SESOIL Application Input Parameters
Application file parameters are used to establish the soil profile characteristics and contaminant load. These properties are used by the SESOIL hydrologic and pollutant cycle sub-models. Values for these parameters are typically obtained as part of the site investigation.
1.1 Load Area
This parameter establishes the areal extent of the contamination in centimeters squared. There is some confusion regarding the importance of this parameter. This is because SESOIL outputs the same concentration no matter the areal extent of the contaminated soil. However, the mass entering groundwater will vary significantly based on the extent of the soil contamination. The importance of this parameter depends on which method is used to determine the resulting groundwater concentration. You do not need to worry about the areal extent when using methods based on the SESOIL leachate concentration. This includes methods such as the dilution factor equation, the SUMMERS model and MODFLOW. However, areal extent is crucial when linking SESOIL to AT123D. This is because the load in AT123D is a as a mass not a concentration.
1.2 Site Latitude
This is the latitude of the site above or below the equator in decimal degrees. Latitude is used along with other parameters to establish monthly soil temperatures.
1.3 Instantaneous/Continuous Release
The instantaneous release option is only available in the top soil layer. In an instantaneous release all of the contamination is released at the start of the month. Using a continuous release option contaminant load will be spread out over the entire month. Essentially the instantaneous option gives highly volatile contaminants an extra month in which to volatilize. As the release option only applies to the top layer where volatilization is highest it can have some impact on the results for highly volatile contaminants. The continuous release option is used in layers 2, 3 and 4 no matter which release option is selected.
1.4 Layer Thickness
This parameter is used to establish the thickness of the soil layers in centimeters. Values are typically based on varying soil properties and by the thickness of the contamination.
1.5 Number of Sub-layers
This parameter is used to establish the number of sub-layers in each of the layers. These sub-layers are used in two ways.
First, the sub-layers can be used in the contaminated zone to define a contaminant profile. For instance if a contaminated layer is 10 feet thick and it was sampled with a two foot spoon, a 10 foot layer would be divided in to 5 sub-layers. This would establish 5 sub-layers each 2 feet thick. Analytical results can then be entered directly to the appropriate sub-layer.
Second, sub-layers are used to "discretize" the migration of contaminants through the soil column. Basically the higher the number of sub-layers the higher the contamination remains in the soil column. This is because mass in SESOIL is evenly distributed throughout a layer or sub-layer as soon as it reaches it. For example if a contaminant front entered a 10 foot thick layer which was divided in to 10 sub-layers the mass would be distributed throughout the upper one foot sub-layer. However, if a contaminant front entered a 10 foot thick layer with only one sub-layer, the mass would be distributed throughout the entire 10 foot layer. The resulting leachate concentration would be higher in the second scenario due to less volatilization from the greater depth. This has little influence on the rate at which the contaminant migrates towards the water table, but it does influence volatilization. In general you want to establish as many layers and sub-layers as possible beneath the soil contamination.
Non SEVIEW users should be careful in setting up the layers and sub-layers in SESOIL. This is because the bottom most layer or sub-layer moisture concentration is dedicated as the leachate concentration. Using the bottom most layer soil moisture concentration as the leachate concentration is problematic for three reasons.
First SESOIL soil moisture concentrations are not directly dependent upon load to groundwater. This means that non SEVIEW users will still see a “leachate concentration” even though there was no load to groundwater.
Second, this wastes a soil layer or sub-layer which could be used to refine the soil profile.
Finally, and perhaps most importantly the thickness of this layer or sub-layer can have a significant influence on the “leachate” (soil moisture) concentration. Basically the thicker the layer or sub-layer the sooner the contaminant will reach it and the sooner it “reaches” groundwater. Non SEVIEW users should be careful in using this approach. SEVIEW users do not need to worry as leachate concentrations are based on the mass leaving the soil column divided by the volume of recharge leaving the soil column. Using this approach means that the leachate concentration is not dependent upon the thickness of the bottom most layer or sub-layer.