Are Rain Gardens Mini Toxic Cleanup Sites?




Research debunks myths about what happens to pollutants swept up in rainwater runoff

“If you’re concerned about water pollution, you’ve likely heard this message: The water that gushes off our roofs, driveways, streets, and landscaped yards is to blame for the bulk of the pollution that dirties Puget Sound and numerous Northwest waterbodies,” writes Lisa Stiffler in the latest in a series of investigative reports released by the Seattle-based Sightline Institute.

“You probably also know about the most popular stormwater solutions, including rain gardens and other green infrastructure that soak up the filthy water, cleaning it before it reaches sensitive waterways that are home to salmon, frogs, orcas, and other wildlife.”

“But those two ideas taken together are making some people anxious. If stormwater is the source of such devastating amounts of petroleum and heavy metals, won’t the rain garden in my front yard become a mini toxic waste site that could harm children and pets?”


Syopsis of the Research

“Rain gardens and similar environmentally friendly stormwater infrastructure are being embraced worldwide because they do their job so well. They sponge up polluted runoff, keeping the foul chemicals out of the places that are home to beloved wildlife and where people like to play and fish,”

“The worry is that these same, very efficient rain gardens that are cropping up in our parking strips and front yards are doing their job so well that they could become residential toxic sites. But in fact are they? Not according to the research that’s available. The bottom line is that the soil in rain gardens is safe for kids and pets.”



To download Lisa’s synopsis of the research findings, click on Are Rain Gardens Mini Toxic Cleanup Sites?

To read the online discussion that Lisa’s work has generated, and to add comments, click here.

To read a story posted previously on the Rainwater Management community-of-interest, and featuring the work of Lisa Stiffler, click on Shifting from Gray to Green: Curbing Polluted Stormwater and Creating Communities in the Pacific Northwest


Acknowledgment: The Sightline Institute is an independent, nonprofit research and communications center. Sightline’s goal is to equip the Northwest’s citizens and decision-makers with the policy research and practical tools they need to advance long-term solutions to the region’s most significant challenges. The work of Sightline includes in-depth research, commentary, and analysis.


Testimonials – What “they” are saying about the research synopsis

This report has generated widespread interest and kudos for Lisa Stiffler from leading practitioners from around North America.

“Great summary – suitable for about everyone I know,” comments Nashville-based Andy Reese – water resources engineer and popular writer, speaker, and co-author of the best-selling textbook Municipal Stormwater Management. To access stories posted elsewhere on the website about articles written by Andy Reese, click on Green Infrastructure and Storm Depth Retention Criteria explained by Andy Reese

“Lisa Stiffler has provided a great summary. I have tracked these findings for years and they are consistent in their conclusion that fears of mini-superfund sites in everyone’s yards are unfounded,” adds Maryland-based Dr. Dan Medina, co-editor of the updated Manual of Practice for Design of Urban Stormwater Controls published jointly by the Water Environment Federation and American Society of Civil Engineers in 2012. was formerly a professor of Civil Engineering at Northeastern University in Boston, Massachusetts.

“Yes, this is an excellent article!,” adds Colorado-based Paul Crabtree.  Among his many accomplishments, he is the leader of the CNU Rainwater in Context Initiative. To access stories posted elsewhere on the website about Paul Crabtree, click on Cross-Border Alignment: Connecting the Dots Between Land Use Planning, Development, Watershed Health AND Infrastructure Management.


The View from British Columbia

“Lisa Stiffler has stimulated a discussion that has great value as it begins to focus upon the magnitude of the issues and puts into context some of the scary things that have been circulating by negative thinkers. We know much of this is not true but the comments about toxic cleanup keep appearing,” comments Jim Dumont, Engineering Applications Authority for the inter-governmental Water Balance Model Partnership, and member of the Rainwater in Context Initiative.
“On the other hand, all is not sugar and spice. There are two points that I believe should be part of the discussion; these being adsorption and plant uptake.”


Adsorptive Capacity

“When we examine the chemical processes that occur, we must recognize the adsorptive capacity of a soil is finite and that each soil has a different capability to adsorb metal ions. Clays have the greatest capacity and coarse sands the very least due to the surface area and the way the electrical charges are distributed in the particles.”

“Alternatively organic media have been used to sequester pollutants however the capacity is finite and eventually the organic media will decompose thus releasing the sequestered material back into the environment. Use of organic material for treatment creates a need for maintenance and replacement of the organic media.”

“One needs to do a few simple tests in soil chemistry to determine the adsorption and sequestration capacity; and then use a realistic loading function to estimate the life of the pollutant removal capacity of any treatment system. It is just good professional practice to do this as part of the design process for municipal systems.”

“Where municipalities rely upon private systems on private property, there should be overall master planning and conceptual designs that examine the potential and the constraints inherent in systems because it is not reasonable to expect a home owner to do the design.”

“As Lisa Stiffler’s research findings demonstrate, the good news is that the loadings for most rain gardens are very small and the useful life is long, roadside systems being a possible exception. The bad news is that when all of the adsorption sites are full then the metals just pass on through. Finding when that might occur is part of a process of understanding and managing risks using a scientific approach.”


Plant Uptake

“Plants absorb nutrients and metals; some of which are food; for example nitrogen and phosphorus are things you want to capture. In a natural setting the cycle is self-sustaining with no need for human intervention.”

“The processes are more complex when we introduce a greater input loading and this is where we must begin to harvest the organic bulk because it will decompose and release its components back into the environment for other plants to utilize or to travel further in the environment.”

“If the loading is high there may not be enough plant material to hold the load. In the case where we rely on plant uptake for treatment we must allow for some harvesting to unload the system of the excess. Along with the benefit of treatment comes the need for some future maintenance of the treatment system.”


What We Are Learning

“The lesson to learn and to take away is that there is more happening than we can see on the surface, and knowledge is important in understanding our environment,” concludes Jim Dumont.


To Learn More:

Jim Dumont is the co-author of Rainwater Management in a Watershed Context – What’s the Goal?, published by Stormwater Magazine in November 2011. The article elaborates on how science-based understanding has informed the process for moving from awareness to action in British Columbia; and was part of Stormwater Magazine’s Green Infrastructure & Community Design Series.

To read a synopsis of the article posted on the Rainwater Management community-of-interest, click here.

“The approach we have taken in British Columbia differs from that of the United States EPA due to the nature of the root problems being solved,” states Jim Dumont. ”The critical issue in British Columbia is the damage and loss of habitat caused by development and erosion of the headwater streams.”