Well Disinfection for Bacterial Contamination

If water from a well is found to be contaminated with coliform bacteria, it may be possible to eliminate  the  contamination  by disinfecting the well. 


Well water systems may be disinfected by adding unscented liquid chlorine bleach to the water in the well.

These quantities are for 100 feet of well depth. Adjust the quantities to fit the depth of your well. 

Use the following amounts as a guide:

Well Casing Diameter Amount of 5.25% (Household) Chlorine Bleach Needed
4 inches Two and one-half (2½) cups
6 inches Five (5) cups
8 inches Seven and one-half (7½) cups
12 inches Twenty (20) cups OR One and one-quarter (1¼) gallons


  • If the water is cloudy, attempt to clear as much as possible by pumping the well to waste.
  • Mix one gallon of household bleach containing 5.25% sodium hypochlorite to 3 gallons of water. Greater amounts or stronger chlorite solutions should be used for wells more than·l00 feet deep.
  • With the pump OFF, pour the chlorine solution into the well. It may be necessary to lift the pump. Some wells have openings, which can be used for this purpose.
  • Do not operate the pump for 30 minutes.
  • After the 30-minute period, surge the well by starting and stopping the pump several times.
  • Open every tap, faucet or hydrant in the water piping system, start the pump·and let water flow until clean water with a strong odor of chlorine comes out.
  • Stop the pump and close all the taps, faucets and hydrants. Allow the mixture to stand in the system for 24 hours, or at least overnight. Disinfectant contact time with bacteria is important.
  • After contact time is accomplished, flush the chlorine mixture from the system by hooking a garden hose to an outside tap and running until no chlorine odor is present.

    Do not flush the mixture into your septic system by running chlorinated water down drains! Your septic system was not designed to handle the large continuous flow of water necessary to remove the chlorine and chlorine is harmful to the beneficial bacteria that make your septic tank function properly. Since chlorine will kill grass and plants, be careful where you run the water outside.
  • When you can no longer smell chlorine in the water, close all taps and faucets and use the system normally.
  • A water sample for bacteriological analyses recheck should be obtained one week or more after disinfecting the well.

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Public Health Division
County of Sonoma
Business Hours
Monday – Friday
8:00 AM – 4:00 PM
Office Location
625 5th Street
Santa Rosa, CA 95404
38.441823, -122.7137698

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