UC Riverside Study Finds Airborne Microplastics Across Sierra Snowpack

Microscopic plastic fibers and fragments are falling with winter snowstorms across even the most isolated peaks of California's Sierra Nevada, according to new research from UC Riverside published in the journal Environmental Science & Technology. Led by UC Riverside environmental toxicologist Aleksandra Karapetrova, scientists collected freshly fallen snow in stainless-steel containers between January and July 2024 across 7 locations along the 400-mile mountain range and detected microplastics at every site.

The findings carry broad significance for watershed monitoring because the Sierra Nevada snowpack provides roughly 30% of California's freshwater, feeding the State Water Project, the federal Central Valley Project, San Francisco's Hetch Hetchy system, and the Los Angeles Aqueduct. The study is the first to quantify microplastic deposition during individual Sierra Nevada snowstorms, demonstrating that falling snowflakes actively capture airborne particles suspended in the atmosphere through a process known as scavenging.

Of the 7 sampling locations—which ranged in elevation from 9,200 to 9,650 feet—6 remote sites were situated 2 to 4 miles from the nearest town of more than 10,000 residents and could only be reached on skis. Those 6 backcountry stations included 3 sites near Pear Lake in Sequoia National Park about 10 miles west of Mt. Whitney, plus Eastern Sierra sites at Earthquake Dome and TJ Lake outside Mammoth Lakes and Yost Meadow near June Mountain. During the height of storms, snowfall at the 6 remote sites deposited between 270 and 3,200 plastic particles per square meter per day.

At the 7th station, located inside the busy Mammoth Mountain ski area and accessed by chairlift, snowfall deposited between 2,600 and 7,500 plastic particles per square meter per day—roughly 3 times higher than the remote backcountry sites. Even at that maximum deposition rate of 7,500 particles per square meter per day, researchers calculated the total mass of plastic at approximately 1.5 grams across 1 acre in a day. Laboratory testing identified polyethylene, polypropylene, polyester, and chemically modified plant-based cellulose derivatives, while atmospheric computer models traced storm air masses traveling across Fresno and Madera counties in the Central Valley.