69
Groundwater Modeling Involving PRBs
Prommer et al. (2008) modeled the geochemical and isotopic changes in a
column experiment for degradation of TCE using ZVI. The researchers considered that incorporation of the details of the degradation pathways of the
organic contaminants had not been a primary concern and had not been
taken into account in models such as Moffett Air Field (Yabusaki et al., 2001)
and Elizabeth City (Mayer et al., 2001). The objective of their study was to
provide a more comprehensive and integrated analysis of experimental data,
including isotopic data, toward the long-term goal of process-based hydrogeochemical modeling for the efficient and economic design of ZVI PRBs.
PHT3D (Prommer et  al., 2003) was used in this study to simulate TCE
degradation by ZVI and corresponding geochemical changes. PHT3D was
developed by coupling a MT3DMS (Zheng and Wang, 1999) and PHREEQC-2
(Parkhurst and Appelo, 1999) to compute the reactive processes. The transport of TCE-contaminated groundwater through the experimental column
filled with the Fe filings was simulated using PHT3D. Fitting the observed
data with reaction rate constants provided by the parameter estimation
tool PEST (Doherty, 2002) was coupled with PHT3D. The TCE degradation
reaction network provided very good agreement between simulated and
observed concentration as shown in Figure 4.2.
Contrary to this observation, poor model calibration results were achieved
with alternative versions of the reaction network, such as when the production of C3–C5 hydrocarbons was omitted, and also when the pathway
TCE
DCE
× 10 –4
× 10 –5
4
× 10 –6
× 10 –5
× 10 –5
× 10 –5
Sim.
Meas
Acetylene
Ethylene
Hydrocarbons
6
4
2
0
2
Conc. (mol L –1
)
Conc. (mol L –1
)
Conc. (mol L –1
)
1.5
1
0.5
0
1.5
1
0.5
0
8
6
4
2
0
8
6
4
2
0
3
2
1
0
Ethane
0
0.1
0.2
0.3
Dist. (m)
0.4
0.5
0
0.1
0.2
0.3
Dist. (m)
0.4
0.5
FIGURE 4.2
TCE degradation reaction network using PHT3D.