Contaminant Density

Immiscible fluids such as oils do not readily mix with water; therefore, they either float on top of the water table or sink into the groundwater depending on their density. Immiscible fluids with densities less than water, also called light nonaqueous phase liquids (LNAPLs) or floaters, form a separate phase that can float on the groundwater table. For example, if a light-bulk hydrocarbon is released from a surface spill as shown in Figure 9.13.3, it migrates downward in the unsaturated zone due to gravity and capillary forces. If the volume of the released hydrocarbon is large, the hydrocarbon reaches the groundwater and forms a pancake on top of the water table. The pancake tends to spread laterally and in the downgradient direction until it reaches residual saturation. A portion of the pancake dis-

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