2019-12-03_HYDROLOGY - M2001107 - Laserfiche WebLink

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NCCI Pit#1 Substitute Water Supply Plan December 3, 2019 Page 2 of 5 inches for effective precipitation. The net depletions of ground water due to evaporation from the 5.96 acres at the site totals 17.5 acre-feet per year, as shown on the attached Table 2. The mining operation has continuously dewatered directly back to the South Platte River since mining operations commenced in 2003. The Applicant estimates a continuous pumping rate of 4 to 10.5 million gallons per day for 2020 for a total maximum pumping of 6,819 acre-feet per year (assuming an average rate of 6.1 million gallons per day). However, since dewatering may fluctuate from month-to-month, an alternate dewatering scenario was evaluated for a total pumping of 6,413 acre-feet per year (5.7 million gallons per day). Dewatering operations must be metered to the satisfaction of the Division Engineer. Meters were installed on the dewatering pumps in May 2012. See Table 3-A for the month-to-month anticipated values for dewatering pumping and see Table 3-6 for the month-to-month alternate values for dewatering pumping. The IDS AWAS stream depletion model was used to determine the lagged depletions to the South Platte River caused by the NCCI # 1 Pit. The following parameters were used in the model: transmissivity (T), 150,000 gallons per day per foot; specific yield (SY), 0.2; distance from the South Platte River to the edge of the alluvium (W), 5,000 feet; and distance from the centroid of the exposed surface water areas to the South Platte River (X), 3,300 feet. The total lagged depletions during 2020 caused by past and projected mining activities at the NCCI # 1 Pit is 6,940.2 acre-feet, as shown on the attached Table 4-A (including anticipated dewatering depletions). The alternate total lagged depletions during 2020 caused by past and projected mining activities at the NCCI # 1 Pit is 6,432.9 acre-feet, as shown on the attached Table 4-13 (including alternate dewatering depletions). After accounting for the 6,819 acre-feet of anticipated dewatering water that is returned directly back to the stream the net depletion for 2020 is 121.2 acre-feet. After accounting for the 6,789.3 acre-feet of alternate values of dewatering water that is returned directly to the stream the net depletion for 2020 is 19.7 acre-feet. Depletions from the mining operation accrue to the South Platte River in Section 19, Township 2 North, Range 66 West. For purposes of this SWSP, the point of depletion will be considered the point where the South Platte River crosses the South line of Section 19, Township 2 North, Range 66 West, which is just upstream of the Meadow Island No. 1 Ditch. Replacements The source of replacement water is fully consumable water from Thornton. Thornton purchased the NCCI Pit #1 site for future reservoir storage and entered into an agreement allowing NCCI to continue mining the site. The agreement requires Thornton to replace depletions caused by NCCI's mining operations. The anticipated quantity of replacement water needed for this plan in 2020 is estimated to be 140.9 acre-feet with an alternate quantity of replacement water needed for this plan in 2020 estimated to be 1,040.9 acre-feet. The water from Thornton will be released from the Metropolitan Wastewater Treatment Plant ("Metro", WDID 0200700). A 0.5% per mile transit loss (14% overall loss for the 28 miles between Metro and the point of depletion) was accounted for in the SWSP. Conveyance loss for delivery of augmentation water is subject to assessment and modification as determined by the water commissioner and/or division engineer. The lagged depletions include depletions resulting from past uses at the site, and the replacements are shown on attached Table 4-A and Table 4-13. Conditions of Approval hereby approve the proposed substitute water supply plan in accordance with Section 37-90- 137(11), C.R.S., subject to the following conditions: