Stormwater News

It’s no wonder that Seattle is one of the United States’ fastest growing cities. Washington’s most populous city continues to attract new residents due to its booming job market and proximity to endless outdoor activities. In fact, from July 1, 2021, to July 1, 2022, Seattle saw a population gain of almost 18,000 individuals. As the population increases, so does the number of motor vehicles on our roads and amount of pollutants released into our waterways. Paved roadways and parking lots accumulate an array of contaminants, including heavy metals, oil, tire debris, and vehicle exhaust. After rainfall, stormwater runoff collects these pollutants and eventually flows into our streams, lakes, estuaries, and the Salish Sea. When these toxicants enter our waterways, they can cause harm to local aquatic species, including Chinook Salmon and the endangered Southern Resident Killer Whales. 

So, how can we help mitigate stormwater pollution and create cleaner waterways in our communities? Dr. Chelsea Mitchell, a Senior Ecotoxicologist of King County, says that the answer lies in implementing green stormwater infrastructure. 

As a Ph.D. student at WSU, Dr. Mitchell’s research focused on evaluating the effectiveness of various stormwater infrastructure systems. She completed her Ph.D. program and began her work with King County earlier this year. Dr. Mitchell’s role involves planning and carrying out research projects on stormwater contaminants, water quality, and sustainable methods of stormwater treatment. “I have my dream job now,” she explained. “I get to perform applied science where I have a direct ability to move the needle on important measures related to green infrastructure.” 

Dr. Mitchell says that there are several different types of stormwater systems that help to filter polluted runoff and ensure cleaner waterways. You may have heard of terms such as rain gardens, bioretention swales, and stormwater parks. What’s the difference between these different kinds of systems, and how do they effectively filter pollutants out of contaminated stormwater runoff? “All of these are bioretention systems build little  ecosystems using soil, sand and plants which serve to capture stormwater runoff and then infiltrate it into the ground,” Dr. Mitchell explains. “While it’s infiltrating, the system also provides treatment of that water and removes pollutants. These systems are living, and contain not only plants but also microbes like bacteria and fungi that can eat some of the contaminants that are captured by the bioretention media.”  

For example, rain gardens are made using a small depression in the landscape and native plants that can handle both flooding and drought. The idea is that rather than having stormwater runoff flow along pavement and into storm drains, the shallow depression of the garden captures the water. There, the water can slowly infiltrate back into the soil as the plants, mulch and soil naturally remove pollutants from the runoff. Dr. Mitchell explains, “The main way we think bioretention removes pollutants is through sorption, a process where chemicals in solution move out of the water and onto the organic particles. Organic contaminants (like hydrocarbons from vehicle emissions and oil, for example) have a really strong affinity for sorbing to the organic components of bioretention media.” 

Bioretention swales are often located alongside roadways, where contaminated runoff from the pavement can be captured rather than flowing directly into a stormwater drain. Swales look like long, narrow ditches with sloped sides that are filled with native grasses. As the water moves horizontally through the swale, the grasses intercept sediments and particles, which drops them out of suspension, cleaning the water. Many harmful pollutants stick to particles and are transported in stormwater that way.  “Just getting these particulates out of the water actually treats a shocking amount of pollution!” Dr. Mitchell shares. 

Much larger than a rain garden or bioretention swale, stormwater parks are meant to centralize and treat an entire basin of stormwater runoff. Stormwater parks allow runoff to pass through soil, which naturally filters out many toxicants. Particles of pollutants stick to the soil or are absorbed by the roots of native plants. Dr. Mitchell highlights, “The really cool thing about stormwater parks is that you get these larger scale treatment benefits, while also having a community space that connects people and educates them about stormwater treatment.” While King County already has implemented many rain gardens and bioretention swales throughout the region, they are currently working towards the goal of creating at least 30 stormwater parks. 

The next time you go out for a walk around your neighborhood, take a good look around. You might be passing by stormwater infrastructure that you’ve never noticed before. Dr. Mitchell has several tips and tricks for spotting filtration systems in your community that might be right under your nose. “The first thing to look for is some kind of slope or depression in the landscape where it looks like water would be able to pool. You’ll also often see tall grasses planted in these depressions, as these plants are resistant to both drought and flooding. If you see lots of tall grasses poking out of a depression near the side of the road, that’s probably a bioretention swale!” 

We can all play an important role in keeping our waterways clean and healthy. One of the best ways to help make a difference is to build your own rain garden on your property. If you live in King County, the Be RainWise rebate programs will help you finance the construction of your own rain garden. In addition to taking stormwater-friendly actions in our daily lives, Dr. Mitchell encourages us all to use our voices and advocate for strong water quality management on a governmental level. Dr. Mitchell says, “It does actually make a big difference when you submit your comments to your local public works department!” Click here to learn more about water quality management and stormwater infrastructure in King County. You can also submit your comments about stormwater management in King County here.