Ontario Ministry of Health and Long-Term Care - Public Information - Public - Water Safety

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Water Safety : Disinfection Instruction Sheet

Cross section of a water well

If your drinking water continues to test positive on repeated submissions, consult your local public health unit , which can help you interpret the results of your tests and provide advice on what measures you can take to safeguard your drinking water.

The first step in identifying the reason for repeated adverse water quality is to conduct a visual inspection of your well. Start with a close look at your well. The area around it should be clear of any potential contaminant sources, such as pets, lawn care products, and gardens. Once you are satisfied that the area around your well is okay, take a good, close look at the well itself. If you have an older well, make sure that the cap and the sealant around the well casing isn’t cracked or damaged. If it is, you need to fix or replace it right away.

If the source of the problem can’t be detected, consult a licensed well contractor right away to identify the source of the problem and eliminate it. You can save yourself a lot of money by doing this instead of rushing out to buy a home treatment device that may be expensive to install, operate, and maintain. And it may not eliminate the source of your trouble.

(If you have a cistern, please talk to your public health unit about disinfection requirements.)

Measure the diameter of the well.
Measure the well depth and the static or resting water level, then calculate the depth of water in the well.
Using the table below , measure out the amount of bleach needed. (The table gives the volume of bleach needed for different well sizes.) Then, pour the mixture into your well.
If possible, mix the water in the well. This can be accomplished by attaching a hose to a tap, running water from the well, through the hose and back into the well.
After adding chlorine to the well, remove or bypass any carbon filters that are in the system for water treatment. If you don’t, these filters will remove the chlorine from the water, and any pipes beyond the filter will not get disinfected. Replace with new filters after chlorination to avoid reintroducing bacteria into the system.
Run water at every faucet in the house (and barn, if you have one) until a strong chlorine odour is detected. Be aware that your nose may lose its ability to detect chlorine.
If there is no chlorine smell or it is very weak, add more bleach to the well and repeat Step 6 above.
Drain the water heater and fill with chlorinated water.
Back flush the water softener and all water filters (except carbon filters).
Let the chlorinated water stand in the system for at least 12 hours.
Clear chlorine from the well by running an outside hose to the ground surface. Then, run clear water through the faucets until the water no longer smells of chlorine.
Avoid putting too much chlorine into the septic system because the bacteria needed for septic decomposition maybe killed.
Do not drink the water without boiling it until test results show the water is safe to drink.

Volume of Bleach to Add for Every 3 Metres (10 Feet) of Water in the Well*
Casing Diameter
Volume of Unscented Bleach
(5.25% solution)
Millimetres
Inches
Millilitres
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
2000 (2 litres)
1200
48
3600 (3.6 litres)

For example :
If you have 6 metres (20 feet) of water in your well and it has a casing diameter of 100 millimetres or 4 inches, you would add 60 millimetres or 2 fluid ounces of bleach.

*For questions or more information on how to disinfect your well, contact your local public health unit .

See also :

For more information

Call the ministry INFOline at 1-866-532-3161
(Toll-free in Ontario only)
TTY 1-800-387-5559
Hours of operation : 8:30am - 5:00pm
(External) Ministry of the Environment
Information Centre : 1-800-565-4923
Water Well Records : 1-888-396-9355

(External) Ministry of Agriculture and Food
OMAF Agricultural Information Contact Centre :
1-877-424-1300

(External)

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