Your well tested positive for coliform bacteriaColiform BacteriaA group of bacteria used as indicators of potential contamination. Total coliform presence suggests a pathway for pathogens to enter your well.EPA MCL: 0 (no coliform should be present)Learn more → (contamination indicators). Shock chlorination is the first fix. It kills bacteria in your well, pipes, and pressure tank.
Cost: about $15 in supplies. Time: 2 hours of work plus a 12–24 hour wait[1].
Quick Answer
Pour a strong bleach solution into your well. Run it through all pipes. Wait 12–24 hours. Flush until the chlorine smell is gone. Retest for bacteria in 2 weeks. If bacteria return, you need a permanent UV disinfectionUV DisinfectionA chemical-free method that uses ultraviolet light to destroy bacteria, viruses, and other microorganisms by damaging their DNA. Does not change water taste or chemistry.Learn more → (ultraviolet light) system.
When to Shock Chlorinate
- After a positive bacteria test.
- After flooding or heavy surface water intrusion.
- After well repair, pump replacement, or any work that opened the well.
- When buying a home with a well that has been sitting unused.
- After installing new plumbing[3].
Supplies You Need
| Item | Details | Cost |
|---|---|---|
| Unscented household bleach | 5.25–8.25% sodium hypochlorite. No "splash-free" or scented. | $4–$6 |
| 5-gallon bucket | For mixing the bleach solution. | $3–$5 |
| Garden hose | To circulate water back into the well. | Already own |
| Rubber gloves and eye protection | Concentrated bleach is caustic. | $3–$5 |
| Funnel (optional) | Makes pouring into the well casing easier. | $2 |
Total cost: about $15.
Step-by-Step Process
Step 1: Calculate bleach amount
Use unscented household bleach. The amount depends on your well depth and diameter[2]:
| Well Diameter | Bleach per 100 ft of water |
|---|---|
| 4 inches | 1.5 cups |
| 6 inches | 3 cups |
| 8 inches | 6 cups |
Don't know your well depth? Check your well report or call your well driller. When in doubt, use 2–3 gallons — more is better than less[3].
Step 2: Mix the solution
Mix the bleach with 5 gallons of water in a bucket. This dilutes it enough to be safe to pour but strong enough to disinfect.
Step 3: Pour into the well
Remove the well cap. Pour the bleach solution directly into the well casing. Use a funnel if the opening is small.
Safety: Wear rubber gloves and eye protection. Do not breathe the fumes. Work outdoors.
Step 4: Circulate the water
Connect a garden hose to an outdoor faucet. Run the hose back into the well. Turn on the water and recirculate for 15–30 minutes[4].
This mixes the bleach throughout the well and coats the casing walls.
Step 5: Run all indoor faucets
Open every faucet in the house — hot and cold. Run each until you smell chlorine. Then turn it off. This pushes chlorinated water into all pipes[5].
Include: kitchen sink, bathroom sinks, showers, tub, laundry, outdoor spigots, and toilets (flush each twice).
Step 6: Wait 12–24 hours
Do not use any water during this time. The chlorine needs contact time to kill bacteria throughout the system.
Plan ahead. Stock bottled water for drinking, cooking, and brushing teeth.
Step 7: Flush the system
After waiting, flush all chlorinated water out of the system. Run an outdoor hose (away from plants, septic, and storm drains) until the chlorine smell is gone. This may take 1–3 hours[3].
Then run each indoor faucet until the smell is gone there too.
Step 8: Retest in 2 weeks
Wait 2 weeks after flushing. Then take a new bacteria sample and send it to a certified lab. This confirms the chlorination worked[1].
What If Bacteria Come Back?
If your retest shows bacteria again, the source is ongoing. Shock chlorination is a one-time reset, not a permanent fix. You need:
- Well inspection: Hire a contractor to check your casing, cap, and grout seal.
- UV disinfection: Install a UV purifier ($400–$800) for permanent bacteria protection.
Frequently Asked Questions
How much bleach do I need?
About 3 cups per 100 feet of water depth for a 6-inch well[2]. When in doubt, use 2–3 gallons.
How long do I wait?
12 hours minimum. 24 hours is better. Do not use water during this time[5]. Flush until the chlorine smell is gone, then retest in 2 weeks.
When should I shock chlorinate?
After a positive bacteria test, flooding, well repair, or buying a home with a well[6]. It is a one-time disinfection, not ongoing treatment.
References
- U.S. EPA. "Disinfection of Wells." Private Wells, 2024. https://www.epa.gov/privatewells
- NC DHHS. "Shock Chlorination of Wells." Division of Public Health, 2023.
- Penn State Extension. "Shock Chlorination of Wells and Springs." 2023.
- Virginia Cooperative Extension. "Disinfecting a Well." Publication 442-663, 2022.
- Minnesota Department of Health. "Well Disinfection." Well Management, 2023.
- National Ground Water Association. "Disinfecting Wells After Flooding." 2024. https://www.ngwa.org