Natural uranium as a tracer in radionuclide geosphere transport studies

Uranium (U) is a common element in the earth's crust. Because of its solubility characteristics it has been moved and will be moved by naturally occurring geochemical processes. Accordingly, study of the mobile fraction of U offers an opportunity to obtain information on geochemical evolution in general and on how the geochemical evolution controls U transport. This interrelation was utilised in the present study to gain an understanding of U transport in crystalline bedrock. Such information is important in assessing the fate of radionuclides possibly escaping from a nuclear waste repository excavated deep in the bedrock. Uranium transport was studied in the network of water-conducting fractures in the vicinity of a small U-Th deposit. Investigations were focused on U fractions susceptible to groundwater, i.e. U on fracture surfaces and in rock pores close to fractures. Uranium-series disequilibria were examined to obtain spatial and temporal information on past transport. Reference U samples prepared in the laboratory and other U-bearing samples were examined to assist the interpretation. Simultaneous carrying out of in situ studies and laboratory experiments assisted the development of experimental methods. Similar observations of U fixation in situ and in the laboratory provided a good insight into the mechanisms by which U is fixed. Information on reversible and irreversible U fixation was obtained. Attachment of U to ferric compounds appeared to take place through mechanisms that fix U more firmly than adsorption. Movement of U isotopes in the rock matrix close to fractures in centimetre scale was observed. Because of the interest in future evolution of an underground waste repository environment, the observations on geologically recent U transport were scrutinised for information they might contain on events in past evolution. Uranium-series disequilibrium signatures that might reflect such events were identified. These signatures could not have been preserved so long unless they had been shielded from present groundwater leaching, and they could not have formed in the first place unless significant amounts of U had been subject to groundwater leaching. Additionally, in order for groundwater leaching to have generated such distinctive disequilibria it must have occurred not later than 300,000 years ago. The findings of the study suggest that the loosely bound U (labile U) that was abundantly present in the samples is a sensitive indicator of the changes occurring in groundwater-rock conditions. Labile U can serve as a valuable tracer of radionuclide migration, and also as an indicator of the possible impacts of processes (e.g. glaciation-deglaciation processes) that might occur in the geochemical evolution under future climatic changes. (orig.)