Geoscience Reference
In-Depth Information
physicochemical properties of the solid phase, as well as changes in the amount of
water in the subsurface as result of natural and human influences, it is difficult to
make generalizations concerning the chemical composition of the subsurface
aqueous solution.
1.3 Subsurface Gaseous Phase
The volume of the subsurface gas phase is controlled by the medium porosity and
moisture content, or in other words, by the ratio between the gas and water phases
occupied in pores. From the point of view of chemical component behavior, the
subsurface gas phase can assist the movement of organic molecules in the vapor
phase or chemicals dissolved in the water; it also can affect microbiological
activity and consequently define chemical persistence mainly in a near-surface
environment. Gaseous transport through pores makes the subsurface gas phase an
important channel for subsurface pollution, particularly by volatile toxic chemi-
cals. From the gaseous phase, chemicals might be adsorbed on solid surfaces or
dissolved in subsurface water. On the other hand, the transport of water as vapor
into pores might lead to the formation of a water layer that coats potentially
available sites for nonpolar gaseous pollutants, thus reducing pollutant fixation on
the solid phase. The presence of gaseous phases in the region near the water table
(capillary fringe) may be of particular importance for geochemical transformations
of pollutants.
The subsurface gas phase is composed mostly of CO
2
,N
2
, and O
2
, which are
the major gases in the atmosphere. Gases arising from microbiological activity,
such as nitrogen oxides, may be present at any time, but because of their high
reactivity with subsurface components and their susceptibility to microbiological
activity, they usually are transitory (Paul and Clark
1989
). In general, in well-
aerated soil, the amount (by volume) of O
2
is around 20 % and that of CO
2
is
between 1 and 2 %. In clay-dominated material, with a high water content, the
CO
2
concentration may reach values as high as 10 %.
The composition of the subsurface gas phase may change as a result of gas
dissolution into the liquid phase. The solubility of gases in water depends on the
type of gas, temperature, salt concentration, and the partial pressure of the gases in
the atmosphere. The most soluble gases are those that become ionized in water
(CO
2
,NH
3
,H
2
S), while O
2
and N
2
are much less soluble (Table
1.2
).
The CO
2
concentration in the subsurface may be different in small and large
pores and varies as a function of the aerobic or anaerobic activity of the microbial
population. Paul and Clark (
1989
) showed that a change from aerobic to anaerobic
metabolism occurs at an O
2
concentration of less than 1 % (by volume). The
overall aeration of the soil layer is not as important as that of individual aggre-
gates. Calculations show that water-saturated aggregates larger than 3 mm in
radius have no O
2
in their center (Harris
1981
). This means that aerobic and
