Distribution Systems

Contamination Vulnerabilities

Distribution systems are vulnerable to a variety of external contamination events such as cross connections, permeation and leaching, and intrusions and infiltrations.

Cross-Connection Control

Distribution systems contain locations-cross connections-where non-potable water can be accidentally cross connected to potable sources.

Backflow occurs when there's reduced pressure in the distribution system (termed backsiphonage) or when there's increased pressure from a non-potable source (termed backpressure). Both situations act to change the direction of water, which normally flows from the distribution system to the customer, so that non-potable and potentially contaminated water from industrial, commercial, or residential sites flows back into the distribution system through a cross connection.

The risk posed by cross-connection backflow can be mitigated through preventive and corrective measures; for example, the installation of backflow-prevention devices and assemblies and formal programs to seek out and correct cross connections within the distribution system. Corrective measures include flushing and cleaning the distribution system after a detected incident. This may help mitigate any further adverse health effects from any contaminants that may remain in the distribution system.

There are no national reporting requirements for backflow incidents, but EPA compiled data on 459 reported backflow incidents that occurred in the U. S. between 1970 and 2001. During these reported incidents of backflow, chemical or biological contaminants have caused illness and deaths, with contamination affecting a number of service connections. This number of reported incidents is believed to be a small percentage of the total number of backflow incidents in the U.S.

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Permeation and Leaching

Distribution system infrastructure and its components (piping, linings, fixtures, and solders) can react with the water supply as well as the external environment. Permeation and leaching are two events that can result in the degradation of the distributed water.

Permeation is the passage of contaminants external to the pipe, through porous, non-metallic materials, into the drinking water. Volatile organic compounds present in the groundwater or vadose zone (from the top of the ground surface to the water table) can permeate plastic piping and gaskets. Permeation is most severe for small-diameter, low-flow pipes. Smaller water lines contain the highest ratio of mass transfer surface area to pipe volume, and are often associated with stagnant or low-flow conditions. Permeated plastic piping must be replaced since the piping retains its swollen porous state after permeation.

Leaching is the dissolution of metals, solids, and chemicals into drinking water. Leaching from cement linings can occur in soft, aggressive, poorly buffered waters. Under static conditions, metals such as aluminum, arsenic, barium, chromium, and cadmium can leach from cement linings, even when approved materials are used and linings are applied according to standard. Unidirectional flushing can be used to rid the distribution system of stagnant, contaminated water, but additional research is needed to determine the fraction of heavy metals and organics that can be removed through flushing.

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Intrusion and Infiltration

When a rapidly closed valve suddenly stops water flowing in a pipeline, pressure energy is transferred to the valve and pipe wall. Shock waves are set up within the system and they travel backward until they encounter the next solid obstacle. This results in a series of forward and backward movements. The shock wave's velocity is equal to the speed of sound, so it bangs as it travels back and forth, until dissipated by friction losses. This is called a water hammer.

A less severe form of water hammer is called surge. Surge happens when a slow-motion, mass oscillation of water is caused by internal pressure fluctuations in the system. If these surges are not controlled, they can damage pipes, fittings, and valves. This creates the opportunity for intrusion and infiltration of contaminated water into the pipe from outside.

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