Bioretention Monitoring - Six Full Scale Bioretention Swales

The City of Redmond is in search of a bioretention soil media design standard that can demonstrate good stormwater runoff treatment for wide scale use prior to discharge to surface water or to groundwater. The treatment performance goals: - For discharge to surface waters, remove dissolved copper, dissolved zinc, and total suspended solids. - For discharge to groundwater that is used as a drinking water resource, remove nitrogen. - For discharge to phosphorous sensitive lakes, remove phosphorous. - Remove oils and grease. Bioretention soil must also: - be easy to specify, - be readily available, - be nonproprietary and economical, and - support plant growth. A previous study in Redmond showed poor treatment performance for the Ecology standard mix design of 60% sand and 40% compost. Redmond received a grant from Ecology to perform monitoring of six new full scale bioretention swales using the Technology Assessment Protocol – Ecology (TAPE) , that is used in Washington State for evaluation of emerging technologies for stormwater treatment. The objective of this project is to use TAPE to prove that bioretention can be an effective nonproprietary method of removing pollutants from stormwater runoff. Redmond constructed six swales, each sized to Ecology standards. Each swale was constructed in such a way that stormwater influent and effluent can be monitored. The mix designs to be studied are: - 60/40 Mix. Repeat the 2012 study to see if Redmond got a “bad batch” of compost in that study. - Loamy Sand. Loamy sand is hard to specify, but is commonly used on other parts of the country in bioretention. - Loamy Sand 2. Obtain loamy sand from a second source to identify some variability in performance - Redmond BSM. 60% sand, 15% compost, 15% biochar, 10% shredded bark - Redmond BSM 2. Replicate the Redmond BSM swale. - Redmond BSM 3 (saturated) Replicate the Redmond BSM swale, but incorporate a saturated underdrain zon