Low-Temperature Mobility of Rare Earth Elements (REE), U, and Th at the Olkiluoto Site, SW Finland

Abstract

ABSTRACTThis 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.