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When the news media announce a "boil water emergency," reporters often speak of a "total coliform violation." Coliforms are a group of bacteria, most of which are harmless. At first glance, it might seem strange that a harmless group of bacteria such as coliforms could cause such commotion. But like police tape and chalk outlines, coliform bacteria are often found at the scene of a crime even though they are not themselves criminals.

There are a variety of bacteria, parasites, and viruses which can cause health problems when humans ingest them in drinking water. Testing water for each of these germs would be difficult and expensive. Instead, water quality and public health workers measure coliform levels. The presence of any coliforms in drinking water suggests that there may be disease-causing agents in the water.

The Total Coliform Rule (published 29 June 1989/effective 31 December 1990) set both health goals (MCLGs) and legal limits (MCLs) for total coliform levels in drinking water. The rule also details the type and frequency of testing that water systems must do.

The coliforms are a broad class of bacteria which live in the digestive tracts of humans and many animals. The presence of coliform bacteria in tap water suggests that the treatment system is not working properly or that there is a problem in the distribution system that moves treated water from the treatment plant to customer homes. Although many types of coliform bacteria are harmless, some can cause health problems which include diarrhea, cramps, nausea and vomiting. Together these symptoms comprise a general category known as gastroenteritis. Gastroenteritis is not usually serious for a healthy person, but it can lead to more serious problems for people with weakened immune systems, such as the very young, elderly, or immuno-compromised.

In the rule, EPA set the maximum contaminant health goal (MCLG) for total coliforms at zero. Since there have been waterborne disease outbreaks in which researchers have found very low levels of coliforms, any level indicates some health risk.

EPA also set a legal limit on total coliforms. Systems must not find coliforms in more than five percent of the samples they take each month to meet EPA's standards. If more than five percent of the samples contain coliforms, water system operators must report this violation to the state and the public.

When a system finds coliforms in drinking water, it may indicate that the system's treatment system is not performing properly or that there is a problem in the distribution system. To avoid or eliminate microbial contamination, systems may need to take a number of actions, including repairing the disinfection/filtration equipment, flushing or upgrading the distribution system, and enacting source water protection programs to prevent contamination.

If a sample tests positive for coliforms, the system must collect a set of repeat samples within 24 hours. When a routine or repeat sample tests positive for total coliforms, it must also be analyzed for fecal coliforms and Escherichia coli (E. coli), which are a type of coliform bacteria that are directly associated with fresh feces. A positive result to this last test signifies an acute MCL violation, which necessitates rapid state and public notification because it represents a direct health risk.

The number of coliform samples a system must take depends on the number of customers that it serves. Systems which serve fewer than 1000 people may test once a month or less frequently, while systems with 50,000 customers test 60 times per month and those with 2.5 million customers test at least 420 times per month.

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