How to Disinfect Your Well Water: Step-by-Step Shock Chlorination
Yes, you can disinfect most private wells yourself. The process is called shock chlorination: introduce a concentrated bleach solution into the well, circulate it through the entire system, hold it for 12–24 hours, then flush it out. When done correctly, shock chlorination eliminates most bacterial contamination. A certified lab retest confirms the water is safe again.
Before you start, check our well water quality guide for a full overview of water testing and treatment options for private well owners.
When to disinfect your well (and when to call a pro)
Shock chlorination is appropriate in several situations. Disinfect your well if:
- A certified lab test returned a positive coliform or E. coli result
- Your well was affected by flooding or heavy rain
- You recently completed a repair on the well, pump, or casing
- You moved into a home with a private well and have no testing history
- You noticed a sudden change in water taste, color, or smell
Standard shock chlorination is a manageable DIY task for most homeowners. Call a licensed well contractor instead if:
- Bacteria persists after two rounds of shock chlorination (likely a structural problem)
- Your well is artesian or flowing (these require specialized disinfection methods)
- The well casing is cracked or the cap is damaged
- Flooding caused debris to enter the well
The EPA notes that after flooding, it’s often wise to get assistance from a well or pump contractor to clean and disinfect the well before turning on the pump. For standard post-test disinfection, DIY shock chlorination is the accepted first-line treatment.
What you need
Gather these materials before you start. Having everything ready prevents delays once you begin:
- Unscented liquid bleach (5.25–8.25% sodium hypochlorite, standard household bleach). Do not use scented, gel, or splash-less formulas; these contain additives that can contaminate the well.
- Bleach amount: varies by well depth and casing diameter (see table below)
- Garden hose attached to an outdoor faucet
- 5-gallon bucket for mixing bleach with water before adding to the well
- Well cap removal tool or appropriately sized wrench
- Rubber gloves and eye protection (bleach splashes are serious)
- Bottled water for drinking and cooking during the 24-hour hold period
- Certified lab address or mailing materials for post-treatment retest
Bleach amounts by well specifications:
| Well Depth | 4-inch Casing | 6-inch Casing | 8-inch Casing |
|---|---|---|---|
| Up to 50 ft | 1 pint | 1 quart | 2 quarts |
| 50–100 ft | 1 quart | 1 gallon | 2 gallons |
| 100–200 ft | 2 quarts | 2 gallons | 4 gallons |
| 200+ ft | Contact a well contractor | Contact a well contractor | Contact a well contractor |
Note: These are starting estimates. The goal is to achieve a chlorine concentration of roughly 200 parts per million (ppm) in the well water. If you don’t know your well depth or casing diameter, check your well driller’s log or call your local health department; they often have records on file.
Certified lab testing for bacteria runs $20–$300+ depending on the lab and test panel. We recommend budgeting for a full panel retest, not just a single bacteria screen.
Step-by-step: shock chlorinating your well
Work through these steps in order. The sequence matters; skipping steps reduces effectiveness.
1. Turn off and bypass your water treatment systems. UV disinfection units, water softeners, activated carbon filters, and iron filters must all be bypassed or turned off before introducing chlorine. Chlorine damages most filter media and UV lamp coatings. Consult your system manual for bypass procedures. Also turn off your water heater or set it to vacation mode so hot water doesn’t draw chlorinated water through the heating element while you work.
2. Turn off the well pump. Switch off the pump at the breaker before opening the well. This prevents the pump from pulling water out while you’re adding bleach. Locate the pressure switch that controls your pump; the breaker for the pump circuit is typically labeled in your panel.
3. Mix the bleach before adding to the well. Pour the measured bleach into a 5-gallon bucket and fill it halfway with water. This prevents concentrated bleach from coming into direct contact with your pump and well components. Never pour undiluted bleach directly onto the pump motor.
4. Open the well and add the bleach solution. Remove the well cap carefully. Pour the diluted bleach slowly into the well opening. Replace the cap.
5. Circulate the chlorine through the system. Turn the pump back on at the breaker. Attach a garden hose to an outdoor faucet and run it back into the well opening. Run the hose for 30 minutes; this circulates the chlorinated water from the well into the pressure tank and back. During this process, open each indoor faucet one at a time until you can smell chlorine. Once you detect the smell, close that faucet and move to the next one. Do the same for outdoor faucets. The goal is chlorinated water throughout every branch of the plumbing system.
6. Turn off the pump and let the system sit for 12–24 hours. The chlorine must remain in contact with the water long enough to kill bacteria. We recommend 24 hours where possible. During this period, do not use tap water for any purpose. Use your bottled water supply.
7. Flush the system. After the hold period, run all outdoor faucets (or connect the hose to an outdoor spigot) until you can no longer detect a chlorine smell. Route this water to the street, driveway, or a spot in the yard away from your lawn; chlorine at these concentrations kills grass. Do not route flush water to your septic system drain field. Chlorine kills the beneficial bacteria that make a septic system function, and a single large dose can damage an otherwise healthy system.
Once outdoor flushing is complete, run each indoor faucet until the chlorine smell disappears. Run the water heater cold supply until the smell clears, then flush two or three tankfuls of hot water to clear the heater.
8. Reactivate your water treatment systems. Reinstall filter cartridges (replace them, since they were likely exposed to some chlorine), restart UV units, and bring softeners and iron filters back online according to their manuals.
The 24-hour hold: what to do and what not to do
The hold period is straightforward but requires some discipline.
Do:
- Use bottled water for all drinking, cooking, baby formula, and brushing teeth
- Keep pets on bottled or alternative water sources
- Note the time you finished adding chlorine (you’ll want the full 24 hours, not 18)
Do not:
- Run the dishwasher or washing machine (this drains chlorinated water and shortens the hold)
- Take showers or baths; skin exposure to 200 ppm chlorine over extended periods can irritate sensitive skin, though brief contact is not typically dangerous for healthy adults
- Flush toilets more than necessary; each flush draws chlorinated water and routes it to the septic system
- Use a water filter or softener during the hold period
If you open a faucet briefly, you’ll likely smell bleach. That’s normal. A strong chlorine smell confirms the system is properly chlorinated.
Flushing the system and retesting
After the 24-hour hold, flushing takes time. Plan for 1–3 hours depending on your system size and well depth.
Begin outdoors. Run a garden hose at full flow until you detect no chlorine smell. This may take 30–90 minutes for deeper wells. Once outdoor flushing is complete, flush each indoor faucet, starting with the closest to the pressure tank and working outward.
When no chlorine smell remains anywhere in the system, test your well water with a certified state laboratory. Skip DIY test strips for the post-treatment retest; strips miss low-level bacterial counts that labs detect. The only passing result is zero bacteria detected. There is no acceptable threshold.
Wait 7–10 days after flushing before submitting your retest sample. This allows sediment disturbed during treatment to settle and gives you a more accurate baseline reading. The EPA and CDC both require testing through state-certified laboratories. Your local health department can provide a list of certified labs, and some offer free or reduced-cost testing.
If bacteria persist after disinfection
A single failed retest is not unusual. Run a second round of shock chlorination using the same procedure. Many bacterial contamination cases resolve after two treatments.
If bacteria are still present after two rounds, the problem is likely structural, not a treatment issue. The most common causes:
- A cracked or deteriorated well casing allowing surface water to continuously enter
- A damaged or improperly seated well cap creating an entry point for insects, animals, or rainwater
- Surface water intrusion from poor grading around the wellhead (water should drain away from the well, not toward it)
- A nearby septic system failure; older properties may have wells closer to septic systems than modern codes require
At this point, a licensed well contractor should physically inspect the well structure. This is not a DIY repair; casing work and grouting require specialized equipment. A structural repair may run $500–$2,000 depending on the extent of damage and your region.
For ongoing bacterial contamination, continuous treatment systems (UV disinfection at $300–$800 installed, or a continuous chlorination system) provide long-term protection while you address the underlying structural issue. These are stop-gap measures, not permanent solutions when a structural defect exists.
For broader well infrastructure issues, see our guide on well pump repair if you suspect the contamination is related to pump or casing damage from a recent repair.
External resources
- EPA Private Well Protection Guidelines{:target=“_blank”}: official guidance on well protection and disinfection
- CDC Well Water Testing Guidelines{:target=“_blank”}: testing frequency and certified lab requirements
- EPA Shock Chlorination for Private Wells{:target=“_blank”}: detailed federal guidance on the chlorination procedure
FAQ
How much bleach do I need to disinfect my well?
The amount depends on your well’s depth and casing diameter. For a 6-inch casing at 100 feet deep, use approximately 1 gallon of unscented bleach mixed with water before adding to the well. Shallower wells and smaller casings require less; a 4-inch casing at 50 feet needs roughly 1 quart. Always mix bleach with water in a bucket before adding to the well to prevent concentrated chlorine from damaging pump components.
Can I use my well during shock chlorination?
No. During both the treatment process and the 24-hour hold, tap water should not be used for drinking, cooking, or bathing. Use bottled water for all consumption needs. Brief toilet use is acceptable but should be minimized to reduce chlorinated water flowing to your septic system.
How long does it take to disinfect a well with bleach?
The full process takes 24–48 hours. Adding bleach and circulating it through the system takes 1–2 hours. The hold period is 12–24 hours (we recommend the full 24 hours). Flushing takes 1–3 hours depending on system size. Plan to be without tap water for at least one full day.
Will shock chlorination hurt my septic system?
Yes, if done incorrectly. Chlorinated water routed to your septic system during the hold period or flushing can kill the beneficial bacteria that make the system function. Route all flush water to the street, driveway, or a planted area away from the drain field. Limit toilet flushing during the hold period. The chlorine kills septic bacteria just as effectively as it kills well bacteria.
When should I retest my well after disinfection?
Wait 7–10 days after completing the flush before submitting a water sample for retesting. This gives disturbed sediment time to settle and provides a more accurate baseline. Use a state-certified laboratory. DIY test strips are not reliable enough to confirm successful disinfection. The only passing result is zero bacteria detected.