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INTRODUCTION TO AT123D
AT123D, analytical, transient One, Two, and ThreeDimensional Model, is an analytical groundwater transport model. AT123D computes the spatialtemporal concentration distribution of wastes in the aquifer system and predicts the transient spread of a contaminant plume through a groundwater aquifer. The fate and transport processes accounted for in AT123D are advection, dispersion, adsorption, and decay. AT123D estimates all the above components on a monthly basis for up to 99 years of simulation time.
AT123D can be used as an assessment tool to help the user estimate the dissolved concentration of a chemical in three dimensions in groundwater resulting from a mass release over a source area. AT123D can handle: two kinds of source releasesÐinstantaneous, continuous with a constant loading or timevarying releases; three types of wasteÐradioactive, chemicals, heat; four types of source configurationsÐa point source, a line source parallel to the x, y, zaxis, an area source perpendicular to the zaxis, a volume source; four variations of the aquifer dimensionsÐfinite depth and finite width, finite depth and infinite width, infinite depth and finite width, infinite depth and infinite width.
AT123D assumes that the aquifer is a homogeneous and isotropic aquifer with groundwater flowing almost horizontally. When describing points at which the chemical enters the aquifer or points at which concentrations are to be estimated, AT123D uses a tripleaxis system. Groundwater flow and pollutant spread occur primarily in the direction of the xaxis. The yaxis describes the width of the release source or the plume in the horizontal or the transverse direction. The depth of the chemical plume from the surface of the aquifer is described using the zaxis.
AT123D INPUT AND OUTPUT DATA
Input data consists of three types: simulation parameters, source configurations, and soil and waste properties. AT123D output data contains the model input and predicted concentrations at a userspecified set of positions (x,y,z coordinates) for specified output times. The concentration tables report dissolved pollutant in parts per million (ppm) which correspond to milligrams per liter. In the output file, the concentration of sorbed chemicals is not directly reported. Instead, AT123D prints a ratio for calculating the sorbed concentration above each table of dissolved concentrations.
Table  Input Parameters Required by AT123D

AT123D DATA CATEGORY

AT123D PARAMETERS

AT123D INTERFACE

Simulation Parameters

Simulation Parameters:
Simulation length, time step;
Accuracy tolerance, number of roots
Output controls:
Number of points in x, y and z directions;
First/last output time step numbers;
Print interval

Simulation parameters
can be obtained from
the SSOUT###.ATX
file.

Source Configuration

Release:
Instantaneous or continuous;
Loading rates
Waste:
Radioactive waste, chemicals, or heat
Type:
A point, a line, an area, or a volume source
Media:
Finite/infinite depth and width

Loading rates can be
obtained from the
SSOUT###.ATX file

Soil & Waste Properties

Aquifer:
Bulk density, porosity;
Hydraulic conductivity, hydraulic gradient;
Dispersivities in x, y, and z directions
Chemical:
Adsorption coefficient, firstorder decay rate,
Heat exchange, molecular diffusion coefficient

Density, porosity, and
hydraulic conductivity
can be obtained from the
Soils5 Database


AT123D can be used alone or it can be linked to SESOIL so that the mass loading to the ground water predicted by SESOIL is automatically transferred to AT123D. The AT123D Postprocessor creates a graphic data file which may be used by any spreadsheet or graphics program that accepts ASCII files, i.e., Excel, Surfer. The graphic data file contains all the data required for Isopleths or 3D plots. AT123D also allows the user to directly access RISKPRO.
AT123D Requirements
Pentium running Microsoft Windows 95/98/2000/NT.




