Quantifying Radionuclides Migration From U-series in an Unconfined Aquifer

HR: 08:30h
AN: H41I-03 [Abstracts]
TI: Quantifying Radionuclides Migration From U-series in an Unconfined Aquifer
AU: * Hubert, A
EM: ahubert@ipgp.jussieu.fr
AF: CEA/DASE Laboratoire d'HydrogＰchimie et d'Etude de Sites, BP 12, Bruyeres-le-Chatel, 91680 France
AU: * Hubert, A
EM: ahubert@ipgp.jussieu.fr
AF: IPGP Laboratoire deGeochimie et Cosmochimie, 4 place Jussieu Tour 14-24, Paris Cedex, 75252 France
AU: Bourdon, B
EM: bourdon@ipgp.jussieu.fr
AF: IPGP Laboratoire deGeochimie et Cosmochimie, 4 place Jussieu Tour 14-24, Paris Cedex, 75252 France
AU: Pili, E
EM: eric.pili@cea.fr
AF: CEA/DASE Laboratoire d'HydrogＰchimie et d'Etude de Sites, BP 12, Bruyeres-le-Chatel, 91680 France
AB: With the increase of contaminant flux of radionuclides in the environment, there is a need to understand and model the processes that control the distribution of uranium and its daughter products during transport within aquifers. We have used U-series disequilibria as an analogue for the transport of anthropogenic nuclides in a small watershed of a chalk aquifer in Eastern France. We have measured naturally occurring U and Th isotopes in rock and water phases in order to determine transport mechanisms at water/rock interface and quantify parameters controlling the migration of radionuclides. The studied aquifer has a double porosity and is characterised by a saturated matrix up to 40m above the water-table and variably saturated fractures. Measurements of U and Th isotopic composition in the water phase were performed on a MC-ICPMS which enable in particular high precision measurements of (230Th/232Th) activity ratio. Water samples were obtained from wells both in low and high water-table conditions. Chalk samples were collected at the same wells and were analysed also for U-series and Sr isotopes. We observed seasonal precipitation of uranium ((234U/238U) activity ratio above 1) in the zone of water-table fluctuation linked with the oversaturation of water with respect to calcite during low water-table period. (230Th/232Th) activity ratios in water are lower than in the substratum meaning that the time of water infiltration is not long enough to allow an equilibration of groundwater with rock. Water acquires its Th isotopic composition when interacting with another phase of the aquifer (soil) and joins the water-table mainly through fractures. The complete data set obtained on solid and liquid phases of the aquifer allow us mass balance calculations and to model the behaviour of uranium and thorium during transport. The 1-D model we have developed considers a transient behaviour of radionuclides in the aquifer and for the first time allows the calculation of the evolution of rock composition with time. We have then calculated a global uranium weathering rate within the aquifer of 2.5 10-6 y-1.
DE: 1832 Groundwater transport
DE: 1886 Weathering (1625)
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting