United States
Environmental Protection
Agency
Solid Waste and
Emergency Response
(5102G)
EPA 542-N-98-003
February 1998
Issue No. 28
CONTENTS
Heating Technologies and
SVE in Hydraulic Fractures
to Remove Hydrocarbon
Fuels page 1
In Situ Thermal Blankets
and Wells for PCB Removal
in Tight Clay Soils page 2
In Situ Electroremediation
of Chromate-Contaminated
Soil page 3
Feasibility Study on
Electrokinetic Processing
Systems
page 3
The Applied Technologies
Newsletter for Superfund
Removals & Remedial
Actions & RCRA
Corrective Action
This issue highlights in situ
remediation technologies
using various forms of
electrokinetics and
electro-heating.
TECH TREWS
in
to
by Kathy Balshaw-Biddle, AATDF
Recent field testing under the Advanced
Applied Technology Development Facility
(AATDF) program at Rice University
showed effective removal of JP-8 jet fuel
from tight clay soils through the use of
hydraulic fracturing, steam injection or
electro-heating, and soil vapor extraction
(SVE). Soil heating reduced total petroleum
hydrocarbons (TPH) concentrations to less
than 250 mg/kg, which is below target
levels. This remediation technology project
is one of 12 that have been field-tested
through the U.S. Department of Defense
AATDF program.
The test site is located adjacent to a tank
farm on the Robert Gray Army Airfield, Fort
Hood, TX. Underlying soil consists of
argillaceous limestone and calcareous clay
with some oyster shells. Shallow ground
water in the area is unconfined and flows
through clay fractures. Hydraulic conduc-
tivity in soil samples ranges from 3.3 x 10"8
to 2.1 x 10"9 cm/sec. Separate phase
hydrocarbons were identified up to 4.8 feet
in thickness in monitoring wells prior to
remediation TPH concentrations in soil
samples before treatment were as high as
35,500 mg/kg, and concentrations greater
than 1,000 mg/kg occurred consistently
between the depths of 10-20 feet.
The initial remediation strategy included
hydraulic fracturing followed by steam
injection in three 25 x 25-foot test cells.
The fractures were mapped using soil
borings, and the heating and SVE wells
were installed to intersect the fracture. Field
evaluation determined that the radial size
and lateral continuity of the fractures were
not sufficient in two of the three test cells to
allow effective steam injection. The original
strategy was then modified to include a
comparison of electro-heating [alternating
current (AC)] and steam injection in the two
moderately fractured cells.
Comparison of the pre-fracture and post-
fracture permeabilities in the soil was
affected negatively by heavy rains. Pre-
fracture vent testing on relatively dry clay
soil indicated an air permeability of
approximately 4.9 x 10"9 cm2. Post-fracture
vent testing occurred after five intervening
months of unexpectedly heavy rains during
which the water table rose 8-10 feet and
had to be pumped back down in the test
cells. The post-fracture air permeability of
the water-saturated clay soils was approxi-
mately 2.9 x 10"9 cm2, probably related to
hydrationof the clay soil.
Steam-enhanced hydrocarbon recovery at
the Fort Hood site produced maximum soil
temperatures of 180-200°F. (Soil tempera-
ture generally depends on steam injection
pressure (typically 5-20 psi), which is a
function of the soil type and depth of the
target zone.) Injection of steam into the
hydraulically fractured clay transferred heat
to the soil via both convection along
fractures and intersecting discontinuities,
and conduction to clays adjacent to the
fractures. Latent heat of condensation
from the steam increased the vapor
pressure and volatilization rate of volatile
and semi-volatile constituents of the JP-8
jet fuel. Increased temperatures also
enhanced fluid mobility of semi-volatile
constituents. The system produced a large
volume of vapors, mobile TPH, and
condensate that were removed using SVE
and ground water extraction.
The electro-heating technology used at Fort
Hood employed a combination of
multiphase electric heating and SVE
extraction, with six electrodes and
electrodic irrigation (water drip into the
electrode's carbon matrix to maintain soil
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moisture) installed in a hexagonal array to
deliver 3-phase AC power to the soil. The 3-
phase current consisted of three voltage
sources having the same amplitude and
frequency but displaced from each other by
120 degrees. Heat in the AC cells was not
dependent upon SVE operation or dewater-
ing, but occurred more slowly than in the
steam cell. Heating was less effective in
areas where inadequate hydraulic fracturing
hampered both steam injection and water
extraction from the soil.
Overall, preliminary mass removal rates in
the steam-heating cell were an order of
magnitude higher than in the electrode-
heating cell. The AATDF is issuing a final
report on this demonstration in early 1998.
For more information, contact Kathy
Balshaw-Biddle (AATDF) by fax at 713-
285-5948. Project information is available
on the AATDF internet site
(www.ruf.rice.edu/~AATDF/index.htm).
in
far
in
by Pauletta France-Isetts,
EPA Region 7
A recent field demonstration conducted at
the Missouri Electric Works (MEW), Cape
Girardeau, MO, Superfund site indicated
that a new in situ thermal desorption (ISTD)
process can reduce concentrations of
polychlorinated biphenyls (PCBs) in tight
clay soils to non-detect levels of less than 33
ppb. The ISTD process employed thermal
blankets to remove contaminants from soil at
shallow depths (0-18 inches) and thermal
wells for removal at lower depths. The PCB
(Aroclor 1260) destruction and removal
efficiency (ORE) for both tests at the MEW
site was greater than "six nines" (>99.9999%).
Both the ISTD-thermal blanket and ISTD-
thermal well technologies employ a
fundamental process involving heat flow,
fluid flow, phase behavior, and chemical
reactions (as shown in Figures 1 and 2). In
each form of the ISTD process, heat is
applied to soil from a high-temperature
surface in contact with the soil, thereby
allowing for effective radiation and thermal
conduction heat transfer near the heat
source, and thermal conduction and
convection in the bulk of the soil volume.
Thermal conduction accounts for over 80%
of the heat transfer. A very high temperature
(>1,000°F) is created near the heat source,
which causes rapid destruction of the
contaminants before they exit the soil.
(Aroclor 1260, which was removed at the
MEW site, has a boiling point range of 720-
780°F, the highest of all PCBs.)
The ISTD-thermal blanket (Figure 3) is an
8-foot by 20-foot steel box covering 160
square feet of 100-kw heating rods that raise
near-surface soil temperatures to 1,400-
1,600°F. Thermal monitoring devices
(thermocouples) are installed at the ground
surface and at various depths to monitor
spatial heating of the treatment area.
Multiple blankets are installed next to one
another to treat large areas. Heating
elements are covered by a 12-inch-thick
thermal insulation layer, which reduces
upward heat loss to less than 10%. A
barrier is installed over the insulated
blanket frame and vapors are collected.
High temperatures near the heating
elements convert the majority of contami-
nants to CO2 and water vapor.
The ISTD-thermal well process utilizes an
array of heater/vacuum wells emplaced
vertically in the ground in triangular
patterns. The wells are equipped with high-
temperature electric heaters and connected
to a vacuum blower. As heat is injected and
soil temperatures rise (to 1,400-1,600°F near
the wells and >1,000°F between the wells),
the vaporized formation fluids, including
contaminants, are collected by the vacuum
drawn at the wells. Off-gases are treated in
surface facilities to remove residual contami-
nants that have not been destroyed in situ.
At the MEW site, two heater blankets were
emplaced and 12 heater/vacuum wells were
constructed in a multiple triangular array with
a 5-foot well spacing to a depth of 12 feet. The
demonstration consisted of soil heating and
vacuum extraction of vapors for 42 days.
Temperatures above 1000°F were achieved in
the interwell regions. PCB concentrations in
the treated area were reduced from a maxi-
mum concentration of 20,000 ppm to less than
33 ppb. Peak and continuous emission
monitoring by a mobile process unit ensured
that the discharge of PCBs and combustion
byproducts resulting from treatment complied
with ambient air requirements. The off-gas
treatment system consisted of a flameless
thermal oxidizer with >99.99% ORE, followed
by two carbon beds in series.
Hydrogen chloride (HC1) stack emissions,
which are byproducts of PCB conversion,
were used to monitor stack emissions and
indicate when the remediation process was
complete. Although the majority of HC1
reacted at high temperatures with the iron
and carbonate minerals in the soil, lime-
stone was used in treatment carbon beds as
an additional scrubber. Emission stack
sampling by EPA methods demonstrated
that the discharge of PCBs and combustion
byproducts complied with state and federal
ambient air requirements.
For more information on the MEW demonstra-
tion, contact Pauletta France-Isetts (EPA
Region 7) at 913-551-7701.
Figure 1: ISTD-ThermnI Blankets
Silt/Thermal tor (0-18")
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EPA initiated a 3-part study on the feasibil-
ity of using electrokinetic processing
systems to remediate radioactive and
hazardous mixed wastes. The first part of
the study resulted in a resource document
entitled, "Electrokinetic Laboratory and
Field Processes Applicable to Radioactive
and Hazardous Mixed Waste in Soil and
Groundwater from 1992 to 1997" (publica-
tion # EPA-402-R-97-006). The document
lists and describes all published work on
electrokinetic remediation conducted
between 1992 and 1997. This work
includes electrokinetic remediation used
commercially or on a conceptual, bench,
pilot, or field scale. Resource information
includes the technology developers' name
and address, technical description, status,
cost, and illustration (if available). This
resource document may be downloaded
from the Internet at http://www.epa.gov/
radiation/technology/rttcpubs.htm.
In the second part of the feasibility study,
researchers will collect radioactive and
hazardous mixed waste soil samples from a
selected U.S. Department of Energy (DOE)
facility. An electrokinetic facility at DOE's
Idaho National Engineering Laboratory will
conduct a bench-scale analysis of the
applicability of electrokinetic processing to
the collected soil samples. If bench-scale
analysis proves successful, part 3 of the
feasibility study will consist of a field
demonstration at the DOE site. Contact
Robin Anderson (EPA Office of Radiation
and Indoor Air) at 202-564-9385 for
additional information.
Of
by RonaldF. Probstein, PhD., MIT
Preliminary results from field tests initiated
in December 1997 at a Jersey City, NJ,
landfill indicate that in situ
electroremediation effectively removes
hexavalent chromium in the form of
chromate from unsaturated soils. The
removal rate per operating day is averaging
about one percent of the 5,000 ppm of the
mean chromate concentration found at the
test site (see Figure 4). Continuous
monitoring and adjustments of the electroki-
netics-based process are underway to
achieve an average daily removal rate of 2%
of the chromate found at the test site.
Researchers from the Massachusetts
Institute of Technology (MIT), Corrpro
Companies, Inc., and EPA Region 2 are
collaborating on this project, which is
sponsored by the Northeast Hazardous
Substance Research Center.
In electroremediation, contaminants are
removed from soil under the action of an
electric field. A direct-current potential
applied across pairs of electrodes placed in
the soil causes dissolved chromate ions in
the pore liquid to move toward the anode by
the process of ionic migration. Once
contaminants are at the electrode well, they
are dissolved and pumped to the surface.
The Jersey City field tests are based on the
findings of earlier laboratory studies and
computer modeling. The test site is a 6 x
12-foot plot extending to the depth of the
water table (approximately 6 feet below
ground surface). The site as originally
configured contains a rectangular electrode
array of 18 cathodes and 5 anodes. The
cathodes are 6 foot-long, 0.5 inch-diameter
rods driven into the soil on 2-foot centers
around the periphery of the treatment area.
The iron-pipe anodes are contained in wells
that accommodate recirculating purge solution
and serve as a sink for the chromate ions.
The system's purge solution is continuously
recirculated between the anode chambers
and a treatment drum, where the pH is
checked and adjusted as required. Precipi-
tating agents are added to the purge solution
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and will be disposed as a hazardous waste at
the conclusion of the test demonstration. A
diaphragm pump provides the suction
needed to draw the pure solution from the
anode wells, pull in the solution from the
treatment drum, and maintain a slight
vacuum to prevent loss of liquid into the soil
through the porous walls of the well.
During 1998, researchers will design field
demonstrations using the results of these
field tests. Process variables will address
data collected on relationships among the
rate of dissolution, solubility limit, pH,
temperature, and background electrolyte
composition, and will reflect computer
model refinements.
Researchers also have
found that variations of
this technology are
successful for application
in fully saturated soils and
ground waters and for
other waste types,
including radionuclides
and soluble organics. For
more information, contact
Dr. Ronald Probstein
(MIT) at 617-253-2240.
Fig
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tire 4: Percent of Chromate in Soil
ved as a Function of Operating Time
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0 5 10 15 20
Time (days)
United States
Environmental Protection
Agency
Solid Waste and
Emergency Response
(5102G)
EPA 542-N-98-003
February 1998
Issue No. 28
EPA TECH TRENDS
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