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Attachment A -Water information <br />Proposed Two Rivers Mine Site <br />Introduction <br />The Two Rivers Mine site is located in Weld County north of the Town of Milliken near the <br />confluence ofthe Big Thompson and Little Thompson Rivers. The site is located in the NW %< of <br />the SE'/, and the NE '/. of the SW '/. of Section 2, Township 4 North, Range 67 West, 6th P.M. <br />The consumptive use of groundwater from proposed mining activities will create depletions to <br />the Big Thompson River, which can be mitigated without injury to senior water rights. <br />Mining of the Two Rivers Mine will last approximately 10 yeazs. The site will not be mined <br />continuously and will be dewatered approximately two months each year. A portion of the <br />mining site will be excavated and dewatered to facilitate dry-mining. Sufficient mined material <br />will be stockpiled at the site for several months of construction at an adjacent development. After <br />sufficient material is stockpiled the pumps will be turned off and the groundwater lake will be <br />allowed to fill. When the existing aggregate stockpiles are exhausted, Western Equipment and <br />Truck, Inc, staff will install pumps and dewater the azea while additional area is mined for the <br />next phase of construction. Reclamation of the site will include a lined reservoir of <br />approximately 25 acres. <br />The mining operation will impact groundwater systems hydrologically connected to the Big <br />Thompson River. The depth to groundwater is approximately 6 feet and, therefore, the operation <br />will extract sand and gravel deposits from the unconfined Big Thompson River alluvial aquifer. <br />Water Requvements <br />The annual consumptive use for this site is estimated at 15.7 acre-feet. Consumptive uses result <br />from evaporative losses and operational losses. The evaporative losses aze dependent on amount <br />of exposed water surface area during the mining operation. Operational losses include water used <br />for dust control and water retained in the mined product transported off site. <br />The annual evaporative loss is estimated at 10.3 acre-feet. Total annual evaporative loss is the <br />product of net annual evaporation and total exposed groundwater surface. Net annual <br />evaporation is the difference between gross annual evaporation and effective annual <br />precipitation. Since the site will be dewatered approximately two months and then allowed to fill, <br />the exposed surface area at the Two Rivers Mine will be approximately 4.0 acres during mining. <br />The NOAA Technical Report NWS 33, Evanoration Atlas~or the Contiguous 48 United States <br />(U.S. Department of Commerce) was used to determine gross annual evaporation. For this <br />location, gross annual evaporation was estimated at 4] inches. The Colorado Climate Center <br />climatic data for Greeley LTNC was used to estimate annual precipitation. For this station, <br />average annual precipitation was reported as 14.29 inches. Effective annual precipitation was <br />assumed to be 70% of average annual precipitation and was calculated to be 0.8 feet. The net <br />annual evaporation for this site is 2.6 acre-feet per acre. <br />