Permeation in Water Distribution Systems
Drinking Water Distribution Systems:
Assessing And Reducing Risks
NATIONAL RESEARCH COUNCIL OF THE NATIONAL ACADEMIES 2006
Permeation in water distribution systems occurs when contaminants external to the pipe materials and non-metallic joints pass through these materials into the drinking water, Permeation is generally associated with plastic non-metallic pipes (Holsen et al., 1991). The contaminants that are most commonly found to permeate plastic pipes are organic chemicals that are lipophilic and non-polar such as highly volatile hydrocarbons and organic solvents (Holsen et al., 1991: Burlingame and Anselme, 1995), These chemicals can readily diffuse through the plastic pipe matrix, alter the plastic material, and migrate into the water within the pipe.
The most common example of permeation of water mains and fittings is associated with soil contamination of the area within which the pipe was placed (Glaza and Park, 1992), The majority of permeation incidents appear to be associated with gasoline related organic chemicals, These incidents have occurred at high-risk sites, such as industrial sites and near underground chemical storage tanks, as well as at lower risk residential sites (Holsen et al., 1991), In some cases the integrity of the pipe has been irreversibly compromised, requiring the complete replacement of the contaminated section,
Although there is the potential for water quality degradation as a result of the permeation of plastic pipe, especially in the water's taste and odor aspects, the health impacts associated with such permeation arc not expected to be significant. In some permeation incidents, the concentrations of certain chemicals have been shown to reach levels in the low parts per million, which are well above their respective MCLs (AWWA and EES. Inc,, 2002a,). However, these MCLs are based on long-term exposure, and the short-term risk levels for these chemicals are generally much higher. In the case of permeation by gasoline components, the taste or odor thresholds of the majority of these chemicals are below the levels that would pose a short-term risk (EPA, 2002e,f,g,h). Since the taste and odor thresholds for some contaminants may he above the MCLs, consumers could be unaware of contamination without monitoring in the distribution system, and some long-term exposures could result, unless otherwise detected. Therefore, the magnitude of long-term exposures resulting from permeation is unknown. In addition, these high concentrations would be expected to occur during worst case situations where water has been in contact with the affected pipe for a considerable length of time, During periods of normal water use these concentrations would expected to be much lower.
Appropriate measures can be taken to minimize the occurrence of permeation, such as issuing regulations or guidelines that define the conditions under which plastic pipe should be used, For example, the State of California precludes the use of plastic pipe in areas subject to contamination by petroleum distillates (California Code of Regulations, Title 22, Division 4, Chapter 16, Article 5, Section 64624f).
After assessing the potential health impacts associated with permeation, the committee has concluded that the potential health impacts are low and that distribution systems can best be protected through measures that minimize the conditions under which permeation can occur.
All materials in the water distribution system undergo reactions that introduce substances to the water via a process known as "leaching." Pipes. fittings. linings, and other materials used in joining or sealing pipes leach at least some substances to water through corrosion, dissolution. diffusion. or detachment. Internal coatings in water storage facilities can also leach substances. Most known substances leaching to water from materials in the distribution system do not appear to pose a public health threat due the fact they are non-toxic, present only at trace levels, or are in a form unlikely to cause health problems. Taste and odor complaints arc possible, however (sec Choi et al., 1994, and Khiari et al.. 2002. for examples).
Under some circumstances. leaching of toxic contaminants occurs at levels that pose a substantial health threat. PVC pipes manufactured before about 1977 are known to leach carcinogenic vinyl chloride into water at levels above the MCL (AWWA and EES, Inc., 2002a). It should be noted that the MCL is based on a measurement of samples at the treatment plant and not within the distribution system. To protect against a health problem from this source, sampling in the distribution system would have to he required alter installation of new PVC pipe. Cement materials have, under unusual circumstances. leached aluminum to drinking water at concentrations that caused death in hemodialysis and other susceptible patients (Berend et al.. 2001). Because levels of aluminum normally present in drinking water can also threaten this population, the FDA has issued guidance for water purification pretreatments in the U.S. for dialysis and other patients (Available on-line at http://www.gewater.com/library/tp/1111_Water_The.jsp). Finally, excessive leaching of organic substances from linings, joining and sealing materials have occasionally been noted in the literature, and asbestos fibers in water are regulated with an MCL. Potential problems with lead and copper leaching to water are managed via the LCR and, thus, are not considered further here.
Problems from older distribution system materials can be managed by monitoring of contaminant leaching in the distribution system. adjustments to water chemistry, or by costly replacement of the material. Lead leaching to water from old lead pipe is managed in this way via the LCR. For new materials, thc NSF establishes levels of allowable contaminant leaching through ANSI/NSF Standard 61. It should be noted that ANSI/NSF Standard 61. which establishes minimum health effect requirements for chemical contaminants and impurities does not establish performance, taste and odor, or microbial growth support requirements for distribution system components. There is uncertainty regarding what the states require and systems use in terms of the application and testing of ANSI/NSF Standard 61, particularly for cement materials. Therefore, father investigation is needed to determine the potential public health implications of cement materials used in distribution systems. Research has shown that distribution system components can significantly impact the microbial quality of drinking water via leaching. For example. pipe gaskets and elastic sealants (containing polyamide and silicone) can be a source of nutrients for bacterial proliferation. Colbourne et al. (1984) reported that Legionela were associated with certain rubber gaskets. Organisms associated with joint-packing materials include populations of Pseudomanas aeruginosa, Chromobacter spp., Enterobacter aerogenes. and Klebsiella pneumoniae (Schoenen, 1986; Geldreich and LeChevallier. 1999). Coating compounds for storage reservoirs and standpipes can contribute organic polymers and solvents that may support regrowth of heterotrophic bacteria (Schoenen. 1986; Thofern et al.. 1987). Liner materials may contain bitumen, chlorinated rubber, epoxy resin, or tar-epoxy resin combinations that can support bacterial regrowth (Schoenen, 1986). PVC pipes and coating materials may leach stabilizers that can result in bacterial growth. Studies performed in the United Kingdom reponed that coliform isolations were four times higher when samples were collected from plastic taps than from metallic faucets (cited in Geldreich and LeChevallier, 1999). Although procedures are available to evaluate growth stimulation potential of different materials (Bellen et al., 1993). these tests are not applied in the United States by ANSI/NSF. Standards or third-party certification that establishes performance, taste and odor, or microbial growth support requirements for distribution system components could be considered. In spite of these limitations and occasional problems. It is currently believed that leaching is a relatively low priority relative to other distribution system problems.
Undoubtedly part of the reason that certain bacteria associate with certain pipe types is because materials may leach compounds that support bacterial ...
Drinking Water Distribution Systems: Assessing and Reducing Risks
By National Research Council (U.S.), National Academies Press (U.S.
Published by National Academies, 2006
ISBN 0309103061, 9780309103060