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 kills bacteria in the well casing, pump, and water distribution system. It is the standard first response to a positive coliform test.

1. Purchase unscented liquid household bleach (5.25% or 8.25% sodium hypochlorite).
2. Calculate the amount needed: approximately 1 quart per 100 gallons of water in the well (well depth in feet × casing diameter squared × 0.41 = gallons).
3. Pour bleach directly into the well casing or through the vent hole.
4. Turn on all faucets until you smell chlorine at each one.
5. Let the chlorinated water sit for 12–24 hours without use.
6. Flush all faucets until the chlorine smell is gone (may take several hours of flushing).
7. Wait 2 weeks, then retest with a certified lab.

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.