tures. An earth dam can of course be breeched with conventional earthmoving equipment, but this would require unrestricted access for many hours.

About half the U.S. water supply comes from groundwater, generally unfiltered. Wellheads are easier targets than dams, because they are dispersed and little protected, but their physical destruction would not be a threat to life; the response time for such disruption could be days to months. The principal threat lies in the potential for introducing contaminants at the wellhead, not in physical destruction.

The waters collected at dams or wellheads are transferred over long distances in pipelines and aqueducts, typically by gravity with occasional pumping stations. For example, the San Francisco water-supply aqueducts from the Sierra Nevada transport water 150 miles. Most aqueducts are covered, but not all. The California Aqueduct carrying water from the Sacramento delta to southern California is an open channel for much of its 400-mile length. Aqueducts are designed to withstand hazards such as earthquakes, and some have systems for monitoring such natural disasters and responding to them, if necessary. These systems could be enhanced to handle attacks.

Sanitary collection systems are also vulnerable and pose the threat of significant disruption to normal societal functioning, if not to loss of life. Metropolitan areas cannot long function without the prompt and efficient removal of sanitary wastes. Loss of sewer services can make cities essentially uninhabitable, possibly requiring large-scale vacating of homes and businesses.

Gasoline or other flammable or explosive liquids allowed to flow into the sewer system pose the potential for explosions. Such an event killed 200 people in Guadalajara in 1992 (Eisner, 1992). Sewer explosions caused by the illegal or inadvertent release of flammable liquids are not uncommon in the United States.

More threatening than physical disruption is the potential chemical, biological, or radiological contamination of the water supply. Deininger (2000) discusses biological agents and industrial chemicals that could be used to taint drinking water. Even if the mortality or morbidity caused by contamination were minimal, the psychological effect of a credible threat to the water supply could be significant. No one willingly drinks water suspected to have even trace contamination.

The potential points of contamination of the water supply are the following: upstream of the intake of a water supply, at the water intake or wellhead, at the treatment plant, or at a point in the distribution system. The threat of upstream or collection point contamination is limited by the large volumes of water and thus the dilution involved at that stage, and by the effect of filtering and sanitation at the treatment plant. Yet, certain biological agents or their toxins may be very hazardous at low concentrations, and water treatment plants are designed to remove only a special set of contaminants, typically those found in nature. A further concern is that the water supply in several U.S. cities is not filtered. Thus,



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