Coliform Bacteria in Well Water — What to Do

A positive coliform test result is alarming — but it does not necessarily mean your water is unsafe to drink permanently. Here is what the result means, what to do immediately, and how to prevent recurrence.

What Coliform Bacteria Means

Coliform bacteria are a group of organisms used as indicators of water contamination. Finding coliform in your well does not mean you have a specific pathogen — it means there is a pathway for contamination to enter the well. Something is wrong with the well's barrier to surface water or soil infiltration.

Total Coliform vs E. coli — Important Difference

Total coliform positive, E. coli negative: Contamination pathway exists but not necessarily from sewage. May be from surface water, soil, or harmless environmental bacteria. Still requires action but lower immediate health risk.

E. coli positive: Fecal contamination is confirmed. This is a direct health risk. Do not drink the water untreated. Boil all water used for drinking and cooking until the problem is resolved.

Step 1: Shock Chlorinate the Well

Shock chlorination is the standard first response to a positive coliform test. See the full How to Shock Chlorinate a Well guide for the complete step-by-step process, bleach dosing table by well size, and safety notes.

Step 2: Find the Source

Shock chlorination treats the symptom, not the cause. If bacteria return, the well has a structural problem. Common causes:

• Cracked or deteriorated well casing — allows surface water and soil to enter
• Missing or damaged well cap — insects and surface water enter through the top
• Shallow well depth — water table too close to surface contamination
• Nearby septic system — drain field too close to well or failing
• Flooding — recent flood event drove surface contamination into the well

Step 3: Consider a Continuous Treatment System

If bacteria recur after shock chlorination, or if your well is in a high-risk location (near agriculture, flood-prone area, or with an aging casing), a continuous treatment system provides ongoing protection:

• UV disinfection system — uses ultraviolet light to kill bacteria without chemicals. Cost: $300–$800 installed. Most effective and maintenance-friendly option for most homeowners.
• Chlorine injection system — continuously doses small amounts of chlorine. Cost: $500–$1,500. Requires maintaining chlorine supply and removes all bacteria including beneficial types.

Frequently Asked Questions

Is it safe to shower if my well tests positive for coliform bacteria?

Showering and bathing are generally fine, but you should switch to bottled water for drinking, cooking, and brushing teeth immediately, and not wait to retest — treat the well right away.

What's the difference between total coliform and E. coli in a well test?

Total coliform positive with E. coli negative means a contamination pathway exists but isn't necessarily from sewage — it could be surface water, soil, or harmless environmental bacteria, and carries lower immediate health risk. E. coli positive results indicate likely fecal contamination and are a higher-priority health concern.

How do you shock chlorinate a well?

Use unscented liquid household bleach (5.25% or 8.25% sodium hypochlorite), calculate roughly 1 quart per 100 gallons of water in the well, pour it into the casing or vent hole, run all faucets until you smell chlorine, let it sit 12-24 hours, then flush thoroughly and retest with a certified lab after 2 weeks.

If shock chlorination doesn't fix a coliform problem, what does that mean?

Shock chlorination treats the symptom, not the cause. If bacteria return after treatment, it usually points to a structural problem with the well — such as a cracked casing, poor seal, or nearby contamination source — that needs to be found and fixed.

How soon should I retest after treating my well for coliform bacteria?

Wait about 2 weeks after shock chlorination and flushing before retesting, and use a certified lab for the follow-up test to confirm the treatment worked.