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Water: Water Quality Standards Academy

Deriving the CCC: Calculation of the FCV

Aquatic Life Criteria

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A three-step calculation process results in a Final Chronic Value (FCV), which is then used to derive the Criterion Continuous Concentration. These FCV calculation steps are described briefly below.

Step 1. Calculate Acute-Chronic Ratios (ACRs).

ACRs are calculated for each set of parallel acute and chronic tests by dividing the acute value by the chronic value. That is, ACR = AV ÷ CV.  At least three species with a specified taxonomic diversity must be addressed by studies with parallel testing to calculate a valid final ratio.

Step 2. Develop a Final Acute-Chronic Ratio (FACR).

The geometric mean of the ACRs is then determined. That is, the ACRs are averaged to arrive at the FACR.

Step 3. Calculate the Final Chronic Value (FCV).

The FCV is then calculated from the Final Acute Value (FAV)—which was calculated in deriving the CMC—by dividing it by the FACR. That is, FCV = FAV ÷ FACR.

Thus, if the side-by-side tests show that, on average, chronic effects occur at one-half the concentration of acute effects, the chronic criterion will be one-half of the FAV, which is the same as the acute criterion.

Key Point. A chronic criterion is applied as a limit on the 4-day average concentration in the environment. Both the acute and chronic criteria are values that are not to be exceeded more than once in 3 years. In other words, the criteria specify a magnitude, duration, and frequency to be met in order to provide protection of aquatic life.

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

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