contaminants that are not neutralized by chlorination can pass through these systems into distribution.

The greatest vulnerability to contamination is at the distribution level. Downstream of the water treatment and sanitation works, any contaminant that enters the system has the potential for traveling unimpeded to end users. A scenario of concern to many water districts is the potential for backflow into the distribution system from any household connection or hydrant. This might affect a few thousand households (Dreazen, 2001). These agents could arrive in concentrations high enough to be harmful and would be subject to only residual levels of chlorine in the water. The contamination could be targeted to specific end users, such as those in a government building.

Water treatment involves hazardous chemicals in large quantities, specifically chlorine. At the time of the Pentagon 9/11 attack, a string of railroad tanker cars filled with liquid chlorine sat across the Potomac at the Blue Plains treatment works. Chlorine, sulfur dioxide, and other dangerous chemicals are routinely used at every water treatment plant.

Implementation Issues for Existing Technology

Deferred Maintenance

It makes little sense to improve the security of our water system against terrorism without addressing the history of deferred maintenance of the water infrastructure. One of the best and most cost-effective ways to make the water infrastructure more robust against malicious threats is to return its physical condition to a satisfactory level of repair. Initiatives by the federal government to develop a nationwide process and a plan for funding of rebuilding water-supply systems are necessary steps.

Water Industry Slow to Change

The water industry has not traditionally been fast-moving. When the U.S. Environmental Protection Agency (EPA) proposes rules, compliance typically spans a decade or more. Outreach and communication is needed to reduce the “time constant” for change in the water sector. Meanwhile, add-ons to existing technology may provide the best opportunity for improvement because they are more easily accepted by the industry than radically new technology.

Facilities Open to the Public

Many parts of the water-supply infrastructure are highly accessible, partly as a result of multiuse provisions written into public funding legislation. However, control of public access to components of the water system is critical for security

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