This paper discusses a computerized source water assessment tool that uses digital elevation model (DEM) information and other geographic information system (GIS) databases to assist drinking water watershed managers in assessing the susceptibility of surface water supplies to pollution from current and future activities in the watershed. Surface water source protection assessors and managers may be aided in their work by being able to visualize the locations of potential pollution sources and the routes that pollutants may follow in the event of a pollutant release. The tool is designed to use scientific information and professional experience in the pollution susceptibility assessment process while minimizing the need for new data collection by the user. A first approximation estimate of pollutant concentration reaching the drinking water treatment plant point of diversion and the time-of-travel of the pollutant are calculated. The pollutant transport path and changes in pollutant concentration due to dilution, volatilization, and degradation are output in a grid format and can be overlain other GIS coverages of a watershed to help in visualization. Pollutant degradation is estimated using first-order kinetics. Threats from either point or nonpoint sources of pollution under various storm intensities can be analyzed. The influences of shallow groundwater quality (e.g., petroleum product contaminated groundwater or septic system contaminated groundwater) on surface water sources can be simulated. "What if" questions can be easily and quickly analyzed. The assessment tool consists of four components: a GIS spatial database that includes DEM data, gridded annual precipitation grid data, gridded land use data, and river reach files; a watershed inventory database that includes pollutant source descriptions, pollutant properties, nonpoint source loading rates, and accident probability data; a graphical user interface; and, the Utah Pollutant Transport Model (UPTraM). UPTraM was developed using concepts and approaches developed for the Terrain analysis using Digital Elevation Models effort. The assessment tool has been applied to the Ogden River watershed in northern Utah and may be adapted to other watersheds. Includes 23 references, table, figures.