⚠ Safety First
Chlorine bleach is a skin and eye irritant. Wear rubber gloves, eye protection, and old clothing. Work in ventilated conditions. Keep children and pets away from the work area. Never mix bleach with ammonia or other cleaners.
When to Shock Chlorinate
- After a positive coliform or E. coli bacteria test
- After any flooding that reached or surrounded the wellhead
- After well repairs, pump replacement, or any work inside the casing
- After a prolonged period of non-use (vacation property startup)
- When moving into a home with a well and no recent test history
- After any drilling or deepening work
What You’ll Need
- Unscented household liquid bleach — 5.25% or 8.25% sodium hypochlorite (standard grocery store bleach, no fragrance, no “splash-less” formula)
- Measuring cup
- Rubber gloves and eye protection
- Garden hose (to recirculate water into the well)
- Permanent marker and tape
- Chlorine test strips (optional, to confirm flushing is complete)
💧 Chlorine Quantity by Well Depth and Diameter
Use unscented liquid bleach at 5.25–6% concentration. The amount depends on the standing water in your well (depth × diameter). A 6-inch diameter well with 50 feet of water needs about 1 pint of 5.25% bleach to achieve 200 ppm. For 8.25% bleach, use about 2/3 that amount. Your state health department or well contractor can provide exact quantities for your specific well.
Chlorine Quantity Reference Table
| Well Diameter | Gallons per 10 ft of water | Bleach needed (5.25%) per 50 ft of water |
|---|---|---|
| 4 inches | 0.65 gal | ~1/3 pint |
| 6 inches | 1.47 gal | ~1 pint |
| 8 inches | 2.61 gal | ~1.5 pints |
| 12 inches | 5.88 gal | ~3.5 pints |
| 24 inches (large bored/dug) | 23.5 gal | ~1 gallon |
These quantities are approximate targets for 200 ppm residual chlorine. If uncertain, consult your state health department or well contractor for precise calculations based on your well log.
Step-by-Step Procedure
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Notify household members and find alternative water supply
The water will be unusable for 12–24 hours during treatment plus several hours of flushing. Have bottled water available. Notify everyone in the household the water will be off for drinking and cooking during this period.
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Mix the bleach solution
In a clean bucket, mix the calculated amount of unscented bleach with several gallons of clean water. Mixing first prevents concentrated bleach from sitting on the pump equipment.
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Pour the bleach solution into the well casing
Remove the well cap carefully. Pour the bleach solution slowly into the well casing. Follow with 1–2 buckets of clean water to move the bleach down the casing and into the screen area. Replace the cap loosely.
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Recirculate with the garden hose
Connect a garden hose to an outdoor hose bib (cold water only) and place the other end back into the well opening. Run the pump and allow water to recirculate from the tap back into the well for 15–30 minutes. This distributes the chlorine throughout the well and adjacent formation and treats the pump and all pipes. You should be able to smell chlorine at the hose.
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Run chlorinated water to every tap in the house
Open every cold water tap—sinks, showers, hose bibs, laundry hookup, ice maker line—until you smell chlorine at each one. This ensures all pipes and fixtures are treated. Then close all taps.
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Let it sit for 12–24 hours
Leave the chlorinated water sitting in the well and all pipes for a minimum of 12 hours. 24 hours is better for heavily contaminated situations. Do not use any water during this period.
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Flush the system to waste
Using a garden hose directed away from any garden, garden pond, or water feature (chlorine is toxic to plants and fish), run water from the hose bib until the chlorine smell is no longer detectable at the tap. This may take several hours and many hundreds of gallons for a deep well. Test with chlorine test strips — flush until residual chlorine is below 0.5 ppm. Do not discharge heavily chlorinated water to a septic system all at once.
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Flush interior taps
Once the exterior hose is running without chlorine odor, run interior taps until the chlorine smell clears from all fixtures. This typically takes 10–20 minutes of flow per tap.
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Wait 7–14 days before collecting a re-test sample
This is the step most people skip too quickly. Testing too soon gives a false negative — residual chlorine in the system suppresses bacteria. Wait at least 7 full days from the completion of flushing. 10–14 days is better for high-risk situations.
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Collect and submit a re-test sample
Follow proper sample collection procedure. Correct sample collection is critical → Submit to a state-certified lab. Resume normal water use when the re-test returns negative.
After Two Failed Shock Chlorinations
If the well tests positive after two rounds of shock chlorination, the contamination source is structural — surface water is entering the casing through a defect that chlorination cannot fix. You need a licensed well contractor to camera-inspect the casing, assess grout integrity, and recommend whether casing repair, grouting, or a new well is needed.