Soil diffusion

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 ... 

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.

Air-soil diffusion thus appears to be much slower than air-water diffusion because of the slow migration in the soil matrix. In practice, the result will be a nonuniform composition in the soil with the surface soil (which is much more accessible to the air than the deeper soil) being closer in fugacity to the atmosphere. 

The C02 flux at the atmosphere vegetation cover boundary is determined in many respects by the soil processes involved in organic matter transformation. To better understand the biotic and abiotic mechanisms that control C02 emission from the soil, Jassal et al. (2005) compared measured C02 fluxes in a forest with their distribution profile in the soil of a 54-year-old coniferous forest on the eastern coast of Vancouver. It was established that C02 concentration grows at all depths of the soil layer with rising temperature and humidity. This is explained by soil diffusion reduction and changes in soil ecosystem functioning. It was noted that more than 75% of C02 emitted from the soil was generated at a depth of 20 cm, and almost total C02 flux forms from the 0 cm-50 cm layer. 

Notably, the 5 0 of soil water affects more than just the isotopic composition of respired CO2. It also affects the isotopic composition of ambient CO2, which makes contact with soil water by diffusing into and out of the soil. Depending upon its residence time in the soil, the diffusing ambient CO2 can partially or fully equihbrate with the soil water. This diffusion-equilibration-retro-diffusion process is termed the invasion effect (Tans, 1998) and was demonstrated experimentally by Miller et al. (1999). The impact of this effect on atmospheric 5 0 is defined in Equation (43) as Fi(5a — SJ/Ca. This non-biological invasion flux, Fi, is a function of the CO2 concentration near the soil surface, c and the speed with which CO2 in the air above the soil diffuses into the soil and equilibrates with soil water, v (the piston velocity with values for typical soil of —0.012 cm s and effective penetration of invading CO2 of —3 cm) ... 

Finally, we consider acidification mechanisms specific to the continents so that ocean acidification is not an issue. Both enhanced respiration by surviving plant roots and bacterial decomposition of dead biomass witliin soils following Uie impact may have increased soil carbonic acid concentrations and soil weathering. (Dead biomass is also a source of alkalinity as Ca and other cations are released into the soil solution, but this process neutralized only a fraction of Uie total carbonic acid produced.) The subsurface soil biomass presently contains 2 x 10 moles C, which, if multiplied by -4 in the late Cretaceous, may liave been able to supply just enough carbonic acid to explain the foram Sr data. However, nearly all of the subsurface soil biomass would liave to have been decomposed by bacteria. Furthermore, the vast majority of CO2 released in soils diffuses out of the soil and joins the atmosphere [27], The numbers for plant root respiration are even less favorable. Presently, respiration accounts for 0.5 X 10 mol C yr released in soils. In the post-K/T impact atmosphere photosyntliesis was very likely interrupted for at least several months by dust and aerosols [7,8], so surviving plants would have had to respire at four times present biomass. One year of respiration yielded 2 x 10 mol CO2, not enough to weather Sr even if the CO2 remained in the soil. 

Microbial activity in the soil diffusion from underlying oil- or gas-bearing strata... 

Where is the whole sediment or soil diffusion coefficient in terms of area of sediment or soil per unit time, and Dq is free ion diffusion coefficient measure in bulk solution (Berner, 1980). 

Figure 24.1. Conductive heating of 1-, 2-, and 3-m-thick impermeable layers from both sides. Steam temperature above and below the layer is 120 °C, ambient temperature is 10 °C, and soil diffusivity is 5.82 x 10 mVs.

Compared with the relatively unobstructed path for the diffusion of solvents in the atmosphere, diffusion coefficients for solvents in soil air will be less because of the tortuosity of the soil matrix pathways. Several functional relationships have been developed that relate the soil diffusion coefficient (DJ to various soil properties (see Roy and Griffin ), such as the Millington Equation ... 

Table 17.1.5. Estimated soil diffusion coefficients Dg (from Roy and Griffin )...

In addition, many remediation processes require information concerning the diffusion of various gases through the soil. Diffusion may possibly be a rate-limiting step in aerobic biodegradation as these processes require sufficient amounts of oxygen in the soil. Brusseau (1991) noted that diffusion is often the rate-limiting step in contaminant sorption and desorption from soil. 

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