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COLORADO DEPARTMENT OF PUBL/C HEALTH AND ENVIRONMENT, Water Quality Corstrol Division Rationale -Page 5, Permit No. CO-0044776 Table VI-2 -Federal Standards (40 CFR 434) Parameter Limitations 30 Day Avg Daily Max pH, s. u. (minimum -maximum) ---6.0 - 9.0--- Total Suspended Solids, mg/P* 35 70 Total /ton, mg/P* (NSPS**) 3.0 6.0 Settleable Solids, mP/P* N/A 0.5 * These parameters subject to Burden of Proof Requirements (available for surface runoff only -see next section) ** New Source Performance Standards -applies to those facilities constructed after 5/4/84 Alternate Limitation Burden of Proof Requirements: /n conformance with 40 CFR 434.63, the permittee has the burden of proof when requesting relief from total suspended solids (ISS), total iron and/or settleable solids limitations, as appropriate, for discharges of surface runoff. Relief shall be granted only when necessary and shall not be granted when rite permittee has control over the discharge. The permittee should endeavor to meet the primary limitations whenever possible. Relief is not available for mine drainage alone, nor when the majority of a discharge consists of mine drainage. For rainfall, to waive TSS and total iron limitations, it is necessary to prove that discharge occurred within 48 hours rafter measurable precipitation has stopped. !n addition, to waive settleable solids [imitations, it is necessary to prove that discharge occurred within 48 hours after precipitation greater than rite 10-year, 24-hour event has stopped. For snowmelt, to waive TSS and total iron limitations, it is necessary to prove that discharge occurred within 48 hours after pond inflow has stopped. /n addition, to waive settleable solids limitations, it is necessary to prove that discharge occurred within 48 hours after pond inflow volume greater titan the 10-year, 24-hour even! has stopped. Should a precipitation event or snowmei[ occur, the permittee shall submit ndeguate proof in order for an exe»tprion to be claimed. Weather station data sitou[d be submitted where appropriate. Documentation tltar rite treatment facilities were properly operated and maintained prior to and during the storm event must 6e submitted wit/t any request for relief. Additionally, jot outfall 002. t/te permir[ee must demons[rate that ti:e majorit~of the discharge for which the exemption is claimed consists of surface rttno(f Said proof shall be submitted as an attachment to the Discharge Monitoring Report (DMR) jot the appropriate period. Tire Division shall determine the adequacy of proof. As part of this determination, the Division shall evaluate whether the pernrittee could have controlled the discharge in such a manner that primary limitations could have been met, whether proper sediment storage levels were maintained and rite ponds had su~cient water and sediment capacity for the storm event plus other relevant factors. A[l manual pond dewatering must meet TSS and total iron limitations unless previous approi•al iras been granted far ponds that stave no other method of dewnterirsg. For ponds with automatic dewatering structures, only discharges through these structures are eligible for the exemption. c) Calculation of Water Quality Based Effluent Linrltations: Because outfa[[ 002 may contain ntirie drainage, a mass balance equation ryas used to deternine the effluent concentrations that would not violate rite allowable in-strenm concentrations defined by the water gttaliry standards for the Nortft Fork of rite Gunnison River. The mass balance equation is: M3Q3- MrQr Mz = Qr