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April 2021 MNGW--1 pH Memo♦1 <br /> 1.0 INTRODUCTION <br /> This Memorandum has been prepared for Climax Molybdenum Company's Henderson Mine <br /> (Henderson) to present hydrogeologic data related to a field investigation of pH values in <br /> Point of Compliance (POC) monitor well MNGW-1. Figure 1 presents a regional map <br /> showing the location and geographic setting of the Henderson Mine,which is located west of <br /> Empire,in Clear Creek County, Colorado. <br /> 1.1 BACKGROUND AND OBJECTIVES <br /> This assessment was initiated following the measurements of pH values less than 6.5 s.u. in <br /> POC well MNGW-1 in 2012 and 2013,which were reported to the Division of Reclamation, <br /> Mining, and Safety (DRMS) per the requirements in the approved Henderson Groundwater <br /> Management Plan (GWMP), formally submitted as Technical Revision 16 (TR-16) to the <br /> Henderson Mine and Mill Reclamation Permit No. M-1977-342 (Climax Molybdenum <br /> Company, 2012). Monitoring of water quality and water levels at MNGW-1 has been <br /> performed generally on a monthly basis since summer 2013.A field investigation was initiated <br /> in September 2014 in accordance with recommendations in the Groundwater Quality <br /> Assessment Technical Memorandum submitted to DRMS in December 2013 (2013 <br /> Groundwater Quality Memorandum; AJAX/Clear Creek, 2013). The investigation included <br /> the drilling, logging, installation, and testing of two new monitor wells, MNGW-3, and <br /> MNGW-4,in the shallow alluvial/colluvial deposits north and west of MNGW-1 (Figure 2). <br /> Installation and testing activities associated with the installation of the new monitor wells was <br /> presented in the April 6,2015 status update letter to DRMS (Climax Molybdenum Company, <br /> 2015). In 2017, additional sampling was conducted upstream of the Mine along No Name <br /> Gulch, which conveys water from Red Mountain around the south side of the Mine to the <br /> West Fork of Clear Creek. South of the mine,No Name Gulch is a natural stream.At the base <br /> of the mountain, just south of the Mine facilities, No Name Gulch becomes an unlined <br /> diversion ditch which routes water around the southern and eastern edge of the surface <br /> facilities.This Memorandum presents and summarizes monitoring results for the groundwater <br /> wells and surface water locations through December 2020. <br />