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

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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 ,

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                        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,

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  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

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