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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division • Rationale - Page 5. Permit No. CO-0000213 EFFLUENT LIMITATIONS: Table 5 Final Limitations - Post-Mining (d is. pts. 002, 003, 004 and 005) Parameter Limitation Basis Flow (MGD) Report b/ Monitoring Only pH (s.u.) 6.5-9.0 a/ WQS 031 and Grease (mg/1) 10 b/ SES Settleable Solids (ml/1) 0.5 b/c/ BAT/NSPS Total Dissolved Solids (mg/1) Report b~/ Monitoring Only a/ Daily minimum - Daily maximum b/ Daily maximum c/ Applicability of limit may be based upon precipitation exemption, subject to burden of proof requirements. d/ Monitoring for electrical conductivity (EC) may be used instead of TDS, based on certain conditions. See footnote g/ in the permit - Part I.C. DISCUSSION OF PERMIT LIMITATIONS AND CONDITIONS: A. Active Mining (Discharge Points 001 and 006) • Limitations for pH and oil and grease apply to all discharges, regardless of precipitation conditions. Additionally, TSS and total iron limitations apply unless exempted ae described herein. The permittee may submit documentation proving that discharge was precipitation induced. Upon acceptance by the Division of said proof, the settleable solids limitation will apply in place of the TSS and total iron limitations. Similarly, upon acceptance of proof that discharge was caused by precipitation greater than the 10-year, 24-hour event (or equivalent snowmelt volume), compliance with TSS, total iron and settleable solids limitations will be exempted. This means such a discharge would be subject [o pH and oil and grease limitations only. The Division allows these exemptions for a period up to 48 hours after measurable rainfall has stopped or, for snowmelt, within 48 hours after inflow to the sedimentation pond has ceased. After such time, TSS and total iron limitations shall be met. B. Post Mining (Discharge Points 002, 003, 004 and 005) Since active mining has ceased at these discharge points and surface areas have been reclaimed, these discharges are subject to limitations other than those specified for active mining. These post-mining limitations shall remain in effect until the Division is notified by the MIRD of bond release for the specific site. As with active mining, pH and o11 and grease limitations apply to all discharges regardless of precipitation conditions. But, for post~ining . discharges, the settleable solids limitation applies unless ezemp[ed. Exemption is only available when the discharge is caused by precipitation greater than the 10-year, 24-hour event (or equivalent snowmelt volume).