2012-09-20_HYDROLOGY - M1987049 - Laserfiche WebLink

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Jeffrey Clark RMCC Combined SWSP - AMENDMENT June 7, 2012 Page 2 of 7 are not expected to hit the river. Additionally, the successful installation of the proposed slurry wall will prevent the pit from refilling with ground water in the future. Therefore, it is assumed lagged depletions will not result from the initial dewatering flow rate of 44.2 acre -feet per day. Steady state dewatering will be required until the slurry wall is installed and disconnects the site from the alluvial aquifer. Cell 3 is bounded to the north by cell 2 which is used by RMCC as a silt pond, to the east by the three unmined parcels, to the south by North Tower Pit, and to the west by the South Platte River. Due to the proximity of cell 3, it is expected the majority of this dewatered volume will come from the unmined parcels to the east or from a near direct connection to the river system to the west. Water pumped during this dewatering period will be discharged directly to the river as well as into the unlined North Tower Pit to the south. The North Tower Pit will act as a recharge basin with lagging parameters near identical to cell 3's dewatering activities. It is expected that the dewatering rate will reduce in correspondence with the slurry wall construction progress. Any cone of depression created in the unmined parcels to the east will be disconnected from the alluvial aquifer upon completion of the slurry wall. The slurry wall is scheduled for completion by the end of 2012. For these reasons, it is assumed that there will be no lagged depletions hitting the river once the dewatering operation ceases. Depletions In addition to the new dewatering operations discussed above, RMCC is increasing its mining production projection starting September 2012 from that originally projected in the SWSP. Similar to the original SWSP although some mining will occur above the ground water surface and some will occur below the ground water surface, to be conservative all projected mining is assumed to be below the ground water surface and has been charged 4% water loss by weight. Actual accounting will track where the mining occurs and only assess 2% water loss by weight where mining is above the ground water surface. For September 2012 through July 2013, RMCC projects 1,340,000 tons of gravel projection at this site. This projects to a maximum water loss of 39.4 acre -feet. Due to the dewatering of cell 3 evaporative water Toss will be less than originally projected in the SWSP. Table 1A shows the reduction of exposed ground water areas starting at 121.5 acres in September 2012, tailing down to 53.6 acres by January 2013, and remaining at 53.6 acres through the rest of the plan period. This amendment also increases the gross evaporation rate from 43 inches per year to 44.5 inches per year to be consistent with the North Tower engineering report revised on March 9, 2012. Using the previous effective precipitation of 10 inches per year and historical North Tower Pit ET credit of 2.8 acre -feet per year, the projected net evaporative Toss from September 2012 through July 2013 is 170.6 acre -feet The total consumptive use at this site from September 2012 through July 2013 is 210.0 acre -feet. These depletions do not hit the river instantaneously and thus must be lagged back to the river. The IDS AWAS program was used to determine the lagging factors to be used at the site. The input parameters are as follows: S, specific yield = 0.2; W, distance from the stream to the alluvial boundary = 5,300 feet; T, transmissivity = 100,000 gallons per day per foot; and X, distance from the centroid of the exposed ground water to the stream = 977 feet. This analysis showed that over 95% of the first months consumptive use impacts the river within the first 12 months. The depletions from this analysis were normalized to 100% over 12 months in order to define the depletions factors for exposed ground water and mining depletions, as shown in the following table: