Introduction to Field Sampling - Field Sampling: Principles and Practices in Environmental Analysis - page 8

1.3. WATER AND AIR
Water and air are completely different from soil, although they make up a significant part
of the soil environment. They are more mobile, and contaminants diffuse and are mixed
with them more readily. Another difference is that the movement of water and air is
associated with currents, which are responsible for the mixing and movement of
components (either dissolved or suspended) in them. In the case of air these currents may
be called wind. Water and air carry soil and contaminants (both with and without
associated suspended particles) for long distances. Eventually materials carried by water
and air are deposited either on the soil surface or at the bottoms of bodies of water. The
potential and real movement and deposition of soil and other components carried by
water and air is extremely important in sampling.
One area in which soil, water, and air are similar is that all three have layers or
horizons that have different characteristics. In addition to currents and wind, the layers or
horizons from which samples are taken are thus important. However, because of the
differences among soil, water, and air, separate specialized sampling tools are used for
each. In many cases sample containers for these three will also be different.
In addition to their individual existences, air and water are also integral and active parts
of soil. Under idealized conditions a soil sample is half solid and half void space. Under
the same conditions soil is envisioned as having half the void space filled with water and
the other half filled with air. Figure 1.4 gives a graphical representation of this idealized
composition of soil. There is an inverse relationship between soil water and soil air; that
is, when it rains the void space is filled with water and there will be little or no air and
when the soil dries the space occupied by water is replaced by air. The result is that both
soil air and water content are highly variable.
Soil air itself is also highly variable. In general it contains the same constituents as
atmospheric air; however, these are in different concentrations in soil. Plant roots and soil
microorganisms respire, taking up oxygen and releasing carbon dioxide into soil air. The
soil atmosphere is thus higher in carbon dioxide and lower in oxygen than atmospheric
air. In addition, it is possible to find highly reduced gaseous species in soil air even when
the soil is aerobic. For instance, it is not unusual to find methane in the soil atmosphere
along with nitrogen, oxygen, carbon dioxide, and water vapor.
Soil water is variable in concentration, depending on the minerals and amount of water
present. In terms of ions the types present are determined by the mineral composition of
the soil and so are less variable in any one soil. Soil pH is also determined by the
minerals present and the amount of water available for leaching the soil profile. The more
water leaching through the soil, the more acidic it will be.