Appendix F - Conversion Factors to Change Total Recoverable Metal Values to Dissolved Values and Equations For Hardness Dependent Metals
(a) Conversion Factors. Aquatic life values for the following metals are based on the dissolved amount of each substance. The recommended aquatic life value was calculated by using previous 304(a) aquatic life values expressed in terms of total recoverable metal and multiplying it by a conversion factor (CF). The conversion factors provided below are necessary to convert a metal value expressed as the total recoverable fraction in the water column to the dissolved fraction in the water column.
The toxicity of these metals also varies with hardness and the total recoverable value must be calculated based on the hardness (mg/L of CaCO3) prior to multiplying by the conversion factor (CF).
(i) The conversion factors for the following metals are constants:
| Metal | Acute Value | Chronic Value |
| Chromium (III) | 0.316 | 0.860 |
| Copper | 0.960 | 0.960 |
| Nickel | 0.998 | 0.997 |
| Silver | 0.85 | n/a |
| Zinc | 0.978 | 0.986 |
(ii) The conversion factors (CF) for cadmium and lead are not constant but vary with hardness (mg/L of CaCO3). Conversion factors can be calculated using the following equations, although when an ambient hardness of less than 25 mg/L (as CaCO3) is used to establish criteria for lead or cadmium, the conversion factor should not exceed one(a):
(A) Cadmium Acute: CF = 1.136672 - [(ln hardness)(0.041838)]
(B) Cadmium Chronic: CF = 1.101672 - [(ln hardness)(0.041838)]
(C) Lead Acute and Chronic: CF = 1.46203 - [(ln hardness)(0.145712)]
(b) Equations for Hardness Dependent Metals. Aquatic life values at various hardness(b) concentrations can be calculated using the formulas below. The formulas include the conversion factors to derive dissolved metal values:
| Parameter | Acute 1-Hour Average Concentration (µg/L) | Chronic 4-Day Average Concentration (µg/L) |
| Cadmium | e(1.0166[ln(hardness)] - 3.924)(CF) | e(0.7409[ln(hardness)] - 4.719)(CF) |
| Chromium (III) | e(0.8190[ln(hardness)] + 3.7256)(0.316) | e(0.8190[ln(hardness)] + 0.6848)(0.860) |
| Copper | e(0.9422[ln(hardness)] - 1.700)(0.960) | e(0.8545[ln(hardness)] - 1.702)(0.960) |
| Lead | e(1.273[ln(hardness)] - 1.460)(CF) | e(1.273[ln(hardness)] - 4.705)(CF) |
| Manganese | e(0.7693[ln(hardness)] + 4.4995) | e(0.5434[ln(hardness)] + 4.7850) |
| Nickel | e(0.8460[ln(hardness)] + 2.255)(0.998) | e(0.8460[ln(hardness)] + 0.0584)(0.997) |
| Silver | e(1.72[ln(hardness)] - 6.52)(0.85)(0.5)(c) | n/a |
| Zinc | e(0.8473[ln(hardness)] + 0.884)(0.978) | e(0.8473[ln(hardness)]+0.884)(0.986) |
(a)Based on Guidance on the Calculation of Hardness-Dependent Metals Criteria presented in: U.S. EPA. 2002. National Recommended Water Quality Criteria. EPA-822-R-02-47.
(b)Hardness as mg/L CaCO3. Hardness values used in these equations must be less than 400 mg/L. For hardness values greater than 400 mg/L, use 400.
(c)Criterion multiplied by 0.5 to be comparable with other acute values derived using an averaging period. Value does not need to be multiplied by 0.5 if criterion is to be used as an instantaneous maximum or end of pipe value, as the original criterion was derived using EPA's 1980 guidelines as a not to be exceeded instantaneous maximum.
Amended, Eff. 4/24/2018.