EPA/540/2-91/003
                                          February 1991
     SOIL VAPOR EXTRACTION TECHNOLOGY

            Reference Handbook
                    By
    Tom A. Pedersen and James T. Curtis
        Camp Dresser & McKee, Inc.
      Cambridge, Massachusetts  02142
          Contract No. 68-03-3409
              Project Officer

               Chi-Yuan Fan
Superfund Technology Demonstration Division
   Risk Reduction Engineering Laboratory
         Edison, New Jersey  08837
                    U.S. Environ;--r-  .  "
                      \'.^ii 5, Libvai-y
                    •ioJ S. Dearborn Strc.
                    Chicago,  IL  .60604
   RISK REDUCTION ENGINEERING LABORATORY
    OFFICE OF RESEARCH AND DEVELOPMENT
   U.S. ENVIRONMENTAL PROTECTION AGENCY
          CINCINNATI, OHIO  45268
                                    Printed on Recycled Paper

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                                 DISCLAIMER
      The information in this document has been funded by the  U.S.
Environmental Protection Agency (EPA)  under Contract No.  68-03-3409  to COM
Federal Programs Corporation. It has been subjected to the Agency's  peer and
administrative review,  and has been approved for publication.   Mention of
trade names or commercial products does not constitute endorsement  or
recommendation for use.
                                      11

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                                   FOREWORD
      Today's rapidly developing and changing technologies and industrial
products and practices frequently carry with them the increased generation of
materials that, if improperly dealt with, may threaten both human health and
the environment.  The U.S. Environmental Protection Agency (EPA) is charged by
Congress with protecting the nation's land, air, and water resources.  Under a
mandate of national environmental laws, the agency strives to formulate and
implement actions leading to a compatible balance between human activities and
the ability of natural resources to support and nurture life.  These laws
direct the EPA to perform research to define our environmental problems,
measure the impacts,  and search for solutions.

      The Risk Reduction Engineering Laboratory is responsible for planning,
implementing and managing research, development, and demonstration programs to
provide an authoritative, defensible engineering basis in support of the
policies, programs and regulations of the EPA with respect to drinking water,
wastewater, pesticides, toxic substances, solid and hazardous wastes, and
Superfund-related activities.  This publication presents information on
current research efforts and provides a vital communication link between the
researcher and the user community.

      An area of major concern to the Risk Reduction Engineering Laboratory is
the impacts associated with uncontrolled releases of petroleum hydrocarbons
from underground storage tanks. This document provides information on soil
vapor extraction corrective action technologies for cleaning up soils
contaminated with petroleum products.


                                    E. Timothy Oppelt, Director
                                    Risk Reduction Engineering Laboratory
                                      111

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                                  ABSTRACT
      Soil vapor extraction (SVE)  systems are being used at an increasing
frequency for remediation of soils contaminated by volatile organic compounds
(VOCs).  The growing interest in this technology is due in part to its
demonstrated effectiveness for removing volatile compounds, its relatively low
cost, and the apparent "simplicity" of system design and operation.  Although
SVE technology may be "simple" in concept,  vapor behavior in the soil
environment is quite complex.  Recognition of this complexity led the EPA
Superfund Technology Demonstration Division (STDD),  Risk Reduction Engineering
Laboratory (RREL),  Releases Control Branch (RGB) to convene a workshop to
discuss the state-of-the-art of SVE technologies and to identify additional
research needed in the areas of site characterization,  pilot systems, full
scale system design and operation, attainment of cleanup criteria and post
closure monitoring.  This document serves as an assessment of the state-of-
the-art and as a summary of that workshop.   The introduction to this document
discusses the background of the expert workshop, summarizes the activities and
discussions that took place, and lists the recommendations of those present at
the workshop regarding future research directions.

      Part I of the document presents an overview of six major SVE topics:
principles of soil vapor behavior; site investigations; system design; system
operation and maintenance; secondary emission controls; and cost.

      Part II contains selected papers that were presented at the SVE
workshop.  These papers were chosen to represent the wide diversity of topics
presented at the workshop.  These papers include a survey of SVE technology,
two case studies, results of modeling studies, discussion of soil gas surveys,
evaluation of biodegradation that occurs concurrent with SVE, and a discussion
of research developments in SVE.

      This report was submitted in fulfillment of Contract No. 68-03-3409 by
CDM Federal Programs Corporation under the sponsorship of the U.S.
Environmental Protection Agency.  This report covers a period from July, 1989
to February, 1990,  and work was completed as of July, 1990.
                                      IV

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                                  CONTENTS
Section

Foreword	
Abstract	       iv
Figures	     viii
Tables	       ix
Abbreviations Used	        x
Acknowledgments	       xi

      1.  INTRODUCTION	        1
            Background	        1
            Workshop	        3
            Research and Development	        4
            Regulatory Considerations	        8
            Format	       12

      2 .  PRINCIPLES OF SOIL VAPOR BEHAVIOR	       15
            Introduction	       15
            Contaminant Characteristics	       15
            Soil Environment	       25
            Gaseous Flow in Subsurface Environments	       31
            Air Permeability Test Methods	       36

      3 .  SITE INVESTIGATIONS	       41
            Introduction	       41
            Site History Review	       42
            Preliminary Site Screening	       42
            Detailed Site Characterization	       47
            Characterization of Contaminants	       48
            Pilot Testing	       49

      4.  SYSTEM DESIGN	       50
            Introduction	       50
            Extraction System Options	       50
            Well Configuration	       56
            Air Flow Control	       59
            Equipment	       63

      5 .  SYSTEM OPERATION AND MONITORING	       70
            Introduction	       70
            System Operations	       70
            Enhanced Biotreatment	       71
            System Monitoring	       71
            Vapor Stream Characteristics	       75
            Clean Up Attainment Determination	       77

                                     v

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                                   CONTENTS
                                 (Continued)

  Section                                                              Page

      6 .  EMISSION CONTROL	        83
            Introduction	        83
            Granular Activated Carbon Adsorption	        85
            Thermal Incineration	        87
            Catalytic Oxidation	        88
            Internal Combustion Engines	        91
            Packed Bed Thermal Processor	        95
            Biotreatment	        95
            Direct Discharge	        96

      7 .  COST OF SOIL VAPOR EXTRACTION	        97
            Introduction	        97
            Site Investigation Costs	        97
            Capital Costs	       104
            Operations and Maintenance Costs	       116

References	       119
Bibliography	       129
Appendices
      A.   Review of Soil Vapor Extraction System Technology by N.J.
          Hutzler, B.E. Murphy, and J . S . Gierke	       136

      B.   Applicability and Limits of Soil Vapor Extraction for
          Sites Contaminated with Volatile Organic Compounds by J.
          Danko	       163

      C.   Soil Gas Surveys in Support of Design of Vapor
          Extraction Systems by H.B. Kerfoot	       171

      D.  In-Situ Biodegradation of Petroleum Distillants in the
         Vadose Zone by R.E. Hinchee, D.C. Downey, and R.N. Miller      186

      E.  A Practical Approach to the Design, Operation, and
         Monitoring of In-Situ Soil Venting Systems by P.C.
         Johnson, M.W. Kemblowski,  J.D. Colthart, D.L. Byers, and
         C.C. Stanley	       195

      F.  Design of Soil Vapor Extraction Systems - A Scientific
         Approach by M.C. Marley, S.D. Richter, B.L. Cliff, and
         P. E. Nangeroni	       240

      G.  Modeling Applications to Vapor Extraction Systems by L.R.
         Silka, H. Cirpili, and D.L. Jordan	       252

                                       vi

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                                   CONTENTS
                                  (Continued)
      H.  Performance of In Situ Soil Venting System at Jet Fuel
         Spill Site by D.W.  DePaoli, S.E.  Herbes,  and M.G.  Elliott      260

      I.  In Situ Vapor Stripping Research Project:   A Progress
         Report by R.D. Mutch,  Jr.,  A.N.  Clarke,  D.J.  Wilson,  and
         P.O.  Mutch	      273

      J.  Soil Vapor Extraction Research Developments by G.E.  Hoag.      286

      K.  Soil Cleanup Criteria	      300

      L.  Air Discharge Criteria	      306

Glossary	      310
                                     vii

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                                   FIGURES

Number                                                          Page

1     SVE Applicability Nomograph	   16
2     Unsaturated Zone Contaminant Phases 	   18
3     Effect of Weathering on Gasoline Composition	   24
4     Variation of Hydrocarbon Distribution in Extracted Gas....   26
5     Air and Water Permeability as a Function of Water Content.   30
6     Illustration of VOC Adsorption Under Three Moisture Regimes 32
7     Relationship of Flow and Induced Vacuum	   37
8     Typical Soil Gas Sampling Apparatus	   45
9     Location of Soil Gas/Groundwater Sampling Points and
      Concentration Contours of Total Hydrocarbons in
      Soil Gas (MG/L)	   46
10    SVE System Design Options	   51
11    Typical Extraction Well Construction	   53
12    Water Table Upwelling in Response to Applied Vacuum	   54
13    Trench Construction	   55
14    Field Air Permeability Test Results	   57
15    Effect of Air Injection on Flowpaths	   60
16    Effects of Surface Seal on Vapor Flowpaths	   62
17    Groundwater Depression Pumping to Control Upwelling	   64
18    Typical Soil Vapor Extraction System Schematic	   65
19    Sidestreams Resulting from Vapor Pretreatment	   68
20    Effect of "Pulsed" Operation	   72
21    Oxygen Increase Following SVE System Startup	   73
22    Effect of Biodegradation on Overall Removal Rate	   74
23    Applicability of Vapor Treatment Options	   84
24    Benzene Isotherms	   86
25    Explosimeter Readings	   89
26    Catalytic Oxidation Schematic	   90
27    Activated Carbon Systems Costs Comparison	  114
                                      viii

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                                   TABLES
Number                                                                 Pa§e


1     Chemical Properties of Hydrocarbon Constituents	 20
2     Composition of Fresh and Weathered Gasolines	 23
3     Effect of Temperature on Water and Air Properties and Air
      Conductivity	 27
4     Vapor Diffusion-Parameter Relations in Chemical Transport
      and Loss to Atmosphere	 34
5     Cleanup Levels Published for Petroleum Contaminated Soils	 79
6     Petroleum Hydrocarbon Constituents That Occur Naturally in Soils  81
7     Destruction Efficiencies of ICEs	 93
8     Monitoring Equipment Costs	 99
9     Worker Protective Equipment Costs	100
10    Site Investigation Equipment Costs	102
11    Typical Analytical Costs	103
12    SVE System Components Capital Costs	106
13    SVE Vapor Treatment Unit Costs	Ill
                                      IX

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                              ABBREVIATIONS USED
ABNs   - Acid-Base Neutrals
API    - American Petroleum Institute
BTEX   - Benzene, Toluene, Ethylbenzene,  and Xylenes
CEC    - Cation Exchange Capacity
CERCLA - Comprehensive Environmental Response, Compensation, and
         Liability Act of 1980 ("Superfund")
CFR    - Code of Federal Regulations
EM     - Electro-magnetics
EPA    - Environmental Protection Agency
FID    - Flame lonization Detector
GAG    - Granular Activated Carbon
GC     - Gas Chromatograph
GPR    - Ground Penetrating Radar
LEL    - Lower Explosive Limit
NAPL   - Non-Aqueous Phase Liquid
PID    - Photoionization Detector
PVC    - Polyvinyl Chloride
RGB    - Releases Control Branch
RREL   - Risk Reduction Engineering Laboratory
SARA   - Superfund Amendments and Reauthorization Act
SCS    - Soil Conservation Service
SITE   - Superfund Innovative Technology Evaluation
SVE    - Soil Vapor Extraction
IDS    - Total Dissolved Solids
THA    - Total Hydrocarbon Analyzer
TPH    - Total Petroleum Hydrocarbons
UEL    - Upper Explosive Limit
USGS   - United States Geological Survey
UST    - Underground Storage Tank
VOCs   - Volatile Organic Compounds

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                               ACKNOWLEDGMENTS

      The Soil Vapor Extraction Technology Workshop,  from which much of the
information compiled in this document was obtained,  and the preparation of
this report were completed under the direction of Technical Project Monitor
Chi-Yuan Fan, P.E., of the USEPA Superfund Technology Demonstration Division,
Risk Reduction Engineering Laboratory, Releases Control Branch in Edison,  New
Jersey.

      This report was prepared for the U.S. Environmental Protection Agency
(EPA) Office of Research and Development by Camp Dresser & McKee Inc. (COM)
under the direction of Tom A. Pedersen, CPSS,  Project Manager for Work
Assignment No. 3-09 under EPA Contract No. 68-03-3409 with COM Federal
Programs Corporation.  The principal authors of this report were Tom A.
Pedersen and James T. Curtis.  Additional material was provided by Paul B.
Blais, Kathleen G. Murphy, Katharine Sellers,  Yvonne L. Unger, and Michael L.
Whitehead.  Technical review was provided by Warren Lyman, Ph.D. and William
Glynn, P.E.

      CDM would like to acknowledge the guidance and assistance provided by
Anthony N. Tafuri, ORD's Project Officer; Chi-Yan Fan, ORD's Technical Project
Monitor for this work assignment; and Robert L. Stenberg, Ph.D. of USEPA,  Risk
Reduction Engineering Laboratory, Cincinnati,  Ohio,  who provided a complete
technical review of the final document.

      Special thanks is extended to the following individuals who presented
papers at the workshop that are included as appendices to this document:

      Hasan Cirpili, Hydrosystems,  Inc., Dunn Loring, VA
      Joseph Danko, CH2M Hill, Corvalis, OR
      David W. DePaoli, Oak Ridge National Laboratory, Oak Ridge, TN
      Captain Michael Elliott, Tyndall Air Force Base, FL
      Robert E. Hinchee,  Battelle,  Columbus, OH
      George E. Hoag, University of Connecticut, Storrs, CT
      Neil J. Hutzler, Michigan Technological Univ.,  Houghton, MI
      Paul C. Johnson, Shell Development Corporation, Houston, TX
      Henry B. Kerfoot, Kerfoot & Associates,  Las Vegas, NV
      Michael C. Marley,  VAPEX, Philadelphia,  PA
      Robert J. Mutch, Eckenfelder, Inc., Mahwah, NJ

      The presentations by the following workshop attendees are also greatly
appreciated:

      Glenn Batchelder, Groundwater Technology, Norwood, MA
      Bruce Bauman, American Petroleum Institute, Washington, DC
      Richard A. Brown, Groundwater Technology, Inc., Mercerville,  NJ
      Paul de Percin, USEPA RREL, Cincinnati,  OH

                                      xi

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      Paul Lurk,  USATHAMA,  Aberdeen,  MD
      Mark Johnson,  Chas.  T.  Main,  Boston,  MA
      James J.  Malot,  Terra Vac,  San Juan,  PR
      Frederick Payne,  Midwest Water Resources,  Inc.,  Charlotte,  MI

      The contributions of the workshop participants listed below are also
acknowledged:

      Ronald J. Backsai, Envirosafe Technologies,  Inc.
      Michael Broder,  New York University,  New York, NY
      John E. Brugger,  USEPA RREL,  Edison,  NJ
      Paul C. Chan,  New Jersey Institute of Technology,  Newark,  NJ
      Jim Ciriello,  Terra Vac, Princeton,  NJ
      Joseph G. Cleary, CH2MHill,  Parsippany, NJ
      Gordon Dean, Florida Dept.  of Environment Regulation, Tallahasee,  FL
      Dominic C.  DiGiulio,  USEPA,  RSKERL,  Ada, OK
      Joseph C. Foglio, Environmental Applications,  Inc.,  Waltham, MA
      Iris Goodman,  USEPA OUST, Washington, DC
      Robert Hillger,  USEPA RREL,  Edison,  NJ
      Peter R.  Jaffe,  Princeton University, Princeton,  NJ
      Martin S. Johnson, Amoco Corporation, Tulsa,  OK
      William Librizzi, New Jersey Institute of Technology, Newark, NJ
      Arthur E. Lord Jr.,  Drexel University, Philadelphia, PA
      Joel W. Massman,  Michigan Technological University,  Houghton, MI
      Emil Onuschak, Delaware Dept. Nat. Res. & Env. Control, New Castle,  DE
      Myron Rosenberg,  Camp Dresser & McKee Inc.,  Boston,  MA
      John Schuring, New Jersey Institute of Technology, Newark,  NJ
      David E.  Speed,  IBM,  Hopewell Junction, NY
      Mary Stinson,  USEPA RREL, Edison, NJ
      David J.  Wilson,  Vanderbilt University, Nashville, TN

      Special thanks are also extended to Gloria Veliz and Kathy Popplewell,
who performed the word processing of the original manuscript; Valerie
Zartarian, who contributed greatly to the production of the final report;  and
A. Russell Briggs, who prepared the original graphics,  all of Camp Dresser &
McKee Inc.
                                      XII

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                                   SECTION  1

                                 INTRODUCTION

BACKGROUND

      Over two million underground storage tank (UST) systems located at
700,000 facilities exist nationwide.  The U.S. Environmental Protection Agency
(EPA) has indicated that about 25 percent of existing UST systems fail
tightness testing and may be leaking (EPA,  1988a).   Faced with the significant
environmental impact to groundwater resources that releases from USTs pose,
EPA, through their Office of Underground Storage Tanks (OUST), has embarked on
an aggressive regulatory program under Subtitle I  of the Hazardous and Solid
Waste Amendments of 1984 (PL 98-616).  Section 9003 of Subtitle I required EPA
to promulgate regulations,  applicable to all owners and operators of UST
systems, to protect human health and the environment.  Section 9003(h) was
added by the Superfund Amendments and Reauthorization Act of 1986 (SARA) to
give EPA and the states, under cooperative agreements with EPA, authority to
clean up releases from UST systems or to require owners and operators to do
so.

      On September 23, 1988, in response to Section 9003, EPA promulgated
final rules (40 CFR 280) on the technical requirements for underground storage
tank systems (EPA, 1988a).   The regulations describe a two-stage process for
cleaning up petroleum or hazardous substances released from UST systems.  The
first stage addresses corrective actions taken in response to confirmed
releases of regulated substances to mitigate immediate dangers posed by the
release.  The second stage addresses long-term corrective action measures
undertaken if the implementing agency determines that additional corrective
action is required to protect human health and the environment.

      Since most UST releases result in contaminated soil, the regulations
deal in part with soil treatment technologies, including:

      •     Excavation with on-site or off-site treatment or disposal;

      •     Biological treatment;

      •     Soil flushing;

      •     Recovery of non-aqueous phase  liquid (NAPL); and

      •     Soil vapor extraction.

      Soil vapor extraction (SVE) or vacuum extraction is an accepted,
cost-effective technique for removing volatile organic compounds (VOCs) and
motor fuels from contaminated soil.  This  technology is known in the industry

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by various names, including soil vapor extraction,  vacuum extraction, soil
venting, aeration, in situ volatilization, and enhanced volatilization.   In
this report the term soil vapor extraction (SVE) is used.  Vapor extraction
systems have many advantages that make this technology applicable to a broad
spectrum of sites:


      •     SVE is an in situ technology that can be implemented with a
            minimum of site disturbance.  In many cases, normal business
            operations may continue throughout the cleanup period.

      •     SVE has potential for treating large volumes of soil at reasonable
            costs, in comparison to other available technologies.

      •     SVE systems are relatively easy to install and use standard,
            readily-available equipment. This allows for rapid mobilization
            and implementation of remedial activities.

      •     SVE effectively reduces the concentration of volatile organic
            contaminants in the vadose zone, which in turn reduces the
            potential for further transport of contaminants due to vapor
            migration and infiltrating precipitation.

      t     SVE can serve as an integral component of a complete remedial
            program, which may include groundwater extraction and treatment.

      •     Discharge vapor treatment options allow design flexibility
            required to satisfy site specific air discharge regulations.

      The technical information available for designing, constructing and
operating an extraction system is largely held by the SVE technology
developers.  Furthermore, it appears that engineering practices employed by
vendors are based in large part on each developer's experiences.  Therefore,
an attempt has been made in this document to provide SVE technology design and
operational information that would be of use in ensuring the appropriate and
cost-effective application of these technologies.

      The ease with which SVE systems can be installed and operated to achieve
removal of volatile organic contaminants from the subsurface environment
belies  the complexity of vapor behavior in site specific subsurface settings.
Some SVE users may be able to achieve significant VOC removal even without an
in-depth understanding of subsurface vapor behavior.  Nevertheless, further
elucidation of the means by which SVE systems operate should lead to
enhancement of system efficiency and increased confidence in attainment of
clean-up standards.

      The impacts resulting from past UST product releases, as well as those
that will inevitably occur due to system failures and accidental discharges,
present a significant remediation challenge.  The EPA Office of Research and
Development (ORD) is currently undertaking research into the applicability of
a number of approaches for dealing with the contaminants released from UST
systems.  Soil vapor extraction is one  such corrective action alternative that

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has been demonstrated effective in removing volatile contaminants from
unsaturated soils.

      One of the goals of this project was to develop technical assistance
documents to assist in establishing design and operational parameters for
vapor extraction systems, and to evaluate the effectiveness of the technology
for removing the major gasoline constituents from contaminated subsurface
zones.  Parameters such as the layout and spacing of extraction and injection
wells, induced vacuum/extracted flow relationships, transmission of the
induced vacuum, vacuum pump capacity, air flow paths, and the effect of soil
conditions (air permeability, temperature, moisture content, contaminant
concentration, etc.) on system applicability are being considered and
evaluated as part of these efforts.

      Although SVE has become a widely used technology, few guidelines exist
for the optimal design, installation, and operation of soil vapor extraction
systems.  The paucity of standardized design and operational criteria is due
in part to the limitations posed by site specific factors including:

      •     contaminant characteristics and degree of weathering;

      •     extent of contamination;

      •     soil characteristics and stratigraphy;

      •     depth to groundwater;

      •     emission control requirements; and

      •     soil cleanup criteria.

      ORD recognizes that SVE technology  is being aggressively marketed by
vendors and that data on the efficacy of  these systems are  sometimes not
available because of the proprietary nature of the technology.  Likewise, the
evaluation of  a site specific system application may not provide data that can
be extrapolated to other sites due  to variability  in soils, geology,
contaminant characteristics  and SVE  technologies.  However, significant
strides have been made recently in  computer modeling of vapor phase transport
of contaminants in unsaturated soils.  In addition,  laboratory soil column
studies have yielded insight into the migration of contaminants released from
leaking underground storage  tanks.   The synthesis  of the results of computer
modeling, laboratory studies and field data in a comprehensive, yet concise
manner, and the distribution of these results, would afford greater uniformity
in the evaluation and application of SVE.  In turn,  this should provide
greater confidence  in the use of the technology for  remediation of
contaminated  soils where preliminary site conditions indicate a high
probability of success.

WORKSHOP

      The initial step in developing this document was the  convening of SVE
experts for a  two day workshop in Edison, New Jersey on June 28 and 29, 1989.

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Data on case studies, design factors and site evaluation approaches were
presented and discussions were held to disseminate technology and to determine
the most appropriate course of action for ORD with regard to technology
dissemination and future SVE research.  The data and information collected
during the workshop are being used to develop an understanding of SVE
technology and its limitations, while making this information available to
individuals involved in site remediation.  Specific issues suggested to ORD
for future investigation include:

      •     Preparation of a technical assistance document that could be used
            to assess whether SVE technology is appropriate for implementation
            at specific UST sites.  [Assessing UST Corrective Action
            Technologies: Site Assessment and Selection of Unsaturated Zone
            Treatment Technologies (EPA/600/2-90/011,  1990) has since been
            prepared by RREL-RCB];

      •     Preparation of design and operation guidance that would provide
            technical procedures for the application and evaluation of SVE
            technology;

      •     Development of a site screening procedure that could be used to
            assess whether SVE technology would be a viable option for site
            specific remediation;

      •     Performance of site screening procedures and field testing to
            assess SVE usefulness;

      •     Generation of data that could be used to estimate the site
            specific cost for site evaluation, screening, design and operation
            of SVE systems.

      The following  section addresses more fully the research and development
ideas discussed during the workshop.

RESEARCH AND DEVELOPMENT

      The ORD staff  organized the SVE workshop reported on herein to develop a
picture of the current state-of-the-art  of SVE and to identify areas where
additional research  is needed.  This section provides a brief synopsis of the
thoughts expressed during the workshop on research and development needs.  The
experts generally agreed that ORD should undertake additional studies  in the
following three areas:

      •     Basic Research

      •     Applied  Research and  Development

      •     Regulatory Support

Basic Research

      Workshop participants agreed that  basic research into  the behavior of

                                       4

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contaminants in the subsurface environment is warranted.   The type of research
projects that should be given consideration are those that are predicated on
the scientific method.  Development of experimental designs that provide for
generation of data to which statistical confidence limits and values can be
assigned are necessary to allow for further clarification of soil vapor
functions.  Scholarly basic research serves as the foundation on which
engineering advancements are built.  The soundness of the engineering
applications rests on the basic research finding.   Failure of the engineering
structure is often the result of faults in basic research.

      Basic research in the following areas was recommended by workshop
participants:

      •     Interaction of immiscible phase liquids in the capillary zone with
            unsaturated zone infiltration and saturated zone transport.

      •     Correlation of soil vapor and soil contaminant concentrations.

      •     Contaminant fate and transport in the unsaturated zone.

      •     Enhancement of biotreatment by SVE.

      •     Refinement of mathematical models to address complex soil
            environments.

      •     Identification of removal mechanisms mediating SVE clean-up.

      •     Factors affecting contaminant diffusion and retention in soils.

      •     Moisture content impacts on effective porosity and contaminant
            migration.

      •     Pulsed venting and desorption kinetics.

      •     The influence of micromorphological features on vapor migration.

      •     Ganglion formation and residual characterization.

Applied Research and Development

      Applied research serves as a test of the findings of basic research.
Field verification can be used to identify the limitations of extrapolating
data obtained under controlled laboratory conditions to complex field
situations.  Applied research is undertaken not to refute theoretical findings
of basic research but rather to obtain data that can be used to define the
practical limitations of a system.

      Applied research may also yield information on techniques that could
potentially be applicable to SVE systems.   The recent advancement in optical
fiber technology provides an example of such an application.  Fiber  optics are
gaining acceptance for use in analytical applications.   Use of fiber optics in
soil gas survey and monitoring of SVE systems is one area where applied

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research and development could yield advancement in engineering of SVE
systems.  Another example is the use of high-frequency radio waves for soil
decontamination (Dev and Downey, 1988).

      Most SVE vendors attending the workshop were adamant in their opinion
that EPA should not be in the "business" of developing technologies.   The
development of technologies should be left to private enterprise,  according to
the vendors.   They felt that divulging proprietary information would not be
beneficial for advancement of SVE technology.  Although it was recognized that
some firms currently offering SVE services may not be cognizant of all the
intricacies of soil environment or contaminants, or the limitations of the
systems, the general consensus of the vendors was that the private sector
should be allowed to advance the technology.  In a free market, they contend,
the SVE vendor offering the "better mousetrap" would be in the best position
to advance the state-of-the-art.

      Field performance evaluation of SVE has been undertaken at several sites
with varying success.  In some cases field tests have resulted in data that
have not yielded meaningful conclusions regarding the system's effectiveness.
These shortcomings may be due to the limited understanding of the fundamentals
of vapor behavior and limitations of field and laboratory instrumentation and
analytical techniques.  The design and operation of adequately characterized
evaluations would allow results to be extended to better understand SVE
behavior at all sites.

      During the workshop roundtable discussion it was noted that field
demonstrations have in some cases preceded the basic research needed to
adequately design the field tests.  It was suggested that demonstration
projects that yield inconclusive results may actually hamper future SVE
efforts.  The vendors who took part in the discussions were especially
sensitive to the ramifications resulting from poorly designed field
demonstration projects that could adversely  influence regulatory and public
perceptions about SVE technology.  To forestall generation of faulty data,
field demonstration projects should be undertaken only when the techniques and
methods used will yield valid data.

      Based on the discussions held during the workshop it was concluded that
SVE applied research and development efforts should be concentrated in the
following areas:

      •     Development of  standardized SVE  system monitoring and data
            interpretation  approaches.

      •     Definition of temporal variations in overall extracted vapor
            quality, as well as the relative proportions of individual
            constituents.

      •     Determination of residual  levels of contaminants  in the soil at
            the conclusion  of SVE.

      •     Determination of the  relationship between extracted gas flow and
            the resultant zone  of  influence  and upon cleanup  times.

-------
      •      Refinement  of  mathematical models  to permit modeling of more
            complex soil conditions  such  as  layered, heterogeneous soil
            systems.

      •      Evaluation  of  the  effect of refrigerated condenser units  on gas
            stream humidity and granular  activated  carbon  life, and the
            quality of  condensate  from the  refrigerated unit.

      •      Development of novel approaches  and enhancements  for SVE.

      •      Identification of  variables that affect the rate  of contaminant
            extraction  and correlation of these variables  with extraction
            rates.

      •      Evaluation  of  SVE  effectiveness  for petroleum  products and dense
            halogenated products.

      •      Development of strategies to  maximize  saturated zone dewatering  in
            the vicinity of immiscible phase liquids.

      The American Petroleum Institute  (API) is currently  sponsoring  a number
of research projects related to SVE that  may provide  information on  these
topics.   API research currently underway  includes:

      •      Field evaluation of SVE, including measurement of the  residual
            hydrocarbon content in the  soil prior  to  and  following vapor
            extraction.

      •      Quantification of mass balance hydrocarbon losses from sand
            columns.

      •      Pilot scale evaluation of soil venting.

      •      Field scale evaluation of SVE systems  and venting well
            configuration.

      •      Quantification of the efficacy of subsurface  venting in
            controlling vapor migration using multiple extraction vents.

Regulatory Support

      Research to support regulatory development and implementation was
identified as a major area of concern by SVE vendors,  contractors,
researchers, and regulators.  The frustration of not being able to define an
acceptable range of values for  site clean up or even to define the parameter
to be measured, was evident in  the discussions during the workshop.   It was
agreed,  despite the complexity  of risk assessment and site specific
conditions, that efforts  should be made to standardize techniques for
measuring contaminant concentrations in soil vapors and in soils that could be
used as guidance by state regulators in setting standards.  The lack of any
clear guidance has led to the development of widely divergent cleanup
standards.  The areas requiring emphasis include:

                                      7

-------
      •     Establishment of risk based cleanup criteria for contaminants in
            soils under differing conditions.

      •     Standardization of soil gas sampling techniques.

      •     Standardization of laboratory analytical techniques for soil
            petroleum contaminant quantification.

      •     Guidance on acceptable sampling frequencies or statistical
            confidence intervals and limits for determining attainment of
            cleanup criteria and standardization of data reporting units.

REGULATORY CONSIDERATIONS

      An important aspect of SVE design for each site is establishing a
cleanup goal and a protocol that will be applied to ensure attainment of that
goal, prior to commencement of the remediation effort.   The clean up goal
serves initially to guide the selection of the most appropriate remedial
method and will also signal when site remediation has been achieved.  This
section provides an overview of the regulatory climate for both soil cleanup
and air discharge.

Soil Criteria

      Federal UST regulations (40 CFR 280) do not include specific soil
cleanup standards; however, cleanup standards are suggested.  A brief synopsis
of the relevant subparts of the federal regulations governing UST corrective
action plans follows.

Corrective Action Plan (40 CFR 280.66) --

      (a)   Upon review of information from 40 CFR 280.61 - 280.63, the agency
            may require owners and operators to submit additional information
            or to develop a corrective action plan.

      (b)   Plan must provide for adequate protection of human health, safety
            and environmental protection, and should include:

            •     characteristics of released substance, including toxicity
                  and mobility

            •     hydrogeologic characteristics of facility and area

            •     proximity, quality, and present and future uses of nearby
                  surface and ground water

            •     potential effects of residual contamination on surface and
                  ground water

            •     exposure assessment

-------
            •     any information compiled during fulfillment of these
                  requirements

      (c)    Upon approval,  implement plan;  monitor,  evaluate, and report
            results to the  satisfaction of the implementing agency

      (d)    Corrective action may begin prior to approval if:

            •     notify agency of intention to begin cleanup

            •     comply with agency conditions

            •     incorporate these measures into corrective action plan
                  submitted to agency for approval.


Public Participation (40 CFR 280.67) --

      (a)    If corrective action plan is required, the implementing agency
            must notify affected public

      (b)    Agency must make information and decisions about corrective action
            plan available to the public

      (c)    Before corrective action plan approval,  agency may hold a public
            meeting to consider comments if sufficient interest exists

      (d)    If corrective action plan has not achieved target, and termination
            of the plan is being considered, agency must notify the public.


      Soil cleanup targets are determined on a site specific basis based upon
state and local regulations and guidelines.  Site specific criteria may be
influenced by the location of the site as it affects human health and the
environment.  Remediation guidelines are generally set based on the risk
associated with the contamination of public and private drinking water
supplies or other exposure pathways (e.g., vapor exposure).  Factors that are
considered in the establishment of clean-up standards include:  location of
drinking water supplies; public utilities (such as sewers) in the area that
may provide a pathway for future migration, and nearby buildings in the area
that may be at risk of an explosion.

      The determination of when a site remediation system is to be shut down
and the site considered "clean" may require a site specific risk assessment.
The following procedures serve as a guide for development of the risk
assessment (Hinchee et al., 1986):

      • Site characterization - soil types, groundwater levels and flow
        direction, bedrock formations.

      • Hazard identification - hazardous substances at the  site that
        could pose a threat to human health and environment.

-------
      « Fate analysis -  pathways through which these hazardous  substance
        may migrate (i.e.  ground water,  surface water,  direct,  air,  etc.)

      • Exposure assessment -  level and degree of exposure to the
        hazardous substances via the site of the receptors.

      • Risk determination - Risk is generally reported as a value related to
        the number of individuals exposed or at risk.   For example,  a value of
        15-6 indicates that one out of one million people exposed is at risk.

      • Risk management -  levels to which contamination is reduced to
        achieve "acceptable" risk.   "Acceptable risk" is typically
        dejfined as falling between IE-5 and IE-7.

      This risk assessment approach can be standardized on a computer or other
method to rank each pathway for "risk," from some minimum to some maximum.
The State of California uses its "LUFT" (Leaking Underground Fuel Tank) field
manual for this purpose (Daugherty, 1984).  These risks are then weighted and
summed to form a hazard ranking, with high risk sites receiving more priority
and low ranking sites receiving lower cleanup priority.  Since many more sites
exist than can be addressed immediately, prioritization is necessary, and a
risk based system seems most reasonable.  Low risk sites may best be addressed
by limited initial response followed by a determination of the need for long
term remediation (Duchaine, 1986).

      Appendix K provides a listing of soil cleanup criteria for 50 states and
the District of Columbia.   This table was compiled through a telephone survey
conducted in July and August,  1989.  The table lists the office in each state
that oversees soil cleanup, and the name and telephone number of a contact in
that office.

      This Appendix also lists the allowed residual.  Each state falls into
one of several groups:  states that have no regulations; states that determine
the allowed residual on a site-by-site basis; and sites that have specific
attainment standards, usually based on total petroleum hydrocarbon (TPH)
residual concentration.

      Eleven states have yet to adopt formal regulations:  Alaska, Arkansas,
Colorado, Connecticut, Florida, Idaho, Iowa, Kentucky, Maine, Pennsylvania,
and Wyoming.  Many states surveyed indicated that no specific standards have
been established but rather each site is treated on a case-by-case basis.
This approach, while more subjective, allows for prioritization of the sites
based upon perceived risks.  Three states -- California, Oregon, and Virginia
-- use risk assessment models to prioritize sites based on a ranking system
that takes into account toxicity, exposure pathways, and other criteria.
These models have also accounted for remedial action costs.

      Twenty two states have established specific criteria for allowable
residual in the soil.  The most common standards are based on total petroleum
hydrocarbon (TPH).  The allowable limits range from 1 ppm (Ohio) and 2 ppm
(Nebraska) to 500 ppm in Oklahoma.  Ten states use 100 ppm TPH, making it the

                                      10

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most common attainment standard.  Michigan and New Hampshire use total
volatile organics rather than TPH as a standard.  Three states use specific
compounds as cleanup criteria.  Illinois has allowed levels for BTEX:  benzene
(5 ppb),  toluene (2 ppm),  ethylbenzene (680 ppb),  and xylenes (1400 ppb).   New
Jersey specifies levels for three compounds:  benzene (0.07 mg/1), toluene
(14.4 mg/1), and dichloroethylene (0.1 mg/1).  Texas uses 500 ppm of BTX as
the indicator of cleanup.

Air Discharge Criteria

      The removal of contaminants from the subsurface and their subsequent
discharge into the atmosphere without treatment, while sometimes feasible and
legal, fails to adequately address the core issue.  Prior to 1986, though,
emission of toxic air pollutants was only minimally regulated.  In that year,
Section 112 of the Clean Air Act required EPA to establish national emission
standards for hazardous air pollutants (NESHAPS).   Since that event, some
toxic pollutants have received NESHAPS.  The Emergency Planning and Community
Right to Know Act of 1986 has further increased public awareness of air
toxics, which may lead to further and increasingly complex air discharge
standards (Stever, 1989).

      Appendix L lists the air discharge standards for 50 states and the
District of Columbia.  These  data were compiled from a state-by-state
telephone survey conducted in July and August, 1989.  Nearly half  (24 of 51)
of the states have no statewide air discharge standards and rely on federal
standards.  California has no statewide standards.  Rather, each county has
established its own limits.   Many states do not recognize SVE systems as major
discharge points and regulate these systems as general emission sources.
Since many of the general emission source laws were written primarily for
large sources like power plants, SVE systems  (which are by comparison small
sources) sometimes do not require treatment under present regulations.

      Nine states have not established specific discharge regulations but do
require permits for SVE systems.  Of these nine,  two states, Kansas and
Oregon, require permits only  for sites that discharge more than 10 tons of
VOCs per year.

      Seventeen states have discharge limits  expressed on a mass per time
basis.  The states have widely varying allowable  limits, however;  for example,
North Carolina allows up to 40 pounds per day, while the District  of Columbia
allows only 1 pound per day to be discharged.   Other states have  compound
specific emission limits.  Connecticut, for  example, lists over one hundred
compounds with an allowable limit based on both an 8-hour average  and a
30-minute average.  Some states  (e.g., Vermont) require  that  the vapor
concentration in the  influent stream be reduced by up  to 85 percent.

       In summary, the states  vary widely  in  their air  emission  regulations,
from  little or no formal regulation to contaminant-specific mass  discharge
rates.  Some base their standards on the  concentration at the nearest
receptor, while others  treat  each site on a  case-by-case basis.
                                       11

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FORMAT OF THIS REPORT

      The main text of this report is an assessment of the state-of-the-art of
soil vapor extraction technology.   It was written specifically for state and
local regulators, agency staff, environmental managers,  remedial contractors
and consultants who desire a basic understanding of SVE principles,
applicability, operation, arid cost.

      A general overview of the theoretical considerations applicable to soil
vapor extraction is provided in Section 2.  This section includes discussions
of the effect on SVE of contaminant properties,  including vapor pressure,
solubility, Henry's law constant,  boiling point, soil sorptlon coefficient,
contaminant composition and weathering and soil  properties such as structure,
moisture content, texture, air permeability, and temperature.  Section 2 also
discusses gaseous flow in the subsurface environment, including the equations
that govern subsurface vapor flow.  Finally, several field methods of
determining the soil's air permeability are presented.

      Section 3 provides an overview of site investigation approaches that can
be used to obtain data necessary to determine if vapor extraction is a viable
remedial option and, if so, obtain critical design information.  This section
also includes references to field techniques and equipment used to evaluate
the site specific feasibility of vapor extraction.

      General design approaches, Including the determination of the air
permeability, well selection and system configuration, are described in
Section 4.  In addition, this section discusses  the components that comprise
an SVE system.  The purpose of this section is to provide the reader with a
qualitative analysis of the design procedure and the individual components to
aid in the preliminary design of such a system.

      Operation, maintenance, and monitoring of SVE systems are discussed in
Section 5.  This section also includes discussions of enhanced biotreatment
due to SVE, clean up attainment determination, including new methods for
measuring residuals; and other issues related to system operation.

      Section 6 discusses emission control methods available to treat the
extracted vapors.  Discussions are included on activated carbon adsorption,
thermal and catalytic incineration, internal combustion engines, packed bed
thermal processors, biotreatment, and direct discharge to the atmosphere.

      The costs related to SVE implementation and operation are discussed in
Section 7.  This section discusses costs  for a site investigation,
component-by-component capital costs for  SVE equipment, costs for prepackaged
units, and operations and monitoring costs for these systems.

      Ten appendices contain selected papers presented at the workshop held  in
Edison, New Jersey on June 28 and  29, 1989.  Papers reprinted here were
selected  as representative of  the wide range of topics discussed.  Appendix A
is a review of existing  SVE operations by N.E. Hutzler, B.E. Murphy, and J.S.
Gierke.   This section reports on various  aspects of SVE operations,  including
number, type, and layout of wells, type of blower or pump used, emission

                                      12

-------
control units and additional operational information.   The section will give a
reader a sound historical basis with which to view other sections.

      In Appendix B, J. Danko discusses the applicability and limitations of
SVE operation.  This paper describes the advantages of SVE and discusses, from
an engineering viewpoint, some practical observations  and advice.

      Appendix C contains a report by H.B. Kerfoot on  the use of soil gas
surveys in the design of SVE systems.  Soil gas surveys are frequently used
during the site investigation phase to help to delineate the extent of
contamination and determine the types and relative concentrations  of compounds
in the ground.  With this information, a judgment can  often be rendered
regarding the applicability of SVE for that site.

      Appendix D, by R.E. Hinchee,  D.C. Downey, and R.N. Miller, discusses the
enhancement of biodegradation that accompanies the use of soil vapor
extraction.

      P.C. Johnson, M.W. Kemblowski, J.D. Colthart, D.L. Byers,  and C.C.
Stanley contribute "A Practical Approach to the Design, Operation, and
Monitoring of In Situ Soil Venting Systems" in Appendix E.  This report
presents a structured logical approach that forms a "decision-tree" for
deciding if SVE is appropriate to be used and, if so,  describes the steps to
be taken during system design, operation, and monitoring.

      Appendix F contains a scientific approach to SVE design in a paper by
M.C. Marley, S.D. Richter, B.L. Cliff, and P.E. Nangeroni.  This paper
describes, among other things, the use of a computer model to calibrate data
obtained during a field air permeability test.

      L.R. Silka, H.D. Cirpili, and D.C. Jordan discuss in Appendix G modeling
of subsurface vapor flow and the applications of modeling to SVE.

      D.W. DePaoli, S.E. Herbes,  and M.G. Elliot describe, in Appendix H, the
performance of SVE at a jet fuel spill site in Utah.  This paper contains
knowledge and experience gained during SVE operation,  operational  results, and
a discussion of various aspects of SVE.

      Appendix I also contains actual case history results for an industrial
site that has contamination from several volatile organic and base neutral
compounds, in a report by R.D. Mutch, Jr., A.N. Clarke, D.J. Wilson, and P.O.
Mutch.  This interim report focuses on the measured zone of influence of the
extraction well, the composition of the extracted gas  and its changes with
time, the treatability of the extracted vapor by granular activated carbon,
temperature variations that occur in the system, and groundwater upwelling due
to the induced vacuum. The authors also describe the use of a mathematical
model in their work.

      A report by G.E. Hoag in Appendix J comments on  recent SVE research
developments and research needs.   These discussions follow a summary of SVE
"research milestones".


                                      13

-------
      Appendices K and L to this document contain responses to a
state-by-state survey  conducted in August,  1989 regarding the allowable soil
residual and air discharge criteria.
                                       14

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                                   SECTION  2

                      PRINCIPLES OF SOIL VAPOR BEHAVIOR
INTRODUCTION

      Products released into the subsurface environment from UST systems are
acted upon by numerous forces that influence the degree and rate at which they
migrate from the source.  The extent to which the released products partition
into the vapor phase is dependent upon the characteristics of the product, the
nature of the subsurface environment, and the elapsed time since the release
occurred.  The manner by which the released product behaves in the subsurface
will have a significant bearing on whether soil vapor extraction could be an
appropriate corrective action for the site under consideration.

      Figure 1 is a nomograph that uses the soil's permeability to air flow,
the contaminant vapor pressure, and the time since release to predict the
likelihood of success of a soil vapor extraction system.   To use the
nomograph, start at the time since release and draw a horizontal line to the
appropriate soil air permeability.  At the match line, draw a straight line to
the vapor pressure.  The point where this line intersects the continuum
indicates how likely SVE would be at a site with that set of conditions.

      This section provides a theoretical overview of the factors that
influence contaminant fate and vapor phase transport in the vadose zone.  The
basic principles that govern soil vapor behavior and transport are identified
to provide a sound basis for decision making with regard to site
investigations, pilot testing, system design, operation arid monitoring.

      This section consists of three subsections:  contaminant
characteristics, soil environment and vapor transport.  Contaminant
characteristics considered include the physical arid chemical properties of the
released compounds, the composition of major petroleum classes,  and how those
properties affect their behavior in soil.   Characteristics of the soil
environment include those properties that affect the fate and transport of the
released products.   The section on vapor transport integrates the effects of
the contaminant characteristics and the soil environment to show how vapor
movement depends on those characteristics.  The discussion focuses on how the
product and soil factors influence the applicability of SVE systems, and also
discusses the equations governing vapor flow under the influence of a vacuum.

CONTAMINANT CHARACTERISTICS

      The physical and chemical properties of the released product control,  to
great extent, the movement and ultimate fate of that product in the
subsurface.   A product's properties affect the distribution of the product

                                      15

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VAF
PRES
|
Butane — ^-
Pentane — ^-
Benzene — ^~
Toluene — ^-
Xylene — **•
Phenol — ^"
Naphthalene — ^~
Aldicarb — ^-
-
S\
LIKEL
'OR 0
SURE SUC(
Lo4
-103
-102
-i^"^
-10"1
-1C'2 I
-ID'3
-io-4
/E
HOOD
F J
:ESS PER
l
SUCCESS
VERY
LIKELY
•^ 	
SUCCESS
SOMEWHAT
LIKELY
: SUCCESS
LESS
LIKELY
SOIL AIR
MEABILITY
i
•'f^-
' i -'"
Wf'f" < '
¥|i;
>:?y
10
-' t^-i
, &%
- *.> *
$,'Z&,t,,l',
t
•nJtmmi
> :
> .
t
r :
t
f :
t !


HIGH
(gravel,
coarse
sand)
MEDIUM
(fine sand)
LOW
(clay)
TIME
SINCE
RELEAS
Weeks
Months
Years
Weeks
Months
Years
Weeks
Months
Years

                     Match Point
Figure 1. SVE Applicability Nomograph
               16

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among the four main phases in which the product may exist once released to
soil (EPA, 1988a):   (1) vapor, (2) dissolved in pore water,  or (3)  sorbed to a
soil particles and colloids,  and (4) as a non-aqueous phase  liquid (NAPL)
(Figure 2).  The product's properties and other factors then affect the
movement of each phase through the soil zone and how the compound's properties
and movement change with time.

      The distribution of a compound among the four phases can be described by
several parameters.  The degree to which a compound partitions into the vapor
phase is described by that compound's vapor pressure and Henry's law constant.
The soil sorption coefficient, K^, describes the tendency of a compound to
become adsorbed to soil.  The solubility describes the degree to which a
product will dissolve into water.   The distribution of a product among the
four phases varies with changes in site-specific conditions  and will also
change over time in response to weathering.  It is important, however, to
understand the mode in which a product exists prior to predicting transport of
that product.

      A product's volatility is directly associated with how much of that
product will partition into the vapor phase in the soil gas.  Highly volatile
products, such as gasoline and certain chlorinated solvents, have a greater
tendency to exist in the vapor phase than do other, less volatile products
such as heating oil.  The volatility of a product is perhaps the most
important characteristic affecting the applicability of soil vapor extraction
to that compound.  The parameter that best describes a compound's volatility
is its vapor pressure.

Vapor Pressure

      All solids and liquids possess a vapor pressure, which is a measure of
the tendency of the substance to evaporate.  Conceptually, vapor pressure can
be considered analogous to the solubility of the material in air at a given
temperature.  Higher vapor pressures reflect an increased tendency to
volatilize.

      Vapor pressure is the force per unit area exerted by the vapor of the
chemical in equilibrium with its pure solid or liquid form (Weast,  1981).  For
example, vapors from gasoline in a container will evaporate  from the surface
of the liquid gasoline and diffuse throughout the space occupied initially by
air until an equilibrium is reached.  The gasoline vapor that occupies the
space above the liquid surface at equilibrium will exert a pressure on the
container - the vapor pressure.  The pressure within the container can be
measured by the height to which it raises a column of mercury.  Therefore
vapor pressure is typically expressed in terms such as millimeters of mercury
(mm Hg).

      Chemicals with vapor pressures of less than IE-7 mm Hg would generally
not be expected to volatilize to a significant degree under  ambient conditions
(Dragun, 1988).  Chemicals with vapor pressures of greater than 0.5 mm Hg
would be expected to volatilize to a significant degree when released from an
underground storage tank; these chemicals would be expected  to respond to SVE
technology (Bennedsen et al., 1985).  Several of the major components of

                                      17

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SAND
PARTICLES
SILT
CLAY
ORGANIC
MATTER
            GROUNDWATER
            TABLE
                                                       UNSATURATED
                                                       ZONE
                                                        CAPILLARY ZONE
                           SATURATED
                           ZONE
                                 BEDROCK
   PETROLEUM
i  i PRODUCT
   VAPORS
   IN PORE
   SPACE
                                                                SORBED TO
                                                                      SOIL
                                                                  PARTICLE
                                                                   ©
                                 PETROLEUM
                                   PRODUCT
                                  DISSOLVED
                                     IN SOIL
                                   MOISTURE
                 CLEAN
                 SOIL
         SOIL CONTAMINATED BY
       PETROLEUM HYDROCARBONS
                RELEASE
                 Figure 2. Unsaturated Zone Contaminant Phases
                                   18

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gasoline (as shown in Table 1) have vapor pressures greater than 0.5 mm Hg.
These chemicals will have a much greater fraction of vapors in the subsurface
environment.  Thus, sites where release of these chemicals have occurred are
likely candidates for remediation by SVE.

      Temperature has a strong influence on the vapor pressure of a
compound, with the vapor pressure increasing dramatically as temperatures
increase.  Jury et al.  (1987) state that for most intermediate molecular
weight organic compounds, vapor pressure increases three to four times for
each 10 degree C increase in temperature.   In general, vapor pressure may be
approximated by:

                              VP(T)  =  A exp(-B/T)

where T is the temperature in degrees Kelvin and A and B are constants
characteristic of the substance. Vapor pressure must always be reported with
the temperature at which the pressure was measured.  Table 1 lists vapor
pressures at a temperature of 20 degrees C.

      The temperature of the subsurface environment therefore has an important
influence on SVE technology.  Research is currently underway to evaluate the
effectiveness of steam injection or soil heating to increase volatilization
and thereby increase the efficiency of soil vapor extraction systems (Hunt et
al., 1986; Lord et al., 1987).

Water Solubility

      The solubility controls the degree to which a product dissolves into
ground water and pore water present in the vadose zone.  Constituents present
in petroleum products have widely varying solubilities.  Table 1 shows that
phenols, for example, are highly soluble while the heavier alkanes are only
sparingly soluble.

      Water solubility has an important impact on the movement of released
product  in  the  soil.  Soluble products are likely to  dissolve in infiltrating
precipitation and move away  from the  source.  Also, soluble products will move
into the saturated zone and be  transported away from  the site, provided that
the release is  of  significant volume.  A fluctuating water table is another
means of inducing  groundwater contamination from soluble products.

Henry's  Law

      The volatilization of product dissolved in water  is governed by Henry's
law, which  states  that the concentration of a volatile  chemical in a liquid  is
proportional to the partial pressure  of the volatile  chemical at low
concentrations.  The proportionality  constant that relates concentration in
solution to the partial pressure is known as Henry's  law constant (Ky).
Henry's  constant may be reported as "dimensionless" or  as atm*m /mole.

      The Henry's  law constant  describes the relative tendency for a
contaminant in  solution  to exist in the vapor phase.  It is analogous to the
vapor pressure, which describes the partitioning behavior between a pure

                                       19

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      TABLE 1.  CHEMICAL PROPERTIES OF HYDROCARBON CONSTITUENTS
Chemical Class
n-Alkanes
C4
C5
C6
C7
C8
C9
CIO
Mono- aroma tics
C6
C7
C8
C8
C9
CIO
Phenols
Phenol
Cl-phenols
C2-phenols
C3-phenols
C4-phenols
Indanol
Di-aromatics
Representative Liquid Density
Chemical (g/cm.3)
@ 20°C

n-Butane
n-Pentane
n-Hexane
n-Heptane
n-Octane
n-Nonane
n-Decane

Benzene
Toluene
m-Xylene
Ethylbenzene
1 ,3 ,5-Trimethylbenzene
1 ,4-Diethylbenzene

Phenol
m-Cresol
2,4-Dimethylphenol
2,4 ,6-Trimethy Iphenol
m-Ethylphenol
Indanol
Naphthalene

0.579
0.626
0.659
0.684
0.703
0 718
0.730

0.885
0.867
0.864
0.867
0.865
0.862

1.058
1.027
0.965
NA
1.037
NA
1.025
Henry's
Law
Constant
(dim.)

25.22
29.77
36.61
44.60
52.00
NA
NA

0.11
0.13
0.12
0.14
0.09
0.19

0.038
0.044
0.048
NA
NA
NA
NA
Water
solubility
(mg/L)
@ 25°C

61.1
41.2
12.5
2.68
0.66
0.122
0.022

1780
515
162
167
72.6
15

82000
23500
1600
NA
NA
NA
30
Pure Vapor
Pressure
(mmHg)
@ 20°C

1560
424
121
35.6
10.5
3.2
0.95

75.2
21.8
6.16
7.08
1.73
0.697

0.529
0.15
0.058
0.012
0.08
0.014
0.053
Vapor Density
(gAn3)
@ 20°C

4960
1670
570
195
65.6
22.4
7.4

321
110
35.8
41.1
11.4
5.12

2.72
0.89
0.39
0.09
0 53
0.1
0.37
Soil Sorption
Constant (Koc)
(L/kg)
@25°C

250
320
600
1300
2600
5800
13000

38
90
220
210
390
1100

110
8 4
NA
NA
NA
NA
690
NOTE:  NA - Not available



SOURCE:  EPA, 1990.
                                          20

-------
substance (rather than a contaminant In solution)  and its vapor phase.
Similar to vapor pressure, Henry's law constant is highly temperature
dependent and increases with increasing temperatures.  Munz and Roberts (1987)
state that each rise of 10 degrees C in temperature corresponds to an increase
in the Henry's constant of 1.6 times.

      The Henry's constant may be the more appropriate partitioning constant
outside of the free product zone, where product is likely to exist in solution
with pore water (Stephanatos,  1988).  Henry's constant also applies in regards
to volatilization of contaminants from the groundwater into soil gas.

      Table 1 lists the Henry's law constants for several common hydrocarbons
present in gasoline.  The constituents listed in this table all have Henry's
law constants greater than 0.01 (dimensionless),  the level at which compounds
have significant volatility making soil vapor extraction attractive (Danko,
1989; Hutzler et al.,  1989a).   Gasoline is particularly well-suited to SVE due
to its high composite volatility  (for fresh gasoline, KH = 32).  Other
petroleum products, such as fuel oil No. 6, are less amenable than gasoline to
removal by SVE due  to their lower volatility.  However, researchers have
reported on the successful removal of petroleum products other than gasoline
through SVE; for example, DePaoli et al. (1989) report the successful removal
of JP-4 from a site in Utah.

Boiling Point

      At a compound's boiling point, its vapor pressure equals the vapor
pressure of the atmosphere.  At sea level the pressure exerted by the
atmosphere is 760 mm Hg.  As elevation  increases above sea level, atmospheric
pressure decreases  rapidly; for example, at an elevation of 15,000 ft msl the
atmospheric pressure is 451 mm Hg.  With a decrease  in atmospheric pressure
comes a reduction in the boiling point.  At a pressure of 451 mm Hg, for
example, water boils at a temperature of 86 degrees  C, significantly less than
the boiling point at sea  level (100 degrees C).

      The relationship of boiling point to vapor pressure has  important
implications with regard  to SVE.  This  relationship  describes  a driving force
for movement of vapors from the liquid  to the gaseous phase during vacuum
extraction.  Inducing a vacuum in the soil causes the pressure in the soil
pore space to decrease.   This, in effect, depresses  the boiling point and
assists in driving  the contaminant  into the vapor phase.  While the actual
magnitude of boiling point depression due to the induced vacuum is not always
a major factor in its volatilization, understanding  this phenomenon should
provide an appreciation of the factors  that might influence SVE operation.

Soil Sorption Coefficient

      Sorption of contaminant liquids to soil particles and organic matter  is
a very important factor:  it controls the distribution of released products  in
the  soil zone and has a very strong effect on the movement of  the product
through the vadose  zone.  In many cases the majority of the released product
may  exist in the sorbed phase.  Hinchee et al. (1987) state that for a 1000
gallon release at a "typical" site  (as  defined in the paper),  the distribution

                                      21

-------
of the gasoline may be 962 gallons in the soil,  25 gallons in the soil gas,
and 13 gallons in the groundwater.   While these  values are theoretical and
dependent on many assumptions,  the values do indicate the importance of
considering the product that remains in the soil.

      The sorption of a product to soil and organic matter is described by the
contaminant's soil sorption coefficient,  K^.  Values for K^ are not always
readily available, so the more common octanol-water coefficient, KQW,  is often
used as a surrogate for the soil sorption coefficient.  The sorption
coefficient describes the tendency for a product to sorb to the soil or
organic matter.  Table 1 lists the octanol-water coefficients for common
hydrocarbon constituents.  As this table shows,  a strong relationship exists
between the number of carbon atoms and the sorption coefficient, with larger
molecules having a much greater tendency to sorb.   This explains in part why
compounds such as No. 6 fuel oil, which is high  in these "heavy fractions",
are very immobile in the subsurface (viscosity also plays an important part).

Contaminant Composition

      Petroleum hydrocarbons are the most common products released from
underground storage tanks (EPA, 1988a).  Efforts to address the remediation of
sites contaminated by releases from USTs often focus on the properties of the
particular motor fuel that leaked, and on how the properties of that product
affect the fate, mobility, and persistence in the ground.  Petroleum
hydrocarbons, however, are composed of many different compounds, each with
different chemical and physical properties.  Gasoline, for example, is a
mixture of up to 200 compounds (COM, 1986).  Similarly, diesel fuel, heating
oil, and other types of petroleum contain many different compounds in varying
fractions.  The characteristics of the bulk product (e.g., gasoline) will
reflect the characteristics of the compounds that comprise the bulk product
and the fraction of the whole that each constituent comprises.

      Table 2 lists gasoline constituents, excluding additives, and the
fraction each constituent comprises in a typical mixture, for both fresh and
"weathered" gasoline.  "Weathering" refers to the changes in the nature of a
chemical mixture after its release into the environment.  Weathering over time
changes the product composition and will affect the ease with which that
product is removed using SVE.  Figure 3 shows the change in the composition of
gasoline following weathering.

      For example, each constituent of gasoline possesses a vapor pressure
unique to that compound: its pure chemical vapor pressure.  The vapor pressure
of the gasoline is equal to the weighted average (by mole fraction) of the
vapor pressures of all of the constituents, according to Raoult's law.
Raoult's law states that the partial pressure of a volatile component (i)
above a liquid mixture is given by:

                                 Pi = Pi'  * Xi

where p^' is the vapor pressure of pure component i and X^ is the mole
fraction in the liquid.


                                      22

-------
       TABLE 2.  COMPOSITION OF FRESH AND WEATHERED GASOLINES
Compound  Name
                                       MW (el
                                                        Fresh  Gasoline
                                                                                Weathered Gasoline
propane
isobutane
n-butane
trans-2-butene
cis-2-butene
3 -methyl- 1 -butene
isopentane
1 -pentene
2-methyl-l -butene
2-methyl- 1 ,3 -butadiene
n-pentane
trans-2-pentene
2-methytl-2-butene
3 -methyl- 1 ,2-butadiene
3 ,3 -dimethyl- 1 -butene
cyclopentane
3 -methyl- 1 -pentene
2,3 -dimethylbutane
2-methylpentane
3-methylpentane
n-hexane
methylcyclopentane
2,2-dimethylpentane
benzene
cyclohexane
2,3-dimethylpentane
3 -methylhexane
3-ethylpentane
n-heptane
2,2,4-trimethylpentane
methylcyclohexane
2,2-dimethylhexane
toluene
2,3 ,4-trimethylpentane
3 -methylheptane
2-methylheptane
n-octane
2,4,4-trimethylhexane
2,2-dimethylheptane
ethylbenzene
p-xylene
m-xylene
3,3,4-trimethylhexane
o-xylene
2,2,4-trimethylheptane
n-nonane
3 ,3 ,5-tnmethylheptane
n-propylbenzene
2,3,4-trimethylheptane
1,3,5-trimethylbenzene
1 ,2,4-ttrimethylbenzene
n-decane
methylpropylbenzene
dimethylethylbenzene
n-undecane
1 ,2,4,5 ,-tetramethylbenzene
1,2,3,4,-tetramethylbenzene
1 ,2,4-trimethyl-5-ethylbenzene
n-dodecane
naphthalene
n-hexylbenzene
methylnaphthalene
44.1
58.1
58.1
56.1
56.1
70.1
72.2
70.1
70.1
68.1
72.2
70.1
70.1
68.1
84.2
70.1
84.2
86.2
86.2
86.2
86.2
84.2
100.2
78.1
84.2
100.2
100.2
100.2
100.2
114.2
98.2
114.2
92.1
114.2
114.2
114.2
114.2
128.3
128.3
106.2
106.2
106.2
128.3
106.2
142.3
128.3
142.3
120.2
142.3
120.2
120.2
142.3
134.2
134.2
156.3
134.2
134.2
148.2
170.3
128.2
162.3
142.2
0.0001
0.0122
0.0629
0.0007
0.0000
0.0006
0.1049
0.0000
0.0000
0.0000
0.0586
0.0000
0.0044
0.0000
0.0049
0.0000
0.0000
0.0730
0.0273
0.0000
0.0283
0.0083
0.0076
0.0076
0.0000
0.0390
0.0000
0.0000
0.0063
0.0121
0.0000
0.0055
0.0550
0.0121
0.0000
0.0155
0.0013
0.0087
0.0000
0.0000
0.0957
0.0000
0.0281
0.0000
0.0105
0.0000
0.0000
0.0841
0.0000
0.0411
0.0213
0.0000
0.0351
0.0307
0.0000
0.0133
0.0129
0.0405
0.0230
0.0045
0.0000
0.0023
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0069
0.0005
0.0008
0.0000
0.0095
0.0017
0.0021
0.0010
0.0000
0.0046
0.0000
0.0044
0.0207
0.0186
0.0207
0.0234
0.0064
0.0021
0.0137
0.0000
0.0355
0.0000
0.0447
0.0503
0.0393
0.0207
0.0359
0.0000
0.0343
0.0324
0.0300
0.0034
0.0226
0.0130
0.0151
0.0376
0.0056
0.0274
0.0012
0.0382
0.0000
0.0117
0.0000
0.0493
0.0705
0.0140
0.0170
0.0289
0.0075
0.0056
0.0704
0.0651
0.0000
0.0076
0.0147
0.0134
TOTAL
                                                         1.0000
                                                                                 1.0000
SOURCE:  Johnson et al. (1989)
                                           23

-------
                             (10%)
       (45%)
                 FRESH GASOLINE
               WEATHERED GASOLINE







                 [2]  OLEFINS




                 ^  AROMATICS




                 PI  ALIPHATICS
                                        (45%)
                                        90%
SOURCE: R.L JOHNSON, 1989
Figure 3. Effect of Weathering on Gasoline Composition
                        24

-------
      Gasoline released into the subsurface will change composition as time
passes because the more volatile components will partition into the vapor
phase.  This phenomenon will result in the remaining gasoline having a lower
vapor pressure because it is relatively higher in the low-volatility
compounds.  Similarly, the more soluble components will preferentially
dissolve versus the less soluble components, leaving the remaining gasoline
less soluble overall.

      The concept of weathering applies to SVE due to the effect SVE has on
the composition of the residual product.  At startup, the vapors removed by
SVE are likely to be composed mainly of the more volatile, lighter-end
fractions (molecules with five carbon atoms or fewer).   After the system
operates for some time, the extracted vapor will likely be depleted in the
lighter-end fractions and will be composed mostly of heavier compounds.
DePaoli et al. (1989) describe the occurrence of this phenomenon at a jet fuel
spill site in Utah.  Figure 4 shows how the composition of the extracted
vapors changed as operation progressed.  The figure shows that, initially,
heavier molecules (Cy and above) were a minor part of the extracted vapor, but
became an increasingly large fraction as removal continued.   This change is
due to the loss of the more volatile compounds and an increase in the partial
pressures of the less volatile compounds as their relative mole fractions in
the liquid increase.

      These concepts also have important implications with regard to SVE
applicability as a corrective action alternative.  SVE generally removes only
the more volatile contaminants, often leaving in the soil those compounds that
are less volatile.  R.L. Johnson (1989) indicates that 20 to 35 percent of the
product as total petroleum hydrocarbons (TPH) will remain in the soil.  This
residual will be composed predominantly of C^Q and above hydrocarbons.  These
residual components may or may not be important from a risk standpoint, but it
should be recognized that, depending on the standards for allowed residual,
SVE may not remove all product constituents to an acceptably low level.

      The air permeability of the soil incorporates the effects of several
soil characteristics, the most important of which are soil structure,
stratigraphy, air-filled porosity, particle size distribution, water content,
residual saturation, and the presence or absence of macropores.

SOIL ENVIRONMENT

      The soil environment, like the product characteristics, has a
significant effect on the fate and transport of products released into the
subsurface.  Differences in the soil type, structure and stratigraphy, the
particle density, and the porosity will affect the ease with which vapors will
pass through the soil and will determine the air permeability of the soil
zone.  Other parameters, such as moisture content and organic matter content,
will affect air movement through the soil zone and, more importantly, may
retard the transport of contaminants in the soil gas by adsorption onto the
soil or dissolution into pore water.  Because soil properties have such an
important effect on the movement of soil gas, the efficacy and design of soil
vapor extraction systems are largely dependent on the soil properties.


                                      25

-------
                                     WEIGHT PERCENT
          §
          -o
          B)
          O
         
-------
Air Permeabili-ty

      The  soil's permeability  to  air flow is simply  a measure of the  ability
of vapors  to flow through porous  media.  It is analogous to the permeability
to water flow in the saturated zone.  The permeability of the soil  to air flow
is perhaps  the single most  important parameter with  respect to the  success of
soil venting.   It is a key  parameter not only in  deciding if SVE is a feasible
remedial option, but also for  the SVE system design  (Johnson et al.,  1989).

      The  air-filled porosity  is  a basic determinant of the volume  available
for vapor  transport.  Water content, or the percentage of the porosity filled
with water,  has a great effect on the pore space  available for vapor
transport,  as discussed above.  The air permeability is significantly
influenced by the density and  viscosity of the soil  gas (Krishnayya et al.,
1988).  The density and viscosity are in turn affected by temperature.   Table
3 shows the effect of temperature on water and air properties.  The table
developed  from theoretical  considerations, shows  the ratio of hydraulic
conductivity to air conductivity  at various temperatures, and how  this ratio
increases  with increasing temperatures.  This ratio  can be used to  estimate
the air permeability of a soil from its hydraulic conductivity.

Porosity

      Vapor migration in the subsurface occurs principally through  air filled
pore spaces.  Diffusion of  gases  through liquid filled pore spaces  does occur,
albeit at  rates a thousand  times  less than that which occur through air filled
pores.  Factors influencing vapor migration include  the air-filled  porosity
(which is  affected by the water content) of a soil and the orientation of the
soil pores.
                     TABLE 3. EFFECT OF TEMPERATURE ON WATER AND
                          AIR PROPERTIES AND AIR CONDUCTIVITY
         DYNAMIC VISCOSITY
Temperature        (kg/m.S)
         Water x IP'3   Air x IP"5
                               DENSITY AT STANDARD
                               ATMOSPHERIC PRESSURE
                                    (kg/m3)
                               Water       Air
 HEAD AT STANDARD
ATMOSPHERIC PRESSURE
       (m)
 Water       Air
0
5
10
20
30
40
1.79
1.52
1.31
1.00
0.80
0.65
1.71
1.74
1.77
1.83
1.86
1.90
999.868
999.992
999.728
998.234
995.678
992.247
1.2930
1.2697
1.2472
1.2047
1.1649
1.1200
10.333
10.332
10.335
10.350
10.377
10.413
7991
8137
8284
8576
8869
9225
RATIO OF HYDRAULIC
CONDUCTIVITY TO AIR
   CONDUCTIVITY
      (Kr)

      7.4
      9.0
      10.8
      15.2
      19.9
      25.9
  NOTES:    1) Standard Atmospheric Pressure = 10.332 kg/m2
           2) Ratio of Hydraulic Conductivity to Air Conductivity:
              Kr = (viscosity of air/viscosity of water) (Density of water/Density of air)
           3) Properties of water and air are from Weast (1910)
  SOURCE:   Krishnayya et al. ,1988
                                        27

-------
Soil Structure

      The soil type affects the fate and transport of products released from
USTs and, therefore, the applicability of SVE.   The porosity,  or the fraction
of voids in the soil, is important to SVE.   Soils with higher porosities will
allow a higher flow for the same induced vacuum.  Michaels and Stinson (1989)
report that porosity appears to be an important parameter, based on their SITE
(Superfund Innovative Technology Evaluation) demonstration.  At that site,
highly permeable sands and impermeable clays (both with porosities of 40 to 50
percent) exhibited good removal rates via SVE.   Porosity is related to the
particle size distribution of the soil.  Coarse-textured soil will generally
have higher permeabilities than fine-textured soils.

Residual Saturation

      Product migrating from an UST leak may coat the soil through which it
passes.  When the release ceases, the product may continue to migrate under
the influence of gravity, soil matric potential and pressure gradients.  That
product which remains in the soil after the saturation front has passed and
after free gravity drainage has occurred, is referred to as "residual
saturation".  The amount of product that is retained at residual saturation
will vary depending on the soil characteristics and the product composition.

      Jury and Ghodrati (1989) estimate that 5 to 10 percent of available pore
space may be occupied by non-aqueous phase liquids  (NAPL) after the wetting
front moves through the soil.  Hoag and Marley  (1986) also report on the
results of column tests showing how residual saturation varies with soil type
and moisture content.  Coarse particles held less product than fine particles
at residual saturation.  Soils that were wet originally were shown to retain
from 20 to 30 percent less (for medium sand) up to  60 percent less (fine sand)
product than soil that was originally dry.  In general, residual saturation
values ranged from  12 to 60 percent saturated.

      NAPL remaining in the soil under these conditions is immiscible and not
subject to significant migration in response to wetting fronts from
infiltrating rainfall or induced wetting fronts.  This factor tends to favor
the use of SVE systems over in situ soil flushing options,  SVE induces
product to volatize  into the vapor phase, where contaminants are far more
mobile.  Thus, otherwise immobile residual liquids  are volatilized into the
vapor phase, where  they are more easily removed than as NAPL.  Soil flushing
attempts to mobilize liquid and sorbed contaminants through dissolution and a
pressure gradient.  Researchers focusing on petroleum residual saturation have
noted that once  the products have sorbed to soil, they are often extremely
difficult to remove  as a liquid.

Ganglia Formation

      Soil structure and the physical  arrangement of pore  spaces in the
subsurface influence the manner  in which contaminants migrate from a source.
Product released from an UST system will move  through soil under the influence
of  gravity and other gradients  to varying degrees and at  differing rates
depending on the soil characteristics.   In  a situation where  the soil

                                       28

-------
surrounding an UST is a uniform sand, one would expect migration at a greater
rate than in a clayey soil, unless significant cracks or macropores exist in
the clay.  Migration of fluids in soils has been shown to predominate in
macropores.  The extraction of vapor from a soil would therefore be expected
to occur initially from the larger continuous pores rather than from small
continuous pores.  As products migrate from a source they may collect in
subsurface pools, cracks, or fissures.  The isolated globules of product that
may form in soil have been referred to as "ganglia" (Jury and Ghodrati, 1989).
Ganglia may be especially prevalent in soils where immiscible petroleum
products have been released as compared to sites where miscible products have
leaked.
Subsurface Conduits

      Products released from an UST do not generally flow through soils in a
uniform wetting front.  Flow will be diverted due to variations in soil
structure and horizons.  The presence of subsurface features that are less
restrictive to flow may result in preferential flow in directions different
than what would be anticipated from a review of localized flow patterns or
soil strata.  For example, subsurface utility conduits (such as electric,
telephone, sewer, or water lines) may be bedded in gravel or materials more
permeable than surrounding soils.  Liquids and gases may migrate along these
preferential flow paths for significant distances, whereas migration in the
bulk soil mass may be limited.  Because the majority of USTs have been
installed in areas where subsurface disturbance has occurred, it is important
to consider these features in assessing vapor migration.   In general, any
feature that favors liquid product transport would also serve as a
preferential flow path for vapors.

Water Content

      Water content of the soil has competing effects on the air permeability.
The primary effect of pore water is to reduce the air-filled pore space of a
soil.  Stephanatos (1988) concludes that the movement of soil gas is reduced
as water content increases due to the physical reduction in available air
pathways.  Figure 5 shows the relationship between air permeability and water
permeability.  Stonestrom and Rubin (1989) relate the air permeability to
matric pressure and trapped air,  and also show the reduced air permeability
for higher matric pressure.  This body of research indicates that SVE would be
more successful at lower water contents since a greater percentage of the pore
space is air-filled and available for vapor transport, and thus the induced
air flow is greater for a given vacuum.

      The water content also has a significant effect on the success of SVE
through its effect on the sorption characteristics of organic compounds.
Researchers have concluded that the reduction in air-filled pore space that
occurs as water content increases serves to reduce volatilization (Farmer et
al., 1980; Aurelius and Brown, 1987).  Lighty et al.  (1988) and Houston et al.
(1989) show that the soil sorption coefficient is greater for drier soils; as
water content increases, sorption of contaminants to soil decreases as water
displaces contaminant molecules.   Reible (1989) showed that electrostatic

                                      29

-------
    100
ffl
<
UJ

cc
UJ
Q.

UJ



I

UJ
DC
                                        WATER

                                     PERMEABILITY
                        Surface soil


                     D  Subsurface soil
                10
20
30
40       50       60

  Water Saturation %
90
100
         Source: Adapted from Corey, 1957 by permission of Soil Science Society of America, Inc.



                Figure 5. Air and Water Permeability as a Function of Water Content
                                                 30

-------
force increases for drier soils, leading to reduced volatilization from the
dissolved phase into the soil gas.  Figure 6 shows schematically the effect of
varying soil moisture regimes on soil sorption.   Davies (1989) reports that
the soil sorption coefficient may be four orders of magnitude higher for dry
soils than for wet soils.

      The optimal soil moisture regime for SVE applications is a water content
low enough to ensure adequate air permeability yet wet enough to reduce
electrostatic sorption force.  Davies (1989) states that the critical moisture
regime for SVE applications is 94 to 98.5 percent relative humidity in the
soil gas.  Below this range volatiles are more tightly bound to soil and may
not be as readily volatiled.  In regions such as the southwestern United
States, where ambient soil moisture is typically very low, the use of
humidified air may be warranted if the soil moisture is low enough that
adsorption forces become significant (Davies, 1989).  Some researchers (Hunt
et al., 1988; Lord et al. ,  1987) have investigated steam injection to
volatilize contaminants, both those sorbed to soil and those that exist as
non-aqueous phase liquid (NAPL) in the soil.  The results show potential for
this type of operation, although its applicability for wide scale use has not
been proved.

Preferred Flow Paths

      Preferred flow paths, such as macropores formed by cracks, root casts,
or earthworms, are highly permeable, continuous voids that may transmit a
significant quantity of the vapor or liquid through a soil (Levy and Germann,
1988).  Researchers have shown that these macropores often control the rate of
infiltrating water in the soil.  This has major implications with regard to
vapor extraction because significant quantities of flow may be realized in
continuous macropores leading to the vapor extraction system.  Because of this
short circuiting phenomenon the contaminants adhering to soils through which
limited air flow occurs may not be cleaned up as rapidly as would those
particles through which a large volume of air flows.  Removal of contaminants
from soil particles in dead end pores (i.e., pores that are connected to other
pores only at one end) would therefore be more closely related to diffusion
effects.  Removal of product from these dead end zones may not be enhanced by
increased vacuum or pumping rates, but rather removal will be limited by
diffusion from these zones to the more continuous flow paths.  Diffusion of
contaminant vapors occurs much more slowly than advection.  Soils with lower
air permeability values are therefore likely to be less amenable to soil vapor
extraction or will at least require higher vacuums.

GASEOUS FLOW IN SUBSURFACE ENVIRONMENTS

      Air flow rates and subsurface permeabilities have historically been used
in the petroleum industry to estimate extraction of natural gas from oil- and
gas-producing geological formations.  The petroleum industry tests typically
are conducted in the field, employ gas extraction wells, and are performed on
confined subsurface strata.  Gas producing strata are generally found at
significant depth and have-much higher natural gas pressures and temperatures
than the situations encountered at UST sites where SVE technology could be
applied.  Soil gas permeability estimates have also been used in agricultural

                                      31

-------
  VAPOR
  PHASE
   0
                                      NON POLAR ORGANIC
                                        o

  ADSORBED
  LAYER
                      \X\\\\\\\\\Vs
                      VNSOLID SURFACED
                       \\S\\\\\\\\\ N
                        a) DRY
   n
_Q.
oono
                                        O
o
OOP
                        b) DAMP
 onoLJOooo
                   OLJOOOOOOOO
Source: Reible, 1989
                        c)WET
     Figure 6. Illustration Of VOC Adsorption Under Three Moisture Regimes
                           32

-------
research to determine the amount of atmospheric nitrogen and oxygen available
to plants in the root zone.  Agricultural soil gas permeabilities are usually
determined from laboratory measurements or in situ air injection tests.

      While the situations explored by petroleum and agricultural researchers
do not correspond exactly to most SVE situations, the fundamental vapor
transport principles used by researchers in these fields do apply to transport
of contaminant vapors for SVE theory and practice.  This section provides a
brief description of the basic principles controlling vapor flow in porous
media, as well as available methods and techniques to measure air
permeabilities .

      Many factors influence contaminant vapor transport and diffusion.  Table
4 lists several parameters that influence diffusion and loss to the
atmosphere, including water content, adsorption site density, chemical
concentration, temperature, and Henry's law constant.

Vapor Transport Fundamentals

      Vapor flow through soil is dependent on soil characteristics ,  including
porosity and permeability; gas properties, such as viscosity and density; and
pressure gradients.  Gas is a fluid and as such its flow rate through porous
media is commonly characterized by Darcy's law.  Darcy's law is valid for
laminar, isothermal flow that is uniformly distributed across a given cross-
sectional area.   The general formulation of Darcy's law for saturated fluid
flow in one dimension is:

                              Q -  (kA/AO(dP/dm)                    (1)
where :
                        o
      Q  = flow rate (cm°/sec)
      k  = permeability (cm )
      A  = cross-sectional area (cm )
      IJL  = viscosity (g/cm*sec)
      dP/dm = pressure gradient ( (g/cm*sec^)/cm)

The use of Darcy's law is restricted to laminar flow situations, where
individual fluid particles are considered to flow parallel to the soil pore
walls.  Fluid flow is characterized by laminar or turbulent flow based upon
its Reynolds number.  Generally, laminar flow conditions exist when the
Reynolds number (R) is less than about 2100:

                                                                   (2)
                                      M

where :
      R  = Reynolds number (dimensionless)
      V  = velocity of the fluid (cm/sec)
      p  = density of the fluid (g/cm3)
      d  = effective particle diameter (cm)
      H  = viscosity of the fluid (g/cm*sec)
                                      33

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              TABLE 4. VAPOR DIFFUSION-PARAMETER RELATIONS IN
                 CHEMICAL TRANSPORT AND LOSS TO ATMOSPHERE
      Parameter
Water content (0)
Adsorption site density
Chemical concentration
Temperature
Henry's Constant
                   Influence

Decreases effective porosity for vapor flow;
also, vapor diffusion decreases strongly with
increasing water content.  A frequently used
model assumes that the soil vapor diffusion
coefficient is proportional to (0-0) exp 3.33,
where 0 is porosity.

Adsorption decreases gaseous chemical
concentration and decreases vapor diffusion.
Most volatile organic chemicals are nonpolar
and adsorb primarily to organic matter.

For chemicals whose vapor density is not
saturated, increasing chemical concentration
will increase vapor density and increase vapor
diffusion.  The increase may be greater than
proportional if the chemical vapor adsorption
isotherm is nonlinear.

Increasing temperature significantly increases
vapor density for a given amount of chemical
in soil, thereby increasing vapor diffusion.
However, soils typically remain at a constant
temperature at a particular site.  The vapor
diffusion coefficient increases nonlinearly
with increasing temperature, proportional to
T exp 1.75 (Kelvin).

Henry's constant (ratio of saturated vapor
density to solubility) is an index of the
partitioning of a chemical between dissolved
and gaseous phases.  Compounds with larger K^
values are more likely to move by vapor
diffusion as opposed to liquid diffusion.
Source:  Jury and Valentine, 1986
                                      34

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      Johnson et al .  (1990) developed vapor flow rate and air permeability
estimation methods assuming laminar, steady- state vapor flow conditions for a
soil venting well.  The basic governing equations used to model this flow are
the continuity equation and Darcy's law.  The continuity equation states that
the mass flow rate of a fluid through a given cross -sectional area remains
constant with time.  The forms of these equations that can be used in this
analysis are (Johnson et al . , 1990):

                            d(gpm) ..V(pM|i)                        (3)
                               at
                                         £)                         (4)

where :
      ? = vapor-filled void fraction  (0
-------
      • changes in the air-filled porosity due to contaminant removal are
        neglected.

Flow Under Vacuum Conditions

      Under normal conditions,  many factors influence the movement of gases
through the unsaturated zone.   Diffusive transport of gases takes place in
response to a concentration gradient, according to Pick's law.  Transport can
also occur due to density differences in pore gases;  meteorological changes
such as changes in temperature, barometric pressure,  and wind; infiltration of
rainfall; and a fluctuating water table.

      All of these factors, however, are dominated by the pressure gradients
induced by a vacuum well in the vicinity of that well.  Advective flow
resulting from the applied vacuum is far greater in magnitude than diffusive
flow, and thus, the above factors have a negligible effect on the flow to a
well during SVE.

      The application of a vacuum to a well will cause a pressure gradient
(actually, negative pressure)  to propagate throughout the zone in proximity to
that well.  This zone extends radially away from the well for some distance;
this distance is known as the radius of influence.  Many factors affect the
radius of influence.  These include the strength of the applied vacuum; soil
properties like porosity and permeability; site features such as the
stratigraphy and the presence of an impermeable surface barrier or air inlet
wells; and other factors, which are discussed elsewhere.

      Within the radius of influence, the vacuum is strongest at the well and
decreases as the distance from the well increases.  Monitoring probes or wells
spaced throughout this zone allow the determination of the pressure field from
which isobars (contours of equal pressure) can be developed.  Figure 7
illustrates airflow paths for an idealized extraction well application.

      Flow occurs to the well in direct response to the vacuum.  Flow lines
are usually perpendicular to the isopotentials; that is, flow occurs in the
direction of steepest pressure gradient.  Pressure gradients are always
greatest near the well and least at a distance from the well.  Therefore, a
surface seal might be of benefit in reducing air flow short circuiting near
the well  (resulting in cleanup of a greater volume of soil) because it would
force air to be drawn from greater distances and to travel through more soil
to reach the extraction well.

AIR PERMEABILITY TEST METHODS

      One critical factor used to determine the feasibility of SVE technology
is the vapor flow rate that can be induced at a particular site.  The vapor
flow rate is directly dependent upon the air permeability  (along with the
applied vacuum).  Air permeability describes how easily vapors flow through
the soil.  Since  the air flow rate and  the air permeability are linearly
dependent, a higher air permeability will result in a higher  flow rate at the
same vacuum.  Thus, there  is a greater  likelihood that SVE will be a feasible
remedial  technology in soils with higher air permeability values.

                                      36

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                    EXTRACTION WELL
                                             SURFACE SEAL
         1"
               2"
                                                             2"
.37.
                            14"
20" WATER
 VACUUM

14     i

J	I.
                                                                         ISOBARS
                                       WATER
                                       TABLE
                                          .v
                 Figure 7. Relationship of Flow and Induced Vacuum

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Field Methods

      Numerous types of in situ methods have been employed for the
determination of air permeability.   All of these methods rely on measuring the
difference between the ambient atmospheric pressure and the air pressure in
the soil during vapor transport.  Methods used include:  air injection testing;
oil field tests, such as pressure buildup and drawdown tests; and the
procedure described by Johnson et al.  (1990).

      Air Injection. These methods rely on measuring the pressure differential
created when air is injected into the soil.  Equipment used for evaluation of
surficial agricultural soils consists of a compressed air tank and a gas
flow/pressure regulator attached to a cylinder that is inserted into the soil
(Evans and Kirkham, 1949; Grover, 1955; Van Groenewoud,  1968).  The pressure
differential is measured by comparing the air pressure before and after a
known volume of air is injected through the cylinder into the soil over a
given time period.  The soil air permeability is estimated using Darcy's law
with both measured and known values for the pressure differential, air flow
rate, cross-sectional area of the cylinder, and viscosity of the air.  Air
injection testing has also been used to test permeability in a packed-off
section of a drill hole in granitic rock following a contained nuclear
explosion (Boardman and Strove, 1966).  The advantages of surficial air
injection testing for air permeability are the portability of measurement
equipment, rapid measurement, and low cost.

      There are several limitations associated with using air injection
testing to determine the air permeability for SVE tests.  First, the
permeability measured by injecting air into the soil may not be the same as
that observed during implementation of soil vapor extraction.  When air is
injected into the soil, the soil particles will tend to expand and increase
the apparent permeability.  By  contrast, the surficial soil may collapse
during vacuum conditions experienced during SVE.  Second, SVE requires air
permeability of soils at depth, whereas air injection measures the
permeability of the surficial soils.

      Oil Field Test Methods. The petroleum industry has used several in situ
test methods to determine soil  air permeability.  These tests and associated
gas flow models are similar to  the SVE vapor transport model presented by
Johnson et al.  (1990).  Most of the tests are performed using one or more gas
production wells.  The natural  gas from production wells is usually contained
at higher pressures and temperatures than the gas subject to SVE.  Thus, the
modeling of natural gas flow and air permeability estimates of natural gas
reservoirs usually  incorporates parameters for gas compressibility and
temperature not typically found in SVE vapor transport modeling.  There are
two commonly used methods of determining air permeability of gas producing
formations: the pressure buildup test  and  the draw down test.

      The pressure  buildup  test is conducted by closing a gas well that has
been producing  at  a constant rate for  a given period of time and monitoring
the  increase  in down hole pressure after the well is closed  (Donohue and
Ertekin, 1982).  The pressure buildup  is considered to be the result of two
superimposed  effects:   the  pressure drawdown caused by the initial gas flow

                                       38

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from the well and the increase in pressure that occurs when the well is
closed.  The pressure increase is modelled as a gas injection with the flow
rate equal in magnitude but opposite in sign to the flow rate that was
occurring during the well's active lifetime.  The final pressure versus time
relationship is approximated by the following equation (Donohue and Ertekin,
1982):

                Pw2 = Pr2 - (1637MzTQ)/(km) {log[(t + &t)/At]}          (7)

where:
      Pw = pressure in well (psia)
      Pr = static reservoir pressure (psia)
       jit = gas viscosity (cp)
       z = compressibility factor (0Mcr2)) - 3.23+0.87S]   (9)


Additional parameters:
       - porosity (fraction)
      c = gas compressibility  (1/psi)
      r - radius of well (ft)
      S = skin effect
                                                                         n
      For the drawdown test, the  permeability is estimated by plotting  Pw
versus log (t) such that the slope  is the same as in equation 8.  Permeability
is determined from this equation.
                                       39

-------
      The mechanics and modeling of the drawdown test are similar to those
used by Johnson et al.  (1990), described below.  The gas compressibility and
skin effects may be unnecesssary to determine the permeability during SVE air
permeability tests.  Pressure buildup and drawdown tests can be performed
using a single well or, for increased accuracy, using both an extraction well
and a monitoring well.

      The soil air permeability methodology proposed by Johnson et al.  (1990)
for SVE technology can be considered as a special case of the drawdown test.
In this case, the drawdown or vacuum pressure, P', is measured in a monitoring
well at a distance, r,  from the extraction well while a constant flow rate, Q,
is extracted.  The equation that approximates expected pressure changes over
time, taken from Bear  (1979), is:

           P'  =  Q/(47rm)k/M)   {-0.5772  -  ln[(r2e^)/(4kPatm)]  + ln(t)}    (10)
where:
      P'= vacuum measured in  soil
      Q = extracted flow rate
      m = stratum thickness
      k = permeability
      fjL = vapor viscosity
      r = distance to vapor monitor
      e = vapor-filled void fraction
   ^atm = atmospheric pressure
      t = time from start of  test

      As in the drawdown test, the permeability is measured  from the slope  of
P' versus ln(t).  Permeability is determined from the equation for the slope:

                                A - Q/47Tm(k//i)                          (11)

      To enhance the likelihood of obtaining meaningful soil air permeability
data for SVE  technology evaluation:

      • take  field measurements of vacuum pressure from monitoring wells
        around a vapor extraction well  to allow for measurements at natural
        soil  moisture  content and to account for  lateral heterogeneity of soil
        type  and structure;

      • maintain a constant air flow rate from the well; and

      • conduct tests  at several locations throughout the region to be
        remediated for increased confidence in the results.

Appendix E reprints the entire text of  a paper by Johnson et al. (1989), which
discusses this method  in greater detail.
                                       40

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                                  SECTION 3

                             SITE INVESTIGATIONS
INTRODUCTION

    Federal underground storage tank regulations (40 CFR 280),  promulgated on
September 23, 1989, require that a site investigation be conducted when a
release of product from an underground storage tank (UST) system has occurred
or is suspected.  The objective of the investigation is to characterize and
delineate the area of soil and ground-water affected by the release.

    The site investigation should address the following issues:

    • The type of contaminants released (gasoline,  fuel oil,  solvents,
      etc.) and quantities released.

    • The extent of product migration and routes for further migration.

    • Product behavior in the subsurface environment (i.e., sorbed to
      soil, non-aqueous phase liquid (NAPL),  dissolved in groundwater,
      or in vapor phase).

    • Receptors subject to impact.

    Johnson et al. (1989)  have developed an investigation sequence that
includes the following steps:  (1) site history review;  (2) preliminary site
screening; (3) detailed site characterization; and (4) contaminant assessment.
This, generalized approach to investigation of leaking UST sites is one that
regulatory staff and contractors should recognize as being similar to that of
site assessment and remedial investigation approaches developed by EPA for
uncontrolled hazardous waste sites.

    Pursuant to the Comprehensive Environmental Response, Compensation, and
Liability Act of 1980  (CERCLA) and the Superfund Amendments and
Reauthorization Act of 1986  (SARA), technical assistance documents have been
developed that provide a wealth of information on site investigation
methodologies (EPA, 1987a; EPA, 1988c) and protocols (EPA, 1987b) .  These
guidance documents address hazardous waste sites that may pose a more
complicated  investigation problem than a typical UST site due  to the presence
of numerous  types  of contaminants  (caustics,  metals, chlorinated solvents,
PCBs, etc.)  over a large area.  Nevertheless, the investigatory approaches
developed for these Superfund sites have application to  leaking UST sites
because they present a sound basis for data collection and assessment.  This
section provides a brief description of site investigation approach for
leaking UST  sites where implementation of SVE technology may occur.

                                      41

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SITE HISTORY REVIEW

    The objective of the site history review is to assimilate available data
that will be useful in guiding subsequent investigation activities.   Data
obtained should include historical records,  site plans, engineering drawings,
interviews with site personnel,  meteorological data,  boring logs,  aerial
photos, a. soil survey and United States Geological Survey (USGS) maps.   Record
sources include the Town Hall, police and fire departments,  EPA, state  and
local agencies, generator records and media reports.   This information is
useful in identifying the types of products stored in the USTs and the  leakage
and spill history of the site.  Site plans and engineering drawings should be
consulted to identify subsurface structures, pipes, and utility lines prior to
any excavation, auguring and boring.

    Soil survey maps, which can be obtained from the Soil Conservation Service
(SCS), provide information regarding the types of soils at the site and
environs.  Many sites, however,  have been altered and are composed of
heterogeneous fill materials.  USGS surficial and bedrock topographic
quadrangle and geologic maps will indicate elevation contours and geology for
the area.  Geologic maps are useful for assessing potential migration routes.

    The identification of potential receptors that may be impacted by the
release is critical to any investigation.  During the site history review
phase, information on public and private water supplies in the area must be
obtained.  Locations of all municipal water supply wells should be noted on a
site vicinity map to show their proximity to the UST site.  The location of
private water supply wells and surface water supplies should also be shown on
a vicinity map.

    Development of a generalized geologic cross section map will also be
valuable during the initial investigation phases.  The cross section should
depict the on-site geologic strata and any nearby receptors such as public
water supply wells.  Even cross sections that lack detail provide a graphical
presentation that helps to conceptualize potential migration routes and rates.

    The site review should provide a basic understanding of the site to allow
better use of time, equipment and laboratory analyses during the preliminary
site screening and detailed site characterization phases.  The site review
should also aid in the assessment of health and safety procedures to be used
by the field investigative team.

PRELIMINARY SITE SCREENING

    The primary objective of  the preliminary site survey is to identify the
nature and extent of contamination.  Preliminary site screening should
include, at a minimum, a general site survey using field monitoring equipment,
the development of a generalized site sketch and a subsurface environment
cross section.  The preliminary site screening may also entail a soil gas
survey and groundwater sampling with mini-well points.

      During conduct of the site survey  the general location of the
contaminated soils and the primary source of contamination should be noted as

                                      42

-------
well as the physical setting of the site.  During the site survey,  potential
human health risks should be identified.  In addition,  the general survey of
the site will provide some idea of the ground elevations of various
structures, locations of catch basins, location of possible sources of
contamination other than those initially identified,  and soil properties such
as color and texture.  Ambient air volatile organic compounds (VOCs) should be
monitored to determine explosion risks and as an aid in locating the
contaminant source.  These surveys are conducted using field monitoring
equipment such as a flame ionization detector (FID) or photoionization
detectors (PID) (Spittler, 1980).  Gas chromatography (GC) can identify
specific compounds, but is often unnecessary at this  preliminary stage.

    The presence of catch basins on and off the site  should be noted, as they
offer a convenient method for spilled fuel products to enter the soil zone.
Contaminants may also migrate along drainage culverts,  ditches,  sewer lines or
other conduits and then enter the soil zone through cracks or open joints.
Other potential sources of contamination, such as nearby service stations or a
user of hazardous compounds (such as a dry cleaning facility),  should also be
identified during this stage.

    Soil morphological features such as color and texture are useful
indicators of subsoil conditions.  Soils can be readily sampled to a depth of
six feet with a hand auger, and even greater depths (up to 20 feet) in sandy
soils.  Soil samples from various depths should be examined for color, texture
and morphological features, such as mottles.  Using visual assessment
techniques, the location of petroleum-stained soils,  water table depth (if
shallow), and the presence of restricting layers or horizons in the soil
profile may be identified.

    Determing the depth to water table is a key parameter in site screening
activities, since SVE is most effective at sites where the water table is 20
feet or greater in depth.  Where the water table is nearer the surface (less
than 5 to 10 feet), SVE may not be appropriate or, if used, may require some
means to lower the water table (Danko, 1989).  Gleying and mottling (variable
splotches of color in the soil) will indicate whether the water table is
permanent at that depth or fluctuating, respectively.

    Values of hydraulic conductivity (Mishra et al.,  1989) and residual
saturation of gasoline (Hoag and Marley, 1986),  both  important parameters in
system design, may be estimated from a particle size  distribution analysis.
Preliminary field determination of soil texture can be used to make a crude
estimate of the permeability of the soil. Generally,  soils such as sands and
gravel are highly permeable and amenable to SVE.  Fine textured soils high in
silt and/or clay are more slowly permeable and SVE may or may not be
applicable.  The site investigator should note that contamination in the soil
profile at an UST site will probably include several  different phases:

    • Residual trapped in the porous media as the leaked product moved
      downward via gravity;

    • Product volatilized into soil vapor;


                                      43

-------
    • Free product floating on the surface of the water table;  and

    • Soluble components of contaminants in the groundwater or  pore
      water.

    Headspace analysis of soil samples is an additional means available for
delineating subsurface contaminants.   Soil samples may be collected with an
auger and placed in wide mouth jars and sealed with aluminum foil.  VOCs in
the headspace of the sample can be determined by field monitoring equipment
after the equilibration of the sample.  Headspace analysis provides
approximation of the concentrations of soil VOCs, however, and may not be
highly correlated to soil contamination.  Smith and Jensen (1987) found a poor
correlation between field monitoring data and total petroleum hydrocarbon
(TPH) concentrations in the soil.  Even in view of these deficiencies, the
approximate concentrations determined from headspace analyses will give some
indication of the degree of contamination that must be addressed.

    Soil gas is a screening tool that can be used to rapidly and cost
effectively identify and delineate VOCs in the subsurface.  A soil gas survey
measures the VOC vapor phase concentration within the soil pore space.  Figure
8 shows a typical soil gas sampling apparatus.  Soil gas is most often used to
optimize the placement of monitoring wells, to more precisely define an area
designated for remedial action, or determine the most likely location of
contaminant source areas (Thompson and Marrin, 1987) (Figure 9).   Pitchford et
al. (1988) used soil gas surveys at four sites to successfully map solvents,
gasoline and JP-4 contamination.  According to Kerfoot (1989),  soil gas data
is best used for planning the placement of SVE extraction wells.

    The site specific usage of a soil gas survey should be based on factors
such as soil type, depth to the water table and characteristics of
contaminants as discussed by Marrin (1988).  Coarse textured soils with low
organic carbon content are typically more amenable to soil gas investigations.
Fine textured soils (high in silt and/or clay) may retard vapor transport
(diffusion and convection) due to their low air-filled porosity.   Organic
carbon in the soil will also retard vapor contaminant transport due to
sorption of polar compounds onto the organic carbon.  Soil gas is more
applicable to sites where the depth to water is greater than 15 feet than to
sites where the depth to the water table is shallower.  This is due to a very
steep chemical concentration gradient in areas with a shallow water table.  In
these areas, a slight variation in the ground elevation or depth to the water
table can result in large variations in the measured concentrations of VOCs.
Contaminants most applicable to soil gas surveys should have a vapor pressure
of 1.0 mmHg or greater  (20 degrees C).  Additionally, contaminants should have
an adequately low water solubility.

    Soil gas measurements are limited in their capacity to predict soil and
groundwater contamination.  Bradford et al. (1989) found the predicative
capabilities of soil gas concentrations to be valid only 1 to 8 feet from the
soil or groundwater samples.  Predictive capabilities also decreased with
increased time between  soil gas and soil or groundwater sampling.  Karably and
Babcock (1987) found that soil gas concentrations vary temporally and with
climatic changes.  Barriers in the soil profile, such as saturated clay

                                      44

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                                                                                     TYGON

                                                                                     TUBING
                                                                                            AIR

                                                                                         SAMPLING

                                                                                           PUMP
       DRIVE PLATE
                                                          SAMPLING PORT
             . «  . s . s . s . V . s . s • i S . S
                                                                              fmfmfmfmfmfmfmfmfmfmfmfmf9f
 •ff
 *.•*.*
   ffff
               • S S • V- S • S • % • S • S •
      s,.',. -if if if i .' !.' .V .V "V »V • V •.'

                f if if if if if if • ,•

s • s • s • s • s • s • % • s • s • s • s • s • ^ •
•ff'f'f'f-f'f'f; f; f; f; f; f; Sr f; f
SAMPLING

  PORTS
flf'f'f'f'f'f'f'f'f'f-f'f'f'f'f'f'f'f'f'f'f'f'f'f'f'f'f'f
                        .
 f-f'fmflf'f'f'f'f-f'ffS'f'f'f'f'.'
 a"_BB_BBHaBiB*BBBBaBB>%a^B%aS*BBB%BBBB^B^"
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S* B. . B. . B. . *.V*. • B. . S . V* '. " S " VB VB B. " •, • B. B B. "
7_B • _B If If Iflf Iflfmf If If if If if If'f' •"
          "
                  i
              V. s • B. • S V
           ff -ff -ff •
           f'f-f'f-f-f-f-f-f-f-f:

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•fflfff-flfff
           [^^^^^^^m^^ PROBE TIP

           •f&&?^
           ••"i7i>v.v.177v.S77i17vw77-7vw.•Si'.!'.17A7.-7y^y^^^yv^^yyyryyyyyyyyyyyi
           l\%.%BS.S>S\S>S>.SBSB/.>SBSBVB%BS\SBiVBS\BBBS\SB%B^
                .•SBS«SBSBS'SBS«S-V«S-S'SBSBSBS-V"V«VB1
      -. -. -• -. -. -w ^f*f*f*fmf*f*f*f*f*f*f*f*f*f*f*fmf*f»f*
    rBBs-.s-.s-.S'SBS-sVsVsVsVsVsIsBS'SBS-s.>VsVsVsVsVsV-.VB.V
     .••.•.f.f.fnf.f.fnf.f.f.f*f.f.f,f,f.f.f.fmf.f.f*f.f,f*
     liVBSBS>S*S>>*SBS>SB.S*.S-S>SB.S*.SB.S*.S-.S*.S>S«SB.S*.S-.SB.S>'
                        Figure 8. Typical Soil Gas Sampling Apparatus
                                                    45

-------
                                                                      OSOlOO
                                                                  LEGEND

                                                                   .-g-Conanlrotion Contours of
                                                                  /f^   Total Hydrocarbon* In
                                                                        Soil Gat 
-------
lenses, perched water bodies, pavement and buildings can affect soil gas
concentrations (Kerfoot, 1989).  Interpretation of results can be aided by
preliminary modeling of diffusion transport (Silka,  1988).

    Mini-well points allow for the in situ collection of groundwater samples
prior to (or in place of) the use of monitoring wells.   Micro-well points can
also aid in the characterization of hydrogeologic conditions and assist with
the optimum placement of permanent monitoring wells (Edge and Cordry, 1989).
Well points can be quickly inserted into the soil and samples obtained for
further analysis.

DETAILED SITE CHARACTERIZATION

    If the preliminary site screening indicates that the product released is
present in soil pore spaces and the product's volatility suggests that SVE
technology may be an appropriate remedial action alternative, then additional
site characterization geared towards SVE may be warranted.  Methods for soil
and groundwater sampling are outlined in the American Petroleum Institute's
(API), "Manual of Sampling and Analytical Methods for Petroleum Hydrocarbons
in Groundwater and Soil" (Kane, 1987).

    Subsurface soil conditions may be determined from soil samples collected
by an auger, split spoon sampler or Shelby tubes.  Soil samples can provide
information on soil texture and structure, density, and other diagnostic
features.  The SCS soil survey provides a detailed evaluation of the shallow
soils, but only to a depth of six feet.  Soil test pits provide a means to
evaluate the horizonation of soils, to collect soil samples and to obtain soil
cores for determining hydraulic conductivity in both the horizontal (K^) and
vertical (K^.) orientations.  SVE is most effective in soils with hydraulic
conductivity values above IE-3 cm/sec.  Contaminant removal, however, has been
demonstrated in soils with hydraulic conductivities ranging from IE-3 to IE-6
cm/sec (Danko, 1989).

    Moisture characteristic curves can be used to evaluate the characteristics
of the soil under various soil matric potentials.  Air-filled porosity can be
determined from the moisture content and bulk density measurements.  SVE is
most effective in soils with a porosity between 40 to 50 percent (Danko,
1989) .   Additional soil tests to characterize the site with regard to SVE may
include in situ hydraulic conductivity measurements and column studies.
Hydraulic conductivity can be determined in situ using slug test techniques,
pump tests, air entry parameters, or Guelph permeameters  (Elrick et al.,
1989).  Soil cores may be used to evaluate the pore volumes required to remove
specific contaminants as well as to determine air permeability on a microscale
basis.

    Geotechnical borings are generally necessary to identify deep strata and
to allow sampling of soil and bedrock materials at depths of greater than five
feet.  Soil samples may be submitted to the laboratory for particle size
analysis, bulk density testing, porosity determination, and chemical
characterization.  Geophysical techniques such as ground penetrating radar
(GPR), electromagnetics (EM), resistivity, seismic methods, gravity, metal
detection, and magnetometry can aid in the evaluation of subsurface conditions

                                      47

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(Benson, 1988).  Site hydrogeology,  including depth to water table,
groundwater gradient and aquifer permeability,  can be determined from
monitoring wells and examination of the cores from split spoon sampling.

    Monitoring wells are typically used to monitor VOCs in the groundwater
along with other groundwater quality parameters.  Groundwater is collected by
either a bailer (stainless steel or teflon) or some type of pump (centrifugal,
submersible, peristaltic or bladder).  VOC samples are typically collected
first, followed by collection of ABNs (semi-volatiles) and TPH (total
petroleum hydrocarbons).  Field determined parameters include temperature,
redox potential, dissolved oxygen, pH, electrical conductivity and depth to
water.  Metals concentrations and general groundwater quality parameters such
as total dissolved solids (IDS) and chloride are also readily collected.

CHARACTERIZATION OF CONTAMINANTS

    The composition and concentration of contaminants in the soil, soil gas,
and groundwater should be determined as part of the site investigation.  These
analyses will  indicate the applicability of SVE as a remedial technology.  Use
of SVE for soil remediation is best suited to VOCs that exhibit a vapor
pressure of at least 1 mmHg at 20 degrees C and a Henry's Law constant of at
least 0.01  (dimensionless)(Danko, 1989).  A survey of SVE use (Hutzler et al.,
1989) has indicated that SVE is most commonly applied to gasoline and its
volatile constituents (benzene, toluene, ethylbenzene, and xylenes),
industrial solvents, including the dichloroethylenes, trichloroethylene (TCE),
tetrachloroethylene (PCE), methyl ethyl ketone  (MEK), and many others.  Sites
at which the contaminants are primarily high molecular weight petroleum fuels,
such as fuel oil no. 6, or "weathered" gasoline are less amenable to SVE.

    Monitoring often focuses on indicator compounds, such as BTEX  (benzene,
toluene, ethylbenzene, and xylenes), to narrow  the scope of the analyses.
Such indicator compounds should be easy to detect and be representative of the
compounds at the site.  Monitoring for certain  indicator compounds is
considerably easier than monitoring for the range of compounds that may be
found at one site.  Methods for various analyses are outlined in Kane  (1987).

    Alternately, a boiling point distribution can be measured for a
representative sample.  Compounds elute from a  GC packed column in the order
of increasing  boiling point.  A boiling point distribution curve can be
constructed by grouping all unknowns that elute between two known peaks  (e.g.,
between n-hexane and n-heptane)  (Johnson et al., 1989) to determine the
relative proportion of contaminant distribution.

    Contaminants present  in the soil should be  characterized to determine
applicability  of SVE for remediation and overall contamination.  Since some
contaminants may not be removed by SVE, analysis of soils for less volatile
contaminants  (ABNs and TPH) may  indicate the need for remediation by other
methods  (biotreatment, thermal desorption, soil washing or excavation).  The
cleanup  goals  must take into account  the fact that only volatile compounds
will be  removed by SVE.
                                      48

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    Characterization of contaminants in the soil gas will provide critical
data regarding the contaminants present at the site.  Often these data will
provide clues about the release itself.  By identifying the contaminants
present in the soil gas the released product may be identified.  This process
may help to pinpoint the source of contamination - a leaking UST, a past spill
at the site, or perhaps an off-site source.  Identifying the source allows the
contamination to be characterized more quickly and makes the site
characterization more efficient.  If an off-site source is identified,
responsibility for clean-up could be assigned to a different party than was
originally identified.
PILOT TESTING

    If the site and contaminant characterizations indicate that SVE is an
appropriate remedial measure, pilot testing may be performed to determine the
site-specific design parameters for full scale design and implementation.  The
data obtained from the pilot test should enable determination of the radius of
influence, initial and final exhaust concentrations, obtainable flow rates,
water level changes, and vacuum well pressures.  Collection of this
information will be used to design the full scale system by determining:  (1)
number and location of extraction wells; (2) number and location of injection
or inlet wells (if required); (3) equipment requirements, such as blower size
(flow rate and vacuum);  (4) aquifer parameters and groundwater pumping system
specifications (if required); and (5) likely mass removal rates, and thus, the
length of operation.

    Data valuable for the design and operation of the SVE system should
already have been collected during the site investigation.  This data should
indicate soil texture, organic carbon content, moisture content, bulk density,
permeability and porosity from soil cores,  groundwater elevations and the
extent, concentration of the contamination in the soil, groundwater and soil
gas.   Soil textural data is needed to determine extraction and injection well
design parameters such as filter pack media and well screen lot size.
Textural data may also be used to estimate the hydraulic conductivity and
residual saturation of the soil.  Bulk density measurements can be used to
estimate porosity.  Permeability data can be used to estimate blower
requirements and the radius of influence.  The water table elevation relative
to the contaminants should be used to determine the necessity of pumping
equipment, based on an estimate of groundwater upwelling from SVE operation.
The relative location of contaminants in the soil profile (shallow vs. deep)
will determine the necessity for flow control devices such as surface seals
and injection wells.
                                      49

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                                  SECTION 4

                                SYSTEM DESIGN
INTRODUCTION

    A major advantage of soil vapor extraction technology is the relative
simplicity of the design of these systems.   In addition,  the equipment that
comprises the systems consists of commonly-used and widely available devices
such as PVC piping,  valves, and pumps.   These factors impart an advantage to
soil vapor extraction over other techniques (e.g.,  biotreatment or soil
flushing) that may require more complex design or single-purpose equipment.
Simplicity of design, however, does not imply that a logical,  reasoned, and
informed design procedure has been followed for all site  specific
installations.  Maximum system efficiency and contaminant removal will occur
only through a thorough understanding of the site and the SVE process.

    The objective of a well-thought out and reasoned design process is to
construct a soil vapor extraction system that removes the greatest degree of
contamination from the site in the most efficient,  timely, and cost-effective
manner.  The attainment of that objective will occur through an understanding
of the three main determinants of system effectiveness (Johnson et al., 1989):
the composition and characteristics of the contaminant;  the vapor flow path
and flow rate; and the location of the contamination with respect to the vapor
flow paths.  Design of an SVE system is basically a process to maximize the
intersection of the vapor flow paths with the contaminated zone.  Operation of
the system should be done to maximize the efficiency of the contaminant
removal and reduce costs.

    This section discusses several aspects of SVE system design.  Several
options are available for layout of systems, including wells,  trenches, and
above ground soil piles.  After selection of the appropriate system option,
the number and placement of wells or trenches, the applied vacuum and pumping
rate, the use of a surface seal or other types of air flow control, and the
depth and size of the screened interval are all decisions that are made to
maximize system effectiveness.  Selection of the appropriate equipment type
will also affect the system effectiveness.  These aspects are discussed fully
below.

EXTRACTION SYSTEM OPTIONS

    Several options are available for extraction system layout.  Figure 10
shows the most common methods:  vertical wells, trenching or horizontal wells,
and excavated soil piles.  These variations are discussed separately below.
                                      50

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CONTAMINATED
    LAYER
CONFINING
   UNIT
      (a) Vertical Well Screened in
         Contaminated Zone
                                                           SURFACE SEAL
               EXTRACTION
                 PIPING
CLAY

GRAVEL
BACKFILL
                                                   CONTAMINATION
                             (b) Trench
                                           AIR INLET VENTS   SURFACE SEAL
                                                          CLEAN BACKFILL
          (c) Horizontal Drilling
                             (d) Soil Pile
                    Figure 10. SVE System Design Options
                                    51

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Vertical Wells

    Vertical wells are the most widely used SVE design method (Hutzler et al.,
1989).   This method is the only feasible option at sites where the
contamination extends far below the land surface.  Bennedsen (1987) suggests
that horizontal wells or trenches may be more practical than vertical wells
where the depth to groundwater is less than 12 feet.  Vertical wells are
generally inappropriate for sites with a shallow water table due to the
potential upwelling of the water table that may occur after the application of
a high vacuum.

    Extraction wells are similar in construction to monitoring wells and, in
many cases, existing monitoring wells have been used as extraction wells.
Construction of an extraction well is straightforward (Figure 11).  The bore
hole is augured or drilled, PVC casing and screening (usually 2 inches to 12
inches diameter but depends on flow rate) are placed in the hole and the
annular space is filled.  Keech (1989) suggests that six-inch diameter
extraction wells are more effective for removal of volatiles than the
commonly-used two- or four-inch diameter wells.  Slots are usually sized as
small as possible to reduce silt entrainment.  A highly permeable sand or
gravel packing is placed around the screen for optimal gas flow to the well.
Above the pack, bentonite is used to seal the hole.  A cement-bentonite grout
is typically used to seal the annular space to the surface.

    The extraction well is typically located to intercept the center of
contamination.  Where multiple wells are used, they are placed so that the
flow zone intercepts the contaminated zone.  The screened interval should also
coincide with the depth of highest product concentration.  Often, this is just
above the water table for products lighter than water like petroleum.  The
screened interval should be extended into the water table to allow for the
possibility of a fluctuating water table.  Also, the application of a vacuum
will result in upwelling of the water table  (Figure 12); if not counteracted,
the wells may remove less vapor and more water, depending on the magnitude of
the upwelling.

Trenches

    Where the water table is near the surface, trenches or horizontal wells
may be installed.  Horizontal wells minimize the upwelling of the groundwater
and allow coverage of a greater area than vertical wells.  Installation of
this type of well is accomplished quickly and easily where no surface or
subsurface  impendiments exist.  A PVC drain pipe, wrapped in filter fabric to
prevent fine material from clogging the drain, is placed at the base of the
trench and backfilled with gravel.  The surface  is typically sealed with
bentonite,  asphalt, or a manmade liner to prevent air short-circuiting and
infiltrating rainwater.  The result is simply a  dry trench or french drain
(Figure 13).

    Connor  (1988) reports on the use of horizontal wells at a site where the
depth to groundwater was less than six feet.  DePaoli et al.  (1989) used
vertical wells, horizontal wells, and wells  in an excavated soil pile at a
jet-fuel spill site.  Horizontal wells were  installed in the excavated area

                                      52

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                                      Header
*
Valve
  Pipe  to  Blower
                                                     Concrete Cap
                                               '.'Cemeni — Benionlie  Grout
                                                -v Bentonite  Pellets
             .  .  Slotted  PVC
Source: Hutzler, 1989
                                              i .. • Coarse Sand
                 Figure 11. Typical Extraction Well Construction

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o
 CM
I
                                                          D Vacuum increase
                                                          O Water table upwelling
                                                                T	1	1   I I  I I
       .1                        1                         10                       100
                                       Time (min)
              NOTE: (ft H20) denote vacuums expressed as equivalent water column heights
         Source: Johnson et al.,1989
             Figure 12. Water Table Upwelling in Response to Applied Vacuum
                                           54

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  (a) Excavate Trench
(b) Prepare Trench
                                   SLOTTED^
                                     PVC
                                    PIPING'
   CONTAMINATED
        SOIL
                         (c) Cover with Containinat
    id Soil
                        CONTAMINATED
                             SOIL
(d) Operate
                                                                   GRAVEL
                                                                   BACKFILL
                CONTAMINATED
                     SOIL
                                         CONTAMINATED
                                              SOIL
                        Figure 13. Trench Construction
                                     55

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prior to backfilling to remediate any remaining contaminant.   These wells have
been operated only sparingly, however, so little data is available.  Malot and
Wood (1985) also report the use of horizontal wells.   Horizontal wells can
also be installed without excavation and backfill using special drilling
techniques.  Conventional drilling uses rigid drilling assemblies, whereas
horizontal (lateral) drilling uses jointed, flexible drive pipe.  While not
widely practiced, there are companies such as Eastman Christensen (Houston,
Texas) that specialize in horizontal drilling.  Horizontal drilling possesses
several advantages including those mentioned above, the ability to intercept
specific zones, and the possiblility of intercepting more vertical fractures.

Excavated Soil Pile

    Soil vapor can also be extracted from above-ground piles of excavated
soil.  This option is often used to remediate the soil removed from the
leaking tank area at UST sites.  The usual procedure is to excavate the
contaminated soil and place it in a pile over one or more PVC pipes, which are
packed in gravel and encased in filter fabric.  An impermeable liner may be
used to cover the excavated soil to prevent contaminants from volatilizing to
the ambient air in an uncontrolled fashion and to prevent infiltration of
precipitation.  Prior to the system operation the cover is removed to allow
air to be drawn into the pile.

    Brown and Harper (1989) describe the use of vapor extraction  in
conjunction with biotreatment for excavated soil piles.  A single, slotted PVC
pipe was placed through the center of a six to eight foot high mound.  DePaoli
et al. (1989) report treatment of 52,000 ft3  (1,500 m3) of soil excavated
during tank removal operations.  The pile constructed was 160 feet in length
and had a triangular shape, 43 feet wide at the bottom and 12 feet in height.
At this site, eight vents were used, spaced 18 feet apart at a height of 5
feet.

WELL CONFIGURATION

    Two main issues must be  addressed with regard  to the configuration of  the
extraction well(s).  First,  the number of wells required and their proper
spacing and placement must be determined.  Second, the extraction vents need
to be sized and placed for optimal removal.   Each  of these topics  is
considered below.

Spacing and Placement

    The number and  locations of extraction wells required at a  remediation
site  is highly site-specific and depends on many factors, including  the extent
of the zone of contamination, the physicochemical  properties of the
contaminants,  the soil type  and characteristics  (especially the air
permeability of  the soil), the depth  of contamination, and discontinuities  in
the  subsurface.  The radius  of influence is the primary design variable and
incorporates many of the  above parameters.  The radius of influence  is the
zone  in which  the effect  of  the vacuum is  felt.  Keech  (1989) states  that  a
vacuum of  0.1  inch  of water  or more  in a monitoring well  indicates  that an
extraction well has an influence at  that point.

                                      56

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    The initial step in placing extraction wells is to determine the radius of
influence of one well.  This is done by performing a field air permeability
test, as described by Marley et al.  (1989).  The air permeability  is the
fundamental design parameter (Baehr et al., 1989), and is required to predict
the effective area influenced by that well.

    The objective of the test is to determine the pressure distribution
throughout the vadose zone.  To do this,  one vapor recovery well should be
located in an area likely to be remediated; this well can later be used during
the actual cleanup.  An existing monitoring well is often chosen.  This
recovery well should be surrounded by vapor monitoring wells or, more simply,
soil probes, located at various distances and directions from the  recovery
well, and at varying depths.  The object  is to measure the vacuum  induced at
each location and compare that pressure to the flow rate at the recovery well.
A simple method to do this is to start at the maximum test flow rate and
decrease the flow slightly, stepwise, taking pressure measurements at the soil
probes or monitoring wells at each step.  Alternatively, the flow  can be
maintained at a constant rate and the vacuum measured against time.  The
results of this air permeability test are then plotted as shown in Figure 14.
0>
13

o
o
o

OB
D>
Q)
 03
 o
 tn
 tn
 Q>

                       Extraction System Withdrawal Rate, Q (cfm)

                     Figure 14. Field Air Permeability Test Results
                                       57

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     The slope of the line would be directly proportional to the air
permeability for one-dimensional flow conditions.  For most SVE applications,
however, radial flow conditions predominate and the determination of the
permeability is more complex.  These results can then be used to determine the
effective radius of influence for each well.  This is best done using a
steady-state flow model and calibrating it against measured values from the
field area permeability test.  The results of the air permeability tests will
allow a preliminary estimation of the radius of influence of one well.

     The radius of influence of a well varies widely from site to site;
Hutzler et al. (1989) report survey results that show the typical radius of
influence ranges from 15 feet to over 100 feet.  Generally, sandy soils result
in smaller radii of influence than do clayey soils and require more
closely-spaced wells at a given flow rate.  However, the vacuum or flow may be
increased for sandy soils.  Johnson and Sterrett (1988) suggest that the
radius of influence decreases as the bulk density increases and the soil
porosity decreases.  Krishnayya et al.  (1988) modeled the effects of the
suction head, the depth of the vadose zone, and the presence of a surface seal
on the radius of influence.  Results showed that the radius of influence
increased linearly with increasing suction head.  The depth of the vadose zone
was also shown to highly affect the radius of influence; the radius increased
non-linearly for increasing vadose zone depths.  An impermeable surface seal
was also modeled to determine the effects on the vapor flow paths.  Results
show that the presence of an impermeable surface seal increases the radius of
influence, forcing air to travel longer horizontal distances and contact more
soil than it would otherwise.  Wilson et al. (1989) indicate that, as a rule
of thumb, extraction wells should be spaced at two times the depth to which
they are installed (e.g., if wells are 40 feet deep they should be spaced 80
feet on centers).  This is highly site-specific, however.  The location of the
screened interval also affects the radius of influence and the vapor flow
paths.

Extraction Vents

    Extraction vents may be screened, slotted, or gravel packs.  In addition
to the vent construction, two other design decisions must be considered:  the
length of the vent and its location with respect to the unsaturated zone.

    A goal of proper well design is to induce the air to flow through the zone
of contamination to maximize cleanup efficiency.  This is controlled by both
well spacing and layout and by vent location.  SVE operators use widely
varying approaches to vent design, ranging from screening the entire depth
from near the ground surface to the water table, to having a short interval at
a particular depth corresponding to the zone of contamination.  The location
and length of screening will depend upon the stratification of the soil and
the distribution of the contaminants in the soil.  In many cases, the greatest
concentration of petroleum vapors is immediately above the water table,
especially at sites with a free product lens on the water table.
Determination of the concentration gradient throughout the vadose zone can be
accomplished with soil gas survey techniques.
                                      58

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    In cases where the contaminant concentrations are greatest at the water
table, the vents should be located close to or into the water table for
optimal removal efficiency.  In areas where the water table is expected to
fluctuate throughout the period of venting, the screen length can be increased
to ensure that venting can continue during periods of high water table.

    The extraction wells are normally grouted or sealed with bentonite in the
annular space to prevent ambient air from entering directly along the
borehole/well interface.  In some cases, the annular space is grouted down to
the screened interval.

AIR FLOW CONTROL

    In addition to the placement of the vacuum extraction wells, several other
methods are available to control the flow paths of the extracted vapor to
result in more efficient contaminant removal.  These methods include the use
of air injection or passive inlet wells, impermeable surface seals, and
groundwater depression pumping.  Each of these topics is discussed below.

Air Injection Systems

    To enhance air movement through the soil, inlet or injection wells can be
placed in strategic locations.  Inlet wells are open to the atmosphere and
allow air to be drawn passively into the soil from the surface.  Injection
wells use forced air to control the movement of air through the soil.  Figure
15 shows the influence of inlet/injection well on the movement of air through
soil.

    Inlet or injection wells provide several advantages to SVE system design
versus systems without air inlet wells.  A major advantage is the ability to
control the air pathways and thus, the zones of the soil to be affected by an
SVE system.  Connor (1989) stated that inlet wells act as a type of "vapor
barrier"; that is, the radius of influence does not propagate beyond an inlet
well in that vicinity.  This allows the SVE system to be designed to give
intensive treatment of a specific small area, rather than less intensive
treatment of a larger area.  The use of inlet or injection wells may also
allow more rapid cleanup - by allowing greater flow rates - than would
otherwise be possible.  Injection of the extracted air may eliminate the need
to obtain an air discharge permit.  Also,  if air is injected below the water
table, volatiles may be "stripped" or volatilized from the dissolved phase
into the soil gas.  Disadvantages of their use include the added cost
associated with construction of additional wells (although this would not be
the case if inlet wells were just inactive extraction wells) and the added
energy cost of the compressor for injection wells.

    Connor (1988) reports the use of passive inlet trenches to increase the
air flow rate to the recovery trenches.  Gasoline recovery increased by a
factor of three due to the improved pathway for air to enter the soil  (ambient
air had been restricted from entering the  system through the use of an
impermeable cap).  Zenobia et al. (1987) describe a system with four wells
that can be used alternately as extraction, injection, or inlet wells as the
situation dictates.

                                      59

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                      t
          VAPOR FLOW


          EXTRACTION WELL
                                        t

                                                       AIR FLOW
                                                       STAGNATION
                                                       ZONE
         EXTRACTION WELL


         INJECTION WELL
                                       o
Source: Johnson et al, 1989

                   Figure 15. Effects of Air Injection on Flowpaths
                                     60

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    Typically, injection and inlet wells are similar in construction to
extraction wells, although injection wells may have a longer screened interval
to provide uniform air flow.  In fact, a well designed SVE system allows wells
to act as extraction, injection, and/or inlet wells depending on the system
requirements.  Injection wells should be placed so that contamination is not
forced away from the extraction wells in a manner that will result in
bypassing the vapor treatment system.

    Often, neither inlet nor injection wells are used, and air is drawn
directly through the surface soils (Malot and Wood, 1985;  Camp Dresser & McKee
Inc., 1988).  Using ambient air drawn through the soils is easy and
inexpensive, but the air flow pathways are not readily controlled.
Short-circuiting along pathways of product movement may occur, which would
decrease the degree of contact between the contaminated zone and the fresh
air.  Short-circuiting refers to air being drawn down around the well without
passing through smaller pores of the contaminated zone. In these cases, the
use of impermeable barriers or surface seals may help to control vapor flow
paths.

Surface Seals

    An impermeable seal may be used where minimization of inflow from the
surface is required (Figure 16).  An impermeable surface seal prevents air
from entering from near the extraction well (where the pressure gradient is
the greatest) and forces air to be drawn from a greater distance and,
ultimately, to contact a greater volume of soil.  Surface seals may also
prevent infiltration of rainfall, reducing the amount of water removed by the
extraction well, thereby minimizing the production of air-water separator
sidestreams.  Surface seals also reduce fugitive VOC emissions from the soil
to the air.  Johnson et al. (1989) state that the effects of a surface seal
are reduced when the screened interval is greater than 25 feet below the land
surface.  Krishnayya et al. (1988) modeled the effect of a surface seal and
determined that the radius of influence increased dramatically but that the
flow velocities decreased due to the lower pressure gradients (at a
constant applied vacuum).

    Depending on the characteristics of the site, different materials can be
used as an impermeable cap.  A flexible membrane lining can be rolled out on
the site and can easily be removed when the treatment is complete.  These
membranes are available in a variety of materials, with high density
polyethylene (HOPE) being the most common.  These membranes are often used in
landfill applications and are available from several companies.  Selection of
a synthetic liner that does not require backfill on top of the liner and is
not susceptible to degradation by ultraviolet light will be most successful.
For commercial or industrial sites, this option may not be feasible due to
ongoing operations.

    An alternative to a synthetic membrane is a clay or bentonite layer
(Anastos et al., 1985).  These natural liners can be applied to varying
thicknesses. The drawbacks to clay liners are that they are not as easily
removed as are the synthetic liners and they are more susceptible to damage
from personnel and equipment.   A third alternative, the most common at

                                      61

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 OPEN SOIL SURFACE
                     __    /-"SHORT-CIRCUITING"
     IMPERMEABLE SEAL ,—i
Figure 16. Effect of Surface Seal on Vapor Flowpaths
                      62

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commercial or industrial sites, is the use of concrete or asphalt as a cap.
This alternative is normally used only at a site that is already paved (Camp
Dresser & McKee, Inc., 1988) or will be paved (e.g., a filling station).

Groundwater Depression Pumping

    The vacuum induced in the vadose zone by the extraction wells will cause
upwelling of the water table in the vicinity of the extraction wells about
equal in magnitude to the vacuum at that point (expressed as inches of water).
This may result in the groundwater being entrained in the vapor and being
drawn up by the vacuum pump.  This water must then be removed from the vapor
stream via an air-water separator prior to passing the vapor stream through
the pump or vapor treatment device.  To prevent the entrainment of
groundwater, Kemblowski (1989) suggests that a maximum vacuum of 20 inches
should be used to minimize upwelling.  Alternately, pumping wells may be used
to depress the water table in the vicinity of the extraction wells (Figure
17).  Pumping wells serve a dual purpose in cases where contamination exists
near the water table.  The depression of the water table will directly expose
to air flow those contaminants in the capillary fringe and just below the
original water table, allowing for volatilization of these contaminants.   This
method of operation makes SVE an especially effective complement to
groundwater pump and treat schemes.

EQUIPMENT

    The basic equipment for SVE systems consists of pumps or blowers to
provide the motive force for the applied vacuum; the piping, valves, and
instrumentation to transmit the air from the wells through the system and to
measure the contaminant concentration and total air flow; vapor pretreatment
to remove soil and water from the vapor stream;  and an emission control unit
to concentrate or destroy the vapor phase contaminants.  Figure 18 shows a
schematic diagram of a typical SVE system.  This equipment is discussed below.

Piping/Blowers

    The driving force for the creation of a vacuum in the soil is a positive
displacement blower, a centrifugal blower, or a vacuum pump.  Data presented
by Hutzler et al.  (1989) indicate that centrifugal blowers are about twice as
common as vacuum pumps in SVE applications.   Each method has particular
advantages and disadvantages that should be considered at each site.

    Several factors influence blower choice and design.  Most important is the
vacuum necessary to remove the design flow rate required to attain site
cleanup within the time frame agreed upon.  This depends on the areal extent
of contamination and the air permeability of the soil.   Pressure losses
through the pump,  piping,  and the collection system in general will also
affect the blower design.   Many,  widely varying values for the level of vacuum
have been published, from just a few to over 100 inches of water (0.2 to 10
inches Hg).   Weber (undated) gives general guidelines for a vacuum of 2 inches
to 4 inches Hg for gravelly and sandy soils, 3 inches to 8 inches Hg for thick
topsoil,  and up to 14 inches Hg for clayey soil to produce an equivalent air
flow rate.  Vendors of blowers and pumps should be consulted for assistance to

                                      63

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A) WITHOUT GROUNDWATER PUMPING
        WATER TABLE
  Y7   UPWELLING
                                            VACUUM WELL
                                                    ORIGINAL WATER
                                                    TABLE
 B) WITH GROUNDWATER PUMPING
                                                   VACUUM WELL
                           GROUNDWATER
                          EXTRACTION WELLS
                                                       DRAWN
                                                       DOWN
                                                       WATER
                                                       TABLE
     Figure 17. Groundwater Depression Pumping to Control Upwelling
                                64

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 SECONDARY
> EMISSIONS
                              WATER COOLED
                             HEAT EXCHANGER
    PRESSURE
     GAUGE
       VAPOR
     TREATMENT
        UNIT
       I
                                                         7 L
ANNUBAR
                                         WATER

                                     AIR-WATER
                                     SEPARATOR
                VACUUM
         BALL    GAUGE
        VALVE
                                                      STRAINER
PRESSURE
 RELEASE
  VALVE
                   SILENCE
                   MUFFLER
nt/AUtri
IMPERMEABLE ja,
SURFACE SEAL
\
BENTONITE X
CEMENT/
GROUT
4
\7 X
Si
•:




:Iv
:.v



s
«l


x
L

•==• /
SAND
PACK
W=
IXJ=
Jfxi







\ ^ rrr -r
JO, I^"N | | \y 	 ^



0.020 SLOT
^SCREEN




~~\








I—I 1 1
PUMP


SUBMERSIBLE
/ PUMP TOV\

	

— fu) 	
l\
BLOWER
fATER
/ y TREATMENTSYSTEM
sf ^

                FigurelS. Typical Soil Vapor Extraction System Schematic

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help assess the size and number of pumps or blowers required.

    Safety should be a prime consideration at any site where explosive gases
are present.  Motors that prevent or minimize vapor leaks should always be
employed.  If a vacuum pump is employed, the reduced pressure causes a
reduction in the flash point temperature, so extra care is needed.
Liquid-sealed vacuum pumps prevent metal-metal contact and reduce spark
potential, further reducing explosion potential,  but produce a wastewater
stream.

    The influent header from the wells to the pump should be equipped with a
vacuum indicator, a manual flow control valve, and an ambient air bleed valve.
The manual valve is used to control the blower and off-gas temperature to
prevent overheating of the blower and discharge piping.  Each individual well
should have shutoff valves, vacuum indicators, sampling parts, and may be
equipped with an air flow measurement device.  Each type of blower and pump
has its own temperature rise characteristics, and the choice of a blower with
low temperature rise is often best.  The effluent pipeline should be equipped
with a pressure indicator, temperature indicator and automatic discharge
valve.  Hutzler et al.  (1989) describe the type and rating of the pumps and
blowers used at several SVE installations.
    The piping used to connect the wells to the blower and emission control
device is termed the manifold.  Manifold piping may be very simple for a
single well SVE system, but becomes increasingly complex for systems employing
several extraction wells or systems with injection wells.

    Manifold piping is constructed of either polyvinylchloride (PVC) (Schedule
40 or Schedule 80), polypropylene, HDPE, or stainless steel.  The pipe
diameter depends on the amount of total flow from all wells; six-inch piping
is common.  The piping can be above-ground or buried, which is common at
active service stations.  In the northern states, the piping may be insulated
to prevent freezing of condensed water.

    Proper piping design will have the riser sloped either back toward the
well or towards liquid traps and the manifold piping sloped to liquid traps or
sumps at various intervals.  Entrained water and condensate (especially in
cases where the above ground temperature is below the ground temperature),
must be removed from the liquid traps occasionally, depending on the
condensate produced.

    The manifold system should also contain flow and pressure meters, to  allow
measurement during system operation, and flow control valves.  Valves can be
used to control which wells are active at any one time.  One method of "pulsed
venting" is to operate wells only periodically.  This allows for vapor to
diffuse into the larger pore areas.  This may be accomplished simply by
switching valve settings.  Valves may also be used to bleed in ambient air to
reduce vapor concentrations to comply with air discharge regulations or to
maintain concentrations below unsafe levels.
                                      66

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Vapor Pretreatment

    Vapors exiting the extraction wells may contain moisture and fine silt
particles that may impair mechanical devices and vapor treatment operations.
Air/water separators (knock-out drums and condensers), which often use
demisting fabric and centrifugal force, are used to reduce moisture entering
the vacuum pump and vapor treatment unit.

    Knock-out drums (Figure 19) decrease the velocity of the incoming vapor
stream, allow gravity to separate dirt and the heavier liquid molecules from
the lighter vapor stream and change the flow direction to impinge particles on
the side of the tank.  Condensers use the same technology, and in addition
refrigerate the vapor stream to condense any moisture in the vapor.
Condensation may remove more contaminants than knock-out tanks if the
concentrations of volatiles are above 5000 ppmv, but the added cost is often
not justified.

    Experience has shown that of the two components removed during vapor
pretreatment - water vapor and water liquid - liquid water is by far more
important.  Thermodynamic calculations show that the amount of water vapor
condensed from an air stream, assuming 98% relative humidity and a temperature
reduction of 20 degrees C, will be approximately 0.1 gal/day/scfm.  For a 100
scfm SVE system, the upper limit would be roughly 10 gal/day of condensate.

    Liquid water entrained in the vapor during extraction may far exceed this
amount.  Malot (1989) reports liquid recovery of 1,500 gal/hr at a flow rate
of 500 scfm.  This equals 72 gal/day/scfm, almost three orders of magnitude
higher than the theoretical maximum amount of condensate.  The amount of
liquid removed is highly variable and depends most directly on the proximity
of the screened interval to the water table and the amount of infiltration.
Application of a vacuum from the extraction well causes the water table to
rise, especially in soils with high clay content; if the capillary fringe or
water table rises enough to intersect the screen, significant quantities of
water can be removed.  Other important factors are the water content and
porosity of the soil, extraction rate employed, and the size of the well.

    Air/water separators extend the life of the vapor extraction system.
Removing the moisture from the vapor stream will prevent the pump parts from
rusting quickly.  One vendor (Greene, 1990) suggests inclusion of a separator
whenever the entrained water exceeds three gallons per minute (gpm).   If
activated carbon is being used as a vapor treatment process, reducing the
moisture content is essential since high humidity will decrease the carbon's
capacity to remove contaminants, vastly increasing carbon replacement costs.
Piping and other systems are also susceptible to damage from extreme
temperatures, although they can be protected by wrapping them with insulation.
Air-water separators also provide an important barrier prior to the pump to
prevent sediment and gravel from entering the blower machinery.  This is
especially important for blowers that operate at close tolerances.

    A disadvantage of separating the moisture from vapor stream is that the
side stream water must be treated.  Many sites employ groundwater pump and
treat schemes, such as air strippers, along with soil vapor extraction since

                                      67

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         INCOMING
          VAPOR
         STREAM
AIR WATER SEPARATOR
    CONDENSATE
                                  TO VAPOR TREATMENT
•DEMISTING FABRIC
                          SIDE STREAM
                                                        TO EXISTING
                                                        GROUNDWATER
                                                        TREATMENT FACILITIES
                                                        DISCHARGE TO SEWERAGE
                                                        SYSTEM
                               ON-SITE TREATMENT
                                •AIR STRIPPING
                                •GAG
                                                        COLLECTION FOR TRANSPORT TO
                                                        OFF-SITE TREATMENT FACILITY
               Figure 19. Vapor Pretreatment Sidestreams

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the two methods complement one another.  In such cases,  the side stream water
can be treated on-site by the facilities used for the groundwater treatment.
An advantage of not separating the incoming stream is that this entrained
water helps to cool the internal blower temperature,  reducing the temperature
rise through the blower or pump.

    Discharge to a sewer system is another possibility for disposal of the
side stream water.  This option is viable only if there is a sewer line
nearby, and if the concentrations of contaminants in the side stream water
comply with local discharge regulations and treatment plant capacity
limitations.  The quality of this sidestream is poorly defined, however, with
no references found in the literature.  Permits must typically be obtained for
this option.  When neither of these two disposal options is available, the
side stream water must be treated on-site separately or transported off-site
by tanker truck for proper treatment and disposal.

Emission Control

    Contaminants removed from the subsurface through SVE are not destroyed but
rather only transferred to a different location.  Many states and localities
realize that without some type of treatment of the off-gas, soil vapor
extraction may be simply substituting an air quality problem for a soil and
water quality problem.  For this reason, regulations regarding the disposition
of the vapor phase contaminants are becoming increasingly widespread.  The
regulatory climate in the states and on the federal level regarding air
discharge from SVE operations is discussed in the Introduction to this report.
In general, treatment is now or soon may be required for all vapor streams
except those that discharge minor amounts of contaminants into the atmosphere.
Section 5 discusses in greater detail the most common options for treating the
vapor phase contaminants.

Pre-assembled Systems

    Some vendors now sell preassembled systems that can be hooked up to a well
or manifold piping and incorporate all the above equipment.  Often these units
are trailer- or skid-mounted and can be brought directly to the site.  Such
systems have air/water separation, emission control,  and may be operated by
computer.  These systems can usually be rented, leased or purchased outright.
                                      69

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                                  SECTION 5

                       SYSTEM OPERATION AND MONITORING
INTRODUCTION

      Once installed, SVE systems can be operated in a flexible manner for
optimal removal efficiencies.  The systems should be operated to optimize both
efficiency and effectiveness so that the greatest amount of contamination is
removed in the shortest time period at the lowest cost.   Monitoring systems,
which are essential for determining SVE system success and cleanup attainment,
are composed of equipment familiar to most operators.  Interpretation of the
monitoring data, however, is sometimes a complicated task.  The clean up
attainment determination depends on having set cleanup goals prior to system
startup and proper monitoring procedures and interpretation of those data.
This section provides a brief overview of SVE system operation and monitoring
considerations.

SYSTEM OPERATIONS

      Once the SVE system is designed, constructed, and installed, the
start-up consists of turning on the SVE blower(s) or vacuum pumps and, as
appropriate, opening the system air inlet wells to the atmosphere.  Vacuum
gauges installed at various locations on the wells and manifold network are
monitored during start up so that flows and pressures can be adjusted to be
compatible with the system design.  Several hours to several days of system
operation are  required to establish steady-state flow conditions, depending on
the air permeability of the formation (Johnson et al., 1990).  After the
start-up period, the SVE system may be left in continuous operation
essentially unattended except for daily checks to make sure the water level in
the air-water  separator does not rise above the safe level and occasional tank
draining.  In  addition, the blower must be serviced periodically by checking
the drive belts and lubricating the bearings.  The emission control unit may
require more extensive attention, especially during the early stages of
operation.  In general, maintenance requirements are highest at system startup
and decline over time.

      The VOC  extraction rate is measured by sampling VOC concentrations  in
the exhaust air and measuring flow.  Removal rates, typically expressed in
units of mass  per time, equal the concentration multiplied by the volumetric
flow rate.  Numerous researchers have shown that the rate of VOC extraction is
high initially, but decreases with time.  This decrease may signal the
transfer to a  diffusion limited system.  In other words,  the saturated vapors
present in the soil at system startup are quickly removed.  Removal of
contaminants thereafter may be diffusion-limited.  Since  diffusion rates  are
slower than advection, removal rates drop with time of continuous operation.

                                      70

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      To maximize VOC removal while minimizing pumping, the vacuum pump may be
shut off and the soil vapor allowed to re - equilibrate.   This method is known
as "pulsed venting".  Alternately, different combinations of wells may be
vented to change the flow field and accomplish the same goal.  VOC
concentrations increase after a temporary shutdown, although to a level lower
than the concentration at the initiation of vapor extraction.  Figure 20 shows
an example of data collected during pulsed venting.

      Several studies have indicated that intermittent venting from individual
vents is more efficient than continuous operation in terms of mass of VOC
extracted per unit of energy expended (Hutzler et al.,  1989; Crow et al.,
1987; Oster and Wenck, 1988; Payne and Lisiecki, 1988).  Optimal operation of
an SVE system may involve taking individual vents in and out of service to
allow time for liquid and gas diffusion and to change air flow patterns in the
region being vented (Hutzler et al.,  1989).  For example, blowers can be
equipped so that they could be moved easily from one well to another for a
cost effective means of operation.

ENHANCED BIOTREATMENT

      An additional benefit of vapor extraction systems is the enhancement of
biodegradation of organic contaminants.  The increased amount of oxygen in the
soil pores that results from SVE operation seems to be responsible for the
increased biological activity.  Figure 21 shows the oxygen increase following
SVE startup for one site (Hinchee et al.,  1989).  Biodegradation can be
monitored by measuring the ratio of oxygen to carbon dioxide (02/C02)
(Johnson, R.L., 1989).  Connor (1988) showed that soil temperatures increased
in the zone of contamination, presumably as a result of biodegradation.  At a
jet fuel spill clean-up in Utah (DePaoli et al., 1989), C02 and 02 levels were
measured in the soil gas.  Initially, C02 levels were high (11 percent) and 02
levels were low (1 percent).  As venting continued, the C02 levels decreased
and 02 levels increased.  Carbon dioxide levels continued to be an order of
magnitude higher than background.  Considering that the hydrocarbons are the
only source of carbon at this site (the site was underlain by sandy soils) and
that half of the hydrocarbon consumed by bioactivity is converted to C02 (the
other half is converted to biomass),  it was calculated that bioactivity
contributed 27.5 percent of total'hydrocarbon removal at this site (DePaoli et
al., 1989).  Similarly, Fall and Pickens (1989) report that at another site
biodegradation accounted for approximately 40 percent of removal.  Figure 22
shows how biodegradation contributed to the overall mass removal at a site in
Utah.  Hinchee (1989) suggests designing SVE systems to maximize degradation
and minimize volatilization of contaminants to decrease vapor phase treatment
costs.

SYSTEM MONITORING

      SVE performance must be monitored to insure efficient operation and to
determine when it is appropriate to  shut off the system.  Johnson et al.
(1989)  recommend that at a minimum,  the following parameters should be
measured and recorded:
                                      71

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    300-
~  200'


Q.
O^

UJ
Q
Q.
    100-
SYSTEMSHUT

 DOWN OF 48 •

  HOURS
                                   150       200      250


                          TOTAL OF HOURS OF SVE OPERATION
                      300
350
        Source: Zenobiaetal., 1987
                        Figure 20. Effect Of "Pulsed" Operation
                                        72

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           10      20
     Distance (feet)

30     40     50     60      70      80      90
 10 -
 20
 30 -
 40 -
 50 -
 60 -
                   W
                         IT
                                        IK
            Vent
           ' Well #7
                                    IM

                                    IQ
                              PRIOR TO
                              VENTING
 70
                          Distance (feet)
           10     20     30     40     50     60     70     80     90
                                     Vent
                                     Well #7
                                                     FOLLOWING
                                                      VENTING
Source: Hinchee, 1989


        Figure 21. Oxygen Increase Following SVE System Startup
                               73

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o
m
DC
<
u
V)
CO

o
m
DC
<
u
o
cc
o
•>
I

"3T

8
CO
Q
o
D.
       14
       12 -
10 -
 8 -
 6 -
        4 -
 2 -
                O REMOVAL VIA VOLATILIZATION



                D REMOVAL VIA BIODEGRADATION
                              JANUARY                                  FEBRUARY


           Source: Hincheeetal. ,1989



                           Figure 22. Effect of Biodegradation on Overall Removal Rate

-------
      •     Date and time of measurements.

      •     Vapor flow rates at each extraction well and injection well.
            Measurements can be made by a variety of flow meters, including
            pitot tubes, orifice plates, and rotameters.

      •     Pressure readings at each extraction and injection well can be
            measured with manometers and magnehelic gauges.  Pressure should
            also be monitored at each soil gas probe location.

      •     Vapor concentrations and composition from each extraction well.
            Vapor concentrations can be measured by an on-line total
            hydrocarbon analyzer calibrated to a specific hydrocarbon.  This
            information can be combined with vapor flow rate data to calculate
            removal rates (mass/time) and the cumulative amount of contaminant
            removed.  Soil gas measurements should be made periodically at
            different radial distances at soil gas probes to monitor the
            reduction in contaminant vapor concentration.

      •     Temperature of the soil and ambient air.  (Connor (1988) predicted
            through monitoring soil temperatures that biodegradation was
            occurring in the zone of contamination.)

      •     Water table elevation (for soils with a relatively shallow water
            table).   Water level measurements can be made with electronic
            sensors located in air tight monitoring wells.

      •     Meteorological data, including barometric pressure,  precipitation,
            and similar data.

      In addition to these parameters, product thickness on the groundwater
should be measured if more than one-eighth inch exists on the water table.
Capillary rise in the soil can result in an apparent free product thickness 25
percent or more greater than the actual free product thickness (Hoag and
Cliff, 1985).

      Regalbuto et al.  (1988) observed that where the vadose zone is made up
of clean, coarse-grained material, most of the removal will typically occur
within the first three months of operation.  This is due to the removal of the
most volatile fraction of the product early in the system operation.  The
extraction rate was not necessarily highest immediately after SVE start-up.
During the first few months of operation the VOC concentrations varied as much
as an order of magnitude from one day to the next.


VAPOR STREAM CHARACTERISTICS

      The physicochemical properties of the contaminant-laden vapors change as
the vapors move from the soil through the SVE system until they are ultimately
discharged to the atmosphere.  This section discusses those properties and how
they vary through the system.


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Extraction Wells/Manifold Piping

      Subsurface soil vapors are usually saturated with water in the eastern
portion of the U.S.  In the arid regions in the Southwestern United States,
the soil vapors may not be as moist,  but relative humidities (RH) of 60
percent or more are common.  This is  particularly true when the water table  is
relatively close to the area of the screen in the venting wells.  If the
ambient air temperature is lower than the subsurface temperature (e.g., winter
conditions), the water in the soil vapor will begin to condense as the
extracted vapor reaches the surface piping of the SVE system, resulting in
free water droplets in the vapor stream.  Piping from the extraction well may
be sloped back towards the extraction well at a shallow slope of approximately
one percent to induce flow of accumulated moisture back into the well.
Alternately, the condensate can be collected in liquid traps and then treated.
In vapor wells screened close to the water table that have high vapor
extraction velocities, water droplets may carry-over as a mist into the
manifold piping due to the mechanical energy of the vapor stream.  This free
water in the stream must be removed because it may cause operational problems
with the blower, and reduce the effectiveness of air emissions control
processes such as carbon adsorption units.

Air/Water Separator

      The free water in the air stream is usually removed with the use of an
air/water separator (Figure 19).  This unit usually consists of a tank with a
demisting fabric in the inlet.  The air/water separator in the line works by
decreasing the velocity of the vapor stream, thus allowing the free water and
sediment particles to drop out and collect in the tank.  The demisting fabric
helps the suspended droplets collect and drip into the tank without being
carried over past the air/water separator.  Although, the air leaving the
air/water separator is usually still saturated (relative humidity = 100%), the
free water In the line is removed using the separator.  The air/water
separator also serves to collect sediment and gravel that may be removed from
the wells during pumping.

Vacuum Pump/Blower

      The pump or blower causes a pressure differential between the inlet and
outlet streams that results in a vacuum in the subsurface.  The heat of
compression in the blower causes an increase in temperature and a
corresponding decrease in relative humidity (although the total mass of water
remains constant)  in the vapor stream as it exits the blower.  This
temperature increase may cause operational and efficiency problems  in GAG
systems if not reduced.

Heat Exchanger

      The heat of  compression of the vapor through the blower causes  the vapor
stream to increase in temperature to a  level which may reduce the efficiency
of  the carbon adsorption units.  This effect is often minimized by  cooling the
vapor stream with  a heat exchanger to a temperature  that can achieve  an
optimal condition between  air temperature and level  of saturation.  The heat

                                      76

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exchanger can be designed to cool the vapor with cooling water,  with forced
air, or with another liquid such as brine or glycol.   Heat exchangers are
often used to preheat incoming vapors using the heat  of exhausted vapors that
have passed through thermal incineration or catalytic oxidation systems.

Emissions Control System

      The emissions control system is the mechanism by which the contaminants
in the vapor stream are treated prior to discharge to the atmosphere.  Vapor
combustion units use heat to combust flammable contaminants.  Incineration is
recommended for use on vapor streams containing high  concentrations (>10,000
ppmv) of contaminants such as BTEX compounds.  Destruction efficiencies are
typically 95 percent or greater.

      Catalytic oxidation units operate by passing the heated vapor stream
over a catalyst bed which facilitates combustion.  Destruction efficiencies
are typically greater than 95 percent.  When this method is used for vapor
streams containing more than about 3000 ppmv (depending on the heat value of
the influent stream) of volatiles, the vapor stream must be diluted with fresh
air because more concentrated vapors can cause catalyst bed melt-down.
Although best suited for petroleum hydrocarbon compounds,  catalysts have been
developed which can break down halogenated hydrocarbons as well.  The
technology for chlorinated hydrocarbons is relatively new, and further
treatment of the emissions stream may be required with scrubbers to prevent
releases of acidic by-products.

      Activated carbon adsorption is a proven technology which has been used
for many years for treatment of vapor streams containing organic compounds.
The process works by passing the vapor stream through a bed of activated
carbon.  The carbon has a high specific surface area  (ratio of area to
volume), which allows a large surface to adsorb contaminant compounds.  For
optimal efficiency, the relative humidity should be below 50 percent.
Contaminant saturated carbon is potentially hazardous and must be handled with
care.

CLEAN-UP ATTAINMENT DETERMINATION

      The determination that a site has been "cleaned up" involves both having
a specific cleanup target and then actually measuring or monitoring for that
target level in the field.

Target Levels

      Generally, soil clean-up target levels are pre-determined on a site by
site basis and depend on state and local regulations  and guidelines.
Confirmatory soil borings and, in some cases, soil gas samples are required
prior to closure.  Soil analysis is usually expensive and may be disruptive to
the site; therefore it is judicious to determine where soil borings should be
taken.  Key criteria for determining when the system  can be shut down include:
cumulative amount of contaminant removed, extraction  well vapor
concentrations, extraction well vapor composition, soil gas contaminant
concentration and composition (Johnson et al.,  1989), or remaining soil

                                      77

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concentration.  Appendix K contains the results of a survey of the states'
requirements regarding soil cleanup attainment.  It appears that most states
limit the allowed residual that may remain in the soil (typically expressed as
TPH).   Cleanup is considered complete when soil samples indicate that the
residual product is below the regulated limit.  When setting clean-up
standards for SVE sites, it should be noted that immobile,
high-molecular-weight compounds will remain in the soil and will be identified
during soil analysis.  These compounds are the least mobile compounds,
however, and pose the lowest exposure risks (via the groundwater pathway) of
all petroleum compounds.

      Measuring the vapor concentrations in the extracted vapors gives an idea
of the efficacy of the system; however, a decrease in vapor concentrations is
not necessarily strong evidence that soil concentrations have decreased.
Decreases in vapor concentrations can also be attributed to other phenomena
such as water table upwelling, increased mass transfer resistance due to
drying, diffusion-limited mass transfer flow from dead-end zones, or leaks in
the extraction system ("short-circuiting") (Johnson et al., 1989).  Monitoring
of the extraction well vapor composition as well as the concentration gives
more insight into the effectiveness of the system.  If the total vapor
concentration decreases without a change in composition then it is most likely
due to one of the phenomena listed above.  If the decrease in concentration is
accompanied by a shift in composition to less volatile compounds, then it is
most likely due to a change in the residual contaminant concentration.
DePaoli et al. (1989) show how the composition of the extracted soil gas
changed over the course of system operation (Figure 4).  As the total volume
of gas extracted increased, the fraction of lighter-end components decreased
while the fraction of heavier components increased markedly.  The figure shows
that the lighter, more volatile constituents  (such as butane and propane) are
removed first.  As more vapor is removed, the heavier, less volatile
constituents predominate due to the absence of the light fraction.  This
phenomenon influences cleanup criteria in some states. For example, a residual
gasoline cleanup operation might operate a vapor extraction system until
benzene, toluene and xylenes were not detected in the vapors.  The remaining
residual would then be composed of larger, less mobile molecules, which would
not volatilize or leach as easily and therefore may pose less of a health
threat  (Johnson et al. 1989).

Residuals Measurement

      The residual petroleum in the soil can be measured directly, by
retrieving a  sample  and then determining how much petroleum remains, or
indirectly, by monitoring the soil gas concentration and composition.  Often,
the soil gas measurements are made throughout  the course of the cleanup, and
that data is used to determine where the final soil samples should be taken.

      Residuals analysis typically means the  quantification of either Total
Petroleum Hydrocarbons  (TPH), total Volatile  Organic Compounds  (VOCs), or one
or more indicator compounds  (e.g., benzene, toluene, ethylbenzene, and xylenes
(BTEX)).  Most state and local regulations are based on these measurements
(Table  5).  These parameters are determined in different ways, each of which
has its particular limitations.

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     TABLE 5.  CLEANUP LEVELS PUBLISHED FOR PETROLEUM CONTAMINATED SOILS

 State     TPH  (ppm)                            Notes
 AK       NE             Clean  to background or use LUFT  to justify  level.

 CA       10-1,000       Leaching analysis  potential  - LUFT manual.

 CT       NE             Level  necessary  to protect groundwater.

 PL       5              Sum of BTEX <100 ppb.

 HI       NE             Function of water  quality standards.

 IL       NE             Function of health considerations

 KS       NE             Function of groundwater standards.

 KY       NE             To background or detection limit.

 ME       20-50          Level  dependent on water quality factors.

 MD       NE             Case-by-case basis.

 MA       100            Or 10  ppm total organic volatiles as benzene.

 MI       NE             To background levels.

 MS       NE             Function of water  quality standards.

 NH       <10            BTEX <1 ppm for gasoline and diesel.

 NJ                     BTEX - Groundwater based standards

 NY       NE             Function of water  quality considerations.

 PA       NE             Case-by-case basis.

 RI       50-100         TPH level function of site specific factors.

 SC       NE             Caser-by-case basis, function of water quality.

 TN       100            10 ppm total BTX in soil (except for gasoline.)

 TX      NE             Function of water quality standards.

 UT      NE             Standards under development.

 VT       <10            May apply water quality standards.

 WI       <10            Groundwater standards for benzene and toluene.

 WY       <10,<100       Depends on depth to groundwater

NE = None established                  *Adapted from Bell et al. 1989
                                         79

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      The measurement of TPH begins with the extraction of oil and grease from
the sample using Freon 113,  which is the trade name for a fluorocarbon
solvent.  The oil and grease is composed of both fatty materials from animal
and vegetable sources and hydrocarbons of petroleum origin.   The Freon 113 is
then passed through a polar silica gel,  which adsorbs the fatty materials,
leaving the hydrocarbons in the Freon 113 solvent (Kopp and McKee, 1979).

      The hydrocarbons are then quantified by gravimetric techniques or
infrared (IR) spectroscopy.   The IR spectra of petroleum hydrocarbons contain
a variety of bands that are indicative of specific molecular structures.  The
advantage of the IR method is that instrumentation costs are modest (Potter,
1989).  Unfortunately, precision and accuracy vary greatly depending on the
petroleum products undergoing analysis.   For example, the more volatile
components of gasoline or light fuel oils may be lost in the solvent
concentration step.  The recoveries for heavier distillates are often low
because many of the distillate constituents are only sparingly soluble in
Freon (Potter, 1989).  Additionally, environmental concern over the use of
chlorofluorocarbons is growing and the long-term availability of Freon is
uncertain.

      Quantification of volatile compounds such as BTEX is usually carried out
using gas liquid chromatography or gas liquid chromatography coupled with mass
spectrometry, which can detect specific organic cOompounds.  Volatile
compounds from the sample may be identified and quantified using "purge and
trap" techniques.  Limitations of this analytical method include the detection
of non-target compounds and poor chromatographic resolution with packed and
capillary columns  (Potter, 1989).  BTEX are typically used as indicator
chemicals for assessing the extent of leaking gasoline USTs (CDM, 1987) .
However, if  the dissolved gasoline plume in the ground is older than one to
two years and free product no longer exists, the BTEX compounds may no longer
be present because they are readily biodegraded.  Under these conditions,
other gasoline compounds such as methyl-tertiary butyl ether  (MTBE), an octane
enhancing additive found in some gasolines, could be used to locate and
delineate the plume.

      Soil humic materials can influence the results of both TPH and BTEX
analytical tests.  Many hydrocarbon compounds that are found in various
petroleum products occur naturally in soil (Table 6).  Aromatic hydrocarbons
such as benzene, toluene and xylene occur naturally  in soils at concentrations
of 1-5 ppm (Dragun and Barkuch, 1989).

      Given  the dynamic nature of the soil's biota and chemical reactivity,
the petroleum products themselves undergo alteration.  As petroleum products
weather in the soil environment they may not reach a mineralized endpoint.
Instead, the degradation process may result in relatively stable aliphatic  and
aromatic compounds, which are integral parts of soil humus  (Alexander,  1977).
Because both humic and fulvic acids, fractions of humic material, are composed
of random polymers of aromatic and aliphatic substituents (Tate, 1987),
petroleum hydrocarbon degradation processes may increase the amount of  soil
humic material.
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               TABLE 6. PETROLEUM HYDROCARBON CONSTITUENTS
                       THAT OCCUR NATURALLY IN SOILS
         acetic acid
         benzene
         benzoic acid
         carbazole
         2,6-dimenthylundecane
         n-dotriacontane
         eicosanoic acid
         ethylbenzene
         n-hemeicosane
         heptacosane
         heptanoic acid
         n-hexadecane
         methane
         methanol
         n-nonacosane
         nonanoic acid
         n-octadecane
         pentacosane
         pentanoic acid
         phenanthrene
         n-tetracosane
         tetradecanoic acid
         n-triacontane
         p-xylene
alkanes
1,2-benzofluorene
butanoic acid
decanoic acid
n-docosane
n-eicosane
ethanol
formic acid
n-hentriacontane
n-heptadecane
hexacosane
hexadecanoic acid
me thanethiol
naphthalene
n-nonadecane
n-octacosane
octanoic acid
n-pentadecane
perylene
propanoic acid
n-tetradecane
toluene
n-tricosane
o-xylene
Source: Dragun and Barkach, 1989.
                                      81

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      Given the potential for the enhancement of soil humic material via
petroleum degradation, questions arise concerning what the determination of
TPH is measuring.  Is it measuring actual petroleum product hydrocarbon
compound and/or the end products of biotic and abiotic transformations of
petroleum products?  There appears to be no published research describing
compounds to which petroleum hydrocarbons may be transformed or methods to
distinguish between humic material and petroleum hydrocarbons.  New techniques
and methods that could address some of these issues are discussed below.

      The EP Toxicity Test is designed to simulate the leaching to which a
waste would be subjected if disposed of at an improperly designed sanitary
landfill.  This procedure can be applied to solids, liquids, and multiphasic
materials.  The application of this test will identify wastes that have the
potential to leach significant concentrations of hazardous compounds.
Following the extraction procedure the leachate is analyzed.  This method,
since it is designed to extract only inorganics, is not applicable to
hydrocarbon spill sites.  This disadvantage has led to the development of an
extraction method that is capable of leaching both inorganics and organics
from a waste sample.

      The Toxicity Characteristic Leaching Procedure (TCLP) was designed to
improve the EP toxicity protocol and to expand its applicability to a greater
number of contaminants that could leach from waste materials.  This procedure
is used to extract semi-volatile organic compounds, pesticides and metals
using sample containers similar to those used in EP toxicity analysis.
However, with the utilization of a "zero headspace extractor", volatile
organic compounds can be extracted (EPA, 1986b). The key difference between
the EP toxicity method and the TCLP method is that TCLP uses a more acidic
leaching medium for moderate to highly alkaline wastes.  This ensures that the
leaching potential is not underestimated.  The leaching medium is acetic acid
in a sodium acetate buffer solution at a pH of 2.9 or 5.0.  For moderately to
highly alkaline wastes the 2.9 pH solution is used, while the 5.0 pH solution
is used for other wastes.  The extraction procedure is identical to EP
Toxicity, except that the agitation step requires the use of a rotary
end-over-end shaker,  the extraction time is 18 hours, and the extraction
procedure must employ a "zero-headspace extractor".

      The limitations or sources of variability associated with any leaching
procedure includes:

      •     chemical  differences in the laboratory leaching media and the
            actual leaching  solution at the site,

      •     liquid:solid ratio, and

      •     number and  time  of extraction(s).

      The advantage  of using a leaching test  is that the question of
identifying the  substance is secondary and the  issue of risk  to the
environment is directly addressed.
                                       82

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                                  SECTION 6

                          SECONDARY EMISSION CONTROL
INTRODUCTION

      SVE systems that do not incorporate a treatment system for the extracted
vapors do little other 'than transfer the contaminant from the subsurface
environment to the atmosphere.  One may argue that the environmental risks
associated with atmospheric discharge of the extracted vapors are less than
those associated with leaving the vapors in the soil.  Dispersal of these
vapors in the atmosphere renders the concentrations harmless, this argument
holds, while leaving the contaminants in the soil means a continued risk of
migrating to water supplies.  This argument is spurious, however, considering
the air quality problems being encountered in some areas, the global impacts
now being attributed to hydrocarbon and chlorinated organics releases, and the
availability of effective vapor treatment technologies.  Regulatory agencies
are becoming increasingly stringent with regards to SVE air discharge and
permitting requirements as the technology becomes more widely used.

      This section discusses the six most common alternatives for dealing with
the evacuated vapor stream from an SVE system:

      • adsorption on granular activated carbon;

      • thermal incineration;

      • catalytic oxidation;

      • internal combustion engine;

      • biodegradation; and

      • direct discharge.

      Many of these methods have been used in industrial applications to
control point source VOC emissions.  A new treatment method, using a packed
bed of ceramic beads to thermally destroy contaminants, is also discussed.
Figure 23 shows the general ranges of applicability for these options.  The
figure shows that most of these alternatives may be used over a range of
concentrations that spans several orders of magnitude.  Usually, however, each
option is cost effective over a small part of that range.  For example, GAG
adsorption could be used to treat a vapor stream containing 10,000 ppm of
hydrocarbon vapors, but the cost for carbon regeneration would be prohibitive.
Rather, an incineration technique would likely be used.  Likewise, an internal
combustion engine could be effective at reducing a vapor stream containing

                                      83

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CO
-p-
                     LEGEND

                   COST EFFECTIVE RANGE

                   TECHNOLOGY EFFECTIVE;
                   COST MAY BE PROHIBITIVE
         DIRECT
         .DISCHARGE]
                                     GAG
                                                                                   INTERNAL
                                                                                COMBUSTION
                                                                                     ENGINE
                                          CERAMIC BEADS
                                                                  THERMAL INCINERATION
                               CATALYTIC
                               .OXIDATION
                  0.1
1            10          100         1,000

   EXTRACTED VAPOR CONCENTRATION (ppmv)
10,000      100,000
                           Figure 23. Applicability of Vapor Treatment Options

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less than 1000 ppm, but this technology would require additional fuel and
would probably not be the most appropriate vapor treatment choice.

GRANULAR ACTIVATED CARBON ADSORPTION

      Adsorption refers to the process by which molecules collect on and
adhere to the surface of an adsorbent solid (EPA, 1988b).   This adsorption is
due to chemical and/or physical forces.  Physical adsorption (the more common
type in this application) is due to Van der Waals'  forces, which are common to
all matter and result from the motion of electrons.   Activated carbon is used
as an adsorbent because of its large surface area,  a critical factor in the
adsorption process since the adsorption capacity is  proportional to surface
area.  "Activated" carbon has significant surface area due to its internal
pore structure; commercially available GAC typically has a surface area 1,000
to 1,400 m2/gram.

      Granular activated carbon (GAC) is the most common method of vapor phase
treatment.  GAC is popular for several reasons, including its relative ease of
implementation and operation, its reputation as a commonly-used treatment, its
ability to be regenerated for repeated use, and its  applicability to a wide
range of contaminants at a wide range of flow rates.   Many vendors sell or
lease prefabricated, skid-mounted units that can be  procured and implemented
in a matter of days.  Carbon is economical only for  relatively low mass
removal rates, however; when the vapor concentration is high, carbon
replacement or regeneration may be prohibitively expensive.

      The adsorption capacity of the carbon depends  on several factors,
including influent vapor temperature and relative humidity and, most
importantly, the influent VOC types and concentrations.  Isotherms, which show
the mass of contaminants that can be adsorbed per unit mass of carbon, are
available to predict the contaminant-specific adsorption capacity for a
specific type of carbon.  Figure 24 shows isotherm data for benzene at various
temperatures and pressures for a specific charcoal-based carbon.  Isotherms
for common compounds are available from carbon vendors.  GAC generally has a
high affinity for volatile molecules, such as hydrocarbons or chlorinated
compounds, which are the most likely types of compounds to be removed via SVE;
however, some hydrocarbons such as isopentane have relatively low adsorption
capacities.

      Although GAC has a very high surface area for  adsorption of
contaminants, the mass of contaminants removed via SVE may quickly exceed the
carbon's capacity.  At sites with high mass removal  rates, due to high
concentration, high flow rate, or both, the carbon may quickly become spent
(i.e., the capacity is filled).  Replacement and disposal of spent carbon can
become very expensive, especially if the spent carbon must be treated as a
RCRA waste.

      An alternative to replacement of the carbon with off-site disposal or
reactivation is on-site regeneration of the carbon.   Such systems regenerate
the carbon in place, using steam or hot air to desorb the contaminants.  The
contaminants recovered in liquid form may then be disposed or, in some cases,
may be used as fuel to produce steam.

                                      85

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                                              98
                                       CAPACITY, WEIGHT %
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-------
      Connor (1989) states that this type of system has been able to achieve
recovery of 36 gallons of product per day.  That system has also shown that
the steam generator unit requires about six gallons of fuel (gasoline) for
every five gallons of product recovered.  Thus, operating at maximum capacity,
this unit would consume about fifty gallons of gasoline weekly for steam
generation. This system becomes economical as the mass removal rate increases.

      The relative humidity of the incoming vapor stream may severely limit
the effectiveness of and increase the cost of the GAG.  Water vapor will
preferentially occupy adsorption sites, thereby decreasing the capacity of the
carbon to remove contaminants from the air stream.  For this reason, entrained
water should be removed from the incoming vapor stream by use of an air/water
separator.  Vendors typically recommended that the vapor should be treated
further to reduce the relative humidity to below 50 percent, usually by
heating the air.  Placement of the blower prior to the GAG unit is a way to
utilize the heat of compression produced by the blower to increase the vapor
temperature and thereby increase water's solubility in air, reducing the
relative humidity.  The GAG may be placed prior to the vacuum pump or blower
to create a greater negative pressure through the GAG and to prevent
overheating the GAG unit, although adsorption may be reduced under vacuum.

      The heat generated by pumping systems and by the compression of vapors
often results in an exhaust stream of elevated temperature.  The off-gases
from some vacuum systems must be cooled from 200 to 80 degrees F for efficient
treatment before entering the carbon adsorption units.  Figure 24 shows how
the adsorption capacity decreases as temperature increases.  Cooling may be
accomplished by vertical, chilled water coils.  Condensate generated during
vapor cooling must then be .treated.

      Information on GAG design parameters is available from the carbon
vendors.  Calgon Carbon Corporation (Pittsburgh, PA) and Carbtrol Corporation
(Westport, CT),  among many others, supply pressure drop curves for the various
GAG types they supply.  The pressure drop curves are developed as a function
of flow rate.  Standard size containers are available depending on the
expected air flow rate.  Many vendors supply modular, prefabricated GAC units
of 200 to 2000 pounds of activated carbon that may accommodate flow rates from
below 400 scfm to over 1,000 scfm.

THERMAL INCINERATION

      Several types of incineration options may be used to destroy vapor-phase
contaminants resulting from SVE.  Incineration ideally converts compounds to
carbon dioxide and water.  Complete destruction of contaminants requires very
high temperatures, typically 1000 to 1400 degrees F.  The destruction of the
contaminants is a major advantage of this technique over carbon adsorption,
which serves only to concentrate the contaminants onto the carbon, which must
then be disposed.

      Thermal incineration is the most basic of all incineration techniques.
The vapors are heated to a very high temperature, usually in a combustion
chamber, although some methods use a direct flame.  To reach the required
temperatures, the influent vapor is preheated and then enters the combustion

                                      87

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unit.  In cases where the influent concentration is very high (on the order of
percent by volume), the incineration may be self-sustaining.   Fall and Pickens
(1989) reported that vapor concentrations in the range of 50,000 to 90,000 ppm
sustained a flame.  Concentration of volatiles in the air stream might be
increased by intermittent blower operation (the "pulse method")  or by
intermittently operating different extraction vents (Hutzler et  al.,  1989).
For the case cited above, the flame was self-sustaining only when the system
was operated eight hours per day; if operated continuously,  a stable flame
could not be maintained.  For most vacuum extraction applications, the vapor
concentrations are not high enough to sustain a flame and a supplemental fuel
must be used to maintain the necessary temperatures.  Natural gas or propane
normally serves as the supplemental fuel.

      For safety reasons, influent concentrations are normally limited to 25
percent of the lower explosive limit (LEL) (EPA, 1986).  The LEL for gasoline
is between 12,000 ppm and 15,000 ppm, depending on the gasoline's grade (A.D.
Little, 1987) (Figure 25).

      Direct incineration is not appropriate for influent vapor streams
containing chlorinated compounds.  Partial or incomplete combustion of
chlorinated compounds could result in the production of chlorine gas and other
PICs  (products of incomplete combustion).

CATALYTIC OXIDATION

      Catalytic oxidation is a variation of thermal incineration.  In this
process, the vapor stream is heated and passed through a combustion unit in
contact with a catalyst.  The catalyst unit is generally a metallic mesh,
ceramic honeycomb, or packed bed consisting of catalyst-impregnated pellets
(EPA, 1986).  The catalyst is typically composed of a precious metal
formulation  (e.g., palladium or platinum) that facilitates the transformation
of the contaminant molecules into carbon dioxide and water.  Trowbridge and
Malot  (1990) describe a catalytic oxidation unit with a non-precious metal
catalyst that may be used for chlorinated air streams.  Figure 26 shows a
schematic diagram of a catalytic incinerator unit.

      In this process, a catalyst accelerates the chemical reaction without
undergoing a chemical change itself.  The catalyst increases the incineration
reaction by adsorbing the contaminant molecules on the catalyst surface.  The
higher concentration of reactive materials serves to increase the reaction
rate, thereby facilitating the oxidation process (Hardison and Dowd, 1977).
Careful monitoring of extraction gas concentration is required,  and the air
stream must be diluted to be kept below  25 percent LEL (3,000 ppm) since
higher concentrations may cause overheating, resulting in the catalyst
melting.

      The main advantage of catalytic incineration versus thermal incineration
is the much  lower  temperature required with a catalyst.  These systems
typically operate  at 600 to 900 degrees  F  (GSM Systems, 1989), versus
temperatures of 1400 degrees F or higher for thermal incineration.  The lower
temperature  results in lower fuel costs.  Natural gas or propane are typical
fuels used for vapor streams that do not contain sufficient heat value for a

-------
     LEL
    12,000
TOO LEAN
TO BURN
 GASOLINE
COMBUSTION
   RANGE
                         UEL
                        15,000
TOO RICH
 TO BURN
               Figure 25. Explosimeter Readings

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               I
PREHEAT SECTION
                   CATALYST
                                      \*
IZ
                                HEAT
                              EXCHANGER
                                        CONTAMINATED
                                          AIR INLET
               TREATED
                AIR
               OUTLET
        Figure 26. Catalytic Oxidation Schematic

-------
self-sustaining incineration.  Energy costs can be further reduced by
reclaiming heat from the exhaust gases, using the exhaust to preheat the
influent vapor stream, as shown in Figure 26.

      Catalytic oxidation units require careful monitoring to prevent
overheating of the catalyst, which would result in its deactivation of the
catalyst.  While the exact limitations depend on the heat value of the
influent vapor stream, concentrations over 3000 ppm VOCs are normally diluted
with ambient air to control the temperature in the catalytic unit.  Safety is
also a concern with these units, as with any incineration method.   Maximum
permissible Total Hydrocarbons (THC) concentrations vary by site,  but are
usually below 25 percent of the LEL.  THC concentrations are measured before
operation of the catalytic unit to determine the necessity of sidestream
dilution of the vapor stream.  New technologies potentially capable of
treating chlorinated compounds by catalysis are currently under development
(Trowbridge andMalot, 1990).

INTERNAL COMBUSTION ENGINES

      Internal combustion engines (ICEs) have been used for years to destroy
landfill gas.  The application of this method to hydrocarbon destruction is
recent, with the first operational unit having been installed in 1986.
Currently, over one hundred of these units are operating in southern
California and providing good destruction and removal efficiencies.

      The internal combustion engine used for this technique is simply an
industrial or automotive engine with its carburetor modified to accept vapors
rather than liquid fuel.  Virtually any make of engine can be used:
Volkswagen, Audi, Ford, Chevrolet and others have all been reported as having
been used.  The size of the engine (expressed in cubic inches) reportedly
greatly affects the flow rate of air through the engine, with larger capacity
engines able to handle larger flow volumes.  For example, RSI (Oxnard, CA)
sells an internal combustion device that uses a 1988 Ford 4-cylinder, 140
cubic inch engine, which is rated to accept 30 to 60 scfm of inlet flow.  By
contrast, Wayne Perry Inc. (Buena Park, CA) employs a 460-inch engine that
accepts 400 scfm.  RENMAR, a manufacturer of self-contained ICE system,
reports a correlation of roughly 100 scfm flow-through per 300 cubic inches of
engine capacity without loading the engine (i.e, running it at idling speed).
This flow rate could be increased greatly by loading the engine (i.e., forcing
the engine to work harder and do more work by, for example,  generating
electricity with the unused work potential of the engine).  Differences in
destruction efficiencies based on engine type are unknown at present.

      A second required modification to the engines is the addition of a
supplemental fuel input valve.  When the intake hydrocarbon concentration is
too low to sustain complete combustion, a supplemental fuel source must be
added to ensure complete combustion.  Propane is the fuel used almost
universally, although one vendor reported that tests with natural gas showed
greatly reduced (by 50 to 75 percent) energy costs.  The concentration below
which supplemental fuel needs to be added is uncertain at this time.  The
engines are also equipped with a valve to bleed in ambient air to maintain the
required oxygen concentration.  Because soil vapor may have very low

                                      91

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concentrations of oxygen,  especially during the initial stages of operation,
ambient air is added to the engine,  via an intake valve,  at a ratio sufficient
to bring the oxygen content up to the stoichiometric requirement for
combustion.  One vendor suggested a ratio of 4 parts ambient air to one part
extracted vapor at start up, with the required ratio of ambient air decreasing
as the soil oxygen content increases.  This requirement for bleed-in air
reduces the effective flow rate that the engine can treat;  for example, at a
flow rate of 100 cfm and a bleed-in ratio of 3:1, only 25 cfm would be
extracted soil vapors, while 75 cfm would be ambient air.

      A catalytic converter is an integral component of the system, providing
an important polishing step to reach the low discharge levels required by many
regulatory agencies.  A standard automobile catalytic converter, using a
platinum-based catalyst, is normally used.  Data from the South Coast Air
Quality Management District (SCAQMD), the air quality regulatory body for Los
Angeles and the surrounding area, show that the catalyst reduced
concentrations of total petroleum hydrocarbons (TPH) from 478 ppm to 89 ppm
and from 1250 ppm to 39 ppm, resulting in important additional contaminant
removal (Millican, 1989).   SCAQMD requires a catalytic converter to permit
this type of system.  Catalysts have a finite life span (typically expressed
in hours of operation) and must be monitored as that time approaches to ensure
that the catalyst is working properly.  The length of operation of the
catalyst depends on the vapor concentration, whether lead is present, and the
amount of propane added.  A range suggested by one equipment vendor was 750
hours to 1500 hours (about one to two months) of operation.  Upon
deactivation, they can be replaced easily with any automobile catalytic
converter, available at most auto parts stores.

      To date, the use of ICEs appears to be limited to California, mostly in
the South Coast Air Quality Management District in southern California, which
has among the most stringent air discharge regulations in the country.  SCAQMD
has permitted over 100 ICEs for use  in their district.  RSI, Inc.  (Oxnard, CA)
has installed more than thirty ICE systems, all in California.

      Data obtained from ICE operators and regulators, summarized  in Table 7,
show that ICEs are capable of destruction efficiencies of well over 99
percent.  They have been especially  useful in radically reducing incoming
vapor streams with very high concentrations of TPH (up to 30 percent volume)
to levels below 50 ppm.  Results of  tests for specific compounds (BTEX) show
not detected in some cases and below 1 ppm in many other situations.  The
total destruction capacity may be expressed as mass removal rate.  One ICE
operator reported a mass removal and destruction rate of over one  ton per day
(about 12 gallons per hour) (Perry,  1989).

      No information has been obtained to indicate how the destruction
efficiency varies with changes in influent type and concentration, engine type
and operation, the effects of manual versus automatic control, and the effects
of physical parameters like relative humidity, temperature, contaminant
concentration, and other variables.  Landfill gas experience may shed  light on
some of these issues.
                                      92

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TABLE 7.  DESTRUCTION EFFICIENCIES OF ICEs
Parameter
THC

THC
Benzene
Ethylbenzene
Toluene
Xylenes
TPH (non-methane)
Methane HCs
Benzene
Toluene
Xylenes
Ethylbenzene
TPH



Benzene



THC
Benzene
Toluene
Ethylbenzene
Total Xylenes
THC
Benzene
Toluene
Ethylbenzene
Total Xylenes
Initial After Removal
Concentration Catalytic Converter Efficiency
(ppm) (ppm) (%)
38,000
200,000
318,832
995
125
1005
1550
49,265
741
380
400
114
18
65,450
34,042
30,500
39,000
1,094
470
785
730
58,000
1,400
720
77
320
26,000
960
840
91
360
89
39
16 ppm
ND (<10 ppb)
ND (<10 ppb)
0.014
<11.5 ppb
225
109
0.8
1.1
0.7
<0.5
30
14.5
1.4
4.7
67
1.6
0.63
0.056
160
0.13
0.024
0.062
0.13
140
0.024
0.020
ND (0.02)
0.080
99.76
99.98
99.99
99.99
99.99
99.99
99.99
99.56
85.29
99.79
99.73
99.39
—
99.95
99.96
99.99
99.99
93.88
99.66
99.92
99.99
99.72
99.99
99.99
99.92
99.96
99.46
99.99
99.99
100.00
99.98
Reference
Millican, 1989

Wayne Perry, 1989
it M
ii ii
ii ii
ii ii
RSI, 1989
n ti
ii n
ii n
n n
n n
Rippberger, 1989
n n
n n
n n
Rippberger, 1989
n n
n ii

RSI, 1989
n n
n ii
n n
n n
n n
n n
n n
n n
n n
                  93

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      The use of internal combustion engines as a vapor treatment device for
extracted soil vapors may possess several advantages over conventional
treatment methods (carbon, thermal oxidation,  or catalytic oxidation),  at
least for certain instances.   Perhaps the most important advantage that this
method has over other vapor treatment methods  is that the ICE produces an
easily harnessed source of power that can be used to produce useful work.  In
fact, some vendors sell self-contained units that use the ICE to power the
vacuum pump that is the driving force behind vapor removal.   An added benefit
of this system is that vapors cannot be extracted unless treatment is also
occurring, eliminating the possibility of vapors bypassing the treatment
system.  RENMAR, an equipment supplier, contends that the vacuum extraction
system consumes only about 25 percent of the useful work produced by the
engine.  Other ideas for using the power could include lighting the site,
heating a field trailer, or similar ideas.  The engine could also be used as
injection air for the SVE system.

      The ICE is able to handle very high concentrations of extracted air,
such as would likely be found over a large free product plume.  Reports of
inlet vapor concentration have gone as high as 300,000 ppm (Millican, 1989).
By contrast, catalytic incinerators and thermal incinerators sometimes require
dilution with bleed-in air to a safe level, reducing their upper limit.  ICEs
can also accept fairly low concentrations (to below 1000 ppm), although
supplemental fuel use increases and the cost effectiveness decreases greatly
at reduced intake concentrations.  Further, the removal efficiency compares
favorably with other treatment methods, based on the limited data available
from actual system installations.

      Another advantage of ICEs is their portability.  Typically, the
self-contained units are skid-mounted or put on a trailer and can go from site
to site very easily.  The site requirements may also favor ICEs over other
oxidation methods.  ICE units are reportedly smaller and less noticeable than
direct thermal incineration units and may be more appropriate for areas that
wish to remain low profile.

      Some disadvantages of ICEs have been noted.  The primary drawback may be
that the method requires a fairly high degree of manual supervision,
especially when the system is being started up.  Mainly, the air to fuel ratio
must be adjusted to maintain the proper conditions for complete combustion.
Microcomputers are available to monitor and adjust the air to fuel ratio and
add propane as needed; however, immediately after system startup, the
characteristics of the extracted vapors may change so quickly that manual
adjustment is required.  As the system operates for a longer period, manual
attendance may no longer be required.

      Another potential disadvantage of this technique is the somewhat limited
flow rates that can be removed from the soil and treated.  The flow rate is
limited especially by the ambient air that must be added to maintain required
oxygen levels.  The use of oxygen, rather than air, might increase the
relative proportion of extracted vapors that the engine treats.  A third
potential disadvantage of ICE is the noise level.  Users have reported that
these devices are quite loud and their use may be restricted  in certain
neighborhoods.

                                      94

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PACKED BED THERMAL PROCESSOR

      A technology that may have application to the control of vapor emissions
is the packed bed thermal processor.  This system consists of a bed packed
with ceramic beads that are heated to 1800 degrees F, through which the vapor
stream passes and the contaminants are destroyed.   The patented packing
geometry and the uniformly high temperature that the ceramic beads are able to
result in nearly complete destruction of the influent vapors,  without the use
of a flame.

      IN-Process Technology (Sunnyvale, CA) has used these systems to destroy
vapors at several chemical and other industrial plants.  The company is
currently investigating its applicability to the remediation market
(Fredricks, 1990).  According to Fredricks, this technology has several
characteristics that may allow it to be used successfully as an emission
control system for remediation streams.  First, the removal efficiency is
extremely high and very reliable.  Tests have shown efficiencies of 99.99+
percent, and this removal is attained continuously.  The residence time can be
adjusted to attain any necessary removal efficiency.  Other incineration
techniques use a flame, which leads to non-uniform heating and non-uniform
removal efficiencies. This method does not use a flame, and the geometry of
the packed ceramic beads results in complete, uniform combustion, so that no
NOX compounds are produced.  Once the beads reach the proper temperature of
1800 degrees F, which is reached by electrically heating the ceramic, no
additional energy input is required if the heat value of the vapors is
sufficient.  This point is near a concentration of 2000 ppmv.   If the
concentrations are below this value, natural gas or propane can be bled in
with influent to maintain the proper temperatures.

      As with any incineration technique, excess air needs to be added to
dilute the concentration to safe levels if the influent is too rich.  This
method has handled concentrations at the percent level.  Throughput levels
depend on the model selected (presently 100, 200,  and 500 scfm) with higher
flow rates met by combining two or more of the modular units.

      Perhaps the greatest advantage of this technique, according to the
vendor, is its ability to destroy chlorinated compounds without the production
of hazardous by-products and without degradation of the ceramic beads.  This
ability sets this method apart from most other incineration techniques.

BIOTREATMENT

      Biofilters have been used for many years to treat odors (Carlson and
Leiser, 1966; Prokop and Bohn,  1985; and others).   Typically,  these soil beds
are used to control malodorous gases resulting from sewage treatment plants or
industrial plants.  Biofilters have also been used recently to treat vapor
phase VOCs prior to atmospheric discharge.  Pilot studies indicate that
significant VOC removal may be possible with this method (Johnson, M., 1989).
This process operates by introducing the VOC-laden vapor to a soil bed, which
serves as a growth medium for microorganisms.  As the contaminants flow up
through the soil they sorb onto the soil surface,  where they are degraded by
microbes.  The process whereby the contaminant adsorbs onto the soil is the

                                      95

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same process that occurs during GAG adsorption.   Soil has a much smaller
surface area than activated carbon and therefore has a smaller adsorption
capacity; however, the degradation activity of the soil microbes serves to
oxidize the contaminants and allow further contaminant sorption at those
sites.  The theory behind biofilters is analogous to biological wastewater
treatment.  That is,  by providing an environment with suitable amounts of
oxygen, nutrients, pH, and temperature, microbiological degradation will
occur.

      Soil biofilter construction is relatively straightforward.  A network of
perforated piping is buried in the soil bed.  The exhaust gas is pumped
through the piping network, from which the gas flows up through the soil.
Adsorption and subsequent degradation occur on the soil surface.  The soil bed
must be sufficiently porous to allow large volumes of the exhaust air to move
expeditiously through the bed; suitable media include sandy loam soils or
mature compost (Johnson, M.,  1989).  The beds must be designed to avoid short
circuiting due to drying and cracking.  Clay or organic matter serve to give
the soil matter sufficient sorption sites.  Aerobic conditions must be
maintained to permit oxidation of the organics and survival of the microbes.
Aerobic conditions are maintained by ensuring that the exhaust air contains
sufficient oxygen.  The air permeability must be maintained at a level that
permits the discharge of the vapor extraction flow rate.  The moisture content
plays a crucial role in biofilter operation.  Soil moisture should be
maintained at a level to maximize biological activity.  A leachate collection
system may be necessary where large quantities of water are being added.  Neff
(1989) listed several other operational parameters and their optimal ranges:
retention time (>15 seconds); temperature (15 to 45 degrees C); pH (7 to 8);
moisture content  (50 to 70 percent by weight); media porosity (80 to 90
percent); and influent gas relative humidity (60 to 80 percent).

      The advantages of the soil bed treatment system are the low cost and the
complete destruction of the contaminants.  A disadvantage is the acclimation
period required and the potential for system upset following changes in
influent concentration.  This type of system is widespread in deodorization
applications, and it is now being applied to VOC control.  Experience will
tell whether this technology will be applicable to remediation sites.

DIRECT DISCHARGE

      In some cases treatment of the extraction well vapor stream may not be
required and direct discharge of vapors to the atmosphere may occur.  The
concentration of  the contaminants, the flow rate of the SVE system, and the
presence or absence of nearby receptors are generally taken into consideration
when evaluating direct discharge options.
                                      96

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                                 SECTION 7

                       COST OF SOIL VAPOR EXTRACTION

INTRODUCTION

    The costs for a soil vapor extraction system may be apportioned into three
general categories.  Site investigation costs include site history review,
site assessment, and pilot testing.  Capital costs consist of costs for
procuring and installing the system components,  as well as design and
engineering fees, permitting costs, and contingencies.   Operation and
monitoring (O&M) costs are those associated with the continued operation of
the system, including power and labor, system monitoring,  and clean up
attainment analytical costs.

SITE INVESTIGATION COSTS

    The basic costs to perform a site investigation include sampling and
monitoring equipment, laboratory analysis, personal protective equipment
(PPE),  and labor.  These costs are discussed below for each segment of a site
investigation.

Site History Review

    During the site history review, all available data pertinent to the
site should be located and collected.  This data normally includes maps
from the U.S. Geological Survey and the Soil Conservation Service, aerial
photographs, site plans, and operational records.  Interviews with persons
familiar with the site are often helpful.

    The cost of the site history review will depend primarily on the labor
expended to obtain the information and, to a lesser extent, the cost
associated with obtaining maps or other information.  The time spent on the
initial site investigation may range from only one or two days for a
service station UST release that has a clearly identified leak history
(e.g.,  a vapor monitor detects a leak that inventory records indicate began
two weeks earlier) to one month or more, continuing even as field work
begins, for a larger, more complex site (a tank farm or a manufacturing
facility).  The cost of map procurement may range from nil to over $1,000 if
original aerial photography is required.  The total cost of the initial site
investigation may range from $5,000 to $20,000.

Preliminary Site Screening

    The objective of a preliminary site screening is to assess, rapidly and
cost-effectively, the nature and extent of contamination.   During a site
survey, key features of the area (such as the presence of sewers and utility

                                      97

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lines) are noted, and health and safety conditions for the site are
determined.  A soil gas survey and groundwater sampling are often performed to
gain additional data. . Sampling of soil, soil gas, groundwater and air
normally occurs in this step.

    The preliminary site screening requires, at a minimum, an organic vapor
monitor to identify total VOCs and a combustible gas indicator (CGI) to
identify combustible gas concentrations and oxygen-deficient atmospheres.  The
Foxboro OVA (organic vapor analyzer) 128 GC and the HNu PI 101 model are the
most commonly used models.  The OVA model uses a flame ionization detector
(FID) while the HNu uses a photo ionization detector (PID).  It is usually
beneficial to incorporate both an FID and a PID vapor monitor in the survey,
because these instruments respond differently to various gases and
environmental conditions (e.g., the FID is sensitive to methane, while the PID
is not).

    This preliminary site screening is usually conducted in level D safety
equipment, which requires a minimum of personal protection equipment (PPE),
unless otherwise indicated.  However, the first step to occur during this
screening  is to monitor for vapors that may be immediately dangerous to life
or health  (IDLH), other conditions that may result in death, such as explosive
vapor concentrations, and oxygen deficient atmosphere. Containers, drums,
spills and other potential safety hazards should be identified at the
beginning of the site screening.

      Subsequent tasks might include subsurface investigations using ground
penetrating radar (GPR), metal detection, and soil gas surveys.  At this
stage of the site investigation, the soil gas survey should be rapid.
Laboratory GC equipment should be avoided until the detailed site
characterization.  Portable  field monitoring equipment such as the OVA and
the Photovac 10S50 are suitable for the soil gas survey if both quantitative
and qualitative  data are required.  Soil gas systems are also available from
other vendors such as K.V. Associates,  Inc. and Xitech.

    The total cost for the preliminary  site screening will depend on the
cost for labor and mobilization, monitoring equipment, personal protective
equipment, sampling  equipment and supplies.  Laboratory analytical costs
should be negligible during  this stage  of the site investigation because
analyses should  be done primarily in the field.  The labor cost will depend
on the number of personnel required, site conditions (both size and ease of
access), weather, and the level of personal protection required
(significantly more  time is  required for greater levels of protection).

    The cost for monitoring  equipment will depend on the  type of equipment
used and whether it  is purchased or rented.  At a minimum, a vapor monitor and
CGI are required.  The cost  for this equipment is presented in Table 8.  The
costs range from $4600 for the HNu 101  with a calibration kit to $7,300 for
the Foxboro OVA  128  GC.  This equipment can also be rented at a cost of
approximately $350 to $550 for a ten day period or $900 to $1,200 per month
(CAE Instrument  Rental, Inc.).  The CGI may be purchased  for $650 to $1,950,
or can be  rented for $50 to  $220 for a  ten day period.


                                      98

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        TABLE 8.  MONITORING EQUIPMENT  COST ESTIMATES
 Model
Vendor
Ranee fanm')
Cost ($)
Notes
OVA-128
OVA-128GC
OVA-108
OVA-108GC
HNu PI-101
Micro TIPII
OVM 580A
HNu-311
10S50
10S70
Sniffer 302
1314
Sentinel 4
Foxboro 0-1000
Foxboro 0-1000
Foxboro 1-10000
Foxboro 1-10000
HNu Systems 1-2000
Photovac 0.1-2000
Thermo Env. 0.1-2000
HNu Systems
Photovac
Photovac
Bacharach
GasTech
Bacharach
6100
7300
1125/month
6100
7300
1125/month
4250
900/month
4400
900/month
4700
935/month
14000
15500
18500
940
1100
1450
FID
w/GC
rental
FID
w/GC
rental
rental
PID
w/data logging
rental
PID
rental
w/ data logging
GC/PID
GC/PID
GC
w/data logging
02/LEL
tt
Personal
Monitor
 NOTE: Costs are estimates (1989). Contact vendor for actual price quotes.
    The cost for PPE varies depending on  the  level  of  protection (Table 9).
For example, a suit for Level B costs $100, while the  Level  A suits cost from
$440 to $4,000, depending on the type of  suit required.   The NIOSH Pocket
Guide to Chemical Hazards (U.S. Department  of Health and Human Services, 1985)
is available to determine the appropriate PPE based on the contaminants
present.  Organic vapor monitor badges, which may be worn during the on-site
work, can be purchased for $125 (box of 10).

                                      99

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TABLE 9. WORKER PROTECTIVE EQUIPMENT COSTS
Tvpe
Suit
tyvek
saranex
Top security suit
Frontline Suit
Responder Encapsulating
Responder Encapsulating
Aluminum FBI/
Kevlar
Butyl
Viton
PVC Chemical
Teflon/Nomex Encapsulating
Gloves
Viton
Butyl
Silver Shield
Neoprene
Argus
PVA
Latex
Rubber
Boots
PVC
Neoprene
Hazmex Latex
PVC Shoe/Boot Protector
Half-Mask Facepiece
Full Facepiece
Cartridges
Powered air Purifying
Respirator
Pressure Demand
Airline Respirator
Continuous How
SCBA
Air Pump
Vendor

Dupont
Dupont
Kappler
Kappler
Lifeguard
Lifeguard
Conners Env.
Conners Env.
Conners Env
Lifeguard

Conners Env.
Conners Env.
Conners Env.
Pioneer
Playtex
Edmont
Conners Env
Edmont
Environmental
Conney
Conney


North
North
North
North
North
North
North
Allegro
Allegro
Level of
Protection

C
B
B
B
A
B
A
A
A
A













C
C
C
C
B/A
B/A
B/A
B/A
B/A
B/A
Costs C$1



100
115
440
160
660
1,450
3,300
1,350
4,000
(pr.)
37-47
18-25
3
6-16
7.50
24
13/50 pair
18/dozen
13-23
53
7
3.50
21
175
25/box of 6
560
620
400
1,725
2,350
850
2,400
Notes

not water resistant
water resistant

chemical resistant
chemical resistant
chemical resistant
fire resistance/outer cover
general purpose-chemical
PCBs, chlorine, hydrocarbons
chemical hazards
chemical resistant

PCBs
gas or water
disposable
PCBs 22 to 30 min
PCBs, oils, acids
aromatic and chlorinated solvents
disposable
acids, alkalis, ketones
Waterproof, Chemical resistant
Chemical & petroleum resistant
"chicken boots"



organic vapor
requires cartridges


30 min
60 min
2 min
6 min
                         100

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Detailed Site Characterization

    Additional site characterization geared specifically toward SVE is
normally undertaken if the preliminary screening results indicate that SVE may
be an appropriate remedial technology.  This step includes more detailed soil
and contaminant analyses to determine specific parameters to be used during
remedial design.

    Soil samples may be collected by a hand auger,  auger drill, split spoon
sampler, or Shelby tubes.  These samples can be used to determine various soil
properties such as the permeability, porosity, bulk density, moisture content,
pH, cation exchange capacity (CEC),  and organic carbon content.  In situ
measurements of hydraulic conductivity and infiltration rate can be made with
a Guelph Permeameter and an infiltrometer,  respectively.  The determination of
other soil parameters, such as residual saturation and leaching
characteristics, are possible with soil column testing apparatus.  Groundwater
samples are taken via monitoring wells or mini-well points.

    Table 10 lists costs for soil, groundwater, soil gas, air and other
environmental measurement equipment.  Costs on this table are estimates and
the vendors should be contacted for actual equipment costs.  Table 11 lists
typical analytical costs for various analyses.  Geological evaluations are
required to characterize media at depths below about 15 feet, the rough limit
of hand equipment, or into shallow rock structures.  Common geologic sampling
equipment includes split-spoon samplers and Shelby tubes.  Soil and rock
samples should be evaluated for texture, density, organic carbon content, and
contaminants.  Samples may be "screened" by headspace analysis on field GC
equipment in lieu of laboratory analysis.

    Geologic cross sections, which will be used in the evaluation of
contaminant migration pathways, can be developed at the site from boring logs.
Geophysical techniques may be required to evaluate specific subsurface
conditions.  These techniques may include seismic refraction and reflection,
magnetometry, electromagnetics (EM), resistivity and gravity.  Seismic
measurements can be performed to determine the thickness and number of
subsurface layers, depth to bedrock, and the presence of fractures and
cavities.  Magnetometry detects the presence of buried ferrous metals and is
normally used to detect drums.  Electromagnetic measurements can be made to
assess lateral variations in soil and rock, such as fractures and karst
features, as well as to locate shallow drums.  Resistivity measurements are
conducted to evaluate the depth and thickness of soil and rock layers and the
depth to the water table.  Gravity determinations are used to map major
geologic features over a large area or for detecting local fractures and
cavities.

      The cost for the geologic investigation depends on the costs for
drilling and geophysical analyses.  Well costs are approximately $300 to
$1,000 for deployment, $20/foot of drilling, $75 for decontamination equipment
(steam cleaner), $100/hour for decontamination (labor), and $50 per sample
(split-spoon).  Geophysical equipment can be purchased from vendors at prices
shown in Table 10.  The conductivity meter for shallow depths (less than 20
feet) costs $12,200, while the meter for greater depths costs $18,200.  A

                                      101

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               TABLE 10.   SITE  INVESTIGATION EQUIPMENT COSTS
SOIL
Equipment

Auger (manual)
Auger (power)

Soil core sampler
Retaining cylinders
Split-spoon sampler
Thin-walled tubes

GROUNDWATER

Bailer (economy)
Bailer (PVC)
Bailer (stainless steel
Bailer (teflon)
Pump (portable)
Tubing
Pump (portable)
Dedicated well sampler

SOIL GAS

Gas sampler system
Gas sampler system
Gas sampler system
Gas sampler system

AIR

Sampling pump
Sampling pump
        Yen
AMS (Forestry Suppliers)
Little Beaver
Hoffer PH980
AMS
AMS
SoilTest Inc.
SoilTest Inc.
Forestry Suppliers
Forestry Suppliers
Forestry Suppliers
Forestry Suppliers
Masterflex
Masterflex
Geoguard
Geoguard
Xitech/Vista 4000
K-V Associates/Macho
K-V Associates/Basic
K-V Associates/Hefty
SoilTest
SoilTest
ENVIRONMENTAL MEASUREMENTS
Double ring infiltrometer
Permeameter
Conductivity meter
Resistivity meter
Resistivity meter
Seismograph
Groundwater Quality
Groundwater Quality
ORP electrode
CI electrode
SoilTest
Soilmoisture  Guelph Permeameter
SoilTest
SoilTest
SoilTest/I.P.  System
SoilTest
YSI 33 S-C-T
YSI 3560
Orion
Orion
Cost(S)

 100
 1225
 360
 250
 10
 500
 800/doz
 55
 60
 90 - 140
 130 - 220
 500 - 560
 44/roll
 4000
 600 - 700
 2500
 2575
 1675
 3600
 330
 35
 1550
 1100
 12200 - 18200
 2715
 12800
 4250
 500
 2000
 100
 375
 Notes
11 hp
85 cc
w/slide hammer
stainless steel
stainless steel
stainless steel
350 cc
36"xl.66"  O.D.
Johnson

peristaltic pump
silicon, roll of 25'
bladder pump
also ground water
electric
manual
infiltration rate
soil  permeability
197'  soil; <20' rock
<100 meters
induced polarization
signal enhanced
                                               102

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                    TABLE 11.  TYPICAL ANALYTICAL COSTS


      Analysis              Soil/Sediments         Groundwater         Soil Gas

      VOCs                     $225                $200              $250
      ABNs                    500-550                525
      TPH                     75-125                100
      BTX                       180                 150
      Pesticide/PCBs              175                 150              150
      Texture                   75- 125
      Moisture                  10-20
      pH                       5-15                  10
      CEC                     75-85
      Organic Matter             10-40
      Permeability               335-495

resistivity meter and an automatic  resistivity-induced polarization system are
sold for $2,715 and $12,800,  respectively, by Soil  Test.  A signal-enhanced
seismograph costs $4,250.

Characterization of Contaminants

     A contaminant characterization helps to  determine the applicability of
SVE by identifying the compounds  present and  their  concentrations.  A Quality
Assurance/Quality Control(QA/QC)  sampling plan,  addressing soil,  groundwater,
and soil gas sampling should be in  place to ensure  data precision and
representativeness.  Wastes (e.g.,  lagoons),  surface water and sediments
should also be sampled if appropriate.

    The soil should be analyzed for VOCs, acid/base/neutrals (ABNs) or
semi-volatile organic compounds,  and total petroleum hydrocarbons (TPH).
VOC analysis will determine the concentration of volatile  compounds that may
be amenable to SVE.  EPA Method 8240 is used  to  evaluate VOCs.   Targeted
analyses for specific, indicator  compounds such  as  benzene,  toluene and
xylenes will reduce analytical costs.   Analysis  of  the soils for ABNs (EPA
Method 8270) will identify less volatile compounds  that are less likely to be
removed by SVE.  ABN analysis may be augmented by the less expensive TPH
analysis (EPA method 418.1),  which  does not  identify specific compounds, but
gives only a total of the petroleum compounds present in the soil. Table 10
lists sampling equipment costs and  Table 11  lists typical  analytic costs.
These constitute most of the costs  of this part  of  the site investigation.

    During this phase, groundwater  analyses may  be  required.  Bladder pumps
are preferred over bailers for collecting groundwater samples,  although pumps
are more expensive.  Geoguard, for  example,  sells both dedicated and portable
bladder pump systems.  The portable pump system  (pump,  compressor, controller,
and 100 feet of tubing) costs $4,000.   The portable compressed air source for
dedicated sampler systems costs $300.   The cost  per well for the sample is
$600 to $700.   Glass sampling bottles will cost  approximately $3 each and
vials for VOA are $2.50 each.   Laboratory analysis for groundwater
contaminants will cost $200-250 for VOCs (EPA Method 624), $400-500 for ABNs
(EPA Method 625), $75-100 for TPH (EPA Method 418.1),  and  $150 for
pesticides/PCBs (EPA Method 608).

                                     103

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CAPITAL COSTS

      Capital costs for SVE systems include the cost of procuring and
installing all the equipment,  piping,  and instrumentation.   Design and
engineering fees, permitting costs, and contingencies are also included under
capital costs.  Many different components may comprise a complete system
(Figure 18),  but in general the capital costs can be divided into three main
groups:

    o  Vapor capture, including costs  associated with extraction and
       injection well installation, impermeable surface seals, and
       groundwater level control;

    o  Vapor removal. including costs  associated with getting the vapors
       out of the ground and to the vapor treatment device and including
       the costs of the vacuum pumps/blowers, piping, valves, mufflers and
       monitoring equipment; and

    o  Vapor treatment.  including vapor pretreatment (air-water separator),
       sidestream treatment, air conditioning, and the vapor treatment
       device.

    Within each category, some components may not be necessary for every
site; for example, groundwater depression pumps would be used only at sites
where the contamination exists at or near the water table.   Vapor treatment is
normally used only where the discharge limits are above allowable levels.

      The costs presented below were developed from a vendor survey conducted
in October, 1989 and are typical of those typically encountered by SVE users.
These costs generally do not include delivery, installation, and trouble-
shooting (unless indicated), which may add significantly to the overall cost.
Appendix C is a partial listing of vendors who supply SVE equipment.  No
judgment of any vendor should be inferred or implied based on inclusion or
exclusion from this list.

Extraction Wells

      Extraction wells are normally constructed from schedule 40 PVC
(polyvinylchloride) piping of various  diameters (2" to 12").  Polypropylene
(PP) or chlorinated polyvinyl chloride (CPVC) are more rigid and may be used
where stronger piping is required.  A typical 30 feet deep extraction well
installation will usually cost from $2,000 to $4,000.  Of this cost, materials
such as casing (riser), well screen, plugs, filter pack materials, bentonite,
and cement grout may range from $500 to $2,000 per well, depending on the
method of construction.  Table 12  shows the range of costs for various
extraction well components.  PVC piping, for example, costs as little as $2
per linear foot  (If) for 2" diameter casing to up to $12/lf for 6" diameter
casing.  Similarly,  PVC screens cost from $2 to $15 per linear foot depending
on diameter.  Ball valves  (PVC) cost $60 for a 2" riser to $300 for a 6"
riser.
                                      104

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Injection Wells

      Injection wells and inlet wells are constructed similarly to extraction
wells and have very similar costs.  They are normally used to enhance vapor
flow or control vapor paths.  Existing monitoring wells are often used as
injection or inlet wells.

Impermeable Surface Seal

      Impermeable surface seals may be used to control vapor flow paths toward
the extraction wells and reduce the entry of atmospheric air in the area of
the well.  At sites where a surface seal is present, the radius of influence
of the extraction wells will increase as vapors are drawn from greater
horizontal distances, permitting the removal of contaminants from a larger
volume of soil.  Surface seals can be constructed of high-density polyethylene
(HDPE) or low-density polyethylene, bentonite clay, or asphalt.  Material
costs depend most directly on the area covered and are normally expressed in
dollars per square yard ($/yd^).   HDPE, which is often used in landfill
closure, costs about $5.00/yd^ for 40 mil thickness.  Polyethylene (10 mil
thickness) costs about $2.25/yd .   Bentonite clay can also be used as an
impermeable barrier.  Costs range from $2.22/yd^ for a 4" layer to $3.33/yd^
for a 6" application.  Asphalt paving (2" layer) costs about $9.24/yd .   This
material would likely be used only for sites that will remain paved, such as a
service station.  Often, industrial and commercial SVE operations are
implemented at sites that are already paved.  For all methods, installation
costs may add considerably to the total overall cost.

Groundwater Level Control

      Extraction wells are normally screened through the contaminated layer.
At sites where the contamination exists at or near the water table (as is
common for petroleum and other products that are lighter than water), the
application of a vacuum in a well will cause upwelling of the water table,
especially in silty or clayey soils.  This may result in the extraction wells
drawing up large quantities of water or free product, which must be removed
from the vapor stream.  To minimize liquid entrainment, pumping wells may be
installed near the vapor extraction wells to lower the water table.

      Where free product exists on the water table, product recovery equipment
is usually employed.  These devices often use water table depression as  a
means of capturing free product and thus, can be used in conjunction with
extraction wells as a means of groundwater level control.

    Groundwater depression systems are available from vendors such as R.E.
Wright Associates,  Inc., who sell  the Auto Skimmer, and Del Harlow
Enterprises,  Inc.,  who sell the Reclaimer System.   These systems are capable
of both water level depression and recovery of floating hydrocarbons.  The
water level depression system marketed by R.E.  Wright Associates,  Inc. is
capable of removing 45 to 95 gallons per minute (gpm) and sells for $3,700,
uninstalled.
                                      105

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TABLE 12. SVE SYSTEM COMPONENTS
    CAPITAL COSTS
COMPONENT TYPE
EXTRACTION WELL CONSTRUCTION
CASING PVC


SCREEN PVC


SAND PACK
GRAVEL PACK
PIPING PP


PVC


CPVC



VALVES (BALL) PVC
single union


JOINTS (ELBOW) PVC
90 degrees --slip

WATER TABLE
DEPRESSION PUMPS


SURFACE SEALS BENTONITE 6 in
BENTONITE 4 in
POLYETHYLENE 10 mil
HOPE 40 mil
ASPHALT 2 in
BLOWER (FAN)






centrifugals blowers


SIZE

2 IN
4 IN
6 IN
2 IN
4 IN
6 IN


2 IN
4 IN
6 IN
2 IN
4 IN
6 IN
2 IN
4 IN
6 IN

2 IN
4 IN
6 IN

2 IN
4 IN
6 IN









1 hp
1.5 hp
2 hp
3 hp
5 hp
10 hp
30 hp
2.5 hp
25 hp
50 hp
CAPITAL
COSTS ($)
20-40/FT
2-3/FT
3-5/FT
7-12/FT
2-4/FT
5-7/FT
10-15/FT
15-20/CU FT
20/CU YD
1/FT
3.50/FT
8/FT
1/FT
3/FT
5.25/FT
4/FT
12.50/FT
24/FT

65
300
700

2.50
16
51

3700
3000

3.33/SQ.YD.
2.22/SQ YD
2.25/SQ YD
5/SQ YD
9.24/SQ YD
1700
2000
2200
2700
3300
5000
6000
600
12000
42000
NOTES

SCH. 40 PVC



SCH. 40 PVC,
ANY SLOT SIZE





SCH. 40 PVC


SCH. 80 PVC


vendor- M&T Plastics
SCH. 40 PVC
2in & 4in threaded socket
6in flange end connection
M&T Plastics
SCH. 40 PVC
threaded, socket
end connections

R.E.Wright Assoc.
w/Explosion Proof Pump
Motor Control System





Environ Instruments







includes installation

        (continued)
                106

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                     TABLE 12. SVE SYSTEM COMPONENTS
                          CAPITAL COSTS (continued)
COMPONENT TYPE
AIR/WATER SEPARATOR
knockout pots
INSTRUMENTATION
VACUUM GUA6E (MAGNEHELIC)
FLOW (ANNUBAR)
SAMPLING PORT brass T
CONCRETE PAD
HEATEXCANGER & housing unit
FIBERGLASS SHED 8'x10'
FLAME ARRESTOR w/o ss element
w/ ss element
SIZE
800 gal
20 gal
35 gal
65 gal
105 gal
130 gal






CAPITAL
COSTS ($)
1500-2400
1 1600
1470
1560
1750
2150
2350
50-75
300
20-30
450/CY
1400
8500
665
735-930
NOTES
vendor-Water Resources
installation 33%
of capital costs





vendor- Stafford Tech.
     AIR RELIEF VALVE

     SOIL GAS PROBE

     ENGINEERING/DESIGN

     DIFFUSER STACKS
      225

     30-50

8-15% of system cost
vendor- Stafford Tech.

 vendor- K.V. Assoc.
CARBON STEEL
STAINLESS STEEL
4 IN
6 IN
4 IN
6 IN
$8/FT
$10/FT
S30/FT
S40/FT
Add 40% for installation
Blowers/Vacuum Pumps

      A vacuum pump or  a positive displacement blower may be used  to provide
the power for the  SVE system.   Fans and pumps are characterized by the  flow
rate that can be achieved,  the horsepower,  and the vacuum that can be induced.
Vacuum is usually  expressed in terms of inches of water (in ^0) or
millimeters of mercury  (mm  Hg).   A large selection of commercial blowers
exists and numerous vendors provide these blowers (see Appendix C).  The  price
for this equipment varies with the fan size and power, with a wide variety of
combinations available.

    Blowers often  must  be spark and explosion-proof, which will raise the
price considerably.  Explosion-proof blowers range in price from $1,700 (1 hp)
to $6,000 (30 hp).  In  contrast,  blowers that are not so designed  range from
$300 (1 hp) to $1,900 (10 hp).  Prices for the blowers are normally determined
by both fan size  (6" blades,  8"  blades, 10" blades,  etc.) and by rating (flow
rate).  Prices for specific units should be obtained from vendors.  Table 12
shows some representative prices.
                                      107

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Monitoring Equipment

      Monitoring of the extracted vapor stream is a vital part of SVE design
and operation.  Monitoring equipment should allow the determination of the
vacuum air flow, and vapor characteristics and concentrations.

    The vacuum can be measured with a magnehelic gauge.   Gauges are typically
located at each extraction well and prior to the blower.   The cost of
magnehelic gauges can range from $50 to $75.  Gauges at each well may be
substituted by a quick-coupling sampling port.  Air flow, expressed in
standard cubic feet per minute (scfm) to normalize flow readings taken at
different pressures, can be measured in-line by an annubar flowmeter or at
flow ports using portable equipment.  Air flow should be measured at each well
and upstream of the blower.  Annubar flow meters cost about $300. Quick-
coupling sampling ports with two or three connection can be purchased for $25.

    Monitoring of the concentrations and composition of the extracted vapors
is critical in determining vapor treatment alternatives and operation
procedures.  Quantitative vapor concentration can be determined using an
organic vapor analyzer (OVA),  total hydrocarbon analyzer (THA) or a
combustible gas indicator  (CGI).  Vapor components and concentrations can be
determined using a gas chromatograph (GC).  The vapor concentration is usually
monitored between the demister (or knockout pot) and the blower.  In carbon
adsorption systems, monitoring may also occur in the exhaust from the carbon
bed.

    The choice of a specific monitor for vapor concentration will depend on
cost, operational range, and sensitivity required.  An OVA or an HNu is
normally used in conjunction with quick-coupling sampling ports.  The OVA uses
a flame ionization detector (FID) to determine the total hydrocarbons present.
The Foxboro Company offers two portable OVA units.  The 108 GC is capable of
monitoring VOCs in the range of 0 to 10,000 ppm  (0 to 1 percent), while the
128 GC can monitor vapor streams of 0 to 1,000 ppm (0 to 0.1 percent).  A 1/10
dilution is available for  these OVA units that allows for monitoring of a
vapor stream of up to 100,000 ppm (10 percent) with the OVA 108 GC model.
Vapor monitoring systems such as the Foxboro OVA models 108 and 128 sell for
$6,100 and $7,300  (units have GC capability).  The dilutor costs an additional
$450.   The HNu, which is  also used for monitoring VOCs, is not practical for
use with SVE systems because the moisture in the vapor stream, which is
present even after moisture reduction from a demister, renders the HNu
inoperative.

    Total hydrocarbon analyzers can be set in line for continuous sampling.
THAs also use an FID for contaminant detection.   J.U.M. Engineering
manufactures several types of total hydrocarbon  analyzers.  For example, the
model VE7 has a range of 0 to 100,000 ppm (0 to  10 percent) and has a maximum
sensitivity of  1 ppm as methane.  These range in price from $15,000 to
$21,000.  Thermo Environmental Instruments Inc.  offers the Model 810 THC
analyzer, which is capable of monitoring vapor stream in the  range of 0.1 to
10,000 ppm.  Other vendors include Byron Instruments and Analytical Instrument
Development,  Inc.


                                      108

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    The combustible gas indicator measures the lower explosive limit (LEL)
of the vapor stream.  The LEL is the lowest concentration by volume in air at
which a combustible gas or vapor will explode, ignite, or burn when there is
an ignition source.  This equipment is far less sensitive to vapor
concentration than the aforementioned FIDs.   Limitations of CGI use include:

    •  dependence on temperature;

    •  sensitivity is based only on calibration gas; and

    •  filament may be fouled by leaded gasoline and halogens.

    Gastech manufactures a portable CGI (Model 1314) .   Stationary models are
also available.  Portable CGIs from Gastech (Model 1314) cost approximately
$1,000.  Stationary models sell for $1,000 to $2,000.

     Gas chromatography equipment is capable of determining the concentrations
of specific components. Both portable and laboratory grade units are
available.  GC units cost from $20,000 to over $30,000.

    Sampling probes may be used to monitor soil gas and/or vacuum at locations
within the periphery of remediation.  Probes are available to monitor vacuum
and for vapor sampling.  K-V Associates Inc. offers a vapor monitoring probe
that can be installed  in the vicinity of the SVE system to monitor cleanup.
This system is capable of sampling both the soil gas and groundwater and sells
for $30.  Hollow steel probes are also available, which are driven into the
ground for soil gas sampling and vacuum measurements.

Vapor Pretreatment

      The extracted vapors are normally pretreated prior to the emission
control unit. Air water separators  ("knock-out pots")  decrease the velocity of
the vapor stream and allow water droplets and sediment to fall out via
gravity.  They can be very simple (e.g., a 55-gallon drum) or may incorporate
level controls and other instrumentation.  The size depends on the flow rate
(to reach a minimum residence time); a typical size range is 800 to 1200
gallons.  Construction materials vary but may be cast iron, stainless steel,
or similar material.  Demisters are often incorporated into the vapor
pretreatment process.  These screens are capable of removing particles down to
microns in size by coalescing these droplets on the demister material.

    Duall Industries,  Inc. manufactures a variable sized demister ranging in
cost from $700 to $1,000 for flow volumes of 100 to 1,000 scfm.  Water
Resources Associates,  Inc. manufactures knockout pots for use with their
complete thermal incineration SVE systems.  The cost for knockout pots may
range from $1500 to $2500 according to size and flow rate capabilities.

Sidestream Treatment

      Liquids that accumulate in the air-water separator must be treated
on-site, disposed off-site (to a sewer line), or removed by truck.  On-site
groundwater treatment  is often occurring simultaneously with SVE. In such

                                      109

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cases, the air-water separator sidestream is added to the groundwater for
treatment, typically by air stripping or granular activated carbon (GAG).
Where the volume of the sidestream is small compared to the flow of the
groundwater treatment method, the marginal cost of treating this sidestream is
minimal.  Where on-site  treatment is desired and groundwater treatment is not
occurring, liquid phase GAG can be installed.  Carbon is available in small,
easily installed units that are appropriate for the flows expected from the
vapor pretreatment units. The sidestream can also be removed off-site by
tanker trucks.

Emission Control

      The vapors removed from the subsurface must normally be treated to
reduce the vapor concentration prior to release to the atmosphere, depending
on local regulations.  Several options are available for vapor treatment,
including carbon adsorption, catalytic oxidation, thermal incineration,
combination systems, and internal combustion engines.   Where vapor treatment
is not required, diffuser stacks are used to allow safe dispersal of the
extracted vapors.  These options are appropriate for different ranges of vapor
phase concentration.

    The cost of vapor treatment can be a significant portion of the total
SVE system cost.  Care must be taken to ensure that the most cost-effective
vapor treatment option is used.  This determination must be based on the
vapor discharge standards, the extracted vapor concentration, the expected
mass removal over the life of the system, and several other variables.  The
operations cost of vapor treatment may dominate the calculation of the system
cost, especially for GAG treatment systems.  For this reason, the forecast for
expected removal rate becomes even more important.

Carbon adsorption is a widely used vapor treatment method in industrial and
SVE settings.  It is applicable to a variety of vapor contaminants and can
achieve very high removal rates when required.  Carbon is economical only for
relatively low mass removal rates; however, when mass removal rates are high,
the cost of replacing or regenerating the carbon may be prohibitive.

    Carbon adsorption systems are available  in a large variety of sizes from
numerous vendors; Table  13 shows a partial list of vendors with their
respective products.  These systems are available in very small systems  (55
gallon drums holding less than 200 pounds of carbon) through larger,
skid-mounted systems (up to 5,700 pounds of  carbon).  For very large
installations, vendors can customize carbon  to the specific requirements of
the site.  Carbtrol offers the G-l, G-2, G-3, and G-5 canisters that are rated
for various air flows.   These systems are modified drums containing 200, 170,
140 and 2000 Ibs of activated carbon, respectively.  The G-l system, rated at
100 scfm, costs $660, the G-2  (300 scfm) costs $985, the G-3 (500 scfm) costs
$1100 and the G-5 (600 scfm) costs $11,000.  TIGG Corporation offers a Nixtox
series N500 DB, N750 DB  and N1500 DB (deep bed) systems that contain 1900,
3200, 5700 pounds of virgin carbon, respectively.  Calgon Carbon Corporation
offers a large variety of carbon adsorbers.  The Ventsorb canister can handle
average flows up to 100  cfm or high flows from 400 to 11000 cfm.


                                      110

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                TABLE 13. SVE VAPOR TREATMENT UNIT COSTS*
MAXMUM
TREATMENT VENDORS MODEL FLOW (scfm)
CARBON ADSORPTION CARBTROL SVX



CONTINENTAL MANUAL BEDS
RECOVERYSYSTCMS 1 BED
2 BEOS
3 BEDS
4 BEDS
5 BEDS
6 BEDS
AUTOMATIC
REGENERABLEBEDS
CALGCN HIGH FLOW VENTSORB
28 in dia
36 in dia
4B in dia
CATALYTIC OXIDATION DEDERTCORP. CATOX


CSM MODEL 2BTORVEX
SYSTEMS MODEL 5A+heat ex.
MODEL 58 TORVEX
MODEL 10BTORVEX

CRS CATALYTIC SCAVENGER
1282001
1282002
105

250
500










400
600
1100
1000

5000
200
500
500
1000


200
500
CAPITAL
COSTS ($)
18400

21900
30000

20000
26000
32000
38000
44000
50000

149000

5400
8550
11000
85000

200000
37500
60000
50000
70000


63000
78000
RENTAL LEASE
$/month PERIOD OPERATION EQUIPMENT INCLUDED
1540 1 yr.f deposit hp x kw/hr a skid mounted system including
a blower, demister, controls.
1830 1 yr.-t- deposit guages, valves, & flow ammeter
1900 1 yr.+ deposit

2400 5 hp includes controls, blower &
2900 steam regenerator
3325
3650
3875
4000

7500 6 mo fully remote monitored trailer

0.5 hp skid mounted system includes
3 hp fan, flexible connector, & damper
5hp
fuel skid mounted system includes
reactor, blower, heat exchanger,
boiler, & water heater
skid mounted system includes
burner, catalyst, control panel.
& blower.



6600/11584 12mo/6mo 20kw ;kid mounted includes installation
35kw heater, control module, heat
NOTES
(uses G-1 or
or G-5 carbon)


















treats chlorinated
solvents
air dillution
system*$20.000
trailer . $8500

no chlorenated
solvents
1282008
                 300
                                 90000
                                                15000       1 mo
                                              9900/17377  12mo/6mo
              exchanger, &  catalyst          catalyst
                                    replacement  « $2800

20kw       additionally includes (2) 5hp
           blowers,(2)catalysts, piping,
           filters, guages, valves, flame
           arrestor,  enclosure &  trailer
                                  (continued)

-------
                                                             TABLE 13. SVE VAPOR TREATMENT UNIT COSTS*
                                                                                  (continued)
TREATMENT VENDORS MODEL
WATER RESOURCES AB15-5-SVS
ASSOCIATES AB19-10-SVS
AB22-10-SVS
AB24-10-SVS
AB22-15-SVS

AB15-5-SVS
AB19-10-SVS
AB22-10-SVS
AB24-10-SVS
AB22-15-SVS
COMBINATION SYSTEMS HASSTECH MMC-5
MMC-2 w/trailer
MMC-3


MAXIMUM
FLOW (scfm)
100
210
320
420
570

100
210
320
420
570
100
30
1000


CAPITAL
COSTS ($)
11200
15300
18400
20100
22900

23000
28000
32000
36000
40000
60000




RENTAL LEASE
$/month PERIOD






3850
4675
5350
6000
6675





OPERATION EQUIPMENT INCLUDED






fuel includes burner, blower, tlame
1.5 hp arrester, guages, valves, filters,
knockout pot, & sampling port
skid mounted w/enclosure, fence
& control panel
fuel includes thermal incinerator,
catalytic oxidizer, vacuum pump
compressor, control Sanalylical
instruments, valves, gauges, &
fuel system
NOTES
No warranty or
process efficiency
is extended when
flow rates are in
excess of design
capabilities of not
more than
12,000 ppm
total hydrocarbons







                           CEMI
                                             RANGER
                                                              200
                                                                              55000
                                                                                                                       fuel       skid  mounted  includes installation
                                                                                                                   maint. contract  (2)carbon beds, heat exchanger,
                                                                                                                      $7600/yr     thermal incinerator,controls,
                                                                                                                                 blower, valves, piping,VOC sensor,
                                                                                                                                   regeneration  steam blower
CARBON CANNISTERS CARBTROL G-SERIES cannisters
G-1 200 Ib
G-2 170 Ib
G-3 140 Ib
G-5 2000 Ib
CALGON VAPOR-PAC
1800 Ib
VENTSORB cannister
HIGH FLOW VENTSORB
cannisters 28 in
36 in
48 in
TIGG NIXTOX SERIES
N500DB
N750DB
N1500DB
BOXSORBER6X6
BOXSORBER8X8

100
300
500
600

1000
100

400
600
1100

500
750
1500
2200
4000

650


11000

5600
764

1700
4000
6400

7050
11850
19150
13750
20500

• - carbon can be
reactivated at 3-13%
discount from origina
purchase price








Deep bed units




•1989  Estimates. Contact vendor for actual  prices.

-------
The high flow model is also available as a skid-mounted unit that includes a
fan, flexible connectors, and a damper.  The canisters range in price from
$760 to $6,330, while the skid-mounted models cost from $5,400 to $10,700.

    The carbon may be virgin (unused) or reactivated.  Purchase of
reactivated carbon usually saves three to thirteen percent off the price of
virgin carbon.  For example, the virgin G-l (200 Ib) canisters offered by
Carbtrol sell for $660, while a reactivated canister sells for $640.
Larger containers are usually charged on a weight basis.  Environtrol
reactivates carbon for $1.15 Ib plus transportation costs.  A one time RCRA
Toxic Characteristics Leaching Procedure (TCLP) test is required ($2800 to
$3000) for hazardous materials.

    An alternative to the replacement of canisters and off-site
reactivation is a recycling carbon system.  Such systems regenerate the
carbon in place, usually using steam to desorb the contaminants.  The
contaminant/steam mixture is then drawn off and treated or disposed.
Continental Recovery System Inc. offers this type of system.  The system
comes in several sizes using from one to six carbon beds.

    Manually-operated systems cost from $20,000 (one bed)  to $50,000 (six
beds).  A fully automated, remotely-monitored, trailer-mounted system costs
$150,000 or leases for $7,400/month on a 6-month lease.  The cost
effectiveness of the system depends on the mass removal rate (Figure 27).
The system is initially more costly than non-regenerative  systems,  but
reduced carbon usage may make it a cheaper option on a life-cycle basis.

    Use of carbon for vapor treatment may necessitate the  need for a heat
exchanging unit to cool extracted vapors because temperatures increase due to
the heat of compression from the blower.  This will ensure maximum contaminant
uptake.  Alternatively, the GAG can be placed prior to the blower in the
treatment train.

Thermal incineration of contaminant vapors is often an excellent treatment
option when vapor concentrations are high.  Vapors are combusted at
temperatures of 1000 to 1400 degrees F or higher,  leading  to destruction of
over 95 percent of the influent contaminant concentration.

    Supplemental fuel may be required to maintain the requisite temperatures
for adequate removal.  The amount of supplementary fuel depends on the vapor
concentration; some vendors report that at gasoline concentration above 12,000
ppm, the flame is self-sustaining; at concentrations below this figure,
increasing amounts of fuel are needed in proportion to the contaminant
concentration.  The operating cost of a thermal incineration system is
obviously greatly affected by the amount of supplementary  fuel required.
Propane,  which costs about $1.00/gallon, is often used for this purpose.

    While higher contaminant concentrations make this method cheaper,
safety concerns increase with higher concentrations.  Highly volatile
contaminants (such as gasoline) become explosive in certain concentration
ranges (Figure 25).   This range is limited by the lower explosive limit (LEL)
and the upper explosive limit (UEL).   When the extracted vapors are at very

                                      113

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    6000-
    5000-
~  4000-
w
to
o
    3000-
•o   2000-
03

ts
"*   1000H
                            10
                                                                 DUAL BED, ON SITE
                                                                   REGENERATION
                                                      AUTOMATIC
                                                   REGENERABLE BEDS
                                                  Note: Capital costs amortized over five years
                                                I
                                               20
                                                         I
          I
30
40
                                     VOC Recovery (Ibs/day)
                       Figure 27. Activated Carbon Systems Cost Comparison

-------
high concentrations, fresh air must be mixed with the vapors to reduce the
concentration to a safe level.

    Table 13 shows the cost for various incineration units.  These units
are available in prepackaged units that include the burner unit, blowers,
sampling valves, and other appurtenances.   Capital costs depend on the flow
rate to be treated, and range from $23,000 (for 100 scfm) to $40,000 (570
scfm) from one vendor.  A smaller unit (70 scfm) is available for $12,000.
A heat recovery system, which uses the exhaust to preheat the incoming vapors,
can result in substantial energy and cost savings.

Catalytic oxidation systems employ a catalyst to facilitate the oxidation of
contaminants, and thus operate at much lower temperatures (600 to 800 degrees
F) than direct thermal incineration while achieving destruction and removal
efficiencies (DREs) above 95 percent.  The catalyst is a precious metal
formulation  (typically platinum or palladium) and can be in the form of either
beads or a honeycomb bed.  Like thermal incineration, this method
traditionally has not been appropriate for chlorinated organics due to the
potential for forming chlorine gas.  The new catalyst described by Trowbridge
and Malot (1990), however, may allow catalytic oxidation of vapor streams
containing chlroinated solvents.

    Catalytic oxidation requires careful monitoring to prevent overheating
of the catalyst, resulting in its destruction.  If the concentration of
vapors in the extracted air is above about 3000 ppm, the vapor stream must be
diluted with fresh air to remain below this cutoff level.  At low
concentrations supplemental fuel (propane) may be needed to maintain the
required temperatures.  Safety is also a concern for catalytic oxidation.
This method  is best suited for concentrations below ten percent of the LEL.

    Catalytic oxidation units are available that can handle flows as little
as 30 to 40  scfm to more than 50,000 scfm for large, permanent industrial
facilities.  Hasstech offers a trailer mounted unit (MMC-2) that can handle
30 to 40 scfm.  ORS offers the Catalytic Scavenger in a 20 kw model (200
scfm) and 35 kw model (500 scfm) that sell for $60,000 and $75,000,
respectively.  Installation and training will cost $3000 for these units.
GSM Systems, Inc. produces the Torvex series Model 5A, 5B (500 scfm) and
Model 10B (1000 scfm) that sell for $50,000 and $70,000, respectively.  A
trailer ($8,500) and ADS dilution system ($20,000) are available for these
models.  Larger catalytic oxidation systems are also available from GSM and
Dedert Corporation.  Dedert sells "field ready" units rated at 5000 scfm for
$200,000.

Combination vapor treatment systems that combine different treatment
options are  available from some vendors.  These systems often have higher
capital costs than simpler systems but may be more economical on a life-cycle
basis, depending on the mass removal rate and other considerations.
Combination  systems often provide some means to destroy (incinerate) the
contaminants.  This is an important consideration and reduces the exposure to
liability arising from transporting contaminants off-site.
                                      115

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    Hasstech Combination Systems,  Inc.  offers a trailer mounted system that
incorporates both catalytic oxidation and thermal incineration.  This
system can accommodate the efficiency of thermal incineration at high
contaminant concentration and allows for the use of catalytic oxidation as the
concentration in the vapor stream decreases over time.   Hasstech's hybrid
system (MMC-5) is sold for $60,000.   Supplemental fuel (propane or natural
gas) is required for operation.

    GEM Inc. (CEMI) markets a unit that uses both carbon adsorption and
thermal incineration.  VOCs are adsorbed onto the carbon,  which is then
regenerated by steam.  The VOC-laden stream is then oxidized by thermal
incineration.  This system allows for on-site elimination of VOCs and
reduces the fuel costs often associated with thermal incineration.  This
system operates at a flow from 40 to 500 scfm and sells for $50,000, and
requires either natural gas or propane for start up and heating of the
water to strip the carbon.  No additional fuel is required to combust the
contaminants in most cases.

Internal combustion engines have also been used to destroy vapor phase
concentrations.  Several vendors sell or rent such units.   RSI Inc. sells a
unit powered by a Ford, 4-cylinder,  63 hp,  140 cubic inch capacity engine
capable of accepting 30 to 60 scfm.   The engine itself sells for $56,500,
which includes 8 hours of a technician's time during setup.  RSI also
leases this engine for $4,800 per month with a six-month minimum.  RENMAR
sells a Ford 6-cylinder, 100 hp, 300 cubic inch engine with a design flow
rate of 100 scfm.  This engine sells for $45,000 or rents for $4,000/month.
Wayne Perry Construction rents their system, which is powered by a 460
cubic inch Ford engine reportedly capable of passing 400 scfm, for
$7,500/month.  This price also includes some consulting services such as
geological services.

Diffuser stacks are usually constructed from either carbon steel or
stainless steel.  They merely direct vapors into the atmosphere.  This
system is simple and inexpensive,  but only an option where treatment of the
vapors is not required.  Diffuser stacks should be designed to minimize health
risks.  The cost of diffuser stacks depends on the height required and the
material of construction.

Other Costs

    Implementation of an SVE system will entail other costs that are not
strictly capital costs or O&M costs,  and include system design,
engineering, permit acquisition, contingencies and other miscellaneous
costs.  These costs are often treated as capital costs.  Engineering and
design fees are often approximated by 10 to 15 percent of the system cost, as
are contingencies.  These and other costs are highly site-specific, however,
and the figures quoted here are arbitrary.

OPERATION AND MONITORING COSTS

    The operation and monitoring (O&M) costs may comprise a significant
portion of the overall remediation cost.  These costs are composed mainly of

                                      116

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power requirement for the blowers and condenser (if necessary);  vapor
treatment, including fuel costs for incineration methods and GAG
regeneration/replacement; monitoring and analyses for SVE progress and
cleanup attainment determination; and other on-going costs such as labor,
which depends most highly on whether the system is operated manually or with a
microprocessor.  These costs are discussed separately below.

Power Requirements

    The cost for electric power depends on the horsepower of the fan or
blower, the hours of operation, and the local cost of electricity.  The
formula for determining the cost is:  (0.75) x (fan horsepower)  x (electricity
cost in $/kw-hr) x (hours of operation).  For example, if a 10-hp blower was
operated continuously and electricity averaged $0.10/kw-hr, the daily cost
for power would be 10 x 0.75 x $0.10 x 24 = $18.00 per day.  Pulsed
operation -- operating the blowers intermittently -- would save power costs
by decreasing the hours of operation.   Power may also be required if heat
exchangers are used.

Vapor Treatment

    The operating cost of the vapor treatment depends on the treatment
method, the concentration of contaminants, and the flow rate.  Generally,
the cost for GAG adsorption increases while the cost for incineration and
oxidation decreases with higher vapor concentrations.   GAG treatment costs
will be dominated by carbon replacement and regeneration; incineration and
oxidation treatment will be dominated by fuel costs to sustain incineration.

    Carbon adsorption.  Adsorption of contaminants from the vapor phase
concentrates the contaminants onto the carbon.  When the carbon's capacity
to hold contaminants is used up, the carbon is considered "spent" and must
be replaced or regenerated.  Obviously, higher mass removal rates (flow
rate x concentration) will result in more frequent carbon replacement and
higher costs.

    Carbon costs vary depending on the type and quantity ordered, and may
range up to $2.00/lb.  Regenerated carbon costs 87 to 97 percent of virgin
carbon cost.  One vendor quoted $1.15/pound as the cost of regenerated
carbon for large orders.  Table 13 shows costs for virgin carbon units.
Hinchee et al.  (1987) state that, as a rule of thumb,  carbon costs about $20
per pound ($130 per gallon) of gasoline removed.

    Where carbon use is desired and mass removal rates are high, on-site
regeneration may become economical.   Continental Recovery Systems offers a
unit that uses steam to regenerate carbon in place.   Other vendors offer
units that regenerate the carbon and then incinerate the contaminants.
These combination units are more costly initially but save on O&M costs;
Figure 27 shows the mass removal rates when the system are cheaper on a
life-cycle cost basis.  The determination of the most cost-effective option
is site-specific and is normally found through pilot system results.
                                     117

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    Thermal incineration.  Thermal incineration requires supplementary fuel
for vapor concentrations below about 12,000 ppm.   This fuel is typically
propane  or LPG, which cost about $1.00/gallon.  When the BTU value of the
contaminants is not sufficient to sustain the required temperature (about 1400
to 1600 degrees F),  fuel must be added to maintain proper temperatures.

    Catalytic oxidation.  This method requires much lower temperatures (600 to
800 degrees F) than thermal incineration and therefore is less costly to
operate.   Optimal vapor phase concentration for catalytic oxidation is about
3000 ppm; higher concentrations require dilution (to protect the catalyst from
destruction) while lower concentration may require supplemental fuel.  ORS
states that a 200 scfm Catalytic Scavenger costs about $800 per month to
operate with no incoming hydrocarbons (i.e., just air);  as the hydrocarbon
concentration increases, the supplemental fuel requirements decrease.

    Internal combustion engines.  Like other incineration techniques
discussed above, internal combustion engines require supplemental fuel if
the vapor phase concentrations are too low to sustain the proper air-fuel
ratio.  Supplemental fuel is propane or, perhaps, natural gas.  RSI reports
that 20 gallons of propane per day is typical, with the range from zero to
an upper end of 35 gallons.  Propane costs about $1.00/gallon.  RSI reports
that if natural gas is used, the equivalent cost would be about $0.75/gallon.

Monitoring and Analyses

    Laboratory sampling for soil, groundwater, and vapor contaminant
concentrations is relatively costly.  It is therefore judicious to determine
carefully the appropriate samples to be collected and the proper methodology
for sample collection and preparation.  Soil sample analyses will generally
cost $75-125 for TPH, $1,100-1,600 for TCLP, $200-250 for VOCs, $100 for BTEX,
$450-550 for ABNs, $100-200 for a petroleum "fingerprint" identification, and
$70 for routine soil parameters, which include organic carbon and particle
size distribution.

    Analysis for groundwater sampling costs $100 (TPH),  $225  (VOCs), $100
(BTEX), $400-500  (ABNs), and $50 for general groundwater quality parameters.
Soil gas analysis using a GC for determination of total hydrocarbons and
specific contaminants may cost as much as $250 if sent to a laboratory.

    These price ranges reflect the variation among different  laboratories and
should be used as a guide.  Large orders or preferred customers may qualify
for discounts.  Priority orders that require short turnaround times are
surcharged; analyses can often be performed on a 48-hour turnaround, but at
double the cost.
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                                      129

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Chiou, C.T., D.E.  Kile,  and R.L.  Malcolm.  1988.   Sorption  of Vapors of Some
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                                     130

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Hoag, G.E. and B. Cliff. 1985.  The Use of the Soil Venting Technique for the
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                                      131

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                                      132

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                                      134

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                                  APPENDIX A
              REVIEW OF  SOIL VAPOR EXTRACTION SYSTEM TECHNOLOGY
             Neil J. Hutzler3, Elaine E. Murphyb,  John S.  Gierke3
ABSTRACT
     Soil vapor extraction is a cost-effective technique for the removal of
volatile organic chemicals (VOCs) from contaminated soils.   Soil air
extraction processes cause minimal disturbance of the contaminated soil and
can be constructed from standard equipment.   There is demonstrated experience
with soil vapor extraction at pilot- and field-scale, the process can be used
to treat larger volumes of soil than can be  practically excavated, and there
is a potential for product recovery.

     A soil vapor extraction system involves extraction of air containing
volatile chemicals from unsaturated soil.  Fresh air is injected or flows into
the subsurface at locations around a spill site, and the vapor-laden air is
withdrawn under vacuum from recovery or extraction vents.  A typical system
consists of: (1) one or more extraction vents, (2) one or more air inlet or
injection vents (optional), (3) piping or air headers, (4)  vacuum pumps or air
blowers, (5) flow meters and controllers, (6) vacuum gauges, (7) sampling
ports, (8) air/water separator (optional), (9) vapor treatment (optional), and
(10) a cap (optional).

     A large number of pilot- and full-scale soil vapor extraction systems
have been constructed and studied under a wide range of conditions.  Based on
a review of 17 studies, a number of conclusions can be drawn.  Soil vapor
extraction can be effectively used to remove a wide range of volatile
chemicals over a wide range of conditions.  The design and operation of these
systems is flexible enough to allow for rapid changes in operation, thus,
optimizing the removal of chemicals.  Air injection and the capping of a site
can control air movement, but injection systems need to be carefully designed
to avoid spreading contamination.  Incremental installation of vents, while
probably more expensive, allows for a greater degree of freedom in design.
While a number of variables intuitively affect the. rate of chemical
extraction, no extensive study to correlate variables to extraction rates has
been identified.


Department of Civil and Environmental Engineering,  Michigan Technological N
 University, Houghton,  Michigan 49931
bWoodward-Clyde Consultants,  Chicago,  IL 60603

                                      136

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INTRODUCTION

      Soil may become contaminated with volatile organic chemicals such as
 industrial solvents and gasoline components in a number of ways.  The sources
of contamination at or near the earth's surface include intentional disposal,
leaking underground storage tanks, and accidental spills.   Contamination of
groundwater from 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 are
washed from the contaminated soil and migrate towards groundwater.

     Alternatives for decontaminating unsaturated soil include excavation with
on-site or off-site treatment or disposal, biological degradation, and soil
washing.  Soil vapor extraction is also an accepted, cost-effective technique
for the removal of volatile organic chemicals (VOCs) from contaminated soils
(Bennedsen, 1987; Malot and Wood, 1985; Payne et al.,  1986).  With vapor
extraction, it is possible to clean up spills before the chemicals reach the
groundwater table.  Soil vapor extraction technology is often used in
conjunction with other clean-up technologies to provide complete restoration
of contaminated sites (Malot and Wood, 1985; Oster and Wenck, 1988; CH2M-Hill,
1987).

     Unfortunately, there are few guidelines for the optimal design,
installation, and operation of soil vapor extraction systems (Bennedsen,
1987).  Theoretically-based design equations which define the limits of this
technology are especially lacking.  Because of this, the design of these
systems is mostly empirical.  Alternative designs can only be compared by the
actual construction, operation, and monitoring of each design.

     One of the major objectives of this paper is to critically review
information that describes current practices.  The information is summarized
in several tables, which form the basis for a discussion of the design,
installation, and operation of these systems.

SCOPE OF STUDY

     As a part of this investigation, information on 7 pilot-scale, and 10
full-scale studies have been reviewed with respect to the design and
operational variables.  These sites along with their location,  the study type,
the duration of study or date the study began,  and the project status are
listed in Table A-l.  While this list is by no means complete,  it provides a
means for discussing the range of system variables for current installations.

     This technology has been referred to by several names,  including
"subsurface venting", "vacuum extraction", "in situ soil air stripping", and
"soil venting", as well as "soil vapor extraction".   The term "soil vapor
extraction" is used throughout this paper.  Soil vapor extraction technology
seems rather simple in concept, but its application appears to be relatively
recent as indicated by the dates of the available reports.   There is a wide
variety of system designs and operating conditions.
                                      137

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Table A-l.  List of Typical Pilot and Field Soil
            Vapor Extraction Systems.
SITE
FUEL MARKETING
TERMINAL

VALLEY
MANUFACTURING
INDUSTRIAL
TANK FARM
TIME OIL
COMPANY
SOLVENTS
STORAGE TANK
TCAAP
PILOT 1
TCAAP
PILOT 2
TCAAP
SITE D
TCAAP
SITE G
GAS
STATION
UNION 76
GAS STATION
SOUTH PACIFIC
RAILROAD
CUSTOM
PRODUCTS
ELECTRONIC
MANUFACTURING
PAINT
STORAGE
THOMAS SOLVENT
COMPANY
HILL AFB
VERTICAL VENTS

HILL AFB
LATERAL SYSTEM
HILL AFB
SOIL PILE
LOCATION
Granger
Indiana

Grove I and
Massachusetts
San Juan
Puerto Rico
Tacoma
Washington
Cupertino
California
New Brighton
Minnesota
..

..

it

unknown

Bellview
Florida
Benson
Arizona
Stevensvi I le
Michigan
Santa Clara
Valley, CA
Dayton
Ohio
Battle Creek
Michigan
Hill AFB

(3 parallel)
(extraction)
(systems)


STUDY
SCALE
pilot


pilot

pilot/
full
pilot/
full
pilot/
full
pilot

pilot

full

full

full

full

full

full

full

full

full

full


II

II

DATE OR
DURATION
12 days
10 days
15 days
Jan-Apr 88

30 months

11 days
(Aug 1985)
several
months
67 days

78 days

Feb 1986

Feb 1986

7

7 months

7 months

Dec 1988
>280 days
3 yrs

since
July 1987
since
Jan 1988
since
Fall 1988

II

II

1988
STATUS
completed


completed

completed?

pilot
completed
completed?

completed

completed

ongoing

ongoing

completed?

ongoing

completed

completed7

completed?

ongoi ng

ongoing

ongoing






REFERENCES
Crow et al., 1987
Amer Petr Inst, 1985

Envi response, 1987

Malot & Wood, 1985
Malot, 1985
Woodward- Clyde, 1985

Bennedsen, 1987

Anastos et a I., 1985

Anastos et a I., 1985

Wenck, 1985
Oster & Uenck, 1988
Wenck, 1985
Oster & Wenck, 1988
Malot & Wood, 1985

Camp, Dresser, &
McKee, 1987, 1988
Johnson, 1988
Johnson & Sterrett, 1988
Payne et a I., 1986
Payne & Lisiecki, 1988
Bennedsen, 1985

Payne & Lisiecki, 1988

CH2M-HiU, 1987

Oak Ridge National
Lab, 1988
Radian, Corp., 1987




NAME USED FOR
SYSTEM
Subsurface
Venting

Vacuum
Extraction
Vacuun
Extraction
Soi I Gas Vapor
Extraction
Soi I Gas Vapor
Extraction
In-si tu
Vent i ng
n

•i

n

Vacuum
Extraction
Vacuum
Extraction
In-situ Soil
Air Stripping
Forced Air
Circulation
Vapor
Extraction
Enhanced
Volati lization
Vacuum
Extraction
Soil Venting


11

n

                       138

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     From Table A-l,  it can be seen that soil vapor extraction systems  have
been installed at locations across the United States and have  been observed
over periods ranging from several weeks to several  years.   Projects ranging  in
status from being complete to being in the preliminary design  stage have  been
identified.  Some of the studies were too short to  fully assess the
effectiveness of this technology.

PROCESS DESCRIPTION

     A soil vapor extraction system involves the extraction of air containing
volatile chemicals from unsaturated soil.   Fresh air is injected or flows into
the subsurface at locations around a spill site, and the vapor-laden air  is
withdrawn under vacuum from recovery or extraction  vents.   A typical soil
vapor extraction system, such as the one shown in Figure A-l,  consists  of:  (1)
one or more extraction vents, (2) one or more air inlet or  injection vents
(optional), (3) piping or air headers, (4) vacuum pumps or  air blowers,  (5)
flow meters and controllers, (6) vacuum gauges,  (7)  sampling ports,  (8)
air/water separator (optional),  (9) vapor treatment (optional),  and (10)  a cap
(optional).  The design of each of these components is discussed below.

SYSTEM VARIABLES

     A number of variables characterize the successful design  and operation  of
a vapor extraction system.  They may be classified  as site  conditions,  soil
properties, chemical characteristics,  control variables,  and response
variables (Anastos et al., 1985; Enviresponse,  1987).  Table A-2 lists
specific variables that belong to these groups.
Injection
Well
Blower
                                     Air/Water
                                     Separator
                                                    Blowerr
                                                       Vapor
                                                       Treatment
                            Header
                                                           Clean
                                                      ...   .SoU  .
                 Figure A-l.  Soil Vapor Extraction System.
                                     139

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Site Conditions

      Most site conditions can not be changed.   The extent to which VOCs are
dispersed in the soil, vertically and horizontally, is an important
consideration in deciding if vapor extraction is preferable to other methods.
Soil excavation and treatment is probably more  cost effective when only a few
hundred cubic yards of near-surface soils are contaminated (Bennedsen,  1987).
If the spill has penetrated more than 20 or 30  feet or has spread through an
area over several hundred square feet at a particular depth or if the spill
volume is in excess of 500 cubic yards,  then excavation costs begin to exceed
those associated with a vapor extraction system (CH2M Hill,  1985;  Payne et
al., 1986).
              TABLE  A-2.   SOIL VAPOR EXTRACTION SYSTEM VARIABLES.
    Site Conditions
       Distribution of VOCs
       Depth to groundwater
       Infiltration rate
       Location of heterogeneities
       Temperature, humidity
       Atmospheric pressure
       Location of structures
       Rainfall
       Barometric pressure

    Soil Properties
       Permeability (air and water)
       Porosity
       Organic carbon content
       Soil structure
       Soil moisture characteristics
       Particle size distribution

    Chemical Properties
       Henry's constant
       Solubility
       Adsorption equilibrium
       Diffusivity (air and water)
       Density
       Viscosity
Control Variables
   Air withdrawal rate
   Vent configuration
   Extraction vent spacing
   Vent spacing
   Ground surface covering
   Pumping duration
   Inlet air VOC concentration
      and moisture content
Response Variables
   Pressure gradients
   Final distribution of VOCs
   Final moisture content
   Extracted air concentration
   Extracted air moisture
   Extracted air temperature
   Power usage
                                      140

-------
     The depth to groundwater is also important.   Where groundwater is at
depths of more than 40 feet and the contamination extends to the groundwater,
use of soil vapor extraction systems is one of the few ways to remove VOCs
from the soil (Malot and Wood, 1985) .   Groundwater depth in some cases may be
lowered to increase the volume of the unsaturated zone.

     Heterogeneities influence air movement as well as the location of
chemical, making it more difficult to position extraction and inlet vents.
There generally will be significant differences in the air conductivity of the
various strata of a stratified soil.  A horizontally-stratified soil may be
favorable for vapor extraction because the relatively impervious strata will
limit the rate of vertical inflow from the ground surface and will tend to
extend the influence of the applied vacuum horizontally from the point of
extraction.

     The specific location of the contaminant on a property and the type and
extent of development in the vicinity of the contamination may favor the
installation of a soil vapor extraction system.  For example, if the
contamination extends across property lines, beneath a building or beneath an
extensive utility trench network, vapor extraction should be considered.

      Temperature affects the performance of soil vapor extraction systems
primarily because of its influence on chemical properties such as Henry's
constant, solubility, and sorption capacity.  In most cases, extraction
systems are operated at ambient temperatures.

Soil Properties

     The soil characteristics at a particular site have a significant effect
on the applicability of vapor extraction systems.  Air conductivity controls
the rate at which air can be drawn from soil by the applied vacuum.  Grain
size, moisture content, soil aggregation, and stratification are probably the
most important properties (Bennedsen et al., 1985; Hutzler et al.,  1988).  The
soil moisture content or degree of saturation is  also important in that it is
easier to draw air through drier soils.  As the size of a soil aggregate
increases, the time required for diffusion of the chemical out of the immobile
regions also increases.  However, even clayey or  silty soils may be
effectively ventilated by the usual levels of vacuum developed in a soil vapor
extraction system (Camp, Dresser, and McKee, 1987; Terra Vac, 1986b).   The
success of the soil vapor extraction in these soils may depend on the presence
of more conductive strata, as would be expected in alluvial settings,  or on
relatively low moisture contents in the finer-grained soils.

Chemical Properties

     In conjunction with site conditions and soil properties, chemical
properties will dictate whether a soil vapor extraction system is feasible.  A
vapor-phase vacuum extraction system is most effective at removing compounds
that exhibit significant volatility at the ambient temperatures in soil,
compounds exhibiting vapor pressures over 0.5 mm  of mercury (Bennedsen et al.,
1985) and compounds which have values of dimensionless Henry's Law constants
greater than 0.01.   Compounds which have been effectively removed by vapor

                                      141

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extraction include trichloroethene,  trichloroethane,  tetrachloroethene, and
most gasoline constituents.  Compounds which are less easily removed include
trichlorobenzene, acetone, and heavier petroleum fuels (Payne et al., 1986;
Bennedsen et al., 1985; Texas Research Institute,  1980).

     Soluble compounds tend to travel farther in soils where the infiltration
rate is high.  The movement of chemicals with affinity for soil organic
material or mineral adsorption sites will be retarded.  In drier soils,
chemical density and viscosity have the greatest impact on organic liquid
movement, however, in most current systems, the contamination is old enough
that no further movement of free product occurs.

Control Variables

     Soil vapor extraction processes are flexible in that several variables
can be adjusted during design or operation.  Higher air flow rates tend to
increase vapor removal because the zone of influence is increased and air is
forced through more of the air-filled pores.  More vents allow better control
of air flow but also increase construction and operation costs.  The water
infiltration rate can be controlled by placing an impermeable cap over the
site.  Intermittent operation of the blowers allows time for chemicals to
diffuse from immobile water and air and permits removal at higher
concentrations.

Response Variables

     Parameters  responding to soil vapor extraction system performance
include: air pressure gradients, VOC concentrations, moisture content, and
power usage.  The rate of vapor removal is expected to be primarily affected
by the chemical's volatility, its sorptive capacity onto soil, the air flow
rate, the distribution of air flow, the initial distribution of chemical, soil
stratification or aggregation, and the soil moisture content.

SOIL VAPOR EXTRACTION SYSTEM DESIGN

     Tables A-3, A-4, and A-5 summarize the design and operation of the major
components of  the pilot- and field-scale systems reviewed for this paper.
These include  extraction vent design and placement, piping and blower  systems
and the miscellaneous components discussed previously.

Vent Design  and  Placement

Extraction vents  - -

     Typically,  extraction vents are designed  to fully penetrate the
unsaturated  soil zone or  the geologic  stratum  to be cleaned.  An extraction
vent usually is  constructed of slotted plastic  pipe placed in a permeable
packing  as shown in Figure A-2.  Vents may be  installed vertically or
horizontally.  Vertical alignment is typical for deeper contamination  zones
and results  in radial  flow patterns.   If the depth of the contaminated soil  or
the depth to the groundwater table  is  less  than 10 to 15 feet,  it may  be more
practical to dig a trench  across the area  of contamination and  install

                                      142

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        Table  A-3.   Pilot  and  Field  Soil Vapor  Extraction  Systems  --
                            Vent  Design and Placement.

SITE
FUEL MARKETING
TERMINAL
VALLEY
MANUFACTURING
INDUSTRIAL
TANK FARM
TIME OIL
COMPANY
SOLVENTS
STORAGE TANK
TCAAP
PILOT 1
TCAAP
PILOT 2
TCAAP
SITE D
TCAAP
SITE G
GAS
STATION
UNION 76
GAS STATION
SOUTH PACIFIC
RAILROAD
CUSTOM
PRODUCTS
ELECTRONIC
MANUFACTURING
PAINT
STORAGE
THOMAS SOLVENT
COMPANY
HILL AFB
VERTICAL VENTS
HILL AFB
LATERAL SYSTEM
HILL AFB
SOIL PILE
EXTRACTION VENTS
NUMBER AND
TYPE
2 vents
8 vents
4 sh, 4 deep
3 vents
7 vents
1 vent
9 vent
grid
9 vent
grid
39 vents
89 vents
vertical &
horizontal
6 vents
3 sh, 3 deep
79 vents
1 vent
1 to 2
vents
over
20 vents
14 vents
15 vertical
vents
6 laterals
8 laterals
VENT
MATERIAL
2" PVC
4" PVC
7
2" PVC
7
3" PVC
3" PVC
3" PVC
3" PVC
9
4" PVC
2" PVC
2" galv.
steel
2" diaro.
galv.
steel
4" PVC
4" PVC
4" poly-
ethylene
4" poly-
ethylene
VENT VENT
CONSTRUCTION SPACING
screened 20, 40,
14 to 20 ft BLS & 100 ft
up to 20 ft
30 ft deep
25 to 75 ft BLS ?
& at 300 ft BLS
screened 40-90 ft
6 to 25 ft BLS
? na
grav. pack, slotted 20 ft
5 to 20 ft BLS
grav. pack, slotted 50 ft
5 to 20 ft BLS
grav. pack, slotted 25 ft
5 to 25 - 35 ft BLS
grav. pack, slotted 25 ft
5 - 25 to 35 ft BLS
•> •)
slotted 14-50 ft
10 to 15 ft
15 to 25 ft deep variable
gravel pack 50-70 ft
8 to 25 ft BLS
? ?
7 •>
? ?
screened 20 and
10 to 30 ft BLS 40 ft
20 ft BLS 15 ft
5 ft above 18 ft
pile bottom
NUMBER AND
TYPE
4 air inlet
vents
surface
surface
surface
1 air inlet
vent
4 vents
4 vents
surface
or air inlet
surface
or air inlet
surface
surface
surface
£ injection
6 air
inj. vents
1 to 2
air inlets
large no.
of vents
surface
surface
or air inlet
surface
or air inlet
surface
AIR INPUT
VENT
MATERIAL
2" PVC
na
na
na
7
3" PVC
3" PVC
vents can be
air inlets
vents can be
air inlets
na
na
21 vents were
used as A1V
1.25"
PVC
2" diam.
poly-
ethylene
na
vents can be
air inlets
or injection
laterals can
be inlets
na

VENT
CONSTRUCTION
screened
14 - 20 ft BLS
na
na
na
7
slotted
15 - 20 ft BLS
slotted
15 - 20 ft BLS
same as
extraction
same as
extraction
na
na
same as
extraction
gravel pack
15 to 25 ft BLS
?
?
na
same as
extraction
same as
extraction
na
AIV -- air inlet vent
BLS -- below land surface
GWT -- ground water table
na -- not applicable
? -- no information
sh -- shallow
                                          143

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 Table A-4.   Pilot  and Field Soil Vapor Extraction Systems --
                    Piping and Blower  Systems.

SITE
FUEL MARKETING
TERMINAL
VALLEY
MANUFACTURING
INDUSTRIAL
TANK FARM
TIME OIL
COMPANY
SOLVENTS
STORAGE TANK
TCAAP
PILOT 1
TCAAP
PILOT 2
TCAAP
SITE D
TCAAP
SITE G
GAS
STATION
UNION 76
GAS STATION
SOUTH PACIFIC
RAILROAD
CUSTOM
PRODUCTS
ELECTRONIC
MANUFACTURING
PAINT
STORAGE
THOMAS SOLVENT
COMPANY
HILL AFB
VERTICAL VENTS
HILL AFB
LATERAL SYSTEM
HILL AFB
SOIL PILE
PIPING
1 & 2" PVC
PVC manifold
heated
?
2" PVC
manifold
i
3" PVC grid
insulated
3" PVC grid
insulated
8 to 18" steel
insul. manifold
heated
12 to 24" steel
insul . mam fold
heated
7
manifold
4" PVC
man i f o I d
2" galv.
steel
duct
galv. St., heat
manifolds
7
10-16"
metal
manifold
same
same
VACUUM
SOURCE
2 liquid ring
vacuum pumps
blower
vacuum
pump
blower
blowers
2 blowers
1 extr., 1 inj.
2 blowers
1 extr., 1 inj.
up to 4 blowers
variable speed
up to 4 blowers
variable speed
vacuum
pump
vacuum
pump
3 blowers
separate systems
rotary vane
vac . pump
2 blowers
8 blowers
blower
common source
2 rotary lobe
blowers
up to 1000 cfm
each
250 cfm
aux. blower
AIR
FLOW
23 cfm
18 cfm
40 cfm
3 to 800 cfm?
18 cfm
150 cfm
210 cfm
30 cfm/well
10 cfm
100 cfm
40 - 55 cfm
200-220 to
100 to 50 cfm
2200 cfm
per blower
5700 cfm
per blower
7
7
86 - 250
cfm
10.2 cfm
10 cfm to
100 cfm
7
?
up to
2000 cfm
GAS FLOW
VACUUM METER
0.4" Hg pi tot tube w/
0.3" Hg diff. press, meas
0.9" Hg
0-29" Hg X
25-30" Hg ?
? pi tot tube u/
diff. press, meas
0.24" Hg ?
6" Hg
? X
? X
1 .8" Hg totalizing
flow meter
1.8" Hg totalizing
flow meter
7 ?
7 7
0.7-0.6" Hg none
4.5" Hg X
0.2 to ?
3" Hg
? ?
? ?
up to orifices
8" Hg with
Magnehelic
1 to 2"Hg differential
normal pressure gauges
or U-tube
manometers
X -- listed component present, no detailed information
' •- no information
                                  144

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  Table A-5.  Pilot and Field Soil Vapor Extraction Systems --
                     Miscellaneous  Components.

SITE
FUEL MARKETING
TERMINAL
VALLEY
MANUFACTURING
INDUSTRIAL
TAN< FARM
TIME OIL
COMPANY
SOLVENTS
STORAGE TANK
TCAAP
PILOT 1
TCAAP
PILOT 2
TCAAP
SITE D

TCAAP
SITE G

GAS
STATION
UNION 76
GAS STATION

SOUTH PACIFIC
RAILROAD
CUSTOM
PRODUCTS
ELECTRONIC
MANUFACTURING
PAINT
STORAGE
THOMAS SOLVENT
COMPANY

HILL AFB
VERTICAL VENTS

HILL AFB
LATERAL SYSTEM

HILL AFB
SOIL PILE
IMPERMEABLE
CAP
plastic
membrane
none

none

none

none

7

7

18" clay


18" clay


concrete
pavement
existing
pavement

none

6 mi I -poly-
ethylene
none

clay cover
& concrete
none


80'x HO'
plastic

concrete
tank pad

none

AIR/WATER
SEPARATOR
none

500 gallon

condenser

55 gallon
tank
55 gallon
tank
none

none

none


none


condenser'

gas/water
separator

none

I iquid
trap
none

trap w/pump
to tank
none


160 gallon
knock-out
drum

ii

ii

VAPOR
TREATMENT
none

GAC

recovery
tank
none

none

GAC

GAC

none


none/GAC


none

none


none

GAC

none

combustion

GAC


2 catalytic
incinerators
1 - SOOcfm
fluidized bed
1 - 1000cfm
fixed bed


GAUGES
vacuum
temperature
vacuum

?

vacuum
temperature
7

vacuum
temperature
vacuum
temperature
vacuum


vacuum


7

?


7

vacuum

vacuum

vacuum
temperature
vacuum
temperature

vacuum
temperature
humidity

H

"

SAMPLING
PORTS
vent heads
exhaust port
vent head
system lines
?

vent heads
exhaust port
7

inlet ports
exhaust port
inlet ports
exhaust port
vent heads
central header

vent heads
central header

7

->


7

before and
after GAC
exhaust

vent heads
vapor, water
exhaust
GAC outlets

vent heads
exhaust


H

H

TYPES OF
MONITORING
monitoring well
vapor probes
exhaust gas
monitoring wells
exhaust gas
monitoring we I Is
soil borings
exhaust gas
exhaust gas

soil borings
air monitoring
soil borings
air monitoring
soi 1 vapor
air monitoring
exhaust gas
soi I vapor
air monitoring
exhaust gas
monitoring wells

monitoring wel Is
soil borings
vapor probes
monitoring wells
soil borings
exhaust gas
soi I samples
exhaust gas

monitoring wel Is
soil borings
monitoring wells
soil borings
air monitoring
pressure
monitoring
wells

soil
borings


GAC -- granular activated carbon
 ? - - no information
                                 145

-------
perforated piping in the trench bottom versus installing vertical extraction
vents (Oak Ridge National Lab,  1988;  Connor,  1988).   The surface of the
vertical vents or the trench for horizontal vents is usually grouted to
prevent the direct inflow of air from the surface along the vent casing or
through the trench.  Usually several  vents are installed at a site, especially
if soil strata are highly variable in terms of permeability.  In stratified
systems, more than one vent may be installed in the same location,  each
venting a given strata (Camp, Dresser, and McKee, 1987, 1988).   Extraction
vents can be installed incrementally  starting with installation in the area of
highest contamination (Payne and Lisiecki, 1988;  Johnson and Sterrett, 1988).
This allows the system to be brought  on-line as soon as possible.

     Vent spacing is usually based on an estimate of the radius of influence
of an individual extraction vent (Malot and Wood, 1985; Wenck,  1985; Oak Ridge
National Lab, 1988).  In the studies  reviewed, vent spacing has ranged from 15
to 100 feet.  Johnson and Sterrett (1988) suggest that vent spacing should be
decreased as soil bulk density increases or the porosity of the soil
decreases.
                                                    Valve
         Figure A-2.  Typical Extraction/Air Inlet Vent Construction.

                                      146

-------
     One of the major differences noted between systems was the soil boring
diameter.  Larger borings are preferred to provide air/water separation within
the packing.

Air Input

     In the simplest soil vapor extraction systems, air flows.to an extraction
vent from the ground surface as depicted in Figure A-3.  To enhance air flow
through zones of maximum contamination, it may be desirable to include air
inlet vents in the installation.  Injection or inlet vents may be located at
numerous places around the site.  The function of inlet vents and caps is to
control the flow of air into a contaminated zone.  Air vents are passive.
Injection vents force air into the ground and can be used in closed-loop
systems (Payne et al.,  1986).  Injection vents are installed at the edge of a
site, as depicted in Figure A-4, so as not to force contamination away from
the extraction vents.   In addition, injection vents are often installed
between adjacent extraction points to ensure pressure gradients in the
direction of the extraction vents (Payne et al.,  1986).  Typically, injection
and inlet vents are similar in construction to extraction vents.  In some
installations, extraction vents have been designed so they can also be used as
air inlets (Wenck, 1985; Oak Ridge National Lab,  1988).

     Usually, only a fraction of extracted air comes from air inlets (American
Petroleum Institute, 1985; Crow et al., 1987; Ellgas and Marachi,  1988).  This
indicates that air drawn from the surface is the predominant source of clean
air.
        y////////////////^^^^^
            Figure A-3.  Air  Flow  Patterns  in Vicinity  of  a  Single
                          Extraction Vent --No Cap.

                                      147

-------
                 Figure A-4.  Air Flow from Injection Vents.

     Thornton et al.  (1984)  investigated  the  effects  of air flow rate,  and the
configuration of the  inlet and extraction vents  on gasoline recovery from an
artificial aquifer.  They determined that screening geometry only had an
effect at the low air flow rates.   At low flow rates,  higher recovery rates
resulted when the screen was placed near  the  water table versus  being screened
the full depth of the aquifer.  A similar assessment  was made by Woodward-
Clyde Consultants (1985) at the Time Oil  Company site.   Woodward-Clyde
engineers suggested that the vents should be  constructed with approximately 20
feet of solid pipe 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.

Piping and Blower Systems

     Table A-4 summarizes information on the  design of piping systems and the
selection of blowers for vapor extraction systems.

Piping --
    Piping materials connecting the vents to  headers as well as  the headers
themselves are usually plastic or steel.   Wenck (1985) suggests  that headers
be constructed of steel for durability, especially in colder climates.
Headers may be configured as a manifold or in a grid, although,  manifold
construction appears to be the most common.  Pipes and headers are usually
buried or wrapped with heat tape and insulated in northern climates to prevent
freezing of condensate  (Wenck, 1985).

Valving - -
     A control/shut-off valve is usually installed at each venthead and at
other critical locations, such as lateral/header connections, to provide
operational flexibility and optimize extraction rates.  Typically, ball or
butterfly valves are used because they provide better flow control.
                                      148

-------
Vacuum Source --
    The vacuum for extracting soil air is developed by an ordinary positive
displacement industrial blower, a rotary blower,  vacuum or aspirator pump, or
a turbine.  There are a large number of commercially available blower models.
In the studies reported herein, the blowers have had ratings ranging from 100
to 6,000 cubic feet per minute at vacuums up to about 30 inches Hg gauge as
shown in Table A-4.  Ratings of the electric drive motors are usually 10
horsepower or less.  The pressure from the outlet side of the pumps or blowers
is usually used to push the exit gas through a treatment system and can be
used to force air back into the ground if injection vents are used (Payne et
al., 1986), although, it is more common to use a separate blower for injection
(Anastos et al., 1985).  Vapor treatment efficiency can be improved by
installing the blower between the moisture separator and the vapor treatment
system to take advantage of the heat generated by the blower.  The blower or
blowers are usually housed in a temporary building on-site.

Gas Flow Meter --
     A flow meter should be installed to monitor the volume of extracted air.
This measurement is used in conjunction with gas analysis to determine the
total mass of vapor extracted from the soil.  Flow measurements from
individual vents are useful for optimizing extraction system operation.  A
flowmeter consisting of an orifice plate and manometer, together with the
appropriate rating curve, will yield the system discharge air flow rate.

Miscellaneous Components

     In addition to the basic vent, piping, and blower components, a soil
vapor extraction system may require a cover, air/water separator, and vapor
treatment.  Table A-5 summarizes the range of design of miscellaneous
components of the various pilot- and field systems.

Impermeable Cap - -
     The surface of the entire site may be sealed with plastic sheeting, clay,
concrete, or asphalt as indicated in Table A-5.  If movement of the air toward
the extraction vent is desired to be more radial than vertical, then an
impermeable cap should be added.  The cap controls the air flow pathway so
that clean air is more likely to come from air vents or injection vents.
Without the cap (Figure A-3),  a more vertical movement of air from the soil
surface takes place.  But when an impermeable cap is in place, the radius of
influence around the extraction vent is extended (Figure A-5).  Thus, more of
the contaminated soil may be cleansed by the air flow.

     The use of a polyethylene cover will also prevent or minimize
infiltration, which, in turn,  reduces the moisture content and further
chemical migration.  With little or no infiltration, water is less likely to
be extracted from the system,  thus reducing the need for an air/water
separator.  In very dry climates,  a reduction of moisture content below which
partial drying of the soil occurs, extraction system efficiency may be reduced
due to increased adsorption capacity of the dry soil (Johnson and Sterrett,
1988).
                                      149

-------
Air/Water Separator --
     If water is pulled from the extraction vents,  an air/water separator is
required to protect the blowers or pumps and to increase the efficiency of
vapor treatment systems.  The condensate may then have to be treated as a
hazardous waste depending on the types and concentrations of contaminants.
The need for a separator may be eliminated by covering the treatment area with
an impermeable cap.  In some cases, a gasoline/water separator may be used in
conjunction with a combination vapor extraction/pumping system for gasoline
product recovery (Malot and Wood,  1985;  Thornton et al., 1984).

Vapor Treatment - -
     Air emission problems should not be created while solving a soil
contamination problem.  Vapor treatment may not be required for systems that
produce a very low emission rate of easily degradable chemicals.  The decision
to treat vapor must be made in conjunction with air quality regulators.  There
are several treatment systems available that limit or control air emissions.
These include liquid/vapor condensers, incinerators, catalytic converters,  and
gas-phase granular activated carbon (GAG).

     If air emissions control or vapor treatment is required for an
installation, a vapor phase activated carbon adsorber system probably will be
the most practical system depending on chemical emission rates and VOC levels,
although catalytic oxidation units have produced favorable results (Bennedsen,
1985).  Gas-phase GAG may require heating of the extracted air to control the
relative humidity in order to optimize the carbon usage rate.  As the fraction
of water increases, the capacity for the target chemical decreases and the
carbon replacement rate increases.  The spent carbon may be considered as a
hazardous waste depending on the contaminants (Enviresponse, 1987).
                      Cap
         Figure A-5.   Air Flow Patterns with Impermeable Cap in Place.

                                      150

-------
     On one project, where the initial extraction rate of volatiles was over
200 pounds per day, the extracted gas was able to be piped to the combustion
air intake zone of a nearby industrial boiler that was in continuous operation
(Bennedsen et al., 1985).  Laboratory analyses did not detect unwanted
volatiles in the boiler emissions.  Incineration can be self-sustained
combustion if the vapor contains high concentrations of hydrocarbons or
combustible volatile chemicals.  Usually there is a lag time to achieve a high
concentration of combustibles.  Concentration of volatiles in the air stream
might be increased by intermittent blower operation or by intermittently
operating different extraction vents.  Some systems have auxiliary fuels to
maintain a desired exhaust temperature.

Pitot Tubes and Pressure Gauges - -
     Various monitoring devices such as sampling ports, vacuum gauges, and
pitot tubes are required for estimating vapor discharges.   Pressure gauges are
used to monitor the pressure losses in the overall system and to optimize air
flows.

Sampling Ports --
     Sampling ports are usually installed at each vent head, at the blower,
and after gas treatment.  The basic measurements required to assess soil vapor
extraction system performance are the system air flow rate and the
concentration of volatile organic chemicals in the extracted flow.  A gas
chromatograph equipped with an appropriate detector for the compounds expected
to be present in the exhaust gas is typically used to provide VOC
concentration data.

Monitoring Systems --
     Vapor and pressure monitoring probes may be placed in the soil
surrounding the extraction system to measure vapor concentrations and the
radius of influence of the extraction vents.  The monitoring wells are usually
required to assess the final clean-up of a particular site.

SITE CONDITIONS

Soil and Geological Conditions

     Table A-6 briefly summarizes the geologic conditions at the various pilot
and field sites.   Although it has been suggested that soil vapor extraction
systems should be used primarily in highly permeable soils, they have been
installed in soils with a wide range of permeabilities.  The range of areas
and volumes of soil vented is large.  Soil vapor extraction systems have been
used in shallow as well as deep unsaturated zones.  Much of the information
needed to fully assess the effects of soil properties (moisture content,
organic carbon content, and porosity) on vapor extraction is not available.

     As the permeability of the soil decreases,  the time required for
extraction and decontamination increases.  In addition to permeability, the
presence of heterogeneities make it more difficult to position inlet and
extraction vents.  For example, the effect of a clay lens at the Groveland
site resulted in perched water table.  During high rainfall periods, the
contaminant seeped over the lip of this clay lens and spread further.

                                      151

-------
      Table A-6. Pilot and Field Soil Vapor  Extraction  Systems --
                     Soil and Geological Conditions.

SITE
FUEL MARKETING
TERMINAL
VALLEY
MANUFACTURING
INDUSTRIAL
TANK FARM
TIME OIL
COMPANY
SOLVENTS
STORAGE TANK
TCAAP
PILOT 1
TCAAP
PILOT 2
TCAAP
SITE D
TCAAP
SITE G
GAS
STATION
UNION 76
GAS STATION
SOUTH PACIFIC
RAILROAD
CUSTOM
PRODUCTS
ELECTRONIC
MANUFACTURING
PAINT
STORAGE
THOMAS SOLVENT
COMPANY
HILL AFB
VERTICAL VENTS
HILL AFB
LATERAL SYSTEM
HILL AFB
SOIL PILE
SOIL/ GUT
GEOLOGY DEPTH
sand and fine sand layers 25 ft
w/ traces of clay and silt
5 - 12 ft of sand over 27-52 ft
5 - 10 ft of clay
over glacial till
40 - 210 ft clayey silts 300 ft
900 ft limestone
sand and gravel >30 ft
with some silt
unknown 85 ft
4 - 6 ft sand and loamy sand 170 ft
f i 1 1 over stained low
permeability sediments over sand
same as Pi lot 1 "
same as Pi lot 1 "
up to 135 ft sand over 130 ft
glacial till and sand
6 - 12 ft clayey soil 8-10 ft
grading to silt & sand
18 - 21 ft clayey sand over 48-53 ft
5-13 ft gumbo clay over 28-42 ft
silty sand over limestone
20 - 25 ft silt & sand, gravel 240 ft
layers, 50 ft silty clay 3 40 ft
30 ft of fine sand 30 ft
al luvial clayey 90 ft
si Its and sands
sandy soil with clay strata 40-50 ft
sands and gravels
a sand and gravel alluvial 22 ft
deposit over sandstone
4 ft si Ity sand 600 ft
underlain by 16 - 31 ft
of sand underlain by
discontinuous "
sand and clay layers
mixture of sand "
and si Ity sand
SOIL HYDRAULIC MOISTURE
POROSITY CONDUCTIVITY CONTENT
0.38 10"3 cm/s ?
? permeable to perched
impermeable water
? very ?
permeable
? 3x1 O"3 cm/s ?
•> ? ?
? very ?
permeable
7 ii •>
7 ii 7
? very ?
permeable
•> impermeable'' '
' ? ?
0.1 - 0.3 10"4 cm/s 2 - 5%
•> •? 7
7 relatively ?
impervious
•>'•>•>
' sand - 0.1 cm/s ?
bedrock-
0.06 cm/s
? permeable to perched
impermeable water
^ ii ii
? permeable ?
AREA
AFFECTED
two 60 ft2
areas
?
4,400,000
cu yds
30,000 sq
unknown
3800 to
33000
cu yds
u
II
7
•>
unknown
60 x 70 ft
50 acres
7
?
7
200 x 120 ft
7
 "> -- no information
GWT -- groundwater table
                                    152

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Extraction vents had to be installed below this clay lens to assure an
effective extraction operation.  Varying strata were also a concern at the gas
station site in Bellview, Florida (Camp, Dresser, and McKee,  1987).  Some
layering of soil can make it easier to extract VOCs from soils where
horizontal air channeling occurs through sand layers with subsequent VOC
diffusion from less permeable layers.

     The soil moisture content or degree of saturation is also important in
that it is easier to draw air through drier soils.  A case in point is that of
the South Pacific Transportation site in Arizona where the soil was relatively
dry (2 to 5% moisture content) (Johnson, 1988; Johnson and Sterrett, 1988).
After seven months, 6500 kg of dichloropropene had been extracted using a
moderate air flow rate of 85 to 250 cfm.  Higher air flow rates tend to
increase vapor removal because the radius of influence increases and more air
is forced through the air filled pores.   In addition,  more air is pulled
through the soil in a shorter time period.

Types and Magnitude of Contamination

     The types and magnitude of chemical contamination encountered at the
various sites are summarized in Table A-7.  The common chemical contaminants
extracted were trichloroethylene, 1,1,1-trichloroethane,  methylene chloride,
carbon tetrachloride,  tetrachloroethylene, dichloroethylene,  toluene, 1,3-
dichloropropene,  and gasoline constituents (benzene, toluene, ethylbenzene,
and xylene).   Most chemicals that have been successfully extracted have a low
molecular weight and high volatility.  Another common screening tool is the
air-water partitioning coefficient,  expressed in dimensionless terms as
Henry's Law constant (See Table A-8).  Most of the compounds have values of
Henry's Law constants greater than 0.01.  Vapor extraction has removed large
quantities of volatile chemicals as demonstrated at several sites.

EXTRACTION SYSTEM OPERATION

     At most sites, the initial VOC recovery rates were relatively high and
then decreased asymptotically to zero with time (Oster and Wenck, 1988; Payne
et al., 1985; Payne and Lisiecki, 1988;  Terra Vac, 1987b) .   Vapor extraction
is more effective at those sites where the more volatile chemicals are still
present than when the spill is relatively recent.  Several studies have
indicated that intermittent venting from individual vents is probably more
efficient in terms of mass of VOC extracted per unit of energy expended (Crow
et al., 1987; Oster and Wenck, 1988; Payne and Lisiecki,  1988).  This is
especially true when extracting from soils where mass  transfer is limited by
the rate chemicals diffuse out of immobile air and water.  Optimal operation
of a soil vapor extraction system may involve taking individual vents in and
out of service to allow time for liquid and gas diffusion and to change air
flow patterns in the region being vented.

     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.
Mass balances using initial and final soil borings have not been particularly
successful in predicting the amount of chemical actually removed in a system
(Anastos et al.,  1985;  Camp, Dresser, and McKee, 1988).  Soil vapor

                                     153

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Table A-7.  Pilot and Field Soil Vapor Extraction Systems  --
           Types and Magnitude of Contamination.

CHEMICALS SPILL
SITE IDENTIFIED VOLUME
FUEL MARKETING gasoline > 100000 gal
TERMINAL hydrocarbons
VALLEY TCE, PCE, MC unknown
MANUFACTURING OCE, TCA
INDUSTRIAL carbon 200,000
TANK FARM tetrachloride Ibs
TIME OIL TCE, PCE, TTCA unknown
COMPANY MC, TCA, DCE
SOLVENTS TCA, TCE unknown
STORAGE TANK DCA, DCE
TCAAP TCE, TCA unknown
PILOT 1 OCE, toluene
+ others
TCAAP
PILOT 2 " "
TCAAP " "
SITE D
TCAAP TCE, TCA unknown
SITE G DCE, toluene
+ others
GAS gasoline unknown
STATION
UNION 76 benzene, toluene unknown
GAS STATION xylene, HCs
SOUTH PACIFIC dichloropropene 150,000 Ibs
RAILROAD
CUSTOM PCE <5000
PRODUCTS cu yd soil
ELECTRONIC TCA unknown
MANUFACTURING chl. solvents
PAINT acetone, ketones over 400,000
STORAGE toluene, xylenes cu yds soil
THOMAS SOLVENT PCE, TCE, TCA '
COMPANY
HILL AFB jet fuel >25,000 gal
VERTICAL VENTS (JP4) total
HILL AFB " "
LATERAL SYSTEM
HILL AFB " "
SOIL PILE
INITIAL CONTAMINATION FINAL CONTAMINATION
LEVELS LEVELS
1.6 ft product on GWT ?
60-110 ppmv a 16 ft, 3500-28000
a 20 ft, 11000-51000 a 21 ft
max cones: 2500mgTCE/kg being measured
40 mgPCE/kg, 12 mgDCE/kg
70% of carbon tet ?
contained in unsat. zone initial rate = 250 Ib/day
from 5 ppm at 30 ft current status
to over 1000 ppm at 6 in unknown
>10 mgTCA/m , 1 mgTCE/m unknown, extraction rate
decreased with time
5-50 mgVOC/kg not determined
stained sediments 4-40 ft BLS
TCE up to 8000 mg/kg
M
H
M »
>1000 mgVOC/kg not determined
AMOUNT
EXTRACTED
190 gallons
being
measured
>70% of spi 1 1
volume
240 Ibs
1 lib/day
"1000 Ibs
•)
>84,000 Ibs
VOCs
>85,000 Ibs
VOCs
up to 10 in of gasoline no free product 1200 Ibs of
on GUT, no HC a 18 ft BLS 98% reduction of HC in GU gasoline
0.2 to 12.4 mgBTEX/kg
highest cone, at 15 ft
30 to 60% of initial spill less than 10 ppm in 40
remaining in soil soil samples
8 to 5600 maPCE/kg soil 17 ugPCE/kg soil
92000 mg/m in exhaust after 280 d
2000 ppmv organics 50 ppmv in exhaust
in initial extracted gas (target is 20 ppmv)
Total VOC in GU from Total VOC in GW from
1 to 620,000 ug/L not detected to 10 ug/L
1700 Ibs VOC in 1984 not determined
up to 6200 mg/kg fuel system still in
in upper 5 ft of soil operational
200 - 900 mg/kg
between 5 - 10 ft deep
below detection
below 50 ft deep
soil vapor cone, up to
80000 ppb in top 10 ft
22,000 Ibs
in 123 days
90,000 Ibs
62 - 76 kg
in 35 days
> 12000 Ibs
VOCs
>7800 Ibs
after 165 days
7
1600 Ibs
in one-
well vent
test
BTEX -- benzene, toluene, ethylbenzene, and xylene TCA -- trichtoroethane
DCA -- dichloroethane TCE -- trichloroethene
DCE -- dichloroethene TTCA -- tetrachloroethane
HC -- hydrocarbon VOC -• volatile organic chemical
MC -- methylene chloride na -- not applicable
PCE -- tetrachlorethene (perch loroethylene) ppmv -• parts per million by volume
                            154

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         Table  A-8.  Dimensionless  Henry's  Law Constants for Typical
                                Organic Compounds.
Component
                            10°C
15°C
20°C
25°C
30°C
nonane
n-hexane
2-methylpentane
cyclohexane
chlorobenzene
1,2-dfchlorobenzene
1,3-di chlorobenzene
1 ,4-di chlorobenzene
o-xylene
p-xylene
m-xylene
propy I benzene
ethylbenzene
toluene
benzene
methyl ethylbenzene
1,1-dichloroethane
1 ,2-dichloroethane
1,1, 1-trichloroethane
1 ,1 ,2-trichloroethane
cis- 1 ,2-dichloroethylene
trans -1 ,2-dichloroethylene
tetrachloroethylene
trichloroethylene
tetralin
decalin
vinyl chloride
chloroethane
hexach loroethane
carbon tetrachloride
1 ,3,5-trimethylbenzene
ethylene dibromide
1, 1-dichloroethylene
methylene chloride
chloroform
1 , 1 ,2,2-tetrachloroethane
1 ,2-dichloropropane
dibromochloromethane
1,2, it- tri chlorobenzene
2, 4-di methyl phenol
1 , 1 , 2-trichlorotrif I uo roe thane
methyl ethyl ketone
methyl isobutyl ketone
methyl cellosolve
t r i ch 1 or of I uoromethane
17.21519
10.24304
29.99747
4.43291
0.10501
0.07015
0.09511
0.09124
0.12266
0.18076
0.17689
0.24446
0.14030
0.16397
0.14203
0.15106
0.15838
0.05035
0.41532
0.01678
0.11620
0.25390
0.36410
0.23154
0.03228
3.01266
0.64557
0.32666
0.25522
0.63696
0.17344
0.01291
0.66278
0.06025
0.07403
0.01420
0.05251
0.01635
0.05552
0.35678
6.62785
0.01205
0.02841
1.89798
2.30684
20.97643
17.46626
29.35008
5.32869
0.11884
0.06048
0.09769
0.09177
0.15267
0.20427
0.20976
0.30915
0.19073
0.20807
0.16409
0.17762
0.19200
0.05498
0.48635
0.02664
0.13787
0.29815
0.46943
0.28208
0.04441
3.53977
0.71049
0.40515
0.23641
0.80776
0.19454
0.02030
0.85851
0.07147
0.09854
0.00846
0.05329
0.01903
0.04441
0.28504
9.09260
0.01649
0.01565
1.53517
2.87580
13.80119
36.70619
26.31372
5.81978
0.14175
0.06984
0.12222
0.10767
0.19704
0.26813
0.24859
0.36623
0.24983
0.23071
0.18790
0.20910
0.23404
0.06111
0.60692
0.03076
0.14965
0.35625
0.58614
0.35002
0.05654
4.40641
0.90207
0.45727
0.24568
0.96442
0.23736
0.02536
0.90622
0.10143
0.13801
0.03035
0.07898
0.04282
0.07607
0.41986
10.18462
0.00790
0.01206
4.82210
3.34222
16.92131
31.39026
33.72000
7.23447
0.14714
0.06417
0.11649
0.12957
0.19905
0.30409
0.30409
0.44143
0.32208
0.26240
0.21581
0.22807
0.25545
0.05763
0.71119
0.03719
0.18556
0.38625
0.69892
0.41690
0.07643
4.78211
1.08313
0.49456
0.34129
1.20575
0.27507
0.02657
1.05860
0.12098
0.17207
0.01022
0.14592
0.04823
0.07848
0.20150
13.03840
0.00531
0.01594
1.26297
4.12815
18.69235
62.70981
34.08841
8.96429
0.19014
0.09527
0.16964
0.15637
0.25164
0.37988
0.35656
0.55072
0.42209
0.32480
0.28943
0.30953
0.31194
0.06995
0.84819
0.05346
0.23114
0.48640
0.98487
0.51454
0.10773
• 7.99952
1.12556
0.57484
0.41405
1.51951
0.38711
0.03216
1.27832
0.14512
0.22270
0.02814
0.11497
0.06110
0.11939
0.15074
12.90375
0.00442
0.02734
1.53277
4.90423
Adapted from Howe et al. (1986)
                                        155

-------
measurements in conjunction with soil boring and groundwater monitoring may be
useful in determining the amount of chemical remaining in the soil.   Risk
analysis has been used to evaluate final clean up in at least one system
(Ellgas and Karachi,  1988).  Payne and Lisiecki (1988) suggest intermittent
operation near the end of clean up.   If there ceases to be a significant
increase in vapor concentration upon restart, one can assume the site has been
decontaminated.

     Malot and Wood (1985) discuss use of in-situ soil air extraction in
conjunction with groundwater pumping and treatment as a low-cost alternative
for the clean up of petroleum and solvent spills.  Large quantities  of organic
chemicals can be retained in the vadose zone by capillary forces,  dissolution
in soil water, volatilization,  and sorption.  If this product can be removed
before it reaches the groundwater then the problem is mitigated.  Since vapor
transport is diffusion-controlled in the absence of air extraction,  the vapor
spreads horizontally, and a concentration gradient is established in the
vertical direction as vapor diffuses back to the surface.  Malot and Wood
(1985) indicate that vapor extraction is effective in removing organic
chemical vapor,  sorbed chemical, and free product at the water table.  This
suggests that the soil should be decontaminated by vapor extraction before
groundwater clean up can be completed.  Vapor extraction becomes more cost-
effective as the depth to groundwater increases, primarily because the cost of
excavation becomes prohibitive.

     The design and operation of soil vapor extraction systems can be quite
flexible, allowing for changes to be made during the course of operation, with
regard to vent placement or blower size, and air flows from individual vents.
If the system is not operating effectively, changes in the vent placement or
the capping the surface may improve it.  At one site, the blowers were housed
in modules with quick disconnect attachments.  This allowed for portability,
thus improving the removal efficiency by allowing for the blowers to be moved
about the site to particular locations where extraction was required the most.

CONCLUSIONS

     Based on the current state of the technology of soil vapor extraction
systems, the following conclusions can be made:

      1.  Soil vapor extraction can be effectively used for removing a wide
          range of volatile chemicals over a wide range of condi.tions.

      2.  The design and operation of these  systems is flexible enough to
          allow for rapid changes in operation,  thus, optimizing the removal
          of chemicals.

      3.  Intermittent blower operation is probably more efficient in terms  of
          removing the most chemicals with the  least energy, especially  in
          systems where chemical transport  is limited by diffusion through  air
          or water.

      4.  Volatile chemicals can be extracted from clays and silts but at a
          slower rate.  Intermittent operation  is certainly more efficient

                                      156

-------
    under these conditions.

5.   Air injection and capping a site have the advantage of controlling
    air movement, but injection systems need to be carefully designed.

6.   Extraction vents are usually screened from a depth of from 5 to 10
    below the surface to the groundwater table.  For thick zones of
    unsaturation, maximum screen lengths of 20 to 30 feet are specified.

7.   Air/water separators are simple to construct and should 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 vents, while probably more expensive,
    allows for a greater degree of freedom in design.  Modular
    construction, where the most contaminated 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 than use of soil borings alone.  It
    is usually impossible to do a complete materials balance on a given
    site because most sites have an unknown amount of VOC on the soil
    and in the groundwater.

11.  Soil vapor extraction systems are usually only part of a site
    remediation system.

12.  While a number of variables intuitively affect the rate of chemical
    extraction, no extensive study to correlate variables to extraction
    rates has been identified.
                               157

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Radian Corp.  1987.  Installation Restoration Program Phase II  Draft Report.
      U.S. Air Force, Hill AFB, UT.   July 1987.

Rollins, Brown, and Gunnell, Inc.   1985.   Subsurface Investigation and
      Remedial Action, Hill AFB JP-4 Fuel Spill, Provo, Utah.   U.S. Air Force,
      Hill AFB, UT.  December 1985.

Terra Vac, Inc.  1987.  Demonstration Test Plan In-Situ Vacuum  Extraction
      Technology.  Enviresponse No.  3-70-06340098.   Terra Vac,  Inc.,  SITE
      Program, Groveland Wells Superfund Site,  Groveland, MA.   November, 1987.

Terra Vac, Inc.  1987.  Union 76 Gas Station Clean-up, Bellview,  Florida.
      Florida Department of Environmental Regulation, Tallahassee, FL.

Texas Research Institute.  1986.  Examination of Venting For Removal of
      Gasoline Vapors From Contaminated Soil.  American Petroleum Institute,
      March, 1980,  (Reprinted in 1986).

Thornton, S.J. and W.L. Wootan.  1982.  Venting for the Removal of Hydrocarbon
      Vapors from Gasoline Contaminated Soil.  J.  Environmental Science and

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      Health, A17(1):31-44.

Thornton, S.J., R.E. Montgomery, T. Voynick, andW.L. Wootan.  1984.  Removal
      of Gasoline Vapor from Aquifers by Forced Venting.  Proceedings of the
      1984 Hazardous Materials Spills Conference, Nashville, TN.  pp. 279-286.
      April 1984.

Towers, D.S., M.J. Dent, and D.G. Van Arnam.  1988.  Evaluation of In Situ
      Technologies for VHOs Contaminated Soil.  Proceedings of the 5th
      National Conference on Hazardous Wastes and Hazardous Materials, HMCRI,
      Las Vegas, NV. April 19-21, 1988.

Treweek, G.P. and J. Wogec.  1988.  Soil Remediation by Air/Steam Stripping.
      Proceedings of the 5th National Conference on Hazardous Wastes and
      Hazardous Materials, HMCRI, Las Vegas, NV.  April 19-21, 1988.

U.S. Army.  1986a.  "Twin Cities Army Ammunition Plant In-Situ Volatilization
      System, Site G,  First Week Operations Report", Twin Cities Army
      Ammunition Plant, New Brighton, MN, March 1986.

U.S. Army.  1986b.  "Twin Cities  Army Ammunition Plant In-Situ Volatilization
      System Site D, Operations Report," Twin Cities Army Ammunition Plant,
      New Brighton, MN, September 8, 1986.

U.S. Army.  1987a.  "Twin Cities Army Ammunition Plant In-Situ  Volatilization
      System Site D Operations Report," Twin Cities Army Ammunition Plant,  New
      Brighton, MN, September 1, 1987.

U.S. Army.  1987b.  "Twin Cities  Army Ammunition Plant In-Situ Volatilization
      System Brighton,  MN, October 2, 1987.

U.S. Army.  1987c.  "Twin Cities  Army Ammunition Plant In-Situ Volatilization
      System Site G, Emissions Control System Operations Report," Twin Cities
      Army Ammunition Plant,  New Brighton, MN, September 1, 1987.

U.S. Army.  1987d.  "Twin Cities Army Ammunition Plant  In-Situ Volatilization
      System Site G, Emissions Control System Operations Report,"  Twin Cities
      Army Ammunition Plant,  New Brighton, MN, October 2,  1987.

Wenck Associates, Inc.   1985.   "Project Documentation:  Work Plan, ISV/In-Situ
      Volatilization,  Sites D and G, Twin Cities Army Ammunition Plant,"
      Federal Cartridge Corporation, New Brighton,  MN.  September 1985.

Weston, Roy F., Inc.  1985.  Appendices -- Task 11, In-Situ Solvent Stripping
      From Soils Pilot  Study.   Installation Restoration General Environmental
      Technology Development Contract DAAK11-82-C-0017.   U.S. Army Toxic and
      Hazardous Materials Agency, Aberdeen Proving Grounds, MD.   May 1985.

Woodward-Clyde Consultants.  1985.  Performance Evaluation Pilot Scale
      Installation and  Operation Soil Gas Vapor Extraction System Time Oil
      Company Site Tacoma, Washington, South Tacoma Channel, Well 12A Project.
      Work Assignment No.  74-ON14.1, Walnut Creek,  CA.  December 1985.

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Wootan, W.L. and T.  Voynick.   1984.   Forced Venting to Remove Gasoline Vapor
      from a Large-Scale Model Aquifer.   Texas Research Institute,  Inc.,  Final
      Report to American Petroleum Institute.
                               ACKNOWLEDGEMENTS

This paper is based on work supported by the United States Environmental
Protection Agency under assistance agreement CR-814319-01-1.   This paper has
not been peer-reviewed.  Any opinions, findings,  and conclusions are those of
the authors and do not necessarily reflect the views of the Environmental
Protection Agency.  Mention of trade names or commercial products does not
constitute and endorsement or recommendation for use.
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                                  APPENDIX B
                         APPLICABILITY AND LIMITS OF
               SOIL VAPOR EXTRACTION FOR  SITES CONTAMINATED WITH
                          VOLATILE ORGANIC COMPOUNDS
                              Joseph Danko,  P.E.a
INTRODUCTION

      Many communities throughout the United States and the rest of the world
have discovered that their soil and groundwater are contaminated with volatile
organic compounds (VOCs).  This contamination results from activities such as
poor disposal practices; careless handling of VOCs at transfer and storage
facilities that results in surface spillage; and leaking underground storage
tanks (USTs).

      Soil vapor extraction (SVE) is an in situ technology currently used for
the removal of VOCs from vadose zone soils.   The use of this technology has
grown significantly over the last 5 years because it generally costs less than
other alternatives (especially excavation with subsequent disposal or
treatment), is easy to implement, and has the potential for significant
removal rates.  However, many criteria need to be evaluated before selecting
this technology for a VOC site.  Without this evaluation, ineffective or
noncompliant cleanup goals could result.  These criteria include identifying
the contaminants and their characteristics;  defining the nature, extent, and
volume of contamination; investigating in detail the vadose zone
characteristics affecting vapor transport; determining the depth to and nature
of the underlying saturated zone; and evaluating air emissions requirements
and vadose zone cleanup standards.

      The purpose of this paper is to provide a list of criteria to evaluate
the applicability of soil vapor extraction at a potential site.   Wherever
possible, rules of thumb and limitations are provided to assist the user in
this assessment.

SVE DESCRIPTION

      Soil vapor extraction is a physical means of removing VOCs from
contaminated soil.  The typical SVE system consists of a network of vacuum
extraction wells screened in the contaminated zone.  The extraction wells are
joined together by a common header pipe, which is connected to a vapor water
aCH2M Hill, P.O. Box 428, Corvalis, OR 97339

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separator where water is removed.   The separator is then connected to a
positive diplacement blower,  which provides a negative pressure gradient in
the subsurface.  Discharge from the blower is vented to the atmosphere or
connected to an offgas treatment system,  depending upon air emissions
requirements and the nature and extent of VOC contamination.

      The subsurface vacuum created by the blower pulls VOC-laden vapors
through the subsurface into the extraction wells.   Pulling air through soil
voids disrupts the equilibrium concentration between liquid or sorbed
contaminants and VOCs in the gas phase.  A concentration gradient is es-
tablished from liquid or sorbed contaminants in soil interstices and
micropores and VOCs present in the gas phase.  Evaporation of contaminants to
the gas phase occurs in the same manner in which air stripping removes
contaminants from groundwater.  The vacuum also decreases pressure in soil
voids, thereby causing the release of additional VOCs.

ADVANTAGES OF SVE

      The current increase in the use of SVE stems from the advantages,
including ones related to regulatory factors, associated with the in situ
nature of the technology.  These advantages include the following:

      •     SVE is minimally intrusive to contaminated soils.  During
            construction and operation, the potential release of VOCs to
            onsite and offsite receptors is insignificant when compared to
            excavation and removal of contaminated soils.

      •     SVE is not a complicated technology to implement.  As described
            earlier, the typical system contains extraction wells, piping,
            positive displacement blowers, and standard instrumentation and
            controls. However, optimal sizing and operation of this equipment
            on a medium-sized or larger site does require assistance from
            experienced personnel.  Also, if flammable VOCs are present, care
            must be taken to avoid fire and/or explosion in the SVE system.

      •     The use of SVE can result  in a relatively quick reduction in VOCs.
            In addition, full-scale SVE systems have been successful at many
            sites across the country,  and the systems can be constructed
            safely with proper instrumentation and controls.

      •     Vadose zone VOC contamination often acts as a source  input  to
            groundwater contamination.  SVE can reduce or effectively
            eliminate the vadose zone  source input, thereby drastically
            decreasing the time required for saturated zone pump-and-treat
            alternatives.

      •     SVE, when applicable,  is more cost effective than other  in  situ
            technologies.  When compared to excavation costs  (with subsequent
            disposal or treatment), its costs can easily be an order of mag-
            nitude lower.

      •     With the land ban on solvent tainted soils, SVE offers a viable
            alternative technology to  excavation with disposal or treatment.


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APPLICABILITY AND LIMITATIONS OF SVE

      With all of the advantages of this technology,  it is easy to understand
that its use is on the rise in site cleanups.  However, there are many
criteria that need to be evaluated before assessing the limitations and
applicability of this technology for a given site.   The following text
describes criteria to be evaluated when considering this technology for site
remediation.

Volatility of Contaminants

      SVE is applicable if VOCs are the primary contaminants in the soil. As a
guideline, a compound is a likely candidate if it has both of these
characteristics:

      •     Vapor pressure (P*) of 1.0 mm or more of mercury at 20°C

      •     Henry's Law constant greater then 100 atmospheres/mole fraction
            (in the moderate range), or dimensionless Henry's Law constants
            greater than 0.01

Examples of VOCs amenable to SVE are listed in Table B-l.

                       TABLE B-l.  VOCs AMENABLE TO SVE
Compound
1, 1,1-Trichloroe thane
Trichloroethylene
Tetrachloroethylene
1 , 1-Dichloroethylene
1 , 2-Dichloroethane
Benzene
Toluene
Henry's Law Constant a
1,535/100
590/60
1,261/14
1,086/500
60/61
302/76
367/22
/P*b







aHenry's Law = atm./mole fraction
bP* = mm of Hg at 20°C
      Compounds that are more difficult to extract would be trichlorobenzene
and diesel and .other large molecular weight petroleum fuels.  "   "

Nature. Volume, and Extent of Contamination

      SVE is an especially attractive option if the contamination is beneath a
building or surrounding the support structure of a building.  In this case,
excavation could unearth the existing support structure of the building or ex-
tensive demolition of the building would be required before excavating the
contaminated soil.

      The quantity of contaminants spilled is important to quantify through
investigations of site history and site operations and by conducting soil gas
and soil boring analyses.  If the total quantity of contaminants is low and


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there is no apparent exposure pathway or regulatory requirement to expedite
soil cleanup, it may be more cost effective to leave the contaminated soil in
place and allow natural transport and degradation processes to govern the fate
of contaminants.

      Another viable alternative for a point source of contamination may be
excavation with disposal and treatment, depending upon the type and
concentration of contaminants present.

Groundwater Depth

      SVE usually is more effective than other alternatives if groundwater
depth is greater than 20 feet and vadose zone contamination extends to the
groundwater table.  If groundwater is less than 5 to 10 feet,  then soil
washing or excavation without SVE could be more cost effective, depending on:

      •     Quantity to be excavated for disposal or treatment (smaller
            quantities would favor excavation);

      •     Whether state and federal applicable or relevant and appropriate
            requirements (ARARs) would allow excavation and disposal or
            treatment.

      In addition, SVE usually is a good alternative for sites where the
majority of contamination is in the vadose zone.  Sites where the majority of
contaminants are in the saturated zone must be evaluated to compare dewatering
and subsequent SVE with groundwater pump and treat in conjunction with SVE.
This evaluation should consider:

      •     Saturated zone characteristics

      •     Overall cleanup schedule

      •     Regulations governing discharge.

Characteristics of Contaminated Soil

      SVE is typically more applicable to cases where the contaminated
unsaturated zone is relatively permeable (hydraulic conductivities in excess
of 10"3  or  10"2 cm3/cm2  sec)  and  uniform.  Sands  and gravels are especially
amenable to SVE.   However, the technology has been used in less permeable
clayey or silty soils with some success.  Agrelot et al.,  1985 and Applegate
et al, 1988, have demonstrated removal of contaminants in soils with
conductivities ranging from 10"3 to  10"6.  This success could be due to the
presence of more permeable sand and gravel strata typically found in alluvial
settings or the relatively low moisture contents in the finer-grained soils
(Bennedsen, 1987).

      Michaels and Stinson  (1989) have found that porosity appears to be a
more important characteristic to consider when evaluating the applicability of
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SVE.  These conclusions are based on the results of the SITE program
demonstration test of Terra Vac's vacuum extraction system in Groveland,
Massachusetts.  Significant VOC removal rates were achieved in relatively
impermeable clays (hydraulic conductivities of 10"8 cm3/cm2-sec)  and more
permeable sands (hydraulic conductivities of 10"*  cm3/cm2-sec).   Both soil
strata had porosities between 40 and 50 percent.

      It is important to note that many other physical characteristics that
will influence vapor transport must be investigated at the site before a
system can be designed and constructed.  These characteristics include
stratigraphy, particle size distribution, moisture content, bulk density, and
particle density.  Such parameters are currently used by SVE vendors and other
design professionals in flow models, and empirically to provide additional
information on the airflow and vacuum requirements of the system.

Emission Controls

      More and more states are now requiring SVE offgas treatment to comply
with stringent ambient air quality criteria, rather than allowing direct
discharge to the atmosphere. Offgas treatment methods include:

      •     Vapor phase GAG with offsite or onsite regeneration or offsite
            disposal

      •     Condensation

      •     Incineration or flaring.

      The need for offgas treatment can be determined by investigating state
and federal air emissions requirements.  If offgas treatment is required, the
most cost-effective technology needs to be determined by evaluating the type,
concentration and quantity of contaminants; permit requirements; and
availability of required utilities.  If expensive and complex offgas treatment
systems are needed, the SVE is less favorable than other alternatives.  It is
critical that estimates of total costs for the SVE system include offgas
emission requirements, if any, to allow a valid comparison with other in situ
alternatives and with the excavation alternative.

Schedule for Cleanup

      Soil vapor extraction is neither the fastest nor the slowest method of
VOC removal from the vadose zone.  The Table B-2 lists a range of vadose zone
remedial alternatives and qualitative estimates of cost and cleanup rates.

      The selection of an alternative depends upon the relative magnitude of
the cost difference for the specific site, and the impact of immediate removal
(excavation) vs. slower removal rates.  Soil vapor extraction is more
applicable when site conditions and regulatory requirements can accommodate a
removal rate taking weeks or months at moderately contaminated sites, to
several months or years at heavily contaminated sites.
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                TABLE B-2.  VADOSE ZONE REMEDIAL ALTERNATIVES
   Alternatives                	Cost	           Cleanup Rate

Natural Attenuation            Minimal                 Slow
Soil Washing                   Low to Moderate         Slow to Moderate
SVE                            Low                     Moderate
Excavation                     High                    Fast
Vadose Zone Cleanup Standards

        The site's vadose zone cleanup standard performance objective  (PO)
has a significant bearing on the applicability of SVE.   The following text
describes two aspects of the vadose zone cleanup standard--concentration
limits and PO testing--that need to be considered.

Concentration limits--

        Soil vapor extraction is not as applicable if the concentration
limits are low compound-specific limits (eg.,  5 ug/kg tetrachloroethylene or
10 ug/kg trichloroethylene) to be achieved in a short duration of time.  The
performance of SVE at such low levels has not been widely demonstrated,
especially in nonhomogeneous soils.

        However,  the  site  is a good candidate  for SVE if  the concentration
limits to be achieved are high ug/kg limits (e.g.,  total VOCs greater than
500 ug/kg), or if the PO is vapor analysis from the extraction well system
(e.g., 750 ug/1 tetrachloroethylene).

PO Testing--

Soil vapor extraction is not as applicable if the PO is to be verified by a
statistical soil sampling grid with low compound-specific concentrations.
Less permeable hot spots could remain at higher concentrations for a
significant amount of time beyond the time required to remove the majority of
contaminants.  However, SVE would be a likely candidate if the PO is to be
measured by vapor stream analysis from the combined extracted vapor.

        This criterion cannot be overlooked when considering the
applicability of SVE at a site.   If the site is a good candidate after
considering all of the factors discussed above, one could implement the
technology and remove 95% or more of the contaminants present.   This removal
probably would be a significant environmental benefit,  but the responsible
party could be left with a liability if low compound-specific criteria are not
obtained.  This situation is highly probable given the low cleanup
concentration limits in some states, and is a predicament none of us want to
experience.
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SUMMARY

        Soil vapor extraction has been demonstrated as an effective
technology for the removal of VOCs from soils contaminated from leaking USTs
and surface spills.   There are many advantages to the use of SVE due to the in
situ nature of the technology,  the effective removal of VOCs,  and the ease of
implementation and operation.  However,  many criteria must be evaluated before
selecting SVE for a VOC-contaminated site.   Failure to evaluate these criteria
before selecting the technology could result in ineffective or noncompliant
cleanup goals.
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                                    REFERENCES

Agrelot, Jose C.,  James J.  Malot,  and Melvin J.  Visser.  Vacuum Defense System
  for Groundwater VOC Contamination.  Presented at the Fifth National Symposium
  on Aquifer Restoration and Ground Water Monitoring, Columbus,  Ohio,  May 21-24,
  1985

Applegate,  Joseph, John K.  Gentry,  and James J.  Malot.  Vacuum Extraction of
  Hydrocarbons from Subsurface Soils at a Gasoline Contamination Site.  1988.

Bennedsen,  M.B. et al.   Use of Vapor Extraction Systems for In Situ Removal of
  Volatile Organic Compounds from Soil.   1987.

Michaels, P.A. and Stinson, M.K.   Technology Evaluation Report.   SITE Program
  Demonstration Test Terra Vac In Situ Vacuum Extraction System Groveland,
  Massachusetts.  Contract No. 68-03-3255.  1989.
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                                  APPENDIX C
                   SOIL GAS SURVEYS IN SUPPORT OF DESIGN OF
                           VAPOR EXTRACTION SYSTEMS
                                H. B. Kerfoot3

ABSTRACT

      Soil-gas surveys have proven to be a useful tool in preliminary
delineation of subsurface contamination by volatile organic compounds.
Delineation of soil-gas concentrations is also frequently used to choose the
locations of vapor extraction systems for cleanup of vadose zone contami-
nation by volatile organic compounds.  The subsurface fate and transport
of these compounds can drastically affect static soil-gas concentrations of
them and must be considered in evaluation of soil-gas survey results.  In
addition, static soil-gas conditions can be different than those expected
during operation of a vapor extraction system.  However,  within these con-
straints, soil-gas surveys can provide valuable data in support of design and
installation of vapor extraction systems for vadose-zone  cleanup.  At a site
in California, an areal soil-gas survey was used to choose the locations of
vapor-extraction wells, and vertical borings with soil-gas depth profiles were
used to select the withdrawal depth and to install monitoring devices and the
actual vapor wells.

INTRODUCTION

      Soil-gas sampling and analysis is a technique that  has recently found
many applications in responses to soil and ground-water contamination.
Although soil-gas sampling techniques have been available since the turn of
the century, the technology has only recently been applied to subsurface
contamination.  However, due to the cost-effectiveness and rapidity of the
technology relative to traditional soil or ground-water sampling and analysis,
its use has grown rapidly.  Although the technology is limited to volatile
analytes, those are frequent problem compounds.  Table C-l lists the 25 most
frequently encountered ground-water contaminants at CERCLA (Superfund) sites
from a CERCLA Section 301 list.  It can be noted that the majority of them are
volatile and thus are amenable to application of this technology for detection
and delineation of contamination.  In addition to these compounds, gasoline
and jet fuels are quite volatile.  The high volatility of these compounds
makes the use of soil vapor extraction an attractive potential alternative to
soil removal for cleanup.  In fact, a soil-gas survey to  delineate the extent
of contamination is frequently an initial step in design  of soil vapor

aKerfoot and Associates,  3057  Hebard Drive,  Las Vegas,  NV 89121

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               Table C-l.   Most Frequently Reported Groundwater
                     Contaminants  at 546  Superfund Sites.
     Rank                         Compound                        Percent
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Trichloroethene(TCE)b
Lead
Toluene13
Benzeneb
Pol/chlorinated Biphenyls(PCBs)
Chloroform5
Tetrachloroethene(PCE)b
Phenol
Arsenic
Cadmium
Chromium
1,1, 1-Trichloroe thane"
Zinc
Ethylbenzene"
Xylene
Methylene Chloride
trans-l,2-Dichloroetheneb
Mercury
Copper
Soluble Cyanide Salts
Vinyl Chloride"
1 , 2-Dichloroe thaneb
Chlorobenzeneb
1 , l-Dichloroethaneb
Carbon Tetrachlorideb
33
30
28
26
22
20
16
15
15
15
15
14
14
13
13
12
11
10
9
3
8
8
8
8
7

"Source:   CERCLA 101 C Study
bCompounds amenable  to detection  by  soil-gas analysis
extraction system.   However,  both soil-gas surveys and soil vapor extraction
are also subject to influences from vadose-zone properties that can hamper
their effectiveness.  In this paper,  factors are discussed that can influence
soil-gas survey data and thus parameters used in the design of soil vapor
extraction systems, cleanup criteria,  and monitoring of the cleanup process.
Two case studies are presented;  one is intended to demonstrate the effect of
subsurface transformations on soil-gas surveys to delineate fuel
contamination, while the other presents a soil-gas survey that was used to
provide real-time data for installation of a soil vapor extraction system and
a monitoring system.  Although soil-gas surveys provide information about a
site that describes it without bulk gas flow, which is a fundamental aspect of
a soil-venting system, data from a soil-gas survey can be interpreted to
provide information to be used in design of an in situ soil-stripping system.


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BASIS OF THE TECHNOLOGY

      Soil-gas survey technology is based on transport of volatile compounds
from contaminated soil or water through the vadose zone to the atmosphere.
This mass-transport can be described by a three-step model.  The first step in
the mass transport is phase transfer from the condensed phase (soil or water)
to the gas phase.  Once in the gas phase the contaminant travels through the
vadose zone in response to a concentration gradient.  Once the contaminant
reaches the atmosphere it is rapidly carried away.  Under relatively static
subsurface conditions (e.g., in the absence of bulk gas flow), contaminant
transport through the vadose zone is by diffusion.  Only at shallow depths
(<0.3m) with uncovered, high-porosity soils does wind have an effect.

Subsurface Diffusion

      Diffusion of gases in soils has been studied since the early 1900's by
soil scientists in the study of oxygen, nitrogen, and carbon dioxide transport
and respiration.  In addition, diffusion in porous media is a subject of
considerable interest to chemical engineers in modeling the actions of
catalysts in pellet form.  Diffusion can be described by Pick's Law:

            dm/dt = -D(d2C/dz2)                                           (1)

where dm/dt is the mass flux through a unit area, D is the diffusion coeffi-
cient (cm2/sec),  C is concentration and z is distance.   Under steady-state
conditions;

          dC/dz - constant                                            (2)

indicating a linear concentration profile, increasing towards the vapor
source.

      Typically, the soil-gas diffusion coefficient, Ds,  is expressed as  the
diffusion coefficient for the compound in air, Da,  multiplied by a correction
for the fraction of the volume occupied by soil, water and solids and a factor
to correct for the tortuosity of the diffusion path.  Table C-2 lists several
published tortuosity relationships between Ds and D3.   It  can be noted that
all of the relations incorporate the air-filled porosity, Da, to at least the
first power.  Since the correction for the solid-filled volume also includes
the air-filled porosity, these relations point out the sensitivity of soil-gas
transport to that parameter.  The data in Table C-2 are presented only to
demonstrate the significant dependence of soil-gas transport on fa,  to point
out the fact that minimal soil-gas diffusion occurs at air-filled porosities
below 5 to 10 percent, and to indicate the inconsistent and often non-linear
relationship between Da and Ds.  The  first  two of  these points can be useful
in evaluating the applicability of soil-gas-survey technology at a given site,
and the last point provides insight into one cause of uncertainty in modelling
the risk to ground-water quality due to gas-phase contaminant migration from
soil to ground water.  Such risk assessments can be an important factor in
setting vadose-zone cleanup criteria.

      The steady-state approximation for soil-gas diffusion can be useful as a
limiting situation and agrees with observations in several instances.


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                     TABLE C-2.  TORTUOSITY RELATIONSHIPS
      Author
                                Ds/Da*
                             Comments
Blake and Paige

Van Bavel

Marshall

Millington

Wesseling

Grable and Siemer
0.62fa to  0.80fa

     0.61fa



  (fa/f)2 fa4'3

  0.9f. -  0.1
                                                   Ds }  0 when fa  <10%
                                                   Ds }  0 at fa _~ 11%

                                                   Ds/Da  ~~ °-02 when J
Lemon and Erickson
                                                   Ds/Da ~~ 0.005 when fa _- 5%
* fa = air-filled porosity;  f = total  porosity

However, shallower water tables, greater water infiltration, higher degrees of
saturation, and higher organic carbon content of soils may hamper the
achievement of steady state.

     Surface or subsurface barriers to soil-gas transport can drastically
affect soil-gas concentrations above and below them.  From the discussion
above, it can be seen that barriers to soil-gas transport are zones with air-
filled porosity below 5 to 10 percent.  Such zones would include saturated
clay lenses, perched water bodies, pavement, and buildings.  Standard mass-
transport considerations indicate that such barriers will elevate soil-gas
concentrations on one side (upstream in the mass flow) of them and will
depress them on the other.  Such barriers can also promote horizontal
diffusion of the contaminants.

SUBSURFACE TRANSFORMATIONS

     Organic contaminants can undergo a variety of subsurface transformations,
both microbiologically catalyzed and chemical.  Fuels are composed of reduced
organic compounds and present a potential source of energy for subsurface
microorganisms.  Through oxidation of these compounds in several steps by
several microorganisms, the organic carbon in the fuels becomes mineralized to
inorganic carbon or carbon dioxide.  Because of the high energy yield from
these oxidations, microorganisms can use energy not just for respiration but
for reproduction.  The resulting microbial "population explosion" can
obliterate hydrocarbons from soil gases when sufficient water, other
nutrients, and electron acceptors are present.  Workers at the U.S. EPA lab in
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Ada, Oklahoma, have observed subsurface biooxidation of creosote that was
limited only by the rate of oxygen diffusion from the atmosphere to the
creosote.  In zones of residual fuel saturation, such biodegradation can be
inhibited because of the lack of water and the toxicity of the fuels to the
microorganisms responsible for it.  However, when the supply of oxygen is
increased, such as would be the case in a soil-venting effort, microorganisms
in nearby areas with sufficient water can be stimulated to increased levels of
mineralization of these compounds over that which takes place under static gas
mass conditions.  Work to investigate this issue by the U.S. Navy Civil
Engineering Laboratory in Port Hueneme, California, is currently underway.

     The impact of the potential for subsurface biodegradation of fuel
hydrocarbons on soil-gas surveys is to create a significant potential for
false negative results, where soil-gas hydrocarbons are not detected above
ground-water contamination or near shallow soil contamination.  Figure C-l
shows the total hydrocarbon concentrations at a site with four 23-foot
diameter waste jet fuel tanks.  Ground water is encountered at 80 to 90 feet
and the regional ground-water flow direction is to the southeast, and three
monitoring wells were installed during the soil-gas survey in hopes of
obtaining a RCRA permit to close the site.  Table C-3 lists the analytical
results from those three wells.  It should be noted that one of the down-
gradient wells had 8 feet of jet fuel present, and Table C-3 shows analytical
results for the ground water and the fuel layer.


                    TABLE  C-3.  MONITORING  WELL DATA (mg/L).
     WELL #
                           ETHYLBENZENE XYLENES    BENZENE   TOLUENE

       11                  0.3          ND          ND            ND
       13                  ND           ND          ND            ND
       12                  6000        9200         440          1510
       12 (8' NAPL)         520        4000         820           900
     Because it was apparent that the contamination could well extend past the
extent indicated by the soil-gas hydrocarbon concentrations,  and
biodegradation could be creating false negative results,  a soil-gas survey
measuring soil-gas carbon dioxide concentrations was performed downgradient.
Figure C-2 shows the results of that survey.  These data indicate a
significant soil-gas carbon dioxide anomaly downgradient of the well with pure
product in it.   When a monitoring well was installed in that anomaly, a 4-foot
layer of fuel was encountered.

     These results are presented only to demonstrate the high degree of
variability in soil-gas hydrocarbon behavior encountered at the same site.
Near the source (leaky tanks),  floating fuel was adequately indicated by soil-


                                      175

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                               •Mfr
                             15 (»t
                Monitoring Hell
Figure C-l.  Total  Hydrocarbon Concentrations at  a  Site with
             24-foot Diameter Waste Jet  Fuel Tanks.
                             176

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                100 feet
      *  C02 Sampling Locations

     ;«8$J! 100 ng/cc Total Petroleum Hydrocarbons

        Figure C-2. Results of Soil-Gas Survey Measuring  Carbon Dioxide
                                Concentrations.
gas hydrocarbon concentrations while  some distance away false negative results
were obtained from consideration of soil-gas hydrocarbon concentrations as
indicators of underlying  contamination.   Because of the numerous factors  that
can influence these in  situ microbial processes and our lack of detailed
knowledge of them at any  site, it is  not possible to guess what may have  been
the factors responsible for this difference in behavior.  In fact, due to the
probable lack of knowledge of subsurface conditions at any site, prediction  of
biodegradation of hydrocarbons or conservative behavior is probably not
                                      177

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possible.  Therefore, soil-gas QA/QC for hydrocarbons should incorporate
components to deal with this potential situation so that it will not destroy
the usefulness of the data.

USE OF SOIL GAS DATA FOR VES PLANNING

     Soil-gas concentrations can be very useful in planning a VES.   Because
optimum performance is obtained when the vapor-withdrawal well of the VES
removes vapors from the center of mass of the contamination, the soil-gas
survey data can be used to estimate the optimum horizontal location for it.
At that point, vertical exploration can be undertaken to optimize the vertical
zone of withdrawal.  Because the goal of a VES system is to treat soil con-
tamination, and not underlying ground water, it is important to be able to
interpret the soil-gas data to differentiate between soil and ground-water
contamination as a vapor source.  This can be done through consideration of
both the magnitudes and horizontal gradients of soil-gas concentrations.  Near
soil contamination, horizontal concentration gradients will be very steep and
concentrations will be high, since they are from pure product and have not
been diluted by diffusion of the atmosphere to as large a degree as those
resulting from deeper contamination.

     At a major aerospace firm in southern California, a vapor degreaser had
leaked TCE and PCE into the vadose zone for an unknown period of time.
Because of contamination of nearby municipal supply wells, a leak-detection
program was implemented and the leak was noted.  Due to that, a soil-gas
survey at that part of the site was conducted.  The site is an industrial
facility that has been operated parties since the 1930s.  The vadose zone is
approximately 120 feet thick and is covered totally by pavement or buildings.

     The first part of the survey was a survey to estimate the areal extent of
contamination, in order to plan the soil-venting system to be installed there.
Figure C-3 shows a map of the site, and Figure C-4 shows the soil-gas PCE
concentrations measured at a depth of 4 feet.  Several features of Figure C-4
are noteworthy.  First, a second source area was indicated near Building 183
and a possible third source area can also be seen.  Building 183 is a
hazardous-waste storage shed with a gravel-filled drainage basin surrounding
it.  Surface  spills could be hosed from the inside floor out to drain through
these basins, since a 4-inch ventilation gap between the bottoms of the
building's walls and the floor exists there.  A second point that can be noted
is that there is considerable horizontal vapor migration evident from these
source areas, although they both exhibit the steep horizontal soil-gas
concentration gradients characteristic of shallow soil contamination.  A third
point that can be noted is the seemingly artificial division between the high-
concentration zone in the center of Building 175 and the one at the western
end.  This distinction was made on the basis of soil-gas data for other
compounds.  Figure C-5 shows the 1,1,1-trichloromethane (TCA) soil-gas data.
Because of the much lower concentrations of TCA encountered, the presence of
three distinct sources of VOC vapors can be clearly seen.   In addition, it can
be seen that  the TCA/TCE ratios are not the same among the  source areas. Based
on the results of  the above areal delineation of contamination, evaluation of
the vertical  extent of contamination at locations indicated  to be sources was
planned.  This effort had several objectives:  the primary  goal was to
estimate as closely as possible the vertical center of mass  of contamination


                                      178

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                                                         CJ
                                                         o
                                                         CQ
                                                         o

                                                         w
                                                         Q>


                                                         CO

                                                         ID
                                                         W
                                                         CO
                                                         JZ.
                                                         CL
Figure C-3.   Site Map of  Industrial Facility.
                      179

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'399

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    004
    pV.
                        Figure C-5.  TCA Soil-Gas Data.

and to obtain other data valuable for design of the soil-venting system.  A
secondary goal was to obtain actual soil samples,  in order to determine what
correlation (if any) exists between the soil-gas data and traditional soil-
analysis results.  A third objective was to evaluate "background" soil-gas and
soil concentration profiles.

     Borings were installed at the five locations  shown in Figure C-3.
Borings 1 and 2 were placed to investigate conditions at those two source
areas, while boring 5 was placed to investigate the other anomaly.  Due to
limited access to classified areas indoors, boring 5 had to be placed outside
of Building 175.  Because of the potential for a source inside Building 180,
boring number 3 was drilled just to the south of it.  Building 180 was also a
highly classified facility.  Boring number 4 was to evaluate "background" soil
and soil-gas concentrations at the site.

     The borings were made with a hollow-stem auger; each 10 feet a soil-gas
probe was attached to A-rod and hammered into the  vadose zone 3 feet past the
auger bit.  Soil-gas samples were withdrawn and analyzed until duplicates were
                                      181

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within 20 percent of each other.   After soil-gas samples were taken and
analyzed, soil samples were taken, the soil characterized,  and samples wrapped
and stored on ice.  Samples selected on the basis of the soil-gas data were
brought to a laboratory for analysis.   At all of the borings, 1/4-inch O.D.
tubes attached to a 1-inch perforated metal sampling manifold were installed
between two 1 1/2-foot bentonite  layers in a 5-foot layer of sand at the
bottom of the borehole for future monitoring of subsurface pressure andsoil-
gas concentrations and 20 feet above the zone of soil-gas removal.  At borings
1, 2, 3, and 5 PVC vapor wells were installed for future soil-venting work.

     Table C-4 lists the soil-gas and soil analytical data for boring 1.  It
can be very clearly seen that the maximum contamination, as indicated by soil-
gas concentrations, was at 70 feet.  Therefore, 1 1/2-foot bentonite layers
were put at depths of 81 and 55 feet with a capped 4-inch PVC pipe terminating
at 80 feet, and slotted for 20 feet.  As mentioned above, a 1/4-inch O.D.
metal tube led to sampling manifolds at 30 feet and 105 feet in 5-foot layers
of sand with 1 1/2-foot bentonite layers above and below.

                 TABLE  C-4.   BORING #1  SOIL AND SOIL-GAS  DATA.
                     Soil-Gas Concentrations           Soil Concentrations
         Soil Type  	(ng/cc)	         	(mg/kg)	
Depth(ft)  (USCS)    TCA       TCE       PCE         TCA        TCE       PCE
3
10
20
30
40
50

60
70
80
90
100
110
SW
sw
sw
sw
sw
sw

sw
SP
SP
SP
SP
ND
0.6
.30
2
18
40
N/R

<3
N/R
N/R
<0.6
2
<0.2
<0.03
<0.03
<3
<3
<0.3
N/R

<3
N/R
N/R
<0.6
<0.3
<0.3
67
15
380
790
930
1200

31000
108000
160
370
30
15
<0.01
N/A
N/"A '
<0.01
N/A
<0.01/

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      Boring number 2 was drilled to 100 feet and Table C-5 shows the soil-gas
and soil data.  Boring number 2 was completed similarly to boring number 1
with the placement of the monitoring devices and the 20-foot screened section
of PVC pipe based on the soil-gas results.

      Boring number 3 was augured to a depth of 70 feet and was completed
similarly to borings number 1 and number 2,  except that the screened interval
of PVC pipe was between 25 and 45 feet.   Soil-gas concentrations of TCA and
PCE were consistent with remote shallow soil contamination as the source of
these compounds.  Boring number 5 was drilled to 70 feet also and the slotted
interval of PVC pipe was at 20 feet to 40 feet.  Table C-6 shows the boring
number 3 soil-gas and soil data.

      It should be noted that, at boring number 5, soil-gas PCE concentrations
of nearly 100 ng/cc were measured at locations with non-detectable soil
concentration (below 10 g/kg), while soil-gas PCE concentrations of 5 ng/cc
was present at a depth with a soil concentration of 22 g/kg.   It is not clear
what the reason is for this inconsistent behavior, but it may be related to
PCE being partitioned more into the soil-sorbed phase in boring number 3 or to
it being trapped in pores where it is not available to contribute to the
observed soil-gas concentrations.  In addition, the possibility of

                 TABLE  C-5.   BORING  #2 SOIL AND SOIL-GAS DATA.
                     Soil-Gas Concentrations
          Soil Type  	(ng/cc)	
Depth(ft)  (USCS)    TCA       TCE       PCE
    40

    50

    60

    72

    80

    90
SW/SM

GP/SP

GP/SP

GP/SP

GP/SP

GP/SP
<2

<4

<4



<2

<0.4
14

33

28



<3

<0.8
 4200

11000

 5700



  180

    3
                                            Soil Concentrations
                                           	(mg/kg)	
                                          TCA
 N/A

<0.01

 N/A

<0.01

 N/A

<0.01
                                           TCE
 N/A

<0.01

 N/A

<0.01

 N/A

<0.01
                                           PCE
1
10
20
30
SP
SP
SP
SW/SM
0.04
<2
<4
<2
0.3
<3
<8
<3
10
2500
3900
1400
N/A
<0.01
N/A
<0.01
N/A
<0.01
N/A
<0.01
N/A
<0.01
N/A
0.01/
0.017
 N/A

<0.01

 N/A

<0.01

 N/A

<0.01
                                      183

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                TABLE C-6.  BORING #3 SOIL AND SOIL-GAS DATA.
                     Soil-Gas Concentrations
          Soil Type  	(ng/cc)
Depth(ft)  (USCS)    TCA       TCE
                                         PCE
                                          Soil Concentrations

                                         	(mg/kg)	
                                        TCA
                                          TCE
                                           PCE
     2

    10

    20

    30

    40

    50

    60

    70
SP

sw

sw

SM

SM

SW

SW

SW
2

5

7

2

5

0.04

0.2

0.07
<0.01

<0.01

<0.04

<0.04

<0.04

<0.04

<0.1

<0.04
                                         I

                                         2

                                         5

                                         3

                                         5

                                         0.4

                                         1

                                         0.6
 N/A

 N/A

0.011

 N/A

<0.01

<0.01

 N/A

<0.01
 N/A

 N/A

<0.01

 N/A

<0.01

<0.01

 N/A

<0.01
 N/A

 N/A

0.022

 N/A

<0.01

<0.01

 N/A

<0.01
experimental error must also be considered.   However,  from the data it can be
seen that this approach allowed for immediate placement  of the slotted
interval in the zone of highest soil-gas and soil contamination so as to
remove contaminant from the center of mass of the contamination.   The above
data do not describe the complete Initial Remedial Measures that were underway
or any interactions between the VES and the ground-water cleanup that was in
operation at the site.   However, these results do not show the utility of
real-time data in planning and installing a Vapor Extraction System.

CONCLUSIONS

      Soil-gas sampling and analysis is a potentially rapid and cost-effective
approach for delineation of the areal and/or vertical extent of subsurface
contamination by volatile organic compounds,  and can provide useful data for
planning in situ soil stripping systems.  Soil-gas data can be used to map
plumes, indicate sources, and evaluate subsurface gas-phase dynamics.
However, the relationship of soil-gas concentrations to pore-water
concentrations, soil-sorbed concentrations,  or the presence of residual
contaminant are not simple and are probably greatly influenced by subsurface
heterogeneity and kinetic factors.

      Static soil-gas concentrations can be affected by compound-specific
factors, such as biodegradability, as well as site-specific ones, such as
barriers to vapor transport.  For these reasons, it is important to obtain
data describing the relationship between soil-gas and other concentrations on
a site-specific basis.   In addition, static soil-gas conditions may be quite
                                      184

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factors, such as biodegradability, as well as site-specific ones,  such as
barriers to vapor transport.  For these reasons,  it is important to obtain
data describing the relationship between soil-gas and other concentrations on
a site-specific basis.  In addition,  static soil-gas conditions may be quite
different than those that will exist during a soil vapor extraction effort.

      The typical soil-gas survey for placement of vapor extraction systems
involves an areal survey with vertical profiling at selected locations.  If
this vertical profiling is deep enough, the real-time data obtained can be
used to select the depths from which vapors will be withdrawn and hardware for
that purpose can be left in the hole.  Because of the potential for signifi-
cant variability in the relationships between soil-gas and soil concentrations
within a single site, simplistic use of soil-gas concentrations to estimate
the total mass of contaminant present in the vadose zone should be avoided in
favor of a carefully planned program to obtain the data needed to make an
estimate with a known degree of uncertainty.

      Further work is required to more fully understand this technology so
that it can be effectively applied.   In particular,  the mechanism of cleanup
(stripping vs. mineralization) for hydrocarbons should be investigated to
evaluate the potential for use of the technology for low-volatility
hydrocarbons and the effects of temperature should be evaluated.   In addition,
methods to set cleanup criteria for these types of cleanup should be developed
om the basis of the mechanism of a VES cleanup, so that the efficiency of the
system as gauged by process-control monitoring data can be used,  rather than
calculations from unvalidated models.  Further progress in the design of VES
systems is required to deal with the problem of slow water-phase mass
transport of contaminants in the capillary fringe and may be dealt with by
pulsed sparging of underlying ground water.
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                                  APPENDIX D

               IN SITU BIODEGRADATION OF PETROLEUM DISTILLANTS
                              IN THE VADOSE ZONE

          Robert  E.  Hincheea,  Douglas C. Downeyb, and Ross N. Miller0


      Thousands of underground storage  tank sites are contaminated with
petroleum hydrocarbons.   Most of these  hydrocarbons  are biodegradable if
naturally occuring microorganisms are provided an adequate supply of oxygen
and basic nutrients.  Conventional methods of enhancing natural biodegradation
use water to carry oxygen or an alternative electron acceptor to the
contamination.   However,  this method requires substantial amounts of water,
and is frequently unsuccessful in unsaturated soil because of poor oxygen
distribution.

      Using air as the oxygen source is a potentially cost-effective way to
increase the microbial degradation of fuel hydrocarbons in unsaturated soil.
Biodegradation of hydrocarbons in unsaturated zones  through forced aeration
has been observed at several field sites.   Air must  flow through hydrocarbon-
contaminated soil at rates and configurations that ensure adequate oxygenation
and minimize hydrocarbon-contaminated offgases.   Adding nutrients and moisture
may be desirable to increase biodegradation rates.

INTRODUCTION

      As a result of regulations requiring investigation of underground
storage tanks,  literally thousands of sites have been identified as
contaminated with petroleum hydrocarbons.   To date,  much attention has been
given to pump-and-treat remedial technologies, but this technique leaves a
substantial fuel residue in the capillary fringe or  vadose zone.  Methods of
uniformly destroying fuel hydrocarbons  in situ,  without excessive groundwater
pumping or toxic releases to the atmosphere, need to be developed.  This paper
focuses on one such emerging technology.

      Petroleum distillate fuel hydrocarbons are generally biodegradable if
naturally occuring microorganisms are provided an adequate supply of oxygen
and basic nutrients.  Although natural  biodegradation does occur, and at many
sites will eventually minimize most fuel contamination, the process is
frequently too slow to prevent the spread of contamination.  Such sites
require rapid removal of the contaminant source and groundwater treatment to
a Battelle Memorial Institute,  505 King Avenue,  Columbus,  Ohio 43201
b HQ, AFESC/RDVW,  Tyndall Air Force Base,  Florida 32403
c 649 N. 200 E., Kaysville,  Utah 84037


                                      186

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protect sensitive aquifers.  At these sites,  an acceleration or enhancement of
the natural biodegradation process is desired.

      Important in any in situ remediation is an understanding of the
distribution of contaminants.  Most of the residue of hydrocarbons at a fuel
contaminated site is found in the unsaturated zone soils in the capillary
fringe and immediately below the water table.  Typically,  seasonal water table
fluctuations spread residues in the area immediately above and below the water
table.  Any successful bioremediation effort  must treat these areas.

CONVENTIONAL ENHANCED BIODEGRADATION

      Over the past two decades the practice  of enhanced biodegradation has
increased, particularly for treating soluble  fuel components in groundwater
(Lee et al., 1988).  Until recently, less emphasis was given to enhancing
biodegradation in the unsaturated zone.   The  current conventional enhanced
bioreclamation process uses water to carry the oxygen or an alternative
electron acceptor to the contamination,  whether it occurs in the groundwater
or unsaturated zone.

      A recent field experiment at a jet fuel-contaminated site using
infiltration galleries and spray irrigation to introduce oxygen, nitrogen and
phosphorous to unsaturated, sandy soils  was unsuccessful due to rapid H202
decomposition and resulting poor oxygen distribution (Hinchee et al., 1989a).
A study being conducted by the U.S. Environmental Protection Agency and the
U.S. Coast Guard at Traverse City, Michigan,  uses deep well injection to raise
the water table in order to supply oxygen-enriched water to the contaminated
soils.  Pure oxygen and hydrogen peroxide have been used as oxygen sources,
and recently nitrate has been added as an alternative to oxygen.  Preliminary
results indicate better hydrogen peroxide stability than achieved by Hinchee
et al. (1989a).  Some degradation of the aromatic hydrocarbon appears*to have
occured;   however, total hydrocarbon contamination levels appear unaffected
(Ward, 1988).

      In most cases where water is used as the oxygen carrier, oxygen is the
limiting factor for biodegradation.  If pure  oxygen is utilized and 40 mg/1 of
dissolved oxygen is achieved, approximately 25,000 Ibs of water must be
delivered to the formation to degrade a single pound of hydrocarbon.  If 500
mg/1 of hydrogen peroxide is successfully delivered, then 4,000 Ibs of water
are necessary.  As a result, even if hydrogen peroxide can be successfully
used, substantial volumes of water must be pumped through the contaminated
formation to deliver sufficient oxygen.

ENHANCED BIODEGRADATION THROUGH SOIL VENTING

      A system engineered to increase the microbial degradation fo fuel
hydrocarbons in the vadose zone using forced air as the oxygen source is a
potentially cost-effective alternative to conventional systems.  This process
stimulates soil-indigenous microorganisms to  aerobically metabolize fuel
hydrocarbons in unsaturated soils.  Depending upon air flow rates, volatile
compounds may be simultaneously removed from contaminated soils.
                                      187

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      By using air as an oxygen source,  the minimum ratio of air pumped to
hydrocarbon degraded is approximately 13 Ib to 1 Ib.   This compares to well
over 1000 Ib of water per Ib of hydrocarbon for a conventional waterborne
enhanced bioreclamation process.  An additional advantage of using an airborne
process is that gases have greater diffusivity than liquids.  At many sites,
geological heterogeneities present an added problem with a waterborne oxygen
source, because fluid pumped through the formation is channeled into the more
permeable pathways.  For example,  in an alluvial soil with interbedded sand
and clay, virtually all of the fluid flow will take place in the sand.  As a
result, oxygen must be delivered to the less permeable clay lenses through
diffusion.  In a gaseous system, this diffusion can be expected to take place
at a rate several orders of magnitude greater than in a liquid system.  While
it is probably not realistic to expect diffusion to aid in water-based
bioreclamation, in an air-based application, diffusion may be a significant
mechanism for oxygen delivery.

      The first documented evidence of unsaturated zone biodegradation
resulting from forced aeration was reported by the Texas Research Institute,
Inc., in a study for the American Petroleum Institute.  A large-scale model
experiment was conducted to test the effectiveness of a surfactant treatment
to enhance  recovery of spilled gasoline.  The experiment accounted for only 8
of the 65 gallons originally spilled and raised questions about the fate of
the gasoline.  A subsequent column study was conducted to determine a
diffusion coefficient for soil venting.   This column study evolved into a
biodegradation study in which it was concluded that as much as 38% of the fuel
hydrocarbon was biologically mineralized.  Researchers concluded that venting
would not only remove gasoline by physical means, but also could enhance
microbial activity (Texas Research Institute, 1980; Texas Research Institute,
1984).

      Wilson and Ward (1986) suggest that using air as a carrier for oxygen
could be 1000 times more efficient than transferring it to water,  especially
in deep, hard-to-flood unsaturated zones.  They made the connection between
soil venting and biodegradation by observing that, "soil venting uses the same
principle to remove volatile components of the hydrocarbon."  In a general
overview of the soil venting process,  Bennedsen et al. (1987) concluded that
soil venting provides large quantities of oxygen to the vadose zone, possibly
stimulating aerobic degradation.  He states that water and nutrients would
also be required for significant degradation and encourages additional
investigation into this area.

      Biodegradation enhanced by soil venting has been observed at several
field sites.  Investigators at a soil venting site for remediation of gasoline
contaminated soil claim significant biodegradation as measured by a
temperature rise that they attributed to biodegradation.  They claim that the
pile was cleaned up during the summer primarily by biodegradation (Conner,
1988).  However, they did not control for natural volatilization from the
above ground pile, and not enough data was provided to critically review the
biodegradation claim.

      Researchers at Traverse City, Michigan, measured toluene concentration
in vadose zone soil gas as an indicator of fuel contamination in the vadose
zone.  They assumed absence of advection and attributed toluene loss to


                                      188

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biodegradation.  The investigators conclude that,  because toluene
concentrations decayed near the oxygenated ground surface, soil venting is an
attractive remediation alternative for biodegrading light volatile hydrocarbon
spills (Ostendorf andKambell, 1989).

      Hinchee et al. (1989b) documented biodegradation of JP-4 at a
conventional soil venting site.  Biodegradation accounted for at least 15% of
the hydrocarbon removal.  Their study compared 02  and C02  invented gases from
the contaminated area to that from an uncontaminated area.  Additionally,  they
measured the stable carbon isotope makeup of the C02 carbon to confirm the JP-
4 as its source.

      Ely and Heffner (1988) of the Chevron Research Company patented a
process for the in situ biodegradation of spilled hydrocarbons using soil
venting.  Experimental design and data are not provided,  but findings are
presented graphically.  At a gasoline and diesel oil contaminated site,
results indicated that about 2/3 of the hydrocarbon removal was by
volatilization and 1/3 by biodegradation.  At a site containing only fuel
oils, approximately 20 gal/well/day were biodegraded, while vapor pressures
were too low for removal by volatilization.  Ely and Heffner (1988)  claim that
the process is more advantageous than strict soil venting because removal is
not dependent only on vapor pressure.   In the examples stated in the patent,
C02 was maintained between 6.8% and 11% and 02 between 2.3% and  11%  in  vented
air.  The patent suggests that the addition of water and nutrients may not be
acceptable because of flushing to the water table,  but nutrient addition is
claimed as part of the patent.  The patent recommends flow rates between 30
and 250 cfm per well and states that air flows higher than those required for
volatilization may be optimum for biodegradation.

APPLICATIONS

      The use of an air-based oxygen supply for enhancing biodegradation
relies upon air flow through hydrocarbon contaminated soils at rates and
configurations that will ensure adequate oxygenation of aerobic biodegradation
and minimize or eliminate the production of a hydrocarbon contaminated offgas.
The addition of nutrients and moisture may be desirable to increase
biodegradation rates.  Figures 1 and 2 illustrate possible air
injection/withdrawal configurations.  Dewatering is illustrated in the
figures, but this may not always be necessary depending upon the distribution
of contaminants relative to the water table.  However,  it is required at many
fuel hydrocarbon contaminated sites.  A key feature not illustrated is the
narrowly screened soil gas monitoring points that sample only a short vertical
section of the soil.  These points are required to determine local oxygen
concentrations.  Measurements of oxygen levels in the vent are not
representative of local conditions.  Nutrient and moisture addiction typically
may take any of a variety of configurations.

      The configuration in Figure D-l is more like a conventional soil venting
installation where air is drawn from a vent well in the area of greatest
contamination.  The advantage of this configuration is that it generally
requires the least air pumping;  the disadvantage is that hydrocarbon offgas
concentration is probably maximized and all of the capillary fringe
contamination may not be treated.  Figure D-2 illustrates a configuration in


                                      189

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061

-------
     Nutrient Application
Rgure 2.  Potential Configuration for Enhanced Bioreclamation Through Soil Venting
         (Air Withdrawn From Clean Soil)

-------
which air is injected into the contaminated zoneand withdrawn from clean
soils.  This configuration allows the more volatile hydrocarbons to degrade
prior to being withdrawn and thereby eliminates contaminated offgases.   The
optimal configuration for any given site will,  of course,  depend upon site-
specific conditions and remedial objectives.

      The significant features of this technology include:

      •     Optimizing air flow to reduce volatilization while maintaining
            aerobic conditions for biodegradation.

      •     Monitoring local soil gas conditions to assure aerobic conditions,
            not just monitoring vent gas composition.

      •     Adding moisture and nutrients as  required for air/contaminant
            contact.

      The U.S. Air Force Engineering and Service Laboratory presently is
sponsoring a field-scale pilot test of the enhanced biodegradation technology.
The site is JP-4 contaminated and air is being introduced and withdrawn in
configurations similar to those shown in Figures 1 and 2.   The effect of
varying nutrients and soil moisture levels is being evaluated.  The use of
conventional soil venting to enhance biodegradation is being practiced on a
limited basis at a few other sites; however,  at most soil venting sites, the
biodegradation effects are ignored and undocumented.

RECOMMENDATIONS

      Soil venting alone, with no nutrient or moisture addition, typically
results in some stimulation of in situ biodegradation.  The following
recommendations are made for documenting biodegradation when conducting
conventional soil venting of fuel hydrocarbon contaminated soils :

      1.    Prior to venting, determine soil gas hydrocarbon, C02,  and  02
            profiles.

      2.    Determine the end point for venting.  A mixed hydrocarbon fuel
            such as JP-4 has a fraction too heavy to volatilize, and it is
            possible that biodegradation may continue after the light end has
            volatilized.

      3.    Develop an estimate of noncontaminant respiration.  This may be
            accomplished either through background measurements of C02  and 02
            in an uncontaminated location or by means of carbon isotopic
            analysis.


      To further pursue the development of an enhanced in situ soil-venting
technology, the following studies are recommended:

      1.    Fuel biodegradation in unsaturated soils to develop a better
            understanding of variables such as oxygen content, nutrient
            requirements, soil moisture, contaminant levels  (both high end for


                                      192

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      possible toxic effects and low end for treatment limits),  and soil
      types

2.     Gas transport in the vadose zone to allow adequate design of air
      delivery system

3.     Nutrient and moisture delivery systems,  including possible gaseous
      nutrient injection (i.e.,  NH3) ;  means  of engineering moisture
      addition in deeper stratified formations; and nutrient
      formulations to allow adequate nutrient mobility in pore water
                               193

-------
                                  REFERENCES

Bennedsen, M.B., Scott, J.P.,  and Hartley,  J.D.  1987.  Use of Vapor Extraction
      Systems for In Situ Removal of Volatile Organic  Compounds from Soil.
      Proceedings of National  Conference on Hazardous  Wastes and Hazardous
      Materials, Washington,  D.C. pp.  92-95.

Conner,  R.J. 1988. Case Study  of Soil Venting.  Poll.  Eng. 7:74-78.

Ely, D.L. and Heffner, D.A.  1988. Process for In Situ  Biodegradation of
      Hydrocarbon Contaminated Soil.   U.S.  Patent Number 4,765,902.

Hinchee, R.E.,  Downey, D.C.,  Dupont,  R.R.,  and Arthur,  M.F.  1989a.  Enhanced
      Biodegradation through Soil Venting.  Unpublished Report Submitted to  the
      U.S. Air Force Engineering and Services Laboratory.

Hinchee, R.E.,  Downey, D.C.,  Slaughter,  J.K., and Westray,  M. 1989b.Enhanced
      Biorestoration of Jet Fuels; A Full Scale Test  at Elgin Air Force Base,
      Florida.   Air Force Engineering and Services Center Report ESL/TR/88-78.

Lee, M.D., et al.  1988. Biorestoration of Aquifers Contaminated with Organic
      Compounds. CRC Critical  Reviews in Env. Control,  Vol.  18 pp. 29-89.

Ostendorf, D.W. and Kambell,  D.H. 1989.  Vertical Profiles and Near Surface
      Traps for Field Measurement of Volatile Pollution in the Subsurface
      Environment. Proceedings of NWWA Conference on New Techniques for
      Quantifying the Physical and Chemical Properties of Heterogeneous
      Aquifers, Dallas, TX.

Texas Research Institute. 1980. Laboratory Scale Gasoline Spill and Venting
      Experiment.  American Petroleum Institute,  Interim Report No. 7743-5:JST.

Texas Research Institute. 1984. Forced Venting to Remove Gasoline Vapor from a
      Large-scale Model Aquifer. American Petroleum Institute.  Final Report
      No. 82101-F:TAV.

Ward, C.H. 1988. A Quantitative Demonstration of the  Raymond Process for In
      Situ Biorestoration of Contaminated Aquifers.  Proceedings of NWWA/API
      Conference on Petroleum Hydrocarbons and Organic Chemicals in
      Groundwater, Houston,  TX pp 723-746.

Wilson,  J.T., and Ward, C.H.  1986. Opportunities for  Bioremediation of
      Aquifers Contaminated with Petroleum Hydrocarbons. J.  Ind. Microbiol.,
      27:109-116.
                                      194

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                                  APPENDIX E
        A PRACTICAL APPROACH TO THE DESIGN, OPERATION, AND MONITORING
                        OF  IN-SITU  SOIL VENTING SYSTEMS
              P.  C. Johnson*,  M. W. Kemblowskia,  J.  D.  Coltharta,
                       D.  L.   Byersa,  and  C.  C.  Stanleyb
INTRODUCTION
      When operated properly, in-situ soil venting or vapor extraction can be
one of the more cost-effective remediation processes for soils contaminated
with gasoline, solvents, or other relatively volatile compounds.  A "basic"
system, such as that shown in Figure E-l, couples vapor extraction (recovery)
wells with blowers or vacuum pumps to remove vapors from the vadose zone and
thereby reduce residual levels of soil contaminants.  More complex systems
incorporate trenches, air injection wells, passive wells,  and surface seals.
Above-ground treatment systems condense, adsorb, or incinerate vapors; in some
cases vapors are simply emitted to the atmosphere through diffuser stacks.
In-situ soil venting is an especially attractive treatment option because the
soil is treated in place, sophisticated equipment is not required, and the
cost is typically lower than other options.

      The basic phenomena governing the performance of soil venting systems
are easily understood.  By applying a vacuum and removing vapors from
extraction wells, vapor flow through the unsaturated soil zone is induced.
Contaminants volatilize from the soil matrix and are swept by the carrier gas
flow (primarily air) to the extraction wells or trenches.   Many complex
processes occur on the microscale, however, the three main factors that
control the performance of a venting operation are the chemical composition of
the contaminant, vapor flowrates through the unsaturated zone, and the
flowpath of carrier vapors relative to the location of the contaminants.

      The components of soil venting systems are typically off-the-shelf
items,  and the installation of wells and trenches can be done by most
reputable environmental firms.  However, the design, operation, and monitoring
of soil venting systems is not trivial.  In fact, choosing whether or not
venting should be applied at a given site is  a difficult question in itself.
If one decides to utilize venting, design questions involving the number of
3Shell Development/bShell Oil Company, Westhollow Research Center
 Houston, TX 77152-1380

                                      195

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                                          Vapor Treatment
                                                Unit
                             Vacuum
                              Pump
                        [ni    W^
Vapor
 Flow
             41UUI-
                                Vapor Extraction Well
Contaminated
     Soil
Vapor
Flow
                                                      Free-Liquid
                                                      Hydrocarbon
     Groundwater Table
                                      Soluble
                                       Plume
               Figure E-l.   "Basic"  In-Situ Soil Venting System

wells,  well spacing,  well location,  well construction,  and vapor treatment
systems must then be answered.   It is the current state-of-the-art that such
questions are answered more by instinct than by rigorous logic.   This is
evidenced by the published soil venting "success stories" (see Hutzler et al.1
for a good review),  which rarely include insight into the design process.

      In this paper we suggest a series of steps and questions that must be
followed and answered in order to a) decide if venting is appropriate at a
given site, and b) to design cost-effective in-situ soil venting systems.
This series of steps and questions forms a "decision tree" process that could
be easily incorporated in a PC-based expert system.   In the development of
this approach we will attempt to identify the limitations of in-situ soil
venting, and subjects or behavior that are difficult to quantify and for which
future study is needed.
                                      196

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THE "PRACTICAL APPROACH"

      Figure E-2 presents a flowchart of the process discussed in this paper.
Each step of the flowchart is discussed below in detail, and where
appropriate, examples are given.

The Site Investigation

      Whenever a soil contamination problem is detected or suspected, a site
investigation is conducted to characterize and delineate the zone of soil and
groundwater contamination.  Often the sequence of steps after initial response
and abatement is as follows:

      (a) background review: Involves assembling historical records, plot
plans, engineering drawings (showing utility lines), and interviewing site
personnel. This information is used to help identify the contaminant, probable
source of release, zone of contamination, and potentially impacted areas
(neighbors, drinking water supplies, etc.).

      (b)  preliminary site screening: Preliminary screening tools such as
soil-gas surveys and cone penetrometers are used to roughly define the zone of
contamination and the site geology.  Knowledge of site geology is essential to
determine probable migration of contaminants through the unsaturated zone.

      (c) detailed site characterization: Soil borings are drilled and
monitoring wells are installed based on the results from steps (a) and (b).

      (d) contaminant characterization: soil and groundwater samples are
analyzed to determine contaminant concentrations and compositions.

      Costs associated with site investigations can be relatively high
depending on the complexity of the site and size of the spill or leak.  For
large spills and complex site geological/hydrogeological conditions, site
investigation costs are often comparable to remediation costs.  In addition,
the choice and design of a remediation system is based on the data obtained
during the site investigation.  For these reasons it is important to insure
that specific information is collected, and to validate the quality of the
data.

      If it is presumed that in-situ soil venting will be a candidate for
treatment, then the following information needs to be obtained during the
preliminary site investigation:

      (a) site geology - this includes soil type and subsurface stratigraphy.
While they are not essential, the moisture content, total organic carbon, and
permeability of each distinct soil layer also provides useful information that
can be used to choose and design a remediation system.

      (b) site hydrogeology - the water table depth and gradient must be
known, as well as estimates of the aquifer permeability.

      (c) contaminant composition, distribution and residual levels - soil
samples should be analyzed to determine which contaminants are present at what


                                      197

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    Process
      or Spill Discovered
           Site
       Investigation
    Screen Treatment
       Alternatives
No
  Air Permeability Test
 Groundwater Pump Test
 No
        Yes
      System Design
     System Operation
                            Yes
      Output
Site Characteristics:

• soil stratigraphy
• characteristics of distinct soil layers
  (permeability estimates)
• depth to groundwater & gradient
• aquifer permeability (estimate)
• residual levels of contaminants
• distribution of contaminant
• composition of contaminant
• soil & above-ground temperature
• soil vapor concentrations (optional)
                                        • removal rate estimates
                                        1 vapor flowrate estimates
                                        • final residual levels & composition
 air permeability of distinct soil layers
1 radius of influence of vapor wells
• initial vapor concentrations
• aquifer properties (gradient,
 transmissivity, storativity)
• number of vapor extraction wells
 vapor well construction
1 vapor well spacing
• instrumentation
1 vapor treatment system
' flowrate (vacuum) specifications
• groundwater pumping system specifics
                                         • venting recovery rates
                                         • changes in vadose zone contamination
                                               "clean" site
                      (target levels based on
                      exposure assessment)
                                  System Shut-Off
    Figure E-2.   In-Situ  Soil Venting System Design Process,
                                  198

-------
levels. Recommended analytical methods should be used to identify target
compounds (i.e., benzene, toluene, or xylenes) and total hydrocarbons present.
For soil analyses these methods are:

            EPA 8240 - volatile organic chemicals
            EPA 8270 - semi-volatile organic chemicals
            EPA 418.1 - total petroleum hydrocarbons

 The corresponding water analyses methods are:

            EPA 624 - volatile organic chemicals
            EPA 625 - semi-volatile organic chemicals
            EPA 418.1 - total petroleum hydrocarbons

      With the current high cost of chemical analyses it is important to
intelligently select which analyses should be performed and which samples
should be sent to a certified laboratory.  Local regulations usually require
that a minimum number of soil borings be performed,  and target compounds must
be analyzed for based on the suspected composition of the contamination.
Costs can be minimized and more data obtained by utilizing field screening
tools, such as hand-held vapor meters or portable field GC's.   These
instruments can be used to measure both residual soil contamination levels and
headspace vapors above contaminated soils.  At a minimum, soil samples
corresponding to lithology changes or obvious changes in residual levels
(based on visual observations or odor) should be analyzed.

      For complex contamination mixtures, such as gasoline, diesel fuel, and
solvent mixtures, it is not practical or necessary to identify and quantify
each compound present.  In such cases it is recommended that a "boiling point"
distribution be measured for a representative sample of the residual
contamination. Boiling point distribution curves, such as shown in Figure E-3
for "fresh" and "weathered" gasoline samples, can be constructed from GC
analyses of the soil residual contamination (or free-product)  and knowledge of
the GC elution behavior of a known series of compounds (such as straight-chain
alkanes).   Compounds generally elute from a GC packed column in the order of
increasing boiling point, so a boiling point distribution curve is constructed
by grouping all unknowns that elute between two known peaks (i.e. between
n-hexane and n-heptane).   Then they are assigned an average boiling point,
molecular weight, and vapor pressure.  Use of this data will be explained
below.

      (d)  temperature - both above- and below-ground surface.

            The cone penetrometer, which is essentially an instrumented steel
      rod that is driven into the soil, is becoming a popular tool for
      preliminary site screening investigations.  By measuring the shear and
      normal forces on the leading end of the rod, soil structure, and hence
      permeability can be defined.  Some cone penetrometers are also
      constructed to allow the collection of vapor or groundwater samples.
      This tool has several advantages over conventional soil boring
      techniques (as a preliminary site characterization tool): the subsurface
      soil structure can be defined better, no soil cuttings are generated,
      and more analyses can be performed per day.


                                      199

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                                 o
                                 c.
                                       a
                                       x
                                       o
o
i
Q.
u
                                                 o
a
c
a
o
o
-a
u
c
crj
O
O
-a
c
o
c
ra
o
u
T3
0
                  1.0-

    Cumulative
      Weight     0.8-
     Fraction

                  0.6-
                 0.4
                 0.2
                 0.0
                                               Weathered  Gasoline
                          "Fresh" Gasoline
                                    X
                           0    40     80    120   160    200   240
        Figure  E-3.   Boiling  Point Distribution Curves  for  Samples  of
                      "Fresh"  and "Weathered" Gasolines.

      Results from the preliminary site investigation should be summarized in
contour plots, fence diagrams, and tables prior to analyses.

Deciding if Venting is Appropriate

      As stated above, the three main factors governing the behavior of any
in-situ soil venting operation are the vapor flow rate,  contaminant vapor
concentrations,  and the vapor  flowpath relative to the contaminant location.
In an article by Johnson et al.2 simple mathematical  equations were  presented
to help quantify each of these factors. Below we illustrate how to utilize
these "screening models" and the information collected during the preliminary
site investigation to help determine if in-situ soil venting is appropriate at
a given site.  In making this  decision we will answer the following questions:

      (1)  What contaminant vapor concentrations are likely to be obtained?

      (2)  Under ideal vapor flow conditions (i.e.  100 -  1000 scfm vapor
          flowrates), is this  concentration great enough to yield acceptable
          removal rates?
                                      200

-------
      (3) What range of vapor flowrates can realistically be achieved?

      (4) Will the contaminant concentrations and realistic vapor flowrates
           produce acceptable removal rates?

      (5)  What are the vapor composition and concentration changes?  What
           residual, if any, will be left in the soil?

      (6)  Are there likely to be any negative effects of soil venting?

Negative answers to questions (2), (3), or (4) will rule out in-situ soil
venting as a practical treatment. method.

(1) - What contaminant vapor concentrations are likely to be obtained?

      Question (1) can be answered based on the results of soil vapor surveys,
analyses of headspace vapors above contaminated soil samples, or equilibrium
vapor models2.   In some cases just knowing which compounds  are present is
sufficient to estimate if venting is feasible.  In the absence of soil -vapor
survey data, contaminant vapor concentrations can be estimated.  The maximum
vapor concentration of any compound (mixture) in extracted vapors is its
equilibrium or "saturated" vapor concentration, which is easily calculated
from knowledge of the compound's (mixture's) molecular weight, vapor pressure
at the soil temperature, residual soil contaminant composition, and the ideal
gas law:
                        Cest = L  -                    (E-l)
                               L      RT

where :
      Cest   - estimate of contaminant vapor concentration [mg/1]
      Xj^    = mole fraction of component i in liquid-phase residual
               (xt - 1 for single compound)
      Pjv    = pure component vapor pressure at temperature T [atm]
      M^   - molecular weight of component i [mg/mole]
      R '    = gas constant - 0.0821 l-atm/mole-°K
      T     — absolute temperature of residual [°K]

      Table E-l presents data for some chemicals and mixtures often spilled in
the environment.   There are more sophisticated equations for predicting vapor
concentrations in soil systems based on equilibrium partitioning arguments,
but these require more detailed information (organic carbon content, soil
moisture) than is normally available.  If a site is chosen for remediation,
the residual total hydrocarbons in soil typically exceed 500 mg/kg.  In this
residual concentration range the majority of hydrocarbons will be present as a
separate or "free" phase, the contaminant vapor concentrations become
independent of residual concentration (but still depend on composition) ,  and
Equation E-l is applicable2.   In  any case,  it should be noted that these  are
estimates only for vapor concentrations at the start of venting, which is
when the removal rates are generally greatest.  Contaminant concentrations in
the extracted vapors will decline with time due to changes in composition,
residual levels,  or increased diffusional resistances.  These topics are


                                      201

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        Table E-l.  Selected Compounds and Their Chemical Properties.


Compound Mw
fa/mole)
n-pentane
n-hexane
trichloroe thane
benzene
cyclohexane
trichloroe thylene
n-heptane
toluene
tetrachloroethvlene
n-octane
chlorobenzene
p-xylene
ethylbenzene
m-xylene
o-xylene
styrene
n-nonane
n-propylbenzene
1,2,4 trimethylbenzene
n-decane
DBCP
n-undecane
n-dodecane
napthalene
tetraethyllead
gasoline1
weathered gasoline^
72.2
86.2
133.4
78.1
84.2
131.5
100.2
92.1
166
114.2
113
106.2
106.2
106.2
106.2
104.1
128.3
120.2
120.2
142.3
263
156.3
170.3
128.2
323
95
111
Tb (1 atm) Pv° (20°C)
f°O (atm)
36
69
75
80
81
87
98
111
121
126
132
138
138
139
144
145
151
159
169
173
196
196
216
218
dec. @200C
-.
"
0.57
0.16
0.132
0.10
0.10
0.026
0.046
0.029
0.018
0.014
0.012
0.0086
0.0092
0.0080
0.0066
0.0066
0.0042
0.0033
0.0019
0.0013
0.0011
0.0006
0.00015
0.00014
0.0002
0.34
0.049
Csat
fms/1)
1700
560
720
320
340
140
190
110
130
65
55
37
40
35
29
28
22.0
16
9.3
7.6
11
3.8
1.1
0.73
2.6
1300
220

1  Corresponds  to  "fresh"  gasoline  defined  in  Table  E-2 with boiling point
  distribution shown in Figure E-3.
2  Corresponds  to  "weathered"  gasoline  defined in  Table E-2 with boiling point
  distribution shown in Figure E-3.
                                     202

-------
discussed be.i.ow in more detail.

(2) - Under ideal vapor flow  eoaj.itions  (i.e.  100 -  1000 scfm vapor
      flowrates) , is this  ooii'ontration  great  enough to yield acceptable
      removal rates?

      Question (2) is ansvered bv multiplying  the concentration estimate Cest,
by a range of reasonable llowraces,  Q:


                  Rest - c,,t  Q                                     (E-2)
      Here Rest denotf s  the  t-tjujied removal rate, and Cest and Q must be
expressed in consistent  units,   l"i, r  reference,  documented venting operations
at service station sites  typically report vapor flowrates in the 10 - 100 scfm
range1,  although 100 - 1000 scfm flowrates are  achievable for very sandy soils
or large numbers of extra tio-i  wells.   At this  point in the decision process
we are still neglecting  t h<- i  vapor concentrations decrease during venting due
to compositional changes  and  mass  transfer resistances. Figure E-4 presents
calculated removal rates  Kt,r  !kg/d]  for a range of Cest and Q values.  Cest
values are presented  in  [rap,/1 ]  and  [ppmCH4] units, where  [ppmCH4] represents
me thane-equivalent parts-per-rd 11 ion volume/volume (ppn^) units.   The [ppmCH4]
units are used because fie]c  analytical tools that report [ppmj values  are
often calibrated with metnane.   The  [mg/1]  and [ppmCH4] units are related by:

              [ppmCH4] *  160GOinp,-CHA/mole-CHA  *  10'6
      [mg/1] =	         (E-3)
              (0.0821 l-atm/°K-moie) *  (298K)
      For field instruments  calibrated with other compounds (i.e., butane,
propane) [ppm^] values are (.inverted to  [mg/1] by replacing the molecular
weight of CHA in Equation K-'< bv the molecular weight  [mg/mole] of the
calibration compound.

      Acceptable or desirable removal  rates Racceptabie >  can ^>e determined by
dividing the estimated spill >.'i.;:-;s  MsplU, by  the maximum  acceptable clean-up
time T:

            Raccept.bie  ~ Msp,jL-                                       (E-4)


      For example, if 1'iOU kg (~500  gal)  of gasoline had been spilled at a
service station and we wished 10 complete the clean-up within eight months,
then Racceptabie  = 6.3 kg/'d.  Base-.i on Figure E-4,  therefore, Cest  would have  to
average >1.5 mg/1  (2400  ppnc.;i)  fror Q=2800 1/min  (100 cfm) if venting  is to  be
an acceptable  option.  Gener,-:', I y,  removal rates  <1 kg/d will be unacceptable
for most spills, so soils containt^ted with compounds  (mixtures) having
saturated vapor concentrations  less  than 0.3 mg/1 (450 ppmCH4) will not be
good candidates for venting, "nless  vapor flowrates exceed 100 scfm.  Judging
from the compounds listed n: J'uuLe E-l,  this corresponds to compounds with
boiling points (Tb)>150°C,  or ruic  component vapor pressures <0.0001 atm


                                       203

-------
                                           CH.
                                       1530
                       15300
153000
         10
Removal
  Rate
 (kg/d)
          .1
         .01
        .001
                      ! I 1 11 I	!	t  t ! I I 1 11
                                             1  |_._ 1 1 I I 11
           .01
            1             10

Vapor Concentration (mg/1)
   100
             * (ppm   ) - concentration in methane-equivalent ppm (voL/vol.) units
                   CH
        Figure E-4.   In-Situ Soil Venting Removal Rate Dependence on
               Vapor Extraction Rate  and Vapor Concentration.
                                     204

-------
evaluated at the subsurface temperature.

(3) - What range of vapor flowrates can realistically be achieved?

      Question (3) requires that we estimate realistic vapor flowrates  for  our
site specific conditions.  Equation E-5, which predicts the flowrate per  unit
thickness of well screen Q/H [cm3/s],  can be used for this purpose:

       Q    k      [l-(Patm/Pw)2]
      -- 7T— Pw -                                 (E-5)
       H    i
where :
      k     =• soil permeability to air flow  [cm2] or [darcy]
      M     = viscosity of air = 1.8 x 10"*  g/cm-s  or 0.018 cp
      Pw    - absolute pressure at extraction well  [g/cm-s2] or [atm]
      ^Atm   = absolute ambient pressure « 1.01 x 106 g/cm-s2 or 1  atm
      Rw    = radius of vapor extraction well [cm]
      Rx    = radius of influence of vapor extraction well  [cm]

      This equation is derived from the simplistic  steady- state radial  flow
solution for compressible flow2,  but should provide reasonable estimates for
vapor flow rates.  If we can measure or estimate k,  then the  only unknown
parameter is the empirical  "radius of influence" Rx .  Values ranging from 9 m
(30 ft) to 30 m  (100 ft) are reported in the literature for a variety of soil
conditions, but  fortunately Equation E-5 is not very sensitive to large
changes in Rz.   For estimation purposes,  therefore, a value of Rx=12 m (40  ft)
can be used without a significant loss of accuracy.  Typical vacuum well
pressures range  from 0.95 - 0.90 atm (20 - 40 in H20 vacuum).   Figure E-5
presents predicted flowrates per unit well screen depth Q/H, expressed  in
"standard" volumetric units Q*/H (= Q/H(Pw/PAtm) ) for a 5.1 cm  radius (4"
diameter) extraction well, and a wide range of soil permeabilities  and  applied
vacuums.  Here H denotes the thickness of the screened interval, which  is
often chosen to be equal to the thickness of the zone of soil contamination
(this minimizes  removing and treating any excess "clean" air) .  For other
conditions the Q*/H values in Figure E-5 can be multiplied by the following
factors :

      R,, = 5.1  cm (2")   Rx = 7.6 m  (25')   -  multiply by Q*/H by 1.09
      Rw - 5.1  cm (2")   Rj. = 23 m  (75')    -  multiply by Q*/H by 0.90
      R, = 7.6  cm (3")   Rx = 12 m  (40')    -  multiply by Q*/H by 1.08
      R« - 10 cm (4")    Rx = 12 m  (40')    -  multiply by Q*/H by 1.15
      Ru = 10 cm (4")    Rx - 7.6 m  (25')   -  multiply by Q*/H by 1.27

      As indicated by the multipliers given above,  changing the radius  of
influence from 12 m (40 ft) to 23 m (75 ft) only decreases the predicted
flowrate by 10%.  The largest uncertainty in flowrate calculations  will be due
to the air permeability value k,  which can vary by  one to  three orders  of
magnitude across a site and can realistically only be estimated from boring
log data within an order of magnitude.   It is prudent, therefore, to choose a
range of k values during this phase of the decision process.  For example, if
boring logs indicate fine sandy soils are present,  then flowrates should be


                                     205

-------
          100
           10
 (m  /m-min);
           .1
          .01
         .001.
        .0001
                 R  =5.1 cm (2")

                 R,= I2m(40')
                  P = 0.40 arm = 2-
                   w

               P  = 0.60 atm = 13.6
                w
             .01
                                         P  =0.80atm = 6.8ftH0O
                                          w                  *•
                P = 0.90 atm = 3.4 ft H,O
                 w                 •£

              P  = 0.95 atm =1.7 ft H_O
               w                 i
                  clayey
                   sands
      fine
      sands
medium
 sands
coarse
sands
                                                                    1100
                                          _  no
                                             n
                                                Vapor
                                              Flowrate
                                              (scfm/ft)
                                                                  - 1.1
                          _ o.n
                                                                  - 0.011
                                            0.0011
.1          1         10        100

    Soil Permeabilty (darcy)
                        1000
           [ft HO] denote vacuums expressed as equivalent water column heights
Figure E-5.   Predicted Steady-State Flowrates (per unit well  screen depth)
         for  a Range of Soil Permeabilities and Applied Vacuums  (Pw) .
                                     206

-------
calculated for k values in the range 0.1
-------
                      r
vapor flow
vapor flow
                                                            vapor flow
                  side view
                          top view
      b)
                                                         vapor concentration
                                                              profile
            vapor flow
                                 impermeable layer
                                                                   liquid contaminant
       c)
                               "wet" zone with residual contamination
              Figure  E-6.   Scenarios for Removal Rate Estimates.
                                         208

-------
      ^Atm   = absolute ambient pressure = 1.016 x 106 g/cm-s2
      Pw    - absolute pressure at the venting well  [g/cm-s2]
      Rj
-------
            Rest = 7T(R  - R )CestD/5(t)                          (E


where D is the effective porous media vapor diffusion coefficient  (as
calculated above from Equations E-8 and E-9) and Cest is  the  estimated
equilibrium vapor concentration (Equation E-l) .  With time 6(t) will grow
larger.  In the case of a single component system the dry zone  thickness can
be calculated from the mass balance:
                                                                   (E-ll)
            PbCs— = CestD/5(r)
                dt
where Cs is the residual level of contamination in the low permeability zone
[ g-contamination/g- soil ], and all other variables are defined above.  The
solution to Equations E-10 and E-ll yields the following equation  that
predicts the change in removal rate with time:

                   [2CestDt]1'2


                                                                   (E-12)
                     2    2    [CestDCsPb]1/2
            Rest = 7T(R - R ) -
                     2    1       2T

As an example, consider the case where benzene (Cv = 3.19 x 10~A  g/cm3 @20°C)
is being removed from a zone extending from Rx = 5 . 1 cm to R2 = 9 m.   The
initial residual level is 10,000 ppm (0.01 g-benzene/g-soil) , pb = 1.6 g/cm3,
D° = 0.087 cm2/s,  and  6T = 6A  =0.30.  Figure E-7 presents the predicted
removal rates and  "dry" zone thickness d(t) as a function of time.  Note that
it would take approximately one year to clean a layer 1.5 m  (5 ft) thick,  for
a compound as volatile as benzene.  Equation E-12 predicts very  high  initial
removal rates; in practice, however, the removal rate will be limited
initially by the vapor-phase diffusion behavior described above  for Figure
E-6b.

      Mixture removal rates for the situations depicted in Figures E-6b  and E-
6c are difficult to estimate because changes in composition  and  liquid-phase
diffusion affect the behavior.  Presently there are no simple analytical
solutions for these situations, but we can postulate that they should be less
than the rates predicted above for pure components .

      The use of equilibrium-based models to predict required removal rates is
discussed below under the next question.

(5) - What are the vapor composition and concentration changes?  What  residual,
      if any, will be left in the soil?

      As contaminants are removed during venting, the residual soil
contamination level decreases and mixture compositions become richer  in  the


                                      210

-------
          1000-q
"Dry" Zone
 Thickness
      est
   (kg/d)
                benzene (20  C)
                R, =5.1
                                200      300
                                   Time (d)
     Figure E-7.   Estimated Maximum Removal  Rates  for  a Venting Operation
                            Limited by Diffusion.

less volatile compounds.  Both of these  processes result in  decreased vapor
concentrations,  and hence,  decreased removal rates with   time.  At  low
residual soil contamination levels  (<500  ppm)  Equation E-l  becomes  less valid
as sorption and dissolution phenomena begin  to affect  the soil residual -
vapor equilibrium.   In the  limit of low residual contamination levels,
contaminant equilibrium vapor concentrations are expected to become
proportional to the residual soil contaminant concentrations.  As venting
continues and residual soil levels  decrease,  therefore, it  becomes  more
difficult to remove the residual contamination.  It  is important to realize
that, even with soil venting,  there are practical  limitations on the final
soil contamination levels that can  be achieved.  Knowledge  of these limits is
necessary to realistically  set clean-up criteria and design effective venting
systems.

      The maximum efficiency of a venting operation  is limited by the
equilibrium partitioning of contaminants  between the soil matrix and vapor
phases.  The maximum removal rate is achieved when the vapor being  removed
from an extraction well is  in equilibrium with the contaminated soil.  Models
for predicting this maximum removal rate  have been presented by Marley and
Hoag4 and Johnson et al.2  The former considered only compositions  in a
residual free-phase, while  the latter also considered  the effects of sorption
and dissolution processes.   A complete  discussion  of the  development of these
models is not appropriate here,  but we  will  discuss  use  of the predictions.
                                     211

-------
      The change in composition,  vapor concentration,  removal rate,  and
residual soil contamination level with time are functions of the initial
residual composition,  vapor extraction well flowrate,  and initial soil
contamination level.   It is not necessary to generate  predictions for every
combination of variables,   however,  because with appropriate scaling all
results will form a single curve for a given initial mixture composition.
Figure E-8a presents the results computed with the model presented by Johnson
et al.2 for the "weathered" gasoline mixture whose composition is given by
Table E-2.  The important variable that determines residual soil levels, vapor
concentrations, and removal rates is the ratio Qt/M(t=0),  which represents the
volume of air drawn through the contaminated zone per  unit mass of
contaminant.  In Figure E-8, the scaled removal rate (or equivalently the
vapor concentration) decreases with time as the mixture becomes richer in the
less volatile compounds.

      While a detailed compositional analysis was .available for this gasoline
sample, an approximate composition based on a boiling  point distribution curve
predicts similar results.   Figure E-8b presents the results for the
approximate mixture composition also given in Table E-2.

      Model predictions, such as those shown in Figure E-8 for the gasoline
sample defined by Table E-2, can be used to estimate removal rates (if the
vapor flowrate is specified),  or alternatively the predictions can be used to
estimate vapor flowrate requirements (if the desired removal rate is
specified).  For example,  if we wanted to reduce the initial contamination
level by 90%, then Figure E-8 predicts that «100 1-air/g-gasoline will be
required.  This is the minimum amount of vapor required, because it is based
on an equilibrium-based model.  The necessary minimum  average vapor flowrate
is then equal to the spill mass times the minimum required vapor flow/mass
gasoline divided by the desired duration of venting.  Use of this approach is
illustrated in the service station site example provided at the end of this
paper.

      Figure E-8 also illustrates that there is a practical limit to the
amount of residual contaminant that can be removed by venting alone.  For
example, it will take a minimum of 100 1-vapor/g-gasoline to remove 90% of the
weathered gasoline defined in Table E-2, while it will take about 200
1-air/g-gasoline to remove the remaining 10%. In the case of gasoline, by the
time 90% of the initial residual has been removed the  residual consists of
relatively insoluble and nonvolatile compounds.  It is important to recognize
this limitation of venting, and when setting realistic clean-up target levels,
they should be based on the potential environmental impact of the residual
rather than any specific total residual hydrocarbon levels.

(6) - Are  there likely  to  be any negative effects of soil venting?

      It is possible that venting will induce the migration of off-site
contaminant vapors towards the extraction wells.  This is likely to occur at a
service station, which  is  often in close proximity  to other service stations.
If this occurs, one could  spend a lot of time and money to unknowingly
clean-up someone else's problem.  The solution is to establish a "vapor
barrier" at the perimeter  of the contaminated zone.  This can be accomplished
by allowing vapor flow  into any perimeter groundwater monitoring wells, which


                                      212

-------
    TABLE E-2.  COMPOSITION OF "FRESH" AND  "WEATHERED" GASOLINES
 Compound Name
Fresh Gasoline
propane
isobutane
n-butane
trans -2-butene
cis-2-butene
3 -methyl- 1 -butene
isopentane
1 -pentene
2-methyl- 1 -butene
2-methyl- 1,3 -butadiene
n-pentane
trans-2-pentene
2-methytl-2-butene
3-methyl-l,2-butadiene
3 ,3 -dimethyl- 1 -butene
cyclopentane
3-methyl-l -pentene
2,3 -dimethylbutane
2-methylpentane
3-methylpentane
n-hexane
methyl cyclopentane
2,2-dimethylpentane
benzene
cyclohexane
2,3-dimethylpentane
3 -methylhexane
3-ethylpentane
n-heptane
2,2,4-trimethylpentane
methylcyclohexane
2,2-dimethylhexane
toluene
2,3,4-trimethylpentane
3-methylheptane
2-methylheptane
n-octane
2,4,4-trimethylhexane
2,2-dimethylheptane
ethylbenzene
p-xylene
m-xylene
3,3,4-trimethylhexane
o-xylene
2,2,4-trimethylheptane
n-nonane
3,3,5-trimethylheptane
n-propylbenzene
2,3,4-trimethylheptane
1,3,5-trimethylbenzene
1 ,2,4-ttrimethylbenzene
n-decane
methylpropylbenzene
dimethylethylbenzene
n-undecane
1 ,2,4,5,-tetramethylbenzene
1,2,3,4,-tetramethylbenzene
1 ,2,4 -trimethyl-5 -ethylbenzene
n-dodecane
naphthalene
n-hexylbenzene
methylnaphthalene
44.
58.
58.
56.
56.
70.
72.2
70.1
70.1
68.1
72.2
70.1
70.1
68.1
84.2
70.1
84.2
86.2
86.2
86.2
86.2
84.2
100.2
78.1
84.2
100.2
100.2
100.2
100.2
114.2
98.2
114.2
92.1
114.2
114.2
114.2
114.2
128.3
128.3
106.2
106.2
106.2
128.3
106.2
142.3
128.3
142.3
120.2
142.3
120.2
120.2
142.3
134.2
134.2
156.3
134.2
134.2
148.2
170.3
128.2
162.3
142.2
0.0001
0.0122
0.0629
0.0007
0.0000
0.0006
0.1049
0.0000
0.0000
0.0000
0.0586
0.0000
0.0044
0.0000
0.0049
0.0000
0.0000
0.0730
0.0273
0.0000
0.0283
0.0083
0.0076
0.0076
0.0000
0.0390
0.0000
0.0000
0.0063
0.0121
0.0000
0.0055
0.0550
0.0121
0.0000
0.0155
0.0013
0.0087
0.0000
0.0000
0.0957
0.0000
0.0281
0.0000
0.0105
0.0000
00000
0.0841
0.0000
0.0411
0.0213
0.0000
0.0351
0.0307
0.0000
0.0133
0.0129
0.0405
0.0230
0.0045
0.0000
0.002.3
0.0000
0.0000
0.0000
0.0000
0.0000
0.0000
0.0069
0.0005
0.0008
0.0000
0.0095
0.0017
0.0021
0.0010
0.0000
0.0046
0.0000
0.0044
0.0207
0.0186
0.0207
0.0234
0.0064
0.0021
0.0137
0.0000
0.0355
0.0000
0.0447
0.0503
0.0393
0.0207
0.0359
0.0000
0.0343
0.0324
0.0300
0.0034
0.0226
0.0130
0.0151
0.0376
0.0056
0.0274
0.0012
0.0382
0.0000
0.0117
0.0000
0.0493
0.0705
0.0140
0.0170
0.0289
0.0075
0.0056
0.0704
0.0651
0.0000
0.0076
0.0147
0.0134
TOTAL
                                              1.0000
                                                              1.0000
                                          213

-------
   a;


               .1'.
  QC/QC(t=0)  :
changed from 4-phasc to
   3-phase system
                                   T100
                 Weathered Gasoline

                  T = 20°C

                  10% moisture content

                  C(t=0) = 222 mg/1
             .001
            .0001
 •80
    % removed
                                                             •60
                                                             •40
                                                             •20
                               100            200
                               Qt/m(t=0) (1/g)
                                    0
                                  300
 b)
              .1 -
QC/QC(t=0)
                                         Weathered Gasoline

                                         T-20°C
                                         10% moisture content
changed from 4-phase to   C(t=0) = 270 mg/1
   3-phase system
            .01  '
            .001
           .0001
                   Approximate Composition
                                    100
-80

  % removed

-60


•40


•20
                              100           200
                              Qt/m(t=0) (1/g)
                                    0
                                 300
Figure  E-8.   Maximum Predicted Removal Rates  for a Weathered Gasoline.
            a)  full composition,  b) approximate composition.
                                     214

-------
then act as passive air supply wells.  In other cases it may be necessary to
install passive air injection wells, or trenches, as illustrated in Figure E-
9a.

      As pointed out by Johnson et al.2 the application of a vacuum to
extraction wells can also cause a water table rise.  In many cases
contaminated soils lie just above the water table and they become water
saturated, as illustrated in Figure E-9b.  The maximum rise occurs at the
vapor extraction well, where the water table rise will be equal to the vacuum
at the well expressed as an equivalent water column height (i.e., in or ft
H20).   The solution to this  problem is to install a dewatering system,  with
groundwater pumping wells located as close to vapor extraction wells as
possible.  The dewatering system must be designed to insure that contaminated
soils remain exposed to vapor flow.  Other considerations not directly related
to venting system design, such as soluble plume migration control and
free-liquid product yield, will also be factors in the design of  groundwater
pumping system.

Design Information

      If venting is still a remediation option after answering the questions
above, then more accurate information must be collected.  Specifically, the
soil  permeability to vapor flow, vapor concentrations, and aquifer
characterics are required.  These are obtained by two field experiments: air
permeability and groundwater pump tests.  These are described briefly below.

Air Permeability Tests

      Figure E-10 depicts the set-up of an air permeability test.  The object
of this experiment is to remove vapors at a constant rate from an extraction
well, while monitoring with time the transient subsurface pressure
distribution at fixed points.  Effluent vapor concentrations are also
monitored.  It is important that the test be conducted properly to obtain
accurate design information. The extraction well should be screened through
the soil zone that will be vented during the actual operation.  In many cases
existing groundwater monitoring wells are sufficient, if their screened
sections extend above the water table. Subsurface pressure monitoring probes
can be driven soil vapor sampling probes (for shallow <20 ft deep
contamination problems) or more permanent installations.

      Flowrate and transient pressure distribution data are used to estimate
the soil permeability to vapor flow. The expected change in the subsurface
pressure distribution with time P'(r,t) is predicted2 by:
                      ^     TT
            P'	11	 dx                            (E-13)
                  47Tm(k//i)      x
                            2
                                      215

-------
   a)
    Vapor Extraction
         Well
                                                             Off-Site
                                                          Contamination
                                  Passive Air Injection Well
                                            or
                            Perimeter Groundwater Monitoring Well
   b)
    Unsaturated
     Soil Zone
                                          Vapor Extraction
                                               Well
     Saturated
     Soil Zone
Water Table Upwelling
  Caused by Vacuum
Figure  E-9. a) Use of Passive Vapor  Wells to  Prevent Migration  of Off-Site
   Contaminant Vapors,  b) Water  Table Rise Caused by the Applied Vacuum.
                                       216

-------
          Pressure
           Gauge

  Vapor Sampling
       Port
Vapor Flowmeter
                     Vapor Treatment
             Vacuum       Unit
              Pump         1
Vapor
Flow
                                Vapor Extraction Well
      Contaminated
           Soil
Vapor
Flow
                            Pressure Sampling Probes
            Figure E-10.  Air  Permeability Test System.
                                217

-------
For (r2 /4kPAtmt)<0.1,  Equation E-13 can be approximated by:

                     Q                   r2
            P' -- [-0.5772 - ln( - ) + ln(t)]          (E-14)
                  47Tm(k//i)
where :
      P'    - "gauge" pressure measured at distance r and time  t
      m     - stratum thickness
      r     = radial distance from vapor extraction well
      k     = soil permeability to air flow
      M     = viscosity of air = 1.8 x 10"*  g/cm-s
      e     = air-filled soil void fraction
      t     = time
      Q     - volumetric vapor flowrate from extraction well
      ^Atm   = ambient atmospheric pressure = 1.0 atm = 1.013 x  106
                 g/cm-s2

      Equation E-14 predicts that a plot of P' -vs- ln(t) should be a  straight
line with slope A and y- intercept B equal to:

            Q             Q                    r2e/i
      A --  B --  [-0.5772 -  ln( - )]                 (E-15)
          47Tm(k/M)     47Tm(k//Lt)               4kPAtm

      The permeability to vapor flow can then be calculated from the data by
one of two methods.  The first is applicable when Q and m are known.   The
calculated slope A is used:
            k = 	                                                (E-16)
                  4A7Tm

      The second approach must be used whenever Q or m is not known.   In  this
case the values A and B are both used:

            r26/i       B
      k- 	exp(—+0.5772)                                     (E-17)
            4PAt»       A

      Equation E-13 can also be used to choose the locations of subsurface
pressure monitoring points before conducting the air permeability  test, given
an estimation of k and the flowrate to be used.

      Vapor samples should be taken at the beginning and end of the  air
permeability test, which should be conducted for a long enough time  to extract
at least one "pore volume" Vp of vapor from the contaminated soil zone.  This
insures that all vapors existing in the formation prior to venting are
removed.  The vapor concentration at the start of the test is representative
of the equilibrium vapor concentration, while the concentration measured  after
one pore volume has been extracted gives an indication of realistic  removal
rates and the mixing or diffusional limitations discussed in association  with


                                      218

-------
Figure E-6.  The time Tp for one pore volume to be removed is:

            TP = Vp/Q =  eA7TR2H/Q                                        (E-18)

where R, H, €A, and Q are the radius of the zone of contamination, vertical
thickness of the zone of contamination, air-filled void fraction, and
volumetric vapor flowrate from the extraction well.  For example, consider the
case where R=12 m, H=3 m, eA=0.35,  and Q=0.57 m3/min (20  ft3/min) .   Then  rp=475
m3/0.57 m3/min=833  min=14 h.

Groundwater Pump Tests

      To achieve efficient venting the hydrocarbon-contaminated soil has to be
exposed to air flow, which in turn requires that the water table be lowered to
counteract the water upwelling effect caused by the  decreased vapor pressure
in the vicinity of a venting well (Johnson et al.2) and to possibly expose
contaminated soil below the water table.  Thus the groundwater pumping system
has to have a sufficient pumping rate and be operated for a long enough  time
period to obtain the required drawdowns. Since most venting systems are
installed above phreatic aquifers,  two aquifer parameters are needed for the
design: average transmissivity T and effective porosity S.  These parameters
can be estimated using the results of the standard transient groundwater pump
test with a constant pumping rate (Bear5) .   Using the estimated values the
required pumping rate may be calculated as follows:


                  Q = 47TTS(r,t)/W(u)                        (E-19)


where: W(u) is the well function5 of u — Sr2/4Tt,  and s(r,t)  is  the  required
drawdown at distance r and pumping time equal to t.

System Design

      In this section we discuss the questions that must be answered in order
to design an in-situ soil venting system.   It is not our intention  to provide
a generic "recipe" for soil venting systems design; instead we suggest a
structured thought process to guide in choosing the number of extraction
wells, well spacing, construction,  etc.  Even in a structured thought process,
intuition and experience play important roles.  There is no substitute for a
good fundamental understanding of vapor flow processes, transport phenomena,
and groundwater flow.

- Choosing the number of vapor extraction wells

      Three methods for choosing the number of vapor extraction wells are
outlined below.  The greatest number of wells from these three methods is then
the value that should be used. The objective is to satisfy removal  rate
requirements and achieve vapor removal from the entire zone of contamination.

      For the first estimate we neglect residual contaminant composition and
vapor concentration changes with time.  The acceptable removal rate Racceptabie
is calculated from Equation E-4, while the estimated removal rate from a


                                      219

-------
single well Rest is estimated from a choice of Equations E-2, E-6,  E-7,  or E-
12 depending on whether the specific site conditions are most like Figure E-
6a, E-6b, or E-6c.  The number of wells Nwell  required to achieve the
acceptable removal rate is:
                                                                   (E-20)
            Nwell = ^acceptable/Rest

      Equations E-2, E-6, and E-7 require vapor flow estimates, which can be
calculated from Equation E-5 using the measured soil permeability and chosen
extraction well vacuum Pw.   At  this point one must determine what blowers and
vacuum pumps are available because the characteristics of these units will
limit the range of feasible (PW,Q)  values.  For example,  a blower that can
pump 100 scfm at 2 in H20 vacuum may only be able to pump 10 scfm at 100  in
H20 vacuum.

      The second method, which accounts for composition changes with time,
utilizes model predictions, such as those illustrated in Figure E-8.  Recall
that equilibrium-based models are used to calculate the minimum vapor flow to
achieve a given degree of remediation.  For example, if we wish to obtain a
90% reduction in residual gasoline levels, Figure E-8 indicates that «100
1-vapor/g-gasoline must pass through the  contaminated soil zone.   If our  spill
mass is 1500 kg (=500 gal), then a minimum of 1.5 x 10s 1-vapor must pass
through the contaminated soil zone.  If our target clean-up period is six
months, this corresponds to a minimum average vapor flowrate of 0.57 m3/min
(=20 cfm).  The minimum number of extraction wells is then equal to the
required minimum average flowrate/flowrate per well.

      The third method for determining the number of wells insures that we
remove vapors and residual soil contamination from the entire zone of
contamination Nmln. This  is simply equal  to  the  ratio of  the  area of
contamination Acontaminatlon,  to  the area of influence  of  a  single  venting well
7rRx2:
                   A
                   "contamination
                        2
                      7TR
                        I
      This requires an estimate of Rz , which defines the zone in which vapor
flow is  induced.   In general, Rx depends on soil properties of the vented
zone, properties of surrounding soil  layers, the depth at  which  the  well is
screened, and  the  presence  of any impermeable boundaries  (water  table,  clay
layers,  surface seal, building basement, etc.).  At  this point  it  is useful to
have some understanding  of  vapor flow patterns because, except  for certain
ideal cases6, one  cannot accurately predict vapor flowpaths without
numerically  solving vapor flow equations .  An estimate for Rj can be obtained
by  fitting radial  pressure  distribution data from  the  air  permeability test to
the steady-state radial  pressure distribution equation2:
                                      220

-------
            P(r) = Pw[l-Kl-( - )  ) - ]1/2                    (E-22)
where P(r) ,  PAtm, Pw, and Rw are the  absolute  pressure measured at a distance r
from the venting well, absolute ambient pressure, absolute pressure applied at
the vapor extraction well, and extraction well radius, respectively. Given
that these tests are usually conducted for less than a day, the results will
generally underestimate Rx.  If no site specific  data is  available, one can
conservatively estimate Rj based on the published reports from in-situ soil
venting operations.  Reported  Rz values for  permeable soils (sandy soils) at
depths greater than 20 ft below ground  surface,  or shallower soils beneath
good surface seals, are usually 10 m - 40 m.1  For less permeable soils
(silts, clays), or more shallow zones Rx is usually less.

- Choosing well location, spacing, passive wells, and surface seals

      To be able to successfully locate extraction wells, passive wells, and
surface seals one must have a good understanding of vapor flow behavior.  We
would like to place wells so that we insure adequate vapor flow through the
contaminated zone, while minimizing vapor flow through other zones.

      If one well is sufficient, it will almost always be placed in the
geometric center of the contaminated soil zone, unless it is expected that
vapor flow channeling along a preferred direction will occur.  In that case
the well will be placed so as to maximize air flow through the contaminated
zone .

      When multiple wells are used it is important to consider the effect that
each well has on the vapor flow to all other wells.  For example, if three
extraction wells are required at a given site, and they are installed in the
triplate design shown in Figure E-lla, there  would be a "stagnant" region in
the middle of the wells where air flow would be very small in comparison to
the flow induced outside the triplate pattern boundaries.  This problem can be
alleviated by the use of "passive wells" or "forced injection" wells as
illustrated in Figure E-llb (it can also be minimized by changing the vapor
flowrates from each well with time) .   A passive well is simply a well that is
open to the atmosphere;  in many cases groundwater monitoring wells are
suitable.  If a passive or forced injection well is to have any positive
effect, it must be located within the extraction well's zone of influence.
Forced injection wells are simply vapor wells into which air is pumped rather
than removed.  One must be very careful in choosing the locations of forced
injection wells so that contaminant vapors are captured by the extraction
wells, rather than forced off -site.   To date  there have not been any detailed
reports of venting operations designed to study the advantages/disadvantages
of using forced injection wells. Figure E-llc presents another possible
extraction/injection well combination.  As illustrated in Figure E-9, passive
wells can also be used  as vapor barriers to  prevent on-site migration of
off -site contamination problems.

      For shallow contamination problems (<4  m below ground surface) vapor
extraction trenches combined with surface seals may be more effective than
                                      221

-------
a)
                                extraction
                                  wells
 b)
   c)
            vapor flow
              lines
           injection
             well
                              _ extraction
                              * * wells
.--"    I  V
                             \
        injection
         wells
                      \
extraction
  wells
                  Figure  E-ll.   Venting Well Configuration.
                                      222

-------
vertical wells. Trenches are usually limited to shallow soil zones because the
difficulty of installation increases with depth.

      Surface seals, such as polymer-based liners and asphalt, concrete, or
clay caps, are sometimes used to control the vapor flow paths.  Figure E-12
illustrates the effect that a surface seal will have on vapor flow patterns.
For shallow treatment zones (<5 ra) the.surface seal will have a significant
effect on the vapor flow paths, and seals can be added or removed to achieve
the desired vapor flowpath.   For wells screened below 8 m the influence of
surface seals becomes less significant.

- Well screening and construction

      Wells should be screened only through the zone of contamination, unless
the permeability to vapor flow is so low that removal rates would be greater
if flow were induced in an adjacent soil layer (see Figure E-6).   Removal rate
estimates for various mass-transfer limited scenarios can be calculated from
Equations E-7 and E-12.

      Based on Equation E-5, the flowrate is expected to increase by 15% when
the extraction well diameter is increased from 10 cm (4 in) to 20 cm (8 in).
This implies that well diameters should be as large as is practically
possible.

      A typical well as shown in Figure E-13a is constructed from slotted pipe
(usually PVC).   The slot size and number of slots per inch should be chosen to
maximize the open area of the pipe.  A filter packing, such as sand or gravel,
is placed in the annulus between the borehole and pipe .  Vapor extraction
wells are similar to groundwater monitoring wells in construction but there is
no need to filter vapors before they enter the well. The filter packing,
therefore, should be as coarse as possible.  Any dust carried by the vapor
flow can be removed by  an above-ground filter.  Bentonite pellets and a
cement grout are loaded above the filter packing.  It is important that these
be properly installed to prevent a vapor flow "short-sircuit".  Any
groundwater monitoring wells installed near the extraction wells must also be
installed with good seals.

- Vapor treatment

      Currently there are four main treatment processes available.  Each Is
discussed below.

      - vapor combustion units:  Vapors are incinerated and destruction
efficiencies are typically >95%.  A supplemental fuel, such as propane, is
added before combustion unless extraction well vapor concentrations are on
the order of a few percent by volume.  This process becomes less economical as
vapor concentrations decrease below «10,000 ppm,,.

      - catalytic oxidation units:  Vapor streams are heated and then passed
over a catalyst bed.  Destruction efficiencies are typically >95%.  These
units are used for vapor concentrations <8000 ppnv,.   More concentrated vapors
can cause catalyst bed temperature excursions and melt-down.
                                      223

-------
   a)
                 "open" soil surface
b)
                     impermeable seal
          Figure  E-12.   Effect of Surface  Seal  on Vapor Flowpath.
                                     224

-------
                     cement cap
  a)
                     slotted pipe
                       section
            t
                                              ccment/bentonile
                                                  grout
 coarse packing
"" material
b)
    air-tight monitoring well
   cap/water sensor assembly
                                                     pressure gauge
                                                      connection
       electronic water
           sensor
                                               wire to sensor
                                                                        double teflon
                                                                       inner septa seal
                                                                   monitoring
                                                                    well cap
   Figure E-13.
a) Extraction Well  Construction,  and b) Air-Tight
Groundwater  Level Measuring System.
                                      225

-------
      - carbon beds:  Carbon can be used to treat almost any vapor streams,
but is only economical for very low emission rates (<100 g/d)

      - diffuser stacks:  These do not treat vapors,  but are the most
economical solution for areas in which they are permitted.   They must be
carefully designed to minimize health risks and maximize safety.

      - Groundwater pumping system

      In cases where contaminated soils lie just above or below the water
table, groundwater pumping systems will be required to insure that
contaminated soils remain exposed.  In designing a groundwater system it is
important to be aware that upwelling (draw-up) of the groundwater table will
occur when a vacuum is applied at the extraction well (see Figure E-9b).
Because the upwelling will be greatest at the extraction wells, groundwater
pumping wells should be located within or as close to the extraction wells as
possible.  Their surface seals must be airtight to prevent unwanted
short-circuiting of airflow down the groundwater wells.

      - System integration

      System components (pumps, wells, vapor treating units, etc.) should be
combined to allow maximum flexibility of operation.  The review by Hutzler et
al.1 provides descriptions of many reported systems.   Specific requirements
are:

      - separate valves, flowmeters, and pressure gauges for each extraction
and injection well.

      - air filter  to remove particulates from vapors upstream of pump and
flow meter.

      - knock-out pot to remove any liquid from vapor stream upstream of pump
and flow meter.

Monitoring

      The performance of a soil venting system must be monitored in order  to
insure efficient operation, and to help determine when to shut-off the system.
At a minimum the following should be measured:

       - date and time of measurement.

       - vapor flow  rates from  extraction wells and into  injection wells:
these can be measured by a variety of  flowmeters  including pitot tubes,
orifice plates, and rotameters.   It is important  to have calibrated these
devices at  the field operating pressures and  temperatures.

       - pressure readings at each extraction  and  injection well can be
measured with manometers or magnahelic gauges.

       - vapor concentrations and  compositions  from extraction  wells:  total
hydrocarbon concentration can  be  measured by  an on-line  total  hydrocarbon


                                      226

-------
analyzer calibrated to a specific hydrocarbon. This information is combined
with vapor flowrate data to calculate removal rates and the cumulative amount
of contaminant removed.  In addition, for mixtures the vapor composition
should be periodically checked.  It is impossible to assess if vapor
concentration decreases with time are due to compositional changes or some
other phenomena (mass transfer resistance, water table upwelling, pore
blockage, etc.) without this information. Vapor samples can be collected in
evacuated gas sampling cylinders, stored, and later analyzed.

      - temperature: ambient and soil.

      - water table level (for contaminated soils located near the water
table): It is important to monitor the water table level to insure that
contaminated soils remain exposed to vapor flow.  Measuring the water table
level during venting is not a trivial task because the monitoring well must
remain sealed.  Uncapping the well releases the vacuum and any effect that it
has on the water table level.  Figure E-13b illustrates a monitoring well cap
(constructed by Applied Geosciences Inc., Tustin, CA) that allows one to
measure simultaneously the water table level and vacuum in  a monitoring well.
It is constructed from a commercially available monitoring well cap and
utilizes an electronic water level sensor.

Other valuable, but optional measurements are:

      - soil gas vapor concentrations and compositions: these should be
measured periodically at different radial distances from the extraction well.
Figure   E-14 shows the construction of a permanent monitoring installation
that can be used for vapor sampling and subsurface temperature measurements.
Another alternative for shallow contamination zones is the use of soil gas
survey probes.

      This data is valuable for two reasons: a) by comparing extraction well
concentrations with soil gas concentrations it is possible to estimate the
fraction of vapor that is flowing through the contaminated zone f=Cextraction
weii/Csoii gas>  anc^ b)  ^ ^s  possible to determine if the zone of contamination
is shrinking towards the extraction well, as it should with time.  Three
measuring points are probably sufficient if one is located near the extraction
well, one is placed near the original edge of the zone of contamination, and
the third is placed somewhere in between.

      These monitoring installations can also be useful for monitoring the
subsurface vapors after venting has ceased.


When To Turn Off The System?

      Target soil clean-up levels are often set on a site-by-site basis, and
are based on the estimated potential impact that any residual may have on air
quality, groundwater quality, or other health standards.  They may also be
related to safety considerations (explosive limits).   Generally, confirmation
soil borings, and sometimes soil vapor surveys, are required before closure is
granted. Because these analyses are expensive and often disrupt the normal
business of a site, it would be valuable to be able to determine when


                                      227

-------
                                                     Ground Surface
     1/8" OD Teflon Tubing
                                              Box Containing Vapor Sampling
                                                  Ports &Thermocouples
                                         TPVCPipe
                                              coarse packing
                                              cement/bentonite
              Figure E-14.   Vadose Zone Monitoring Installation.

confirmation borings should be taken.   If the monitoring is done  as suggested
above, then the following criteria can be used:

      - cumulative amount removed: determined by integrating the  measured
removal rates (flowrate x concentration) with time.   While this value
indicates how much contaminant has been removed,  it is usually not very useful
for determining when to take confirmation borings unless the original spill
mass is known very accurately.  In most cases that information is not
available and can  not be calculated accurately from soil boring  data.

      - extraction well vapor concentrations: the vapor concentrations  are
good indications of how effectively the venting system is working, but
decreases in vapor extraction well concentrations are not strong  evidence that
soil concentrations have decreased.  Decreases may also be due to other
phenomena such as water table level increases, increased mass transfer
resistance due to drying, or leaks in the extraction system.

      - extraction well vapor composition: when combined with vapor
concentrations this data gives more insight into the effectiveness of the
system.  If the total vapor concentration decreases without a change in
composition, it is probably due to one of the phenomena mentioned above, and
is not an indication that the residual contamination has been significantly
                                      228

-------
reduced.  If a decrease in vapor concentration is accompanied by a shift in
composition towards less volatile compounds,  on the other hand, it is most
likely due to a change in the residual contaminant concentration.   For
residual gasoline clean-up, for example,  one might operate a venting system
until benzene, toluene, and xylenes were not detected in the vapors.  The
remaining residual would then be composed of larger molecules, and it can be
argued that these do not pose a health threat through volatilization or
leaching pathways.

      - soil gas contaminant concentration and composition:  this  data is the
most useful because it yields information about the residual composition and
extent of contamination.  Vapor concentrations can not be used to  determine
the residual level, except for very low residual levels (<500 mg/kg).

Other Factors

- increased biodegradation

      It is often postulated that because the air supply to the vadose zone is
increased, the natural aerobic microbiological activity is increased during
venting.  While the argument is plausible and some laboratory data is
available7,  conclusive evidence supporting this  theory has yet to  be
presented.  This is due in part to the difficulty in making such a
measurement.  A mass balance approach is not likely to be useful because the
initial spill mass is generally not known with sufficient accuracy.  An
indirect  method would be to measure C02  levels  in the extraction  well vapors,
but this in itself does not rule out the possibility that 02 is converted to
C02 before the vapors pass through the contaminated soil zone.   The best
approach is to measure the 02/C02  concentrations  in  the  vapors  at  the  edge  of
the contaminated zone, and in the vapor extraction wells.  If the  C02/02
concentration ratio increases as the vapors pass through the contaminated
soil, one can surmise that a transformation is occurring, although other
possible mechanisms (inorganic reactions) must be considered.  An  increase in
aerobic microbial populations would be additional supporting evidence.

- in-situ heating/venting

      The main property of a compound that determines whether or not it can be
removed by venting is its vapor pressure, which increases with increasing
temperature. Compounds that are considered nonvolatile, therefore, can be
removed by venting if the contaminated soil is heated to the proper
temperature.  In-situ heating/venting systems utilizing radio-frequency
heating and conduction heating are currently under study8.   An alternative  is
to reinject heated vapors from catalytic oxidation or combustion units into
the contaminated soil zone.

- air sparging

      Due to seasonal groundwater level fluctuations,  contaminants sometimes
become trapped below the water table.  In some cases groundwater pumping can
lower the water table enough to expose this zone, but in other cases this is
not practical.  One possible solution is to install air sparging wells and
then inject air below the water table.  Vapor extraction wells would then


                                      229

-------
capture the vapors that bubbled up through the groundwater.   To date, success
of this approach has yet to be demonstrated.   This could have a negative
effect if foaming, formation plugging,  or downward migration of the residual
occurred.

Application of the Design Approach to a Service Station Remediation
      In the following we will demonstrate the use of the approach discussed
above and outlined in Figure E-2 for the design operation,  and monitoring of
an in-situ venting operation at a service station.

Preliminary Site Investigation

      Prior to sampling it was estimated that 2000 gal of gasoline had leaked
from a product line at this site. Several soil borings were drilled and the
soil samples were analyzed for total petroleum hydrocarbons (TPH) and other
specific compounds (benzene, toluene, xylenes) by a heated-headspace method
utilizing a field GC-FID.  Figure E-15 summarizes some of the results for one
transect at this site.  The following relevant information was collected:

      - based on boring logs there are four distinct soil layers at this site
between 0 - 18 m (0- 60 ft) below ground surface (BGS).   Figure E-15 indicates
the soil type and location of each of these layers.

      - depth to groundwater was 15 m, with fine to medium sand aquifer soils

      - the largest concentrations of hydrocarbons were detected in the sandy
and silty clay layers adjacent to the water table.  Some residual was detected
below the water table.  Based on the data presented in Figure E-15 it is
estimated that  - 4000 kg of hydrocarbons are present in the lower two soil
zones.

      - initially there was some free-liquid gasoline floating on the water
table, and this was subsequently removed by pumping.  A sample of this product
was analyzed and its approximate  composition (-20% of the compounds could not
be identified) is listed in Table E-2 as the "weathered gasoline".  The
corresponding boiling point distribution curve for this mixture has been
presented in Figure E-3.

      - vadose zone monitoring installations similar to the one pictured in
Figure E-14 were installed during the preliminary site investigation.

Deciding if Venting is Appropriate

For the remainder of the analysis we will focus on the contaminated soils
located just above the water table.

      - What contaminant vapor concentrations are likely to be obtained?

      Based on the composition given  in Table E-2, and using Equation E-l, the
predicted saturated TPH vapor concentration for this gasoline is:
                                      230

-------
                               Cest =  220 mg/1

      Using the "approximate" composition listed  in Table  E-2  yields a value
of 270 mg/1.  The measured soil vapor concentration obtained from the vadose
zone monitoring well was 240 mg/1.  Due to composition  changes with time,  this
will be the maximum concentration obtained during venting.

      - Under ideal flow conditions is this concentration  great enough to
        yield acceptable removal rates?

      Equation E-4 was used to calculate Racceptabie' Assuming Msplll  = 4000 kg
and t = 180 d, then:


                              Racceptabie = 22 kg/d

Using Equation E-2, Cest = 240 mg/1, and Q = 2800  1/min  (100 cfm):

                                Rest = 970 kg/d

which is greater than Racceptabie.

      - What range of vapor flowrates can realistically be achieved?

      Based on boring logs the contaminated zone  just above the water table is
composed of fine to medium sands, which have an estimated permeability 1< k < 10
darcy.   Using Figure  E-5,  or  Equation E-5,  the predicted flowrates for an
extraction well vacuum Pw - 0.90 atm are:

      0.04 < Q < 0.4 mVm-min Rw = 5.1 cm,  Rx  = 12 m
      0.43 < Q < 4.3 ft3/ft-min      Rw - 2.0  in,  R: - 40 ft

      The thickness of this zone and probable  screen thickness  of an extraction
well is about 2 m  (6.6  ft).   The total flowrate per well  through  this  zone is
estimated to be 0.08
-------
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HB-10 V HB-5 HB-3
                                       HB-21
                                    [Ground Water
                                    Recovery Well)
                                     \
                                    HB-25
                                    [Vapor
                                 Recovery Well]
                                                             5C/U.E (ft)
                                                                10
                                                        20
 Figure  E-15.
Initial  Total Hydrocarbon Distribution  [rag/kg-soil]  and
    Location of  Lower Zone Vent Well.
                                      232

-------
      - What residual, if any, will be left in the soil?

      A target clean-up level for most gasoline spill sites is <1000 mg/kg TPH
residual. If our initial residual level is -10,000 ppm, then we need to remove
at least 90% of the initial residual.  According to the curves in Figure E-8,
which represent the maximum removal rates for the gasoline analyzed at this
site, approximately 100 1-vapor/g-residual will have to pass through the
contaminated zone to achieve this target.  Based on our estimated initial
residual of 4000 kg TPH, 4 x 108 1-vapor are required.   Over a six month
period this corresponds to an average flowrate Q=1.5 m3/min (54 cfm).   Recall
that since this corresponds to the maximum removal rate, it is the minimum
required flowrate.

      - Are there likely to be any negative effects of soil venting?

      Given that the contaminated soils are located just above and below the
water table, water table upwelling during venting must be considered here.

Air Permeability Test

      Figure E-16 presents data obtained from the air permeability test of
this soil zone.  In addition to vapor extraction tests, air injection tests
were conducted.  The data is analyzed in the same manner as discussed for
vapor extraction tests.  Accurate flowrate (Q) values were not measured,
therefore, Equation E-17 was used to determine the permeability to vapor flow.
The k values ranged from 2 to 280 darcys, with the median being ~8 darcys.


System Design

      - Number of vapor extraction wells:

      Based on the 8 darcys permeability, and assuming a 15 cm diameter (6 in)
venting well,  a 2 m screened section, Pw = 0.90 atm (41 in H20  vacuum) and
Rx=12 m,  then  Equation E-5 predicts:

                            Q = 0.7 m3/min = 25 cfm

      Based on the  discussion above, a minimum average flowrate of 1.5 m3/min is
needed to reduce the  residual to 1000 ppm  in 6 months.  The  number  of wells
required is then 1.5/0.7 - 2, assuming that 100% of the vapor flows through
contaminated  soils.    It  is  not  likely that  this will  occur,  and a  more
conservative estimate  of 50% vapor  flowing through contaminated  soils  would
require that twice as many wells (4)  be installed.

      A single vapor extraction well  (HB-25)  was  installed in this soil layer
with the knowledge  that more wells were likely to be required.   Its location and
screened interval are shown in Figure E-15.   Other wells were installed in the
clay layer and upper sandy zone, but  in this paper we will only discuss results
from treatment of the lower contaminated zone.  A groundwater pumping well was
installed to maintain a 2 m drawdown below the static water level. Its location
is also shown in Figure E-15.
                                      233

-------
        Pressure  -- -
        Decrease
         (inH,0)   _


                  -6-

                  -8-
                 -10
       C  HB-7D  (r=3.4m)
       A  HB-6D  (r=16. m)
       -  HB-14D (r=9.8 m)
                                             n  D
                                10           100
                                  Time (min)
                                           1000
  b)
                  50
        Pressure
        Increase
        (inH20)
   40-
                  30 J
                  20 J
                  10-
3 HB-7D (r=3.4m)
A HB-6D (r=16. m)
0 fiB-14D (r=9.8 m)
+ HB-10 (r=7.6m)
                        2
                        U
                      rO
                      #
                    tfP    +

                       ^>°
                    +  o"
                                           +
                                             ^
                                 10           100
                                  Time (min)
                                           1000
Figure E-16.
Air Permeability Test Results:   a)  vapor extraction test,
           b) air  injection test.
                                    234

-------
System Monitoring

      Three vadose monitoring wells similar in construction to the one pictured
in  Figure  E-14  were  installed  so   that  the  soil  temperature,  soil  gas
concentrations, and subsurface pressure distribution could be monitored at three
depths.  One sampling port is located  in  the zone adjacent to  the aquifer.  The
vapor flowrate from HB-25 and vapor concentrations were measured frequently, and
the vapor composition was determined by GC-FID analysis.   In  addition, the water
level in the groundwater monitoring wells was measured with  the system pictured
in Figure  E-13b.    The  results  from  the first four  months  of  operation are
discussed below.

      In Figure E-17a the extraction well vacuum and corresponding vapor flowrate
are presented.  The vacuum was maintained at  0.95 atm (20  in H20 vacuum), and
the flowrate was initially 12 scfm. It gradually decreased to  about 6 scfm over
80 d.   For comparison, Equation (5)  predicts that  Q=12 cfm for  k-8  darcys.
Increasing the applied vacuum to 0.70  atm (120  in H20 vacuum) had little effect
on the flowrate.   This  could be  explained by  increased water table upwelling,
which would act to  decrease the vertical cross-section available for vapor flow.
The  scatter  in  the  flowrate  measurements  is  probably  due to  inconsistent
operation  of  the  groundwater  pumping operation,   which  frequently  failed to
perform properly.
HB-25
Vacuum
(inH O)
2
120-
lOO-i
80-
60-
40-
"70-
n.
*
*!•;
\ '£
1 » ?• *

1 Vacuum)
-,— _ plowracc
^ / *>
^ T* \ ?
'«! \A'
\' * V

** A,
\ * *

i 	 ,
«
> * ; y
^/\/
<* i
^
-10
"5
- r>
                                                               Flowrate
                                                                (SCFM)
                    0     20    40    60     80     100    120
                                   Time (d)
             [in H O] denote vacuums expressed as equivalent water column heights
        Figure E-17.   Soil Venting Results:  a) Vacuum/Flowrate Data,
                                      235

-------
      Figure E-17b presents the change in vapor concentration with time.  Fifteen
specific compounds  were  identified during  the  GC-FID vapor analyses;  in this
figure we present  the total concentration of known and unknown compounds detected
between five boiling point  ranges:
            methane - isopentane  (<28°C)
            isopentane - benzene  (28  -  80°C)
            benzene - toluene (80  - 111°C)
            toluene - xylenes (111  -  144°C)
            >xylenes (>144°C)

      There was a shift  in composition towards less volatile  compounds in the
first 20 d, but after that period the  composition  remained relatively constant.
Note that there is still a  significant  fraction of volatile compounds present.
Within  the  first  two  days  the  vapor  concentration decreased by  50%,  which
corresponds to the time period for the removal of  the first pore volume of air.
Comparing the subsequent vapor concentrations with the  concentrations measured
in  the  vadose zone  monitoring  wells  indicates  that  only  (80  mg/l)/(240
mg/l)*100=33% of  the vapors are flowing  through contaminated soil.

      Figure E-18a presents calculated removal rates  (flowrate x concentration)
and cumulative amount (1 gal = 3 kg) removed during the first four months.  The
decrease in removal rate  with  time  is  due to a  combination of decreases  in
flowrate and  hydrocarbon vapor  concentrations.  After  the first  four months
approximately one-fourth  of the  estimated residual has been  removed from this
lower zone.

      On day  80 the vacuum was increased  from  20 -  120  in H20 vacuum and the
subsequent increase in subsurface vacuum and water table upwelling was monitored.
Figure E-18b presents the  results.  Note that the water table rise paralleled the
vacuum increase, although the water table did not  rise the same amount that the
vacuum did.
                  100-

      b)
           Vapor
           Cone.  60
           (mg/1)
Xylenes
                     0     20    40    60     80     100    120
                                   Time (d)

                 Figure E-17b. Concentration/Composition Data.
                                      236

-------
    a)
              60
              50-
    Removal 40i
      Rate
      (kg/d)   30-
                 0     20
              40    60     80
                  Time (d)
100
                                         400
                                                       •300
                                             Cumulative
                                             Recovered
                                                (gal)
b)
0.5
               0.4-
      ftH  O
         2     0.3 -I
               0.2-
               0.1 -
               0.0
                       - vacuum increase

                     *  - water table upwclling
                  .1
                    n crP
                1            10

                  Time (min)
           [ft HO] denote vacuums expressed as equivalent water column heights
   Figure E-18,
    Soil Venting Results:  a) Removal Rate/Cumulative
     Recovered, b)  Water Table  Rise.
                                   237

-------
      Figure  E-19  compares  the  reduced  measured  TPH vapor   concentration
C(t)/C(t=0)  with  model  predictions.     C(t=0)  was  taken  to  be  the  vapor
concentration after one pore volume of air had passed through  the contaminated
zone (=80 mg/1), m(t=0)  is equal to the estimated spill mass (=4000 kg), and V(t)
is the total volume of air that has passed through  the contaminated zone.   This
quantity  is  obtained  by integrating the  total vapor  flowrate with  time,  then
multiplying it by the fraction of vapors  passing through the contaminated zone
f (=0.33).   As  discussed, the quantity f was  estimated by comparing  soil  gas
concentrations  from  the  vadose  zone  monitoring installations  with  vapor
concentrations in  the  extraction well  vapors.   As  can  be  seen,   there  is  good
quantitative agreement between the measured and predicted values.

      Based on  the  data presented in Figures  E-15  through  E-19 and  the  model
predictions in Figure E-8, it appears that more extraction wells  (-10 more)  are
needed to remediate the site within a reasonable amount  of time.

CONCLUSIONS
      A structured,  technically based approach has been presented for the design,
construction, and  operation of venting  systems.   while we  have attempted  to
explain the process in detail for those not familiar  with venting operations  or
the underlying governing phenomena,  the most effective and efficient systems can
only be  designed and operated by  personnel with a  good  understanding of the
fundamental processes  involved.   The  service  station spill example  presented
supports the validity and usefulness of  this approach.

      There are  still  many  technical  issues  that  need to be  resolved in the
future.  In particular, we must be able to estimate removal rates for  non-ideal
situations,  demonstrate  that biodegradation  is  enhanced by  venting,  and
investigate novel ideas for  enhancing  venting  removal rates.
                     1.0
                     0.8-
             C(t)/C(t=0)  '

                     0.6-
                                                Qt)/C(t=0) predicted
                                                C(l)/C(t=0) measured
                          weathered gasoline
                          m(t=0) - 4000 kg
                     0.4-
     Figure E-19.
               V(t)/m(t=0) (1/g)
Comparison of Model Predictions and Measured Response.
                                      238

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                                  REFERENCES

Bear, J., Hydraulics of Groundwater,  McGraw-Hill,  1979.

Dev, H., G.  C.  Sresty,  J. E. Bridges  and  D.  Downey,  Field Test of  the Radio
      Frequency  in-situ  Soil  Decontamination  Process,   in  Superfund  '88:
      Proceedings of the 9th National Conference,  HMCRI,  November 1988.

Hutzler, N.  J., B. E. Murphy,  and J. S. Gierke,  State of Technology Review:  Soil
      Vapor Extraction Systems,  U.S.E.P.A., CR-814319-01-1,  1988.

Johnson, P.  C., M. W. Kemblowski, and J.  D.  Colthart, Practical Screening Models
      for   Soil  Venting   Applications,  NWWA/API  Conference   on   Petroleum
      Hydrocarbons and Organic Chemicals in Groundwater,  Houston,  TX, 1988.

Marley, M.C., and G.  E.  Hoag,  Induced Soil Venting for the Recovery/Restoration
      of Gasoline  Hydrocarbons in  the  Vadose Zone, NWWA/API Conference  on
      Petroleum Hydrocarbons  and Organic Chemicals in Groundwater, Houston, TX,
      1984.

Millington,  R. J.,  and J.  M.  Quirk,  Permeability of Porous Solids,  Trans.
      Faraday Soc.,  57:1200-1207,  1961.

Salanitro, J. P., M.  M.  Western,  and M. W. Kemblowski, Biodegradation of Aromatic
      Hydrocarbons in Unsaturated Soil Microcosms.  Poster paper presented at the
      Fourth National Conference on Petroleum Contaminated Soils, University of
      Massachusetts, Amherst, September 25-28,  1989.

Wilson, D. J., A. N.  Clarke, and  J. H. Clarke, Soil  Clean-up by In-situ Aeration.
      I.  Mathematical  Modelling,  Sep.  Science Tech.,  23:991-1037,  1988.
                                     239

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                                  APPENDIX F
                 "DESIGN OF SOIL VAPOR EXTRACTION SYSTEMS --
                            A SCIENTIFIC APPROACH"

         Michael C. Marley,  Scott D. Richter, Bruce L. Cliff, P.E.,
                            and Peter E.  Nangeronia


INTRODUCTION

      The fate and transport of volatile, semi -volatile,  and gaseous
contaminants released into the sub-surface environment, whether accidentally
or intentionally,  has become a subject of primary concern this decade.  It has
been estimated that up to 20 percent of the approximately 2 million federally
regulated underground storage tanks in the United States may be leaking.1
Spilled product migrates through the unsaturated soil  zone, under the
influence of gravitational and capillary forces, to the water table.
Corrective action generally includes an effort to physically remove the
product by bailing and pumping as well as pumping and  treating contaminated
ground water.  The product retained in the unsaturated zone, however, usually
is a significant portion of the total spill.   Natural  transport models have
demonstrated the potential of long-term groundwater contamination due to vapor
and solute transport emanating from the trapped immiscible plume.2'3

      In the past few years the need to remediate these contaminated
unsaturated soils as part of an overall and cost effective site clean-up has
been emphasized.  There are a number of methodologies  commonly utilized in the
remediation of contaminated unsaturated soils including:

            excavation and off site disposal
            excavation and on-site treatment
            biodegredation
            in-situ soil washing
            in-situ vapor excavation (soil venting, air stripping, enhanced
            volatilization)

      In general,  it is recognized that where applicable, vapor extraction is
the most cost effective alternative.4"10
aVAPEX Environmental Technologies,  Inc.
480 Neponset Street
Canton, MA 02021
                                      240

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      The cost effectiveness of utilizing vapor extraction has been diminished
by somewhat less than optimal employment of the technology.  Often, vapor
extraction systems have been designed and implemented based on less than a
full understanding of the physical/chemical principles governing the process.
The focus of this paper is to present a more scientific approach to the design
and implementation of vapor extraction systems.  This scientific approach is
based on several years of research, including the development and application
of computer transport models and field sampling and analysis protocols
specific to the design, implementation, and understanding of vapor extraction
technology.2'5'6'11"15  The air flow models referred to in the text have been
developed as part of the Ph.D. dissertation of one of the co-authors.

VAPOR EXTRACTION TECHNOLOGY

      The volatility of the released substances establish a premise for
contaminant removal based on inducing air movement in the unsaturated zone.
In the unsaturated zone, an air flow field can be established with
combinations of injecting and withdrawing boreholes or trenches.  Air laden
with contaminant vapors would move along the induced flow path toward the
withdrawing system where it is analyzed, treated, and/or released to the
atmosphere.  The remediation of contaminated soils by vapor extraction/soil
venting in the past few years has demonstrated the potential effectiveness and
economics of this technology.""10

A SCIENTIFIC APPROACH TO DESIGN IMPLEMENTATION

Site Assessment -- Investigation

      As with any remediation project, it is important to fully characterize a
site in order to develop an optimal remedial approach.  On vapor extraction
projects, it is important to focus investigative effort on the unsaturated
zone.  A review of available data on site history and conditions, including
site plans (surface and subsurface structures), drilling logs, soils and
ground water quality data, potential ground water and vapor receptors, and any
site drawings locating utilities.  Existing physical and chemical soil
analyses will be examined to help evaluate the variability of soil conditions.

      Shallow soil gas surveys are effective in providing a preliminary
characterization of the degree and extent of soil contamination.  Where
geologic units, confining to natural vapor transport, prevent contaminant
vapors from reaching the capture zone of the shallow soil gas survey, it is
necessary to perform a deep soil gas survey.  It is important to characterize
the contaminant within each separate geologic unit.

      Upon data review and analysis, a risk assessment is conducted for the
site.  Based on the results of a risk assessment, a course of action will be
recommended, which, in the majority of cases, constitutes an initiation of the
necessary regulatory permitting processes in conjunction with a Phase 2
feasibility study -- conceptual design.

Feasibility Study -- Conceptual Design

      A client's attraction for utilizing vapor extraction to remove VOC's


                                      241

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from the subsurface is due to the relatively low cost of implementing the
technology in conjunction with the higher contaminant removal rates achievable
in comparison to standard pump and treat techniques.  The high removal rates
achievable are related to the properties of the soil matrix,  the advective air
phase and the physical/chemical properties of the contaminants.   The success
of the method depends on the rate of contaminant transfer from the immiscible
and water phases into the air phase and, in particular,  the ability to
establish an air flow field that intersects the distributed contaminants.

General Design Approaches --

      A more general approach to vapor extraction system design follows the
pattern where, the information obtained from the evaluation of the degree and
extent of contamination is utilized to provide assumptions of uniform
contamination over a specified soil zone.  Intraphase equilibrium is assumed
between the contaminants and the air and water phases.  Uniform air flow
fields are assumed and extrapolations of the remediation process are
performed.  However, following immiscible fluid flow in porous media, the
remaining, immobilized, immiscible fluids may exist as a few large globs of
liquid, or a large number of smaller globs.14   The  geometry of the  fluid
distribution depends on the nature of the capillary forces between the fluids,
the pore sizes and geometry, and the history of fluid movement in the medium.
Although intraphase equilibrium may exist at the pore scale,  the heterogeneous
distribution of the immobilized immiscible organics within the pores may make
the overall equilibrium assumptions inappropriate.   It should be noted that
where a uniform distribution of residual contamination does exist, the
assumption of a dynamic equilibrium between the advective air phase and the
immiscible contaminant provides a good approximation of the physical/chemical
processes. 5-6'11  The  intraphase  transfer  of contaminants  should be considered
in terms of mass transfer limitations.  At this time, few utilize transport
models which consider the potential mass transfer limitation.3

      However, of greater importance than the potential mass transfer
limitations/equilibrium assumptions is a knowledge of and capability to
control the airflow pathways to optimize contact with the contaminants.
Without the aid of air flow models, it would be difficult to evaluate the air
flow pathways for all but the most simplistic of cases (homogeneous, dry
sands, closed to the atmosphere, with no subsurface structures within the
extraction system zone of influence).

Air Permeability and Air Flow Modeling  --

      Compressible flow in porous media has been a  subject of investigation for
many  years  in petroleum  reservoir engineering.   Mathematical models  of air
movement in unsaturated porous media have been calibrated with air pressure data
in previous investigations to provide determinations of in-situ air permeability.
Muskat and Botset (1931) developed a one-dimensional (radial)  air  flow model  to
evaluate  the  horizontal permeability  of gas  reservoirs.16  Boardman and  Skrove
(1966) injected air into packed-off sections of drill holes and observed  radial
pressure distributions to obtain horizontal fracture permeability  of  a granitic
rock mass.17 Stallman and Weeks (1969)  and Weeks  (1977) describe the use of depth
dependent air pressure to calculate vertical air permeability in  the unsaturated
zone.18'19  Rozsa and others  (1975)  document an application of  this technique  to


                                      242

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determine vertical air  permeabilities  of nuclear chimneys  at  the Nevada Test
Site.20  As another historical note, soil scientists have utilized injected air
and pressure measurements  to evaluate  soil permeability but  these techniques
provide estimates over small  regions of soil and are not directly applicable for
unsaturated zone evaluation.21"2*

      The application of such models  to aid in  the design of a vapor extraction
system is exactly analogous and can be  thought of in two steps:

      •     Evaluate, in-situ, the air permeability tensor for the
            contaminated unsaturated zone by calibrating a steady-state air
            flow model with pressure measurements obtained during pumping
            tests.

      •     Utilize the air permeability values and a steady state air-flow
            model to determine the well spacings, screened intervals of wells
            in the unsaturated zone and the size and type of pumps required to
            generate the desired air movement.

      An in-situ determination of the air-phase  permeability tensor is preferred
over laboratory determinations to account  for variations in prevailing soil-water
conditions, the presence of  the  immiscible organic  liquid,  and anisotropy and
heterogeneity in air phase  permeability.  Further, permeability evaluations are
sensitive to the soils'  bulk  density  and  structure, which are generally altered
in  the  disturbed soil  samples  taken for  laboratory analysis.   Steady-state
pumping tests,  which require less data than transient analyses, are sufficient
for this application because only the  in-situ air permeability  is  needed for
design purposes.

      The governing equation defining conservation of mass for compressible
flow is given as:
                         3  (6. )
                      	--~-   + V '  (Pa  qa)   =  0
                         o t               ~
where
     p   = air density

     ©a  =  air filled porosity
     qa = specific discharge vector


Expressing density as a function of pressure in accordance with the ideal gas
law and by Darcy's law:

              P  W
     Pa   -     a  a
     a
                R T
where
                  Pa = air pressure
                  Wa = molecular weight of air
                  R = universal gas constant
                                      243

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                  T = temperature
                  k = intrinsic permeability tensor
                  Ha = air viscosity

yields a partial differential equation in terms of air phase pressure.  The
selection of a coordinate system and appropriate boundary conditions, together
with equation (1) defines the air-flow model.

      Commonly,  hydraulic conductivity values  are  available  from ground water
studies performed prior  to  the  vapor  extraction feasibility  study.   Where
applicable (e.g., uniform, dry,  medium-coarse,  sands), these values may provide
an accurate evaluation of the intrinsic horizontal air permeability that could
be  used in  the design  process.    In the  majority  of  cases,   however,  this
assumption is invalid for one or more of the following reasons:

            gas slippage  (Klinkenberg effect 25)  is ignored,
            anisotropy is neglected,
            swelling soils are present,
            the variable water saturation in the unsaturated zone is ignored,
            the presence of an oil phase is ignored,
            the groundwater test may be in a different strata,
            the scale of the ground water test may invalidate the parameter
            evaluations.

Field Permeability Test

Air Flow System  - -

      A one to two day in-situ evaluation of the air permeability  tensor in each
soil strata of concern is  typically performed.   Additionally,  the  data collected
will allow a  characterization  of  the surface  boundary condition, an important
parameter in  the development  and control of the airflow pathways.   The field
evaluation normally consists of the installation of one vapor extraction well and
several permanent installation vapor  probes.   The wells and probes installed as
part of the feasibility study are located specifically,  as to  become an integral
and efficient part of the overall  full scale remediation design.   In a number of
cases, this single well installation  has  proven sufficient to effect full scale
site remediation.  The permanently installed vapor probes  allow, over the entire
course of the project, for the collection of data on the level of VOC's in the
soil gas and hence, to properly document the progress of the cleanup.

      The vapor  extraction well  and  vapor  probes are installed using standard
hollow stem auger techniques.   Split  spoon sampling is performed to provide soil
samples for characterization,  initial jar headspace screening and  for laboratory
analysis by EPA  Series 8000 tests.   The split spoon sampling is  also necessary
to  detect changes  in  the stratigraphy that can have  significant  impact on the
developed air flow pathways.   This is clearly  demonstrated in a field parameter
evaluation by Baehr and Hult (1988).26

      Further,  soil samples may be taken to evaluate moisture contents and to
develop a moisture content profile.  The moisture content data may be used in
conjunction with the  air  permeability evaluations as input data  to fit a
Corey-Brooks  (1966),  Parker et al. (1987) or equivalent parametric model to


                                      244

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allow interpolation of phase relative permeability-saturation
relationships.27'28   The  parametric  model  may  then be  used as  part  of numerical
air flow models, in the development of the air permeability grid.

      In the analysis of the field parameters, the  physical characteristics of
the site and the field test layout are used as inputs to the air flow models.
The field test is operated at two or more air flow rates; this allows for both
the initial calibration of the model (i.e., parameter evaluations using the
collected field data at one air flow rate) and verification of the model
(i.e., the model is set to simulate the system for  the other air flow rates
using the parameters established in the calibration made, and comparison is
made between the predicted air flow rates and pressure distributions at the
well/probes by the model, and the actual pressure data measured at the
well/probes at the other air flow rates).  Figures  F-l to F-3 demonstrate
model calibration,  verification and simulation runs for an actual field test
where the site geometry and stratigraphy were relatively simplistic.
Following calibration/verification, the air flow models are then utilized to
establish the optimal, site specific vapor extraction system design, based on
achieving the desired airflow rates and pathways.

      The operation of a vapor extraction system is accompanied by water
movement in the subsurface through:

      •     Vaporization/condensation to and from the advective air phase;
      •     The simultaneous water flux in the unsaturated zone induced by the
            advective air flow; and
      •     The local ground water mounding due to  the negative pressure
            created within the zone of influence of the vapor extraction well.

      Where appropriate, transport models are utilized to evaluate the water
movement to ensure a complete extraction system design.  In addition to the
data collected for the air flow models during the field test, samples are
collected and analyzed from the vapor probes and the extraction system
discharge gas to better define the degree and extent of contamination and to
allow an accurate evaluation of the optimal off-gas treatment system needed in
the full scale design.  Field technicians and field calibrated instruments
(including a portable gas chromatograph/PC system)  are used to obtain an in-
situ qualitative and quantitative evaluation of the contaminants present.

      The selection of off-gas controls (required by most states when exhaust
gas contamination exceeds a certain level) is based on estimates of the mass
and type of contamination in the subsurface.   Using the data generated from an
air permeability test and determining the minimum airflow required to clean up
the zone of contamination, it is possible to determine whether emission
controls will be required at a specific site.  If possible, design a low flow
extraction system that eliminates the need for emission controls.   If controls
are necessary, either carbon adsorption,  catalytic  incineration, or thermal
incineration will be recommended.

Laboratory Vent Test --

      To aid in the evaluation of the feasibility of applying vapor extraction
technology and time to clean up, and in consideration of the above discussion


                                      245

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                                                                                                                                                            NORMALIZED PRilSSURf:  (Pr/Potm)
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       0.9 -
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 U
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                           RADIAL DISTANCE FROM VACUUM WELL (ft)

                    Figure F-3.   Model Simulations for VW3.
                      Simulations (§15,  10.5, and 20 cfm.

on the mass transfer limitation/equilibrium  assumptions,  a  laboratory vapor
extraction test may be performed on a 2 to 3 kg  soil  sample, commonly obtained
from the 'highest contaminated area of the site.  The  laboratory vent test,
which is essentially a controlled, accelerated vapor  extraction process,
provides data on:

      •     the expected  level of clean up achievable utilizing vapor
            extraction technology.
      •     a quantitative and qualitative  (GC/MS) evaluation  of  the
            contaminants  present in the soil system.
      •     An approximation of the composite intrinsic air
            permeability.
      •     An estimate of the air flow volume necessary  to achieve
            the regulatory required clean up levels imposed on the
            site, and hence in conjunction with  the air flow
            modelling an  estimate of the time required for  site
            remediation.

Mobilization and Completion of Installation

      Following receipt of regulatory approval,  optimally-sized vacuum
pump(s)/blower(s),  manifold piping, emission control  (if  necessary),  and
additional wells and probes (if necessary) are installed.
                                      247

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System Operation.  Maintenance and Monitoring

      Regular monitoring of the system is recommended to insure its
effectiveness and to document and optimize the progress  of cleanup.  Vapor
samples and air flow readings taken from the soil vapor  monitoring probes and
system exhaust sampling ports are used to monitor the progress  of cleanup, to
estimate the volume of hydrocarbons removed by the system,  and  to establish a
timetable for completion of the project.

      Where appropriate, ground water samples will be taken periodically for
analysis by EPA 600 series tests; this will allow an evaluation of the
remedial effect of vapor extraction technology on the ground water
contamination due to accelerated intraphase transfer from the ground water to
the overlying soil gas.

System Shut Down and Demobilization

      When monitoring indicates that remediation goals have been achieved, the
system will be shut down.  The pump and all aboveground  piping  will be removed
from the site.  All vapor probes, extraction wells, and  below grade manifold
piping are typically left in place.  If necessary, confirmatory soil samples
may be taken, and an ongoing analysis of soil vapors at  vapor probe locations
can be undertaken to ensure cleanup.

COST COMPARISON

The remediation of contaminated soils by vapor extraction/soil  venting in the
past few years has demonstrated the potential effectiveness and economics of
this technology.  Generally, vapor extraction can be applied for as little as
$10 to $50 per cubic yard. This compares quite favorably to other treatment
and disposal alternatives.  For example, excavation and  landfill or
incineration disposal can cost as much as $200 - 350/yd3.
                                      248

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                                  REFERENCES

1  Porter,  J.  Winston,  1989.  "Superfund Progress":  Hazardous Material Control,
      Volume 2, No.  1,  Page 48.

2.  Baehr,  A.L., 1987,   "Selective Transport of Hydrocarbons in the Unsaturated
      Zone Due to Aqueous and Vapor Phase Partitioning":  Water Resources
      Research, Vol.  23,  No.  10,  Page 1926-1938.

3.  Sleep,  B.E., and Sykes,  J.F.,  1989,  "Modelling the Transport of Volatile
      Organics in Variably Saturated Media": Water Resources Research,  Vol.
      25,  No.  1, Page 81-92.

4.  Thornton, J.S., and Wooton, W.L.,  1982,  "Venting for  the Removal of
      Hydrocarbon Vapors  from Gasoline Contaminated Soil",  Journal of
      Environmental Science and Health,  A 17,  Page 31-44

5.  Marley,  M.C., and Hoag,  G.E.,  1984,  "Induced Soil Venting for
      Recovery/restoration of Gasoline Hydrocarbons in the  Vadose Zone";
      Proceedings of the  National Water Well Association  American Petroleum
      Institute Conference on Petroleum Hydrocarbons and  Organic Chemicals in
      Groundwater, Nov. 5-7,  Houston, TX.

6.  Baehr,  A.L. Hoag,  G.E.,  and Marley,  M.C., 1989 "Removal  of Volatile
      Contaminants from the Unsaturated Zone by Inducing  Advective Air Phase
      Transport": Journal of Contaminant Hydrology, Vol.  4  Feb., Pages 1-26.

7.  Krishnayya, A.V.,  O'Connor, M.J.,  Agar,  J.G.,  and King,  R.D., 1988 "Vapour
      Extraction Systems  -  Factors Affecting their Design and Performance,":
      Proceedings of the  National Water Well Association  -  American Petroleum
      Institute Conference on Petroleum Hydrocarbons and  Organic Chemicals in
      Groundwater, Nov.,  Houston, Texas, Pages 547-569

8.  Regalbuto,  D.P.,  Barrera,  J.A., and Lisieki,  J.B.  1988,  In-situ Removal of
      VOC's By Means of Enhanced Volatilization":  Proceedings of the National
      Water Well Association - American Petroleum Institute Conference on
      Petroleum Hydrocarbons and Organic Chemicals in Groundwater,  Nov.,
      Houston, Texas,  Pages 571-591.

9.  Johnson, P.C., Kemblowski, M.W., and Colthart, J.D.,  1988,  "Practical
      Screening Models for Soil Venting Applications": Proceedings of the
      National Well Water Association -  American Petroleum  Institute
      Conference on Petroleum Hydrocarbons  and Organic Chemicals in
      Groundwater, Nov.,  Houston, Texas, Pages 521-547.

10. Towers, D., Dent,  M.J., and Van Arnam,  D.G.,  1989, "Choosing a Treatment
      for  VHO-Contaminated Soil", Hazardous Material Control,  Vol.  2, No. 2,


                                     249

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      Page 8.

11.  Marley,  M.  C.,  1985,  Quantitative and Qualitative  Analysis  of Gasoline
      Fractions Stripped by Air from the Unsaturated Zone";  M.S.  Thesis,
      University of Connecticut,  Department of Civil Engineering,  Page 87.

12.  Bruell,  C.J.,  and Hoag,  G.  E.,  1984, Capillary and Packed Column Gas
      Chromatography of Gasoline  Hydrocarbons  and EDB.   Proc. National Water
      Well Association/American Petroleum Institute Conference  on Petroleum
      Hydrocarbons and Organic  Chemicals in Groundwater,  Nov. 87,  Houston,  TX,
      Pages 234-266.

13.  Cliff, B.L. 1988,  "Method Development for  the Gas  Phase  Analysis of
      Gasoline and Solvent Contaminated Soil," M.S.  Thesis,  University of
      Connecticut,  Department of Civil Engineering.

14.  Hoag, G.E., and Marley,  M.C.  1986 "Gasoline Residual Saturation in
      Unsaturated Uniform Aquifer Materials."  ASCE,  Environmental Eng.
      Division, Vol. 112, No. 3,  Pages 586-604.

15.  Bruell,  C.J.,  1987, The Diffusion of Gasoline -  Range Hydrocarbon Vapors
      in Porous Media. Ph.D. Dissertation, University  of Connecticut,  Civil
      Engineering Department, 157 pages.

16.  Muskat,  M., and Botset,  H.G.,  1931,1. "Flow of Gas through  Porous
      Materials":  Physics, Vol. 1,  Pages 27-47.

17.  Boardman,  C.R., and Skrove, J.W., 1966, "Distribution in Fracture
      Permeability of a Granitic Rock Mass Following a Contained Nuclear
      Explosion":   Journal of Petroleum Technology,  Vol.  181, No.  5, Pages
      619-623.

18.  Stallman,  R.W., and Weeks,  E.P., 1969, "The Use of Atmospherically Induced
      Gas Pressure Fluctuations for Computing Hydraulic Conductivity of the
      Unsaturated Zone": Geological Society of American Abstracts with
      Programs, Pt. 7, Page 213.

19.  Weeks, E.P, 1977, Field Determination of Vertical  Permeability to Air in
      the Unsaturated Zone:  U.S.  Geological Survey Open-file report 77-346  92
      P-

20.  Rosza, R.B., Snoeberger, D.F.,  and Bauer,  J., 1975, Permeability of a
      Nuclear  Chimney and Surface Alluvium:  Livermore Lab Report UCID-16722,
      11 p.

21.  Kirkham, D., 1946, "Field Methods for Determination of Air  Permeability of
      Soil in  its Undisturbed State": Soil Science Society of America
      Proceedings, Volume 11, Page 93-99.

22.  Evans, D.D., and Kirkham, D., 1949,  "Measurement of Air Permeability of
      Soil In-situ":  Soil  Science Society of America  Proceedings, Volume 14,
      Page 65-73.
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23.  Grover,  B.L.,  1955,  "Simplified Air Permeabilities for Soil in Place":
      Soil Science Society of America Proceedings,  Volume 19,  Pages 414-418.

24.Tanner, C.B.,  and Wengel,  R.W.,  1957,  "An Air Permeameter for Field and
      Laboratory Use":   Soil  Science Society of America Proceedings,  Volume
      21,  Pages 663-664.

25.  Klinkenberg,  L.J.,  1941,  The Permeability of Porous Media to Liquids and
      Gases:   American Petroleum Institute Drilling and Production Practice.

26.  Baehr, A.L.,  and Hult, M.F.  1988,  "Determination of Air-phase Permeability
      at the Bemidji Research Site":  USGS 4th Toxic Substances Hydrology
      Technical Meeting,  Sept.,  Phoenix,  Arizona.

27.  Brooks,  R.H.,  and Corey,  A.T.  1966, "Properties of Porous Media Affecting
      Fluid Flow": Journal of the Irrigation and Drainage Division,
      Proceedings of A.S.C.E., June Pages 61-88.

28.  Parker,  J.C.,  Lenhard, R.J., and Kuppusamy,  T.  1987,   "A Parametric Model
      for Constitutive Properties Governing Multiphase Flow in Porous Media":
      Water Resources Research,  Vol. 23,  No.  4,  Pages 618-624.
                                     251

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                                 APPENDIX G
                           MODELING APPLICATIONS TO
                           VAPOR EXTRACTION SYSTEMS
            Lyle R.  Silka3, Hasan D. Cirpilia, and David L. Jordan3
INTRODUCTION

      Vapor extraction systems (VESs)  can be an effective tool in remediating
unsaturated soils contaminated by volatile organic compounds (VOCs).   The
operating principles are straightforward.   Contaminated soil is flushed with
fresh air via a vacuum extraction well,  drawing VOCs from the soil.  As the
contaminant is drawn off, more VOCs go into the vapor phase to regain
equilibrium, and are again drawn off by the vacuum.   In a clean,  homogeneous
material this procedure should work quite well to remove the VOCs.

      However, there are potentially severe limitiations for a VES operation.
Heterogeneous soil may contain isolated or dead-end pores and have variable
saturation, and thus not be strongly affected by the VES.  Since advective
movement of the VOC in these stagnant areas is negligible,  diffusion of the
contaminant will dominate.  Diffusive movement tends to be much slower than
advective movement, and thus would impede the efficiency of a VES.

      In a soil where some regions allow transport of VOCs by advection and
other regions limit VOC transport to diffusion, the advection dominated
portions of the soil would be quickly flushed of VOCs, while flushing the
diffusion dominated portions would take much longer.  The VOC extraction rate
from such a soil would be high in the early stages of operation,  during the
advection dominated portion of flushing.  Once the advection dominated region
had been flushed out, the extraction rate would tend to exhibit considerable
tailing, as VOCs slowly diffused out of the stagnant regions of the soil and
into the advection dominated region.  Therefore, the extraction rate of a VES
in the early stages of operation should not be extrapolated to the entire
flushing period, since it may only reflect the rate of contaminant removal
from the advection dominated area of the soil.  Thus, there is a distinct need
to quantify the VES process, and especially the effects of a diffusion
dominated region.
aHYDROSYSTEMS,  P.O.  Box 348,  Dunn Loring,  VA 22027
                                      252

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PREVIOUS STUDIES

      Previous studies include a one-dimensional model developed by Baehr and
Hoag,   (1987) which describes advection and diffusion of gasoline vapors in an
unsaturated sand column.  They found that the  rate of volatilization of
organics was high relative to the rate of air  flushing in areas of fairly open
porosity.  However,  they also mention that this may not hold true in areas
where  vapor extraction is diffusion limited.   Their results  also show that
even in an advection dominated system, the extraction rate will exhibit
exponential decay,  due to a shift in the organic phase to less volatile
components, as the more volatile components are flushed out  first.

      Bowman (1987)  cites empirical results from a VES installed at the Twin
Cities Army Ammunition plant in Minnesota, which displays an exponential
decline in extraction rate (Figure G-l).  From the figure, it is apparent that
the extraction rate is quite high in the early stages of operation, but drops
off quickly and approaches a steady state value asymptotically.  Even after
adding another blower at 120 days, the rate of extraction remained essentially
constant at the asymptotically low value throughout the remainder of
operation.  The flattening of the curve is probably due to both fractionation
of VOCs and diffusion dominated transport.
       600
 0

X

v^>
LJ
      400
      200
   LU
   CC
        0
                                                 T	'	1
                 HYDROCARBON  REMOVAL
                                             - TCE
                                             - TOTAL VOCs
                                                     3  blowers
       0      40      80     120     160     200
                      DAYS  OF  OPERATION
                                                       240
280
         Figure G-l.  Hydrocarbon Removal from the Twin Cities site,
                                   253

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      Woodward and Clyde (1985) also present empirical data from a
contaminated site in Tacoma, Washington (Figure G-2) .   Their data indicate
that the extraction rate reached an equilibrium value.  One possible
explanation of their data is that the rate of air flushing is lower than the
rate of volatilization, thus the vapor being extracted was continually being
re-saturated with VOCs.  Therefore, the system remained in the advection
dominated stage, and did not reach the stage where the rate of VOC removal
would be limited by diffusion.   The Lauck's Laboratory dichloroetheylene (DCE)
curve does show that the rate of extraction drops off significantly after an
initial plateau, and remains fairly constant for three days.  This may
represent a transition from the advection dominated to the diffusion dominated
regime.

CONCEPTUAL MODEL

      The proposed conceptual model consists of a three layer case, shown in
Figure G-3.  A high porosity, low soil moisture middle layer (Region 1) is
bounded by two low porosity, saturated layers (Region 2).  We assume that a
vacuum applied at the right hand boundary creates a constant flux of soil gas
out of the system.  Also, we assume that contaminant transport in Region 1 is
advection dominated, and that contaminant transport in Region 2 is diffusion
dominated.  VOCs in the soil gas in Region 1 should be flushed from the system
fairly rapidly since the water content in Region 1 is low and the soil
porosity is high.  As Region 1 is flushed and a concentration gradient is
established from Region 2 into Region 1, VOCs will begin to diffuse from
Region 2 into Region 1, volatilize into the soil gas, and be advectively
flushed from the system.  We used trichloroethylene (TCE) as our generic VOC
in this study.

      For the purposes of this modeling, a simple one -dimensional mass balance
approach is used.  An  initial contaminant concentration is given for the
Region 2.  The concentration of VOC in the soil gas in Region 1 is then given
by Henry's law
where CG is the concentration in the soil gas in Region 1, CL is the
concentration  in the liquid in Region 2, and KH is Henry's constant.

      The  change in mass of contaminant  in each of the two regions  is acounted
for by a simple finite -difference equation.  In Region 1, both  diffusion  and
advection  are  considered, although advection will be the  dominant transport
process.   Using a  simple finite-difference formulation, the  change  in VOC
concentration  in Region 1 is:

                            dCx/dt = DjjdCi/dz + vq

where Ci is the concentration in Region  1,  DE is  the  effective diffusion
coefficient, v is  the velocity of the soil gas, and z is  the vertical
direction.  Silka  (1988) derives DE, which is an effective diffusion
coefficient for the VOC which considers  partitioning, adsorption, and
tortuosity.
                                      254

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            300
       >

       E
       O
       O
            100
             50
            30
             10
            0 5
            03
            0.1
                  1.1.2.2-Tnri.
                     • Onute Anjlyiu ppm (V) vj. Time



                     O Laucks laboratory Anilysis
                                      456

                                        TIME (days)
                                                             1,1.2.2 -

                                                             Trtr*chk>roeth«ne
                                                               TCE
                                                               TCE
                                                                1.1.2.2'Tiua.
                                                               DCE
                                                              ,DCE
B     9
           10
Figure G-2.   Concentration of Extracted Soil  Gas  from  the Tacoma Site,

                      (After Woodward and Clyde,  1985).
                                        255

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            convection
                                                            region 2
                                                             region i
                                                                     q
                                                                        out
      Figure G-3.  Conceptual Model Used for the Numerical Modeling
                  The  dark  regions are diffusion dominated,  while the light
                  region is convection dominated
      Diffusion from the  liquid layer in Region 2 to the gas layer in Region 1
is assumed to be controlled by the concentration gradient between the two
layers.   The diffusion coefficient is given by a turbulent diffusion
coefficient (Thibodeaux,  1979) for diffusion from a liquid into a gas.   Thus
the mass balance equation for the liquid layer in Region 2 is:

                              dC2/dt = DtdC2/dz

where C2 is  the  liquid  concentration in the Region 2 layer,  and Dt is the
turbulent diffusion coefficient.

Assumptions

      The following assumtions were made in the course of developing this
model and the associated  modeling results.  These are:

      (1)   There is no volatilization loss to the atmosphere (i.e., the top
            of the system is an impermeable boundary).  This could be realized
            in the field  by sealing the ground surface with plastic or
            asphalt.

      (2)   The adsorbed  phase is neglected (i.e., it is assumed that VOCs
            will not desorb back into the system from organic matter or soil
            solids).  There is no partitioning between soil solids and soil
            gas or water.
                                     256

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      (3)   Diffusion in the Region 2 layer is Fickian.

      (4)   All soil properties are homogeneous in each soil region.

      (5)   VOC soil-gas concentrations achieve equilibrium instantaneously
            (i.e., the rate of extraction is significantly slower than the
            rate of volatilization of VOCs).

      (6)   Model geometry is linear and one-dimensional.

Results

      Results of the numerical model are shown in Figures G-4a and G-4b.
Reasonable values of extraction rate, q, were chosen and soil gas velocities
were calculated from these.  The extraction values chosen seemed small (1CTA  -
10"3  cfm),  compared with values used  at  several actual  sites  (typically  101 -
102 cfm).   However,  much of the  flux in the actual field tests  may be due to
short-circuiting with the atmosphere, and thus may not be representative of
advection through the porous media.  In our model, we assume that all of the
gas flux in the system acts to extract the soil gas.

      Note the general character of the concentration curves, which show an
initial region of rapid extraction, followed by a steep decline in the rate of
extraction.  The total concentration in the extracted soil gas approaches an
asymptotic value, which is controlled by diffusion in the liquid layers.  The
initial character of the concentration curve is controlled mainly by the
extraction rate, q.   A low extraction rate (depicted in Figure G-4) obviously
takes longer to flush out the initial soil gas in Region 1.

      Both curves compare favorably with results cited in Bowman (1987), shown
in Figure G-l.  The model results have the same initially high rate of
extraction, followed by a rapid exponential decline in the rate of extraction.
However, the model only compares favorably with one curve from the Woodward
and Clyde data (1985), the DCE data from Lauck's Laboratory.   This curve shows
an initial high concentration in the system, followed by a rapid decrease in
concentration, dropping off to a residual value of about 0.1 ppm.  The other
Woodward and Clyde curves do not compare as favorably to model predictions.
This may be due the much higher volatility of DCE and lower mass of DCE in the
soil, or the possibility that the TCE curves depict situations where TCE
remains in the region where the high porosity zone is still being flushed out,
and rate limiting diffusion has not come into effect.

CONCLUSIONS

      Numerical modeling results of a vapor extraction system qualitatively
agree with results from several empirical studies.  The proposed model
consists of two zones: an unsaturated, high porosity region,  which is
advection dominated,  and a saturated, low porosity zone, which is diffusion
dominated.   The response of such a system to a VES has been shown to consist
of an initial period of rapid vapor extraction,  whereby the advective zone is
flushed of soil gas.   The advective period is followed by a sharp exponential
decrease in the extraction rate as the system becomes diffusion dominated.
                                     257

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a
a
u
o

6
o

UJ
u
t-

in
<
O

o
u
                                                              100
                                                                        120
      250
 E
 a
 a.
 Ul
 u
 O
 u
 a
 u
 u
 <
 cc
240 -



230.-



220 -



210 -



200



190

    i

180 -j


170 -



160 -


150 -



140 -


130 -J



120 ->



1,0 J



100 -r
          0
                    2C
                               40
                                   60



                                £. hours
                                                    80
                                                              100
     Figure  G-4.
             Numerical  Modeling Results, (a)  q  = 0.0001  cfm,

             (b) q =  0.001 cfm, initial concentration is 500

             ppm TCE  in the saturated zone.
                                       258

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      Thus, it is important to consider the diffusion stage when designing and
pilot testing a VES,  since predictions of extraction time for a given remedial
action based on the early response of the VES can be erroneous.

      One other factor should also be considered in designing a VES.   In a
highly adsorptive soil, such as a soil with a high organic content,  a large
amount of contaminant would be resident in the solid matrix of the soil.   This
contaminant could easily be desorbed and released to the water and/or gas
phases in the soil, and thus re-contaminate the soil gas.  This would
necessitate further soil gas extraction.   The amount of contamination in the
different phases of the soil could easily be measured to gauge the magnitude
of these effects. Effects of this situation are currently being investigated.
                                     259

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                                 APPENDIX H
                 PERFORMANCE OF IN SITU SOIL VENTING SYSTEMS
                           AT JET FUEL SPILL SITES

                  David W. DePaolia,  Stephen E.  Herbesa,  and
                     Michael G.  Elliott,  Capt,  USAF,  BSCb
INTRODUCTION

      The Air Force Engineering and Services Center and Oak Ridge National
Laboratory (ORNL) are performing a field demonstration of in situ soil venting
at a 27,000-gal jet fuel spill site at Hill Air Force Base (AFB),  Utah.   In
situ soil venting is a soil cleanup technique that uses vacuum blowers to pull
large volumes of air through contaminated soil.   The air flow sweeps out the
soil gas, disrupting the equilibrium existing between the contaminants on the
soil and in the vapor.  This causes volatilization of the contaminant and
subsequent removal in the air stream.   In situ soil venting has been used for
removing volatile contaminants such as gasoline and trichloroethylene, but a
full-scale demonstration for removing jet fuel from soil has not been
reported.

This paper briefly describes the jet fuel spill site and the design and
results to date of our full-scale in situ soil venting system.

DESCRIPTION OF JET FUEL SPILL SITE

      On January 9, 1985, in a fuel yard at Hill AFB, Utah,  -27,000 gal
(102,000 L) of jet fuel (JP-4) spilled on the ground after an automatic
filling system malfunctioned and underground storage tanks overfilled.  JP-4
is made by blending various proportions of distillate stocks such as naphtha,
gasoline, and kerosene to meet military and commercial specifications.  In
general, it has more heavy molecular weight hydrocarbons and is less volatile
than gasoline and other contaminants which have previously been investigated
for remediation by in situ soil venting.

      An initial surface cleanup effort at the spill site resulted in the
recovery of about 1000 gal (3800 L) of JP-4 with the remaining portion
Engineering Development Section,  Chemical  Technology  Division,  Oak Ridge
 National Laboratory*, Oak Ridge,  Tennessee 37831-6044
bAir Force Engineering and Services  Center, United  States Air  Force,  Tyndall
 Air Force Base, Florida 32403-5001
^Operated by Martin Marietta Energy  Systems,  Inc.,  for the  U.S.  Department of
 Energy under contract DE-AC05-840R21400

                                      260

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infiltrating into the soil.  A soil sampling study conducted in December 1985
delineated the areas of soil having >1% fuel in the soil.  Based on this
study, a decision was made to excavate the highly contaminated soil near the
underground storage tanks and then place the tanks in an aboveground concrete
enclosure.

      To determine if the JP-4 had the potential to reach the water table,
further investigations were completed to evaluate the geohydrological
characteristics and the contaminant level in the soil of the site.
Investigations included seismic and resistivity tests, soil vapor surveys, and
core-boring analysis.

      Based on past geophysical investigations at Hill AFB, it was known that
the Provo formation comprises the surface strata beneath the spill site.  The
Provo formation consists of medium to fine sands with thin, interbedded layers
of silty clay.  Regionally, these sands are underlain by clay layers that
extend to a depth of 600 ft (180 m) below land surface (BLS) at a well which
is located 500 ft (150 m) south of the spill site.

      A total of 43 soil borings were performed for characterization of the
spill site.  The lithologic logs describe a surface layer of brown silty sand
about 4 ft (1.2 m) thick, underlain by brown sand to a depth of 23 to 35 ft (7
to 10.7 m) throughout the spill area.  Variable-spaced clay layers were
reported at depths between 23 and 42 ft (7 and 12.8 m).

      The Delta aquifer, at an average depth beneath Hill AFB of -600 ft
(180 m),  is the regional aquifer of greatest significance as a water-bearing
unit because of its high permeability.  The Sunset aquifer, at a mean depth of
300 ft (90 m) BLS is less permeable, and no wells of large volume draw from
this unit.  Both aquifers are isolated from the surface by impermeable
formations which give rise to artesian flows in some wells in the area.

      Local perched groundwater is found above the clay layers that confine
the regional aquifer.  Perched groundwater was encountered in one borehole
near the fuel tanks at a depth of 32 ft (9.8 m) BLS.  Perched water was also
encountered at a depth of 51 ft (15.5 m) BLS, while water was present at a
depth of 57 ft (17.4 m) BLS in a monitoring well.  Saturated conditions were
found at the clay layer [-27 ft (8.2 m) BLS] in several boreholes beneath the
excavated tanks.  The seismic and resistivity tests indicated the presence of
perched groundwater at a depth of 43 to 46 ft (13.1 to 14.0 m) BLS.  However,
data from soil borings suggest that the perched groundwater is variable in
depth and probably not continuous throughout the spill site.

      A soil gas survey was conducted at the spill site in 1986,  with probes
installed to a depth of 10 ft (3.0 m).  Highest values extended from the point
of fuel spillage west across the spill area, approximately along the path of
fuel flow.  A second survey was conducted in September 1987.  Probes installed
to a depth of 1 ft (0.3 m) within and outside the originally defined plume
boundaries resulted in profiles of fuel vapor distribution in the soil that
were virtually identical in areal extent to the earlier results.   No fuel
vapors were detected west of the fence that bounds the fuel storage area.
                                      261

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      Analytical results from the soil boring samples showed the residual fuel
concentrations to be highest between the surface and a depth of 10 ft (3.0 m)
in the area of the spill.   Little lateral movement appears to have occurred
since the spill.  Residues and fuel odors have been detected in two boreholes
in the western portion of the spill area, indicating that downward migration
of vapors and/or fuel to 50 ft (15.2 m) BLS has occurred in at least several
locations.  In the eastern portion of the spill area, fuel appears to have
migrated to the surface of a clay layer [33 ft (10 m) BLS].   Beneath the tank
excavation, fuel levels are highest between 18 and 23 ft (5.5 and 7.0 m) BLS
and fuel was detected as deep as 33 ft (10.0 m) BLS.

      Based on the site investigations, it was concluded that high levels of
fuel hydrocarbons were present to depths less than 50 ft (15.2 m).  Also,
because no continuous confining layer was identified at the spill site,  a
possibility exists for downward migration of the JP-4.  Therefore, a no-action
alternative was not applicable at the site, and a remediation technique must
be implemented to prevent the contamination of groundwater.   Since the soil is
very sandy and high permeabilities for air flow were expected, the Hill AFB
spill site provided an ideal setting for investigation of in situ soil venting
as a remediation technique for the JP-4 spills.

DESIGN OF FULL-SCALE IN SITU VENTING SYSTEM

      Based on information from the site characterization and a one-vent pilot
test that was conducted in January 1988,-1 a full-scale in situ soil venting
system was designed for collection of data and remediation of the JP-4
contaminated soil.  The venting system was designed to consist of the three
subsystems  (Figure H-l):  (1) a vertical vent array in the area of the
spill,(2) a lateral vent system under the new concrete pad and dike for the
tanks, and (3) a lateral vent system in the pile of soil that was retained
after excavation for the tanks.  This design includes features that permit
evaluation of several factors affecting contaminant transport and subsurface
air flow.

      The vertical vent subsystem consists of 16 vents and 31 pressure
monitoring points, covering an area of 120 x 100 ft  (36.6 x 30.5 m).  Half of
the vertical venting area is covered by a plastic liner for comparison of flow
patterns with and without a surface barrier.  The vertical vents were located
based on the best knowledge of the contaminant distribution, allowing
flexibility in operation to investigate different venting strategies.  The
vents are arranged in a square grid with a 40-ft (12.2-m) spacing.  The center
line of vents has a 20-ft (6.1-m) spacing and is aligned from the existing
vent installed for the single-vent pilot test to the point at the tank where
the spill occurred.  The 20-ft  (6.1-m) spacing is not intended to
be an optimized vent spacing; rather,  it is used to  allow operation of  several
vent configurations.  Each vent  is valved separately  to allow each  to act as
either an extraction vent or as  a passive  inlet vent.  The vents were
constructed of 4-in.-(10.2-cm-)ID schedule 40 PVC screen  [slot width of
0.02 in.  (.05 cm)] and were installed  in a 9 5/8-in.  (24.4-cm) augered hole.
Flush-joint schedule 40 PVC was  used for riser pipes.  They were  screened
between  10 and  50 ft (3.0 and 15.2 m)  BLS  and capped at the lower end.
                                      262

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 CONCEPTUAL DRAWING OF IN SITU SOIL VENTNG DEMONSTRATION SYSTEM
                                 HILL AFB
             ANALYTICAL TRAILER
                                               EMISSIONS CONTROL
                          LATERAL VENT ARRAY
    VERTICAL VENT ARRAY
Figure H-l.  Conceptual Drawing of In Situ Soil  Venting Demonstration System
                                   Hill AFB
      The pressure-monitoring points for the  vertical  vent  subsystem were
located to map the pressure distribution at various  depths  in order to
determine air flow patterns resulting from different venting strategies.   The
pressure monitoring points may be broken down into three  groups:   (1) points
surrounding the vents in the area with a surface  barrier,  (2)  points
surrounding the vents in the area without a surface  barrier,  and  (3) points
surrounding the entire spill system to determine  areal vacuum influence.   The
configuration of the pressure monitoring points  is such to  provide  pressure
distribution data while minimizing the influence  of  soil  inhomogeneities.
Each pressure monitoring point is installed in a  separate borehole  to avoid
uncertainty invited by boreholes with multiple completions.   The  pressure-
monitoring wells were constructed of 1-in.-(2.54-cm-)OD flush-joint PVC with a
2-ft (0.6-m) screened section, capped at the  lower end.

      Large diameter PVC pipe [6 to 8 in. (15.2  to 20.3 cm)]  and  fittings  were
installed to direct flow from the vents to the blower.  Pipes were  run on
stands along the ground, meeting a manifold at the center of the  vertical
system.  The pipes throughout the system were heat traced and insulated with
fiberglass insulation, wrapped with aluminum  covering.

      The lateral vent subsystem under the new concrete pad was installed  at
the time of the tank excavation.  This system is  being used to investigate
behavior of vents in horizontal geometry while decontaminating the  soil
beneath the tanks that was not removed during the tank excavation.   The
                                     263

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subsystem consists of six lateral vents constructed from 35 ft (10.7 m) of 4-
in. (10.2-cm) perforated polyethylene drainage pipes that lie at a depth of
about 20 ft (6.1 m) ELS, and at a distance of about 15 ft (4.6 m) apart.  The
vents induce air flow in the soil between a plastic liner at a depth of 15 ft
(4.6 m) and an underlying clay layer at a depth near 26 ft (7.9 m).   Each vent
pipe is valved separately to allow each to act either as an extraction vent or
as a passive air-inlet vent.  Thirty-two soil-gas monitoring probes  were
placed in the soil at the time of the tank excavation.  The probes are used
for monitoring of pressure and sampling of soil gas below the new concrete
dike.

      The venting system in the excavated soil pile consists of a series of
lateral vents placed in the contaminated soil that was removed during the tank
excavation.  Approximately 52,000 ft3 (1,500 m3)  of  this  soil  (initially
within the zone contaminated to greater than 1 wt % of hydrocarbons) was
removed and formed into a noncompacted pile.  The pile is approximately 160 ft
(48.8 m) long with a nearly triangular cross section that is 43 ft (13.1 m) at
the base and 12 ft (3.7 m) high.  Eight vents were placed in the pile at a
nominal level of 5 ft (1.5 m) high and 18 ft (5.5 m) apart.  The length of the
vents within the pile is about 36 ft (11 m).  To prevent erosion of the pile
due to wind and rain, the pile was dressed and covered with a geotextile
matting.  The matting is a woven wood fabric with netting on both sides which
allows for air flow but prevents soil from escaping.  The lateral vents for
both the pile and under the new concrete pad were constructed from 4-in.
(10.2-cm) perforated polyethylene drainage pipe wrapped in filter fabric.

      A blower/emission control system was installed for inducing air flow to
three subsystems and for treating emissions as necessary to meet regulatory
requirements.  The two rotary-lobe blowers provide the capability for
extraction of up to 2000 ft3/min (57 std m3/min)  of  gas  from  the  three
subsystems.  In order to protect against potential hazards presented by
combustible gas mixtures, flame arresters were installed at the inlet to each
blower; the blowers are controlled by an automatic shutdown system based on a
combustible gas detector.  Two catalytic oxidation units are used for
conversion of the jet fuel hydrocarbons to carbon dioxide and water before
discharge into the atmosphere.  The propane-fired units differ in the
configuration of their catalyst beds, one having a fluidized-bed design and
the other containing a fixed-catalyst bed.  The units are being evaluated in
terms of economics and reliability, as well as hydrocarbon destruction
efficiency.  A knock-out drum, flowmeters, and gas monitors were also included
in the system.

RESULTS

      Operation of the Hill AFB full-scale in situ soil venting system began
in December 1988.  As of June 15, 1989, -70,000 Ib  (32,000 kg) of hydrocarbons
have been extracted from the JP-4-contaminated soil in 42 million ft3 (1.2
million std m3) of gas.   Nearly all of the hydrocarbons were removed from the
vertical vent system area, since the lateral system has been operated
sparingly to this point and the pile has retained little contamination.

      Progress made in decontamination of the site may be seen by examination
of Figures H-2 through H-4.  These  figures present contours of the depth-


                                      264

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120

1  10

'100

  90

  80

  70

  60

  50

  40

  30

  20

  10
     0
20
40
60
80
100
120
                                                                          140
             Figure  H-2.   Soil  Gas  % LEL  Contours  - February 2, 1989.
  averaged soil-gas  concentration  in  the vertical vent area at two-month
  intervals.   The  data were  collected by extracting gas from each vent
  separately  and measuring the hydrocarbon  levels in percent of the  lower
  explosion limit  (LEL).  The numbers on the plots refer to percent  LEL, with
  100 percent LEL  equal  to 13,100  ppm JP-4.  The scale in each direction of the
  plots  is in feet,  with the labelled asterisks denoting the position and
  hydrocarbon concentration  of each vent.   As expected, the contours show a
  trend  of decreasing soil-gas hydrocarbon  concentration, with the largest
  changes  occurring  during the initial portion of venting.

        Figure H-5 presents  the hydrocarbon concentration measured in the
  extracted gas  as a function of the  extracted gas volume.  The data are
  characteristic of  soil venting operations, with initially high hydrocarbon
  levels and  a rapid decrease in concentration.  Two discontinuities of interest
  are present in the plot.   The first is an abrupt decrease in concentration
  from -35,000 ppm hexane equivalent  to 24,000 ppm, which occurred during a two-
  week shutdown  after about  42,000 ft3 (1,200 m3)  of  gas had been  extracted.
  This decrease  is similar to a concentration decrease that was measured after
                                       265

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    0
20
40
60
80
100
120
140
             Figure H-3.  Soil Gas % LEL Contours - April 2, 1989.
-50,000 ft3 (1,400 m3) of gas was extracted during the pilot test at the site.
It is postulated that this change in concentration is the result of the soil
pore space in the zone of influence of the well being swept by the atmospheric
air.  The equilibrated light fractions in the vapor are removed, and further
hydrocarbon extraction is caused by volatilization.   The second discontinuity
marks the point at which single-vent extraction was shifted from vent 7 (the
third vent from the left in the center line of vents) to vent 10 (the second
vent from the right in the center line).   The concentration increase is due to
the fact that the latter well is centered in the most highly contaminated soil
zone.  Other vents in the center line have since been included in operation
with vent 10.

      The concentration decrease shown in Figure H-5 deviates markedly from
the straight-line logarithmic behavior that was projected for the system by
noting the results of other researchers at single-component spills.2"4  This
observed behavior may be explained in terms of the contaminant mixture present
in the JP-4-contaminated soil.   Figure H-6 shows how the contaminant mixture
affects the extracted gas hydrocarbon concentration by presenting the relative
weights of different hydrocarbon fractions as measured in gas chromatographic
                                      266

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            120
            100  -
                      20      40     60     80     100     120    140
           Figure H-4.   Soil Gas % LEL Contours - June  10,  1989.
    50,000
          r  °      °   cc


                         0  o   o,
O
UJ

LU
Z
<
X
UJ
I   10,000
a.
CL
z
O
O
z.
O
O
     1000
o  03
                 o
                   °
             6?    o
         0.003  0.010         0.1           1.0           10

                              3AS EXTRACTED (106 ft3)

              Figure H-5.  Hill  Air Force Base  Soil Venting

                 Extracted Gas Hydrocarbon  Concentrations.
                           100
                                     267

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    0.20
    0 10
 2
 o
 2
 UJ
 O
 2
 O
 o
     0.01
                   A
                        A
                           A   A
                                   A
                                           A
                                                             AA
         0.1
1.0           10           100          1000

    VOLUME OF EXTRACTED GAS (103 m3)
5000
               Figure H-6.   Comparison of Equilibrium Model and
                       Hydrocarbon Concentration Data.

analysis of gas samples.   As was noted by Thornton et al.5  in their pilot-
scale study of venting of gasoline from sand,  the  lighter hydrocarbon
fractions are a large portion of the  hydrocarbons  in the initial samples.   As
the contaminant plume is  depleted of  the lighter fractions, the hydrocarbon
distribution in both the  soil and the extracted gas is shifted  in favor of the
heavier fractions, lowering the total concentration in the gas  as a result of
the lower vapor pressures.

      A simple equilibrium model similar to that of Marley and  Hoag6'7 was
used for projection of system behavior. The model  assumes perfect contact of
the gas with the hydrocarbons and equilibrium,  as  calculated by Raoult's law,
controlling transport to  the vapor phase at each point in the soil.   The
results obtained by input of a spill  size of 26,000 gal (98,000 L) of jet fuel
with a distribution of components as  measured in soil samples taken in October
1987 are shown in Figure  H-7.  Agreement with the  actual results, in
particular in matching the shape of the curve,  is  quite good, given the
simplicity of the model and the uncertainties in the input parameters.   Model
results are expected to improve with  the input of hydrocarbon distributions
that were derived from soil samples taken during vent installation by a
sampling technique which  was revised  to prevent losses of lighter fractions.
                                     268

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                                    1
                                     !„
              184
 ISO    212     218     243     490

VOLUME OF EXTRACTED GAS (103 m3)
                                                           748
I
EUD ce-c9
^^ C5"C6 l[- 	 l C8"C7 ^^ C7~C 8
     Figure H-7.  Variation of Hydrocarbon Distribution in Extracted Gas.

It is expected that an equilibrium model may give good results in the earlier
that were derived from soil samples taken during vent installation by a
sampling technique which was revised to prevent losses of lighter fractions.
It is expected that an equilibrium model may give good results in the earlier
portion of venting, but it may deviate greatly from reality as soil
contamination is lowered, increasing the importance of (1)  partitioning both
on soil and in the aqueous phase, and (2) airflow bypassing contaminated soil
zones.

      A potentially significant means of hydrocarbon removal due to in situ
soil venting is enhanced biodegradation.  The increased oxygen levels in the
soil gas due to infiltration of atmospheric air may considerably stimulate
biological activity.  To evaluate this factor, carbon dioxide and oxygen are
being measured in the extracted gas.  Initially, high C02 (11%)  and low oxygen
(1%) levels were measured in the soil gas.   As venting continued, the C02
levels decreased and oxygen levels increased, with an abrupt change in both
occurring at the same extracted gas volume noted for the change in hydrocarbon
concentration.  Carbon dioxide levels have continued to be an order of
magnitude higher than background.  From the start of operation until May 26,
approximately 40,000 Ib (18,000 kg) of C02  had been extracted (after
subtraction of background C02 levels).   If  one considers  the hydrocarbons as
the only carbon source (a reasonable assumption for the sandy soil), the C02
corresponds to about 12,700 Ib (5,800 kg) of hydrocarbons.  Considering that
half of the hydrocarbons consumed by bioactivity is converted to C02 and half
converted to biomass,8 biodegradation may account for destruction of about
25,400 Ib (11,500 kg) of hydrocarbons.  This corresponds to 38% of the
hydrocarbons removed by volatilization.   Thus, bioactivity contributed 27.5%
of total hydrocarbon removal.
                                      269

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      A two-week shutdown of the system was performed in early June to
evaluate biological activity.  Gas samples taken from each vent after this
period were analyzed for carbon dioxide and oxygen.   The resulting contour
plots are shown in Figures H-8 and H-9.  Comparison of these plots with Figure
H-4 shows that soil gas is depleted of oxygen and rich in C02 in areas where
hydrocarbon levels are high, whereas a small change in oxygen from atmospheric
levels and little or no C02 generation is  seen in areas  with low remaining
hydrocarbon contamination.  These results are important because they show that
biological activity may continue to be significant despite no addition of
nutrients.  Biodegradation may prove to be the means for decontamination of
heavier compounds that are not readily volatilized.

      The results obtained thus far in the Hill AFB in situ soil venting
demonstration have shown that this technique is very effective for removal of
large amounts of jet fuel from soil in a very short period of time.  It
remains to be seen if the technique may achieve complete remediation of a jet
fuel spill site.  Our continued testing is aimed at answering this question as
well as determining the importance of various factors in hydrocarbon removal.
We will continue to sample the extracted gas to determine both the total
hydrocarbon levels and hydrocarbon distribution.  The effects of moisture on
volatilization and bioactivity will be determined by monitoring soil moisture
and extracted gas humidity.  The effect of heat addition to the soil for need
volatilization will be tested by routing heated air from the catalytic
oxidation units to vents acting as air inlets.  Also, soil sampling
will be conducted in October 1989 to determine the extent of hydrocarbon
removal to that point.  This upcoming data should provide insight into whether
in situ soil venting is a viable remediation technique for jet fuel-
contaminated soil and will be valuable for modeling and application of venting
to other sites.
                                      270

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   120
   100
                      '+<*>      60      80     100     120    140
    120
         Figure H-8.   Soil Gas % Oxygen - June  10,  1989.
LU
UJ
    100  -
     80  -
60  -
     40
     20
        0      20      40     60      80      100     120    140

                                 FEET

     Figure H-9.   Soil  Gas % Carbon Dioxide  - June  10,  1989.
                                271

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                                  REFERENCES

1.   Elliott,  M.  G.  and D.  W.  DePaoli,  "In Situ Venting of Jet  Fuel-
    Contaminated Soil," presented at the 44th Purdue  Industrial Waste
    Conference,  May 10, 1989,  West Lafayette,  Ind.

2.   Anastos,  G.  J., P. J.  Marks,  M.  H.  Corbin,  and M.  F.  Coia,  In Situ Air
    Stripping of Soils Pilot  Study,  Final Report,  AMXTH-TE-TR-85026,  October
    1985.

3.   Payne,  F. C.,  C.  P. Cubbage,  G.  L.  Kilmer,  and L.  H.  Fish,  "In Situ
    Removal of Purgeable Organic  Compounds from Vadose Zone  Soils,"  In:
    Proceedings of the 41st Purdue University Industrial  Waste Conference,
    West Lafayette, Ind.,  May 14,  1986.  pp.  365~69.

4.   Johnson,  J.  J., and R. J.  Sterrett,  "Analysis  of  In Situ Soil Air
    Stripping Data,"  In: Proceedings of the 5th National  Conference  on
    Hazardous Wastes  and Hazardous Materials,  HMCRI,  Las  Vegas, Nev.,  April
    19-21,  1988.

5.   Thornton, J. S.,  R. E. Montgomery,  T. Voynick,  and W. L.  Wootan.   "Removal
    of Gasoline Vapor from Aquifers by Forced Venting," 1984 Hazardous
    Material Spills Conference Proceedings,  April  1984.

6.   Marley, M. C.  and G. E. Hoag,  "Induced Soil Venting for
    Recovery/Restoration of Gasoline Hydrocarbons  in  the  Vadose Zone," In:
    Proceedings of NWWA/API Conf.  on Petroleum Hydrocarbon and Organic
    Chemicals in Groundwater - Prevention, Detection  and  Restoration,  pp.
    473-503,  November 1984.

7.   Marley, M. C.  "Quantitative and Qualitative Analysis  of Gasoline Fractions
    Stripped by Air from the Unsaturated Soil Zone,"   University of
    Connecticut, Master's Thesis,  1985.

8.   Thornton, J. S. and W. L. Wootan,  Jr., "Venting for the Removal of
    Hydrocarbon Vapors from Gasoline from Gasoline Contaminated Soil," J.
    Environ.  Sci.  Health A17(1),  pp. 31-44, 1982.
                                      272

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                                  APPENDIX I

                   IN  SITU  VAPOR  STRIPPING RESEARCH PROJECT:
                              A  PROGRESS REPORT
           Robert D. Mutch, Jr, P.Hg.,P.E.a, Ann N.Clarke, Ph.D.'
                  David J.  Wilson,  Ph.D.b,  and Paul D. Mutch*
      A two-year long in situ vapor stripping research project is being
conducted at the Ciba-Geigy Plant in Toms River, New Jersey.  The research is
being conducted by AWARE Incorporated and is co-funded by the USEPA Small
Business Innovative Research Program and Ciba-Geigy Corporation.   The research
project, which began in August of 1988, involves a closely monitored
installation of in situ vapor stripping.  The research program calls for the
vapor stripping facilities to operate for a period of one year.  This paper
reports on findings through the first five months of operation.

      The general objectives of the research program are to improve the
scientific foundation for this remedial technology, better define its
technical limitations and further refine the mathematical model of the
process, developed in an earlier phase of the research program (AWARE, 1987).
A further objective of the research is to evaluate the performance of
granulated activated carbon as a treatment agent for the extracted vapors.

      The Ciba-Geigy Toms River Plant lies in the Atlantic Coastal Plain
Physiographic Province in Toms River, New Jersey.  The site is underlain by
the Cohansey Sand, a geologic formation consisting predominantly of moderate
to high permeability sand, interbedded with finer-grained, often lenticular,
strata of silt and clay.  The site chosen for the research program lies within
the central production area of the 1200 acre plant site at the location of
several recently demolished chemical process buildings.  Soil contamination
was detected in the razing of the buildings, presumably from underground
storage tank and process pipeline leaks.

INITIAL SOIL CONTAMINATION LEVELS

            A drilling program was undertaken in order to characterize initial
levels  of soil contamination within the study area.  The program consisted of
26 exploratory borings and collection and analysis of 40 soil samples.  The
complete menu of organic priority pollutant analyses was run.  Table 1-1
aECKENFELDER,  Incorporated.   1200 MacArthur Boulevard,  Mahwah,  NJ  07430
bSenior Research Associate,  ECKENFELDER,  Incorporated;   Professor  of
  Chemistry, Vanderbilt University, Nashville,  TN

                                      273

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summarizes the specific chemicals detected and their respective concentration
ranges (including limits of detection).   Not all chemicals were identified as
present at all probe locations or at all depths at a given location.   No acid
extractable compounds were detected at a limit of detection of 2.0 ppm (mg/kg)
with the exception of one sample which exhibited a phenol value of 3.0 ppm.
The most prevalent soil contaminants in the study area are
1,1-dichlorobenzene,  1,3-dichlorobenzene, 1,4-dichlorobenzene, and  1,2,4-
trichlorobenzene.

RESEARCH PROGRAM FACILITIES AND EQUIPMENT

      One of the initial tasks of the research proposal was the drilling of a
four-inch diameter extraction well in the approximate center of the area of
contamination.  The well consisted of a four-inch diameter, five-foot long,
factory-slotted PVC screen which was set slightly above the water table on a
four inch diameter PVC casing.  In the area of the project, the water table is
at a depth of approximately 20 feet.  A series of 38 soil gas probes were
installed at radial distances of approximately 20, 40, 60, and 80 feet from
the extraction well.   A number of the probes were constructed as clusters with
individual probes at depths of 5, 10, and 15 feet below ground surface.  A
sketch of a typical soil gas probe is illustrated in Figure 1-1.  The probes
were constructed of Teflon tubing and were installed by means of a truck-
mounted hollow stem auger rig.  The screened section of the probe was  sand-
packed and the remaining annular space was sealed by bentonite pellets  and
grout.  The probes allow for measurement of in situ soil vacuum and also
permit sampling of soil gas quality.  Twelve of the probes were fitted with
temperature thermisters to permit measurement of in situ soil temperature.

      Extraction of the soil gas vapors and much of the monitoring is
performed by AWARE's In Situ Vapor Stripping Pilot Unit.  The eight-foot by
12-foot long pilot unit trailer houses two New York Blower Model 2606-A
pressure blowers.  The blowers utilize a 26-inch aluminum compression fan
blade encased in a steel-frame housing.   They are powered by 7-1/2 hp, 460
volt, 3-phase motor so at the rated 3500 rpm fan speed, the two blowers
produce 50.5 inches water column pressure on the outlet at a flow rate of 400
scfm.  The blowers and associated ducts are configured for individual, series,
or parallel operation, depending upon flow rate/pressure requirements.

      The pilot unit trailer also contains a baffled demister to remove water
droplets from the air stream and instrumentation and controls for operation of
the system and the monitoring of system performance.  A layout of the pilot
unit trailer is illustrated in Figure 1-2.  There are five sampling ports in
the duct work to allow sampling of extracted gas quality at various points in
the system.  Measurements of temperature and pressure can also be remotely
taken at each sampling port.  Treatment of the extracted gas  is accomplished
by use of granulated activated carbon.  A carbon canister is  set up outside
the trailer as indicated in Figure 1-2.  An HNU Model PI-201 photoionization
monitor with an Esterline Angus Model 410 chart recorder is utilized to
continuously record gas quality.  An electronic control panel, in conjunction
with a Masterflex pump, automatically samples each of the five gas monitoring
probes and a calibration gas cylinder once every hour.  The automatic
sequencing can be overridden  if manual readings are desired.  The HNU
photoionization detector output data is  stored on the chart recorder for


                                      274

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1/4" TEFLON-
 TUBING
                                                 PRESSURE-VACUUM
                                                     U ANOMETER
                                 2CO UESH  S1LKSCREEN
 Figure 1-1.  Sketch  of Typical Soil Gas  Probe.
                       275

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           DUCT SYSTEM OVERVIEW  LAYOUT
                            CONTPOL DOXES
                             FOR BLOWEnS
        EXHAUST
                                                                      PITOT TUBE
                                                                  — INSULATED EXHA3T

                                                                       DUCT
EXTRACTION WELL
                      VLEXIOLE DUCT
                       NOT TO SCALE
 A VALVES

t I  BLOWERS

 -  FLEXIOLE DUCT CONUCCTION

 Ul SAMPLE PORTS

  'LOCATED BELOW TRAILER FLOOR
   DIRECTLY BELOW DCMISTCR

  'OPTIONAL SYSTEM SPECIFIC
   TO EACH APPLICATION
     Figure 1-2.   Layout of In Situ Vapor  Stripping Pilot Scale
                             Research Trailer.
                                     276

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        TABLE  1-1.   Summary  of  Chemicals  Identified  at  the  Study Site.
Family
Volatile
Base Neutral
Chemical (limit of detection)
   Range of
Concentrations
                (ppb)
    Trichloroethylene (10)
    1,2-dichlorobenzene (10)
    1,1,2,2-tetrachloroethane (10)
    Tetrachloroethylene (10)
    1,1,1-trichloroethane (10)
    Chloroethane (50)
    Methylchloride (50)

                (ppm)
    Benzidine (3)
    Di-n-butylphthalate (1)
    Fluoranthene (1)
    Phenanthrene (1)
    Pyrene (1)
    1,2,4-trichlorobenzene (1)
    Bis(2-ethylhexyl) phthalate (1)
    Fluorene (1)
    Indeno (1,2,3-cd) pyrene (1)
    Anthracene (1)
    Benzo(a) anthracene (1)
    Benzo(a) pyrene (1)
    Benzo(g,h,i) perylene (1)
    Benzo(k) fluoroethene (1)
    Chrysene (1)
    1,3-dichlorobenzene (1)
    1,4-dichlorobenzene (1)
    Naphthalene (1)
    Nitrobenzene (1)
     (ppb)
     ND-21
     ND-3,800,000
     ND-23
     ND-11
     ND-36.5
     ND-257
     ND-34

     (ppm)
     ND-5.2
     ND-3.4
     ND-13
     ND-15
     ND-11
     ND-294
     ND-10
     ND-1.4
     ND-2.2
     ND-3.1
     ND-5.3
     ND-4.3
           I
     ND-4.2
     ND-5.0
     ND-100
     ND-161
     ND-2.4
     ND-1.4
                                                                  ND-1.1
manual interpretation.  The flow rate of the system is monitored by means of a
Dwyer pitot tube and micromanometer.

      An air permit was obtained from the New Jersey Department of
Environmental Protection in order to operate the vapor stripping system.  The
permit established a maximum discharge concentration of 50 parts per million.

PRELIMINARY FINDINGS

      The preliminary findings of the research project center upon the
measured  zone of influence of the extraction well, the quality of the
extracted soil  gas with time, the treatability of the extracted gas by means
of the granulated activated carbon system, temperature variations occurring in
the  system, and the observed rise of the groundwater table induced by the
vacuum  extraction.  The preliminary findings in each of these areas are
briefly discussed as follows.
                                      277

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Zone of Influence

      Mathematical modeling of the in situ vapor stripping process indicates
that in an isotropic soil the zone of influence of an extraction well screened
near the base of the unsaturated zone should produce a zone of influence with
a radius approximately equal to the depth of the unsaturated zone (Wilson, et
al, 1988).  In a soil with vertical anisotropy, the radius of the zone of
influence is increased in proportion to the degree of anisotropy.  Because the
Cohansey Sand was expected to have a vertical anisotropy of two to three, the
in situ soil gas monitoring probes were set out at radial distances of ID, 2D,
3D, and 4D, where "D" equals the depth to the water table area.

      Soil gas extraction was commenced on September 6,  1988, at a rate of
180 cfm.  In situ soil gas vacuum levels were observed almost immediately
throughout the study area and reached a steady-state condition in less than 15
minutes.  The in situ vacuum levels have remained essentially constant
throughout the course of the research program.  Contours of in situ vacuum
levels are depicted in plan view and in cross-section in Figures 1-3 and 1-4,
                 Figure 1-3.   Contours of In Situ Soil Vacuum.
                                      278

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                                                   UJ  to
                                                   o  _
                                                   o  S
                                                   c
o to
z 2

t:?
                                                   10  Z > to
Figure 1-4.  Cross-Sectional In Situ  Soil Vacuum Contour Map

                      (inches of water).
                             279

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respectively.  As indicated in these figures,  a wider zone of influence was
established than anticipated,  even considering the vertical anisotropy of the
Cohansey Sand.  It can be extrapolated from the measured in situ vacuum levels
that the effective radius of influence of the  soil gas extraction system is
approximately 150 feet.  This  is more than twice the anticipated radius of
influence.  The apparent reasons for this occurrence can be tied to the
surficial conditions.  As mentioned earlier,  the research project is sited at
the location of previous production buildings.  Approximately 25 percent of
the surface of the study area  is comprised of  ten-foot square,  five-foot
thick, concrete footings.  Moreover, the intervening soil around the footings
is to a large extent fill material of a finer-grained character
than the underlying native soils of the Cohansey Sand.  Consequently,  the
upper five feet of soil acts to impede the influx of atmospheric air from the
surface, causing the zone of influence to spread laterally beneath this
surficial layer.

Extracted Soil Gas Quality

      The quality of soil gas  extracted during the initial months of the
research program, measured by  means of the photoionization detector, is
presented in Figure 1-5.  The  soil gas quality is presented with respect to
days  of system operation.  Days of system shut down for maintenance and
installation of additional granulated activated carbon canisters are omitted
from the graph.  Extracted soil gas concentrations were initially in the range
of 110 to 140 parts per million and have fairly steadily declined to current
levels of approximately 60 to  70 ppm.  Chemical analysis of the extracted soil
gas reveals that the principal constituents are:  1,1-dichloroethane;  1,1,2,2-
tetrachloroethane; 1,1,1-trichloroethane;  trichloroethylene;  1,2-
dichlorobenzene; and 1,3-dichlorobenzene.

      Figure 1-6 presents a graph of discharge gas quality after granulated
activated carbon treatment.  The three peaks  in the graph represent
progressive exhaustion of the  granulated activated carbon canisters.  The
first and third peaks on the graph represent  exhaustion of the typical 1200
pound granulated activated carbon canisters used in the project.  The
intermediate peak represents a more rapid exhaustion of a standby granulated
activated carbon system consisting of two parallel 55-gallon drum carbon
canisters.

      The treatment efficiency of the granulated activated carbon has been
significantly diminished by sorption of water  vapor in the carbon.  The
demister has removed relatively little water  since the water occurs in the
form of water vapor rather than as a mist.  The demister is currently being
replaced with refrigerated condenser unit, designed to remove approximately 75
percent of the water vapor from the gas stream.  It is anticipated that this
unit will prolong the useful life of the carbon by a factor of approximately
four.

Temperature Variations

      The temperature of the extracted gas, ambient air, and the 12 in situ
probes have been measured throughout the course of the study.  Figure 1-7
depicts the variations in extracted soil gas  temperature and ambient


                                      280

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                  20      30     to      50      60      70
            Figure  1-5.  Extracted Gas Quality.
/o
20  -
10  -
                 20      30      40      50





                        OAT'S Of OITRAIION
            Figure 1-6.  Discharge Gas Quality.
                              281

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temperature.  The temperature of the extracted soil gas was initially
approximately 18°C and has  steadily  declined during the fall  and beginning of
winter to current temperatures of between 11°C and 12°C.  Figure  1-8 presents
a  graph of in situ soil temperature at probe 1.   The temperature variations
observed in probe 1 are characteristic of soil temperature variations
occurring within the study area.  The graph illustrates that, initially, soil
gas temperatures were in the range of 18°C to 22°C.  Also,  the deeper soil
probe exhibited a consistently lower temperature  than the intermediate and
shallow probes.  This is not surprising considering the time of the year.
With the onset of fall and winter, in situ soil temperatures declined and
reversed their relative positions.  The deeper probe, probe ID,  exhibited the
highest temperature and the shallow probe, probe  IS, the coolest temperature.

Groundwater Quality

      A rise in groundwater levels beneath in situ vapor stripping facilities
has been both predicted and observed.  The phenomenon results from the fact
that the groundwater table represents the point in the subsurface where the
voids in the soil or rock are not only fully saturated, but also at
equilibrium with atmospheric pressure.  Consequently, if soil gas pressures
are reduced to below atmospheric pressure, a corresponding rise in the
groundwater table should result.  The magnitude of groundwater table rise (in
inches) should coincide with magnitude of the pressure drop below atmospheric
occurring at any point in the system (in inches).  Monitoring of groundwater
levels during the course of the research study confirms that the water table
does indeed indeed rise a level commensurate with the soil vacuum levels
produced by the extraction well.  The maximum rise in the water table of
nearly 2.5 feet occurred immediately beneath the extraction well.
MATHEMATICAL MODELING

      A mathematical model has been developed for predicting various aspects
of  a full-scale in situ vapor stripping system.  This model will be verified
using data from the pilot scale study site discussed in this paper.  The model
was originally calibrated using laboratory data generated from specially
designed equipment which simulated actual field parameters and operating
conditions.  A critical model parameter is the determination of lumped
partition coefficients for the chemical constituents of interests.  This
coefficient addresses the chemical interaction with water, soil and other
chemicals present.

      The model can be used to generate important design criteria and optimize
operating parameters from pilot scale studies for use in full-scale
remediations.  The model can be run on a PC.  A list of the model capabilities
is provided in Table 1-2.
                                      282

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  0-t-Sop
           24- Sop
     +  EXTRACTED GAS
  03 -Nov    23-Nov

DAYS 01' OPCRAIION
    a  fXll RMAL AMHII Ml
                                                         02-Jon
Figure 1-7.   Extracted Gas and Ambient Temperatures.
  0-t- Sop    24—Sop
     D  PR013E IS
                             03-Nov
                                      ZJ-Nov
                                                13-Doc
                                                         02-Jan
                           DAYS OF OPERATION
                         -  PRODL- II
                                            o  pnonr ID
       Figure  1-8.  Soil and Ambient  Temperature,
                              283

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         Table 1-2.   List  of Model  Capabilities  for  the  Prediction of
         Design and  Operating Parameters  for  Full  Scale  In  Situ  Vapor
                                 Stripping.

     1.  Predict clean-up time to  reach  a  target  level  of residual
        contamination.
     2.  Predict residual contaminant levels after  a  given period of
        operation.
     3.  Predict location of hot spots through diagrams of contaminant
        distribution.
     4.  Develop system design:
                     horizontal well placement
                     vertical well  placement
                     screen placement
     5.  Predict clean-up levels around  buried debris from various  system
        designs.
     6.  Predict impact of ambient air temperatures on  removal.
     7.  Calculate the anisotropy  of the soil  or  rock.
     8.  Predict recontamination time of the remediated vadose  zone from slow
        moving contaminated groundwater.
     9.  Predict the  rate of remediation of floating  pools of LNAPLs.
FUTURE RESEARCH OBJECTIVES

      While the preliminary findings have answered a number of questions
concerning the behavior of in situ vapor stripping systems, the majority of
the research objectives remain to be accomplished.   These objectives include
the following:

      1.    Describe the temporal variations in overall gas quality, as well
            as the relative proportions of individual constituents within the
            gas stream.

      2.    Determine residual levels of various contaminants in the soil at
            the conclusion of the project.

      3.    Describe the relationship between extracted gas flow and the
            resultant zone of influence and upon cleanup times.

      4.    Refine the mathematical model to permit modeling of more complex
            soil conditions such as layered heterogeneity.

      5.    Evaluate the performance of the refrigerated condenser unit upon
            gas stream humidity and granulated activated carbon life.

      6.    Determine the quality of condensate from the refrigerated unit.
                                      284

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                                  REFERENCES

AWARE Incorporated,  Phase I Zone 1 Soil Decontamination Through In Situ Vapor
      Stripping Processes,  USEPA Contract No.  68-02-4446,  April,  1987.

Wilson, D.J., A. N.  Clarke, and J. H.  Clarke.   "Soil Clean-Up by In Situ
      Aeration, I.   Mathematical Modeling,"   Separation Science and
      Technology, 23(10 & 11),  pp 991-1037,  1988.
                                     285

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                                  APPENDIX J


                 SOIL VAPOR EXTRACTION RESEARCH DEVELOPMENTS



                               George E.  Hoaga

INTRODUCTION

      Recently, in situ subsurface remediation processes have been the focus
of significant attention by the scientific community involved with the clean-
up of volatile and semi-volatile  environmental contaminants.   Of the in situ
processes researched to date,  vapor extraction holds perhaps  the most
widespread application to the  remediation of these types of organic chemicals
frequently found in the subsurface.  The vapor extraction process has been
successfully employed at many  types of sites as a stand alone technology and
may also be considered a synergistic technology to other types of in situ
subsurface remediation technologies, such as,  bioremediation and groundwater
pump, skim and treat.

      In the past five years,  in situ vapor extraction has been applied at
many sites by means of significantly different approaches.  These range from
"black box" design while you dig techniques to those utilizing sophisticated
numerical models interfaced with laboratory, pilot and full-scale parameter
determination for design purposes.  The extent of success in field application
of vapor extraction is varied, in many cases related to monitoring and
interpretive limitations employed before, during and after the remediation.
Because application of the technology is quite recent, many remediations are
still in progress,  thus final  results interpretation and publishing in
refereed scientific journals is limited.

RESEARCH MILESTONES IN VAPOR EXTRACTION

      Thorton and Wootan (1982) introduced the concept of vertical vapor
extraction and injection wells for the removal of gasoline product, as well
as, vapor probe monitoring for the quantitative  and qualitative analysis of
diffused hydrocarbon vapors.  A further enhancement of this research was
published by Wootan and Voynick (1984),  in which various venting geometries
and subsequent air flow paths  were hypothesized and tested in a pilot sized
aDirector,  Environmental Research Insitiute and Associate Professor of Civil
Engineering, University of Connecticut, Storrs, CT 06269
aSenior Technical Consultant,  VAPEX Environmental Technologies,  480 Neponset
Street, Canton, MA 02021

                                      286

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soil tank.  In their first study, 50 percent gasoline removal was achieved,
while in their second study up to 84 percent removal of gasoline was observed.

Local Equilibrium Concept

      In controlled laboratory soil column vapor extraction experiments by
Marley and Hoag (1984) and Marley (1985), one hundred percent removal of
gasoline at residual saturation was achieved for various soil types (0.225 mm
to 2.189 mm average diameters), bulk densities (1.44 g/cm to 2.00 g/cm),
moisture contents (0 percent to 10 percent v/v) and air flow rates (16.1
cm3/(cm2-min)  to 112.5  cm3/(cm2-min.)  They also successfully developed an
equilibrium solvent-vapor model using Raoult's Law to predict concentrations
of 52 components of gasoline in the vapor extracted exhaust of the soil
columns.

      Baehr and Hoag (1985) adapted a one dimensional three phase (immiscible
solvent,  aqueous and vapor phases) local equilibrium transport model developed
by Baehr (1984) to include air flow as described by Darcy's Law for
compressible flow.  This first deterministic one dimensional model effectively
predicted the laboratory vapor extraction results of Marley and Hoag (1984)
and provided the basis for higher dimensional coupled air flow contaminant
models for unsaturated zone vapor extraction.

Porous Media Air Flow Modeling

      Because local equilibria prevailed in the above studies, a higher
dimensional model, developed by Baehr, Hoag and Marley (1988) was used to
model air flow fields under vapor extraction conditions.   The three
dimensional radially symmetric compressible air flow model is used to design
vapor extraction systems using limited lab and/or field air flow pump tests.
A steady state in situ pump test determination of air phase permeability is
preferred over laboratory tests because an accounting is possible of the
presence of an immiscible liquid, anisotropy, soil surface, variations in soil
water conditions and heterogeneity in air phase permeabilities.   The numerical
solution developed can simulate flow to a partially screened well, and allow
determinations of vertical and horizontal air phase permeabilities.
Heterogeneous unsaturated zones can also be evaluated using the numerical
simulation.  Analytical solutions to radial flow equations, such as one
developed early by Muskat and Botset (1931) are generally restricted to
determination of average horizontal air permeability determination for
impervious soil surfaces.

Removal of Capillary Zone Immiscible Contaminants

      Hoag and Cliff (1985) reported that an in situ vapor extraction system
was effective in removing 1330 L gasoline at residual saturation and in the
capillary zone at a service station and achieved clean up levels to below
3 ppm (v/v) in soil vapor and below detection limits in soils.  The entire
remediation took less than 100 days.  A groundwater elevation and product
thickness log for the time period of before, during and after the vapor
extraction remediation is graphically shown in Figure J-l.   On day 250 only a
skim of gasoline was present in this well and on day 290 no skim was detected.
One year after the vapor extraction remediation took place, groundwater


                                      287

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           Product Thickness
                             Producl Elevation
                 Ground-Water Elevation
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— 1 	 1 	
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                               TIME (IN DAYS)

   Figure J-l.  Apparent Product Thickness in Ground Water Monitoring Well.

samples were non detected for gasoline range hydrocarbons, reflecting that  at
least advective dispersive transport and possible natural microbiological
activity in the groundwater were mechanisms responsible for this  effect.

Field Application of Porous Media Air Flow Models

      Baehr, Hoag and Marley (1988) utilized the above site for a field air
pump test to determine the horizontal air phase permeability and  to  simulate
the sensitivity of the model to changes in air phase permeabilities  utilizing
site geometries and boundary conditions.  Based upon a full-scale air flow
pump test, the air phase permeability for the site was predicted  to  be  k -  7.0
x 10~8  cm2 for a mass air withdrawal rate of 11.1 g/sec and a normalized
pressure of Ps/Patm =0.9.  For reference 11.1 g/sec, assuming an  air phase
density of 1.2 x 10"3 g/cm3, equals about 555 L/min.

      To illustrate the sensitivity of the model to a range of air phase
permeabilities, the above service station vapor extraction well geometry,
depth to water table,  and appropriate boundary conditions were used  as  input
and air phase permeability and mass air withdrawal rates were varied.   In
Figure J-2, the normalized air phase pressure in the well, for various  mass
air withdrawal rates and air phase permeabilities are shown.  Significant
increase in the vacuum developed in the wells can be observed for order of
magnitude decreases in air phase permeabilities and small increases  in  the
mass air withdrawal rates.  Review of Figure J-2, indicates that  if  a mass  air
withdrawal rate of 40 g/sec was used at the service station site  (k  - 7.0 x
10"8) ,  the  the normalized air phase pressure  at  the  well  would be
approximately Ps/Patm =0.6, within an acceptable range of operating
conditions.  A limitation of the model developed by Baehr and Hoag (1988) is
that it is not coupled to contaminant transport.   However, for the  design  of
vapor extraction systems for volatile contaminants, this generally is not a
fundamental need and can be accomplished by either laboratory venting tests,
similar to those developed by Marley and Hoag (1984) or by utilization  of the
one dimensional coupled model developed by Baehr and Hoag (1985) .
                                      288

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RESEARCH NEEDS

      A fundamental need of vapor extraction modeling occurs in the area of
capillary zone/unsaturated zone interaction when immiscible phases are present
on or in the capillary fringe.  While air phase modeling alone is probably
adequate for most vapor extraction system system design purposes, particularly
if a full three-dimensional model is used with optimization modeling, a
rigorous modeling effort to couple air phase flow and immiscible contaminant
transport, particularly in the capillary zone, will provide strategic insight
to vapor extraction operation and planning.

      To assess research needs in this area, two basic vapor extraction
systems applications should be considered: 1.  Immiscible contaminant with
density less that 1.0 (petroleum range hydrocarbons); and 2. Immiscible
contaminants with density greater than 1.0 (halogenated compounds).
                 0.5
                             Qm, Mass Rate of Air Withdrawal

                                      (g/sec)



         Figure J-2.   Normalized Air-Phase Pressure at a Single Well,
                                      289

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      Generalized subsurface phase distributions for immiscible  liquids with
densities  less  than that of pure water are illustratively shown  in Figure J-3.
A typical  vapor extraction system installation in this type of subsurface and
contaminant  condition is found in Figure J-4.  In the case study presented by
Hoag and Cliff  (1985),  as detailed above, pump and skim was employed at the
site for the first 210  days of the remediation with only 300 L gasoline
removed  (i.e. mostly through manual bailing).  Thus, an important question
should be: Was  vapor extraction alone necessary in this case or  were both pump
and skim and vapor extraction required for optimal or even effective
remediation  of  immiscible contaminants?  To answer this question requires an
understanding of air-immiscible liquid-water three phase conduction and
distribution in the porous media, particularly in the capillary  fringe areas
at a site.   Additionally, the site history of groundwater fluctuation and
immiscible contaminant  behavior in the capillary fringe is essential
information  necessary to answer the above question.  Parker, Lenhard and
Kuppusamy  (1986)  and Lenhard and Parker (1986) provide a parametric model for
three-phase  conduction  and measurements of saturation-pressure relationships
for immiscible  contaminants in the unsaturated and capillary zones.   However
to date, this author is not aware of the coupling of these types of models to
air phase  and contaminant transport models.

      A more in depth hypothetical examination of the possible relationships
near the capillary fringe will illustrate the importance of this zone in
determining  the need for pump and treat and the importance of solute mass
transfer from the capillary zone into the saturated flow regime.   In the case
of a recent  spill of an immiscible contaminant with density less than water,
when relatively steady  groundwater flow prevails, a zone may exists on the
capillary  fringe of floating product, as shown in Figure J-5.
                                       SURFACE GRADE LEVEL-
Unsaturated Zone
  Capillary Fringe
  Saturated Zone
                                  Residual Saturation
                                  In Capillary Zone
                                                              t-J"l"? of Vapor Phase and Soil
                                                              Moisture Components
                                                                Groundwater Elevation
                                                           Limits of Dissolved Components
      Figure J-3.  Generalized Subsurface Phase distribution for Immiscible
               Contaminants with Densities Less  than Pure Water.
                                       290

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Figure J-4. Typical Vapor Extraction Pump and Treat  In-Situ Subsurface
            Remediation System for Immiscible Contaminants  with
            Densities Less than Pure Water.  (Tt represents  the
            groundwater table prior to pump  and skim and  T2  is  after
            pump and skim).
                                291

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                                                                INFILTRATION
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                 '  •  •   '	•  * i	  jS^  *	___*		
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                                         Solute  Concentration

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      Infiltrating water, under draining conditions will reach an equilibrium
with the immiscible liquid resulting in a saturated solute condition.  For
hypothetical purposes only, if it is assumed that only vertical groundwater
flow exists in the capillary zone, the rate of solute input to the saturated
zone will be limited by the rate of infiltration and Cs.   If it assumed,  again
for illustrative purposes, that a horizontal flow boundary exists at the
groundwater table, then mass transfer of solute from the capillary zone into
the saturated zone will have only limited effects on the rate of solute input
into the saturated zone.  When considering the quantities of water
infiltrating through the capillary zone per year in comparison to saturated
flow rates, the above assumptions may be valid.  The result is relatively,
inefficient transfer of solutes from  the capillary fringe zone to the
saturated zone.  Thus,  in this scenario, pump and treat systems may not be
necessary to remove the immiscible contaminants and advective dispersive
dilution may be adequate to protect groundwater resources.  Without knowledge
a priority of the immiscible liquid distribution and interaction with the
capillary zone, and advective-dispersive transport characteristics at a site,
this approach may be risky.  An alternative, however, may be close monitoring
of groundwater in the saturated zone near the spill area, as vapor extraction
proceeds.  If the scenario in Figure J-5 exists at a site then solute
concentrations in groundwater will decrease with time and no pump, treat and
skim system may be necessary, to achieve desired levels of remediating in soil
and groundwater.  If near field transport of solutes from the spill area
increases steadily with time, then groundwater pumping may be necessary to
employ at that time.

      In the case of an immiscible contaminant with a density less than water
with impingement on the saturated zone by penetration of the capillary zone,
the potential for solute transfer from the unsaturated zone to the saturated
zone is greatly increased.  This scenario may result from the depression of
the capillary zone in a spill event where considerable quantities of an
immiscible contaminant are spilled,  such as that shown in Figure J-3.
Alternatively, fluctuating groundwater tables may result from a rise in the
groundwater table through wetting (imbibition) of the capillary zone as
described by characteristic curves for a given porous media and immiscible
contaminant.  Remembering that immiscible contaminants become immobilized
once at residual saturation, the result of wetting the capillary zone may
result in the condition shown in Figure J-6.  The net result of this scenario
is that saturated solute concentrations exists at the top of the horizontal
flow zone of the saturated zone.  This boundary condition enables
substantially greater mass transfer of solute into the saturated zone,
principally resulting from the upper flow boundary being the immiscible
contaminant itself.  In Figure J-5,  the upper boundary was only solute at less
than Cs,  and solubilization was  limited to that achieved through infiltration.
Clearly,  the difference in these two situations greatly affects the rate of
solute input into the saturated zone and should affect remedial action
responses.  Unless the solute transport phenomena from an immiscible phase
into the saturated zone is understood and physically defined at a site, then
neither optimal remediation systems can be designed nor saturated zone solute
transport models can be effectively utilized to predict the impact of
immiscible liquid remediation on saturated zone solute transport.
                                     293

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Figure J-6. Generalized Subsurface Condition  of Immiscible Contaminant
            with  Density Less Than Water  Impinging on the Water Table.
                                 294

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      Immiscible phase boundary conditions presented in Figures J-5 and J-6
also greatly effect the vapor diffusive flux rates from the capillary zone
into the unsaturated zone.  Bruell (1987) and Bruell and Hoag (1986)
rigorously investigated the effect of immiscible liquid phase boundary
conditions on subsequent hydrocarbon diffusive flux rates of benzene.  For a
given column geometry (diffusive path length of 47.6 cm), diffusive flux rates
for benzene at 20 °C for an  immiscible phase boundary condition  similar  to
that shown in Figure J-5,  resulted in benzene diffusive flux rates of 24.9
mg/cm2-min and 6.1 mg/cm2-min,  for dry and wet  (i.e. field  capacity moisture
content) concrete sand, respectively.  Thus,  moisture content played a
significant role in reducing the effective porosity of the concrete sand. When
residual saturation immiscible liquid phase boundary conditions were
investigated the maximum benzene diffusive flux rate was 26.6 mg/cm2-min,
however the diffusive path length was only 22.4 cm.  The moisture content in
the residually saturated zone was 3.2 percent(v/v).  When capillary zone
immiscible liquid phase boundary conditions were investigated, the maximum
benzene diffusive flux rate was reduced to 4.8 mg/cm2-min with a diffusive
path length of 22.4 cm.  The moisture content in the capillary zone reflected
saturated conditions (i.e.,  9W = n).   This  research demonstrated that the
immiscible liquid phase boundary condition greatly affects the diffusive flux
rates of hydrocarbons that occur in the unsaturated zone.  As the moisture
content increases, then the diffusive flux rates of contaminants will
decrease.  The net result of these boundary conditions affects the
concentrations of hydrocarbon vapors detected using soil gas assessment
techniques and the rates of hydrocarbon recovery utilizing in situ vapor
extraction.

      With reference to Figure J-6 and knowing that advective air flowrates
also decrease with increasing moisture content, creates a circumstance in the
area of the capillary zone where advective air flow may not be in direct
contact with the immiscible phase.    Thus,  diffusion in this case, will be the
controlling mechanism of contaminant removal during vapor
extraction.  In the case depicted in Figure J-5, it is likely that some
advective air flow will contact the immiscible phase,  greatly increasing vapor
extraction efficiency.

      In the case of an immiscible liquid with a density greater than that of
water, contaminant distribution is significantly different given a
hypothetical spill to the subsurface.   Penetration of the capillary and
saturated zones by the immiscible liquid is likely, given sufficient spill
volumes as shown is Figure J-7.  Of great importance is the occurrence of
groundwater flow through the immiscible liquid phase in the saturated zone,
resulting in substantially greater solubilization rates of the immiscible
phase and greater groundwater contamination potential than in the cases
presented in Figures J-5 and J-6.

      A typical in situ remedial action response to the dense immiscible
liquid phase contamination is given in Figure J-8.  Simultaneous vapor
extraction and groundwater pumping are necessary to expose immiscible phase
contaminants to advective air flow and to increase diffusive flux rates of
contaminants in the vicinity of the groundwater table at time =  T2.   In this
case, dewatering of the saturated zone in the area of immiscible phase
contaminants is desirable.  Long-term plume management interceptor pumping


                                     295

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                                                        SURFACE GRADE LEVEL
  Unsaturated Zone
 Capillary Fringe
Residual Saturation In
Saturated Zone
                                 Residual Saturation In
                                 Unsaturated Zone
Limits of Immiscible
Phase Components
                                                           Limits of Vapor Phase
                                                           and Soil Moisture contaminants
                                                     Residual Saturation in Capillary Zone
                                                           Saturated Ground Water Flow
                                                           with Dissolved Contaminants
                                                                Impervious Boundary
    Figure J-7.   Generalized Subsurface Phase  Distribution for Immiscible
                 Contaminants with Densities Greater  than Pure Water.

strategies,  such as those developed by Ahlfeld,  Mulvey, Finder and  Wood (1988)
and Ahlfeld, Mulvey and Pinder (1988) should be  implemented to optimally
circumvent uncontrolled groundwater contamination and to maximize groundwater
contaminant  recovery rates.  Strategies to  maximize saturated zone  dewatering
in the vicinity  of the immiscible phase liquids  must be developed to properly
implement this approach.   Additionally, in  situ  bioremediation may  be
considered as an additional technology to further degrade the immiscible
liquid, if complete subsurface dewatering is not possible.

SUMMARY

      Significant advances have been made in the past five years in the
understanding of volatile and semi-volatile contaminant behavior as related to
vapor extraction technologies.   Coupled modeling of both contaminant behavior
and advective air flow, however remains limited  to one dimensional  systems.
Given the significant hydrogeological complexity of porous media and
subsequent heterogeneous distributions of immiscible phase contaminants, the
design utility of higher dimensional coupled models is questionable.   Higher
dimensional  advective air flow models are being  used to design vapor
extraction systems.  These models are generally  dependent on site specific
parameters best  determined in field air pumping  tests, unless uniform
hydrogeologic conditions prevail with quantifiable boundary conditions
necessary for model design predictions.  Three dimensional models are being
                                       296

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               VAPOH EXTRACTION PUMP
                                                           j Impervious Boundary
                               Groundwater Pump ing Wells
   Figure J-8.
Typical  In-Situ Remedial Actions  to  Immiscible  Liquid Phase
Contaminants with Densities Greater than Water.
adapted to deal with non-radial symmetry and will be necessary to rigorously
model multiple extraction well and extraction well/injection well
applications.

      Significant modeling and experimental research is needed to further
understand immiscible contaminant behavior in the capillary zone and adjacent
boundary conditions.  The interaction of immiscible phase liquids in the
capillary zone with unsaturated zone infiltration and saturated zone transport
must be the focus of this research.  The approach should include both
hydrological characteristics and testing procedures necessary to determine  the
influencing factors.  Chemical fate and transport in the unsaturated zone
under natural and advective air flow conditions must also be better understood
to wore effectively apply optimal in situ remediation processes.

      Emphasis should be placed on basic research in the above areas,  to be
followed at the appropriate time by demonstration level projects. When
demonstration level projects precede basic research needs, as has frequently
been the case in the past five years, the result generally do not properly
reflect necessary parameter control or monitoring and either inconclusive or
misleading results may be generated.
                                      297

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                                  REFERENCES

Ahlfeld, D. P., Mulvey, J.  M. ,  and Finder,  G.F.   1988.Contaminated Groundwater
      Remediation Design Using Simulation,  Optimization and Sensitivity
      Theory: 2, Analysis of Field Site.   Water  Resources Research.
      24(3):443-452.

Ahlfeld, D. P., Mulvey, J.  M.,   Finder,  G.F, and Wood,  E. F.1988.
      Contaminated Groundwater Remediation Design Using Simulation,
      Optimization and Sensitivity Theory:  1,  Model Development.   Water
      Resources Research.  24(3):431-442.

Baehr, A.  L.  1984.  Immiscible Contaminant Transport in Soils with an
      Emphasis on Gasoline Hydrocarbons.   Ph.D.  Dissertation,  Dept.  of Civil
      Eng., University of Delaware.

Baehr, A.  L., and Hoag, G.  E.   1985.   A Modeling and Experimental
      Investigation of Venting Gasoline Contaminated Soils.   In: E.  J.
      Calabrese and P. T. Kostecki,  (editors), Soils Contaminated  by
      Petroleum: Environmental and Public  Health Effects, Wiley, New York,  458
      pp.

Baehr, A.  L., Hoag, G. E.,  and Marley,  M.  C.  1988.  Removing Volatile
      Contaminants from the Unsaturated Zone by  Inducing Advective Air-Phase
      Transport.  Journal of Contaminant Hydrology, 4:1-26.

Bruell, C. J.  1987.  The Diffusion of Gasoline-Range Hydrocarbons in Porous
      Media.  Ph. D. Dissertation, Environmental Engineering,  University of
      Connecticut, Storrs.

Bruell, C. J., and Hoag, G. E.   1987.   The Diffusion of Gasoline-Range
      Hydrocarbon Vapors in Porous Media,  Experimental Methodologies.
      Proceedings of Petroleum Hydrocarbons and  Organic Chemicals  in Ground
      Water: Prevention, Detection and Restoration.  National Water Well
      Association and the American Petroleum Institute, Houston.   420-443.

Hoag, G. E., and Cliff, B.   1985.   The Use of the Soil Venting Technique for
      the Remediation of Petroleum Contaminated  Soils.   In:  E. J.  Calabrese
      and P. T. Kostecki, (editors),  Soils Contaminated by Petroleum:
      Environmental and Public Health Effects, Wiley, New York, 458 pp.

Lenhard, R. J., and Parker, J.  C.   1986.   Measurement and Prediction of
      Saturation-Pressure Relationships in Air-Organic Liquid-Water-Porous
      Media Systems.  Virginia Poly.  Tech.  and State Univ.

Marley, M. C.  1985.  Quantitative and Qualitative Analysis of Gasoline
      Fractions Stripped by Air from the Unsaturated Zone.  M.S. Thesis,
      Department of Civil Engineering,the  University of Connecticut, 87 pp.


                                     298

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Marley, M. C., and Hoag, G. E.  1984.   Induced Soil Venting for Recovery and
      Restoration of Gasoline Hydrocarbons in the Vadose Zone.   Proceedings of
      Petroleum Hydrocarbons and Organic Chemicals in Ground Water:
      Prevention, Detection and Restoration.   National Water Well Association
      and the American Petroleum Institute, Houston.  473-503.

Muskat, M.,  and Botset, H. G.  1931.  Flow of Gas Through Porous Materials.
      Physics.  1:27-47.

Parker, J. C., Lenhard, R. J., and Kuppusamy.  1986.  A Parametric Model for
      Constitutive Properties Governing Multiphase Fluid Conduction in Porous
      Media.  Virginia Poly. Tech. and State Univ.

Thorton, J.  S., and Wootan, W. L.,  1982.  Venting for the Removal of
      Hydrocarbon Vapors from Gasoline Contaminated Soil.  J. Environ. Sci.
      Health, A17(l);31-44.

Wootan, W. L., and Voynick, T.  1984.   Forced Venting to Remove Gasoline Vapor
      From A Large-Scale  Model Aquifer.  American Petroleum Institute. 82101-
      F:TAV.
                                      299

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                                                                       APPENDIX K
                                              STATE REGULATORY AGENCY CONTACTS AND SOIL CLEANUP CRITERIA
u>
o
o
STATE
ALABAMA
ALASKA
ARIZONA
ARKANSAS
CALIFORNIA
OFFICE
DEPARTMENT OF ENVIRONMENTAL MANAGEMENT
GROUNDWATER SECTION
AIR & SOLID WASTE MANAGEMENT
UNDERGROUND STORAGE TANK COMPLIANCE
UNDERGROUND STORAGE TANKS
STATE WATER RESOURCES--
CONTACT
SUSAN CHAMLIES
JEFF MACH
KIM MACAFEE
LINDA GRESCHEN
BETTY MORANO
TELEPHONE
(205)271-7832
(907)465-2653
(602)257-2300
(501)562-7444
(916)739-4436
ALLOWED RESIDUAL
SITE SPECIFIC-<1 ppm TPH
NO STATE REGULATIONS
TPH - 100 ppm
BENZENE -.130 ppm
TOLUENE - 200 ppm
ETHYL BENZENE - 68 ppm
XYLENE -44 ppm
NO STATE REGULATIONS
BASED ON MODELS-
              UNDERGROUND STORAGE TANK DIVISION




COLORADO      DEPARTMENT OF ENVIRONMENTAL HEALTH

CONNECTICUT    OIL AND CHEMICAL SPILLS
            DELAWARE
            DISTRICT OF
              COLUMBIA

            FLORIDA
                                                               SCOTT WINTERS      (303)320-8333

                                                               JAMES SANPACROCE  (203)566-4633
              UNDERGROUND STORAGE TANKS
              EPA GUIDANCE
              BUREAU OF WASTE CLEANUP
EMILONUSCHAK
(302)323-4588
HIGHEST- BENZENE-1 ppm
EACH TEX--50 ppm
TPH--1000 ppm
LOWEST-TPH-10 ppm

NO STATE REGULATIONS

NO MINIMUMS; CLEANUP
TO LIMITS OF PROPERTY
LINE; GROUNDWATER LIMITS

SITE BY SITE
BTEX -1 ppm
ANGELOTOMPROSE    (202)783-3194    100 ppm TPH
TODD ALLEN
(904)487-3299
NO FORMAL REGULATIONS-
>500 ppmTPH-CHECK GROUND-
 WATER IMPACT
<10 ppm NO CLEANUP REQUIRED
<500 ppm THC
<100 ppb TOTAL AROMATICS

-------
U)
o
         STATE
GEORGIA




HAWAII


IDAHO


ILLINOIS




INDIANA




IOWA

KANSAS



KENTUCKY

LOUISIANA


MAINE


MARYLAND
                                                                    APPENDIX K
                                           STATE REGULATORY AGENCY CONTACTS AND SOIL CLEANUP CRITERIA
                                                                      (continued)
              OFFICE
                                     CONTACT
TELEPHONE
ALLOWED RESIDUAL
                        ENVIRONMENTAL PROTECTION DEPARTMENT--   CLIFF TRUSSELL
                        INDUSTRIAL WASTE MANAGEMENT PROGRAM
                        UNDERGROUND STORAGE TANKS              LIZ ALBAZ
                        DEPARTMENT OF ENVIRONMENT-WATER QUALITY
                                                  BUREAU

                        EPA-DIVISION OF LAND POLLUTION CONTROL   KELLY DUNBAR
                                                        (404)669-3297
                                                        (808)548-8837
                                                        (208)334-5839
                                                        (217)782-6761
DEPARTMENT OF ENVIRONMENTAL MANAGEMENT MANUELLA JOHNSON    (317)243-5060




DEPARTMENT OF NATURAL RESOURCES        JIMHORN            (515)281-8964

DEPARTMENT OF HEALTH AND ENVIRONMENT    MARY JANE STELL     (913)296-1500
                        DIVISION OF WASTE MANAGEMENT

                        OFFICE OF SOLID AND HAZARDOUS WASTE
                        MANAGEMENT

                        DEPT.OF ENVIRONMENTAL MANAGEMENT
                        WASTE MANAGEMENT

                        DEPARTMENT OF ENVIRONMENT
                                     DOUG BONK

                                     GEORGE GULLET


                                     RON SEVERENCE


                                     HERBNEAD
(502)564-3382

(504)342-7808


(207)289-2651


(301)631-3442
GASOLINE-SHOULD CLEAN TO
10 ppm-BTEX, IF BTEX >10 ppm,
CLEAN TO BACKGROUND GENERALLY
(PRACTICE NOT LAW)

VISIBILITY, ODOR, SOME
SOIL SAMPLING

RCRA RULES (HAZARDOUS)
TO BACKGROUND IF FEASIBLE (NON-HAZARDOUS)

BENZENE- 5 ug/kg
TOLUENE- 2mg/kg
ETHYL BENZENE-680ug/kg
XYLENES- 1400 ug/kg

SITE BY SITE
TPH TEST FOR DETERMINATION
>100 ppm CLEANUP IS REQUIRED
BACKGROUND IF POSSIBLE

NO STATE REGULATIONS

TPH-100  mg/kg
BENZENE--1.4 mg/kg
1,2   DCE-8 mg/kg

NO STATE REGULATIONS

SITE BY SITE
BTEX 50-100 ppm

NO STATE REGULATIONS -
 SITE SPECIFIC

TPH <100 ppm, IF LOW
FLASHPOINT.MIXWITH
CLEAN SOIL

-------
CO
o
                                                                    APPENDIX K
                                           STATE REGULATORY AGENCY CONTACTS AND SOIL CLEANUP CRITERIA
                                                                      (continued)
         STATE
              OFFICE
                                     CONTACT
                   TELEPHONE
                 ALLOWED RESIDUAL
         MASSACHUSETTS DEPT. OF ENVIRONMENTAL PROTECTION
MICHIGAN


MINNESOTA




MISSISSIPPI



MISSOURI



MONTANA

NEBRASKA
         NEVADA
LAND AND MANAGEMENT DEVELOPMENT


POLLUTION CONTROL AGENCY




BUREAU OF POLLUTION CONTROL



DEPARTMENT OF NATURAL RESOURCES



SOLID AND HAZARDOUS WASTE BUREAU

DEPARTMENT OF ENVIRONMENTAL CONTROL
CHARLIE TUTTLE



DON PARSONS


SHIELAGROW




JACK McCORD



FRED HUTSON



DOUGROGNESS

JIM BOROVICH
              DIVISION OF UNDERGROUND STORAGETANKS    ALAN BIAGGI
         NEW HAMPSHIRE  DEPARTMENT OF ENVIRONMENTAL SERVICES   TIM DREW
         NEW JERSEY
              DIVISION OF WATER RESOURCES
                                     ANALAB, INC
(617)292-5903



(517)373-1170


(612)297-2316




(601)961-5062



(314)751-7326



(406)444-3454

(402)471-4230
                   (702)885-5872


                   (603)271-3306


                   (609)984-3156
SITE SPECIFIC
DEVELOPING NEW POLICY,
AS  OF (10/4/89)

10 mg/kg TOTAL VOC WITH
 <15% ABOVE 1 mg/kg

GUIDELINES-<10 ppm TPH, with
ORGANIC VAPOR ANALYZER, OK;10-100ppm
CLEAN UP NECESSARY.FUELOIL
CLEAN TO 1ppm OVER BACKGROUND

1 COMPOSITE SAMPLEATANK
ACTION--100 ppm(mg/kg)
<10%LEL

SITE SPEC--GENERALLY
<10 ppm TPH
<1 ppm BTEX

SITE SPECIFIC

TPH 2 ppm
BENZENE .005 ppm
TOLUENE 2.42 ppm
ETHYLBENZENE 1.4 ppm
XYLENE 0.4 ppm

100 ppm  TPH--*RCRA-IGNITIBILITY,
                LEAD CONTENT

*TOTAL VOLATILES;
LEAD

BENZENE - 0.07 mg/l
TOLUENE - 14.4 mg/l
DCE - 0.1 mg/l

-------
                                                                  APPENDIX K
                                         STATE REGULATORY AGENCY CONTACTS AND SOIL CLEANUP CRITERIA
                                                                    (continued)
       STATE
               OFFICE
                                     CONTACT
TELEPHONE
ALLOWED RESIDUAL
LO
O
LJ
       NEW MEXICO      DEPARTMENT OF HEALTH AND ENVIRONMENT
                      ENVIRONMENTAL IMPACT DIVISION

       NEW YORK       DEPARTMENT OF UNDERGROUND STORAGE TANKS FRANK PEDUDO
       NORTH CAROLINA DEPARTMENT OF HEALTH AND HUMAN SERVICES BILLJEETER
NORTH DAKOTA

OHD

OKLAHOMA



OREGON
                      HEALTH DEPARTMENT
                                     GARYBERRETH
EPA OFFICE OF SOLID & HAZARDOUS WASTE    TOM FORBES
MANAGEMENT
CORPORATION COMMISSION                TANA WALKER
                      DEPARTMENT OF ENVIRONMENTAL QUALITY     MIKE ANDERSON
(505)827-2894


(518)457-2462





(919)733-5083




(701)224-2366

(614)752-7938

(405)521-3107



(503)229-5731
VISUAL & OLFACTORY-
OBSERVE AFTER AERATION

SITE SPECIFIC - GENERALLY DRINKING
WATER STANDARDS, WITH TOXICITY
CHARACTERISTICS OF LEACHATE
POTENTIAL TEST, IF 100 ppmTPH REMEDIATE
10-100 ppm MONITOR
<10 ppm CONSIDERED CLEAN
THC < 100ppm

SITE SPECIFIC

BTEX 2 ppb; TPH 1 ppm

ACTION LEVELS
TPH-500  ppm
BTEX-10 ppm

SfTE SPECIFIC MODELS
LEVEL 1-TPH 40 ppm
LEVEL 2-TPH 80 ppm
LEVEL 3-TPH 130  ppm
       PENNSYLVANIA  NON-POINT SOURCES AND STORAGE TANKS     JOHN BORLAND
                                                                      (717)787-2666
                                                                        NO STATE REGULATIONS

-------
                                                                   APPENDIX K
                                          STATE REGULATORY AGENCY CONTACTS AND SOIL CLEANUP CRITERIA
                                                                     (continued)
        STATE
              OFFICE
                                     CONTACT
                   TELEPHONE
ALLOWED RESIDUAL
OJ
o
RHODE ISLAND


SOUTH CAROLINA

SOUTH DAKOTA

TENNESSEE


TEXAS


UTAH


VERMONT


VIRGINIA



WASHINGTON
                                                           DAVID SHELDON
                                                        (401)277-2808
                      DEPTOF HEALTH & ENVIRONMENTAL CONTROL

                      DEPT OF WATER AND NATURAL RESOURCES

                      DEPTOF GROUNDWATER PROTECTION
                      WATER COMMISSION AND DEPT OF HEALTH
                      CORRECTIVE ACTION

                      BUREAU OF SOLID AND HAZARDOUS WASTE
                      AGENCY OF NATURAL RESOURCES
STATE WATER CONTROL BOARD
                      ECOLOGY DEPT
       WEST VIRGINIA

       WISCONSIN



       WYOMNG
              UNDERGROUND STORAGE TANKS

              BUREAU OF SOLID & HAZARDOUS WASTE MGMT



              DEPARTMENT OF ENVIRONMENTAL QUALfTY
                                     PRESTON CAMPBELL   (803)734-5331

                                     DICKPIEFER         (605)773-3351

                                     DONGILMORE         (615)741-4094
                                     DAN AIREY
                                     BOBFORD
                   (512)463-7972
                   (801)538-6121
                                     PAUL VAN HOLIBEKE   (802)244-5674
STEVE WILLIAMS      (804)367-0970
                                     JOE HICKEY
                   (206)867-7000
                                     PAT BOYD
                   (304)348-5935

                   (608)266-1327
                                     DAVE MONTAGUE      (307)777-7781
VISUAL OBSERVANCE-- SHIPPED
TO STATE APPROVED FACILITY

THC-100 ppm

TPH-10 ppm

10 ppm BTEX (GASOLINE)
100 ppmTPH

<500 ppm BTX ;
IGNITIBILITY

SITE SPECIFIC
100 mg/l THC IN SOIL

SITE SPECIFIC-SOIL MAY BE PLACED
ON SITE IF <20 ppm AS BENZENE

SITE SPECIFIC THROUGH
RISK ASSESSMENT
100 ppm (GENERALLY)

TPH 200 ppm
BENZENE 660 ppb
TOLUENE 143ppm
ETHYLBENZENE 14 ppm
XYLENE 900 ppm

BACKGROUND

SITE SPECIFIC-10-50 ppm THC
OR LOWER DEPENDING ON
GROUNDWATER LEVELS

NO STATE REGULATIONS
OLFACTORY LEVELS

-------
                                                                  APPENDIX K
                                          STATE REGULATORY AGENCY CONTACTS AND SOIL CLEANUP CRITERIA
                                                                    (continued)
                      LEGEND:
                       REFERENCE: EPA SURVEY OF STATE PROGRAMS PERTAINING TO CONTAMINATED SOILS (1988)
                       TPH - TOTAL PETROLEUM HYDROCARBONS
                       BTEX - BENZENE, TOLUENE, ETHYLBENZENE, XYLENE
                       LEL - LOWER EXPLOSIVE LIMIT
                       TEX - TOLUENE, ETHYLBENZENE, XYLENE
                       VOC - VOLATILE ORGANIC COMPOUND
                       GW - GROUNDWATER
                       THC- TOTAL HYDROCARBON
                       DCE - DICHLOROETHYLENE
                       RCRA - RESOURCE CONSERVATION RECOVERY ACT
o
Ln

-------
                                                                     APPENDIX L
                                          STATE REGULATORY AGENCY CONTACTS AND AIR DISCHARGE CRITERIA
                                                                                                AIR QUALITY
o
STATE
ALABAMA

ALASKA
ARIZONA
ARKANSAS
CALIFORNIA
COLORADO

CONNECTICUT

DELAWARE
DISTRICT OF
COLUMBIA
OFFICE
AIR QUALITY

AIR & SOLID WASTE MANAGEMENT
AIR QUALITY
AIR QUALITY
AIR RESOURCES BOARD
AIR POLLUTION

AIR COMPLIANCE

UNDERGROUND STORAGE TANKS
AIR MONITORING
CONTACT
TIM OWENS

JOHN SANSTEND
CARROLL DEKLE
J.B. JONES
BOB FLETCHER
JOHN PLOG

JOHN GOVE

EMILONUSCHAK
DONWAMBSGANS
TELEPHONE
(205)271-7861

(907)465-2666
(602)257-2300
(501)562-7444
(916)739-8267
(303)331-8500

(203)566-2690

(302)323-4588
(202)767-7370
EMISSIONS
EMISSION OF AIR TOXICS
>0.1 Ibs/hr STATE REQUIRES
MORE INFORMATION ON SOURCE
40 CFR 60
40 CFR 60
NO STANDARDS
EACH COUNTY HAS OWN
REASONABLE CONTROL
TECHNOLOGY
COMPOUND SPECIFIC
per 8 hr average
2.4 Ibs/day of VOCs
1 Ib/day of VOCs
      FLORIDA
      GEORGIA
      HAWAH


      IDAHO

      ILLINOIS
AIR QUALITY
                     AIR PROTECTION
DEPARTMENT OF HEALTH
POLLUTION CONTROL BRANCH

ENVIRONMENTAL QUALITY

AIR QUALITY PLANNING
                                                             BARRY ANDREWS
                                                             LYNN RHODES
TYLER SUGIHARA


MARTIN BAUER

JOHN REED
(904)488-1344      GUIDANCE POLICY-MAXIMUM
                IMPACT vs. ACCEPTED FRACTION
                OFTLV

(404)656-6900      PERMIT REQUIRED FOR ALL
                EMISSIONS; IF PROCESS IS IN-
                CINERATION, 90% OXIDATION
                REQUIRED

(808)543-8205      NSPS - 40 CFR 60
(208)334-5834      NO LIMITS - SITE SPECIFIC

(217)782-7326      VOCs CONSIDERED PHOTOCHEMICAL-
                LY REACTIVE (RULE 66) - 8 Ib/hr
                or 85% CONTROL

-------
                                                               APPENDIX L
                                    STATE REGULATORY AGENCY CONTACTS AND AIR DISCHARGE CRITERIA
                                                                (Continued)
STATE
INDIANA
IOWA
OFFICE
AIR QUALITY STANDARDS AND PLANNING
DEPARTMENT OF NATURAL RESOURCES
CONTACT
ANN BLACK
REX WALKER
TELEPHONE
(317)247-5110
(515)281-5145
AIR QUALITY
EMISSIONS
SITE BY SITE - MUST REGISTER
IF CARCINOGEN INVOLVED AND THE
KANSAS
KENTUCKY
LOUISIANA
                AIR EMISSIONS
AIR QUALITY
AIR PERMITS
                                                       GENESALLEE
                                                           (913)296-1500
MARJORIEMULLIN    (502)564-3382
KARLOSTERTAHLER  (504)342-9047
MAXIMUM CONCENTRATION AT
GROUND LEVEL >10(-6)

MODELED FOR BENZENE
REPORTING REQUIREMENTS
IF >10 TONS EMISSION/YEAR
PERMITS

>85% REMOVAL OF INPUT
BY WEIGHT

NO STANDARDS - MAKE A MODEL
& COMPARE TO 1/42 OF TLV
MAINE
MARYLAND
MASSACHUSETTS
MICHIGAN
MINNESOTA
MISSISSIPPI
MISSOURI
MONTANA
NEBRASKA
AIR QUALITY
AIR MANAGEMENT
AIR QUALITY
AIR QUALITY
AIR STANDARDS
AIR QUALITY
PLANNING
AIR QUALITY
AIR QUALITY
FRED LAV ALEE
BONNIE WATTS
RICH DRISCOLL
STEVE KISH
AHTO NIEAMIEJA
JACK McCORD
TODD CRAWFORD
BOB RAUSCH
JOE FRANCIS
(207)289-2437
(301)631-3285
(617)292-6630
(517)373-7023
(612)296-7802
(601)961-5171
(314)751-7929
(406)444-2821
(402)471-2189
NO REQUIREMENTS
PERMITS REQUIRED
MODELED; PERMITS REQUIRED
PCE - 0.0024 PPM
TCE - 0.0073 PPM
NO EMISSIONS STANDARDS
COMPLY WITH FEDERAL STANDARDS
NATIONAL EMISSIONS
STANDARD HAZARDOUS AIR
POLLUTANTS (NESHAPS)
NO STANDARDS
REPORTING REQUIRED
                                                                                           IF>151bs/hrOR
                                                                                           lOOlbs/day VOCs

-------
                                                                      APPENDIX L
                                           STATE REGULATORY AGENCY CONTACTS AND AIR DISCHARGE CRITERIA
                                                                       (Continued)
      STATE
               OFFICE
                                                              CONTACT
                                                                           TELEPHONE
                                                                                                  AIR QUALITY
                                                                                                  EMISSIONS
U)
o
oo
      NEVADA

      NEW HAMPSHIRE
               AIR QUALITY

               AIR QUALITY
      NEW JERSEY
                      UNDERGROUND STORAGE TANKS
NEW MEXICO


NEW YORK

NORTH CAROLINA



NORTH DAKOTA

OHIO


OKLAHOMA


OREGON
ENVIRONMENTAL IMPROVEMENT BUREAU


UNDERGROUND STORAGE TANKS

AIR QUALITY



AIR QUALITY

AIR POLLUTION CONTROL
MANAGEMENT

AIR QUALITY


AIR QUALITY
                                        BAYMcCLEARY      (702)885-5065

                                        ANDY BARDENARIK   (603)271-1370
                                                              DIANE PUPA
       PENNSYLVANIA   AIR QUALITY
                                                              LARRY BYRUM
                                                              RAY POTTS
                                                              JOHNCLARKE
                                                                           (609)292-6383
BOB KIRKPATRICK    (505)827-0070


ANTHONY KARWIL    (518)457-2462

BILLJEETER         (919)733-5083



TOM BACHMAN       (701)224-2348

BILL JURIS           (614)644-2270
                   (405)271-5220
                   (503)229-6411
                                                                           (717)783-9248
NO STATEWIDE REGULATIONS

ACTION LEVELS
TOLUENE - 0.8 ppm
ETHYLBENZENE - 0.1 ppm
XYLENE - 0.55 ppm
TPH as BENZENE
1 ppm/24 hr average
At Nearest Public Receptor

MAXIMUM EMISSION RATE IS
15% OF INPUT BY WEIGHT
@ 7 Ibs/hr SOME GREATER
RESTRICTIONS OCCUR FOR
HIGHER CONCENTRATIONS
SEE NJ TITLE 7:CH 27: SUB 16

0.19 ppm TPH/3hr Average
FOR SCREENING SITES ONLY

NO STATE REGULATIONS

40 Ibs/day VOCs
THEN IT DEPENDS ON
REGIONAL OFFICES

NO SPECIFIC REGULATIONS

8 Ibs/hr; 40 Ibs/day VOC
PERMITS NECESSARY

REGULATIONS FOR VOCs ARE
BEING CHANGED AS OF 10/4/89

NO STANDARDS-PERMITS REQUIRED
IF >70 TONS YEAR EMITTED

ODOR STANDARDS @ PROPERTY
LINES

-------
                                                                     APPENDIX L
                                          STATE REGULATORY AGENCY CONTACTS AND AIR DISCHARGE CRITERIA
                                                                      (Continued)
      STATE
                OFFICE
                                        CONTACT
                                                                                TELEPHONE
                                                                                           AIR QUALITY
                                                                                           EMISSIONS
U)
o
RHODE ISLAND

SOUTH CAROLINA

SOUTH DAKOTA

TENNESSEE

TEXAS


UTAH

VERMONT


VIRGINIA

WASHINGTON


WEST VIRGINIA

WISCONSIN

WYOMING
AIR & HAZARDOUS MATERIALS

AIR QUALITY PERMITS

AIR POLLUTION CONTROL

AIR POLLUTION CONTROL

AIR QUALITY
CORRECTIVE ACTION

AIR STANDARDS

HAZARDOUS MATERIALS


AIR QUALITY

AIR QUALITY


AIR POLLUTION CONTROL

AIR AND WASTE MANAGEMENT

AIR QUALITY
                                                             CHRIS JOHN         (401 )277-2808

                                                             PRESTON CAMPBELL  (803)734-4541

                                                             TIM ROGERS         (605)773-3151

                                                             BILL CLELAND       (615)741 -3651

                                                             KEITH COPELAND     (512)463-7786
JEFF MINUS

CHUCK SCHMER


BILLSYDNOR

DAN HOVAC


BOB WEISER

LARRY BRUSS

BERNIE DAILEY
(801)538-6108

(802)244-5674


(804)367-0970

(206)867-7100


(304)348-4022

(608)266-7718

(307)777-7391
EPA TOXICITY LEVELS

NO EMISSIONS STANDARDS

NO EMISSIONS STANDARDS

SITE-SPECIFIC - NEED PERMITS

TPH 100 ppm
TOTAL BTEX 30 ppm

<1 TON HYDROCARBONS/SITE

90% REMOVAL
PERFORMANCE STANDARD

NO STATEWIDE REGULATIONS

NO STATEWIDE REGULATIONS
CONTACT LOCALITIES

40 CFR 60 - NSPS

3 Ibs/hr; 15 Ibs/day VOCs

40 CFR 60 -NSPS
                      LEGEND:
                      VOC - VOLATILE ORGANIC COMPOUNDS
                      40 CFR 60 - CODE OF FEDERAL REGULATIONS, Vol. 40, Part 60
                      TLV- THRESHOLD LIMIT VALUE
                      NSPS - NEW SOURCE PERFORMANCE STANDARDS
                      PCE-PERCHLOROETHYLENE
                      TCE - TRICHLOROETHYLENE
                      UST - UNDERGROUND STORAGE TANKS

-------
                                  GLOSSARY

Adsorption.  The attraction of ions  or compounds to the surface of a solid.

Advection.  The process of transfer  of fluids (vapors or liquids)  through a
      geologic formation in response to a pressure gradient that may be caused
      by changes in barometric pressure,  water table levels,  wind fluctuations
      or rainfall percolation.

Aerobic.  In the presence of oxygen.

Air Permeability.  A measure of the  ability of a soil to transmit gases.   It
      relates the pressure gradient  to the flow.

Air/Water Separator.  A device to separate,  through additional retention time,
      physical means, or cooling, entrained liquids from a vapor stream.

Aliphatic.  Of or pertaining to a broad category of carbon compounds
      distinguished by a straight, or branched, open chain arrangement of the
      constituent carbon atoms.  The carbon-carbon bonds may be saturated or
      unsaturated.

Anaerobic.  In the absence of oxygen.

Anisotropy.  The dependence of property upon direction of measurement (e.g.,
      hydraulic conductivity, porosity, compressibility, dispersion, etc.).

Aromatic.  Of or pertaining to organic compounds that resemble benzene in
      chemical behavior.

Bentonite.  A colloidal clay, largely made up of the mineral sodium
      montmorillonite, a hydrated aluminum silicate.

Biodegradation.  A process by which microbial organisms transform or alter
      through enzymatic action the structure of chemicals introduced into the
      environment.

Bulk Density.  The oven-dried mass per unit volume  (including pore space) of
      soil.

Capillary  Fringe.  The zone of a soil above the water table within which most
      of  the soil is  saturated, but  is at less  than atmospheric pressure.  The
      capillary fringe is considered to be part of  the vadose zone but not of
      the  unsaturated zone.

Catalyst.  A substance that increases the rate  of a chemical reaction and may
      be  recovered essentially unaltered in form and amount at the end of the

                                      310

-------
      reaction.

Coarse-Textured Soils.  Soils comprised primarily of particles with relatively
      large diameters (e.g., sand, loam).

Darcy's Law.  An empirical relationship between hydraulic gradient and the
      viscous flow of water in the saturated zone of a porous medium under
      conditions of laminar flow.  The flux of vapors through the voids of the
      vadose zone can be related to pressure gradient through the air
      permeability by Darcy's Law.  See hydraulic conductivity, air
      permeability, hydraulic gradient, pressure gradient, laminar flow,
      vadose zone,  saturated zone.

Density.  The amount of mass per unit volume.

Diffusion.  The process by which molecules in a single phase equilibrate to a
      zero concentration gradient by random molecular motion.  The flux of
      molecules is from regions of high concentration to low concentration and
      is governed by Pick's Second Law.

Dispersion.  The process by which a substance or chemical spreads and dilutes
      in flowing groundwater or soil gas.

Emissions Control Device.  The equipment used to remove pollutants from the
      exhaust stream of a soil vapor extraction system.

Enhanced Biotreatment.  The phenomenon sometimes noticed after SVE whereby
      naturally-occurring soil microbes degrade the soil contaminant at an
      increased rate, perhaps due to higher available soil oxygen.

Entrainment.  A process in which suspended droplets of liquid are carried in
      the vapor stream.

Pick's Second Law.   An equation relating the change of concentration with time
      due to diffusion to the change in concentration gradient with distance
      from the source of concentration.  See diffusion, effective diffusion
      coefficient.

Field Capacity.  The percentage of water remaining in the soil 2 or 3 days
      after gravity drainage has ceased from saturated conditions.

Fine Textured Soils.  Soils comprised primarily of particles with small
      diameters (e.g.,  silt, clay).

Flow Lines.  Lines  on a cross-sectional diagram that show the direction of
      flow of air through the soil.   Flow lines are used in the design of
      vapor extraction systems.

Flux.   The rate of movement of mass  through a unit cross-sectional area per
      unit time in response to a concentration gradient or some advective
      force, having units of mass per area per time (g/cm2-sec).


                                     311

-------
Free Product.  A contaminant in the unweathered phase,  where no dissolution or
      biodegradation has occurred.   See non-aqueous phase liquid.

Fugacity.  Escaping tendency of a chemical substance from a particular phase.
      Fugacity is measured in units of pressure,  and the higher the fugacity
      the greater tendency of chemical to escape from a phase.

Gas Chromatography.  The process in which the components of a mixture are
      separated from one another by volatilizing the sample into a carrier gas
      stream.  Different components move through a bed of packing or a coated
      capillary tube at different rates, and so appear one after another at
      the effluent end, where they are detected and measured by thermal
      conductivity changes, density differences,  or ionization detectors.

Gasoline.  A mixture of volatile hydrocarbons suitable for use in a
      spark-ignited internal engine and having an octane number of at least
      60.  The major components are branched-chained paraffins,
cyclaparaffins,  and aromatics.

Henry's Law.  The relationship between the partial pressure of a compound and
      the equilibrium concentration in the liquid through a constant of
      proportionality known as Henry's Law Constant.  See partial pressure.

Heterogeneity.  The dependence of property upon location of measurement (e.g.,
      hydraulic conductivity, porosity, compressibility, dispersion, etc.).
      Heterogeneity may be due to grain size trends, stratigraphic contacts,
      faults, and vertical bedding.

Homogeneity.  The independence of property with location of measurement.

Hydraulic Conductivity.  The constant of proportionality in Darcy's Law
      relating the rate of flow of water through a cross-section of porous
      medium in response to a hydraulic gradient.  Also known as the
      coefficient of permeability, hydraulic conductivity is a function of the
      intrinsic permeability of a porous medium and the kinematic viscosity of
      the water which flows through it.  Hydraulic conductivity has units of
      length per time  (cm/sec).

Hydrocarbon.  Any of a large group of compounds composed only of carbon and
      hydrogen.

Hysteresis.  The dependence of the state of a system on direction of the
      process leading to it; a non-unique response of a system to stress,
      responding differently when the stress is released.  Compressibility,
      moisture content, soil adsorption and unsaturated hydraulic conductivity
      exhibit hysteretic behavior.

Immiscible.  Incapable of being mixed, such as oil and water.

Infiltration.  The downward movement of water through a soil from rainfall or
      the application of artificial recharge in response to gravity and
      capillarity.

                                      312

-------
Injection Well.   A well used during soil vapor extraction into which air is
      forced under pressure.

Inlet Well.  A well used during soil vapor extraction through which air is
      allowed to enter the soil passively.

Isopotential lines.  Lines indicating areas of equal pressure.  On a
      cross-sectional diagram in an isotropic medium, isopotential lines
      intersect flow lines at right angles.

Isotherm.  A relationship between the amount of solute adsorbed (expressed as
      a mass percentage of adsorbent) and the concentration in the influent
      vapor stream, at a given temperature and pressure.

Isotropy.  The independence of a property with direction of measurement (e.g.,
      hydraulic conductivity, porosity, compressibility,  dispersion, etc.).

Lower Explosive Limit.  The concentration of a gas below which the level is
      insufficient to support an explosion.

Microorganisms.   Microscopic organisms including bacteria, protozoans,  yeast
      fungi, viruses and algae.

Macropore.  A large pore in a porous medium which may be formed by physical
      phenomena or biological activity, and through which water, or other
      fluids, flows solely under the influence of gravity, unaffected by
      capillarity.

Moisture Content.  The amount of water lost from the soil upon drying to a
      constant weight, expressed as the weight per unit weight of dry soil or
      as the volume of water per unit bulk volume of the soil.  For a fully
      saturated medium, moisture content equals the porosity; in the vadose
      zone, moisture content ranges between zero and the porosity value for
      the medium.  See porosity, vadose zone, saturated zone.

Mottling.  The reticulate coloring pattern of soils which is indicative of
      alternating oxidizing/reducing conditions.  Mottling implies seasonally
      wet soil conditions.

NAPL.  Non-aqueous phase liquid.

Oxidation.  A chemical reaction that increases the oxygen content of a
      compound,  or raises the oxidation state of an element.

Oxidation Potential.  The difference in potential between an atom or ion and
      the state in which an electron has been removed to an infinite distance
      from this atom or ion.
Partial Pressure.  The portion of total vapor pressure due to one or more
      constituents in a vapor mixture.

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Particle Density.   The amount of mass of a substance per unit volume of the
      substance.

Percolation.  The downward movement of water through soil.   Especially, the
      downward flow of water in saturated or nearly saturated soil at
      hydraulic gradients of the order of 1.0 or less.

Permeability.  A measure of a soil's resistance to fluid flow.  Permeability,
      along with fluid viscosity and density, is used to determine fluid
      conductivity.

Piezometer.  An instrument used to measure pressure.  Often used in reference
      to tubes inserted into the soil for measuring soil pressure or water
      table depth.

Porosity.  The volume fraction of a rock or unconsolidated sediment not
      occupied by solid material but usually occupied by water and/or air.
      Porosity is a dimensionless quantity that is expressed as a percent or a
      decimal.

Pressure Gradient.  A pressure differential in a given medium, such as water
      or air, which tends to induce movement from areas of higher pressure to
      areas of lower pressure.

Pulsed Venting.  A method of operation in which the vacuum pump or blower is
      operated intermittently.  During periods when the vacuum is off, the
      contaminant vapors re-equilibrate.  When the system is turned back on,
      extracted vapors have higher concentrations.  Pulsed venting is cheaper
      than continuous venting due to lower power consumption.

Radius of Influence.  The maximum distance away from a vacuum source that is
      still affected by the vacuum.

Residual Saturation.  The amount of water or oil remaining in the voids of a
      porous medium and held in an immobile state by capillarity and dead-end
      pores.

Saturated Zone.  The zone of the soil below the water table where all space
      between the soil particles is occupied by water.

Short Circuiting.  As it applies to SVE, the entry of ambient air into the
      extraction well without passing through the contaminated zone by, for
      example, entering a utility trench.

Site Characterization.  Documentation of all site characteristics that may
      impact the design of a subsurface venting system.

Soil Gas Survey.  Investigation of the distribution of soil gas concentrations
      in three dimensions.  The term may apply to the map or  to data
      documenting the soil gas concentrations.


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Soil Sorption Coefficient.  A measure of the preference of an organic chemical
      to leave the dissolved aqueous phase in the soil and become attached or
      adsorbed to soil particles and organic carbon.

Soil Texture.  Refers to the particle size distribution of the soil.  Particle
      size classes of the USDA system depend on the relative proportions of
      sand (2-0.05 mm), silt (0.05-0.002 mm), and clay particles (<0.002 mm).

Soil Vapor Extraction.  A soil remediation technique that involves removing
      contaminant-laden vapors from the soil under a vacuum.  Also known as
      vacuum extraction, soil venting, soil stripping, and enhanced
      volatilization.

Solubility.  The amount of mass of a compound that will dissolve into a unit
      volume of solution.

Sorption.  A general term used to encompass the process of absorption,
      adsorption, ion exchange, and chemisorption.

TPH.  See total petroleum hydrocarbons.

Total Petroleum Hydrocarbons.   A measure of the mass or concentration of all
      the petroleum constituents present in a given amount of air, soil, or
      water.

Tortuosity.  The ratio of path length through a porous medium to the
      straight-line flow path which describes the geometry of the porous
      medium.  Tortuosity is a dimensionless parameter which ranges value from
      1 to 2.

Unsaturated Zone.  The portion of a porous medium, usually above the water
      table in an unconfined aquifer, within which the moisture content is
      less than saturation and the capillary pressure is less than atmospheric
      pressure.  The unsaturated zone does not include the capillary fringe.

Upper Explosive Limit.  The concentration of a gas above which the gas will
      not explode.

UST.  See underground storage tank.

Underground Storage Tank.  By statutory definition, any tank that is used to
      contain an accumulation of regulated product [basically, petroleum or
      hazardous substances] and the volume of which is 10 percent or more
      underground.

Vacuum.  The existence of below-atmospheric pressure.

Vadose Zone.   The portion of a porous medium above the water table within
      which the capillary pressure is less than atmospheric and the moisture
      content is less than saturation.  The vadose zone includes the capillary
      fringe.
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Vapor Density.  The amount of mass of a vapor per unit volume of the vapor.

Vapor Pressure.  The equilibrium pressure exerted on the atmosphere by a
      liquid or solid at a given temperature.  Also, a measure of a
      substance's propensity to evaporate or give off flammable vapors.  The
      higher the vapor pressure, the more volatile the substance.

Volatilization.  The process of transfer of a chemical from the water or
      liquid phase to the air phase.  Solubility, molecular weight, and vapor
      pressure of the liquid and the nature of the air-liquid/water interface
      affect the rate of volatilization.  See solubility, vapor pressure.

Water Content.  See moisture content.

Water Table.  The water surface in an unconfined aquifer at which the fluid
      pressure in the voids is at atmospheric pressure.

Weathering.  The process where a complex compound is reduced to its simpler
      component parts, transported through physical processes, or biodegraded
      over  time.

Well Screen.  The segment of well casing which has slots to permit the flow of
      liquid or air but prevent the passage of soil or backfill particles.
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