Capillary fringe

In the lower region of the unsaturated zone, immediately above the water table, is the capillary fringe, where water is drawn upward by capillary attraction. Above the capillary fringe, moisture coats the solid surfaces of the soil or rock particles. If the liquid coating becomes too thick to be held by surface tension, a droplet will pull away and be drawn downward by gravity. The fluid can also evaporate and move through the air space in the pores as water vapor. 

In the upper unsaturated zone (above the capillary fringe), multiphase movement and transformation are typical. Vapor-phase gasoline becomes more important gasoline adsorption by soil, dissolution in pore water, and free product in the pore space can also be significant. 

May be used to remediate the capillary fringe and smear zone... 

MPE provides several advantages when compared with the use of SVE or pump-and-treat alone. MPE provides for an increase in groundwater recovery rates, an increase in ROI in individual ground-water recovery wells, and recovery of shallow free product. By depressing the groundwater table in the vicinity of the extraction wells, MPE provides for remediation of the capillary fringe and smear zone, and remediation of volatile, residual contaminants located above and below the water table.46... 

Capillary fringe The zone immediately above the water table which is essentially saturated this is the height that can support saturated conditions at negative pressure (due to soil suction). 

Liquid phase migration and retention — NAPL held suspended by the water table or capillary fringe or perched above low permeability zones (water wet soil) in the unsaturated zone. 

As more LNAPL enters the soils, it increases the pressure on the moving front, which allows increasing displacement of water. If a sufficient quantity of sufficiently mobile oil is available, it can produce a pressure head large enough to displace water through the funicular zone and to form a distinct interface at the top of the capillary fringe. 

Once formed, the LNAPL is referred to as a pool that is really a continuous accumulation of LNAPL. Subpools are individual accumulations of relatively uniform free-product types based on geochemistry (API gravity, specific constituents, isotope ratios, etc.) that have coalesced, reflecting multiple-source accumulations with time. The LNAPL pool, once formed, also maintains a capillary fringe. The anticipated capillary rise of LNAPL can be calculated as follows ... 

FIGURE 6.3 Sandbox model showing LNAPL overlying capillary fringe, and apparent vs. actual LNAPL thickness. Saturated conditions (water table) are represented by the straight horizontal line. 

Since hydrocarbon and water are immiscible fluids, free-phase recoverable LNAPL can simplistically be viewed as being perched on the capillary fringe above the actual water table with the understanding that what is being referred to as actual NAPL thickness is what one could perceive as being equivalent to the approximate thickness of that portion of the zone of hydrocarbon saturation that is considered mobile. The physical relationships that exist are illustrated in Figure 6.5. This discrepancy can be a result of one or a combination of factors or phenomena. Some of the more common factors or phenomena are schematically shown in Figure 6.6 and include ... 

Grain size differences reflected in varying heights of the capillary fringe ... 

The measured or apparent hydrocarbon thickness is not only dependent on the capillary fringe but also on the actual hydrocarbon thickness in the formation (Figure 6.6b). In areas of relatively thin LNAPL accumulations, the error between the apparent well thickness and actual formation thickness can be more pronounced than in areas of thicker accumulations. The larger error reflects the relative difference between the thin layer of LNAPL in the formation and the height it is perched above the water table. The perched height is constant for thick and thin accumulations however, a thick accumulation can depress and even destroy the capillary fringe as illustrated in Figure 5.1. 

In addition, monitoring wells screened across LNAPL within confined aquifers will exhibit an exaggerated thickness. This exaggerated thickness reflects relatively high confining pressure, which forces the relatively lower density fluid upward within the borehole. Thus, the measured thickness is a function of the hydrostatic head and not the capillary fringe, which has been destroyed by the confining pressures. 

Classification3 Minimum Apparent Thickness, cm Formation Factor (F), cm Capillary Fringe Height,b cm U.S. Standard Sieve Size Rang... 

The resulting LNAPL thickness is conservative in that it incorporates both the actual thickness of LNAPL in the adjacent formation and the height of the capillary fringe. For most practical purposes, this level of accuracy is sufficient, although the more complex and extensive the site conditions, the more sophisticated approaches may be warranted, as discussed later in this chapter. 

In the capillary fringe, a third phase (air) can be present, further reducing the value... 

A correction factor is then applied for capillary fringe effects. This factor can be empirically derived reflecting the corrected depth to water as shown below ... 

Soils, typically, are not fully saturated by water the soil layer and the region reaching to the water table contain water contents below full saturation. These regions usually are referred to as the vadose zone and said to be unsaturated, but they are more correctly considered partially saturated. The degree of saturation is the ratio of the volume of water to the pore volume within the porous medium. Saturation levels usually are a few percent at land surface (or even zero in perpemaUy dry arid zones) and increase slowly with depth until the region of the capillary fringe (water table), where it increases rapidly to 100%. 

The capillary fringe (CE), which is the region where saturation increases quickly, near the water table, deserves special consideration. How through the vadose zone commonly is characterized by a mean vertical flow, with substantial temporal and... 

Throughout the following sections, we consider mechanisms relevant to both the (partially saturated) vadose and capillary fringe zones as well as to saturated... 

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