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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division Rationale - Page 14. Permit No. CO-0027154 VI. TERMS AND CONDITIONS OF PERMIT A. Effluent Limitations (continued) Settleable Solids Limitation - This limitation may apply to discharges resulting from precipitation less than or equal to the 10-year, 24-hour event for the facility. This limitation applies to active and post mining areas. See the next section for an explanation of the exemption procedure. Alternate Limitation Burden of Proof Requirements - In conformance with 40 CFR 434.63, the permittee has the burden of proof when requesting relief from total suspended solids (TSS), total iron and/or settleable solids limitations. Relief shall be granted only when necessary and shall not be granted when the permittee has control over the discharge. The permittee should endeavor to meet the primary limitations whenever possible. For rainfall, to waive TSS and total iron limitations, it is necessary to prove that discharge occurred within 48 hours after measurable precipitation has stopped. In addition, to waive settleable solids limitations, it is necessary to prove that discharge occurred within 48 hours after precipitation greater than the 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. In addition, to waive settleable solids limitations, it is necessary to prove that discharge occurred within 48 hours after pond inflow volume greater than the 10-year, 24-hour event has stopped. The Division shall determine the adequacy of proof. As part of this determination, the Division shall evaluate whether the permittee could have controlled the discharge in such a manner that primary limitations could have been met. All manual dewatering must meet TSS and total iron limitations. 2. Water Ouality Standard Based Effluent Limitations: A mass balance equation was used to calculate potential effluent limitations for outfalls 001, 005 and 007 for total recoverable iron, total recoverable manganese, potentially dissolved copper and potentially dissolved silver. Based on a previous request by the permittee to establish a waste load allocation-based total recoverable manganese limitation, an "administrative" outfall - number 019, equivalent to the combined discharges from outfalls 001, 005 and 007, was established to which the limitation was applied. This administrative outfall will continue for manganese. Because copper and silver are based on TVS - the table value standard equation specified in Table III of the Basic Standards and Methodoloeies for Surface Water, it was necessary to determine instream hardness. However, without the discharges from this facility the low flow in Foidel Creek is zezo and no instream hardness data are available. In lieu of instream data, effluent hardness data provided by the permittee were utilized to calculate an adjusted mean hardness value. This adjusted mean hardness value was used 1n the TVS equation to generate hardness-based standards for copper and silver.