Environmental Engineering Reference
In-Depth Information
Fig. 18.15 Schematic
illustration of the stream tube
model ( left ), (Toride et al.
1995 )
The stream tube model was implemented into the CXTFIT 2.0 code (Toride
et al. 1995 ) for a variety of transport scenarios in which the pore water velocity
in combination with either the hydrodynamic dispersion coefficient, D h , the dis-
tribution coefficient for linear adsorption, K d , or the first-order rate coefficient for
nonequilibrium adsorption,
α k , are stochastic variables (Toride et al. 1995 ).
18.3.5 Multicomponent Reactive Solute Transport
The various mathematical descriptions of contaminant transport presented thus far
all considered contaminants that would move independently of other contaminants
in the subsurface. In reality, the transport of reactive contaminants is more often
than not affected by interactive physico-chemical and even biochemical processes.
Simulating these processes requires a more comprehensive approach that couples
the physical processes of water flow and advective-dispersive transport with a range
of biogeochemical processes. The liquid phase is always a mixture of many ions
which may be involved in mutually dependent chemical processes, such as complex-
ation reactions (Lichtner 1996 ; Yeh and Tripathi 1990 ), cation exchange (White and
Zelazny 1986 ), precipitation-dissolution (Šimunek and Valocchi 2002 ), sorption-
desorption, volatilization, redox reactions, and degradation, among other reactions
(Šimunek and Valocchi 2002 ). Transport and transformation of many contaminants
is further mediated by subsurface aerobic or anaerobic bacteria. Bacteria catalyze
redox reactions in which organic compounds (e.g., hydrocarbons) act as the elec-
tron donor and inorganic substances (oxygen, nitrate, sulfate, or metal oxides) as the
electron acceptor. By catalyzing such reactions, bacteria gain energy and organic
carbon to produce new biomass. These and related processes can be simulated
using integrated reactive transport codes that couple the physical processes of water
flow and advective-dispersive contaminant transport with a range of biogeochemical
processes (Jacques et al. 2003 ;Šimunek et al. 2006b ).
Once the various chemical reactions are defined, the final system of governing
equations usually consists of several partial differential equations for contaminant
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