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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division <br />Rationale - Page 7. PERMIT NUMBER: COG-850000 <br />• <br />Pont Mining <br />As with active mining, pH and oil and grease limitations apply to aII <br />discharges regardless of precipitation conditions. But, for post~ining <br />discharges, the settleable sollds limitation applies unless exempted as <br />described above. Exemption is only available when discharge is caused by <br />precipitation greater than the 10-year, 24-hour event (or equivalent snowmelt <br />volume). <br />Example 2: The post-mining area nerved by pond and outfall 004 receives <br />snowmelt volume during one 24-hour period (as measured by continuous <br />influent recorder) greater than the 10-year, 24-hour equivalent volume. <br />The permittee collects samples while inflow !a occurring to the pond. The <br />permittee submits the results along with flow totalizer data and requests <br />an exemption from the settleable solids limitation. IIpon acceptance of <br />the proof, the division will determine compliance with the pH and oil and <br />grease limitations only. <br />Snowmelt greater than 10-year 24-hour event <br />Section 40 CFR 434.63(d)(2) states that the exemption from TSS, total iron and <br />eett].eable solids limitations is available only for discharges "caused by <br />precipitation within any 24-hour period greater than the 10-year 24-hour <br />precipitation event (or snowmelt of equivalent volume)." Thua for snowmelt, • <br />pond inflow volume must be greater than the 10-year, 24-hour event equivalent <br />volume regardless of effluent volume). Thus, the permittee must be able to <br />submit documentation showing that the total volume over a 24-hour period into a <br />pond is greater than the 10-year, 24-hour event equivalent volume. Continuous <br />inflow volume monitoring is typically required for this demonstration. The <br />division allows this exemption for an additional 48 hours to allow the pond to <br />stabilize. <br />Other WQS-Based Parameters <br />Surface runoff discharges from coal mining facilities are intermittent and <br />normally occur during periods when receiving waters are well above low flow. <br />The Colorado Mined Land Reclamation Division and the State Engineers Office <br />- typically require ponds to be dewatered within 24 to 36 hours after a storm <br />event. For these reasons no WQS-based limitations other than pH are deemed <br />necessary at this time to protect inatream beneficial uses. <br />Total Suspended Solids <br />The State Effluent Limitation Regulations dictate that where Federal Effluent <br />Guidelines exist they shall take precedence over SRS. This is the reason for <br />selection of BPT-based TSS limitations. <br />• <br />