Microplastics Are Falling With the Sierra Snow That Feeds Folsom Lake. Here's What That Means at Your Tap.

Snow falling over the Sierra Nevada eventually helps supply Folsom Lake, the American River, and many Sacramento-area water systems. A new UC Riverside study, published this week, measured something else arriving with that snow: plastic. Not in the ocean or a river downstream, but in freshly fallen flakes at remote mountain sites that a researcher had to ski to reach.

What the researchers found

Melted snow sample in a beaker next to a microscope.

UC Riverside environmental toxicologist Aleksandra Karapetrova collected fresh snow at seven Sierra Nevada locations during storms. One site was reached by chairlift; the rest required skiing in. As she put it, “no matter how far out the site, I always found microplastics there.”

The numbers, reported in the journal Environmental Science & Technology and announced October 28, are specific. At the remote sites, snow delivered roughly 270 to 3,200 plastic particles per square meter per day at the height of a storm. Near the Mammoth Mountain ski area, closer to people and traffic, the rate was about three times higher, between 2,600 and 7,500 particles per square meter per day. At the top end, that works out to about 1.5 grams of plastic landing on every acre, every day the snow is falling.

The most common plastics were polyethylene, polypropylene, and polyester (PET), the materials in packaging and clothing. Snow turns out to be an efficient collector: as flakes fall, they scavenge particles suspended in the air, which is why far more plastic arrived during storms than in dry weather. Tracing the storm air masses backward showed most had passed over Fresno and Madera counties, pointing toward the Central Valley as one likely contributor. The researchers could not identify exact sources or rule out material transported from farther away.

Why a mountain study matters in a Sacramento-area kitchen

Foothill reservoir with the Sierra Nevada in the distance.

The Sierra snowpack supplies roughly 30 percent of California's freshwater, according to the UCR release. For many Sacramento-area homes, the connection is direct: Sierra snowmelt feeds the American River and Folsom Lake before treatment and delivery by local utilities.

The study measured what lands on Sierra snow, not what comes out of a faucet. It did not test treated tap water, and it did not assess health effects. Drinking-water treatment removes particulate matter, but this study did not measure how much of the deposited plastic reaches source water or remains after treatment. It also did not test rain-fed reservoirs in Sonoma or Marin counties. What it does establish is that microplastics are present in the atmosphere over the Sierra Nevada and can settle with snowfall.

What the science says so far

Microplastics are a young field, and the honest answer on health is that researchers are still working it out. The World Health Organization reviewed the evidence in 2019 and concluded that microplastics in drinking water “don't appear to pose a health risk at current levels,” while calling for more research, especially on the smallest particles. WHO also noted that particles larger than about 150 micrometers are unlikely to be absorbed by the body, and that conventional drinking-water treatment can remove particles smaller than one micrometer.

California decided not to wait for the science to settle. Under a 2018 state law (SB 1422), the State Water Resources Control Board adopted an official definition of microplastics in drinking water in 2020, then adopted a policy handbook establishing standardized testing and four years of monitoring and public reporting in 2022. The program is intended to build evidence about microplastics in California drinking water; it is not a regulatory limit or a finding that a particular local utility is in violation.

How to check your own water

Homeowner filling a glass of water at her kitchen sink.

Your water utility publishes a Consumer Confidence Report every year, generally by July 1, listing regulated contaminants it tested for and the results. Start with our guide to reading your water-quality report, then check your utility's current report and notices. Microplastics may not appear in the standard contaminant table because California's work is a monitoring and reporting program, not a drinking-water limit. The Environmental Working Group's Tap Water Database can also help you look up utility data by ZIP code, but its health-based benchmarks are not the same as federal or state regulatory limits.

Testing must match the question. Family First Water's complimentary in-home screening checks chlorine, sediment, and visible water characteristics; it does not identify or quantify microplastics and is not a substitute for accredited laboratory analysis. If you need contaminant-specific results, use an appropriately accredited laboratory and the correct sampling method.

Treatment should likewise be selected from verified water results and the specific product's performance documentation. Whole-home filtration and point-of-use reverse osmosis serve different purposes; our whole-home filtration and reverse-osmosis guide explains the distinction without assuming that every system is certified to reduce every contaminant.

Curious about chlorine, sediment, hardness, taste, or odor in your water? Book a complimentary in-home screening — about 30 minutes, no obligation.

Healthier water, healthier family.