Transport processes soil systems

Fig. 14-4 Schematic representation of the transport of P through the terrestrial system. The dominant processes indicated are (1) mechanical and chemical weathering of rocks, (2) incorporation of P into terrestrial biomass and its return to the soil system through decomposition, (3) exchange reactions between soil interstitial waters and soil particles, (4) cycling in freshwater lakes, and (5) transport through the estuaries to the oceans of both particulate and dissolved P.

In all of the workshops, but especially in the FAT and Exposure Assessment workshops, the need for better understanding and model representation of soil systems, including both unsaturated and saturated zones, was evident. This included the entire range of processes shown in Table II, i.e., transport, chemical and biological transformations, and intermedia transfers by sorption/desorption and volatilization. In fact, the Exposure Assessment workshop (Level II) listed biological degradation processes as a major research priority for both soil and water systems, since current understanding in both systems must be improved for site-specific assessments. 

Although there exists a good understanding of the chemistry of phosphorus in soil-water systems, the hydrologic pathways linking spatially variable phosphorus sources, sinks, temporary storages, and transport processes in landscapes are less... 

Upon release to surface waters, di- -octylphthalate is expected to partition mainly to sediments and to suspended particulates. In a pilot-scale waste-water treatment system, di- -octylphthalate partitioned mainly to primary treatment sludge (Petrasek et al. 1983). The compound strongly adsorbs to organic matter contained in soils and sediments adsorption is probably the most important transport process for the... 

A number of investigators have modeled solute transport in soils assuming an equilibrium occurs between solution and solid phases. This assumption is often not valid in heterogeneous soil systems, and has been the impetus for the development of a number of nonequilibrium models. Some researchers have assumed that the nonequilibrium is caused by stagnant zones, which result in tortuous diffusional processes between solution and sorbed phases (Rao et al., 1979). Other researchers have attributed the nonequilibrium to kinetic effects. 

In a soil system, the relevant coupled processes that influence groundwater flow and solute transport arise from hydraulic, chemical, electrical and thermal driving forces, see Table 1. 

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. 

Figure 2 Conceptual model of carbon cycling in the litter-soil system. In each horizon or depth increment, SOM is represented by three pools labile SOM, slow SOM, and passive SOM. Inputs include aboveground litterfall and belowground root turnover and exudates, which will be distributed among the pools based on the biochemical nature of the material. Outputs from each pool include mineralization to CO2 (dashed lines), humification (labile slow passive), and downward transport due to leaching and physical mixing. Communition by soil fauna will accelerate the decomposition process and reveal previously inaeeessible materials. Soil mixing and other disturbances can also make physically protected passive SOM available to microbial attack (passive slow).

Surface Complex Formation, Ion Exchange, and Transport in Ground-water and Soil Systems The retardation equation can also be applied to inorganic soluble substances (ions, radionuclides, metals). But here we have to consider, in addition to the sorption or ion exchange process, that the speciation of metal ions or ligands in a multicomponent system influences the specific sorption process and varies during the pollutant transport in the groundwater chemistry then becomes an important part of the transport. 

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