Answers About Pollutants Accumulating in Natural Drainage Systems

At the October 16 Broadview Community Council meeting several audience members asked about the accumulation of pollutants in the bottom of the natural drainage swales that Seattle Public Utilities has installed and plans to install in Broadview.  Below is a message from SPU project manager Don Anderson about that.

Eugene and Sue, good morning –

I wanted to get back to you all on the concerns of pollutants in natural drainage systems which arose during our meeting with the Broadview community Tuesday night.  Please share with your group the information below from our technical experts.

SPU consults experts in bioretention research and has conducted a thorough literature review that is used to design and build natural drainage systems.  In addition, SPU is active in national conferences to remain current on the latest research regarding bioretention water quality.  In general, the body of knowledge supporting Seattle’s program reveals good treatment performance for conventional contaminants and their expected safety.  One of the more easy-to-read and often cited articles on these topics and the overall safety of natural drainage systems is attached to this email for your use:  “Are Rain Gardens Mini Toxic Cleanup Sites.”

Regarding the lifespan of the amended soil and geotechnical sand, we have conservatively estimated that the soil may need to be replaced as early as every 15 years.  A study in Toronto determined replacement of highway ditch soils every 20 years.  In Prince George County, they anticipated replacement in 15 years, but haven’t needed to yet (based on monitored function).  In Seattle, metal levels in roadside rain gardens built about 13 years ago in the Broadview neighborhood show small accumulations with no cause for concern.  Given these timelines as conditions warrant, SPU will determine maintenance or replacement needs.

In short, SPU values public safety and assures that natural drainage systems are safe to prune, weed or garden with regard to contaminant or heavy metal exposure;  short of ingesting the soil there is no worry with gardening activities.

If there remain other questions on this issue, please let me know.  Also, if I can plug our web sites a little here, more information is available on both project web sites, Venema Creek NDS, and the Broadview Projects (this site has the basin map we discussed in both presentations).

I hope this info is helpful.  Please let me know if you have additional questions on this topic, or any concern on the Venema project.  Thanks for having us to your meeting!



Don Anderson, P.E.

Project Manager

Seattle Public Utilities

Project Delivery Branch

700 5th Ave., Suite 4900, Seattle 98124-4018

Phone: (206) 233-1086, E-mail:


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