Porosity soil, effective

Spunbonded fabrics are effective filters in that they are layered stmctures of relatively fine fibers, the three-dimensional stmcture of which creates a torturous path. Even relatively thin spunbonded fabrics (eg, 0.2—0.25 mm) present a significant challenge to the passage of soil fines and are suitable for use in some filtration appHcations. The porosity of geotextile fabrics is classified by means of several procedures such as flux (volume flow/area per time) and equivalent opening size (EOS), which is a measure of the apparent pore size of the openings in the fabric. The flux measures the porosity to Hquid water, and the EOS measures the porosity to soHd particles of a known diameter. Literature is available on limitations of particular styles of fabrics within an apphcation (63). 

The term porosity refers to the fraction of the medium that contains the voids. When a fluid is passed over the medium, the fraction of the medium (i.e., the pores) that contributes to the flow is referred to as the effective porosity of the media. In a general sense, porous media are classified as either unconsolidated and consolidated and/or as ordered and random. Examples of unconsolidated media are sand, glass beads, catalyst pellets, column packing materials, soil, gravel and packing such as charcoal. 

As previously mentioned, a minimum level of soil moisture is necessary for successful biodegradation. The continuous circulation of air during bioventing results in the evaporation of soil moisture. For this reason, the design of these systems must include an appropriate installation for adding water to the contaminated zone. Care must be taken to avoid the addition of excess water. If soil moisture is significantly increased, e.g., above the limit of 85%, air circulation is no longer effective due to the decrease in free soil porosity. 

Winters and Lee134 describe a physically based model for adsorption kinetics for hydrophobic organic chemicals to and from suspended sediment and soil particles. The model requires determination of a single effective dififusivity parameter, which is predictable from compound solution diffusivity, the octanol-water partition coefficient, and the adsorbent organic content, density, and porosity. 

If the liquid uniformly passes through all the pores in the soil, then the effective and total porosities are equal. However, if the flow takes place in only a small percentage of the total pore space, for example, through fractures or macropores, the effective porosity will be much lower than the total porosity. Judging the effective porosity is one of the problems in estimating seepage velocities. 

If effective porosity and other parameters are known, the time of travel (TOT) for a molecule of waste transported by flowing water through the soil liner can be calculated. TOT equals the length of the particular flow path times the effective porosity, divided by the hydraulic conductivity times the hydraulic gradient (Figure 26.10). 

A plot called a breakthrough curve shows the effluent liquid concentration c divided by the influent liquid concentration c0 as a function of pore volumes of flow (see Figure 26.11). One pore volume of flow is equal to the volume of the void space in the soil. The effective porosity of the soil is determined by measuring a breakthrough curve. 

Many of the waste constituents in the leachate are attenuated or retarded by the soil. For example, lead migrates very slowly through soil, whereas chloride and bromide ions migrate very quickly. With no retardation or attenuation, breakthroughs would occur at a c/cG of 0.5-1 pore volume of flow and below. With effective and total porosities being equal, a much delayed breakthrough of chemicals that have been absorbed or attenuated by the soil could be expected. 

The best way to determine effective porosity is to perform a test using a tracer ion that will not be absorbed significantly by the soil, such as chloride or bromide. If the breakthrough occurs in one pore volume of flow, the effective and total porosity is equal. If, instead, the breakthrough occurs at half a pore volume of flow, then the effective porosity is half the total porosity. 

Velocity hydraulic conductivity x hydraulic gradient effective soil porosity... 

Sites suitable for conventional SVE have certain typical characteristics. The contaminating chemicals are volatile or semivolatile (vapor pressure of 0.5 mm Hg or greater). Removal of metals, most pesticides, and PCBs by vacuum is not possible because their vapor pressures are too low. The chemicals must be slightly soluble in water, or the soil moisture content must be relatively low. Soluble chemicals such as acetone or alcohols are not readily strippable because their vapor pressure in moist soils is too low. Chemicals to be removed must be sorbed on the soils above the water table or floating on it (LNAPL). Volatile dense nonaqueous liquids (DNAPLs) trapped between the soil grains can also be readily removed. The soil must also have sufficiendy high effective porosity (permeability) to allow free flow of air through the impacted zone. 

Pneumatic or hydraulic fracturing can occasionally be used to enhance the effective porosity (permeability) of unsaturated zones for SVE or air sparging. Successful soil fracturing is as much an art as a science. Pilot testing by experienced professionals is strongly recommended before performance guarantees are granted. 

Table 2.2 Effect of cultivation at different soil-water states on components of percentage porosity in a Vertic Tropaquept clay soil...

Figure 6.6 Effect of cortical porosity of primary root and fraction of root covered with laterals on (a) maximum primary root length, (b) absorbing root surface per unit root mass, and (c) absorbing root surface per primary root as a function of net O2 consumption, and (d) O2 consumed in root respiration and loss to the soil. Numbers on curves are porosities other parameters have standard values (Kirk, 2003). Reproduced by permission of Blackwell Publishing...

These findings broadly agree with experimental observations. Measured rates of CH4 oxidation in the rice rhizosphere range widely from 5 to 90% of the CH4 transported (Holzapfel-Pschom et al 1985 Epp and Chanton, 1993 van der Gon and Neue, 1996). This agrees with the model. Rates of O2 flow through rice roots to the rhizosphere are of the order of a few mmol 02m (soil surface) h (Section 6.4), which is sufficient to account for the rates of oxidation calculated with the model. Measured differences in emissions between rice cultivars are largely due to differences in root biomass (Lu et al., 1999) the effects of differences in root porosity are smaller (Aulakh et al., 2001a,b). 

The effect of aggregation of the subsurface solid phase on kerosene volatilization was studied by Fine and Yaron (1993), who compared the rate of aggregation in two size fractions of a vertisol soil the <1 mm fraction and 2 mm aggregates. The total porosity of these two fractions was similar (53% and 55% of the total volume, respectively). Differences in aggregation are reflected in the air permeability that is, their respective values were 0.0812 0.009 cm and 0.145 0.011 cm Figure 8.10 presents the volatilization of kerosene as affected by the soil aggregation, when the initial amount applied was equivalent to the retention capacity. The more permeable fraction releases kerosene faster and thus enhances volatilization. 

Fig. 8.9 Effect of porosity on composition of kerosene during 14 days of volatilization from fine, medium and coarse sand, as seen from gas chromatograph analyses. Reprinted from Galin Ts, Gerstl Z, Yaron B (1990) Soil pollution by petroleum products. IB Kerosene stability in soil columns as affected by volatilization. J Contam Hydrol 5 375-385. Copyright 1990 with permission of Elsevier...

Low-permeability soils are difficult to treat with soil flushing. Surfactants can adhere to soil and reduce effective soil porosity. Reactions of flushing fluids with soil can reduce contaminant mobility. 

The HAVE System is a simple, effective, and low-cost technology that completely destroys the contaminants that it removes. However, variables such as pile temperature, soil characteristics, soil moisture, and porosity can negatively affect the performance of HAVE. 

A variety of factors affect the horizontal and vertical migration of PAHs, including contaminant volume and viscosity, temperature, land contour, plant cover, and soil composition (Morgan Watkinson, 1989)- Vertical movement occurs as a multiphase flow that will be controlled by soil chemistry and structure, pore size, and water content. For example, non-reactive small molecules (i.e., not PAHs) penetrate very rapidly through dry soils and migration is faster in clays than in loams due to the increased porosity of the clays. Once intercalated, however, sorbed PAHs are essentially immobilized. Mobility of oily hydrophobic substances can potentially be enhanced by the biosurfactant-production capability of bacteria (Zajic et al., 1974) but clear demonstrations of this effect are rare. This is discussed below in more detail (see Section 5 5). 

According to a review by Magdoff and Weil (2004), increased SOM can counteract the ill effects of too much clay or too much sand. Increasing the SOM content usually increases total porosity and therefore decreases bulk density. Within a limited range of SOM contents, the relationship for a given soil is nearly linear (Weil and Kroontje, 1979). However, across a wider range of SOM levels, the relationship... 

---