2022-12-16_PERMIT FILE - M2021067

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Stiles Mining Resource Groundwater Monitoring and Mitigation Plan December 2022 Page 1 1.0 INTRODUCTION Asphalt Specialties Co., Inc. (ASCI) has prepared this Groundwater Monitoring and Mitigation Plan for implementation at the Stiles Mining Resource (Site) located at 17085 County Road 394 in Evans, Weld County, Colorado(Figure 1). This plan details the activities ASCI will conduct to monitor for potential adverse impacts to groundwater quality and quantity. In addition, this plan defines the approach ASCI will take to mitigate potential adverse impacts identified, if necessary. Mining operations at the Site have not yet begun as ASCI is in the process of collecting background groundwater quality and quantity data requested by the Division of Reclamation, Mining, and Safety (DRMS) prior to approval of ASCI's permit application (File No. M-2021-067). Groundwater monitoring activities addressed in this plan include: 1) Determining pre-operational ambient/background groundwater quality and quantity. 2) Establishment of site-specific groundwater quality benchmarks. 3) Post-operational data evaluation and procedures for reporting data(both routine and if potential issues are identified) 4) Triggers for identifying potential adverse off-site groundwater quality or quantity impacts to nearby wells once mining operations begin and potential mitigation measures. This plan has been developed in accordance with the requirements set forth in the Construction Materials Rule No. 3, Section 3.1.7 and based on correspondence with DRMS to ensure the appropriate data required for protection of groundwater quality and quantity is obtained. 2.0 BACKGROUND AND HISTORIC DATA Mining activities at the Site will occur within the unconfined alluvial aquifer of the South Platte River(i.e.,uppermost groundwater bearing unit[GWBU])which is unclassified in this area by the Water Quality Control Commission (WQCC). The alluvial deposits consist of sands and gravels with an approximately thickness of 38 feet across the Site. Beneath the alluvial aquifer is consolidated bedrock stratum composed primarily of generally impermeable shale and claystone. No historic groundwater level, flow direction, or groundwater quality data was readily available to ASCI prior to beginning pre-operational monitoring activities. ASCI will provide adjacent well owners within 600-feet of the Site the opportunity to have their wells evaluated for water quality and quantity prior to the start of mining. 3.0 GROUNDWATER QUALITY MONITORING 3.1 MONITORING WELL NETWORK Seven (7) groundwater monitoring wells were completed within the unconsolidated alluvial GWBU at the Site to monitor for groundwater quality. Borings were installed utilizing 4-1/4" ID hollow stem auger(HSA) drilling method from ground surface to the top of underlying bedrock.