluated two new bioretention facilities, an expanded and new combined detention stormwater treatment wetlands (wetland complex), and the regional facility as a whole, for their ability to improve water quality and to reduce peak flows of stormwater runoff. The study also evaluated the in-stream water quality and benthic macro-invertebrates in North Fork West Hylebos Creek before and after the retrofit.
Study findings
Eighteen storms between March 2016 and April 2017 were sampled using compositors. Flows were measured continuously. Both bioretention facilities, the expanded wetland complex, and the system as a whole reliably attenuated stormwater flows by reducing and delaying the timing of peak flows. The bioretention facilities and the wetland complex had mixed water quality treatment results, they were able to treat some targeted pollutants but not others. The system as a whole reduced total suspended solids, organic contaminants, and total metals (zinc, lead, copper and cadmium). However, the system increased concentrations of nutrients and dissolved lead. The bioretention facilities were a source of nutrients.
Water quality and benthic community data from the North Fork of West Hylebos Creek were collected downstream of the retrofit and expansion before and after the project to assess overall performance of the system. Turbidity improved, though the data were not significant, and no changes in the benthic community were not observed. It is likely too early to detect a change in the benthic communities in the creek.