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Water: Water Quality Standards Academy

Deriving the CMC: Factoring in Water Characteristics

Aquatic Life Criteria

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Basic Course: Supplemental Topics

Water hardness (the amount of calcium and magnesium in the water) is known to moderate the toxicity of some metals. In particular, the toxicity of cadmium (Cd), chromium III (Cr III), copper (Cu), lead (Pb), nickel (Ni), silver (Ag), and zinc (Zn) decreases as water hardness increases.

For these materials, the relationship of toxicity to hardness is calculated. Then, the CMC is adjusted accordingly. The same approach is applied for characteristics associated with other materials as appropriate.

EPA has been working cooperatively with various organizations to develop a model called the biotic ligand model, or BLM. The BLM predicts acute toxicity based on site-specific water quality parameters like pH, hardness, and dissolved organic carbon (DOC). This model accounts for the major variables that effect toxicity rather than only accounting for hardness.

For informational purposes only–Not official statements of EPA policy.

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