Geology Reference
In-Depth Information
leaded solder to seal food cans. A dramatic reduction in gasoline lead
usage occurred at the end of 1985 when the maximum permissible lead
content of petrol was reduced from 0.4 g L
1
to 0.15 g L
1
, and there
has been a steady reduction in lead use since, with the increases in
market penetration of unleaded fuel, until it became mandatory in the
year 2000. Despite the ability of a vehicle emitting lead to cause direct
lead exposure through the atmosphere, as well as indirect exposure
through contamination of food and water, the lack of any obvious step
change in blood lead associated with the reduction of lead in petrol
shows clearly that at that time leaded petrol was not a major source of
direct exposure for the general population.
7.5 BEHAVIOUR OF LONG-LIVED ORGANIC CHEMICALS IN
THE ENVIRONMENT
A very large number of organic chemicals are manufactured and have a
wide variety of uses. In most cases of manufacture, distribution or use,
some of the chemical becomes released and dispersed in the environment.
In some instances, the chemical is highly reactive and degrades very
rapidly; often the main mechanism is breakdown by bacteria or other
organisms and there are standard tests to evaluate the biodegradability of
synthetic chemicals. On the other hand, many such chemicals, in order to
be useful in products, are relatively long-lived and hence may be persistent
within the environment. This raises the question of which environmental
compartment (air, water, soil, etc.) the chemical will partition into pre-
dominantly. Such knowledge is critical to predicting possible environ-
mental effects and directing monitoring programmes. Over the past few
years Mackay and colleagues
28
have developed methods for predicting the
behaviour of chemicals released into the environment and at the simplest
level, these are based solely upon an assumption of equilibrium parti-
tioning between the various environmental compartments. The basic
principles are described in Chapter 6, and this section is concerned with
how these methods are applied in a practical way.
The approach depends upon assuming a model environment consist-
ing of specified volumes of air, water, soil, and sediment (see Table 5).
Within those compartments, there are other components such as parti-
cles in water, fish, or soil solids, and some of these are assumed to
contain an organic fraction of defined abundance (see Table 5). The
presence of this organic matter is important since most organic com-
pounds will preferentially partition into an organic-rich medium.
Having defined a model environment, partitioning between the vari-
ous compartments is predicted on the basis of equilibrium relationships,
