## Report DOE/ER/82329-2 Abstract

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For the proposed Yucca Mountain site for a nuclear waste depository,
the prediction of flow for the transport of contaminants is critical.
Common standard methods of calculating vertical, unsaturated,
single-phase flow between model grid cells can produce predicted
flows in error up to six orders of magnitude, in the wrong direction,
or with effectively misplaced hydrostatic conditions. This raises
valid questions about assuring the eternal safety of any depository
through such modeling.

Calculating Darcian mean flows between grid cells, derived directly
from the modeling assumptions, depending on characteristics of
both the medium and the model, addresses this issue. This provides
a new mathematical and modeling framework with which to judge
the appropriateness of such means and their approximations.

Using a presumed composite relative conductivity relation for
Topopah Spring welded tuff, this work develops true Darcian means
over many modeling regimes, and the viable approximations for
general exponential and Brooks-Corey conductivity relations necessary
for running models.

This demonstrates how very differently matrix and fracture flow
must be treated in a model. It illustrates how a composite relation
itself may be responsible for poor-posedness in the problem and
suggests that separate, linked models for fracture and matrix
flow may be in order.

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