2012-06-22_HYDROLOGY - M1983067 - Laserfiche WebLink

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Mr. Kathryn Cloutier Stagecoach SWSP June 5, 2012 Page 2 dewatering trench and sump. Gross evaporation at this site is estimated to be 45 inches (3.75 feet) per year with an effective precipitation amount of 0.83 feet. This equates to an annual evaporative Toss of 0.15 acre -feet for the 0.05 acres of ground water exposed at the Site. It is estimated that 437,100 tons of aggregate will be mined during 2012. Assuming a moisture content of 4% by weight, the moisture loss for water entrained with the mined product is estimated to be 12.86 AF per year. The Applicant has estimated 1 acre -foot of ground water will be used for dust control at the site during 2012. See Tables 1 and 2 for the monthly breakdown of these projected depletions. Depletions at the Site do not hit the river instantaneously and must therefore be lagged back to the river. The Applicants engineer estimated the lagged depletions that occur due to gravel mining by using the Glover equation (Glover and Balmer, 1954). The distance between the exposed water at the pit and the stream edge is 810 feet as measured from the centroid of the slurry wall to the perpendicular distance to the South Platte River. A transmissivity value of 100,000 gallons per day per foot was used as well as a specific yield of 0.2. For the purposes of this SWSP it was conservatively assumed that the depletions at the exposed water at the Site would occur for the duration of this SWSP which is longer than the anticipated period to complete and have fully certified the slurry wall. The equation was calculated from t = 0 to t = 10 years. The river depletion analysis shows that the Stagecoach Stop Pit's stream depletion will equal approximately 12.09 acre -feet during 2012. The estimated monthly lagged depletions are provided in Table 2. Replacements The source of replacement water for this plan will be provided by leased fully consumable water from the City of Aurora. Aurora will deliver the water to the river as effluent from the Metro Wastewater Treatment Plant located approximately 12 miles upstream of the mining operation. As such, a transit loss of 6% (0.5% per mile) is assessed on all deliveries made by Aurora to be used at the Stagecoach Stop Pit. A copy of the May 17, 2011 lease, and the May 22, 2012 amendment has been provided to this office and is attached to this letter. This lease is also shared with the neighboring Worthing Pit and provides sufficient replacement water for Stagecoach Pit in 2012 with the exception of months April, May, November, and December. Therefore the duration of this SWSP approval is only for June 1, 2012 through October 31, 2012. See Table 3 for the monthly replacement schedule. It is noted that the pending slurry wall leak test should be completed prior to October 31, 2012. Should the wall be found to leak in excess of allowable rates, an amendment will be required to this SWSP. Long Term Augmentation Requirements In accordance with the letter dated April 30, 2010 from the Colorado Division of Reclamation, Mining, and Safety ( "DRMS "), all sand and gravel mining operators must comply with the requirements of the Colorado Reclamation Act and the Mineral Rules and Regulations for the protection of water resources. Specifically gravel pit operators are required to show that they can comply with any long term augmentation requirements once mining operations are completed. The reclamation plan for the Site includes lining the site with a slurry wall thereby creating a water storage reservoir. Once the slurry wall is approved by the State Engineer's Office as meeting the design or performance leakage rate, native ground water trapped within the slurry wall may be removed from within the lined area and returned to the stream system through surface flow or ground water recharge without need for replacement, so long as the operator does not put the water to beneficial use. Water that is legally stored in the lined reservoir will not be assessed evaporative losses that must be replaced to the river. Therefore long term augmentation will not be required. AFS holds a bond through DRMS in the amount of $231,191 which can be used to complete the reclamation plan in the unlikely event the operator walks away from the Site before reclamation is complete.