2015-09-22_HYDROLOGY - M1980110 - Laserfiche WebLink

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Austin Malotte, P. E. September 17, 2015 Page 2 2016. In the event that backfilling of the site is not completed as estimated the Applicant or its successor in ownership must provide information on the steps taken towards the completion of the backfilling of the site. Depletions The depletions that will be covered by this SWSP are limited to evaporation from the ground water exposed to the atmosphere, after December 31, 1980. According to the information submitted, no water surface was exposed within the reclamation permit boundary prior to January 1, 1981. A survey of the Sandy Acres Pit was conducted by Western Water Surveying on June 14, 2012 indicating that the maximum area of the pit is equal to 33.02 acres. In the previous SWSP the evaporative calculation were based on a surface area of 33.02 acres. On August 8, 2015, R. W. Bayer Associates re -surveyed the partially backfilled pond and determined an updated maximum surface area of 25.21 acres. Therefore for the purpose of this SWSP the evaporation depletions were based on an exposed surface area of 25.21 acres. Depletions from past mining operations have already depleted the stream system. The net evaporative depletions to the South Platte River will be calculated as the difference between gross evaporation and the effective precipitation, where the effective precipitation is equal to 70% of the total precipitation. The gross evaporation will be calculated based upon evaporation atlases in NOAA Technical Report NWS 33 and the monthly distribution using the State Engineer's monthly evaporation distribution for elevation below 6,500 feet. The annual gross evaporation rate for this location is approximately 44.69 inches (3.72 feet). The annual gross precipitation rate for this location is approximately 13.80 inches, based on average annual precipitation from 1974 through 2013, taken from the Brighton 1 NE weather station. The effective precipitation is 9.66 inches (13.80 inches X 70%). The anticipated consumptive use for this SWSP is 70.1 acre-feet per year, as shown on the attached Table 1. Computation of evaporation under this plan was reduced during the ice covered period. You have assumed the ice covered period to occur during the months of December and January, based on the average temperatures of 29.81'F for December and 29.03'F for January; taken from the Brighton 1 NE weather station. However, for the purpose of this SWSP, the Applicant shall replace the net evaporation depletions from the exposed ground water surface area that may occur during the assumed ice covered period (the months of December and January) for any time that the pit is not completely covered by ice. Depletions resulting from evaporation will be calculated using the Alluvial Water Accounting System ("AWAS") model developed by the IDS Group at Colorado State University. The AWAS - alluvial aquifer model uses four aquifer input parameters as follows: 1) X -distance from the well (pond) to the river, 2) W -distance from the aquifer boundary through the well (pond) to the river channel, 3) T-transmissivity of the alluvial aquifer (in gallons per foot/day) between the well (pond) and the river, and 4) S -specific yield. All distances were measured from/through the centroid the pond exposed surface area to the South Platte River. The parameters used in the model are shown in the table below.