2021-07-01_HYDROLOGY - M2015030 - Laserfiche WebLink

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Chavers Mining Resources Substitute Water Supply Plan Page 2 of 7 July 1, 2021 mined product. The life expectancy of the Chavers Mining Resources site is approximately 10 to 12 years, with 5 to 7 years remaining. Approximately 700,000 tons are to be excavated from the pit each year during mining operation. The proposed reclamation of the site is a lined reservoir through the construction of a compacted clay liner around the mining area. Fully consumable water from the City of Aurora will be used to replace depletions from the mining operation during this plan period. In accordance with the letter dated April 30, 2010 (copy attached) from the Colorado Division of Reclamation, Mining, and Safety ("DRMS"), all sand and gravel mining operators must comply with the requirements of the Colorado Reclamation Act and the Mineral Rules and Regulations for the protection of water resources. The April 30, 2010 letter from DRMS requires that the Applicant provide information to DRMS to demonstrate they can replace long term injurious stream depletions that result from mining related exposure of groundwater. The DRMS letter identifies four approaches to satisfy this requirement. In accordance with approach nos. 1 and 3, the Applicant has indicated that a bond has been obtained for $1,233,955.00 through the DRMS for lining of this site to assure that depletions from groundwater evaporation do not occur in the unforeseen event, or events 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, dust suppression, dewatering, and concrete production. Evaporation The Applicant anticipates that a total of 3.23 acres of water surface will be exposed at the site in the pump basin (0.50 acres), dewatering trench (0.48 acres) and wash ponds (2.25 acres). The Applicant proposes to replace evaporation from exposed ground water at the site based upon evaporation atlases in NOAA Technical Report NWS 33 and the SEO monthly distribution factors for sites below 6,500 feet, as shown in attached Table 2. Gross annual evaporation at the gravel pit location is estimated to be 44.00 inches per year. Net evaporation is defined as gross evaporation minus 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 1 NE (record 1974-2020) NOAA weather station. The net evaporation from the estimated exposed water surface is 2.63 acre-feet per year for the pump basin and dewatering trench, and 6.07 acre-feet per year for the wash ponds, as shown in Table 2, columns 8 Et 9, totaling a net evaporation of 8.70 acre-feet per year. Computation of evaporation under this SWSP was reduced during the ice covered period. It is 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 1 NE (record 1974-2020) 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 for any time that the pit is not completely covered by ice. Computation of the net evaporation during any time that the pit is not completely covered by ice shall be determined as the pro-rata amount of the monthly gross evaporation rate distribution amount identified in the State Engineer's General Guidelines for Substitute Supply Plans for Sand and Gravel Pits, subtracting the pro-rata amount of the effective precipitation for that period.