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Well Water Quality

As a private well owner, it is your responsibility to maintain your well and test your well water frequently each year. Find out about drinking water contaminants, possible health effects of drinking contaminated water and how to keep your drinking water safe.

Water quality can change over time. Test your well water for bacteria frequently to stay informed about the safety of your family’s drinking water.

Consider testing for chemicals if contamination is suspected, or if chemicals are known to occur naturally at elevated levels in the area groundwater.

What are some examples of well water contaminants?


Bacteria are found in animal waste, soil, vegetation and feces. They can get into well water if the well is not protected from the entry of surface water, or there is a source of contamination such as a septic system.

While many bacteria are harmless, some can be a health concern when found in drinking water.



Arsenic is an element found in some types of soil and rock. Arsenic can occur naturally in well water if soil or rocks that contain arsenic are present in the area. Less commonly, arsenic may be in well water as a result of human activities such as mining or agriculture.

Health issues related to arsenic depend on the amount of exposure and individual factors such as age, pre-existing illnesses, lifestyle, genetics, and other chemical exposures.

For most Canadians, the main source of arsenic exposure is through food, followed by drinking water, soil and air. There are no known health effects from exposures to arsenic in drinking water through bathing, hand washing or laundry.

Arsenic is known to cause cancer. Ingesting water that contains elevated levels of arsenic over a long period of time may increase the risk of lung, skin, bladder and other cancers. The risk of developing cancer from arsenic depends on how much you are exposed to, how often you are exposed and for how long.

Arsenic poisoning is rare. It may occur as a result of ingesting water with very high arsenic levels. These levels are much higher than typically found in Ontario.

Ontario’s drinking water standard for arsenic is 0.010 milligrams per litre (mg/L). Daily ingestion of water with an arsenic level of 0.010 mg/L is associated with a lifetime risk of developing arsenic related cancer of around 1 in 10,000. Therefore, levels in drinking water should be as low as possible.

In drinking water, potentially harmful levels of arsenic may have no taste or odour. It can only be detected through chemical testing.

Have your well water tested by an accredited laboratory to find out how much arsenic, if any, is in your well water. A list of laboratories licensed to test drinking water in Ontario is available. The laboratory will provide you with a sample bottle and instructions on how to take a sample.

You should test your well for arsenic:

  • Every three years in areas known to have elevated levels. See the question below “How do I know if arsenic has been found above the drinking water standard in my community?”
  • Frequently if your arsenic levels are near the drinking water standard.
  • If you have a treatment system or device to remove arsenic from your water, test the treated water yearly to ensure it is working properly.

You are responsible for ensuring your well water is safe to drink.

If your well water has levels of arsenic above the standard of 0.010 mg/L, you should take action to reduce the arsenic in water used for drinking and preparing food. One option is to use a treatment system or device to remove arsenic, or use another source of drinking water, such as bottled water from an approved source or water from a public system.

Several treatment methods can remove arsenic from drinking water. The one most commonly used is reverse osmosis. Treatment can be at the point-of-entry (POE) to treat all of the water entering a household or at the point-of-use (POU) to treat the water from a single tap used for drinking or cooking. Speak to a water treatment professional about treatment options.

Before a treatment device is installed, the well water should be tested to determine the general water chemistry and to verify the arsenic level. The testing should also assess the presence and levels of competing ions (e.g. fluoride, iron, sulphate, silicate) and organic matter, which could interfere with arsenic removal.

Pre-treatment with an oxidation step should be considered to ensure both species of arsenic (trivalent and pentavalent arsenic) are removed. Or, choose a treatment system that removes both arsenic species.

When purchasing a treatment device, check that it has been certified by a credible standards organization such as NSF International (NSF), Underwriters Laboratory (UL), or Water Quality Association (WQA). The device should be certified to meet the NSF/American National Standards Institute (ANSI) standards for arsenic removal. Be sure to follow the manufacturer’s instructions for use and maintenance of the device.

Based on the available groundwater data for Halton Region, arsenic levels appear to vary across the Region and range from below detection to around 0.030 mg/L.

The Health Department is aware of four general locations where arsenic has been found above the drinking water standard of 0.010 mg/L:

  • Highway 7 and Fourth Line, Halton Hills
  • Tremaine Road and Campbellville Road, Milton
  • Highway 25 and Louis Saint Laurent Ave, Milton
  • Sixth Line and Burnhamthorpe Road, Oakville

Arsenic levels in well water can vary widely from one well to the next. Testing is the only way to know the arsenic level in your well water.

More information on groundwater monitoring in Ontario may be found on the following websites:

Herbicides and pesticides

Herbicides and pesticides from agricultural and household use can contaminate wells if used improperly. Always follow the manufacturer's label for proper use.


Nitrate is a form of nitrogen that is found naturally in groundwater, and in plants such as fruits and vegetables. Other sources include fertilizers, industrial waste water and septic system leaks. Nitrates may also be added as a preservative to foods such as cold cuts.

Most people are not exposed to levels of nitrate in drinking water that would cause health problems. Exposure to high levels of nitrate can cause a condition called methemoglobinemia. In infants, this has also been called blue baby syndrome. Methemoglobinemia interferes with oxygen delivery to cells in the body. The most obvious symptom is bluish skin colour, particularly around the eyes and mouth.

Elevated levels of nitrate in groundwater are often localized and due to human activities. This means that you could have high nitrate in your water even if your neighbour does not.


Sodium is an essential nutrient found in many foods and water. The human body needs sodium to maintain blood pressure, control fluid levels and for normal nerve and muscle function.

Too much sodium can lead to high blood pressure, a major risk factor for stroke, heart disease and kidney disease.

Sodium can be found naturally in groundwater that supplies water wells as a result of salt deposits and weathering of rocks. Human activities, such as the storage and use of road salt and fertilizers, livestock farming and releases from sewage systems can also increase sodium levels in groundwater.
A health-based maximum has not been set for sodium in drinking water. When sodium is above 200 milligrams per litre (mg/L), the average person can detect a salty taste. Sodium levels above 20 mg/L are reported to the Health Department by operators of public drinking water systems. This information is passed on to local physicians so they can inform their patients who have health conditions that may require a sodium-restricted diet.
Sodium in drinking water is not a health concern for most people. Drinking water usually provides a small fraction of the sodium in a normal diet. Healthy adults between the ages of 14-50 years should eat 1500 milligrams (mg) of sodium per day, and no more than 2300 mg per day. Healthy children need only 1000 to 1500 mg of sodium per day. Drinking up to two litres of water a day having a sodium level of 20 mg/L can add 40 mg of sodium to the diet. This is about 3 percent of the recommended daily intake for a healthy adult. When sodium levels are above 20 mg/L, drinking water can become a significant source of sodium for some people. Individuals and parents of infants who have health conditions that may require them to be on a sodiumrestricted diet should speak to their family physician.
If you are concerned, have your well water tested for sodium by an accredited laboratory. A list of laboratories is available on the Ministry of the Environment, Conservation and Parks website.
Home water softeners can increase sodium levels in drinking water. Some softeners reduce the hardness in the water by replacing the minerals that make the water hard, such as calcium, with sodium. A separate, un-softened water supply should be used for cooking and drinking purposes.
In Halton Region, sodium levels in groundwater range from around 3 mg/L to 313 mg/L. This data is based on the 2021 results of the Provincial Groundwater Monitoring Network. For more information about the province’s groundwater monitoring program, visit the Ministry of the Environment, Conservation and Parks website.

Using best practices to manage road salt use and plowed snow storage may improve sodium levels over the long-term.

Regulatory documents developed under the Province’s Clean Water Act, 2006 called Source Protection Plans specify mandatory actions to accomplish this. These actions include improved monitoring and use of road salt by the municipality and the public.

  • Shovel or plow your snow instead of using road salt.
  • Store road salt in a sealed container.
  • Store snow away from paved surfaces that drain towards catch basins or ditches.
  • Aim downspouts away from walkways and driveways, if possible.
  • Consider using materials that contain less sodium and chloride, such as sand/salt blends.
  • In colder temperatures, use materials such as sand to improve traction.
  • If you hire a contractor for winter maintenance, ensure they are Smart about Salt certified.


What can happen if I drink water contaminated with bacteria?

In some cases, drinking water that is contaminated with bacteria can result in no physical symptoms; in others, it can cause severe gastrointestinal illness. Gastrointestinal illness symptoms can include some or all of the following:

  • nausea;
  • vomiting;
  • diarrhea;
  • muscle aches;
  • headache; and/or
  • low-grade fever.

Symptoms can start in a few hours, days or weeks after consuming the water. In rare cases, drinking contaminated water may result in serious illness or death.

Anyone can get sick from drinking contaminated water. However, children, older adults and people with weak immune systems are at a higher risk of the harmful effects.

Can I tell if my water is contaminated by the way it tastes, smells or looks?

Contaminated water may be odourless, colourless, and tasteless. Testing your well water is the best way to ensure the safety of your well water supply.

How can I protect my well from contamination?

Well location

You should ensure that:

  • your well is located a safe distance from sources of contamination such as septic systems, barnyards and roads. Note: a dug well should be at least 30 metres away from a septic system and a drilled well should be at least 15 metres from a septic system.
  • the land slopes away from the well to prevent the entry of surface water.

Well construction

You should ensure that:

  • the well is covered with either a watertight cover or a manufactured cap that prevents the entry of animals and insects (depending on the type of well).
  • ground around the well is mounded to direct surface water away from the well.
  • the well casing is watertight and extends at least 40 centimetres above the ground.
  • the annular seal (material that fills the gap between the well casing and the surrounding earth) for drilled wells is watertight.
  • the connection at the well casing for the pump and electrical lines is watertight.

Water supply protection

Contaminants spilled onto the ground or introduced into septic systems can eventually get into a nearby well water supply. Take action to protect your well water supply:

  • Ensure liquid wastes, garbage and manure piles do not drain towards the well.
  • Prevent spillage of chemicals onto the ground by using chemicals such as petroleum products responsibly.
  • Do not apply pesticides or fertilizers around the well.
  • Reduce the entry of sodium from road salt into groundwater.
  • Use the Put Waste In Its Place tool to learn how to properly dispose of detergents, cleaners, chemical and oils. Do not flush these items down the toilet or sink.

Visit Halton Region’s Source Protection web page to learn more about protecting drinking water sources.