How to Disinfect Your Well

Before starting

It’s recommended that you find the source of the contamination of the well and correct it, if possible. Some possible causes could be:

  • Faulty well construction (i.e. loose lid, unscreened well vents, unsealed side walls, broken seal or well casing not extending above flood level)
  • System contamination (i.e. hot water tank or water softener)


  1. You can disinfect your well with household chlorine bleach, such as Javex®. Use fresh chlorine bleach to ensure an effective disinfection of the well.
    • Chlorine can evaporate and lose half of its strength in 6 months.
    • Don’t use scented bleach, to avoid adding extra chemicals to your water.
  2. Collect water for personal use while your system is being disinfected. This could include:
    • Filling the bathtub with water to be used to flush the toilet
    • Filling clean containers for drinking water. Drinking water can be disinfected by boiling it for at least one minute or by adding chlorine bleach (add 2 drops of bleach per litre of water and let sit for 15 minutes). You can also use bottled water.
  3. Determine the volume of water in the well.
    • Establish the diameter (distance across) of the well and depth of the water in the well.
    • Calculate the amount of chlorine bleach needed to disinfect the well (see Table below).
  4. Remove any carbon filters from the water system. Carbon will remove the chlorine. If this isn’t done, any pipes beyond the filter won’t be adequately disinfected.

Amount of chlorine bleach required

Diameter of Well Casing Amount of Chlorine Bleach per 3 Metres (10 ft) of Water Depth
10 cm (4 in.) 56 mL (2 oz.)
15 cm (6 in.) 113 mL (4 oz.)
20 cm (8 in.) 227 mL (8 oz.)
30 cm (12 in.) 454 mL (16 oz.)
40 cm (16 in.) 795 mL (28 oz.)
50 cm (20 in.) 1250 mL (44 oz.)
61 cm (24 in.) 1760 mL (61 oz.)
76 cm (30 in.) 2784 mL (98 oz.)
91 cm (36 in.) 3976 mL (140 oz.)
122 cm (48 in.) 7100 mL (250 oz.)

NOTE: Chlorine bleach comes in different concentrations, with the standard being 5.25%. The formula in the table above is based on 5.25% chlorine bleach. It’s important to verify the concentration of your bleach before starting to disinfect your well as it will impact the effectiveness of the disinfection.


  1. Mix the amount of chlorine bleach, determined in step 3 of Preparation, with some water and add to the well. Distribute the bleach through your water system.
    • Turn on ALL household cold water faucets to distribute the bleach throughout the water system until you detect a chlorine odour.
    • If there is no odour, add more chlorine until an odour is detected.
    • Drain the hot water tank and fill with chlorinated water.
    • Back flush the water softener and all water filters (except carbon filters).
  2. Let the treatment stand overnight (6 to 12 hours). Don’t turn on any taps or flush toilets at this time.


  1. Use an outside house tap to drain the water system until no chlorine odour is detected. Be sure that the water is being drained away from the septic, leaching or tile-bed system. Chlorine can damage septic systems.
  2. Install new carbon filters after chlorination to avoid introducing bacteria back into the water system.
  3. Retest the water about one week after disinfecting. To ensure that the treatment worked, take 3 consecutive samples one to three weeks apart.

If your well water continues to show bacterial contamination after you have disinfected the well twice or more, the groundwater supply could be contaminated. You may need a permanent water treatment device (such as an ultra-violet light unit, automatic chlorinator, or ceramic filters).

Brochures on water treatment devices are available at your local EOHU office.

For more information, please consult What to do if your well is contaminated with bacteria and Wells Regulation – Well Disinfection (technical bulletin).