VADOSE ZONE MODULE

COMPOSITION TAB

WP-CLASS--Partially saturated zone soil texual classification
Units: none

The U.S. Department of Agriculture (USDA) soil textural classification scheme is used by the MEPAS modules. This scheme is based on the relative proportions of sand, silt and clay by weight normalized to 100 percent. Refer to site documents to determine an appropriate soil textural classification.

Selecting a soil texture results in the assignment of default values for percent sand (WP-SAND), percent silt (WP-SILT), percent clay (WP-CLAY), percent organic matter (WP-OMC), and percent iron and aluminum (WP-IRON). If "Autofill" is enabled (i.e., checked) under the "Options" menu, default values are also assigned for soil type coefficient (WP-SOILCOEF), total porosity (WP-TOTPOR), field capacity (WP-FIELDC), saturated hydraulic conductivity (WP-CONDUC), and dry bulk density (WP-BULKD). The default values are the same as those listed in Table 2.1. These assignments are not necessarily permanent - they can be changed to site specific values at any time.

WP-SAND--Percent sand of partially saturated zone
Units: percent (%)
Range: 0 to 100

If the percentage of sand is not available from site documents, a typical value can be selected from Table 2.1 for the soil textural classification determined in Section WP-CLASS above. The percentage of sand is only used for the estimation of subsurface adsorption coefficients (WA-SUBKD) for organic constituents. The sum of percent sand (WP-SAND), percent silt (WP-SILT), percent clay (WP-CLAY), percent organic matter (WP-OMC), and percent iron and aluminum (WP-IRON) must be 100.

WP-SILT--Percent silt of partially saturated zone
Units: percent (%)
Range: 0 to 100

If the percentage of silt is not available from site documents, a typical value can be selected from Table 2.1 for the soil textural classification determined in Section WP-CLASS above. The percentage of silt is only used for the estimation of subsurface adsorption coefficients (WA-SUBKD) for organic constituents. The sum of percent sand (WP-SAND), percent silt (WP-SILT), percent clay (WP-CLAY), percent organic matter (WP-OMC), and percent iron and aluminum (WP-IRON) must be 100.

WP-CLAY--Percent clay of partially saturated zone
Units: percent (%)
Range: 0 to 100

If the percentage of clay is not available from site documents, a typical value can be selected from Table 2.1 for the soil textural classification determined in Section WP-CLASS above. The percentage of clay is only used for the estimation of subsurface adsorption coefficients (WA-SUBKD) for both organic and inorganic constituents. The sum of percent sand (WP-SAND), percent silt (WP-SILT), percent clay (WP-CLAY), percent organic matter (WP-OMC), and percent iron and aluminum (WP-IRON) must be 100.

WP-OMC--Percent organic matter content of partially saturated zone
Units: percent (%)
Range: 0 to 100

Obtain the percentage of organic matter from site documents, if possible. Often times, the fraction of organic carbon (foc) will be available. Since the carbon content of organic matter in environmental systems is generally about 58 percent, the percentage of organic matter can be estimated from the fraction of organic carbon as follows:

Organic Matter (%) = 1.7 x (Fraction Organic Carbon) x 100 (%)

If the percentage of organic matter is not available from site documents and cannot otherwise be determined, assume it is zero.

The percentage of organic matter is only used for the estimation of subsurface adsorption coefficients (WA-SUBKD) for organic constituents. The sum of percent sand (WP-SAND), percent silt (WP-SILT), percent clay (WP-CLAY), percent organic matter (WP-OMC), and percent iron and aluminum (WP-IRON) must be 100.

WP-IRON--Percent iron and aluminum in partially saturated zone
Units: percent (%)
Range: 0 to 100

If the percentage of iron and aluminum is not available from site documents, assume it is zero. The percentage of iron and aluminum is only used for the estimation of subsurface adsorption coefficients (WA-SUBKD) for inorganic constituents. The sum of percent sand (WP-SAND), percent silt (WP-SILT), percent clay (WP-CLAY), percent organic matter (WP-OMC), and percent iron and aluminum (WP-IRON) must be 100.

WP-SOILCOEF--Soil type coefficient
Units: none
Range: 0 to 15

This is an empirically determined coefficient in an equation relating unsaturated hydraulic conductivity to soil moisture content. Unless a measured value is available, use the values from Table 2.1 for the appropriate soil-textural classification determined in Section WP-CLASS. See the following document for the equation in which this coefficient is used:
Whelan, G., J. P. McDonald, and C. Sato. 1996. Multimedia Environmental Pollutant Assessment System (MEPAS): Groundwater Pathway Formulations. PNNL-10907, Pacific Northwest National Laboratory, Richland, Washington.

SOIL CHARACTERISTICS TAB

WP-PH--pH of pore water in the partially saturated zone
Units: none
Range: 1 to 14

If pH is unavailable from site documents, use pH = 7 (i.e., neutral conditions). The pH is only used for the estimation of subsurface adsorption coefficients (WA-SUBKD) for inorganic constituents.

WP-TOTPOR--Total porosity of partially saturated zone
Units: percent (%)
Range: 0.1 to 99.9

If total porosity is not available for the partially saturated zone, use the values from Table 2.1 for the appropriate soil-textural classification determined in Section WP-CLASS. If you must choose between two categories, choose an average value. This value should correlate to the dry bulk density identified in Section WP-BULKD as follows:

Porosity = 1 - (Dry Bulk Density/Specific Weight)

If the specific weight is unknown, it can be approximated as 2.65 g/cm3.

WP-FIELDC--Field capacity of partially saturated zone
Units: percent (%)
Range: 0.1 to WP-TOTPOR

If a field capacity is not available for the partially saturated zone, use the typical value from Table 2.1 for the appropriate textural classification determined in Section WP-CLASS. Field capacity must be less than or equal to total porosity (WP-TOTPOR). Field capacity is used as a lower limit on the moisture content calculation by the vadose zone module.

WP-CONDUC--Saturated hydraulic conductivity
Units: centimeters per day (cm/day)
Range: greater than 0.0

Use a site-specific value, if available. Otherwise, select a value from Table 2.1 for the appropriate soil-textural classification determined in Section WP-CLASS. Saturated hydraulic conductivity is used in the moisture content calculation by the vadose zone module.

WP-THICK--Thickness of partially saturated zone
Units: centimeters (cm)
Range: 0.1 to 304,800

Determine the number of distinct geologic layers in the unsaturated zone beneath the waste site, and represent each one using a separate glyph (i.e., partially saturated zone icon) in the FRAMES conceptual model diagram. Minimize the number of layers by requiring each layer to have very distinct or different characteristics (percent sand, silt, and clay). If you are uncertain about distinguishing between one or two layers, make one and use the more conservative parameters (i.e., use the parameters yielding the fastest travel time for constituent passing through the zone). Layers must be continuous beneath the site as the model does not simulate partial layers.

The example cross-sectional system illustrated in Figure 2.20 can be simplified into five layers represented by G1 through G5. Layers L1, L2, and L3 are not treated as layers because they are discontinuous. Sandy aquifers S1, S2, and S3 can be combined into a single aquifer whose physical properties are based on a weighted average of the individual layers. Try to keep the number of layers at a site to five or less.

The thickness of each major, distinct geologic layer should be obtained from site data, such as well logs or fence diagrams. The total thickness of all layers must equal the thickness of the unsaturated zone (distance from the bottom of the waste pit, pond, or landfill to the water table). If a clay or admix liner is present, it constitutes a layer.

WP-LDISP--Longitudinal dispersivity of partially saturated zone
Units: centimeters (cm)
Range: greater than 0.0 and less than WP-THICK

Enter the longitudinal dispersivity for the layer from site documents. The following equation is used for the estimate:

DL = 0.01 (WP-THICK)

where DL is the longitudinal dispersivity and WP-THICK is the thickness of the partially saturated layer.

WP-BULKD--Dry bulk density of partially saturated zone
Units: grams per cubic centimeter (g/cm3)
Range: 1.00 to 3.00
Typical environmental range: See Table 2.1

If a dry bulk density value is not available from site documents, typical values may be selected from Table 2.1 for the appropriate soil-textural classification determined in Section WP-CLASS. If you are uncertain which of two values to choose, use the smallest value to be conservative. This value should correlate to the total porosity identified in Section WP-TOTPOR.

CONSTITUENT PROPERTIES TAB

FS-CNAME--Name of constituent(s) for current transport scenario

This field shows the constituents that are selected for the scenario. Use the arrow buttons to scroll through the list of constituents and enter an adsorption coefficient and water solubility for each one, and a degradation/decay half-life for non-radionuclides. If a constituent has decay products, they will be shown on the bottom part of this screen.

WA-SUBKD--Subsurface adsorption coefficients
Units: milliliters per gram (ml/g)
Range: greater than or equal to 0.0

WA-SURFKD represents the equilibrium (partition or distribution) coefficient (Kd) of constituent between the soil particles and the soil moisture. Use laboratory or field-derived Kd values if they are available from site documents or reports of studies using similar soil and constituent characteristics. Theoretically, Kd is defined as:

Kd = Cp/Cd

where Cp is the particulate concentration (i.e., weight or activity of adsorbed constituent per weight of solid) and Cd is the dissolved concentration or activity. Although the Kd value of a constituent of concern can be determined in the laboratory or obtained from published literature, such values can vary considerably depending upon the experimental or environmental conditions (e.g., characteristics of soil and water) and technique used. When choosing between Kd values, use the value which will provide the conservative results (e.g., highest risk). For example, a receptor who is living on or near the waste site and ingesting contaminated soil would have high risk with the large Kd value because the constituent would not be readily leached from the soil and would be retained in soils for an extended period of time. On the other hand, low Kd value would pose a high risk to the offsite receptor who ingests water because the constituent is readily dissolved into water and mobilized. Note that because of the uncertainty associated with Kd, it is preferably used as a calibration parameter to adjust the constituent velocity.

Selecting Adsoption Coefficient - The vadose zone module can provide estimates of Kd for all constituents. For inorganic constituents, typical Kd values reported in the peer reviewed literature are provided by the user interface, via a lookup table, based on the soil water pH (WP-PH), percent clay (WP-CLAY) and percent iron and aluminum (WP-IRON). For organic constituents, the percent sand (WP-SAND), percent silt (WP-SILT), percent clay (WP-CLAY), and percent organic matter (WP-OMC) are used in an empirical equation to provide a Kd estimate.
The dropdown box gives the user the choice to select a Kd from
  1. Database Value - provided by the connected chemical database module
  2. Table Lookup Value - provided by the "KD_DATA.CSV" file. Primarily contains metals and radionuclides
    Kd = lookup(casid,col_index) where column index is calculated by
    sum = WP-OMC + WP-CLAY + WP-IRON
    If PH >= 9.0 Then
       If sum >= 30.0 Then col_Index = 3
       else If sum >= 10.0 Then col_Index = 2
       else col_Index = 1
    else If PH >= 5.0 Then
       If sum >= 30.0 Then col_Index = 6
       else If sum >= 10.0 Then col_Index = 5
       else col_Index = 4
    else
       If sum >= 30.0 Then col_Index = 9
       else If sum >= 10.0 Then col_Index = 8
       else col_Index = 7
  3. Estimated Value - calculated by the equation
    Kd = 0.0001 * koc * (57.735 * WP-OMC + 2.0 * WP-CLAY + 0.4 * WP-SILT + 0.005 * WP-SAND)

WP-SOL--Water solubility
Units: milligrams per liter (mg/l) or picocuries per milliliter (pCi/ml)
Range: greater than or equal to 0.0

Enter the aqueous solubility for the constituent. For most organic constituents, this value is available in the constituent database or in chemical property books. For inorganic constituents, this value will have to be estimated based on the geochemistry of the waste zone. If a value is available in the constituent database, it will be used as an initial default value. If a database value is not available, the solubility is set to 0, which is used as an indicator meaning an unknown solubility. The vadose zone module only uses the water solubility for a comparison with the concentration of constituent entering the layer. If that concentration exceeds the solubility, a warning message is written to a warning message file (casename.WRN). If the solubility is 0, this comparision is not performed.

WP-GHALF--Half-life in ground water
Units: years
Range: greater than 0.0

This parameter is the half-life of the constituent in the aquifer. For radionuclide and non-radionuclide constituents, this value defaults to the data in the constituent database, but the value can be changed to a site-specific value. This parameter can be used to implement other degradation rates (e.g., biodegradation, photo degradation, etc...) when they are converted to equivalent half-lives.