Soil systems transport directions

Fungi clearly play significant direct and indirect roles in effecting and modulating transport of a wide range of elements in soil systems. The principal transport modes that they are implicated in can be summarized as dispersion, concentration, inter-organism and bulk transfer (Fig. 3.5). 

Mycorrhizas are critical to plant survival and production in arid soils. Hyphae transport water in both directions. Water from patches of moist soils to the plant is provided by fungal hyphae and rhizomorphs that explore large volumes of soil ranging outward from the canopy edge. Mycorrhizal hyphae even extend from deep roots into the bedrock to access sources of water that cannot be reached or transported in the time scales necessary to be important to plant water balance. Mycorrhizal hyphae also benefit from hydraulically lifted water from plants that have deep root systems that reach groundwater or perched water tables. 

Figure 1.1. Schematic of the implementation of in situ electrochemical remediation systems. The electrodes are inserted into the soil and a direct electric field is applied to the contaminated site, which indnces the transport of the contaminants toward the electrodes. The electrode solntions are pumped, treated, and circulated for contaminant removal. Selected electrode conditioning solutions may be used to induce favorable chemistry at the electrodes and in the soil.

Electrophoresis is the transport of charged particles of colloidal size and bound contaminants due to the application of a low direct current or voltage gradient relative to the stationary pore fluid. Compared to ionic migration and electroosmosis, mass transport by electrophoresis is negligible in low-permeability soil systems. However, mass transport by electrophoresis may become significant in soil suspensicm systems, and it is the mechanism for the transportation of biocolloids (i.e., bacteria) and nucelles. 

Since there is no effective mechanism for transporting molecules from a buried source to the surface when the soil is very dry (see Section 4.3.1.3) when faced with such conditions, the system operator may need to become innovative. In addition, unless there is some mechanism for horizontal transport of molecules in moist soil, the operator should expect the strongest surface concentration to appear directly above the buried source. 

Nitrous oxide is important not only as a greenhouse gas but, as discussed in Chapter 12, as the major natural source of NC/ in the stratosphere, where it is transported due to its long tropospheric lifetime (Crutzen, 1970). The major sources of N20 are nitrification and denitrification in soils and aquatic systems, with smaller amounts directly from anthropogenic processes such as sewage treatment and fossil fuel combustion (e.g., see Delwiche, 1981 Khalil and Rasmussen, 1992 Williams et al., 1992 Nevison et al., 1995, 1996 Prasad, 1994, 1997 Bouwman and Taylor, 1996 and Prasad et al., 1997). The use of fertilizers increases N20 emissions. For pastures at least, soil water content at the time of fertilization appears to be an important factor in determining emissions of N20 (and NO) (Veldkamp et al., 1998). 

Thus, under equilibrium conditions, the emf of the double electrode-pair system is determined solely by electric potential differences developed at the two liquid junctions that involve KC1 salt bridges. The two Ej may differ because of the effect of soil colloids. Thus the fact that this emf can develop is known as the suspension effect.40 Only ionic transport processes across the liquid junctions need be taken into account in order to evaluate E. Ionic transport processes across the semipermeable membrane between the suspension and the solution are not germane. Moreover, since neither Ej nor Ej can be calculated by strictly thermodynamic methods, the interpretation of E must be made in terms of specific models of ionic transport across salt bridges contacting suspensions and solutions. Thus the relation between E and the behavior of ions in soil suspensions is not direct. 

The numerical solution to the advection-dispersion equation and associated adsorption equations can be performed using finite difference schemes, either in their implicit and/or explicit form. In the one-dimensional MRTM model (Selim et al., 1990), the Crank-Nicholson algorithm was applied to solve the governing equations of the chemical transport and retention in soils. The web-based simulation system for the one-dimensional MRTM model is detailed in Zeng et al. (2002). The alternating direction-implicit (ADI) method is used here to solve the three-dimensional models. 

The purpose of this article is to consider the nature of soils, how soils are contaminated by human activities, how these contaminants are transported and transformed in the soil column, and the types of human activities that could result in human exposure to soil contaminants. Soils are complex systems that exist at the interface among atmosphere, biosphere, hydrosphere, and lithosphere. A true soil includes gas, water, mineral, and organic components. Potential human contacts with soil can result in inhalation, ingestion, and dermal uptake of soil contaminants through both direct and indirect exposure pathways. The magnitude and persistence of exposure depends not only on the level of soil contamination, but also on the physical and chemical properties of soil, the chemical properties of the contaminant, and the frequency and duration of human activities such as occupational and recreational activities or use of home-grown food, which result in direct and indirect soil contacts. Toxicologists should be aware of the complex nature of soils, of the potential of soil contamination, and of types of direct and indirect contacts that human populations have with soil. 

---