United States
Environmental Protection
Agency
Office of
Research and
Development
Off ice of Sol id Waste
and Emergency
Response
EPA/540/S-92/003
February 1992
&EPA Ground Water Issue
In-Situ Bio remediation of
Contaminated Ground Water
J.L. Sims3, J.M. Suflita", and H.H. Russellc
An emerging technology for the remediation of ground water is
the use of microorganisms to degrade contaminants which are
present in aquifer materials. Understanding the processes
which drive in-situ bioremediation, as well as the effectiveness
and efficiency of the utilization of these systems, are issues
which have been identified by the Regional Superfund Ground
Water Forum as concerns of Superfund decision makers. The
Forum is a group of ground-water scientists and engineers,
representing EPA's Regional Superfund Offices, organized to
exchange up-to-date information related to ground-water
remediation at Superfund sites.
Although in-situ bioremediation has been used for a number of
years in the restoration of ground water contaminated by
petroleum hydrocarbons, it has only been in recent years that
this technology has been directed toward other classes of
contaminants. Research has contributed greatly to
understanding the biotic, chemical, and hydrologic parameters
which contribute to or restrict the application of in-situ
bioremediation, and has been successful at a number of
locations in demonstrating its effectiveness at field scale.
This document is one in a series of Ground Water Issue
papers which have been prepared in response to needs
expressed by the Ground Water Forum. It is based on
findings from the research community in concert with
experience gained at sites undergoing remediation. The intent
of the document is to provide an overview of the factors
involved in in-situ bioremediation, outline the types of
information required in the application of such systems, and
point out the advantages and limitations of this technology.
For further information contact Dr. Hugh Russell, RSKERL,
FTS 743-2444, commercial number (405) 332-8800.
Summary
In-situ bioremediation, where applicable, appears to be a
potential cost-effective and environmentally acceptable
remediation technology. Suflita (1989) identified
characteristics of the ideal candidate site for successful
implementation of in-situ bioremediation. These characteristics
included: (1) a homogeneous and permeable aquifer; (2) a
contaminant originating from a single source; (3) a low
ground-water gradient; (4) no free product; (5) no soil
contamination; and (6) an easily degraded, extracted, or
immobilized contaminant. Obviously, few sites meet these
characteristics. However, development of information
concerning site specific geological and microbiological
characteristics of the aquifer, combined with knowledge
concerning potential chemical, physical, and biochemical fate
of the wastes present, can be used to develop a
bioremediation strategy for a less-than-ideal site.
Introduction
In-situ bioremediation is a technology to restore aquifers
contaminated with organic compounds. Organic contaminants
found in aquifers can be dissolved in water, attached to the
aquifer material, or as freephase or residual phase liquids
referred to as NAPLs which are liquids that do not readily
dissolve in water (Palmer and Johnson, 1989c). Generally,
Soil Scientist, Utah Water Research Laboratory,
Utah State University
Professor, Dept. of Botany and Microbiology, University
of Oklahoma
Research Microbiologist, Robert S. Kerr Environmental
Research Laboratory
Superfund Technology Support Center for
Ground Water
Robert S. Kerr Environmental
Research Laboratory
Ada, Oklahoma
Technology Innovation Office
Office of Solid Waste and Emergency
Response, US EPA, Washington, DC
Walter W. Kovalick, Jr., Ph.D.
Director
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NAPLs are subdivided into two classes: those that are lighter
than water (LNAPLs density <1.0), and those with a density
greater than water (DNAPLs density >1.0). LNAPLs include
hydrocarbon fuels, such as gasoline, heating oil, kerosene, jet
fuel, and aviation gas. DNAPLs include chlorinated
hydrocarbons, such as 1,1,1-trichloroethene, carbon tetra-
chloride, chlorophenols, chlorobenzenes, tetrachloroethylene,
PCBs, and creosote.
In this discussion, a technical approach is presented to assess
the potential implementation of bioremediation at a specific
site contaminated with an organic compound. The approach
consists of (1) a site investigation to determine the transport
and fate characteristics of organic waste constituents in a
contaminated aquifer, (2) performance of treatability studies to
determine the potential for bioremediation and to define
required operating and management practices, (3) develop-
ment of a bioremediation plan based on fundamental
engineering principles, and (4) establishment of a monitoring
program to evaluate performance of the remediation effort.
The pattern of contamination from a release of contaminants
into the subsurface environment, as would occur from an
underground leaking storage tank containing NAPLs, is
complex (Figure 1) (Palmer and Johnson, 1989c; Wilson et al.
1989). As contaminants move through the unsaturated zone,
a portion is left behind, trapped by capillary forces. If the
release contains volatile contaminants, a plume of vapors
forms in the soil atmosphere in the vadose zone. If the
release contains NAPLs less dense than water (LNAPLs),
they may flow by gravity down to the water table and spread
laterally. The oily phase can exist either as a free product,
which can be recovered by pumping, or as a residual phase
after the pore spaces have been drained. Contaminants
associated with NAPLs can also partition into the aquifer's
solid phase or in the vapor phase of the unsaturated zone. If
the release contains DNAPLs, these contaminants can
penetrate to the bottom of an aquifer, forming pools in
depressions. In either case, when ground water comes into
contact with any of these phases, the soluble components are
dissolved into the water phase.
There are a number of techniques available to remediate
ground water contaminated with organic compounds including:
physical containment such as slurry walls, grout curtains, and
sheet pilings (Ehrenfield and Bass, 1984); hydrodynamic
control using pumping wells to manipulate the hydraulic
gradient or interceptor systems (Canter and Knox, 1985);
several methods of free product recovery (Lee and Ward
1986); and (4) extraction of contaminated ground water
followed by treatment at the surface (Keely, 1989; U.S. EPA,
1989b).
Alternatively, contaminated ground water can be treated in
place, without extraction using in-situ chemical treatment or
biological treatment with microorganisms (Thomas et al.,
1987c). An advantage of in-situ treatment strategies is that
treatment can take place in multiple phases.
In-situ chemical treatment techniques are similar to methods
used to treat contaminated materials after withdrawal or
excavation, but are directly applied to the materials in place
(Ehrenfield and Bass, 1984). Chemical treatment may involve
neutralizing, precipitating, oxidizing or reducing contaminants
Leaking Underground
Storage Tank
Dissolved
Hydrocarbon
Figure 1. Regions of contamination in a typical release from an underground storage tank (Wilson et al., 1989).
-------
by injecting reactive materials into a contaminated leachate
plume through injection wells, but may be limited by mass
transport and concentration dependence. For treatment of
shallow contaminated aquifers, permeable treatment beds
containing reactive materials such as activated carbon or ion
exchange resins may be constructed downgradientto
intercept and treat the contaminated plume.
Biological in-situ treatment of aquifers is usually accomplished
by stimulation of indigenous microorganisms to degrade
organic waste constituents present at a site (Thomas and
Ward, 1989). The microorganisms are stimulated by injection
of inorganic nutrients and, if required, an appropriate electron
acceptor, into aquifer materials.
Most biological in-situ treatment techniques in use today are
variations of techniques developed by researchers at Suntech
to remediate gasoline-contaminated aquifers. The Suntech
process received a patent titled Reclamation of Hydrocarbon
Contaminated Ground Waters (Raymond, 1974). The process
involves the circulation of oxygen and nutrients through a
contaminated aquifer using injection and production wells (Lee
et al., 1988). Placement of the wells is dependent on the area
of contamination and the porosity of the formation, but are
usually no more than 100 feet apart. The nutrient amendment
consists of nitrogen, phosphorus, and other inorganic salts, as
required, at concentrations ranging from 0.005 to 0.02 percent
by weight. Oxygen for use as an electron acceptor in
microbial metabolism is supplied by sparging air into the
ground water. If the growth rate of microorganisms is 0.02 g/l
per day, the process is estimated to require approximately 6
months to achieve 90 percent degradation of the hydro-
carbons present. Cleanup is expected to be most efficient for
ground waters contaminated with less than 40 ppm of
gasoline. After termination of the process, the numbers of
microbial cells are expected to return to background levels.
In addition to stimulating indigenous microbial populations to
degrade organic waste constituents, another technique, which
has not yet been fully demonstrated, is the addition of
microorganisms with specific metabolic capabilities to a
contaminated aquifer (Lee et al., 1988). Populations that are
specialized in degrading specific compounds are selected by
enrichment culturing or genetic manipulation. Enrichment
culturing involves exposure of microorganisms to increasing
concentrations of a contaminant or mixture of contaminants.
The type of organism (or group of organisms) that is selected
or acclimates to the contaminant depends on the source of the
inoculum, the conditions used for the enrichment, and the
substrate. Examples of changes that may occur during an
acclimation period include an increase in population of
contaminant degraders, a mutation that codes for new
metabolic capabilities, and the induction or derepression of
enzymes responsible for degradation of specific contaminants
(Aelionetal., 1987).
It is important to note that the inoculation of a specialized
microbial population into the environment may not produce the
desired degree of degradation for a number of reasons
(Goldstein et al., 1985; Lee et al., 1988; Suflita 1988b).
Factors that may limit the success of inoculants include
contaminant concentration, pH, temperature, salinity, and
osmotic or hydrostatic pressure. They may act alone or
collectively to inhibit the survival of the microorganisms. The
subsurface environment may also contain substances or other
organisms that are toxic or inhibitory to the growth and activity
of the inoculated organisms. In addition, adequate mixing to
ensure contact of the organism with the specific organic
constituent may be difficult to achieve at many sites.
Successful inoculation of introduced organisms into simpler,
more controllable environments (e.g., bioreactors such as
waste-water treatment plants) to accomplish degradation has
been demonstrated. However, effectiveness of inoculation into
uncontrolled and poorly accessible environments such as the
subsurface is much more difficult to achieve, demonstrate and
assess (Thomas and Ward, 1989).
Genetic manipulation of microorganisms to produce
specialized populations to degrade specific contaminants
involves the acceleration and focusing of the process of
evolution (Kilbane, 1986; Lee et al., 1988). Genetic
manipulation can be accomplished by exposure of organisms
to a mutagen, followed by enrichment culturing to isolate a
population with specialized degradative capabilities, or by the
use of DMA recombinant technology to change the genetic
structure of a microorganism. The use of genetically engi-
neered organisms in the environment is illegal without prior
government approval (Thomas and Ward, 1989). In addition,
the introduction of genetically engineered organisms into the
environment would meet the same kind of barriers to success
as organisms developed by enrichment culturing, or more.
Additional methods that have been suggested to enhance
biodegradation include: cross acclimation, which involves the
addition of a readily degradable substrate to aid in the
biodegradation of more recalcitrant molecules; and analog
enrichment, which involves the addition of a structural analog
of a specific contaminant in order to induce degradative
enzyme activity that will affect both the analog and the specific
contaminant (Suflita, 1989a).
In most contaminated aquifers, the hydrogeologic system is so
complex, in terms of site characteristics and contaminant
behavior, that a successful remediation process must rely on
the use of multiple treatment technologies (Wilson et al.,
1986). The combination of several technologies, in series or
in parallel, into a treatment process train may be necessary to
restore ground-water quality to a required level. Barriers and
hydrodynamic containment controls alone serve as only
temporary plume control measures, but can be integral parts
of withdrawal and treatment or in-situ treatment measures.
A possible treatment train might consist of: (1) source removal
by excavation and disposal; (2) free product recovery to
reduce the mass in order to decrease the amount of
contaminants requiring treatment; and (3) in-situ treatment of
remaining contamination. When applicable, biological in-situ
treatment offers the advantage of partial or complete
destruction of organic contaminants, rather than transfer or
partitioning of contaminants to different phases of the
subsurface.
In-Situ Bioremediation Technical Process
The in-situ bioremediation technical process consists of the
following activities:
1. performance of a thorough site investigation;
2. performance of treatability studies;
-------
3. removal of source of contamination and recovery of
free product;
4. design and implementation of the bioremediation
technology; and
5. evaluation of performance of the technology through
a monitoring program (Lee and Ward, 1986; Lee et
al., 1988).
A thorough site investigation in which biological, contaminant,
and aquifer characterization data are integrated, is essential
for the successful implementation of a bioremediation system.
Biological characterization is required to determine if a viable
population of microorganisms is present which can degrade
the contaminants of concern. An assessment of waste
characteristics provides information for determining whether
bioremediation, either alone or as part of a treatment train, is
feasible for the specific contaminants at the site. Aquifer
characteristics provide information on the suitability of the
specific environment for biodegradative processes, as well as
information required for hydraulic design and operation of the
system.
Bioremediation of an aquifer contaminated with organic
compounds can be accomplished by the biodegradation of
those contaminants and result in the complete mineralization
of constituents to carbon dioxide, water, inorganic salts, and
cell mass, in the case of aerobic metabolism; or to methane,
carbon dioxide, and cell mass, in the case of anaerobic
metabolism. However, in the natural environment, a
constituent may not be completely degraded, but transformed
to an intermediate product or products, which may be equally
or more hazardous than the parent compound. In any event,
the goal of in-situ bioremediation is detoxification of a parent
compound to a product or products that are no longer
hazardous to human health and the environment.
In 1973 a review of ground-water microbiology was published
by researchers at the U.S. EPA Robert S. Kerr Environmental
Research Laboratory (RSKERL) (Dunlap and McNabb, 1973)
that stimulated research into microbiology of the subsurface.
Previously, biological activity in the subsurface environment
below the root zone was considered unlikely and that
microbial activity in the subsurface could not be of significant
importance (Lee et al., 1986). However, as methods for
sampling unconsolidated subsurface soils and aquifer
materials without contamination from surface materials
(Dunlap et al., 1977, Wilson et al., 1983, McNabb and Mallard,
1984) as well as methods to enumerate subsurface microbial
organisms (Ghiorse and Wilson, 1988) were developed,
evidence for microbial activity in the subsurface became
convincing.
Bacteria are the predominant form of microorganisms that
have been found in the subsurface, although a few higher life
forms have been detected (Ghiorse and Wilson, 1988; Suflita,
1989a). The majority of microorganisms in pristine and
uncontaminated aquifers are oligotrophic, because organic
materials available for metabolism are likely present in low
concentrations or difficult to degrade. Organic materials that
enter uncontaminated subsurface environments are often
refractory humic substances that resist biodegradation while
moving through the unsaturated soil zone.
Many subsurface microorganisms are metabolically active and
can use a wide range of compounds as carbon and energy
sources, including xenobiotic compounds (Lee et al., 1988).
Compounds such as acetone, ethanol, isopropanol, tert-
butanol, methanol, benzene, chlorinated benzenes,
chlorinated phenols, polycyclic aromatic hydrocarbons, and
alkylbenzenes have been shown to degrade in samples of
subsurface aquifer materials.
The rate and extent of biotransformation of organic
compounds at a specific site are controlled by geochemical
and hydraulic properties of the subsurface (Wilson et al.,
1986). Populations of microorganisms increase until limited by
a metabolic requirement, such as mineral nutrients, substrates
for growth, or suitable electron acceptors. At this point, the
rate of transformation of an organic material is controlled by
transport processes that supply the limiting factor. Since most
subsurface microorganisms are associated with the solid
phase, the limiting factor must be delivered to the microbes by
advection and diffusion through the mobile phases. Below the
water table, all transport must be through liquid phases, and
as a result, aerobic metabolism may be severely limited by the
very low solubility of oxygen in water. As oxygen becomes
limiting, aerobic respiration slows. However, other groups of
organisms become active and continue to degrade contamina-
ting organic materials. Under conditions of anoxia, anaerobic
bacteria can use organic chemicals or several inorganic
anions as alternate electron acceptors (Suflita, 1989a).
Even though microorganisms may be present in a
contaminated subsurface environment and have
demonstrated the potential to degrade contaminants in
laboratory studies, they may not be able to degrade these
contaminants without a long period of acclimation. Acclimation
results in development of the capability to accomplish
degradation.
In summary, the rate of biological activity in the subsurface
environment is generally controlled by:
1. the concentration of required nutrients in the mobile
phases;
2. the advective flow of the mobile phases or the
steepness of concentration gradients within the
phases;
3. opportunity for colonization in the subsurface by
metabolically active organisms or groups of
organisms capable of degradation of the specific
contaminants present;
4. presence, availability, and activity of appropriate
enzymes for degradation of specific contaminants
present; and,
5. toxicity exhibited by the waste or co-occurring
material(s) (Wilson et al., 1986; Suflita, 1989a).
Methods to Collect Biological Samples
Traditionally, unconsolidated soils or sediments are sampled
through a hollow-stem auger with a split-spoon core barrel or
a conventional thin-walled sample tube (Acker 1974, Scalf et
al., 1981; Wilson etal., 1989). The hollow-stem auger acts as
a temporary casing to keep the borehole open until a sample
can be acquired. A borehole is drilled down to the depth to be
sampled and a core barrel is inserted through the annular
opening in the auger and driven or pushed while rotating the
auger into the earth to collect the sample. These tools are
effective in both unsaturated and saturated cohesive
-------
materials, but are not as effective in unconsolidated sands as
it is difficult to keep aquifer material out of the hollow stem
auger (a phenomenon referred to as "heaving") and to keep
the sample in the core barrel while the sample is being
retrieved to the surface. In recent years there have been
many improvements in sampling the subsurface, particularly
with respect to heaving materials (Zapico et al., 1987; Leach
etal., 1988)
Just as it is important to protect the integrity of samples while
coring, it is as important to assure integrity while transferring
sample material to containers which are to be returned to the
laboratory for analysis. To prevent contamination of aquifer
material samples from introduced microorganisms and to
protect samples from the atmosphere to prevent injury of
anaerobic microorganisms, samples are extruded inside a
nitrogen-filled glove box (Figure 2). The glove box is prepared
for sample collection by filling it with the desired number of
sterile sampling jars and sterile paring devices, sealing the
box, and then purging it with nitrogen gas. A slight positive
pressure of nitrogen is maintained in the box by purging during
extrusion and collection of the samples.
Biological Characterization
A wide variety of methods are available to detect, enumerate,
and estimate biomass and metabolic activities of subsurface
microorganisms. These methods include: direct light and
epifluorescence microscopy, viable counts (e.g., plate counts,
most probable number counts, and enrichment culture
procedures), and biochemical indicators of metabolic activity
such as ATP, GTP, phospholipid, and muramic acid (Ghiorse
and Wilson 1988). Levels of microorganisms ranging from 106
to 107 cells/g of dry aquifer material have been reported from
uncontaminated shallow aquifers (Ghiorse and Balkwill, 1985;
Lee et al., 1988). Often the distribution of microorganisms in
aquifers, as it is in soils, is sporadic and nonuniform, indicating
the presence of micro-environments conducive to growth and
activity.
Waste Characterization
The source of contamination is usually the primary object of
remedial activities (Wilson et al. 1989) as the treatment of
plume areas will not be effective if the source continues to
release contaminants. Information concerning: (1) the areal
location of the source area and contaminant plumes; (2)
amounts of contaminants in the source area; and (3) amounts
of contaminants released into the subsurface are required to
select and apply an appropriate remediation technology and to
determine cost and time requirements for completion of a
remedial action. If in-situ bioremediation is selected as the
remedial technology, information concerning the amount and
distribution of contamination is used in conjunction with
hydrogeological site characteristics to locate injection and
extraction wells and to optimize pumping rates and
concentrations of amendments, such as nutrients and
alternate electron acceptors.
The use of conventional monitoring wells can generally
accurately define the geometry of the ground-water plume
(Palmer and Johnson, 1989a; Wilson et al., 1989). However,
there are important factors that control the quality of
information collected from a network of monitoring wells,
which include the amount of well purging done prior to
sampling (Barcelona and Helfrich, 1986), method of sampling
(Stolzenburg and Nichols, 1985), and method of well
construction and installation (Keely and Boateng, 1987).
Methods for ground-water sampling are presented by Scalf et
al. (1981), Ford et al. (1984), and Barcelona et al. (1985).
Other methods used for detecting contaminant plumes in the
subsurface include geophysical techniques such as surface
resistivity and electromagnetic surveys, chemical time-series
sampling tests (Palmer and Johnson, 1989a), and vapor
Sample Head
Space Analysis
Vent
Flushing Vent
Flow Regulator
and Indicator
Sample Tube
from Extrudei
Iris Port
Figure 2. Field sampling glove box (Wilson, et al., 1989).
-------
monitoring wells (Devitt et al., 1988; Palmer and Johnson,
1989b).
The distribution of the source area and the extent of
contamination should also be characterized by collecting
cores of the solid aquifer materials. Precise information is
required to define the vertical extent of contamination so that
nutrients, oxygen and other amendments injected into the
aquifer contact the contaminants. Injection into a clean part of
the aquifer is a wasted effort and may give the false
impression that the region of aquifer between the injection and
recovery wells is clean (Figure 3).
Additional characteristics of waste contaminants present at a
specific site that should be considered are related to their
environmental fate and behavior in specific aquifer materials
(Armstrong, 1987; Johnson et al., 1989). These character-
istics include physical and chemical properties that determine
recalcitrance, reactivity, and mobility of contaminants at the
site. Information concerning partitioning of contaminants
between aquifer solids and water is especially important. This
information is used to evaluate the extent and rate of release
of contaminants into the ground water, their mobility, and the
quantity of electron acceptors and inorganic nutrients that
must be supplied to support in-situ bioremediation.
Aquifer Characterization
Important geological characteristics of an aquifer that should
be considered during a site investigation include the
composition and heterogeneity of aquifer material, specific
yield, hydraulic connections to other aquifers, magnitude of
water table fluctuations, ground-water flow rate and direction,
hydraulic conductivity distribution, permeability, bulk density,
and porosity (Lee et al., 1988; Palmer and Johnson, 1989a).
Hydraulic conductivity (K) is an especially important
characteristic since the aquifer must be permeable enough to
allow the transport of electron acceptors and inorganic
nutrients to the microorganisms in the zone of contamination.
Permeable aquifer systems, i.e., aquifers with K values of 10"4
cm/sec or greater, are usually considered good candidates for
in-situ bioremediation (Thomas and Ward, 1989).
Hydraulic conductivity of an aquifer can be determined by a
variety of methods (Thomas et al., 1987b, Palmer and
Johnson, 1989a). Knowledge of K values at multiple locations
is necessary because of the heterogeneity of aquifer
materials. Laboratory methods are also available for
determining hydraulic conductivity, but field-measured values
represent average properties over a larger volume and utilize
less disturbed materials (Palmer and Johnson, 1989a).
Aquifer characteristics play an extremely important role in
determining the effectiveness of in-situ bioremediation. Even
in the presence of organisms acclimated to the specific waste
constituents present in an aquifer, biodegradation of
contaminants may be limited by unfavorable aquifer
characteristics that affect microbial activity including:
1. insufficient concentrations of dissolved oxygen for
aerobic metabolism of compounds susceptible to
aerobic degradation;
2. excessive oxygen that inhibits anaerobic
biodegradation of many halogenated compounds in
the subsurface;
Injection
Well
\
Land Surface
Water
Table -»^
^
— •^^
Contaminated
Interval
i
;
;
df> Direction of Flow d^ d^
V V V
^> Wasted ^>
^N
1 — /
Figure 3. The value of accurately locating the contaminated interval (Wilson et al., 1989).
-------
3. lack of a suitable alternative electron acceptor, if
oxygen is unavailable or not usable;
4. insufficient inorganic nutrients, such as nitrogen,
phosphorus, and trace minerals;
5. presence of toxic metals or other toxicants; and
6. other aquifer characteristics, such as pH, buffering
capacity, salinity, osmotic or hydrostatic pressures,
radiation, sorptive capacity, and temperature
(Armstrong, 1987; Lee etal., 1988).
Treatability Study
Atreatability study is designed to determine if bioremediation
is possible at a specific site, and whether there are any
biological barriers to attaining clean-up goals. Even though
the scientific literature may indicate that a specific chemical is
likely to biodegrade in the environment, a treatability study
using site specific variables should be used to confirm that
contention (Suflita, 1989a). Microcosms are generally used to
conduct treatability studies. Pritchard (1981) defined a
microcosm as "a calibrated laboratory simulation of a portion
of a natural environment in which environmental components,
in as undisturbed a condition as possible, are enclosed within
definable physical and chemical boundaries and studied under
a set of laboratory conditions." Microcosms may range from
simple batch incubation systems to large and complex flow-
through devices (Suflita, 1989a).
Results of a treatability study can also provide an estimate of
the rate and extent of remediation that can be attained if
microorganisms are not limited by the rate of supply of an
essential growth factor or by the presence of an unfavorable
environmental factor.
Treatability studies to determine inorganic nutrient and
electron acceptor requirements of subsurface microorganisms
present at a specific site should be conducted using samples
of subsurface solids as well as the ground water. Nutrient and
electron acceptor requirements that will enable indigenous
microorganisms to efficiently degrade organic contaminants
present at a specific site can be determined by incubating
contaminated subsurface materials with combinations of levels
of inorganic nutrients and electron acceptors. Studies should
be performed under conditions that simulate field
environmental conditions. Results of the studies are used to
design the bioremediation program as well as to optimize the
treatment strategy.
Design and Implementation of an In-Situ
Bioremediation System
Before implementation of an in-situ bioremediation system, the
source of contamination in the soil and in the ground water
should be removed as much as possible. In the case of a
liquid fuel spill, source removal may consist of recovery of
LNAPL free product from the ground water. Depending on the
characteristics of the aquifer and contaminants, free product
can account for as much as 91 percent of the spilled
hydrocarbon, with the remaining hydrocarbon (accounting for
9-40 percent of the spill) sorbed to the soil or dissolved in the
ground water (Lee et al., 1986).
Physical recovery techniques, based on the fact that LNAPL
hydrocarbons are relatively insoluble in and less dense than
water, are used to remove free product from a contaminated
site. Physical recovery often accounts for only 30 to 60
percent of spilled hydrocarbon before yields decline.
Continued pumping of recovery wells may be used to contain
a spill while in-situ bioremediation is being implemented. If a
spill is comprised of DNAPLs, which may sink to the bottom of
the aquifer, physical recovery may be considerably more
difficult to achieve.
Information from the performance of site characterization and
treatability studies may be integrated with the use of
comprehensive mathematical modeling to estimate the
expected rates and extent of treatment at the field scale
(Javandel, 1984; Keely, 1987). The specific model chosen
should incorporate biological reaction rates, stoichiometry of
waste transformation, mass-transport considerations, and
spatial variability in treatment efficiency (U.S. EPA, 1989a).
After assessment of site characterization and treatability
studies, if results indicate that in-situ bioremediation is
applicable to the site and will be an effective clean-up
technology, the information collected is used to design and
implement the system.
When in-situ bioremediation of a contaminant ground-water
plume involves using methods to enhance the process, such
as the addition of nutrients, additional oxygen sources, or
other electron acceptors, the use of hydraulic controls to
minimize migration of the plume during the in-situ treatment
process may be required (Thomas et al., 1987c; U.S. EPA,
1989a). In general, hydraulic control systems are generally
less costly and time consuming to install than physical
containment structures such as slurry walls. Well systems are
also more flexible, for pumping rates and well locations can be
altered as the system is operated over a period of time.
Pumping-injection systems can be used to: (1) create
stagnation zones at precise locations in a flow field; (2) create
gradient barriers to pollution migration; (3) control the
trajectory of a contaminant plume; and (4) intercept the
trajectory of a contaminant plume (Shafer, 1984). The choice
of a hydraulic control method depends on geological
characteristics, variability of aquifer hydraulic conductivities,
background velocities, and sustainable pumping rates (Lee et
al. 1988). Typical patterns of wells that are used to provide
hydraulic controls include: (1) a pair of injection-production
wells; (2) a line of downgradient pumping wells; (3) a pattern
of injection-production wells around the boundary of a plume;
and (4) the "double-cell" hydraulic containment system. The
"double-cell" system utilizes an inner cell and an outer
recirculation cell, with four cells along a line bisecting the
plume in the direction of flow (Wilson, 1984).
Well systems also serve as injection points for addition of the
materials used for enhancement of microbial activity and for
control of circulation through the contaminated zone. The
system usually includes injection and production wells and
equipment for the addition and mixing of the nutrients (Lee et
al., 1988). Atypical system in which microbial nutrients are
mixed with ground water and circulated through the
contaminated portion of the aquifer through a series of
injection and recovery wells is illustrated in Figure 4 (Raymond
et al., 1978; Thomas and Ward, 1989).
Materials can also be introduced to the aquifer through the
use of infiltration galleries (Figure 5) (Brenoel and Brown,
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To Sewer or
Red rcu late
Nutrient
Addition
Tank
Air Compressor
Coarse Sand
Production Well
Water Table—i
Water Suppl
• Injection We
Clay
Figure 4. Typical schematic for aerobic subsurface bioremediation (Thomas and Ward, 1989).
Air Compressor or
Hydrogen Peroxide
Tank
Nutrient Addition
Infiltration Gallery
Trapped Hydrocarbons
V
Monitoring Well
Water Table
Recovery Well
Figure 5. Use of infiltration gallery for recirculation of water and nutrients in in-situ bioremediation (Thomas and Ward, 1989).
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1985; Thomas and Ward, 1989). Infiltration galleries allow
movement of the injection solution through the unsaturated
zone as well as the saturated zone, resulting in potential
treatment of source materials that may be trapped in the pore
spaces of the unsaturated zone.
Amendments to the aquifer are added to the contaminated
aquifer in alternating pulses. Inorganic nutrients are usually
added first through the injection system, followed by the
oxygen source. Simultaneous addition of the two may result in
excessive microbial growth close to the point of injection and
consequent plugging of the aquifer. High concentrations of
hydrogen peroxide (greater than 10%) can be used to remove
biofouling and restore the efficiency of the system.
Operations Monitoring
Both the operation and effectiveness of the system should be
monitored (Lee et al., 1988). Operational factors of impor-
tance include the delivery of inorganic nutrients and electron
acceptor, the point of the delivery within the aquifer in relation
to the contaminated portion of the plume, and the effective-
ness of containment and control of the contaminated plume.
Measurements of dissolved oxygen and nutrient levels in
ground-water samples are recommended to assess whether
or not bioremediation is being accomplished. Increases in
microbial activities in samples of aquifer materials may be
quantified relative to plume areas prior to treatment, areas
within the plume that did not receive treatment, and control
areas outside the plume. Carbon dioxide levels in ground-
water samples may also be a useful indicator of microbial
activity (Suflita, 1989b).
Measurement of contaminant levels should indicate that
concentrations of contaminants are decreasing in areas
receiving treatment and remaining relatively unchanged in
areas that are not. If degradation pathways of specific
contaminants are known, measurement of presence and
concentrations of metabolic products may be used to
determine whether or not bioremediation is occurring. Both
soil and ground-water samples should be collected and
analyzed to develop a thorough evaluation of treatment
effectiveness. The use of appropriate control samples, e.g.,
assays of untreated areas or areas outside the plume, is
highly recommended to confirm the effectiveness of the
bioremediation technology (Suflita, 1989b).
The frequency of sampling should be related to the time
expected for significant changes to occur along the most
contaminated flow path (U.S. EPA, 1989a). Important
considerations include time required for water to move from
injection wells to monitoring wells, seasonal variations in water
table elevation or hydraulic gradient, changes in the
concentration of dissolved oxygen or alternative electron
acceptor, and costs of monitoring.
Advantages and Limitations in the Use of In-Situ
Bioremediation
There are a number of advantages and disadvantages in the
use of in-situ bioremediation (Lee et al., 1988). Unlike other
aquifer remediation technologies, it can often be used to treat
contaminants that are sorbed to aquifer materials or trapped in
pore spaces. In addition to treatment of the saturated zone,
organic contaminants held in the unsaturated and capillary
zones can be treated when an infiltration gallery is used.
The time required to treat subsurface pollution using in-situ
bioremediation can often be faster than withdrawal and
treatment processes. A gasoline spill was remediated in 18
months using in-situ bioremediation, while pump-and-treat
techniques were estimated to require 100 years to reduce the
concentrations of gasoline to potable water levels (Raymond
et al., 1986). In-situ bioremediation often costs less than other
remedial options. The areal zone of treatment using
bioremediation can be larger than with other remedial
technologies because the treatment moves with the plume
and can reach areas that would otherwise be inaccessible.
There are also disadvantages to in-situ bioremediation
programs (Lee et al., 1988). Many organic compounds in the
subsurface are resistant to degradation. In-situ
bioremediation usually requires an acclimated population of
microorganisms which may not develop for recent spills or for
recalcitrant compounds. Heavy metals and toxic
concentrations of organic compounds may inhibit activity of
indigenous microorganisms. Injection wells may become
clogged from profuse microbial growth resulting from the
addition of nutrients and oxygen.
In-situ bioremediation is difficult to implement in low-
permeability aquifers that do not permit the transport of
adequate supplies of nutrients or oxygen to active microbial
populations. In addition, bioremediation projects require
continuous monitoring and maintenance for successful
treatment.
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