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