If your lab result shows that your water is unsafe, you will need to disinfect your well. A one-time (shock) chlorination of your well is one option that may make your well safe again. This procedure is not recommended if the well is dug (because dug wells are at a higher risk for contamination), or if there is a known source of contamination. In these cases, you need a more permanent solution such as a new well or treatment device such as a chlorinator or an ultraviolet light. Consult a licensed well driller or plumber for help. Similarly, if shock chlorination does not work, consider adding a treatment device or constructing a new well.
Any work or maintenance done on your well should be completed by a well contractor licensed under the
Ontario Water Resources Act.
To find a licensed contractor, visit the
licensed well contractors directory.
How much household bleach will I need for my well?
Dug wells, 1 m (3 ft.) in diameter: Add 1 L (1 qt.) of household bleach for every 1.5 m (5 ft.) of water depth.
Drilled wells, 15 cm (6 in.) in diameter: Add 85 mL (3 oz.) of household bleach for every 7.5 m (25 ft.) of water depth.
Well points, 5 cm (2 in.) in diameter: Add 85 mL (3 oz.) of household bleach for every 3 m (10 ft.) of water depth.
Do not drink the water until you retest your water and the results show zero bacteria in the water.
Please follow the instructions below; you can also print
this disinfection instruction sheet.
- Pour household bleach (do not use lemon-scented or laundry bleach as listed on the label) down the well. Make sure the bleach contains at least 5.25% chlorine. If you do not know how deep the water is in the well, use the well depth to estimate how much bleach to add.
- Remove sanitary well cap if possible or use venthole in a drilled well to pour in the chlorine.
- Disconnect any filters/treatment devices, run water through all taps and let sit for 12 hours. Turn on each tap—inside and outside your house—one at a time. Let the water run until you smell a strong odour of chlorine. If you can't smell chlorine, add more to the well.
- Let the chlorinated water stay in the plumbing system for at least 12 hours. Do not run any water at this time.
- After 12 hours, starting from outside, run the treated water through an outside hose away from the septic system until you no longer smell chlorine. This will reduce the impact the chlorine has on your septic system.
- Continue to run and flush the water for each tap in the house until there is no longer a strong odour of chlorine.
- Wait a minimum of 48 hours and resample your well water for testing.
- Following a shock chlorination, we recommend that you submit two samples for testing, each collected one to three weeks apart. If both sample results show zero bacteria, that indicates a bacteriologically-safe supply of water, and that the treatment has been effective.
You can also use Public Health Ontario's
Well Disinfection Tool to help you calculate the amount of chlorine product that is needed to disinfect a well.
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