Environmentalists battling San Francisco's use of Hetch Hetchy Valley as the city's main drinking water cache are taking a new tack - raising questions about the water's quality in light of higher-than-normal rates of certain parasitic infections.

But officials with the city's water and public health agencies insist that the water is safe and that the group's alarms have a political goal.

The source of debate lies miles away among the rocks and fissures of Yosemite National Park. Hetch Hetchy Valley, once a rich landscape of meadows and trees, was submerged in 1923 after San Francisco won its controversial bid, with congressional approval, to create a stable water supply and built O'Shaughnessy Dam on the Tuolumne River.

Restore Hetch Hetchy, as its name suggests, aims to return that valley to its natural state by demolishing the dam and releasing the billions of gallons of water that lie behind it. Since San Francisco receives about 85 percent of its water from the Hetch Hetchy reservoir system, the group argues for storing water in another spot downstream, such as Don Pedro Reservoir.

But Hetch Hetchy is unique: Due to its remote and pristine location, federal regulators do not require the water to be filtered. It is chemically treated with chlorine and lime.

Filtration system

Storing the river water at another location would mean building a large-scale filtration system, a project estimated to cost $310 million to $515 million, according to a 2006 Hetch Hetchy restoration study by the state.

Here's where the giardia and cryptosporidium - the parasites - come in. Folks at Restore Hetch Hetchy contend that San Francisco's water should already be filtered because rates of infection by giardiasis and cryptosporidiosis are higher in San Francisco and San Mateo (also served by Hetch Hetchy) than almost anywhere else in the state. So, they say, the costs behind building a filtration plant shouldn't be one of the arguments against removing O'Shaughnessy Dam and allowing the valley to revert to its verdant past.

"This is a serious public health issue that needs to be addressed by the city of San Francisco, regardless of if we restore Hetch Hetchy or the Tuolumne River," said Mike Marshall, executive director of Restore Hetch Hetchy. "Hence, when evaluating the cost, a filtration system shouldn't be factored in."

Whereas Restore Hetch Hetchy figures the restoration would cost about $1 billion, the state report by the Department of Water Resources said it would cost between $3 billion and $10 billion.

For years, Restore Hetch Hetchy has vowed to place an initiative to dismantle the dam and re-establish the valley on the ballot in San Francisco; so far, the group has not succeeded.

'Great water'

Officials at the San Francisco Public Utilities Commission, the city's water agency, acknowledge Hetch Hetchy's contentious past - environmental giant John Muir vehemently opposed the valley's flooding. This latest criticism of the water system, however, has "crossed the line," said PUC spokesman Tyrone Jue.

"We encourage debate and dialogue, but I'm ashamed that they're spreading disinformation about our great water in San Francisco," Jue said. "This is nothing more than a smoke screen for the fact that they've never come up with a source of funding for the $10 billion needed to tear down the dam.

"The public understands we have a great water supply and that we're fortunate to have it."

That said, San Francisco's rates of giardiasis and cryptosporidiosis, the intestinal diseases caused by exposure to their waterborne parasitic counterparts, are higher than in other regions of California.

Twenty-one residents out of every 100,000 contracted giardiasis in San Francisco in 2008, compared with fewer than 6 in Contra Costa County and fewer than 4 in Los Angeles. Since 2001, giardiasis rates in San Francisco have declined; but they remain above the rates in other areas. The disease is usually caused by ingestion of fecal matter, either through contaminated water, anal to oral contact, changing diapers, handling animals or through food touched by infected food workers. Symptoms include abdominal cramps, diarrhea, vomiting and fever.

Reasons for rates

Three factors are behind the elevated rates, city health experts say: San Francisco's strong disease-surveillance program, broad access to health care, and a relatively high population of people with suppressed immune systems - specifically, gay men with HIV/AIDS.

"This is not a drinking water problem," said June Weintraub, senior epidemiologist with the San Francisco Department of Public Health. "San Francisco is off the charts" with other sexually transmitted diseases and those common among people with compromised immune systems.

Rod Adam, a giardia expert at the infectious disease department at the University of Arizona, concurs. Adam said the prevalence of the disease in men between the ages of 25 and 54, according to the city's 2008 annual report on communicable diseases, hints that giardiasis is being passed from partner to partner in the gay community.

"It really does suggest a lot of this could be sexual transmission," he said.

Mark Cloutier, former executive director of the San Francisco AIDS Foundation, isn't convinced the problem lies in San Francisco's water supply, either. But he believes the issue needs more scrutiny.

"If the rates are higher in San Francisco, we have to look at what makes San Francisco different," he said. "There needs to be more analysis."