2023-07-19_REVISION - M2021052 (7)

Home Browse Search

Rifle Gravel Pit #] July 2023 In determining demands, SGM referenced the guidelines from the State Engineer's Office dated April 1, 2011, "General Guidelines for Substitute Water Supply Plans for Sand and Gravel Pits" (SEO Gravel Pit Guidelines). Item 13 of the SEO Gravel Pit Guidelines provides direction for estimating water lost with mined aggregate based on multiple factors (such as washed vs. not washed and mined above the groundwater table vs. mined below the groundwater table). Material that is mined below the groundwater table that has been washed is charged 4% moisture content by weight, because the entire 4% is a groundwater diversion. Essentially, when mined below the groundwater table, the aggregate already has a 4% by weight moisture content which is not changed by the process of washing. The entire 4 percent by weight moisture content is then lost with the mined aggregate. Scott estimated for the entire property they could meet a production rate of 105,000 tons of aggregate per year, or 13,125 tons per month assuming operations 8 months of the year. Applying the standard assumption of 4% water by weight results in total annual demand of 3.09 acre-feet (AF) for 105,000 tons of aggregate. 8.7.3 Water Used for Dust Control Dust control water will be captured from the pond, defined as a well per the DWR-SEO. Scott estimates that the active mining operations will use 25,000 gallons per day for dust suppression. Based on 30-31 operational days per month, this results in an annual demand of 18.72 AF. 8.7.4 Exposed Groundwater Lost Due to Evaporation Evaporation from exposed groundwater during active mining will occur as mining operations progress. 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 and February for Rifle. A copy of the DWR GWS Form 27 Permit application is included in Attachment G. Groundwater will be exposed to the surface during the wet mining operations. The final surface area of the pond is estimated to be 9.70 acres. It is conservatively estimated that the total surface area of the pond will be exposed within the first year of operation. The total annual evaporation of the pond was calculated at 44.28 AF/year, based on a previous version of the mining proposal that involved a larger pond surface of 12.56 acres. Given that the pond has been reduced in size, annual evaporation will be lower in fact, but IHC Scott has elected not to revise the loss calculation downwards since augmentation/supplementary supply plans have already been prepared. 8.7.6 Water Used Indoors (Domestic and Commercial) Scott does not anticipate using water from the site for indoor water demands. As such, there will be no groundwater demands or depletions associated with domestic or commercial water depletions. Potable water is not available from City of Rifle or Town of Silt and drilling a well for potable water on the property is not feasible given minimal domestic needs and short operating period. Instead, Scott will truck potable water onto the site to supply employees with potable 51