Bacteria & Coliform in Well Water
Total coliform and E. coli are two different results with different implications. Here's how to tell them apart and what to do next.
Private wells
Short answer: Total coliform present but E. coli absent usually points to a maintenance or construction issue, not necessarily sewage contamination. E. coli present is a more serious signal that requires prompt action. Either result calls for disinfection and retesting, following your lab's specific guidance.
What the two results mean
- Total coliform: a broad group of bacteria found in soil, surface water, and human/animal waste; presence alone often points to a well construction or seal issue.
- E. coli: a more specific indicator of fecal contamination; presence is a more urgent signal.
Common causes
- A damaged or missing well cap or seal
- A well casing that allows surface water intrusion
- Recent well work that introduced contamination
- Proximity to a septic system, in some cases
What to do after a positive result
- Follow your lab's specific disinfection (shock chlorination) instructions.
- Retest after the recommended waiting period to confirm the issue is resolved.
- If bacteria persist after disinfection, have a well professional inspect the casing, cap, and seal for physical issues.
- Avoid drinking the water until you have a clear retest result, following your lab's or health department's guidance.
Common mistakes
- Assuming total coliform presence automatically means sewage contamination
- Skipping retesting after disinfection
- Attributing a result to a specific neighbor, farm, or septic system without evidence
Official sources
- Delaware Public Health Laboratory and certified laboratories
- U.S. EPA private wells guidance
Related
Can I keep using my water while waiting for a retest?
Follow your lab's or local health department's specific guidance for your result — this varies by situation and should not be generalized from this page.
This page provides general educational information and is not medical or laboratory advice.
Last reviewed: 2026-07-11. Report a correction.