Testing Drinking Water for Bacteria (Total Coliform and E. coli)

Testing drinking water for presence of Total Coliform and E. coli is a way to assess risk that waterborne pathogens are present. These tests will only identify the presence of bacteria if they are alive in the sample. For this reason, it is important to get samples to the lab as quickly as possible after collection.

The amount of time between sample collection and analysis is called the “hold time” for the sample.

A hold time limit of 30 hours (as well as transport on ice) is required for testing of public water supplies, where strict regulations are in place. There are no hold time requirements for private well samples.

Bacteria results will be more reliable if they reach the lab within 30 hours and if they have not been exposed to extreme hot temperatures during transport.

If there is an “H” in the “Qualifier” column, the sample reached the lab too late and an “Absent” or “Safe” result is not reliable. In this case, consider retesting.

For results from the Well Educated program, an “H” in the “Qualifier” column (see image below) indicates that the sample arrived at the lab after the 30 hour ideal hold time and that results may have limited reliability. You can determine the hold time by comparing the “Collection Date” to the “Received Date” at the top right of the “Laboratory Analytical Report.” If the hold time was only a few hours more than 30, the results may be reliable, but if the hold time is more than 48, the results are likely not reliable.

A test result indicating bacteria are “Present” in your sample, should be reliable regardless of the hold time, so the “H” does not matter if bacteria were found “Present” or “Unsafe.” In other words, bacteria cannot become present in a sample due to long hold time (i.e. false positives do not occur).

The Qualifier column on your Well Test Results is to the right of the Safe/Unsafe column. An "H" indicates the sample was received past holding time.

Retesting for Bacteria (Total Coliform and E. coli)

If your bacteria results came back as "Absent" or "Safe" with an "H" in the Qualifier column, consider retesting.

The most reliable way to ensure a bacteria test will be reliable, is to deliver the sample to the lab yourself immediately following collection.

See the list of certified labs in Montana, most of which can conduct bacteria analysis (E. coli and Total Coliform). You can communicate directly with the labs to get sample bottles for a test.

If you do not want to deliver the sample to the lab yourself, communicate with the parcel service you plan to use to ensure they are confident the sample will reach the lab in less than 30 hours. See our mailing samples page for more information.

You are welcome to participate in the Well Educated program for a bacteria test, although the simplest package includes bacteria and nitrate. Well Educated samples are analyzed in Billings, and the samples need to arrive there on time, which usually involves a parcel service. Using a parcel service will always come with some risk that samples will arrive past the recommended hold time.