BGTK 2.0: A 3D Partial-equilibrium model to simulate coupled hydrogeological, microbiological and geochemical processes in subsurface systems
BGTK (BioGeochemical Transport - Thermodynamic and Kinetic, pronounced “bigtek”) is a three-dimensional multi-component reactive transport model that can handle mixed kinetic/equilibrium descriptions (Phanikumar and McGuire, 2004). BGTK combines the ability to efficiently simulate microbial transport and degradation reactions using kinetic descriptions based on the RT3D model (Clement et al., 1998) with the ability to simulate equilibrium reactions using the geochemical speciation model PHREEQC-2 and its reaction databases (Parkhurst and Appelo, 1999). The new model offers enormous flexibility in simulating a wide range of complex biogeochemical reactions. The model uses the sequential operator-splitting (SSO) strategy to solve the sub-models for the advection, dispersion and reaction processes separately. An advantage of using RT3D includes the ability to specify user-defined reaction modules for rate expressions and Jacobians and use of the ODE solver LSODA (Hindmarsh, 1983) to solve systems of both stiff and non-stiff reaction differential equations.
BGTK was used to simulate the spatial and temporal evolution of redox zonation in a contaminated aquifer. The problem is adapted from the test problem of Walter et al. (1994) who simulated metal mobility in aquifers impacted by acidic-mine tailings discharge. The hydrologic system used in the present problem is the same as that considered by Walter et al. (1994). The simulation is done in the two-dimensional vertical cross section following the water table gradient. The source region of tailings in Walter et al. (1994) is replaced with a constant source of complex organic matter called “Corg”.
Figure 1 shows the evolution of the redox plumes at 5, 10, 20 and 42 years. As the contaminant “Corg” migrates away from the source, it is fermented to a generic product “Cace” which serves as electron donors. These electron donors are then coupled to the sequential reduction of electron acceptors oxygen, nitrate, ferric iron, and sulfate based on thermodynamic arguments. Figure 2 shows the time history of the concentrations at a point in the aquifer below the source zone (x = 19.75, z = 9.375, shown by an asterisk in Figure 2) and this sequence is illustrated. The temporal evolution shown in Figure 2 is also consistent with published results (Mayer et al., 2002). Once these electron acceptors are depleted, methane is produced by methanogenesis.
Future versions will provide additional solvers as well as built-in modules/templates to simulate a number of “standard” microbiological and geochemical problems.
Participants:
Figure 1. Temporal evolution of redox zones in the aquifer for “Corg”, Oxygen, Sulfate, pH, Nitrate, Methane and Iron (II) at four different times (top to bottom: 5, 10, 20 and 35 years).
Figure 2. Time history of concentration evolution at a point in the aquifer:
x = 19.75 m, z = 9.375 m (shown with a * in Figure 1)
References:
M.S.
Phanikumar and J.T. McGuire, A 3D Partial Equilibrium Model to Simulate
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mobility in aquifers impacted by acidic mine tailings discharge, Water
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Parkhurst, D.L. and C.A.J. Appelo, Users’ guide to PHREEQC (Version 2),
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Denver, Colorado (1999).