Applied numerical modeling of saturated / unsaturated flow and ...

The process object has access to all
required data structures and functions, and
thus is self configuring, executing and
destructing.
Contaminant transport problems usually
involve a number of different processes as
outlined in sections 2.1 – 2.3. The resulting
multi-field problems can be approached by
a multi-process algorithm, where one instance
of the process object is created for
each process considered. Solution of any
number of flow or transport equations is
fully automatic and encapsulated, guaranteeing
high efficiency and flexibility.
In GeoSys / Rockflow saturated and unsaturated
flow as well as conservative transport
are solved using standard Galerkin FE. A
non-iterative operator splitting technique
for the coupling of conservative transport
and (bio-)chemical reaction processes is
used (Xie et al., 2006; Bauer et al., 2006b).
First, the flow field is solved followed by
conservative transport for all species. In the
third step the calculation of kinetic
biochemical reactions is performed. Finally
chemical equilibrium reactions are calculated.
This approach allows an easy handling
of any number of species and reaction
processes as well as employing optimised
mathematical methods for the solution of
the corresponding equation systems. The
non-iterative approach, however, is limited
to small time steps in order to avoid
numerical instabilities. It is also known not
to converge necessarily to the exact solution
(Carrayrou et al., 2004). These limitations
can be overcome using a computationally
more demanding iterative operator splitting
approach (e.g. Kinzelbach et al., 1991).
3. Modeling applications
In this chapter three application examples
of numerical models for flow and reactive
transport as established in chapter 2 are
presented. Section 3.1 introduces the Virtual
Aquifer (VA) concept, in which numerical
modeling is used for the evaluation of
investigation strategies for contaminated
sites. In section 3.2 the VA concept is
applied to derive and test a novel method
for the estimation of biodegradation kinetic
parameters from measured field data. Section
3.3 uses numerical modeling as a tool
to predict the environmental impact of demolition
waste used in road constructions
3.1. Evaluation of investigation
strategies for contaminated
aquifers using the Virtual
Aquifer concept
Due to the limited accessibility of the
subsurface, measurements of piezometric
heads and pollutant concentrations at contaminated
sites are sparse and may not be
representative of the heterogeneous hydrogeologic
conditions. Any site investigation
is thus subject to uncertainty, reflecting the
limited knowledge on aquifer properties
and the extent of the contamination. Three
main sources of uncertainty can be identified
for site investigation, which are illustrated
in Fig. 5. Conceptual model errors
result from an incorrect identification of the
governing processes at a site. Heterogeneity
of the site causes an incomplete or wrong
description of the relevant parameter distributions.
For variables measured at observation
wells like heads or concentrations
measurement errors are inevitable. Due to
this uncertainty, field investigation methods
for plume screening and measuring of
hydraulic conductivity or degradation rates
can hardly be tested or verified in the field.
The VA approach is particularly aimed to
overcome this problem. Its basic idea is the
computer based evaluation of the performance
and reliability of field investigation
methods by application in heterogeneous
synthetic (i.e. virtual) aquifers. In this it
resembles the concept of “virtual realities”
(Schäfer et al., 2002) which are used e.g. in
car industry (“virtual crash test”), education
(flight simulators) or medicine (interactive
operation planning).
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