Environmental Engineering Reference
In-Depth Information
analogous to assimilative capacity used in surface waters.
It describes all of the naturally occurring processes, both
biologic and abiotic, that act to decrease contaminant levels.
For example, contaminant concentrations can decrease to
acceptable levels by dilution. This can occur when clean
water is added to contaminated groundwater, such as occurs
during recharge through uncontaminated unsaturated-zone
sediments and lateral groundwater inflow from upgradient
areas or adjacent aquifers. Additional important processes
include volatilization of contaminants from the water table
to the unsaturated zone, sorption onto natural organic matter,
or trapping in pore water of clays. These processes, however,
may act to transfer aqueous-phase contaminants to the soil or
atmosphere, and do not solve the overall problem or con-
taminant removal. Attenuation processes may, however,
lower the overall risk posed by contaminants by lowering
the concentrations to acceptable levels at points of exposure.
A brief history of natural attenuation in relation to
groundwater remediation is warranted. In the early 1970s,
around the time of the Love Canal contamination investiga-
tion, contaminant remediation consisted of engineered
approaches, such as dig-and-haul for sediments, and pump-
and-treat for groundwater. In the early 1990s, evidence
mounted that particularly for pump-and-treat approaches
that initially decreased contaminant levels in the aquifer,
the concentrations rarely approached regulatory levels and
never approached zero but, rather, leveled off at some higher
value following an asymptotic pattern (National Research
Council 1994). As a consequence, many researchers began
to investigate the hypothesis that rather than contaminants
accumulating in the groundwater environment, as was the
implication for inefficient contaminant removal exhibited by
pump-and-treat systems, contaminants could be decreased
by in situ processes.
Initial evidence that natural processes were occurring at
contaminated sites came in the form of monitoring data. The
best example occurred at the Borden site in Canada. At that
site, a plume of BTEX in groundwater was expected to
continue to migrate downgradient from the source area and
undergo attenuation by dilution and sorption. The monitor-
ing data indicated that the measured plumes were smaller
than would be predicted by sorption and dilution alone. It
was theorized that subsurface heterotrophic microorganisms
were oxidizing the contaminants.
That subsurface microbes would play a role in contami-
nant degradation was a radical idea at the time. This is
because knowledge of the presence and distribution of
microbes in the subsurface below the O layer was still in
its infancy the late 1980s and early 1990s. Up until then,
microbial numbers were known to decrease with depth in the
soil column as concentrations of organic matter and oxygen
decreased. Deeper sediment-water systems were considered
to be as oligotrophic as the deep oceans. That mindset
changed, however, when deep subsurface cores collected
and were analyzed for the presence of bacteria that had
been assumed to be there based on geochemical evidence
(Chapelle 1993).
Most bacteria that inhabit groundwater are a group
of fungi or simple plants that lack chlorophyll and, there-
fore, rely on external sources of energy for growth. They
are either parasites, heterotrophs, or saprophytes. Some
species reduce CO
2
to make organic compounds, such as
methanogenic bacteria, much like plants reduce CO
2
to
make sugars and starches. Some bacteria, however, are auto-
trophic and can make organic compounds from simple inor-
ganic compounds, such as CO
2
,H
2
S, Fe(II), or H
2
.
The application of microbes to restore contaminated
soils, water, and groundwater has a rich history. First, it
could start with wetlands where, as we noted earlier, plant-
based remediation is important, but the majority of remedia-
tion is due to microbes. Microbial-based contaminant degra-
dation processes rely on external sources of energy, in which
organic compounds are broken down to release energy to
drive growth. This is opposite of plants, which use the
energy of the sun to make organic compounds that are used
within the plant to support growth. Hence, there is a need for
many more enzymes in heterotrophic bacteria to deal with
external organic compounds that are not required by plants.
Whereas bacteria can derive energy and growth from these
compounds, plants do not. More on this difference is
discussed in Chap. 12.
An interesting result of a study performed by Schnoor
et al. (1995) was that plant enzymes were detected in
contaminated soils that had been characterized as
undergoing attenuation. This may be due to the increased
role of microbial processes in the rhizosphere or the direct
effects of the release of enzymes, as is discussed in Chap. 12.
There is a relation between bioremediation and phyto-
remediation of contaminated groundwater. They should not
be confused, however. The presence of plants tends to
stimulate the number of microbes relative to unplanted
areas. Plants can interact with the atmosphere, and change
redoxbyallowingoxygentodiffuse into potentially anoxic
soils. Phytoremediation processes are directly plant-
oriented processes, not just plant-stimulated processes.
Much work has been done examining the effect of aquifer
microorganisms on contaminated groundwater. Examina-
tion of the effect of microbes in relation to plants on
groundwater contamination has intensified, particularly
into soil remediation, but the extension to groundwater
restoration is less well developed. Plants have many
advantages over microbes when it comes to biodegrada-
tion. Whereas microbes need to derive energy from the
contaminants, plant roots obtain their energy from respira-
tion of the food they make and, therefore, have more energy
available to remediate contaminants.
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