Rare Earth Elements: A Tool for Understanding the Behaviour of Trivalent Actinides in the Geosphere
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0985-NN11-06
Rare Earth Elements: A Tool for Understanding the Behaviour of Trivalent Actinides in the Geosphere Belen Buil , Paloma Gómez, Antonio Garralón, and M. Jesús Turrero Medioambiente, CIEMAT, Avda. Complutense 22, Madrid, 28040, Spain
ABSTRACT Rare earth element (REE) concentrations have been determined in groundwaters, granite and fracture fillings in a restored uranium mine. The granitoids normalized REE patterns of groundwaters show heavy rare earth elements (HREE)-enrichment and positive Eu anomalies. This suggests that the REE are fractionated during leaching from the source rocks by groundwaters. Preferential leaching of HREE would be consistent with the greater stability of their aqueous complexes compared to those of the light rare earth elements (LREE), together with the dissolution of certain fracture filling minerals, dissolution/alteration of phyllosilicates and colloidal transport. INTRODUCTION Rare earth elements (REE) have been extensively used as a tool for solving various geological and geochemical problems, since their chemical behaviour is a good geochemical analogue of trivalent actinides such as Am3+, Cm3+, Pu3+ etc. The study of the geochemistry of these elements in waters circulating through U, Th and REE-enriched granitic materials might shed some light on the response of the deeply buried radioactive waste to potential interaction with the groundwaters. These groundwaters exhibit REE patterns that closely resemble those of the rocks through which they have flowed [1]. However, the sensitivity of the REE to adsorption/complexation processes, colloidal and suspension particle transport and redox changes can result in extensive fractionation between the REE-groundwaters and that of the parent rocks [2]. This paper describes the factors that control the REE signature in the groundwaters of a uranium mine named “Ratones” (dug granite) located in the Albalá Granitic pluton (southwest sector of the Iberian Massif, Spain). The evaluation of the specific roles played by each factor is carried out by the interpretation of host rocks and fracture fillings-normalized REE patterns of groundwater samples. METHODOLOGY Five boreholes, with depths varying from 76 to 501 m, were drilled around the mine in order to make a comprehensive hydrogeological and hydrogeochemical study of the water circulating through the main structures. The data set of rock and groundwater samples, considered in this study, comes from three boreholes, SR2 (75m), SR3 (120m) and SR5 (501m).
REE and other trace element concentrations were determined using a Perkin-Elmer SciexElan 5000 inductively coupled plasma-mass spectrometer, using Rh as an internal standard. Finally, the speciation of the major cation, anion and REE, as well as the saturation indices, were calculated using the geochemical modeling code EQ3NR REE DISTRIBUTION OF INTEGRAL PHASES IN THE URANIUM MINE The study of REE geochemical behaviour in a natural system entails a comprehensive study of the characteristics of all the phases: solid (as source or sink
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