Coliform bacteria and E. coli in well water
Coliform bacteria in a well test mean surface water or soil has a path into your well. E. coli means human or animal waste is getting in, and you should boil drinking water until it's fixed. Test every year. Fix the well cap and casing first, then shock chlorinate and retest.
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What you need to know about coliform bacteria
| The limit | None should be present. A positive result means at least 1 coliform bacterium in a 100 mL sample. |
|---|---|
| Who it hurts | Germs that come in with coliforms can cause diarrhea, cramps, nausea and fever. Children and older adults get sick most often. |
| Where it comes from | Soil, surface water, septic systems and animal waste getting in through a loose cap, a buried or cracked casing, or a shallow well. |
| How often to test | Every year, plus after floods, well repairs, or any change in taste, color or smell. |
| What removes it | Repairs to the cap and casing, then shock chlorination. If it keeps coming back, a UV light or a chlorinator that runs all the time. |
| What doesn't | Shock chlorination alone, if the way in isn't fixed. The bacteria come back. |
These counties see the most coliform bacteria in their wells
Ranked by the share of tested wells with a result over the limit. We only list counties with at least 25 tested wells, so a few bad results can't skew the ranking.
| County | Wells tested | Wells over the limit |
|---|---|---|
| Stearns County, MN | 30 | |
| Whatcom County, WA | 38 | |
| Collier County, FL | 196 | |
| Anoka County, MN | 58 | |
| Sedgwick County, KS | 52 | |
| Cumberland County, PA | 33 | |
| Harvey County, KS | 172 | |
| Dauphin County, PA | 35 | |
| Minidoka County, ID | 343 | |
| Washington County, MN | 64 |
See the full rankings and download the data
Bacteria get in through gaps at the top of the well
Coliform bacteria live in soil, surface water and the guts of animals. Most kinds are harmless. Labs test for them because they're easy to find: if coliforms are in your well, there's a path that can also carry germs that do make people sick.
E. coli is one kind of coliform that lives in the intestines of people and animals. Penn State Extension says an E. coli positive is much more serious, because it means human or animal waste is reaching the water.
The usual ways in are a loose or basic well cap, a casing buried below ground level, ground that slopes toward the well, a failing septic system, and animal waste near the wellhead. Springs and shallow wells get contaminated more often, since soil and rock filter out less. Deep wells (over 100 feet) can still be affected if surface water runs down along the casing.
It's a common problem everywhere. A Penn State survey of 450 private wells found coliforms in about 35% and E. coli in about 15%. A national USGS study found bacteria or other microbes in about 1 in 3 of the roughly 400 wells it tested for them.
E. coli means waste is reaching your water
Drinking water with coliforms doesn't always make people sick. When disease-causing germs are present, Penn State says the usual symptoms are stomach upset, fever, cramps and diarrhea. The EPA adds nausea and headaches. Children and older adults get sick most often.
A few strains of E. coli make a strong toxin and can cause severe illness.
If your test shows E. coli, stop drinking the water as it comes from the tap. Boiling it for about 1 minute kills bacteria, according to Penn State. Use boiled or bottled water for drinking and cooking until the well is fixed and retests clean.
A total coliform positive with no E. coli is less urgent. It still means something is getting in, so plan to find and fix it.
Test every year, after floods, and after any well work
The CDC says to test every year for total coliform bacteria, nitrate, pH and total dissolved solids. Test again after a flood, after work on the well or pump, or if the water's taste, color or smell changes.
Get a sterile bottle from a state-certified lab and follow its instructions closely. Penn State says a total coliform test generally costs $10 to $30.
Some states cover it. Iowa's Private Well Grants program tests private wells for bacteria for free. Ask your state or county health department what yours offers.
Results come in a few forms:
| Result | What it means |
|---|---|
| Absent, or 0 per 100 mL | No coliforms found in this sample |
| Present, or any count above 0 | Coliforms got in. Ask the lab about an E. coli test if it wasn't included |
| TNTC (too numerous to count) | Usually more than 200 per 100 mL. Send a new sample for an exact count |
| Confluent | Other bacteria overgrew the plate. Send a new sample |
Timing matters. Penn State found that bacteria show up most after several weeks of rain and least in cold, dry weather. A clean test in a dry spell doesn't rule out a problem.
Fix the way in first, then disinfect
Penn State Extension lays out these steps:
- Confirm the result. Retest with a sterile bottle from a certified lab before you spend money. Ask for E. coli if the first test was only total coliform.
- Fix the well. Extend a buried casing above ground and slope the soil away from it. Replace a loose cap with a sanitary well cap. If E. coli was found, check your septic system and move animal waste away from the well.
- Shock chlorinate. A high dose of chlorine for a short time disinfects the well and pipes. Penn State recommends having a well professional do it. Our shock chlorination calculator shows how much chlorine a well takes. Retest in 10 to 14 days and again a few months later.
- Add continuous treatment if it comes back. If bacteria return after shocking, the source is still there. A UV light or a chlorinator treats all the water entering the house.
| Option | What it does | Typical equipment cost | Things to know |
|---|---|---|---|
| Shock chlorination | Disinfects the well once | Varies; ask a well contractor | Doesn't stop new contamination |
| UV light | Kills bacteria as water passes through | $400 to over $1,000 | Needs clear water, often a sediment filter ahead of it; bulb changed yearly; no protection past the unit |
| Chlorinator | Keeps chlorine in the water | Ask for quotes | Needs about 30 minutes of contact time and regular refills |
UV costs are from Penn State Extension. Penn State doesn't recommend UV when total coliforms are over 1,000 per 100 mL or fecal coliforms are over 100.
Questions people ask about coliform bacteria
Why has nobody in my family gotten sick from this water?
Most coliforms are harmless, so a positive test doesn't mean the water will make you ill. Penn State also notes that people can build up some immunity to germs in their own water. Guests who drink it may get sick when you don't.
Should I retest before I spend money?
Yes. Penn State suggests confirming any positive with a second sample in a sterile lab bottle. Ask the lab to count the bacteria too, since a number tells you more than "present."
Why did my well test positive after heavy rain?
Bacteria live near the surface and move into groundwater when rain soaks in. Penn State found that fewer than half of positive wells were still positive when retested in cold, dry weather.
How long after shock chlorination should I retest?
Penn State says 10 to 14 days after, and again a few months later. If both come back clean, it was probably a one-time event. If bacteria return, look at continuous treatment.
Where this comes from
- U.S. EPA, National Primary Drinking Water Regulations
- CDC, Guidelines for Testing Well Water
- Penn State Extension, Coliform Bacteria
- Penn State Extension, Tips for Buying Water Treatment Equipment
- USGS, Domestic (Private) Supply Wells
- Minnesota Department of Health, Bacteria, Viruses, and Parasites in Drinking Water
- Iowa DNR, Private Well Testing