Agriculture Reference
In-Depth Information
water vapor), and most other solutes represent parts per million (greater than
300 ppm of carbon dioxide). Thankfully, most contaminants in air and water,
if found at all are found 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 compo-
sition of each fraction is highly variable. For example, soil gas concentrations
are different from those in the atmosphere and change profoundly with depth
from the surface. Table B7.2 illustrates the inverse relationship between carbon
dioxide and molecular oxygen. Sediment is really an underwater soil. It is a
collection of particles that have settled on the bottom of water bodies.
Table B7.2
Composition (Percent 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.
Ecosystems are combinations of these media. For example, a wetland sys-
tem 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 resulting from the reduced conditions. Air is important not only at the
water and soil interfaces, but is a vehicle for nutrients and contaminants de-
livered to the wetland. The groundwater is fed by the surface water during
high-water conditions and feeds the wetland during low-water conditions.
So another way to think about these environmental media is that they
are compartments, each with boundary conditions, kinetics, and partitioning
relationships within a compartment or among other compartments. Chemicals,
whether nutrients or contaminants, change as a result of the time spent in each
compartment. The designer's challenge is to describe, characterize, and predict
the behaviors of various chemical species as they move through media in a
way that makes best use of them. When something is amiss, the cause and cure
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