Coliform and E. coli in Well Water: What a Positive Test Means

On this page

1. What coliform bacteria are
2. Total coliform vs E. coli
3. How common is it?
4. What to do after a positive test
5. Why shock chlorination doesn’t always last
6. Continuous disinfection options
7. Prevention
8. Testing for bacteria
9. Sources

What coliform bacteria are

Coliform bacteria live in human and animal guts, but also in soil, on plants and in surface water. According to CDC, they “generally will not make you sick.” Labs test for them because they get into water the same way disease-causing germs do, for example from a sewage leak, and they’re much easier to detect. EPA’s health goal for total coliforms in drinking water is zero (EPA).

Total coliform vs E. coli

CDC notes that a standard E. coli indicator test doesn’t tell you whether the dangerous strains are present. Detecting those takes a more specific test.

Some reports say “TNTC” or “confluent.” Penn State Extension explains that TNTC (“too numerous to count”) generally means more than 200 colonies per 100 mL. “Confluent” means other bacteria overgrew the test plate. Ask the lab whether you need to resample.

How common is it?

In one Penn State survey of 450 Pennsylvania private wells, about 35% had coliform bacteria and about 15% had E. coli (Penn State Extension). That’s one state’s data, not a national rate, but it shows a positive result is common and fixable. It isn’t a rare disaster.

What to do after a positive test

1. Switch to a safe water source for drinking. CDC advises considering bottled water or another safe source if harmful germs are found, and contacting your local health or environmental department.
2. Retest to confirm, following the lab’s sampling instructions exactly. EPA recommends confirming any exceedance before choosing treatment.
3. Inspect the wellhead. Look for a missing or loose cap, cracked casing, settling around the surface seal, or ground that slopes toward the well (EPA).
4. Fix the cause, then disinfect. Have a licensed well contractor repair defects. Then shock chlorinate the well, either by hiring a contractor or by following official instructions. How to Shock Chlorinate a Well, Step by Step
5. Retest after disinfection. CDC says to wait at least 7–10 days, until all the chlorine is flushed out, then test again. Follow up with tests 2–4 weeks and 3–4 months later (CDC).

Why shock chlorination doesn’t always last

Shock chlorination kills the bacteria that are in the well now. It doesn’t stop new bacteria from getting in. A Penn State study of coliform-contaminated wells found that shock chlorination plus a sanitary well cap kept the bacteria away for a year in only 15% of wells (Penn State Extension). The University of Georgia likewise says shock chlorination is not recommended for recurring bacteria problems. Instead, a licensed contractor should find and fix the source, or you should install continuous disinfection.

Shock chlorination works best after one-time events: new well construction, pump or plumbing work, or flooding (Penn State, UGA).

Continuous disinfection options

If bacteria keep coming back and the source can’t be fixed:

UV disinfection

Look for systems certified to NSF/ANSI 55 Class A. That class is designed to inactivate bacteria, viruses, Cryptosporidium and Giardia in contaminated water. Class B systems are only for supplemental treatment of water that’s already considered safe (ANSI summary of NSF/ANSI 55). Penn State Extension explains that water entering a UV unit must be completely clear and free of sediment so all the bacteria are exposed to the light; a sediment filter is often installed first. UV doesn’t leave any disinfectant in the plumbing after the unit. Penn State also says UV isn’t recommended where total coliform exceeds 1,000 colonies per 100 mL or fecal coliform exceeds 100 per 100 mL. The bulb typically needs replacing about once a year. Manufacturers list their own pretreatment limits (for iron, hardness and so on), so check them against your test results.

Options: SpringWell: UV systems → · US Water Systems: UV systems →. Before buying, confirm the specific model’s NSF/ANSI 55 Class A listing in NSF’s certified-product directory.

Continuous chlorination

A feeder injects chlorine ahead of a holding tank. Penn State notes that you generally need about 30 minutes of contact time, which often means a larger holding tank than a standard pressure tank, plus a sediment filter upstream and regular maintenance.

Compare systems: Well Water Treatment Options: UV vs RO vs Softener vs Whole-House Filter

Prevention

From EPA:

• Keep a sealed, intact well cap.
• Slope the ground so runoff drains away from the well.
• Maintain your septic system on the schedule your health department recommends.
• Never cut the casing below ground level.
• Test at least once a year (CDC).

Testing for bacteria

Bacteria samples need sterile bottles and must reach the lab quickly, so follow your lab’s instructions exactly. Your county health department or a local state-certified drinking water lab is often the most practical choice (EPA lab directory). Mail-in option: Tap Score’s Advanced Well Water Test ships overnight to the lab with an insulated cooler, according to the company Tap Score: Advanced well water test →.