Debate brewing over Sacramento pollution impact on Lake Tahoe

Dennis Murphy, a leading researcher at the University of Nevada, Reno, is among those who believe increased ozone levels in the growing areas of Stockton and the foothills east of Sacramento are playing a significant role in recent losses in Tahoe clarity.

The air pollution translates into nitrogen and phosphorus, which fuel algae growth in the lake.

"We're swallowing the pollutants that are coming out of the rapidly growing Sacramento Valley," said Murphy, who was team leader of a recent study of the entire Tahoe Basin watershed.

Tom Cahill, a longtime expert on Sierra air pollution expert at the University of California-Davis, disagrees.

Cahill believes the vast majority of pollution generated at Lake Tahoe comes from the cars, road dust and wood smoke surrounding Lake Tahoe itself. He also worked on the federal environmental assessment of the lake to be released soon.

Tahoe's clarity is declining at about one foot a year. A white dinner plate visible at 100 feet just 30 years ago can now be seen only at depths of about 70 feet.

Cahill acknowledged some members of the watershed assessment team suspect nitrate particles blown into the basin during the summer may settle into the lake, contributing to algae growth. The assessment cites the need for more studies into the question and one is planned this year.

"We want to resolve this very soon," Cahill said.

Wally Miller, another UNR scientist invovled in the study, says they both may be right.

"My feeling is we don't have all the answers," Miller said. "I would suggest the potential is there for both in-basin and out-of-basin pollution."

South Lake Tahoe is the only city in California where ozone concentration has increased over the last 20 years.

Of the estimated 418 metric tons of nitrogen loaded in the lake over a year, more than half comes from atmospheric deposition. Of the 45.7 metric tons of phosphorus loaded into the lake, 27 percent comes from the atmosphere and 29 percent comes from streams, the new study shows.

If pollution from outside the basin is ever is determined to be a significant factor, that could limit the ability to restore Tahoe's clarity, Murphy said.

"Even if we do the most thorough job possible here in the basin, that's not going to remove the source of some of these pollutants," he said. "If the contribution of outside-the-basin (algae) nutrients proves to be great, there will be limitations to our ability to restore the lake's prehistoric clarity."

In the meantime, halting the lake's loss in clarity - primarily through erosion control projects that keep sediment from the lake - must be a top priority, Murphy said.

"Even if we can't return the lake to the clarity it had," he said, "simply arresting the current decline is very much worth the investment. It's worth every penny."

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