The numerical model CompFlow is used to study the mechanisms controlling vacuum extraction, coupled with air sparging, as a means for remediation of heterogeneous formations contaminated with dense nonaqueous phase liquids (DNAPLs). Two dominant mechanisms are demonstrated to control this remediation technology. First, at early times, the gas phase directly contacts the DNAPL, particularly in the unsaturated zone, causing relatively rapid transfer of contaminant from the nonaqueous phase to the gas phase and subsequent removal by the vacuum extractor. Second, at later times, remediation is controlled by the transfer of contaminant from the nonaqueous phase to the aqueous phase below the water table. During this time the vacuum extractor pumps both liquid and vaporized water in the aqueous and gas phases. This causes the contaminant that is dissolved in the aqueous phase to migrate vertically upward across the permeability layers toward the vacuum extractor where it is removed. This intermediate to late time removal mechanism is shown to be controlled by contaminant dissolution, which is a slower transfer process than the direct DNAPL vaporization that occurs at early time. Our analysis indicates that as long as both air and water are actively flushed through the DNAPL zone, both early-time vaporization and intermediate- to late-time dissolution are effective mechanisms leading to the removal of the DNAPL. We show that it may be possible to design the remedial system so as to reduce its performance sensitivity to geologic heterogeneity. A lack of sensitivity of a remedial design to heterogeneity is highly desirable because a robust design implies that the degree of site characterization required for reasonable success will be less than that needed for a less robust scheme. ¿ American Geophysical Union 1995