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

Determination of first order degradation rate constants from
monitoring networks ∗
Christof Beyer, Cui Chen, Jan Gronewold, Olaf Kolditz, Sebastian Bauer
Center for Applied Geoscience, Eberhard-Karls-University of Tübingen,
Sigwartstraße 10, D 72076 Tübingen, Germany
Tel.: +49 7071 29 73176; Fax: +49 7071 5059. E-mail: christof.beyer@uni-tuebingen.de
Abstract
In this paper different strategies for estimating first order degradation rate constants from measured field data are
compared by application to multiple synthetic contaminant plumes. The plumes were generated by numerical simulation
of contaminant transport and degradation in virtual heterogeneous aquifers. These sites then were individually and
independently investigated on the computer by installation of extensive networks of observation wells. From the data
measured at the wells, i.e. contaminant concentrations, hydraulic conductivities and heads, first order degradation rates
were estimated by three one-dimensional center line methods, which use only measurements located on the plume axis,
and a two-dimensional method, which uses all concentration measurements available downgradient from the
contaminant source. Results for both strategies show that the true rate constant used for the numerical simulation of the
plumes in general tends to be overestimated. Overestimation is stronger for narrow plumes from small source zones with
an average overestimation factor of about 5 and single values ranging from 0.5 to 20, decreasing for wider plumes, with
an average overestimation factor of about 2 and similar spread. Reasons for this overestimation are identified in the
velocity calculation, the dispersivity parameterization and off-center-line measurements. For narrow plumes the onedimensional
and the two-dimensional strategies show approximately the same amount of overestimation. For wide
plumes, however, incorporation of all measurements in the two-dimensional approach reduces the estimation error. No
significant relation between the number of observation wells in the monitoring network and the quality of the estimated
rate constant is found for the two-dimensional approach.
Introduction
Although detailed mathematical descriptions of contaminant degradation in the subsurface are available (e.g. Rittmann
and VanBriesen 1996; Wiedemeier et al. 1999; Islam et al. 2001), in field studies often simplified approaches are used for
contaminant transport modeling. This is mainly because the identification of a large number of parameters and processes
from field data often is impossible. Due to its mathematical simplicity, its easy implementation into transport models and
the necessity of determining only a single parameter, the biodegradation model most frequently used is first order kinetics
(Wiedemeier et al. 1999). Field methods for the determination of biodegradation rates in ground water comprise a variety
of approaches, including mass balance calculations, in-situ microcosm studies and the so called center line method
(Chapelle et al. 1996). With the center line method, only concentration measurements in observation wells located on the
plume axis are evaluated and additional information possibly at hand (e.g. well data downgradient from the source but
not on the center line) is not explicitly accounted for in the rate estimation. Thus, the center line method is an essentially
one-dimensional approach to rate constant estimation, as flow, transport and degradation are only evaluated for a single
streamline. Assuming that contaminant biodegradation can be approximated by a first order kinetics, the degradation rate
constant can be calculated with analytical transport models to yield the sought for first order rate constant. An overview
of common methods for estimating degradation rate constants from field data within the context of monitored natural
attenuation is given by Newel et al. (2002). The authors discuss approaches based on (point-) concentration over time
data and different concentration vs. distance relations, where dispersion is completely neglected as well as accounted for.
Important to note is that the different approaches yield different types of degradation rates that should be used for
destined purposes only. Rate constants estimated from concentration vs. time data for single monitoring wells for
example represent point decay rates that are typically used to assess source decay or the time required to reach defined
remediation goals at a particular location (Newell et al. 2002). Complete neglect of dispersion in the rate estimation
procedure for concentration vs. distance data along the center line of the plume yields bulk attenuation rates, which
quantify the reduction of contaminant concentrations with distance from the source due to the combined effects of
dispersion, dilution (e.g. by recharge) and degradation processes. The frequently used method of Buscheck and Alcantar
∗
Beyer, C., Chen, C., Gronewold, J., Kolditz, O., Bauer, S. (2007): Determination of first order degradation rate constants from monitoring networks.
(accepted by Ground Water.).
The manuscript was accepted for publication in the Journal Ground Water. Copyright © 2007 Blackwell Publishing. The definite version will be
available at www.blackwell-synergy.com.
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