Groundwater contamination as a result of the subsurface presence of nonaqueous phase liquids (NAPLs) is ubiquitous in today’s society. Sources of NAPLs include the release of crude oil and refined petroleum-related products from aboveground and underground storage tanks (USTs), pipeline corridors, dry wells, and accidental spills. Since the 1980s, much focus has been placed upon USTs, although over the past few years emphasis has shifted to other industries and operations such as petroleum refining, bulk liquid storage terminals, major pipeline networks, gas production, steel industry, and coking and wood treating. NAPLs are referred to in the federal regulations as “free product,” and as previously discussed can occur in the subsurface in two forms: lighter than water (LNAPLs) and denser than water (DNAPLs) (Figure 5.1). As NAPLS migrate vertically downward through the soil column and groundwater is encountered, an LNAPL will tend to form a pool overlying the capillary fringe and water table. DNAPLs behave much the same as LNAPLs in the vadose zone; however, once groundwater is encountered, DNAPLs will tend to continue to migrate vertically downward through the water column until a significant permeability contrast is encountered, providing a sufficient volume of DNAPL was released. The subsurface presence of NAPLs for the most part can range from essentially unnoticed small releases over very long periods of time to episodic releases due to a significant breach of a storage or transportation unit.