Hydraulic Conductivity Distributions for Anisotropic Systems and Application to Tc Transport at the U.S. Department of Energy Hanford Site
Abstract: At the United States Department of Energy Hanford Site a spill of radioactive Technetium has been migrating horizontally in the vadose zone rather than flowing vertically to the water table. This result has been interpreted as being due to horizontal anisotropy in the hydraulic conductivity, K, (a tendency for fluids to migrate more easily in the horizontal direction) due to high horizontal connectivity of sedimentary deposits with a tendency for larger values of K. Such layers have larger components of silt and clay than the predominantly sandy soils at the Hanford site. It is generally accepted that effects of such anisotropy tend to be greater at smaller length scales, probably because of the lack of perfect correlations at large length scales. It has also been suggested that this anisotropy in K is maximized under relatively dry conditions when finer soils (with smaller pores) trap moisture more effectively than sands and gravels. The random component of the distribution of the Hanford flood deposits requires a probabilistic framework for the calculation of K. The work on this project had two main components: 1) to use continuum percolation theory applied to random fractal models to produce a general framework for calculating distributions of K under anisotropic conditions and as a function of system scale, 2) to apply the scheme for calculation to the Hanford site. The results of the general calculation (submitted for publication in Philosophical Magazine) are that the mean horizontal and vertical K values become equal in the limit of large system size (in agreement with general perception above) while the distributions of K values cause significant overlap of expected experimental values of K in the vertical and horizontal directions already at intermediate length scales. In order to make these calculation specific to the Hanford site, however, values of the appropriate length scales to describe the Hanford subsurface as well as to describe the maximum anisotropy in K (largest ratio of horizontal to vertical values) must be provided. The data analyzed so far (received from Pacific Northwest National Laboratory Researchers, Dr. Glendon Gee and Dr. Mark Rockhold) has suggested that the majority of the finer soils in the Hanford subsurface have particularly low values of K at low moisture contents rather than particularly large values as expected by most researchers. Preliminary results suggest that about 50% of the Hanford Site subsurface may contain soils which preferentially retain water under dry conditions, however, only about 20% of these appear to be highly conductive as well. Thus only about 10% of the Hanford subsurface may actually contain soil which not only retains water preferentially under generally dry conditions, but which also has unusually high hydraulic conductivity. If this is the case, then the argument that such highly conductive regions may tend to be connected, even for short distances in the horizontal direction, may fall apart. So several alternate hypotheses have to be considered: 1) that the very small proportion (ca. 10%) of soils which retain water and are highly conductive is sufficient to percolate, even though typical percolation probabilities tend to be 15% or higher, 2) the subset of soils analyzed (from the DOE sponsored Vadose Zone Transport Study Site) is not a representative sample, 3) the horizontal transport is occurring at higher moisture contents. Further research on this particular aspect is thus required, and will be performed in the summer of 2006.
- Research Organization:
- Wright State University, Dayton, OH
- Sponsoring Organization:
- USDOE - Office of Energy Research (ER)
- DOE Contract Number:
- FG02-05ER64067
- OSTI ID:
- 862059
- Report Number(s):
- DOE/ER/64067-1; TRN: US0806196
- Country of Publication:
- United States
- Language:
- English
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