Shock Disinfection of a Water Supply

When a water supply has been exposed to bacterial contamination or is newly constructed, it is advisable to disinfect the system using a commercial chlorine compound such as chlorine bleach.

Disinfection should occur after: construction of a new water supply, repairs are made to an existing water supply, a positive coliform or E. coli test, or any time the well cap or lid has been removed.

The disinfection process outlined below is intended to eliminate the effects of previous contamination, but will not continue to disinfect or render safe a water supply which is continuously or intermittently contaminated. Therefore, before disinfecting the water supply system, all sources of pollution should be eliminated and proper repairs should be made. Contact a water system specialist or the WPA for advice.

The most convenient source of chlorine is ordinary household bleach. Chlorine bleach contains about 5.25% chlorine (sodium hypochlorite) and is available at most grocery stores. Note that "Ultra" Chlorine Bleach products contain 6% chlorine. If possible, use NSF approved chlorine, as other types have additives in them. DO NOT use scented bleach.

During the procedure the water may be used for sanitary purposes, but should not be used for drinking purposes.

There are various methods for disinfecting individual water supplies. This method is recommended for most individual homeowners as the simplest and as a largely reliable disinfection procedure. Contact the WPA for more detailed or alternative disinfection procedures.

Shock Disinfection Procedure:

1. Ensure that you have potable drinking water (bottled water or disinfected well water) available. Your well water will be heavily chlorinated for 1-2 days after this procedure, so you may consider doing laundry and showering before you begin.

2. Disconnect carbon or charcoal filters. Chlorinated water should be allowed to pass through water treatment devices such as softeners, iron filters and water sand filters to disinfect them. Water softeners can be shocked separately by adding 1/2C household bleach into the salt (brine) solution. Check with the manufacturer to ensure chlorine will not damage the water treatment equipment. To save energy during this procedure, turn your hot water heater to pilot if it is gas, or off if it is electric.

3. Clean the well and any storage tanks. Using a hose, rinse down the inside of the casing of the well. Then run the hose away from the well until the water is clear.

4. The depth of the well and the volume of water to be disinfected determines the amount of chlorine to be used. As a rule of thumb, a shallow well (< 50 ft) should use ½ gallon, an average well (50 ft to 150 ft) should use 1 gallon and a deep well (>150 ft) should use 1 ½ to 2 gallons. Use the table below if also disinfecting a storage tank.

5. Dilute the chlorine in a clean 5 or 10 gallon bucket with water, then pour this dilution into the well casing, dug well, reservoir, or other structure to be disinfected.

6. Run the hose into the well again for at least 10 minutes to mix and re circulate the chlorine solution, and to re rinse the inside of the casing. Cover the well.

7. Turn on each indoor and outdoor tap, one at a time. When the water coming out has a strong chlorine odor, turn that tap off and proceed to the next one. Allow the chlorine solution to sit in the distribution piping for a minimum of 12 hours. This step sanitizes the distribution piping.

8. Flush the chlorinated water from all the lines after the contact time has elapsed, at a slow steady rate of about 1 gallon per minute. You should run water through a hose to a location away from the well or septic system before turning on the indoor faucets. This is not required, but prevents the exposure of your septic system to high levels of chlorine. Repeated exposure to elevated chlorine levels can negatively impact the functioning of your septic system. Also, be aware that heavily chlorinated water can be harmful to your lawn, trees, or garden.

9. Reconnect any water conditioning equipment you may have disconnected. Continue to use the water for all household purposes except those intended for consumption (drinking, ice-making, washing of fruits and vegetables to be eaten raw).

10. Contact the WPA to collect a water sample for bacteriological testing. It is critical when re-sampling that no chlorine be present in the water.

11. It is recommended that wells be tested every two to three years to ensure safe drinking water standards.

Water In Gallons
Chlorine Concentration 100
100 PPM
(10 hr contact)
3 C 1 1/2 Q
2Q 1G 1 1/2 G
9 1/2 G
19 G
250 PPM
(4 hr contact)
2 Q 1G 1 1/4 G
2 1/2 G
3 3/4 G
10G 25G 50G

KEY: C=cups; Q=quarts; G=gallons

Note: Caution should be exercised when handling bleach solutions. If chlorine accidentally gets on your skin, immediately flush the area with clean water.