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Hydraulic Head and Salinity Calibration of the Paris Basin Hydrogeological Model Developed by IRSN


Congress title :IHLRWM - 12th International High-Level Radioactive Waste Management Conference
Congress location :Las Vegas
Congress date :07/09/2008

Summary

IRSN is conducting studies and research as a support of safety evaluation for a potential HLW repository in the deep argillaceous formation of the - Meuse/Haute-Marne - site, currently studied by ANDRA (the French National Radioactive Waste Management Agency) by means of the Bure underground research laboratory. In particular, IRSN modeled radionuclide migration from this potential repository site to surface outlets. This modeling exercise required to determine the behaviour of underground water flows at a much larger scale, consisting of the whole Paris sedimentary basin. IRSN first built a geological model of this sedimentary basin. Then, IRSN and the Paris School of Mines elaborated a hydrogeological model, calibrated on the hydrodynamical parameters measured on-site, which allowed the identification of radionuclide pathways, the estimation of transfer times associated with those pathways and the location of potential outlets. The preferred strategy to derive the hydrogeological model from the measured hydrogeochemical and hydrogeological parameters obtained throughout the Paris basin was a simultaneous calibration of the hydraulic heads and salinity values, as presented in this paper. The Keuper halite formation (Triassic) located in the eastern part of the basin was considered as the imposed salt source for the model. The calibration required, in particular, to take into account the basin's major faults which allowed vertical hydraulic connections between aquifers and thus allowed salt water fluxes toward the shallower formations. In addition, because the presence of pure halite in high quantities in the Keuper formation can generate brine close to salt saturation and thus yield fluid density variations up to 25%, the density effects, coupled with the temperature effects on density variation, were taken into account in the flow and transport modeling. The results presented in this paper show that when considering these "thermohaline" effects, the model satisfactorily simulates the observed hydraulic heads and salinity values throughout the entire Paris basin. This double calibration allows to further validate the model, therefore reducing the uncertainties on flow rates and directions, and thus on radionuclide migration itself.
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