Many groundwater contamination incidents begin with the release of a petroleum liquid (e.g. gasoline or diesel fuel) into the subsurface. Once there, such a nonaqueous phase liquid (NAPL) poses a long-term threat to large quantities of groundwater because of a combination of interphase mass transfer and transport processes, namely, volatilization, dissolution, gas-water partitioning, advection in the gas and aqueous phases, as well as diffusion. These processes can lead to contamination of soil and groundwater far beyond the extent of the original NAPL spill. Therefore, a complete removal of the NAPL is required to successfully clean up the contamination.