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Drinking Water

Nitrates in Drinking Water

Nitrates are colourless, odourless chemicals that occur naturally throughout the environment or as a result of human activities.  Nitrate is a compound that is formed naturally when nitrogen combines with oxygen or ozone.  Nitrogen is essential for all living things, but high levels in drinking water can be harmful to health, especially for infants.

The Ontario Drinking Water Quality Standard for nitrate is 10mg/L as nitrate-nitrogen. This is the maximum acceptable concentration (MAC) for drinking water.

Nitrate can occur naturally in surface and groundwater at low levels that do not cause health problems. However, high levels of nitrate in surface and groundwater can result from agricultural runoff, refuse dump runoff, improper well construction, or well location and contamination with human or other animal wastes. Sources can include: fertilizers, leachate from garbage dumps, animal feedlots, municipal and industrial waste water, and septic systems. Wells may be more vulnerable to such contamination after flooding, particularly if the wells are shallow, have been dug or bored, or have been submerged by flood water for long periods of time.

Potential health effects of nitrates in drinking water depend on how much nitrate a person is exposed to, how long they were exposed, their age, and pre-existing health conditions. The level of nitrates most people are exposed to would not cause adverse health effects.

An elevated level of nitrate in drinking water is primarily a health concern for bottle-fed infants less than six months of age who have not yet developed the ability to properly digest nitrates. This can lead to a rare but very serious condition called methemoglobinemia or blue-baby syndrome. The inability to digest nitrate leads to the production of nitrite which affects the amount of oxygen carried in the blood, resulting in a bluish skin colour. If the level of nitrate in your water is above 10mg/L, you should use a different water supply to prepare baby formula and food or use ready-to-use formula. Breastmilk is not affected by nitrates and is considered safe.

Nitrates have also been classified as a possible carcinogen by the International Agency for Research on Cancer (IARC) if they undergo changes in the body that result in the formation of N-nitroso compounds. The extent to which this reaction occurs in the body is influenced by long-term consumption of high levels of nitrates in exceedance of the drinking water guidelines and diet. Research continues to explore the effects associated with long-term consumption of nitrates and some cancers.

The only way to know whether there are nitrates in your drinking water is to have it tested by a licensed laboratory. Municipal drinking water systems are tested regularly, but if you draw your drinking water from a well, the health unit recommends testing your well water for nitrates at least once a year. Visit the Ministry of the Environment, Conservation and Parks' website for a complete list of accredited laboratories.

It is also recommended that you test your well water for bacteria at least three times a year. Bacterial contamination can be an indicator that surface water is entering your well which may also contain nitrates. Bacterial testing is free for private homeowners.

Here are a few ways you can help keep your water safe:

  • Improve agricultural practices such as fertilizer storage and manure application.
  • Inspect your septic system every 3-5 years.
  • Avoid activities such as mixing or storing chemicals around water wells.
  • Regularly inspect your well for cracks and evidence of flooding.
  • If you are further concerned about nitrates in your drinking water, it is recommended that you contact a professional about installing an effective water treatment device to reduce the amount of nitrates in your drinking water. We recommend using equipment that has an approval stamp from NSF International or ANSI (American National Standards Institute). Boiling water will not remove nitrates and will only increase the concentration of nitrates in the water.

Once the level of nitrates in your drinking water is below 10mg/L, it is safe to resume using water for formula and food preparation for infants less than six months of age.

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