Coliform in Well Water Treatment (2026): Shock Chlorination & Long-Term Fixes

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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Act now

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.

Calculate well volume: depth (feet) × diameter squared × 0.4 = approximate gallons
Use unscented household bleach (5.25–8.25% sodium hypochlorite)
Dose: approximately 1 cup of bleach per 50 gallons of well water
Pour diluted bleach into the well casing while running a hose into the well to mix
Run each faucet until you smell chlorine, then shut off
Let chlorinated water sit for 12–24 hours minimum
Flush all faucets until chlorine smell is gone
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 disinfection	Best for most homes. 99.99% kill rate for bacteria, viruses, cysts. No chemicals. Install after sediment pre-filter on main line.
Continuous chlorination	Chemical dosing pump + carbon filter. Effective but requires ongoing bleach supply and monthly chemical cost.
Point-of-use RO	For 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.