Coliform Bacteria in Drinking Water

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.

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

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.

There is no wrong time of year to test your water for bacteria, but the time of year and weather conditions can affect the occurrence and number of coliform bacteria in wells. Coliform bacteria like to live near the surface of the earth and prefer warm temperatures. This means that the highest number of bacteria will be found when testing your well shortly after several weeks of rainy weather, and the fewest bacteria will be found when testing during dry, cold conditions in the winter. Be aware of these weather and season variations when testing your water supply for bacteria.

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

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 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:

• A chlorinator, which injects exact amounts of chlorine into your plumbing system when the water is being pumped to the home.
• 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.

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.