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COLORADO DEPAR77NENT OF PUBL/C HEALTH AND ENVIRONMEM; Water Qua[iry Comrol Division <br />Rationale -Page 8, Permit No. CO-0027746 <br />Where.' <br />Q, = Upstream low flow (30E3) from Pan IH.B.2. (412 cfs) <br />QZ = 30-day average effluent flow, combined for a!! jour outfa[Is (0.86 cfs) <br />Q, =Combined downstream flow (Q, + Q~ (412.86 cfs) <br />M, = Upstream background pollutant concentration from Table III-1 (1.6 mg/Q) <br />Mt = Unbtown; Maximum allowable effluent pollutant concemration calculated using mass balance equation <br />M, =Maximum downstream allowable pollutam concentration (stream standard = 2.0 mg/Q) <br />The term M,Q, represents the Total Maximum Daily Load (I'MDL) of a pollutant that can be carried by the receiving <br />dream without violating the stream standards. The term M,Q, represents the pollutam load that already exists <br />instream above the discharge point in question. The difference between the two loads, which is M,Q, - M,Q, <br />represents the loading that can be allocated to the discharge. <br />Ir may be appropriate in certain situations to perform a more detailed analysis that would identify, quantify and <br />develop control strategies for addirional individual pollutam sources that might exist above and below the permittee's <br />discharge points. /n this case, it was determined that limitations for the permittee's discharge points can be <br />calculated independently from any other pollutant source because there are no known point source dischargers in the <br />vicinity discharging to these receiving waters that have iron in their discharge. And, this is the only parameter of <br />concern for which this analysis is necessary. Thus, appropriate limitations were calculated using the mass balance <br />equation and associated factors described herein. <br />This rationale and accompanying permit serve as a Total Maximum Daily Load (fMDL) under Section 303(d) of the <br />Clean Water Act, and this TMDL will be submitted to the US EPA Region VI/I for review and approval. The <br />parameters in Table VI-4 have been evaluated for this 7MDL. <br />Because of the mathematical relationship between flow, pollutant concemration and pollutant mars, concentration <br />limitations calculated using this method implicitly limit instream pallutam mass to the maximum allowable level. Also, <br />only flow and concentration limitations need to be specified in the permit. Mars limitarions are not required. <br />The values for Q„ taken from section I/1.B.2. of this ratonale; Q=,, taken from secrion II.G.; and Q, for chronic <br />limitations and used in the calculations for water quality limited parameters follow: <br />( ) : .... <br />Flow ! ; :.::.' Chronic 30E3 <br />Q, 412 cfs <br />Qt 0.86 cfs <br />Q, 412.86 cfs <br />Values jot M, and M, vary depending on the background stream quality data (M,) and the applicable water quality <br />standard (M~. These values are also shown in Table VI-4 along with the calculated effluent limitations (M~. <br />Table V!~ -Summary ojMass Balance Calculations jot OutfaBs 001, 002 and 004 <br />.. . <br />Parameter::; :.. .. ,Stream Standard; (M3), EjfltteruConientratwe, (M21, . <br />Acute/Chronic Acute/Chronii <br />Because this concemration is above the federal effluent guideline (FEG) based effiuem limitations for total iron of <br />3.5/7.0 mg/Q, the FEG limitations shall apply (see section V/.A.3.6)). <br />3. Di.cc,%csinn of Ellluea~ Limitations: <br />a) j~ulatians for uem Limitations: The Regulations for EJfhtent Limitations (S CCR 1002-61), apply to the <br />conventional pollutants. For this jaciliry the limitations for Oil and Grease are based on this regulation. <br />