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

74 C. Beyer et al. / Journal of Contaminant Hydrology 87 (2006) 73–95
1. Introduction
One major requirement for the implementation of natural attenuation (NA) as a remedial and risk
reduction strategy for contaminated aquifers is an assessment of the dimensions of contaminant
plumes and to predict their fate. Down gradient contaminant concentrations, i.e. plume lengths, must
be calculated or estimated to identify potential receptors and predict exposure levels. For this purpose
analytical and numerical solute transport models (e.g. Bioscreen (Newell et al., 1996), Biochlor
(Aziz et al., 2000), Bioplume III (Rifai et al., 1998)) are routinely used. The rate of contaminant
removal through biodegradation is a key parameter, as concentrations and the modeled plume
lengths are highly sensitive to the degradation rate (McNab, 2001; Suarez and Rifai, 2004). Although
very detailed mathematical descriptions of contaminant degradation in the subsurface are available
(Baveye and Valocchi, 1989; Rittmann and VanBriesen, 1996; Wiedemeier et al., 1999; Islam et al.,
2001), for applications in the field, usually simplified approaches are used because the identification
of a large number of parameters and processes from field data is often impossible. Due to its
mathematical simplicity, its ease of implementation into transport models and the necessity of
determining only a single parameter, the most frequently used degradation model is first order
kinetics (Wiedemeier et al., 1999). Field methods for the determination of biodegradation rates in
ground water include mass balance calculations, in-situ microcosm studies and the center line
method (Chapelle et al., 1996; Wiedemeier et al., 1999). The latter is frequently used for plume
monitoring and degradation rate evaluation (e.g. Chapelle et al., 1996; Wiedemeier et al., 1996;
Zamfirescu and Grathwohl, 2001; Suarez and Rifai, 2002; Wilson and Kolhatkar, 2002; Bockelmann
et al., 2003), and is based on contaminant concentrations measured in observation wells
installed along the presumed center line of a plume. This approach, however, is only applicable for
plumes that have reached a (quasi-) steady state, i.e. the plume is neither shrinking nor expanding and
the measured concentrations do not change with time. The concentration-distance relations thus
obtained for a steady state plume can be used to estimate the first order rate constant λ. This
parameter can then be used with an appropriate transport model to estimate the contaminant
distribution in the aquifer. However, as the spatial variability of aquifer properties has a substantial
influence on the distribution of contaminants and plume development, also the results of such an
assessment are affected. Wilson et al. (2004) point out that the approach is prone to errors because the
center line of a plume may be missed by monitoring wells if the inferred ground water flow direction
is incorrect or the contaminant plume meanders in all three dimensions due to macro-scale
heterogeneities. McNab and Dooher (1998) demonstrated that, even in a homogeneous aquifer,
transverse dispersion can produce center line concentration profiles of recalcitrant compounds that
exhibit characteristics consistent with first order degradation; this can easily lead to misinterpretation
of the monitoring data. The result of these complicating factors is that the degradation potential may
be severely overestimated, causing underestimation of plume length or contaminant mass and an
over optimistic prognosis of down gradient concentrations and exposure levels. Moreover, it is well
known that the use of first order kinetics may be problematic in some situations, as it is a poor
representation of the processes occurring in contaminated aquifers. Usage of a first order model
outside its range of validity may result either in significant under- or overestimation of the
attenuation potential at a site (Bekins et al., 1998). In a numerical experiment, Schäfer et al. (2004a)
demonstrated that for specific points in time, first order kinetics may be able to approximately
reproduce mass and dimensions of contaminant plumes that follow from a far more complex
degradation model. For a long term prognosis, however, the first order approximation proved
inappropriate, resulting in an underestimation of plume length and contaminant mass. Recently,
Bauer et al. (2006) performed a sensitivity study on the influences of aquifer heterogeneity on first