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

conceptional
model error:
wrong process
description
10
measurement
error
resulting
investigation error
conceptual site model
investigation
virtual aquifer
Fig. 5: Virtual aquifer concept and possible sources of investigation error.
An application of the VA concept requires
the definition of a synthetic site model and
its translation into a numerical model for
the simulation of the identified relevant processes.
Synthetic site models are generated
based on statistical properties of natural
aquifers. A defined contaminant source is
then introduced and the evolution of aquifer
contamination is simulated by numerical
modeling, thus generating a realistic
contaminant distribution in the synthetic
aquifer. In comparison to the "real world",
the unique advantage of the synthetic aquifer
is that the spatial distribution of all
physical and geochemical properties and
parameters as well as contaminant concentrations
are exactly known. Once the
synthetic contaminated aquifer is generated,
it can be studied by standard monitoring
and investigation techniques, e.g. by emplacement
of observation wells. Although
the parameter distribution of the synthetic
aquifer is known a priori, only the data
“measured” at wells (i.e. hydraulic heads or
concentrations) are used and interpreted.
This is done because in a real site investigation
also only a limited amount of measured
data would be available. Finally, the results
are compared to the “true” parameter distribution
known from the synthetic aquifer,
allowing an evaluation of the accuracy of
the investigation method used. Using the
VA concept, sources of uncertainty or error
can be considered individually and the
heterogeneity:
incomplete or
wrong description
of parameter
distribution
sensitivity of investigation results on these
can be studied. Stochastic approaches like
the Monte-Carlo method are applied to
study the propagation of parameter variability
and uncertainty into the investigation
results. The VA has been first introduced
by Schäfer et al. (2002) and was applied by
Schäfer et al. (2004, 2006b), Bauer and
Kolditz (2006), Bauer et al. (2005 [EP 1];
2006a [EP 2], 2007 [EP 4]) and Beyer et al.
(2006 [EP 3], 2007a [EP 5]). An overview
of VA applications is given in Bauer et al.
(2006b) and Schäfer et al. (2006a).
Fig. 6: Virtual investigation of a heterogeneous
contaminant plume by the center
line method (Bauer et al., 2005 [EP 1]).
The VA concept is used here to study errors
and uncertainties in degradation rate constants
estimated from data typically collected
by site investigation with the so called