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Chemical Transport in Fractured Rock
Abstract Transport of dissolved species in fractured rock has become an area of special interest in recent years when deep lying crystalline rocks have become potential sites for repositories for nuclear waste. In Sweden, research was started in 1977 to investigate the flow and transport in low permeability crystalline rocks such as granites and gneisses. About 10 different potential sites have been investigated by surface mapping and by deep (700 m) drillings. The rock even at large depths is fractured. The conductivity of the fractures range from below measurement limit, which is near the permeability of the rock matrix, up to 4–5 orders of magnitude higher values in the more permeable fractures. The fracture frequency is usually 1 to several fractures per meter, but only a few of the visible fractures carry measurable amounts of water. The frequency of permeable fractures is often one in 5 to 10 or more. The range of conductivities of the conductive fractures is very large. The flow in individual fractures seems to take place in permeable sections making up only a minor part of the fracture. The matrix of the rock is porous and dissolved species can move in and out of the stagnant water in the pores by diffusion. Dissolved species may thus move at a very different rate than the mobile water. Those species which interact by sorption with the large inner surfaces of the matrix are even more retarded in relation to the mobile water. The sparseness of fractures and very large variability in conductivity cast doubts on the applicability of the Advection-Dispersion equation. To understand and model the flow and transport in such rock a series of field experiments as well as laboratory experiments have been performed. Field experiments with tracer migration between injection and with withdrawal boreholes at distances of 10 and up to 51 m have been performed and analysed. Tracer experiments in natural fractures over short distances have been made in the laboratory under well controlled conditions. Tracer tests in the Stripa mine at 360 m depth have been made in 2 natural fractures over distances of 5 and 10 m with sorbing and non-sorbing tracers. A large scale tracer experiment is in progress at the same site where tracers are injected above a 75 m long drift. The water flow and tracers can be collected in more than 350 different sampling sections. Diffusion experiments in the rock matrix have been performed in laboratory as well as in undisturbed rock at 360 m depth in Stripa granite. Laboratory measurements and in situ measurements of the diffusion of sorbing species have also been performed. Models describing the transport of dissolved species have been designed and tested against the experiments. The models include such mechanisms as advection, dispersion, channeling, diffusion into stagnant water in the fractures as well as diffusion into the rock matrix with arbitrary geometries and sorption on the inner surfaces. The models have been used to predict radionuclide transport in crystalline rock. In that context matrix diffusion was shown to be the dominating mechanism for retardation. Channeling was shown to have adverse effects because the fast channels may carry the nuclides at a rate at which they will have less time to decay.
Chemical Transport in Fractured Rock
Abstract Transport of dissolved species in fractured rock has become an area of special interest in recent years when deep lying crystalline rocks have become potential sites for repositories for nuclear waste. In Sweden, research was started in 1977 to investigate the flow and transport in low permeability crystalline rocks such as granites and gneisses. About 10 different potential sites have been investigated by surface mapping and by deep (700 m) drillings. The rock even at large depths is fractured. The conductivity of the fractures range from below measurement limit, which is near the permeability of the rock matrix, up to 4–5 orders of magnitude higher values in the more permeable fractures. The fracture frequency is usually 1 to several fractures per meter, but only a few of the visible fractures carry measurable amounts of water. The frequency of permeable fractures is often one in 5 to 10 or more. The range of conductivities of the conductive fractures is very large. The flow in individual fractures seems to take place in permeable sections making up only a minor part of the fracture. The matrix of the rock is porous and dissolved species can move in and out of the stagnant water in the pores by diffusion. Dissolved species may thus move at a very different rate than the mobile water. Those species which interact by sorption with the large inner surfaces of the matrix are even more retarded in relation to the mobile water. The sparseness of fractures and very large variability in conductivity cast doubts on the applicability of the Advection-Dispersion equation. To understand and model the flow and transport in such rock a series of field experiments as well as laboratory experiments have been performed. Field experiments with tracer migration between injection and with withdrawal boreholes at distances of 10 and up to 51 m have been performed and analysed. Tracer experiments in natural fractures over short distances have been made in the laboratory under well controlled conditions. Tracer tests in the Stripa mine at 360 m depth have been made in 2 natural fractures over distances of 5 and 10 m with sorbing and non-sorbing tracers. A large scale tracer experiment is in progress at the same site where tracers are injected above a 75 m long drift. The water flow and tracers can be collected in more than 350 different sampling sections. Diffusion experiments in the rock matrix have been performed in laboratory as well as in undisturbed rock at 360 m depth in Stripa granite. Laboratory measurements and in situ measurements of the diffusion of sorbing species have also been performed. Models describing the transport of dissolved species have been designed and tested against the experiments. The models include such mechanisms as advection, dispersion, channeling, diffusion into stagnant water in the fractures as well as diffusion into the rock matrix with arbitrary geometries and sorption on the inner surfaces. The models have been used to predict radionuclide transport in crystalline rock. In that context matrix diffusion was shown to be the dominating mechanism for retardation. Channeling was shown to have adverse effects because the fast channels may carry the nuclides at a rate at which they will have less time to decay.
Chemical Transport in Fractured Rock
Neretnieks, Ivars (author) / Abelin, Harald (author) / Birgersson, Lars (author) / Moreno, Luis (author) / Rasmuson, Anders (author) / Skagius, Kristina (author)
1987-01-01
78 pages
Article/Chapter (Book)
Electronic Resource
English
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