Coliform in Well Water: What a Positive Test Means

A positive total coliform result means bacteria from soil or surface water have a way into your well, and disease-causing organisms could follow the same path. A positive E. coli result is more serious: it means human or animal waste is reaching the water. In both cases, stop drinking the water (or boil it at a rolling boil for one full minute), shock chlorinate the well, and retest before going back to it.

Most coliform bacteria are harmless themselves. Labs test for them because they are easy to detect and they travel the same routes as the pathogens that are hard to test for.

Total coliform vs fecal coliform vs E. coli

The drinking-water goal for all three is zero. The CDC notes that a routine E. coli test does not tell you whether a dangerous strain is present; a more specific test is needed for that.

What to do today

1. Stop using the water for drinking, cooking, brushing teeth and ice. Boil it at a full rolling boil for one minute, or switch to bottled water, per the Minnesota Department of Health (MDH).
2. Confirm with a second sample if the first one was a surprise. Penn State Extension recommends confirming a positive and following any total coliform positive with an E. coli test. A careless sample can produce a false positive; see the sampling steps in our well water testing guide.
3. Look at the wellhead. A loose or missing cap, a cracked casing, or a casing that ends at or below grade are the usual suspects.
4. Plan the disinfection, either yourself using the steps below or through a licensed well contractor.

Why wells test positive

Penn State Extension surveyed 450 private wells and found coliform bacteria in about 35 percent and E. coli in about 15 percent. The common causes it lists:

• Surface water running down along a poorly sealed casing
• A loose or inadequate well cap, or an unsealed spring box, that lets in water, insects or small animals
• A failing septic system or animal waste close to the well
• One-time events, such as heavy rain or a pump replacement

Springs and shallow wells are contaminated far more often than deep ones, but a well deeper than 100 feet can still test positive if it was built poorly. MDH recommends retesting whenever floodwater reaches within 50 feet of the well or the well is opened for service.

Results also vary with weather. In a Penn State study, fewer than half of 38 wells that were positive in a normal year were still positive when retested during a dry, cold spell, so one clean winter test does not prove the problem is gone.

How to shock chlorinate a well

Shock chlorination puts a high dose of chlorine through the well and every pipe for a few hours, then flushes it out. These steps condense MDH’s well disinfection procedure; read the full MDH sheet before starting. If you are not comfortable working around the pump’s electrical supply, hire a licensed well contractor.

Bleach and amounts

Use unopened, unscented household bleach with no additives, made within the last six months, labeled 6% or 8.25% sodium hypochlorite. Do not use pool or hot-tub chlorine, which can contain algicides. MDH targets 50 to 200 ppm chlorine and warns that going above 200 ppm reduces effectiveness.

MDH’s dose table, in cups of bleach mixed into a 5-gallon pail about three-quarters full of water:

“Water in the well” is total depth minus the static water level. If you do not know the static level, use total depth. Add more for large storage, multiple buildings, flooding or a larger-diameter well.

Steps

1. Bypass treatment equipment. Put softeners, reverse osmosis units and filters on bypass and remove filter cartridges.
2. Shut off the pump breaker and confirm the power is off.
3. Open the well cap (or the vent on a sanitary seal) and check the cap, casing and wiring for damage. Fix defects first.
4. Pour in the bleach solution slowly through a funnel, keeping it off wires and metal cap parts.
5. Recirculate. Turn the pump on, run a hose until you smell chlorine and the water is clear, then put the hose back down the well and recirculate for about 30 minutes. Confirm at least 50 ppm with chlorine test strips; add bleach and repeat if it is lower.
6. Reassemble the well, rinsing components with unchlorinated water.
7. Pull chlorine to every fixture. Open each hot tap, then each cold tap, toilets, showers, appliances and outdoor spigots until strips read at least 50 ppm.
8. Let it sit at least 2 hours, and preferably 6 hours or overnight, with no water use.
9. Flush through an outdoor spigot, directed away from the septic system, plants and surface water. MDH says this can take 30 minutes to 24 hours or more. Then flush the water heater and every fixture until strips read 0 ppm.
10. Disinfect treatment devices per their manuals and return valves to service.

Safety

Wear goggles and rubber gloves. Never mix bleach with other chemicals. Work with ventilation; gases can collect in well pits. Keep children and pets away until the job is finished.

When to retest

Once no chlorine shows at the tap, have a certified lab test for total coliform before anyone drinks the water. MDH then recommends a second test 2 to 4 weeks later, because coliform can regrow. Penn State Extension suggests retesting within 10 to 14 days and again several months later. Use whichever your state health department specifies.

If coliform comes back, repeat the disinfection and look harder for the entry point. Penn State’s study found that shock chlorination plus a sanitary well cap fixed about 15 percent of contaminated wells, and success was more likely when the original count was under 10 colonies per 100 mL. A high count, or a positive that keeps returning, points to a structural problem or a contaminated aquifer, and that calls for continuous disinfection.

When shock chlorination is not enough

Ultraviolet (UV) disinfection

A UV unit kills bacteria as water passes a lamp, with no chemicals added. Penn State Extension lists its limits:

• Not recommended where total coliform exceeds 1,000 colonies per 100 mL or fecal coliform exceeds 100 colonies per 100 mL
• Water must be clear, so a sediment filter usually goes ahead of it
• The quartz sleeve must be kept clean, and hard water scales it
• The lamp typically needs replacing about once a year
• It leaves no residual disinfectant in the pipes after the unit

Ohio’s private-water-system rule requires UV units used for continuous disinfection to meet NSF/ANSI 55 Class A, to sit after any softener or iron filter, to have a 5-micron or finer prefilter, and to have an automatic shutoff or alarm. When comparing whole-house units in our UV water filter roundup, check each one’s Class A listing before its flow rate. If your water also has iron, sort that out first; see iron in well water.

Continuous chlorination

A chlorine feed pump injects chlorine ahead of a storage tank. Penn State Extension recommends about 0.3 to 0.5 mg/L of residual chlorine and roughly 30 minutes of contact time, which usually needs a larger holding tank than a standard pressure tank. Iron, hydrogen sulfide and organic matter use up chlorine, so the dose rises with them. It takes regular maintenance and careful chemical handling, but unlike UV it leaves a residual in the plumbing.

Common questions

Can I shower in it? The sources above focus on drinking, cooking and food preparation. If the result is E. coli positive or anyone in the house is immunocompromised, ask your local health department before using it for bathing.

Does a water softener or carbon filter fix coliform? No. Neither is a disinfection device.

What about nitrate? Wells that let in bacteria often let in nitrate from the same sources. Test for both; see nitrates in well water.

Can I use a home kit for the retest? Use a certified lab for the result you rely on. Home bacteria kits are covered in our well water test kit comparison.