The Feasibility of USD Method in Tracking Past Hydrogeochemical Changes in the Fennoscandian Shield
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7KH)HDVLELOLW\RI86'0HWKRGLQ7UDFNLQJ3DVW+\GURJHRFKHPLFDO&KDQJHVLQWKH )HQQRVFDQGLDQ6KLHOG Kari Rasilainen1, Juhani Suksi2, Timo Ruskeeniemi3, Petteri Pitkänen4 1 VTT Processes, P.O. Box 1608, FIN-02044 VTT, Finland; 2University of Helsinki, P.O. Box 55, FIN-00014 HY, Finland; 3Geological Survey of Finland, P.O .Box 96, FIN-02151 ESPOO, Finland; 4VTT Building and Transport, P.O. Box 1800, FIN-02044 VTT, Finland $%675$&7 Uranium-series disequilibrium (USD) method is the only paleohydrogeological method that for the time scale relevant for performance assessments can detect and directly date past hydrogeochemical changes. Such changes cause U mobility which is seen as uranium-series disequilibrium. We have reviewed the method taking into account experimental and interpretation aspects and identified pros seem to outweigh identified cons. Reported USD studies in Finland and Sweden have earlier been mostly qualitative but a quantitative methodology has been developed for future studies. ,1752'8&7,21 Many countries have chosen deep geological disposal for their nuclear waste management option, as bedrock is thought to preserve current favourable chemical conditions for the repository. Performance assessments of the disposal must estimate the effects of possible hydrogeological changes for hundreds of thousands of years into the future. In an effort to help performance assessments scope possible changes paleohydrogeological studies have been conducted as a means to understand past conditions in the bedrock. The uranium-series disequilibrium method (USD) is the only paleohydrogeological method that can at the same time detect changes in hydrochemistry and directly date them for the time scale relevant for performance assessments. Its classical application has been dating in Quaternary geology and the method is one of the cornerstones in current paleoclimatology [1]. In this paper the USD method is discussed with an eye on possible changes in the Fennoscandian Shield of which glaciations are the most extensive. It appears that the USD method has not been used to its full potential in reported studies discussing past hydrogeochemical changes [2,3,4,5]. Direct mineralogical observations and qualitative USD interpretations have been covered well, but quantitative testing of alternative U mass flow scenarios has been largely lacking. The purpose of this paper is to critically review the feasibility of the USD method covering experimental and interpretation viewpoints. Our aim is to promote balanced use of the method that avoids over- and understatements. + 1 and U accumulation by Th-230/U-234 < 1. When this activity ratio is not equal to 1, U has been either accumulating or releasing within 380 000 years and, conversely, when it is equal to 1, the U accumulation or release event took place at least 380 000 years ago. Activity ratios provide qualitative information of the nature of the U mobility. If no contact U-234 2,5.105 a
U-238 4,5.109 a Pa-234m 1,2 min Th-234 24,1 d
Th-230 7,6.104 a Ra-226 1,6 .103 a
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