2011-08-09_HYDROLOGY - M1987049 - Laserfiche WebLink

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Jeffrey Clark August 8, 2011 Depletions Page 3 of 8 The depletions that result from the mining operation at the Bromley Lakes site consist of evaporation from exposed surface area and water removed in the mined product. The exposed surface area at this site is 24.4 acres at Bromley Lakes Cell 2, which is projected to be completely filled in by the end of September, 2011, 46.5 acres at Bromley Lakes Cell 3, 25.4 acres at Ready Mixed Pit No. 1, and 5 acres is conservatively projected to be exposed at the Arends Pit beginning in August 2011. Depletions at the North Tower Pit consist of evaporation from 28 acres of exposed ground water. Net evaporative depletions were calculated using a gross annual evaporation of 43 inches from the exposed water surface, with a credit of 10 inches for effective precipitation based on data from the Brighton weather station and NOAA Colorado Annual Average Precipitation 1951 -1980 map. The gross evaporation at the North Tower Pit was reduced by an additional 2.8 acre -feet due to the phreatophyte credit for the 7.4 acres of pasture grass that occupied the site prior to mining, using a depth of 4 feet to the ground water table. The net depletion of ground water due to evaporation at the Bromley Lakes site and North Tower Pit was calculated to be 304 acre -feet during the first year of this plan and 286 acre -feet during the second year of this plan, as shown on the attached Table 1A. The Applicant anticipates mining 500,000 tons of material from the Bromley Lakes site during each year of this plan period. The material is mined below the water table and is not washed; therefore the water retained in the mined product is considered to be 4.0% of the mined material by weight. The water removed in the mined product is estimated to be 14.7 acre -feet per year. The total consumptive use of ground water at the Bromley Lakes site and North Tower Pit (including evaporative and operational losses) is estimated to be 318.3 acre -feet for the first year of this plan period and 300.3 acre -feet for the second year of this plan period. The Integrated Decision Support Alluvial Water Accounting System (IDS AWAS) stream depletion model using the Glover alluvial aquifer condition was used lag depletions to the South Platte River. The lagging methodology has been revised since the previous SWSP to calculate the travel time from the centroid of the exposed ground water area rather than from the western edge of the pits. Depletions from past and proposed operations at the Bromley Lakes site and North Tower Pit were lagged to the stream system using the following aquifer parameters: transmissivity (T) = 100,000 gallons per day per foot, specific yield (SY) = 0.2, the distance from the centroid of the exposed ground water to the river (X) = 1,250 feet, and the distance from the parallel impermeable boundary to the stream (W) = 5,300 feet. The results showed that over 13 months, about 95 percent of the first month's slug depletions would impact the river. The depletions were normalized to 100 percent over the 13 months to determine the lagging for the Bromley Lakes site. The total lagged depletions for the Bromley Lakes site and North Tower Pit are estimated to be 271.6 acre -feet for the first year of this plan period and 305.7 acre -feet for the second year of this plan period, as shown on the attached Table 1 B. Replacements The Applicant proposes to replace the depletions with consumptive use credits from 70 Fulton Ditch shares, water previously stored in Kenneth Mitchell Lake, water leased from the City of Brighton, and water stored under the Erger Pipeline water right.