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Protecting Your Well By Shock Chlorination

Shock chlorination is a way to protect a private well from coliform, iron, and sulfur bacteria. "Shock" involves putting a chlorine solution directly into the well. If someone in your family becomes ill and you suspect bacteria in your well water is the problem, contact your county health department to have your water tested.

Coliform bacteria in a well indicate disease-causing bacteria and viruses are likely to be present, too. Although iron and sulfur bacteria are not a health hazard, they produce noticeable odor, taste, and color and plugging problems in the water system.

Coliform and other bacteria can enter a well from several sources -- floodwater over the top of a well, a septic tank with a leach field too close, an abandoned well near the affected well, or an improperly sealed well. Bacteria can live in water, on casing pipe, in plumbing lines, or inside fixtures.

It is important to remember, while shock chlorination corrects immediate bacteria problems in a well, it does not correct the source of the bacteria. If bacteria are entering your well from a septic system or other source, you should correct the problem. Otherwise, the bacteria will reoccur.

How Shock Chlorination Works

Shock chlorination involves introducing a strong chlorine solution into the water source and plumbing system and letting it disinfect the system for 12 to 24 hours. You can use regular household bleach that contains 5.25 percent sodium hypochlorite, which kills bacteria and certain viruses.

Shock chlorination is recommended after a new well is constructed and installed, any time a well is opened for repairs, or if floodwater has entered a well. Be sure to store enough fresh water to last 12 to 24 hours while the well and water system are being disinfected.

Shock chlorination is recommended for all bacterial contamination. Flushing after treatment is required to reduce the chlorine concentration; flush until all chlorine odor is gone. Also, use an in-line cartridge water filter to remove iron or sulfur bacteria that may dislodge from plumbing lines during chlorination. Be sure to change the filter regularly to keep it from becoming clogged.

How To Use Shock Chlorination

To use shock chlorination effectively, you must introduce the chlorine solution directly into the water supply. Depending on how your well is constructed, some pipe disconnections or mechanical adjustments may be needed. If you need help doing this, contact a licensed well driller or plumber in your area.

First, clean the well, spring house, or storage reservoir. Remove debris and scrub or hose off any dirt or other deposits on interior surfaces. Pump the well to remove any suspended solids or foreign matter in the water. Scrub interior surfaces with a strong chlorine solution of 1/2 gallon of chlorine laundry bleach (nondetergent and unscented) in each 5 gallons of water.

To know how much chlorine solution to use to disinfect your well, you must know the number of gallons of water in your well. This is determined by the diameter of the well casing and the depth of the water in the well. Follow directions in Table 1 to calculate the number of gallons of water in your well.

Add chlorine bleach into your well until you reach a concentration of 200 parts per million (ppm). This concentration should kill all bacteria. You can use Table 1 to help you calculate how much chlorine to use to reach this concentration.

You can use other chlorine sources instead of common household bleach. Be sure to handle them with extreme caution since they are much stronger. Wear protective gloves, clothing (aprons), eyewear, and shoes. Chlorine is toxic and corrosive, and it can burn your skin or irritate your eyes. Rinse off any exposure immediately and if irritation persists, consult your doctor.

Table 1 will help you determine the amount of water contained in a water well of varying diameter, for each 1 foot of depth.

Table 1.

Well casing diameter
Water per 1-foot depth
2 0.16
3 0.37
4 0.65
5 1.02
6 1.47
8 2.61
10 4.08

To figure the number of gallons of water in your well, multiply the gallons per foot by your well water depth. For example: If your well casing is 5 inches in diameter, there are 1.02 gallons of water for every foot of water. If your well water depth is 100 feet, your well contains 102 gallons of water (100 x 1.02 = 102 gallons). Contact a well driller if you need help finding your well water depth.

Table 2 will help you in estimating the amount of 5.25 percent chlorine bleach (nondetergent, unscented) needed for different amounts of water to equal 200 ppm (parts per million).

Table 2.

Amount of water in your well
Bleach to add in your well
25 3/4
50 1 1/2
75 2 1/4
100 3
125 3 3/4
150 4 1/2
175 5 1/4
200 6

Common conversions:
1 cup = 8 ounces
1 pint = 16 ounces
1 quart = 32 ounces
1 gallon = 128 ounces

After determining the amount of water in your well, use Table 2 to find how much 5.25 percent chlorine bleach to use to disinfect your well. You also may use other chlorine sources, but using household bleach may be easiest.

The mixed chlorine solution must be poured directly into the well. The best way to add chlorine to a drilled well is to fill a tank or other container that holds more water than is stored within the well casing. Mix the chlorine solution with the water in the tank, and then let the tank contents flow into the well. Or, put the required chlorine tablets in a weighted porous sack and lower and raise it within the entire water depth until the tablets are dissolved. Some wells may require different dispensing methods, depending on their construction.

After adding chlorine, attach a hose to the nearest faucet. Turn on the pump to recirculate the chlorinated water. Use the hose to wash down the well casing and drop pipe as the water is returned to the well through the hose. For the process to be effective, the returning water must have a strong chlorine odor. If it doesn't, add more chlorine to the well. If you use common household bleach, be sure the bleach is nondetergent and unscented.

Drain the water system accessories, such as the water heater, and refill with chlorinated water. Release the air from the pressure tank (except for tanks with a permanent air cushion) to fill the tank completely with chlorinated water.

Before disinfecting the water lines leading to, and in, your home, temporarily remove or bypass any carbon filter in the system. Then, one at a time, open each faucet and hydrant in your home. Let the water run until it has a strong chlorine odor at each faucet. Add more chlorine solution at the well if the chlorine odor is not apparent at any faucet.

Once the chlorine adequately reaches all faucet points, let the chlorinated water stand in the well and household plumbing at least 12 hours, preferably 24 hours, to kill bacteria completely. Open outside hydrants and let them run until all chlorine odor is gone. Next, flush all lines inside your home until all chlorine odor is gone. Follow this order of elimination to reduce the chlorine effect on your septic tank. Strong chlorine solutions can kill the bacteria that make a septic system work. Do not allow more than 100 gallons of shock chlorinated water to flow through faucets and drains that lead to a septic tank. Do not use this water for a garden or lawn.

Strongly chlorinated water will not harm livestock, but they will refuse to drink it unless very thirsty. Fill livestock tanks before starting to shock chlorinate if the strong chlorine solution is to be left in the system for a long period.

Follow-up Treatment

After you have thoroughly pumped the well to remove the chlorine, use the water for a week and then have another water sample tested. Two, or even three, consecutive tests give you more confidence the problem has been corrected. No bacteria test is perfect and the results of only one test can be misleading. If tests continue to show bacteria, you may need to retreat and retest your water. You also may need to have your well site evaluated to help determine the source of the disease-causing bacteria or iron and sulfur bacteria problems.

Continuous Chlorination

If several shock chlorinations do not remove the bacteria, your well may require continuous chlorination to eliminate the bacteria problem. This requires equipment to add chlorine to the water as it is drawn from the well. The chlorine must be thoroughly mixed with the water and have time to kill all bacteria. This equipment may be purchased from a water-treatment company, a local well driller or a swimming pool dealer.


Shock chlorination is a good way to treat coliform, iron, and sulfur bacteria in your private well. Always find and correct the source of any bacteria contamination in or around your well. For more information on water quality, contact your county Extension office.


  • Small Water System Operation and Maintenance, California State University -- Sacramento, School of Engineering, Applied Research and Design Center, Adopted by Environmental Protection Agency, 1987.
  • Manual of Individual and Non-Public Water Supply Systems, Environmental Protection Agency, Office of Water, May 1991.

This publication was produced with United States Department of Agriculture/Extension Service funds from grant number 90-EHUA-1-0014.

By James G. Thomas, Leader, Extension Agricultural Engineering, Jack W. Carroll, Coordinator, Energy Extension Center, and Terry S. Holder, Extension Water Quality Program Assistant

Mississippi State University does not discriminate on the basis of race, color, religion, national origin, sex, age, disability, or veteran status.

Publication 1865
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director

Copyright by Mississippi State University. All rights reserved.

This document may be copied and distributed for nonprofit educational purposes provided that credit is given to the Mississippi State University Extension Service.