Environmental Engineering Reference
In-Depth Information
so small—colloidal in size—that they will remain in suspension and not settle out.
Particles that are heavier than water can be modeled in a manner similar to modeling the
movement of the same particle in air. Particles that float or that initially float absorb
water and then sink can also easily be modeled.
Particles that remain suspended can be modeled, but the modeling is more complex.
Some components in water will be in equilibrium with the particles; others will be
irreversibly bound to them. In both cases modeling these particles is essential because
they carry contaminants.
There is one other common characteristic of air and water in regard to contaminants.
Gaseous and soluble contaminants will mix with and be dispersed throughout the air or
water in which they are mixed. There are initially concentration gradients, but over time
the solute or suspended material will be evenly distributed through the solvent [13].
Movement of components through soil, regolith, decomposed rock, fractured rock, and
whole rock is entirely different. In all these cases an agent (air or water) is necessary to
carry the component through the media. The movement will be tortuous, and solutes will
interact with the solid media through which the solvent is moving. The interaction may
be chemical, such as bonding to surface groups, or it may be physical, such as diffusion
into pores or filtering out. In either case the solute is slowed in its movement. Sometimes
its chemical composition is changed by interaction with solids or by being attacked by
microorganisms in the environment. Modeling this movement is thus very complex and
subject to a great deal of variability.
7.6. DYNAMIC MODELS
Dynamic modeling is complex in that it considers all the eventual fates of a component in
the environment. This includes the reservoir of the component, the sources that add to it,
and the mechanisms by which it is dissipated. If there are any feedback loops or
conditions, these will also be included. Simply knowing the components of the system
does not make a model dynamic. What does make it dynamic are the rates of all its
transformations, including feedback. Knowing the rates of the various transformations,
additions, and deletions allows the calculation of the fate and longevity of the component.
Several possible dynamic systems are illustrated in Figures 7.5 through 7.10. These
cases show in a general way how components interact with the environment. Figure 7.5
shows a logarithmic increase, which is very common for microbial growth when an easily
decomposed compound is added to soil. Figure 7.6 illustrates the increase with a
subsequent decrease in a component in the environment. This type of change is common
when an easily decomposed compound is added to soil, and microorganisms produce a
component toxic to themselves.
Figure 7.7 shows a cyclic change in a component's occurrence in the environment.
Many phenomena follow this pattern, particularly plant and animal populations. Figure
7.8 shows a periodic decay that is characteristic of a microbial population decomposing
one component and releasing another that can be used by a different population. The
second population breaks down this component, releasing yet another component used by
another population. This process continues until all the material has been broken down
 
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