NOV 41994
United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada OK 74820
Research and Development
EPA/600/SR-94/120 September 1994
EPA Project Summary
Evaluation of Technologies for
In-Situ Cleanup of DNAPL
Contaminated Sites
Dennis G. Grubb and Nicolas Sitar
Ground-water contamination by
nonaqueous phase liquids poses one
of the greatest remedial challenges In
the field of environmental engineering.
Denser-than-water nonaqueous phase
liquids (DNAPLs) are especially prob-
lematic due to their tow water solubil-
ity, high density, and capillary forces
arising from Interfaclal tension between
the DNAPLs and water. As a result,
conventional pump-and-treat tech-
nologies have met poor success in
remediation of DNAPL-contaminated
aquifers. In certain situations, conven-
tional pump-and-treat methods may
actually extend existing contamination
into previously uncontamlnated areas.
The problems associated with current
pump-and-treat remedial approaches
have served as the Impetus to develop
alternative technologies to accelerate
In-situ DNAPL contamination remedia-
tion. This report provides a review and
technical evaluation of /n-s/fu technolo-
gies for remediation of DNAPL con-
tamination occurring below the ground
water table. Various In-situ technolo-
gies are reviewed and are evaluated on
the basis of their theoretical back-
ground, field implementation, level of
demonstration and performance, waste,
technical and site applicability/limita-
tions, and cost and availability.
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
This report assesses in-situ treatment
technologies as they pertain to the treat-
ment, mobilization, and recovery of
DNAPLs from the subsurface. It identifies
in-situ technologies that remediate
DNAPLs below the water table; second-
ary importance is placed on contaminants
dissolved in the aqueous phase. Reme-
dial options are controlled by technology
evaluation and selection, site consider-
ations, regulations, cost, extent of con-
tamination, and presence of other waste
types.
DNAPL Fate and Transport
Processes
A DNAPL is a sparingly soluble hydro-
carbon having a specific gravity greater
than that of water at a typical soil tem-
perature, usually less than 20°-25°C. The
distribution of a DNAPL within the subsur-
face results from chemical and physical
interactions among the DNAPL, pore wa-
ter, pore gases, and porous media. Four
phases can be present in the subsurface:
the gas phase (in the vadose zone); the
solid phase (rock, soil grains, soil organic
matter); the aqueous (polar) phase; and
the DNAPL (nonpolar phase). For the
DNAPL to migrate as a separate phase in
any direction, both the capillary pressure
resisting DNAPL fbw and the DNAPL re-
tention capacity of the soil must be ex-
ceeded.
The report predicts how hydrophobia
compounds will partition in a complex sub-
surface environment, and to what extent
in-situ technologies will affect partitioning.
Two classes of equilibria problems exist:
(1) those where only sparingly soluble hy-
drocarbons are present, in which the ob-
jective is to predict the evolution of the
composition of the multicomponent non-
aqueous phase liquid pool over time, con-
sidering all partitioning that may take place;
and (2) those where natural or synthetic
surfactants or hydrophilic organic solvents
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such as alcohols, ethers, ketones, amines,
nitriles are also present, in which the ob-
jective is to predict their influence on the
resulting chemical equilibria.
Injection, extraction, observation wells,
other invasive monitoring, sampling, and
remedial structures locally disrupt the
stratigraphy and therefore introduce bias.
Sampling data can often be misleading
relative to the nature and extent of con-
tamination, principally in the delineation of
DNAPL in the subsurface. Frequently the
importance of mutticomponent-multiphase
equilibria and interphase transport phe-
nomena has been ignored or underesti-
mated.
Soil heterogeneity affects DNAPL fate
and transport. The site stratigraphy af-
fects the distribution of the DNAPL in the
subsurface, and the contaminant distribu-
tion then plays a critical role in the selec-
tion of the overall approach for site
remediation. Ultimately, the success of any
passive or active in-situ technology is
largely associated with its susceptibility to
soil heterogeneities and its ability to favor-
ably alter the DNAPL properties to facili-
tate recovery or remediation.
Successful technologies also have to
be able to adapt to other site-specific con-
ditions such as depth to the water table,
depth of the contaminated zone, volume
of contaminated soil, site access, and man-
made structures.
Remedial goals often require that a
baseline aqueous contaminant concentra-
tion be attained or that in excess of 99%
of the DNAPL be treated or recovered.
This standard poses a challenge to many
technologies.
Technology Evaluation
Several of the evaluated technologies
were not originally developed for remedia-
tion of contaminated sites, much less
DNAPLs. As a result, some of the tech-
nologies have not yet been demonstrated
on DNAPLs, and, owing to their develop-
mental stage, have not been demonstrated
in the field below the water table. Some
in-situ technologies that have potential ap-
plicability to remediation of DNAPLs oc-
curring below the water table have been
demonstrated in the vadose zone only.
However, the evaluation of technologies
used to clean up contamination in the
vadose zone is not included in this report.
Also, several in-situ technologies have
been fully demonstrated only in non-envi-
ronmental applications and are currently
being adapted for environmental applica-
tions. In all cases, the applicability to re-
mediation of DNAPLs occurring below the
water table is nonetheless considered.
Biological Processes
In-situ biodegradation is a process in
which aqueous phase organic compounds
are completely or partially metabolized by
microorganisms situated in the subsurface.
Bacteria are largely responsible for the
biological transformations that occur in
porous media and are generally consid-
ered as a stationary phase either through
attachment to solid surfaces or via ag-
glomeration. These organisms convert
natural and xenobiotic organic compounds
into energy and end products and use a
portion of the organic material for cell syn-
thesis.
Metabolic processes of aerobic and
anaerobic microbial consortia are distin-
guished by the nature of carbon substrate
use, and three metabolic processes are
recognized: primary metabolism, second-
ary metabolism, and cometabolism. The
metabolic use of a compound depends on
its molecular structure, concentration, en-
vironmental conditions, bioavailability of
nutrients, presence of competing or inhibi-
tory substrates, the nature of the micro-
bial consortia and the enzymes and
cofactors they possess, and toxicity ef-
fects.
Primary metabolism of an organic com-
pound occurs when it
yields sufficient energy for cell main-
tenance and growth and
is present at concentrations large
enough to sustain the microbial
population.
Petroleum hydrocarbons are good ex-
amples of primary substrates, while com-
pounds such as ammonia can serve as a
primary energy source but not a carbon
source. Many stoichiometric relationships
describing the oxidation and reduction of
organic compounds by microbes are
known. From the stoichiometric relations,
nutrient demands can be estimated and
Monod kinetics can be used to relate the
growth and decay of the microbial consor-
tia to the degradation reactions.
Secondary metabolism describes the
use of trace organic compounds that
cannot sustain microbial growth.
Cometabolism occurs when nonspecific
microbial enzymes or cofactors biotrans-
form organic compounds that provide
insignificant energy and organic carbon
for growth. Cometabolism is one of the
major mechanisms in the transformation
of chlorinated hydrocarbons and pesti-
cides.
Electrolytic Processes t
In-situ electrolytic processes use applied
electric fields to enhance organic contami-
nant removal. The effectiveness of these
processes in soils is controlled by coupled
flow phenomena. Usually the flow results
from the presence of fluid, heat, electrical,
and chemical flow potentials; any of these
potentials may be created even though
only one driving force is applied.
Containment and Ground
Modification
Containment systems and ground modi-
fication methods are used to contain and
immobilize dissolved contaminants and,
in certain cases, DNAPLs. Containment
systems are usually placed on the periph-
ery of the contaminated area so that the
encompassed area becomes isolated from
its surroundings. Impermeable barriers and
ground-water injection/extraction systems
are examples of containment systems. The
ground modification methods are usually
confined to DNAPL source areas and im-
mobilize or neutralize the contaminants.
Stabilization/solidification (S/S), vitrification,
and permeable treatment walls are ex-
amples of ground modification. Contain-
ment and ground modification can be either
passive or active; the distinction is madi
on the required energy expenditure afte.
installation.
Immobilization of contaminants is
achieved by neutralization, precipitation,
sorption, and physical encapsulation of
the contaminants within a solidified soil
matrix. The major issues surrounding in-
situ S/S are chemical compatibility and
the durability and teachability of the treated
soil mass.
In-situ permeable treatment walls are
granular backfill walls that provide treat-
ment of dissolved contaminants but no
containment or immobilization. The com-
position of the porous backfill can pro-
mote favorable conditions for in-situ
biodegradation, precipitation, and chemi-
cal oxidation or reduction. The major is-
sues regarding in-situ permeable treatment
walls pertain to changes in ground-water
flow direction, clogging, long-term perfor-
mance, and incomplete treatment of
wastes.
Soil Washing Processes
In-situ soil washing (or fluid flushing/
flooding) relies on fluid-fluid displacement
processes to enhance contaminant re-
moval. Fluids can be injected into the po-
rous media to mobilize the resident pore
fluids, water, and DNAPL. This is done b)
a combination of physical forces that can
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be aided by favorably altering chemical
partitioning so that bulk fluid properties
change. The exact nature of the displace-
ment and the prevailing physical and
chemical behavior occurring in these sys-
tems depends on the liquid properties and
environmental conditions.
Air Stripping Processes
In-situ air stripping processes rely on
the air circulation through the subsurface
to remove volatile DNAPLs from the sub-
surface. The applications considered here,
in-situ air sparging, vacuum extraction, and
vacuum vaporizer wells, differ from con-
ventional air stripping and soil vapor ex-
traction in the vadose zone in that they
operate in both the saturated and unsat-
urated zones.
Air sparging and vacuum extraction en-
tail the injection of clean air directly into
the saturated zone. Stripping occurs in
the porous medium, and volatilized con-
taminants are recovered by vapor extrac-
tion wells nested in the vadose zone.
Vacuum vaporizer wells, or UVBs, create
water recirculation cells in the porous me-
dia. Stripping is performed "in-well," and
contaminant-laden vapors are collected at
the top of the well. Water is recycled back
into the aquifer. UVBs can also simulta-
neously recover soil vapors from the va-
dose zone.
Both processes apply to the recovery of
volatile and semi-volatile DNAPLs only.
Sparging may also result in uncontrolled
migration of DNAPL out of the treatment
zone. Enhanced biostimulation may be a
beneficial byproduct of both processes.
Both technologies are commercially avail-
able and used.
Thermal Processes
Thermal and thermally enhanced pro-
cesses deliver thermal energy into the sub-
surface: the CROW® process uses hot
water and/or low qualify steam injection;
in-situ steam enhanced extraction (SEE)
relies on high quality steam injection; and
radio frequency heating and in-situ vitrifi-
cation (ISV) facilitate heating using micro-
wave and electrical arrays, respectively.
During these processes, steam and hot
water progress through cool porous me-
dia and heat the interstitial fluids and po-
rous media. These fluid-fluid displacement
processes are analogous to liquid-liquid
displacement processes with the added
complexity of heat transfer. The contami-
nants can be recovered as vaporized
gases and as dissolved- and separate-
phase liquids.
The effectiveness of the CROW® pro-
cess and SEE is controlled by the thermo-
dynamics and hydrodynamics of hot wa-
ter and steam displacement in porous
media. Thus, the thermal properties of
both the porous media and the pore fluids
become important. The orientation and
shape of the propagating steam fronts are
governed by the matrix heterogeneities,
geometry of the aquifer, initial moisture
and boundary conditions, steam quality,
injection rates, and the ratio of buoyancy
to viscous forces. In saturated homoge-
neous isotropic porous media, the ratio of
buoyancy to viscous forces is important in
terms of gravity override and effective
sweep-out. The same principles hold for
condensation fronts propagating through
layered media, but the temperature pro-
files and fronts will be curved at layer
interfaces owing to intrinsic permeability
differences. When gravity effects are neg-
ligible, the behavior of propagating fronts
can be predicted and controlled.
Radio frequency heating achieves sub-
surface heating by using an electrode ar-
ray system to transmit electromagnetic
waves through the porous media. In-situ
moisture is converted to a steam front
that propagates through porous media thus
displacing other pore fluids, including
DNAPLs.
ISV also employs an electrode array
system but for the purposes of current
flow. Large current flows cause electrical
resistance (joule) heating of the soil to the
melting point. During this process, DNAPLs
can be volatilized and pyrolized.
The CROW®, SEE, and radio frequency
heating processes have their origins in
the enhanced oil recovery business. ISV
was developed for the S/S of wastes con-
taining radionuclides. All of these tech-
nologies have been demonstrated at the
pilot scale, but only CROW® and SEE
have been successfully demonstrated in
the saturated zone. A full-scale demon-
stration of SEE is in progress.
Results and Discussion
This study was conducted between De-
cember 1991 and May 1993. No actual
experiments were conducted. Approxi-
mately 400 references were compiled dur-
ing this study. Information was collected
from journal articles, conference proceed-
ings, vendor and manufacturer fact sheets
and literature, and federal, state, and lo-
cal agency reports and publications. The
authors also attended a number of confer-
ences to obtain information that was as
current as possible.
To supplement these sources of infor-
mation, an "In-situ DNAPL Remediation
Technology Description Questionnaire"
was developed in cooperation with EPA
personnel at the Robert S. Kerr Environ-
mental Research Laboratory. The ques-
tionnaire was sent to professionals working
in the area of DNAPL cleanup. These
questionnaires were first mailed in Febru-
ary 1992. Positive responses were fol-
lowed up with letters and personal
contacts. As the project progressed, the
correspondence was expanded.
Descriptions of the relevant in-situ tech-
nologies were then prepared. The follow-
ing aspects of each relevant in-situ
technology were evaluated: theoretical
background, field implementation, level of
demonstration and performance, applica-
bility/limitations, and cost and availability.
Several technologies have been demon-
strated.
Limitations of the Report
The technology descriptions included in
the report cannot be considered exhaus-
tive because of the following limitations:
• short time—18 months
poor literature reporting
gaps due to unavailability of infor-
mation
nature of proprietary research and/
or confidential information
• stage of development of technol-
ogy
Therefore, the expected performance of
these technologies can be difficult to in-
terpret in the context of DNAPL cleanup.
While this report can help identify po-
tentially applicable in-situ technologies for
cleanup of DNAPL-contaminated sites, it
should not be the sole basis for selecting
a technology for a particular DNAPL at a
given site. The report is not a substitute
for engineering judgement, analysis, and
design. Potential in-situ technologies must
be further evaluated by contacting tech-
nology developers and by performing
bench- and/or pilot-scale treatability tests
as necessary under site-specific condi-
tions. This is especially true for
undemonstrated technologies and for tech-
nologies whose success depends heavily
on the characteristics of the waste matrix.
Conclusions
The remediation of DNAPLs faces chal-
lenges posed by the site stratigraphy and
heterogeneity, the distribution of the con-
tamination, and the physical and chemical
properties of the DNAPL. A successful
technology has to be able to overcome
the problems posed by the site complexity
and be able to modify the properties of
the DNAPL to facilitate recovery, immobi-
lization, or degradation. In addition, the
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methodology must be adaptable to differ-
ent site conditions and must be able to
meet the regulatory goals.
Thermally based technologies are
among the most promising. Among ther-
mal technologies, SEE is probably the
most promising candidate, the CROW®
process relies on similar mechanisms;
however, it is not dear whether the injec-
tion of hot water and low quality steam
offers an advantage over SEE. Radio fre-
quency heating, which relies on in-situ
steam generation to be most effective,
has only been tested in the vadose zone.
The next group of promising technolo-
gies are the soil washing technologies be-
cause they can manipulate chemical
equilibria and reduce capillary forces. A
blend of alkalis, cosolvents, and surfac-
tants is probably the best combination for
a soil washing application, and each is
important for its own reasons; alkalis can
saponify certain DNAPLs and affect wet-
tability and sorption, cosolvents pro-
vide viscous stability and enhance solubility
and mass transfer between the aqueous
phase and the DNAPL, and surfactants
have the largest impacts on solubility and
interfacial tension reduction. Water flood-
ing is best applied in highly contaminated
areas as a precursor to these methods.
The thermal and soil washing technolo-
gies are best suited for areas that are
highly contaminated with DNAPLs. How-
ever, these techniques by themselves still
may not be able to achieve the currently
mandated regulatory cleanup standards.
Thus, consideration should be given to'
using these technologies in combination
with the technologies suitable for long-
term plume management. The bioreme-
diation techniques and permeable
treatment walls hold the best promise.
A special problem is posed by mixed
wastes, heavy metals and radionuclides
mixed with DNAPLs since recovery at the
ground surface may not be desirable in
many instances. In such instances, S/S
and vitrification are the most viable in-situ
technologies. Excluding radionuclides, in-
situ S/S is the most promising candidate
because of its broadly demonstrated ef-
fectiveness, cost, and applicability to the
saturated zone.
Dennis G. Grubb and Nicolas Sitar are with the Department of Civil Engineering,
University of California, Berkeley, CA 94720.
Stephen G. Schmelllng is the EPA Project Officer (see below).
The complete report, entitled "Evaluation of Technologies for In-Situ Cleanup of
DNAPL Contaminated Sites," (Order No. PB94-195039; Cost: $27.00; 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 Officer 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
Official Business
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