Fault zones are potential paths for release of radioactive nuclides from radioactive-waste
repositories in granitic rock. This research considers detailed maps of en echelon fault zones
at two sites in southern Sweden, as a basis for analyses of how their internal geometry can
influence groundwater flow and transport of radioactive nuclides.
Fracture intensity within these zones is anisotropic and correlated over scales of
several meters along strike, corresponding to the length and spacing of the en echelon steps.
Flow modeling indicates these properties lead to correlation of zone transmissivity over
similar scales.
Intensity of fractures in the damage zone adjoining en echelon segments decreases
exponentially with distance. These fractures are linked to en echelon segments as a
hierarchical pattern of branches. Echelon steps also show a hierarchical internal structure.
These traits suggest a fractal increase in the amount of pore volume that solute can access by
diffusive mass transfer, with increasing distance from en echelon segments. Consequences
may include tailing of solute breakthrough curves, similar to that observed in underground
tracer experiments at one of the mapping sites.
The implications of echelon-zone architecture are evaluated by numerical simulation
of flow and solute transport in 2-D network models, including deterministic models based
directly on mapping data, and a statistical model. The simulations account for advection,
diffusion-controlled mixing across streamlines within fractures and at intersections, and
diffusion into both stagnant branch fractures and macroscopically unfractured matrix.
The simulations show that secondary fractures contribute to retardation of solute,
although their net effect is sensitive to assumptions regarding heterogeneity of transmissivity
and transport aperture. Detailed results provide insight into the function of secondary
fractures as an immobile domain affecting mass transfer on time scales relevant to field
characterization and repository safety assessment.
In practical terms, secondary fractures in these en echelon zones are not indicated to
limit release of radiation to the surface environment, to a degree that is significant for
improving repository safety. Thus en echelon zones are to be regarded as detrimental geologic
features, with potentially complex transport behavior which should be considered in the
interpretation of in-situ experiments. / Graduation date: 2005
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/29561 |
| Date | 10 December 2004 |
| Creators | Geier, Joel E. |
| Contributors | Haggerty, Roy D. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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