This study presents results from monitoring the performance of six full scale bioretention systems in Redmond, Washington from 2013 to 2015. The results from this study corroborate other recent studies in the region indicating that bioretention systems which utilize a BSM which meets the Ecology 60/40 specification will act as a pollutant generating source of nutrients and copper. These findings indicate that the previous assumptions regarding metals and nutrient reductions in bioretention, as summarized in two recent local literature reviews (Geosyntec 2013; Taylor Aquatic Science and Policy and Cardno TEC 2013), are not accurate for the 60/40 mix installed to the 2012 Stormwater Management Manual for Western Washington specifications. For systems installed with underdrains which discharge to sensitive receiving waters there is a potential for increasing downstream nutrient and copper loading. For such applications, bioretention projects should use sand with low metal and nutrient concentrations and find organic alternatives to compost, such as coconut coir. Follow up studies at both the bench scale and field scale are currently being conducted with the goal of formulating a BSM that does not export pollutants, while still reducing influent pollutant concentrations.
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Final Report: City of Redmond Six Swales Bioretention Monitoring
Reference: Herrera. 2015. Final Report: City of Redmond Six Swales Bioretention Monitoring. Prepared for City of Redmond Department of Public Works, by Herrera Environmental Consultants, Inc., Seattle, Washington.