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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division Rationale - Page 5. Permit No. CO-0036684. VI. TERMS AND CONDITIONS OF PERMIT b) Metals: (Cont.) of Iron, no metals limits will be applied. In the ease of Iron, the WQS-based limit must be compared to the federal limit, and the stricter of the two applied. A mass balance equation was used to determine the effluent concentrations for Total Recoverable Iron. The mass balance equation is: M3Q3 - M1Q1 M2 Q2 Where: Q1 Upstream low flow from Part III.B.1 Q2 Effluent flow from Part II.F Qg = Combined downstream flow (Q1 + Q2) Ml = Upstream background pollutant concentration M2 = Unknown; effluent pollutant concentration Mg = Water Quality Standard For this facility: Ql = 0.9 MGD (chronic) Q2 0.46 MGD Ml = 0.54 mg/1 Total Recoverable Iron M3 = 1.0 mg/1 Total Recoverable Iron Values for Ml vary depending on the background stream quality data. Although there was instream data for Iron in the Division data base, the latest data points are from 1973, and thus not very reliable. The permittee submitted instream data with the permit application. The sample result for Total Recoverable Iron, dated 9/20/88, was 0.54 mg/1. This result was used for the background concentration. Therefore, the calculated limit for Total Recoverable Iron is 1.9 mg/1. The actual calculations are in the permit file and are available upon request. The WQS-based limit is stricter than the federal limits (see Section VI.A.2, below), and so is applied to the effluent. Since the Iron limit is calculated using the effluent flow, a maximum effluent flow limit (0.46 MGD) must also be applied. Based on monitoring data provided by the permittee, no problems are expected in meeting the limits. 2. Applicable Federal Effluent Guidelines and Standards: The federal guidelines which apply to this type of facility are found under 40 CFR 434, the Coal Mining Point Category. The limitations which apply are: