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Bacteria Treatment

Coliform in Well Water: Treatment Guide

You've got a positive coliform test. Here's exactly what to do — in order — to treat it and prevent it coming back.

Step 1: Test your water

Verify coliform presence before shock chlorination

Don't shock-chlorinate based on a hunch. Get a confirmed test result first — Tap Score's coliform bacteria test rules out false positives from at-home strips and identifies whether you're dealing with total coliform or the more serious E. coli. Test again 1-2 weeks after treatment to verify the well is clear.

Order Coliform Bacteria Test (~$60) →
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Stop drinking unfiltered well water until the issue is resolved

A positive coliform test means bacterial contamination is confirmed. Use bottled water for drinking and cooking until you've completed treatment and a follow-up test comes back negative.

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:

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

Step 3: Install permanent disinfection

UV disinfectionBest for most homes. 99.99% kill rate for bacteria, viruses, cysts. No chemicals. Install after sediment pre-filter on main line.
Continuous chlorinationChemical dosing pump + carbon filter. Effective but requires ongoing bleach supply and monthly chemical cost.
Point-of-use ROFor drinking water only. Not a whole-house solution.

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.

Calculator

Shock Chlorination Calculator

Calculates the bleach dose needed to shock your well to 200 ppm chlorine — the EPA-recommended treatment concentration for coliform contamination.

Stop drinking the water until retesting confirms it is safe

Shock chlorination treats the symptom. It does not fix the cause — a cracked casing, a failed seal, surface intrusion. Retest 1–2 weeks after treatment. If coliform returns, find the pathway first.

Your well dimensions

Static level = where water sits with the pump off. From the well log or measured with a tape.

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