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Gold HW MW Evatwratlon for Modified Taillntp Pond 12/18/95 Several changes were made to the drainage around the mill and tailings paid that have resulted in a decreased amount of storm water going to the tailingg pond. Pelts of Areas 3 end 4, which previously drained into the tailings pond, are now routed to a sediment trap to the southwest of the tailings pond. This storm water is now disclaaged to the surface draunages as overland flow if it excocds storage capacity in the sediment trap.. The table below shows the arcs, flow distance, elevation charge, and CN rrutnber for the drainages. Total nuroff volume fora 100-year, 24hour Type 2 storm was calculated asirg Roasted Methods, Inc. "Stone" program. The discharge volume for the CN munber for each area was calculated and en equivalent area was beck calculated far a CN of 100 for areas that now drain to the tailings pond. The 100-year, 24hoia slam natoff into the tailitgs pond is now 0.45 acre-feet (Antes 1,2 end SW Roof). TABLE i RUNOFF AND EQUIVALENT AREA CAI,CUI,ATIONS AREA ACRES DELTA ELEV. FLOW DISTANCE CN NUMBER RUNOFF ACRE/FT EQUIVALENT AREA TO POND •1 0.471 20 ]64 85 0.12 0.313 •2 0.762 10 NA 100 0.29 0.762 3 0.225 15 100 85 0.06 4 0.748 45 312 85 0.19 'S.W. ROOF .100 NA NA 100 0.04 .100 N.E. ROOF .120 NA NA 100 0.05 TOTAL .75 0 . 5 1 17 • Areas draining to tailings pond. , The equivalent area (CN=100) was used for Area 1 and the souf}rwest side of the mill building roof end the total tailings pond area (Area 2) to colr..ulate evaporation from the tailings pond (see table on page 2). The table used for evaiporetian lvas presented in the Gold Hill permit responses of July 18, 1995. With the decreased stormwater inflow from the decreased runoff area into the pond (using an equivalent area of 1.175 saes), the evaporation excoeds nmoff for all but one month of s normal rainfall year. The temporary embanlrntent raise that was oompleted during the summa added a minimum of two feet to the embankment height. The decant risers wero erRended so that they would discharge et a height of 6-inches below the lowest point on the raised embankment. A volume survey will have to be completed, but a rough estimate of present storage volume below the e~errded decent risers is 10,700 cubic feet W the top of the pnxxisting embankment (Allen Sorenson survey of 2/28/95) plus 29,700 cubio-feet (0.455 acre pond area) for the additional 18- inches of height to the risers. This gives a total pond capacity to the risers of 0.927 acre-feet. This capacity is more twice the 0.45 acre-foot naroff into the tailings pond from the ] 00.year, 24- hourstorm.