multidrug-resistant typhoid fever (8,901 cases of typhoid fever and 95 deaths) was reported in the city of Dushanbe, Tajikistan, which affected about 1 percent of the city’s population. Low water pressure and frequent water outages had contributed to widespread increases in contamination within the distribution system (Hermin et al., 1999). More recently (April 2002), a Giardia outbreak was reported at a trailer park in New York State causing six residents to become seriously ill (Blackburn et al., 2004). Contamination was attributed to a power outage, which created a negative pressure transient in the distribution system. This allowed contaminated water to enter the system through either a cross-connection inside a mobile home or through a leaking underground pipe that was near sewer crossings. During the same period (February 2001 to May 2002), a large-scale case-control study conducted in England of the risk factors for sporadic cryptosporidiosis suggested a strong association between self-reported diarrhea and reported low water pressure events (Hunter et al., 2005).

Intrusion events can be controlled or prevented by developing and implementing best distribution system operational practices such as the requirement for maintaining a sufficient water pressure and an adequate level of disinfectant residual throughout the distribution system, leak detection and control, replacement and rehabilitation of nearby sewer lines, proper hydrant and valve operations, redesign of air relief venting (above grade), routine monitoring program (a sudden drop in the chlorine residual could provide a critical indication to water system operators that there is a source of contamination in the system), use of transient modeling to predict and eliminate potential weak spots in the distribution system, and more rigorous applications of existing engineering standards.

Although there are currently insufficient data in the literature to indicate whether intrusion from pressure transients is a substantial source of risk to treated distribution system water quality, nevertheless intrusion is inherently a subset of backflow events, has health risks and is, therefore, an important distribution system water quality maintenance and protection issue.

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