Proposed Adoption of the Canadian Water Quality Guideline for Cadmium as the Provincial Water Quality Objective

This notice was originally posted in the old Environmental Registry under number PA8E0023.

Description of policy

Cadmium occurs naturally in the environment as a minor constituent in mineral sulphides. The two oxidation states of cadmium are the metallic (Cd 0) and divalent (Cd2+). The divalent state predominates in most natural deposits. Several cadmium salts are freely soluble.

The water quality guidelines for the protection of aquatic life were developed based on the 1991 CCME protocol (CCME 1991).

The interim guideline value for cadmium for the protection of freshwater life is 0.017 ug/L. A cadmium concentration of 0.17 ug/L impaired fecundity after 16 d in soft water. This concentration was multiplied by a safety factor of 0.1 to arrive at a water quality guideline for the protection of freshwater life of 0.017 ug/L. However, water hardness can have a major influence on cadmium toxicity to freshwater organisms. To allow for the derivation of a site-specific, hardness-related guideline, a linear relationship was established between water hardness and acute toxicity (LC50 data - concentration of the contaminant which is lethal to 50% of the tested species after a specified time) for cladocerans, the most cadmium-sensitive aquatic species. The regression is based on 40 data points, with significance of P = 0.0007 and correlation coefficient (r) of 0.66. A preferred regression line based on chronic data was impossible to establish with the available data, because of variations in experimental design, endpoints, exposure durations, and experimental conditions in the studies considered. Therefore, it was assumed that the relationship between cadmium acute toxicity and water hardness observed for cladocerans is similar to that observed for other freshwater organisms. It was also assumed that the slope of the regression line for the acute toxicity relationship is similar to that for the chronic relationship. Using the water quality guideline of 0.017 ug/L, the regression line was normalized with respect to a water hardness of 48.5 mg/L. This allows the derivation of the interim water quality guideline on a site-specific basis according to the local water hardness. The equation is:

CWQG = 10exp{0.86[log(hardness)]-3.2} (Equation 1)
where the CWQG is in ug/L and hardness is measured as calcium carbonate equivalents in mg/L.

The recommended Provincial Water Quality Objective is derived directly from the above equation for the appropriate surface water hardness.

Purpose of policy

Contaminants that impair the aquatic environment are an important environmental concern. Provincial Water Quality Objectives are used to assist in managing these contaminants. The policies and procedures which govern their uses are contained in the booklet - Water Management (1994) - which deals with all aspects of Ontario's water management policy. PWQOs are used to: i) classify receiving waters for water management purposes; ii) assess contaminant discharges to the aquatic environment; and iii) derive water quality-based effluent limits which may be included in Certificates of Approval which are issued to regulate effluent discharges.