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

©2001 CRC Press LLC
CHAPTER 6
Advection and Reactions
6.1 INTRODUCTION
In Level I calculations, it is assumed that the chemical is conserved; i.e., it is
neither destroyed by reactions nor conveyed out of the evaluative environment by
flows in phases such as air and water. These assumptions can be quite misleading
when determining of the impact of a given discharge or emission of chemical.
First, if a chemical, such as glucose, is reactive and survives for only 10 hours
as a result of its susceptibility to rapid biodegradation, it must pose less of a threat
than PCBs, which may survive for over 10 years. But the Level I calculation treats
them identically. Second, some chemical may leave the area of discharge rapidly as
a result of evaporation into air, to be removed by advection in winds. The contamination
problem is solved locally, but only by shifting it to another location. It is
important to know if this will occur. Indeed, recently, considerable attention is being
paid to substances that are susceptible to long-range transport. Third, it is possible
that, in a given region, local contamination is largely a result of inflow of chemical
from upwind or upstream regions. Local efforts to reduce contamination by controlling
local sources may therefore be frustrated, because most of the chemical is
inadvertently imported. This problem is at the heart of the Canada–U.S., and Scandinavia–Germany–U.K.
squabbles over acid precipitation. It is also a concern in
relatively pristine areas such as the Arctic and Antarctic, where residents have little
or no control over the contamination of their environments.
In this chapter, we address these issues and devise methods of calculating the
effect of advective inflow and outflow and degrading reactions on local chemical
fate and subsequent exposures. It must be emphasized that, once a chemical is
degraded, this does not necessarily solve the problem. Toxicologists rarely miss an
opportunity to point out reactions, such as mercury methylation or benzo(a)pyrene
oxidation, in which the product of the reaction is more harmful than the parent
compound. For our immediate purposes, we will be content to treat only the parent
compound. Assessment of degradation products is best done separately by having
the degradation product of one chemical treated as formation of another.