The Flow of Nonaqueous Phase Liquids

Transport in unsaturated porous media is sufficiently complex when only two fluid phases, air and water, are present flow becomes even more complicated when a third fluid phase, such as an immiscible organic fluid, is involved. This third fluid phase (NAPL) arises when liquid hydrocarbon fuels or solvents are spilled accidentally on the ground surface or when they leak from underground storage tanks. The resulting subsurface flow problem then involves three fluids, air, water, and NAPL, each having different interfacial tensions with each other, different viscosities, and different capillary interactions with the soil. The adequate description of three-phase flow is still a topic of active research, but a few qualitative generalizations can be drawn. 

Small quantities of NAPL can move into porous media under both gravity and capillary effects and can become essentially immobilized, due to discontinuities that develop in the NAPL as it spreads out these discontinuities are very much like the ones formed in water films in very dry porous media. Discontinuities prevent flow of NAPL from one region to another the amount of NAPL present when flow stops is called the residual saturation. The effect can be likened to that of water breaking into discontinuous, discrete droplets on the bottom of a greasy kitchen sink, thereby preventing flow of the last drops of water down the drain. (In this domestic example the water, not the grease film, is the discontinuous phase.) Residual saturation depends on soil texture and on the surface tension (cr) between the NAPL and water this surface tension is approximately 30 to 50 dyn/cm for many immiscible solvents and fuels. Residual saturation also depends on the initial water content of the porous media when the NAPL is introduced. 

especially when distributed discontinuously, may show little or no tendency to move unless the applied hydraulic gradients are extremely high, beyond what is normally achieved in the field (Fig. 3-25). The same problem of residual saturation occurs during the recovery of oil from a petroleum reservoir a significant fraction of the petroleum cannot be recovered even with the most advanced techniques. In coarse soils, NAPL with sufficient vapor pressure can sometimes be recovered below residual saturation by volatilization into the air in pore spaces, and subsequent removal of the air 

In the case of a dense NAPL (DNAPL), whose density exceeds 1 g/cm3 (Table 1-3), downward flow takes place, until a condition of residual saturation occurs or an aquiclude or aquitard is reached (see Fig. 3-26 see also Schwille (1988)). Such a DNAPL is exceedingly difficult to recover or even to 

FIGURE 3-27 Typical schemes to recover a floating NAPL, such as gasoline, from a phreatic aquifer. In each case a cone of depression is created by pumping to encourage the NAPL to flow toward the collection point. Either a NAPL/water separator or two pumps are required. 

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High airflow rates may result in unintended fracturing leading to nonuniform flow or short-circuiting of injected air in the subsurface, or may result in unintended mobilization of contaminants as nonaqueous phase liquids (NAPL), dissolved in groundwater, or in soil gas. 

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