Contaminant-transport models study

We have argued that molecular-scale understanding of the structure and composition of mineral surface-aqueous solution sorption complexes is vital to development of robust reactive contaminant transport models. This argument suggests that significant error may be expected in predicted environmental behavior in the absence of such knowledge. In an attempt to test this claim, we have used the results of our XAFS studies of Co(II) sorbed by alumina to refine a quasi-thermodynamic uptake model, then used the model to evaluate the sensitivity of predicted Co(II) partition coefficients to the choice of reactions included in the sorption model. 

Experiments on the scale of 1 to 1 are often used to study the local ventilation around an operator s workplace. Tracer gas is used to simulate the contaminant transport, and a high concentration level of the model tracer gas makes it possible to work with a convenient level of concentration for the measurements. Figure 12.31 shows an enclosure with an emission source S and a laboratory. setup with a model source 5,. The dimensionless concentration c/cg is... 

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. 

An important and recently reported issue, namely slow sorption/desorption rates, their causes at the intra-particle level of various solid phases, and how these phenomena relate to contaminant transport, bio availability, and remediation, is also discussed and evaluated. A case study showing the environmental impact of solid waste materials which are mainly complex organic mixtures and/or their reuse/recycling as highway construction and repair materials is presented and evaluated from the point of view of sorption/desorption behavior and data modeling. 

Hites RA, Lopez-Avila V. 1980. Sedimentary accumulation of industrial organic compounds discharged into a river system. In Baker Ra, ed. Contaminants and sediments. Vo1. 1. Fate and transport case studies, modeling, toxicity. Ann Arbor, Ml Ann Arbor Sci., 53-66. 

Most studies of contaminant transport in slurry walls have relied on the equilibrium assumption, either for convenience or because of the long residence times associated with diffusion-dominated transport. As an alternative, Rabideau and Khandelwal (1998a) have proposed the simple two-compartment mass transfer model to describe nonequilibrium sorption in soil/bentonite systems ... 

Steen, W.C., Paris, D.F., Baughman, G.L. (1982) Effects of sediment sorption on microbial degradation of toxic substances. In Contaminants and Sediments Fate and Transport, Case Studies, Modeling, Toxicity. Vol. 1, Baker, R.A., Editor, p. 477, Ann Arbor Science, Ann Arbor, Michigan. 

The transport associated with a line source of contaminant was also studied for the same data set (anisotropic media, isotropic dispersion). The model, besides providing numerical details of the simulation (maximum concentration value) also shows (Figure 3.10) the three-dimensional distribution of the contaminant for two cutting planes. The migration of the contaminant concentration for the line source has a wider distribution in the horizontal and transversal planes as shown in the figure. The contaminant concentration meets the CMC and CCC criteria just at 50%... 

Because actual exposure measurements are often unavailable, exposure models may be used. For example, chemical emission and air dispersion models are used in air quality studies to predict the air concentrations for down-wind residents. Residential wells located down-gradient from a site may not currently show signs of contamination, but they may become contaminated in the future as chemicals in the groundwater migrate to the well site. In these situations, groundwater transport models may estimate the period of time that chemicals of potential concern will take to reach the wells. 

Laboratory experiments, transport modeling, field data, and engineering cost analysis provide complementary information to be used in an assessment of the viability of an MNA approach for a site. Information from kinetic sorption/ desorption experiments, selective extraction experiments, reactive transport modeling, and historical case analyses of plumes at several UMTRA sites can be used to establish a framework for evaluation of MNA for uranium contamination (Brady et al, 1998, 2002 Bryan and Siegel, 1998 Jove-Colon et al, 2001). The results of a recent project conducted at the Hanford 100-N site provided information for evaluation of MNA for a °Sr plume that has reached the Columbia River (Kelley et al, 2002). The study included strontium sorption-desorption studies, strontium transport and hydrologic modeling of the near-river system, and evaluation of the comparative costs and predicted effectiveness of alternative remediation strategies. 

There are also several methods to determine patterns of fate and transport of pollutants in the environment. In some cases, microcosms and me-socosms are used to study fate, biodegradability, bioavailability, and transport within compartments. Field surveys may also be used to study fate and transport of pollutants in contaminated environments. Such studies involve collection and analysis of biota, water, air, soil, or sediment. In some cases, radioactively labeled contaminants ( tracers ) may be introduced in mesocosms or noncontaminated environments in order to determine their fate and patterns of transport. Finally, mathematical models are often used to produce computer simulations to... 

Actually, the use of groundwater geochemistry to delineate flow patterns is not limited to clean systems the same principles apply for contaminated systems. The knowledge of the groundwater flow field at a contaminant site is a prerequisite for development of transport models (National Research Council, 1990). Therefore, contamination problems can also benefit from studies of the movement of dissolved chemicals and isotopes that are not toxic. 

Mathematical models are useful to better understand the processes that occur under electric field and predict remedial performance in field application. Compared with laboratory studies, only few studies have been reported on the mathematical modeling of electrochemical processes and remediation. Generally, electrochemical remediation models should incorporate the contaminant transport, transfer, and transformation processes and dynamic changes in electrical conductivity, pH, and geochemical reactions. Recognizing this as a complex task, researchers have developed some simple models based on a set of simplified assumptions (Chapters 25 and 26). 

To accurately model contaminant transport when Al, Fe, and Mn oxide minerals are present, intraparticle diffusivities are needed. Additionally, as we tried to point out in this ehapter, there are a number of implieations in using the diffusion model with amorphous oxides. Some of these implications of intraparticle diffusion have been observed by researchers in macroscopic studies of both model and real systems. However, as only a small number of studies have been conducted on metal eontaminant diffusion in aqueous oxide systems, many implications need yet to be addressed sueh as the long-term effect of contaminants sorbed in micopores of metastable minerals and desorption of contaminants from both coprecipitated oxides and oxides exposed to contaminants over long periods of time. Therefore, future studies are needed to study and improve our understanding of this slow sorption proeess, intraparticle diffusion. 

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