AT123D, analytical, transient One-, Two-, and Three-Dimensional Model, is an analytical ground-water transport model. AT123D computes the spatial-temporal concentration distribution of wastes in the aquifer system and predicts the transient spread of a contaminant plume through a ground-water aquifer. The fate and transport processes accounted for 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 ground water resulting from a mass release over a source area. AT123D can handle: two kinds of source releases-instantaneous, continuous with a constant loading or time-varying releases; three types of waste-radioactive, chemicals, heat; four types of source configurations-a point source, a line source parallel to the x-, y-, z-axis, an area source perpendicular to the z-axis, a volume source; four variations of the aquifer dimensions-finite 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 ground water 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 triple-axis system. Ground-water flow and pollutant spread occur primarily in the direction of the x-axis. The y-axis 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 z-axis.

Table 1 - Input Parameters Required by AT123D

DATA CATEGORY | 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, first-order decay rate, Heat exchange, molecular diffusion coefficient | Density, porosity, and hydraulic conductivity can be obtained from Soil 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 3-D plots. AT123D also allows the user to directly access RISKPRO.

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