United States
Environmental Protection
Agency
Office of
Research and
Development
Office of Solid Waste
and Emergency
Response
EPA/540/S-93/501
May 1993
&EPA Engineering Issue
In Situ Bioremediation of Contaminated Unsaturated
Subsurface Soils
J.L. Sims*, R.C. Sims*, R.R. Dupont*, J.E. Matthews** and H.H. Russell*
Introduction
An emerging technology for the remediation of unsaturated
subsurface soils involves the use of microorganisms to
degrade contaminants which are present in such soils.
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 Engineering Forum as
concerns of Superfund decision makers.
The Regional Superfund Engineering Forum is a group of
EPA professionals, representing EPA's Regional Superfund
Offices, committed to the identification and resolution of
engineering issues impacting the remediation of Superfund
sites. The Forum is supported by and advises the Superfund
Technical Support Project.
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
in situ systems have been directed toward contaminants in
unsaturated subsurface soils. 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 engineering issue papers
which have been prepared in response to needs expressed
by the Engineering 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 John Matthews (405) 436-
8600 or Dr. Hugh Russell (405) 436-8612.
Background
Bioremediation of contaminated surface soils using in situ
systems, prepared bed, and above-ground bioreactors, has
been previously addressed with regard to characterization,
environmental processes and variables, and field-scale
applications (Sims et al.,1989). This paper will address
processes which are currently being utilized or are in
development to treat contaminated unsaturated subsurface
soils in place.
In situ biological remediation of subsurface soils
contaminated with organic chemicals is an alternative
treatment technology that, in certain cases, can meet the
goal of achieving a permanent cleanup at hazardous waste
sites. Use of such alternatives is encouraged by the U.S.
Environmental Protection Agency (U.S. EPA) for
implementing the requirements of the Superfund
Amendments and Reauthorization Act (SARA) of 1986.
Bioremediation of subsurface soils is consistent with the
philosophical thrust of SARA, for it involves use of naturally
occurring microorganisms to degrade and/or detoxify
hazardous constituents in the soil to protect public health and
the environment. Use of in situ subsurface bioremediation
Utah State University
Robert S. Kerr Environmental Research Laboratory,
U.S. EPA
Superfund Technology Support Center for
Ground Water
Technology Innovation Office
Office of Sofia Waste and Emergency
Response, US EPA, Washington, 0,C.
Robert S. Kerr Environmental
Research Laboratory
Ada, Oklahoma
U.S. Environmental Protection Agency
Region 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Floor
Chicago, IL 60604-3590
Walter W, Kovallck, Jr., Ph.D.
Director
Printed on Recycled Paper
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techniques in conjunction with chemical and physical
treatment processes, i.e., "treatment trains," is an effective
means for comprehensive site-specific remediation (Ross et
al., 1988; Sims, 1990). For instance, bioremediation may be
utilized to lower the concentration of organic contaminants in
a soil matrix before stabilization or solidification is used as a
remedial alternative for metals.
Bioremediation has been shown effective in reducing the
overall mass of a variety of organic contaminants. Full scale
systems have been utilized to remediate soil contaminated
with both crude and refined petroleum hydrocarbons (i.e.,
diesel fuel, gasoline), creosote, and pentachlorophenol. To
date, it has not been shown effective at removing highly
structured, highly insoluble compounds such as
polychlorinated biphenyls and dioxins.
For the purposes of this document, subsurface soil refers to
unsaturated soil within the vadose zone at depths greater
than three feet below the land surface. The vadose zone
extends from the ground surface to the upper surface of the
principal water-bearing formation (Everett et al., 1982). The
vadose zone usually consists of three to six feet of topsoil
(weathered geological materials) which gradually merges with
deeper underlying earth materials such as depositional or
transported clays or sands. In this zone, water primarily
coexists with air, though saturated regions may occur.
Perched water tables may develop at interfaces of layers
(soils having different textures) of soil having less hydraulic
conductivity. Prolonged infiltration also may result in
transient saturated conditions. In some regions, the entire
vadose zone may be hundreds of feet thick and the travel
time of constituents to ground water can be hundreds or
thousands of years. Other regions may be underlain by
shallow potable aquifers that are especially susceptible to
contamination due to short transport times and reduced
potential for pollutant attenuation by soil materials and
processes.
This document addresses specific environmental processes,
factors, and data requirements for characterizing and
evaluating the application of subsurface in situ
bioremediation, and describes selected field-scale
applications of recovery and delivery systems to enhance in
situ subsurface soil bioremediation.
Overview: In Situ Subsurface Microbial Processes
and Controlling Environmental Factors
The rate and extent of biodegradation of organic chemicals
during subsurface in situ bioremediation are influenced by
several site-specific factors. These include type and activity
of microbial populations; chemical environmental factors;
bioavailability of the target chemical(s) and other substrates
required for co-metabolism, i.e., electron donor; mass
transport of moisture, nutrients, and oxygen (the terminal
electron acceptor in aerobic metabolism); toxicity; and
stratigraphy, heterogeneity, and geochemistry of the surface
or subsurface environment. A detailed discussion of the
impact of these and other factors on bioremediation can be
found in 'Transport and Fate of Contaminants in the
Subsurface" (EPA/625/4-89/019) and "Bioremediation of
Contaminated Surface Soils" (EPA/600/9-89/073).
Microbial Populations
Successful in situ bioremediation depends on the presence of
appropriate microbial populations which can be stimulated to
degrade contaminants of concern by modifying or otherwise
managing environmental conditions at a site. Results of
microbial characterization of deep subsurface materials have
indicated that: (1) microorganisms are present at populations
sufficient to change the chemistry of the environment when
stimulated; (2) the microbial communities are diverse and
carry out a wide range of chemical transformations; (3) a
majority (>95%) of the microbes are chemotrophic bacteria
that degrade organic chemicals to obtain energy; and (4)
environmental characteristics identified previously (oxygen
concentration, nutrient status, moisture content) are
important in influencing microbial activity and degradation
patterns (Fliermans and Hazen, 1990).
Microbial communities in the subsurface are diverse and
adaptable. Microbial populations at older sites are usually
acclimated to the contaminants of concern. Therefore, levels
of critical nutrients or electron acceptors, toxicity, and
adverse environmental conditions most often are the major
factors which limit the extent and rate of in situ
bioremediation.
Critical Environmental Conditions
There are several environmental conditions that affect activity
of soil microorganisms. These factors, along with individual
soil and waste characteristics, all interact to affect microbial
activity at specific contaminated sites. Many of these
conditions can be managed to enhance biodegradation of
organic constituents in subsurface soils. Optimum ranges for
the most critical of these factors are presented in Table 1.
Water content of soil is an important factor which regulates
microbial activity. Soil water serves as the transport medium
through which many nutrients and organic constituents
diffuse to the microbial cell, and through which metabolic
waste products are removed. Soil water also affects soil
aeration status, nature and amount of soluble materials,
osmotic pressure, pH of the soil solution, and unsaturated
hydraulic conductivity of the soil (Paul and Clark, 1989). The
water content of deeper subsurface soils may vary greatly.
Unsaturated soil samples have been obtained even from
cores collected below the water table in deep subsurface
environments, and the low water content was shown to
adversely affect microbial activity (Kiett et al., 1990).
Biodegradation rates often depend on the rate at which
terminal electron acceptors can be supplied. A large fraction
of the microbial population within soils are aerobes which use
oxygen as the terminal electron acceptor. Oxygen can be
easily depleted in subsurface soils where there is an oxygen
demand due to plant root respiration or due to normal
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Table 1. Critical environmental factors for mlcroblal activity (Sims et al., 1984; Huddleston et al., 1986; Rochkind and
Blackburn, 1986; Paul and Clark, 1989)
Environmental Factor
Optimum Levels
Available soil water
Oxygen
Redox potential
pH
Nutrients
Temperature
25%-85% of water holding capacity; -0.01 MPa
Aerobic metabolism: Greater than 0.2 mg/l dissolved
oxygen, minimum air-filled pore space
of 10%;
Anaerobic metabolism: 02 concentrations <1%
Aerobes and facultative anaerobes: greater than
50 millivolts; Anaerobes: less than 50 millivolts
5.5-8.5
Sufficient nitrogen, phosphorus, and other nutrients so not
limiting to microbial growth
Suggested C:N:P ratio of 100:10:1
15°C-45°C (Mesophiles)
microbial activity throughout the depth of the unsaturated
zone. Oxygen levels tend to decrease in soils having high
clay and organic matter content. Clayey soils tend to retain
higher moisture content, which restricts oxygen diffusion,
while organic matter may increase microbial activity and
deplete available oxygen. Under these circumstances,
oxygen may be consumed faster than it can be replaced by
diffusion from the atmosphere, and the soil may become
anoxic.
Facultative anaerobic organisms (which can use oxygen or
alternative electron acceptors such as nitrate or sulfate in the
absence of oxygen) and obligate anaerobic organisms then
become the dominant populations under such conditions.
The sequence of use of various electron acceptors is
determined by the redox potential and the electron affinity of
the electron acceptors present (Zehnder and Stumm, 1988).
The potential of alternative electron acceptors has been
evaluated with nitrate at field scale for contaminants
(including benzene, toluene, and xylene) in an aquifer
environment (Hutchins et al., 1991).
Redox potential also affects metabolic processes in
subsurface microbial populations (Paul and Clark, 1989).
Redox potential provides a measurement of electron density
and is related to the oxygen status of a subsurface soil. As
oxygen is removed and a system becomes more reduced,
there is a corresponding increase in electron density,
resulting progressively in an increased negative potential.
Soil pH affects growth and activity of subsurface soil
microorganisms. Fungi are generally more tolerant of acidic
soil conditions (below pH 5) than bacteria. Solubility of
phosphorus, a critical nutrient in biological systems, is
maximized at a pH value of 6.5. A specific contaminated soil
system may require management of soil pH to achieve levels
that maximize microbial activity. Control of pH to enhance
microbial activity may also aid in the immobilization of
hazardous metals in a subsurface soil system (a pH level
greater than 6 is recommended to minimize metal transport).
Subsurface soil pH may be managed through addition of an
aqueous phase containing pH adjusting chemicals through
gravity delivery systems such as infiltration galleries or
surface irrigation systems.
Microbial metabolism and growth depends upon adequate
supplies of essential macro- and micro-nutrients. Critical
nutrients such as nitrogen and phosphorous must be present
and available to microorganisms in: (1) usable form; (2)
appropriate concentrations; and (3) proper ratios (Dragun,
1988). If wastes are high in carbon (C), and low in nitrogen
(N) and phosphorus (P), biodegradation will cease when
available N and P are depleted. Therefore, fertilization of
subsurface soils may be required as a management
technique to enhance microbial degradation.
Soil temperature affects microbial growth and metabolic
activity. Biodegradation rates decrease as temperature
drops and essentially cease at temperatures below 0° C.
While surface soils exhibit both diurnal and seasonal
variations in temperature, changes of temperature decrease
with depth. Generally, only the top 30 feet of the subsurface
profile are affected by seasonal variations in temperature;
temperature is generally constant and corresponds to the
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mean annual air temperature of the locality (Kuznetsov et al.,
1963; Matthess, 1982). In the United States, temperatures in
this zone range from 3°C to 25° C (Dunlap and McNabb,
1973). Due to the high specific heat of water, wet soils are
less subject to larger diurnal changes than dry soils (Paul and
Clark, 1989).
Bioavailability is a general term which refers to the
accessibility of contaminants by degrading populations.
There are two major components involved: (1) a physical
aspect related to phase distribution and mass transfer
limitations of the contaminant, and (2) a physiological aspect
related to the suitability of the contaminant as a substrate.
Major factors which affect bioavailability include water
solubility and sorption. Target chemicals may occur in one or
more of the four phases comprising the subsurface soil
environment: (1) soil solids, including organic matter and
inorganic sand, silt, and clay particles; (2) soil water; (3) soil
gas; and (4) often a nonaqueous phase liquid (NAPL). In
general, chemicals that distribute to the water phase (more
soluble) are more bioavailable than chemicals that either sorb
strongly to solid phases or occur in a NAPL phase. NAPLs
are generally degraded from the waterNAPL interface inward
since the aqueous phase contains nutrients, oxygen, and
moisture required for microbial life processes. The
bioavailability of a NAPL phase may be increased by
increasing the surface area to volume ratio of NAPL
elements. This increases mass transfer of nutrients,
moisture, and oxygen; and decreases toxicity by decreasing
interfacial concentrations (Symons and Sims, 1988).
Substrate chemicals in the gas phase have also been found
to be bioavailable (Dupont et al., 1991; Miller et al., 1991).
Generally, chemicals that are highly sorbed, such as high
molecular weight PAHs present in creosote, petroleum, and
manufactured town gas plant wastes, are found to be
degraded at slower rates than chemicals that are only slightly
sorbed. Since the majority of the mass of target constituents
at many contaminated sites is associated with NAPL and/or
solid phases, these represent the greatest challenge with
regard to in situ bioremediation.
Bioavailability is also a function of the biodegradability of the
target chemical, i.e., whether it acts as a substrate,
cosubstrate, or is recalcitrant. The target chemical may be
physically available (i.e., water soluble and/or not sorbed to
solids) but not useful as a metabolic substrate.
Contaminants of concern may not be the dominant organic
substrate in a system. When the target chemical cannot
serve as a substrate (source of carbon and energy) for
microorganisms, but is oxidized in the presence of a
substrate already present or added to the subsurface, the
process is referred to as cooxidation and the target chemical
is defined as the cosubstrate (Keck et al., 1989; Sims et
al.,1989). Cooxidation processes are important for the
biodegradation of high molecular weight polycyclic aromatic
hydrocarbons (PAHs), and some chlorinated solvents,
including trichloroethylene (TCE). Contaminants with
complex molecular structures or high degrees of toxicity may
not be degradable, and may persist or be recalcitrant under
aerobic conditions. Examples of recalcitrant compounds
include highly oxidized halogenated compounds such as
polychlorinated biphenyls (PCBs), pesticides such as
toxaphene, and dioxin contaminants present in wood-
preserving wastes.
The toxicity of the environment may be reduced by
decreasing the concentration of a toxic waste (e.g., creosote)
or chemical (e.g., pentachlorophenol) within one or more
subsurface phases. Concentrations of toxic chemicals in the
gas phase may be reduced through soil vacuum extraction; in
the water phase through soil flushing; in the NAPL phase
through soil flushing with water containing viscosifiers, or with
solvents or surfactants; and in the soil solid phase by
inducing partitioning of contaminants from solid to fluid
phases. All mobile phases in the subsurface have potential
for escape; therefore, containment strategies are often
necessary while the constituents within the phase are
biodegraded.
Heterogeneity of the subsurface environment limits the rate
and extent of in situ bioremediation. Restrictive layers (e.g.,
clay lenses), although more resistant to contamination, are
also more difficult to remediate due to poor permeability and
low rates of diffusion. Clay soils have larger porosities than
silty or sandy soils and therefore larger storage capacities for
contaminants, but have greater resistance to fluid flow
including aqueous, gas, and NAPL phases. Also clay layers
with poor hydraulic conductivity are less permeable to
nutrients and oxygen. In sites that have substantial clay and
silt deposits, more permeable soils will become preferential
conduits for remedial fluids, and the clay/silt deposits will
require much longer time frames for remediation. For
example, heterogeneity of the subsurface with respect to soil
layering and chemical parameters at a gas works site in the
United Kingdom presented constraints on the feasibility of
utilizing in situ bioremediation (Thomas et al., 1991).
Enhancement of In Situ Subsurface Bioremediation
The method of enhancing in situ bioremediation efforts
depends on the four phases in which contaminants can
occur, heterogeneity of subsurface matrix, and the types of
delivery and recovery systems utilized. Removing limiting or
controlling factors and establishing favorable conditions are
the primary goals of recovery and/or delivery systems.
Enhancement may be achieved by increasing bioavailability;
reducing toxicity; increasing delivery of moisture, nutrients,
and oxygen; and/or by introducing substrates that stimulate
indigenous microbial degradative activity.
A variety of strategies may be implemented to maximize
biodegradation activity in contaminated subsurface soils. The
success of in situ bioremediation efforts is often determined
by the effectiveness of the recovery and delivery systems
used to remove major sources of contaminants and to
transport nutrients and electron acceptors to the location of
the remaining contaminants. Establishing optimum levels of
essential nutrients and electron acceptors at specific
subsurface locations is often driven by physical limitations of
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the subsurface matrix on transport of fluids (liquids or gases)
used to deliver these amendments. Overcoming these
limitations is the primary goal of a delivery system, and the
development of adequate delivery technologies continues to
be the major challenge of in situ bioremediation. A summary
of delivery and recovery techniques commonly used to
manage subsurface remediation is provided in Table 2.
Making the Saturated Zone Unsaturated
Advantages of Unsaturated Systems
Because hydraulic conductivity is a function of soil moisture
content, changing a saturated soil into an Unsaturated soil
greatly reduces the hydraulic conductivity and therefore the
downward transport of chemicals in the water phase to the
ground water. Also, because oxygen diffuses through air
10,000 times faster than through water, an Unsaturated
environment may be maintained in an aerobic condition more
easily than a saturated environment in the presence of
oxygen-demanding chemicals (Table 3). Soil pore space that
contains a gas phase also allows removal of volatile contam-
inants (via soil vacuum extraction) in a direction that is away
from the ground water. Therefore, management of a site to
change the saturated zone to an Unsaturated condition may
reduce potential for ground-water contamination as well as
enhanced oxygen delivery to stimulate in situ biodegradation.
Physical Containment
There are a variety of approaches to establishing and
maintaining dewatered conditions. In order to adequately
dewater the subsurface, it is often necessary to physically
isolate the treatment zone. Impermeable subsurface barriers
can prevent the migration of ground water by preventing
uncontaminated water from entering the contaminated site
and stopping contaminated water from leaving. Extraction
systems or drains must then be used to remove the ground
water to create an unsaturated zone.
Commonly used barriers include slurry walls, grout curtains,
and sheet piling cutoff walls to retard the flow of water under
and through a site (Devinny et al., 1990).
Ground-Water Removal
Ground-water removal can be accomplished by hydraulic
pumping and/or drainage trenches. Hydraulic pumping using
a well-point system is one such technique (Devinny et al.,
1990) using short lengths of plastic or Teflon well screen
placed in the saturated zone.
Ground water can also be removed using subsurface drains
or drainage ditches. Subsurface drains are constructed by
excavating a trench to the desired depth, partially backfilling
the trench with highly permeable sand or gravel, placing a
plastic or ceramic drain tile in the sand and gravel bed, and
completing the backfilling (Devinny et al., 1990).
Drainage ditches or surface drains are similar to subsurface
drains except that no collection pipes or tiles and backfills are
used. They may be used at sites underlain by poorly
permeable soils (Devinny et al., 1990).
Recovery and Delivery Technologies for Subsurface
Bioremediation
Recovery and delivery technologies are those that facilitate
transport of materials either out of or into the subsurface
(Murdoch et al., 1990). Recovery technologies are primarily
utilized for contaminant source reduction. High levels of
contamination present as either trapped residuals or NAPLs
can severely limit success of bioremediation attempts.
Therefore, removal of as much of this initial contaminant
mass as possible is a prerequisite to in situ bioremediation
efforts.
Specific recovery and delivery technologies for enhancing in
situ bioremediation of subsurface soils are identified in
Table 2. Each identified technology is discussed below with
regard to its applications and limitations, and current status.
Recovery Technologies
The principal recovery technologies used for subsurface
remediation depend on the ability to move fluids. Also
involved is the ability to move contaminants by altering their
solubility or sorption characteristics (Murdoch et al., 1990).
These techniques are used to move materials from the
subsurface soil environment in order to enhance in situ
bioremediation by addressing one or more limiting factors
identified in Tables 1 and 2, including: soil vacuum extraction,
soil flushing, steam stripping, and radio frequency heating.
Soil vacuum extraction
Soil vacuum extraction (SVE) (also referred to as subsurface
or forced air venting, in situ air stripping, or soil vapor
extraction) involves the removal of contaminants carried in
the soil gas phase by reduction of the vapor pressure within
the soil pores by applying a vacuum. As clean air is drawn
through the soil, the contaminants are removed. This
process is driven by concentration differences between solid,
aqueous, and NAPL phases and the clean air that is
introduced through the soil vacuum extraction process.
Vacuum extraction is most applicable to sites contaminated
with highly volatile compounds, such as those associated with
gasoline and solvents (e.g., perchloroethylene,
trichloroethylene, dichloroethylene, trichloroethane, benzene,
toluene, ethylbenzene, and xylene).
Important soil characteristics that should be measured or
estimated to determine the feasibility of vacuum extraction at
a specific site include physical factors that control the rate
and extent of air flow through contaminated soil, and
chemical factors that determine the amount of contaminant
that partitions from soil to air. These factors include: bulk
density (weight per volume); total porosity (void spaces
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Table 2. Management strategies for addressing factors limiting In situ bioremediation of subsurface soils
Limiting Factor
Management Response
Delivery or Recovery Technique
Bioavailability limited due to NAPL
Reduce NAPL mass
Gravity or forced delivery; Soil flushing,
Steam stripping, Hydraulic fracturing
Bioavailability limited by sorption or slow
mass transport through soil matrix
Reduce sorption, Increase mass transport Soil flushing, Steam stripping, Hydraulic
fracturing
Moisture
Add water or water saturated air
Gravity or forced delivery; Bioventing, Cyclic
pumping
Nutrients
Add nutrients in water or as ammonia gas Gravity or forced delivery; Bioventing, Cyclic
pumping
Oxygen/Redox
Toxicity
pH
Add air
Remove chemicals
Adjust soil pH
Bioventing, Hydraulic fracturing, Cyclic
pumping, Radial drilling, Kerfing
Soil vacuum extraction, Soil flushing, Steam
stripping
Gravity or forced delivery
Temperature
Increase temperature
Radio frequency heating, Steam stripping
Substrate Addition
Add in water or air
Gravity or forced delivery; Bioventing,
Hydraulic fracturing
Heterogeneity
Add or withdraw material in more restrictive
layers
Cyclic pumping, Hydraulic fracturing, Radial
drilling, Kerfing
Table 3. Carrier fluid oxygen supply requirements (Dupont et a)., 1991)
Carrier
g Carrier/g O2
Water
Air Saturated
Pure O2 Saturated
500 mg/L H2O2 (100% Utilization)
110,000
22,000
2,000
Air (20.0% 02)
13
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between soil grains) and air-filled porosity (that portion of the
total porosity filled with air); diffusivity of volatiles (amount of
volatiles which move through an area over time); soil
moisture content (percentage of void spaces filled with
water); air phase permeability (ease with which air moves
through soils); texture; structure; mineral content; surface
area; temperature; organic carbon content; heterogeneity;
depth of air permeable zone; and depth to water table
(Metcalf & Eddy, Inc., 1991). Soils at sites where vacuum
extraction is used should be fairly homogeneous and have
high permeability, porosity, and uniform particle-size
distributions (Metcalf & Eddy, Inc., 1991). Soil vapor
transport can be severely limited in a soil with high bulk
density, high soil water or high NAPL content, low porosity,
and low permeability. In heterogeneous soils, air flows
preferentially through more permeable zones, leaving less
permeable zones untreated.
Contaminant characteristics that affect the feasibility of
vacuum extraction include the extent and degree of
contamination, vapor pressure, Henry's law constant,
aqueous solubility, diffusivity, and partition coefficients. Due
to the high solubility of many organic contaminants in NAPL
phases, the presence of NAPL in subsurface soil systems
may significantly affect the distribution of the compounds in
various phases, and their fate in SVE systems. Specific
contaminant and soil conditions that determine the feasibility
of vacuum extraction are presented in Table 4.
The efficiency of a vacuum extraction system can be
enhanced in several ways. For example, a system of air
injection wells can be installed at the perimeter of a
contaminated area (Metcalf & Eddy, Inc., 1991) which can
be connected to air blowers to force air into the soil or
remain open to the atmosphere. Use of air injection wells
can result in increased soil air flow rates and a larger area
through which clean air can move.
Pulsed pumping may be used to give contaminants time to
desorb from solid surfaces, diffuse from restricting layers,
and volatilize from residual saturation (NAPL) in the soil pore
space. Using pulsed pumping for recovery of contaminants
allows a lower volume of air with higher concentrations of
contaminants to be recovered.
If ground water is at or near the zone of soil contamination,
water table rise may occur due to reduced air pressure near
extraction wells (Metcalf & Eddy, Inc, 1991). Ground-water
Table 4. Conditions affecting feasibility of use of vacuum extraction (U.S. EPA, 1990; Metcalf & Eddy, Inc., 1991)
Condition
Favorable
Unfavorable
Contaminant:
Dominant form
Vapor pressure
Water solubility
Henry's Law Constant
Soil:
Temperature
Air conductivity
Moisture content
Composition
Surface area of soil matrix
Depth to ground water
Vapor phase
> 100 mm of mercury
<100mg/l
>0.01 (dimensionless)
>20°C (usually will require external heading
of soils)
>10-" cm/s
<10% (by volume)
Homogeneous
<0.1 m2/gof soil
>20m
Solid or strongly sorbed to soil
<10mm of mercury
>1,000mg/l
<0.01 (dimensionless)
<10°C (common in northern climates)
<10~6 cm/s
>10% (by volume)
Heterogeneous
>1.0m2/gof soil
<1 m
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pumping may be used to counteract the water table rise, as
well as to expose additional contaminated soil that can be
treated by vacuum extraction.
Horizontal extraction wells (wells drilled parallel to ground
surface) have been used for deep subsurface contamination
at the U.S. Department of Energy Savannah River facility to
access larger areas of the contaminated site (Hazen,1992).
This use of horizontal wells may be a means to reduce costs
associated with deep subsurface remediation since only a
single hole may be required to access contaminated areas
instead of many vertical wells.
The performance of a vacuum extraction system is monitored
by system operational characteristics and by treatment
efficiency characteristics (Metcalf & Eddy, Inc., 1991).
System characteristics include strength of vacuum applied,
air flow rate, and contaminant concentrations and moisture
content in the vented gas. Wells are used to monitor
pressure in the contaminated area. Efficiency of treatment is
monitored by soil gas analyses, and soil core analyses to
determine residual concentration of contaminants. For more
detailed discussions of soil venting evaluation, see
"Evaluation of Soil Venting Application" (EPA/540/S-92/004).
Since soil vacuum extraction is an in situ treatment technique
that requires only addition of ambient air to the subsurface, it
can be applied with little disturbance to existing facilities and
operations (Metcalf & Eddy, Inc., 1991). SVE can be used at
sites where areas of contamination are large and deep, or
when the contamination is beneath a building. The system
can be easily modified, depending on additional analytical
and subsurface characterization data and/or changing site
conditions. Even if vacuum extraction can be implemented at
a site, most of the conditions listed in Table 4 must be met, or
the cost and time for cleanup will be prohibitive.
The use of SVE at remedial sites has been reviewed by the
U.S. EPA (1989a) and classified as a developed technology
for remedial applications. It is currently the most commonly
used in situ remedial technology (Murdoch et al., 1990). Soil
vacuum extraction may be used to reduce toxic concen-
trations of contaminants to levels which are more conducive
to bioremediation. In addition, it will also deliver oxygen to
the subsurface which is required by aerobic bacteria.
So/7 flushing
In situ soil flushing is used to accelerate movement of
contaminants through unsaturated materials by solubilizing,
emulsifying, or chemically modifying the contaminants. A
treatment solution is applied to the soil and allowed to
percolate downward and interact with contaminating
chemicals. Contaminants are mobilized by the treatment
solution and transported downward to a saturated zone
where they are captured in drains or wells and pumped to the
surface for recovery, treatment, or disposal (Murdoch et al.,
1990). In combination with bioremediation, the flushing
solution may be amended with nutrients to enhance biological
activity (Metcalf & Eddy, Inc, 1991).
Treatment solutions are delivered to the contaminated zone
by using either gravity or forced methods. Forced delivery
consists of various pumping techniques. Gravity delivery
methods include surface flooding, ponding, spraying, ditching
and subsurface infiltration beds and galleries (Amdurer et al.,
1986). Barriers, such as slurry walls, may be required to
prevent the transport of contaminants away from the site
(Metcalf & Eddy, Inc. 1991). A ground-water extraction
system must be used to capture the flushing solution and
associated contaminants, in some cases, the flushing
solution may be treated to remove the contaminants and
reused, and in others it may require disposal.
Efficiency of soil flushing is related to two processes: the
increase in hydraulic conductivity that accompanies an
increase in water content of unsaturated soil, and the
selection of treatment solutions with regard to the
composition of the contaminants and the contaminated
medium. The hydraulic conductivity of soils decreases
markedly with decreases in water content; therefore, the flow
of liquids through unsaturated soils is extremely slow and the
recovery of contaminants by conventional pumping
techniques is not possible. With soil flushing, the water
content and consequently the hydraulic conductivity of the
soil is increased (Murdoch et al., 1990). However,
heterogeneities in soil permeability may result in incomplete
removal of contaminants.
At sites where water-soluble contaminants are present, water
can be used to flush or mobilize the contaminants (Metcalf &
Eddy, Inc., 1991). Surfactants can be added to increase the
mobility of hydrophobic organic contaminants, such as oils
and petroleum. Examples of other flushing solutions include:
acidic aqueous solutions (for the removal of metals and basic
organic constituents including amines, ether, and anilines),
basic solutions, chelating agents, oxidizing agents, and
reducing agents. Toxicity of flushing solutions to soil
microorganisms should be considered when followed by
bioremediation of residual contamination. The flushing
solution may change physical and chemical properties of the
soil environment that affect bioremediation potential.
The level of treatment that will be achieved is dependent on
selection of an appropriate flushing solution, extent and time
of contact between the solution and waste constituents, soil
partition coefficients of the waste constituents, and the
hydraulic conductivity of the soil (Metcalf & Eddy, Inc., 1991).
Soil flushing is not applicable to soils with low hydraulic
conductivities (e.g., less than 1 ft/day), or for contaminants
that are strongly sorbed to the soil (e.g., PCBs, dioxin).
Soil flushing has been classified by the U.S. EPA as a
developed technology used for recovery in remedial
applications (Murdoch et al., 1990). Although the technology
has been tested at field-scale, soil flushing has not yet been
used extensively in large-scale clean-up operations. As with
SVE systems, soil flushing may be utilized with bioremed-
iation as a coupled technology. Soil flushing may initially be
utilized to lower toxic or extreme concentrations of contam-
inants to a manageable level for biological processes which
-------
may be utilized as a polishing step to remove those
contaminants which were not removed through the flushing
process. If biological processes are used during or after soil
flushing, the compatibility of the soil flushing solution with
subsurface bacteria must always be considered.
Delivery Techniques
The major limiting factor to the bioremediation of amenable
compounds is the delivery of required nutrients, co-oxidation
substrates, electron acceptors or other necessary enhancers
of microbial growth. Delivery techniques are used to add
required materials to the subsurface environment to enhance
in situ bioremediation by addressing one or more limiting
factors identified in Tables 1 and 2. A variety of delivery
techniques are in use or are being developed (Figures 1-3).
These include soil venting, gravity and forced hydraulic
delivery, hydraulic fracturing of low permeability zones, radial
drilling, and cyclic pumping. Of these, only gravity and forced
hydraulic delivery and venting systems are in common use at
sites. The other three approaches are still in developmental
stages.
In Use: Gravity/Forced Hydraulic Delivery and
Bioventing
Gravity and Forced Hydraulic Delivery
Irrigation technologies were among the first utilized for
enhancing in situ biodegradation. Gravity methods are used
to deliver water and amendments to the contaminated
subsurface by applying the solutions directly over the
contaminated area. Applied solutions then percolate
downward through the subsurface to contaminated zones.
Application methods consist of both surface and subsurface
spreading (Amdurer et al., 1986).
Surface application methods include flooding, ponding,
ditches, and sprinkler systems. These methods are generally
applicable to contamination at depths less than 15 feet.
Flooding is a surface application method in which the solution
is spread over the land surface in a thin sheet. Flooding is
applicable to sites that are flat or gently sloped (i.e., less than
3 percent slope), uniform, without gullies or ridges, and have
soils with high hydraulic conductivities (i.e, greater than 10"3
cm/sec; such as those found in sands, loamy sands, and
sandy loams).
Ponding can be used to increase the infiltration rate of the
applied solution above that achieved by flooding. Ponds are
constructed by excavating into the ground or by constructing
low berms. The depth of the solution in the pond becomes
the driving force to increase infiltration rates. Ponding can be
used in sandy or loamy soils and in flat areas.
The ditch method of surface spreading utilizes flat-bottomed,
shallow, narrow ditches to transport the solution over the land
surface; allowing for infiltration of the solution into the ground
through both bottom and side surfaces. Gradients in the
ditches are kept small to prevent erosion as well as to allow
residence time for infiltration. Ditches may be constructed by
excavating surface materials or by building small
embankments. Ditches are used at sites where it is not
desirable to completely cover an entire area with the solution.
\ .flGravel Bed ^E:.;
Figure 1. Schematic of a sprinkling system used to deliver nutrients to contaminated subsurface soil.
-------
Extracted Air
Injected Air
Extracted Air
Top Soil
Figure 2. Schematic of a bioventing system designed to deliver air to contaminated subsurface soil.
^Contaminated
Figure 3. Schematic of a ponding system used to deliver nutrients to contaminated subsurface soil.
10
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Sprinkler systems can be used to deliver solutions uniformly
and directly to the ground surface. These systems are less
susceptible to topographical constraints than flooding and
ponding. Sprinkler systems have been used successfully to
deliver nutrients and moisture to bioventing systems where
the site was contaminated to a depth of 50 feet (Dupont et
al., 1991).
Subsurface gravity delivery systems include infiltration
galleries (or trenches) and infiltration beds. These systems
are applicable to sites where there is deep contamination or
where the surface layers have low permeability. Subsurface
systems consist of excavations filled with a porous medium
(e.g., coarse sands or gravels) that distribute solutions to the
contaminated area. An infiltration gallery consists of a pit or
trench that is filled with gravel or stones. The solution fills the
pores in the gallery and is distributed to the surrounding soils
in both the vertical and horizontal directions. This system is
most applicable to sites with sandy or loamy soils. In sites
with silty soils, an infiltration gallery can be used but
application rates will be reduced. Solutions can be
introduced into the gallery by injection at locations along the
length of the gallery or through perforated distribution pipe.
Infiltration galleries can be used in sites with steep slopes
(i.e., up to 25 percent slope) and uneven terrain. Infiltration
beds are similar to galleries but are wider and contain more
than one perforated distribution pipe. Infiltration occurs
almost entirely through the bottom, with little infiltration
through sidewall surfaces. This system is applicable to soils
with sandy and loamy textures, but limited to sites where the
topography is relatively flat (i.e., with less than 5 percent
slope) and the terrain is even. Beds can saturate larger
areas than a single trench and are easier to install than a
multi-trench system.
Forced systems deliver fluids under pressure into a
contaminated area through open end or slotted pipes that
have been placed to deliver the solution to the zone requiring
treatment (Amdurer et al., 1986). These systems are
generally applicable to soils with hydraulic conductivities
greater than 10~4 cnVsec (i.e., fine sandy or coarse silty
materials) and high effective porosities (i.e., ranging from 25
to 55 percent). A maximum injection pressure must be
established to prevent hydraulic fracturing and uplift in the
subsurface, which would cause the fluid to travel upward
rather than through the contaminated area. Unlike gravity
systems, a forced delivery system is theoretically
independent of surface topography and climate.
Design considerations for gravity and forced delivery systems
are presented in Amdurer et al. (1986). Application of gravity
delivery systems in subsurface bioremediation systems has
been demonstrated in bioventing systems (Dupont et al.,
1991; Miller etal., 1991). In Russia, methane-oxidizing
bacteria grown in fermenters have been injected into lateral
core holes in a coal mine (Fliermans and Hazen, 1990). This
process has been shown to reduce methane concentration in
the air by 50-60 percent in one month, thus reducing the risk
of explosions and fire.
So/7 Bioventing
Soil bioventing incorporates soil vacuum extraction processes
to deliver oxygen to the subsurface to enhance in situ
bioremedialion of organic contaminants. The large amounts
of oxygen-saturated water required for bioremediation often
cannot be delivered due to hydraulic conductivity limitations.
For example, benzene and hexane, which are common
hydrocarbon contaminants, require more than 3 g O2 per g of
hydrocarbon for mineralization. Soil bioventing is applicable
to remediation of contaminants of low volatility and can also
reduce concentrations of volatile contaminants in off-gases,
thus reducing the amount of contaminants requiring off-gas
treatment.
To accomplish bioventing, soil vacuum extraction processes
are operated at lower than usual air flow rates to reduce
vapor extraction quantities and maximize vapor retention
times. Soil moisture levels necessary for biological activity
are usually higher than those recommended for optimum
vacuum extraction operations. The addition of nutrients may
also enhance bioremediation. Nutrient addition can be
accomplished by surface application, incorporation by tilling
into surface soil, and transport to deeper layers through
applied irrigation water. Increased soil temperatures have
been shown to enhance biodegradation rates in bioventing
systems (Miller et al., 1991). Possible means of increasing
soil temperature include the use of heated air, heated water,
or low-level radio-frequency heating. High temperature
should be avoided, since this can result in decreased
microbial populations and/or activity.
Soil bioventing has been demonstrated in several field
applications (Dupont et al., 1991; Hinchee et al., 1991;
Hoeppel et. al., 1991; Miller et al., 1991; van Eykand
Vreeken, 1991; Urlings et al., 1991). At Hill Air Force Base in
Utah, a JP-4 jet fuel spill occurred in January 1985 that
resulted in the contamination of approximately 0.4 hectares
(1 acre) to a depth of approximately 50 feet with approxi-
mately 25,000 gallons of JP-4 (Dupont et al., 1991). Soil total
petroleum hydrocarbon (TPH) concentrations at the site were
as high as 15,000 mg/kg, with average TPH levels of 1,500
mg/kg. Site soil consists of mixed coarse sand and gravel
deposits with interspersed, discontinuous clay stringers to a
confined ground-water table located approximately 600 feet
below ground surface. Prior to initiating a full-scale vacuum
extraction project, the fuel tanks were excavated,
refurbished, and installed in an above-ground concrete
cradle. Excavated soil was placed in a pile and subjected to
vacuum extraction.
An SVE system consisting of 15 wells in the undisturbed soil
and 10 wells in the excavated soil pile and under the tanks
was installed to provide access to the contaminated soil and
allow flexibility in the operation of the venting system. The
system was operated in a conventional mode to maximize
the recovery of volatile components of the JP-4 through
volatilization. Venting was initiated on December 18, 1988, at
a rate of 1,270 ft3/hr (approximately 0.04 pore volumes/day),
11
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and gradually increased to approximately 74,000 ft3/hr
(approximately 2.5 pore volumes/day) as the hydrocarbon
levels in the vent gas decreased over time. The venting rate
during the start-up period was limited by the operating
conditions of the catalytic incinerator used to treat the
collected vent gas. This high-rate operating mode was
maintained from December 18, 1988, through September 15,
1989 with approximately 340 pore volumes (245 x 106 ft3) of
soil gas extracted from the site.
In situ respiration tests conducted during the high-rate SVE
operating period indicated that significant respiration was
occurring without nutrient or moisture addition, and that
enhancement of biodegradation might be possible under
modified site management conditions. Biodegradation was a
significant removal mechanism during the initial high-rate
venting, accounting for 15 to 25 percent of the recovered
hydrocarbon. To assess the potential for enhancing
biodegradation rates, a series of laboratory and field
biotreatability studies were conducted to evaluate moisture
and nutrient additions. The effect of SVE system operational
parameters on biodegradation rates was also evaluated by
decreasing air flow rates and increasing flow path length.
A number of in situ respiration tests were conducted during
the field studies to assess the impact of different engineering
management options on microbial activity. A total of three
tests were conducted to monitor the effect of different
management approaches,including: (1) flow rate and
operating configuration modifications , (2) moisture addition,
and (3) moisture and nutrient addition. Biodegradation
reactions were estimated based on cumulative oxygen
consumption and carbon dioxide production. All
biodegradation calculations were normalized to background
CO2 and O2 concentrations so that the effects of field
management techniques could be isolated from changes in
background respiration taking place during the study.
The results of these studies indicated that moisture addition
and operational modifications significantly enhanced
biodegradation rates. Based on analyses of O2 uptake rates,
moisture addition (35% to 50% field capacity) was shown to
statistically accelerate in situ respiration at the site. However,
nutrient addition generally did not statistically increase the
degradation rates of residual JP-4 constituents. The
operational modifications (reduced air flow rate, increased
path length) significantly improved biodegradation rates. Fuel
removal due to biodegradation increased to greater than 80
percent, resulting in an additional 12,000 Ib of total petroleum
hydrocarbons being degraded during the bioventing portion of
the study. Initial hydrocarbon (on a carbon equivalent basis)
removal rates of 70 Ib/day were maintained at an average
rate of greater than 100 Ib/day following system operating
modifications.
Soil bioventing was also investigated at Tyndall Air Force
Base in Florida to remediate sandy soils contaminated by
past jet fuel storage activities (Miller, 1990; Miller et al.,
1991). Hydrocarbon concentrations in the soil ranged from
30 to 23,000 mg/kg. The contaminated area was dewatered
prior to system installation. The impact of moisture and
nutrient addition was investigated during a 7-month period.
Moisture addition had no significant effect on biodegradation
rate in this system. Nutrient addition also did not affect
biodegradation rate, since naturally occurring nutrients were
present in adequate quantities to support the amount of
biodegradation observed. Biodegradation rates were shown
to be affected by soil temperature and followed predicted
rates based on the van't Hoff-Arrhenius equation. Fifty-five
percent removal was attributed to biodegradation during the
period of study, but a series of flow rate tests showed that
biodegradation could be increased to 85 percent by
decreasing air flow rates. The optimal air flow conditions were
found to be the removal of 0.5 air flow volumes per day. The
contaminated gas phase was drawn through clean soil to
increase gas residence time within the soil. This augmented
in situ biodegradation and eliminated the need for off-gas
treatment as well as reducing exposure to off-gas.
Research: Hydraulic Fracturing, Radial Drilling
Research areas are focusing on methods to increase the
capacity of current systems to deliver increased
concentrations of required solutions to the subsurface. Two
of these systems are discussed below.
Hydraulic fracturing
Hydraulic fracturing is a technique that involves using
hydraulic pressure to induce cracking in rock or clay/silt
lenses in the vicinity of a borehole, which develops a larger
framework of interconnected pore space. The newly created
pore space is filled with solid, granular materials, which can
act as permeable channels to increase the rate and area of
delivery of fluids containing nutrients or oxygen to the
subsurface (Murdoch et al., 1990; Murdoch et al., 1991;
Davis-Hoover et al., 1991). The hydraulic fractures may be
filled with granules of slow-dissolving nutrients or oxygen-
releasing chemicals, which may provide a reservoir of these
compounds for the enhancement of bioremediation. This
technique could also potentially be used in recovery systems,
e.g., by increasing extraction of vapor phases in soils with low
permeabilities, or by forming horizontal sheet-like drains to
capture leachates in soil flushing systems.
Hydraulic fracturing has been successfully utilized in
petroleum engineering in many types of geologic materials,
ranging from granite to poorly consolidated sediments. For
remedial applications, it has been demonstrated in soft clay
soils at shallow depths, but has not yet been demonstrated in
a wide range of soils or at waste sites. For use in remedial
applications, hydraulic fracturing has been classified by the
U.S. EPA as an emerging technology (i.e., research on its
use is in progress) (Murdoch et al., 1990).
Radial drilling
Radial well technology consists of drilling horizontal wells
radially outward from a central borehole. This enhances
access to a contaminated subsurface environment by
12
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increasing the volume serviced by each vertical well
(Murdoch et al., 1990). Radial wells can be placed at the
same level or on multiple levels in the same borehole. The
use of horizontal wells allows access to fracture zones that
are perpendicular to the ground surface and allows
contaminated areas to be entered laterally rather than
vertically.
Radial wells have been installed in both consolidated rock
and unconsolidated materials (Murdoch et al., 1990). In
unconsolidated formations, drilling rates range from 5 to 120
ft/min, while in very hard, homogeneous basalt, rates range
from 0.10 to 0.50 ft/min. For use in remedial applications,
radial well drilling has been classified by the U.S. EPA as an
emerging technology (i.e., research on its use is in progress)
(Murdoch et al., 1990).
Waste, Soil, and Site Information Requirements for
Evaluation and Management of In Situ
Bioremediation
Adequate site characterization including: surface and
subsurface soil characteristics, hydrogeology, and
microbiological characteristics, serve as the basis for rational
design of any subsurface soil bioremediation system. A
thorough site characterization is necessary to determine both
the three-dimensional extent of contamination as well as
engineering and management constraints which may limit the
rate and extent of remediation. Specific characterization
information regarding waste, soil, and hydrogeology is
required in order to assess the potential effectiveness of
bioremediation. Specific waste characterization information
required includes the relative aerobic biodegradability of
waste chemicals under optimum conditions. Important
hydraulic, physical, and chemical properties of soils that
affect the behavior of organic constituents in the vadose zone
are presented in Sims et al. (1989). Subsurface soil
characterization information required includes identification of
limiting soil environmental factors identified in Table 1.
Required site characterization information includes
identification of potential limiting factors with regard to relative
ease of delivery and recovery listed in Table 2.
Based upon waste, subsurface soil, and site characterization
information, appropriate containment strategies need to be
considered for the mobile contaminant phases associated
with the subsurface (Figure 1). Naturally occurring
containment may be sufficient with regard to preventing
escape of mobile phases under existing site conditions.
However, other containment strategies may need to be
considered if materials are to be added or removed from the
subsurface to stimulate microbial activity. These may include
volatiles removed in vacuum extraction, water used to add
oxygen and nutrients, or NAPLs if soil flushing is carried out.
For each chemical (or chemical class), the information
required is summarized as: (1) characteristics related to
potential leaching, e.g., water solubility, octanol/water
partition coefficient, solid sorption coefficient; (2) volati-
lization, e.g., vapor pressure, relative volatilization index;
(3) Henry's Law Constant; (4) potential biodegradation, e.g.,
half-life, degradation rate, biodegradability index; and (5)
chemical reactivity, e.g., hydrolysis half-life, soil redox
potential (Sims et al., 1984; Sims et al., 1989).
Information from waste and site characterization studies, and
laboratory evaluations of biodegradation may be integrated
by using appropriate mathematical models to predict: (1) the
potential for bioremediation of and (2) the potential for cross-
contaminating other media (i.e., ground water under the
contaminated area, atmosphere over the site or at the site
boundaries, surface waters, etc). The models used will be
highly dependent on site characteristics and contaminants of
interest. These may range from "back-of-the-envelope"
calculations to sophisticated fate and transport computer
models.
Mass Balance Approach to In Situ Subsurface
Bioremediation
Successful subsurface bioremediation depends upon
thorough characterization and management of each
subsurface phase with regard to containment, stimulation of
microbial activity, and monitoring strategies. The chemical
mass balance approach provides a framework for evaluating,
managing, and monitoring subsurface soil bioremediation
(Sims, 1990). Mass balance helps obtain specific information
that is needed to determine fate and behavior, evaluate and
select management options for in situ bioremediation, and
monitor treatment effectiveness for specific chemicals in
specific subsurface phases. The information needed to
construct a mass balance for subsurface contamination
simultaneously addresses site characterization and
biodegradation rates.
A necessary first step in mass balance requires
characterizing each phase present in the subsurface (Figure
1) with regard to location, amount, and heterogeneity of the
subsurface environment to assess which chemicals are
associated with which phase(s). This information allows
determination of the relative bioavailability of chemicals. For
example, chemicals associated with aqueous and gas phases
are generally more bioavailable than chemicals associated
with solid and NAPL phases. In addition, chemicals
associated with aqueous and gas phases are more prone to
migration. This information also allows determination of the
need for containment by defining where contamination is
migrating under the influence of natural processes. The
problem can be defined in the context of mobility versus
biodegradation for chemicals. Is the rate of biodegradation
(either natural or enhanced) such that chemicals which are
prone to leaching or volatilization degrade before either
occurs? Using mathematical models or other tools,
chemicals can be ranked in order of their relative tendencies
to leach, volatilize, or remain in-place under subsurface site-
specific conditions. Containment and management options
can then be selected that address specific escape and
attenuation pathways. For example, SVE may be
appropriate as a managerial tool to remove highly volatile,
biologically recalcitrant chemicals from soil before switching
13
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to a bioventing mode to remove less volatile, easily
biodegraded compounds. Specific waste phases may be
addressed at specific times during bioremediation. Finally,
comprehensive monitoring programs can be designed to
track specific chemicals in specific phases in the subsurface
at specific times.
After a phase is contained through natural or managed
processes, techniques to enhance microbial activity may be
applied. Monitoring strategies can then be designed to
ensure that the rate and extent of biodegradation within each
phase, as well as transfer of chemicals between phases, are
measured. Biodegradation rates of organic compounds in
soil systems are generally measured by monitoring their
disappearance in a soil through time. Rates of degradation
are often expressed as a function of the concentration of one
or more of the constituents being degraded. This is
accomplished by measuring at specific time intervals the
concentration of contaminants of interest (in the medium of
interest, i.e., soil phase, gas phase, etc.), through a properly
designed sampling and analysis plan. This sampling and
analysis plan should be statistically valid and provide
sufficient information to determine the rate of disappearance
of contaminants of interest or appropriate surrogates, such
as petroleum hydrocarbons (TPH). Care should be taken to
ensure that transfer or partitioning of contaminants from one
phase to another is not misinterpreted as biodegradation
within the source phase. Abiotic losses such as volatilization
and leaching must be defined in order to accurately
determine biodegradation rates. Identification of metabolic
transformation products is also necessary since metabolites
may be more mobile or toxic than the parent compounds. In
addition, measuring only for parent compounds and not
metabolites may tremendously overestimate extent of
biodegradation. In addition, identification of metabolites is
warranted when known daughter products are toxic.
Recommendations
There is currently a lack of information concerning some
aspects of in situ bioremediation of subsurface soils. Specific
areas where additional information is required include site
characterization with regard to effects of physical, chemical,
and hydrologic properties on microbial distribution, numbers,
and activity. Field research to obtain these types of
information is currently limited; however, this information is
required in order to estimate the feasibility of bioremediation
for subsurface contamination. Implementation of subsurface
remediation is currently limited to a significant extent by the
difficulty of establishing adequate systems for delivery and
recovery of chemicals for augmenting biological activity. As
research continues, these difficulties may be overcome as
more information becomes available concerning the
applicability of innovative technologies in the remediaiton of
contaminated soil.
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