Page 5: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Trihalomethanes

3.0 Application of the guidelines

Note: Specific guidance related to the implementation of the guidelines should be obtained from the appropriate drinking water authority in the affected jurisdiction.

Guidelines for drinking water contaminants are usually developed using the results of animals studies. The guideline for total THMs is based on the health effects of chloroform, which is now classified as a possible human carcinogen (changed from its previous classification of "probable" human carcinogen). It incorporates uncertainty factors to account for a range of issues, including differences within and across species, deficiencies in the database and some limited evidence of carcinogenicity. Because of the limitations in current scientific methodology, it is not possible to quantify the increased risk to human health when a drinking water supply exceeds the guideline value.

The guideline for THMs is also designed to take into consideration exposure and potential health effects related to other disinfection by-products (DBPs), on which very little is known. It represents a level of exposure that is acceptable throughout life (70 years) and that will not cause an increased risk to health. It takes into account all exposures from drinking water (whether by ingestion, inhalation or dermal absorption). The guideline is measured as a locational running annual average of quarterly samples, because THM levels can vary significantly over time, including seasonally, with factors such as the levels of organic matter in the raw water and temperature. Although individual measurements may exceed the guideline value, this would be of concern only if they caused the running average of quarterly samples to exceed the guideline value.

BDCM is a probable human carcinogen, which means that exposure to any level in drinking water may increase the risk of cancer. The guideline is established at a level at which the increased cancer risk is "essentially negligible" when humans are exposed at that level over a lifetime (70 years). In the context of drinking water guidelines, Health Canada has defined this term as a range from one new cancer above background per 100,000 people to one new cancer above background per 1 million people (i.e., 10-5 to 10-6) over a lifetime. Exposure to BDCM at levels higher than the guideline value has also been linked to a possible increase in reproductive effects (increased risk for spontaneous abortion or stillbirth) above what can normally be expected in the population although further studies are required to confirm these effects. However, where BDCM exceeds the guideline value, a jurisdiction may decide to take action to reduce levels of BDCM and, may choose to take further precautionary steps on the basis of these studies to protect populations believed to be vulnerable. The guideline for BDCM is protective of both cancer and non-cancer health effects.

Both drinking water guidelines are protective against health effects from lifelong exposure. However, as with all guidelines, any significant exceedance should be a signal to investigate the situation in order to take remedial action and to consult the authority responsible for public health. For significant exceedances above the guideline value, it is suggested that a plan be developed and implemented to address these situations.

Given the potential health effects from THMs, including BDCM, and the limited information on the risks and uncertainties of other chlorinated disinfection by-products (CDBPs), it is recommended that treatment plants strive to maintain THM and BDCM levels as low as reasonably achievable (or ALARA) without compromising disinfection. Treatment plants also need to ensure that any effort aimed at reducing disinfection by-products, such as changing disinfection strategies, does not inadvertently increase the levels or leaching of other contaminants, such as lead, in the distributed water.

3.1 Monitoring

At minimum, quarterly monitoring of treated water from surface water and groundwater sources is recommended for both THMs and BDCM. Increased frequency may be required for facilities using surface water sourcesFootnote 2 during peak by-product formation periods. It is also recommended that monitoring samples be taken at the water treatment plant and at the point in the distribution system with the highest THM formation potential. These points generally represent the areas in the distribution system with the longest disinfectant retention time, which are typically at the far end of the distribution system.

Monitoring/reporting may be reduced if drinking water monitoring does not show elevated levels of CDBPs within the distribution system.

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