Transport of Passive Contaminants

A tremendous amount of research has been devoted to quantifying and modeling transport processes in the vadose zone, with readily available scientific literature (journals and textbooks) extending over the last half century. Modeling is used to quantify the dynamic redistribution of chemicals along the near surface and deeper subsurface profile, which often also is subject to reactive chemical processes including sorption, dissolution or precipitation, and volatilization. 

In this chapter, we examine the various mechanisms that influence chemical redistribution in the subsurface and the means to quantify these mechanisms. The same basic principles can be applied to both saturated and partially saturated porous media in the latter case, the volumetric water content (and, if relevant, volatilization of NAPL constiments into the air phase) must be taken into account. Also, such treatments must assume that the partially saturated zone is subject to an equilibrium (steady-state) flow pattern otherwise, for example, under periods of heavy infiltration, the volumetric water content is both highly space and time dependent. When dealing with contaminant transport associated with unstable water infiltration processes, other quantification methods (e.g., using network 

Berkowitz et al. Contaminant Geochemistry Transport and Fate in the Subsurface Environment. 

The vast majority of literature on quantifying transport processes has been considered in the framework of laboratory experiments. Field experiments, which often display fundamental differences in transport behavior relative to laboratory experiments, are inevitably subject to serious uncertainties, relating to initial and bonndary conditions, medium heterogeneity, and experimental control. A major aspect— and difficulty—lies in integrating laboratory and field measurements and upscaling small-scale laboratory measurements to treatment of field-scale phenomena. 

Throughout the following sections, we consider mechanisms relevant to both the (partially saturated) vadose and capillary fringe zones as well as to saturated 

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Contaminant redistribntion in the subsurface, as a result of transport (in dissolved form, as an immiscible-with-water phase, or adsorbed on colloids) is discussed in Part IV. These phenomena do not occur in a static domain, and contaminants are redistributed, usually by flowing water, from the land surface, through the partially saturated subsurface down to the water table, and within the fully saturated aquifer zone. After a basic presentation of water movement in the subsurface environment (Chapter 9), we focus on transport of passive contaminants (Chapter 10) and reactive contaminants (Chapter 11). 

Reactor Component Development — Corrosion of heat-transport tubing — Growth of passive oxides — Hydrogen embrittlement — Surface chloride contamination... 

In order for a polyimide to be useful as an interlevel dielectric or protective overcoat (passivant), additional demanding property requirements must be met In the case of the passivant, the material must be an excellent electrical insulator, must adhere well to the substrate, and must provide a barrier for transport of chemical species that could attack the underlying device. It has been demonstrated that polyimide filrns can be excellent bulk barriers to contaminant ion motion (such as sodium) [10], but polyimides do absorb moisture [11,12], and if the absorbed moisture affects adhesion to the substrate, then reliability problems can result at sites where adhesion fails. However, in the absence of adhesion failure, the bulk electrical resistance of the polyimide at ordinary device operating temperatures and voltages appears to be high enough to prevent electrochemical corrosion [13]. 

Lipid-soluble contaminants diffuse passively through the thin alveolar-vascular cell barrier of the alveolar sac and then dissolve into the blood according to the ability of the contaminant to partition between alveolar air and circulating blood. Substances that are very soluble in blood are rapidly transported into the bloodstream. For these substances, like styrene and xylene, the amount absorbed will be greatly enhanced by increasing the rate and the depth of respiration, as is likely to happen when doing strenuous physical work. On the other hand, substances that are poorly soluble in blood have limited capacity for absorption due to rapid saturation of blood. For these substances, like the solvents cyclohexane and methyl chloroform, the amount absorbed may be increased only by... 

PIMs have shown considerable potential for passive sampling of specific contaminants in waters [87]. In this application, low membrane diffusion coefficients can be an advantage. Passive samplers are generally left immersed in a river, lake, or contaminated water site for several days, and a PIM can slowly and selectively accumulate and transport the analyte to the receiver phase within the passive sampler. 

Unlike organic contaminants, nutrient and metal ions are generally actively transported into roots since the rate of passive transport of charged compounds across hydrophobic root membranes is limited. Plants take up most nonnutrient metals incidentally while acquiring nutrients for growth (Pulford and Watson, 2003 Saison et al., 2004). We do not address uptake of metals and nutrients in this chapter. The reader is referred to the above references and to phytoremediation reviews by Salt et al. (1995), Raskin and Ensley (2000), and Weis and Weis (2004). 

After emission, contaminants may be partitioned among the terrestrial, aqnatic, and various atmospheric phases, and those of sufficient volatility or associated with particles may be transported over long distances. This is not a passive process, however, since important transformations may take place in the troposphere during transit so that attention should also be directed to their transformation products. 

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