2015-04-02_REVISION - C1981014 (4)

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Table III — Footnotes (1) Metals for aquatic life use are stated as dissolved unless otherwise specified. Where the hardness -based equations in Table III are applied as table value water quality standards for individual water segments, those equations define the applicable numerical standards. As an aid to persons using this regulation, Table IV provides illustrative examples of approximate metals values associated with a range of hardness levels. This table is provided for informational purposes only. (2) Metals for agricultural and domestic uses are stated as total recoverable unless otherwise specified. (3) Hardness values to be used in equations are in mg /I as calcium carbonate and shall be no greater than 400 mg /I. The exception is for Al, where the upper cap on calculations is a hardness of 220 mg /I. For permit effluent limit calculations, the hardness values used in calculating the appropriate metal standard should be based on the lower 95 per cent confidence limit of the mean hardness value at the periodic low flow criteria as determined from a regression analysis of site - specific data. Where insufficient site - specific data exists to define the mean hardness value at the periodic low flow criteria, representative regional data shall be used to perform the regression analysis. Where a regression analysis is not possible, a site - specific method should be used, e.g., where hardness data exists without paired flow data, the mean of the hardness during the low flow season established in the permit shall be used. In calculating a hardness value, regression analyses should not be extrapolated past the point that data exist. For determination of standards attainment, where paired metal /hardness data is available, attainment will be determined for individual sampling events. Where paired data is not available, the mean hardness will be used. (4) Both acute and chronic numbers adopted as stream standards are levels not to be exceeded more than once every three years on the average. (5) Unless the stability of the chromium valence state in receiving waters can be clearly demonstrated, the standard for chromium should be in terms of chromium VI. In no case can the sum of the instream levels of Hexavalent and Trivalent Chromium exceed the water supply standard of 50ug /1 total chromium in those waters classified for domestic water use. (6) FRV means Final Residue Value and should be expressed as "Total" because many forms of mercury are readily converted to toxic forms under natural conditions. The FRV value of 0.01 ug /liter is the maximum allowed concentration of total mercury in the water that will present bioconcentration or bioaccumulation of methylmercury in edible fish tissue at the U.S. Food and Drug Administration's (FDA) action level of 1 ppm. The FDA action level is intended to protect the average consumer of commercial fish; it is not stratified for sensitive populations who may regularly eat fish. A 1990 health risk assessment conducted by the Colorado Department of Public Health and Environment indicates that when sensitive subpopulations are considered, methylmercury levels, in sport- caught fish as much as one -fifth lower (0.2 ppm) than the FDA level may pose a health risk. In waters supporting populations of fish or shellfish with a potential for human consumption, the Commission can adopt the FRV as the stream standard to be applied as a 30 -day average. Alternatively, the Commission can adopt site- specific ambient based standards for mercury in accordance with section 31.7(1)(b)(ii) and (iii). When this option is selected by a proponent for a particular segment, information must be presented that (1) ambient water concentrations of total 58