2023-08-30_HYDROLOGY - M2001046 - Laserfiche WebLink

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Nix Gravel Pit SWSP August 30, 2023 Page 2 of 8 replacements for water consumption for the slurry wall construction will be made by Dixie Water, LLC under a separate SWSP that was approved on May 17, 2022 and will expire on April 30, 2024. A slurry wall around Cells 4-6 is anticipated to be constructed in approximately 5-10 years. The depletions associated with this SWSP include evaporation from exposed groundwater, water lost with the mined product, and dust control. 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 you provide information to DRMS to demonstrate you 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, you have indicated that a bond has been obtained for $6,964,000.00 through the DRMS for lining or backfilling 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, concrete and asphalt batching and first fill. This SWSP anticipates that a maximum of 22.7 acres of water surface will be exposed at the site in the overflow and freshwater ponds during the period of this SWSP. The Applicant proposed 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. Gross annual evaporation at the gravel pit location is estimated to be 42.00 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 Northern Colorado Water Conservancy District's weather station at Gilcrest (record 1994-2004). The net evaporation from the exposed water surface is 43.52 acre- feet for the SWSP period (Table 1). While the projected evaporation depletion amount assumes no ice cover throughout the year, the evaporation in the accounting will be set to zero for periods where the exposed groundwater is completely covered by ice during actual operations. 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. In addition to the evaporation, water is lost with the mined product removed from the mine site. The Applicant projected that they will produce 800,000 tons of gravel during the SWSP period and 1.7 million tons/year in subsequent years. Approximately 20 percent of the mined material is anticipated to be mined above the groundwater table. Beginning in April 2023, it was assumed all material mined above the groundwater table will be washed, and will therefore be charged 2% by weight. Material mined below the groundwater table will be charged 4 percent by weight whether washed or not. The total consumption of water removed with the product during the approval period is estimated to be 21.19 acre-feet (Table 2).