Agriculture Reference
In-Depth Information
In water systems, sediment has the same type of matrix. The matrix contains
liquids (
substrate
to the chemist and engineer) within its interstices. Much of the
substrate of this matrix is water with varying amounts of solutes. As a general
rule, sediment is more highly saturated with water than are soils. However, some
soils can be permanently saturated, such as the muck in wetlands.
At least for most environmental conditions, air and water are solutions of
very dilute amounts of compounds. For example, air's solutes represent small
percentages of the solution at the highest level (e.g., water vapor) and most other
solutes represent parts per million (a bit more than 300 ppm of carbon dioxide).
Thankfully, most “contaminants” in air and water are in the parts per billion
range. On the other hand, soil and sediment themselves are conglomerations of
all states of matter.
Soil is predominantly solid but frequently has large fractions of liquid (soil
water) and gas (soil air, methane, carbon dioxide) that make up the matrix. The
composition of each fraction is highly variable. For example, soil gas concen-
trations are different from those in the atmosphere and change profoundly with
depth from the surface. Table 4.2 shows the inverse relationship between carbon
dioxide and oxygen. Sediment is a collection of particles that have settled on the
bottom of water bodies.
Ecosystems are combinations of these media. For example, a wetland system
consists of plants that grow in soil, sediment, and water. The water flows through
living and nonliving materials. Microbial populations live in the surface water,
with aerobic species congregating near the water surface and anaerobic microbes
increasing with depth due to the decrease in oxygen levels caused by the reduced
conditions. Air is not only important at the water and soil interfaces but is a vehicle
for nutrients and contaminants delivered to the wetland. The groundwater is fed
by the surface water during high-water conditions and feeds the wetland during
low water.
Table 4.2
Composition (% Volume of Air) of Two Important Gases in Soil Air
Silty Clay
Silty Clay Loam
Sandy Loam
Depth from Surface (cm)
O
2
CO
2
O
2
CO
2
O
2
CO
2
30
18.2
1.7
19.8
1.0
19.9
0.8
61
16.7
2.8
17.9
3.2
19.4
1.3
91
15.6
3.7
16.8
4.6
19.1
1.5
122
12.3
7.9
16.0
6.2
18.3
2.1
152
8.8
10.6
15.3
7.1
17.9
2.7
183
4.6
10.3
14.8
7.0
17.5
3.0
Source
: V. P. Evangelou,
Environmental Soil and Water Chemistry: Principles and Applications
, Wiley, New York,
1998.
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