2024-05-09_HYDROLOGY - M2001107 - Laserfiche WebLink

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NCCI Pit #1 Substitute Water Supply Plan - Amendment Page 2 of 4 May 9, 2024 and dewatering that occurred prior to the liner approval in November 2023. Unlagged evaporative and dewatering depletions were 851.3 acre-feet as shown on the attached "Stream Depletions" table. 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 in 2024 caused by past evaporation and dewatering at the NCCI #1 Pit are estimated at 88.9 acre-feet, as shown on attached Revised Table 2. The Applicant proposes to wrap the remaining depletions from 2025 and 2026, which are 2.5 acre-feet or 2.7% of remaining lagged depletions into 2024, as shown on attached Revised Table 2. Therefore, the total remaining lagged depletions for 2024 are 91.4 acre-feet. The lagged depletions from January 1, 2024 through April 30, 2024 were already replaced, therefore the remaining depletions from May 1, 2024 through December 31, 2024 are 35.6 acre-feet. No depletions from past activities extend beyond 2026. Depletions from the mining operation accrue to the South Platte River in Section 19, Township 2 North, Range 66 West, 6th P.M. 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 (attached) allowing NCCI to continue mining the site. The agreement requires Thornton to replace depletions caused by NCCI's mining operations. The water from Thornton will be released from the Metropolitan Wastewater Treatment Plant ("Metro", WDID 0200700). Thornton will provide 46.5 acre-feet of replacement supplies in 2024 as shown on the attached Revised Table 2. 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. Note that free river conditions existed during the majority of January, February, and March of 2024 and the first ten days of April of 2024 during which replacement was not required; therefore, the total amount of replacement water provided by Thornton is less than the total lagged depletions. Conditions of Approval The proposed substitute water supply plan is hereby approved in accordance with section 37-90-137(11), C.R.S., subject to the following conditions: 1. This SWSP amendment shall be valid for the period of January 1, 2024 through December 31, 2024, unless otherwise revoked or superseded by decree. 2. Well Permit 74792-F has been issued for this pit, and this permit remains valid. The permit covers up to 111.14 acres of exposed surface area and dewatering activities. Since mining at the site is completed, there is no more groundwater exposed at the site and the site is lined, the Applicant, shall submit a Well Abandonment Report (form GWS-09) which is available for download at https://dwr.state.co.us/eforms. 3. Approval of this plan is for the purposes as stated herein. Any use of this water must first be approved by this office. 4. The storage structure/reservoir must be equipped with outlet works capable of passing all out-of- priority inflows to the nearest natural watercourse. All out-of-priority inflows to the storage