Understanding microbial indicators for drinking water assessment - revised

Boil water advisories are issued when water testing detects higher than accepted amounts of microbial indicators or when there are deficiencies with water treatment. The fear of not issuing a boil water advisory when there is truly a health risk associated with water consumption can lead water operators to call for boil water advisories as a precautionary measure. Yet, issuance of boil water advisories can have unintended consequences. Repetitive boil water advisories affect their effectiveness and can translate into a loss of confidence in drinking water systems. There is also an economic impact at the local level, causing a financial loss for the community.

Often practitioners interpret the presence of microbial indicators to mean that there is a public health risk, when this is not always true. This document aims to assist environmental health officers in interpreting drinking water laboratory results by describing the types of microbial indicators used, the ways in which they are reported, and their limitations. This document is meant to help EHOs in their decision-making regarding whether or not to intervene (e.g., calling some type of advisory).