Using a Waste Product to Improve Rain Garden Performance

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Stormwater pollution is a growing problem for the people of 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 (LID) 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 stormwater 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, a waste product created through the treatment of clean drinking water) 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 stormwater, 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.