Characterization of water pathways in low permeable rocks at the rock matrix scalemethodological review

Resumen

Esta publicación metodológica está relacionada con la caracterización de las vías de circulación del agua, a escala de la matriz rocosa (o ¿intact rock¿), en rocas cristalinas candidatas para almacenar definitivamente residuos radioactivos de alta actividad. A esta escala la circulación del agua en estas rocas está condicionada básicamente por su capacidad de fluir a través de la porosidad abierta efectiva. Es decir, a través de los poros y de las fisuras conectadas. Por consiguiente, se resumen los diversos procedimienISSN: 1698-6180www.ucm.es /info/estratig/journal.htmJournal of Iberian Geology 32 (2) 2006: 197-213 198Montoto and Mateos / Journal of Iberian Geology 32 (2) 2006: 197-213ping. Therefore, this is a methodological paper related to this subject, not one providing specific results and conclusions concerning a specific geological site.It is obvious that two basic scales of study coexist in any geological scenario: the rock mass scale (with fractures from a few m up to several km long) and the intact rock scale (with fissures and cracks to the order of µm to dm). The last one was defined by the ¿International Society for Rock Mechanics¿ referring to a rock volume free of massif discontinuities. The integration of data from both scales is a question that in Engineering Geology requires further development and many authors are aware of this problematic geological reality. The attempts for reproducing this situation in the laboratory, combining information from both scales, are very promising (Montoto, 2003). For instance, under this approach rock cores which include rock mass features such as fractures are studied; basically, non-standardized tests are being used for characterizing the hydraulic properties, and non-destructive procedures are applied for a preliminary characterization of rock anisotropy and heterogeneity.At the intact rock scale, water circulation is basically conditioned by its capacity to flow through the connected pores and fissures (Fig. 1). In consequence, the different types of porosity have to be evaluated and hydraulically interpreted. The key aspects required for a deeper study and understanding of the hydrogeological behaviour of the rock-matrix and to characterize the textural and mineralogical location of water pathways, can be found in Montoto (2003). Among them, the following stand out: a) the two really different scales of porosity and water pathways coexisting in any geological scenario: the rock mass and the intact rock scale, b) fractographic parameters, such as fissure trace orientation, fissure volume, fissure range aperture, fissure network connectivity, fissure tortuosity, fractal dimension of fissure trace profiles, 1. IntroductionFinding a reliable solution to protect the biosphere from the radiotoxicity irradiated by high level radioactive wastes (HLW) is the main problem concerning those wastes. The internationally accepted solution for this problem is the isolation of these wastes in the interior of rock masss, under long-term requirements of stability and water tightness, until that radiotoxicity will be decayed to human acceptable doses.That isolation is based on a ¿multibarrier concept¿ constituted by different engineered and natural systems; basically: canister, buffer and rock. In this multibarrier system, the selected rock mass plays one of the most important roles. In fact, for the very long-term basis, it is the final effective barrier for the wastes stored there.As long as the principal medium for radioelement transport is water, hydrogeological studies are indispensable for the full characterization of the disposal site. According to all the studies realised up to now, a better understanding of the mechanisms governing radioelement migration in a geological scenario has been obtained. In any case, uncertainties still exist about the in-situ character of transport pathways at the intact rock scale. Therefore, more studies are required, particularly those related to the in-situ characterization of the pore space and the fluid flow (Frieg et al., 1998). As a consequence, in the mentioned framework of HLW studies, the water movements and their pathways through the rock mass deposit is one of the key questions to be understood and predicted. The objective of this publication follows that goal but focused on the intact rock scale. It tries to summarize the hydraulic function of the pore space and the procedures developed for the imaging and characterization of the water pathways, as well as their textural and mineralogical location. That is, their maptos desarrollados para la obtención de imágenes y cartografía de estas vías de circulación, entre los que destacan: microscopía de fluorescencia, microscopía láser confocal, microscopía electrónica de barrido (electrones secundarios y retrodispersados, análisis por energía dispersiva de rayos X) e impregnación con 14C-polimetilmetacrilato. Se mencionan las dos técnicas esenciales para la reconstrucción tridimensional de la estructura del espacio poroso, tomografía informatizada de rayos X y microscopía láser confocal y se incluyen diversos ejemplos de esta última. Además se describe la caracterización ¿in-situ¿ de la porosidad y la identificación de antiguas vías de circulación del agua mediante trazas de fisión del uranio. Se resumen y se incluyen ejemplos de los principales parámetros petrográficos relacionados con la estructura geométrica del espacio poroso, ya que esta información es básica para la interpretación petrofísica de la función hidráulica de la porosidad de las rocas cristalinas. Finalmente, se incluyen ejemplos de perfiles petrofísicos que contienen información secuencial a escala granulométrica (mm-cm), al objeto de documentar detalladamente la capacidad del agua y los radioelementos para introducirse y circular por la matriz rocosa a través de las fracturas hidráulicamente activas del macizo rocoso.