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Treatment of Petroleum-Contaminated Soils
Petroleum products are only slightly soluble in water. Petroleum is considered immiscible in water, or a nonaqueous phase liquid (NAPL). Because petroleum is less dense than water, it is often called a light nonaqueous phase liquid, or LNAPL. The immiscibility of petroleum largely controls the movement of petroleum underground. The following discussion briefly describes the transport and fate of LNAPL in unsaturated soils.
The subsurface is usually divided into three distinct zones: the unsaturated zone (sometimes referred to as vadose zone), the capillary fringe (or nearly saturated zone), and the saturated zone. Once a LNAPL is released to unsaturated soil, two forces act on the fluid: gravity and capillary pressure. Gravity will be the predominant force as free-phase LNAPL moves down toward the water table. As free phase LNAPL moves toward the capillary fringe, capillary pressure will cause the LNAPL to spread laterally. Lateral spreading depends on the method through which the petroleum was released (a catastrophic sudden release or a slow continuous release). A sudden release will result in more lateral spreading than a slow continuous release, such as a release from a leaking underground storage tank. Even though methods of investigating contaminated sites are not covered in this manuscript, understanding how the contaminant was released to the soil is important when determining the extent of the contamination in the soil.
Water saturation is high in the capillary fringe zone, which has relatively low permeability to LNAPL. Once free-phase LNAPL reaches this zone, the pattern of spreading is complex. In this zone, the tendency is for free-phase LNAPL to spread laterally near the top of the capillary fringe. Free-phase LNAPL will flow near the top of this zone until a critical depth of free-phase LNAPL is achieved. The direction of flow coincides with the gradient of the water table. Once the depth of free-phase LNAPL is sufficient near the top of the capillary fringe, the LNAPL will move to the water table.
As mentioned previously, the free-phase LNAPL flow characteristics depend on the method by which the liquid is released to the soil. If a sufficiently large volume of LNAPL is suddenly released, the critical pressure required for LNAPL to penetrate the capillary fringe is reached with minimal lateral spreading. Free-phase LNAPL flows toward the water table with relatively little resistance. Once free-phase LNAPL reaches the water table, it will spread laterally in the direction of groundwater flow. The extent of free-phase LNAPL flow depends on such factors as the volume released, the gradient, the characteristics of the porous medium, the rise and fall of the groundwater table, and other factors.
When the supply of free-phase LNAPL from the release is exhausted, LNAPL in the unsaturated zone will drain until, for all practical purposes, a minimum volume of LNAPL remains in the unsaturated soil. This minimum volume is often referred to as residual saturation and depends on the soil type and the heterogeneous nature of the porous medium. At residual saturation, LNAPL exists in the pores of the medium as discrete volumes disconnected from LNAPL contained in neighboring pores. Under these conditions, the LNAPL is in a discontinuous phase that is not conducive to flow. Within the unsaturated porous media, a fraction of the LNAPL will partition into existing soil water, adsorb onto the surface of soil particles, and volatilize into pore space. These reactions were discussed previously.
In any waste treatment operation, characterizing the waste is important to designing the treatment system. The description of immiscible fluids in porous media is just an introduction to a very complex topic. For a more thorough description, the reader is referred to: Mercer and Cohen (1990), Bedient, Rifai, and Newell (1999), and Charbeneau (2000).
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