Coliform Bacteria in Well Water: What a Positive Test Means and What to Do

A positive total coliform test means a pathway exists for surface contamination to reach your well; it doesn't mean the water will definitely make you sick. A positive E. coli or fecal coliform test is more serious: it means human or animal waste is getting in. Either way, stop drinking the water, use bottled or boiled water, find the entry point, disinfect, and retest until the results are clean. Here's how to read your result and what to do, in order.

What do total coliform, fecal coliform, and E. coli results mean?

Coliform bacteria live in soil, surface water, plants, and animal waste. Labs test for them because they're easy to detect and they travel the same routes as the germs that actually cause disease. Penn State Extension calls them "indicator organisms." Penn State says none of the three should be present in drinking water.

• Total coliform: the broad group. Most types are harmless. A positive means something at the surface found a way into the well or plumbing. CDC notes that the higher the count, the more likely harmful germs are present too.
• Fecal coliform: a subgroup that lives in the intestines of warm-blooded animals, including people. A positive points to sewage or animal waste.
• E. coli: a specific fecal coliform and the most telling result. Most strains are harmless, but a few produce a toxin that can cause severe illness and death, per Penn State. The standard lab test doesn't tell you which strain you have, so treat any E. coli positive as dangerous.

Many labs report total coliform and E. coli together. If yours only ran total coliform and it came back positive, Penn State recommends a follow-up test for E. coli to judge how serious the problem is.

What to do after a positive coliform test

Your next step depends on what the lab found. Use this as a starting point, and follow your county or state health department if it gives you different instructions.

Is it safe to drink, cook, or shower with coliform in well water?

Not for drinking or cooking until a retest is clean. The Minnesota Department of Health tells well owners with a positive bacteria test to use boiled water, bottled water, or another safe source for cooking and drinking until the problem is fixed. That applies to a total coliform positive too, not just E. coli.

Other household uses follow CDC's boil-water advisory guidance:

• Showers and baths: allowed, but don't swallow any water. CDC suggests sponge baths for babies and young children so they don't swallow it.
• Brushing teeth: bottled or boiled water only.
• Dishes: a dishwasher is generally fine if it has a sanitizing cycle or a final rinse of at least 150°F. Wash and rinse hand-washed dishes in hot water, then soak them for at least 1 minute in 1 teaspoon of unscented bleach per gallon of warm water and let them air dry.
• Food and pets: prepare and cook food with bottled or boiled water, and give pets bottled or boiled water that has cooled.

Minnesota's health department lists symptoms of waterborne illness as diarrhea, vomiting, cramps, nausea, headaches, and fever. Infants, children, older adults, and people with weakened immune systems are at higher risk. Penn State also notes that people who drink the water every day can build some tolerance, so visitors may get sick when you don't. If anyone in the house is ill, tell their doctor about the test result.

What are the first steps after a positive result?

1. Switch water sources today. Bottled or boiled water for drinking, cooking, teeth, and formula.
2. Check the sample. Penn State recommends confirming the result before spending money: make sure a state-certified lab ran it and that the sample was collected exactly per instructions in a sterile bottle. A non-sterile bottle or careless sampling can introduce bacteria and cause a false positive, so resampling is reasonable. Our well water testing guide covers how to sample correctly.
3. Ask for E. coli and a count. If the lab only reported total coliform as present or absent, ask for an E. coli test and a colony count. The number tells you how bad the problem is and which treatment fits.
4. Inspect the wellhead before disinfecting. Disinfecting a well with an open defect just buys you a few weeks. Look before you open anything: the pump's power wires run through the cap, so turn off the well's breaker before lifting a cap, or leave that to a contractor.
5. Call your county health department. Many have well staff who will explain your result and point you to licensed contractors.

How does coliform bacteria get into a well?

Coliform bacteria come from the surface, so the question is how surface water or debris is reaching the water. Minnesota's health department notes that surface bacteria usually don't travel more than a few feet into soil, so a positive often points to a problem at or near the top of the well. Check these, roughly in order:

• Well cap or sanitary seal. Loose caps let insects, small animals, and runoff in. Penn State recommends replacing a loose basic cap with a tight sanitary well cap, and EPA tells well owners to install a cap or sanitary seal.
• Casing height and cracks. EPA warns against cutting off a casing below the land surface. A buried or cracked casing lets shallow water in. Minnesota lists openings or cracks in the upper casing as a cause. See our well casing repair guide for diagnosis and costs.
• Surface drainage. Ground should slope away from the wellhead, per EPA. Puddles around the casing after rain are a red flag. A well that turns cloudy after rain is getting surface water.
• Flooding. EPA calls flooding a common cause of private well contamination and says not to drink or wash with water from a flooded well until it has been cleaned and disinfected. Our flood recovery guide covers the well and septic sequence.
• Septic system. Penn State says to check your septic system when E. coli shows up. Minnesota's well code requires at least 50 feet between a well and a septic tank or a typical home drainfield, and 100 feet from the drainfield for a "sensitive" well with less than 50 feet of watertight casing. Other states set their own distances, so check yours. Look also for signs of septic failure.
• Animals. Penn State advises removing or diverting animal waste near the well. Minnesota also sets minimum distances from feedlots and animal buildings.
• Recent plumbing or well work. Penn State lists installing a new submersible pump as a typical one-time contamination event, and CDC and EPA both say to test after any repair to the well system.

Deep wells aren't immune. Penn State notes wells deeper than 100 feet can still be contaminated if construction lets surface water run down along the casing into the aquifer. That's a grouting or casing problem, and it needs a licensed well contractor.

Does shock chlorination get rid of coliform bacteria?

It does for one-time contamination, such as a heavy storm or pump replacement. Shock chlorination floods the well and plumbing with a strong chlorine solution, holds it for 12 to 24 hours, and flushes it out. Our shock chlorination guide has the bleach doses, the 8 steps, and the mistakes that make it fail. A licensed contractor typically charges about $150 to $300. With an E. coli positive, or if anyone in the house is immunocompromised, hire the professional.

Then retest. Minnesota's health department advises testing once the chlorine is fully flushed, then retesting 2 to 4 weeks later because coliform can regrow, and keeping bottled water in use until results are clean. Testing while chlorine is still in the lines can give a false pass. Penn State adds a second test several months later. If that one is also clean, you probably had a one-time event.

Keep expectations realistic. A Penn State study found that only about 15% of wells with coliform bacteria were fixed by shock chlorination plus a sanitary well cap, especially wells with low counts (fewer than 10 colonies per 100 mL). Minnesota notes it is not unusual to disinfect a system more than once, and that a well may need to be cleaned as well as disinfected.

When do you need continuous disinfection?

If bacteria come back after you've repaired what you can and shocked the well, Penn State's advice is a treatment system that disinfects all the water entering the house. The two common choices:

• Ultraviolet (UV) light. No chemicals, which makes it a popular choice for homes. Water flows past a UV lamp in a quartz sleeve. The water must be clear, so a sediment prefilter goes ahead of it, and Penn State notes hard water can scale the sleeve, which may mean routine cleaning or a softener. Penn State says UV is not recommended where total coliform exceeds 1,000 colonies per 100 mL or fecal coliform exceeds 100 per 100 mL. The bulb needs replacing about once a year, and UV leaves no disinfectant in the pipes.
• Continuous chlorination. A feed pump injects chlorine ahead of a holding tank. Penn State gives the target as a 0.3 to 0.5 mg/L chlorine residual with about 30 minutes of contact time, which usually means a larger tank than a standard pressure tank. It leaves a residual in the plumbing but needs more upkeep and careful handling of the chemical. Water should be clear here too, so a sediment filter usually goes first.

For costs, our water treatment cost guide puts UV disinfection at $700 to $1,500 installed plus $80 to $120 a year for lamps, and a sediment prefilter at $150 to $600. Penn State estimates a UV unit adds $2 to $4 a month in electricity. Treat these as estimates; local prices vary. Treatment doesn't replace fixing the well. If the casing is cracked or the well sits too close to a drainfield, a contractor may recommend repair or a new well instead.

Penn State also lists ozone, but says it's generally not recommended for home disinfection because it costs much more than chlorination or UV.

How often should you test well water for coliform?

Once a year at minimum. CDC, EPA, and Penn State Extension all recommend testing private wells for total coliform annually, and CDC and EPA put nitrate on the same yearly list. Test sooner after:

• Flooding, or water pooling around the wellhead
• Any repair or replacement of the pump, casing, cap, or plumbing
• A change in taste, smell, or color
• Repeated stomach illness in the household
• New activity nearby, such as land disturbance or a new septic system

EPA also suggests testing more often if young children, older adults, or pregnant or nursing people live in the home. If you're buying a house with a well, get a coliform and E. coli test before closing. Keep every report. A history of clean annual tests is the best evidence your well is sound, and a sudden positive after years of clean results usually points to something that just changed.