2018-12-03_HYDROLOGY - M2008082 - Laserfiche WebLink

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Ms. Jennifer S. Lindahl Page 4 of 9 December 3, 2018 A slurry wall will be constructed around Stage 6C/6D using water from Stage 6A. The water used for the slurry wall construction is estimated to be 1,000,000 gallons per month from approximately February 2018 through June 2019 or 15.3 acre-feet, as shown in Table 2, column 5. The estimated water used for dust suppression is 14.4 acre-feet in 2019, as shown in Table 2, column 6. RMCC currently uses water from Stage 6A for dust suppression purposes but plans to switch to Stage 2 for these uses in 2019. RMCC will dewater Stages 6A, 6B, 3 and 4 during the period of this SWSP. Additionally, dewatering at Stage 6C/6D will begin during 2019. The dewatering pumping volume is projected to be 14 ac-ft per day at each unlined pit and 0. 5 ac-ft per day at each lined pit and will be continuous at one or more stages throughout the SWSP request. The dewatering volumes shown in Table 2, column 7 represent the combined dewatering for all stages. Stage 6B was filled with water to a depth of 12 feet in March 2018 during free river conditions. The diversions to storage were direct from the river thus no lagged depletions occurred. RMCC plans to increase the excavated in Stage 2 in January 2019 from 15 to 20 feet, and it is expected to increase its ground water surface area from 7 acres to 14 acres. When dewatering at Stage 2 stops in January of 2019, the additional fill will create a lagged depletion to the river requiring replacement. RMCC estimates that the pit is currently 15 feet deep over 7 acres, or 105 acre- feet. In January of 2019 RMCC plans to allow the pit to fill to a depth of 20 ft, resulting in a surface area of 14 acres. Therefore, the additional fill volume that will likely occur in January 2019 is 175 acre-feet (280 acre-feet-105 acre-feet). The total consumptive use at the site for the period of this SWSP is 344.5 acre-feet. The dewatering pumping volume to be discharged to Big Dry Creek during the period of this SWSP is estimated at 23,485 acre-feet, as shown on the attached Table 2, column 7. The IDS AWAS stream depletion model was used to determine the lagged depletions from dewatering, evaporation and operational losses at the Morton-Holton Lakes site to Big Dry Creek and the South Platte River, using the following Glover parameters: • specific yield = 0.2 • transmissivity = 150,000 gpd/ft based upon pumping test data in Colorado Ground Water Circular No. 11 • distance from centroid of the Stage 1 mining area to Big Dry Creek = 300 feet • distance from centroid of the Stage 2 mining area to Big Dry Creek = 1,150 feet • distance from centroid of the Stage 6A mining area to the South Platte River = 1,360 feet • distance from centroid of the Stage 6B mining area to the South Platte River = 1,720 feet • distance from centroid of the Stage 3 mining area to South Platte River = 700 feet • distance from centroid of the Stage 4 mining area to South Platte River = 1,000 feet • distance from centroid of the Stage 6C/6D mining area to South Platte River = 640 feet • distance from Big Dry Creek to alluvial boundary = 7,250 feet for Stage 1 and 2, and 8,557 feet for Stage 6A, 8,680 feet for Stage 6B, 7,830 feet for Stage 3, 8,400 feet for Stage 4, and 9,160 feet for Stage 6C/6D based upon CDSS South Platte River alluvial aquifer boundary mapping Based upon this analysis, 95 percent of the depletions accrue to the stream within 5 months for Stage 1, within 14 month for Stage 2, within 19 months for Stage 6A, within 22 months for Stage 6B, within 11 months for Stage 3, within 16 month at Stage 4, and within 12 months at Stage