2017-02-03_REVISION - C1980004 (6)

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C. Sedimentation Ponds There is one sedimentation pond at the mine, located between the portal bench area and the office facilities area, on the south side of the haul road. The pond treats runoff from the portal bench disturbed area, the upper segment of the haul road surface and cut slopes, and a small supply storage area. The pond is somewhat oversized (storage volume adequate to contain the sediment and runoff from multiple 10 year, 24 hour precipitation events below the level of the gated 8" primary discharge pipe). In addition to the primary discharge pipe, the pond has an open channel emergency spillway designed for the 25 -year, 24-hour precipitation event. The sedimentation pond is discussed in Section 2.4.4 of the text, and designs are depicted on Figure 2.2- 3. The sediment pond was filled in with sediment by a 100 year magnitude storm event in August 2001 that had buried and overwhelmed one of the primary upland diversion culverts. In association with excavation of sediment, the inside slopes of the sediment pond were reduced to approximately 2H:1V. The re -constructed pond was properly certified by a professional engineer (see "Impoundment Yearly Inspection" of March 29, 2002). The pond embankment failed in 2004, apparently as result of leakage from the buried primary discharge pipe. The embankment was reconstructed and the primary discharge pipe was replaced. Embankment reconstruction was properly certified (see "Sediment Pond Certification" of December 14, 2004). D. Surface and Ground Water Monitoring GROUND WATER Information provided by the applicant with respect to ground water in the permit area can be found in 4.2.2 through 4.2.4 of Volume I of the permit application. The monitoring plan is described in Section 4.2.4, and monitoring parameters are specified in Table 4.2-15. As discussed in the Probable Hydrologic Consequences (PHC) section of this document, no significant effects to bedrock or alluvial ground water systems are anticipated for this operation. The applicant will conduct monitoring of ground water in a manner approved by the Division to verify the projections of the PHC. The ground water monitoring plan is as follows (4.05.13(1)): Quarterly monitoring of alluvial wells GW -1 and GW -9 for static water level, pH, temperature, and specific conductance. 2. Water quality sampling of mine water discharges as required by CDP S permit. Monitoring of all non -seep inflows into the mine to include source, quantity, duration and quality (pH, electrical conductance, and temperature). 23