Coliform in Well Water: What a Positive Test Means

The short answer

Coliform bacteria are a group of bacteria that live in soil, on plants, in surface water and in the guts of people and animals. Most of them don't make people sick. Labs test for them because they get into a well the same way as germs that do make people sick, such as through a crack in the well or seepage from a septic system, and they are much easier to test for. So a positive coliform test is a warning sign that the well may be letting something in.

E. coli is different. It is one kind of coliform that lives in the guts of people and animals, so finding it in well water likely means feces have gotten in, from sewage or animal waste.

If your well tests positive for either one, health agencies say to stop drinking the water as it is, use boiled or bottled water for drinking and cooking, disinfect the well, and test again.

What a positive result means

EPA describes total coliform as "not a health threat in itself": it is used to show whether other, possibly harmful bacteria may be present. Public water systems are covered by EPA's Revised Total Coliform Rule; private wells are not.

CDC puts it this way: if the total coliform count is high, it is likely that harmful germs, including some viruses, bacteria and parasites, are in the water too. A positive E. coli or fecal coliform result likely means feces, and the germs in feces, have reached the well.

The germs carried in human and animal waste can cause diarrhea, cramps, nausea and headaches, according to EPA. The Minnesota Department of Health adds vomiting, fever and fatigue to that list. EPA and the Minnesota Department of Health say infants, young children and people with weakened (EPA says severely compromised) immune systems face greater risk, and Minnesota also names older people.

Two things a standard test can't tell you. It doesn't say which germs are in the water, only that something from the surface has probably found a way in. And CDC notes that most E. coli are harmless but some types can make people seriously sick, and a routine E. coli test won't show whether those types are there.

What to do if your well tests positive

1. Stop drinking the water as it is. The Minnesota Department of Health says to use boiled or bottled water, or water from another source, for drinking and cooking until the problem is fixed. CDC's boiling guidance is a rolling boil for 1 minute, or 3 minutes above 6,500 feet. One warning: if nitrate or arsenic was also found in your well water, boiling can make those more concentrated, so use bottled water instead. Boiling also doesn't remove fuel or chemicals.

2. Call your local or state health department. EPA and CDC both point well owners there first. Staff can help you read the result and plan next steps, and CDC notes they may ask you to test more often.

3. Disinfect the well. This is often called shock chlorination: chlorine bleach is added to the well and the plumbing, left to sit, then flushed out. CDC's steps leave the chlorine in the system for at least 12 hours before draining it away from plants and water bodies. Both CDC and the Minnesota Department of Health warn about electric shock, fumes and well pits, and CDC says a well or pump contractor should do the work. If you do it yourself, follow your state's written instructions exactly. Minnesota's instructions start by bypassing treatment devices such as softeners and reverse osmosis systems and removing filters.

4. Test again. CDC says to wait at least 7 to 10 days after disinfecting, so the chlorine has fully flushed out, and to keep boiling or using bottled water until the new result comes back. The Minnesota Department of Health says you can use the water again without boiling once the well has been disinfected and the water no longer tests positive. Because coliform can grow back, both the Minnesota Department of Health and CDC recommend testing again 2 to 4 weeks later, and CDC suggests another test in 3 to 4 months.

5. If it keeps coming back, find the cause. If bacteria return after disinfection, CDC says to disinfect again and retest, then contact your health department if they still show up. The Minnesota Department of Health lists likely causes: small animals or debris in the well, cracks or openings near the top of the casing, or soil that isn't filtering surface water well. A licensed well contractor may need to repair the well, and in some cases a new well is the answer. EPA notes that adding disinfection to a well, such as chlorine, ozone or ultraviolet light, can sometimes control high bacteria levels. Treatment doesn't fix a damaged well, so find the cause first.

How and when to test

When: EPA and CDC both say to test a private well at least once a year for total coliform bacteria, nitrates, total dissolved solids and pH. CDC also says to test again after flooding or land disturbance near the well, after any part of the well system is repaired or replaced, when the water's taste, color or smell changes, and when someone in the home becomes pregnant or a child comes to live with you. EPA adds a coliform test if stomach illness keeps coming back in the household.

How: use a state-certified laboratory, as CDC and EPA advise. The Minnesota Department of Health says to contact the lab for sample containers and instructions. Many labs test total coliform and E. coli together; the Wisconsin lab below, for example, reports them as present or absent.

Cost: prices vary by state and lab. EPA notes your local health department may test private wells for free, so ask there first. As one example, the Wisconsin State Laboratory of Hygiene lists a total coliform and E. coli test at $35 (prices effective July 1, 2026). Bigger panels that add nitrate, metals or other chemicals cost more.

Why testing your well is up to you

City water and private wells follow different rules. A public water system must test its water and report the results under federal rules. A private well has no utility behind it. In EPA's words, the quality of water from private wells is "not regulated by the Federal Government under the Safe Drinking Water Act nor by most state governments and laws." CDC says the same: EPA's public-water rules don't apply to privately owned wells, and no government official treats or monitors that water.

That makes the owner responsible for testing. It also means there is no published record for your well. PurityMap's well-water pages show modelled arsenic and nitrate by state and county, but those figures describe an area, not your well, and they don't cover bacteria. Only a lab test of your own water shows whether coliform is there.

What to do next

Start on PurityMap's private well water page: pick your state to see county-level arsenic and nitrate estimates and the yearly tests health agencies recommend. Each state has its own well page, for example Minnesota or Wisconsin, the two states whose health and lab guidance this article draws on. If you already have results, the water test results reader explains what each line means.

For the test itself, your health department or a state-certified lab comes first. PurityMap's water test kit guide compares mail-in lab kits and home screening kits, and the kits we track that include a bacteria test are listed below. For a result you'll act on, check that the lab is certified in your state. Those kit links may be affiliate links: PurityMap may earn a commission from qualifying purchases, at no cost to you, and it never changes what we report. If you're weighing ongoing disinfection, PurityMap's UV water purifier guide explains how UV treatment works; it doesn't fix a damaged well.

Health disclaimer

This article summarizes public guidance from EPA, CDC and state health agencies on coliform bacteria in private wells. It is not medical advice. If anyone in your home becomes ill and you think your water may be the cause, contact a healthcare provider, and call your local health department about your well.