United States
Environmental Protection
Agency
National Risk Management
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/540/SR-95/513 July 1995
v>EPA Project Summary
Review of Mathematical
Modeling for Evaluating Soil
Vapor Extraction Systems
David L. Jordan, James W. Mercer, Robert M. Cohen
Soil vapor extraction (SVE) is a com-
monly used remedial technology at
sites contaminated with volatile organic
compounds (VOCs) such as chlorinated
solvents and hydrocarbon fuels. Mod-
eling tools are available to help evalu-
ate the feasibility, design, and
performance of SVE systems. These
models provide a means by which to
quantify some of the important SVE
operating processes. This report pro-
vides information on SVE model selec-
tion, data requirements, design, and
application; describes the equations
governing flow and transport pro-
cesses; and highlights model limita-
tions.
This Project Summary was developed
by the EPA's National Risk Manage-
ment 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 infor-
mation at back).
Introduction
Soil vapor extraction (SVE), a demon-
strated technology, enhances the removal
of volatile chemicals from the subsurface
through application of a vacuum at an
extraction well to induce air flow through
the subsurface toward the well. As of
1991, SVE comprised 13% of selected
remedies at Superfund sites, and approxi-
mately 7% of leaking underground stor-
age tanks. The flow of air enhances
volatilization of compounds from the re-
sidual NAPL phase in soil pores and from
the dissolved phase in soil pore water.
The technology is particularly applicable
to relatively volatile organic compounds
(Henry's law constant > 10"3 atm-m3/mole)
residing in the vadose zone. The technol-
ogy may also be applicable for removal of
volatile light non-aqueous phase liquids
(LNAPLs) floating on the water table or
entrained in the capillary fringe, if the
chemicals of concern have high vapor
pressures (e.g., benzene). During SVE,
contaminant removal is expected to be
enhanced by decreasing soil moisture. As
the percent of moisture decreases, air per-
meability increases. Increased organic
carbon content will increase sorption to
the soil matrix, decreasing SVE efficiency.
Heterogeneous flow conditions also affect
the efficiency of contaminant removal, with
higher flow zones (preferential flow zones)
cleaning up faster than low flow zones
(less-permeable zones).
Air sparging, another SVE-related tech-
nology, generally involves the use of in-
jection wells to inject gas (typically air)
into the saturated zone below areas of
contamination. Ideally, dissolved, sepa-
rate-phase and sorbed contaminants will
partition into the injected air, effectively
creating an in-situ air-stripping system.
This can take place within a single-well
system, or the stripped contaminants can
be transported in the gas phase to the
vadose zone and collected by SVE wells.
The advantage of such a system is that
the treatment of groundwater and soil takes
place in-situ, reducing the need for dis-
posal of treated material. Although air
sparging is a physical/chemical treatment
process, the addition of air has the poten-
tial to promote biodegradation.
Printed on Recycled Paper
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The SVE process involves installation
of vacuum extraction wells or trenches at
strategic locations and depths. Air extrac-
tion can also be combined with air injec-
tion. The spacing of wells or trenches
depends on soil properties such as per-
meability and porosity. Where the objec-
tive is to remove both air and water, dual
vacuum extraction wells may be used.
The injection wells for air sparging can be
vertical or inclined, ranging to horizontal.
Effective design and prediction of system
performance can be difficult, depending
on site conditions.
Tools are now available in the form of
numerical models, that allow one to both
screen for the potential feasibility of SVE,
and design and estimate performance of
the system. While modeling should not
be considered an end in itself, it provides
a means by which to quantify some of the
important SVE operating processes. Mod-
eling can provide estimated answers for
numerous questions concerning the feasi-
bility and usage of SVE. Screening mod-
els can be used in conjunction with site
characterization data and best professional
judgment to determine the potential feasi-
bility of SVE at a contaminated site. Flow
and transport models can then be used to
enhance the system design process and
estimate performance. This review in-
cludes a summary of critical information
required in a SVE application. It also
includes a model selection process, model
usage guidelines, and case studies.
Methodology
At an "Integrated In Situ Treatment Sys-
tem Design Workshop" on August 10 and
11, 1993 in Edison, NJ, a need was iden-
tified to provide environmental managers
with guidance on how models may be
used to: (1) determine the viability of us-
ing SVE, (2) help design the SVE system,
and (3) estimate system performance.
The methodology used to provide this re-
port was a literature review and analysis
of the various codes that may be applied
to SVE. The literature review, and basic
information on SVE system design, are
provided. This includes introductory ma-
terial, model selection tips, and example
applications. In addition, information is
provided on flow and transport theory.
Applicable codes were divided into the
categories of screening, air flow, and com-
positional flow and transport. For each of
these categories, currently available mod-
els were compiled and reviewed. Several
example applications utilizing a number of
the codes are presented, along with three
case studies.
Results
The result of this review is a technical
document that highlights the following top-
ics and guides the user through the pro-
cesses of selecting and applying models
to SVE sites. Technical information is
provided in order to: (1) determine the
types of problems that can be addressed
by modeling; (2) highlight the methods
that are commonly used to solve such
problems; (3) determin the need for mod-
eling at the site and, selecting a model
for the site; (4) identify and illustrate the
major processes governing air flow and
contaminant transport in the subsurface;
(5) present a discussion of model data
needs; (6) review available commercial
and public domain codes; and (7) present
a suite of model applications and case
studies.
Conclusions
Modeling can provide estimated answers
for numerous questions concerning the
feasibility and usage of SVE. Screening
models can be used in conjunction with
site characterization data and best profes-
sional judgment to determine if SVE at a
contaminated site is feasible. Flow and
transport models can be used to enhance
the system design process and estimate
performance. In some cases, no complex
model is necessary, and decisions can be
made based on simple analytical solu-
tions and/or best professional judgment.
Geographical information systems (GIS)
can provide valuable assistance in orga-
nizing and presenting site data graphically
in order to enhance the remedial alterna-
tive selection process.
A total of six computer programs were
evaluated, including the screening, air flow,
and compositional flow and transport
codes. For screening, these models in-
clude the Hyperventilate and VENTING
codes, as well as other analytical solu-
tions. Air flow models available at this
time include AIRFLOW, CSUGAS, and
AIR3D. For compositional flow and trans-
port, the VENT2D/VENT3D model is avail-
able and capable of simulating contaminant
transport and removal via SVE.
The selection and application of any
model will ultimately lie with the model
user. This document attempts to provide
the potential model user through a deci-
sion-making process that is intended to
help decide how and when to select a
model, to make users aware of the pro-
cesses governing flow and transport in
the vadose zone, and highlight the limita-
tions of model results.
The full report was submitted in fulfill-
ment of Contract No. 68-C2-0108 by
GeoTrans, Inc., under the sponsorship of
the U.S. Environmental Protection Agency.
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David L. Jordan, James W. Mercer, and Robert M. Cohen are with GeoTrans,
Inc., Sterling, VA 20166
Chi-Yuan Fan is the EPA Project Officer (see below).
The complete report, entitled "Review of Mathematical Modeling for Evaluating
SVE Systems," (Order No. PB95-243051; Cost: $36.50, 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:
National Risk Management Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
Penalty for Private Use
$300
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EPA/540/SR-95/513
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