By the Closer Look Inspectors team · Facts checked October 8, 2026 against MDH, MPCA and EPA sources · About our inspectors
Coliform bacteria are common on the ground surface but do not usually occur more than a few feet into the soil. Finding them in a well suggests surface contamination has reached the water and disease-causing organisms may be present. The Minnesota Department of Health (MDH) treats them as an indicator of well sanitary protection and recommends testing a private well every year and after any well work.
All the water tests we offer
Bacteria
Shows whether surface water or waste is reaching the well.
- Surface contamination reaching the wellNone should be present (MDH)
- Fecal contamination from septic, manure or wildlifeNone should be present (MDH)
Health chemicals
No taste, color or smell. These are the long-term health checks.
- Fertilizer, septic and manure; highest risk to infants10 mg/L (EPA)
- Natural arsenic from Minnesota rock and soil10 µg/L (EPA)
- Lead service lines, solder and brass fixturesNo safe level (MDH)
- ManganeseNatural manganese; a concern for infants100 µg/L for infants (MDH guidance)
- FluorideNatural fluoride; dental health4.0 mg/L (EPA)
- UraniumNatural radionuclide in groundwater30 µg/L (EPA)
Petroleum & solvents
Fuel and chemical spills from old tanks or nearby sites.
Household quality
Staining, scale and taste, and choosing the right treatment.
- IronRusty staining and metallic taste0.3 mg/L, aesthetic (MDH)
- HardnessScale buildup; sizing a water softenerNo health standard
- pHCorrosive or scaling water6.5 to 8.5, aesthetic (EPA)
- SulfateTaste and odor; diarrhea risk for infants250 mg/L, aesthetic (EPA)
Limits from MDH, EPA and MPCA. Your lab report compares each result to these values.
Health standard: MDH: any coliform detected in private well water may be harmful. EPA public-system limit: no more than 5.0% of monthly samples positive.
Where coliform bacteria come from in Minnesota wells
Coliform bacteria are common on the ground surface and usually do not travel more than a few feet into soil, so finding them in a well suggests surface contamination has entered the water. The Minnesota Department of Health (MDH) lists higher-risk wells as those built before 1974, dug wells with leaking walls or rotten cover boards, wells at the bottom of old frost pits, wells with corroded casing holes, and wells too close to sewers, septic systems, or feedlots. Damaged or cracked casings, a loose or missing well cap, and small animals or debris getting into the well are other routes in.
Health effects and who is most at risk
Most coliform bacteria do not cause disease themselves, but their presence suggests that disease-causing microorganisms may also be in the water. MDH notes that such microorganisms can cause illnesses including diarrheas, dysenteries, salmonellosis, hepatitis, and giardiasis. Symptoms can include diarrhea, vomiting, cramps, nausea, headaches, fever, and fatigue, and death can occur in some cases. Infants, children, elderly people, and people with weakened immune systems are more likely to become seriously ill. Because you cannot see or smell most contaminants, a laboratory test is the way to know.
When to test for coliform bacteria
MDH recommends testing a private well for coliform bacteria every year. It also recommends testing after any well work and after any change in the taste, smell, or appearance of the water. CDC adds other triggers: nearby flooding or waste problems, someone in the home becoming pregnant, or a child moving in. For a home purchase, a coliform test gives a snapshot of how the well is performing at that time. MDH recommends using an accredited laboratory, which can supply sample containers and instructions.
If your result is positive: disinfection and retesting
MDH advises using boiled or bottled water, or another safe source, for drinking and cooking until the problem is fixed. Do not boil if nitrate or arsenic was also found, because boiling concentrates them. The usual fix is shock chlorination of the well, which a licensed well contractor can do or which owners can do by following the MDH well disinfection instructions. Retest through an accredited lab before using the water, and again about two to four weeks later, since coliform can regrow. If repeated attempts fail, a licensed well contractor may need to clean or repair the well, or construct a new one.
Facts checked October 2026 against: Minnesota Department of Health, Minnesota Department of Health, Minnesota Department of Health, cdc.gov.
Coliform Bacteria water testing in pictures
Minnesota private well testing timeline
| Test | Year 1 | Year 2 | Year 3 | Year 4 | Year 5 | Year 6 |
|---|---|---|---|---|---|---|
| Coliform bacteria | ||||||
| Nitrate | ||||||
| Arsenic | ||||||
| Lead |
Test again any time after flooding, after work on the well, or when the water's taste, color or smell changes, and always when you buy a home with a well.
What a properly finished wellhead looks like in Minnesota
- Casing at least 12 inches above the ground, slab or well house floor.
- Vent ends at least 12 inches above ground, screened, opening facing down.
- Ground graded so water drains away. A casing ending below ground, as in a well pit, is not allowed.
A short casing or a buried well pit lets surface water in. A coliform bacteria test shows whether it is reaching the water. Get a free instant quote
How far a well must be from common hazards in Minnesota
- Building (closest projection)3 ft
- Lake, river, stream or pond35 ft
- Septic tank50 ft
- Drainfield (soil dispersal area)50 ft (100 ft sensitive well)
- Animal feedlot or building (over 1 animal unit)50 ft
- Cesspool or seepage pit75 ft (150 ft sensitive well)
- Unroofed feedlot, 300+ animal units100 ft
- Unprotected petroleum tank, 1,100+ gallons150 ft
Septic, feedlot or fuel tank near the well? A coliform and nitrate test shows whether it is reaching the water. Get a free instant quote
Quick answers
What does a positive total coliform result mean in well water?
Coliform bacteria are common on the ground surface, so a positive result suggests surface contamination has entered the well. Most coliform bacteria do not cause disease, but their presence suggests disease-causing microorganisms may also be in the water. MDH advises using boiled or bottled water until the well is disinfected and a retest comes back negative.
How often should I test my well for coliform bacteria in Minnesota?
MDH recommends testing private wells for coliform bacteria every year. It also recommends testing after any well work and after any change in the water's taste, smell, or appearance. Use an accredited laboratory, which can provide a sample container and instructions for collecting the sample.
How do you get rid of coliform bacteria in a well?
The usual method is shock chlorination, also called well disinfection. A licensed well contractor can do it, or owners can follow the MDH well disinfection instructions. Afterward, have an accredited lab retest the water. MDH suggests retesting again about two to four weeks later because coliform can regrow, and repeating disinfection if it returns.
Can I boil well water that tested positive for coliform?
MDH says boiled or bottled water can be used for drinking and cooking until the problem is fixed. However, do not boil the water if nitrate or arsenic was also detected, because boiling can increase their concentration. In that case use bottled water or another safe source until the well is treated and retested.
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