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COLORADO LEGACY LAND <br /> $CHWARTZWALDER MINE <br /> arsenic after mine filling to 13 ppb mean arsenic now in no way affects the effectiveness of RO at being <br /> capable of producing compliant water for discharge,as 4 years of successful dewater demonstrates. <br /> • The chloride concentrations increased after the continuous injection of RO concentrate, which is <br /> amended with barium chloride prior to being returned to the mine pool. <br /> Further demonstration of the chemical stabilization of the mine pool can be seen with a comparison of data <br /> collected in the past 3.5 years to historical data.A hydrologic evaluation was conducted in anticipation of mine <br /> closure,which included a table that summarized results from water quality samples collected from the mine <br /> pool from June 2000 to July 2007(2007,Whetstone)'.A summary of results for water quality samples collected <br /> from the mine pool from March 2018 to July 2021 is provided in Table E-3. These data are comprised of the <br /> quarterly routine monitoring samples collected and provided to DRMS and the increased frequency of samples <br /> collected during the in-situ treatment in 2020. The mean concentrations from the historical hydrologic <br /> evaluation are provided in the last column in Table E-3. A comparison of historical mean concentrations to <br /> current mean concentrations indicates an overall decrease in concentrations with the exception of an increase <br /> in concentrations for the general parameters,which are most likely from the treatment process, and arsenic, <br /> copper,iron,and magnesium.A summary of these increases follows: <br /> • The increase in arsenic and iron concentrations may be the result of either the in-situ treatments or <br /> natural variation in the water entering the mine.The 2007 data had 73%and 75% of non-detects for <br /> dissolved and total arsenic,respectfully,while the current data had 3%and 6%,respectfully.The 2007 <br /> data also had 80% non-detects for dissolved iron and the current data had 0%. Iron is typically <br /> insoluble in oxidized and neutral conditions observed when the mine initially filled and transitioned <br /> to slightly soluble conditions when the mine became mildly reducing from being sealed and the <br /> introduction of in situ treatment.This should be considered a positive development that is consistent <br /> with a lack of increase in sulfate as not iron increasing due to iron sulfide oxidation,but rather from <br /> reductive solubility increase.Arsenic concentrations generally follow this same trend—low solubility <br /> in oxidized conditions with iron present, and slightly more soluble in mildly reducing conditions. <br /> However,a change from 3.6 ppb mean arsenic after mine filling to 13 ppb mean arsenic now in no way <br /> affects the effectiveness of RO at being capable of producing compliant water for discharge,as 4 years <br /> of successful dewater demonstrates. <br /> • The increase in total copper concentrations may be the result of the large percentage of non-detects <br /> (68%)used in the calculation of the mean.The 2007 calculation of the mean only included 29%of non- <br /> detects. <br /> • The increase in magnesium concentrations may not be an increase at all. For example,the 2007 data <br /> listed magnesium under the general parameters without differentiating total or dissolved.If the 2007 <br /> mean magnesium concentration is compared to the current mean dissolved magnesium concentration <br /> ' Whetstone Associates, Inc. Schwartzwalder Mine Hydrologic Evaluation of Mine Closure and Reclamation. <br /> (2007) November 7. <br /> AUGUST 2022 27 AMENDMENT 6 <br />