2018-12-03_HYDROLOGY - M2008082 - Laserfiche WebLink

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Ms. Jennifer S. Lindahl Page 3 of 9 December 3, 2018 33 and the SEO monthly distribution factors for sites below 6,500 feet, as shown in attached Table 1. Gross annual evaporation at these gravel pit locations is estimated to be 44.27 inches per year. Net evaporation is defined as gross evaporation less the consumptive use of water by vegetation that naturally occurred at the site prior to construction of the pit. The historical consumptive use was assumed to be equal to the effective precipitation, which was estimated based on the data from the Fort Lupton 2SE (ID 3027, record 1950-1976) and Brighton 1 NE (ID 950, record 1976-2014) NOAA weather stations. The net evaporation rate for the exposed water surface is estimated to be 2.92 acre-feet/acre, as shown on the attached Table 1. The projected maximum exposed ground water surface area at each stage is described below. Stage 1-As described above the exposed ground water surface area in the southern portion of Stage 1 is currently 13 acres which will continue through the end of this SWSP period. Stage 2 - Stage 2 currently exposes 7 acres excavated at a depth of 15 feet. RMCC would like to increase the depth in Stage 2 in January 2019 from 15 to 20 feet, and it is expected to increase its ground water surface area from 7 acres to 14 acres. Stage 6A – Mining at Stage 6A is completed. The exposed ground water surface area will remain at 1 acre through the end of December 2019. Stage 6A will remain dewatered throughout the SWSP approval period. Stage 6B - Stage 6B currently exposes 3.8 acres of ground water and is expected to increase to 4.0 acres beginning in January 2019, which will remain throughout the period of this SWSP. Stage 6B will continued to be dewatered throughout the period of this SWSP. Stage 3 - Stage 3 currently exposes 1.0 acre of ground water and is expected to be backfilled with overburden from other stages by the end of 2019 and will result in no permanent surface areas or first fill depletions. Stage 4 – RMCC constructed a slurry wall around Stage 4. The ground water surface area at Stage 4 is currently 1 acre and will remain at 1 acre through the end of this SWSP approval period. Stage 4 will continued to be dewatered throughout the period of this SWSP. Stage 6C/6D – Mining at Stage 6C/6D is expected to begin in 2019 and will expose up to 6 acres of ground water by mid-2019. Stage 6C/6D will be dewatered throughout the period of this SWSP. The projected maximum net evaporation volume during the period of January 2019 through December 2019 is 110 acre-feet, as shown in the attached Table 2, column 2. In addition to the evaporation, water is lost with the mined product removed from the mine sites. RMCC projected that they will produce 2,023,580 tons of gravel during the requested SWSP period, as shown in Table 2, column 3b. All of the gravel mined will be from within previously lined gravel pits within Stage 4 and starting in July 2019 also from Stage 6C/6D (provided the slurry wall construction for Stage 6C/6D is completed and liner approved) and therefore, the production uses are limited to water used to wash the mined product, which is estimated to be 2 percent of the gravel weight for all production during the period of this SWSP. The water used for gravel washing is projected to total 29.8 acre-feet, as shown in Table 2, column 4.