The chemical state of rare earth elements (REEs) in groundwater was determined by a combination of ultrafiltration techniques maintaining in–situ pressures and anaerobic conditions, speciation considering contribution of natural organic matter, and fingerprinting using REE patterns of the stability constants for probable complexes of REEs in groundwater. Groundwater samples were collected from a horizontal borehole at a depth of 200 mbGL (meter below ground level) in the Mizunami Underground Research Laboratory (MIU), Gifu, Japan. Spectroscopic analysis for colloidal matter on a membrane filter was also conducted to estimate chemical properties of colloidal ligands. Results of this study suggest that REEs–humic complexes are the dominant chemical states of REEs in this groundwater. The presence of humic substances in groundwater was also confirmed by spectroscopic analysis. Thus, it is expected that chemical behavior of REEs mainly is dependent on humic substances. The information of chemical state of REEs is useful for prediction of behavior of trivalent actinides in groundwater.

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