Independent Review of Simulation of Net Infiltration for Present-Day and Potential Future Climates Page: 39 of 45
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the log-normal distribution routinely used in the field of hydrology. No convincing
justification for the use of log-uniform was given in the documentation of the MASSIF
model. Log-uniform distributions, to the review panel's knowledge, have not been
reported previously for any subsurface hydraulic properties. It is the opinion of the
review panel that a log-normal distribution is a more appropriate and realistic
approximation of the variation of hydraulic properties. However, a uniform distribution is
adequate if it extends far enough; it does increase the probability of sampling the extreme
values compared to a normal distribution. Perhaps if more site-specific data were
available, this issue could be resolved.
More importantly, it is well-documented that spatial variability in soil properties is not
spatially independent, but rather correlated due to natural processes (e.g., Russo and
Bouton, 1992; Russo et al., 1997; Jury et al., 1991). The epistemic uncertainty in this
modeling report, however, mainly deals with the variance of the saturated hydraulic
conductivity and some other parameters without spatial correlations. In other words, the
soil properties in the study should have been treated as a spatial stochastic process; the
probability distributions assigned to the parameters (p. 6-220) should have been joint
probability distributions (i.e., the parameters should have been treated as spatially random
fields). This oversimplified assumption leads to unrealistic estimates of net infiltration
and uncertainty associated with the estimates.
In Sec. 184.108.40.206, "Soil Properties," field capacity, permanent wilting point, and saturated
moisture content were determined from the moisture retention curves provided in the
analogous database from Hanford, WA. A pedotransfer function using these derived
parameters from Hanford is used to predict hydraulic properties. However, use of the
Hanford-derived pedotransfer function is problematic, as acknowledged by the authors:
"The pedotransfer approach introduces uncertainty due to the fact that the Hanford soil
property database represents soils in a location and depositional environment that is
different from Yucca Mountain" (p. 1-3). The measured data for all soil property
development in the model is particle size distribution and fraction of rock fragments. The
Hanford soils are fluvial, while Yucca Mountain soils are colluvial with a much larger
rock fragment fraction and a much higher probability of the rock fragments being able to
retain water. The predicted conductivities from the pedotransfer function do not appear to
be compared with any measured data for verification, which represents a serious
weakness in the approach for characterizing the site's soil properties.
The soil saturated and unsaturated hydraulic conductivities developed from the
pedotransfer function model are quite similar or higher in magnitude to the bedrock
conductivities used in the model simulation, particularly for the soils above the repository
footprint. In addition, saturated hydraulic conductivity varies significantly with little
change in porosity and pore size distribution. No sensitivity analyses have been
conducted to show how saturated and unsaturated hydraulic conductivity would impact
net infiltration when assumed constant, as opposed to its possible dynamic nature over
10,000 years at Yucca Mountain (because of pore plugging or enhancement due to
hydrologic, geochemical, and biologic activities). This will be a key parameter for
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Oak Ridge Institute for Science and Education. Independent Review of Simulation of Net Infiltration for Present-Day and Potential Future Climates, report, August 30, 2008; Oak Ridge, Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc897028/m1/39/: accessed May 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.