E. coli and Coliform Bacteria: What a Positive Result Means

Three terms, three meanings

Total coliform is a broad family of bacteria found in soil, plants, and the digestive tracts of animals. Detecting total coliform in drinking water doesn't necessarily mean the water is dangerous — but it means something has entered the water that shouldn't have. The well or distribution system is not as sealed off from the environment as it should be.

Fecal coliform is a subset of total coliform that lives specifically in animal intestines. A positive fecal coliform result tightens the focus: contamination is biological and likely from a warm-blooded animal.

E. coli is a specific species within fecal coliform. A positive E. coli result is the alarm. It indicates recent fecal contamination from humans or animals, and the water is unsafe to drink until the problem is identified and resolved.

If you get a positive result

Total coliform positive, E. coli negative: don't panic. Re-test to confirm — false positives happen if the sample bottle was contaminated during collection. If the second test is also positive, inspect the well casing, cap, and any nearby surface drainage. Shock chlorination (described below) is often appropriate.

E. coli positive: stop drinking the water immediately. Use bottled water for drinking and cooking, including for pets. Don't rely on a standard pitcher filter — neither carbon filtration nor most pitcher filters reliably remove bacteria. Boiling at a rolling boil for at least one minute kills E. coli.

How contamination happens

Common causes: a damaged or missing well cap, surface water entering through a cracked casing, septic system failure or proximity, livestock waste runoff (especially after heavy rain), and a back-siphon event during a pressure loss (rare on city water but possible).

On city water, a coliform positive at the tap usually points to something inside the home — a contaminated faucet aerator, a back-siphon, or a connection between treated and untreated water.

Shock chlorination

For wells, shock chlorination is the standard response to coliform contamination. The well is treated with a strong chlorine solution that's circulated through the casing and all home plumbing, left to sit (typically 12-24 hours), then flushed out completely.

Shock chlorination is a corrective action, not a substitute for finding the cause. If the source of contamination isn't identified and fixed, the coliform will return.

After shock chlorination, retest in 1-2 weeks once chlorine has fully cleared. Then retest again at 60-90 days to confirm the fix is durable.

Continuous treatment

If contamination keeps returning, or if your well has a structural vulnerability that can't be fully fixed, continuous treatment may be appropriate.

UV disinfection: Kills bacteria and viruses as water passes through. Requires clear water (sediment must be filtered out first).

Chlorination injection: Adds a small amount of chlorine continuously, like a city utility does. Requires maintenance.

Ozonation: Effective but uncommon in residential settings.