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

(1995) yields a “hybrid” rate constant between a bulk attenuation and a pure biodegradation rate constant, as it accounts
for longitudinal dispersion whereas the effects of transverse dispersion are still reflected in the rate constant. A pure
biodegradation rate constant can be estimated analytically by normalizing contaminant concentrations to concentrations
of a recalcitrant tracer, if such a compound is emitted from the same source zone as the contaminant of concern
(Wiedemeier et al. 1996). If this is not the case, an approach of Zhang and Heathcote (2003) may be used, in which the
Buscheck and Alcantar (1995) method is extended to account for dispersion in two or three dimensions. Pure
biodegradation rate constants exclusive of dispersion or other attenuation processes (and only those) may be used in
contaminant transport models for prognoses of plume trends. It is important to note that these center line based rate
constant estimation approaches are only applicable to contaminant plumes that have reached steady state conditions, as
for still expanding plumes the rate constant would be overestimated.
Although it is known, that biodegradation rate estimates obtained from an investigation of the plume center line are
subject to substantial uncertainty, this strategy is frequently used in practice. Even in homogeneous aquifers vertical and
horizontal transverse dispersion can produce center line concentration profiles of recalcitrant compounds that could be
mistaken as following from first order degradation (McNab Jr. and Dooher 1998). Moreover, the plume axis may easily
be missed by monitoring wells when the inferred ground water flow direction is incorrect, changes over time due to
transient flow behavior or when the contaminant plume shows large scale meandering due to aquifer heterogeneity
(Newell et al. 2002; Wilson et al. 2004). Therefore, biodegradation rate constants obtained from such field data should be
taken as rough estimates only (Chapelle et al. 2003). As first order rates calculated from center line data include all
effects and processes that lower local contaminant concentrations, the precarious result of the different sources of
uncertainty is that the degradation potential may be severely overestimated (Rittmann 2004), causing underestimation of
plume length and contaminant mass as well as a too optimistic prognosis of down gradient concentrations and exposure
levels. These aspects were recently studied in-depth by Bauer et al. (2006a) and Beyer et al. (2006) by means of
numerical experiments in two-dimensional synthetic heterogeneous contaminated aquifers. The numerical experiments
were based on coarse monitoring networks with 6 to 8 wells, which were all placed along an inferred plume center line.
In reality, however, monitoring networks typically are designed to suit multiple and sometimes conflicting requirements
and objectives, which may also change with time, depending on the stage of the site investigation. Objectives in this
context are the detection of ground water contamination (e.g. Storck et al. 1997), site characterization and spatial
delineation of the contamination (e.g. McGrath and Pinder 2003) or the long term monitoring of the plume behavior (e.g.
Wu et al. 2006). These aims require spatially more extensive monitoring networks with larger numbers of wells
compared to the relatively simple center line well configurations necessary to estimate a degradation rate constant.
Recently, Stenback et al. (2004) demonstrated that additional off center line measurements can be incorporated in the
estimation of the degradation rate, when a two-dimensional analytical transport model is fitted to contaminant
concentrations of all monitoring wells of an extensive monitoring network downgradient from the source. In a field
application example Stenback et al. (2004) showed, that accounting for the additional information on contaminant
concentrations and distribution significantly reduced rate constant estimates obtained from the conventional center line
approach to about 50 %, pointing out the well known problem of rate constant overestimation with the 1D center line
method.
This paper therefore studies the performance of several rate constant estimation approaches based on center line
investigation data for sites with extensive monitoring networks and compares the results to the two-dimensional approach
of Stenback et al. (2004). Adopting the terminology of Newell et al. (2002) the types of degradation rate constants
regarded here comprise bulk attenuation, biodegradation and “hybrid” rate constants. Point decay rates are not addressed
in this paper. Both strategies for rate constant estimation, i.e. the investigation of the plume center line and the approach
of Stenback et al. (2004), are applied to a set of synthetic sites with extensive monitoring networks, which were
independently designed and installed by individual test persons engaged in hydrogeological research and consulting. The
networks were installed for a general characterization and quantification of the contaminant plume (Chen et al. 2005;
Bauer et al. 2006b), and are used here as a basis for degradation rate estimation. Estimated rate constants for both
strategies are compared with regard to magnitude of errors and variability to draw conclusions on their limitations in
view of the monitoring network used. Thereby the studies of Bauer et al. (2006a) and Beyer et al. (2006) are considerably
extended, as the monitoring networks used in this paper are representative of real field situations by using all installed
observation wells, and are not restricted to an one-dimensional plume center line. Moreover, this analysis allows an
evaluation of the “human factor” on estimated rate constants resulting from individual notions of “sufficient accuracy” in
plume investigation.
Background and Scope
This study is based on a set of synthetic contaminated two-dimensional aquifers, generated by multiple stochastic
simulations of heterogeneous hydraulic conductivity fields and subsequent numerical simulation of contaminant
spreading from a defined source zone in the synthetic aquifers. The evolved virtual plumes were independently
investigated by a number of German scientists engaged in hydrogeological and environmental research using an
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