Contaminant-transport models description

To construct models of this sort, we combine reaction analysis with transport modeling, the description of the movement of chemical species within flowing groundwater, as discussed in the previous chapter (Chapter 20). The combination is known as reactive transport modeling, or, in contaminant hydrology, fate and transport modeling. 

To understand the behavior of the movement of the contaminant in ground-water, people solve Eq. (1) forward in time. In solving this equation forward in time, one assumes that the plume is originated from somewhere and will travel through the porous media due to advection and dispersion. The conventional procedure to solve Eq. (1) is to use finite difference or finite element methods. For simple cases, closed-form solutions exist. Quantitative descriptions of the processes forward in time are well understood. Multidimensional models of these processes have been used successfully in practice [50]. Numerical solute transport models were first developed about 25 years ago. When properly applied, these models can provide useful information about transport processes and can assist in the design of remedial programs. 

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. 

Unfortunately, cation exchange does not lend itself to simple mathematical description as does idealized sorption of hydrophobic organic compounds. For low concentrations of a contaminant ion in a constant background of other ions, however, the ion exchange process often is treated approximately as a linear partitioning process, and use of a simple retardation factor in a transport model may be justified. Distribution coefficients for ionic contaminants in an aquifer are usually determined experimentally. Typically, a batch test is performed in which the ionic concentration on a fixed volume of aquifer solids and the ionic concentration in the associated pore waters are analyzed the distribution coefficient is taken as the ratio of the concentrations. Because sorption by ion exchange is affected by the concentrations of all other ions in the groundwater, the... 

Notwithstanding the natural heterogeneity of the subsurface, we can usefully consider homogeneous (bulk, effective) descriptions for at least some problems, especially for water flow (but less so for contaminant migration see Sect. 10.1). Therefore, two basic approaches to modeling generally are used to describe and quantify flow and transport continuum-based models and pore-network models. We discuss each of these here. 

Overall, geochemical computer models can be extremely useful in the description of chemical equilibria occurring in the aquatic environment. In some cases, predictions about reaction kinetics and transport of species can also be made. The application of geochemical models is not limited to natural aquatic systems but has been usefully extended to predict the eflfectiveness of certain remediation strategies in the treatment of waters emanating from contaminated sites." ... 

Exposure has been defined as the concentration of toxic materials in space and time at the interface with target populations (Travis et aL, 1983). The respective parameters required for environmental hazard assessments relate to the spatial and temporal abundance of the contaminants in the various compartments of the ecosystem and hence their bioavailability. Reference environmental descriptions must include the biota exposed to the released chemicals and the hydrological, topographical, geological and meteorological characteristics of the environment that affect the transport and transformation of the contaminants. The key step in exposure assessment is the use of transport and transformation models to quantify the movement of contaminants from the source through the environment to the target populations. 

Several dispersion models have been developed. These models are mathematical descriptions (equations) of the meteorological transport and dispersion of air contaminants in a particular area that allow estimates of contaminant concentrations, either at ground level or elevated (Carson and Moses, 1969). User-friendly modeling programs are available now that produce quick, accurate results from the operator s pertinent data. 

A detailed and comprehensive theoretical description of dry particle deposition and wet deposition processes can be found in Seinfeld and Pandis [19], and treatments with a specific focus on trace contaminants are provided in the texts by Mackay [6], Thibodeaux [20], and Scheringer [21], In this section, we review this transport theory and place special emphasis on a method to approximate the effect of intermittent rainfall events in a steady-state model. 

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