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",•; <br />GeoScience Services '~ IggC ;-'rr~ 2263 %ingston Road <br />Grand !unction, CO 81503 <br />FEB 0 8 2006 ~ (970)314-3356 <br />bawd .dmat~nn 1 ~/ <br />Date: February 7, 2006 ,~,, e! ~ ~ , sJ'Q - M - / 9 7 7 - 3 06 <br />To: Colorado Department of Mining and Geology (DMG), Grand Junction Office <br />Re: Response to the January 17, 2006 letter from the Colorado Department of <br />Mining and Geology (DMG) <br />The purpose of this letter is to respond to a request for additional inforrnation presented <br />in the Colorado Department of Mining and Geology (DMG) letter dated January 17, <br />2006. First, it is important to clarify any potential misunderstandings regarding the <br />model used in the groundwater simulations. The term "simple dilution model" used by <br />the DMG to describe transport simulations maybe misinterpreted. PORFLOW is not a <br />dilution model but a multiphase transport model that simulates the effects of transport on <br />contaminant concentrations. Since none of the simulations showed infiltrating water <br />from the waste piles entering the upper most aquifer, there is in essence no dilution by <br />groundwater but rather an attenuation of the species of concern by hydrodynamic <br />dispersion and diffusion. A 2-dimensional model aligned parallel to the dip of the <br />geologic units was used to simulate subsurface transport. Since no dispersion or <br />diffusion is allowed in the third dimension, resulting in higher predicted concentrations of <br />species of concern, the 2-D model is inherently conservative. <br />Second, the term conservative modeling approach is intended to provide a reasonable <br />mazgin of error for predicting the rate and concentration of the subsurface migration of <br />species of concern. Commonly, the degree of conservatism is taken to such an extreme <br />that the simulations are no longer realistic. For example, ignoring sorption for transport <br />simulations ignores the fact that without sorption by the geologic material underlying the <br />waste piles, there would be no mineralization in the Uravan Mineral Belt. As noted by <br />Pyrih and Associates (1982), the geochemical mechanisms through which clay-bearing <br />material immobilizes ions from solution are analogous to the mechanisms that aze <br />credited with forming uranium ore bodies in sedimentary rocks. They further state that <br />these materials will function as a geochemical trap and immobilize potential chemical <br />and radiochemical contaminants. The Colorado Department of Public Health and <br />Environment (2000) reports that tests on Salt Wash shales indicate Cation Exchange <br />Capacity (CEC) values ranging from 9.9 meq/100g to 32 meq/100g and a pH of <br />approximately 8.4. These tests indicate that the Salt Wash shales would attenuate <br />contaminants contained in repository seepage. A realistic modeling approach would use <br />the mean sorption value for the species of concern for transport simulations while a <br />conservative approach would use the lower end of the sorption range-an approach used <br />for modeling predictions at radionuclide disposal sites (U. S. Department of Energy, <br />2004). The modeling simulations used in the Cotter mine reports used a zero value for <br />sorption that is unrealistic but still results in contaminant migration rates that failed to <br />reach the water table. <br />RECEIi/ED <br />FEB 2 12006 <br />pivisioa ai Minerah and Geolopr <br />