An Introduction to Environmental Chemistry

high up the food chain such as seals and caribou, for their food source. Animals

like seals are rich in fat and thus people living in the arctic can have very high

exposure to POPs through their diet. For example, a Beluga whale washed up on

the St Lawrence seaboard contained PCBs in excess of 50 mg kg-lipid^1. This PCB

concentration classified the whale as toxic waste. It is sadly ironic that POPs con-

taminate arctic environments, ecosystems and peoples who were never involved

in their manufacture or use.

7.4.2 Global persistent organic pollutant equilibrium

The manufacture and use of many exotic organic compounds (see Section 1.4)

has now been discontinued because of their persistence, potential health effects

and global mobility. For example, PCBs were first manufactured and used in the

1930s and their usage increased until the early 1970s. Thereafter PCB usage was

banned in many instances or subject to restrictions. However, PCBs did not

immediately disappear from the environment. Even today PCBs are present in

all of Earth’s environmental reservoirs and they are distributed globally. What

has changed, following discontinuation of widespread PCB use, is the equilib-

rium between PCB concentrations in air, water and soil. In the 1970s, when PCB

concentrations were at their peak, PCBs in the atmosphere were at much higher

concentrations than today. These high atmospheric concentrations caused a net

flux of PCBs from the atmosphere to the Earth’s surface waters and soils to estab-

lish equilibrium between these reservoirs (Fig. 7.28). Today, PCB concentrations

in the atmosphere are much lower. This has disturbed the equilibrium between

soils and water and the overlying air such that the net flux of PCBs is now out

of the soil and water into the atmosphere to re-establish equilibrium (Fig. 7.28).

This situation is termed re-emission or secondary emission.

Unequivocal evidence for the re-emission of POPs has been demonstrated in

an elegant study of the Great Lakes that border the USA and Canada. This study

focused on the POP a-hexachlorocyclohexane (a-HCH), a chiral compound (Box

278 Chapter Seven

1970

PCB air concentration higher

PCB

into soil

PCB

out of soil

Air

Soil

2000

PCB air concentration lower

PCB

into soil

PCB

out of soil

Air

Soil

Fig. 7.28Environmental equilibration of polychlorinated biphenyls (PCBs) between soil and air.