2015-05-12_HYDROLOGY - M2003091 - Laserfiche WebLink

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Mr. David Heintz Duckworth SWSP May 6, 2015 Depletions Page 2of10 Evaporation and Mining Operations Water uses at the Duckworth Pit site include evaporation from the exposed surface are of the dewatering trenches and silt ponds, dust suppression and water lost with the mined product. From August 2014 through July 2015 only the south half of the Phase 2 area will be actively mined. The dewatering trenches that surround the perimeter of the south half of the Phase 2 area are approximately five feet wide by 2,950 feet long, which is equal to a total of 0.34 acres of exposed surface area. Net evaporative depletions are calculated using a gross annual evaporation of 40 inches per year per NOAA Technical Report NWS -33. This is reduced by applying a credit of 9.4 inches for effective precipitation which is 70% of the average annual precipitation based on Longmont 2, CO (IDii5116) NOAA weather station. Based on a projected exposed acreage of 0.34 acres from August 2014 through March 2015, the total net evaporative depletion will be 0.44 acre-feet for this plan period, shown on Table 1. The Applicant has estimated that 4.35 acre-feet of water wilt be used for dust suppression at the site and will occur during the months of aggregate production. Mining is expected to occur during this SWSP period with a total amount mined of 308,000 tons. The aggregate will be crushed, not washed and mined in a dewatered state. Pursuant to paragraph 13 of the General Guidelines for Substitute Water Supply Plans for Sand and Gravel Pit (April 2, 2011), a 2% moisture content by weight is charged as a ground water diversion. Therefore, 4.53 acre-feet of water is expected to be consumed from the aggregate production during this approval period. The Alluvial Water Accounting System ("AWAS") model was used with the alluvial aquifer boundary condition option to lag depletions to Boulder Creek. The following parameters were used in the model: transmissivity (T) = 35,000 gallons per day per foot, distance (X) from the centroid of the surface of the exposed ground water to the river = 4,400 feet, distance (W) from the aquifer boundary through the exposed ground water to the river channel = 8,800 feet, and specific yield (SY) = 0.2. The location of the stream depletion is assumed to be perpendicular to the river. The lagged depletions due to evaporation and mining operations during the SWSP period are shown in Table 1 and are estimated to total 10.0 acre-feet during this approval period. Dewatering The Applicant will dewater the site to allow for dry mining operations using collection ditches that will surround the pit. Under the previous SWSP, the Applicant has pumped the dewatering water to two unlined silt ponds located in the northwest corner of the Duckworth Property as shown in attached Figure 2. Dewatering water that was pumped to the silt ponds was assumed to return to the stream in the same timing as the dewatering depletions. Therefore no credit was taken for dewatering operations at the site and no lagged depletions accrued to the stream system. With the completion of the liner on the north half of Phase 2 area the silt ponds will be removed. For this reason, dewatering water is expected to be pumped to the Cottonwood Pond, which is an unlined pond. As seen in Figure 2, the Cottonwood Pond is located just south of the Duckworth Pit. The Cottonwood Pond is owned and operated by MMM and therefore no agreement is necessary to deliver the Duckworth dewatering water to the pond. The DRMS number for the site is M-1988-042. Depletions at the Cottonwood Pit are replaced pursuant to an agreement with District 6 Water Users Association. Due to the close proximity of the pond to the Duckworth Pit it is assumed that deliveries to the Cottonwood Pond recharge the aquifer and are assumed to return to the stream in the same timing as the dewatering depletions. The Cottonwood Pond was flooded in