Basics of Environmen

Environmental interactions, cycles, and systems

Inquisitive children sometimes ask whether the air they breathe was once breathed by a dinosaur. It

may have been. The oxygen that provides the energy to power your body has been used many times

by many different organisms, and the carbon, hydrogen, and other elements from which your body is

made have passed through many other bodies during the almost four billion years that life has existed

on our planet. All the materials found at the surface of the Earth, from the deepest ocean trenches to

the top of the atmosphere, are engaged in cycles that move them from place to place. Even the solid

rock beneath your feet moves, as mountains erode, sedimentary rocks are subducted into the Earth’s

mantle, and volcanic activity releases new igneous rock. There is nothing new or original in the idea

of recycling!

The cycles proceed at widely differing rates and rates that vary from one part of the cycle to another.

Cycling rates are usually measured as the time a molecule or particle remains in a particular part of

the cycle. This is called its ‘residence time’ or ‘removal time’. On average, a dust or smoke particle

in the lower atmosphere (the troposphere) remains airborne for a matter of a few weeks at most

before rain washes it to the surface, and a water molecule remains in the air for around 9 or 10 days.

Material reaching the upper atmosphere (the stratosphere) resides there for much longer, sometimes

for several years, and water that drains from the surface into ground water may remain there for up

to 400 years, depending on the location.

Water that sinks to the bottom of the deep oceans eventually returns to the surface, but this takes very

much longer than the removal of water molecules from the air. In the Pacific Ocean, for example, it

takes 1000 to 1600 years for deep water to return to the surface and in the Atlantic and Indian Oceans

it takes around 500 to 800 years (MARSHALL, 1979). This is relevant to concerns about the

consequences of disposing industrial and low-level radioactive waste by sealing it in containers and

dumping them in the deep oceans.

Those monitoring the movement of materials through the environment often make use of labelling,

different labels being appropriate for different circumstances. In water, chemically inert dyes are

often used. Certain chemicals will bond to particular substances. When samples are recovered, analysis

reveals the presence or absence of the chemical label. Radioisotopes are also used. These consist of

atoms chemically identical to all other atoms of the same element, but with a different mass, because

of a difference in the number of neutrons in the atomic nucleus. Neutrons carry no charge and so take

no part in chemical reactions, the chemical characteristics of an element being determined by the

number of protons, with a positive charge, in its atomic nucleus.

You can work out the atmospheric residence time of solid particles by releasing particles labelled

chemically or with radioisotopes and counting the time it takes for them to be washed back to the

ground, although the resulting values are very approximate. Factory smoke belching forth on a rainy

day may reach the ground within an hour or even less; the exhaust gases from an aircraft flying at

high altitude will take much longer, because they are further from the ground to start with and in

much drier air. It is worth remarking, however, that most of the gases and particles which pollute the

air and can be harmful to health have very short atmospheric residence times. Sulphur dioxide, for

example, which is corrosive and contributes to acid rain, is unlikely to remain in the air for longer