Characterization of pollutant flushing from a newly installed bioretention system: 185th Ave NE, Redmond, WA

Abstract

From February 2012 through September 2013, we conducted hydrologic and water quality monitoring of a bioretention stormwater filtration system at 185th Ave NE in Redmond, Washington. This monitoring was conducted to obtain data that will be used to evaluate if bioretention swales could be used to treat runoff prior to infiltration within a drinking water aquifer recharge area. The study bioretention swale was constructed using the Washington Department of Ecology specified soil mix that consisted of 40 percent compost and 60 percent sand. Eighteen inches of soil mix was installed on top of a six-inch sand blanket underlain by gravel that housed a perforated underdrain pipe. The entire system was lined with an impermeable plastic liner which isolated the underdrain flow from interflow and groundwater entering the swale. Flow weighted composite samples of runoff were collected at an inlet to the swale and from the underdrain. During the first year of study significant export from the bioretention system was observed for all measured nutrients, chloride, hardness, total copper, dissolved copper, and total lead. Bioretention system export during the second year was also characterized by non-significant increases in total Kjeldahl nitrogen, hardness, chloride, and dissolved copper. In addition, one grab sample indicated a large export of methylene chloride in excess of the state groundwater quality standard. During the first six months of operation, the study system exported nitrate+nitrite concentrations in excess of the state groundwater quality standard. The groundwater criterion for fecal coliform bacteria was exceeded at the effluent station during every sampled storm event, even though the study system had a high degree of fecal coliform bacteria removal. No other measured parameters with applicable groundwater criteria were found to exceed criteria in the effluent of the study system. Water extractions of soil mix ingredients (compost, sand, potting soil installed with the plants, and bark mulch) indicated that the compost was the greatest source of nutrients, zinc, and copper, while the sand also contributed levels of concern for zinc and copper. Future bioretention projects should attempt to use sand with lower metals concentrations and composts with lower nutrient and metals content