Perturbation induced by a nitrate plume on diffusion of solutes in a large-scale clay rock sample

Abstract

Mudstones are widely studied in the context of waste landfill leachates or as geological barriers for waste management. Some of the technological wastes undergo biogeochemical degradation, releasing sulfate or nitrate plumes. The concomitant increase of salts concentration and ionic strength may perturb the geological media and affect transport properties. The study investigated the effect of a nitrate plume on the diffusion of ions. The confinement of ionic solutes was quantified in Callovian-Oxfordian mudstone. A 0.5molL−1 NaNO3 solution was injected in a large-scale sample with a low water/rock (≪1.0Lkg−1) close to environmental conditions. The in-diffusion of HTO, 36Cl, NO3−, 22Na, 137Cs was monitored during the perturbation. A minor impact was observed on poral water chemistry and on ions diffusion. The anion exclusion was only reduced by 19% for chloride and effective diffusion of cations reduced by 20 to 30%. Diffusion data of NO3− was quantified and anion exclusion found to be intermediate between that of halogen anions and that of water. Values of diffusion anisotropies were adjusted to a(36Cl)=1.9 and a(22Na)=2.5, close to data in sound sample. All these results confirmed a limited perturbation by 0.5molL−1 nitrate plume on the confinement properties of the clay rock. Additional data is provided on major ions released from the rock and diffusing towards the injection solution. It may be useful for further chemistry-transport modeling including co-diffusion of ions, surface diffusion, anion exclusion and effect of ionic strength.