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Well Water Safety

Disinfecting Your Well (Drilled Well)

Shocking is a temporary method of disinfection used to eliminate a one time case of bacterial contamination. Shocking should not be used routinely or repeatedly. It is not a substitute for eliminating an ongoing source of contamination or a defect in your well.

Shock chlorination involves adding large amounts of chlorine to the water in the well and pumping it through the system. The chlorinated water is left in the system long enough to ensure complete disinfection.

Drilled Well Disinfection

To disinfect the water in your well, you will need to shock chlorinate the system with a concentration of plain bleach (5.25% sodium hypochlorite) at approximately 250 mg/L. The following chart will assist you in determining the amount of chlorine to add to your well. If you don't know how high the water stands in your well it is recommended that the well depth be used to estimate the amount of bleach to add. For specifics on your well you should contact the Ministry of the Environment.

Chlorine required for drilled wells casing diameter up to 15 centimetres or 6 inches

Water Depth
Meters

Water Depth
Feet

Household Bleach 5.25%
Millilitres

Household Bleach 5.25%
Ounces

7.6

25

140

5

15.0

50

280

10

22.8

75

420

15

30.0

100

560

20

38.0

125

700

25

45.0

150

840

30

53.0

175

980

35

61.0

200

1,120

40

(Note: All Conversions Are Approximate)

Note: Be careful when handling bleach solutions. It is recommended you wear rubber gloves, goggles, and a protective apron. If chlorine accidentally gets on your skin you should immediately flush your skin with clean water.

Never mix chlorine with other cleaning agents because toxic gases may be formed. Regular household bleach (Sodium Hypochlorite 5.25%) must be used to chlorinate your well. Do not use "fresh scent" bleach or other products to disinfect wells.


  1. Prior to chlorinating your well, ensure that you obtain a sufficient amount of potable drinking water to last over the period of time it takes to disinfect the well. Disinfected well water or alternative potable water is recommended.
  1. Disconnect carbon or charcoal filters. Other treatment devices including water softeners, iron filters and sand filters should be chlorinated. Check first with the manufacturer's literature to ensure that chlorine will not damage the treatment equipment.
  1. Using the chart as a reference, determine the amount of chlorine to disinfect the volume of water in your well. Adding the suggested quantity of chlorine to your well will be sufficient to shock chlorinate the system and provide an approximate concentration of 250 parts per million.
  1. Mix the appropriate amount of chlorine with water before applying it to the well. Mixing of the chlorine solution can be performed using a ratio of 1 unit volume of bleach to 10 units of water (e.g. 1 litre of bleach for every 10 litres of water). This procedure can be performed using a large, clean and sanitary container. For a drilled well, chlorine may be added through the vent pipe or by removing the sanitary seal on the well head. If possible, connect a hose from faucet on the discharge side of the pressure tank to the top of the well casing. Start the pump. Rinse down the sides of the well casing for 5-10 minutes. This procedure will circulate the chlorine solution throughout the water system to ensure total disinfection.
  1. Turn on all faucets to distribute chlorinated water throughout the plumbing system until you can smell chlorine. You can turn off the heating element on your hot water heater to save energy during this process.
  1. Let the water remain in the system for 12 to 24 hours.
  1. Flush the system of remaining chlorine. Do not drain chlorinated water into your septic system. Connect a hose to an outside tap and drain the water onto a remote location on the property away from the septic system and well. Once the smell of chlorine dissipates you may turn on the indoor faucets until the smell of chlorine is completely removed.
  1. Wait one (1) week before collecting a water sample for bacteriological testing so that all traces of chlorine have been flushed from the system.
    If shock chlorination treatment does not eliminate the bacteriological problem or you experience chronic contamination in your well, the Health Department suggests you consider a water treatment device.

For more information please refer to the Homeowners Well Maintenance Checklist (PDF)