Density Viscosity Differences

Wastes having different densities or viscosities (tendency to resist internal flow) than the injection zone fluids will tend to concentrate in the upper (lower density/viscosity) or lower (higher density/ viscosity) portions of the injection zone. Frind62 and Larkin and Clark63 examined the basic requirements for the mathematical simulation of density-dependent transport in groundwater. Miller and colleagues64 described a density-driven flow model designed specifically for evaluating the potential for upward migration of deep-well-injected wastes.

Expected response to a step change in concentration, C

Expected response to a step change in concentration, C

0

Time relative to mean residence time of water, t/tw

'water

FIGURE 20.3 Effects of dispersion, adsorption, and biodegradation on the time change in concentration of an organic compound in an aquifer observation well. (From U.S. EPA, Assessing the Geochemical Fate of Deep-Well-Injected Hazardous Waste: A Reference Guide, EPA/625/6-89/025a, U.S. EPA, Cincinnati, OH, June 1990.)

Time relative to mean residence time of water, t/tw

'water

FIGURE 20.3 Effects of dispersion, adsorption, and biodegradation on the time change in concentration of an organic compound in an aquifer observation well. (From U.S. EPA, Assessing the Geochemical Fate of Deep-Well-Injected Hazardous Waste: A Reference Guide, EPA/625/6-89/025a, U.S. EPA, Cincinnati, OH, June 1990.)

20.3.5 Interaction of Partition, Transformation, and Transport Processes

The actual movement of a specific deep-well-injected hazardous substance depends on the types of processes that act on the waste and on the ways in which different processes interact. Figure 20.3 shows the expected change in concentration over time of a deep-well-injected organic compound in an observation well at an unspecified distance from the original point of injection.

With only dispersion operating, low concentrations are observed before the arrival of a fluid exhibiting ideal plug flow, but dispersion also serves to delay the time it takes for 100% of the initial concentration to be observed. Adsorption combined with dispersion delays the arrival of the compound, and eventually the contaminant will reach full concentration when adsorption capacity is reached. When biodegradation occurs, initial concentrations might well be governed by dispersion alone, until sufficient time has passed for an acclimated bacterial population to establish itself and become large enough to change the organic concentration significantly. If this occurs, the concentration would decrease and level out at some minimum value. When adsorption acts with biodegradation, the arrival of the contaminant is delayed, as with adsorption alone; then the concentration of the contaminant rises to a maximum level below that of the original concentration and declines as biodegradation becomes active.

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