Optimizing Bioretention Soil Media

Municipalities use stormwater bioretention as their main tool to protect our water resources, so it’s essential that strong science underpins bioretention designs. In 2013, Herrera’s research started showing evidence that the current mix in filter media (60 percent sand and 40 percent compost) was actually exporting pollutants (copper, nitrogen, and phosphorus) to treated stormwater. With funding from Washington State Department of Ecology, we did a study to seek out other media mixes that would provide the treatment benefits of the “60/40 mix” without the export issues. Collaborating with students from Seattle University, we conducted column experiments on seven different media mixes and found one that appeared to outperform the others. The new mix contains clean sand, coconut coir, and a high-carbon wood ash additive, and it will be particularly useful for bioretention facilities installed in basins that drain to sensitive water bodies.