Volatile compounds, soil diffusion

The last two assumptions are the most critical and are probably violated under field conditions. Smith et al. (3) found that at least a half-hour was required to achieve adsorption equilibrium between a chemical in the soil water and on the soil solids. Solution of the diffusion equation has shown that many volatile compounds have theoretical diffusion half-lives in the soil of several hours. Under actual field conditions, the time required to achieve adsorption equilibrium will retard diffusion, and diffusion half-lives in the soil will be longer than predicted. Numerous studies have reported material bound irreversibly to soils, which would cause apparent diffusion half-lives in the field to be longer than predicted. 

NPP is the net carbon gain by vegetation over a particular time period— typically a year. It is the balance between the carbon gained by photosynthesis and the carbon released by plant respiration. NPP includes the new biomass produced by plants, the soluble organic compounds that diffuse or are secreted by roots into the soil (root exudation), the carbon transfers to microbes that are symbiotically associated with roots (e.g., mycorrhizae and nitrogen-fixing bacteria), and the volatile emissions that are lost from leaves to the atmosphere (Clark et al., 2001). 

Figure 4 shows pesticide volatilization as affected by soil depths of 1, 5, and 10 cm with water evaporation (E) equal to 0.25 cm/d. Since the concentration is inversely proporational to the depth of soil containing the 1 kg/ha of pesticide, the ratio of the concentrations roughly explains the initial relative volatilization rates. The very water soluble (Category III) compounds appear to approach a constant volatilization rate regardless of depth because their volatilization is controlled by diffusion of the chemical through the boundary layer above the soil surface as well as by the rate of movement upward to the soil surface. 

The fate of chemicals in the environment depends not only on processes taking place within compartments, but also by chemical partitioning between compartments. For example, there may be exchange of chemicals between air and water or soil. Movement from the water or soil into the air is accomplished by volatilization and evaporation of volatile or semivolatile compounds. Movement of chemicals from the air to water or soil is accomplished by deposition or diffusion into the water. Chemicals can also move from water to soil or sediment and vice versa. If a solid chemical in the soil or sediment dissolves into the water, this is called dissolution , while the opposite is called precipitation . If a chemical dissolved in water attaches to a soil or sediment particle, this is called adsorption , while the opposite is called desorption . The fugacity of a chemical, that is, its tendency to remain within a compartment, is affected by the properties of that chemical, as well as the chemical and physical properties of the environments such as temperature, pFF, and amount of oxygen in water and soil. Wind or water currents, wave action, water turbulence, or disturbance of soil or sediment (through the action of air or water currents, animals, or human activities) may also affect partitioning of chemicals. 

The vapors of volatile contaminants, such as radon and volatile organic compounds, can be transported through diffusion from the soil pore spaces into buildings. Three principal factors are needed to define the ratio of contaminant concentration in indoor... 

Volatilization causes contaminants to transfer from the dissolved phase to the gaseous phase. In general, factors affecting the volatilization of contaminants from ground water into soil gas include the contaminant concentration, the change in contaminant concentration with depth, the Henry s Law constant and diffusion coefficient of the compound, mass transport coefficients for the contaminant in both water and soil gas, sorption, and the temperature of the water. ... 

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