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Disinfecting Your Home Well

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Disinfecting a Well

Shock chlorination is a simple procedure for disinfecting wells. Use a solution made of calcium hypochlorite containing 65% to 70% available chlorine such as HTH granules. DO NOT use household bleach as it is too weak. All new wells should be disinfected before use. Existing wells may not be safe sources of water for drinking or cooking if disease-causing bacteria have entered the water. This can happen when the well is opened, such as to replace a pump. It can also happen if the well was surrounded by floodwaters, or if the ground nearby was flooded. Poorly-constructed wells can allow contaminated water to enter, and animal waste on the ground near a well can be carried in.

Note: Do not drink untreated water from heavily contaminated wells, or wells in flooded areas, until testing by your local health department determines that the water is safe.

To kill disease-causing organisms in the well, the well must be disinfected. Shock chlorination is a simple procedure for disinfecting a well using a solution made of calcium hypochlorite containing 65% to 70% available chlorine such as HTH granules. The goal is to add enough chlorine to raise the concentration in the well high enough to kill potentially harmful bacteria and viruses.

This procedure does not protect the well from future contamination. It also has no effect on contaminants other than bacteria, such as gasoline or iron in the water.

Before you begin:

1. If water is cloudy or discolored, run an outdoor faucet until the water runs clear.

2. Do not chlorinate activated carbon or charcoal filters. Use the “bypass” valve on the filter if there is one. Otherwise, disconnect the filter temporarily during shock chlorination.

3. Use rubber gloves, goggles and a protective apron when handling chlorine solutions. If chlorine gets on the skin, flush immediately with fresh water.

4. Never mix chlorine solutions with other cleaning agents, especially ammonia, because toxic gases may be formed.

5. Use a solution made of calcium hypochlorite containing 65% to 70% available chlorine such as HTH granules. Do not use scented or color-safe bleach or other special laundry products to disinfect a well.

Shock chlorination procedure:

1. Select a time when well water will not be used for at least 24 hours. You may wish to store enough drinking water for this period or do the procedure before you leave for a short trip.

2. Determine the volume of water in your well. This depends on the diameter of your well and the height of standing water in the well.

  • First, you need to determine the height of the standing water. The height of standing water is the difference between the depth of your well and the distance from the top of the well down to the water level. For example, if the well depth is 150 feet and the water level is 50 feet down from the top, then subtract to find that the height of standing water is 100 feet.
  • Then, you need to determine the diameter of your well casing. You can do this with a flexible measuring tape. Wrap the tape around your well casing and measure the circumference. Be sure you measuring your WELL CASING and NOT the well cap or collar on the top. Divide your circumference by 3.14. The result is the outside diameter. To determine your inside diameter, you’ll want to subtract  5/8″ from that. Your result should be close to one of the standard sizes – 2, 4, 4.5, 5, 6, 6.25, 7 or 8 inches.
  • Having these two figures can help you determine the volume of water per foot of well depth. This table can help:
    Well Casing Diameter (in inches) Water Volume per Foot of Water Depth
    2 0.163
    4 0.65
    6 1.47
    8 2.61
    10 4.08
    12 5.88
  • Multiply your volume per foot by your height of standing water to determine your total volume of water in the well.
  • Add to that figure an estimate of total water volume in your piping and distribution system (including hot water heater) to get the total gallons in your system and well.

The well should be disinfected with a solution made of calcium hypochlorite containing 65% to 70% available chlorine such as HTH granules. DO NOT use household bleach as it is too weak. DO NOT use calcium hypochlorite containing fungicides, algicides, or other disinfectants.

3. When using high-test calcium hypochlorite, you would want to mix 4 ounces to the gallon per 100 gallons of well water volume. Mix the proper amount of chlorination with water in a five-gallon or larger container and pour the entire solution directly into the well.

4. Turn on the outdoor faucet nearest the well and let the water run until a strong odor of chlorine is detected. Add more bleach if a strong odor is not present.

5. Turn the faucet off. Connect a garden hose to the faucet and attach a spray nozzle to the end of the hose. Thoroughly wash down the entire inside surface of the well casing with the spray nozzle for at least 15 minutes.

6. After washing the inside of the well casing, turn on all outdoor and indoor faucets one at a time until a strong chlorine odor is detected at each location. Turn each faucet off when the chlorine odor is detected.

7. Let the chlorinated water stand in the well and plumbing for at least 24 hours. Do not drink the chlorinated water during this period. You may flush the toilets, but try to minimize the number of flushes.

8. After 24 hours, completely flush the system of chlorine by turning on all outdoor faucets and running them until the chlorine odor can no longer be detected. Do not run the indoor faucets until the odor dissipates to prevent damage to the septic system. Note: When you flush the outdoor faucets, you can attach a hose and use this chlorine solution for cleanup.

9. Finally, turn on the indoor faucets until no chlorine odor is detected. A residual chlorine taste and odor may persist in the water for a few days. Note: You can collect the chlorinated water when you are flushing the indoor faucets, and use this for cleaning, also. This will reduce the amount of water going through your septic system (this is extra important if the ground is saturated with water already).

It is wise to test the water for bacteria a few weeks after shock chlorination to determine if you have a recurring problem. Contact your local health department or county Extension Center for information on water testing and well protection.