How to Shock a Well
To shock a well, pour a measured dose of plain unscented bleach into it, run each tap until you smell chlorine, and leave the water unused for at least 12 hours (CDC). Then flush it out through an outdoor hose and retest; the CDC says to wait at least 7 to 10 days before sampling.
Shock chlorination disinfects the well and the house plumbing with a strong chlorine solution. UGA Extension recommends it when a new well is completed, when the well system is opened for installation, repair or maintenance, when a yearly test finds bacteria, and whenever flood water has surrounded the well.
This page covers the steps. The amount of bleach depends on how much water is standing in your well; our shock chlorination calculator works that out from the well's depth, water level and casing size.
Shock chlorination: what each source says
| Source | Chlorine strength | Contact time | Wait before retesting |
|---|---|---|---|
| CDC | More than 100 mg/L | At least 12 hours | At least 7 to 10 days |
| US EPA, after a flood | Not stated | 6 to 24 hours | 7 to 10 days |
| Minnesota Department of Health | 50 to 200 ppm, and not more than 200 | At least 2 hours, preferably 6 hours or overnight | 2 to 4 weeks |
| Nebraska Extension | 200 ppm | At least 2 to 3 hours, preferably overnight | 1 to 2 weeks |
| UGA Extension | 3 pints of bleach per 100 gallons of water in the well | 12 to 24 hours | 5 to 7 days |
| Kansas State Extension | 500 ppm | 24 to 72 hours | After 2 weeks of water use, once a chlorine test is negative |
| Penn State Extension | Not stated | Not stated | About 10 to 14 days |
mg/L and ppm are the same measure. If your state or county health department publishes its own procedure, follow that.
Before you start
- Use the right bleach. The CDC says to use only unscented household liquid chlorine bleach. The Minnesota Department of Health specifies an unopened bottle with no additives.
- Protect yourself. UGA Extension says to wear goggles, a protective apron, and rubber gloves and boots. Minnesota's health department says not to mix bleach with other chemicals, which may form harmful gases.
- Cut the power first. UGA Extension says to turn the pump circuit breaker off before removing the well cap.
- Protect water treatment equipment. UGA Extension says to disconnect all carbon filters and reverse osmosis units, and Kansas State Extension says to set treatment equipment to bypass. Minnesota's health department says to remove all filters from devices and appliances.
- Set aside water. UGA Extension says no one in the home should use the water for any purpose during the 12 to 24 hour treatment.
How to shock a well, step by step
- Mix the bleach. The CDC says to mix the measured bleach with 3 to 5 gallons of water in a 5-gallon bucket.
- Pour it into the well. The CDC says to remove the vent cap and pour the mixture in with a funnel, avoiding all electrical connections.
- Recirculate. Run a clean hose from the nearest outside faucet back into the well. UGA Extension says that once you smell chlorine at the hose, wash down the inside of the well casing for about 15 minutes. Minnesota's health department says to recirculate for about 30 minutes after you first smell bleach.
- Bring chlorine to every tap. UGA Extension says to open each faucet, outside then inside, hot and cold, one at a time, closing each after a strong chlorine odor is detected, and to flush the toilets one at a time.
- Let it sit. Contact times in the table run from 2 hours to 72; the CDC's minimum is 12 hours. Nobody uses the water during this time.
- Flush it out through an outdoor hose. The CDC says to drain the chlorinated water onto an area without plants, such as a driveway, until the chlorine odor disappears, and to avoid draining into streams or ponds. Then run the indoor faucets until the odor is gone.
- Keep it out of the septic system. Minnesota's health department says not to discharge chlorinated water into or near a septic system. Nebraska Extension allows no more than 50 gallons and UGA Extension no more than 100.
- Retest. Wait the time in the table, then send a sample for total coliform. The CDC says to boil the water or use another source until it has been tested, and Nebraska Extension says not to drink it until the lab confirms no coliform or E. coli bacteria are present.
When shock chlorination won't fix the problem
It is a one-time fix for a one-time contamination. UGA Extension says shock chlorination is not a recommended method for treating recurring bacteria problems, and that the source should be identified and eliminated by a licensed well driller or contractor. In a Penn State study of wells contaminated by coliform bacteria, shock chlorination plus a sanitary well cap removed the bacteria for one year in 15 percent of the wells.
It does not remove chemicals. Nebraska Extension says chlorination will not remove nitrate from water, and the EPA says well disinfection will not protect against pesticides, heavy metals and other non-biological contamination. UGA Extension adds that it may temporarily increase arsenic levels in areas where the aquifer sediments are high in arsenic.
Should you hire a well contractor?
Agencies lean that way. The EPA says drilled, driven and bored wells are best disinfected by a well or pump contractor, because it is difficult for a private owner to disinfect them thoroughly. The CDC says working on a well after a natural disaster can be hazardous and should be done by a well or pump contractor.
The National Ground Water Association goes further: it does not recommend household bleach for chlorinating a well and recommends that qualified water well professionals do the disinfection. Minnesota's health department leaves the choice to you, giving homeowner instructions and noting that you can hire a licensed well contractor instead.
Frequently Asked Questions
How long should bleach sit in a well?
Sources differ. The CDC says a minimum of 12 hours and UGA Extension 12 to 24 hours. The Minnesota Department of Health says at least 2 hours, preferably 6 or overnight; Nebraska Extension says at least 2 to 3 hours, preferably overnight; and Kansas State Extension says 24 to 72 hours.
How long after shocking a well can you drink the water?
Not until a retest comes back clean. The CDC says to wait at least 7 to 10 days after disinfection before sampling, and to boil the water or use another source until it has been tested. Nebraska Extension says not to drink it until laboratory results confirm no coliform or E. coli bacteria are present.
How much bleach does it take to shock a well?
It depends on how much water is in the well. UGA Extension's rule is 3 pints of bleach for every 100 gallons of water in the well, and the CDC publishes tables by water depth and casing diameter. Our shock chlorination calculator works out the amount for your well.
Can you use too much bleach?
Yes, according to the Minnesota Department of Health, which says not to use a bleach solution stronger than 200 ppm. Kansas State Extension's procedure uses 500 ppm, so follow the procedure your own state or county publishes.
Will shocking a well get rid of bacteria for good?
Often not. In a Penn State study, shock chlorination and a new sanitary well cap removed coliform bacteria for one year in 15 percent of wells. UGA Extension says recurring bacteria call for finding and eliminating the source, or installing continuous disinfection.
General information, not a diagnosis of your system. Figures last checked against the sources below on 2026-11-13.
Sources
- CDC: How to Disinfect Wells After an Emergency
- US EPA: What to Do After the Flood (816-F-05-021)
- Minnesota Department of Health: Well Disinfection
- Nebraska Extension: Drinking Water Treatment: Shock Chlorination (G1761)
- UGA Extension: Disinfecting Your Well Water: Shock Chlorination (C 858-4)
- Kansas State Research and Extension: Shock Chlorination for Private Water Systems (MF911)
- Penn State Extension: Shock Chlorination of Wells and Springs
- Wellowner.org (NGWA): Well Disinfection
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