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COLORADO DEP.iRTiL1ENT OF PURL HF~ILTH AND EN'/IRONMENT, Watrr Qu. ~ Control Division <br />Rationale -Page 7, Permit iVo. CO-0042161 <br />2. Calculation oFWnter Oualirv Bared Eflluenr Limitations <br />' ~ Amass balance equation was used to determine the ejJluem cancemratians that would not violate the allowable <br />in-stream concentrations defined by the water quality standards (except in the case of pN, whoe the Limitations are <br />set directly from stream standards or effluent regulations without using a mass balance approach J. The mars <br />balance equation is: <br />Mt - MtQt - MtQr <br />Qt <br />Where: <br />Qr = Upstream low flow (IE3 or 30E3J from Part 1/LB.2 <br />Q7 =Average daily ejfluem flow (design capacity) <br />Qt =Combined downstream Jlow (Qt + Q1J <br />MJ = Upstream background pollutant concentration from IILB.I <br />M= = Unbwwn; Maximum allowable e_Qluent pollutant concentration calculated using mass balance equation <br />Mt = Maximum downstream allowable pollutant concentration (stream standard) <br />Because of the mathematical relationship between flow, pollutant ocncemration and pollutant mass, cancenration <br />limitations calculated using Ibis method implicitly limit instream pollutant mass to tht maximum allowab4 kveL <br />Also, only flow and concentration limitations need to bt specified in the permit. Mass limitations art not required. <br />A summary of the mars balance calculations rf any) is shown and discussed in VLA.3. <br />3. Discussion of Effluent Limitations <br />The basis jot the effluent limitations for ourfall 002 is unchanged from the previous permit. Please see previous <br />rationale jot the discussion. <br />aJ Reeulations for Effluent Limitations: The Regulations jot Effluem Limitations (10.1.0), apply to the <br />conventional pollutnnu. For otujall OOlI the limitation jot Oil and Grease and jot ourjall 002 t/u limitations jot <br />BODs and 7SS art based on this regulation. <br />bJ Aonlicable Federal Effluent Guidelines and Standards: The federal guidelirtet which apply to this tYPt offaciliry <br />are found under 40 CFR 434, titled Coal Minine Effluent Guidelines. The limitations which apply are: <br />Table YI-3 - Fedeml Standards 140 CFR 434) <br />• New Source Performmree Stmdutdr -applies to those pads eolvoructed after Sl19B1 <br />cJ Pollutants Limited by Water Oualirv Standards: For the parameters shown in Table VI-4, the mass balance <br />equation shown in V/-A.2. was used to calculate the allowable efJluem limitations that would not cause the <br />water qualirv standards to be violated. There limittuioru are shown as the values for Mr The values for Qr, Qt <br />and Qt for chronic and acute limitations, taken from section 11LB.2. of this rationale, and used in the <br />calculations for water quality limited parameters follow. <br />