South Lake Tahoe - Crushed road sand and other "super small" particles of sediment are the biggest contributors to the loss of clarity in Lake Tahoe, the latest scientific data indicate.
The particles apparently remain suspended in the lake for years. Researchers at this point don't know for how long, but they are saying suspended sediment has a greater impact on lake clarity than algae.
Algae growth, long thought to be the primary culprit in Lake Tahoe's clarity loss - about 30 feet in as many years - is fueled by nutrients such as nitrogen and phosphorous that get washed into the lake mainly through soil erosion.
But a clarity model developed by the University of California Davis Tahoe Research Group has allowed scientists to make strides in their study of the lake, which started in 1959.
"We're able to model or calculate what the response will be for phosphorous, nitrogen and sediment loading," said Dave Roberts, an environmental scientist for the state of California. "So we're able to take out the sediment load and see what the lake's response would be to that.
"The model indicates that sediment has a greater impact on clarity. Super small particles are causing the biggest problem."
Last week, Roberts presented to the governing board of the Tahoe Regional Planning Agency preliminary results from what he said will ultimately be a $12 million study. Called the "total maximum daily load," it aims to determine how much sediment, nitrogen and phosphorous the lake can absorb without losing clarity.
Sand and other abrasives used by transportation departments at the lake to combat winter weather create some of the fine sediment that ends up clouding the lake. But action like eroding stream banks are also a factor, Roberts said.
Solving the problem will take time and likely involve some control measures for fine sediments and dissolved nutrients that could be proposed by this fall or early next year, said Roberts, who works for the Lahontan Regional Water Quality Control Board.
Transportation departments at the lake have not been ignoring water quality issues created by road dust and erosion. Caltrans and the Nevada Department of Transportation use dustless street sweepers in the Lake Tahoe Basin and fund a variety of erosion control projects.
One of South Lake Tahoe's three street sweepers has dustless technology. The city bought it about a year and a half ago for $160,000. A regular sweeper costs about $90,000.
Air quality funds collected by the Tahoe Regional Planning Agency, which regulates building and environmental policy in the basin, covered the extra cost.
"It's been working out quite well," said Jose Mercado, fleet manager for the city. "It uses filters to filter out the dust being emitted and picks up material with a vacuum. A mechanical broom picks up material embedded into asphalt."
Caltrans bought its first dustless sweeper in 1998. The transportation agency estimates the sweepers recover at least 55 percent of the salt and sand spread around the basin each winter. NDOT uses state-of-the-art sweepers and buys a coarser, more expensive sand.
"It allows better traction and doesn't break down as much and create as much dust," said NDOT spokesman Scott Magruder. "We get out and sweep it up as soon as we can."