2018-11-15_HYDROLOGY - M1980244 (6)

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Executive Summary In a letter dated August 3, 2018, the Colorado Division of Reclamation, Mining and Safety (DRMS) requested that CC&V submit a revised demonstration that CC&V is in compliance with WQCC Regulation No. 41 requirements. The August 3 letter specifically requested that CC&V compare all available site groundwater data to the most restrictive standards set forth in Tables 1 through 4 of WQCC Regulation No. 41, for all parameters with Numeric Protection Limits (NPLs) previously approved by DRMS. DRMS attached Table A, entitled"DRMS Approved Numeric Protection Levels for Compliance Groundwater Monitoring Wells at the Cresson Project Compared with Most Restrictive Table Value Standards Set by CDPHE WQCC's Regulation No. 41 —The Basic Standards for Ground Water." CC&V is providing the following information in response to DRMS's request. In the following demonstration of compliance with Regulation 41, CC&V has also provided an analysis of the mine-site conditions to support previously-approved, site-specific groundwater numeric protection limits (NPLs) for Arequa Gulch,Vindicator Valley, Poverty Gulch,Wilson Creek, and Grassy Valley. The data and analysis presented herein indicate that groundwater in Arequa Gulch, Grassy Valley, Poverty Gulch and Vindicator Valley has been impacted by historic mining and milling activities. Precipitation has mobilized soluble species from historical mine dumps/tailings, and likely the native soils, and transported these into the groundwater. In Squaw Gulch, it appears that long-term naturally elevated dissolved constituents in groundwater chiefly stem from the fact that Squaw Gulch was the natural discharge point from the diatreme prior to construction of the various drainage tunnels, and it is probable that historic mining activity has also contributed to the chemical load in groundwater. The solutes transported from the mineralized zones within the diatreme and historic dumps/tailings, have enriched the shallow aquifer materials. Due to limited recharge, these solutes remain elevated in the groundwater. The sources of the elevated constituents in these drainages existed well before 31 January 1994, and the data presented in the report show that mining/processing activities undertaken since 1994 have not resulted in any new or increased sources of groundwater contamination. Therefore, consistent with Regulation 41, the data collected after 1994 "shall be presumed to be representative of existing quality as of January 31, 1994." 1. Introduction In December of 2016, DRMS issued a request to CC&V for Demonstration of Compliance with WQCC Regulation No. 41 —The Basic Standards for Groundwater. The request required that CC&V provide a review of the currently approved groundwater monitoring plan and available site groundwater monitoring data compared against the WQCC Regulation 41 Interim Narrative Standard requirements. CC&V provided a monitoring plan review and data to DRMS on June 22 2017. Subsequently, DRMS issued a request in August 2018 for additional information to demonstrate compliance with the parameters not included within the June 22 2017 submission. These Interim Narrative Standard Requirements are presented below. 1