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COLORADO LEGACY LAND $CHWARTZWALDER MINE E.S.2.2. In-Situ Treatments Prior to the in-situ treatment of the mine pool in 2020,there were three previous in-situ treatments in 2013, 2015,and 2017.These in-situ treatments are discussed in the following paragraphs. The initial in-situ mine pool treatment was completed in May 2013 as a pre-treatment step to the RO system in which molasses and methanol were injected into the mine pool.Molasses was injected in the#1 Shaft(-800 ft deep),the#2 Shaft(-1,160 ft deep),and the open hole behind the bulkhead.The injection location points are shown on Figure E-2. Three subsequent monthly injections of methanol followed, and a second injection of molasses and methanol was made six months after the start of treatment. The in-situ treatment was interrupted by a 1,000-year rainfall event in September 2013 that prohibited access to the Schwartzwalder Mine from September 2013 until the summer of 2015. Although truck access to the Schwartzwalder Mine was still limited in 2015 and a tanker could not make it to the site, totes of MicroC® (a carbon source)and molasses were brought to the Schwartzwalder Mine and injected into the mine pool in June and September,respectively. In December 2017, the in-situ treatment of the mine pool consisted of an injection of phosphoric acid and molasses into the#2 Shaft. The 25-hp pump was operating and the mine pool effluent was reinjected into the #2 Shaft after being processed through the RO units. The operation of the pump allowed for a mixing of the mine pool to distribute the molasses and phosphoric acid.The pump was shut down on December 22, 2017, and not restarted until January 15,2018. In December 2019,the 60-hp was turned off and was not restarted until April 2020.On January 28,2020,beet molasses,phosphoric acid,and the tracer Rhodamine WT were injected in the#2 Shaft at 410 ft below the Steve Level. On January 29, 2020, alcohol and the tracer Fluorescein were injected in the#2 Shaft at 1,100 ft below the Steve Level.The alcohol was injected into the mine pool at a lower level of the workings to mix the mine pool water as the alcohol bubbled up. The tracers were used to evaluate the mixing of the mine pool. These tracers were water soluble, low toxicity, reasonably stable in normal water environments, and highly detectable. It is expected that the use of tracers will only be required once.A discussion on the conclusion of the tracer test is provided in Section E.5.4 and a complete discussion is provided in Appendix 2. E.S.2.3. In-Situ Treatment Results During the in-situ treatments,the water quality of the mine pool was frequently monitored at a sample port in the WTP just before the RO units.The effectiveness of the in-situ treatments is illustrated by a decrease in the uranium and molybdenum concentrations shown on Figures E-3 and E-4,respectively.As shown on Figure E-3, dissolved uranium concentrations decreased from approximately 23 mg/L before the first in-situ treatment in 2013 to a minimum of approximately 2 mg/L, an approximate 90% reduction in concentration. There was a rebound in dissolved uranium concentrations in 2014,which decreased again after the injections of MicroC® and molasses in 2015.The dissolved uranium concentrations again rebounded to a maximum of approximately 40 mg/L before the second in-situ treatment in 2017 followed by a decrease to a minimum of approximately 4 mg/L,an approximate 90%reduction in concentration.The decreasing uranium concentrations indicate that JUNE 2021 19 AMENDMENT 6