Soil Transport Models

Several types of computer models have been developed for estimating the expected concentrations of the chemicals of interest as they move away from the source. Soil transport models attempt to estimate the expected concentration at the surface above buried sources. Plume transport models attempt to estimate the concentrations within a plume, along with its shape and position. A different form of model is designed to guide a search pattern for employing a sensing system to trace a plume. 

Current multimedia models are inadequate in many respects. Description of intermedia transport across the soil-air and unsaturated soil-saturated soil zones suffers from the absence of a suitable theory for multiphase transport through the multiphase soil matrix. These phenomena are crucial in describing pollutant migration associated with hazardous chemical waste sites. Existing unsaturated-zone soil transport models fail to include mass transfer limitations associated with adsorption and desorption and with absorption and volatilization processes. Rather, most models assume equilibrium among the soil-air, soil-solid, solid-water, and soil-contaminant phases. 

One of the first complete, continuous simulation models was the pesticide mnoff transport model (PRT) (56). Improvements in the PRT modelled to the hydrologic simulation program—FORTRAN model (57). A number of other models have been developed (58,59). These models represent a compromise between the avadable data and the abiHty to encompass a wide range in soils, climates, and pesticides. These models have had mixed success when extended beyond the data with which they were caHbrated. No model has yet been developed that can be proven to give accurate predictions of... 

Models of chemical reactions of trace pollutants in groundwater must be based on experimental analysis of the kinetics of possible pollutant interactions with earth materials, much the same as smog chamber studies considered atmospheric photochemistry. Fundamental research could determine the surface chemistry of soil components and processes such as adsorption and desorption, pore diffusion, and biodegradation of contaminants. Hydrodynamic pollutant transport models should be upgraded to take into account chemical reactions at surfaces. 

In the next chapter (Chapter 27) we show calculations of this type can be integrated into mass transport models to produce models of weathering in soils and sediments open to groundwater flow. In later chapters, we consider redox kinetics in geochemical systems in which a mineral surface or enzyme acts as a catalyst (Chapter 28), and those in which the reactions are catalyzed by microbial populations (Chapter 33). 

In this chapter, we build on applications in the previous chapter (Chapter 26), where we considered the kinetics of mineral dissolution and precipitation. Here, we construct simple reactive transport models of the chemical weathering of minerals, as it might occur in shallow aquifers and soils. 

For the evaluation of long-range atmospheric transport and deposition of POPs, a multi-compartment transport model EMEP/MSCE-POP is used (Mantseva et al 2004). It includes such media as the atmosphere, soil, seawater and vegetation (Figure 1). A multi-compartment approach is conditioned by the ability of POPs to be accumulated in soil, seawater and vegetation with subsequent re-emission. Apart from atmospheric transport the model also takes into account the transport of pollutants by sea currents. 

Jury WA, Fluhler H (1992) Transport of chemicals through soil Mechanisms, models and field applications. Adv Agron 47 142-202... 

Based on their measurements of North African dust transported to the Barbados, Li et al. (1996) estimate that over a 10-year period, dust contributed about 56% of the total light scattering. Similarly, Tegen et al. (1997) estimate using a global transport model that scattering and absorption of light by submicron soil... 

Modeling results of subsurface pressure gradients were used to simulate subsurface soil gas velocity throughout the unsaturated zone profile. Figure 15 shows vertical profiles of unsaturated-zone air velocities for 12-hr time periods for August and October 1996. Results show that subsurface airflow is almost never zero, as is assumed in a diffusion-only transport model. Air-phase solute transport models based solely on diffusion would therefore not be able to accurately predict contaminant flux from the subsurface. 

Failing to incorporate soil-gas advection induced by barometric pumping into gas-phase subsurface transport models may, under certain conditions, under predict contaminant flux to the atmosphere. As previously described, Smith et al. (1996) compared TCE vapor fluxes measured with a chamber device to TCE in groundwater being removed by a pump-and-treat system and discharge into a surface-water receiving body at the same site. These researchers found VOC removal rates by flux to the atmosphere comparable in magnitude to both of the other attenuation pathways. 

A number of kinetically based models have appeared in the literature that describe organic and inorganic reactions in soils. Transport and nontransport models have been used that assume reversible and/or irreversible kinetic reactions. 

Transport Models That Assume Two Types of Phosphorus Sorption Sites. Mansell and Selim (1981) assumed applied soil phosphorus can react reversibly with two broad kinds of sorption sites—those exhibiting high rates, and those showing low rates. Such a model can be represented as... 

Mansell, R. S., and Selim, H. M. (1981). Mathematical models for predicting reactions and transport of phosphorus applied to soils. "In Modeling Wastewater Renovation (I. K. Iskander, ed.), pp. 600-640. Wiley, New York. 

Jardine, P. M., Parker, J. C., and Zelazny, L. W. (1985b). Kinetics and mechanisms of aluminum adsorption on kaolinite using a two-site nonequilibrium transport model. Soil Sci. Soc. Am. J. 49, 867-873. 

Geochemical models of sorption and desorption must be developed from this work and incorporated into transport models that predict radionuclide migration. A frequently used, simple sorption (or desorption) model is the empirical distribution coefficient, Kj. This quantity is simply the equilibrium concentration of sorbed radionuclide divided by the equilibrium concentration of radionuclide in solution. Values of Kd can be used to calculate a retardation factor, R, which is used in solute transport equations to predict radionuclide migration in groundwater. The calculations assume instantaneous sorption, a linear sorption isotherm, and single-valued adsorption-desorption isotherms. These assumptions have been shown to be erroneous for solute sorption in several groundwater-soil systems (1-2). A more accurate description of radionuclide sorption is an isothermal equation such as the Freundlich equation ... 

Thibodeaux LJ, Lipsly D. 1985. A fate and transport model for 2,3,7,8-tetrachlorodibenzo-p-dioxin in fly ash on soil and urban surfaces. Hazard Waste Hazard Mater 2 225-235. 

Jury, W.A., Ghodrati, M. (1989) Overview of organic chemical environmental fate and transport modeling approaches. In Reactions and Movement of Organic Chemicals in Soils. SSSA Special Publication No. 22, Sawhney, B.L., Brown, K., Editors, pp. 271-304, Soil Science Soceity of America and Society of Agronomy, Madison, Wisconsin. 

Calculations of treatment costs for supercritical soil remediation were made with a computer model that evaluates the capital and operating costs depending on plant capacity, carbon dioxide conditions for extraction and separation, operating conditions, soil transport and pretreatment and other boundary conditions like maintenance, depreciation or insurance. Some additional important parameters for the plant design are given in table 1. 

Among several analytical methods for the prediction of movement of dissolved substances in soils, one model (Leij et al., 1993) was developed for three-dimensional nonequilibrium transport with one-dimensional steady flow in a semi-infinite soil system. In this model, the solute movement was treated as one-dimensional downward flow with three-dimensional dispersion to simplify the analytical solution. Another model (Rudakov and Rudakov, 1999) analyzed the risk of groundwater pollution caused by leaks from surface depositories containing water-soluble toxic substances. In this analytical model, the pollutant migration was also simplified into two stages predominantly vertical (one-dimensional) advection and three-dimensional dispersion of the pollutants in the groundwater. Typically, analytical methods have many restrictions when dealing with three-dimensional models and do not include complicated boundary conditions. 

In order to avoid the restrictions to complicated adsorptive reactions in the MOC3D, Selim et al. (1990) developed a simulation system based on the multireaction model (MRM) and multireaction transport model (MRTM). The MRM model includes concurrent and concurrent-consecutive retention processes of the nonlinear kinetic type. It accounts for equilibrium (Freundlich) sorption and irreversible reactions. The processes considered are based on linear (first order) and nonlinear kinetic reactions. The MRM model assumes that the solute in the soil environment is present in the soil solution and in several phases representing retention by various soil... 

In this chapter we present a general-purpose transport model of the multireaction type. The model was successfully used to predict the adsorption as well as transport of several heavy metals in soils (Selim, 1992 Hinz and Selim, 1994 Selim and Amacher, 2001). Multireaction models are empirical and include linear and nonlinear equilibrium and reversible and irreversible retention reactions. A major feature of... 

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