The effects of molecular diffusion on groundwater solute transport through fractured tuff

Persistent Link:
http://hdl.handle.net/10150/191102
Title:
The effects of molecular diffusion on groundwater solute transport through fractured tuff
Author:
Walter, Gary R.
Issue Date:
1985
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Abstract:
Theoretical and experimental studies of the chemical and physical factors which affect molecular diffusion of dissolved substances from fractures into a tuffaceous rock matrix have been made on rocks from G Tunnel and Yucca Mountain at the Nevada Test Site (NT8). Although a number of physical/chemical processes may cause nonadvective transport of dissolved species from fractures into the tuff matrix, diffusion in these rocks is controlled by the composition of the groundwater through multicomponent effects and several rock properties. The effective molecular diffusion coefficient of a particular species in the tuff can be related to its free aqueous diffusion coefficient by Dₑ = θ(m)(α/τ²)D₀ where bm is matrix porosity, α is the constrictivity, and τ is the tortuosity. The porosities of the samples studied ranged from 0.1 to 0.4. The parameter (α/τ²) ranged from 0.1 to 0.3, and effective matrix dif— fusion coefficients were measured to be between 2 to 17. x 10⁻⁷ cm²/s for sodium halides and sodium pentafluorobenzoate. Total porosity was found to be the principle factor accounting for the variation in effective diffusion coefficients. The constrictivity— tortuosity factor was found to have a fair correlation with the median pore diameters measured by mercury intrusion. Measurements of bulk rock electrical impedance changes with frequency indicate that the constrictivity factor, a, has a maximum value of 0.8 to 1, but may be smaller. If the larger values are correct, then the diffusion paths in tuff are more tortuous than in granular media. The diffusion coefficient matrix computed for various tracers in J-13 well water from the NTS indicates coupling of the diffusion fluxes of all ionic species. Multicomponent diffusion is a second order effect, however, which does not significantly affect experimental results. The results of a bench—scale fracture flow experiment revealed that the transport of ionic tracers (SCN ⁻ and pentafluorobenzoate) was affected by diffusion into the tuff matrix. The transport of a particulate tracer did not appear to be affected by diffusion.
Type:
Dissertation-Reproduction (electronic); text
Keywords:
Hydrology.; Groundwater flow.; Groundwater tracers.; Volcanic ash, tuff, etc.
Degree Name:
Ph. D.
Degree Level:
doctoral
Degree Program:
Hydrology and Water Resources; Graduate College
Degree Grantor:
University of Arizona

Full metadata record

DC FieldValue Language
dc.language.isoenen_US
dc.titleThe effects of molecular diffusion on groundwater solute transport through fractured tuffen_US
dc.creatorWalter, Gary R.en_US
dc.contributor.authorWalter, Gary R.en_US
dc.date.issued1985en_US
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en_US
dc.description.abstractTheoretical and experimental studies of the chemical and physical factors which affect molecular diffusion of dissolved substances from fractures into a tuffaceous rock matrix have been made on rocks from G Tunnel and Yucca Mountain at the Nevada Test Site (NT8). Although a number of physical/chemical processes may cause nonadvective transport of dissolved species from fractures into the tuff matrix, diffusion in these rocks is controlled by the composition of the groundwater through multicomponent effects and several rock properties. The effective molecular diffusion coefficient of a particular species in the tuff can be related to its free aqueous diffusion coefficient by Dₑ = θ(m)(α/τ²)D₀ where bm is matrix porosity, α is the constrictivity, and τ is the tortuosity. The porosities of the samples studied ranged from 0.1 to 0.4. The parameter (α/τ²) ranged from 0.1 to 0.3, and effective matrix dif— fusion coefficients were measured to be between 2 to 17. x 10⁻⁷ cm²/s for sodium halides and sodium pentafluorobenzoate. Total porosity was found to be the principle factor accounting for the variation in effective diffusion coefficients. The constrictivity— tortuosity factor was found to have a fair correlation with the median pore diameters measured by mercury intrusion. Measurements of bulk rock electrical impedance changes with frequency indicate that the constrictivity factor, a, has a maximum value of 0.8 to 1, but may be smaller. If the larger values are correct, then the diffusion paths in tuff are more tortuous than in granular media. The diffusion coefficient matrix computed for various tracers in J-13 well water from the NTS indicates coupling of the diffusion fluxes of all ionic species. Multicomponent diffusion is a second order effect, however, which does not significantly affect experimental results. The results of a bench—scale fracture flow experiment revealed that the transport of ionic tracers (SCN ⁻ and pentafluorobenzoate) was affected by diffusion into the tuff matrix. The transport of a particulate tracer did not appear to be affected by diffusion.en_US
dc.description.notehydrology collectionen_US
dc.typeDissertation-Reproduction (electronic)en_US
dc.typetexten_US
dc.subjectHydrology.en_US
dc.subjectGroundwater flow.en_US
dc.subjectGroundwater tracers.en_US
dc.subjectVolcanic ash, tuff, etc.en_US
thesis.degree.namePh. D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineHydrology and Water Resourcesen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.identifier.oclc213415605en_US
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