In today’s modern world, plastics are everywhere. They are so ubiquitous that it can be difficult to imagine a time when plastic was not a part of everyday life. For this reason, it may surprise you to know that mass production of plastic did not begin until the 1950s. Since that time, plastic production has exploded. Today, there is 44 million tons of plastic produced each year globally. Americans alone generate 10.5 million tons of plastic per year with only one to two percent of that plastic being recycled. Unfortunately, a significant amount of plastic ends up in our oceans and waterways each year. According to the Rochester Institute of Technology, more than 22 million pounds of plastic end up in the Great Lakes each year. This adversely impacts aquatic life as well as the animals (including humans) that consume fish from Great Lakes.
Unfortunately, a significant amount of the plastic we use in our daily lives becomes waste at some point. This is because the most common plastic products are made from hydrocarbons that come from fossil fuels, meaning that they are not biodegradable. This means that plastic waste enters the environment rather than breaking down. Plastic waste enters the environment via atmospheric deposition, treated wastewater effluent, agricultural runoff, industrial wastewater, and stormwater runoff.
Recent literature suggests that stormwater is the most significant source of ocean microplastic pollution. The following article will discuss microplastic pollution in stormwater and its impact on human health and the environment.
What are microplastics?
According to the National Oceanic and Atmospheric Administration (NOAA), plastic is the most prevalent type of marine debris found in our oceans and the Great Lakes. Plastic debris can come in all shapes and sizes, but those that are less than five millimeters in length (or about the size of a sesame seed) are called “microplastics.”
The California State Water Resources Control Board has adopted a broad definition of microplastics in drinking water, which includes all solid polymeric particles ranging from 1 nm – 5 mm, excluding certain natural materials. Particles made of materials such as rubber and cellulose acetate, which are found in cigarette butts, can be considered microplastics under this definition.
Primary Microplastics
Primary microplastics are plastics directly released into the environment in the form of small particulates called nurdles. Primary microplastics can also come from the abrasion of large plastic products during manufacturing, driving, or washing.
Secondary Microplastics
Secondary microplastics are microplastics originating from the degradation of larger plastic items into smaller plastic fragments once exposed to the marine environment. This happens through photodegradation and other weathering processes. For example, discarded plastic bags breaking down or lost fishing nets breaking down.
Microplastic Pollution in the Ocean
The International Union for Conservation of Nature estimates that 28 percent of microplastics worldwide end up in the ocean, primarily from rubber tires. Once in the ocean, microplastics can either float and disperse throughout the ocean or sink. For example, acrylic is denser than seawater and will eventually sink to the ocean floor. In 2014, researchers found microplastics in the stomach of a new sea creature that was discovered four miles below sea level in the Pacific Ocean. Dr. Alan Jamieson, a Senior Lecturer in Marine Ecology at Newcastle University said, “We decided on the name Eurythenes plasticus as we wanted to highlight the fact that we need to take immediate action to stop the deluge of plastic waste into our oceans.”
This is a disturbing discovery because this speaks to the extent of the problem.
Microplastic Runoff in Stormwater
It has been known for a while that stormwater is one pathway for microplastic pollution, but new studies have shown that stormwater is most likely the largest source of microplastic pollution. Various studies have shown that about 80% of plastic found in the ocean and waterways comes from the land and the key mechanism in which plastic pollution is getting to our oceans and waterways.
A recent study (Werbowski et al., 2021) collected samples from 12 different watersheds within Northern California. All of these stormwater runoff samples contained anthropogenic microparticles, including microplastics, with concentrations ranging from 1.1 to 24.6 particles/L. Fibers and black rubbery fragments (potentially tire and road wear particles) were the most frequently occurring morphologies, comprising ∼85% of all particles across all samples.
This study also involved sampling stormwater from the inlet and outlet of a rain garden during three storm events. They found that the rain garden successfully removed 96% of anthropogenic debris on average and 100% of black rubbery fragments, suggesting rain gardens could serve as an effective mitigation strategy for microplastic pollution. This is good news because rain gardens and bio-retention cells are commonly used in low-impact development (LID).
The results of this study indicate the importance of incorporating microplastic pollution mitigation measures into stormwater management plans. It seems like some of the more common stormwater BMPs (like rain gardens) can help in this endeavor. However, solutions should not be limited to existing technologies. It is important that we as engineers, scientists, and concerned residents of Earth explore other ways to address microplastic pollution.
These strategies should involve removing the most common sources of microplastic pollution (e.g., rubber tire fragments, cellulose acetate (cigarette butts), and single-use plastics). These sources of microplastic pollution are discussed in more detail in the following sections.
Sources of Microplastic Pollution
In urban areas, heavy rainfall can wash away the trash, pollutants, and debris that have built up on roads and sidewalks since the last rainfall event. This water, which is called stormwater, carries these pollutants to storm drains and eventually the local waterways or ocean. The following section will discuss the sources of microplastic pollution in stormwater – rubber tire fragments, cigarette butts, and single-use plastics.
Rubber Tire Fragments
Rubber tire fragments are considered the biggest source of global microplastic pollution. Due to our car culture, scientists estimate that the United States has the highest tire particle emissions in the world. The average American generates 7 to 12 pounds of rubber tire particles per year. Tire particles have the potential to harm organisms due to both their physical and chemical characteristics. For example, in the Pacific Northwest, researchers found that a common chemical found in tires has caused mass Coho salmon deaths in Puget Sound, Washington. The toxin responsible for these fish deaths is called 6PPD-quinone which helps preserve the car tire. Tires can contain more than 100 different materials and hundreds of chemicals. It is worth noting that the specific makeup of a tire differs by product and brand.
Another common source of rubber tire fragments is artificial turf. Artificial turf typically relies on a particulate “infill” material to support the plastic turf blades. Many artificial turf fields use infill composed of crumb rubber microplastics. Artificial turf is actually the largest source of microplastic pollution in Europe (accounts for 2/3 of microplastic pollution). This is important to understand because many grassed fields are being converted to artificial turf in the United States in the name of environmental stewardship. This is because artificial turf does not require irrigation/watering. But is it environmentally friendly? I would argue that the negative effects of microplastic pollution far outweigh the water conservation benefits.
Ways to minimize the number of rubber tire fragments in stormwater runoff
There are some ways to reduce the amount of rubber tire fragments making it to our oceans and waterways. The following list discusses some possible solutions.
- Manufacture tires in a way that does not include toxic ingredients and eliminates tire wear ingredients.
- Modify road surfaces to reduce tire wear.
- Educate the public about driving habits and maintenance activities that reduce tire wear (e.g., check their tire pressure regularly, rotate their tires regularly, avoid taking corners too fast, and avoid potholes).
- Another option would be to install wear debris collection systems on vehicles. A device like this was invented by some students at the Imperial College London and the Royal College of Art.
- Increase the frequency of street sweeping.
- Ensure that existing stormwater infrastructure is properly maintained! As previously mentioned, there is evidence that rain gardens and other stormwater BMPs that are already in the ground may be catching microplastic pollution. However, if these structures are not cleaned out regularly, they cannot continue to do their job effectively!
Cigarette Butts in Stormwater Runoff
Cigarette butts are the most common source of beach pollution, so it is not surprising that they also contribute significantly to microplastic pollution in stormwater runoff. This may be because cigarettes are the most littered item in the United States because many smokers mistakenly believe that cigarette butts are made of biodegradable cotton. However, this is not so. Most cigarette filters are made of cellulose acetate which is a non-petroleum-based plastic made from cellulose. Although made from cellulose, which is a natural material, cellulose acetate fibers from cigarette butts are not biodegradable and persist in the environment. Not only do cigarette butts contribute to microplastic pollution in our waterways, but they can also leach toxic chemicals, such as lead and arsenic, that harm aquatic life.
Ways to minimize the amount of cellulose acetate in stormwater runoff
Some ways to reduce the amount of cellulose acetate/cigarettes being washed into our oceans and waterways by stormwater runoff are as follows:
- Provide canisters/receptacles for cigarette butts in highly populated areas.
- Increase public awareness and education to reduce littering.
- Recycle cigarette butts and use them to make park benches, asphalt concrete, or bricks.
- Redesigning cigarettes is another way to minimize the amount of microplastic pollution from cigarette butts. This could involve removing the filter or making the filter out of a more environmentally friendly material.
- Finally, the best option (in my opinion) would be to continue efforts to decrease and maybe even someday eliminate smoking.
Single Use Plastics
Single-use plastics are used once or for a short period of time before being thrown away. Examples of single-use plastics are plastic garbage bags, disposable coffee cups, plastic straws, plastic water bottles, drink stirrers, plastic sandwich bags, plastic cutlery, and containers for soaps and detergents. Single-use plastics that are not disposed of properly or fly out of garbage trucks are eventually washed away into local waterways and storm drains as shown in the image below. This is problematic because these waterways will eventually wash this plastic and trash into the ocean. When this happens, plastic and garbage get caught up in currents and circulate around the globe. The trash in the ocean can end up in a garbage patch, which is a large area in the ocean where trash, plastic, and debris accumulate. For example, the Great Pacific Garbage Patch, which is located between California and Hawaii, is twice the size of Texas.
Ways to reduce single-use plastic pollution in stormwater runoff
One obvious way to reduce single-use plastic pollution in stormwater runoff is to participate in river and beach cleanup efforts. Another way is to install trash capture devices in catch basins and stormwater inlets.
While beach cleanups and removal devices like hydrodynamic separators are great, they cannot solve the microplastic problem. Because microplastics are so small, it is not practical to expect trash capture devices, which are typically designed to capture gross solids, to capture these small microplastics. For this reason, the absolute best way to reduce microplastic pollution is source control, which means we need to stop manufacturing so much plastic. Ideally, this would involve regulators passing legislation banning single-use plastic products. However, there is still a lot individuals can do to reduce plastic use. Some of these actions include using refillable water bottles rather than plastic water bottles, replacing as many plastic items as possible with non-plastic alternatives, and recycling.
Individuals can also support local initiatives for better waste management and green infrastructure.
Regulation
Although more could be done, there are regulations in place around the world that ban plastics. In 2018, the Legislative Assembly of Costa Rica approved a law that would ban plastic straws and plastic bags.
While there is no national plastic bag ban in the United States, the states of California, Connecticut, Delaware, Hawaii, Maine, New Jersey, New York, Oregon, Vermont, and Washington have banned plastic bags.
California Trash Amendments
California is one of four states and territories that have trash total maximum daily loads (TMDLs). California, Maryland, Alaska, and the District of Columbia. In 2015, the California State Water Board proposed what is known as the Trash Amendments. The Trash Amendments address the impacts of trash on the surface waters in California with the exception of those waters within the jurisdiction of the Los Angeles Water Board with trash or debris TMDLs.
SB 54
In June 2022, Governor Gavin Newsom passed SB 54 which requires all packaging in the state to be recyclable or compostable by 2032, cutting plastic packaging by an estimated 25 percent in 10 years and requiring 65 percent of all single-use plastic packaging to be recycled in the same timeframe.
Los Angeles and San Diego have passed multiple ordinances that limit single-use plastics. The Los Angeles City Council voted to ban the sale or distribution of Styrofoam products. Starting 4/23/2023, no food, beverage, or retail establishment with more than 26 employees can use polystyrene products. Los Angeles is also expanding on the single-use plastic bag ordinance passed in 2013 to include apparel stores, hardware stores, farmer’s markets, and food and beverage establishments. San Diego passed a ban on polystyrene foam products in 2019 and it will take effect in early 2023. The ordinance also states that food vendors may now only offer utensils made from plastic upon request.
Where You Can Learn More
If you are interested in learning more about microplastic pollution in stormwater, I recommend checking out the following resources.
- Ocean Protect Podcast
- Surfrider Foundation
- Studies from the San Francisco Estuary Institute