2023-05-11_REVISION - M2004051 - Laserfiche WebLink

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Wattenberg Mine, Exhibit G—Water Information May 2023 gross evaporation and effective precipitation. The NOAA Technical Report NWS 33, Evaporation Atlas for the Contiguous 48 United States (U.S. Department of Commerce)was used to determine the site's average annual gross evaporation of 45 inches. Precipitation was obtained for the nearby City of Brighton and the total annual precipitation for this site was determined to be 15.6 inches. Effective precipitation was calculated as 70 percent of total precipitation; thus, the average annual effective precipitation was determined to be 10.9 inches. The resulting evaporative loss rate is therefore 29.4 inches. The maximum total annual evaporative consumptive use during the short period of additional mining at the site is estimated at 7 acre-feet, which is primarily a function of the water used for dust control and the water retained within removed materials. 8.0 MONITORING AND MITIGATION PLAN This Groundwater Monitoring and Mitigation Plan is prepared as part of Holcim's application to the DRMS for a revised permit for the Wattenberg Lakes in Weld County, Colorado. This plan presents the methods and locations for monitoring of groundwater during gravel mining and site reclamation activities. Although adverse impacts to other local users of groundwater are not expected due to activities at the site, this plan addresses how any adverse effects to groundwater would be mitigated, should they occur. If needed, Holcim will submit a Temporary Substitute Water Supply Plan to the State Engineer's Office for approval. The Temporary Substitute Water Supply Plan is designed to protect senior vested water rights and mitigate potential depletions of flows in adjacent waterways. Mining Plan The 6.2 acres of additional mining will occur on the north and west edges of the currently ponded area of Struck Pond. Some dewatering will be required to complete this phase but will be temporary and short term in nature. Dewatering is planned for a period of approximately 10 months to accommodate earthwork activities, within which mining is planned over a period of 2 months. The mined aggregate will be placed on the conveyor(north side of Struck Pond) and conveyed to Holcim's Platte Valley Plant, and temporary stockpiles will be placed to the west and north of the current Struck Pond. Following mining activities, the Struck Pond will be expanded as shown in Figure G-1 and will be surrounded by a slurry wall and perimeter drain. Monitoring The monitoring plan will consist of regular data collection from the set of monitoring wells currently in place around the perimeter of the Wattenberg Lakes permit area (Figure G-3). Data collection activities will include monthly measurement of water levels in wells. Water Quality Parameters As mentioned above, due to the absence of large quantities of potential pollutants on site and the absence of water quality samples collected at this site in the past, additional water quality samples are not proposed. Domestic and Irrigation Water Wells Active domestic and irrigation wells are shown In Figure G-6 and Table 1. In the event of a well owner complaint, either due to water quality or quantity, Holcim commits to reporting any complaints received from well owners to the DRMS within 48 hours, investigating the complaint as soon as practical, and submitting the results to the DRMS for evaluation within 30 days. Dewatering Discharge Based on data collected from monitoring wells, the depth to groundwater is shallow and will likely be around 2 feet below ground surface in the area of the Struck Pond. Due to the absence of large quantities of potential pollutants on site (no on-site processing or concrete or asphalt production), the mining and reclamation operations are not likely to affect groundwater quality on or off the site. OTETRA TECH Page 6 1 8