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AT123D Aquifer and Chemical Input
Parameters
The aquifer and chemical input tab
contains information on the physical
properties of the aquifer and
contaminants. These properties are
used by AT123D to predict the
transport and fate of contaminants in
groundwater. As SEVIEW contains a
chemical database for the most part all
you need to do is find your chemical.
Properties for additional contaminants
can be found on the internet or in
various references. A detailed description of each input parameter is provided below. Additional information on
these parameters can be found in the SEVIEW help file and User's Guide.
Hydraulic Conductivity
The horizontal hydraulic conductivity of the aquifer. Determined from pump tests, slug tests, or estimated based
on the aquifer soil type.
Hydraulic Gradient
The slope of the water table. Determined from potentiometric surface maps of the static water level data from
monitoring wells.
Effective Porosity
Effective porosity is the fraction of interconnected pore space to the total volume of the soil. This parameter is
difficult to measure and is typically estimated based on aquifer soil type. Default values can be found in the
SEVIEW User’s Guide and help documentation.
Bulk Density
Bulk density is the amount of mass per unit volume of the aquifer.
Number of Eigenvalues
The number of eigenvalues establishes the maximum number of terms that will be calculated for a series
solution before truncation occurs.
Dispersivities
The process whereby a contaminant plume spreads out due to mechanical mixing in the aquifer. See the SEVIEW
User’s Guide to learn how to estimate values for this parameter.
Aquifer Dimensions
AT123D simulates both confined and unconfined aquifers.
Water Diffusion Coefficient
Diffusion consists of the random motion of atoms or molecules in a gas or liquid. This motion causes
contaminants to disperse from areas of higher concentration to areas of lower concentration. Diffusion is
controlled by molecular weight, in that lighter atoms or molecules are dispersed at higher rates. This dependency
on weight can be used to establish diffusion coefficients for additional chemicals using the equation below:
Where
D
=Diffusion coefficient of the current molecule in cm
2
/sec
D’
= Diffusion coefficient for a reference molecule in cm
2
/sec
MWT′
= Molecular weight of the reference molecule in g/mole
MWT
= Molecular weight of the current molecule in g/mole
This equation can be used to estimate both air and water diffusion coefficients. Please note that almost any
substance can be used as the reference molecule.
Organic Carbon Content
The organic carbon content is defined as the percent of organic carbon of the aquifer soil. This is one of the most
important site-specific parameters and care should be taken as to which analytical method should be used.
Carbon Adsorption Coefficient (Koc)
The organic carbon adsorption coefficient (K
oc
) is the chemical adsorption coefficient normalized to the organic
carbon content of the soil in (µg/g)/(µg/ml). This parameter establishes the tendency of a substance to bind to
organic matter in the aquifer.
Distribution Coefficient (Kd)
The Kd value can be directly entered if non-zero values for Koc and soil organic carbon content are entered. If
values for both Koc and soil organic carbon content are entered AT123D calculates the soil partition coefficient
(Kd) using the following equation.
Where
K
d
= Soil partition coefficient in (µg/g)/(µg/ml)
K
oc
= Organic carbon adsorption coefficient in (µg/g)/(µg/ml)
oc
= Fraction soil organic carbon content
First-Order Decay Coefficient (Biodegradation)
Biodegradation is the process by which a chemical is converted to a new form via biotic reactions. AT123D
simulates biodegradation as a first-order decay process.
Transport and fate modeling software