A GIS-aided simulation (GISSIM) system is developed for effective management of petroleum-contaminated sites in this study. The GISSIM contains two components: an advanced three-dimensional (3D) numerical model, and a geographical information system (GIS). The modeling component undertakes simulation for the fate of contaminants in subsurface unsaturated and saturated zones. The GIS component is used in three areas throughout the system development and implementation process: (1) managing spatial and nonspatial databases; (2) linking inputs, the model, and outputs; and (3) providing an interface between the GISSIM and its users. The system is applied to a North American case study. Concentrations of benzene, toluene, and xylenes in groundwater under a petroleum-contaminated site are dynamically simulated. Conditions of the contamination in different time stages under a variety of remediation scenarios are predicted. Reasonable outputs have been obtained and presented graphically. Implications of the modeling outputs have been analyzed based on the local environmental regulations. They provide quantitative and scientific bases for further assessment of site-contamination impacts and risks, as well as decisions of practical remediation actions. GISSIM is useful for both industrial and government sectors to make informed decisions on waste management, pollution control, site remediation, and environmental impact assessment.