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Disinfecting private wells

Why a private well would need to be disinfected

  • Safe drinking water must be free of harmful chemicals and disease-producing germs. These germs can cause illnesses such as giardiasis, gastroenteritis, and hepatitis. When there is damage to a well or the well has been flooded, disease causing germs and chemicals can flow into the well water.
  • The purpose of disinfection is to kill or inactivate all disease causing germs that may be present.
  • Chemical contaminants are more difficult to eliminate. Consult the public health department for advice on dealing with chemical contaminants.

What to do if you suspect that well water is contaminated

  • Laboratory tests can identify specific chemicals and a group of bacteria called coliform.
  • The presence of coliform indicates possible fecal contamination and a potential health hazard.

How to get well water tested

Public Health will test water for the presence of coliform, but does not offer testing for chemicals. There is a charge for the test. Please call any of the locations below to ask for current fees, and more information about getting a test.

What you should do when a test of well water comes back as "unsatisfactory"

  • If coliforms are found in the water, the well can be disinfected as described below.
  • Liquid household chlorine bleach is an effective and economical disinfectant. Chlorine bleach is also available in a granule form.

How to disinfect a bored or dug well

Calculate the amount of disinfectant needed by using the calculation table provided below:

 Diameter of well (in feet) Amount of 5.25% laundry bleach per foot of water Amount of 70% chlorine granules per foot of water
1-1/2 cups
1 ounce
3 cups
2 ounces
4-1/2 cups
3 ounces
6 cups
4 ounces
9 cups
6 ounces
12 cups
8 ounces
18 cups
12 ounces
  • Measure the diameter of the well, and round the measurement to the nearest foot. (Example: 5.3 feet = 5 feet; 5.7 feet = 6 feet)
  • Find the amount of chlorine bleach needed to disinfect each foot of water for the diameter well that you have measured. For example, the amount for a 5-foot diameter well would be 4-1/2 cups per foot.
  • Determine the depth of the well in feet.
  • Multiply this measurement by the computed amount of bleach to use. For a 5 foot diameter, 40 foot deep well:
    1. Go to the chart that shows 4-1/2 cups of chlorine bleach per foot of water for a 5-foot diameter well.
    2. Next multiply the 4-1/2 cups of chlorine bleach by the 40-foot depth of the well.
    3. The total amount of chlorine bleach needed would be 4-1/2 times 40 or 180 cups of ?chlorine bleach. This is equal to 11.25 gallons using the conversion of 16 cups in one gallon.
  • Use an appropriately sized, clean container and pour the measured volume of chlorine bleach into the well.
  • While pouring the chlorine bleach, try to splash some around the wall or lining of the well.
  • Seal the well top.
  • Open all faucets and pump the water until a strong odor of bleach is noticeable at each faucet. Then stop the pump and allow the solution to remain in the well overnight.
  • The next day, turn on all the faucets and run the pump until all of the chlorine odor disappears. Be sure to adjust the flow of the water faucets or fixtures to a level that avoids overloading the septic system.
  • After 5 days of normal usage, test water for coliform.

How to disinfect a drilled well

Calculate the amount of water in the well by multiplying the gallons per foot by the depth of the well in feet. For example, a well with a 6-inch diameter contains 1.5 gallons of water per foot. If the well is 120 feet deep, multiply 1.5 gallons per foot by 120 feet to get 180 gallons.

Chlorine bleach for a drilled well
 Diameter of well (in inches) Gallons per foot
 3 0.37
 4  0.65
 5  1.0
 6  1.5
 8  2.6
 10  4.1
 12  6.0

For each 100 gallons of water in the well, use the amount of chlorine bleach (liquid or granules) indicated in the table below.

Amount of disinfectant required for each 100 gallons of water
 Chemical Amount
Laundry bleach (5.25% Chlorine) 3 cups*
Hypochloride Granules (70% Chlorine) 2 ounces**

* 1 cup = 8 ounce measuring cup
** 1 ounce = 2 heaping tablespoons of granules

  • Mix the total amount of liquid or granules with about 10 gallons of water.
  • Remove the access nut on the well casing top.
  • Use a funnel and pour the solution into the top of the well.
  • Connect a hose to a faucet on the discharge side of the pressure tank and insert it into the well casing top.
  • Start the pump. Spray the water back into the well and wash the sides of the casing for at least 15 minutes.
  • Open every faucet in the system and let the water run until the smell of chlorine can be detected.
  • Close all the faucets and seal the top of the well.
  • Let the chlorinated water stand in the system several hours, preferably overnight.
  • After letting the water stand, turn on the pump and open all faucets. Let the water run until all odor of chlorine disappears. Be sure to adjust the flow of water from faucets of fixtures that discharge into septic tank systems to a low flow to avoid overloading the drain field.
  • After 5 days of normal usage, test water for coliform.