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GeoScience Services C/ az63 K;ngsron eoad <br />Grand Junct;on, CO 81503 <br />(970J J1~3356 <br />DEC p 2 2415 [/ <br />Date: November, 28, 2005 a.~,<s . ~+~~:~.~~ "<~~° >.`;a,a.w ~ _. ~ ~ '~ 7 - 3 ~ (~ <br />To: Colorado Department of Mining and Geology (DMG), Grand Junction Office <br />Re: Effects of sorption on transnort <br />The purpose of this letter is to present and discuss the results of an additional modeling <br />simulation requested by the DMG to evaluate the effects of sorption on transport of <br />contaminants from mining-waste piles to the underlying bedrock. Earlier simulations <br />used a conservative approach of a constant source and published values of sorption <br />coefficients from the technical literature. These earlier simulations showed that there is <br />no significant impact to the environment from the mining-waste piles. <br />The attached page shows a comparison of transport for uranium using a realistic sorption <br />value and no sorption. The net result is a doubling of the transport distance for the no <br />sorption scenario. However, even under this extremely conservative approach, there is <br />no significant impact to the underlying groundwater resources. "I'he no-sorption plume <br />fails to reach the water at IE-06 of the initial concentrations (part per billion range). <br />To summarize the results of the initial and subsequent contaminant transport modeling <br />analysis to quantify environmental risks of uranium mining, the important facts and <br />findings are listed below: <br />• A continuous source was used to supply leached "species of concern" to the <br />unsaturated bedrock. Under normal conditions, the source would diminish with <br />time limiting migration. <br />• Initial concentrations of species of concern were leached using acidic solutions <br />not rainwater with a near neutral pH. Consequently, the initial source has an <br />inherent upward bias and would provide initial concentrations for the species of <br />concern significantly higher than under normal conditions. <br />The average crustal abundance of the species of concern provides background <br />concentrations that seepage from the waste pile would actually dilute. For <br />example Wedephol (1968) reports that the crustal average for uranium is 3.4 <br />mg/kg (ppm). Pore-water and the adjacent rock would be in chemical equilibrium <br />resulting in background concentrations in the ppb range. Remember, the mines <br />are located in a mineralized district where background concentrations are <br />expected to be significant. <br />~ The species of concern are generally cations or positively charged ions migrating <br />in the saturated portions of the rock pores. Several hundred feet of the underlying <br />rock consist of shales that contain negatively charged clays that readily adsorb the <br />positively charged ions. The net effect is that the plume would be adsorbed to <br />background equilibrium concentrations-indistinguishabl~y~ the natural <br />environment in the area. C ~/ <br />V <br />AFC o~ FO <br />'~6 <br />.. L~id>jderd/dd <br />Ar`,~ <br />~I <br />