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COLORADO LEGACY LAND
<br /> $CHWARTZWALDER MINE
<br /> data, (ii)the operational periods of the WTP over the last 4 years,(iii) observed gradients in monitoring wells,
<br /> and(iv)active dewatering performed for—6 months of the year,the result is a physically stable mine pool.
<br /> E.5.2. Chemical Stabilization of the Mine Pool Demonstration
<br /> The chemical stabilization of the mine pool water has been accomplished through a number of steps taken to
<br /> optimize the management of the Site, including reducing mine recharge from sumps as reclamation has
<br /> progressed, closure of mine openings,as well as in-situ treatments. Functionally, keeping the mine chemistry
<br /> stable enough in its overall composition to allow the RO to be used to maintain the mine pool in its target
<br /> dewatered range is how achieving and maintaining chemical stabilization should be evaluated.
<br /> The mine chemistry is in a"brackish"water chemistry designation,where dissolved solids are higher than the
<br /> freshwater range,but not so saline that it cannot be readily treated to make fresh water acceptable for other
<br /> uses. Limiting oxidation processes has been accomplished by decreasing oxygen flux into the mine by
<br /> closing/filling the open hole and stopping active ventilation,as well as by adding microbial reagents in situ into
<br /> the mine workings to consume oxygen and reverse historical oxidation that occurred when the mine was open,
<br /> and air was blown through the mine to decrease radon exposure. The amount of in-situ treatment is expected
<br /> to decline as ventilation has been stopped,the mine openings have been closed,and the oxidized rock in the
<br /> saturated workings has now been flooded with biochemically reducing water.The water in the mine has been
<br /> shifted from its oxidizing form (+200 millivolts when initially flooded) to now be consistently negative
<br /> millivolts(from March 2019 to the present,the raw feed into the WTP has ranged from-80 millivolts to-157.9
<br /> millivolts).When the mine water is chemically reducing,fluctuating mine pool levels cannot cause leaching of
<br /> mine rock by oxidative processes.
<br /> In addition to creating a bulk mine water chemistry that is chemically reducing to minimize oxidative leaching
<br /> of mine rock,the in-situ treatment regimen is also designed to create a zone of sulfate-reducing conditions for
<br /> the reduction of soluble uranium species to insoluble uranium species,e.g.,from the hexavalent oxidation state
<br /> (U[VI]) to the tetravalent oxidation state (U[IV]), with a focus on the upper mine workings. Numerous field-
<br /> based studies have documented that a sulfate-reducing environment is conducive to the reduction of U(VI)to
<br /> U(IV) resulting in the decline in observed uranium concentrations (e.g., Anderson et al.,2003; Watson et al.,
<br /> 2013)2, and also sulfate-reducing environments consume oxygen that otherwise could lead to uranium
<br /> oxidation.The reducing environment is essential for the reduction of U(VI)to U(IV)to immobilize uranium and
<br /> Z Anderson,R.T.,Vrionis,H.A.,Ortiz-Bernad,I.,Resch,C.T.,Long,P.E.,Dayvault,R.,Karp,K.,Marutzky,S.,Metzler,
<br /> D.R.,Peacock,A.,White,D.C.,Lowe,M.,Lovley,D.R.(2003)Stimulating the in situ activity of Geobacter
<br /> species to remove uranium from the groundwater of a uranium-contaminated aquifer,Appl. Environ.
<br /> Microbiol.,69,5884-5891.
<br /> Watson, D.B.,Wu, W., Mehlhorn, T., Tang, G., Earles, J., Lowe, K., Gihring, T.M., Zhang, G., Phillips, J., Boyanov,
<br /> M.I., Spalding, B.P., Schadt, C., Kemner, K.M., Criddle, C.S., Jardine, P.M., Brooks, S.C. (2013) In situ
<br /> bioremediation of uranium with emulsified vegetable oil as the electron donor,Environ.Sci.Technol.,
<br /> 47,6440-6448.
<br /> AUGUST 2022 19 AMENDMENT 6
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