Quantifying mixture dissolution is important for assessing of contaminant-plume movement. Typically, a NAPL-phase source is assumed present when the plume’s center of mass (centroid) is stationary. If the center of mass moves along the direction of ground-water flow, then it is assumed no NAPL source exists. However, depending on the mixture composition and the magnitude of the ground-water velocity, a NAPL source can produce a plume with a moving centroid as the NAPL mixture dissolves. Model simulations are used to demonstrate this effect. The model combines a numerical solution for NAPL- mixture source zone dissolution with an analytical solution of a 3-dimensional advection- dispersion equation (Domenico, 1987). The model is used to simulate plume movement from a NAPL-mixture source at the DOE Hanford Site. A moving plume centroid has been used as evidence that a NAPL source does not exist even though other evidence suggests NAPL could be present. The potential NAPL source at the site is a complex mixture comprised of lard oil, butyl-phosphate compounds, and chlorinated solvents, including carbon tetrachloride (CT). The contaminant velocity is calibrated to cause the centroid of the dissolved CT plume to move a distance that is consistent with current plume observations at the site. The calibrated velocity is consistent with site data on groundwater velocities with some retardation of CT.