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Copyright © 2005-2006 Environmental Technical Group, Inc.
Advanced Water
Unlike public water supplies that are regularly tested to ensure the water is safe to drink,
individuals or families using private water supplies are responsible for testing for contamination. If
test results indicate that bacterial contamination is occurring, shock chlorination is the most
widely suggested method of treatment. Shock chlorination is the one-time introduction of a strong
chlorine solution into the entire water distribution system (well, pump, distribution pipeline, hot
water heater, etc.).

When to Shock Chlorinate Your Well
Shock chlorination is recommended:
  • when lab results indicate a presence of bacteria,
  • upon completion of a new well,
  • when the distribution system is opened for repairs or maintenance,
  • following contamination by flood water,
  • to control iron and sulfur bacteria.
Shock chlorination is recommended in these circumstances to ensure that bacterial
contamination is controlled.

How Do I Shock Chlorinate My Water Supply?

Safety Practices:
Before you begin the shock chlorination process, run some fresh water into a five gallon
container. If concentrated chlorine accidently comes in contact with your eyes or skin, use this
fresh water to flush the affected area for 10-15 minutes. If you get some of the chlorine solution
in your eyes, see your doctor after thoroughly flushing the affected eye.

A second safety practice is to wear appropriate safety clothing and equipment. Wear goggles to
avoid contact with the strong chlorine material and your eyes. Wear a pair of rubber gloves to
protect your hands and rubber boots on your feet. To prevent discoloration of your clothing,
wear a waterproof suit, coveralls or a full-length apron.

Begin the shock chlorination procedure by: 1) being sure well construction is adequate to
prevent direct entry of contaminants; 2) finding and eliminating the source of the contamination;
3) disinfecting the well components that could be a source of future contamination; and 4)
isolating portions of the system that may be degraded by the strong chlorine solution.

The best way to prevent a water supply from being contaminated by bacteria or pathogens is to
eliminate the bacteria's access to the water source. Controlling access to the water supply by
contaminants is difficult if the water supply is a pond, spring or other surface water. In some
cases, sealing up cracks in well pits, spring houses (or spring boxes) and other potential points
of entry will suffice. Be sure to remove all debris (leaves, twigs, etc.) from the spring house, well
pit or storage reservoir.

Well Chlorination:
Shock chlorination of the well consists of mixing sufficient chlorine-based chemical with the well
water to create a solution containing 200 milligrams per liter (mg/l), or parts per million (ppm) of
chlorine throughout the entire system (well, distribution pipeline, water heater, pressure tank and
other equipment).

Remember that chlorine is very volatile so it is dangerous to work with in confined areas. Make
sure the work area is well ventilated.  Prepare a mixture of one-half gallon of household bleach
per 5 gallons of fresh water. Disinfect the well pit, spring house or other portions of the
distribution equipment that may contribute bacteria to the water supply (pump, motor, pressure
tank and exposed wiring conduits).

Drain as much water from the system as possible. For systems with pressure tanks containing a
bladder, the rubber air-water separator inside the tank could be damaged by the chlorine
solution. Check manufacturers' recommendations to determine if the pressure tank should be
bypassed. For pressure tanks without bladders, release the air so that the tank can be filled with
chlorinated water. Drain water from the water heater so that chlorinated water can be circulated
through the hot water pipelines.

Backwash and clean water softeners, sand filters and iron removal filters with a strong chlorine
solution. Do not chlorinate activated carbon filters since these filters will remove the chlorine until
they become overloaded. Activated carbon filters should be removed from the distribution system
until after chlorine has been flushed from the system.

Estimate the water volume contained in the well casing using Table I.

Step 1. Determine the depth of water in the well: The company that constructed the well should
be able to provide you with the well depth and water level. For example, let's say that you have a
50 feet deep well, and the water level is at 40 feet. The well contains 10 feet of water (50-40=10

Step 2. Determine the volume of water in the well. You measured the inside diameter of the well
and it was 30 inches. Find the gallons per foot of depth for a 30-inch well in Table I.  For our
example we would multiply the depth of the water in the well (10 feet) by 36.7 gallons of water per
foot of water depth (from Table I) to get 367 gallons of well water (10 x 36.7 = 367 gallons of
water in the well).

Step 3. Estimate the volume of water in the distribution system. Total up the water storage in the
system, including the water heater, pressure tank, etc., and add 50 gallons for the pipeline.  For
example, If you have a 30-gallon hot water heater and a 30-gallon pressure tank, you need to
add 110 gallons for the distribution system.

Step 4. Determine the water contained in the entire system.  Add the water volume in the well to
the water contained in the distribution system, or 477 gallons in our example(367 gallons in the
well plus 110 gallons in the distribution system).

Step 5. Determine the amount of chlorine product required for a 200 ppm solution. Table 2 lists
the product amounts needed to create a 200 ppm chlorine solution using typically available
sources. If you decide to purchase laundry bleach, you will need 3 pints of bleach per 100
gallons of water in the well and distribution system. For our example, you would need to purchase
14 pints or 1.75 gallons of liquid laundry bleach. You would determine this by using the
worksheet at the end of this article (477 gallons divided by 100, multiplied by 3 pints per 100
gallons, and divided by 8 pints per gallon is equal to 1.75 gallons).

Step 6: Introduce the chlorine material into the well and distribution system. The best way to
introduce chlorine material into the well is to dissolve the chlorine in a 5-gallon bucket of fresh
water. Be sure the bucket is plastic and has been thoroughly washed. Then pour the chlorine
solution into the well. Try to splash the solution on the sidewalls of the well casing as much as
possible. Optional: Attach a hose to the water hydrant or faucet nearest the well and run water
through the hydrant and back into the well.

Another method of shock chlorinating a large diameter well is to place tablets or powder in a
weighted porous sack (tightly woven burlap works well). Raise and lower the sack in the well
water (Figure 2). Remember that only the portions of the well coming in contact with the chlorine
will be disinfected. Be sure to allow the sack to touch the bottom of the well during this process.

A less desirable way of introducing chlorine disinfectant into a well is to mix the chlorine in a tank
containing the same volume of water as is held in the well and distribution system (for our
example that would be 477 gallons). Be sure that this tank has not been used to transport
pesticides or fertilizers. Allow the water to drain from the holding tank into the well as you raise
and lower the hose (Figure 3). This will cause the chlorinated water to displace water in the well.

Regardless of how you introduce the chlorine material into your well, start and stop the pump
several times to ensure that the chlorine is thoroughly mixed with well water. Recirculate the
water until a strong chlorine smell has been noted for at least five minutes.

After the chlorine has been placed in the well and the casing, etc., has been washed down, move
around the water distribution system and open each faucet (hot and cold), hydrant or other water
outlet. Allow water to flow until a strong chlorine odor reaches that position in the system. Then
close the valve at that location. Do this with all faucets, hydrants and other outlets in the system.

If a strong chlorine odor is not detected at each site, add more chlorine to the well. This may be
an indication that your well contains iron, hydrogen sulfide or organic materials.

Step 7: Let the chlorine disinfect the system. The most difficult step is to refrain from using water
from the well so that the chlorine can disinfect the system. The system should remain idle for at
least 2-3 hours, preferably overnight.

Step 8: Flush the system to remove the chlorine. After the water system chlorination has been
completed, the entire system must be emptied of chlorine and thoroughly flushed with fresh water
by running water out of each faucet or hydrant until the chlorine odor dissipates. Distribute the
waste water on gravel roads or other areas without plants or aquatic life, which it might harm.

Do not allow more than 50 gallons of chlorinated water to enter the septic system. If possible,
attach a hose to outlets inside the house and distribute the water to a nongrass area away from
the house. The chlorine will eventually evaporate into the atmosphere.

Step 9: Retest the water supply for bacterial contamination. The final step is to retest the water to
ensure that the water source is bacteria free. Take a water sample 1-2 weeks after shock
chlorinating the well, using the same procedures as before. Though most shock chlorination
treatments are successful, do not drink the water until the laboratory results confirm that no
bacteria are present. Retest the well every month for 2-3 months to be sure contamination is not
reoccurring. If test results are negative, an annual water analysis program can be reinstated.

If the water supply continues to develop bacterial contamination problems after being shock
chlorinated, ultraviolet treatment or continuous chlorination may be an option. Other options
include repairing the well, or constructing a new well. It may be necessary to abandon the water

Chlorine compounds are volatile so they will degrade with time. Purchase only what you'll need
and use it all. Always read and follow manufacturers' recommendations. When using chlorine
bleaches, do not purchase bleaches that have scents or other additives.

Do not add other cleaning materials to the chlorine solution. Some combinations of chlorine and
acids or ammonia could produce dangerous gases.

Make sure all work areas are well-ventilated.
Well Casing
Diameter (inches)
Water Volume per
foot of water
depth (gallons)
Table 2.  Amount of chemical required to create a chlorine concentration
of about 200 ppm.
Chemical name   
Amount per 100 gallons of water
Liquid Laundry Bleach (5.25% NaOCl)   
3 pints
Commercial Strength Bleach (12-17%
1 pint
Chlorinated Lime (25% CaOCl2)   
11 ounces
Dairy Sanitizer (30% CaOCl2)  
9 ounces
High-test calcium hypochlorite(65-75%
4 ounces
    Note: Well water containing iron, hydrogen sulfide, or organic substances may require
    more chemical to create a 200 ppm solution. Chlorine combines readily with these
    materials, making some of the chlorine ineffective as a disinfectant.