My well tested positive for coliform bacteria. What now?

A positive coliform result is common. A 2006 survey of 450 private wells, cited by Penn State Extension, found coliform bacteria in about 35% of them and E. coli in about 15%². It’s fixable, but it needs to be taken seriously.

Coliforms are a large group of bacteria, and most of them don’t make people sick². They’re used as a warning light: if they’re in your water, surface water or waste is getting into the well, and germs that do cause illness may be getting in too¹. Minnesota’s Department of Health lists diarrhea, vomiting, cramps, nausea, headaches, fever and fatigue among the possible symptoms, with babies, children, older adults and people with weak immune systems most at risk¹.

What your result means

Result	What it means	What to do
Total coliform absent	No sign of contamination in this sample	Keep testing every year
Total coliform present, E. coli absent	Something is letting surface contamination in	Safe water for now; confirm, disinfect, find the source
E. coli (or fecal coliform) present	Human or animal waste is reaching the water	Stop drinking it; call your health department; check the septic system and animal waste nearby
Positive again after disinfection	The source is still there	Licensed well contractor; consider continuous disinfection

Sources: Penn State Extension, Minnesota Department of Health and Montana DEQ.

E. coli is a type of fecal coliform that lives in the intestines of people and warm-blooded animals, so Penn State calls an E. coli result “much more serious” than coliform alone². Montana DEQ’s advice for E. coli or fecal coliform is not to use the water and to contact your local health department for guidance⁴.

Step 1: Switch to safe water now

Until the water tests clean, use bottled water, boiled water or another safe source for drinking and cooking¹. To boil it, CDC says to bring the water to a full rolling boil for 1 minute, or 3 minutes above 6,500 feet³.

Step 2: Confirm the result

Penn State recommends confirming a positive result with a second sample, collected carefully in a sterile bottle and tested by a certified lab, and adding an E. coli test if it wasn’t included². Bacteria levels in wells change with the weather: they’re most likely after several weeks of rain, and in one Penn State study, fewer than half of the wells that tested positive in a wet year were still positive a year later in dry, cold weather².

That’s useful context, not a reason to wait: keep using safe water while you confirm.

Step 3: Disinfect the well

The standard first fix is shock chlorination: a strong dose of chlorine through the well and the plumbing. Minnesota says a licensed well contractor can do it, or you can do it yourself following its instructions¹. Our shock chlorination guide walks through every step and has a bleach calculator.

Then test again. Penn State suggests retesting 10 to 14 days after disinfection and again several months later². When the water tests negative, Minnesota says you can use it again without boiling¹.

Well shock chlorination calculator How much plain bleach to use for your well’s size, water depth and bleach strength.

Step 4: Find out how the bacteria got in

Disinfection kills what’s in the well today. If the way in is still open, the bacteria come back: in a Penn State study, shock chlorination plus a new sanitary well cap fixed the problem in only about 15% of contaminated wells, mostly those with low bacteria counts². Look for the source:

• The well cap. Is it tight, intact and secured? A sanitary cap keeps insects, small animals and surface water out¹².
• The casing. Look for cracks, holes or corrosion in the part above ground¹. A casing that is buried, or sealed poorly, lets surface water run down into the well².
• The type and age of the well. Minnesota considers dug wells with leaking walls or rotten covers, and wells built before 1974, at higher risk¹. Springs and shallow wells are contaminated more often than deeper wells².
• What’s nearby. Septic systems, livestock and manure are the main sources of E. coli². Minnesota recommends keeping the required distances between wells and septic systems or feedlots¹.
• Recent events. Heavy rain or work on the well, such as installing a new pump, can bring a temporary positive². After a flood, treat the well as contaminated: see the shock chlorination guide.

If E. coli was found, Penn State’s advice is to check that the septic system is working and to remove or redirect animal waste near the well². Our guide to septic pumping and warning signs can help with the first part.

Step 5: If it keeps coming back

When disinfection doesn’t hold, Minnesota recommends working with a licensed well contractor; the fix may mean cleaning debris out of the well or building a new one¹. If the source can’t be removed, continuous disinfection is the next step²:

Option	Good to know
Continuous chlorination	Needs enough contact time and regular upkeep; chlorine is hazardous to handle
Ultraviolet (UV) light	No chemicals and no residual protection; water must be clear; new bulb every year; not for more than 1,000 coliform colonies per 100 mL
Ozonation	Generally more expensive than the alternatives for disinfection alone

Source: Penn State Extension (Coliform Bacteria).

Boiling works in an emergency, but Penn State points out that it isn’t practical as a long-term water supply².