United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/SR-92/173 September 1992
&EPA Project Summary
A Technology Assessment of Soil
Vapor Extraction and Air
Sparging
Mary E. Loden
In recent years, there has been a
strong movement away from the tradi-
tional methods of remediating sites
contaminated with volatile organic
compounds (VOC), (capping the site
and pumping and treating groundwa-
ter), to the more cost effective treat-
ment consisting of in situ air sparging
and soil vapor extraction (SVE). SVE,
by itself, has enjoyed an excellent ac-
ceptance in treating VOC contaminated
vadose zones. Air sparging of the
saturated zone has added an important
new dimension to the in situ treatment
of contaminated sites. Areas below and
in the water table are able to be stripped
of VOCs using this technology, thus
making it possible to substantially de-
crease the length of time required to
achieve site closure.
The full report discusses the basics
of in situ air sparging system design,
presents case studies of documented
applications, includes a section on
process component costs including a
conceptual cost estimate for a hypo-
thetical site, and finally outlines the
research needs required.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see ordering
information at back).
Introduction
Air sparging, also called "in situ air
stripping" and "in situ volatilization," is a
technology used to remove VOCs from
the subsurface saturated zone. It intro-
duces contaminant-free air into an affected
aquifer system; this forces contaminants
to transfer from subsurface soil and
groundwater into sparged air bubbles. The
air streams are then transported into soil
pore spaces in the unsaturated zone where
they can be removed by SVE.
Air sparging systems must operate in
tandem with SVE systems that capture
volatile contaminants stripped from the
saturated zone. Using air sparging with-
out accompanying SVE could create a
net-positive, subsurface pressure that
could extend contaminant migration to as-
yet-unaffected areas and increase the
overall zone of contamination. Without
SVE, uncontrolled contaminated soil vapor
could also flow into buildings (e.g., base-
ments) or utility conduits (e.g., sewers),
creating potential explosion or health haz-
ards.
The effectiveness of combined SVE/air
sparging systems results from two major
mechanisms: contaminant mass transport
and biodegradation. Depending on the
system configuration, the operating pa-
rameters, and contaminant types found
onsite, one mechanism usually predomi-
nates. In both remediation mechanisms,
oxygen transport in the saturated and un-
saturated zones plays a key role.
The nature of air transport affects mass
transfer to and from the groundwater re-
gime. Bubbles exhibit higher surface area
for transfer of oxygen to the groundwater
and for volatile migration to the unsatur-
ated zone than does the area provided by
continuous, irregular air-flow pathways.
{§£> Printed on Recycled Paper
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SVE/AIr Sparging Technology
Mass Transfer
Mass transfer employs several mecha-
nisms that move contaminants from satu-
rated zone groundwater to unsaturated
soil vapors. Figure 1 illustrates the follow-
ing major mechanisms: (a) dissolving soil-
sorbed contaminants from the saturated
zone to groundwater; (b) displacing water
in soil pore spaces by introducing air; (c)
causing soil contaminants to desorb; (d)
volatilizing soil contaminants, and (e) en-
abling soil contaminants to enter the
saturated zone vapor phase. Due to the
density difference between air and water,
the sparged air migrates upwards in the
aquifer. The pressure gradient resulting
from the creation of a vacuum in the un-
saturated zone pulls the contaminant va-
pors toward and into the SVE wells.
Blodegradatlon Mechanism
Aerobic biodegradation of contaminants
by indigenous microorganisms requires the
presence of a carbon source, nutrients,
and oxygen. Air sparging increases the
oxygen content of the groundwater and
thus enhances aerobic biodegradation of
contaminants in the subsurface. Certain
organic contaminants, such as petroleum
constituents, serve as a carbon source for
microorganisms under naturally occurring
conditions. The rate of biodegradation can
be enhanced by optimizing nutrient status
of the system.
Remediation of an aquifer via the bio-
degradation mechanism has distinct ad-
vantages since a portion of the contami-
nants will be biologically degraded to car-
bon dioxide, water, and biomass — yield-
ing a lower level of VOCs in the extracted
air. This in turn can substantially reduce
vapor treatment costs. The possibility of
offsite contaminant vapor migration is also
reduced when sparged vapors entering
the vadose zone contain lower levels of
contaminants.
Certain contaminants, such as chlori-
nated solvents, can undergo biodegrada-
tion under anaerobic conditions. Air
sparging, in these instances, could ad-
versely affect this biodegradation process.
Requirements for Effective Air
Sparging
Applicability of Air Sparging
Some of the conditions that affect the
applicability of this technology are:
• depth to groundwater — a water table
located at a shallow depth (<5 ft) may
increase the difficulty of recovering
vapors with the technology.
• volatility of contaminants — com-
pounds should have a high volatility.
With Henry's Law Constants of at least
10s atm-m3/mol.
• solubility of contaminants — in gen-
eral, compounds that are very soluble
in water are not easily air stripped.
• soil permeability — injected air must
flow freely throughout the saturated
zone to achieve adequate removal
rates. Soil permeability should be at
least 10'3 cm/sec for air sparging to
be effective.
• aquifer type — generally, air sparging
should only be used on sites with
unconfined aquifers.
Air Sparging Apparatus
The major components of an air
sparging system include:
• extraction, sparging, and monitoring
wells.
• mechanical equipment — air com-
pressors and vacuum blowers.
• vapor treatment system including air/
water separator, emissions control
systems such as granular activated
carbon canisters, thermal oxidizers,
and catalytic oxidizers.
• instrumentation — analytical equip-
ment.
The combination of air sparging and
SVE systems provides a cost-effective in
situ technology for the remediation of VOC
contaminated sites.
Future Research
Despite the many air sparging installa-
tions — over 30 in Europe alone — there
is much about the technology that still
requires further investigation. The nature
Soil
Vaporization
*
Saturation Zone
Vapor Phase
Density
Gradient
Unsaturation Zone
Vapor Phase
Pressure
Gradient
Extracted Air
Soil
Dissolution
*
Groundwater
Stripping
t
Saturation Zone
Vapor Phase
Density
Gradient
Unsaturation Zone
Vapor Phase
Pressure
Gradient
Extracted Air
Groundwater
Stripping
*
Saturation Zone
Vapor Phase
Density
Gradient
Unsaturation Zone
Vapor Phase
Pressure
Gradient
Extracted Air
Figure 1. Mechanisms of mass transport during air sparging. (* Mechanisms enhanced by air sparging.)
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of the saturated zone vapor phase re-
quires further definition. Subsurface air
injection requires additional study includ-
ing the researching of phenomena such
as dissolution and partitioning. Validation
of developed mathematical models is an
area of key interest.
Document Contents
The document summarized here pre-
sents an overview and an assessment of
the state-of-the-art in SVE and air sparging
technology. It was written specifically for
state and local regulators, agency staff,
and those involved in remedial design and
operations who desire a basic under-
standing of the technology's principles,
applicability, operations, and cost.
Section 2 provides a description of the
process including subsurface mechanisms
involved in stripping contaminants from
the saturated zone. Various parameters
that affect the applicability of the technol-
ogy are discussed such as depth to
groundwater, volatility of contaminants,
solubility of contaminants, site permeabil-
ity, aquifer type, and soil type.
Section 3 presents a description of and
details on a number of actual air sparging
installations both in the United States and
in Europe.
Section 4 gives an overview of the de-
sign considerations for the various ele-
ments that go into the makeup of an SVE
and air sparging installation including in-
jection well characteristics, configurations,
and radius of influence. The factors that
go into the selection of mechanical equip-
ment are also discussed.
Section 5 discusses the capital costs
and operating costs of the components of
the technology, including well installation,
mechanical equipment such as compres-
sors and vacuum blowers, emission con-
trol equipment, and instrumentation. A
conceptual estimate for a hypothetical site
contaminated by petroleum hydrocarbons
from a leaking underground storage tank
is presented.
Section 6 discusses future research
needs in the areas of further definition of
saturated zone mechanisms and system
design and operations.
The full report was submitted in fulfill-
ment of Contract No. 68-03-3409, by Camp
Dresser and McKee, Inc. under the spon-
sorship of the U.S. Environmental Protec-
tion Agency.
•trv.8. GOVERNMENT PRINTING OFFICE: t993 - 7SO-07I/MHX
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Mary E. Loden is with Camp Dresser and McKee, Inc., Cambridge Center,
Cambridge, MA 02142-1401
Chi-Yuan Fan is the EPA Project Officer (see below).
The complete report, entitled "A Technology Assessment of Soil Vapor
Extraction and Air Sparging," (Order No. PB93-100154/AS; Cost: $ 19.00,
subject to change) will be available only from:
National Technical Information Ser\fice
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
Edison, NJ 08837
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
EPA600/SR-92/173
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EPA
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