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COLORADO DEPARTM£NZ OF PUBL HEALTH AND ENVIRONMENT, Water Qa i Cantro! Division <br />Rationale -Page 7, Permit No. CO-0042161 <br />2. Calculation oFWater Oualirv Based Effluent Limitations <br />• Amass balance equation was used to determine the effluent concerurations that would not violate the allowable <br />in-stream concentrations de, fined by the waver quality stardards (except in the cast of pH, where tlu limitations are <br />set directly from stream standards or effluent regulations withoru using a mass balance approach). Tlu mass <br />balance equation is: <br />Mt = 'NtQt - MtQt <br />Q= <br />Where: <br />Qr = Upstream low flow (IE3 or 30E3J from Part 1/LB.2 <br />Qt =Average daily effluem flaw (design capacity) <br />Qs = Combined downstream flow (Qt + QrJ <br />Mt = Upstream 6adcground pollutant mncemration from 1lLB.1 <br />M7 = Unknown; Maximrtm allowable effluent pollutant oonoemration calculated using mass balance equation <br />Ms =Maximum dawnstrtam allowable pollutant mncemration (stream standard) <br />Because of the mathematical relationship between flow, pollutant mnamration and pollutant mars, toncemration <br />limitations calculated using this method implicitly limit instream pollutant mass to the maximum allowable lttrl. <br />Also, only flow and rnncemration limitations need to be speciflcd in the permit. Mass limitations are not required. <br />A summary of the mass balarttx calculations rf any) is shown and discussed in VI.A.3. <br />3. Discussion of Efjluem Limitations <br />The basis for the efjluem (imitations for outjall 002 is uncfianged from the previous permit. Please see previous <br />rationale for the discussion. <br />• aJ Regulations for Elliuent Limitations: The Regulations for Effluent Limitations (!0. L.OJ, apply to the <br />rnnventional pollutanu. For outfall AOIJ the limitation for Oil and Grease and for orufaU 002 the limitations for <br />BODs and TSS are based on this regulation. <br />b) Applicable Federal E1Fluem Guidelines and Standards: The federal guidelints whidr apply to this type of fanfiry <br />arc found under 40 CFR 434, titled Caal Mininr E/Ruem Guidelines. The lunitationr whiaft apply are: <br />.- <br />Parameter . ,. . <br />30:Dar'Av¢Cortantmtloii .:.::.::.:::..:::.s:::<:::::::::::::.:AailY' as: oneen! a::.::. <br />pH, s. u. (minimum -maximum) -~.0 - 9.0- <br />Tora! Suspended Solids, mg/f 33 70 <br />Total Iron, mg/1 (NSPS*J 3.0 6.0 <br />• New Source Pnfomww~e 5,.._.r....r. - appGer ra those porde eaneouered after Slt9Bt <br />cJ Pollutants Limited by Water Oualirv Standards: Fos the parameters shown in Table Vhf, tlu mass balance <br />equation shown in V/-A.2. was used ro rnlculate the allowable effluent limitations that wordd not muse the <br />water quality standards to bt violated. 7ltese limitations art shown as [he values for M= The values far Qr, Q= <br />and Qt for chronic and acute limitations, taken from section III.B.2. of this rationale, and used in the <br />calculations for water quality limited parameters follow. <br />u <br />