McKay, Donald. "Front matter" Multimedia Environmental Models ...

©2001 CRC Press LLC
CHAPTER 2
Some Basic Concepts
2.1 INTRODUCTION
Much of the scientific fascination with the environment lies in its incredible
complexity. It consists of a large number of phases such as air, soil, and water, which
vary in properties and composition from place to place (spatially) and with time
(temporally). It is very difficult to assemble a complete, detailed description of the
condition (temperature, pressure, and composition) of even a small environmental
system or microcosm consisting, for example, of a pond with sediment below and
air above. It is thus necessary to make numerous simplifying assumptions or statements
about the condition of the environment. For example, we may assume that a
phase is homogeneous, or it may be in equilibrium with another phase, or it may
be unchanging with time. The art of successful environmental modeling lies in the
selection of the best, or “least-worst,” set of assumptions that yields a model that is
not so complex as to be excessively difficult to understand yet is sufficiently detailed
to be useful and faithful to reality. The excessively simple model may be misleading.
The excessively detailed model is unlikely to be useful, trusted, or even understandable.
The aim is to suppress the less necessary detail in favor of the important
processes that control chemical fate.
In this chapter, several concepts are introduced that are used when we seek to
compile quantitative descriptions of chemical behavior in the environment. But first,
it is essential to define the system of units and dimensions that forms the foundation
of all calculations.
2.2 UNITS
The introduction of the “SI” or “Système International d’Unités” or International
System of Units in 1960 has greatly simplified scientific calculations and communication.
With few exceptions, we adopt the SI system. The system is particularly
convenient, because it is “coherent” in that the basic units combine one-to-one to