Environmental Engineering Reference
In-Depth Information
When taking samples from various depths in water, soil, or sediment, however, the
gaseous composition of the material being sampled is important. Samples from
submerged or water-saturated zones will have reduced chemical species, which will
oxidize when exposed to atmospheric oxygen. This means that if the samples are exposed
to air the species identified during analysis will be different from the species actually
present in the environment sampled. Different species have different physical, chemical,
and biological properties, and thus knowing their exact form is important. Ammonia is a
reduced form of nitrogen and is basic. Nitrite is a partially oxidized form of nitrogen and
is toxic, and nitrate is fully oxidized nitrogen and is less toxic than nitrite. A sample from
a reducing environment can have ammonia in it. When this sample is exposed to air the
ammonia can be oxidized to nitrite and nitrate, which can subsequently be reduced to
nitrogen gas if the environment becomes reducing again [7].
2.1.1.1. The Soil Atmosphere
At first glance the soil atmosphere is much like the rest of the atmosphere, and it is, in
that it contains nitrogen, oxygen, argon, carbon dioxide, and water vapor. Two factors
make the soil air different from atmospheric air. First, soil air is constrained in soil pores
and because pores are torturous it moves slowly between soil and atmosphere. There are
three forces that move air from the soil to the atmosphere and vice versa. Two cause a
mass flow of air into and out of soil. The other force causing movement is diffusion.
When the soil warms the air in it expands and moves out of the pores. This happens
during sunny days in accordance with the gas law PV=nRT . Rearranging this becomes
V=nRT/P . This equation states that as the temperature increases the volume of gas
increases. (All other factors nR and P are constant.) During the day when the soil is
warmed air thus expands out of the soil and at night when the soil cools its gaseous
constituents decrease in volume and air moves back into the soil.
Rain fills soil pores with water and air is forced out. The little oxygen remaining in
water-saturated soils is quickly consumed by plant roots and microorganisms. At this
point the soil becomes anaerobic and reducing. When the water moves out of the soil
either by evaporation or percolation, air replaces it. The oxygen content increases and the
soil becomes aerobic.
Soil air is typically higher in carbon dioxide and lower in oxygen than atmospheric air.
Plant roots and soil microorganisms respire, taking in oxygen and giving off carbon
dioxide. Carbon dioxide builds up to levels that may be 10 times higher than in
atmospheric air. Carbon dioxide diffuses from high concentration in soil air to the lower
concentration in atmospheric air. Likewise, oxygen diffuses in the opposite direction [8].
The higher carbon dioxide content in soil air is not insignificant. In soils with pH less
than 7 carbon dioxide dissolves in soil water and produces carbonic acid. The acid
dissolves soil minerals; for example, calcium carbonate. In soils underlain by limestone
this leads to the development of a karst landscape (see Figure 2.2) [9]. In basic soils
carbon dioxide reacts with calcium, forming calcium carbonate or lime. Calcium
carbonate precipitates out of solution, forming compacted layers that inhibit the
movement of water in soils. Heavy metal contaminants may also be precipitated as
carbonates and thus change their availability during analysis.
 
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