Micro-plastics are everywhere. They’ve been discovered in the deepest ocean trench, the most remote arctic ice and, perhaps the most frightening place of all, inside the human body.
Traditionally, microplastic research has centered around the marine environment. Almost everyone is familiar with the Great Pacific Garbage Patch, a collection of marine debris in the North Pacific Ocean that has produced images of straws in the noses of sea turtles and plastic accumulations in the stomachs of seabirds. However, limited research on the presence of microplastics in freshwater ecosystems has been conducted and even less research is available about the impact of microplastics on stormwater runoff.
What is clear is that urban streams are suffering from an onslaught of plastic waste, and the City of Springfield, Missouri, USA, is no exception. A series of rapid trash assessments conducted by city staff found an average of 139 pieces of individual trash per 100 linear feet of stream. Trash was divided into categories that included plastic, metal, glass and paper and ranged in size from small cigarette butts to large items like furniture and tires. Plastic made up 60% of all trash surveyed and of this plastic material, the majority was single-use food-related plastics which was followed in second place by plastic bags.
While the volume of visible plastic was interesting, city staff was primarily interested in plastics that could not be seen.
To better understand the scope of microplastic pollution in stormwater runoff, the city hired an environmental intern who spent six months reading literature, experimenting with testing methods and analyzing samples. A pre-med student whose academic research focused on the inflammation response of mammalian cells to eco-coronated microplastics was selected for the position. Quantifying microplastics in stormwater runoff was a perfect complement to her academic research and her connection to Drury University provided the city with unique access to laboratory equipment and the oversight and expertise of university professors.
City and university staff collaborated to establish a research plan. Sample locations were selected based on their potential to contain plastic pollution. A total of four locations were selected: a plastic recycling facility, an artificial turf field, a low-intensity parking lot and a high-intensity parking lot. Samples were collected in glass jars (Figure 1) using common best practices such as allowing for 72 hours from any previous storm events and collection during the “first flush.” Over the 6-month period, a total of seven storm events were sampled. In the lab, samples were filtered through a series of sieve stacks down to a size of 45 microns. The 45-micron sieve was backwashed with deionized water and collected for analysis. From there, 10-microliter samples were dyed with Rhodamine B dye and heat-fixed to microscope slides for observation under a fluorescent microscope.
A series of images were taken of 10-microliter stormwater samples using the fluorescent microscope. Because we observed that exposure affected the number of particles visible, a series of three photos were taken of each sample, at exposures of 200, 500 and 1,000. Therefore, final estimates are representative of the average of these three exposures. Of the observed microplastics, there were two distinct morphologies: fibrous (Figure 2) and globular (Figure 3). It appeared that globular particles were indicative of rubber material and tire wear, whereas fibers were associated with the breakdown of larger materials, such as textiles and cigarette butts. However, we did not perform Raman spectroscopy testing to determine the makeup of the plastic material due to lack of access to this technology. Instead, our study focused on quantification, and this was accomplished with an image processing software called Fiji. The images in Figures 2 and 3 are the same 10-microliter samples viewed through Fiji software.
Fiji software uses artificial intelligence to count the number of particles in each image and generates the results in a spreadsheet. From there, it’s just a matter of math to estimate the total particles per liter of stormwater runoff. The study determined that, on average, one liter of urban stormwater runoff contains millions of microplastic particles. The high-intensity parking lot, plastic recycling facility and artificial turf field averaged approximately eight million particles per liter, while the low-intensity parking lot averaged roughly half that value (Figure 4).
An important part of the study was to also evaluate the effectiveness of green infrastructure in removing microplastics from stormwater runoff. Using a density separation methodology, sediment samples from a detention basin were evaluated and found to contain microplastic particles. This indicated that some quantity of plastic is removed from stormwater runoff and captured within the stormwater feature. An ex-situ study utilizing a series of five-gallon buckets was performed to estimate the efficiency of removal. They were filled with bioretention media and planted with native vegetation, and holes were drilled on the bottom to allow them to drain (Figure 5). After a period of establishment, water with a known microplastic concentration was run through the buckets. The effluent was analyzed and compared to the influent to estimate removal efficiency, which was found to be approximately 85%. This supported existing research that found bioretention cells to have a median microparticle percent reduction of 84%.1
The good news is potential solutions to help address the macro-problem of microplastic pollution already exist. While bioretention is not going to solve the plastic pollution crisis any time soon, it is certainly one tool in the toolkit, and one that more communities will adopt as they implement green infrastructure to treat stormwater runoff.
The City of Springfield has also installed several trash nets, increased its volunteer stream and road cleanup efforts and focused more on educational efforts related to littering and plastic pollution. An example of this targeted education is the new ballot bin device (Figure 6) installed in downtown Springfield, which educates pedestrians about the connection of cigarette butts to plastic pollution and encourages them to “vote” for an answer with their used butts.
As more pollutants emerge, education is everything. As Maya Angelou said, “Do the best you can until you know better. Then, when you know better, do better.”
Reference
- Smyth K, Drake J, Li Y, et al. Bioretention cells remove microplastics from urban stormwater. Water Research. Vol. 191, 2021. 116785, ISSN 0043-1354.
Note
The City of Springfield thanks Environmental Intern Laiken Cash and her academic advisor, Rachael Day, Ph.D. for their work on the project. Cash was supported by the City of Springfield and Drury University through the Research Experience in the Natural Sciences program, and Day was supported by Drury University.
About the Expert
Sarah Wilkerson, CESSWI is a senior stormwater specialist at the City of Springfield, Missouri. She has over 10 years of experience in the stormwater industry and holds undergraduate and graduate degrees in biology and environmental science.
