Modeling Cl- Concentration And Delta Cl-37 Profiles In Porewater Across A 250m-Thick Indurated Argillite At The Tournemire Url (France)

Dissolved chloride in argillite porewater has been studied as a natural analogue for radionuclides potentially released from radioactive waste disposal. The Tournemire URL intersects impervious and compacted argillite. A previously obtained chloride concentration profile of intact rock is symmetric with a maximum concentration of 0.6 +/- 0.1 g/L, compared to 19 g/L for the original connate seawater. Dissolved chloride shows high delta Cl-37 values, ranging between + 6 and + 8 parts per thousand vs. SMOC. The modeled profile considers diffusive exchange between connate seawater and meteoric freshwater. Transport parameters were obtained by radial diffusion experiments. Numerical modeling was performed with the coupled reactive-transport code Hytec. Simulations suggest a diffusive-exchange time of 85 +/- 10 Ma for Cl, which correlates with a major erosional period. Simulated delta Cl-37 values between 1.002 and 1.003 agree with observed porewater delta Cl-37. This study strongly suggests that the dissolved chloride profile in the argillites results from diffusive exchange and indicates that unfractured argillites can provide good confinement. (C) 2013 The Authors. Published by Elsevier B.V. Selection and/or peer-review under responsibility of Organizing and Scientific Committee of WRI 14-2013