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

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INTRODUCTION
Bringing Groundwater Quality Research to the Watershed Scale (Proceedings of GQ2004, the 4th International
Groundwater Quality Conference, held at Waterloo, Canada, July 2004). IAHS Publ. 297, 2005.
Assessing measurements of first-order degradation
rates through the virtual aquifer approach
SEBASTIAN BAUER, CHRISTOF BEYER & OLAF KOLDITZ
Center for Applied Geoscience, University of Tübingen, Sigwartstrasse 10, D 72076 Tübingen,
Germany
sebastian.bauer@uni-tuebingen.de
Abstract The principal idea behind the “virtual aquifer” is to simulate and
evaluate investigation strategies for contaminated sites by modelling typical
contamination scenarios. In this paper, first-order degradation rates using
various methods were the focus of study. A virtual reality of a contaminated
aquifer was generated by simulating the spreading of a plume, originating
from a defined source zone, subject to first-order degradation. This plume was
investigated through monitoring wells placed along the plume centre-line.
Using information such as head measurements, concentration and hydraulic
conductivity, first-order degradation rates were calculated and compared to the
true predefined value. This comparison was conducted for varying degrees of
heterogeneity, represented by ln(KF), randomly distributed conductivity fields.
It was found that when heterogeneity was increased, “measured” degradation
rates overestimated the true degradation rate by several orders of magnitude.
The range of degradation rates obtained roughly corresponds to the range
stated in literature values.
Key words first-order degradation; modelling; natural attenuation; virtual reality
At a real contaminated site, the true hydrogeological properties (e.g. conductivity,
porosity, recharge rates, source position, degradation rates, etc.) are generally unknown
(spatially). The basic idea behind the “virtual aquifer” is to produce a “virtual” contaminated
site, where the spatial distribution of parameters is exactly known. By using a
process-based flow and transport model, the fate of contaminants in the subsurface can
be simulated, including plume development. The second step for creating a virtually
contaminated site is to examine the virtual aquifer properties using standard investigative
procedures (e.g. interpolating hydraulic head and contaminant concentrations
measured at monitoring wells). The result of this virtual investigation can be compared
to the true hydrogeological property distribution of the virtual aquifer because, contrary
to an actual contaminated site, the exact distribution is known. The investigation
techniques used can thus be tested and evaluated with respect to certain influences, i.e.
sensitivity to aquifer heterogeneity or variation of other parameters. When the virtual
plume is examined, only the data obtained by these investigation techniques are used,
i.e. only the data that would also be measured at a real field site. Data such as hydraulic
head and contaminant concentrations are “measured” in the virtual aquifer by “reading”
the model output. The “virtual aquifer” approach offers the ability to test and evaluate
site investigation techniques, which cannot be performed in the real world. In this
paper, four methods for determining first-order degradation rate constants, all based on
the plume centre-line method, are examined by the “Virtual Aquifer” approach.