There has been an increased emphasis on sampling and testing. Courtesy of Pacific Northwest National Laboratory.

Cities and states are reassessing the safety of their drinking water, probing for weaknesses and shoring up defenses in what experts consider the unlikely event of a terrorist attack on water supplies.

Helicopters, patrol boats and armed guards sweep across the watershed feeding New York City, enforcing a temporary ban on fishing, hunting and hiking. Massachusetts has sealed commuter roads that run atop dams or wind down to the water's edge. And Utah has enlisted the help of the Federal Bureau of Investigation to peer down at reservoirs from planes and satellites, hoping to spot any weak points before crowds gather in Salt Lake City in February for the Winter Olympics.

Since Sept. 11, fears of biological or chemical attack on water systems have spread through e-mail messages that warn consumers to stock up on bottled water, fueling anxious conversations in offices and living rooms.

Yet experts say any threat to public water supplies remains largely remote. In fact, experts on germ warfare say, to cause widespread health problems by contaminating a public water supply verges on the impossible.

"The water threat is mostly science fiction," said Richard Spertzel, a microbiologist who formerly led the United Nations' biological-weapons inspection teams in Iraq.

Poisoning the voluminous rivers and reservoirs nourishing cities would require truckloads of chemicals or biological agents that would be difficult to produce and relatively easy to spot, experts say.

Even if terrorists crashed a Boeing 767 laden with anthrax into a reservoir, the lethal agent might well be destroyed in any resulting fire, or fail to diffuse effectively.

Perhaps most important, most cities could simply close off a contaminated reservoir and draw water from another source. New York City, for example, has nearly 20 reservoirs to from which to choose.

Rather than tainting a city's water supply, experts say, terrorists would be more likely to try to interrupt it entirely, perhaps by destroying dams or aqueducts.

"That is the larger threat we face," said Diane Vande Hei, executive director of the Association of Metropolitan Water Agencies, who has helped coordinate efforts to safeguard water against terrorism.

But she and other experts concede that they are looking at everything differently now, "in light of what we thought could never happen."

Though pipes and spigots might not be so effective as scattering toxins into the air, some researchers say they believe that the water industry may have underestimated the risk that biological and chemical agents could make their way into homes without being detected.

Many cities, with their thousands of miles of pipe, were never designed to prevent terrorists from patching into neighborhood lines and poisoning the water after it has been treated and tested for safety.

"If someone is going to attack us, that's where they would do it," says Dr. Dennis D. Juranek, associate director of the Division of Parasitic

Diseases at the Centers for Disease Control and Prevention. "We're highly vulnerable."

Such an attack, experts advise, could sicken small neighborhoods or large buildings, but would probably not result in the widespread loss of life that terrorists generally seek.

Still, the threat is real enough that security experts checked the White House for similar weaknesses years ago and found it to be vulnerable. Extensive changes were made there to safeguard against poisoning.

Now cities are doing what they can to protect their pipes, putting padlocks on access doors, setting up surveillance cameras and installing alarms to prevent tampering in their tunnels.

They also have dispatched lobbyists to Washington to drum up money for security and, while they are at it, to get financing for fixing leaky pipes and other outdated infrastructure.

In addition, utility companies are pushing to be allowed to keep many details of their security plans a secret. At present, federal law requires utilities to publish reports on how they would tackle their worst emergencies. That information may be important for fire departments and the police, cities say, but also could be useful to anyone interested in causing trouble.

"We don't need to advertise where the weakest links in the armor are," says Tom Curtis, deputy director of the American Water Works Association.

Some cities have greatly increased their testing efforts. In New York City, technicians stare through microscopes at any organisms they find, and pore through computer printouts of chemicals around the clock, making sure nothing out of the ordinary appears.

"This always has been in the back of our minds," reports Thomas Tipa, operations director for the New York City Department of Environmental Protection. "It's finally become a reality. We're on full alert."

But while cities regularly test for dozens of compounds in the water, they still are incapable of screening for many known chemical and biological agents, according to the disease control centers. Even if they had the technology to do so, the time and expense involved in testing for the many organisms that can survive in chlorine would be prohibitive.

As a result, utility companies are looking for cues overseas. Emulating them, many cities hope to place robots in the pipes, armed with computer chips that light up when dangerous microorganisms are encountered.