This work presents a stream tube--based analytical approach to evaluate reduction in groundwater contaminant flux resulting from partial mass reduction in a nonaqueous phase liquid (NAPL) source zone. The reduction in contaminant flux, Rj, discharged from the source zone is a remediation performance metric that has a direct effect on the fundamental drivers of remediation: protection of human health and the environment. Closed form expressions are provided for analyzing remediation performance under conditions of joint spatial variability of both groundwater flow and NAPL content. The performance measures derived here are expressed in terms of measurable parameters. Spatial variability is described within a Lagrangian framework where aquifer hydrodynamic heterogeneities are characterized using nonreactive travel time distributions, while NAPL spatial distribution heterogeneity can be similarly described using reactive travel time distributions. The combined statistics of these distributions are used to evaluate the relationship between reduction in contaminant mass, Rm, and Rj. A portion of the contaminant mass in the source zone is assumed to be removed via in situ flushing remediation, with the initial and final conditions defined as steady state natural gradient groundwater flow through the contaminant source zone. The combined effects of aquifer and NAPL heterogeneities are shown to be captured in a single parameter, reactive travel time variability, which was determined to be the most important factor controlling the relationship between Rm and Rj. It is shown that as heterogeneity in aquifer properties and NAPL spatial distribution increases, less mass reduction is required to achieve a given flux reduction, although the overall source longevity increases. When rate-limited dissolution is important, the efficiency of remediation, in terms of both mass and flux reduction, is reduced. However, at many field sites the combined effects of field-scale heterogeneities and site aging will result in favorable relationships between mass reduction and flux reduction.