In sync with environmental pollution of solutes in nature, from source, process to consequence, geochemical processes (leaching and sorption) and hydraulic transportation of the rare earth elements (REEs) have been investigated at Yucca Mountain (YM), Nevada. This research includes the leaching behavior of trace elements (including REEs) from aquifer rocks, the surface complexation reactions of REEs in synthetic groundwater, and transportation of reactive REEs in the local-scale groundwater system of YM. This dissertation includes three projects. These studies indicate that surface complexation reactions may retard the transportation of REEs along groundwater paths in YM, which suggests a similar behavior of aqueous actinides.

The first project describes the study of leaching characteristics of trace elements (including REEs) from three widely-distributed aquifer host rocks in YM and Nevada Test Site areas. Leachate compositions are significantly impacted by leaching time, pH, grain size, and rock mineralogy. Geochemical modeling was conducted to explore the redistribution and leaching process of trace elements. Leaching factors of various trace elements were derived to quantify the leached trace elements from each of three aquifer rocks.

The second project describes the adsorption behavior of two REEs, ytterbium (Yb) and europium (Eu), using an integrated approach of laboratory experiments and numerical modeling. Experimental results indicate that Yb and Eu show moderately pH-dependent adsorption behavior, which was simulated with surface complexation models (SCMs). Due to their advantage of wide relevance to ionic strength, experimentally derived SCMs can be used for adsorption reactions in transport modeling.

The third project describes the numerical simulation of REE transport in the mountain-scale groundwater model of YM. A three dimensional reactive groundwater model, PHAST3D, was conceptualized for REE transport modeling by coupling with the experimentally derived surface complexation reactions. Modeling results indicate that sorption reactions may retard transport of REEs and attenuate concentrations of dissolved REEs.

The research demonstrates that surface complexation reactions play an important role in the REE transport in the YM area. As the natural analogue of trivalent-actinides, the simulation results of REEs are expected to help us determine the fate of nuclides stored in the high level nuclear repository in YM.