This study was implemented to assess the potential for groundwater contamination from infiltrating stormwater through a bioretention system constructed with a bioretention soil mix (BSM) composed on 60 percent sand and 40 percent compost (60/40 Mix). A new lined bioretention system installed at the southeast corner of 185th Avenue NE and NE Union Hill Road was studied from February 2012 through September 2013. Influent and effluent chemistry was assessed using automated samplers and continuous flow gauges. The study system was effective at treating influent concentrations of total zinc, dissolved zinc, TPH-Oil, and fecal coliform. During the second year of study system operation, the average percent reductions for these parameters were 89, 43, 88, and 77, respectively. Despite these reductions, the study system exported many other constituents. For example, during the first year of study, significant pollutant export was observed for all measured nutrients, chloride, hardness, total copper, dissolved copper, and total lead. During the second year of study, there continued to be significant export of total phosphorus, orthophosphate, and nitrate+nitrite. The results from this study indicate that bioretention systems which use a 60/40 BSM can potentially act as pollutant generating sources of nutrients and metals.
Final Report: 185th Avenue NE Bioretention Stormwater Treatment System Performance Monitoring
Reference: Herrera. 2014. Final Report: 185th Avenue NE Bioretention Stormwater Treatment System Performance Monitoring. Prepared for City of Redmond Department of Public Works, by Herrera Environmental Consultants, Inc., Seattle, Washington.