Low-Temperature Mobility of Rare Earth Elements (REE), U, and Th at the Olkiluoto Site, SW Finland
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Low-Temperature Mobility of Rare Earth Elements (REE), U, and Th at the Olkiluoto Site, SW Finland Nuria Marcos, Helsinki University of Technology, P.O. Box 6200, FIN-02015 HUT, Finland ABSTRACT This work is a first effort to link hydrogeological measurements to structural modeling by using the mobility of rare earth element (REE), U, and Th at the Olkiluoto site. REE, U and Th concentrations were measured in 5 groundwater samples located at depths varying between 132 and 446 m in three drill holes at the Olkiluoto site. The pH of groundwater samples are all about 8 and Eh(Pt) varies from –50 to –200 mV. Also the REE, U and Th concentrations of rock samples collected from core sections in the vicinity of water conducting fractures were measured to examine rock-water interactions in the system. The rock samples were cut into slices parallel to the fracture surface. An aliquot of each slice was leached with 0.5 N acid nitric to examine the readily leachable REE, U and Th fraction of each slice. The groundwater samples were normalized to each of the analyzed rock samples. Although the REE concentrations of the water samples are overall depleted in all the REEs compared to the rocks and leachates, and whole REE spectra could not be obtained for any of the groundwater samples, enrichment in the intermediate REE (IREE) with respect to the light REE (LREE) is observed. An increasing loss of REE and U from rock matrix towards fracture surface is observed in samples at depths of 141, 159, and 466 m. Loss of U is also observed in a sample at about 246 m depth. Loss of Th is only observed at the depths of 159 and 246 m. The loss of U (readily leachable fraction) in the rock samples was found to be linked to the hydraulic conductivity in the related water-conducting fractures. INTRODUCTION REEs are considered as useful chemical analogues for trivalent actinides, and U and Th can be used as analogues for Pu (e.g., 1, 2). This study aims to get information on the behavior of REE, U and Th in a still relatively undisturbed system by linking hydrological measurements to structural modeling. Olkiluoto in Eurajoki (SW Finland) has been selected as the final disposal site for the spent nuclear fuel repository and it is intend to host an Underground Rock Characterization Facility for further investigations [3]. The application of the mobility of the elements to structural and paleohydrological issues is also considered. Background geological and geochemical site information in [4] and references therein are used as a starting point for this study. EXPERIMENTAL The rock samples used in this study were collected from cores from the depth intervals in Table I. Each core section was cut longitudinally. One half was used to prepare petrographic thin sections and the other was used for chemical analysis as indicated in Figure 1. Each section (T ≡ transverse section for whole rock analysis) and slice (S ≡ leachates) was crushed and then powdered with a tungsten carbide mill to a particle-mesh size less than 100 µm. The transverse section (T)
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