A regional-scale particle-tracking method for nonstationary fractured media Page: 4 of 38
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In spite of the great progress made during the last twenty years in characterizing
and describing flow and transport in fractured media, models for field-scale transport are still
preliminary in character in terms of their capability to take site-specific heterogeneity into
account. The present work introduces a new approach for this purpose. We start from the
detailed-scale geological and hydraulic data and employ fracture network modeling to obtain
the relevant flow and transport statistics at some support scale, for which flow, but not
necessarily transport, can be represented by means of a continuum. We then use stochastic
continuum flow simulation in combination with large scale particle tracking to model
transport. Here, particle velocities determined from the network realizations are transferred to
a large-scale model via a specific scaling method. Special attention is given to transport
channeling characteristics; this is modeled by a superimposed "tracer transport persistence,"
N, in our sampling algorithm. Furthermore, to show the feasibility of allowing for realistic
field conditions, a depth trend in flow and transport characteristics, caused by the closing of
fractures with increasing stress, is also included.
In the following, we will first present the approach, and then apply it to a set of
field data, to study a hypothetical scenario related to deep disposal of high-level nuclear
waste. We use data from Sellafield, England, as an example (Andersson and Knight, 2000).
Sellafield is a fractured rock site that has been intensively investigated by Nirex (e.g., Nirex,
1997a-d) in connection with nuclear waste disposal. The database we use is not complete and
is not intended to reflect the characteristics of the site in general, but is taken merely as an
example of a realistic fractured rock database to demonstrate our method.
2. Model for Regional-Scale Transport in Fractured Media
2.1 Overview of the Model
Our objective is to introduce a model for large-scale solute transport in fracture
media that properly honors the fracture-related heterogeneity observed in boreholes via a
fracture network-based upscaling. The suggested approach is inspired by Nordqvist et al.
(1992), who transferred statistics of within-fracture plane channeled transport, evoked by
variable aperture, onto a flow field solved for a constant-aperture fracture network. In their
approach, transit time distributions are first determined by particle tracking in individual
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Ohman, Johan; Niemi, Auli & Tsang, Chin-Fu. A regional-scale particle-tracking method for nonstationary fractured media, article, November 1, 2004; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc788343/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.