Soil diffusion discussion

Dry deposition of chemical associated with atmospheric aerosols to a plant canopy is in many ways similar to the corresponding deposition pathway to water and soil as discussed in Chapter 6. Atmospheric aerosols can be deposited via Brownian diffusion (dominant for small aerosols (< 0.1 p,m) and characterized by intermediate deposition velocities, see Figure 6.4), impaction or interception (medium size aerosols (< 2 p,m), low deposition velocities), or sedimentation (large aerosols (> 2 jim), high deposition velocities). 

In the following sections I discuss the components of the diffusion coefficient so defined in turn. Note all the components of D are altered by flooding the soil. As well as increasing the cross-sectional area for diffusion, represented by flooding affects the geometry and tortuosity of the soil pore network, represented by /l and fs, and solute sorption on the soil solid, represented by dCt/dC. 

Effects of Flooding and Redox Conditions onfs. I know of no published data on this. Bnt it is likely that the natnre of particle surfaces in intermittently flooded soils wonld restrict snrface mobility. For ions to diffuse freely on the surface there must be a continuous pathway of water molecules over the surface and uniform cation adsorption sites. But in intermittently flooded soils the surface typically contains discontinuous coatings of amorphous iron oxides on other clay minerals, and on flooding reduced iron is to a large extent re-precipitated as amorphons hydroxides and carbonates as discussed above, resulting in much microheterogeneity with adsorption sites with disparate cation affinities. 

Effects of Flooding and Redox Conditions on OClAIC. Reductive dissolution reactions of the sort indicated in Figures 2.6 and 2.7 will affect the amount of a solute in diffusible forms in the soil and the distribution of the diffusible forms between the soil solid and solution. These processes are discussed in detail in Chapter 3. 1 here exemplify their effects by reference to a study of phosphate diffusion in a soil under different water regimes. 

As discussed in Chapter 5, in submerged soils nitrification occurs in aerobic sites at the iloodwater-soil and root-soil interfaces. Denitrification occurs upon diffusion of the NO, to the anaerobic bulk soil. Denitrification is favoured over dissimilatory reduction to NH4+(NO, -> NO2 NH4+) because of the large ratio of available carbon to electron acceptors in submerged soils. Denitrification is likely to proceed completely to N2 with little accumulation of N2O because of the very large sink and therefore steep concentration gradient of O2, and because carbon is less likely to be limiting (Section 5.1). 

However, as we discussed above, when the soil becomes very dry, below about 11% saturation, the molecules tend primarily to sorb onto the soil particles. This reduces the effect of vapor diffusion. Therefore the validity of Figure 4.10 is reduced in those conditions. Realizing this, Webb and his colleagues explored it in more detail [14]. This work showed a dramatic decline in effective diffusivity below 15% saturation [1, p. 43],... 

VJ What is important in the project we discussed is that, to my mind, it indicated what chemicals had staying power in soil and spread out certain distances using plants, rain, wind, and diffusion to be picked up by an animal or chemical detector under real world conditions. In the test, the dogs were trained to find only TNT and nothing else. Because Fido could find more chemicals, it could afford to lose on straight TNT detection in this real-world test. 

In groundwater and soil pollution problems, there is sometimes discussion of fast sorption and slow sorption, where the local equilibrium assumption would not be valid. How would you formulate a diffusion equation to deal with both the fast and slow forms of adsorption and desorption ... 

The issue of bioavailability is further clouded by the physical characteristics of soil and the role of a possible mass transfer limitation. Soil constituents are not simply flat surfaces with free and equal access to all bacterial species. The formation of aggregates from sand-, silt-, and clay-sized particles results in stable structures which control microbial contact with the substrate (Figure 2.7). Discussion of sorption mechanisms and binding affinities must include the possible impact of intra-aggregate transport of the substrate. If the substrate is physically inaccessible to the microorganism then both desorption from soil constituents and diffusion to an accessible site are necessary. The impact of intra-aggregate diffusion on degradation kinetics has been modeled for y-hexachlorocyclohexane (Rijnaarts et al., 1990) and naphthalene (Mihelcic Luthy, 1991). 

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