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COLORADO DEPARTMENT OF PUBLIC HEALTH AND THE ENVIRONMEM; Water Quality Control Division Rationale -Page 3, Permit No. CO-0045225 Division has agreed to support this request. The Division has, therefore, concluded that EMPs are appropriate for the stormwater discharges at discharge points 002A, 003A, and 004A. The bases for the Division's conclusion are summarized below. First, all ponds and contributing areas included in this permi[ are within a volcanic diarreme which is made up of extremely fractured and highly permeable rock. And, the majority of disturbed mine areas contributing runoff to the ponds are made up ojhigltly rubbelized rock has a high permeability and large void space. Therefore, not only will the initial infiltration of precipitation be relatively high, but there will likely nor be any seepage from the lower extent oJthe disturbed areas and any water that initially infiltrates into the land surface is expected to cominue to travel downward lino the diatreme. The absence of visible seeps supports this conclusion. the only water reaching point 002A, 003A, and 004A would be storm-related surface runoff which has had a short comacl lime with the material in the contributing drainage areas. With this short contac[ time, these materials are believed !o be too low in mineral content [o leach significant concentrations of pollutants. Because only relatively immediate storm-related surface runoff flows are expected, none ojthe releases authorized by this ptrntit, neither overflow nor regenerative, are process water", as defined in the Division's WQ Policy 17, "Definition of Process Winer and stormwater at Non-Coal Mining Sites'. Indeed, the limited amount ojdata for standing wafer located below the Ironclad Overburden Area shows the water quality to be much better than the federal ELG effluent limits, indicating that the water has not been in contact with waste rock for a significant period. However, due to the size of the mine features a»d their proximity to waters of the state, the Division believes that Enhanced Management Practices ("EMPs) are warramed. For example, larger than normal se[t[ing ponds are being required in this permit. The Division has concluded that these EMPs will result in the control of pollutant discharges to a level equivalent to best available technology economically achievable ('BAT") and any more stringent controls necessary ro meet water quality standards. The volume of eadt pond that will capture storm-related surface runoff at al[ ojthe points included in this permit is greater than the respective volume of runoff resulting from the 10-yrJ24-hr precipitation event. In some cases they are several times larger. Considering that the settling ponds sit within the diarreme, and there is likely to 6e a high degree of infiltration, it may be possible to contain at least nvo successive 10-yr/24-hr events. Additional capacity may also be provided through regenerative releases from the lower pond at point 003A. As a result, it is reasonable to expect that the ability to capture the volume of runoff resulting from a 10-yr/24-hr precipitation event will be cotuinuously maintained. Based upon these co»siderarions, any overflow release from these points would qualify for storm exemptions grained in Part 440 of 40 CFR, if the associated ELG's actually applied to such releases. An additional consideration related to the size of the ponds, is that any releases -either as a result of overflow at 002A, 003A or 004A, or regeneration at 003A, should occur less often than the 1-in-3 year frequency implicitly allowed by the low flow design criteria normally used in the determination of water quality based effluent limitations. And, it is likely that during such overflow or regenerative releases, irtstream flow rates would act to mitigate the impact of arty releases. Also, because any discharge from the crusher area through Poim 002A must travel through a very wide and lengthy diversion ditch, it will likely not reach Arequa Gulch except when there has been an unusually large precipitation event. /n such a case, the dilution of the discharge by waters flowing from unaffected areas would 6e very great and water quality standards would not be expected to be violated. For point 004A, the area of the mine haul road that contributes flow to the pond is small in comparison to the total area (which is predominately undisturbed), and there will be dilution equally as large as that provided for outfall 002A.