mediation (SEAR).Because of the pioneering work of Schwille (1988) in Germany, chlorinated solvents are now recognized by hydrogeologists as dense, nonaqueous phase liquids (DNAPLs), which are believed to play a major role in the failure of pump-and-treat remediation systems to attain healthbased limits of decontamination within short periods of operation (EPA, 1992a;Mackay and Cherry, 1989). It is most probable that groundwater contamination by chlorinated solvents, such as trichloroethene (TCE), and other DNAPLs is most frequently caused by the dissolution of undetected DNAPLs in the subsurface, which have not been identified as such in any soil or rock cores or in any groundwater sample. Johnson (1992) has noted that a dissolved-phase plume of chlorinated solvents, such as is shown in Figure 1, should be interpreted, a priori, as an indicator of an as-yetunlocated DNAPL zone in the subsurface. Johnson and Pankow (1992) have shown that tens to hundreds of liters of DNAPL may require tens to hundreds of years to be dissolved by groundwater extraction or pump-andtreat systems.The objective of this article is to review briefly the processes controlling DNAPL migration and dissolution in the subsurface and show how these principles affect the practice of surfactant-enhanced aquifer remediation.