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• LOCAL SOIL AND CLAY ALLUVIUM <br />Natural clay materials have the capacity to interact with <br />soluble trace elements. Geochemical interaction may result in <br />the sorption and immobilization of trace elements by the clay. <br />Functioning as geochemical traps, natural clays may mitigate <br />trace element migration into surface and groundwaters. <br />To assess the capacity of local clay material .to ion-exchange <br />with potential contaminants, five samples of local soil and clay <br />alluvium were sent to the Colorado School of Mines Research In- <br />statute for cation-exchange capacity measurements. Surface-soil <br />samples were collected from the vicinity of the present coal- <br />waste disposal site. Clay alluvium from below the surface was <br />retrieved from the two test bores drilled at the site. <br />Table 4 summarizes the cation-exchange properties of the <br />clay materials sent for evaluation. The cation-exchange capaci- <br />ties of the five samples are typical of western soils. Undis- <br />turbed surface soil (Sample no. P 0316-5) near North Thompson <br />Creek showed the highest cation-exchange capacity (CEC) and <br />evidence for the presence of sodium-type clays. The cation- <br />ezchange capacities of the other surface soils and subsurface <br />clay alluvium were lower. Montmorillonite, amectite, and ver- <br />miculite clays display the highest cation-exchange capacities <br />ranging between 80 and 150 meq/100g. The cation-exchange <br />capacity of illite commonly varies between 10 and 40 meq/l0og, <br />and that of kaolinite between 3 and 15 meq/100g. <br />30 <br />