2018-08-01_HYDROLOGY - M2008017 - Laserfiche WebLink

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Derr Pit SWSP August 1, 2018 Plan ID 5240 Page 2 of 6 those cells, approximately 5-6 years from now. This SWSP will make replacements to the Cache La Poudre River for depletions resulting from mining operations at the Derr Pit during the 2018-2019 plan year. The source of replacement water to be utilized in this SWSP is a lease of wholly consumable supplies from the City of Greeley. Depletions During this plan period, consumptive use at the Derr Pit will consist of evaporation from exposed groundwater surface area, and operational losses from water used for dust control and water removed with the mined product. A total of 0.75 acres of groundwater are anticipated to be exposed in Phases 1-4 of the original permit area in dewatering trenches and a recharge pond. Net evaporative depletions were calculated to total 2.188 acre-feet per year based on a gross annual evaporation of 45 inches from the exposed water surface, and a credit of 9.97 inches for effective precipitation (see attached Table 1). No credit was claimed for ice covered periods. Operational losses associated with mining activities will include water removed with the mined product and water used for dust control. Broken Arrow estimates that they will mine a total of 300,000 tons of sand and gravel during this plan period. All of the material will be mined below the ground water table, but in a dewatered state. The water retained by the sand and gravel is considered to be 2% of the mined material by weight, resulting in a total groundwater loss of 4.41 acre-feet. The Applicant has estimated that a total of 5.00 acre-feet of water will be required for dust control purposes at the site during this plan period, based on a projected daily usage of 3,150 to 6,300 gallons per day. A water truck will be utilized to apply water for dust control purposes. Water used for dust control purposes is assumed to be 100% consumed. A monthly breakdown of operational consumptive use at the site is shown in the attached Table 2. Evaporative and operational consumptive use will total 11.598 acre-feet during this plan period. The Alluvial Water Accounting System ("AWAS"), which uses the Glover method, was used with the alluvial aquifer boundary condition to determine the lagged depletions to the Cache la Poudre River from past and projected evaporation and operational losses at the site. The following parameters were used in the model: a distance (X) of 2,126 feet from the exposed groundwater surface to the river; a distance (W) of 22,900 feet from the stream to the impermeable boundary; a harmonic transmissivity (T) of 76,056 gallons per day per foot; and a specific yield (S) of 0.2. The estimated lagged stream depletions due to projected operations at the Derr Pit will total 6.18 acre-feet during this plan period, as shown on the attached Table 3. Dewatering The Derr Pit will be continuously dewatered for the entire plan period. As long as the mine site is continuously dewatered, the water returned to the stream system should be adequate to offset the depletions attributable to dewatering operations. Totalizing flow meters must be installed at each discharge location and meter readings must be reported on the submitted accounting. The meter readings will be used in calculating the post-pumping depletions that must be replaced if dewatering ceases at the site during mining operations and/or upon the conclusion of mining operations at the site.