Bacteria

At a glance: Bacteria are one of the most frequent health problems in private wells, and you cannot see, smell or taste them. Test every year for total coliform and E. coli at a state-certified lab. A coliform hit means surface water can reach your well; an E. coli hit means fecal contamination, so stop drinking the water untreated. The first fix is the well itself: disinfect it, repair the entry point and retest. Continuous treatment such as UV comes after that, if bacteria keep returning.

Public water systems disinfect and test for bacteria. This page is mainly for well owners.

What it is and where it comes from

Coliform bacteria live on the ground surface and in the gut of people and animals. MDH explains why labs test for them: they are everywhere at the surface but do not usually occur more than a few feet into the soil, so finding them in well water means surface contamination has found a way in. E. coli is a fecal coliform, so its presence points to human or animal waste.

Common entry points, according to MDH, include corroded casings, poor construction, and wells close to septic systems or feedlots. Flooding, well repairs and damaged well caps are frequent triggers.

It is widespread. A USGS study of more than 2,000 domestic wells found total coliform in about 34% and E. coli in about 8%. A 2006 survey of 450 Pennsylvania private wells cited by Penn State found coliform in about 35% and E. coli in about 15%.

Health and household effects

Total coliform bacteria are “not a health threat in itself,” in EPA’s words, but they indicate harmful organisms may be present. MDH lists the illnesses that disease-causing microorganisms in water can cause, including diarrheal illness, dysentery, salmonellosis, hepatitis and giardiasis, with symptoms such as diarrhea, vomiting, cramps, nausea, headaches, fever and fatigue.

The CDC recommends testing again when someone in the home becomes pregnant or a baby joins the household.

There are usually no household signs. Water with dangerous bacteria can look, smell and taste normal.

Limits and what they mean

Standard	Level	What it means
EPA health goal (MCLG), total coliform	Zero	Any detection is a warning sign
EPA health goal (MCLG), E. coli	Zero	E. coli detection can be an acute violation for public systems
WHO guideline, E. coli	None detectable in any 100 mL sample	International benchmark

For public systems, the Revised Total Coliform Rule uses total coliform detections to trigger an assessment of the system and requires sanitary defects to be fixed. EPA says an E. coli detection can signify an acute violation requiring rapid public notice. For a private well, apply the same standard: any E. coli is unacceptable, and any coliform needs follow-up.

How to test

Use a state-certified lab and its sterile bottle. Do not rinse the bottle or touch the inside of the cap. Coliform samples have to reach the lab quickly (federal rules for public systems allow 30 hours), so collect early in the week and follow the lab’s instructions.

Penn State says the test is inexpensive, generally $10 to $30, and Wisconsin’s state lab charges $35. MDH, the CDC and EPA all recommend testing every year, and again after flooding, well work or any change in taste, color or smell. Our well testing guide covers the full annual panel.

Treatment options

Fix the source first. Continuous treatment on a well with a broken cap treats the symptom and leaves the contamination path open.

Method	How well it works	Certification to look for	Cost range
Shock chlorination of the well	Temporary to permanent, depending on the source	None	Bleach and your time if you do it yourself
UV disinfection, whole house	High for bacteria and viruses in clear water	NSF/ANSI 55 Class A	$250 to $800; $995 to $1,101 for systems we track
Continuous chlorination plus filter	High	None common	$500 to $2,500
Reverse osmosis, point of use	Moderate; cyst claim, not disinfection	NSF/ANSI 58 cyst reduction	$300 to $1,500
Distillation, point of use	High	NSF/ANSI 62	$300 to $1,200

Cost ranges are Minnesota Department of Health estimates based on 2017 and 2018 quotes, low end self-installed and high end professionally installed. Figures for systems we track are prices checked September 2026.

Shock chlorination. Penn State found it could solve the problem in about 15% of wells with coliform when bacteria counts were low. For iron bacteria, MDH advises a concentration close to, but not above, 200 ppm.

UV. NSF/ANSI 55 has two classes. Class A systems are built to disinfect water contaminated with bacteria and viruses; Class B systems are only for supplemental treatment of water already considered safe. UV needs clear water, so a sediment filter usually goes in front, and Penn State notes hard water can coat the lamp sleeve with scale. Penn State advises against UV where total coliform exceeds 1,000 colonies per 100 mL or fecal coliform exceeds 100.

Chlorination also keeps a disinfectant in the plumbing and helps with iron and sulfur, at the cost of more upkeep and a chlorine taste that may need a carbon filter. See our well water filter picks and the filter finder for UV and combined systems.

When to call a professional

Call your county or state health department after any positive E. coli result. They can advise on disinfection and a safe interim water source.

Call a licensed well contractor if coliform keeps returning after disinfection, if the cap, casing or seal is damaged, or if floodwater reached the wellhead. A contractor can find the entry point, which no treatment unit will fix. Once the well is sound, a water treatment professional can size UV or chlorination to your flow and water chemistry.