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Ontario Drinking Water Quality Standards (ODWQS)

The Provincial Government of Ontario had jurisdiction over the drinking water supplies and regulates drinking water quality through the Ministry of the Environment’s Ontario Drinking Water Quality Standards and related documents. The standards reflect the most current expertise and procedures.

The ODWQS serve to protect public health by ensuring that no disease causing organisms, unsafe concentration of toxic chemicals or radioactive substances is found in water supplied for human consumption.

Standards for drinking water are expressed in terms of: Maximum Acceptable Concentration (MAC) and Interim Maximum Acceptable Concentration (IMAC).

MAC (Maximum Acceptable Concentration)
This is a health-related Ontario drinking water quality standard established for contaminants that have known or suspected adverse health effects when above a certain concentration.  The length of time the MAC can be exceeded without injury to health will depend on the nature and concentration of the parameter.

IMAC (Interim Maximum Acceptable Concentration)
This is a health-related Ontario drinking water quality standard established for contaminants when there are insufficient toxicological data to establish a MAC with reasonable certainty, or when it is not practical to establish a MAC at the desired level.

The Technical Support Documentation for Ontario Drinking Water Standards, Objectives and Guidelines
This is a supporting document to the ODWQS and lists the following types of objectives:

AO ( Aesthetic Objective)
AO’s are established for parameters that may impair the taste, odour or colour of water, which do not affect the safety of the drinking water.   The primary goal of these objectives is to provide water that is aesthetically pleasant to the consumers.   For certain parameters, both aesthetic objectives (which are not health-related) and health-related MAC’s have been derived.

OG (Operational Guidelines)
OG’s are established parameters that need to be controlled to ensure efficient and effective treatment and distribution of the water.

Measurements are expressed in terms of:

mg/L (milligrams per litre)
This is a measurement of the concentration of a parameter in water, and expressed in parts per million (ppm).

ug/L (micrograms per litre)
This is a measurement of the concentration of a parameter in water, and expressed in parts per billion (ppb).

ng/L (nanograms per litre)
This is measurement of the concentration of a parameter in water, and expressed in parts per trillion (ppt).