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COLORADO LEGACY LAND <br /> $CHWARTZWALDER MINE <br /> E.5.2.4. Chemical Stabilization of the Mine Pool Conclusion <br /> The concentrations and linear regressions for uranium and molybdenum over three consecutive years are <br /> shown on Figure E-5. The uranium concentrations (dissolved and total) have maintained an average of <br /> approximately 12 mg/L with a slight positive slope.Although the trend indicates a slight positive slope, the <br /> majority of the uranium concentrations are in the 10 to 15 mg/L range. The molybdenum concentrations <br /> (dissolved and total)have maintained an average of approximately 0.6 mg/L with a slight negative slope.These <br /> stable average concentrations indicates that the in-situ treatments have been effective in not only reducing <br /> U(VI)to U(IV)yet maintaining an environment in which U(IV)is not oxidized to U(VI).The January 2020 in-situ <br /> treatment did not show the dramatic decreases in dissolved uranium concentrations as during the first two in- <br /> situ treatments,which is likely an indication that an environment favorable to U(IV)has been achieved. <br /> Further demonstration of the chemical stabilization of the mine pool can be seen with a comparison of data <br /> collected in the past three years to historical data.A hydrologic evaluation was conducted in anticipation of <br /> mine closure,which included a table that summarized results from water quality samples collected from the <br /> mine pool from June 2000 to July 2007 (2007,Whetstone)z.A summary of results for water quality samples <br /> collected from the mine pool from March 2018 to September 2020 is provided in Table E-2. These data are <br /> comprised of the quarterly routine monitoring samples collected and provided to DRMS and the samples <br /> collected during the in-situ treatment during 2020. The mean concentrations from the historical hydrologic <br /> evaluation are provided in the last column in Table E-2. A comparison of historical mean concentrations to <br /> current mean concentrations indicates an overall decrease in concentrations with the exception of a limited <br /> number of increases,specifically arsenic,chloride,and iron,while most of the constituent concentrations have <br /> decreased. Therefore, the decrease in constituent concentrations further demonstrate that chemical <br /> stabilization of the mine pool has been achieved for the last three consecutive years. <br /> 2 Whetstone Associates, Inc. Schwartzwalder Mine Hydrologic Evaluation of Mine Closure and Reclamation. <br /> (2007) November 7. <br /> JUNE 2021 21 AMENDMENT 6 <br />