The Effect of Flooding on Private Drinking Water Systems


2 May 2019

Flooding and a private drinking water system

Every year as snow melts, the run-off (often called spring run-off) results in contaminants entering groundwater. In addition to spring runoff, Ontario often receives record amounts of rainfall, which can lead to flooding in many areas. Flooding combined with annual spring runoff can cause a higher-than-usual risk of contaminants, particularly in private well water.

It’s important to take flooding seriously, understand the risks and know what steps to take to protect you and your family.

What are the risks?

Flooding can affect water quality, and this is a particular concern for rural residents in Ontario on private drinking water systems. Be aware that flooding may cause contamination of private water sources, such as wells.

"Private drinking water systems serve many Ontarians, particularly in rural and remote areas. These systems are vulnerable to contamination with fecal pathogens, particularly during flooding, and should be tested frequently."

— Dr. Anna Majury, Clinical and Environmental Microbiologist, PHO

What should I do?

  • If you use a private well for your water supply, contact your local public health unit for water quality information and to have your water tested by Public Health Ontario. There are eleven Public Health Ontario laboratory sites that test private drinking water supplies for indicators of bacterial contamination. In addition to bacterial contaminants, flood waters may carry chemical and other contaminants in to your well. Should you wish to test for chemical contaminants, a list of accredited laboratories offering chemical testing in Ontario drinking water can be found at:
  • In the event that your water is contaminated, or you suspect contamination because of heavy rainfall or flooding, consider using bottled water or a municipal supply, if available. Do not use contaminated water for drinking, food preparation, or for brushing your teeth. Private well water supplies are vulnerable to contamination events at any time and should be tested frequently.

Flooding and a private sewage system

If your property is at risk of a flood, or flooded, consider the effect on your private sewage system. A flooded sewage system will not function properly and can cause problems such as:

  • sewage backup in your home, and;
  • contamination of your water supply. 

In the event of a flood, contact your local public health unit for guidance regarding how to manage your private sewage system during the flood period.


If you have any questions, or would like to have your water tested, please reach out to your local public health unit.

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Updated 2 May 2019