An EXCEL workbook is presented for calculating the mean residence time of groundwater based on the environmental tracers. tritium, CFC-11 and CFC-12. The program TRACER is written in Visual Basic for Application language and uses piston. exponential. linear, exponential-piston and linear-piston flow types of lumped-parameter models. Input and output data are stored in worksheets and a graph of results that are best fitted to observations is drawn for visual evaluation. Recharge temperature and altitude are used to convert atmospheric partial pressures of CFC-11 and CFC-12 to dissolved concentrations to provide a direct comparison between the models' output and observed data. The model can also be used to check whether an inferred flow type could be valid in the groundwater system being investigated. Other radioactive and gaseous environmental tracers and reactions such as, sorption and degradation can be included either as decay constant or with modifications in the program code. TRACER matches, satisfactorily, the results obtained from other softwares. (C) 2002 Elsevier Science Ltd. All rights reserved.