Effective long-term operation of soil vapor extraction (SVE) systems for cleanup of vadose-zone sources requires consideration of the likelihood that remediation activities over time will alter the subsurface distribution and configuration of contaminants. A method is demonstrated for locating and characterizing the distribution and nature of persistent volatile organic contaminant (VOC) sources in the vadose zone. The method consists of three components: analysis of existing site and SVE-operations data, vapor-phase cyclic contaminant mass-discharge testing, and short-term vapor-phase contaminant mass-discharge tests conducted in series at multiple locations. Results obtained from the method were used to characterize overall source zone mass-transfer limitations, source-strength reductions, potential changes in source-zone architecture, and the spatial variability and extent of the persistent source(s) for the Department of Energy's Hanford site. The results confirmed a heterogeneous distribution of contaminant mass discharge throughout the vadose zone. Analyses of the mass-discharge profiles indicate that the remaining contaminant source is coincident with a lower-permeability unit at the site. Such measurements of source strength and size as obtained herein are needed to determine the impacts of vadose-zone sources on groundwater contamination and vapor intrusion, and can support evaluation and optimization of the performance of SVE operations.