A D K

where:

S = maximum spread of the pancake, m2 F = thickness of the pancake, mm V = volume of infiltrating bulk hydrocarbon, m3 A = area of infiltration, m2 d = depth to groundwater, m

K = constant dependent on the soil's retention capacity for oil

Table 9.13.1 lists K values for different types of hydrocarbons and soil textures.

Immiscible fluids with densities greater than water, also called dense nonaqueous phase liquids (DNAPL) or sinkers, sink through the saturated zone and show a concentration gradient through the aquifer, becoming more concentrated near the aquifer base as shown in Figure 9.13.4. Fingering of the dense fluid into the water can also occur depending on the characteristics of the aquifer and the viscosity of the fluid (Dragun 1988). The downward migration of the sinker can continue until a zone of lower permeability, such as a clay confining layer or a bedrock surface, is encountered. Halogenated hydrocarbons and coal tars are the principal solvents possessing densities greater than that of water. Examples of DNAPLs include methylene chloride, chloroform, trichloroethylene (TCE), tetrachloroethylene or perchloroethylene (PCE), and various Freons.

Another important factor of both LNAPL and DNAPL plume behavior is residual contamination. As the plume migrates downward through the unsaturated or saturated zone, a small amount of fluid remains attached to soil particles and within the soil pore spaces via capillarity forces. This residual contamination can reside in the soil for many

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