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COLORADO LEGACY LAND <br /> SCHWARTZWALDER MINE <br /> The usefulness of in-situ treatment as a feature of this long term strategy is still under consideration <br /> and may be addressed in a subsequent permit revision. <br /> E.Z. INTERCEPTION AND TREATMENT OF ALLUVIAL GROUNDWATER <br /> Although the ore sorter was removed in 1999, legacy contamination associated with the Ore Sorter <br /> Decommissioning Area and other mining-impacted areas remain in the alluvial valley at the site. Material <br /> associated with these mine disturbances is a mix of mine waste rock and naturally occurring soils, alluvial <br /> sands,and gravels. For the purposes of this discussion,these materials are collectively referred to as"alluvial <br /> fill". (The word "pad" has historically been used in reference to these disturbed alluvial fill materials.) <br /> Construction of the pad was altered over the operational history of the Mine as needed to support changes in <br /> mining facilities and operations. Because seasonally varying amounts of groundwater associated with Ralston <br /> Creek inundate portions of subsurface alluvial fill materials,and some of these materials include mineralized <br /> waste rock, water quality in the creek is subject to seasonal impacts to water quality, namely uranium <br /> concentrations that under low streamflow conditions can exceed the applicable water quality standard (0.03 <br /> mg/L, which is the U.S. Environmental Protection Agency's [USEPA] primary drinking water standard for <br /> uranium and the State of Colorado's domestic water supply limit for uranium). Probable leaching mechanisms <br /> include variable cycles of oxidation and formation of soluble uranium salts on waste rock surfaces,combined <br /> with seasonally fluctuating alluvial groundwater tables.Since operational mine dewatering and treatment was <br /> terminated in 2000,water quality monitoring data have consistently demonstrated that uranium is the primary <br /> constituent of concern. Due to geomorphic valley features in the vicinity of the Site,surface water in the creek <br /> seeps into the alluvium in upper portions of the pad to become alluvial groundwater that flows back towards <br /> the creek in the lower portions of the alluvial pad. The surface water monitoring station just below CLL's <br /> property line(SW-BPL)is situated just below a geologic constriction in the valley that forces most of the alluvial <br /> groundwater to the surface and into the creek channel to become surface water. Surface water monitoring <br /> station SW-BPL is considered a point of compliance for Ralston Creek. In 2010,the water treatment plant in <br /> the valley was refurbished with a new ion-exchange water treatment system,along with infrastructure to begin <br /> interception,pumping,and treatment of alluvial groundwater from existing sumps and monitoring wells across <br /> the alluvial pad.These systems were authorized under a Colorado wastewater treatment discharge permit(CO- <br /> 0001244) with the Water Quality Control Division (WQCD) of CDPHE, along with an amendment to the <br /> Radioactive Materials License (RML CO-369-06) and technical revisions (Technical Revision-12, Technical <br /> Revision-15) to the Mine Permit(M-1997-300).All sumps and monitoring wells were individually permitted <br /> with the State Engineer's Office,Division of Water Resources. <br /> E.3. ISOLATION OF RALSTON CREEK FROM SOURCES OF IMPACTS <br /> While the alluvial groundwater interception and treatment system resulted in dramatic reductions in uranium <br /> concentrations in Ralston Creek (2013 Mine Plan Amendment 4), it was not sufficient to eliminate seasonal <br /> low-flow (base groundwater flow) exceedances of water quality standards at SW-BPL, and in 2012 an <br /> engineered upper cut-off wall was constructed to direct up to 8 cubic feet per second (cfs) of unimpacted <br /> upgradient alluvial groundwater and surface water flowing above historic mine facilities into an 18-inch HDPE <br /> pipeline,convey this water past the Site,and release it back into the creek below the Site. <br /> The pipeline diversion was conceived and designed to achieve two basic objectives: 1) prevent upgradient <br /> groundwater and surface water from becoming impacted as it flows past the Site, and 2) to help dewater the <br /> alluvial fill during low-flow conditions such that other remedial measures,such as excavation of contaminated <br /> MAY 2022 11 AMENDMENT 6 <br />