Detailed procedures required to execute AT123D through the use of two data management systems. The report is intended to be a comprehensive guide for users who have no specialized computer knowledge. AT123D is a generalized analytical transient one-, two-, and/or three-dimensional computer model designed for estimating the rate of pollutant (radioactive, chemical, or thermal) transport/transformation in a groundwater system. In the search for closed-form solutions to the partial differential equations describing the groundwater flow and transport phenomena, the application of the Green's function is used to its optimum advantage. As a result, the model has many optional capabilities and accepts a variety of source release patterns and configurations.