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Rutionnle - Pal;c 7.~ .'L ?lI: ..::-i l: 4: i?: .,~. .,,Olio„ ~. Post Minine As with active mining, pH and oil and grease limitations apply to all discharges regardless of precipitation conditions. But, for post-mining discharges, the settleable solids limitation applies unless exempted as descriUed above. Exemption is only available when discharge is caused by precipitation greater Chan the LO-year, 24-hour event (or equivalent snowmelt volume). Example 2: The post-mining area served by pond and outfall 004 receives snowmelt volume during one 24-hour period (as measured by continuous influent recorder) greater thou the 10-yens, 24•hour equivalent volume. The permitcee collects samplers wlii.l.e inClo~~i is occurring co the pond- 1'he permitCee submits the results along with flow totalizer data and requests an exemption from the settleable solids l.imitacivn. .Upon acceptance of the proof, the division '::i11 determine. compliance with the pH and oil and grease limitations only. snowmelt erecter than 10-year 24-hour event Section 40 CFR 434.G3(d)(2) states that the exemption from '155, coral iron and settleable solids limitations is available only for discharges "caused by precipitation within any 24-hour period greater tl~ar< the ZO-year 24-hour precipitation event (or snotairelt of equivalent volume)." Thus Eor snowmelt, pond inflow volume must be greater Chan the 10-year, 24-hour event equivalent volume (regardless of effluent volume). Thus, the permitcee must be able to submit documentation showing chat the coral volume over a 24-hour period into a pond is greater than the LO-year. 24-hour event equivalent volume. Continuous inflow volume monitoring is typically required for this demonstration. The division allows this exemption for an addition:,l 48 hours co allow the pond to stabilize. Other WOS-Based Parameters Surface runoff discharges from coal mining facilities are intermittent and normally occur during periods when receiving t•~a[ers are well above Low flow. The Colorado Mined Land Reclamation Division and the State Engineers Office typically require ponds co be dewatered within 24 co 36 hours after a storm event. For these reasons no WQS-based Limitations other than pH are deemed necessary at this time to protect ins tream beneficial uses- Total Suspended Solids The State Effluent Limitation Regulations dictate chat where Federal Effluent Guidelines exist they shall take precedence over SES. 1'h is is the reason for selection of BPT-based TSS limitations.