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COLORADO DEPARTMENT OF HEALTH, Water Qunlin~ Control Division Rationale -Page 14 Permit No. CO-0045675 Vl. TERMS AND CONDIT/ONS OF PERMIT A. Determination ojEffiuent Limitations (Cont.) 4. Discussion ojE(fluent Limitations (Cont.) c) Pollutants Limited by Water OualiN Standards -For the parameters shown in Table V/-3, the nuts balance equation s/town in VI-A.2. x~as used to calculate the allowable eJjluent limitations that would not cause the wafer quality standards to be violated. These limitations are shown as the values for M, in Table VI-3. The values for Q+, Qt and Q, for chronic and acute lunitanons, taken from section /ll. B.2. of this rationale, and used in the calculations jar water quality [united parameters are: Flow Chronic (30E3) Acule (/E3) Q, 1.8 cjs 1.3 cfs 00la = 0.89 cfs 001a = 0.89 cfs Qt 0016 = 0.557 cjs 0016 = 0.557 cjs 001 c = 0.245 cfs 001 c = 0.245 cfs 001 a = 2.69 cfs 001 a = 2.19 cfs Q~ OOl b = 2.36 cfs 0016 = 1.86 cfs 001 c = 2.045 cjs 00l c =1.545 cfs Values for Mr and M, vary depending on the background slrearn quality data (Mt) and tl+e applicable water quality star+dard (M~. The hardness-based TVS metal smndards were determined xdth a mean total hardness value of 64 rng/l as CaCO,. The M+ and M, values are shown in Table VI-3 along xdth the calculated eJJluent limitations (M~. In addition, Table VI-3 contains the antidegradadon-based water quality standard and limitation where this is applicable. Table V/-3 slows the calculations jar a jaciliN flow limitation jar outfall Q7lb of 250 gpm (or 0.36 MGD, or 0.557 cfs). Far the calculations jar the other tiered flow limits of 400 gpm (001 a) and 110 gpm (00/c), as well as jar the determination of baseline loads for this permit, refer to Appendix B. d) Anddegradadon Review.• Because segment 28 does not include a Use ProtecteA classification, an antidegradation review is required jar this permit since this is a new discharge. As is discussed in Section 31.8 of the Basic Standards and Methodolo ides for Sulface Water (SCCR 1002-31), antidegradation reviews are required to be performed for new dischargers to a reviewable stream segment that is not use protected. As indicated in 31.8(3)(c) of the Basic Standards, the initial step in an antidegradation review is a determination whether dte activity in question is likely to result in signiftcant degradation ojreviewable waters, xdth respect to adopted narrative or numeric smndards. The significance determination is bnsed on the chronic muneric standard and Jlow for the pollutant of concern except jar those pollutants which have only acute numeric standards in which case the acute standard and jiow will be used. The activity shall be considered not to result in significant degradation, as measured in the reviewable waters segment, ij.~ (i) 7heJlow rare or volume of a new or increased discharge uruier review is small enough that it xdll be diluted by 100 to 1 or more at lowJlow, as defined in section 31.9, by water in the stream: or (ii) 77+e new or increased loading jrorn the source under review is less than /0 percent of the existing total load to that segment for critical constituents (e.g. those for x~l+ich there are stream standards set aru/ wl+ich are present in the discharge); provided, that the cumulative impact of increased loadings from all sources shall not erceed 10 percent of the baseline total load established jar the segment (the baseline total load shall be determined at the tune of the j+rst proposed new or increased water quality impacts to the reviewable raters.); or (iii) The new or increased loading from the source under review rill consume, after mixing, less than IS percent ojthe available increment betx•een lowJlow pollutant concentrntions and the relevant standards, for critical constituents; or (iv) Tl+e activity will result in only temporary or short-[enn changes in water quality.