Vulnerabilities of the Drinking Water Distribution System
Drinking water distribution systems have been targeted for years as an area of concern with respect to ensuring safe delivery of drinking water to the end user. In the last On Tap (WC&P February, 2007) we examined the specific issues related to water distribution at the local level (i.e., premise plumbing of single households or businesses) where stagnation, temperature fluctuations and inconsistent periods of non-use contribute to deteriorating water quality. In this issue, we will focus on understanding the impact of the vast water distribution system, from source water treatment to premise plumbing delivery.
Many have pointed out how interesting it is that we treat our drinking water with state-of-the-art municipal treatment technologies to meet stringent quality standards…and then send it to consumers in a leaky, dirty container (the distribution system). Numerous studies have been published on the negative impacts of a vulnerable distribution system1. In 2003, the US EPA completed its third evaluation of the drinking water infrastructure needs in the United States, estimating that an investment of $276.8 billion dollars is needed over the next 20 years for the installation, upgrade and replacement of infrastructure related to providing safe drinking water to the public2.
The distribution system includes approximately one million miles of pumping and piping and an estimated 154,000 finished water storage facilities. An excess of 13,000 miles of new pipes are installed each year in the US. Distribution pipe materials have a finite life span, depending on the type and age of the material and the water quality. Currently, 26 percent of the distribution pipes in the US are thought to be in poor condition. Although the life expectancies of common piping materials range from 75-120 years, pipes in the US are replaced at an average rate of once every 200 years.
Pipes in poor condition may be more likely to leak or break, allowing an increased potential for contaminants from the outside environment to enter into the distribution system. The number of documented main breaks is thought to be on the rise, considering that the annual number in 1970 was 250, compared to 2,200 in 19893. It is estimated that even well-managed water distribution systems experience about 25–30 breaks per 100 miles of piping per year. Using a value of 27 main breaks per 100 miles per year, Kirmeyer et al.4 estimated 237,000 main breaks each year in the US; however, an accurate estimate is difficult to come up with considering the variability between utilities. The public health significance of these breaks in the distribution system is not currently known.
Outbreaks due to distribution system contamination
Even water that is adequately protected and treated is subject to pathogens entering the distribution system. From 1971-2002 there were 133 (17 percent of all outbreaks) documented waterborne outbreaks in the US linked to distribution system contamination. Preliminary data from the 2003-2004 survey period indicates that 38 percent of the reported outbreaks associated with drinking water systems were also associated with distribution systems1.
During the most recently published survey period (2001-2002), five of 25 (20 percent) of the documented waterborne outbreaks were associated with drinking water distribution system deficiencies; of the seven outbreaks reported involving community water systems, four (57.1 percent) were linked to distribution system problem, according to data gathered by the Centers for Disease Control (CDC). Although the overall number of reported outbreaks associated with community water systems has decreased in the last decade, the proportion of outbreaks associated with distribution systems has increased (see Figure 1).
Causes and prevention
Water loss during delivery is one measure of the quality of the distribution system. In a study of 26 US utilities, reported water losses during delivery ranged from less than 10 percent to as high as 32 percent5. At least 20 percent of distribution mains are reported to be below the water table, but it is assumed that all systems have some pipe below the water table for some time throughout the year, thus providing an opportunity for intrusion of exterior water under low or negative pressure conditions. Pipes buried in soil are subject to contamination from the surrounding environment where animal and human waste can impact the water supply. A survey of water utilities in North America found that 28.8 percent of cross-connections resulted in bacterial contamination6.
Maintaining the hydraulic integrity (positive pressure) of water distribution is important given that insufficient pressure has led to disease epidemics worldwide. Negative hydraulic pressure creates a backflow of non-potable water into the potable water supply via back siphonage, where significant pressure drops siphon contaminants into the system at cross-connections or leakage points; or backpressure due to pressures in the system that exceed the supply pressure. Even minor pressure fluctuations create conditions of back siphonage and intrusion. Such events can occur during fluctuating water use patterns (i.e., in apartment buildings, during fire hydrant use, power outages, etc.).
Although water that is disinfected during treatment often maintains a disinfectant residual, this residual will decline relative to many factors, including the distance traveled, water flow velocity, residence time, age and material of pipes and water pressure. Several studies have shown that standard levels of residual chlorine, even when present in the distribution system of treated water, do not provide significant inactivation of microbial contaminants during intrusion events. Intentional contamination events in the distribution system are also a concern where public water supplies are potentially vulnerable to bioterrorism threats.
According to a CDC survey, cross-connections and back siphonage caused the majority (51 percent) of outbreaks linked to the distribution system from 1971-2000, followed by water main contamination (a collective 33 percent) and contamination of storage facilities (16 percent). (See figure 2).
Other post-treatment contamination events can occur during the storage of drinking water. Water quality may degrade in treated water storage facilities due to loss of disinfectant residual, increased temperature and external contamination from birds, insects, animals, wind, rain, algae, etc. Storage tanks are particularly vulnerable to contamination in the absence or failure, of a protective cover or barrier, open hatches and vents; however, birds have been known to contaminate even covered public water supply distribution storage tanks.
Awareness of the role the distribution system plays in the provision of safe drinking water to consumers is increasing and more data is expected to surface regarding associative health risks. As a result, future water quality improvements will likely address our distribution system needs. Because these water contamination events occur post-treatment, point-of-use water treatment devices are a consideration for reducing the risk of exposure to contaminants introduced in the distribution system.
- NRC. Drinking water distribution systems: assessing and reducing risks. 2006. Committee on Public Water Supply Distribution Systems: Assessing and Reducing Risks, National Research Council. National Academies Press. Available at: www.nap.edu/catalog/11728.html. 437 pages.
- USEPA. Drinking water infrastructure needs survey and assessment: third report to congress. 2005. Office of Water. EPA 816-R-05-001.
- AWWASC (2002) Deteriorating buried infrastructure management challenges and strategies. American Water Works Association Service Company, Inc. Available online at http://www.epa.gov/safewater/tcr/pdf/infrastructure.pdf.
- Kirmeyer G, Richards W, Smith CD (1994). An assessment of water distribution systems and associated research needs. Denver, CO: AWWARF.
- Kirmeyer GK, Freidman M, Martel K, Howie D, LeChevallier M, Abbaszadegan M, Karim M, Funk J, Harbour J (2001) Pathogen intrusion into the distribution system. Denver, CO: AWWARF.
- Lee JJ, Schwartz P, Sylvester P, Crane L, Haw J, Chang H, Kwon HJ (2003) Impacts of Cross-Connections in North American Water Supplies. Denver, CO: AWWARF.
- Calderon RL (2004) Measuring benefits of drinking water technology: ten years of drinking water epidemiology. NEWWA Water Quality Symposium, May 20, 2004. Boxborough, MA.
About the author
Dr. Kelly A. Reynolds is an associate professor at the University of Arizona College of Public Health. She holds a Master of Science degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds has been a member of the WC&P Technical Review Committee since 1997. She can be reached via email at firstname.lastname@example.org.