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Geoenvironmental Engineering and Waste Management
CHAPTER 15
REMEDIATION
Basic Concepts
Public concern with polluted soil and groundwater has encouraged the development
of government programs designed to control and clean up this contamination. The
Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) of
1980 other wise known as the Superfund program explicitly addressed the cleanup of
hazardous waste sites. Because of its importance, we will briefly discuss the major
components of Superfund. Its purpose is twofold; to respond to releases of hazardous
substances on land and in navigable waters and to clean up contaminated sites. The former
deals with future releases whereas the latter deals with sites of existing contamination. There
are two types of responses available within Superfund; (1) removal actions, which are
responses to immediate threats. Such as leaking drums; (2) remedial actions, which involve
clean up of hazardous sites.
The first step is to place the potential site in the Superfund site Inventory. Which is a
list of sites that are candidates for investigation. The site is then subjected to a preliminary
assessment and site inspection, which may be performed by a variety of local state, federal or
even private agencies. The two component remedial investigation feasibility study (RI/FS) is
the next step in the process. The selection of the specific remedial action to be used at a
particular site is a very complex process. The goals of the remedial action are to protect
human health and the environment, to maintain protection over time, and to maximize waste
treatment (as opposed to waste containment or removal). A mandates a set of three categories
of criteria of criteria to be used for evaluating and selecting the preferred alternative:
threshold criteria, primary balancing criteria, and modifying criteria. The threshold criteria
ensure that the remedy protects human health and the environment and is in compliance with
applicable or relevant and appropriate requirements (ARARs): the balancing criteria ensure
that such trade off factors as cost and feasibility are considered and the modifying criteria
ensure that the remedy meets state and community expectations. After a remedial action has
been selected, it is designed and put into action. Some sites may require relatively simple
action. Such as removal of waste-storage drums and surrounding soil.
There are three major categories or types of remedial action; (1) containment, where
the contaminant is restricted to specified domain to prevent further spreading; (2) removal,
where the contaminant is transferred from an open to a controlled environment and (3)
treatment, where the containment is transformed to a nonhazardous substance. Since the
inherent toxicity of a contaminant is eliminated only by treatment, this is the preferred
approach of the three. Containment and removal techniques are very important, however,
when it is not feasible to treat the contaminant. Although we will focus on each of the three
types of remedial actions in turn, it is important to understand that remedial action often
consists of a combination of containment, removal and treatment.
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Containment
Containment can be accomplished by controlling the flow of the fluid that carries the
contaminant or by directly immobilizing the contaminant. Here, we will discuss the role of
physical and hydraulic barriers in containing contaminated water.
Physical Barriers
The principle of physical barrier is to control the flow of water, thereby preventing the
spread of contaminant. Usually, the barrier is installed in front (downgradient) of the
contaminated zone (Fig.1); however, barriers can also be placed up gradient or both up and
downgradient of the contamination. Physical barriers are primarily used in unconsolidated
materials such as soil or sand, but they may also be used in consolidated media such as rock
if special techniques are employed. In general, physical barriers may be placed to depths of
about 50 meters. They horizontal extent of the barriers can vary widely, depending on the
size of the site.
One important consideration in the employment of physical barriers is the presence of
a zone of low permeability, into which they physical barrier can be seated or keyed. Without
such a key into a low permeability zone, the contaminated water could flow underneath the
barrier. Another criterion for physical barriers are the permeability of the barrier itself. Since
the goal of physical barrier is to minimize fluid flow through the target zone, the permeability
of the barrier should be as low as practically possible. Another factor to consider is the
potential of the contaminants to interact with the components of the barrier and degrade its
performance. The properties of the barrier material should be matched to the properties of the
contaminant to minimize failure of the barrier.
There are three major types of physical barriers: slurry walls, grout curtains, and sheet
piling. Slurry walls are trenches filled with slurries of clay or mixtures of clay and soil. Grout
curtains are hardened matrices formed by cement-like chemicals that are injected into the
ground. Sheet piling consists of large sheet of iron that are driven into the ground. Slurry
walls are the least expensive and most widely used types of physical barrier, and they are the
simple to install. Grout curtains, which can be fairly expensive, are limited primarily to sites
having consolidated subsurface environments. Sheet piles have essentially zero permeability
and are generally of low reactivity. They can leak, however, because it is difficult to obtain
perfect seals between individual sheets. Moreover, sheet piling is generally more expensive
than slurry walls, and it is difficult to drive sheet piles into rocky ground.
Hydraulic Barriers
The principle behind hydraulic barriers is similar to that behind physical barrier – to
manipulate and control water flow. But unlike physical barriers, which are composed, of solid
material, hydraulic barriers are based on fluid potentials. They are generated by the pressure
differentials arising from the extraction or injection of water. The key performance factor of
this approach is its capacity to capture the contaminant plume, that is, to limit the spread of
the zone of contamination. Plume capture is a function of the number, placement, and flow
rate of the wells or drains. Often, attempts are made to optimize the design and operation of
the containment system so that plume capture is maximized while the volume of
contaminated water removed is minimized.
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The simplest hydraulic barrier is that established by a drain system. Such a system is
constructed by installing a perforated pipe horizontally in a trench dug in the subsurface and
placed to allow maximum capture of the contaminated water. Water can then be collected and
removed by using gravity or active pumping. Drains are, however, effective containment
mechanisms only for shallow contaminant zones.
Wells are more complicated – and more versatile – than drain systems. Both
extraction and injection wells can be used in a containment system. An extraction well
removes the water entering the zone of influence of the well, creating a cone of depression.
Conversely, an injection well creates a pressure ridge, or mound of water under higher
pressure than the surrounding water, which prevents flow past the mound. One major
advantage of using wells to control contaminant movement is that this is the only
containment technique that can be used for deep systems (>50m). In fact, wells can be used
on contaminant zones of any size; the number of wells is simply increased to handle larger
problems. For these and other reasons, wells are the most widely used method for
containment, despite disadvantages that include the cost of long-term operation and pump
maintenance, and the need to store, treat and dispose of the large quantities of contaminated
water pumped to the surface.
Removal
Excavation
A time-honored method for removing contaminants is excavation of the soil in which
the contaminants reside. This technique has been used at many sites and is highly successful.
There are, however, some disadvantages associated with excavation. First, excavation can
expose site workers to hazardous compounds. Second, the contaminated soil requires
treatment and/or disposal, which can be expensive. Third, excavation is usually, feasible only
for relatively small areas. Excavation is most often used to remediate shallow, localized,
highly contaminated source zones.
Pump and Treat
The pump-and-treat method, currently the most widely used remediation technique
for contaminated groundwater, removes contaminated water from the subsurface by using
one or more wells to pump it out. Furthermore, clean water brought into the contaminated
region by the pumping action removes, or “flushes”, additional contamination by inducing
desorption from the solid phase. The contaminated water pumped from the subsurface is
directed to some type of treatment operation, which may consist of air stripping, carbon
absorption, or perhaps the above-ground biological treatment system (Fig. 2).
Usually discussed in terms of its use for such saturated subsurface systems, as
acquifers, the pump-and-treat method can also be used to remove contaminants from the
vadose zone. In this case, it is generally referred to as in situ soil washing. For this
application; infiltration galleries, i.e. a network of porous, underground pipes whose function
is to supplement large areas of underground soil with fluid amendments, can be used is
addition to wells to introduce water to the contaminated zone.
When using water flushing for contaminant removal (as in pump-and-treat),
contaminant-plume capture and the effectiveness of contaminant removal are the major
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performance criteria. Recent studies of operating pump-and-treat system have shown that the
technique is very successful at containing contaminant plums, and, in some case, shrinking
them. However, it appears that pumping is frequently ineffective for completely removing
contaminants from the subsurface. There are many factors that can limit the effectiveness of
water flushing for contaminant removal.
1. Presence of low-permeability zones. When low-permeability zones (e.g. silt/clay lenses)
are present within a sandy subsurface, they create domains through which advective flow and
transport are minimal in comparison to the surrounding sand. The groundwater flows
preferentially around the slit/clay lenses, rather than through them. Thus, contaminant
located within the slit./clay lenses is released to the flowing water primarily by pore-water
diffusion, which can be a relatively slow process. Thus the mass of contaminant being
removed diminishes with each volume of water pumped, thereby increasing the time required
to completely remove the contaminant.
2. Rate-limited desorption. Research has revealed that absorption/desorption of many
solutes by porous media can be significantly rate-limiting. When the rate of desorption is
slow enough, the concentration of contaminant in the groundwater is lower than the
concentrations obtained under conditions of rapid desorption. Thus, less contaminant is
removed per volume of water, and removal by flushing will therefore take longer.
3. Presence of immiscible liquid. In many cases, residual phases of immiscible organic
liquids may exist in portions of the contaminated sub-surface. Since it is very difficult to
displace or push out residual saturation with water, the primary means of removal must be
dissolution into water and volatilization into the soil atmosphere. It can take a very long time,
to completely dissolve immiscible liquid, thus greatly delaying removal. The immiscible
liquid, therefore, serves as a long-term source of contamination.
Because pump-and-treat is a major remedial action technique, methods are being tested to
enhance its effectiveness. One way to improve the effectiveness of pump-and-treat is to alter
the status of the contaminant source zone, that is, the area in which contaminants were
disposed or spilled. If the source zone remains untreated or uncontrolled, it will serve as a
continual source of contaminant requiring removal. Thus, failure to control or treat the source
can greatly extend the time required to achieve site cleanup. It is important, therefore, that the
source zone at a site be delineated and addressed in the early stages of a remedial action
response. Other methods of improving this technique involve enhancing removal, as detailed
below.
Enhanced Removal
Contaminant removal can be difficult because of such factors as low solubility, high
degree of sorption, and the presence of immiscible-liquid Phases, all of which limit the
amount of contaminant that can be flushed by a given volume of water. Approaches are
being developed to enhance the removal of low-solubility, high-sorption contaminants. One
such approach is to inject a chemical into the aquifer, such as a surfactant (e.g. Detergent),
that will promote dissolution and desorption of the contaminant, thus enhancing removal
effectiveness. Such surfactants work like industrial and household detergents, which are used
to remove oily residues from machinery, clothing, or dishes; individual contaminant
molecules are “solubilized" inside of surfactant micelles, which are groups of individual
surfactant molecules ranging from 5 to 10 m in diameter. Alternatively, surfactant molecules
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can coat oil droplets and emulsify them into solution (Fig.3). In laboratory tests, surfactants
have been successfully used to increase the apparent aqueous solubility of organic
contaminants. However, only a few field tests have been attempted and results have been
mixed. A key factor controlling the success of this approach in the field is the ability to
deliver the surfactant to the places that contain the contaminant. This would depend, in part,
on potential interactions between the surfactant and the soil (e.g. sorption) and on properties
of water flow in the soil.
Soil Vapor Extraction
The principle of soil vapor extraction, or soil venting, is very similar to that of pumpand-treat; a fluid is pumped through a contaminated domain to enhance removal. In the case
of soil venting, however, the fluid is air rather than water. There are two key conditions for
using soil venting. First, the soil must contain a gas phase through which the contaminated air
can travel. This condition limits the use of soil venting to the vadose zone. Second,
contaminants must be capable for transfer from other phase (solid, water, immiscible liquid)
to the gas phase. This requirement limits soil venting to volatile contaminants.
Because air is much less viscous than water, much less energy is required to pump air.
Thus, it is usually cheaper to use soil venting for removing volatile contaminants from the
vadose zone. Once the contaminant is removed from the vadose zone, it is either released to
the atmosphere or placed into a treatment system. The major performance criteria for soil
venting are the effectiveness of capturing and removing the contaminant. The effectiveness of
contaminant removal by soil venting can be limited by the same three factors that limit
removal by water flushing.
A technology somewhat related to soil vapor extraction is air sparging, which
involves the injection of clean air into the saturated zone or aquifer. The primary purpose of
this injection is to volatilize contaminants from the soil into air bubbles. Owing to buoyancy
effects, the air bubbles rise and eventually make their way to the vadose zone, where they are
captured with a soil vapor extraction system. In some cases, in situ biodegradation can be
enhanced by air sparging because oxygen is added to the subsurface.
In Situ Treatment
In Situ treatment technologies are methods that allow in-place clean up of
contaminated field sites. There is great interest in these technologies because they can be
used with minimum economic costs. Second, in situ treatment can, in some cases, eliminate
the risk associated with the hazardous form of the contaminant. The two major types of in
situ treatment are biological (in situ bioremediation) and chemical.
Bioremediation
The object of bioremediation is to exploit the naturally occurring biodegradative
processes to clean up contaminated sites. There are several types of bioremediation: In situ
bioremediation is the in-place treatment of a contaminated site; ex situ bioremediation is the
treatment of contaminated soil or water that is removed from a contaminated site; and
intrinsic bioremediation is the indigenous level contaminant biodegradation that occurs
without any stimulation or treatment. All of these types of bioremediation are receiving
increasing attention as viable remediation alternatives for several reasons. These include
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generally good public acceptance and support, good success rates for some applications, and
the comparatively low cost of bioremediation when it is successful. As with any technology,
there are also drawbacks. First, success can be unpredictable, because biological systems are
themselves complex and unpredictable. A second consideration is that bioremediation rarely
restores an environment to its original condition. Often, the residual contamination left after
treatment is strongly sorbed and not available to microorganisms for degradation. Over a long
period of time (years) these residuals can be slowly released, generating additional pollution.
There is little research concerning the fate and potential toxicity of such released residuals;
therefore, both the public and the regulatory agencies continue to be concerned about the
possible deleterious affects of residual contamination.
Domestic sewage waste, which has been treated biologically for many years, is one of
bioremediation’s major successes. From this success, it is easy to see that biodegradation is
dependent on pollutant structure and bioavailability. Therefore, application of bioremediation
to other pollutants depends on the type of pollutant or pollutant mixtures present, and the type
of microorganisms present. The first successful application of bioremediation outside of
sewage treatment was in the cleanup of oil spills, where acrobic heterotrophic bacteria were
used to biodegrade hydrocarbon products. In the past few years, many new bioremediation
technologies have emerged that are being used to address other types of pollutants.
Several key factors are critical to successful application of bioremediation;
environmental conditions, contaminant and nutrient availability, and the presence of
degrading microorganisms. Thus, when biodegradation fails to occur, it is important to isolate
the “culprit” – the limiting factors of bioremediation. This task can be very complex. Initial
laboratory tests on soil or water from a polluted site can usually determine the presence or
absence of degrading micro organisms; such tests may also reveal an obvious environmental
factor that limits biodegradation, such as extremely low or high pH. Often, however, the
limiting factor is not easy to identify. For example, pollutants are often present as mixtures,
and one component of the pollutant mixture can have toxic effects on the growth and activity
of degrading microorganisms. Similarly, low bioavailability, which is another factor that can
limit bioremediation, can be very difficult to evaluate the environment.
Most of the developed bioremediation technologies are based on two standard
practices: the addition of oxygen and the addition of other nutrients.
Addition of Oxygen or Other Gases
One of the most common limiting factors in bioremediation is the availability of
oxygen. Oxygen is required for aerobic biodegradation. Moreover, oxygen is sparingly
soluble in water and has a low rate of diffusion (movement) through both air and water. The
combination of these three factors makes it easy to understand why inadequate oxygen
supplies often limit bioremediation.
Several technologies have been developed to overcome a lack of oxygen (Fig.4a).
This system is used to treat a contaminated aquifer, together with the contaminated zone
above the water table. It contains a series of injection wells or galleries and a series of
recovery wells, thus providing a two-pronged approach to bioremediation. First, the recovery
wells remove contaminated groundwater, which is treated above ground, in this case using a
bioreactor containing microorganisms that are acclimated to contaminant. Following
bioreactor treatment, the clean water is supplied with oxygen and nutrients (which may not be
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needed if the level of contamination is very low), and then reinjected into the subsurface. The
reinjected water provides oxygen and nutrients to stimulate in situ biodegradation. In
addition, the reinjected water flushes the vadose zone to aid in removal of the contaminant for
above-ground bioreactor treatment. This remediation scheme is a very good example of a
combination of physical/chemical/biological treatments that can be used to maximize the
effectiveness of the remediation treatment.
Bioventing is a technique used to add oxygen directly to a site of contamination in the
vadose zone. Bioventing is a combination of soil venting technology and bioremediation. The
bioventing zone, includes the vadose zone and contaminated regions just below the water
table. As shown in Fig.4b, a series of wells have been constructed around the zone of
contamination. To initiate bioventing, a vacuum is drawn on these wells to force accelerated
air movement through the contamination zone. This effectively increases the supply of
oxygen throughout the site, and hence the rate of contaminant biodegradation. In the case of
volatile pollutants, some of the pollutants are removed as air is forced through this system.
This contaminated air can also be treated biologically by passing the air through aboveground soil beds (biofilters) in a process called biofiltration (Fig.4b). In contrast, air sparging
can be used to add oxygen to the saturated zone (Fig.4c)
Methane is another gas that can be added with oxygen into extracted groundwater and
reinjected into the saturated zone. Methane is used specifically to stimulate methanotrophic
activity and cometabolic degradation of chlorinated solvents.
Nutrient Addition
Second only to the addition of oxygen in bioremediation treatment is the addition of
nutrients, nitrogen and phosphorous in particular. Many contaminated sites contain organic
wastes that are rich in carbon but poor in nitrogen and phosphorous.
Stimulation of Anaerobic Degradation Using Alternative Electron Acceptors
Until recently, anaerobic degradation of many organic compounds was not considered
feasible. Now, however, it is being proposed as an alternative bioremediation strategy, even
though aerobic degradation is generally a much more rapid process. Because it is difficult to
establish and maintain aerobic conditions is some saturated subsurface systems, several
alternative electron acceptors have been proposed for use in anaerobic degradation. These
acceptors include nitrate, sulfate, and iron (Fe1+) ions, as well as carbon dioxide. Although
limited in number, field trials using nitrate have shown promise. This relatively new area in
bioremediation will undoubtedly receive increased attention in the next few years.
Addition of Surfactants
While the use of surfactants is being investigated for improving pump and treat,
surfactant addition has also been proposed as a technique to enhance the biodegradation of
contaminants. Surfactants can be synthesized chemically, but many microorganisms also
produce them. Such microbially produced surfactants are known as biosurfactants. Like
synthetic surfactants, biosurfactants increase the solubility and decrease the sorption of
contaminants, thus increasing their bioavailability and enhancing the rates of biodegradation.
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Addition Microorganisms
If appropriate biodegrading microorganisms are not present in soil or if microbial
populations have been reduced because of contaminant toxicity, specific microorganisms can
be added as “introduced organisms” to enhance the existing populations. This process is
known as bioaugmentation. Scientists are now capable of creating superbugs-organisms that
can degrade pollutants at extremely rapid rates. Such organisms can be developed through
successive adaptations under laboratory conditions, or they can be genetically engineered. In
terms of biodegradation, these superbugs are far superior to organisms found in the
environment. One way to take advantage of the superbugs is to use them in bioreactor
systems under controlled conditions. Extremely efficient biodegradation rates can be
achieved in bioreactors that are used in above ground treatment systems.
The problem with use of superbugs is that introduction of a microorganisms to a
contaminated site may fail for two reasons. First, the introduced microbe often cannot
establish a niche in the environment; in fact, these introduced organisms rarely survive in a
new environment beyond a few weeks, microorganisms, like contaminants, can be strongly
sorbed by soild surfaces, so there are difficulties in delivering the introduced organisms to the
site of contamination. Currently, very little is known about the establishment of
environmental niches or about microbial transport; these are areas of active research. Perhaps
in the next few years scientists will gain further understanding of microbial behavior in soil
ecosystems. However, until we discover how to successfully deliver and establish introduced
microorganisms, their addition to contaminated sites will not be feasible method of
bioremediation.
Metal Contaminants
Current approaches to bioremediation of metals are based upon the complexation,
oxidation-reduction (redox), and alkylation reactions. Microbial leaching, microbial
surfactants (biosurfactants), volatilization and bioaccumulation/complexation are all
strategies that have been suggested for removal of metals from contaminated environments.
Unfortunately, the number of accompanying field-based studies has thus far, been small.
Bioleaching: Ex situ removal of metals from soil can sometimes be accomplished by
microbial leaching or Bioleaching. This technique has been used in mining to remove metals
such as copper, lead and zinc from low grade ores. In Bioleaching, metals are solubilized as a
result of acid production by special microorganisms such as Thiobacillus ferrooxide and T.
thiooxidants. Analogously, in bioremediation this process has been used to leach uranium
from nuclear-waste-contaminated soils and to remove copper from copper tailings. Another
potential application is the treatment of sewage sludge earmarked for disposal in soil.
Sludge-amended soils exhibition proved productivity, but also show increased market.
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Fig.1 Physical containment of a contaminant by the use of a slurry wall
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Fig.2 Removal of a contaminant by the pump and treat process
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Fig.3 Two types of interaction between surfectants and organic contaminants
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Fig.4 (a) In situ bioremediation in the vadose zone and ground water (b) Bioventing and
biofiltration in the vadose zone (c) Bioremediation in the ground water by air sparging
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