The process that transfers mass from a subsurface source zone of residual dense nonaqueous phase liquid (DNAPL) to the flowing groundwater is a controlling factor in determining the time required to dissolve the source by noninvasive means. While mass transfer can be kinetic or equilibrium under laboratory conditions, aqueous concentrations in the field are generally found to be below equilibrium levels. To gain insight into the mass transfer process under field conditions, we simulated the dissolution of the emplaced DNAPL source at the Canadian Forces Base Borden, Ontario, which contains a mixture of three DNAPLs. The simulations clearly show that mass transfer at this site is equilibrium-controlled during the 1000-day observation period and that apparent tailing of one of the organic components is due to its declining solubility, rather than mass transfer kinetics. Flow lines passing through the source are focused in a narrow streamtube downstream of the source, and equilibrium concentrations are therefore observed only at the center of the effluent plume. Since the concentration peaks can be easily missed in the sampling, streamline focusing can explain the low concentrations observed in the field. ¿ 1999 American Geophysical Union