Contaminants can migrate through ground water from leaking underground storage tanks, landfills, and tailings ponds to wells or surface water bodies. Cleanup usually includes the removal of sources of contamination and contaminated ground-water using wells followed by treatment. This approach is termed pump-and-treat. It led to perpetual remediation at many contaminated sites, often because many sources of contamination cannot be effectively removed. This is especially true when nonaqueous-phase liquids (NAPLs) such as gasoline (lighter than water, LNAPLs) and chlorinated solvents (denser than water, DNAPLs) remain in the subsurface, acting as ongoing sources of ground-water contamination. DNAPLs are particularly troublesome because the dense liquid can migrate deep below the water table. With the source remaining, pump-and-treat could operate in perpetuity, protecting potential receptors by cutting off the contaminant plume, but presenting ongoing costs. So, alternate remedial strategies were sought. Two additional approaches have emerged.