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Well disinfection tool

Disinfection is generally necessary when:

  • a new well is installed
  • the well or pump is serviced

This resource is intended for use by public health inspectors and well contractors. For additional information on well disinfection refer to chapter 8 of Water Supply Wells – Requirements and Best Management Practices from the Ministry of Environment and Climate Change.


Well diameter

Well depth

Depth to water

Units of measure

Well diameter

Well depth

Depth to water level

Type of Chlorine Disinfectant
Bleach solution typically known as Javex® bleach solution can be industrial (10 to 12 percent available chlorine) or household bleach (3 to 6 percent available chlorine).
Granular calcium hypochlorite - typically comes in granular, tablet, puck or powder form (60 to 70 percent available chlorine).

Desired concentration of chlorine
Suggested range: 50 - 200 ppm.

This range is based on recommendations provided by Ontario's Advisory Council on Drinking Water Quality and Testing Standards.
ppm or mg/L

Concentration of disinfectant
  • Bleach solution - household bleach typically contains 5.25% chlorine; industrial bleach typically contains 12% chlorine.
  • Granular calcium hypochlorite - typically contains 60 - 70% chlorine.

The depth of the well must be greater than the depth to the water level.
The desired concentration of chlorine in the well is equal to or higher than the concentration of chlorine in {{formData.disType| lowercase}}. Adjust your values to see results.
To disinfect the well add:
of {{formData.disType| lowercase}}.
Mix {{formData.disType| lowercase}} in {{waterMixVolume}} of water, and then pour this mixture into the well. The well calculator accounts for the extra {{waterMixVolume}} of water.

Other units of measure:

Volume of water in the well:
{{waterVolume| number: 0}} {{waterVolumeUnit}}
This is the initial volume of water prior to adding {{formData.disType| lowercase}} mixture to the well.

Dilution Ratio:
1: {{dilutionRatio| number : 0}}
The dilution ratio is volume of bleach to volume of water.



View Assumptions

1 drop ≈ 0.05 ml
1 cup ≈ 250 ml
1 teaspoon ≈ 5 ml
1 tablespoon ≈ 15 ml

Equivalent Available Chlorine:
{{equivDisinfectantConc| number : 1}}%

Available chlorine is the amount of chlorine available for disinfection.


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Important Information about Disinfecting Wells 
What are the limitations of simply disinfecting a well?

Well disinfection is not a substitute for water treatment. Well disinfection is used to inactivate pathogens that may have been introduced during maintenance, alterations, repairs or construction, including construction of new wells. If analysis of your water indicates microbiological contamination and it cannot be explained by the aforementioned activities, there are other sources of contamination that you need to address. Perhaps either the groundwater itself is contaminated, or the well is being contaminated from the surface. If the groundwater is contaminated for example by malfunctioning septic systems, a nearby body of surface water, or a farm, a professionally designed water treatment system may be used to produce high quality drinking water. If the well is being contaminated from the surface, a professional inspection may identify integrity problems with, for example, the seal, the cap, the casing, grade or location. The absence of indicator organisms, e.g. Escherichia coliform (“E.coli.”) does not necessarily mean that water is free of contamination.

Before disinfecting your well, determine if ________ and take corrective action:
  1. there are potential contamination sources proximal to well e.g. manure, malfunctioning septic system?
  2. drainage of rainfall, or snowmelt is towards the well head?
  3. the well does not have a vermin proof cap in good condition?
  4. the well casing protrudes less than 30 cm above grade; above the ground or floor?
  5. there are gaps between the well casing, ground, and surface seal?
Procedure for disinfecting a well
  1. Store enough clean water to meet household needs for a minimum of 12 hours.
  2. Bypass or disconnect any carbon filters, water softeners or other water treatment devices or else any pipes located past these filters will not be disinfected. Replace the filters once chlorination is completed. Highly chlorinated water can damage treatment units. It is important to follow the manufacturer’s recommendations to ensure treatment systems are properly disinfected. Be sure that the hot water tank’s heat source is shut off.
  3. Estimate the chlorine necessary to disinfect the water in the buildings plumbing including the hot water tank (calculator provided), and the chlorine necessary to disinfect the water in the well water column (calculator provided). Add them together.
  4. Drain all water out of plumbing including the hot water tank prior to dosing.
  5. Mix the chlorine required to disinfect the well in 25 litres (5 gallons) of water. Note: The well calculator accounts for the extra 25 litres of water used for mixing.
  6. Pour the mixture into your well.
  7. Thoroughly mix the chlorine solution and the water throughout the well column. This can be accomplished by attaching a hose to a tap and running water from the well through the hose and back into the well.
  8. Start the pump and bleed air from the pressure tank. Open all water taps one at a time, including outside hose bibs and cold and hot water taps. Allow the water to run until a chlorine smell is detected from each faucet then turn off each tap. Since chlorinated water can damage the action in a septic system, chlorinated water should not be allowed into the building’s sewage system.
  9. If a strong chlorine odour is not present, return to step 4, add half the amount of chlorine used for the initial treatment to the well and repeat steps 5 and 6.
  10. Let the chlorinated water stand in the system for 12 - 24 hours.
  11. Start the pump and run water through the outside hose away from vegetation until the strong smell of chlorine disappears. Make certain that the water does not enter any watercourse. Finally, open the indoor taps until the system is completely flushed. Taps or fixtures discharging to the septic tank systems should be temporarily diverted to an outside discharge point to avoid affecting the septic system.
  12. Wait 48 hours and then sample the water using the instructions and bottle provided by the laboratory. Two consecutive "safe" tests, performed on samples obtained over a period of one to three weeks, will probably indicate that the treatment has been effective.
  13. If the above steps do not alleviate the problem, it is recommended that the source of the ongoing contamination be determined and corrected, possibly with professional help.
Precautions when working with chlorine products
  • Always follow safety precautions and the manufacturer’s directions when working with chlorine products.
  • Never mix ammonia products with chlorine products. This practice produces chlorine gas - a very toxic gas that can cause severe breathing problems, choking and potentially death.
  • To avoid injury, use appropriate personal protective equipment during handling (read the label and refer to the material safety data sheet).
  • For chlorine to be effective, let the chlorinated water stand in the system for at least 12 hours but not more than 24 hours.
  • During the chlorination period ensure that the chlorinated water is not used or consumed.
  • Always add the chlorine products to water, not water to the chlorine products.
  • Do not use chlorine products that are expired. Chlorine products have a shelf life so check the bottle for an expiry date.
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The well disinfection tool is provided/being made available by Public Health Ontario without any warranties, express or implied, including fitness for a particular purpose. Public Health Ontario, its directors, employees and agents shall not be liable or responsible to any person for any harm, loss or damage that may arise in any connection with the use of the well disinfection tool, including without limitation any direct, indirect, special, third party, or consequential damages.​​​​

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Page updated on [date/time] 07/06/2017 11:32 AM
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