Coliform Bacteria in Well Water: What a Positive Test Means and What to Do
Quick answer: coliform bacteria in a well test mean surface contamination has found a way into your water, so germs that cause illness could be there too. Most coliforms are harmless, but E. coli means human or animal waste is getting in, which is more serious. Until the problem is fixed, the Minnesota Department of Health says to use bottled or boiled water for drinking and cooking. Then find and fix how the bacteria got in, such as a loose well cap or cracked casing, shock-chlorinate the well, and retest. If coliform keeps coming back, you need continuous disinfection, such as UV certified to NSF/ANSI 55 Class A.
Got a positive result? Stop drinking the tap water now. Use bottled water, or water boiled for about one minute (Penn State Extension), for drinking and cooking. If your well also has high nitrate or arsenic, use bottled water: the Minnesota Department of Health warns that boiling concentrates them. Contact your local health department, especially if the test found E. coli.
What coliform bacteria are, and what each result means
Coliforms are a large group of bacteria found in soil, surface water, plants and animal and human waste. Labs test for them because, as the Minnesota Department of Health explains, they live near the ground’s surface and don’t usually occur more than a few feet into the soil. Finding them in a well means surface contamination has reached the water, and disease-causing organisms may have come with it. They’re called indicator bacteria: easy to test for, and a sign of a pathway.
| Result on your report | What it means | How serious |
|---|---|---|
| Total coliform: absent | No coliform found in the 100 mL sample | Good; retest every year |
| Total coliform: present (or a count) | Surface contamination is reaching the water | Treat the water as unsafe until fixed |
| Fecal coliform or E. coli: present | Human or animal waste is getting in | More serious; a few E. coli strains cause severe illness (Penn State) |
| “TNTC” or “confluent” | Too many to count, or other bacteria overgrew the sample | Resample for an accurate result (Penn State) |
Results come as present/absent or as a count of colonies per 100 mL. Penn State notes that counts tell you more about how bad the problem is, and that the drinking water standard is zero coliform. It also found the season matters: coliform is likelier after weeks of warm, wet weather than in cold, dry spells.
Coliform result checker: what to do next
Enter what your lab report says. You’ll get the steps in order, based on the Minnesota Department of Health’s and Penn State Extension’s guidance.
Coliform found: don’t drink the water untreated
Surface contamination is reaching your well, so disease-causing germs may be present too.
- Use bottled or boiled water for drinking and cooking until the well tests clean. Boil for about one minute (Penn State). If your well has high nitrate or arsenic, use bottled water instead: boiling concentrates them (Minnesota Department of Health).
- Ask the lab to test for E. coli and, if possible, to count the coliform. Penn State says E. coli shows how serious the problem is.
- Inspect the well: a loose or missing cap, cracks or corrosion in the casing, a casing that doesn’t stick up above the ground, or ground that slopes toward the well. Fix what you find; Penn State suggests a sanitary well cap.
- Shock-chlorinate the well, yourself or with a licensed well contractor. Our bleach calculator gives the Minnesota Department of Health’s amounts.
- Retest for coliform 10 to 14 days later and again a few months later (Penn State), or 2 to 4 weeks after disinfection (Minnesota Department of Health). Use the water untreated again only after a clean result.
This checker follows state and university guidance and doesn’t replace your health department’s advice. Call them if anyone has symptoms or the test found E. coli.
How bacteria get into a well
Penn State says coliform shows a contamination pathway exists between a source of bacteria and your water supply. Common ones:
- A damaged or loose well cap, or a cracked or corroded casing, letting in surface water, insects or small animals (Minnesota Department of Health).
- A casing that ends below or at ground level, or ground that slopes toward the well, so rain runs in. Penn State suggests extending the casing above ground and sloping the soil away.
- Septic systems and animal waste nearby, especially if the test finds E. coli.
- One-time events like flooding, heavy rain or installing a new pump, which shock chlorination can often fix.
- Higher-risk wells: Penn State notes coliform is much more common in springs and shallow wells than in wells deeper than 100 feet. The Minnesota Department of Health lists dug wells with leaking walls and wells with corroded casings among those at higher risk.
How common is it? A 2006 survey of 450 private wells in Pennsylvania found coliform in about 35 percent and E. coli in about 15 percent, according to Penn State.
How to shock-chlorinate a well
Shock chlorination puts a strong chlorine solution through the well and every pipe in the house for several hours to kill bacteria. Penn State recommends having a well professional do it. If you do it yourself, follow your state’s instructions. Here is a summary of the Minnesota Department of Health’s 13-step procedure; read the full version first.
Safety first: water and electricity together can kill. Turn off the pump’s power and confirm it’s off before opening the well, and if you aren’t confident working near wiring, hire a licensed well contractor. Wear goggles and rubber gloves, never mix bleach with other chemicals, keep children and pets away, and don’t climb into a well pit, where gases can build up.
- Bypass water treatment equipment such as softeners and reverse osmosis, and remove filters. Disinfect them separately, following each maker’s instructions.
- Turn off the pump’s power and open the well cap or vent, keeping wires dry. Inspect the wiring, casing and cap, and fix any defects.
- Mix the bleach solution: fill a clean 5-gallon bucket about three-quarters full with water, then add the amount of unscented household bleach from the calculator below. Use fresh bleach, 6% or 8.25% sodium hypochlorite, made within six months, never pool chemicals.
- Pour it into the well through a funnel, keeping it off wires and metal parts.
- Recirculate: turn the power back on, run a garden hose until you smell bleach, then put the hose back into the well for about 30 minutes. Chlorine test strips should read at least 50 ppm; the target is 50–200 ppm. Stronger than 200 ppm makes disinfection less effective.
- Run every faucet, hot and cold, plus showers, toilets and outdoor taps, until each shows at least 50 ppm on a test strip.
- Wait at least two hours, preferably six or overnight, with no one using the water.
- Flush it out through a hose, away from the septic system, plants and any water body, until a low-range strip shows no chlorine. This can take 30 minutes to 24 hours or more. Flush the water heater and every fixture too.
- Retest for coliform before drinking the water, using an accredited lab.
Well bleach calculator
From the Minnesota Department of Health’s bleach table, for a well and the water system in an average home. Mix this much bleach into a 5-gallon bucket of water about three-quarters full.
4 cups of bleach
For up to 100 feet of water in a 6-inch well, per the Minnesota Department of Health’s table. Check with chlorine test strips that the water reads 50–200 ppm, and add more solution if it’s under 50.
- You may need more if the system serves several buildings, has a lot of piping or storage, or the well was flooded or has nuisance bacteria.
Amounts are for unscented household bleach that is 6% or 8.25% sodium hypochlorite. The calculator rounds up to the next row of MDH’s table, which lists 10, 50, 100 and 300 feet of water.
Retesting after you disinfect
The two guides differ slightly on timing, so plan for both. Penn State says to retest about 10 to 14 days after shock chlorination and again a few months later; if both are clean, it was likely a one-time event. The Minnesota Department of Health says to retest 2 to 4 weeks after disinfection, because coliform can regrow in the system, and notes that it isn’t unusual to need several rounds.
Shock chlorination doesn’t always last. In a Penn State study, shock chlorination plus a sanitary well cap kept bacteria away for a year in only about 15 percent of contaminated wells, mostly those with fewer than 10 colonies per 100 mL and no E. coli. If coliform comes back, move on to continuous disinfection or a well contractor’s repair.
Continuous disinfection: UV or chlorination
When shock chlorination doesn’t solve it, Penn State recommends a system that treats all the water entering the home:
- Ultraviolet (UV) light adds no chemicals and needs only a yearly lamp change in many units. It needs clear water, so a sediment filter usually goes first, and hard water can coat its sleeve. It leaves no disinfectant in the pipes after it. Penn State says UV isn’t recommended when total coliform exceeds 1,000 colonies per 100 mL or fecal coliform exceeds 100.
- Continuous chlorination injects chlorine ahead of a holding tank. Penn State says to aim for 0.3–0.5 mg/L of residual chlorine after about 30 minutes of contact time. It takes more upkeep: refilling chlorine and checking the feeder.
For UV, the certification matters. Under NSF/ANSI 55, Class A systems are designed to disinfect contaminated water, while Class B systems are only for water that’s already safe. Some popular whole-house UV units are Class B, including VIQUA’s VH410-V, per NSF’s listing. Our well water filtration guide explains the difference.
Tools and tests we’d use
VIQUA PRO10 UV System
NSF lists the PRO10 as certified to NSF/ANSI 55 for Class A disinfection, rated at 10 gpm. VIQUA says it inactivates E. coli and fecal coliform, Cryptosporidium, Giardia and rotavirus, and a sensor shows the real-time UV dose.
- Flow: 10 gpm at 40 mJ/cm² (bigger PRO20 and PRO30 models are also Class A)
- Lamp: amalgam, lasting up to 2 years, per VIQUA
- Install: vertical only; 1¼-inch MNPT × 1-inch FNPT connections; 120 W
Good to know: it isn’t sold on Amazon, so this goes to VIQUA, which sells through dealers. Put a sediment filter ahead of any UV unit, and soften very hard water first. Have a professional size it to your well pump’s flow.
See it at VIQUAChlorine test strips, high and low range
The Minnesota Department of Health’s procedure uses chlorine test strips twice: to confirm the water reaches 50–200 ppm, and to confirm it’s flushed back to zero. Most pool strips don’t read that high, so you need two kinds.
- High range: Industrial Test Systems WaterWorks 480024, with color steps at 0, 25, 50, 100, 200, 300, 400, 500 and 750 ppm (listing)
- Low range: Industrial Test Systems SenSafe 481026, reading 0–6 ppm, to check the bleach is gone (listing)
A certified lab, plus home bacteria tests between
Use an accredited or state-certified lab for the tests that decide whether the water is safe; both the Minnesota Department of Health and the CDC recommend one. Between lab tests, a Watersafe bacteria vial is a cheap yes/no check: it can turn yellow (positive) in as little as 6 hours, and you wait 48 hours to confirm a negative. Our well water test kit guide compares lab kits and home tests.
Watersafe bacteria test on AmazonFrequently asked questions
Is coliform bacteria harmful?
Most coliforms aren’t, but they signal that harmful germs could be getting in. The Minnesota Department of Health lists illnesses from contaminated water including diarrhea, dysentery, salmonellosis, hepatitis and giardiasis. Infants, children, older people and those with weakened immune systems are most at risk.
What’s the difference between total coliform, fecal coliform and E. coli?
Total coliform is the whole group, mostly from soil and plants. Fecal coliform is the subgroup from the intestines of warm-blooded animals, and E. coli is one type within it. Penn State says a positive E. coli result is much more serious because it means human or animal waste is reaching the water.
Does boiling kill coliform bacteria?
Yes. Penn State says boiling water for about one minute effectively kills bacteria. It isn’t practical long term, and it concentrates chemicals like nitrate and arsenic.
How long after shock chlorination can I drink the water?
Not until the chlorine is flushed out and a follow-up test shows no coliform, per the Minnesota Department of Health. Keep using bottled or boiled water until then.
Can a water filter remove coliform bacteria?
Ordinary iron, sulfur and carbon filters aren’t made to disinfect. For bacteria, Penn State recommends continuous disinfection: UV (choose one certified to NSF/ANSI 55 Class A) or chlorination, after fixing the well if you can.
How often should I test my well for coliform?
Every year, say the CDC and the Minnesota Department of Health, and any time the well is worked on or the water’s taste, smell or look changes.
Related guides
- Best well water test kits
- Best well water filtration systems
- Why water smells like rotten eggs
- Best water softeners for well water
Sources
- Minnesota Department of Health, Bacterial Safety of Well Water (updated October 2026) and Well Disinfection (updated October 2024).
- Penn State Extension, Coliform Bacteria (updated September 2025) and Shock Chlorination of Wells and Springs (updated May 2025).
- CDC, Guidelines for Testing Well Water.
- NSF, NSF/ANSI 55 Ultraviolet Microbiological Water Treatment Systems listing (checked November 1, 2026); Michael Sheffield, UV Treatment Systems: NSF Standard 55, Water Conditioning & Purification (2017).
- VIQUA, PRO10 UV System.