The alpine lake, which straddles the border between Nevada and California, contains the third-highest amount of microplastics among 38 freshwater reservoirs and lakes around the globe, the researchers found. Lake Tahoe is so full of microplastics that their concentration in its waters—5.4 plastic particles per cubic meter—is greater than the concentrations measured near some of the huge garbage patches swirling in the world’s oceans.
Only two other lakes analyzed in the study had higher concentrations of microplastics: Lake Maggiore and Lake Lugano, which are located next to each other along the border of Italy and Switzerland.
The findings confirm a 2019 study that detected microplastics in Lake Tahoe for the first time.
Though these miniscule bits of plastic have been found just about everywhere—from human blood to fresh Antarctic snow—the researchers were still surprised to find such a high volume in Lake Tahoe, which is known for its natural, pristine beauty.
“Our expectation was that concentrations in Tahoe would be very low, but that’s not what happened,” says study co-author Sudeep Chandra, a biologist at the University of Nevada, Reno, to the San Francisco Chronicle’s Gregory Thomas.
To understand the microplastic problem at Lake Tahoe, a team of researchers got into boats and dragged fine-mesh nets through the water, just a few feet below the surface. Then, they filtered the samples for fragments that were larger than 250 microns, which is roughly the same width as three strands of human hair. They also washed the particles with hydrogen peroxide to remove any organisms or substances that might be sticking to them.
From all the samples, they identified 9,425 total plastic particles, which ranged in size, shape, color and material. The Lake Tahoe findings were compiled with and compared to samples from other bodies of water in 23 countries.
Lake Tahoe may be particularly susceptible to microplastic accumulation because of its popularity among humans, large surface area, high elevation and lack of outflows, according to the researchers.
The only ways for water to leave Lake Tahoe are through the Upper Truckee River and evaporation, which means the average water molecule spends roughly 650 years in the lake. Perhaps as a result, microplastics stick around longer than they do at other lakes, per the paper.
In addition, Lake Tahoe boasts 72 miles of shoreline and has a surface area that spans a massive 200 square miles, which means microplastics floating in the atmosphere have ample space to land in the reservoir.
It’s not entirely clear where the microplastics came from or how they ended up in the lake, but researchers have a few educated guesses. Many of the microplastic particles found in Lake Tahoe were blue—the same color as ropes used to help moor boats, per the Chronicle. Some also likely came from the synthetic clothing worn by tourists and washed and dried at nearby homes and vacation rentals. Other particles likely broke off and floated into the air through dryer vents.
And many probably came from litter and debris. Despite its clear appearance, Lake Tahoe is full of garbage ranging from sunglasses to car tires. Scuba divers working on behalf of the nonprofit Clean Up The Lake pulled more than 25,000 pounds of debris from the lake between 2021 and 2022.
Trash is also a big problem on Lake Tahoe’s beaches, particularly after busy holiday weekends. Over time, that garbage makes its way into the water, where it breaks down into smaller and smaller pieces.
“We still believe that the concentration of plastics is linked to the human presence, whether from leaving trash on the beach or other small ways where plastic can end up in the water,” says first author Veronica Nava, an ecologist at the University of Milano-Bicocca in Italy, in a statement.
Since scientists now have a better grasp of how prevalent microplastics are in Lake Tahoe, they can work toward solutions. Conservation groups and policymakers have previously implemented rules intended to help keep the lake clean while balancing the region’s popularity among tourists. For example, wastewater has been exported—and not dumped into the lake—since the 1970s, per the Chronicle. And, over the last 27 years, the Lake Tahoe Environmental Improvement Program has invested $660 million to better the water quality.
Now, it seems, they’ll need to add microplastics to their growing list of concerns.
“[Lake Tahoe’s] beauty kind of hides the problem,” says Jesse Patterson, chief strategy officer of the nonprofit League to Save Lake Tahoe, to the Los Angeles Times’ Cari Spencer. “Just because we can’t collectively see it, the science shows there’s an issue, and we should respond now, before we can see it with our own eyes.”