Skip Navigation

Disinfecting Your Well

Learn about disinfecting your well.

Well disinfection is a temporary method of eliminating bacterial contamination. It is important to identify and correct the source of contamination before disinfecting your well. This will help ensure the continued safety of your water supply.

After disinfecting your well, test the water before using it for drinking. You should have three test results taken one to three weeks apart showing “no significant evidence of bacterial contamination”.

When to disinfect your well

You should disinfect your well after:

  • well construction;
  • repairs or maintenance to your well;
  • E.coli is found in the well water sample; and/or
  • total coliforms are found in repeated well water tests.

Routine well disinfection is not recommended.

How to disinfect your well

You can follow the steps below for disinfecting a well. A licensed well contractor should be contacted for specific information on well disinfection.

Well disinfection steps

  1. Measure the well diameter.
  2. Calculate the depth of water in the well by subtracting the distance from ground level to the resting water level from the well depth.
  3. Using the table below, measure the amount of household bleach (5.25 per cent solution) needed. Follow safety precautions on the product label.
  4. Mix the chlorine with water in a bucket. Then, pour the mixture down the inner wall of the well casing. If possible, mix the solution in the well. This can be done by attaching a hose to a tap and running water from the well through the hose and back into the well. Stir the well water but avoid stirring up sediment from the bottom of the well into the water column.
  5. Remove or bypass carbon filters on the system to ensure the chlorine reaches the pipes beyond the filter. Replace the filters after disinfection to avoid reintroducing bacteria into the system.
  6. Run water at every indoor and outdoor faucet until you smell chlorine. Turn off the faucets.
  7. Drain the hot water tank and fill with chlorinated water.
  8. Back-flush the water softener and all water filters (except carbon filters).
  9. Let the chlorinated water stand in the system overnight, or for at least 12 hours.
  10. Remove chlorinated water from the well by attaching a hose to an outdoor faucet and running the water until you can no longer smell chlorine. Make sure to direct waste water away from the septic system.
  11. Remove chlorinated water from the plumbing by running all indoor faucets until you no longer smell chlorine.
  12. Test your well water three to five days after disinfection. Do not drink the water until you receive three results, taken one to three weeks apart, showing “no significant evidence of bacterial contamination”.

Well Disinfection using Chlorine Bleach

Volume of bleach to add for every three metres (10 ft.) of water in the well
Well casing diameter Volume of bleach (5.25% solution)
millimetres (mm) inches (in) millilitres (ml)
50 2 6
100 4 30
150 6 60
200 8 100
250 10 200
300 12 250
400 16 400
500 20 650
600 24 900
900 36 2,000 (2 litres)
1,200 48 3,600 (3.6 litres)


Test for bacteria frequently—at least three times a year (spring, summer and fall, especially after a heavy rainfall).

Order water sample bottles