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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 for the presence of bacteria in drinking water using coliform bacteria as an indicator. The presence of any coliforms in drinking water suggests that there may be disease-causing agents in the water.

The Total Coliform Rule (TCR), which was published in 1989, set both health goals (MCLGs) and legal limits (MCLs) for the presence of total coliforms in drinking water. The rule also details the type and frequency of testing that water systems must undertake. The rule applies to all public water systems.

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