Diffusion as the main process for mass transport in very low water content argillites: 1. Chloride as a natural tracer for mass transport—Diffusion coefficient and concentration measurements in interstitial water

Abstract

[1] Argillites are one of the rock types studied by French authorities for their confining properties for the isolation of radioactive wastes. One of the main objectives of such study is the better understanding of water transport through rocks with very low water content and hydraulic conductivity, using modeling of tracer profiles. This article presents the protocol developed and applied for acquiring data on chloride in interstitial water of the Toarcian argillites in Tournemire (southern France). This protocol is based on laboratory experiments involving diffusion process and on modeling. Experimental data obtained during transient and steady parts of diffusion allow for the assessment of the diffusion coefficient and initial concentration in pore water, respectively. Profiles for both have been acquired along the geological sequence; they are used in part 2 of this article for proposing a hydrogeological model where diffusion appears to be the main process for mass transport through the argillites and for comparing deuterium and chloride transport.

Mass release curves as the constitutive curves for modeling diffusive transport within biological tissue

“…5. The mass release curves are continuous, while the true distribution of fluxes are highly irregular, affected not only by chemical interactions,but also by the molecule size; this observation is of particular importance in diffusion through complex porous media as rocks ([3,4]) Accuracy of the prediction of the mass release using the equivalent diffusion coefficient was verified experimentally[47].…”

“…The mass release curves can be determined numerically [10], or experimentally: in [3] and [4] experiments were performed in which the mass flux from a sample of porous rock was measured and diffusion coefficients calculated, while in [12] mass release was measured from a drug delivery system. Mass release constitutive curves have an additional important property with respect to traditional approaches of calculation of the equivalent diffusion coefficients [13][14][15][16][17]: they provide information about the diffusion characteristics over a selected range of concentration, rather than for one concentration value.…”

“…Diffusion as a process of material transport which occurs in soils and porous rocks [1][2][3][4], in technological processes [5,6] and biological systems [7,8], has been thoroughly investigated. Distribution of transported material (solute) inside the pores, filled with fluid, is defined by the concentration field c(x,t), where x is a spatial position within the continuum and t is time.…”

Equivalent diffusion coefficient and equivalent diffusion accessible porosity of a stratified porous medium

“…The analytical solutions are in both cases verified in a two-layer medium with equal layer thicknesses and in stratified media with more than two layers or with layers with unequal thicknesses. All calculated examples have a realistic variability of D and n that corresponds to the natural variability of diffusion parameters observed in low permeability media (e.g., Shackelford and Daniel 1991;Aertsens et al 2004;Patriarche et al 2004). …”

“…Low permeability media are also important as host formations for disposal of municipal (e.g., Jang and Kim 2003), industrial (e.g., Navarro et al 2000) and nuclear waste (e.g., Landais 2004). As low permeability media receive increasing interest, extensive databases of diffusion coefficients and diffusion accessible porosities are collected (e.g., Aertsens et al 2004;Descostes et al 2004;Patriarche et al 2004;Van Loon et al 2003). Figures 1 and 2 show examples of measured diffusion coefficients and diffusion accessible porosities versus depth from two clay layers which are studied in the context of nuclear waste disposal.…”