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Coliform Bacteria in Well Water: What to Do After a Positive Test

Updated October 31, 2026 · General information, not medical or plumbing advice

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The short answer: Stop drinking the water untreated. Boil it for a full 1-minute rolling boil or use another safe source. Call your health department, then have the well disinfected, ideally by a well or pump contractor. Wait 7 to 10 days, retest at a state-certified lab, and keep retesting as CDC advises.

Safety first. Taste, smell and color do not tell you whether water is safe to drink. For any health-related test, use a state-certified lab. Home kits are screening only. If a lab test comes back positive for bacteria or a harmful chemical, follow your health department's advice.

What the result means (CDC)

Total coliform: A high count suggests harmful germs, including certain viruses, bacteria and parasites, may also be in the water.
E. coli or fecal coliform: A positive result likely means fecal contamination.

Your health department can help you understand the results and what to do next, CDC says.

Step 1: Make water safe in the meantime

CDC says to boil water with a rolling boil for 1 minute before drinking, or use another safe source. It also says not to drink or bathe in the water until all the disinfection steps are done. Boiling will not make fuel or chemical contamination safe. If the water smells like fuel or chemicals, call the health department.

Step 2: Disinfect the well (shock chlorination)

Use a pro. CDC says wells should be disinfected by a well or pump contractor because of safety concerns. Turn off power to the well area first, never enter a well pit (gases and vapors can collect there), wear goggles, and have a qualified electrician or pump contractor check wiring before power is restored. The steps below are CDC's outline, so you know what the contractor will do.

Run the pump, or bail by hand, until the water is clear. Drain it through the outdoor faucet nearest the well.
Close the valves to any water softener, since the chlorine may damage it (or follow the maker's bypass instructions).
Measure the unscented liquid household bleach from CDC's table, based on water depth and casing diameter. As an example, a 10-foot water depth in a 6-inch casing calls for 1/2 cup. Mix it with 3 to 5 gallons of water in a 5-gallon bucket.
Pour it into the well through a funnel, avoiding electrical connections, and circulate water back into the well with a hose. Rinse the inside of the casing for 5 to 10 minutes.
Run indoor faucets until you smell strong chlorine at each one.
Leave the solution in the well and plumbing for at least 12 hours.
Drain the chlorinated water onto an area without vegetation, such as a driveway, until the chlorine smell is gone. Keep it out of streams and ponds. Then run all indoor faucets until the smell disappears.

CDC notes that bored or dug wells are harder to disinfect because they are shallow and often unlined. If contamination continues, consider upgrading the well or drilling a new one.

Step 3: Retest on CDC's schedule

When	What to do
7 to 10 days after disinfection	Sample for total coliform and E. coli or fecal coliform, once all chlorine is flushed out.
No coliforms found	CDC says the water can be considered safe to drink.
2 to 4 weeks later, then 3 to 4 months later	Retest again.
After that	Test at least twice a year.
Coliforms found again	Repeat the disinfection and retest. If bacteria persist, contact your local health department.

Source: CDC, How to Disinfect Wells After an Emergency.

Find the cause

Disinfection treats the symptom. If bacteria keep coming back, ask a licensed well contractor to work out how contamination is getting in. CDC's testing guidance says to test sooner if flooding, land disturbance or nearby waste disposal may affect your well, or after you repair or replace any part of the well system.

Start with a test

A state-certified lab test is the starting point for everything on this site. A home kit can be a cheap first screen, but it does not replace a lab test for anything health-related.

What to test for and how often See home screening kits

Common questions

Is water with coliform bacteria safe to drink?

Not until you know more. CDC says a high total coliform count suggests harmful germs, including certain viruses, bacteria and parasites, may also be in the water. Boil the water for a rolling boil of 1 minute or use another safe source until the problem is fixed and a retest is clean.

What does a positive E. coli result mean?

CDC says a positive fecal coliform or E. coli result likely means fecal contamination. The test does not show whether the E. coli type is dangerous; that needs a more specific test.

How long after shock chlorination should I retest?

CDC says wait at least 7 to 10 days after disinfection, until all chlorine is flushed out, then have the water sampled for total coliform and either E. coli or fecal coliform.

Can I use a filter instead of fixing the well?

A filter is not the first step. CDC says most home filters, like pitcher and fridge filters, are not designed to remove germs. Disinfect the well, find the cause and retest.

Sources