Abstract

The Li+ ion is used frequently as an environmentally acceptable surrogate for sorbing radionuclides in field tracer tests, and experiments using Li are an important part of assessing the potential transport of radionuclides in saturated alluvium south of Yucca Mountain, Nevada, the site of a proposed nuclear waste repository. Equilibrium partition constants (Li+ Kds) were measured using batch studies incorporating a wide range of Li+ concentrations and two different grain-size fractions of alluvium samples from multiple depth intervals in two wells. Cation exchange capacity, surface area, bulk mineralogy from quantitative X-ray powder diffraction, and trace Mn- and Fe-oxyhydroxide mineralogy from extractive studies were evaluated as predictors for linearized Li+ Kd values (Klin) in the alluvium. Many of the predictor variables are correlated with each other and this was considered in the analysis. Linearized Kd values were consistently higher for fine particle-size fractions than for coarse fractions. Single and multivariate linear regression analyses indicated that the clinoptilolite + smectite content, taken together as a combined variable, was the best predictor for Li+ sorption in the alluvium, although clinoptilolite content was clearly a better predictor when the two variables were considered separately in simple linear regressions. Even so, Li+ Klin predictions based on clinoptilolite and smectite abundance were accurate only to within about ±100%. This uncertainty suggests that there is either a high inherent variability in Li+ Klin values or that additional alluvium characteristics not measured or evaluated here may play an important role in simple Li+ cation exchange in the alluvium.

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