2021-02-03_REVISION - M2020008 (4)

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Rifle Gravel Pit #1 January 2021 (,Amendment, to File No M-2020-008) 8.6.2 Water Lost with Mined Aggregate SCI estimates a production rate of 200,000 tons of aggregate per year, or 16,667 tons per month assuming a year-round operation. DWR-SEO uses a standard assumption that aggregate production water depletions are four percent of the production rate when washed and mined below the groundwater table, which results in total annual demand of 5.89 acre- feet (AF) for 200,000 tons of aggregate. 8.6.3 Water Used for Dust Control Dust control water will be captured from the dewatering well. SCI estimates that the active mining operations will use 50,000 gallons per day for dust suppression. Based on 20 operational days per month, this results in an annual demand of 36.83 AF. 8.6.4 Water Lost Due to Evaporation During Dewatering Evaporation from exposed groundwater during active mining is expected to be minimal. A series of dewatering wells (exact number unknown at this time but modeled and documented in Attachment B reports) will lower the local groundwater table at the site and allow for dry mining operations. In the instance mining exposes groundwater not pumped by the dewatering well, a perforated pipe will be buried in a trench at least 3 feet below the mining floor, allowing this water to be intercepted and directed to the dewatering well. Hence, it is assumed that no water is lost to evaporation during active mining. 8.6.5 Water Used Indoors (Domestic and Commercial) SCI does not anticipate using water from the site for indoor water demands. Because potable water is not available from City of Rifle or Town of Silt and because drilling a well for potable water on the property is not feasible given the mining plan, SCI is planning to truck potable water onto the site to supply employees with potable water. 8.6.6 Water Requirements During Reclamation Phase (Post Mining) After mining is completed, water depletions from the site will be solely from evaporation of the exposed groundwater in the finished gravel pit pond. Annual gross evaporation for the Rifle area is estimated at 45 inches based on the contours from National Oceanic and Atmospheric Administration Technical Report NWS 33 dated June 1982, Map No. 3. Annual evaporation was distributed annually using the DWR General Guidelines for Substitute Water Supply Plans for Sand and Gravel Pits (DWR Gravel Pit Guidelines) annual distribution for elevations below 6,500 feet. The evaporation was then reduced to account for periods when the pit is expected to be frozen over when the average temperature is below freezing, typically December through February for Rifle. The completed size of the gravel pit pond is not yet known, however for this preliminary analysis 53 acres is estimated, which includes all of the property area within the 25-foot setback. The 53 acres of surface evaporation is estimated to have 179.87 AF of depletions annually (note Sheet C7 of the drawings depicts a 38-acre pond, but the conservative 53- acre area is being used for evaporation estimating). Table 3 presents the post-mining phase water use demand. 32