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1. Introduction
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The design of successful subsurface remediation technologies is based upon the
understanding of the (reactive) transport processes at the smaller scales. This
information can be obtained by extensive experimental characterization, which
is usually very expensive and time-consuming. As such, mathematically based
numerical modeling has provided an indispensable tool to reduce experimental
investigations and to make them more cost-effective.
At the pore scale, a porous medium system consists of a series of void spaces
distributed heterogeneously, and one or more fluid phases present simultaneously
(e.g. as air and water under partially saturated conditions). As a solute
transports within these phases, it may undergo absorption, reaction, and transformation.
These transport processes are further complicated by the heterogeneity
within the subsurface system’s physical and chemical characteristics.
The inherent pore scale heterogeneity, as well as the complexity involved in the
physics of (partially-) saturated systems, result in a significant challenge to the
development of fundamental theories of flow and transport, which are crucial
to the design and investigation of new remediation technologies. To date, many
difficult problems still remain to be resolved, and standard theories which have
been in existence for several decades have proven to be inadequate to solve
these problems. The purpose of this research is to improve the understanding
of (partially-) saturated flow and (reactive) transport in porous media by using
an alternative modeling approach: Pore Network Modeling (PNM). For a better
understanding of the macroscopic modeling, the scale issues in subsurface
systems should be understood first.
1.1 Issues of scale
Since modeling in porous media involves transfer of data over several length
scales, scaling effects are of great importance. If the solute undergoes reaction
and adsorption, reactive parameters must also be included in upscaling processes.
The state of the system (e.g., whether saturated, wether occupied by
different phases) is also a critical factor which can affect the macro-scale behavior
of the system. Indeed, many studies of flow and transport in porous media
were motivated by one central question, namely, how do pore scale processes in
a medium influence the effective upscaled transport parameters? Without good
insight into such influence, accurate forecasting models and sound remediation
techniques cannot be developed. Pore scale modeling and the upscaling process
contain three components: (i) defining or conceptualizing pore scale geometry
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