and depth of contaminant burial. Furthermore,
crantz et al. 1991, 1992) and site sensitivity rank-
the model was highly sensitive to intrinsic per-
ing for Wisconsin soils (Ladwig et al. 1993).
meability for benzene in sand. However, it was
Several computer models are in use to assess
insensitive to intrinsic permeability for treatment
the cleanup levels of hydrocarbon contaminated
of benzene in till. The till was a finer grained envi-
soils. Recently, Odencrantz et al. (1992) reviewed
ronment, with fewer available pore spaces than
the following vadose zone transport models:
the sand.
1. PRZM--Pesticide Root Zone Model
Bonazountas and Kallidromitou (1993) report-
2. CMLS--Chemical Movement in Layered
ed that at the present state of scientific knowl-
Soils
edge, SESOIL is a well-developed soil compart-
ment model. They emphasized that the model
Agricultural Management Systems
must be calibrated for site-specific variables such
4. LEACHMP--Leaching Estimation And Chem-
as biodegradation rate, soil organic carbon con-
istry Model-Pesticide
tent, local climate and depth to the water table.
5. MOUSE--Method Of Underground Solute
Hetrick et al. (1993) recommended that predic-
Evaluation
tions for the hydrology at a given site be calibrat-
6. PESTAN--Pesticide Analytical model
ed to agree with known measurements. They
7. Jury's analytical transport model
suggested that the user must conduct sensitivity
8. MOFAT--Multiphase Organic Flow And
analyses or evaluate results obtained by assign-
Transport
ing distributions to the input parameters (O'Neill
9. SESOIL.
et al. 1982, Gardner 1984, Hetrick et al. 1991).
They recommended the SESOIL and MOFAT
When properly used, SESOIL is an effective
models on the basis of the following vadose zone
screening-level tool for assessing chemical move-
transport criteria:
ment in soil.
1. Needed is a one-dimensional vertical model
Oregon State regulator Anderson (1992) tested
that can be discretized into at least three lay-
the SESOIL model to simulate transport of organ-
ers, with varying soil properties and chemi-
ic contaminants through the vadose zone. He also
cal loading concentrations by layer.
used the MINTEQA1 model (USEPA 1987) to
2. The model must represent one mobile chemi-
simulate inorganic speciation and concentrations
cal component in three phases (adsorbed,
under specified field conditions. Sensitivity anal-
aqueous and gaseous).
yses were performed on organic contaminants--
3. The model must represent biodegradation,
benzene, carbon tetrachloride, ethylbenzene,
volatilization and gaseous diffusion.
methyl chloride, naphthalene, perchloroethylene,
4. The model must represent the variable effects
toluene, 1,1,1-trichloroethane, 1,1,2-trichloro-
of local climate (precipitation, temperature,
ethylene and o-xylene. The results of the tests in-
etc.).
dicated that the most important parameters con-
MOFAT is a two-dimensional finite-element
trolling organic contaminant transport in the
modeling code that transports up to five compo-
SESOIL/AT123D models were the thickness of
nents between four phases (air, water, oil and
the contaminated zone, depth from the contami-
soil) and also allows up to ten soil layers of differ-
nated zone to groundwater, hydraulic conductiv-
ent properties. MOFAT is more powerful but
ity and gradient, and the fraction of the organic
harder to use than SESOIL; therefore, fewer users
carbon in the soil.
have adopted it.
Numerous studies have been conducted to
The Council for Health and Environmental
compare the sensitivity of the SESOIL model with
Safety of Soils' (CHESS) Analysis and Environ-
other models, using field input data (Bonazoun-
mental Fate Committee reviewed several existing
tas et al. 1982; Wagner et al. 1983; Hetrick 1984;
environmental fate models and recommended
Kincaid et al. 1984; Watson and Brown 1985; Mel-
the SESOIL and POSSM models for further con-
ancon et al. 1986; Hetrick et al. 1986, 1989; Hetrick
sideration as tools to aid the risk assessment pro-
and Travis 1988). The model was used in risk
cess (Calabrese and Kostecki 1992). POSSM,
assessments to evaluate direct coal liquefaction
which stands for PCB On-Site Spill Model, is a
(Walsh et al. 1984), incineration of hazardous
contaminant transport model developed to pre-
waste (Holton et al. 1985, Travis et al. 1986), trans-
dict environmental concentrations associated
port of benzene to the groundwater (Tucker et al.
with a chemical spill. It is a modified version of
1986), soil cleanup levels in California (Oden-
the PRZM model and is considered as a one-
2
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