3 pp., 2 tablesIf your well has been flooded, it must be shock chlorinated before it can be used as a source of drinking water. This publication explains how to disinfect a well using either dry chlorine or liquid household bleach

Dozier, Monty

McFarland, Mark L.

Texas A&M University

ER-004 
6-06
Mark L. McFarland, Associate Professor 
and Extension Soil Fertility Specialist
Monty C. Dozier, Assistant Professor and Extension Specialist 
— Water Resources, The Texas A&M University System
R. Craig Runyan, Program Coordinator, 
xtension Plant Sciences Department, New Mexico State University
If your well has been flooded, it will need to be shock chlorinated before 
you can use it as a drinking water source. You will need to add chlorine to 
the well water to kill any disease-causing organisms that may have entered 
it during the flood.
In addition to the well, you should also disinfect most water treatment 
equipment, such as water heaters, softeners and pressure tanks. During 
the shock chlorination process, temporarily disconnect or bypass drink-
ing water filters such as carbon filters and reverse osmosis systems.
During the disinfection process, water from the system is unsuitable for 
consumption; do not allow people or animals to have prolonged contact 
with it. Make the treatment when faucets and toilets will not be in use 
for at least 12 hours, preferably 24 hours. If there is an automatic water-
ing system for animals and irrigation, provide an alternative water source 
during the treatment period. 
The two most common sources of chlorine for well disinfection are dry 
chlorine (65 percent calcium hypochlorite) and liquid household bleach 
(5.25 percent sodium hypochlorite). Do not use bleach with a “fresh scent,” 
lemon fragrance or other additives.
Chlorination procedure
The amount of chlorine needed is determined by the amount of water 
standing in the well. The standing water depth in the well is the depth of 
the well minus the static water level, which is the level of the water table in 
a well when the pump is not operating.
If you do not know the well’s standing water depth, use a volume of bleach 
equal to twice the depth value for the diameter of the well casing (see Ta-
ble 1 or 2). For example, an 8-inch casing diameter with unknown stand-
ing water depth would require 3 gallons of household bleach.
Disinfecting Water Wells 
by Shock Chlorination
Drain as much water from the system as possible. 
If the system has a pressure tank that contains 
a bladder, the chlorine solution may damage the 
rubber air-water separator in the tank. Check 
your manufacturer’s guide to see if the pressure 
tank should be bypassed. If the pressure tank has 
no bladder, release the air to allow the tank to be 
filled with chlorinated water. Drain all hot water 
heaters to allow chlorinated water to circulate 
through the hot water system also.
Remove the plug or screen on the well cap to ac-
cess the inside well casing. Dilute the beach by 
placing the appropriate amount of bleach (see 
Table 1 or 2) in a 5-gallon bucket and filling the 
bucket with clean water. Use a funnel to pour the 
solution around the sides of the well casing.
Connect a garden hose to a nearby faucet and 
wash down the inside of the well. Continue the 
washing process for 10 minutes, and make sure 
you can smell a strong chlorine odor. Then, do 
not operate the water system for 2 hours.
After 2 hours, open the faucet closest to your 
well. Allow the water to run until you can smell a 
strong odor of chlorine, then close the faucet. Go 
to the next faucet and repeat. If you do not smell 
the odor, check the chlorine rate and add more 
chlorine to the well, repeating steps 1 through 4. 
Then, do not operate the water system for at least 
12 hours, preferably 24 hours.
Next, flush the remaining chlorine from the sys-
tem. Begin by turning on the outside faucets and 
letting the water run until the chlorine odor dissi-
pates. Let the water run on the ground to reduce 
the load on your septic system. However, do not 
let the chlorinated water run onto lawns, gardens 
or other plants because chlorine can injure them. 
Place the garden hose so that it drains into a field 
or low-lying area away from desirable plants. Be 
careful not to discharge the chlorinated water di-
rectly into a pond, lake, river or stream.
Turn on the indoor faucets until the system is 
completely flushed. 
Table 1. Amount of chlorine bleach needed for shock chlorination.
Laundry bleach (about 5.25% hypochlorite)
Standing water depth 
in well (in feet)
Casing diameter
4 inches 6 inches 8 inches 10 inches 12 inches
10 1⁄2 cup 1 cup 11⁄2 cup 1 pint 2 pints
25 1 cup 1 pint 2 pints 3 pints 41⁄2 pints
50 1 pint 1 quart 2 quarts 3 quarts 1 gallon
100 1 quart 2 quarts 1 gallon 11⁄2 gallons 2 gallons
150 3 pints 3 quarts 11⁄2 gallons 2 gallons 3 gallons
Table 2. Amount of dry chlorine needed for shock chlorination.
High-test hypochlorite (HTH 65-75% hypochlorite)
Standing water depth 
in well (in feet)
Casing diameter
4 inches 6 inches 8 inches 10 inches 12 inches
10 — — — — —
25 — — — 1⁄4 pound 1⁄4 pound
50 — — 1⁄3 pound 1⁄2 pound 3⁄4 pound
100 — 1⁄3 pound 3⁄4 pound 1 pound 11⁄2 pounds
150 1⁄4 pound 1⁄2 pound 1 pound 11⁄2 pounds 4 pounds
After chlorination, have the well water tested 
again for bacterial contamination. If bacteria are 
still detected in the well water, repeat the chlori-
nation process and then test the water again.
If bacterial contamination is detected a third time, 
check for potential sources of reinfection such 
as an improperly designed wellhead or livestock 
pens or septic tanks near the wellhead. You may 
need to install a continuous chlorination system 
or other continuous disinfection system.  
Information in this publication came from University of Nebraska publication G95-1255A, Auburn University 
publication ANR-790, Texas Natural Resource Conservation Commission publication GI-005, and other sources. 
For more information, see  http://soilcrop.tamu.edu or http://water.tamu.edu.
Produced by AgriLife Communications and Marketing, The Texas A&M System
Extension publications can be found on the Web at: http://AgriLifebookstore.org
Visit the Texas AgriLife Extension Service at http://AgriLifeextension.tamu.edu
Educational programs of the Texas AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, or national origin.
Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as amended, and June 30, 1914, in 
cooperation with the United States Department of Agriculture. Edward G. Smith, Director, Texas AgriLife Extension Service, The Texas A&M System.


English

Disinfecting Water Wells by Shock Chlorination

Texas A&amp;M Repository

ER-004 6-06Mark L. McFarland, Associate Professor and Extension Soil Fertility SpecialistMonty C. Dozier, Assistant Professor and Extension Specialist — Water Resources, The Texas A&M University SystemR. Craig Runyan, Program Coordinator, xtension Plant Sciences Department, New Mexico State UniversityIf your well has been flooded, it will need to be shock chlorinated before you can use it as a drinking water source. You will need to add chlorine to the well water to kill any disease-causing organisms that may have entered it during the flood.In addition to the well, you should also disinfect most water treatment equipment, such as water heaters, softeners and pressure tanks. During the shock chlorination process, temporarily disconnect or bypass drink-ing water filters such as carbon filters and reverse osmosis systems.During the disinfection process, water from the system is unsuitable for consumption; do not allow people or animals to have prolonged contact with it. Make the treatment when faucets and toilets will not be in use for at least 12 hours, preferably 24 hours. If there is an automatic water-ing system for animals and irrigation, provide an alternative water source during the treatment period. The two most common sources of chlorine for well disinfection are dry chlorine (65 percent calcium hypochlorite) and liquid household bleach (5.25 percent sodium hypochlorite). Do not use bleach with a “fresh scent,” lemon fragrance or other additives.Chlorination procedureThe amount of chlorine needed is determined by the amount of water standing in the well. The standing water depth in the well is the depth of the well minus the static water level, which is the level of the water table in a well when the pump is not operating.If you do not know the well’s standing water depth, use a volume of bleach equal to twice the depth value for the diameter of the well casing (see Ta-ble 1 or 2). For example, an 8-inch casing diameter with unknown stand-ing water depth would require 3 gallons of household bleach.Disinfecting Water Wells by Shock ChlorinationDrain as much water from the system as possible. If the system has a pressure tank that contains a bladder, the chlorine solution may damage the rubber air-water separator in the tank. Check your manufacturer’s guide to see if the pressure tank should be bypassed. If the pressure tank has no bladder, release the air to allow the tank to be filled with chlorinated water. Drain all hot water heaters to allow chlorinated water to circulate through the hot water system also.Remove the plug or screen on the well cap to ac-cess the inside well casing. Dilute the beach by placing the appropriate amount of bleach (see Table 1 or 2) in a 5-gallon bucket and filling the bucket with clean water. Use a funnel to pour the solution around the sides of the well casing.Connect a garden hose to a nearby faucet and wash down the inside of the well. Continue the washing process for 10 minutes, and make sure you can smell a strong chlorine odor. Then, do not operate the water system for 2 hours.After 2 hours, open the faucet closest to your well. Allow the water to run until you can smell a strong odor of chlorine, then close the faucet. Go to the next faucet and repeat. If you do not smell the odor, check the chlorine rate and add more chlorine to the well, repeating steps 1 through 4. Then, do not operate the water system for at least 12 hours, preferably 24 hours.Next, flush the remaining chlorine from the sys-tem. Begin by turning on the outside faucets and letting the water run until the chlorine odor dissi-pates. Let the water run on the ground to reduce the load on your septic system. However, do not let the chlorinated water run onto lawns, gardens or other plants because chlorine can injure them. Place the garden hose so that it drains into a field or low-lying area away from desirable plants. Be careful not to discharge the chlorinated water di-rectly into a pond, lake, river or stream.Turn on the indoor faucets until the system is completely flushed. Table 1. Amount of chlorine bleach needed for shock chlorination.Laundry bleach (about 5.25% hypochlorite)Standing water depth in well (in feet)Casing diameter4 inches 6 inches 8 inches 10 inches 12 inches10 1⁄2 cup 1 cup 11⁄2 cup 1 pint 2 pints25 1 cup 1 pint 2 pints 3 pints 41⁄2 pints50 1 pint 1 quart 2 quarts 3 quarts 1 gallon100 1 quart 2 quarts 1 gallon 11⁄2 gallons 2 gallons150 3 pints 3 quarts 11⁄2 gallons 2 gallons 3 gallonsTable 2. Amount of dry chlorine needed for shock chlorination.High-test hypochlorite (HTH 65-75% hypochlorite)Standing water depth in well (in feet)Casing diameter4 inches 6 inches 8 inches 10 inches 12 inches10 — — — — —25 — — — 1⁄4 pound 1⁄4 pound50 — — 1⁄3 pound 1⁄2 pound 3⁄4 pound100 — 1⁄3 pound 3⁄4 pound 1 pound 11⁄2 pounds150 1⁄4 pound 1⁄2 pound 1 pound 11⁄2 pounds 4 poundsAfter chlorination, have the well water tested again for bacterial contamination. If bacteria are still detected in the well water, repeat the chlori-nation process and then test the water again.If bacterial contamination is detected a third time, check for potential sources of reinfection such as an improperly designed wellhead or livestock pens or septic tanks near the wellhead. You may need to install a continuous chlorination system or other continuous disinfection system.  Information in this publication came from University of Nebraska publication G95-1255A, Auburn University publication ANR-790, Texas Natural Resource Conservation Commission publication GI-005, and other sources. For more information, see  http://soilcrop.tamu.edu or http://water.tamu.edu.Produced by AgriLife Communications and Marketing, The Texas A&M SystemExtension publications can be found on the Web at: http://AgriLifebookstore.orgVisit the Texas AgriLife Extension Service at http://AgriLifeextension.tamu.eduEducational programs of the Texas AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, or national origin.Issued in furtherance of Cooperative Extension Work in Agriculture and Home Economics, Acts of Congress of May 8, 1914, as amended, and June 30, 1914, in cooperation with the United States Department of Agriculture. Edward G. Smith, Director, Texas AgriLife Extension Service, The Texas A&M System.

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Disinfecting Water Wells by Shock Chlorination

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Disinfecting Water Wells by Shock Chlorination

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