United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-89/024 Jan, t990
Project Summary
State of Technology Review:
Soil Vapor Extraction Systems
Neil J. Hutzler, Blane E, Murphy, and John S, Gierke
Extracting vapor from soil is a
cost-effective technique for the
removal of volatile organic chemicals
(VOCsf from contaminated soils.
Among the advantages of the soil
vapor extraction process are that it
minimally disturbs the contaminated
soil, It can be constructed from
standard equipment, it has been
demonstrated at pilot- and field-
scale, it can be used to treat larger
volumes of soil than can be
practically excavated, and it has
potential for product recovery.
Unfortunately, there are few
guidelines for the optimal design,
installation, and operation of soil
vapor extraction systems, A large
number of pilot- and full-scale soil
vapor extraction systems have been
constructed and studied under a
wide range of conditions. The major
objectives of the Report summarized
here are to critically review available
documents that describe current
practices and to summarize this
information as concisely as possible.
A typical vapor extraction system is
briefly described, the experience with
existing extraction systems has been
reviewed, and information about each
system is briefly summarized.
Soil vapor extraction can be
effectively used for removing a wide
range of volatile chemicals over a
wide range of conditions. The design
and operation of this system are
flexible enough to allow for rapid
changes in operation, thus optimizing
the removal of chemicals. Although a
number of variables intuitively affect
the rate of chemical extraction, no
extensive study to correlate variables
to extraction rates has been
identified.
This Project Summary was
developed by EPA's Risk Reduction
Engineering Laboratory, Cincinnati,
OH, to announce k&y findings of the
research project that is fully
documented in a separate report of
fne same title (see Project Report
ordering Information at tne back).
Introduction
Soils may become contaminated in a
number of ways with such volatile organic
chemicals as industrial solvents and
gasoline components. The sources of
contamination at or near the earth's
surface include intentional disposal,
leaking undergrojnd storage tanks, and
accidental spills. Contamination of
groundwater Iron these sources can
continue even after discharge has
stopped because; the unsaturated zone
above a groundwater aquifer can retain a
portion or all of the contaminant
discharge. As rain infiltrates, chemicals
elute from the contaminated soil and
migrate toward gioundwater.
Alternatives for decontaminating
unsaturated soil include excavation with
on-site or off-site treatment or disposal,
biological degradation, and soil flushing.
Soil vapor extraction is also an accepted,
cost-effective technique to remove
volatile organic chemicals from
contaminated soils. Among the
advantages of the soil vapor extraction
process are that it minimally disturbs the
contaminated soil, it can be constructed
from standard equipment, it has been
demonstrated at pilot" and field-scale, it
can be used to treat larger volumes of
soil than are practical for excavation, and
-------
it has a potential for product recovery.
With vapor extraction, spiffs can be
cleaned up before the chemicals reach
Ihe groundwater table. Soil vapor
extraction technology is often used with
other clean up technologies to provide
complete restoration of contaminated
sites.
Unfortunately, there are few guidelines
for the optimal design, installation, and
operation of soil vapor extraction system
Theoretically based design equations
defining the limits of this technology are
especially lacking. Because of th-s. the
design of these systems is moslSy
empirical. Alternative designs can only be
compared by the actual construction,
operation, and monitoring of each design
A large number of pilot- and full-scale
soil vapor extraction systems have been
constructed and studied under a wide
range of conditions. The information
gathered from these experiences can be
used to deduce the effectiveness of this
technology One of the major objectives
o< the Report is to review available
reports describing current practices
critically and to summarize this
information as concisely as possible, A
brief description of a typical vapor
extraction system is presented. The
experience with existing extraction
systems has been reviewed and
information about each system is briefly
summartEed in a standard forrr.. Th0
information is further summarized in
several tables, which form the bases for a
discussion of the design, installation, and
operation of these systems. Because soil
vapor extraction is a relatively new sod
remediation technology, this Technology
Review document will evolve as more
information becomes available,
Process Description
A soil vapor extraction, forcad air
venting, or in situ air stripping system
fFigyre it revolves around the extraction
of air containing volatile chemicals from
unsafurated sort Fresh air is injected or
flows into the subsurface at locations
around a spill site, and the vapof-laden
air is withdrawn under vacuum from
recovery or extraction wells-
System Components
Extraction wells are typically designed
to fully penetrate the unsaturated zone to
the capillary fringe, Extraction wells
usually consist of slotted elastic pipe
placed in permeable packing
System Operations
During remediation, the blower is
turned on and the air flow through trie soil
comes to an equilibrium. The flows that
are finally established are a function of
the equipment, the flow control devices,
the geometry of well layout, the site
characteristics, and the air permeability
of the soil. At the end of operation, the
final distribution of VOCs in the soil can
be measured to ensure decontamination
of the site. Wells may be aligned
vertically or horizontally, Vertical
alignment is typical for deeper
contamination /ones and for residue in
radial flow patterns. If the depth of the
contaminated soil or the depth to the
groundwater table is less than 10 to 15 ft,
it imay be more practical to dig a trench
across the area o! contamination and
install horizontal perforated piping in the
trench bottom rather than to install
vertical extraction wells. Usually several
wells are installed at a site
System Variables
A number of variables characterize the
successful design and operation ol a
vapor extraction system: site conditions,
soil properties, control variables,
response variables and chemical
properties. The specific variables
belonging to these groups include.
Site Conditions: distribution of VOCs,
depth to groundwater, infiltration rate,
location of helerogeneities,
temperature, atmospheric pressure.
Soil Properties: permeability, porosity,
organic carbon content, soil struc-
ture soil moisture characteristics,
particle size distribution.
Control Variables: air withdrawal rate, well
configuration, extraction well spacing,
vent well spacing, ground surface
covering, inlet aw VOC concentration
and moisture content, pumping
duration,
Response Variables, pressure gradients,
final distribution of VOCs, final
moisture content, extracted a»r
concentration, exttacted air
temperature, extracted air moisture,
power usage.
Chemical Properties: Henry's constant,
solubility, adsorption equilibrium,
diffusivity (air and water), density,
viscosity
Design and Placement
Well spacing is usually based on
some estimate of the radius of influence
of an individual extraction well. In the
studies reviewed, well spacing has
ranged from 15 to 100 (t. Well spacing
should be decreased as soil bulk density
increases or the porosity of the soil
decreases One of the major differences
noted between systems was the soil
bonng diameter Larger borings am
preferred to minimize extracting liquid
water from the soil
In the simplest soil vapor extraction
system's, air flows to art extraction we!)
from the ground surface. To enhance air
flow through zones of maximum
contamination- it may be desirable to
include air inlet wells in the installation.
These injection wells or air vents, whose
function is to control the flow of air into a
contaminated zone, may be located at
numerous places around the site.
Typically, injection wells and air vents are
constructed similarly to extraction wells
tn some installations, extraction wells
have been designed so they can also bo
used as air inlets. Usually, only a fraction
of extracted air comes from air inlets.
This indicates that air drawn from the
surface is the predominant source of
ctean air.
One study investigated the effects of
air-flow rate and the configuration of the
inlet and extract«on wells on gasoline
recovery from an artificial aquifer. It was
determined thai screening geometry only
had an effect at the low arr-How rates. At
tow flow rates, higher recovery rates
resulted when the screen was placed
near the water table rathe* than when the
well was screened the lull depth of the
aquifer.
Woodward-Clyde made a similar
assessment at the Time Oil Company
site. Their engineers suggested that wells
should be constructed with approximately
20 ft of blank casings between the top of
the screen and the soil surface to prevent
the short circuiting of air and to aid in the
extraction of deep contamination. At most
sites, the initial VOC recovery rates were
relatively high then decreased
asymptotically to zero with time. Several
studies have indicated that intermittent
venting from individual welfs is probably
more efficient in terms of mass of VOC
extracted per unit of energy expended,
This is especially true when extracting
from soils wfere mass transfer is limited
by diffusion out of immobile water,
Optimal operation of a sod vapor
extraction system may involve taking
individual wells m and out of service to
allow time fur liquid diffusion and to
change air f"ow patterns in the region
being vented. Little work has been done
to study this. -J» 4 ,
-------
Vapor
Treatment
&
Inlet
Weil
i i
Itif _
i i --»
* I"!
* i & ~*
Si IS,
traction
Well
Air/Water
Separator
1
.+ -»-} ^ .+ _» _>
T
f
t
*
T'
t
T
t
t
f
t
E
- ; i •
u
< 4.
«-— f 4.
' ! ^
«•- Si
%%%%%%£
IT -- -
^ -* ->| ^
1 T^« $»
f*-**f
L'l Contaminated |-*
£,* So« fy
,. J«,n»..™ «•».•« J ^
l-Sfil^&tt V., X»
j *£^""^ t te
"• iitukSsZa
%% ''•' Tam
figure 1. Soil vapor extraction system,
One of the major problems in the
operation of a soil vapor extraction
system is determining when the site is
sufficiently clean to cease operation,
The design and operation of soil vapor
extraction systems can be quite flexible;
changes can be made during the course
of operation with regard to well
placement, or blower size, or air flows
from individual wells. If the system is not
operating effectively, changes in the well
placement or capping the surface may
improve it.
Conclusions
Based on the current state of the
technology of soil vapor extracSion
systems, the following conclusions can
be made:
1, Soil vapor extraction can be
effectively used for removing a wide
range of volatile chemicals in a wide
range of conditions.
2. The design and operation of these
systems is flexible enough to allow
for rapid changes in operation, tiius,
optimizing the removal of chemicais.
3, Intermittent blower operation is
probably more efficient in terms of
removing the most chemical with the
least energy,
4. Volatile chemicals can be extracted
from clays and silts but at a
slowerrate. Intermittent operation is
cer-tainly more efficient under these
conditions
5, Air injection has the advantage of
controlling air movement, but
injection systems need to be
carefully designed.
6. Extraction wells are usually screened
from a depth of from 5 to 10 ft below
the surface to the groundwater table.
For thick zones of unsaturated soil,
maximum screen lengths of 20 to 30
ft are specified.
7. Air/water separators are simple to
construct and shouid probably be
installed in every system,
8. Installation of a cap over the area to
be vented reduces the chance of
extracting water and extends the
path that air follows from the ground
surface, thereby increasing the
volume of soil treated.
9. Incremental installation of wells.
although probably more expensive,
allows for a greater degree of
freedom n design. Modular
construction where the most con-
taminated zones are vented first is
preferable,
10. Use of soil vapor probes in
conjunction with soil borings to
assess final clean up is less
expensive tsian use of soil borings
atone, Usually a complete materials
balance on a given site is impossible
because most sites have an unknown
amount of VOC in the soil and in the
groundwater,
11. Soil vapor extraction systems are
usually only part of a site
remediation system,
12, Although a number of variables
intuitively affect the rate of chemical
extraction, no extensive study to
correlate variables to extraction rates
has been identified.
The full report was submitted in partial
fulfillment of Cooperative Agreement No.
CH-814319-01-1 by Michigan Techno-
logical University under the sponsorship
of the U.S. Environmental Protection
Agency,
-------
Neil J. Hutzter, Blane f. Murphy, and John S, Gierke are with Michigan
Technological University, Houghton, Ml 49931.
Paul ft, cte Percin is the EPA Project Officer fsee below),
The complete report, entitled "State of Technology Review: Soil Vapor Extraction
Systems," (Order No, PB 89-195 184IAS; Cost: $75.95, subject to change)
wilt be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22W1
Telephone: 703-467-4650
The EPA Project Officer car be contacted at:
Risk Reduction JEngineering 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 S300
EPA/600/S2-89/024
------- |