Page 3: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Arsenic

2.0 Executive summary

Arsenic is a natural element that is widely distributed throughout the Earth's crust. It is often found naturally in groundwater, through erosion and weathering of soils, minerals, and ores. Arsenic compounds are used commercially and industrially in the manufacture of a variety of products and may enter drinking water sources directly from industrial effluents and indirectly from atmospheric deposition.

This Guideline Technical Document reviews the health risks associated with arsenic in drinking water, focussing on inorganic forms of arsenic. It assesses all identified health risks, taking into account new studies and approaches, as well as the limitations of available treatment technology. It considers exposure to arsenic through drinking water only from ingestion, as exposure through inhalation and skin contact is not considered to be significant. From this review, the guideline for arsenic in drinking water is established at a maximum acceptable concentration of 0.010 mg/L (10 µg/L), based on municipal- and residential-scale treatment achievability.

2.1 Health effects

Arsenic is classified as a human carcinogen. As arsenic is a natural contaminant of groundwater, its health effects have been widely studied in humans, most notably in Taiwan. This is particularly significant because the toxic effects of arsenic vary significantly between species, making animal studies an unreliable basis on which to develop a guideline.

The maximum acceptable concentration for arsenic in drinking water was established based on the incidence of internal (lung, bladder, and liver) cancers in humans, through the calculation of a lifetime unit risk. This guideline for arsenic has been set at a level that is higher than the level that would be considered to be associated with an "essentially negligible" risk, based on limitations of available treatment technology.

The health effects of arsenic in humans vary depending on the compound and form. Metallic arsenic is not absorbed from the stomach and does not have any adverse health effects. Although it was generally accepted that the inorganic forms of arsenic were responsible for its toxic and carcinogenic effects and that its organic forms were less toxic, recent evidence is now questioning this assumption.

2.2 Exposure

Arsenic can be found in both surface water and groundwater sources, with levels generally higher in groundwater. Most provinces and territories across Canada report some areas where arsenic can be detected in drinking water supplies. Although levels are generally well below the guideline, elevated arsenic concentrations have been found in areas with natural sources.

Drinking water is considered to be the major source of exposure to arsenic only in populations living near a source of arsenic (either a natural geological source or a site of contamination). For most Canadians, the primary source of exposure to arsenic is food, followed by drinking water, soil, and air.

2.3 Treatment

The establishment of a drinking water guideline must take into consideration the ability to both measure the contaminant and remove it from drinking water supplies. Arsenic can be reliably measured at a concentration of 0.005 mg/L. The selection of an appropriate treatment process for a specific water supply will depend on the characteristics of the raw water supply and many other factors. It is important to determine what, if any, pretreatment is required. Because arsenic is a human carcinogen, every effort should be made to maintain levels in drinking water as low as reasonably achievable.

Arsenic can be effectively treated in municipal-scale treatment facilities through a number of well-documented methods, which typically include both a pretreatment step and a final polishing step. Arsenic can be reduced to levels below the guideline of 0.010 mg/L (10 µg/L) in both large and smaller municipal plants.

Arsenic can also be removed by residential-scale drinking water treatment devices to levels below 0.010 mg/L. Certified devices are frequently designed to remove arsenic to well below this concentration, but certification to the standard verifies only that a final concentration of 0.010 mg/L or less is achieved.

Since treatment technology considerations are a limiting factor in establishing a guideline for arsenic in drinking water, Health Canada and the Federal-Provincial-Territorial Committee on Drinking Water will continue to monitor new developments in treatment technologies to revise and update the guideline and the guideline technical document as required.

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