HR: 11:35h
AN: V11H-06    [PDF]
TI: Decay-Series Disequilibria in a Chalk Aquifer : Characterisation of Water-Rock Interaction
AU: * Hubert, A
EM: ahubert@ipgp.jussieu.fr
AF: CEA, Departement Analyse Surveillance Environnement, Bruyeres le chatel, BP12 91680 France
AU: * Hubert, A
EM: ahubert@ipgp.jussieu.fr
AF: Laboratoire geochimie-cosmochimie, Institut de Physique du globe de Paris, 4, place Jussieu, Paris cedex 05, 75252 France
AU: Bourdon, B
EM: bourdon@ipgp.jussieu.fr
AF: Laboratoire geochimie-cosmochimie, Institut de Physique du globe de Paris, 4, place Jussieu, Paris cedex 05, 75252 France
AU: Pili, E
EM: eric.pili@cea.fr
AF: CEA, Departement Analyse Surveillance Environnement, Bruyeres le chatel, BP12 91680 France
AB: We have studied uranium-series disequilibria in a chalk aquifer and the unsaturated zone above it in order to characterise the time scales of radionuclide migration from the water recharge zone of the aquifer to the nearby river. Our field area is located in Champagne (France). The aquifer is characterized by a double porosity : matrix and fracture, providing both a fast and a slow pathways for water flow. We have collected both carbonate rocks and groundwater samples from boreholes and spring and river water from the same area. Rock/water interaction inside the aquifer induces dissolution and reprecipitation of carbonates, together with a mobilization of uranium, and (-recoil effect results in preferential mobilization of daughter nuclides. We have measured uranium and thorium isotopes for carbonates samples from the aquifer by TIMS and multi-collection ICP-MS. The fractionation of uranium and thorium isotopes is distinctive in the various parts of the aquifer. Rock samples from the saturated zone show a depletion in 234U with a (234U/238U) ratio ranging from 0.945 to 0.993 (ñ 0.005). This indicates that uranium 234U has been released by rock/water interaction over the last million year. Nevertheless, rock samples from the water table oscillation zone display a (234U/238U) activity ratio greater than 1 and range from 1.002 to 1.052 (ñ 0.005), suggesting uranium reprecipitation possibly by a redox front. (230Th/238U) ratios range from 1.25 to 1.59 (ñ 0.03) in both the saturated and vadose zone, whilst (230Th/232Th) ratios vary from 1.89 to 5.68 (ñ 0.05) with the highest values for the water table oscillation zone. The 238U-230Th system suggests the existence of a iron oxihydroxide and/or silicate phases which influence the redeposition and/or adsorption of elements inside the zone of water oscillation. Modelling is in progress in order to determine leaching rate of the radionuclides in the substratum together with the adsorption/desorption rate constants on surface coating. We are also curently analysing radium isotopes which will provide us further insights on the migration timescale of uranium-series nuclides in groundwater.
DE: 1045 Low-temperature geochemistry
DE: 1806 Chemistry of fresh water
DE: 1832 Groundwater transport
DE: 1886 Weathering (1625)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting