E. Coli in Well Water: What It Means and What to Do Today

What does E. coli in well water mean?

Labs report two bacteria results. Total coliform is a large family of bacteria, most of them harmless, that shows surface water is getting in. E. coli is the member of that family that lives in the gut of people and warm-blooded animals. The CDC (July 2024) says a positive E. coli test likely means feces got into the well, and that a standard test will not show whether the water carries the dangerous types. The Wisconsin Department of Health Services puts it the same way: the well may be contaminated with human or animal waste. For public water systems, the EPA's goal for E. coli is zero. That is the right bar for your well too.

What to do today

1. Stop drinking the tap water. Use bottled water, or boil it. The CDC says a full rolling boil for 1 minute, or 3 minutes above 6,500 feet.
2. Use safe water for cooking, rinsing fruit and vegetables, brushing teeth, ice, and baby formula. Wisconsin's health department adds bathing babies to that list, and says to throw out ice and drinks made with the water.
3. Showers are usually fine for healthy adults if no one swallows the water. Give young children sponge baths, as the CDC suggests.
4. A dishwasher with a final rinse of at least 150 degrees F, or a sanitizing cycle, is generally fine, according to the CDC.
5. If the lab also found nitrate or arsenic, do not boil: the Minnesota Department of Health says boiling may not be the safest option then. Use bottled water.

The CDC (May 2024) says most E. coli are harmless, but some types can cause severe illness that can lead to kidney failure. Children under 5, adults 65 and older, and anyone with a weakened immune system are at the highest risk. If someone in the house has diarrhea, vomiting, cramps, or fever, call a doctor and mention the well test.

How does E. coli get into a well?

The EPA (updated July 2026) names human sewage and animal waste as the sources, carried to wells by septic leach fields and by runoff from rain and snowmelt. The Minnesota Department of Health lists wells too close to septic systems, sewers, or feedlots; cracks in the upper casing; and small animals or debris inside the well. Penn State Extension (September 2025) says shallow wells and springs carry far more risk, and that even a well deeper than 100 feet can be contaminated when surface water runs down along the outside of the casing.

Walk the wellhead and look for the doorway:

• A loose, cracked, or missing cap. Penn State Extension (June 2026) recommends a bolted sanitary cap with a gasket and a screened vent.
• Ground that slopes toward the casing. It should slope away in every direction.
• A casing that ends at or below the ground, or a buried well pit.
• A septic tank, drain field, or animal pen close by. North Carolina, for example, requires a 100-foot minimum between a new well and a septic system, with exceptions down to 50 feet on small lots. Your county sets the rule where you live.
• A recent flood, heavy rain, or work on the well or pump.

How to get rid of E. coli in a well

1. Fix the opening first. A treatment without a repair buys a clean month and another positive test.
2. Treat the whole system. Disinfect the standing water in the well, the casing walls, the pressure tank, and every pipe, as our how to treat a well guide lays out. The CDC (May 2025) says to leave the disinfectant in the well and plumbing for at least 12 hours.
3. Flush. Run the water outside, away from plants and the septic system, until no disinfectant is left.
4. Retest in 7 to 14 days. The CDC says wait at least 7 to 10 days; Penn State Extension (May 2025) says about 10 to 14. Keep using safe water until the result is clean.
5. Retest again. The CDC suggests one more test in 2 to 4 weeks and another in 3 to 4 months.

The dose depends on how much water is in your well: the depth, the water level, and the casing size. Our well lookup reads those from your well's drilling record and sizes the treatment for you.

What if E. coli comes back?

Penn State Extension found that a one-time treatment plus a sanitary cap kept bacteria out for a year in only 15 percent of the wells it followed, and says that when bacteria return, a continuous disinfection system will be necessary. That means a UV unit or a chlorinator, which our well water treatment guide compares. A well that keeps failing needs a contractor to inspect the casing, the seal, and the setbacks, because more disinfectant will not fix a crack. Sometimes the answer is a new, deeper well built to today's code.

How common is E. coli in private wells?

A USGS study of domestic wells (2006) found E. coli in untreated water from 8 percent of 397 wells, and total coliform in 33 percent of 405 wells. Rates run higher in some places: Penn State Extension reports a 2006 survey of 450 Pennsylvania wells that found E. coli in about 15 percent. Private wells are not tested by anyone but the owner, so the EPA recommends testing every year for total coliform, nitrate, total dissolved solids, and pH, and again after a flood or any work on the well. Our well water testing guide covers where to send a sample and how to read the report, and the coliform guide covers a positive total coliform result with no E. coli.

Need the numbers for your own well? The well disinfection calculator gives the gallons, the bleach amount, and the kit for it.