AQUIFER MODULE

COMPOSITION TAB

WZ-CLASS--Subsurface soil textural 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 (WZ-SAND), percent silt (WZ-SILT), percent clay (WZ-CLAY), percent organic matter (WZ-OMC), and percent iron and aluminum (WZ-IRON). If "Autofill" is enabled (i.e., checked) under the "Options" menu, default values are also assigned for total porosity (WZ-TOTPOR) and dry bulk density (WZ-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.

WZ-SAND--Percent sand of saturated zone soil
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 WZ-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 (WZ-SAND), percent silt (WZ-SILT), percent clay (WZ-CLAY), percent organic matter (WZ-OMC), and percent iron and aluminum (WZ-IRON) must be 100.

WZ-SILT--Percent silt of saturated zone soil
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 WZ-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 (WZ-SAND), percent silt (WZ-SILT), percent clay (WZ-CLAY), percent organic matter (WZ-OMC), and percent iron and aluminum (WZ-IRON) must be 100.

WZ-CLAY--Percent clay of saturated zone soil
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 WZ-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 (WZ-SAND), percent silt (WZ-SILT), percent clay (WZ-CLAY), percent organic matter (WZ-OMC), and percent iron and aluminum (WZ-IRON) must be 100.

WZ-OMC--Percent organic matter content of saturated zone soil
Units: percent (%)
Range: 0.00 to 9.99

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 (WZ-SAND), percent silt (WZ-SILT), percent clay (WZ-CLAY), percent organic matter (WZ-OMC), and percent iron and aluminum (WZ-IRON) must be 100.

WZ-IRON--Percent iron and aluminum in saturated zone soil
Units: percent (%)
Range: 0 to 99
Typical environmental range: 0 to 10

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 (WZ-SAND), percent silt (WZ-SILT), percent clay (WZ-CLAY), percent organic matter (WZ-OMC), and percent iron and aluminum (WZ-IRON) must be 100.

CHARACTERISTICS TAB

WZ-FRACT--Percent of total flux to aquifer
Units: percent (%)
Range: greater than 0.01 and less or equal to 100.

This option allows the consideration of multiple aquifers by allowing a partial mass flux of constituent to different aquifers. Enter the percent of the total constituent flux that is expected to move into the subject aquifer. Almost always, WZ-FRACT is set to 100.

WZ-PH--pH of the pore water in the saturated zone
Units: none
Range: 0.1 to 14.0

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.

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

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

WZ-EFFPOR--Effective porosity of saturated zone
Units: percent (%)
Range: 0.1 to WZ-TOTPOR

The effective porosity is the pore space that contributes to advective flow. If effective porosity is not available for the saturated zone, interpolate using the following: sand = 25%, silt = 15%, and clay = 10%. The value used must be less than or equal to the value for total porosity given in Section WZ-TOTPOR and Table 2.1. Be sure the value chosen is consistent with the value used in Section WZ-PVELOC (Darcy velocity in the saturated zone).

WZ-PVELOC--Darcy velocity in the saturated zone
Units: feet per day (ft/day)
Range: greater than 0.0

The parameter WZ-PVELOC represents Darcy or bulk velocity of ground water flow (i.e., the velocity that would result for an effective porosity of 100 %). If no value is available from the site data, Darcy velocity (Vd) can be estimated by:

Vd = K (H1 - H2)/d

where K is the saturated hydraulic conductivity (ft/day), and (H1-H2)/d is hydraulic gradient, defined as the difference in hydraulic head at two points in the aquifer, divided by the distance (d) between the two points. This information can be obtained from water table or potentiometric surface maps.
If no value is available for saturated hydraulic conductivity, a typical value can be selected from Table 2.1 for the soil textural classification determined in Section WZ-CLASS.

In contrast with Darcy velocity, the pore water velocity (Vp) can be estimated as Darcy velocity divided by effective porosity (see Section WZ-EFFPOR):

Vp = Vd/ne

where ne is the effective porosity expressed as a fraction. The constituent velocity (Vc) can be estimated by:

Vc = Vp/Rf

where Rf is the retardation factor and is defined as:

Rf = 1 + Kd (Bd/ne)

where Bd and Kd are dry bulk density (see Section WZ-BULKD) and the equilibrium (partition or distribution) coefficient (see Section WA-SUBKD), respectively.

If sampling indicates that constituents have moved from the release site, check to ensure that the calculated constituent velocity (Vc) is consistent with the speed at which the constituent plume is traveling in the aquifer. The constituent cannot be moving faster than the water (except by longitudinal dispersion) but it can move significantly slower, depending on its retardation (equilibrium coefficient, Section WA-SUBKD). If the calculated constituent velocity (Vc) is different from the apparent speed of the constituent plume (due to retardation), adjust the Kd value, then Darcy velocity, so the calculated constituent velocity is equal to the speed of the plume.

WZ-THICK--Thickness of saturated zone
Units: feet (ft)
Range: 0.1 to 9999.9

The parameter WZ-THICK is the aquifer thickness, which is defined as the distance from the water table to the bottom of (the vertical extent of) the aquifer. The saturated thickness should be obtained from the site data.

Figure 2.21 illustrates a cross section of a release site. In this example, the saturated thickness, WZ-THICK, would be 60 feet.

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

If the dry bulk density is not available for the saturated zone, use the value from Table 2.1 for the appropriate soil textural classification determined in Section WZ-CLASS above. To be conservative, choose the smallest value of bulk density when trying to decide between two categories. This value should correspond to the total porosity identified in Section WZ-TOTPOR.

CONCENTRATION LOCATIONS AND FLUX LOCATIONS TAB

WZ-DIST--Travel distance in saturated zone
Units: feet (ft)
Range: greater than 0.0

WZ-DIST must be determined for each downgradient receptor. This distance is measured from the center of the release site to each downgradient point such as a well or a river into which the groundwater discharges. This distance can be determined from the site documents and maps or from 7.5-min USGS topographic maps. Carefully determine the downgradient direction. It will correspond to the direction used to determine the hydraulic head in Section WZ-PVELOC.

Figure 2.22 illustrates an example waste site with two distances "a" and "b" from the release site to the river. In this example, the distance "b" should be used because constituents must flow along the groundwater flow path (from the waste site to Point B). Also consult Figure 2.23 which illustrates a relationship between WZ-DIST and WZ-YDIST. It should be noted that WZ-DIST is not necessarily the distance between the center of source to receptor. If the receptor well is some way off the downgradient flow direction as illustrated in Figure 2.23, WZ-DIST can be determined as follows: first, draw a line through the plume centerline or a line of downgradient flow path through the center of the source area. Second, draw a second line from the receptor well, perpendicular to the plume centerline or line of flow path (i.e., the first line). Then, measure the distance between the crossed point and the center of waste unit, which is WZ-DIST.

The aquifer module is not designed to handle the case where a receptor well occurs within the boundaries of the waste site (i.e., within the actual area where constituent is released). Therefore, all receptor wells should be located no closer to the center of the source than the downgradient edge of the source (i.e., the distance to the receptor should never be less than half the length of the waste site).

WZ-YDIST--Perpendicular distance from groundwater flow centerline to receptor
Units: feet (ft)
Range: greater than or equal to 0.0

This option allows the consideration of transverse variation of groundwater plume concentrations. For evaluation of plume centerline concentrations, enter a value of 0.0 (most conservative). For evaluation of off-centerline plume concentrations, enter the perpendicular distance from plume centerline to the receptor well, as illustrated in Figure 2.23. The plume is assumed to be symmetric, and this parameter is defined as being positive on either side of plume centerline.

WZ-AQDEPTH--Vertical distance below groundwater table
Units: feet (ft)
Range: greater than or equal to 0.0

WZ-AQDEPTH is the depth or vertical distance measured from the surface of the ground water table to the point where constituent concentration is to be computed. Figure 2.24 illustrates examples of WZ-AQDEPTH. Note that the concentration is computed as if the well does not exist; i.e., radial dilution by uncontaminated water is not taken into account. Entering WZ-AQDEPTH = 0 provides the most conservative result.

WZ-LDISP--Longitudinal dispersivity
Units: feet (ft)
Range: greater than 0.0

Enter the longitudinal (or x-direction) dispersivity for the saturated zone from site documents, or use the "Estimate" button if a value is not available. The following equation is used for the estimate:

WZ-LDISP = 0.1 (WZ-DIST)

where WZ-DIST is the longitudinal travel distance.

WZ-TDISP--Transverse dispersivity
Units: feet (ft)
Range: greater than 0.0

Enter the transverse (or y-direction) dispersivity for the saturated zone from site documents, or use the "Estimate" button if a value is not available. The following equation is used for the estimate:

WZ-TDISP = 0.33 (WZ-LDISP)

where WZ-LDISP is the longitudinal dispersivity.

WZ-VDISP--Vertical dispersivity
Units: feet (ft)
Range: greater than 0.0

Enter the vertical (or z-direction) dispersivity for the saturated zone from site documents, or use the "Estimate" button if a value is not available. The following equation is used for the estimate:

WZ-VDISP = 0.0025 (WZ-LDISP)

where WZ-LDISP is the longitudinal dispersivity.

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 water. 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 low Kd value would pose a high risk to an offsite receptor who ingests ground water because the constituent would be 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 aquifer 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 (WZ-PH), percent clay (WZ-CLAY) and percent iron and aluminum (WP-IRON). For organic constituents, the percent sand (WZ-SAND), percent silt (WZ-SILT), percent clay (WZ-CLAY), and percent organic matter (WZ-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 = WZ-OMC + WZ-CLAY + WZ-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 * WZ-OMC + 2.0 * WZ-CLAY + 0.4 * WZ-SILT + 0.005 * WZ-SAND)

WZ-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 aquifer module only uses the water solubility for a comparison with the concentration of constituent entering the aquifer and the concentration computed at the receptor locations. If a concentration exceeds the solubility, a warning message is written to a warning message file (casename.WRN) and to the Waterborne Listing File (casename.WLS) for receptor concentrations. If the solubility is 0, this comparision is not performed.

WZ-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.