The soil air

Some rocks have a natural porosity and, at the interface of the atmosphere and the lithosphere and given the presence of moisture, all rocks tend to weather to a relatively porous soil. In the simplest case, the pore spaces of soil are occupied by atmospheric air, but the very moisture that enhances soil formation is also highly supportive of flora and fauna, which interact with the air in the pores and modify its composition. 

Gases migrating from depth are also constituents of the soil air, their supply to the soil air varying according to proximity to sources and conduits. Sources of these gases include, but are not exclusively, mineral deposits and petroleum accumulations. 

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Jury WA, Winer AM, Spencer WF, et al. 1987b. Transport and transformation of organic chemicals in the soil-air-water ecosystem. Rev Environ Contam Toxicol 99 119-164. 

The movement of air in the subsurface during the application of SVE is caused by the pressure gradient that is applied in the extraction wells. The lower pressure inside the well, generated by a vacuum blower or pump, causes the soil air to move toward the well. Three basic equations are required to describe this airflow the mass balance of soil air, the flow equation due to the pressure gradient, and the Ideal Gas Law. 

The contaminants removal rate RIem can be calculated by multiplying the flow rate of air extracted from all the wells by the concentration of contaminant in the soil air Ca ... 

Compartmental soil modeling is a new concept and can apply to both modules. For the solute fate module, for example, it consists of the application of the law of pollutant mass conservation to a representative user specified soil element. The mass conservation principle is applied over a specific time step, either to the entire soil matrix or to the subelements of the matrix such as the soil-solids, the soil-moisture and the soil-air. These phases can be assumed in equilibrium at all times thus once the concentration in one phase is known, the concentration in the other phases can be calculated. Single or multiple soil compartments can be considered whereas phases and subcompartments can be interrelated (Figure 2) with transport, transformation and interactive equations. 

From the pollutant and biological cycles the processes of advection, diffusion, volatilization (diffusion at the soil-air interface), adsorption or desorption (equilibrium), and degradation or decay, which are also the most important chemical processes in the soil zone. All other processes can be lumped together under the source or sink term of equation (3). 

In its simplest form a partitioning model evaluates the distribution of a chemical between environmental compartments based on the thermodynamics of the system. The chemical will interact with its environment and tend to reach an equilibrium state among compartments. Hamaker(l) first used such an approach in attempting to calculate the percent of a chemical in the soil air in an air, water, solids soil system. The relationships between compartments were chemical equilibrium constants between the water and soil (soil partition coefficient) and between the water and air (Henry s Law constant). This model, as is true with all models of this type, assumes that all compartments are well mixed, at equilibrium, and are homogeneous. At this level the rates of movement between compartments and degradation rates within compartments are not considered. 

The most economical route Is probably to use screening studies to determine the dominant fate processes and then study only those In detail. In this paper we review some simple screening techniques that can be used to quantify volatilization and leaching rates at the soil/air Interface. Volatilization and leaching rates are then compared with estimates of transformation rates to determine the compound s overall fate and Identify the process requiring further study If a more exact fate assessment Is required. 

Soil Diffusion. Water-soluble material In the soil Includes material dissolved In the soil water, material dissolved In the soil air, and material adsorbed to the soil solids. The soil water-soil air equilibrium partitioning Is described by Henry s law ... 

Results of volatilization and leaching estimations are reported for six pesticides that span a wide range of the physical/chemical properties that affect fate at the soil/air interface. The pesticides are Mirex, toxaphene, methoxychlor, lindane, malathion, and dibromochloropropane (DBCP). These particular pesticides were chosen for discussion here because they illustrate the methods for assessing the fate of organics at the... 

Table I shows the results of calculating a soil diffusion coefficient and soil diffusion half-lives for the pesticides. The 10% moisture level specified means that the soil is relatively dry and that 40% of the soil volume is air available for diffusion. Complete calculations were not made for methoxychlor, lindane, and malathion because, based on Goring s criteria for the Henry s law constant, they are not volatile enough to diffuse significantly in the gas phase. This lack of volatility is reflected in their low values of X. These materials would move upward in the soil only if carried "by water that was moving upward to replace the water lost through evapotranspiration at the surface. Mirex has a very high Henry s law constant. On the basis of Goring s criteria, Mirex should diffuse in the soil air but, because of its strong adsorption, it has a very large a and consequently a very small soil air diffusion coefficient. The behavior of Mirex shows that Goring s criteria must be applied carefully.

The partial pressure of C02 in the soil air controls the concentration of both dissolved C02 and undissociated carbonic acid. At 0.003 atm of C02 (g) as a reference level for soils, [H2C03°] is about 1.04 x 10 4 M (Lindsay, 1979). At a normal atmospheric level of 0.0003 atm C02 (g), [H2C03°] is approximately 1.04 x 10 5 M. In most soils, C02 (g) is higher than in the atmosphere. C02 is released from soil and plant root respiration. In flooded soils, C02 (g) partial pressure increases to 0.01-0.3 atm, about 1000-fold higher than normal upland soils due to strong microbiological activity (Lindsay, 1979). 

For diffusion in the soil air-pores, a molecular diffusivity of 0.02 m2/h is reduced to an effective diffusivity using a Millington-Quirk type of relationship by a factor of about 20 to 10-3 m2/h. Combining this with a path length of 0.05 m gives an effective air-to-soil mass transfer coefficient kSA of 0.02 m/h, which is designated as U5. 

Partition coefficient between octanol and air (K0A) is determined by the relationship of chemical concentrations in octanol and air (K0a = Co/CA). The K0A is directly connected with the soil-air partition coefficient of pollutant K A which is determined as the ratio of solid phase to air concentrations (Ksa = Cs/CA) or through other constants (KSA = KSw/KAw). 

Contamination of soil often leads to a situation where the contaminants are not only in the soil but are also in the soil air. These can be analyzed using the same or similar methodologies that are used to analyze natural soil air constituents. 

SVE functions by the establishment of a concentration equilibrium between the contaminant adsorbed on the soil and the contaminant vapor concentration in the passing air. Significant factors involved in SVE are shown in Figure 10.3. Since the equilibrium concentrations are essentially linear, a higher concentration in the soil causes a similarly high concentration in the soil air. Before SVE is started, stagnant air in the soil pores contains VOC at the established equilibrium concentration. When SVE is initiated, two processes occur. Fresh VOC free air is brought into contact... 

To collect a sample, the probe with a SPME fiber installed is inserted into the soil. Air is pumped through the probe, drawing subsurface soil vapors into the probe tip and across the SPME fiber. Pumping air across the fiber increases uptake of target analytes by the SPME fiber relative to what is collected by molecular diffusion alone. Once a sample is collected, the SPME fiber is removed from the probe for analysis. To analyze the sample, the SPME fiber is inserted into a modified inlet system attached to the Fido sensor. The modified inlet serves to heat the SPME fiber, causing rapid and quantitative desorption of trapped molecules of analyte. The vapor-phase analyte is then swept into the sensor for analysis by a flow of carrier gas. 

In soils, electrons are produced by the metabolic activity of soil biota. These electrons are usually accepted by O2 dissolved in the soil solution which is then replaced by O2 from the soil air. Oxygen may, however, become deficient if all pores are filled with water as in waterlogged or compacted soils. Fe in Fe oxides may then function as an alternative electron acceptor and Fe ions will be formed according to eq. (16.3). The electrons are transferred from the decomposing biomass to the Fe oxide by microbially produced enzymes. Other potential electron acceptors in soils are nitrate, Mn and sulphate. 

Neglecting the water as a distinct bulk phase, the fraction of PCE in the soil air is given by (Eq. 3-65) ... 

Ribes S, Van Drooge B, Dachs J, Gustafsson 0, Grimalt JO (2003) Influence of soot carbon on the soil-air partitioning of polycyclic aromatic hydrocarbons. Environ Sci Technol 37 2675-2680... 

Toxaphene emissions for the entire United States were estimated by Li et al. [26], using application rates in the current year and residues carried over in the soil from past years, the latter estimated by assuming a 10-yr dissipation half life in soils. After the final year of toxaphene use (1986), only soil residues contributed to the emissions. Emission factors were calculated on a 1/4° latitude xl/6° longitude grid using the soil-air exchange and canopy models of Sholtz et al. [35,36]. About 80% of the emissions were from the southeastern, delta, and Appalachian states. The total quantity of toxaphene emitted to the atmosphere in 2000 was estimated as 364 tonnes. 

Spencer et al. (14) reported that the degree of reduction in vapor pressure in soil due to adsorption is dependent mainly upon soil water content, the nature of the pesticide, its concentration and soil properties, particularly soil organic matter content. The concentration of the desorbed pesticide in the soil water dictates the vapor density of the pesticide in the soil air in accordance with Henry s law. Hence, soil water adsorption coefficients can be used to calculate relative vapor densities in the soil atmosphere. 

Jury, W.A., Winer, A.M., Spencer, W.F., Focht, D.D. (1987a) Transport and transformations of organic chemicals in the soil-air water... 

Current multimedia models are inadequate in many respects. Description of intermedia transport across the soil-air and unsaturated soil-saturated soil zones suffers from the absence of a suitable theory for multiphase transport through the multiphase soil matrix. These phenomena are crucial in describing pollutant migration associated with hazardous chemical waste sites. Existing unsaturated-zone soil transport models fail to include mass transfer limitations associated with adsorption and desorption and with absorption and volatilization processes. Rather, most models assume equilibrium among the soil-air, soil-solid, solid-water, and soil-contaminant phases. 

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