United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada OK 74820
Research and Development
EPA/600/S2-87/008 Apr. 1987
SERA Project Summary
Leaking Underground Storage
Tanks: Remediation with
Emphasis on In Situ
Biorestoration
J. M. Thomas, M. D. Lee, P. B. Bedient, R. C. Borden, L. W. Canter, and
C. H. Ward
The current literature indicates that
in situ biorestoration has great potential
for remediation of aquifers contami-
nated by leaking underground storage
tanks. In situ aquifer restoration involves
natural microbial action and the en-
hancement of the indigenous microf lora
to more efficiently degrade subsurface
pollutants. Aquifers amenable to en-
hanced biorestoration are usually those
that are perfusable with solutions that
carry the nutrients to the zone of con-
tamination. The presence of naturally
occurring microorganisms that can
degrade subsurface contaminants has
been demonstrated. However, a period
of adaptation is usually required before
the subsurface microflora can degrade
pollutants. Factors that may limit de-
gradation, even in the presence of
adapted organisms, include lack of an
essential nutrient, substrate concentra-
tion, substrate inaccessibility, pH, tem-
perature, and the presence of toxicants.
Biorestoration of contaminated aquifers
often involves the addition of limiting
nutrients such as oxygen, nitrogen, and
phosphorus. Enriching for microor-
ganisms with special metabolic cap-
abilities has been demonstrated in the
laboratory and may be applicable to
some in situ biorestoration schemes.
Mathematical models of biorestoration
have been developed to simulate pro-
gress of the cleanup and provide in-
formation on the kinetics of the process.
The full report emphasizes the state-of-
the-knowledge of In situ biorestoration
techniques available for remediation of
contaminated ground water.
This Project Summary was developed
by EPA's Robert S. Kerr Environmental
Research Laboratory, Ada, OK, to an-
nounce key findings of the research
project that is fully documented In a
separate report of the same title (see
Project Report ordering Information at
back).
Introduction
One ground-water pollution source
category which has recently received in-
creased attention is underground storage
tanks, specifically, the inadvertent leakage
of liquid products from such tanks into
the unsaturated and saturated zones. This
document was requested by the U.S.
Environmental Protection Agency (EPA)
Office of Underground Storage Tanks
(OUST) to aid in planning a program for
restoring ground water contaminated by
leaking underground storage tanks and is
intended to guide OUST in making judge-
ments regarding the efficacy of alternative
technologies for remedial action The full
report specifically focuses on the use of
innovative biological technologies to
remove or attenuate contaminants.
Remedial action technologies for leak-
ing underground storage tanks can be
considered in relation to preventive
measures and contaminant plume man-
agement Preventive measures include
corrosion prevention, proper installation
practices, material selection, and con-
tainment systems. Plume management
techniques may include one or several of
the following:
(1) ground water pumping to extract
water from or inject water into wells
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to capture a plume or alter the
direction of ground-water move-
ment;
(2) subsurface drains consisting of
permeable barriers designed to in-
tercept ground-water systems;
(3) vertical underground barriers made
of low-permeability materials to
divert ground-water flow or mini-
mize leachate generation and plume
movement; and
(4) innovative technologies that bio-
logically or chemically remove or
attenuate contaminants in the
subsurface.
The most recent and innovative tech-
niques for aquifer remediation involve
stimulating the indigenous microflora to
degrade subsurface pollutants. Stimula-
tion of the native microorganisms can
result in the complete destruction of the
contaminants whereas chemical or
physical treatment may result in incom-
plete destruction or transfer of the con-
taminant to another phase of the
environment. Biostimulation is often
achieved by adding limiting nutrients such
as oxygen, nitrogen, and phosphorus. The
addition of specialized microbial popula-
tions to degrade subsurface pollutants
has been incorporated into many remedi-
ation projects; however, the role of these
microbes in biodegradation has not been
distinguished from that of the indigenous
microflora.
Results and Discussion
Remediation/Restoration Plume
Management Techniques
This full report initially presents a
cursory review of conventional contain-
ment and treatment techniques Physical
containment is reviewed relative to re-
moval and to ground-water flow barriers
such as slurry walls, grout curtains, sheet
piling, block displacement and liners.
Surface water control techniques such
as caps, dikes, terraces, channels, chutes,
downpipes, grading, vegetation, seepage
basins, and ditches are also discussed A
simple review of passive and active
hydrodynamic controls used to manipulate
the hydraulic gradient of ground-water
systems is also presented Withdrawal
and treatment techniques are reviewed
in terms of conventional wastewater
treatment processes.
In Situ Biological Treatment
The major emphasis of the full report
concerns in situ biological treatment
which is discussed in detail. Microbial
processes may be used to degrade con-
taminants in situ by stimulating the native
microbial populations. Another in situ
biostimulation technique which is not yet
demonstrated is the inoculation of the
subsurface with a microbial population
that has specialized metabolic capabilities.
Even in the presence of an indigenous
population which is acclimated to organic
contaminants, degradation may be limited
at high contaminant concentrations or by
some environmental factor. Addition of
electron acceptors, such as oxygen, and
inorganic nutrients, typically nitrogen,
phosphorus, and trace metals, may pro-
vide the microflora with essential
nutrients that are limiting in the presence
of high concentrations of pollutants.
Inoculation of a specialized microbial
population may theoretically reduce the
time required for acclimation of the con-
taminants and/or allow the removal of
recalcitrant contaminants. Related pro-
cesses such as the addition of bioemul-
sifiers or surfactants to increase the
availability of subsurface contaminants
to the microflora can also be used. When
applicable, biological processes may offer
the advantage of partial or complete
destruction of the contaminants rather
than simply transferring the pollution to
another phase of the environment.
The full report goes into significant
detail concerning microbial activity in
aquifers. In summary, the subsurface
environment contains microbes that
degrade many of the organic compounds
that contaminate ground water. The sub-
surface microorganisms in uncontami-
nated aquifers are likely to be adapted to
use low concentrations of organic
materials. The majority of the micro-
organisms are associated with soil par-
ticles Even in the presence of adapted
populations, environmental factors such
as temperature, pH, dissolved oxygen
levels, inorganic nutrient concentrations,
and the availability and concentration of
the organic contaminants may limit bio-
degradation of subsurface pollutants.
A number of advantages and disad-
vantages exist in using in situ biorestora-
tion. Compounds ranging from petroleum
hydrocarbons to solvents have been
treated by in situ biorestoration. Unlike
many aquifer remediation techniques, in
situ bioreclamation can often treat con-
taminants that are sorbed to soil or
trapped in pore spaces. In addition to
treatment of the saturated zone, organics
held in the unsaturated and capillary
zone can be treated when an infiltration
gallery or soil flushing is used. Biode-
gradation in the subsurface can be en
hanced by increasing the concentratior
of dissolved oxygen, through the use o
hydrogen peroxide, ozone, or a colloida
dispersion of air (colloidal gas aphrons)
Complete biodegradation (mineralization
of organic compounds usually produce:
carbon dioxide, water, and an increase ir
cell mass. However, incomplete degrada
tion (biotransformation) of organic mate
rials can sometimes produce by-products
that are more toxic than the pareni
molecule. In situ biorestoration relies or
a microflora that contains few pathogenic
organisms. The time required to treal
subsurface pollution using in situ bio-
restoration can often be faster than some
withdrawal and treatment procedures,
since with current technology not all con-
tamination can be physically withdrawn.
In situ biorestoration can also cost less
than other remedial options. The areal
zone of treatment using biorestoration
can be larger than other remedial tech-
nologies because the treatment moves
with the plume and can reach areas that
are otherwise inaccessible.
Advantages as well as disadvantages
of biorestoration are described through-
out the full report. Major advantages and
disadvantages discussed include:
Advantages -
(1) can be used to treat hydrocarbons
and certain organic compounds,
especially water-soluble pollutants
and low levels of other compounds
that would be difficult to remove by
other methods;
(2) environmentally sound because it
does not usually generate waste
products and typically results in
complete degradation of the
contaminants;
(3) utilizes the indigenous microbial
flora and does not introduce po..~..
tially harmful organisms;
(4) fast, safe and generally economical;
(5) treatment moves with the ground
water; and
(6) good for short-term treatment of
organic contaminated ground water.
Disadvantages -
(1) can be inhibited by heavy metalj
and some organics;
(2) bacteria can plug the soil and reduce
circulation;
(3) introduction of nutrients couk
adversely s4'1""* -o^'b-.1 surfac«
waters;
(4) residues may cause taste and odo
problems;
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(5) labor and maintenance require-
ments may be high, especially for
long-term treatment;
(6) long-term effects are unknown; and
(7) may not work for aquifers with low
permeabilities that do not permit
adequate circulation of nutrients.
No technique to remediate environ-
mental contamination is universally
applicable. However, there should be
many incidents of contamination where
biorestoration through a forced co-oxida-
tion is the technology of choice, either
alone or in conjunction with physical
containment. Successful application of
the approach will require an adequate
understanding of the physiology of the
co-oxidation biotransformation, and
quantitative information on the nutri-
tional ecology of the active organisms.
Hydrologlc Considerations and
Mathematical Modeling of
Biorestoration
The full report considers a number of
methods which have been reported in
the literature for containment of con-
taminated ground water through hydrau-
lic control or through injection-pumping
networks of wells. Biorestoration of a
contaminant plume may involve the addi-
tion of nutrients such as dissolved oxygen
or hydrogen peroxide or the addition of
microbes capable of degrading a particular
waste. In order for such additions to be
successful, it may be necessary to use
hydraulic controls to minimize the migra-
tion of the plume during the in situ
treatment process. Thus, hydrologic con-
siderations cannot be neglected in the
biorestoration of aquifers. Hydraulic con-
trols for the containment of ground water
should be carefully considered for any
site where biorestoration is a viable
treatment alternative. In particular, in-
jection-pumping well networks offer ad-
vantages for the creation of stagnation
(no-flow) zones or for the control of the
trajectory of a contaminant plume. Once
the plume has been controlled hydrau-
lically, then application of additional
nutrients, oxygen, or microbes can be
better controlled and evaluated in terms
of biodegradation efficiency.
Mathematical modeling of biorestora-
tion processes is useful in simulating
cleanup progress and can provide insights
into the kinetics of the restoration process.
Modeling of the hydraulics of the site
may also aid in designing the optimum
injection and production system. Develop-
ment of mathematical models of the bio-
restoration process requires: 1) a
description of the kinetics of biodegrada-
tion/transformation in the subsurface; 2)
a description of the abiotic processes
controlling the transport and availability
of the contaminant and other required
nutrients; and 3) an appropriate procedure
for combining the processes and pre-
dicting the effect of the biorestoration
technique. Most attempts at quantifying
the transport and removal of contami-
nants in ground water have relied on a
solution of the classical form of the
advection/dispersion equation. This ap-
proach is inappropriate for many applica-
tions. Biorestoration models and the
kinetics of biodegradation are considered
in some detail in the full report. Many
references and examples of various
aspects of biorestoration modeling are
given.
The current technology for simulating
subsurface biorestoration is still in its
infancy. Some progress has been made
in developing kinetic descriptions of the
biodegradation process and combining
these with available solute transport
models. Unfortunately, little reliable field
data has been available to rigorously test
these models. Considerable uncertainty
exists over the importance of simulating
transport into biofilms or microcolonies.
Also, the effects of variations in aquifer
parameters on the efficiency of bio-
restoration is unknown. At present, the
technology is not available to quantita-
tively predict the efficiency of enhanced
biorestoration but significant advances
are being made in our ability to describe
the process.
Institutional Limitations on
Ground-Water Pollution Control
The development and application of
technologies for the prevention and
regulation of leaking underground storage
tanks is a complex and interdisciplinary
science, as is the abatement and cleanup
of the ground-water contamination such
leaks produce.' This science is further
complicated, however, by the institutional
limitations that control the rate at which
technology can advance and its use in
the development of regulations. Institu-
tional limitations may be the factor in
determining how the technology will be
applied to leaking underground storage
tank regulations and remediation. Institu-
tional limitations addressed in the full
report include: (1) the scientific under-
standing of the nature of leaking under-
ground storage tanks and released
products; (2) public opinion; (3) business
community attitudes; (4) environmental
interest group concerns; and (5) govern-
mental acceptance, use, and implementa-
tion of the remedial technology.
Conclusions
Of the available biological aquifer
remediation techniques, the most effec-
tive methods are enhancement of the
native population and withdrawal and
treatment by various wastewater treat-
ment processes. Before any aquifer
remediation technique can be imple-
mented, a thorough understanding of the
hydrogeology and contamination prob-
lems of the site must be obtained and
used to design the treatment system.
Costs for treatment may range from tens
of thousands up to tens of millions,
depending upon the extent and nature of
the contaminants, the nature of the site,
and the desired cleanup levels.
Addition of oxygen, nitrogen, phos-
phorus, and trace minerals stimulates
the acclimated indigenous microbial
population to aerobically degrade the
contaminants. In situ biorestoration has
been chiefly used to treat gasoline con-
taminated aquifers, but also has been
employed with ethylene glycol and sol-
vents including acetone, tetrahydrofuran,
methylene chloride, n-butanol, dimethyl
aniline, and isopropanol. Biorestoration
effectiveness will be affected by toxic
levels of organics and heavy metals. In
general, in situ bioreclamation has been
effective in reducing the quantity of the
contaminants, but not in completely
eliminating them. The treatment moves
with the plume allowing treatment of
trapped or sorbed contaminants or, by
using soil flushing or an infiltration gal-
lery, microbial treatment can reach areas
that are not accessible by other tech-
niques. Biorestoration has been used in a
number of aquifers, but may be of limited
usefulness in those with low permeabili-
ties. Undesirable metabolic and inorganic
nutrients may escape from the treatment
zone and affect ground water or surface
water quality. Alternative oxygen sources
such as ozone, hydrogen peroxide, pure
oxygen, and air flooding or soil venting
may speed the removal of the organic
contaminants, but their impact on the
microbial population and the geochemis-
try of the site is not fully understood.
Innovative processes such as treatment
beds or land treatment can be used in
some situations. In the presence of an
acclimated microbial population, many
aquifers will be anaerobic because the
microorganisms will have depleted the
dissolved oxygen. It will be possible to
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use anaerobic degradation to remove
contaminants, although the technology
for this treatment has not yet been
developed. Reducing the interfacial ten-
sion between the hydrocarbon and ground
water with surfactants, dispersants, or
emulsifiers will mobilize the contaminants
and may make them available for micro-
bial degradation. Combinations of in situ
biorestoration treatment with other
chemical, physical, or biological treatment
processes have been successfully utilized
in aquifer remediation.
Treatment by biological wastewater
processes is a proven technology. The
biological processes include activated
sludge, lagoons, waste stabilization ponds,
fluidized bed reactors, trickling filters,
rotating biological discs, and sequencing
batch reactors. All of these processes are
dependent upon extraction of the con-
taminated ground water from the subsur-
face. Combinations of conventional
wastewater treatment processes and
other water treatment processes have
also been successful.
Techniques for simulating the subsur-
face biorestoration process are under
development, but little reliable field data
has been generated that can be applied
to these models. Some of the major con-
siderations in simulating transport and
biodegradation of organic contaminants
in the subsurface are poorly understood;
these include the importance of transport
of organics to the bacteria and the vari-
ability in aquifer parameters.
Recommendations
As biorestoration develops as a re-
mediation technology, many technical
research needs can be identified. Exam-
ples of these needs include:
- Can microbial degradation processes
in the vadose zone be optimized
through the controlled addition of
nutrients, enzymes, and bacterial
seed organisms?
What are the optimum nutrient con-
centrations for achieving in situ
biorestoration of different classes of
organic compounds?
What are the optimum environmental
conditions (pH, oxidation-reduction
potential, micronutrients, etc.) neces-
sary to achieve in situ biorestoration
of different classes of organic
compounds?
Can laboratory development of ac-
climated microorganisms enhance in
situ biorestoration, and what labor-
atory tests/procedures are necessary
to achieve this acclimation?
- What design approaches andA
laboratory tests can be used to opt
mize microbial population densitit
for in situ biorestoration?
- What are the best methods fc
achieving in situ mixing of the bai
terial populations, nutrients an
micronutrients, and organics?
- What are the optimum (most cos
effective) combinations of in sit
biorestoration and interdiction wel
and surface treatment to achiev
ground-water remediation?
- Can we predict clean up efficiencie
of biorestoration processes usin
mathematical models that incorporai
rate coefficients for target pollutan
that are determined in site specif
aquifer materials?
J. M. Thomas, M. D. Lee. P. B. Bedient, R. C. Borden. and C. H. Ward are
with National Center for Ground Water Research, Rice University, Houston,
TX 77251; and L. W. Canter is with National Center for Ground Water
Research, University of Oklahoma, Norman. OK 73019.
Jerry N. Jones and Marion R. Scalf are the EPA Project Officers (see below).
The complete report, entitled "Leaking Underground Storage Tanks: Remediation
with Emphasis on In Situ Biorestoration," (Order No. PB 87-168 084/AS;
Cost: $18.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officers can be contacted at:
Robert S. Kerr Environmental Research Laboratory
U.S. Environmental Protection Agency
Ada. OK 74820
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
; " /o-f; j .^-c j _ ^ ^ 1
Official Business
Penalty for Private Use $300
EPA/600/S2-87/008
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