Abstract
Abstract. Clay plays a prominent role as barrier material in the geosphere. The small particle sizes cause extremely small pore sizes and induce low permeability and high sorption capacity. Transport of dissolved species by molecular diffusion, driven only by a concentration gradient, is less sensitive to the pore size. Heterogeneous structures on the centimetre scale could cause heterogeneous effects, like preferential transport zones, which are difficult to assess. Laboratory measurements with diffusion cells yield limited information on heterogeneity, and pore space imaging methods have to consider scale effects. We established positron emission tomography (PET), applying a high-resolution PET scanner as a spatially resolved quantitative method for direct laboratory observation of the molecular diffusion process of a PET tracer on the prominent scale of 1–100 mm. Although PET is rather insensitive to bulk effects, quantification required significant improvements of the image reconstruction procedure with respect to Compton scatter and attenuation. The experiments were conducted with 22Na and 124I over periods of 100 and 25 days, respectively. From the images we derived trustable anisotropic diffusion coefficients and, in addition, we identified indications of preferential transport zones. We thus demonstrated the unique potential of the PET imaging modality for geoscientific process monitoring under conditions where other methods fail, taking advantage of the extremely high detection sensitivity that is typical of radiotracer applications.
Highlights
Natural clay typically has a heterogeneous composition and a spatially variant anisotropic structure
Each frame is depicted as isosurface of the one-tenth maximum value, three horizontal slices through the source region, and the axial maximum projection at the bottom
We report the measurements on one sample (BLT 137/3): Beginning daily, with increasing time lag, we produced a sequence of 20 positron emission tomography (PET) images over a period of 150 days until the tracer was roughly distributed over the core (Fig. 5) (t = 0, 3, 6, 10, 13, 16, 20, 22, 27, 31, 35, 41, 48, 55, 69, 93, 112, 127, 143, 161 days)
Summary
Natural clay typically has a heterogeneous composition and a spatially variant anisotropic structure. Due to grain sizes of the clay fraction in the micrometre range, pore sizes are extremely small. This effectively inhibits advective flow and causes high internal surface area, forming natural geological barriers. As smallest possible size of the representative elementary volume (REV), we consider the size of standard drill cores because their size is just above the largest observed heterogeneities. It still is an unanswered question as to whether a REV exists beyond which the material may be considered as homogeneous, or whether analogical heterogeneities exist on larger scales
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