Handbook of Solvents - George Wypych - ChemTech - Ventech!

23.1 Natural attenuation of chlorinated solvents 1605
23.1.6.2 Modeling chlorinated solvent plumes
Very few models exist (analytical or numerical) which are specifically designed for simulating
the natural attenuation of chlorinated solvents in ground water. Ideally, a model for
simulating natural attenuation of chlorinated solvents would be able to track the degradation
of a parent compound through its daughter products and allow the user to specify differing
decay rates for each step of the process. This may be referred to as a reactive transport
model, in which transport of a solute may be tracked while it reacts, its properties change
due to those reactions, and the rates of the reactions change as the solute properties change.
Moreover, the model would also be able to track the reaction of those other compounds that
react with or are consumed by the processes affecting the solute of interest (e.g., electron
donors and acceptors).
Two models: BIOCHLOR and RT3D for the natural attenuation of chlorinated solvents
have been presented recently in the general literature and will be briefly discussed in
this section.
23.1.6.2.1 BIOCHLOR natural attenuation model
The BIOCHLOR Natural Attenuation Model 109 simulates chlorinated solvent natural attenuation
using an Excel based interface. BIOCHLOR simulates the following reductive
dechlorination process:
k1 k2 k3 k4
PCE ⎯⎯→TCE ⎯⎯→DCE ⎯⎯→VC ⎯⎯→ETH The equations describing the sequential first order biodegradation reaction rates are
shown below for each of the components:
rPCE =−k1 CPCE
r = k C −k
C
[23.1.16]
TCE 1 PCE 2 TCE
[23.1.17]
r = k C −k
C
DCE 2 TCE 3 DCE
[23.1.18]
r = k C −k
C
VC 3 DCE 4 VC
[23.1.19]
rETH = k4 CETH
[23.1.20]
where:
k1,k2,k3,k4 the first order rate constants
CPCE, CTCE, CDCE, CVC and CETH the aqueous concentration of PCE, TCE, DCE, vinyl
chloride, and ethene, respectively.
These equations assume no degradation of ethene.
To describe the transport and reaction of these compounds in the subsurface, one-dimensional
advection, three-dimensional dispersion, linear adsorption, and sequential first
order biodegradation are assumed as shown in the equations below. All equations, but the
first, are coupled to another equation through the reaction term.
R dC
PCE
dt
v dC
D
dx
dC
D
dx
dC
D
dy
d
2
2
2
PCE
PCE C
= − + x + y +
2
2 z
dz
PCE PCE
PCE
2
− kC [23.1.21]
1 PCE