Variations in 234U/238U activity ratios in groundwater: A key to flow system characterisation?

Two key questions concerning geological disposal of high-level radioactive waste are (i) how robust is the barrier system during long term disturbances and (ii) do we know the possible disturbances well enough? One step towards answers is to study how groundwater flow systems have behaved in the past. Studying naturally occurring radioactive tracers in groundwater offers useful information. Uranium isotopes 234U and 238U are oft-used tracers. A characteristic of the 234U/238U activity ratio (AR) is its considerable variation in groundwater. It is known that the reason for variation is isotopic fractionation at the rock–water interface. However, the reason why AR varies in groundwater is not clear. We tried to answer this question by carefully examining the geochemical and nuclear chemical origins of U isotopic fractionation and by studying a large body of U data from Finnish and Swedish investigation sites. We found that despite large variations in AR and U concentrations, the respective 234U and 238U activities show a correlation which contains useful information of the flow system behaviour which is in line with other hydrochemical data.

[1]  K. Rasilainen,et al.  Fracture smectite as a long-term sink for natural radionuclides – indications from unusual U-series disequilibria , 2000 .

[2]  A. Luukkonen,et al.  Geochemical modelling of groundwater evolution and residence time at the Kivetty site , 1998 .

[3]  M. Paananen,et al.  The Palmottu Natural Analogue Project , 1995 .

[4]  Nuria Marcos,et al.  Direct alpha-recoil as a process to generate U-234/U-238 disequilibrium in groundwater , 2006 .

[5]  J. Adloff,et al.  Recoil and Transmutation Effects in the Migration Behaviour of Actinides , 1991 .

[6]  N. Sturchio.  Uranium-series disequilibrium: Applications to Earth, Marine, and environmental sciences , 1993 .

[7]  Ari Luukkonen,et al.  Hydrogeochemical Interpretation of Baseline Groundwater Conditions at the Olkiluoto Site , 2004 .

[8]  K. Rasilainen,et al.  Isotopic Fractionation of U in Rocks Reflecting Redox Conditions around a Groundwater Flow Route , 2000 .

[9]  P. Wikberg,et al.  Äspö, SE Sweden: a natural groundwater flow model derived from hydrogeochemical observations , 1995 .

[10]  R. Harmon,et al.  Uranium-series disequilibrium : applications to earth, marine, and environmental science , 1992 .

[11]  E. Tullborg,et al.  The use of natural uranium decay series studies in support of understanding redox conditions at potential radioactive waste disposal sites , 2003 .

[12]  K. Rasilainen,et al.  Release of uranium from rock matrix--a record of glacial meltwater intrusions? , 2003, Journal of contaminant hydrology.

[13]  J. Adloff,et al.  Chemical Effects of α-Decay in Uranium Minerals , 1989 .

[14]  Pitkanen,et al.  Hydrochemical stability of groundwaters surrounding a spent nuclear fuel repository in a 100 , 000 year perspective , 2001 .

[15]  K. Rasilainen,et al.  A multisystem modeling approach for uranium-series dating , 1997 .

[16]  H. Nordman,et al.  Near-field model REPCOM , 1994 .