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Table 14 (cant.): Metal parameters for designated uses, Colorado (in pg/l) (~) Metals for aquatic life use BrB stated as dissolved unless otherwise specified. (2) Metals for agricultural and domestic usas are stated as total recoverable unless otherwise specified. (3) Hardness values to be used in equations are in mg/l as :caloium carbonate. The hardness values used in caloulating the appropriate metal standard should be based on the lower 95 percent oonfidence limit of the mean hardness value at the periodic low flow oriteria 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 r~gression analysis. Where a regressIon analysis is not appropriate, a site-speoifio method should be used. In oalculating a hardness value, regression analyses should not be extrapolated past the point that data exist. ' (4) Both aoute and ohronio numbers adopted as stream st~ndards are levels not to be exoeeded more than onoe every three years on the average. (5) Unless the stability of the ohromium valenoe state In reoeiving waters can be clearly demonstrated, the standard for chromium should be in terms of chromium VI.: In no oase can the sum of the instream levels of Hexavalent and Trivalent Chromium exceed the water supply standard of 50 pg/l total chromium In those waters classified for domestic water use. (6) FRV means Final Residue Value and should be express~d as "Total" beoause many forms of mercury are readily converted to toxic forms under natural conditions. The FRY value of 0.01 pg/l is the maximum allowed ooncentration of total mercury in the water that will preserh biooonoentratlon or bioaoournulation of methylmercury in edible fish tissue at the U.S. Food and Drug Administration's (FDA) action level of 1 ppm. The FDA aotion 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 oonduoted by the Colorado Department of Health indioates that when sensitive subpopulations are oonsidered, methylmercury levels, in sport-caught fish as much as one-fifth lower (0.2 ppm) t~an the FDA level may pose a health risk. In waters supporting populations of fish or shellfish with a :potential for human oonsumption, the Commission can a<;topt the FRV as the stream standard to be applied as a 3~-day average. Alternatively. the Commission can adopt site-specific ambient based standards for mercury in accordance with Section 3.1.7(1 Hb)(ii) and (iii). When this option is selected by a proponent for a particular segment, iinformation must be presented that -(1) ambient water concentrations of total mercury are detectable and exceed lthe FRV, (2) that there are deteotable levels of mercury in: the proponent's disoharge and that are contributing to the ambient levels and (3) that conoentratlons of methylmeroury in the fish exposed to these ambient levels _do not exceed the maximum levels suggested in the CDH Health Advisory for sensitive populations of humans. Alternatively or in addition the proponent may submit information showing that human consumption of fish from,the particular segment is not ocourring at a level whioh poses a risk to the general population andlor sensitive populations. Taken from Colorado Water Quality Control Commission 1 ~93a 50'