2015-12-30_HYDROLOGY - M1999069 - Laserfiche WebLink

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Wagner/Kaufmann No. 3 SWSP Page 2 of 6 December 21, 2015 Depletions Depletions at the site are limited to evaporation from exposed ground water in the dewatering trench at the northwest pit. The current exposed ground water surface area is 0.03 acres, based on an estimated trench width of 3 feet and length of 420 feet. Although the dewatering trench may be partially backfilled during this plan period, for the purposes of this SWSP it was assumed that there will be no backfilling during 2016. Net evaporative depletions were calculated using a gross annual evaporation of 38 inches from the exposed ground water surface, with a credit of 9.7 inches for effective precipitation. The net depletion of ground water due to evaporation from the 0.03 acres exposed at the W -K #3 Pit totals 0.071 acre-feet for this plan period, as shown on the attached Table 1. There will be no aggregate production or other consumption of ground water at the site during this SWSP approval period. The W -K #3 Pit site will continue to be dewatered during the plan period. Since 2000, all water pumped from the dewatering trench has been discharged into an adjacent unlined 24.7 -acre pond which is part of the Kauffman #1 Pit (DRMS M-1978-327, WDID 0403009). The Kauffman #1 Pit is covered under a separate SWSP (WDID 0402530) and is owned and operated by the Applicant. The current exposed ground water in Kauffman No. 1 Pit was exposed prior to 1981 in connection with sand and gravel mining and as such evaporation from the Kauffman No. 1 Pit is not required to be augmented (see § 37-90-137(11)(b), C.R.S. Et 2009CW49). Due to the large size of the Kauffman # 1 Pit relative to the volume of water projected to be pumped from the dewatering trench, the additional volume attributable to the dewatering flows will not create a measureable increase in evaporation. The dewatering pump discharges into the Kauffman #1 Pit at a location approximately 100 feet from the intake point. You have estimated that water discharged into the Kauffman No. 1 Pit will accrue to the Big Thompson River at approximately the same timing as depletions from pumping the water out of the W -K #3 Pit. So long as dewatering at the W -K #3 Pit is continuous during this plan period and all dewatering flows continue to be discharged into the adjacent unlined Kauffman No. 1 Pit, the assumption that there will be no net depletions as a result of dewatering at the W -K #3 Pit will be accepted for the purposes of this SWSP. The monthly depletions to the Big Thompson River due to past and projected use at the W -K #3 Pit were lagged from the pit site using the AWAS program developed by the IDS Group at Colorado State University. The following parameters were used in the stream depletion model for the period of 2012-2016: a distance from the centroid of the exposed ground water to the river (X) of 2,130 feet; a distance from the river through the site to the no flow aquifer boundary (W) of 4,000 ft; an aquifer transmissivity (T) of 50,000 gallons/ft/day; and a specific yield (S) of 0.2. To be consistent with previous SWSP approvals for this site, lagged depletions caused by mining operations prior to 2012 were determined using the previously approved distance from the exposed water surface area to the river (X value) of 200 feet. The lagged stream depletions due to past and projected use at the site are estimated to total 0.114 acre-feet during this plan period, as shown on the attached Table 4. Replacements Replacement water for this pit will continue to be made available throughout the year from a lease of 65.0 acre-feet of fully consumable water from the City of Loveland ("Loveland"). A copy of the lease is attached to this letter. This leased water is also used to replace depletions at the