V. 2 Issue 3 September, 2003
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SESOIL vs. VLEACH
This issue provides a comparison between the SESOIL and VLEACH vadose zone models. These models are used to establish site-specific cleanup objectives. Rather than providing a technical description of the model capabilities, this review looks at variations in model results for comparable input parameters. Both models are available as public domain versions and have been incorporated in integrated software packages.

SESOIL
SESOIL is a one-dimensional vertical transport screening-level model. It can simulate transport and fate based on diffusion, adsorption, volatilization, biodegradation, cation exchange and hydrolysis. SESOIL contains three submodels that simulate: soil moisture movement, contaminant transport, and erosion. SESOIL was developed for US Environmental Protection Agency (USEPA) in 1981 by Arthur D. Little, Inc. An enhanced version of SESOIL contained in the SEVIEW 6.2 program was used.

VLEACH
Like SESOIL, VLEACH, is a one-dimensional vadose zone transport and fate model. It simulates transport and fate based on partitioning of organic contaminants between soil moisture, soil air, and adsorbed on soil phases. Unlike SESOIL, VLEACH cannot simulate biodegradation nor does it deal with varying soil properties with depth. It was developed by CH2M Hill in 1990 for Region 9 of the USEPA. The public domain version VLEACH 2.2a was used in this review.

Model Parameters
Input parameters were based on the release of benzene in sandy soil. Soil contamination was established from the surface to a depth of 5 feet. Depth to groundwater was varied between 10 and 25 feet.

Climatic Parameters
The SESOIL hydrologic submodel uses climatic information along with other parameters such as soil properties and depth to groundwater to establish infiltration rates. The climatic data for Milwaukee, Wisconsin was used. This data set is included with the public domain version of SESOIL.

Results of the SESOIL hydrologic submodel were used to establish the infiltration rate of 13.28 inches per year in VLEACH. Water content in VLEACH was also established based on SESOIL results. This was done as VLEACH requires the use of some other model to establish these parameters.

SESOIL Results Used as Input to VLEACH

Parameter
VLEACH
Recharge Rate (inches/year)
13.28
Water Content (fraction)
0.07

Chemical Parameters
Chemical parameters for benzene contained in the SEVIEW 6.2 chemical database were used in both models.

Chemical Parameters

Parameter
VLEACH
SESOIL
Water Solubility (mg/L)
1750
1750
Organic Carbon Adsorption Coefficient
(µg/g)/(µg/ml)
61.7
61.7
Henrys Law Constant
(atm-m3/mol)
5.55E-3
5.55E-3
Air Diffusion Coefficient
(cm2/sec)
0.088
0.088

Soil Parameters
Soil parameters were based on sandy soil. Specific parameters were based on information provided in the SEVIEW 6.2 help file and User's Guide.

Soil Parameters

Parameter
VLEACH
SESOIL
Bulk Density (g/cm3)
1.40
1.40
Effective Porosity (fraction)
0.30
0.30
Permeability (cm/sec)
- -
3.2E-4
Soil Pore Disconnectedness Index (dimensionless)
- -
3.7

Soil Profile Parameters
The SESOIL column was divided into 40 sublayers of equal thickness. While the VLEACH vertical column was divided into 40 cells of equal thickness. A contaminant load of 1.0 µg/g was established in the upper 5 feet in both models. Sensitivity analysis was performed by varying depth to groundwater from 10 to 25 feet.

Soil Colum Parameters

Parameter
VLEACH
SESOIL
Load (µg/g)
1.0
1.0
Load Thickness (ft)
5.0
5.0
Number of Layers (Cells)
40
40
Depth to Groundwater (ft)
10 & 25
10 & 25

Results
SEVIEW 6.2 was used to generate hydrologic and pollutant cycle reports to document the SESOIL results. A custom program was developed that extracted VLEACH soil moisture concentration data in the output file and saved this data in an Excel spreadsheet format. Excel was then used to locate the maximum concentration and to plot the results. A summary the results are presented below.

Results 10 Foot Depth to Groundwater

Parameter
VLEACH
SESOIL
Leachate Maximum (mg/L)
1.13
2.12E-3
Year Max
2.0
2.0


Results 25 Foot Depth to Groundwater

Parameter
VLEACH
SESOIL
Leachate Maximum (mg/L)
0.438
8.18E-4
Year Max
4.0
3.0

Discussion
As you can see from the results although both SESOIL and VLEACH are vadose zone models the predicted leachate concentrations are quite different. Such large discrepancies are not unusual, as there is often more difference between vadose zone models than between most groundwater models.

There are many reasons for the differences between the SESOIL and VLEACH results. Perhaps the most important is the way the models handle the movement of soil moisture within the vadose zone. VLEACH assumes steady state recharge downward towards the water table. In reality, steady state infiltration through the vadose zone rarely occurs. Infiltration rates typically vary from month to month based on numerous factors such as precipitation and temperature. Furthermore, much of the precipitation entering the soil column is often returned to the atmosphere via evapotranspiration prior to it ever reaching groundwater. Additional factors influencing soil moisture movement and recharge rates include capillary rise and depth to groundwater. Simply calibrating VLEACH to the overall infiltration rate does not account for these processes.

Conclusions
Based on this review the following conclusions were made:
  • SESOIL allows more than 530 times more contamination to remain in place,
  • SESOIL is easier to uses as it establishes infiltration rates, and
  • Simply calibrating VLEACH to SESOIL infiltration rates does not produce comparable results.
Given these results it may appear that SESOIL may not be conservative enough to meet regulatory requirements, however, this is not the case. In fact numerous regulatory agencies have used SESOIL to develop their baseline cleanup objectives. What this review indicates is that VLEACH is even more conservative than SESOIL. It is not surprising that regulatory agencies will accept site-specific cleanup objectives based on VLEACH. However, this may mean that funds are needlessly being spent cleaning up contamination which poses minimal environmental threat.

SESOIL
VLEACH
Model Parameters
Climatic Parameters
Chemical Parameters
Soil Parameters
Soil Profile Parameters
Results
Discussion
Conclusions


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