The development of the Regionalized Impact Assessment Methodology (RIAM) that is easily linkable to HAZUS-MH, the software for analyzing risks and costs of damages to infrastructure developed by the Federal Emergency Management Agency (FEMA), is herein described. RIAM comprises four submodels: the Source Term Model (STM), the Plume Segmentation Model (PSM), the GIS-Finite Source Contaminant Release and Migration Model (GIS-FSCRMM), and the Remediation Cost Assessment Model (RCAM). STM enables the estimation of the transport time of contaminants as plug flow through the vadose zone, to the water table from where PSM and GIS-FSCRMM can be employed to model plume dimensions for remediation cost estimation using RCAM. ArcGIS' ModelBuilder, Visual Basic (VB) Scripts and Python are innovatively used in 3 integrated steps (over 250 ArcGIS geoprocesses) to enable GIS-FSCRMM to accommodate spatial variabilities in hydrogeological parameters without compromising computational efficiency. Comparisons of configurations and results of GIS-FSCRMM with those of Visual MODFLOW, a widely used groundwater model, indicate reasonable correlation with R² ranging from 0.94 to 0.98, and from 0.63 to 0.78 for synthetically generated homogeneous and heterogeneous subsurface conditions, respectively. Application of RIAM to field scenarios of pulse releases of leachate containing benzene at 0.9 mg/L from 4 landfills ranging in base area from 20,000 to 60,000 m², into 10-20m-thick vadose zones enabled the estimation of contaminant transport rates, plume sizes and remediation costs for 10 to 50 years post-release time-frames. Remediation cost estimates for the use of permeable barriers to treat benzene released by the probable earthquake in El Monte Quadrangle, CA are significant relative to the HAZUS-estimated utility damage costs.