United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-87/077 Dec. 1987
&EPA Project Summary
Technical Assessment of Low
Pressure Pipe Wastewater
Injection Systems
David L. Hargett
The purpose of this study was to
review all technical and practical
experience with Low-Pressure Pipe
Waste Injection Systems (LPP systems)
and to characterize typical systems and
their field performance. A rigorous
review of available information on LPP
systems is presented in this report. To
augment these data, 12 typical LPP
facilities were monitored over the
winter-spring stress period from
October 1982 to July 1983. Detailed
system design specifications, soil
conditions, flow estimates, back-
ground and in-trench moisture condi-
tions, and numerous other perfor-
mance indicators are reported in this
technical assessment. Study methods
are presented, results are discussed and
summarized, with conclusions, and
recommendations are made for further
research.
This Project Summary was devel-
oped by EPA's Water Engineering
Research Laboratory, Cincinnati, OH,
to announce key findings of the
research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering
information at back).
Introduction
The LPP system combines several
innovative subsurface wastewater
absorption system concepts into a unique
onsite design package. Most important
among the features of LPP systems are
very shallow placement, very narrow
trenches, pressure-dosed distribution,
loading on a system area basis, and
rather flexible site criteria. This system
was developed in North Carolina in
response to intense growth and devel-
opment in unsewered areas with soils
unsuitable for conventional systems.
Since about 1977, approximately 1500
LPP systems have been installed in North
Carolina, and continued rapid prolifera-
tion is anticipated.
Unfortunately, only limited documen-
tation of these systems' performance,
other than general observations, is
available from the first several years of
field experience. Further, based on the
recommended sizing procedure for these
systems and their trench configuration,
it is apparent that LPP systems have
somewhat different operational charac-
teristics from conventional systems.
The pu rpose of th is study was to assess
available information and perform field
studies of the LPP system of onsite
wastewater disposal that is now used
extensively in the southeastern United
States in areas where soil and site
conditions exist that preclude the use of
conventional soil absorption systems.
The LPP system incorporates placement
of a pressure-dosed distribution network
in shallow, narrow trenches. The term
"LPP" derives from the use of low
pressures in the range of 2 to 4 ft of
water, as measured at the distal end of
the network. The septic tank effluent
flows by gravity to a pumping chamber,
where a submersible pump conveys it to
the trenches in controllable doses
approximately 2 to 3 times per day.
The following basic components of the
LPP system are illustrated in Figure 1:
two-compartment septic tank
pumping chamber
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Sanitary Tee
Manifold -•.
Access Riser
41 Supply Line^
Two-compartment Pumping Tank
Septic Tank
-Turn-up
-Distribution Lateral
Figure 1. Basic components of a typical low-pressure pipe wastewater absorption
system.
• submersible effluent pump, with high
and low level controls
• high water alarm system
• supply line and manifold
• perforated distribution laterals
• trenches installed in soil of suitable
area, depth and morphology.
The trenches, spaced 5 ft or more
apart, are excavated by a continuous
trenching machine and are typically 6 in.
wide. Depth of excavation varies with site
and soil conditions, but 6- to 18-in.-deep
trenches are most common. In the case
of very shallow applications, appropriate
fill material may be required for cover,
with final grading to enhance runoff
away from the absorption area. In other
situations LPP systems have been
installed entirely in fill.
Where site and soil conditions pre-
clude the use of a conventional system,
alternatives to include LPP or mound
systems may be proposed. North Carolina
Code requires that LPP systems must
have at least 2 ft of suitable or provi-
sionally suitable soil and maintain a
vertical separation distance, from trench
bottom to limiting condition, of at least
1 ft. This is significantly less than the
2 to 4 ft of separation generally recom-
mended to achieve proper renovation.
Sites on which LPP systems have been
successfully applied include the
following:
• soils with seasonally shallow or
perched groundwater
• soils with hydraulically restrictive
horizons at shallow depths
—clayey subsoils with slow
permeability
—compact or cemented horizons
—bedrock
—saprolite
• sandy soils with rapid permeability
• sites yvith steep slopes
If natural soil conditions will support use
of an LPP system, this is usually the
system of choice, as opposed to a mound
that may cost approximately three times
as much as an LPP. Where less than 2
ft of suitable native soil is available, LPP
systems have been installed partially or
totally in fill materials, a common
practice in replacement situations.
Shallow system placement offers
several advantages over deep placement.
The most obvious, and the primary
application of LPP systems, is the util-
ization of the best soil available at
shallow depths while maintaining max-
imum vertical separation above a limiting
condition such as groundwater or a flow-
restricting horizon. Shallow placement
also introduces the wastewater into the
soil's best aerated, most permeable, and
most biologically active zone. Especially
in thermic climates such as North
Carolina's, shallow systems offer an
important advantage in enhanced evap-
otranspiration. This may contribute
significantly to the system's hydraulic
function by increasing the hydraulic
gradient away from the absorption
trench, allowing for trench zone drying
and breakdown of accumulated organic
material during dry summer periods.
History
The use of LPP systems in North
Carolina has enjoyed rapid growth over
the past few years. Approximately 1,400
LPP systems were in operation in North
Carolina by the end of 1983.
As promoted and endorsed by the N.C.
Division of Health Services and North
Carolina State University, the LPP con-
cept is appropriate for a wide range of
site-limiting conditions. More than half
of North Carolina is unsuitable for
conventional septic systems according to
the North Carolina Administrative Code
(NCAC) guidelines. Given its apparent
successful performance thus far, and the
widespread demand for such a solution,
it is clear that LPP technology will
eventually enjoy use throughout the
state. Equally important, these systems
will be constructed on sites that would
otherwise not be utilized for onsite waste
systems owing to combinations of any
or all of the following site limitations:
high groundwater
shallow bedrock
impermeable soil
periodic flooding
excessive slope
The State of New Mexico has adopted
the North Carolina LPP system compre-
hensively in its 1981 onsite waste
guidelines. The design and installation
procedures specified by New Mexico are
substantially the same as those
employed by North Carolina. As yet, very
few LPP systems have been constructed
in New Mexico.
Virginia has adopted the general
concept of a shallow, narrow-trench,
dosed wastewater absorption system in
their 1982 revised code. However, the
Virginia Low-Pressure Distribution (LPD)
systems do differ from LPP systems in
several important ways. The LPD sys-
tems are designed on the basis of trench
bottom area as opposed to system area
for LPP systems and have a much more
conservative loading schedule. Also the
trench configuration and the distribution
specifications of the LPD system vary
somewhat from the LPP system.
During 1983 a community rehabilita-
tion project (the Harney Project), spon-
sored by the U.S. Department of Housing
and Urban Development, was initiated in
Carroll County, Maryland. As part of this
project alternative, individual onsite
wastewater systems are being con-
structed. Because of shallow bedrock and
seasonal perched groundwater condi-
tions, the alternative selected for about
35 homes in the community was the LPP
system. About 15 LPP systems were
constructed and put into operation in fall
1983, with completion of the remainder
expected in spring 1984. LPP systems
have not been approved for general use
by the State of Maryland.
Study Approach
The study involved an amalgamation
of two forms of engineering assessment.
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The first was an intensive search of
readily available and fugitive literature
sources, personal contacts and other
sources of information to assess the
state of the art. The second was a field
monitoring program designed to assess
the subjects least defined by the former
effort, e.g., interactions of wastewater
flows, trench loading rates, and back-
ground conditions. This evaluation
involved the instrumentation and mon-
itoring of 12 LPP systems over the stress
period of winter-spring 1982-83 (10 mo).
All but one of the sites had been in
operation for more than 1 yr. All available
site information was evaluated and
verified in the field, and each was
retrofitted with flow monitoring devices
and monitoring wells around and in the
trenches. System monitoring activities
typically included examination of the
pumping chamber and flow counter,
determination of water levels in each
well, and inspection of general system
conditions. Based on observed waste-
water production, efforts were made to
conduct monitoring at a time of day when
the system would be ready for a pumping
event. In any case, evidence of recent
pumping events was noted to aid in
interpretation of the well data. At least
twice during the monitoring period, the
wells' response to a pumping event was
measured by observing, for an extended
period, the rate of water level change
after the pump had shutoff This provided
an estimate of trench outflow dynamics.
Water meter data was collected where
available. Precipitation was estimated
from data available from nearby National
Weather Service reporting stations.
To assist in data analysis, plan and
section view layout plots were con-
structed for each of the 12 systems.
Likewise, background and in-trench well
levels were plotted over the monitoring
period. Also, trench well dosing response
curves were developed. These data are
presented for four representative sys-
tems to demonstrate typical system
features and performance. Selected
operational and performance indicators
were summarized for all systems.
Results and Conclusions
Technical Review
1. Shallow placement of the LPP
trenches utilizes the most perme-
able and biologically active soil
horizons, accentuates evapotran-
spiration, and permits use on sites
that have insufficient suitable soil
depth for conventional systems,
and thereby increases useful land
area and improves system
performance.
2. Narrow trenches permit inexpen-
sive construction techniques,
reduce site disturbance, and min-
imize soil compaction.
3. Design hydraulic loadings are
approximately 3.8 times that of
conventional systems on a total
land area basis, but are less than
conventional on a trench-bottom-
area basis.
4. LPP trench volumes result in
decreased storage capacity com-
pared to conventional trenches
during stress periods, but also
reduce gravel requirements.
5. Previous surveys have documented
a success rate of over 90% for LPP
systems installed on sites consi-
dered unsuitable for conventional
systems.
6. Monitoring studies have reported
excellent reductions of all waste-
water pollutants 2 ft below LPP
trenches, including viruses.
Field Study
1. LPP trench location in the shallow
horizons result in extreme sensi-
tivity to soil moisture, resulting in
their functioning as shallow
groundwater injection systems
during periods of high ground-
water.
2. LPP systems dry out very rapidly
when groundwater levels recede
and rainfall ceases, displaying
minimal clogging effects on trench
interfaces owing to the combined
effects of pressure dosing, shallow
placement, and narrow trenches.
3. Mechanical problems of LPP sys-
tems were significant, primarily
with level control switches, and
excessive pumping owing to infil-
tration problems will likely result
in shortened pump service life.
4. Almost half of the systems dis-
played temporary surface out-
breaks during saturated periods in
the course of the pumping cycle.
Overall
Although the LPP systems have been
shown to be a successful alternative to
conventional onsite systems under site
conditions that would prohibit the latter,
the LPP design parameters employed and
site conditions recommended for their
proper application need better definition.
Also, the LPP technology should be
evaluated in colder climates to determine
its applicability in locations with more
severe winter conditions.
The full report was submitted in partial
fulfillment of Contract No. 68-03-3057
with Urban Systems Research and
Engineering, Inc., under the sponsorship
of the U.S. Environmental Protection
Agency.
-------
DavidR. Hargett is with RSE Group/Ayres Association, Madison, Wl 53074.
James F. Kreissl is the EPA Project Officer (see below).
The complete report, entitled "Technical Assessment of Low-Pressure Pipe
Wastewater Injection Systems," (Order No. PB88-107 222/AS; Cost: $13.95,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, v'A 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
UNOFFICIAL
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
JSOI08 I
Official Business
Penalty for Private Use $300
EPA/600/S2-87/077
0000329 PS
U S EWVIR PROTECTION AGENCY
8E6IOH 5 LIBRURT
230 S DEARBORN STREET
CHICAGO IL
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United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-87/078 Feb 1988
SEPA Project Summary
Nondestructive Testing (NOT)
Techniques to Detect Contained
Subsurface Hazardous Waste
Arthur E. Lord, Jr. and Robert M. Koerner
A systematic and comprehensive
study was conducted to detect buried
containers with nondestructive testing
(NOT) remote-sensing techniques.
Seventeen techniques were considered
but only four were ultimately selected.
Those four were electromagnetic in-
duction (EMI), metal detection (MD),
magnetometer (MAG), and ground
penetrating radar (GPR). The containers
— both steel and plastic — varying in
size from 5 gal to 55 gal were buried in
known distributions in a wide variety of
soils; also, some were submerged in
water. Five diverse field sites were used.
As a result of the work at the five
field sites, a relatively complete picture
has emerged concerning the strengths
and weaknesses of the four NOT sub-
surface container location techniques.
GPR is the only reliable method to
detect plastic containers, but it has
limitations. GPR, EMI, and MD all suffer
severe loss of detection ability when
the background electrical conductivity
exceeds 40 millimhos/meter. In dry
sandy soil EMI, GPR, and MAG are all
capable of picking up a single 55-gal
steel drum to a depth of at least 10
feet. The MAG method works well for
steel under all subsurface conditions,
and GPR can usually pickup the side
walls of the excavations where waste is
dumped. Application of signal enhance-
ment techniques (background suppres-
sion) can be expected to enhance NOT
utility.
This Protect Summary was developed
by EPA's Hazardous Watte Engineering
Research Laboratory, Cincinnati, Ohio,
to announce key findings ot the research
project that Is lully documented In a
separate report of the same title (see
Protect Report ordering Information at
back).
Introduction
Since there is a vast amount of
hazardous waste buried below the surface
of the soil, it is important to clean up
these wastes before they do additional
damage to the environment. The first
step in any cleanup procedure is to detect
the waste and then determine its spatial
extent. As in any subsurface exploration,
many techniques can be brought to bear.
Test borings and limited excavations are
very valuable but are not without their
problems. The information obtained is
not continuous and the destructive nature
of the test makes it possible that waste
could inadvertently be released during
the probing phase. Therefore, there is an
interest in probing from the surface with
nonintrusive methods.
The goal of this project is to identify
and assess the best possible NOT tech-
niques for detecting and delineating
hazardous waste. Since another EPA
laboratory was performing the same type
task for monitoring hazardous waste
leachate plumes, this work concentrated
on the detection of steel and plastic con-
tainers buried beneath the surface of soil
and water bodies.
Literature Phase
The first phase of this project consisted
of identifying as many NOT techniques as
possible which could have possible ap-
plication to a broad spectrum of hazardous
waste problems. Seventeen such tech-
niques were identified. They were:
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• Microwave-pulsed — also called
ground penetrating radar (GPR)
Microwave-continuous (CWM)
Eddy current - also called metal
detection (MD)
Magnetometer (MAG)
Seismic reflection
Seismic refraction (SR)
Electrical resistivity (ER)
Penetrating radiation (x-rays,
gamma-rays, neutrons, etc.)
Acoustic emission
Liquid penetrant
Infrared radiometry
Pulse-echo ultrasonics
Sonar
Very low frequency electromagnetic
— also called electromagnetic in-
duction (EMI)
Induced polarization
Self-potential
Optical techniques.
A detailed report was prepared on each
of these techniques. (These are available
from the authors.) Information was sought
from the literature, company brochures
and personal communications. The litera-
ture search eliminated a number of the
techniques from further experimental
evaluation. Some of the reasons for
eliminations were:
• prediction of very little chance of
success
• high cost of equipment
• no indication from literature search
of success for container detection
• inaccessibility of equipment.
As a result of this first phase of the
project, the number of techniques con-
sidered was further reduced from seven-
teen to seven. The remaining techniques
were ground penetrating radar, micro-
wave-continuous, metal detection, mag-
netometer, seismic refraction, electrical
resistivity, and electromagnetic induction.
Field Tests
Each of the NOT methods will operate
"ideally" under a prescribed set of soil
types and man-made interferences. The
typical sites where most waste material
containers are buried are far from those
"ideals." Rather than burial in dry
granular soils, drums are usually dumped
in swamps, mudflats, water and the like.
Furthermore, the most successful
methods we have worked with are based
on measuring electrical or magnetic ef-
fects. High electrical conductivity areas,
e.g., near equipment storage areas, junk
yards, or ocean water, can severely in-
fluence the techniques. Soil homogeneity
and water conductivity are major issues.
Quantities of ferromagnetic material (e.g.,
steel objects) can severely affect the MAG
method. With these thoughts in mind,
test sites were obtained, containers of
various sizes were carefully placed at
different depths and geometric arrange-
ments, backfilled, and then located using
the various NOT methods.
The first field site was a nearly ideal
dry sandy soil in an open field, free of
man-made interference. This site provided
an excellent starting point and essentially
narrowed the selection (after careful
literature review) from seven of the pos-
sible NOT methods to the four mentioned
previously. The surviving methods were
MAG, EMI, GPR, and MD. Steel con-
tainers buried to 10-ft depths were ac-
curately located and could possibly have
been located deeper if stable burial pits
could have been excavated. Various steel
container arrays and the boundaries of a
"metal trash dump" were accurately
located. Some plastic containers were
also located, but with poorer results.
The second site was more formidable.
Here a saturated silty clay soil overlying
shallow shale rock was used. Detection
depths with the four methods indicated
techniques were much shallower, ap-
proximately 4 ft, and the results were
influenced by the large amount of back-
ground metal in the areas (e.g., trailers,
equipment, fences, etc.).
The fact that containers are sometimes
dumped directly into water and that the
salinity of the water can range from fresh
to brine, the third study was directed at
drums under water. Containers were
submerged in water and placed on the
bottom sediments at four different sites.
The salinity of the water ranged progres-
sively from fresh to ocean. (The work was
actually performed at various positions
along the Delaware River.) To depths of 3
ft of water above the containers, the
detection and delineation results were
"excellent" to "no good" in direct propor-
tion to the increase in water salinity, i.e.,
electrical conductivity of the water.
Bearing directly on the above three
studies is the extent to which ground
salinity can influence the detecting cap-
ability of the NDT methods used. At this
point, studies were made at a fourth site
with steel containers buried in a soil of
varying electrical conductivity. The ocean
was used as an electrical conductivity
extreme and the conductivity decreased
substantially as the survey moved inland.
The soil was a medium-to-fine granular
sand indigenous to the coastal area. The
sand density ranged from loose (near the
surface) to intermediate (at a depth of 6
ft).
Background conductivities greater than
40 millimhos/meter seriously impaired
the use of those methods based on
electrical conductivity measurements, i.e.,
MD, EMI and GPR. The MAG method
worked much better since it is a method
based on magnetic measurements and
not on electrical conductivity. The bound-
aries of a "trash dump" containing metal
objects were observed with all methods
even though the background conductivity
varied from 25-60 millimhos/meter.
Site 5 was the same location as Site 4
but, in this case, plastic containers were
used instead of steel. The MD, EMI and
MAG did not detect any of the plastic
containers even when these were filled
with salt water. The ability of GPR to pick
up the water table, as well as the con-
tainers, was demonstrated.
Conclusions
Table 1 presents the results obtained at
all five field sites and should be considered
the final results of the project and can
serve as a guide for the practitioner.
Some additional remarks are in order to
help assimilate all the results of these
studies.
In a dry, granular soil with medium
interference, individual typical steel con-
tainers can easily be seen to a depth of at
least 10 ft with all methods except MD,
which detects to 6 ft. Deeper detection is
probably possible, but 10 ft was the limit
of our burial ability. As the soil water
electrical conductivity becomes larger, the
detection ability of the MD, EMI, and GPR
methods suffers. When the background
conductivity rises to 40 millimhos/meters
or above, the detection ability imperiously.
impaired. The MAG method works well
under all granular soil conditions for it is
not affected by high background electrical
conductivity.
In cohesive soils (clays), there are
definite problems with MD, EMI, and
GPR due to the usual high water content
and soil inhomogeneities. A logistical
problem arose with respect to the MAG
data, since work in cohesive soils was
performed in the presence of magnetic
interfering materials (trucks, fences, etc.).
Research should be conducted in an
interference-free cohesive soil using the
MAG method. The use of MD, EMI, and
GPR in relatively uniform, dry cohesive
soils is of interest.
When steel containers were submerged
under water, the MD, EMI and GPR
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Table 1. General Acceptability of Using Various NOT Methods to Locate Typical Sized Buried Containers
fMaximum Penetration Depth Achieved in Parentheses)
Steel Containers
Subsurface
Material
(Reference)
Saturation
Type of
Void Water
Metal Electromagnetic Ground Penetrating
Detector Induction Radar Magnetometer
(MD) (EMI) (GPR) (MAG)
Granular
(sand)
Cohesive
(clay)
0% - 20%
20%- 50%
50% - 700%
50% - 7OO%
fresh
intermediate
ocean
fresh
excellent (6')
excellent (2')
not good
moderate* (4')
excellent (W)
average (4')
not good
moderate* (4')
excellent (W)
excel lent (3')
poor (2')
moderate* (4')
50%-100%
ocean
not good
not good
poor
excellent (W)
excellent (4')
excellent (W)
poor/4')**
Water
Granular
1OO%
10O%
10O%
1O% - 50%
fresh
intermediate
ocean
intermediate
excellent (3')
poor
not good
Plastic Containers
not good
excellent (3')
not good
not good
not good
excel lent (4')
not good
not good
excellent - if
contents con-
ductive (4')
fair — if con-
tents non-con-
ductive
excellent (3' )
excellent (3')
excellent (3')
not good
not good
* Excellent in dry clay.
**Many interfering magnetic objects. Excellent in absence of interference.
methods are only of value in relatively
fresh water. When the water conductivity
rises above 60 millimhos/meter, the three
methods are quite useless. The MAG
method functions well in water of all
conductivities.
Plastic containers are more difficult to
detect than steel containers. The MD,
EMI and MAG methods are useless in
detecting buried plastic containers. The
GPR method works well for typical size
plastic containers, especially if the con-
tainers are filled with electrically-conduc-
tive material. However, the method still
works with non-conductive contents.
These results for plastic containers apply
only for granular soil with relatively low
electrical conductivity. If the granular soil
has high conductivity material in its voids
or if the soil is a wet, non-uniform
cohesive material, then the same limita-
tions apply to GPR as were mentioned
earlier.
While this is a systematic and compre-
hensive study of NOT methods, it is not
complete and a few additional situations
still remain to be studied.
As a brief bottom line, it can be stated:
• MO, EMI, and MAG all work ex-
tremely well in detecting buried steel
containers in dry, granular soil to
any typical depth.
• The MAG method works well under
all subsurface conditions.
MD, EMI, and GPR will suffer severe
loss of detection ability when the
soil's electrical conductivity rises
above about 40 millimhos/meter.
The same conductivity limitations
also apply to the detection ability for
containers submerged under water.
GPR is the only reliable method to
detect buried plastic containers.
GPR can "see" excavation bound-
aries. This is an extremely important
point.
For a preliminary survey of a metal-
container dump site, the MD (instru-
ment costs about $500) is a good
first method, followed closely by the
MAG method (cost about $4000).
More detailed surveys can use the
more expensive instruments' EMI
(cost about $8000) and GPR (cost
about $30,000).
The full report was submitted in ful-
fillment of Cooperative Agreement No.
CR-807777 by Drexel University under
the sponsorship of the U.S Environmental
Protection Agency.
Arthur E. Lord, Jr., and Robert M. Koerner are with Drexel University,
Philadelphia. PA 19104.
John E. Brugger is the EPA Project Officer (see below).
The complete report, entitled "Nondestructive Testing (NOT) Techniques to
Detect Contained Subsurface Hazardous Waste." (Order No. PB 88-102 405/
AS; Cost: $13.95. 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 Officer can be contacted at:
Releases Control Branch
Hazardous Waste Engineering Research Laboratory—Cincinnati
U.S. Environmental Protection Agency
Edison, NJ 08837
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United States Center for Environmental Research BULK RATE
Environmental Protection Information POSTAGE & FEES I
Agency Cincinnati OH 45268 EPA
PERMIT No G-3
Official Business
Penalty for Private Use $300
EPA/600/S2-87/078
0001961 HWE*
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