The EPA requires testing for coliforms because they can indicate the potential presence of more harmful microorganisms. If DC Water detects coliforms in the drinking water, vigorous follow-up sampling and testing is immediately conducted to ensure no harmful microorganisms are present.

The EPA set a trigger level  requiring water systems to do immediate evaluations if more than five percent of the samples collected in a month are positive for total coliform. The graph below shows the percent of monthly samples testing positive for total coliform over the past year. DC Water is consistently below EPA's requirements regarding bacteria levels in the water. However, immune compromised people are at a greater risk in developing illness. DC Water encourages immune compromised individuals to consult with their doctor regarding extra precautions to avoid infection.