**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.