2012-02-28_REVISION - M1980244 (80)

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methodology (known as the SCS Method) and runoff "Curve Numbers" that are used for determining infiltration were adopted and accepted by participants in both the Amendment 8 and MLE1 Amendment 9 review process, and are also used by AMEC for design of storm water in the Squaw Gulch VLF area. Each hydrologic basin (illustrated and summarized on Drawing Sheets 1 and 2) includes sub - watershed areas with different types of surface conditions such as vegetation types or soil characteristics. Each surface condition can also be categorized as distinct land -uses, and is associated with a specific SCS Curve Number. The previously adopted Curve Number criteria are listed below: • Existing wooded areas, Curve Number 66 • Grassed existing areas and closure areas reclaimed with growth media and Revegetation methods, Curve Number 71. • Rock Fill Slopes on the faces of stockpiles before reclamation, Curve Number 50 • Roads and compacted stockpiles with limited permeability, Curve Number 90. The NRCS soil survey illustrates that there are essentially only three significant soil types in the areas around the perimeter of the mine that have not been previously disturbed. One common soil type is RgR —Rogert-Rock outcrop complex, 20 to 60 percent slopes —but the rock outcrops are not a significant factor in storm water management. The other two prevalent soil types are EB- Quander- Bushpark —very gravelly loams, 5 to 40 percent slopes and Sz —Seitz very gravelly loam, 20 to 50 percent slopes. Those soils have a similar stratigraphy but vary in their hydrologic properties. Both include a 12" to 14" surface horizon of gravelly loam overlaying an additional four feet of either cobbly or gravelly clay loam. The surface layer is relatively permeable for both while the underlying clay loams tend to be moderate to high permeability for the Quander - Bushpark soils and moderate to low permeability for the Seitz soils. Detention ponds will generally be constructed in higher permeability Quander - Bushpark soils in Grassy Valley, but in lower permeability Seitz soils on the Poverty Gulch, North Cresson side of the mine. The subsurface soils will affect the percolation of accumulated water from detention ponds into the diatreme and generally the exfiltration rate from the ponds. Client: CC &V Gold Mining Co. 2/2/12 Title: Storm Water Management, MLE2 Page 3