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Coliform Bacteria in Well Water: What a Positive Result Means

Total coliform bacteria indicate a pathway for contamination into your well. Here’s how to interpret a positive result and what to do next.

By Know Your Well Water Editorial TeamReviewed October 26, 20263 min read

Quick Answer

Total coliform bacteria should be absent (0 per 100 mL) in well water. Most total coliforms are not themselves harmful, but their presence signals a pathway by which contamination — possibly including disease-causing organisms — can enter the well. A positive E. coli result is more serious: it indicates recent fecal contamination. Treat a positive E. coli result as a health emergency for your water supply.

Total coliform vs. E. coli

  • Total coliform: a broad indicator group. A positive result means investigate and disinfect — something is providing a route into the well.
  • E. coli: a specific indicator of fecal contamination. A positive result means stop drinking the water and use a safe alternate source immediately.

If E. coli is detected

Do not drink, cook, or make ice with the water. Use bottled or another safe source, contact your local health department, and have the well investigated and disinfected by a professional before retesting.

Common reasons for a positive result

  • A damaged, missing, or unsealed well cap allowing surface water or insects in.
  • Well casing that is cracked, too short above grade, or compromised.
  • Flooding or surface water reaching the wellhead.
  • Nearby contamination sources (septic systems, livestock) with a pathway to the aquifer.
  • Sometimes, contamination introduced during recent repairs.

What to do

  1. 1For E. coli: switch to a safe water source and contact your health department first.
  2. 2Inspect the wellhead — cap, seal, casing height, and grading around it.
  3. 3Have the well shock-chlorinated (disinfected); this is often best done by a professional, especially if the cause is unclear.
  4. 4Retest after disinfection, and again a few weeks later to confirm the fix held.
  5. 5If positives recur, a professional should investigate the well construction and consider continuous disinfection (e.g., UV) after fixing the pathway.

UV and filters are not a substitute for finding the cause

Continuous disinfection like UV can be appropriate, but it should follow — not replace — fixing the contamination pathway and confirming the well construction is sound.

Frequently asked questions

Is it safe to drink water with total coliform but no E. coli?
A positive total coliform result means the well should be investigated and disinfected, then retested, because it indicates a possible route for contamination. Follow your health department’s guidance; many recommend not drinking the water until a clean retest is obtained.
Will boiling make coliform-positive water safe?
Bringing water to a rolling boil can kill microorganisms as a short-term emergency measure, but it is not a fix — it does nothing about the pathway letting bacteria in, and boiling does not remove chemical contaminants. Address the source and disinfect the well.

Sources & references

This guide is documentary research and general information, not a laboratory analysis or professional assessment of your specific well. See our disclaimer.