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Stormwater Management Plan
<br />INTRODUCTION AND PROJECT DESCRIPTION
<br />The May Day Idaho Mine Complex Stormwater Management Plan (SWMP) is prepared in accordance
<br />with the Colorado Department of Health and the Environment (CDPHE) and the Colorado Division of
<br />Reclamation Mining and Safety (CDRMS) criteria. The SWMP is intended to satisfy the requirements of
<br />both the CDPS General Permit for Stormwater Discharges Associated with Construction Activity (CGP)
<br />and the CDPS General Permit for Stormwater Discharges Associated with Metal Mining Operations
<br />(MMGP). The constructed BMPs outlined in the SWMP includes, but is not limited to: this narrative
<br />SWMP, Figures U-1, Little Deadwood Gulch Channel Design and Reclamation Plan, May Day No. 1
<br />Portal Repair, May Day No. 2 Portal Repair, May Day Idaho Mine Access and Roads Plan (collectively,
<br />the "Site Maps"), the Chief Deadwood Gulch Drainage Plan, the May Day No. 1 Work Plan, the May
<br />Day No. 2 Work Plan and the Access Road and Base of Operations Work Plan (Collectively the "Work
<br />Plans"), and the Construction Plans included within the Operator Section 112d-1 mining permit
<br />application, CN -01, File No. M-1981-185 (the "Mining Permit Application"). The operator will file a
<br />Notice of Intent for coverage under the CGP for its initial mine corrective action and construction
<br />activities, including but not limited to the construction activities identified in its Work Plans and
<br />Construction Plans, and will file a Notice of Intent for Coverage under the MMGP for the mining
<br />operations set forth in the Mining Plan filed with its Mining Permit Application. Following completion of
<br />its initial mine corrective action and construction activities, the operator will stabilize areas disturbed by
<br />these activities and file a Notice of Termination for its coverage under the CGP. Thereafter, the operator
<br />will continue to implement its SWMP to comply with the MMGP.
<br />The SWMP identifies potential sources that could contribute pollutants to waters of the State, and Best
<br />Management Practices (BMP)l that, when implemented, will reduce or eliminate possible surface water
<br />quality impacts. The plan is designed to be implemented after the permit to mine has been granted by the
<br />Colorado Department of Reclamation and Mine Safety (DRMS) and when site access is granted. The
<br />stormwater management plan for the site (Attachment U-3, Figure U-1) summarizes plan assumptions,
<br />approach to the design, BMPs, maps, drawings, drainage basin characteristics, culvert, and ditch design
<br />flow calculations for the site specific structures (Attachment U-3, Figure U-1).
<br />Site surface water flow was estimated using methods developed by Wilkes, S.G and E.C. King, 1975
<br />"Procedures for Determining Peak Flows in Colorado", published Soil Conservation Service, U.S.
<br />Department of Agriculture. Input parameters included NOAA 2, 10, 25 and 100 year 24 hour Type II
<br />storm rainfall estimates (U.S. Department of Commerce, 1961); curve number(s) (a runoff coefficients
<br />which considers slopes, vegetation types, and densities), management factors (Wilkes et at, 1975); and
<br />drainage area. The rainfall for the rainfall return rates are as follows: 2 year 24 hour return rainfall is 1.25
<br />inches, 10 year is 2.7 inches, 25 years is 3.5 inches, and 100 years is 3.7 inches. The National Resource
<br />Conservation Service (NRCS) runoff Curve Numbers (CN) used for steep (>16% slope), well drained,
<br />vegetated slopes is estimated to be 58 whereas the runoff Curve Numbers used for the La Plata River
<br />alluvium and terrace deposit and adjacent residential areas and small drainage areas is estimated to be 77.
<br />The drainage areas, surface and groundwater sampling points, points of interest and associated acreages
<br />are illustrated on Exhibit U, Attachment U-3, and Figure U-1. Soils types are presented in Exhibit I
<br />whereas Exhibit J describes site vegetative characteristics.
<br />BMPs include Rock Check Dams, Erosion Control Logs (Wattles), Silt Fences, Erosion Control Mats, Temporary Slope Drains, Vehicle
<br />Tracking Pad, Sediment Basin, Outlet Protection
<br />Sunrise Mining, LLC
<br />Revised August 9, 2017
<br />Page 4 of 30
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