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

Comparing results for source widths of 4 and 16 m yields, that rate constant estimation improves significantly for wider
plumes, as it is less likely, that observation wells are placed off the center line. Estimated rate constants also become
more accurate when more than only local information on hydraulic conductivity is used to approximate the effective
conductivity along the flow path: Taking Kef as the geometric mean of all measured K values from the monitoring
network reduces rate constant overestimation, although wells are not necessarily located on the flow path or even within
the contaminant plume.
For the two-dimensional approach (strategy B), sources of error are inadequate parameterizations of longitudinal and
transverse dispersivities and an erroneous approximation of the mean flow velocity. As all observation wells
downgradient of the source zone are incorporated in the estimation procedure, off center line measurements are not
relevant for this strategy. However, when the contaminant plume shows significant meandering, the applicability of
strategy B is hampered, as the analytical model which is fitted to the concentration measurements presumes a linear
plume axis. This problem is observed for the small source width of 4 m, where overestimation by strategy B is
comparable to the one-dimensional approaches using the global estimate of Kef. For larger source widths like the 16 m
used in this study, however, strategy B yields closer estimates of the degradation rate constant than strategy A on
average.
One drawback of the two-dimensional approach is that source concentration and degradation rate constant have to be
fitted simultaneously to obtain accurate estimates of the degradation rate constant. This proves problematic especially for
the small source width of 4 m where for some realizations the source concentration is overestimated by up to 300 %. For
the source width of 16 m, however, the source concentration is fitted precisely within 5% of the “true” source
concentration. These results suggest that incorporation of off center line information in the estimation of the degradation
rate can improve results of the plume investigation considerably. Especially for wide plumes, the two-dimensional
approach of Stenback et al. (2004) proves superior to the one-dimensional center-line investigation strategy, resulting in
more precise estimates of the degradation rate constant.
Studying the variability of estimated degradation rates due to different monitoring network configurations, i.e. the
personal factor due to different “site investigators”, it is found that for both source widths the variability of investigation
results of one realization ranges from 25 % up to 69 % of the variability observed among different realizations. This
demonstrates that the configuration of the monitoring network following from the investigators individual site
investigation approach represents a substantial part of the uncertainty of the estimated rate constant.
Overestimation of the degradation rate at a contaminated site is a critical point if monitored natural attenuation is
considered as an alternative to conventional engineered remediation measures because the overall NA potential is
assessed too positive. As demonstrated by Beyer et al. (2006) plume lengths calculated with biased degradation rates will
result in an underestimation of the contaminated regions of the aquifer. Such an application, however, might not be of
primary concern, when monitoring networks of sufficient density have already been established over a site. Nonetheless,
a too high rate constant could falsely lead to the conclusion, that a plume is at steady state, when the present day length
observed fits the calculated steady state plume length. In the assessment of contaminated sites, indication for a steady
state plume is an important result, e.g. for the acceptance of NA as a remediation scheme. Also for well investigated sites,
a reliable determination of contaminant degradation rates is important, when the rates are to be used as modeling
parameters, for comparison with other sites and for comparing and optimizing alternatives of remediation measures.
Acknowledgements: This work is funded by the German Ministry of Education and Research (BMBF) under grant
033 05 12 / 033 05 13 as part of the KORA priority program, sub-project 7.2. We wish to thank our project partners at the
Christian-Albrechts-University Kiel Andreas Dahmke and Dirk Schäfer for their support in our research. We
acknowledge the support of Uwe Wittmann, Iris Bernhardt and Ludwig Luckner in coordination of the project work. Last
but not least we are grateful to Bernie Kueper and two anonymous reviewers for their thoughtful comments and
suggestions which considerably improved this paper.
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