2023-11-29_HYDROLOGY - M2005059 - Laserfiche WebLink

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Nelson Mining Resources SWSP November 29, 2023 Page 2 of 6 Depletions The projected depletions for the period of this SWSP consist of net evaporation from exposed groundwater surface area, water lost with the mined product, dust suppression and dewatering. A total of 3.88 acres of water surface are exposed at the site in sedimentation pond #1 (1.25 acres), sedimentation pond #2 (1.5 acres), sedimentation pond #3 (0.85 acres) in the dewatering trenches (0.28 acres). Evaporation and Mining Operations 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, as shown in attached Table 2. Gross annual evaporation at the gravel pit location is estimated to be 41 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 Colorado Decision Support System (CDSS) NOAA USlCOWE0302 weather station and CoCoRaHS Data (record August -2007 through December -2022). The net evaporation from the estimated exposed water surface is 9.12 acre-feet from the sedimentation ponds and 0.71 acre-feet from the dewatering trenches for a total of 9.83 acre-feet for the November 1, 2023 through October 31, 2024 period, as shown in Table 2, column 10. 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 Longmont West 226, NCWCD weather station (record March 2015 through July 2023). 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. 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 250,000 tons of gravel during the SWSP period. Gravel mined will be in a dewatered state and will be washed therefore the ground water lost with the mined product during this period is estimated at 4 percent by weight. The water lost with the mined product is projected to total 7.37 acre-feet during the SWSP period, as shown in Table 3, column 2. The estimated water used for dust suppression during the period of this SWSP total 1.74 acre-feet as shown in Table 3, column 3. The anticipated annual depletions for this plan are estimated at 18.94 acre-feet for up to 3.88 acres of water surface exposed at the site. You have provided a monthly breakdown of the annual depletions made up of 9.83 acre-feet per year of evaporative loss, 1.74 acre-feet per year of water used and consumed for dust control, and 7.37 acre-feet per year of water lost with the mined product, which represents 250,000 tons of aggregate production (Table 5). The IDS Alluvial Water Accounting System (AWAS) analytical stream depletion model, which uses the Glover method, was used to calculate the lagged depletions to St. Vrain Creek. The following parameters were used in the model: transmissivity (T) = 30,000 gallons per day per foot; specific