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Bob Derlet drinks his water straight — without fancy filters or chemical treatments. He leans face down into Delaney Creek, which flows directly down into Tuolumne Meadows from the Sierra Crest, taking healthy gulps from the rushing stream, and then fills his water bottle. It's nearly noon on an early summer day, and temperatures are hovering in the mid-80s. After a rigorous two-mile ascent in altitudes around 9,500 feet, the pristine mountain water is indescribably refreshing: no chemical aftertaste of tap water and chilled to perfection by the Sierra's melting snowpack.

"No one camps above here. There's no livestock or park animals so there's little chance of contamination," says Derlet, gesturing toward Mt. Dana in the distance and the lush, grassy alpine meadow surrounding the creek.

FOR THE RECORD:

Giardia —A story in last week's section about backcountry water advised backpackers and hikers to bury human waste 10 feet from the water. Many sources recommend it be buried at least 100 feet from water; some say 200 feet. Also, one of the captions referred to giardia as a bacterium. Giardia is a protozoan.

Derlet should know. The emergency room physician and professor at UC Davis School of Medicine in Sacramento has spent part of the last five summers hiking about 2,000 miles throughout the Sierra and stopping at spots such as Bubbs Creek in Kings Canyon and Vogelsang Lake in Yosemite to test the water at 100 sites each year for the presence of microscopic miscreants.

It's a Herculean task, but he's driven by a desire to meld his lifelong passion for the outdoors with his expertise as a scientist. Because half of California's fresh water comes from the Sierra Nevada, Derlet is curious about pollution levels in the wilderness and what that would mean for the future of a state whose growth is dependent on clean water. Funded by grants from the Wilderness Medical Society, Derlet's field work is part of a projected 20-year water quality study.

But what he's uncovered already is surprising, both for the seasoned wilderness traveler as well as the day hiker who stares longingly at a gushing river and wonders whether it's safe to take a slug. At many trails and backcountry camps throughout California, signs warn visitors off casual sipping. But are the dangers of Giardia lamblia, E. coli, Cryptosporidium and other bugs that wreak intestinal havoc grossly exaggerated?

Derlet thinks so, and his research reveals that the water is much cleaner than most people believe. His findings thrust him into the middle of a long-simmering controversy that's blatantly at odds with what many state biologists preach and what wilderness classes teach: Purify water before drinking. But is that really necessary? Do those high-priced pumps, chemical disinfectants and elaborate filtration gadgets truly merit a place in the backpack?

"It's a huge debate," says Ryan Jordan, a biofilm engineer at Montana State University in Bozeman who has studied pollution in wilderness areas.

The available scientific evidence, which is admittedly limited because of the scarcity of funding for testing wilderness water quality, confirms Derlet's findings. The threat is comparable to the chances of beachgoers being attacked by a shark, according to University of Cincinnati researchers who studied the danger giardia poses to backpackers, namely "an extraordinarily rare event to which the public and the press have seemingly devoted inappropriate attention."

And yet, some doctors say that backcountry water is not safe to drink, even if it looks clear as glass. Defecating wildlife and encroaching hordes of campers who aren't environmentally savvy have spoiled the lakes, rivers and streams of the pristine wilderness. "Infectious agents don't change the water's appearance. You can't taste, smell or see them," says Dr. Paul Auerbach, an emergency room physician at Stanford University in Palo Alto and author of the standard text "Field Guide to Wilderness Medicine." "All it takes is a few beavers upstream, and you're in big trouble."

The National Park System and the U.S. Forest Service urge backpackers not to drink untreated water, and it has become an accepted article of faith among wilderness travelers that a water cleanser is as indispensable as a tent, compass and boots. Veteran backpackers like Jim Metropulos, who handles water quality issues for the Sierra Club in Sacramento, view water purification devices as an insurance policy that "provides a backup layer of security."

Little wonder people are convinced that drinking untreated water these days is inviting trouble. A bad case of the runs can ruin a backpacking trek, and you can end up chained to the bathroom for weeks if you contract giardiasis, the intestinal scourge that ignited the water purification debate more than two decades ago. "The issue was first widely publicized in the early 1980s," says Derlet. "Because it only takes a small dose, 10 to 25 giardia cysts [infectious particles of the parasite], to become sickened, people were alarmed."

Some point the finger at pump makers for inflating the risks and making backpackers ultra-vigilant about purifying water. "The advent of affordable water filters kick-started this whole debate," says Jordan, who is also editor of Backpacking Light magazine. "There's a lot of money in water filters: They cost anywhere from $40 to $100 a pop, and there are several million backpackers in the United States, so do the math. The water filter industry has instilled in people a mantra of 'you just never know,' rather than trying to educate them about the differences between good water sources and bad ones."

The results of a study conducted in 1993 by researchers at the University of Nevada in Reno and the U.S. Geological Survey in Sacramento were eye-opening. Of 41 backpackers who trekked to the Desolation Wilderness in Eldorado National Forest west of Lake Tahoe, six of them were stricken with cramping, diarrhea, nausea and bloating. Yet lab tests revealed that none of them was infected with giardiasis. Researchers didn't determine exactly which bugs were sickening the backpackers, but they think the culprits were the usual suspects — E. coli, salmonella or Campylobacter jejuni — which they might not have contracted from drinking water.

Taking this research one step further, the scientists analyzed the backcountry water for giardia. The bug was indeed present, but at such low levels of concentration — just a few cysts per 100 gallons — that backpackers, on average, would have to drink 250 gallons a day to become ill.