For the first time, UC Riverside scientists have measured how much microplastic pollution falls on the Sierra Nevada during snowstorms and were surprised to find significant amounts even in the most remote locations.
Lead researcher Aleksandra Karapetrova, a UCR environmental toxicologist, could not access any of the snowfall collection sites by car or even on foot. She expected plastic to be harder to find or even nonexistent in at least a few of these locations, given their distance from human activity.
“One site was accessed by chairlift. I had to ski to the rest of the remote sites. That’s how isolated they are,” Karapetrova said. “But no matter how far out the site, I always found microplastics there.”
Details about the particles, the seven locations at which they were collected, and the method used to verify their chemical properties are described in the journal Environmental Science and Technology.
Though the study did not determine how much plastic ultimately reaches drinking water, the findings have significance beyond the mountain, as the Sierra Nevada snowpack supplies about 30% of California’s freshwater.
The study also describes how snow scavenges airborne plastic from the atmosphere and carries it to the ground, far from population centers and places where the plastic is produced or used.
Microplastics enter the air when plastic products shed fibers or wear down into small fragments. Clothing can emit fibers directly into the atmosphere, including through dryer vents or just daily wear, while wind lifts particles from the ground and transports them elsewhere.
As snowflakes fall, they collect some of these tiny plastic particles suspended in the air. This process, called scavenging, helps explain why researchers measured substantially more plastic arriving with the falling snow than during dry weather.
“We are at a point where microplastics are in our atmosphere, not just the ocean, not just rivers. It comes down with the snowfall, making its way to places humans typically don’t go,” Karapetrova said.
At remote sampling locations, during the height of the storm, the snow deposited between 270 and 3,200 plastic particles per square meter, per day. At one site in the Mammoth Mountain ski area, the amount of pollution was roughly three times that found in remote sites. Between 2,600 and 7,500 particles per square meter per day there deposited in the snow. Nearby human activity and greater snowfall accumulation there are possible explanations.
“I didn’t think the plastics would stay suspended so much in the town,” Karapetrova said.
Even the large particle counts at Mammoth represent small amounts of material. At the highest estimated rate, snowfall over an acre would deliver about 1.5 grams of plastic per day.
To explore where the plastics may have originated, the researchers traced masses of air that ushered in the storms sampled in the study. Most of the storm air passed over Fresno and Madera counties, suggesting the Central Valley may be a region contributing these particles.
Computer models could not identify exact sources or exclude plastic arriving in the Sierra from even farther away.
Earlier research had already established that microplastics occur in Western U.S. snow. This study examined how much is being deposited on the ground during individual storms, allowing researchers to connect measurements with particular weather events.
Confirming the particles’ identities required more than looking through a microscope. Researchers used infrared light to read chemical signatures, then compared those signatures with a reference library to distinguish plastic from soil and plant material.
Most commonly, the researchers found polyethylene, polypropylene, and polyester/PET, which are plastics commonly used in clothing and packaging. The team also found cellulose derivatives, a group of chemically modified, plant-based materials used in consumer products.
This study did not assess health effects of microplastics in the environment. However, some chemicals used in plastics have documented health effects including hormone disruption, inflammation, and cellular damage.
The particles’ familiar ingredients point to the reach of everyday plastic use far beyond the products themselves.
“We’ve reached a point where plastic pollution is falling out of the sky every day, across this entire region, even when it isn’t snowing,” Karapetrova said.
(Cover image: Sonya Maple/CDFW)
