Experts are studying whether air pollution from Sacramento threatens the environmental health of the Lake Tahoe basin.
A study released last week by the Surface Transportation Policy Project, a Sacramento-based, nonprofit organization focused on transportation reform, ranked Sacramento fifth out of the top 10 clean air offending cities in the country.
While scientists agree pollutants from Sacramento are reaching the Tahoe basin 100 miles to the east, they're not sure if it's enough to cause lake damage or health problems.
Using data from the U.S. Environmental Protection Agency, the study ranked metropolitan areas by how many days their air pollution levels exceeded national health standards during a three-year period. All of the top five cities in the nationwide survey were in California, three in the Central Valley.
"Tahoe is definitely being impacted by pollution from Sacramento and the Bay area," Jennifer Quashnick, an Air Quality Program Manager with the Tahoe Regional Planning Agency, told the Tahoe Daily Tribune.
"We've known for years that we can have transport of pollutants in the wind."
A study conducted a few years ago revealed mercury in the lake's floor.
"There was no mercury activities in the basin, so it had to come from outside," said John Reuter, a professor at the University of California, Davis who has studied Lake Tahoe since 1978.
The TRPA has also observed an increase in the ozone around Tahoe at night, an indication that pollution is coming from outside the basin since local ozone normally rises with the heat of the day, Quashnick said.
Scientists say Tahoe's elevation shields it from the brunt of the Central Valley's pollution. Ozone is carried up the western slope of the Sierra Nevada, but by the time it reaches Tahoe, it either blows across the lake high aboveground or is diluted enough not to be a problem at ground level.
"As far as we know, it's not polluting the lake and it's not having a big impact on people," said Tom Cahill, who teaches atmospheric science and physics at UC Davis. "But it's a cautionary tale because it shows how efficiently it can be transported."
But the pollution is affecting Jeffrey pines, Cahill said. Increased ozone levels make the trees'needles age faster, resulting in bare branches with green tips on the end.
Not all pollution found in Tahoe comes from outside the basin. Quashnick said that while southwest summer winds blow in contaminants, in the winter all pollution is generated locally.
The Placer County Air Pollution Control District reported that the majority of pollution in Tahoe comes from wood-burning stoves and diesel engines.
"It's both in-house and out-of-house," said Reuter.
Whether Sacramento's smog is having a major impact on the lake remains to be seen.
"No doubt bad air quality is getting into the lake. The question is - how much and is it a big deal?" Reuter said.
The California Air Resources Board, along with the TRPA, EPA, and the Lahontan Regional Water Quality Control Board, is conducting a $2 million study to track how much pollution is entering the basin from the valley.
Data should be available this winter.