**INTRODUCTION TO AT123D**

**AT123D,**
analytical, transient One-, Two-, and Three-Dimensional Model, is an analytical groundwater 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 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 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 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 triple-axis system. Groundwater 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.

**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 user-specified 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**