Coliform in Well Water: Treatment Guide

Step 1: Shock chlorinate the well

Shock chlorination is the first response to a positive coliform test. It disinfects the entire well and water system with a concentrated chlorine solution.

1. Calculate well volume: depth (feet) × diameter squared × 0.4 = approximate gallons
2. Use unscented household bleach (5.25–8.25% sodium hypochlorite)
3. Dose: approximately 1 cup of bleach per 50 gallons of well water
4. Pour diluted bleach into the well casing while running a hose into the well to mix
5. Run each faucet until you smell chlorine, then shut off
6. Let chlorinated water sit for 12–24 hours minimum
7. Flush all faucets until chlorine smell is gone
8. Wait 1–2 weeks before retesting

Step 2: Identify the source

If your follow-up test is still positive after shock chlorination, the contamination source is ongoing. Common sources:

• Cracked or corroded well casing — allowing surface water intrusion
• Poorly sealed well cap — insects, rodents, or runoff entering
• Proximity to septic system — especially old or failing systems
• Flooding events — surface water overwhelming the well seal
• Old shallow or dug wells — inherently more vulnerable than deep drilled wells

A licensed well contractor can inspect the casing integrity and seals. This is essential if recontamination keeps occurring.

Step 3: Install permanent disinfection

UV is our recommended permanent solution for most homeowners. Low maintenance (annual bulb replacement ~$60–$100), no chemicals, and a whole-house kill rate that protects every tap.

After treatment: when is it safe?

Retest 1–2 weeks after shock chlorination and again 4 weeks later. Two consecutive negative coliform tests indicate the immediate threat is resolved. Once permanent UV is installed, annual testing continues to be best practice.

Related pages

• Well water coliform guide
• Well water testing guide
• Well water UV system