Environmental Engineering Reference
In-Depth Information
dead plants. Beneath the O layer is the A layer, which can
contain some organic matter along with the sediments, and
in which infiltration can remove soluble minerals from the
sediment, called the E layer, and subsequent mineral depo-
sition in the B layer. Below the B layer is the parent or host
bedrock. Plant roots can be present in all soil layers as long
as water, oxygen, and nutrients are not limiting. Roots may
have a hard time penetrating through the B layer where
precipitates of iron oxides, called hardpan, can accumulate.
Anyone who has planted a tree realizes that the physical
structure of the soil in the upper layers has a direct effect on
the installation and growth of the selected plant. In the
Coastal Plain areas of the eastern United States, the surficial
sediments range from unconsolidated sands to impenetrable
clays. In piedmont areas of the same region, brittle clay-rich
weathered bedrock, called regolith or saprolite, is present on
top of parent bedrock though alluvial sediments have been
deposited in some intervalley areas. Bedrock is available to
plant roots only through fractures that create secondary
porosity. Even in unconsolidated sediments, soils made
tight by natural processes, such as weathering or cementa-
tion or compaction, can prohibit the entry of water and air
that are necessary for plant survival. This only can be over-
come through the introduction of events that lead to second-
ary porosity, such as fracturing.
The common denominator that helps determine the via-
bility of a particular soil for plant growth is its porosity and
bulk density, as these parameters are closely related to water
movement and storage. Bulk density is the dry weight of a
soil sample per unit volume. The bulk density of a soil can,
therefore, increase if volume decreases, such as by compac-
tion. Soil bulk density, typically reported in grams per cubic
centimeter (g/cm
3
), is indirectly related to porosity. The
average specific gravity of soil is 2.65 g/cm
3
, but because
soils also contain organic matter, air, and water, the term
bulk density is used to quantify the weight per unit volume
of sediment. A bulk density lower than 2.65 g/cm
3
in an
oven-dried sample indicates the presence of pores, which
can be filled with water or air.
As soil bulk density increases, the potential for root
penetration into the soil decreases. Knowledge of the distri-
bution of soil bulk density with depth, therefore, can provide
information about potential restrictions to the development
of deep root systems and groundwater interaction (Liang
et al. 1999). Bulk densities should be less than 1.4 g/cm
3
,
as these sediment types contain pores large enough to hold
water for plant use. These pores are needed not only to
transmit and store water but also to allow aeration of the
soil. Drainage is necessary to move new water in and old
water out. As we saw in Chap. 3, plants usually die not
because they are overwatered and cannot eliminate the
excess water but because the low solubility of oxygen in
water leads to root death after cessation of root aerobic
respiration.
The aeration capacity of a soil can be examined during
core collection or digging a hole as part of site-assessment
and characterization activities. In general, the red color
characteristic of oxidized iron minerals in the soil profile
indicates that oxygen has been able to reach that depth, at
least at some recent time in the past. Conversely, a gray color
indicates the absence of oxygen, and the transition between
red and grey delineates the maximum depth of oxygen
diffusion. This also tends to indicate the seasonal high-
water table, especially in low-permeability soils. These
techniques are used widely by those in state agencies who
permit septic systems, where the depth to the water table is
required to be a certain distance below a drain field to ensure
that the groundwater will not become contaminated.
One indication of the depth of the water table that can be
deduced at some sites where the water table is shallow is the
presence of evidence of previous or ongoing oxidation-
reduction cycles in the pore water and sediments. If the
pore water in the unsaturated zone or groundwater is anoxic,
any dissolved iron present in solution will precipitate from
the water if exposed to oxygen from the unsaturated zone or
to oxygen dissolved in infiltrating precipitation. The conver-
sion of ferrous to ferric iron is an abiotic reaction that occurs
spontaneously in the presence of oxygen, but iron-oxidizing
bacteria also can produce ferric iron from ferrous iron. This
typically occurs after a period of precipitation has ended, no
precipitation occurs for some time, and the water table
drops. Evapotranspiration that leads to a decline in the
water table also causes this process to occur. Conversely,
when the water table rises again, oxidized iron will redis-
solve back into the pore water or groundwater if these water
sources are anoxic, and iron-reducing bacteria are present
along with reduced labile organic matter. In some cases,
alternating layers of red and grey soil will be found as soil
is brought to the land surface with a hand auger. In other
cases, at sites where this has occurred for a long time, as in
wetland areas where groundwater is naturally anoxic, the
precipitation of iron leads to the formation of a hardpan
deposit, where a layer of the sediment is encountered in
which precipitated iron has cemented the sediment together.
Another factor that can decrease the amount of oxygen
available to roots and, therefore, affect plant health is the
amount of organic matter in the soil and its bioavailability.
Sandy soils that have at least 3% organic matter generally
are considered to be the best to impart drainage, aeration,
and moisture retention. Greater than 3% organic matter can
lead to water repellency. This is because at a molecular
level, organic matter has functional groups that are hydro-
phobic and repel the absorption of polar water molecules.
Water droplets first must be broken up by the attraction of
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