Coliform Bacteria in Drinking Water

If the result you get from the lab says bacteria is “detected” or more than (>) 0 MPN/100mL, the Health Department recommends treating your water.

The “presence/absence” bacterial test kit shows whether there are or are not total coliform and E. coli in your water. The test has two steps:

1. The sample is checked for total coliform, which tells you if coliform bacteria have gotten into the well or spring. The result says if coliform bacteria have been detected or not detected.
2. If the test detects total coliform bacteria, the same sample is checked again to find out if the bacteria are from human or animal waste. The result specifically states whether E. coli bacteria are detected or are not detected.

If you have bacteria in your water, verify the results with a second bacterial test. You can purchase the count bacteria test kit (Kit NU) for $15, which will give you the number of bacteria in the sample. This will help you find out how severe the problem is.

Need help understanding your drinking water test results? Find out how to read your results

The Health Department recommends testing your well or spring for total coliform every year. You can order a total coliform and E. coli (Kit A) from the Health Department Laboratory, which costs $14, or you can use another certified drinking water lab to test for total coliform and E. coli.

The amount of bacteria in your water supply can change based on the weather. Coliform bacteria like to live near the earth’s surface and prefer warm temperatures. Your test result may be higher after several weeks of rain and lower during dry, cold conditions. Keep this in mind when testing your water supply for bacteria. 

Coliform bacteria include a large group of bacteria that can be found in drinking water. Most types of coliform bacteria will not necessarily make you sick, but since these bacteria have found a way into your water system, other disease-causing bacteria could also enter it.

Disease-causing bacteria include fecal coliform, such as E. coli. Fecal coliform from human or animal waste in your drinking water can cause stomachaches, diarrhea and other gastrointestinal diseases.

In some cases, household residents can become immune to the bacteria in their drinking water if they have been drinking it for long periods of time. In this case, visitors to the home that are not immune may become sick after drinking the water.

Almost all surface waters contain some coliform bacteria. Groundwater in a properly constructed and maintained well or spring should be free of coliform bacteria.

However, a recent study of the results from testing for bacteria in drinking water in Vermont showed:

• About two out of three springs had total coliform present.
• One in three wells had total coliform present.
• 1 in 10 families with a spring is drinking water contaminated with E. coli bacteria that can cause diarrhea and other gastrointestinal diseases.

If coliform bacteria are found in a well or spring, it could indicate that surface water has somehow leaked into the well or spring. This could happen from:

• Rain runoff or snowmelt making its way into the well or spring through cracks in ledge outcroppings, gravelly soil or sandy soil
• The pooling of rain runoff or snowmelt around the well or spring casing if the well or spring cover is not watertight, or if the pipe leading to the house is not properly installed
• Poor construction or cracks in the well or spring casing

Other sources of contamination are from:

• Insects, snakes, mice or other animals getting into the well or spring
• Dead-end pipes in household plumbing
• Improperly maintained treatment devices

If your drinking water is contaminated with bacteria:

• Boil for one minute all water to be used for drinking, cooking, washing fruits and vegetables, brushing teeth, and making ice cubes, baby formula and concentrated juices.
• Let dishes dry thoroughly before use.
• Use for bathing, but make sure children do not drink water by sucking on washcloths or sponges.

Follow these precautions until follow-up test results show the absence of bacterial contamination. Note: Licensed restaurants and other facilities have additional requirements.

Treatment recommendations

If you have coliform bacteria in your water, we recommend that you:

1. Submit a second sample to confirm the initial result. Also, if you had a presence/absence test done, you may consider asking the lab to count the bacteria in your water.
2. Visually inspect your well and water system and make any repairs. Look for leaks, unmortared joints, loose caps, ripped vent screens in the well cap, etc. Try to find any sources of bacterial contamination in the area surrounding the well.
3. Disinfect your well using shock chlorination once repairs have been made.
4. Retest your water two to three days after the smell of chlorine is gone.

If problems continue:

• Consider getting the well professionally inspected by a well contractor. They can clean the well or run a camera down to find cracks or other issues. This is generally recommended every 10 years.
• Consider permanent treatment for your water supply if you have disinfected several times and a well inspection does not find the cause of the contamination. Make sure any treatment device is certified by the National Sanitation Foundation (NSF).

Permanent treatment options include:

• Ultraviolet (UV) light treatment with an NSF/ANSI Standard 55 Certification.
• A filter with an NSF/ANSI Standard 53 Certification and is 1 micron (absolute), which is approved to remove disease-causing bacteria.
• A chlorinator, which injects exact amounts of chlorine into your plumbing system when the water is being pumped to the home.

Monitoring and timing of bacteria testing

If bacteria are detected in your water and you cannot find an obvious source of contamination, you will need to take a series of tests to monitor changes in the well water over time. Order the count bacteria test kit for this type of monitoring. We recommend testing the water before and after a heavy rain event to see if there is a change in the number of bacteria in your water. If the numbers increase after a rain event, surface water may have gotten into your well.

If you think the contamination is from a septic system or animal manure, we recommend that you have the water tested for nitrate and chloride. These are part of our recommended inorganic chemical test kit for homeowners.

Learn how to take care of your septic system

Funding is available to help you pay for water treatment or to fix your water system. Learn more about financial assistance programs and eligibility.