A dense non-aqueous phase liquid or DNAPL is a denser-than-water NAPL, i.e. a liquid that is both denser than water and is immiscible in or does not dissolve in water. The term DNAPL is used primarily by environmental engineers and hydrogeologists to describe contaminants in groundwater, surface water and sediments. DNAPLs tends to sink below the water table when spilled in significant quantities and only stop when they reach impermeable bedrock. Their penetration into an aquifer makes them difficult to locate and remediate. Examples of materials that are DNAPLs when spilled include: chlorinated solvents, such as trichloroethylene, tetrachloroethene, 1,1,1-trichloroethane and carbon tetrachloride coal tar creosote polychlorinated biphenyl (PCBs) mercury extra heavy crude oil, with an API gravity of less than 10 certain per- and polyfluoroalkyl substances (PFAS) When spilled into the environment, chlorinated solvents are frequently present as DNAPL and the DNAPL can provide a long term secondary source of the chlorinated solvent to dissolved groundwater plumes. Chlorinated solvents are typically immiscible in water, having low solubility in water by definition, yet still have a solubility above the concentrations allowed by drinking water protections. Therefore, DNAPL which is a chlorinated solvent can act as an ongoing pathway for constituents to dissolve into groundwater. Common use of chlorinated solvents in manufacturing operations began during World War II, with the rate of usage for most solvents increasing into the 1970s. By the early 1980s, chemical analyses becoming available that documented widespread contamination of groundwater with chlorinated solvents. Since that time, a considerable effort has been extended to improve our ability to locate and remediate DNAPL present as chlorinated solvents. DNAPLs that are not viscous, such as chlorinated solvents, tend to sink into aquifer materials below the water table and become much more difficult to locate and remediate than non aqueous phase liquids that are lighter than water (LNAPLs) which tend to float at the water table when spilled into natural soils. The United States Environmental Protection Agency (USEPA) has focused considerable attention on the remediation of DNAPL which can be costly. Removal or in situ destruction of DNAPLs eliminates the potential exposure to the compounds in the environment and can be an effective method for remediation; however, at some DNAPL sites remediation of DNAPL may not be practicable, and containment may be the only viable remedial action. The USEPA has a program to address sites where DNAPL removal is not practicable for