Site Parameters Definitions

Dry Bulk Density
The dry bulk density is the weight of the soil divided by the volume of the soil, including pore volume (Bates and Jackson, 1987). This parameter can be determined through laboratory analysis of soil samples or in the field using a nuclear densometer.

Total Soil Porosity
The total soil porosity is the degree to which soil is permeated with pores or cavities through which water or air can move and is the sum of the water-filled and air-fill porosities (EPA, 1998). This parameter is not an input parameter but is directly related to the air-filled and water-filled porosities. The total soil porosity can be determined either through a laboratory test, a tabled value based on the soil type or can be calculated from the dry bulk density and the soil particle density. Therefore, if a site-specific dry bulk density is used, then a corresponding total soil porosity should be determined.

Water-Filled Porosity
The water-filled soil porosity is the portion of the total porosity containing water. This value can be calculated as the product of the moisture content of a soil times the dry bulk density. This value should be determined site-specifically if a site-specific dry bulk density or moisture content is determined. Variation in this parameter directly effects the air-filled soil porosity; therefore, if this parameter is determined site specifically, the air-filled porosity and total porosity should also be determined site specifically.

Air-Filled Soil Porosity
The air-filled soil porosity is the portion of the total porosity of soil containing air. This value is calculated by subtracting the water-filled porosity from the total soil porosity. If a site-specific total soil porosity or water-filled soil porosity is determined for a site, then the air-filled soil porosity should be reviewed to ensure that the sum of the air-filled and water-filled soil porosities equals the total soil porosity.

Fraction Organic Carbon
The fraction organic carbon is the organic carbon content of a soil expressed as a percentage. This parameter should be determined through laboratory analysis of uncontaminated soil samples using an analytical method such as EPA Method 415.1 or SW-846 Method 9060. The department will not accept results of a soil burn-off test that determines total organics.

The default value is 0.1% for both surface and subsurface soil. This is a conservative value. This parameter can vary from site to site and within one site.

Average Soil Moisture Content
The average soil moisture content is the percent moisture contained in a soil sample as determined as the weight difference between the as-collected weight of soil and the oven-dried weight (Bates and Jackson, 1987). This parameter is used to calculate the water-filled porosity. The default values are 10% for surface soil and 20% for subsurface soil.

Aquifer Thickness
The aquifer thickness is the thickness of the aquifer impacted by leachate from the overlying vadose zone soil. The depth of an aquifer can be determined from boring or monitoring well logs if they were completed at a depth sufficient to determine the depth of the aquifer. Other data, such as knowledge of the depth to permafrost, historical well log data or information from the United States Geographical Survey (USGS), may be used in demonstrating the thickness of an aquifer.

Hydraulic Conductivity
Hydraulic conductivity is the rate at which water can move through a permeable medium (i.e., the coefficient of permeability) in the saturated zone (EPA, 1998). This parameter can be determined in the field using a slug or pump test, in a lab using ASTM D5084 Hydraulic Conductivity of Saturated Porous Material Using a Flexible Wall Permeameter, or from a tabled value based on the aquifer soil type. The soil type should be determined for the aquifer, not the vadose zone. Typically, the hydraulic conductivity for a specific soil type is actually a range of values spanning several orders of magnitude. In general, the department will only accept the most stringent tabled hydraulic conductivity for a particular soil type. The reference for a tabled hydraulic conductivity should be a published, peer-reviewed reference and should be provided to the department for approval. The department will also accept a calculated hydraulic conductivity using the Hazen method for sands with an effective grain size (d10) between approximately 0.1 and 3.0 mm (Fetter, 1994). The effective grain size is typically reported when a sieve analysis is performed on a soil sample in accordance with ASTM D422.

Hydraulic Gradient
The hydraulic gradient is the change in total head with a change in distance in a given direction yielding a maximum rate of decrease in head (modified from Bates, R.L., and Jackson, J.A., 1987). In general, the hydraulic gradient is the direction of groundwater flow due to changes in elevation of the water table. If a site-specific hydraulic gradient is determined, it should be based on a minimum of three water elevation data points placed in a triangular configuration so the direction of maximum flow can be determined. The hydraulic gradient may need to be determined for different hydrogeographical areas at the site or at different times due to seasonal variations in groundwater flow. The hydraulic gradient value that applies is the most conservative hydraulic gradient determined for a site.

Infiltration Rate
The infiltration rate is the amount of precipitation that enters the soil column averaged on a yearly basis, measured in meters per year . A site-specific infiltration rate can be determined by assuming that infiltration is 1/5 of the average annual precipitation rate for the site or using an approved model. The department will accept data on the average annual precipitation rate from the Monthly Station Normals of Temperature, Precipitation, and Heating and Cooling Degree Days for 1961 to 1990 (Owenby and Exell, 1992) or another source approved by the Department.

Source Length Parallel to Groundwater Flow
The source length parallel to groundwater flow is the dimension of the areal extent of soil contamination in the direction of groundwater flow. This parameter can be determined site specifically if there is sufficient subsurface soil data from either soil boring or monitoring well logs to determine the areal extent of subsurface soil contamination. The maximum length of the source parallel to the direction of groundwater flow should be used in calculating a site-specific mixing zone depth and dilution factor.