United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-91/012 Aug. 1991
iSrEPA Project Summary
Feasibility of Hydraulic
Fracturing of Soil to Improve
Remedial Actions
L. C. Murdoch, G. Losonsky, P. Cluxtpn, B. Patterson,
I. Klich, and B. Braswell
Hydraulic fracturing, a technique
commonly used to increase the yields
of oil wells, could improve the effective-
ness of several methods of in situ
remediation. This project consisted of
laboratory and field tests in which hy-
draulic fractures were created in soil.
Laboratory tests conducted using a
triaxial pressure cell showed that hy-
draulic fractures were readily created in
clayey silt, even when it was saturated.
Laboratory observations are explained
using the parameters and analyses of
linear elastic fracture mechanics.
Field tests were conducted during
the summers of 1988 and 1989. During
the 1988 test, hydraulic fractures were
successfully created from cemented
casing at depths of 2 to 4 m. The tests
were limited to one fracture per bore-
hole, and shortcomings resulted from
the use of oil well equipment too large
for our purposes. During the 1989 test,
injection grouting equipment was used
with a new method of*casing to create
as many as four horizontally layered
fractures from the same borehole.
Following the tests, the vicinity of
the boreholes was excavated to reveal
details of the hydraulic fractures. In gen-
eral, they were slightly elongate in plan
view, and they were highly asymmetric
with respect to their parent borehole; In
each case, there was a preferred direc-
tion of propagation. Maximum lengths
of fractures in 1989 average 4.0 m, and
the average areas was 19 m2. Maximum
thickness of sand in individual frac-
tures ranged from 2 to 20 mm, averag-
ing 11 mm.
Results indicate that it should be
feasible to monitor the growth of hy-
draulic fractures at shallow depths us-
ing injection pressure, surface uplift and
surface tilt.
This Project Summary was devel-
oped by EPA's Risk Reduction Engi-
neering Laboratory, Cincinnati, OH, to
announce 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
The recovery of hazardous chemicals
from contaminated ground is often difficult
and sometimes impossible using estab-
lished techniques, so earth scientists have
begun to turn to related fields for innova-
tive ideas. In petroleum engineering, the
problem of recovering hydrocarbons from
reservoirs is similar to the problem of re-
covering contaminants from aquifers. A
wide range of techniques has been devel-
oped to enhance the recovery of oil from
reservoirs, and one of the most effective is
hydraulic fracturing. The basic process of
hydraulic fracturing, as it is used in the
petroleum industry, begins with the injec-
tion of fluid into a well until the pressure of
the fluid exceeds a critical value and a
fracture is nucleated. A granular material,
which is usually sand and is termed a
proppant, is pumped into the fracture as it
grows away from the well. Transport of the
proppanf is facilitated by using a viscous
fluid, usually a gel formed from guar gum
and water, to carry the proppanf grains
Printed on Recycled Paper
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Into the fracture. After pumping, proppant
holds the fracture open while the viscous
gel breaks down into a thin fluid. The
thinned gel is then pumped out of the
fracture, creating a permeable channelway
suitable for either the delivery or recovery
of liquid or vapor.
Statement of the Problem
Experience from the study of oil wells
suggests that hydraulic fracturing could
increase flow rates from wells used to
recover groundwater contaminants. To re-
alize this increase, however, hydraulic frac-
tures would have to be created and filled
with sand under conditions of contami-
nated regions. Oil reservoirs are typically
deeper and are composed of different ma-
terials than contaminated regions, so
the applicability of fracturing methods used
by petroleum engineers is unknown. Con-
taminants commonly occur in soils that are
weaker and more compliant than limestone
or sandstone typical of reservoirs. Effects
of soil properties on hydraulic fractures are
difficult to anticipate based on the results
of previous studies of hydraulic fractures in
rock. Moreover, most contaminants occur
at shallow depths (several meters to sev-
eral tens of meters), so intersecting the
ground surface and venting could severely
limit the length and, thus, the performance
of the fracture. Hydraulic fractures virtually
never vent when they are created in oil
reservoirs, which are several hundred to
several thousand meters deep, so the
practical problem of creating fractures at
shallow depths has yet to be addressed.
Approach
The approach of this research was to
adapt methods proven for hydraulic frac-
turing of rock to applications of hydraulic
fracturing of soil. Laboratory experiments
were conducted by creating hydraulic frac-
tures in rectangular samples of remolded
clayey silt confined in a triaxial pressure
cell. Results of those experiments were
analyzed using methods of linear elastic
fracture mechanics, a branch of elasticity
theory that is widely used to analyze hy-
draulic fractures in rock.
Two sets of field experiments were per-
formed by creating hydraulic fractures in
Pleistocene glacial drift at depths of be-
tween 2 and 4 m. The first set of tests was
conducted during June 1988 in collabora-
tion with a subcontractor who used equip-
ment designed to create hydraulic fractures
from oil wells. The second set was con-
ducted during June and July 1989 by in-
vestigators from the Center Hill Research
Facility (Cincinnati, Ohio), who used
equipment that was either rented or de-
signed for the project. Hydraulic fractures
were successfully created during the field
tests, and then they were exposed on the
walls of trenches dug with a backhoe. De-
tailed descriptions of exposures document
the geometries of the fractures, highlight-
ing the potential that this process should
have in remediation of contaminated soil.
Laboratory Experiments
Laboratory experiments were designed
to create hydraulic fractures by injecting
dyed glycerin into rectangular blocks of
soil confined in a triaxial pressure cell. An
experimental apparatus and a testing pro-
cedure were developed to reveal physical
characteristics of the fractures and to yield
data describing the characteristics of frac-
ture propagation in soil.
Apparatus
The experimental apparatus consisted
principally of a pump system, a fracture
cell, and a data acquisition computer. The
pump system was used to inject fluid at a
constant rate into a sample contained in
the fracture cell. Typically, the pressure of
the injected fluid increased until fracturing
occurred, then decreased during fracture
propagation. The computer v/as used to
monitor injection fluid pressures as a
function of time and to control the flowrate
of the pump.
The fracture cell is a rectangular
chamber with one moveable side that is
used as a loading plate (Figure 1). The
loading plate is transparent so that the
interior of the cell can be inspected during
a test. The other five sides of the chamber
are lined with neoprene bladders. The three
principal stresses on the sample are con-
trolled independently by adjusting air
pressures in the bladders.
Pressure of the injection fluid as a
function of time was the primary data re-
corded during each test. In addition, the
samples were split open and the details of
the fracture surfaces were described.
Physical Characteristics of the
Soils
Most experiments were conducted us-
ing a yellow-brown, colluvial clayey silt
derived from a pit adjacent to the Center
Hill Research Facility. The soil (which will
be termed the Center Hill clay) is a type CL
soil and it behaves as a plastic material in
Atterburg tests over the range of moisture
contents used during the fracturing tests
(moisture contents were between the liquid
and plastic limits). Data describing the
physical characteristics of the material are
in Table 1.
Samples were formed :by compacting
soil of various moisture contents in a rect-
angular mold. The same amount of
compactive effort was used for each
sample. A narrow slot (0.04 mm in aper-
ture) was cut through the middle of each
sample (Figure 1) using a special blade-
like tool. The slots were rectangular in
shape with the long axis of the rectangle
spanning the width of the sample. The
purpose of the slot was to provide a start-
ing fracture that was much larger than
existing flaws in the sample. The slot was
necessary because measurements of criti-
cal stress intensity require knowing the
length of a fracture when it begins to
propagate.
Events occurring during the fracturing
tests followed a consistent pattern. Typi-
cally, pressure increased nearly linearly
with time early in the test. At some point
the slope of the pressure record began to
flatten, reaching zero slope as the pres-
sure peaked^and then becoming negative
as the pressure decreased with continued
injection. :
A thin (on the order of 0.05 mm) frac-
ture trace typically could be first seen on
the surface of the sample roughly at the
time of maximum injection pressure. Most
traces were nearly straight, although in
many cases they consisted of a family of
straight, subparallel segments arranged
either en echelon or staggered. The loca-
tion of the fracture tip was difficult to estab-
lish because the aperture tapered gradu-
ally until it became undetectable. Thus the
tip could be located within approximately 1
cm, but the trace was too tliin to locate the
tip precisely using unaided visual tech-
niques.
A fracturing test was terminated by
stopping the pump and opening a pres-
sure relief valve, which allowed the injection
fluid to flow back into the injection tube as
the fracture closed. The dyed glycerin was
promptly removed from the fracture to in-
hibit staining of the sample by flow unrelated
to the process of hydraulic fracturing.
Results
More than 5 doz. experiments were
conducted with the Center Hill clay using a
variety of loading conditions, sample
preparation techniques, moisture contents
and durations of consolidation. Hydraulic
fractures were created during every ex-
periment, except in a few cases when the
injection tube was plugged with clay as it
was inserted. The typical test produced a
continuous, parent fracture adjacent to the
starter slot. The parent fracture broke into
discontinuous, lobate planes with increas-
ing distance from the injection hole. Dye
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Table 1. Characteristics of Soils Used in the Laboratory Study
Center Hill Clay
Atterburg Limits
(wt. water/wt. solid)
CH1
CH2
Liquid Limit . 0.429 0.438
Plastic Limit 0.198 0.200
Plastic Index 0.231 0.238
Shinkage Limit 0.188
Grain size
Gravel 0 0
Sand 0.03 0.03
Silt 0.61 0.62
Clay 0.36 0.35
Proctor Test (ASTM D698)
Moisture Content of Greatest Density: 0.197
Maximum Dry Density: 1.68 gm/cm3 (104.7 Ib/ff)
Maximum Wet Density: 2.01 gm/cm3 (126.0 Ib/ft3)
AVE
0.433
0.199
0.234
To Pressure Regulator
Bladder ,
k
Loading jfi
Plate
V
To Pump
Figure 1. Cut-away sketch of hydraulic fracturing cell.
staining formed irregular dendritic patterns
near the ends of the lobes (Figure 2), but
the leading edge of the fracture was be-
yond the zone reached by the dyed glycerin
and was unstained. These features define
four distinct zones on a typical fracture
surface: 1) starter slot, 2) parent fracture,
3) fracture lobes, and 4) a pristine or undyed
zone at the leading edge.
The lengths of the undyed tip zones
are roughly linearly related to the lengths
of the parent fractures, according to com-
parisons of multiple tests. The ratio of the
length of the undyed tip to the length of the
parent fracture m was strongly dependent
on the moisture content of the sample: m =
0, moisture < 0.21; m increases from 0 to
0.27 as moisture increases from 0.22 to
0.27; m decreases from 0.27 to roughly
0.06 as moisture increases from 0.27 to
0.32. The length of the undyed zone is
important because it marks a part of the
fracture that is unwetted by the injected
fluid.
Records of driving pressure Pd (the dif-
ference between the fluid pressure and
confining stress normal to the fracture)
were made of all the experiments con-
ducted during this research. In general,
forms of the records from the test were
similar; an initial period of roughly linear
increase in pressure is marked by an abrupt
change in slope and followed by a period
of diminishing positive slope, followed by a
period of decreasing pressure. A series of
tests were terminated at various times to
determine how fracture development cor-
relates to the forms of the pressure records.
The tests indicate that a fracture first started
to form approximately at the first break in
slope of the injection record. Typical ele-
ments of a pressure record and their in-
terpretation are as follows:
Period I: Constant positive slope; repre-
sents inflation of the starter slot.
Period Ik Slope diminishes, but remains
positive; represents stable fracture
propagation.
Period III: Slope is negative; represents
unstable propagation.
These periods could be broadly identi-
fied in most records, although the details
of the individual records were highly vari-
able, depending at least on the length of
the starter slot and the moisture content of
the soil. As the moisture content increased,
for example, the pressure marking the
break in slope and thus the onset of frac-
turing decreased markedly. Moreover, a
decrease in onset pressure was consis-
tently observed when the length of the
starter slot was increased.
Methods of linear elastic fracture me-
chanics, which are commonly used to ana-
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\
10
cm
Figure 2. Surfaces of three hydraulic fractures of various lengths. Lines on
fracturo surfaces are linear features and hatch marks on the lines
indicate the lower side of a step. Fractured, but undyed areas are
stipled. Heavy lines indicate overlap of lobes.
tyze hydraulic fractures in rock, were used
to explain forms of the pressure records
(Rgure 3). Critical stress intensity, a ma-
terial parameter proportional to the product
of Pj and the square root of the fracture
length, appears to adequately predict the
onset of propagation in Center Hill clay. It
Is a strong function of moisture content,
decreasing by nearly an order of magni-
tude because of an increase in moisture
from 0.20 to 0.23. Preliminary analyses of
fracture propagation can reproduce most
of the essential features of the pressure
records from the laboratory experiments.
The growth of an unwetted zone at the
fracture tip was included in the analyses
and helps to explain stable propagation at
the onset of fracturing.
Field Testing: 1988
The 1988 field test was conducted at a
site 10 km north of downtown Cincinnati
on the western side of the valley of Mill
Creek, a southerly flowing tributary of the
Ohio River. The site is on the southeastern
side of an area owned by the ELDA Com-
pany, who currently uses it as a municipal
landfill.
The site is underlain by glacial drift
composed of silty clay till overlain by up-
wardly grading beds of gravel, sand, silt
and clay that are probably outwash de-
posits. Most of the fractures were created
in the silty clay till. Regional ground water
is several tens of meters below the ground
surface — the till was unsaturated during
the tests. Consolidation tests and in situ
hydraulic fracturing tests indicate that the
till is overconsolidated; that is, the lateral
stress exceeds the vertical stress.
Boreholes used during the fracturing
tests were open over the bottom few dm,
and a casing was cemented from the open
interval to the ground surface. Depths of
the boreholes ranged from 1.6 to 3.8 m.
Halliburton Services, a subcontractor, was
hired to create the fractures. They used
equipment and methods designed to hy-
draulically fracture oil wells.
Ten hydraulic fractures were created in
the till during the 1988 tests. The vicinity of
each fracture was excavated and mapped
to reveal size, shape, location, and form.
Fracture forms differ in detail, but they are
characterized by the idealized form in Fig-
ure 4. A subvertical fracture occurs in the
vicinity of the open interval of the borehole,
indicating that shallow notches cut in the
walls of the bores were insufficient to
nucleate a fracture. The major part of the
fracture consists of a planar to trough-like
feature dipping shallowly toward the parent
borehole. Note that the fracture is highly
asymmetric with respect to the borehole;
this asymmetry was common. Most of the
tests terminated when the fracture vented
to the ground surface several meters from
the parent borehole. Fractures reached an
average length of 5.8 m, an average plan
area of 25.5 m2, and an average dip of 20°.
Field Testing: 1989
Field observations during the first test
indicated that it was feasible to create
hydraulic fractures, but they also highlighted
three shortcomings: 1) sand proppant was
sparse or absent in seven out of ten frac-
tures, so those fractures closed completely
and would have had negligible effect on
flow in the subsurface; 2) the maximum
dimensions of the fractures were limited by
venting to the ground surface; and 3) the
test was conducted using sophisticated
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I
UP
Figure 3. Records of driving pressure as a function of time from
experiments and from analytical solution (dashed) for samples
of various moisture contents.
equipment that would be inaccessible to
most environmental engineers.
The method of creating hydraulic frac-
tures was modified to reduce the short-
comings of the 1988 tests. The new method
was tested in the field during June and
July 1989 at the ELDA site and at another
site also underlain by till.
The injection fluid used during this
project is a gel formed from commercially
available guar gum and water. The vis-
cosity of the basic gel is roughly 20
centipoise, but it increases markedly upon
addition of a borate compound called a
crosslinker. The crosslinked gel is a thixo-
tropic fluid with an apparent viscosity of
roughly 200 centipoise. An enzyme is
added that breaks down the gel to roughly
10 centipoise between 12 and 18 hr after
injection. Coarse quartz sand (0.8 to 1.5
mm average grain size) was mixed with
the crosslinked gel to complete the formu-
lation of the injection fluid. Concentrations
of sand ranged to as much as 0.52 (vol
sand/vol gel) during the field tests.
The above-ground system consisted of
pumps and mixers similar to those used
during injection grouting operations. Injec-
tion rates ranged between 20 and 60 L/min
during the tests.
Below ground, a system of isolating an
interval of a borehole was developed spe-
cifically for use in unlithified material. The
system is based around a lance-like de-
vice (Figure 5) composed of a casing and
an inner rod, both of which are tipped at
one end with hardened cutting surfaces
that form a conical point.
During the 1989 tests, the lance was
driven 1 to 2 dm below the bottom of a
borehole (the borehole was either open or
contained a hollow stem auger). A water
jet was used to cut a disk-shaped notch
extending up to 40 cm away from the
borehole.
Hydraulic fractures were created by in-
jecting the sand-laden slurry into the cas-
ing. Lateral pressure of the soil on the
outer wall of the casing effectively sealed
the casing and prevented leakage of the
slurry. The fractures nucleated at the notch
and grew away from the boreholes
During a typical field test, the onset of
pumping was marked by a sharp increase
in pressure of the injection fluid to between
0.1 and 0.4 MPa. The onset of fracture
propagation, however, was marked by an
abrupt decrease in pressure. During propa-
gation, injection pressure^efther was roughly
constant and on the order of 0.06 MPa or
pressure decreased slightly with time. Af-
ter one fracture was created, the rod and
point were inserted and the lance driven 7
to 30 cm lower, where another fracture
was formed. This procedure was repeated
as many as four times for each borehole
(Figure 5).
The method of hydraulic fracturing de-
scribed above was field tested in June
1989 with the creation of 23 fractures at
two sites in Cincinnati, Ohio. Nineteen of
those fractures were created adjacent to
the ELDA Landfill, within 100 m from the
site of the 1988 tests.
Results
Hydraulic fractures exposed by exca-
vation after the 1989 tests were remarkably
similar in form. Three fractures created at
borehole EL6 are a typical example (Figure
6). The fractures are horizontal and equant
to slightly elongate in plan. They are highly
asymmetric with respect to the borehole,
however, with a preferred direction of
propagation roughly parallel to the slope of
the overlying ground surface. The frac-
tures are stacked one on top of another at
a-spacing of 30 cm, which is maintained
from borehole to leading edge (Figure 7).
The major plan axes of the fractures range
from 5.5 to 8.5 m, and the maximum thick-
ness of sand is 1.3 to 1.4 cm. The sand
proppant is thickest near the centers of the
fractures, and it thins as the edges are
approached. Apparently, the distribution
of sand is independent of the location of
the borehole.
Inflow tests were conducted before ex-
cavation using a Guelph* permeameter,
a device that yields the f lowrate required to
hold a constant water level in a borehole.
Water levels were held at 1.0 m above the
bottom of open boreholes during all tests.
The average inflow rate into three bore-
holes in unfractured ground is 0.055 L/min.
The rate of inflow into boreholes intersect-
ing hydraulic fractures was initially 0.25 to
2.5 L/min, but decreased to between 0.175
and 0.5 L/min at steady state. We con-
clude that the steady-state rate of inflow
increased by a factor between 3.2 and 9.1
as a result of the creation of the fractures.
Discussion of Field Tests
The principal shortcomings of the initial
(1988) field tests were reduced using the
method of fracturing described above. All
19 of the excavated fractures were filled
with sand proppant, and the maximum
thickness of the sand in the fractures was
11.2 mm, on average. The fracturing
method thus appears to be a consistent
means of creating permeable layers in the
subsurface, at least for the field conditions
of this test.
'Mention of trade names or commercial products does
not constitute endorsement or recommendation for
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Load DUB to
Backhoe
Backhoe
Vent
Riser
-Notch
H
Zone 2 1
4
H« H
Figure 4. Idealized hydraulic fracture created at the ELD A test site. Inferred from exposures of
fractures created beneath level ground, a) Oblique view, b) Section along major axis
of the fracture.
Hydraulic fractures created during the
1988 tests climbed gently toward the
ground surface where they vented; whereas
the ones created during the 1989 tests
were essentially horizontal and typically
did not vent. There were at least three
aspects of the 1989 fracturing procedure
that could have inhibited the tendency of
hydraulic fractures to vent:
1. Pumping rate was reduced from 75 to
420 L/min in 1988 to 20 L/min in 1989.
2. Density of injection fluid was increased
by increasing the sand content 0.09 to
0.18 during 1988 to as much as 0.52
during 1989. The density increase re-
duces buoyancy effects.
3. Radius of the notch was increased.
Vertical fractures nucleated at the
wellbores during 1988, but the larger
notch caused horizontal fractures to
nucleate during the 1989 tests.
The mixers and pump, key components
of the above-ground fracturing equipment,
were rented from a geotechnical equipment
company who designed them to be used
for injection grouting. Similar equipment
should be widely available and accessible
to most environmental engineers. Although
propped fractures were consistently cre-
ated with this equipment, it was far from
ideal. The mixers and pump were under-
powered for this application, and the pump
suffered excessive wear and had to be
replaced at the end of the tests. Slight
modifications in the design of the pump
and mixer should improve the efficiency of
the field procedure.
The below-ground equipment per-
formed adequately, facilitating the creation
of multiple fractures from a: single bore-
hole. As many as four flat-lying fractures
were stacked at spacings of 30 cm without
intersecting their neighbors. When fractures
were created at a spacing of 15 cm, the
lower fracture would commonly climb and
intersect the overlying fracture several m
from the borehole.
It has long been recognized that the
orientation of a hydraulic fracture depends
largely on the in situ state of stress, with
the plane of the fracture normal to the
direction of least principal compression.
That direction is vertical in shallow bedrock
and overconsolidated soil, which explains
the horizontal orientations of fractures at
the ELDA site. The orientation of hydraulic
fractures at other sites, such as sites un-
derlain by normally consolidated soil or fill
where the direction of least principal com-
pression is expected to be horizontal, may
differ markedly from the orientation of frac-
tures described here.
Conclusions
1. Hydraulic fractures were created at
shallow depths (several m) during field
testing in glacial drift. Fractures were
flat-lying, to gently dipping, slightly elon-
gate in plan, and 6 to 8 m in maximum
dimension. They were filled with coarse-
grained sand to a thickness of roughly 1
cm.
Site conditions, particularly the state of
stress, are expected to markedly, affect
the form of hydraulic fractures. As a
result, hydraulic fractures created at
other sites may differ from those de-
scribed here.
2. A method of creating hydraulic fractures
in unlithified sediment was developed
and tested. The method facilitates cre-
ating multiple fractures from a single
borehole.
3. In the laboratory, hydraulic fractures
were created by injecting dyed glycerin
into samples of silty clay with a range of
moisture contents. Features of the frac-
tures were remarkably similar to fea-
tures of hydraulic fractures in rock, even
when the silty clay was relatively soft
and saturated with water. Analyses us-
ing linear elastic fracture mechanics can
predict hydraulic fracturing of silty clay
in the laboratory.
The full report was submitted in partial
fulfillment of Contract 68-03-3379 by the
University of Cincinnati under the sponsor-
ship of the U.S. Environmental Protection
Agency.
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Figure 5. Hydraulic fracturing procedure. 1. Hollow-stem auger, 2. drive lance, 3. cut notch,
4. inject slurry to create hydraulic fracture, 5. advance lance, 6. advance auger.
Figure 6. Map of three fractures created at borehole EL6. Dotted line is wall of
trench. Dashed line where fracture 2 intersects a neighboring frac-
ture (not shown) was created from borehole EL7.
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t Meter
EL6
Trench B - Easf Wall
Figure 7. Cross-sections of three fractures created at borehole EL6 showing topographic
profile (upper) and details of fracture traces (lower)
L C. Murdoch, G. Losonsky, P. Cluxton, B. Patterson, I. Klich, and B. Braswell are with
the University of Cincinnati, Center Hill Research Facility, Cincinnati, OH 45224.
Michael H. Rouller is the EPA Project Officer (see below).
The complete report, entitled "Feasibility of Hydraulic Fracturing of Soil to Improve
Remedial Actions," (Order No. PB91-181 818/AS; Cost:$39.00, cost subjectto
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:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-91/012
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