Coliform bacteria in well water: what a positive test means
General information, not medical advice. This site cannot tell you whether your water is safe. Ask your certified lab or local health department. Read the full disclaimer.
The CDC warns that a positive test for fecal coliforms or E. coli likely means feces, and the harmful germs in feces, have gotten into your well water. If your report lists coliform bacteria in well water, the exact name on that line matters. Check whether the entry names total coliform, fecal coliform or E. coli. Contact the lab that ran the test or your local health department to understand your result and what to do next.
Standards, local geology and state requirements all vary. Confirm anything on this page with your state or local health department before acting on it.
Follow the CDC guidance if a test shows harmful germs
The CDC says that if your test results show harmful germs or chemicals in your well water, consider drinking bottled water or another safe source of water, and contact your local health or environmental department. That is the CDC's advice, not a diagnosis of your water by this site. Your health department can help you understand your results and what to do next.
Total coliform, fecal coliform and E. coli are groups inside groups
The CDC describes fecal coliform bacteria as a specific kind of total coliform, and E. coli as part of the fecal coliform group.
Figure source: CDC, Guidelines for Testing Well Water. Checked October 3, 2026. The diagram shows group membership, not relative quantities.
What a positive total coliform result can and cannot tell you
A total coliform result covers a wide group of bacteria. The CDC notes that they also live in soil, on plants and in surface water, not only in the gut.
The Minnesota Department of Health warns that the presence of coliform bacteria suggests there may be disease-causing microorganisms in your water.
For a positive coliform test, Minnesota's guidance includes using bottled water or water from a safe alternative source for cooking and drinking until the issue is addressed. Ask your local health department which instructions apply to your well.
A positive total coliform entry needs follow-up with your lab or local health department. This page cannot turn that result into a verdict about your household's water.
Common mistake: treating an indicator test as a strain-identification test
The CDC says a water quality indicator test for E. coli will not show whether the water contains the dangerous types of E. coli. It distinguishes that indicator test from the more specific type of test needed when a dangerous type is suspected. Ask the issuing lab which test was run and what it can tell you.
Questions to ask your lab and local health department
Bring your report to the call. These questions keep the conversation focused:
- Which line on my report is positive, and what does that result mean for my well?
- Based on this result, should my household use another water source for now?
- Do I need a follow-up sample? If so, can you send your sample containers and your collection and delivery instructions?
- Is your laboratory state-certified for this test? The CDC says to use a state-certified laboratory.
- Does my state or county have its own requirements or testing services for private wells?
Minnesota's health department recommends contacting an accredited laboratory for sample containers and instructions, or asking your county environmental or public health services whether they provide well testing. That is Minnesota guidance, so check what your own state and local health department offer.
Public water rules do not decide a private well result
You may see the federal Revised Total Coliform Rule mentioned online. The EPA says that rule applies to public water systems. The CDC states that the EPA rules protecting public drinking water systems do not apply to privately owned wells.
The EPA also says the quality and safety of water from private domestic wells are not regulated by the federal government under the Safe Drinking Water Act. Private well owners are responsible for their households' drinking water, the EPA says. State or local rules may still apply, so confirm with your state or local health department.
Keep reading your report
For help with the rest of your lab report, see How to read well water test results, including the bacteria-result section.
Sources, check dates and limitations
This guide is based on public guidance from the following official sources, checked on October 3, 2026. The original October 2 captures are retained in the review kit:
- CDC, Guidelines for Testing Well Water
- Minnesota Department of Health, Bacterial Safety of Well Water
- US EPA, Private Drinking Water Wells
- US EPA, Learn About Private Water Wells
- US EPA, Revised Total Coliform Rule And Total Coliform Rule
We have not tested any water, used any test kits or reviewed any products. This guide does not interpret your specific report, give a safety verdict or replace advice from a certified lab or your local health department. Minnesota guidance is cited as one state's approach and may not match your state's rules.