2023-10-10_HYDROLOGY - M2016085 - Laserfiche WebLink

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NCCI Bennett Gravel Pit SWSP Page 2 of 5 October 9, 2023 In accordance with approach nos. 1 and 3, the Applicant has indicated that a bond has been obtained for$1,005,627.00 through the DRMS for lining of this site to assure that depletions from groundwater evaporation do not occur in the unforeseen event(s) that would lead to the abandonment of the Pit. Depletions The projected depletions for the period of this SWSP consist of net evaporation from exposed groundwater surface area, water removed in the mined product and dust suppression. Evaporation The Applicant anticipates 0.85 acres of exposed surface area at the site outside of the north cell lined area. Gross annual evaporation at the gravel pit location was estimated to be 44.00 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 Brighton NOAA weather station (record 1978-2017). The net evaporation from the exposed water surface is estimated at 2.8 acre-feet/acre, as shown in attached Table 1. Computation of evaporation under this SWSP was reduced during the ice-covered period. You have assumed the ice covered period will occur during the months of December and January based on average monthly temperatures less than 32°F taken from the Brighton (record 1978-2017) NOAA weather station. However, for the purpose of this SWSP, the Applicant shall replace the net evaporation depletions from the exposed groundwater 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. After accounting for the ice cover months the net evaporation from the exposed water surface is estimated at 2.27 acre- feet during this SWSP period, as shown in attached Table 1. Water Lost from Mined Product In addition to evaporation, water is lost with the mined product removed from the mine site. The applicant projects approximately 500,000 tons of product will be mined within the north cell's lined boundary. Gravel mined will be in a dewatered state and after washing, the groundwater lost with the mined product during this SWSP period is estimated at 2% by weight. The water used for washing the aggregate is projected to total 7.4 acre-feet during the SWSP period, as shown in attached Table 2. Dust Suppression The estimated water used for dust suppression during the SWSP period totals 9.6 acre-feet, as shown in attached Table 2. Total Consumptive Use The total consumptive use at the site during this SWSP period is 19.2 acre-feet as shown on Table 2. The total lagged depletions for this SWSP period from evaporation, operational losses and current dust suppression are 19.4 acre-feet, as shown in attached Table 3. The IDS AWAS stream depletion model was used to determine the lagged depletions from the previous evaporation and operational losses to the South Platte River. The aquifer characteristics used in the model for the alluvial well are: transmissivity (T) = 133,667 gallons per day per foot, specific yield (SY) = 0.2, the distance from the centroid of the gravel pit site to the stream = 1,136 feet, and the location of the parallel impermeable boundary was estimated to be 5,923 feet from the stream. Depletions from this operation will accrue to the South Platte River in the NE '/a of Section 12, Township 2 North, Range 66 West, 61h P.M.