2012-07-20_HYDROLOGY - M2002020 - Laserfiche WebLink

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Ms. Michelle Johnson 25 Ave SWSP July 20, 2012 with DRMS has not been executed as bonds are not required for Municipal entities. Page 2 Depletions The applicant projects approximately 200,000 tons of aggregate per year will be mined under this SWSP. The material will be in a dewatered state and will not be washed. Therefore 2% losses by weight are assessed on the mined material. This amounts to a ground water loss of approximately 2.94 acre -feet per year. Additionally it is estimated that 1 acre -foot per year of water will be consumed for dust control purposes. Gross evaporation at the site was previously determined in water court case 99CW231 and is 3.75 acre -feet per exposed acre. The applicant conservatively estimated a maximum exposed ground water surface at this site of 30.5 acres. Based on this estimation, the maximum evaporative loss is 114.38 acre -feet per year. The 30.5 acres of exposed ground water surface does not include the 30± acres exposed at Reservoir 5. Pursuant to § 37- 90- 137(11)(b), C.R.S. and 2009CW49, a gravel pit operator or property owner does not need to replace depletions that occur due to evaporation from ground water exposed prior to January 1, 1981 ( "pre -81 ") as a result of open mining of sand and gravel, regardless of whether mining continued after December 31, 1980 ( "post -80 "). The State Engineer has previously granted a reallocation of the 45.5 pre -81 acres of ground water to Reservoir 5. The pre -81 credits can no longer be reallocated throughout the mining site and the remaining 15.5 pre -81 acres can no longer be claimed. Flatiron Pits 3 and 4 are both currently full and will be dewatered to allow for dry mining operations. The initial dewatering rate, required to empty the filled pits, will be approximately 6,800 gallons per minute ( "gpm "). This water will be discharged directly to the river with no beneficial use. After the initial high dewatering rate, the applicant projects to have a lower pumping rate to maintain a dewatered state. The applicant projects the maintenance pumping rate to be equivalent to 850 gpm (1,700 gpm at 50% daily duty cycle). Dewatering of the lake will not result in depletions to the stream system so long as the pit remains dewatered thus preventing the refill of the pit. Dewatering of the alluvial aquifer will result in depletions to the stream system. The initial dewatering will be made up of both lake water and alluvial aquifer water. The maintenance dewatering will be made up of only alluvial aquifer water. For the purposes of this SWSP it is assumed that the initial dewatering will deplete the alluvial aquifer at the maintenance pumping rate. Therefore, depletion modeling will show 850 gpm of the initial rate of 6,800 gpm as causing a depletion and the remainder (5,950 gpm) will not be included in the depletion analysis. Should the pit refill with groundwater in the future, additional depletions will result, and the applicant will be required to replace depletions from such "first fill" above and beyond the depletions from the maintenance pumping. It is anticipated that the successful installation of the proposed slurry wall will prevent the pit from refilling with groundwater and thus prevent any such additional depletion in the future. Therefore it is assumed lagged depletions will not result from the initial dewatering, above the maintenance dewatering, as long as such water is returned directly to the stream system without consumption. However, the continual dewatering activity of 850 gpm will create lagged depletions to the stream once the dewatering pumps are turned off, or significantly reduced in their flow rate, as this water is coming from the surrounding alluvial aquifer. The applicant is required to submit a plan documenting their proposed plan to account for and augment the depletions that will result once the dewatering pumps are turned off, or significantly reduced within 3 years of any such planned activity. The combined water loss associated with the plan is approximately 3.94 acre -feet per year of operational consumptive use and 114.38 acre -feet per year of evaporative losses for a 44