Water & Sewer

Arsenic limits in Drinking Well Water

On January 1, 2018 the limit in the Ontario Drinking Water Standard (ODWS) for arsenic in drinking water wells will be lowered from the current limit of 0.025 mg/L to 0.010 mg/L.

What is Arsenic?

Arsenic (As) is a naturally occurring element. Trace amounts of arsenic are found in food, water, soil and air. Breakdown and erosion of arsenic-containing minerals in soils and rock, agricultural run-off, mining operations and industrial processes are all potential sources of arsenic in surface and ground water.  Amounts of arsenic found in drinking water are generally higher in groundwater than surface water sources.

What are the Ontario Drinking Water Standards?

These standards set the maximum concentrations of chemicals that are allowed in public drinking water. They must be met by all public (regulated) drinking water systems and are used as recommended limits for private drinking water wells.

The provincial arsenic standard or limit (0.025 mg/L) is being lowered to align with the more protective federal limit (0.010 mg/L) which is based on the best worldwide arsenic exposure and health information available to Health Canada. The current Health Canada Canadian Drinking Water Quality Guideline limit for Arsenic is already set at 0.010mg/L. The World Health Organization (WHO) limit is also set at 0.010 mg/L.

What are the health effects of high Arsenic levels?

Long-term exposure to arsenic from drinking-water and food can cause cancer and skin lesions. It has also been associated with developmental effects, cardiovascular disease, neurotoxicity and diabetes. (3)

Short term exposure to very high levels of arsenic can lead to abdominal and muscular pain, diarrhea and vomiting, skin rash and numbness.

What is known about the level of Arsenic in well water in Hamilton?

Public Health Services has tested arsenic concentrations in 100 regulated wells in Hamilton. Traces of arsenic were detected in 25 per cent of the samples. Concentrations ranged from 0.001mg/L to 0.020mg/L averaging 0.006 mg/L.

Fourteen Provincial Ground Monitoring Network (PGMN) and 50 Ontario Ministry of the Environment, Conservation and Parks regulated wells were tested for arsenic. None had levels above the new ODWS. Of the wells tested by Public Health a well in each of the following areas had arsenic concentrations above the new ODWS of 0.010mg/L:

  • The intersection of Sulphur Springs Road and Governors Road
  • The Copetown area
  • The Lynden area
  • Trinity Church Road and Tisdale Road  area south of Kirk and Chippewa Roads to Hall Road

Public Health Services advises residents using private wells in these areas consider testing their drinking water for arsenic.

How can I test my water for Arsenic?

Arsenic testing of private drinking water wells can be undertaken via a private licensed environmental laboratory on a fee-for-service basis. There are several in the region. They can be found in the Yellow Pages under ‘laboratories’ or by visiting the Ontario Ministry of the Environment, Conservation and Parks website www.ontario.ca/page/list-licensed-laboratories

Private wells with arsenic concentrations above the provincial standard can be reduced via three recognised options: reverse osmosis, anionic exchange, and iron oxide. All options have advantages and disadvantages and should be considered on a site-specific basis due to variations in groundwater chemistry. Public Health Services advises that a rural water treatment specialist with a good knowledge of water chemistry testing and with the ability to conduct groundwater chemistry testing of the client’s water be consulted on the type of system best suited for a particular well.

Before deciding on an arsenic reduction device, the well water should be tested to determine general water chemistry and to verify the concentration and valency of the arsenic (As III or V). Testing should also include screening for the presence and concentration of any competing ions (e.g., fluoride, iron, sulphate, silicate) and organic matter in the water, which could interfere with arsenic removal.(1)

Costs for arsenic treatment systems and ongoing operation can range depending on water conditions, vendors, and makes/models of systems.  Public Health Services has not priced any of the arsenic reduction systems that are listed below. 

Public Health Services cannot recommend an installer (water treatment specialist), or a system, make/model over another. 

Are there treatment systems that remove Arsenic?  

Reverse osmosis removal involves filtration of water at a molecular level through a semi-permeable membrane. This method is effective in removing arsenic under the right conditions if testing supports its use.


  • RO is very effective at removing inorganic constituents like arsenic, iron, lead, chromium and manganese. When operated under ideal conditions, RO can remove over 95% of As (V).
  • RO requires very little maintenance and no addition of chemicals.


  • Smaller RO point-of-use systems produce only a few gallons of treated water per day. The supply of drinkable water is limited and normally available only in the kitchen area of the home.
  • If As (III) is in the drinking water, it needs to be pre-oxidized to As (V) to allow the RO unit to remove it, which will increase the complexity and cost of the system.
  • If there are significant amounts of iron or manganese in the water, additional pre-treatment equipment to remove those prior to RO treatment will likely be needed.
  • Larger RO systems, called point-of-entry systems, can treat water for the entire house. However, these are much more expensive. More importantly, RO point-of-entry systems can cause corrosion control problems in house plumbing, which can elevate the levels of lead and copper in drinking water.(2)

Anionic exchange systems exchange ions between a resin bed and chemical elements in the water passing through it. These systems are used to soften water, remove iron and manganese, and lower nitrate and arsenic levels. Specific contaminant removal is determined by the type of resin bed used.


  • Anionic exchange requires little maintenance; additional salt is added every few weeks.
  • Systems are typically installed to treat an entire house.


  • Other chemicals in water can compete with arsenic for the resin sites reducing the systems effectiveness. The US EPA recommends the influent water have less than 500 mg/L of total dissolved solids (TDS) and less than 25 mg/L of sulfate.
  • Treated water can have a very low (acidic) pH and high levels of chloride which can cause corrosion control problems and high levels of lead and copper in the treated water.
  • If the system fails, all of the arsenic captured on the resin at that time can be released at once causing a large concentration of arsenic in the treated water.(2)

Iron oxide filters are similar to activated carbon filters in that the filter material has a large amount of surface area and a structure that helps arsenic to stick to its surface. Although these filters are fairly new to the home treatment market, the principals behind them have been used by public water suppliers for many years.

Iron oxide media can be housed in small inline filter cartridges (point-of-use) or in larger point of entry tanks. These filters can be used to enhance the performance of reverse osmosis systems that are not effectively removing As (III).


  • They are effective for both As (III) and As (V) removal.
  • They can be used as point-of-use or point of- entry systems.
  • They remove other inorganic chemicals.
  • They are simple to use and install.
  • They are disposable as non-hazardous waste.


  • The media must be replaced on regular basis.
  • The presence of iron, manganese, sulfate, silica or organic carbon can reduce effectiveness.(2)

What are treatment costs?

Costs can vary depending on the chemical composition of the groundwater, type of arsenic removal required and type of system chosen. Public Health Services provides the following cost for one arsenic reduction system only as reported to us by the installer. As a very rough guide, an iron oxide system designed for and recently installed in a regulated small drinking water system in Hamilton (similar to a large household system) cost approximately $825 plus installation. 

Contact us

Safe Water Infoline at 905-546-2189
Monday to Friday 8:30 am to 4:30 pm


  1. Health Canada: Guidelines for Drinking Water Quality: Guideline Technical Document – Arsenic (PDF, 331 KB)
  2. State of Oregon Health Authority: Arsenic Removal (PDF, 61 KB)
  3. World Health Organisation (WHO): Arsenic Fact Sheet, 2016
  4. Canadian Cancer Society: Arsenic in drinking water
  5. Ontario Ministry of the Environment, Conservation and Parks: Discussion Paper Re: Arsenic MAC Reduction (PDF, 614 KB)
  6. Canadian Environmental Law Association: Arsenic in Drinking Water: Ontario’s Failure to Endorse Health Canada’s Guideline
  7. Canadian Environmental Health Atlas: Arsenic
  8. Canadian Water Network: Arsenic in Canadian Drinking Water - Full Report (PDF, 558 KB)