Microplastics are falling with the snow in California’s Sierra Nevada

At seven sites across California’s Sierra Nevada, snow collected during active storms contained plastic particles, with remote mountain locations receiving between 270 and 3,200 particles per square meter per day at the height of the weather. The finding comes from Aleksandra Karapetrova, an environmental toxicologist at the University of California, Riverside, who sampled fresh snowfall rather than the settled snowpack. The work appears in the journal Environmental Science & Technology.

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The result matters most for where it was found. Several of the collection points sit far from roads and towns, reached only by chairlift or on skis, and plastic turned up at every one. The study therefore treats the atmosphere itself as the delivery route, with each storm depositing particles along with the snow it brings to the range that feeds much of the state’s water supply.

Seven sites sampled during storms

The paper, indexed under DOI 10.1021/acs.est.6c02757 and listed on the journal’s abstract page, has a web address that points to a title beginning Event-Scale Deposition of Atmospheric, which describes the approach: measuring the amount landing during a single storm rather than the accumulated total across a winter. Karapetrova, based at the University of California, Riverside, collected snow while storms were under way, so the samples reflect what was falling at that moment instead of what had settled over weeks.

The Earth.com report on the study dwells on the logistics. By Karapetrova’s account, one site was reached by chairlift and she skied to the remaining remote ones, and no matter how far out the location, she found microplastics at every stop. The journal pages did not load when checked, so the site descriptions and figures here rest on that report.

Particle counts: remote slopes versus Mammoth Mountain

Remote sites received 270 to 3,200 plastic particles per square meter per day at the height of storms. At the Mammoth Mountain ski area, a developed town-and-resort setting, the range rose to 2,600 to 7,500 particles, roughly three times the remote figure. Karapetrova said she had not expected plastics to stay suspended in the air so persistently over the town.

The study reports counts rather than weight, and the difference is large. At the highest estimated deposition rate, snowfall over an acre would deliver only about 1.5 grams of plastic per day, according to Earth.com. Millions of particles can weigh next to nothing, which is why the number of fragments, not their mass, is the striking part of the data.

Context helps with the scale. A rate of several thousand particles per square meter per day sounds large, but each fragment is microscopic, and the researchers’ own mass estimate of about 1.5 grams per acre per day shows how little material is involved by weight. The concern is less about heft than about persistence: plastic does not dissolve, and every storm that crosses the range adds another layer of fragments to a snowpack that will eventually melt.

Polyethylene, polypropylene and PET from the Central Valley

Infrared spectroscopy identified the particles by comparing each sample’s spectrum with a reference library, a method that separates plastic from soil and plant debris. The most common polymers were polyethylene, polypropylene and PET, a polyester, all staples of packaging, bottles, textiles and carpets. Polyethylene and polypropylene dominate films, caps and containers, while PET is the plastic of drink bottles and polyester fabric, so the mix reads like a cross-section of everyday consumer waste. Cellulose derivatives, which are chemically modified plant-based materials used in consumer products, also showed up.

Air-mass analysis points to a regional source. Most storm systems passed over Fresno and Madera counties in California’s Central Valley before reaching the mountains, which would carry agricultural, urban and highway debris toward the slopes. Computer models could not pinpoint the exact origin, so the Central Valley remains a likely pathway and not a proven source. Pinning down the origin would take finer air-trajectory work and sampling of valley air during the same storms.

Snowpack, drinking water and what remains unmeasured

The Sierra Nevada snowpack supplies about 30 percent of California’s fresh water, according to Earth.com’s summary of the work. That is the reason the result draws attention: snow that accumulates plastic in winter melts into reservoirs and rivers in spring. Earlier local coverage of the range, including a CBS News San Francisco report on plastic in fresh-fallen Sierra snow and Lake Tahoe and an NBC Bay Area piece on microplastics in the Sierra snowpack, had already flagged the issue.

The new study stops short of that question. It did not measure what share of the deposited plastic reaches drinking water, and it did not assess health risks or effects on aquatic ecosystems. Those gaps define the next round of work: tracing how particles move from falling snow into meltwater, and whether treatment and reservoir storage remove them before they reach taps.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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