With an enormous wildfire in the Sierra Nevada posing a potential threat to San Francisco's water supply, city officials were busy planning Monday how to divert water from other sources in case their main reservoir -- the Hetch Hetchy, which sits 170 miles east of the city and serves about 2.6 million people -- were to grow clouded with falling ash.
The wildfire, one of the largest in California's history, has posed enough of a threat to San Francisco's utilities that Gov. Jerry Brown declared a state of emergency for the city last week. So far, the water quality in the Hetch Hetchy Reservoir, which supplies 85 percent of San Francisco's water, has not been affected, city officials said. But the situation could deteriorate either in the short term, as the fire continues, or over time, as rainwater washes residue into the water supply.
On Monday, the San Francisco Public Utilities Commission was working on contingency plans should ash and other particles from the smoke make the Hetch Hetchy water -- a source of pride among San Franciscans, who boast of its purity -- unfit for consumption, agency spokesman Charles Sheehan said. The city had been moving water into several closer reservoirs in its system for some time, starting before the fire, he said, because the Hetch Hetchy was full.
"We have several months of water supply locally," Mr. Sheehan said. "They are available should we need to switch over."
Smoke particles that settle on the water increase its turbidity, or cloudiness, which cannot exceed levels established by the state, based on criteria from the federal Environmental Protection Agency. Mr. Sheehan said there had been no increase in turbidity since the fire began Aug. 17, and that if levels started increasing, the utility would shift to alternative supplies long before standards were exceeded. "We're continuously monitoring water quality," he said.
Water that exceeds turbidity standards would have to be filtered. But large fires in watershed areas can also create longer-term problems for water quality, scientists said.
"The bigger impact is later, when there's rain events," said Sheila F. Murphy, a hydrologist with the U.S. Geological Survey in Boulder, Colo. The rain hits the barren, ash-covered slopes, washing the ash into streams leading to reservoirs.
Depending on the type of vegetation that burned, the ash can add large amounts of phosphorous and other nutrients to the reservoir, boosting growth of organic matter, and can add trace elements to the water as well. This can make treatment much more complex and costly, Ms. Murphy said.
Also affected by the fire, Mr. Sheehan said, were hospitals, other municipal facilities and the utility's power system, which provides electricity to the city's transit system -- including its famed cable cars. The blaze had damaged two of three hydroelectric plants near the reservoir, and 12 miles of transmission lines in the area had been taken offline as a precaution. But he said the utility had been able to buy power from other sources.
Almost all of San Francisco's residential and business customers are served by Pacific Gas and Electric, and a utility spokeswoman said no service interruptions were expected.
By midday Monday, the fire had consumed nearly 150,000 acres, including about 15,000 acres in a remote part of Yosemite National Park.
Julie Hutchinson, a battalion chief with the California Department of Forestry and Fire Protection, said the more than 3,600 firefighters and emergency workers battling the fire had made some progress over the weekend in halting its spread, particularly to the west and northwest.
But the fire was still only 15 percent contained, she said, and because of winds, dry conditions and inaccessible terrain, the blaze had great potential to move further east and northeast, further into and around Yosemite.
The park has remained open, and conditions in the Yosemite Valley and other main visitor areas were clear, with hardly a hint of smoke. But concerns increased Monday about the fate of two groves of giant sequoias near the fire zone.
"They're trying everything they can to save that area," said Jake Sparks, a Yosemite ranger, referring to the Merced and Tuolumne Groves, which together contain fewer than 50 of the ancient trees. Firefighters have erected a sprinkler system to be turned on if flames get near the groves, he said. The park's largest and most accessible grove, Mariposa, is not threatened, he said.
Mr. Sparks said giant sequoias had some natural fire protection, as their thick bark and high branches make ignition difficult. "To live 200 years in the forest, you'd probably have to have some resistance to fire," he said.
But the size and intensity of the Rim Fire might create conditions that were hot enough for long enough to cause the trees to burn. "This fire is something else," he said.nation