Using a Waste Product to Improve Rain Garden Performance

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Storm water pollution is a growing problem for the people of the Puget Sound. When contaminants are released into our environment they collect in storm drains and, for the most part, flow untreated into our precious streams, rivers, lakes, and oceans. Contaminants like copper from brake dust, nutrients from fertilizers, and detergents from washing cars, can cause adverse problems to the aquatic life in these water bodies.  "Low Impact Development" designs like rain gardens and bio-retention systems are a viable option to reduce harmful effects, by filtering out many of the pollutants before they enter our aquatic systems. In order to improve the performance of rain gardens we propose adding various waste products from other treatment processes which have the potential to remove common storm water contaminants.  In this research, we compared various soil mixes commonly used in these systems with various amendments such as "Bio-Char and Water Treatment Residual (WTR)" to determine their ability to capture heavy metals and nutrients. To test the amended soil mixes we designed a series of columns packed with these mixes, saturated the columns with synthetic storm water, and monitored the concentrations of pollutants in the influent and effluent to determine contaminant removal capabilities. The constituents tested in this experiment include hydrologic conductivity, forms of Nitrogen and Phosphorus along with metals such as Aluminum, Arsenic, Cadmium, Copper, Lead, Nickel and Zinc. Our preliminary results have been very promising, with our "WTR" amendments showing an improvement of up to approximately 20% reduction in both total and dissolved forms of phosphorus and up to a 10% reduction in metals over standard soil mixes.