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<br />specifically, as described in footnote (3) to Table III of the Commission's regulations <br />(§3.1.16(1)), <br />"Hardness values to be used in equations are in mgll as calcium carbonate. <br />The hardness values used in calculating the appropriate metal standard should <br />be based on the lower 95 percent confidence limit of the mean hardness value <br />at the periodic low flow criteria as determined from a regression analysis of <br />site-specific data. Where insufficient site-specific data exist to define the mean <br />hardness value at the periodic low flow criteria, representative regional data <br />shall be used to perform the regression analysis. Where a regression analysis <br />is not appropriate, asite-specific method should be used. In calculating a <br />hardness value, regression analyses should not be extrapolated past the point <br />that data exist." <br />The Water Quality Control Division has determined for the recent draft of a discharge permit <br />for Arequa Gulch that a mean hardness value is appropriate. As may be determined from the <br />preceding Table 3, the mean hardness ranges from 362 to 768 mg/1 at AG-1 and AG-1.5, <br />and both values must be recognized. The Division used a value of 400 mg/1 for <br />computations in a recent draft discharge permit. In fact, the use of higher mean <br />concentrations are clearly justified (i.e., 758 mg/1). In any event, use of 400 mg/I provides <br />the general site-specific water-quality criteria tabulated in the next Table 4. As is evidenced <br />in the next Table 4, Dr. Posey used a higher hardness value than the Water Quality Control <br />Division recently used and therefore compared the humidity cell data against a higher number <br />than might the Water Quality Control Division. Dr. Posey appears to have used a hardness <br />value of about 652 mg/l for the computations. This value approximates the average values <br />but does not necessarily approximate the low flow values. The result is, under the Division's <br />computations, a higher probability of exceeding the criteria due to ambient conditions. <br />CC&V has explained to the Water Quality Control Division why the hardness value should <br />be higher than 400 mg/1. Dr. Posey's selection more closely approximates ambient <br />concentrations but still does not reflect the prevailing methodology for computing ambient <br />standards. <br />(The following Table 4 lists the standards applied in the Water Quality Control Division's <br />NPDFS Permit Rationale along with CC&V's listing of the standards for Cripple Creek and <br />the exact description of the standards for each segment. A total hardness of 400 mg/1 was <br />used for the computations by the Division.) <br />11 <br />