Coliform in Well Water: What to Do About Bacteria

The short answer

Finding coliform in well water means something has found a way into your well or plumbing. A bacteria result for a well usually covers two things: total coliform bacteria and E. coli. They mean different things, and the steps that follow are different too.

• Total coliform is a warning sign. EPA calls total coliforms a useful indicator of other pathogens in drinking water. CDC says these bacteria generally will not make you sick, but they get into water the same way as germs that do, and they are much easier to test for. A positive result means something has found a way into your well or plumbing.
• E. coli means fecal contamination. CDC says a positive test for fecal coliforms or E. coli likely means that feces, and the harmful germs in them, have gotten into your well water. Stop drinking the water until the well has been disinfected and a follow-up test shows no bacteria.

Either way, the fix follows the same path: confirm the result, find how bacteria are getting in, repair it, disinfect the well by shock chlorination, and retest with a state-certified lab. If your lab found neither total coliform nor E. coli, no treatment is needed based on that result. Keep the report and test again next year.

Not sure your well has been tested recently? Our guide to the water bacteria test explains how to take a sterile sample and get it to a lab in time.

Coliform and E. coli results and what to do

Bacteria results are reported as present or absent, not as an amount. The table below shows the federal rule that applies to each result and what each outcome means for your household.

Some labs also count the bacteria; the Vermont Department of Health suggests asking for a count after a presence/absence result to see how severe the problem is. Our guide on how to read your lab report explains the units and terms you will see on it.

How the federal rules frame bacteria

The federal rules for bacteria are written for public water systems. EPA's Revised Total Coliform Rule handles the two results differently:

• Total coliform triggers a treatment technique. When a public water system has total coliform positives more often than a specified frequency, it must assess its system to find out why. There is no federal maximum contaminant level (MCL) for total coliform under the current rule.
• E. coli has a federal limit. EPA sets an MCL for E. coli for protection against potential fecal contamination, and its health goal (the MCLG) is 0 CFU/100 mL^[2]. EPA notes that an acute violation due to E. coli may result in a "boil water" notice for a public system's customers.

The rule applies to public water systems, which test their own water. If your home is on a private well, you are the one who tests. We compare well results with these federal benchmarks because they are the clearest reference for reading a bacteria result.

Health effects of bacteria in well water

Total coliform bacteria are not the main worry on their own. EPA's drinking water regulations describe total coliforms as not a health threat in themselves; they are used to show whether other potentially harmful bacteria may be present. CDC adds that if the total coliform count in your water is high, it is likely that harmful germs, including certain viruses, bacteria and parasites, are also in the water.

E. coli is different. EPA explains that disease-causing microbes in human or animal waste can cause diarrhea, cramps, nausea and headaches. These germs may pose a special health risk for infants, young children and people with severely compromised immune systems. CDC notes that some specific types of E. coli can make you seriously sick.

A water test can't tell you whether anyone in your home is ill or why. If you or someone you live with feels unwell and you have a positive bacteria result, talk to a health professional and bring your lab report.

We don't call a bacteria result "safe" or "unsafe" on its own. When we compare a result with a benchmark, we name it: the E. coli limit and health goal, or the total coliform treatment-technique trigger.

How bacteria get into a well

EPA's regulations table explains that coliforms are naturally present in the environment as well as in feces, while fecal coliforms and E. coli only come from human and animal fecal waste. A positive result means one of those sources has a path into your water. The Vermont Department of Health says groundwater in a properly constructed and maintained well should be free of coliform bacteria, so total coliform in well water could indicate that surface water has somehow leaked into the well.

Montana State University Extension lists the usual paths:

• Flooding or runoff at the wellhead. Wells placed where they can be flooded by runoff, or where water flows past the wellhead, can draw in surface water.
• A loose or missing cap. Wells that are not properly capped and screened can let in water, animals and insects.
• Septic systems and animal waste. A well located too close to a septic system or an animal waste source is at risk.
• A contaminated sample. Sometimes the well is fine and the sample picked up bacteria during collection. That is one reason to confirm a positive result before you act on it.

Mississippi State University Extension adds two checks to make: whether the well cap is sealed properly, and whether there are problems with the seal between the well casing and the soil around it.

Flooding deserves special attention. CDC lists flooding, land disturbances and nearby waste disposal sites among the problems near a well that should prompt a test. Indiana's Department of Environmental Management advises disinfecting a well after flooding, especially if the water went over the top of the well cap.

When to retest for bacteria

A single positive or negative result is a snapshot. Plan to test again in these situations:

• To confirm a positive result. The Vermont Department of Health advises verifying a bacteria result with a second bacteria test.
• After disinfecting the well. Vermont advises retesting two to three days^[4] after the smell of chlorine is gone. Indiana's fact sheet waits longer, retesting 7–10 days^[5] after the last trace of chlorine odor has gone. Both agree on the key point: the chlorine must be gone first. Indiana warns that any chlorine left in the system may cause a false negative coliform result. Ask your lab or local health department which timing they want.
• To make sure the fix held. After a clean retest, Indiana suggests two more tests, one within 2–4 weeks^[5] and another in 3–4 months^[5].
• On a yearly schedule. CDC recommends testing a well at least once every year for total coliform bacteria. Our well water testing guide covers how often to test well water for the other yearly tests.
• After a change. Test again if you replace or repair any part of the well system, or notice a change in your water's taste, color or smell. A sign is a reason to test, not an answer: our pages on a rotten-egg smell and brown well water explain what those signs of well water problems can and can't tell you.

Use a state-certified lab each time. A follow-up result is only useful if it is as reliable as the one that found the problem.

How to get rid of bacteria in well water

Start with a result from a state-certified lab. If it shows no total coliform and no E. coli, no treatment is needed based on that result. If it shows bacteria, work through these steps in order.

1. Protect your household first

If E. coli was found, stop drinking the water as it comes from the tap. For any bacteria in drinking water, the Vermont Department of Health advises boiling all water used for drinking, cooking, washing fruits and vegetables, brushing teeth, and making ice, baby formula and concentrated juices for one minute^[4], until follow-up test results show no bacteria. CDC suggests bottled water or another water source as an option, and recommends contacting your local health or environmental department. They can help you understand the result and what to do next.

2. Find and fix the cause

Disinfecting without fixing the cause rarely lasts. Montana State University Extension warns that chlorinating will kill the bacteria in the well, but the system will soon be contaminated again if the source is still there. Check the well cap, the casing and the ground around the wellhead, and think about recent flooding or nearby septic and animal waste. Mississippi State University Extension says it is usually most efficient to have a licensed well driller check the well and fix the problem.

3. Disinfect the well by shock chlorination

Shock chlorination means adding a strong chlorine solution to the well and plumbing, letting it sit, then flushing it out. Indiana's Department of Environmental Management says it can eliminate or reduce harmful microorganisms such as bacteria and viruses. It also notes that disinfection will not correct problems caused by other contaminants, such as nitrates, fuels or pesticides.

Indiana recommends hiring a licensed plumber, licensed water well driller or licensed water well pump installer who is familiar with the process. Its fact sheet sets out the procedure in these stages:

1. Bypass and isolate the parts of the system the procedure names before you start.
2. Take the electrical precautions it lists.
3. Remove the well cap or well seal.
4. Mix the chlorine solution.
5. Add the chlorine to the well.
6. Recirculate the chlorinated water.
7. Draw the chlorine solution through to each faucet.
8. Let the chlorinated water sit in the system for 12–24 hours^[5]. During that time, no one should use the water for any purpose, including drinking, cooking or bathing.
9. Remove the chlorinated water. Don't run it into your septic system.
10. Do follow-up water testing.

Follow the full fact sheet or your contractor's instructions for amounts and details. Don't guess at the chlorine mix.

4. Retest, and know when to get help

Once the chlorine is gone, retest with a state-certified lab, using the timing in the retest section above. If the retest is clean, keep following the follow-up schedule. If coliform bacteria are detected again, Indiana advises disinfecting the well again using the same procedure. If tests keep showing bacteria, contact your county health department for help, and have a licensed well contractor look for a cause that the first repair missed.

To find a certified lab near you, look up your state, or build a test plan that covers bacteria and the other tests your well needs. Our well water testing guide explains the yearly well check.