Plans call for nuclear waste to be confined in rock repositories, because diffusion in rock is relatively slow, assuring adequate time for radioactive decay. While fractures in the rock function as high-velocity flow pathways, solutes are both dispersed and retarded via diffusive exchange between the fracture and the rock matrix. If the matrix is even slightly below saturation, imbibition from active fractures will augment the diffusive movement of solutes from fracture to matrix, increasing diffusive retardation. Hu et al. (2002 and unpublished results) examined imbibition of tracer-laced water into rocks, and observed three apparently anomalous results: first, while cumulative imbibition is generally expected to advance in proportion to the square root of time, in some rocks an advance approximately proportional to the fourth root of time was observed. Second, while a tracer concentration profile is expected to resemble the saturation profile during imbibition into initially dry media but show greater dispersion during imbibition into wet media, high apparent dispersion was observed during imbibition into initially air-dry Indiana sandstone. And third, an apparently anomalous increase in porosity was observed at the inlet face during imbibition into welded tuff, even though this face was prepared from the inside of a solid sample rather than from a weathered fracture face. The objective of this research was to investigate pore connectivity as a potential cause of these .three anomalous observations, through comparison of experimental results with pore-scale imbibition simulations.
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