Transport of Reactive Contaminants

It should be emphasized that, to date, the ability to quantify the complex chemical reaction phenomena that occur in the subsurface and also integrate the variability in flow behavior caused by natural heterogeneity and fluctuating boundary (land surface) conditions remains very limited. As a consequence, developing and improving the predictive capabilities of models is an area of active research. 

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The CTRW approach accounts naturally for transport in preferential pathways, with mass transfer to stagnant and slow flow regions the CTRW can account for these physical transport mechanisms, as well as other factors that influence transport of reactive contaminants, such as sorption. 

Aqueous Transport of Reactive Contaminants Filed and Laboratory Studies... 

Brusseau, M. L., Transport of reactive contaminants in heterogeneous porous media , Rev. Geophys., 32, 285-313 (1994). 

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). 

Brusseau (1994) gives an overview on the real behavior of reactive contaminants in heterogeneous porous media. The reader can also find a large updated list of references on transport in porous media. Just a few phenomena are shortly mentioned here ... 

Brown J. G., Bassett R. L., and Glynn P. D. (1998) Analysis and simulation of reactive transport of metal contaminants in ground water in Pinal Creek Basin, Arizona. In Special Issue—Reactive Transport Modeling of Natural Systems (eds. C. I. Steefel and P. van Cappellen). J. Hydrol. 209, 225 - 250. 

The final goal of the presented procedure is to estimate PAH deep seepage at urban and industrial sites. The evaluation should be plausible and therefore rely on a process-based model for the transport of reactive solutes through unsaturated porous media. After we succesfully managed to reconstruct possible soil profiles by means of a conditional stochastic simulation based on Markov theory, we now have to run the process-based reactive transport model (PBRTM) for all combinations obtained by the stochastic simulation. As PBRTM we used the model CARRY (Totsche et al., 1996 Knabner et al., 1996), in its current Version 5.5, which allows to model reactive transport of hydrophobic organic contaminants, for example PAH, in layered soils under unsaturated flow conditions. CARRY considers linear and non-linear, equilibrium and non-equi-... 

Chemical Reactivity - Reactivity with Water Reacts to form flammable hydrogen gas Reactivity with Common Materials Reacts with acids to form toxic, flammable diborane gas. Slowly attacks and destroys glass Stability During Transport Stable unless contaminated with acids or is overheated, thereby forming flammable hydrogen gas Neutralizing Agents for Acids and Caustics Caustic formed by the reaction with water can be diluted with water and then neutralized with acetic acid Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

The short4ived particle reactive radionuclides of the U/Th series also have enormous potential for tracking particle source and transport in ocean margins. Mass balances comparing inventories in sediments with supply can be used to determine import or export of particles to an area. Such approaches are increasingly important in understanding the fates of particle-reactive contaminants whose sources are often enhanced in the coastal ocean. Studies of especially when supplemented by other... 

Groundwater remediation is the often expensive process of restoring an aquifer after it has been contaminated, or at least limiting the ability of contaminants there to spread. In this chapter, we consider the widespread problem of the contamination of groundwater flows with heavy metals. We use reactive transport modeling to look at the reactions that occur as contaminated water enters a pristine aquifer, and those accompanying remediation efforts. 

A reactive contaminant may be adsorbed on the soil surface prior to rainfall then, following rainfall that canses erosion, the soil is transported by rnnoff water in the form of suspended particles redistribnted on the land snrface. In general, the settling velocity distribntion dnring runoff indicates that the finer particles are resettled initially (Proffit et al. 1991), although the details of the settling process are affected by different environmental factors, such as soil type and rainfall rate. 

Many factors affect the transport of contaminants in the subsurface, including the (spatially and temporally variable) hydraulic and physicochemical characteristics of the solid phase and the properties of water and the contaminants themselves. In this chapter, we focus on several specific, representative examples of reactive (nonconservative) contaminant transport. 

The analysis was limited in part by the scarcity of measurements, and clear discrepancies between measured and calculated values may be observed. As discussed in Chapter 10, tailing effects often are due to non-Fickian transport behavior, which was not accounted for in this model. Interestingly, the field-scale retardation coefficient values of the reactive contaminants were smaller by an order of magnitude than their laboratory values, obtained in an accompanying experiment. 

Transport of contaminants by surface runoff is illustrated in the experimental results of Turner et al. (2004), which deal with the colloid-mediated transfer of phosphorus (P) from a calcareous agricultural land to watercourses. Colloidal molybdate-reactive phosphorus (MRP) was identified by ultrafiltration associated with particles between l am and Inm in diameter. Colloidal P compounds can constitute a substantial component of the filterable MRP in soil solution and include primary and secondary P minerals, P occluded or adsorbed on or within mineral or organic particles, and biocolloids (Kretzschmar et al. 1999). 

Wolfe (1989) suggested a model to describe abiotic reduction in sediments, where a nonreactive sorptive site and an independent reactive sorptive site are considered. The nonreactive sorptive sink is consistent with partitioning of the contaminant to the organic carbon matrix of the solids. The model is described by Fig. 13.5 where P S is the compound at the reactive sorbed site P is the compound in the aqueous phase S and S are the sediments, P S is the compound in the nonreactive sink k, k , k , and k are the sorption-desorption rate constants, and k, k, and k are the respective reaction rate constants. If the reaction constants k and k are neglected, two rate-limiting situations are observed transport to the reactive site and reduction at the reactive site. The available kinetic data, however, do not allow one to distinguish between the two mechanisms. 

The products of our chemical processing industries themselves could become the instruments of terrorists because of their flammability, reactivity, toxicity, or notoriety. It is critical to minimize the vulnerability of chemicals or chemical assets to attack, contamination, or diversion for terrorist purposes, particularly as weapons of mass destraction. Critical challenges include the development of systems or chemistries that reduce the amount of or substitute for materials currently at risk, alter the attractiveness of such materials to terrorists, minimize the inventory and transportation of such materials, and that can detect and track the covert production and transportation of such materials. 

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