THE EFFECT OF MICROSCALE PORE STRUCTURE ON MATRIX DIFFUSION - A SITE-SPECIFIC STUDY ON TONALITE

The matrix diffusion of non-sorbing tracers was studied in rocks from the Syyry area, Central Finland (SY1). The effect of alteration and we athering on rock matrix porosity and on the available pore space, whic h affects diffusivity, are discussed. The main rock type in the crysta lline bedrock of Syyry is a slightly foliated, gray tonalite with mica gneiss inclusions as well as minor, more mafic inclusions. The total porosity and the spatial porosity distribution and microstructure of t he rocks were investigated using infiltration of carbon-14-methylmetha crylate, electron microscopy and Hg-porosimetry. The laboratory-scale diffusion experiments were performed using (1) the out-leaching techni que of methylmethacrylate (MMA), (2) the through-diffusion of tritiate d water (HTO) and chloride in the liquid phase and (3) the through-dif fusion of helium in the gas phase. The results of structure investigat ions indicate that alteration and weathering create micrometric-scale pore apertures, visible as fissures and cracks. These migration pathwa ys dominate in laboratory-scale diffusion experiments and result in hi gh effective porosities and effective diffusivities. Alteration also c reates spherical pores, smaller than 1 mu m, especially in heavily alt ered biotite and plagioclase minerals. The altered mineral phases act as quasi dead-end pores hindering the diffusion of non-sorbing tracers . The heterogeneity of the matrix is increased. The heterogeneous spat ial pore structure of altered and weathered rock samples causes uncert ainty in the diffusion coefficients owing to an assumed homogeneous ma trix in Fickian diffusion models. (C) 1997 Elsevier Science B.V.