Making Green Stormwater Infrastructure Greener

Green stormwater infrastructure uses different combinations of soil media and vegetation to store and filter pollution from stormwater runoff. If designed properly, these systems can reduce phosphorus from stormwater that pollutes freshwater systems and can help solve the problem of eutrophication in surface water. “Bioretention” rain gardens have emerged as a popular type of green stormwater infrastructure. These systems use sand- and gravel-based layers of soil media, topped with native perennial vegetation, to soak up stormwater runoff from roads and parking lots.

New research carried out through the Lake Champlain Sea Grant project Application of Drinking Water Treatment Residuals in Green Stormwater Infrastructure for Enhanced Phosphorus Removal provides evidence that adding certain amendments to stormwater media can improve phosphorus removal from the water.

And one of these amendments is essentially a waste product.

In the article Balancing Hydraulic Control and Phosphorus Removal in Bioretention Media Amended with Drinking Water Treatment Residuals published this summer in the American Chemical Society journal Environmental Science and Technology | Water and presented at the University of Vermont (UVM), researchers describe laboratory experiments using drinking water treatment residuals (DWTRs) —an industrial byproduct from drinking water treatment plants—as a component of bioretention soil media. Researchers found that using DWTRs and sand together has the potential to capture the great majority of phosphorus flowing through catchments while not restricting hydraulic conductivity (water flow).

Lead author Michael Ament, who recently defended his PhD in the Plant & Soil Science Department at UVM, described additional results, including both lab and field research, during his August 2021 dissertation defense “Multi-scale Assessment of Drinking Water Treatment Residuals as a Phosphorus Sorbing Amendment in Stormwater Bioretention Systems.”

“We showed that media design matters. The amount, placement, and type of media used in stormwater treatment systems is critical to capture of phosphorus and other pollutants as well as maintaining hydraulic conductivity,” explained Ament.

Ament, who was advised by Drs. Stephanie Hurley and Eric Roy, from UVM’s College of Agriculture and Life Science and Rubenstein School of Environment and Natural Resources, respectively, investigated DWTR impacts at micro-, meso-, field-, and catchment-scales and found strong phosphorus and other pollution-removal potential, given certain management practices.

Managers of green stormwater infrastructure should know:

• At scale, bioretention media specifically designed for phosphorus removal can help states meet phosphorus-reduction goals related to Total Maximum Daily Load (TMDL) limits.
• Low-phosphorus compost was used in the lab and field studies and presumed in the modeling study. Managers should note that conventional phosphorus-rich composts are likely to leach into stormwater outflows, undermining water quality improvements.
• Both high phosphorus-removal media (with amendments like DWTR) and strategic infiltration of runoff into native soils may be needed to simultaneously achieve urban hydrology and water quality goals.
• Consider bioretention media type, soil infiltration capacity, and runoff source areas when deciding between investments in high performance media or additional bioretention systems.

“Examining the phosphorus capture—versus leaching—associated with specific technical components of bioretention media and then extrapolating to ask ‘what would happen with and without the targeted phosphorus removal at a watershed scale?’ allows us to help guide practitioners and regulators… to improve green stormwater infrastructure design and, hopefully, downstream water quality,” said Hurley.

Additional results and guidance will be published over the next year.

This research project was funded through the National Oceanic and Atmospheric Administration Lake Champlain Sea Grant and the U.S. Environmental Protection Agency’s Regional Applied Research Effort.