Introduction - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 5

Atmosphere

Biosphere

Hydrosphere

Lithosphere

FIGURE 1.2 Equilibrium between various environmental compartments, reactions within
compartments, and material exchange between the compartments.

chemical composition, temperature, and pressure and have no tendency to change
their state. Equilibrium models only give us the chemical composition within the
individual compartments and not how fast the system reached the given equilibrium
state. Although, admittedly, a time-variant (kinetic) model may have significant
advantages, there is general agreement that equilibrium partitioning is a starting
point in this exercise. It is a fact that true equilibrium does not exist in the envi-
ronment. In fact, much of what we observe in the environment occurs as a result
of the lack of equilibrium between compartments. Every system in the environment
strives toward equilibrium as its ultimate state. Hence, the study of equilibrium is a
first approximation toward the final state of any environmental system.

1.4.1.2 Fate and Transport Modeling

The environment is a continuum in that as pollutants interact with various phases,
they undergo both physical and chemical changes and are finally incorporated into
the environment. Fate models, based on the mass balance principle, are necessary
to simulate the transport between and transformations within various environmental
media. This is called the multimedia approach . The mass balance principle can be
applied through the use of a multimedia fate and transport model to obtain the rates
of emissions and the relative concentrations in each compartment. A number of such
models already exist, some of which are described in Table 1.2.
Consider a chemical that is released from a source to one of the environmental
compartments(air,water,orsoil).Toassesstheeffectofthepollutanttotheecosystem,
we need to first identify the various pathways of exposure. Figure 1.3 illustrates
the three primary pathways that are responsible for exposure from an accidental
release. Direct exposure routes are through inhalation from air and drinking water