Laserfiche WebLink
• area outside the pit boundary) will be conveyed by ditches to an "in-pit detention/sedimentpnnd" <br />(see Figure 3). The in-pit ponds are sized for storm runoff from the 10-year, 24-hour event and <br />for sediment. Runoff from lazger floods will still be contained within the pit azea, providing <br />even greater protection to areas outside of the mining limits. <br />For mining block years 1 through 5, the drainage area is 85-acres, requiring a pond with <br />storage for 9 acre feet. The pond will be constructed below the pit floor to avoid interference <br />with mining operation. During mining block year 6 through 10, the existing stock pond and <br />Diversion Ditch #3 will limit the drainage area tributary to the active mine area to 210 acres. <br />These facilities minimize the volume of runoff draining into the active mine azea. An <br />intermediate size, in-pit detention/sedimentpnnd can be constructed with a volume of 22 acre <br />feet. As the mining progresses above the existing stock pond, the in-pit detention/sediment pond <br />will be increased to accommodate the lazger drainage area of 470 acres during years 11 through <br />25. This tributary azea requires a pond with a volume of approximately 49 acre feet. <br />When mining progresses to block year 26 through 50, a new, in-pit detention/sediment <br />pond will be constructed. The pond sizes during this mining block can also be staged, according <br />to tributary drainage area. For instance, up to yeaz 40, the eastern branch of Arroyo "I" can <br />continue to bypass the mining operation, reducing the tributazy area for the pond from 740 acres <br />to 500 acres. <br />Control Stormwater Discharges from Active Mine Area. Storm water will be <br />• temporarily stored (for example, 3 to 5 days) within the in-pit detention/sedimentpnnd. Stored <br />runoff will be tested for pH, TSS, oil and grease, COD, per the General Permit, and, potentially, <br />blasting residuals. Runoff will be pumped into reconstructed Arroyo "B" for mining blocks 1 <br />through 25 and reconstructed Arroyo "I" for block years 26 through 50. These arroyos will be <br />reconstructed by excavating a new channel beginning at an elevation coincident with the planned <br />final reclamation surface (anticipated to be approximately 17 feet above the pit floor for Arroyo <br />"B") and extending downstream until the excavation daylights. To minimize stockpiling <br />requirements for overburden materials, a temporary arroyo configuration will be constructed for <br />Arroyo "B", which is narrower than the final arroyo cross section. The final azroyo cross-section <br />will be constructed as part of reclamation through year 25. Figure 7 shows how Artoyo "B" will <br />be reconstructed and Figure 8 shows the temporary and permanent cross sections for the Arroyo. <br />Post Reclamation <br />As part of reclamation, the mining azea surface will beback-filled and contoured, the <br />reconstructed arroyos will be completed, the drainage patterns will be restored to a more natural <br />state, and the upper arroyos will be protected from erosion. As a measure of reclamation <br />success, existing drainage areas and runoffpotential were calculated for all arroyos impacted by <br />the project. Peak flows, runoff volumes grid sediment loads were calculated for the 5-, 25-, and <br />100-yeaz, 24-hour stonn events to size required drainage facilities. To provide reasonable <br />assurances that these variables will not increase after reclamation, surface-water management <br />• facilities for this phase have been designed to meet the following goals and objectives: <br />^ Restore existing arroyo alignment. <br />SWMP BC June 28 2002.doc, OG/28/02 <br />