Confidence in drinking supply is key, though the chance of massive contamination is low

New security measures at treatment plants, reservoirs and dams include round-the-clock security guards, surveillance cameras, better reservoir covers and locks--and plain old fences.

Some systems are testing water quality more often and for more contaminants. A few are buying extra pumps, intakes and other equipment in case of attack.

Physically poisoning the nation’s 54,000-plus water systems is next to impossible.

“Scientists will tell you the potential for contamination of a water system is minimal, but the psychological impact is key,” says Rick Hahn, a security consultant who developed a five-year, $132-million plan to protect the Los Angeles water system. Hahn recognizes the havoc that would result from even a minor attack that caused the public to lose confidence in water purity.

But if Los Angeles is on a fairly high alert against the threat, other cities appear less so.

“It would take quite an effort to contaminate that amount of water, so it is not high on our list of priorities in terms of infrastructure protection,” said Jim Mosley, director of public works in Wilmington, Del.

Robert J.S. Ross, a Clark University sociologist who has studied Sept. 11, said he rode his bicycle 18 miles along a reservoir’s unprotected banks in Worcester, Mass., before encountering four armed and uniformed men guarding the reservoir’s dam.

The physical and electronic systems that control water supplies are also potentially vulnerable. A hacker penetrated the system controlling sewage treatment plants in Queensland, Australia, two years ago and released hundreds of thousands of gallons of sludge into nearby parks and rivers.