2023-08-30_HYDROLOGY - M2001046 - Laserfiche WebLink

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Nix Gravel Pit SWSP August 30, 2023 Page 3 of 8 The estimated water used for dust suppression during the period of this SWSP total 10.17 acre- feet based on an estimated use of one to four 8,000-gallon truckloads of water per workday (Table 2). Water withdrawn from the freshwater pond will be used for concrete and asphalt batching in the future once mining of the slurry lined area begins, but not during this approval period. The Applicant has indicated that Cell 1A (consisting of the unlined freshwater and overflow ponds) will be continuously dewatered at an estimated rate of 3,000 gpm during this approval period. The overflow pond was expected to be completed and filled with water in July 2023, while the freshwater pond is expected to be completed and filled with water in June 2024. Both the freshwater and overflow ponds will be dry mined. Water pumped for dewatering of the overflow pond will be discharged to the freshwater pond area. Once the overflow pond is completed, water pumped to dewater the freshwater pond will be discharged to the overflow pond area. After both ponds are completed, if the freshwater and overflow ponds cannot accept the full volume of water discharged, excess water will be pumped into a proposed 4 acre recharge pond shown in Figure 3 (attached). The quantity of water discharged to the recharge pond will be tracked separately from the water discharged to the freshwater and overflow ponds, and lagged back to the stream. The Applicant anticipates discharging as much water as possible to the freshwater and overflow ponds to minimize net depletions. The SWSP for this approval period estimates 80% of the water discharged from dewatering will be placed in the freshwater and overflow ponds, with 20% placed in the recharge pond. Because the freshwater and overflow ponds will be dry mined, the first fill will occur once the ponds are fully excavated and dewatering of the ponds ceases. According to the "General Guidelines for Substitute Water Supply Plans for Sand and Gravel Pits", water consumption by the mining operation will also include water removed from the tributary stream system by the "first fill" of the gravel pit. The "first fill" is the water that fills the gravel pit and occupies the volume previously occupied by the removed sand and gravel. For the Nix Pit, the "first fill" was estimated based on 640,000 tons of aggregate production below the water table during the period of this SWSP multiplied by the percentage gravel matrix (70% assuming a porosity of 30% and a density of the material of 1.5 tons/cubic-yard) and then subtracting the volume of water lost with the mined product. Based on the above, the water removed by the "first fill" was determined to be 204.37 acre-feet during the period of this SWSP (Table 2). The IDS AWAS stream depletion model was used to determine the lagged depletions from evaporation and operational losses, to St. Vrain Creek. The aquifer characteristics used in the model are: harmonic transmissivity (T) = 47,518 gallons per day per foot, specific yield (SY) = 0.2, the distance from the centroid of the exposed ground water to the stream X=4,000 feet and the location of the parallel impermeable boundary W=8,775 feet from the stream. In addition, as mentioned above the amount of water discharged to the overflow/freshwater ponds and to the recharge area will be tracked separately and lagged back to the stream. Water discharged to the overflow/freshwater ponds will be lagged using the unit response function (URF) shown in Table 4 (attached). Water discharged to the recharge area will utilize a URF (Table 5 attached) developed using the following aquifer parameters: harmonic transmissivity (T) = 54,974 gallons per day per foot, specific yield (SY) = 0.2, the distance from the centroid of the exposed ground water to the stream X=5,377 feet and the location of the parallel impermeable boundary W=8,660 feet from the stream. For the purpose of this SWSP, the final 1 percent of the impact to the stream was wrapped into the first 99% and re-normalized (Table 5). The total water use during the renewal period, including water pumped for dewatering operations is estimated to be 5,131.6 AF as shown in Table 2. Based on a total water use estimate of 5,131.6 acre-feet, the total lagged depletions to St. Vrain Creek during this renewal period are estimated to be 1,093.03 acre-feet. Of these total