Evaluating the application of Archie’s law for ... THE APPLICATION OF ARCHIE’S LAW FOR ARGILLACEOUS ROCKS P.Blum 1,M.Scheunemann1,L.VanLoon2,D.Coelho3,N.Maes 4,P.Grathwohl1 1. University of …

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P/MTPM/31 Page 483INTERNATIONAL MEETING, SEPTEMBER 17...>...18, 2007, LILLE, FRANCE CLAYS IN NATURAL & ENGINEERED BARRIERS FOR RADIOACTIVE WASTE CONFINEMENT EVALUATING…

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P/MTPM/31 Page 483INTERNATIONAL MEETING, SEPTEMBER 17...>...18, 2007, LILLE, FRANCE CLAYS IN NATURAL & ENGINEERED BARRIERS FOR RADIOACTIVE WASTE CONFINEMENT EVALUATING THE APPLICATION OF ARCHIE’S LAW FOR ARGILLACEOUS ROCKS P. Blum1, M. Scheunemann1, L. Van Loon2, D. Coelho3, N. Maes4, P. Grathwohl1 1. University of Tübingen, Center for Applied Geosciences (ZAG), Sigwartstr. 10, 72076 Tübingen, Germany, (philipp.blum@uni-tuebingen.de) 2. Paul Scherrer Institut (PSI), CH-5232 Villingen, Switzerland, (luc.vanloon@psi.ch) 3. ANDRA, 92298 Châtenay-Malabry, France, (daniel.coelho@andra.fr) 4. SCK-CEN, Boeretang 200, B-2400 Mol, Belgium, (nmaes@sckcen.be) INTRODUCTION Diffusion is an important transport process in geological systems. Hence, for the safety assessment of potential radioactive waste repositories that might be build in such geological systems, effective diffusion coefficients of various species need to be known. The effective diffusion coefficients strongly depend on porosity. Moreover, previous studies could demonstrate that the effective diffusion coefficients can be estimated using Archie’s law: De = Daq . εm (1) where De is the effective diffusion coefficient, Daq is the diffusion coefficient in bulk water, ε is the transport porosity and m represents an empirical constant depending on the type of porous medium. Boving & Grathwohl (2001) studied the diffusion of iodide (I-) in different limestone and sandstone rocks. The exponent m that fitted the data the best was approximately 2.2. Another study, which focused on chalk, showed that the data for various inorganic and organic compounds resulted in an exponent of around 2.4 (Blum, 2000). A few studies also considered the application of Archie’s law for diffusion in argillaceous rocks. Grathwohl (1998) studied the diffusion of tetrachloroethene (TCE) in natural clays (Figure 1). A good fit with Archie’s law was obtained using the transport-through porosity. Figure 1: (a) Normalized diffusion coefficient (D’) of TCE versus overall porosity. (b) Normalized diffusion coefficient (D’) of TCE versus corrected porosity based on water adsorbed onto dry samples at a relative humidity of 98.8 %. The lines represent Archie’s law. km5: Triassic clay (Knollenmergel); scs: silty-clayey soil; bav: Jurassic clays (Opalinus Clay, Dogger α); la1: Jurassic clay (Lias α1); aben: activated bentonite. 0.2 0.3 0.4 0.5 0.6 Porosity (ε) 0 0.05 0.1 0.15 0.2 0.25 bav scs la1 aben km5 (a) D ’= D e /D a q D ’ = ε3 /2 D ’ = ε2 D ’ = ε5 /2 0.2 0.3 0.4 0.5 0.6 Transport-Through Porosity (εt ) 0 0.05 0.1 0.15 0.2 0.25 D ’ = ε3 /2 D ’ = ε2 D ’ = ε5 /2 D ’= D e /D a q bav scs la1 aben km5 (b) P/MTPM/31 Page 484 INTERNATIONAL MEETING, SEPTEMBER 17...>...18, 2007, LILLE, FRANCE CLAYS IN NATURAL & ENGINEERED BARRIERS FOR RADIOACTIVE WASTE CONFINEMENT Furthermore, Van Loon et al. (2003) studied effective diffusion coefficients of tritiated water (HTO) in Opalinus Clay and compared their results with effective diffusion coefficients of HTO for a series of argillaceous rocks (e.g. London Clay, Boom Clay, Wellenberg Marl, Tournemire Toarcian shale). They concluded that the effective diffusion coefficients can be satisfactorily described using Archie’s law with an exponent of around 2.5. DATA ANALYSIS Both dissimilar data sets (Grathwohl, 1998; Van Loon et al., 2003) indicate that Archie’s law might be applied for the estimation of the effective diffusion coefficients in argillaceous rocks. However, the value of the accessible or transport-through porosity, respectively, is often not known and therefore Archie’s law will result in imprecise estimates for effective diffusion coefficients. Hence, in the current study, data on effective diffusion coefficients and transport porosities will be collected from diffusion studies (e.g. Henrion et al., 1991; De Cannière et al., 1996; Decostes et al., 2007) performed on various argillaceous rocks (e.g. Opalinus Clay, Callovo-Oxfordian argillites and Boom Clay). RESULTS AND INTERPRETATION Comprehensive data sets on diffusion of different tracers (anions, cations and neutral species) in argillaceous rocks will be presented and the applicability of Archie’s law for clays and clay stones will be thoroughly discussed in context of the diffusion (or transport) accessible porosity. The relevance for safety assessment of potential radioactive waste repositories will be highlighted. References: Blum, P. (2000): “Sorption- and diffusion behaviour of organic compounds in Israelian and European Chalk“. MSc-Thesis, Applied Geology, University of Karlsruhe. Boving, T.B., Grathwohl, P. (2001): “Tracer diffusion coefficients in sedimentary rocks: correlation to porosity and hydraulic conductivity”. Journal of Contaminant Hydrology, 53, 85-100. De Cannière P., Moors H., Lolivier P., De Preter P., Put M. (1996): “Laboratory and in situ migration experiments in Boom Clay”. Nuclear Science and Technology, EUR16927, EC, Luxembourg. Descostes, M., Blin, V., Bazer-Bachi, F., Meier, P., Grenut, B., Radwan, J., Schlegel, M.L, Buschaert, S., Coelho, D., Tevissen, E. (2007): “Diffusion of anionic species in Callovo-Oxfordian argillites and Oxfordian limestones”. Submitted to Applied Geochemistry. Grathwohl, P. (1998): “Diffusion in Porous Media”. Kluwer Academic Publishers, London. Henrion, P.N., Put, M.J., Van Gompel, M. (1991): “The influence of compaction on the diffusion of non- sorbed species in Boom Clay”. Radioactive Waste Management and the Nuclear Fuel Cycle, 16 (1), 1-14. Van Loon, L.R., Soler, J.M., Jakob, A., Bradbury, M.H. (2003): “Effect Effect of confining pressure on the diffusion of HTO, 36Cl− and 125I− in a layered argillaceous rock (Opalinus Clay): diffusion perpendicular to the fabric”. Applied Geochemistry, 18, 1653-1662.

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