- Dense Non-Aqueous Phase Liquid (DNAPL) : A Dense Non-Aqueous Phase Liquid (DNAPL) is Non-aqueous phase liquids such as chlorinated hydrocarbon solvents or petroleum fractions with a specific gravity greater than 1.0 that sink through the water column until they reach a confining layer. Because they are at the bottom of aquifers instead of floating on the water table, typical monitoring wells do not indicate their presence.

---

Other /More definition:
A Dense Non-Aqueous Phase Liquid (DNAPL) is liquid contaminants that are relatively insoluble and heavier than water; also known as "sinkers” because they will sink to the bottom of an aquifer, where they become especially difficult to detect and clean up