&EPA
United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
EPA. 600 2-90/011
March 1990
Research and Development
Assessing LIST Corrective
Action Technologies:
Site Assessment and
Selection of Unsaturated
Zone Treatment
Technologies
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DISCLAIMER
The information in this document has been funded by the U.S. Environmental
Protection Agency under Contract 68-03-3409 to COM Federal Programs Corps.
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, can threaten both public
health and the environment. The U.S. Environmental Protection Agency 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
is one of the products of that research and provides a vital communication
link between the researcher and the user community.
An area of major concern is the health impacts associated with
uncontrolled releases of petroleum hydrocarbons from underground storage
tanks. This document focuses on cleaning up soils contaminated with
petroleum. It provides much needed assistance on how to make an informed
selection of an effective soil treatment technology.
E. Timothy Oppelt, Director
Risk Reduction Engineering Laboratory
111
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ABSTRACT
A methodology is presented for evaluating the likely effectiveness of
five soil treatment technologies at sites where petroleum products have
contaminated the unsaturated zone. The five soil treatment technologies
are: soil venting, biorestoration, soil flushing, hydraulic barriers, and
excavation. The evaluation consists of a site assessment, selection of a
treatment technology, and performance monitoring and follow-up
measurements. The overall focus of the manual is on making a preliminary
screening of what soil treatment technologies would likely be effective at
a given UST site.
This manual identifies basic information about the subsurface
environment and the released product that is needed for a site
assessment. The reader is shown what information is needed and where it
can be obtained; the manual also provides default values for some
parameters if field data are not available or have not yet been collected.
Worksheets are provided to help the reader make a preliminary
determination as to where in the unsaturated zone most of the petroleum
product is likely to be, and further, how likely the petroleum is to move
into and out of the three "phases" considered in this manual: 1) as
contaminant vapors in the pore spaces of the soil; 2) as residual
saturation trapped between soil particles; or 3) dissolved in pore water.
Process descriptions, costs, advantages and disadvantages are provided for
each soil treatment technology.
Factors that are critical to the successful implementation of each
technology are presented and site conditions which are favorable for each
factor are discussed.
Worksheets containing the relevent success factors are provided for
each technology that assist the reader in determining if actual site
conditions are favorable for a given technology. The worksheets can be
compared to screen those technologies most likely to be effective.
IV
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CONTENTS
Foreword ....iii
Abstract iv
Figures vi
Tables vi i
Abbreviations and Symbols ix
Acknowledgement x
1. Introduction 1
Background 1
Purposes of this Manual .1
Approach and Organization. 3
Limitations 5
2. How to Conduct a Site Assessment .. 7
Introduction 7
Gathering Release Information .9
Gathering Site-Specific Information .11
Gathering Contaminant-Specific Information 13
Evaluating Contaminant Phases in Soil 18
Evaluating Contaminant Mobility 30
3. Technology Selection 36
Introduction 36
Soil Venting. 37
Biorestoration. 42
Soil Flushing. 50
Hydraulic Methods 55
Excavation 58
Above-Ground Treatment Methods 59
Summary 62
4. Monitoring and Follow-up Measurements 64
Introduction. 64
Monitoring Performance and Progress. 65
References 70
Appendices
A How to Use this Manual: Sample Problem 73
Glossary 92
v
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FIGURES
Number Page
1 An Overview of the Approach 4
2 Representation of Three Different Phases In Which Petroleum
Can be Found In Unsaturated Zone 8
3 Groundwater Temperatures in the United States (ฐC) at Depth
of 10 to 25 Meters 16
4 Water-holding Properties of Various Soils 17
5 Schematic Diagram of Vacuum Extraction System 38
6 Soil Venting Performance Versus Time 42
7 Schematic Diagram of Biorestoration System 44
8 Schematic Diagram of Soil Flushing System 51
9 Schematic Diagram of Hydraulic Methods .. 56
10 How Concentrations of Hydrocarbon Vapors Change During
Soil Venting .67
11 Determining Area to be Excavated using Hydrocarbon
Concentrations 69
VI
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TABLES
Number Page
1 Basic Release Information Assumed to be Known 10
2 Site Specific Parameters to Gather 12
3 Physicochemical Properties of Rocks and Soil 14
4 Relationship Between Diameter of Particles and Surface Area 15
5 Contaminant Specific Parameters to Gather 15
6 Unweathered Composition of Four Common Hydrocarbon Mixtures....20
7 Physicochemical Properties of Five Common Hydrocarbon
Mixtures 21
8 Properties of the Chemicals Comprising the Four Common
Hydrocarbon Mixtures 22
9 Refractory Index for Common Hydrocarbon Compounds 23
10 Rules-of-Thumb for Determining the Phase of a Contaminant 24
11 Likelihood of Liquid Contaminants Being Present in the
Unsaturated Zone 26
12 Likelihood of Contaminant Vapors Being Present in the
Unsaturated Zone 27
13 Likelihood of Contaminants Dissolved in Pore Water Being
Present in the Unsaturated Zone 28
14 Factors to Evaluate the Extent of Migration of
Liquid Contaminants 31
15 Factors to Evaluate the Extent of Migration of
Contaminant Vapors 33
16 Factors to Evaluate the Extent of Migration of
Contaminants in Pore Water 35
vn
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TABLES (continued)
17 Worksheet for Evaluating the Feasibility of Soil Venting
Being Effective at Your Site 40
18 Worksheet for Evaluating the Feasibility of Biorestoration
Being Effective at your Site 48
19 Worksheet for Evaluating the Feasibility of Soil Flushing
Being Effective at your Site 54
20 Worksheet for Evaluating the Feasibility of Hydraulic
Barriers Working at your Site 57
21 Worksheet for Evaluating the Feasibility of Excavation Being
Effective at your Site 60
22 Summary of Critical Success Factors for Optimum Performance....63
Vlll
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LIST OF ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
API -- American Petroleum Institute
BOD -- biochemical oxygen demand
COM -- Camp Dresser & McKee Inc.
CFM -- cubic feet per minute
COD -- chemical oxygen demand
CSF -- critical success factor
EPA -- U. S. Environmental Protection Agency
NAPL -- Non-aqueous phase liquid
O&M -- operation and maintenance
ORD -- EPA's Office of Research & Development
OUST -- EPA's Office of Underground Storage Tanks
UST -- underground storage tank
cm/sec -- centimeters per second
cPoise -- centipoise
cm/day -- centimeters per day
in/yr -- inches per year
g/cm3 -- grams per cubic centimeter
g/m3 -- grams per cubic meter
m /g -- meters square per gram
mg/L -- milligrams per liter
mm Hg -- millimeters of mercury
L/kg -- liters per kilogram
SYMBOLS
gal
ft
ฐK
mo
yr
o
PH
RI
degrees Celsius
centimeters squared
gallon
feet
degrees Kelvin
month
year
percent
concentration of contaminant in soil [cm3/cm3]
concentration of contaminant in pore water [mg/1]
soil/water partitioning coefficient
octanol/water partitioning coefficient
indicates alkaline or acid conditions in log units
refractory index
IX
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ACKNOWLEDGEMENT
This manual was produced by Camp Dresser & McKee Inc. (COM) under the
supervision of Dr. Myron S. Rosenberg as a work product associated with
Work Assignment No. 107 under Contract 68-03-3409 with EPA's Office of
Research and Development (ORD).
COM would like to acknowledge the guidance and assistance provided by
Mr. Anthony N. Tafuri, ORD's Project Officer, and Mr. Chi-Yuan Fan, ORD's
Technical Project Monitor for this Work Assignment. COM is grateful to Ms.
Iris Goodman of EPA's Office of Underground Storage Tanks for assisting in
developing the overall focus of this manual. Dr. Bruce J. Bauman of the
American Petrolium Institute provided valuable technical review comments.
Stephen Mangion of EPA Region I OUST and Robert Hillger of ORD also
contributed to the technical review of this manual.
This manual is based on research and scientific evaluations developed
by Dr. Warren Lyman of COM, and Mr. William Thompson of PEI Associates,
Inc., with additional material provided by Mr. James T. Curtis,
Mr. Benjamin S. Levy, and Mr. Tom Pedersen of COM. This research effort
was managed by Dr. Warren Lyman. Mr. John Farlow of ORD initiated and
guided this basic scientific research.
This manual was prepared in its final format based on a technical edit by
Mr. David C. Noonan and Mr. James T. Curtis, with additional material
provided by Mr. Patrick Reidy of COM. Mr. Roland Robinson assisted by
Ms. Linda O'Brien and Ms. Catherine Hooper prepared the final manuscript,
and Mr. A. Russell Briggs and Ms. Lisa Cincotta prepared the final
graphics.
x
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SECTION 1
INTRODUCTION
BACKGROUND
Cleaning up releases from underground storage tanks (USTs) typically
involves using several corrective strategies. Short-term emergency
measures can involve actions to control acute safety and health hazards
such as potential explosions. Examples of emergency measures include
draining and removing tanks. After the imminent danger has been addressed,
longer term corrective actions involve cleaning up petroleum product that
has entered the subsurface environment. Petroleum product in the
subsurface may be trapped between soil particles in the unsaturated zone,
floating on the water table, or dissolved in groundwater in the saturated
zone.
The focus of this manual is on long-term strategies for cleaning up
petroleum product in the unsaturated zone. Unlike groundwater treatment
technologies where theories, equations and performance issues are well
understood soil treatment has only recently received widespread
attention. The science associated with soil treatment technologies is not
as well documented or understood as that associated with water treatment
technologies. Despite this uncertainty, soil contamination must always be
addressed, and some kind of soil cleanup is typically necessary when a
spill occurs.
Removing petroleum hydrocarbons from the unsaturated zone can involve
several technologies. Each technology varies with respect to the
constituents removed and the method of removal. Also, each technology is
most effective and efficient when applied under specific conditions. These
conditions, herein called "critical success factors," are specific to each
treatment technology.
This manual identifies and describes the critical factors that help
characterize the contaminants that were released and the physicochemical
properties that determine their movement in air, soil and water. It also
identifies and describes the hydrogeologic parameters that are important to
developing an understanding of how subsurface conditions will affect
treatment performance. Further, this manual provides a framework for
proceeding from site assessment to identification and evaluation of soil
treatment technologies that can be effective in cleaning up a site.
PURPOSES OF THIS MANUAL
The purposes of this manual are: 1) to provide assistance to local
and state decision-makers on how to assess site conditions in the
unsaturated zone at a site where a petroleum product release has occurred;
and 2) to present methods for evaluating various soil treatment
technologies at a given site to determine their potential effectiveness.
The manual describes: 1) what information is needed in a site assessment;
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how to obtain that information; 3) how that information can be used to
determine where in the unsaturated zone most of the released product is
likely to be located; and 4) which of five soil treatment technologies
considered in this manual are likely to be effective in removing petroleum
products from the unsaturated zone at a given site.
The manual directs the user toward final selection of a treatment
technology rather than presenting a methodology that results in selection
of one best technology; and its value may lie as much in identifying which
technologies are not likely to be effective as in identifying those that
are. The user is provided with a framework for better understanding
complex subsurface conditions and identifying key issues in selecting an
appropriate soil treatment technology.
A good understanding of the conditions in the unsaturated zone is
essential when selecting an appropriate soil treatment technology. One way
to gain this understanding is to conduct a site assessment. The term site
assessment, as used here, refers to a desktop analysis of basic hydrologic,
geologic, and chemical data obtained from the literature, previous
experience at similar sites, and some basic, easy-to-get field measurements
collected at the site of interest. It is important to distinguish between
a site assessment and a site characterization, which consists of a more
rigorous and comprehensive field testing program and can include monitoring
wells, sampling soil gas, and collecting soil samples for precise parameter
determinations.
Though not as accurate as a site characterization, a good site
assessment includes enough information to adequately answer the following
questions:
What was released? Where? When?
Currently, where in the unsaturated zone is most of the petroleum
product likely to be?
How much petroleum product is likely to be present in different
locations and phases?
How mobile are the constituents of the contaminant, and where are
they likely to travel and at what rate?
This manual helps the user answer these questions prior to conducting
extensive field studies at the site. Often, a site assessment based on
estimates of important parameters is sufficient to gain a relative
understanding of the potential effectiveness of a given technology at a
given site. This manual does not eliminate the need for site-specific
measurements of important parameters since there is no substitute for
accurate, site-specific, field data. However, it is possible to combine
actual measurements with literature values for other parameters (for which
no field data are presently available) to make qualitative assessments of
site conditions, and to assess which corrective action technologies are
likely to work. Prior to any final decisions, site-specific measurements
should, of course, be made.
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APPROACH AND ORGANIZATION
The intended primary users of this manual are state and local field
and regulatory personnel. Contractors and other parties who have
responsibility for selecting or reviewing corrective action plans for
cleaning up the unsaturated zone may also find the manual useful. The
manual presents a simplified approach to evaluating a site and does not
require a highly technical background.
Figure 1 shows the three main components of the approach: 1) site
assessment: 2) technology selection; and 3) monitoring and follow-up
measurements. The leftmost column of Figure 1 contains questions which
pertain to the boxed text immediately to their right. These questions
provide the focus for each step in the site evaluation process. Each of
the main components is briefly discussed below.
Site Assessment
In Section 2 the reader will be shown what data are helpful in
developing a working hypothesis about subsurface conditions and how those
data can be obtained. Some information is release-related (e.g., what was
released and when) and is assumed to be readily available. Other important
information may be less easily obtained, such as hydrogeologic
characteristics of the site or the physical and chemical properties of the
contaminant. For these data, the manual provides assistance in measuring
and/or estimating values to be used in the assessment.
The data collected in the initial stages of the assessment are used
to gain an understanding of the behavior of the contaminant in the
subsurface. When dealing with the unsaturated zone, the data are primarily
used to determine the mobility of the contaminant and what phase(s) it is
likely to be in. Mobility of petroleum product in the unsaturated zone is
related to: 1) the potential for the various phases of contaminant to move
through the subsurface; and 2) the potential for the petroleum product to
change from one phase into another. Most treatment technologies rely on
mobilizing the contaminant by physically moving the contaminant through the
suburface or by causing transformation from one phase to another, or both.
Understanding the likely subsurface behavior of the contaminant is
therefore very important in evaluating the effectiveness of a given
technology.
Selection
In Section 3, the general understanding of subsurface contaminant
behavior developed in Section 2 is used to help evaluate the five treatment
technologies presented in this manual: 1) soil venting (including vacuum
extraction); 2) biorestoration; 3) soil flushing; 4) excavation; and 5)
hydraulic barriers. An overview of each technology is presented to
familiarize the reader with the basic principles of how the technology
works, and their major advantages and disadvantages.
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KEY
QUESTION
ACTION
WHAT WAS
RELEASED ?
WHERE IN THE
UNSATURATED
ZONE IS IT?
HOW MUCH IS
THERE?
WHERE IS IT
GOING?
WHAT ARE THE
TECHNOLOGIES?
WHAT ARE EACH
TECHNOLOGY'S
ADVANTAGES?
WHAT ARE THE
CRITICAL SUCCESS
FACTORS?
WHAT WILL WORK
AT MY SITE?
EVALUATION OF
SUCCESS &
SELECTION
SITE ASSESSMENT
OBTAIN BASIC RELEASE INFO
SECTION 2.0
OBTAIN BASIC
CONTAMINANTINFOJ
EVALUATE PHASE(S) OF
CONTAMINANT(S) IN SOIL
TECHNOLOGY SELECT ON
SELECT TECHNOLOGY
BASED ON YOUR
SITE AND HOW IT MATCHES THE
TECHNOLOGY'S CSFs
ICE I
SECTION 4.0
PERFORMANCE MONITORING
^mMvmm&mmmmmmmm,
SAMPLING & MEASUREMENT
OF SITE CONDTIONS
COMPARE TO PRE-ESTABLISHED
CLEAN-UP GOALS
YES
NO
CONTINUE TO MONITOR
Figure 1. An Overview of the Approach
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Critical success factors are listed for each technology. These
critical success factors, or CSFs, are incorporated into worksheets
with which the user can systematically evaluate each technology. The
worksheets are completed using the information collected in the site
assessment stage, and a relative comparison of the alternatives can be
made. Not all CSFs are equally important when evaluating a particular
technology at a particular site and the importance of a particular CSF can
vary depending on site conditions. Because of this, persons with some
knowledge of hydrogeology and experience at site assessment should be
consulted before final selection of a remedial action is made.
It should be noted that the selection process presented in this
manual is scientifically based. Final selection of a soil treatment
technology will also be affected by other criteria such as regulatory
restrictions and time available to clean up the site. The cost of
implementing a technology is another factor that often plays a significant
role in final selection, but it is not considered here.
Performance Monitoring & Follow-Up Measurements
After a technology (or technologies) is selected and implemented, it
is important to track its performance and monitor its effectiveness during
cleanup activities. Section 4 discusses parameters that should be
monitored and how these parameters should be measured (i.e., what types of
samples are needed). Monitoring and follow-up are essential because of the
uncertainty about subsurface conditions. If a technology's performance is
poor, the field data and site assessment, including inherent assumptions,
should be re-examined. Poor cleanup performance might be due to a
misinterpreted or incomplete site assessment or an incorrect selection of
the technology. This feedback loop is an important step in the entire
cleanup process.
LIMITATIONS
Users of this manual should be aware of several significant
limitations regarding its content and its precision:
Estimates are for Relative Assessments - One of the manual's
purposes is to provide estimates of how much of the release is
likely to be in each phase in the unsaturated zone, and which
technologies are more likely to be effective. The screening of
technologies should be reviewed in relative terms rather than an
absolute endorsement of one particular technology.
Not for Emergency Response - It is assumed that all necessary
emergency responses have been taken, that the source of the release
(e.g., tank or supply line) has been identified and repaired, and
that proper notification of government agencies (local, state and
federal) has taken place.
Unsaturated Zone Coverage Only - This technical assistance document
addresses site assessment and corrective action for contamination
in the unsaturated zone only. Guidance is not provided for
contamination of the saturated zone. In some instances, an
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assessment of the unsaturated zone might proceed regardless of
potential or actual groundwater contamination, but it should not be
assumed that the groundwater is uncontaminated. The presence of a
floating contaminant layer on the water table, or a contaminant
plume in the groundwater, may ultimately affect the selection of
the unsaturated zone corrective action or actions. Integrated
guidance for site assessment, corrective action, and evaluation for
both the unsaturated and saturated zones contamination is not
addressed in this document.
Focus on Petroleum Hydrocarbons as Contaminants - Because petroleum
products comprise most of the materials stored in USTs, they are
the focus of this manual. Special focus is given to gasoline,
primarily in example calculations. This implicit focus does not
preclude the use of this report for other types of chemical
contaminants, although it clearly is best suited for organic
liquids.
Focus on In Situ Treatment - Other than excavation, all
technologies discussed in the manual are in situ treatment *
techniques. After a contaminated soil has been excavated, the soil
can also be treated above ground using many of the same techniques
described for in situ applications (e.g., biodegradation can be
promoted in the excavated soil). These techniques are only briefly
discussed in this manual.
In Situ treatment means treatment of the soils in place, i.e., they are
notdug up. The excavation of contaminated soils usually adds
significant expense to a site remediation and also can raise
complicating legal questions.
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SECTION 2
HOW TO CONDUCT A SITE ASSESSMENT
INTRODUCTION
To choose an effective clean-up technology for soils contaminated by
petroleum, it is necessary to collect and interpret certain basic
information about both the released product and the subsurface
environment. At a minimum, the following questions should be addressed:
What contaminants were released? - Knowledge of the type of
product released, its physical and chemical properties, and its
major chemical constituents is required. The physical and
chemical properties of the product can provide valuable insight
into subsurface behavior.
Where is the petroleum product currently? - Different soil
treatment technologies may be appropriate depending on how the
petroleum product is distributed within the unsaturated zone,
i.e., as a vapor in soil gas, as a residual liquid, or dissolved
in pore water;
How much petroleum product is in each phase? - In the time
period immediately after a release (weeks to months) most of the
product will exist as a residual liquid. With time, however,
significant fractions will volatilize (enter the vapor phase)
and dissolve in existing pore water or infiltrating rain water.
For old releases, the residual liquid phase may be nearly
non-existent or transformed into a more viscous, gummy material.
Where is the petroleum product going? - Also important in the
site assessment is an estimate of the mobility of the
contaminant. Mobility affects not only the potential areal
extent of a release (as the contaminant moves away from the
release point), but also the potential effectiveness of any
in-situ treatment scheme that depends on mobilizing the
contaminants in order to remove them (e.g., vacuum extraction).
These four questions provide a framework for the types of
information that should be collected. While additional information is
not essential to making a preliminary decision regarding the corrective
action, more information can often improve the selection process.
The guidelines contained in this manual are based on research done
for EPA's Risk Reduction Engineering Laboratory (REEL) office in Edison,
New Jersey. The report summarizing that research identified as many as
13 different conditions under which petroleum product could be found in
the subsurface (EPA, 1988a). For simplicity, this manual assumes that
petroleum product in the unsaturated zone exists only in three phases as
shown on Figure 2: 1) as contaminant vapors in the pore spaces (vapor
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DRY SOIL
PARTICLES
PORE
SPACES
WET SOIL
PARTICLES
UNSATURATED
ZONE
CAPILLARY ZONE
SATURATED
ZONE
BEDROCK
PETROLEUM
PRODUCT
VAPORS
IN PORE
SPACES
RESIDUAL
PETROLEUM
PRODUCT
TRAPPED
BETWEEN
PARTICLES
CLEAN SOIL
PETROLEUM
PRODUCT
DISSOLVED
IN SOIL
MOISTURE
SOIL CONTAMINATED BY
PETROLEUM PRODUCT
RELEASE
Figure 2. Representation of Three Different Phases in which
Petroleum Product can be Found in Unsaturated Zone
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phase); 2) as residual liquid trapped between soil particles (liquid
phase); or 3) dissolved in the pore water that surrounds soil particles
(dissolved phase). For recent petroleum product releases (less than 1
year old) most of the petroleum product is likely to be in the residual
liquid phase rather than vapor and dissolved phases.
A distinction is made here between bulk product and the individual
constituents that make up the bulk product. Examples of bulk product
include automotive gasoline, No. 2 fuel oil, and jet fuel. These bulk
products, in turn, are composed of hundreds of individual constituents,
each with its own set of chemical properties. These individual
constituents dissolve in water and evaporate at rates that are somewhat
different than the rate of the bulk product. (The chemical properties of
the bulk product tend to be a composite of the chemical properties of its
individual constituents.) Default values are provided in this manual for
the chemical properties for some typical bulk products and their
individual constituents.
The following subsections are structured to correspond to the
questions listed above. In each subsection, only the information that is
basic and essential for this manual is listed. An informed and
successful screening of corrective actions can be made using this
information.
GATHERING RELEASE INFORMATION
A site assessment begins with basic information about the release
itself, which may be obtained by asking the questions listed in Table 1.
Answering these questions provides a starting point for understanding the
mobility and phase distribution of the product in the subsurface. For
most above-ground spills, the answers to the questions in Table 1 are
usually easy to obtain. However, if the questions in Table 1 cannot be
readily answered, additional field sampling and testing will be needed to
start off the site assessment. This manual assumes the answers to these
questions have been obtained.
One of the basic questions in Table 1 is: What contaminants were
released? Petroleum products include a variety of fuel types, each with
different physical and chemical properties. In addition, each fuel type
is a mixture of many constituent compounds which have properties that can
be quite different from those of the mixture. Different fuel types and
compounds behave differently in the subsurface in terms of such things as
their evaporation and dissolution into and out of the various phases
(vapor, pure liquid, dissolved in water); their transport through the
unsaturated zone; and their potential to biodegrade into simpler
compounds. A contaminant's physical and chemical properties must be
known to make a judgment about its mobility (e.g., "Will this contaminant
spread quickly and reach the water table?"), its partitioning ("Will this
contaminant vaporize and pose explosion hazards?") and its degradation
potential ("Will this contaminant be biodegraded easily in the
unsaturated zone?"). These factors are an important part of the
technology selection process.
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TABLE 1. BASIC RELEASE INFORMATION
ASSUMED TO BE KNOWN
INFORMATION NEEDED
WHY INFORMATION IS IMPORTANT
What contaminants were released?
* Physical and chemical properties differ for each
contaminant, leading to varying phase
partitioning, mobility, and degradation
characteristics for each contaminant.
Corrective action selection is tied to these
characteristics.
How much was released?
t The amount released directly affects the
phases in which the contaminant may be found.
What was the nature of the
release (quick spill/slow leak)?
Phase partitioning and mobility of the released
contaminant are both affected by the nature of
the release. As a result, selection of
appropriate corrective actions may differ for
quick spills versus leaks over extended period
of time.
How long since the release?
Contaminants "weather over time, that is,
change in composition due to processes such
as degradation, volatilization, and natural
flushing from infiltrating rainfall. This change in
composition directly affects the physical and
chemical properties of the bulk contaminant.
How was the release detected?
May provide insight into above questions and
areal extent and distribution of contamination in
the subsurface.
10
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A related question is: How much petroleum product was released?
Knowing the volume spilled can help the user evaluate whether the
contaminant has reached the saturated zone and estimate the level of
contamination in the unsaturated zone.
The time since release is important because the composition and
properties of the released^ material changes over time; volatile
compounds evaporate, soluble constituents dissolve in infiltrating
rainwater, and some constituents biodegrade. These biochemical changes
that occur over time are called "weathering." Weathering can result in
contamination with dramatically different chemical concentrations than
that originally released. In general, constituents with low molecular
weights will move away from the source more quickly over time (either
through volatilization or natural flushing by infiltrating rainwater);
the more stable and less mobile constituents (typically, high molecular
weight compounds) remain near the source longer.
GATHERING SITE-SPECIFIC INFORMATION
Site-specific information pertains primarily to the hydrologic and
geologic characteristics of the site. Historic hydrologic data is
typically available through state or local climatology offices or through
the local United States Geological Survey (USGS) office. Geologic
characteristics can vary greatly, even over short distances, making
accurate estimates of soil parameters difficult without collecting
numerous and expensive field data. For a site assessment, as opposed to
characterization, representative geologic data such as can be found at
the local USGS office may suffice to gain a relatively good understanding
of the subsurface.
Table 2 lists site-specific data that are needed to conduct a site
assessment. Again, Table 2 does not list all data that could be useful
in an assessment, but rather lists the essential data needed to assess
the site. In many cases additional data or measured data could improve
the precision of the findings although, as will be shown, they may not be
necessary to make a first-cut choice of corrective action technology.
This manual provides default values obtained via tables, figures,
and other data for many of the parameters. These default values
enable users of this manual to make estimates of the critical parameters
on a timely basis for a preliminary site assessment. (An example of the
use of these default values is given in Appendix A.) Estimated values
however, should eventually be validated on a site-by-site basis through a
carefully designed sampling~program that would naturally take place when
a corrective action is implemented.
Collecting field samples is covered extensively in many other
publications and is not discussed in detail in this manual. A Compendium
of Superfund Field Operations (EPA, 1987b) describes proper sampling
procedures to ensure the collection of meaningful data. EPA (1988a)
describes various techniques to determine eight key parameters needed for
a site assessment:
11
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TABLE 2. SITE-SPECIFIC PARAMETERS TO GATHER
PARAMETER (UNITS^
Soil Porosity (%)
Particle Density (g/cm3)
Bulk Density (g/cm3)
Hydraulic Conductivity (cm/sec)
Air Conductivity
Permeability (cm2)
Soil Moisture Content (%)
Local Depth to Groundwater (ft)
Soil Temperature (ฐC)
Soil pH
Rainfall, Runoff, and Infiltration
Rate (cm/day)
Soil Surface Area (m2/g)
Organic Content (%)
Composition
Fractures in Rock
DEFAULT SOURCE
Tables, Figure4
Table 3
Table 3
Table 3
Table 3
Table 3
Figure 4
Easy to Measure or through
Site or Local records
Figure 3
Measure
Local precipitation and
evaporation records
Table 4
Measure
Local Records
IMPORTANT FOR DETERMINING:
Mobility, Phase
Mobility, Phase
Mobility, Phase
Mobility, Phase
Mobility, Phase
Mobility, Phase
Mobility, Phase
Phase
Mobility, Phase
Bacterial Activity
Mobility, Phase,
Composition
Mobility, Phase
Mobility, Phase,
Mobility
12
-------
depth to groundwater;
soil temperature
moisture content;
particle size distribution;
bulk density;
saturated hydraulic conductivity;
unsaturated hydraulic conductivity, and
residual saturation.
Site-specific data should be obtained whenever possible; however, if
a quick initial assessment is desired it is often necessary to evaluate
alternatives without the benefit of field data or when only incomplete
data are available.In these cases, approximations may be useful.
Tables 3 and 4 provide typical values for several of the parameters
listed in Table 2 for various types of soil and rock. These tables can
be used to select default values in the absence of measured values.
Figure 3 gives generalized groundwater temperatures throughout the
continental United States. An estimate of subsurface temperature can be
made by interpolating between contour lines. Figure 4 provides
water-holding properties by soil type. An estimate of the moisture
content of a particular soil can be made by using a typical field
capacity value from the from the range of values shown on Figure 4.
Field capacity is an approximation of the amount of water retained in the
soil spaces as infiltrating rainwater passes down through the formation,
and is expressed as a percentage of the total soil volume. The sample
problems provided in Appendix A refer to these tables and figures for
representative values.
Other sources that can provide suitable estimates of these
parameters include the USGS and the Soil Conservation Service (SCS).
County extension offices may also offer another source of information, as
hydrogeologic data is often categorized by county.
GATHERING CONTAMINANT-SPECIFIC INFORMATION
In addition to release-related and site-related information, a site
assessment should include an understanding of the physical and chemical
properties of the contaminants released. To a large extent, contaminant
properties govern partitioning in the subsurface; what phase(s) it is
likely to reside in, how it is likely to move away from the site, and
whether it is likely to degrade significantly over time.
Table 5 lists contaminant-specific data that are needed to conduct a
site assessment. For a preliminary site assessment, extensive field
sampling and analysis is unnecessary to estimate values for
contaminant-specific parameters. Instead literature values can be used
to estimate contaminant-specific parameter values.
Looking at Table 5, the user can identify from the default source
column where in the manual literature values for each parameter may be
found. These values can be used in the assessment as will be
demonstrated in the sample example in the Appendix. To illustrate,
assume the liquid density of gasoline is needed. The user can locate
liquid density in Table 5 and find that the default source is Table 7.
Moving to Table 7, the typical value for automotive gasoline in the
liquid density column is 0.73 g/cm3.
13
-------
TABLE 3. PHYSICOCHEMICAL PROPERTIES OF ROCKS AND SOIL
Rock/Soil Type
UNCONSOLIDATED
Gravel
Sand
Loam
Silt
Clay
CONSOLIDATED
Sandstone
Shale
Granite
Granite (fractured)
Limestone
Limestone (Karstic)
Basalt (permeable)
Porosity
25-40
25-50
42-50
35-50
40-70
5-30
0-10
0-5
0-10
0-20
5-50
5-50
Particle
Density
(g/cm3)
2.65
2.65
2.65
2.65
2.25
2.65
2.25
2.70
2.70
2.87
2.71
2.96
Bulk
Density
(g/cm3)
1 .59-1 .99
1.33-1.99
1 .33-1 .54
1 .33-1 .72
0.68-1 .35
1 .86-2.52
1 .98-2.25
2.57-2.70
2.43-2.70
2.30-2.87
1 .36-2.57
1 .48-2.81
Saturated
Hyd. Cond
(cm/sec)
10'1 -102
10-5.-|0-1
10-7-10'3
10'10-10-7
10-S-10-4
10'11 -10'7
10'11 -10"8
10'6-10-2
10-7.-|Q-4
10"4-!
10'5-1
Permeability
(cm2)
10-6-10-1
10-9-10'5
10-10.10-6
io-15-io-12
10-13-10'9
io-16-io-12
1Q-16-10-13
1Q-11 -10'7
10'12-10-9
10"9 -10"5
io-10-io-5
Air
Conductivity
(cm/sec)
10-2-10
10"5-10"1
10'6-10-2
10'11 -10-8
10'9-10-5
10'12- -\Q-8
10-12. 10-9
10-7-10'3
10-8. i0-5
10"5-10'1
10'6-10-1
Air conductivity values at 10ฐC. Values are based on the ratio of hydraulic conductivity to air conductivity (Kr); where Kr= (viscosity of air/viscosity of water)
(density of water/density of air)
Source: Adapted from Freeze and Cherry, 1979, and Krishnayya sLal, 1988.
-------
TABLE 4. RELATIONSHIP BETWEEN DIAMETER OF PARTICLES
AND SURFACE AREA
Diameter of
Particles (mm)
1.0-2.0
0.5-1.0
0.25 - 0.5
0.1 -0.25
0.05 - 0.1
0.002 - 0.05
<0.002
Description
Very Coarse Sand
Coarse Sand
Medium Sand
Fine Sand
Very Fine Sand
Silt
Clay
Approximate Surface
Area (rr^/g)
0.001 - 0.003
0.003 - 0.005
0.005 - 0.01
0.01 - 0.03
0.03 - 0.1
0.1 - 1
>1
Source: Adapted from Hillel, 1980
TABLE 5. CONTAMINANT SPECIFIC PARAMETERS TO GATHER
PARAMETER (UNITS1 DEFAULT SOURCE IMPORTANT FOR DETERMINING:
Pure Vapor Pressure (mm Hg) Tables 7,8 Mobility, Phase
Water Solubility (mg/L) Tables 7,8 Mobility, Phase
Liquid Viscosity (cPoise) Tables 7,8 Mobility
Liquid Density (g/m3) Tables 7,8 Mobility
Vapor Density (g/m3) Tables 7,8 Mobility
Soil Sorption Coefficient (L/kg) Table 8 Mobility, Phase
Refractory Index (ratio) Table 9 Degradation
Unweathered Composition (-)* Table 6 Mobility, Phase, Degradation
* Composition will change over time due to "weathering." See discussion in text.
15
-------
APPROXIMATE TEMPERATURE OF GROUND WATER, IN DEGREES CELSIUS, IN THE CONTERMINOUS
UNITED STATES AT DEPTHS OF 10 TO 25 M
Figure 3. Groundwater Temperature in the United States
(C) at Depths of 10 to 25 m (from Heath, 1985}
-------
Porosity
Field
Capacity
0)
E
J2
o
0>
CJ
CD
Q_
30
Wilting
Point
20
10
o I
Sand
Figure 4. Water-Holding Properties of Various Soils on the Basis of Their Texture.
17
-------
Petroleum products are a mixture of many compounds. The physical
and chemical properties of the constituents are different than those of
the mixture. However, at most sites, evaluation will be based on the
properties of the mixture rather than one or more constituents. This is
particularly true of a preliminary assessment. The sample example in the
Appendix of this manual evaluates contaminant mobility and phase
partitioning based on properties of a mixture. However, an assessment
that targets one or more constituents merits discussion.
There are, however, several reasons why individual constituents
would be targeted during site assessment instead of the petroleum product
mixture. Probably principal among these are: 1) natural weathering of
the contaminants; and 2) focus on known health risks. Weathering is the
process where a compound or mixture is reduced to simpler and/or fewer
component parts over time. If a petroleum product has been in the
subsurface for several years, the existing contaminant is likely to be
significantly different than when originally released. In such a case,
evaluation based on an understanding of the behavior of the original
petroleum product could lead to selection of an ineffective remedial
response.
The other reason to target individual constituents is to focus the
assessment on a compound(s) which is thought to present the greatest
potential threat. For instance, it may be desirable to select a remedial
technology that is likely to be effective in removing benzene, a known
human carcinogen and common constituent of gasoline. An understanding of
how benzene behaves in the subsurface would be developed rather than an
understanding of the behavior of the mixture.
Table 6 presents the unweathered composition of three common bulk
hydrocarbon products. This list can be consulted to determine which
constituents are likely to be in a given petroleum product and to
estimate what percentage of the total product they represent.
Table 7 lists default values for various physical and chemical
properties of common hydrocarbon mixtures; Table 8 provides similar
information for individual hydrocarbon constituents. Contaminant-
specific properties of petroleum product constituents in Table 8 are for
pure compounds. These values may be different than would be found when a
given compound is mixed with other organic compounds. For example, the
water solubility of pure benzene is 1,780 mg/L, but in a gasoline/water
mixture the dissolution of benzene is more likely to be in the 20 to 80
mg/L range.
Table 9 lists selected refractory index values for both hydrocarbon
mixtures and individual constituents. The refractory index is a measure
of the relative biodegradability of a compound.
EVALUATING CONTAMINANT PHASES IN THE UNSATURATED ZONE
Liquid petroleum products will partition into various phases when
introduced to the subsurface. As mentioned previously, thirteen phases
have been identified into which a contaminant can partition (EPA, 1988a).
However, liquid contaminants released in the unsaturated zone will exist
primarily in three physical phases: residual liquid contaminant,
contaminants vapors, and contaminants dissolved in pore water.
18
-------
TABLE 6. UNWEATHERED COMPOSITION OF THREE COMMON HYDROCARBON PRODUCTS
Selected Representative Concentrations (%w/w)
Hydrocarbon
Group
n-Alkanes
C4
C5
C6
C7
C8
C9
C10-C14
Branched Alkanes
C4
C5
C6
C7
C8
C9
C10-C14
Cycloalkanes
C5
C6
C7
C8
Others
Olefins
C4
C5
C6
Others
Mono-aromatics
Benzene
Toluene
Xylenes
Ethyl benzene
C3-benzenes
C4-benzenes
Others
Representative
Hydrocarbon
n-Butane
n-Pentane
n-Hexane
n-Heptane
n-Octane
n-Nonane
n-Decane
Isobutane
Isopentane
2-Methylpentane
2-Methylhexane
2,4-Dimethylhexane
2,2,4-Trimethylhexane
2,2,5,5-Tetramethylhexane
Cyclohexane
Methylcyclohexane
1 ,2,4-Trimethylcyclopentane
1 ,1 ,3-Trimethylcyclohexane
1-Butene
1-Pentene
1-Hexene
Benzene
Toluene
m-Xylene
Ethyl benzene
1 ,3,5-Trimethylbenzene
1 ,4-Diethylbenzene
1 2
Automotive #2 Fuel
Gasoline Oil
10.8
4.8
1.9
2.0
0.2
1.3
0.4
0.2
18.8
0.7
8.6
4.6
1.4
1.8
1.2
0.5
3.2
0.2
1.0
0.2
0.2
1.6
5.5
0.9
1.3
0.8
2.5
19.3
0.9
4.0
5.6
1.2
3.2
2.1
1.6
29.6
7.0
4.5
12.9
2.3
0.8
0.8
59.5
2.2
17.3
9.7
8.3
16.7
2.7
2.6
13.7
3.9
1.4
0.7
7.5
13.5
3.3
1.8
7.5
40.9
4.4
6.5
8.8 0.07
1.4 0.03
11.3 0.67
2.6 0.88
5.2
3
Jet Fuel
JP-4
0.12
1.06
2.21
3.67
3.80
2.25
8.73
0.66
2.27
5.48
8.82
3.36
1.35
2.40
3.77
1.35
3.21
0.50
1.33
2.32
0.37
3.59
3.98
(Continued)
19
-------
TABLE 6 (continued)
Hydrocarbon
Group
Phenols
Phenol
C1 -phenols
C2-phenols
C3-phenols
C4-phenols
Indanol
Poly-aromatics
Nitro-aromatics
C1 -Anilines
C2-Anilines
Complex Anilines
Di-aromatics
Carboxylic Acids
Saturated hydro-
Carbons
C8
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
C21
C22
C23
C24
Pristane
Phytane
Waxes
Base Compounds
Unknowns
Representative
Hydrocarbon
Phenol
o-Cresol
2, 4-Di methyl phenol
2,4,6-Trimethylphenol
m-Ethylphenol
Indanol
Fluorene
Quinoline
Naphthalene
Benzoic Acid
n-Octane
n-Nonane
n-Decane
n-Undecane
n-Dodecane
n-Tridecane
n-Tetradecane
n-Pentadecane
n-Hexadecane
n-Heptadecane
n-Octadecane
n-Nonadecane
n-Eicosane
n-Heneicosane
n-Docosane
n-Tricosane
n-Tetracosane
1 2 3
Automotive #2 Fuel Jet Fuel
Gasoline Oil JP-4
0.001
0.01
0.02
0.02
0.01
0.001
0.57
0.003
0.004
0.002
0.7 3.43 1.59
0
0.05
0.20
0.58
0.98
1.14
1.20
1.31
1.42
1.53
1.51
1.31
1.16
0.99
0.51
0.29
0.15
0.05
0.52
0.46
6.6-13.8
NOTE: Blanks indicate the unavailability of data and do not indicate the absence of a particular compound
from the hydrocarbon product.
SOURCES:
Column 1: Hoag et al, 1984; EPA, 1984; Ghassemi et al, 1984.
Column 2: ICF, 1984
Column 3: Smith et al, 1981.
20
-------
TABLE 7. PHYSICOCHEMICAL PROPERTIES OF FIVE COMMON HYDROCARBON MIXTURES
PRODUCT
Automotive
Gasoline
#2 Fuel Oil
#6 Fuel Oil
Jet Fuel (JF-4)
Mineral Base
Crankcase Oil
Air
Saturated Aqueous
Vapor
LIQUID
DENSITY
(g/crrfi)
(0.73)
0.72-0.76 [15.6]
(0.91)
0.87-0.95
(0.91)
0.87-0.95
0.75
0.84-0.96 [15]
1
LIQUID
VISCOSITY
(cPoise)
(0.45)
0.36-0.49 [15.6]
(1.56)
1.15-1.97 [21]
(254)
14.5-493.5 [38]
0.829 [21]
275 [38]
1
WATER
SOLUBILITY
(mg/L)
(158)
131-185 [13-25]
3.2
"'O
<300
insoluble
VAPOR
PRESSURE
(mmHg)
(469)
263-675 [38]
(14.3)
2.12-26.4 [21]
(14.3)
2.12-26.4 [21]
91
N/A
760
17.5
VAPOR
DENSITY
(g/m3)
1950
109
105
400
N/A
1200
17.3
N/A = Not Available
Note: All values for 20ฐC unless noted in brackets [ ].
Note: Values in parentheses are typical of the parameter ()
Note: Values for air and saturated aqueous vapor are included, where applicable, as a means of comparison.
Source: Compiled from various published and unpublished sources.
-------
Table 8 . Chemical Properties of Hydroparbon Constituents
Chemical Class
n-Alkanes
C4
C5
C6
C7
C8
C9
C10
Mono-aromatics
C6
C7
C8
C8
C9
C10
Phancls
Phenol
Cl-phenols
C2-phenols
C3-phenols
C4-phenols
Indanol
Di-aromatics
Liquid
Representative Liquid Density viscosity
Chemical (g/cm.3) (cPoise)
@20ฐC @20ฐC
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-Trimethylphenol
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
0.177
0.224
0.306
0.409
0.542
0.620
0.740
0.638
0.580
0.608
0.666
0.727
0.700
12.7
20.8
NA
NA
NA
NA
NA
Water
solubility
(mg/L)
@25ฐC
61.1
41.2
12.5
2.68
0.66
0.122
0.022
1780
537
162
167
72.6
15
438
26175
NA
NA
NA
NA
31.7
Pure Vapor
Pressure
(mmUg)
@20ฐC
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
(g/m3)
@20ฐC
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 Sorp
Constant
(Ukg!
@2&
250
320
600
1300
2600
5800
13000
38
90
220
210
390
1100
110
8.4
NA
NA
NA
NA
690
NOTE: NA - Not availabe
SOURCE: Compiled from various published and unpublished sources.
22
-------
TABLE 9. REFRACTORY INDEX FOR COMMON HYDROCARBONS
RELATIVELY UNDEGRADABLE
COMPOUND RATIO
Butane 2.0
Ethane 2.0
Heptane ~0
Hexane 2.0
Isobutane 2.0
o-Xylene <0.008
m-Xylene <0.008
Ethylbenzene <0.009
MODERATELY DEGRADABLE
Gas oil (cracked) -0.02
Gasolines (various) -0.02
Mineral spirits -0.02
Nananol >0.033
Undecanol <0.04
1-Hexene <0.044
Dodecanoi 0.097
RELATIVELY DEGRADABLE
p-Xylene <0.11
Toluene <0.12
Jet fuels (various) 2.0.15
Kerosene ~0.15
Range oil 2.0-15
Naphthalene <0.20
Hexanol 2.0.20
Octanol 0.37
Benzene <0.39
Phenol 0.81
The refractory index is the ratio of BODS to COD
Source: Adapted from Lyman et al. 1982
23
-------
In addition to these three phases, petroleum products also become
sorbed to soil particles in the unsaturated zone. This phase is also
discussed in the manual, but to a lesser extent than the residual liquid,
vapor, and dissolved phases.
The rate and degree of partitioning of the residual liquid into the
vapor phase and dissolved phase depend on site-specific and
contaminant-specific factors, as well as time. A contaminant's
volatility (as measured by vapor pressure) is an indicator of how easily
it will move into the air. A contaminant's solubility is a measure of
how easily it will dissolve in water (including pore water).
Selecting a soil treatment technology will follow directly from an
understanding of the phase in which most of the petroleum is to be found.
As discussed in Section 3, certain corrective actions are more effective
if most of the contaminants are located in one phase rather than another.
The phase(s) in which the contaminants are found also affect the mobility
of the contaminants.
Vapor analysis and soil sampling can be undertaken to determine in
which phase most of the contamination resides. Typically, soil is
analyzed for total hydrocarbon concentration, while soil gas is sampled
for evidence of hydrocarbon vapors. General "rules-of-thumb" for
determining the phase which contains most of the contamination are
presented in Table 10.
TABLE 10. RULES OF THUMB FOR DETERMINING IN WHICH PHASE
CONTAMINATION CAN BE FOUND
Evidence of residual liquid contamination;
High concentrations (>1% by weight) of contaminants in
several soil analyses; (i.e., petroleum product makes up >1%
of the weight of the soil sample);
High concentrations (>10% by volume) of pure chemical vapor
density in several soil gas analyses (i.e. contaminant vapors
are above 100,000 ppm).
Evidence of contaminant vapors;
Presence of NALP;
- Significant concentrations in several soil gas analyses.
Evidence for pore water contamination;
Significant concentrations of contaminants in several
analyses of pore water or groundwater. Dissolved
concentrations for typical petroleum product mixtures will
generally be less than 100 mg/L;
Presence of NAPL and a significant soil moisture content.
24
-------
In the absence of site-specific sample data, it may still be
possible to approximate the likelihood of having contaminants in three
phases. Tables 11, 12, and 13 are provided here and may be used for this
purpose. These tables provide a way to estimate the general likelihood
of contamination being present in various phases by examining the range
of values of several parameters. Parameters are grouped by release
information, site-specific information, and contaminant-specific
information. Each parameter and its importance is discussed below.
Release Information
The amount and rate of release will affect the probability of
residual liquid being found in the unsaturated zone. Hydrocarbons will
persist as bulk liquid longer for larger releases than for smaller
releases, raising the likelihood of residual liquid for large release
incidents. Some releases are quick spills with high rates of release
while others are leaks where small amounts of product are released over a
period of time. A slow rate of release results in conditions more
suitable to transfer from residual liquid to other phases. Less residual
liquid is likely for a slow leak than for a quick spill of similar
volume.
The time since the release may also be important. More recent
releases of petroleum would more likely result in a higher proportion of
residual liquid being present in the unsaturated zone. The vapor and
dissolved phases contain incrementally more of the contaminant mass for
older releases. Older releases provide more opportunity for contaminants
to volatilize and dissolve over time.
Site-Specific Information
A shallow depth to groundwater can result in more contaminants
leaving the unsaturated zone than a site with a thicker unsaturated zone.
Thus the greater the depth to groundwater, the greater the likelihood of
finding significant amounts of contaminants in the unsaturated zone in
all three phases.
A soil's hydraulic conductivity directly affects a contaminant's
mobility, or ability to move away from the release site in the NAPL and
dissolved phases; soil air conductivity affects the mobility of
contaminant vapors. Air and hydraulic conductivity vary from formation
to formation in much the same way, with formations of low hydraulic
conductivity generally having low air conductivity as well. Low
conductivity, either hydraulic or air, indicates a greater probability of
finding contaminants in all three phases close to the release site.
A high rainfall infiltration rate can cause contamination to move
from one phase to another. Some hydrocarbons will dissolve in the
infiltrating rain water, thereby reducing the residual liquid portion of
the contamination while increasing the amount of contaminant dissolved in
pore water.
25
-------
TABLE 1 1 . LIKELIHOOD OF LIQUID CONTAMINANTS BEING ftfl
PRESENT IN THE UNSATURATED ZONE VfjT
FACTOR
RELEASE- RELATED
Amount Released
Rate Of Release
Time Since Release
SITE - RELATED
Depth To Groundwater
Hydraulic Conductivity
Rainfall Infiltration Rate
Soil Temperature
Soil Sorptlon Capacity
(Surface Area)
CONTAMINANT- RELATED
Liquid Viscosity
Liquid Density
Vapor Pressure
Water Solubility
UNITS
gallons
months
meters
cm/sec
cm /day
ฐC
m2/g
cP
3
g/cm
mm Hg
mg/L
SITE OF
INTEREST
INCREASING LIKELIHOOD >^
Small
(<100)
0
Slow Release
o
Long
(>12)
O
Medium
(100-1000)
0
o
Medium
(1-12)
O
Large
(>1000)
O
Instantaneous
Release
O
Short
(<1)
O
Shallow
(<1)
O
High
(>10'3)
O
High
(>0.1)
0
Warm
(>20)
0
Low
(<0.1)
O
Medium
(1-5)
O
Medium
(10-s-10'3)
0
Medium
(0.05-0.1)
O
Medium
(10-20)
0
Medium
(0.1-1)
O
Deep
(>5)
O
Low
(<10'5 )
O
Low
(<0.05)
0
Cool
(<10)
O
High
(>1)
0
Low
(<2)
O
High
(>2)
O
High
(>100)
O
High
(>1000)
0
Medium
(2-20)
0
Medium
(1-2)
O
Medium
(10-100)
O
Medium
(100-1000)
O
High
(>20)
O
Low
(<1)
O
Low
(<10)
O
Low
(<100)
O
26
-------
TABLE 12. LIKELIHOOD OF CONTAMINANT VAPORS BEING f5$$5l
PRESENT IN THE UNSATURATED ZONE
FACTOR
RELEASE- RELATED
Amount Released
Rate Of Release
Time Since Release
SITE - RELATED
Depth To Groundwater
Air Conductivity
Rainfall Infiltration Rate
Soil Temperature
Soil Sorption Capacity
(Surface Area)
CONTAMINANT-RELATED
Liquid Viscosity
Liquid Density
Vapor Pressure
Water Solubility
UNITS
gallons
months
meters
cm/sec
cm/day
ฐC
m /g
cP
3
g/cm
mm Hg
mg/L
SITE OF
INTEREST
INCREASING LIKELIHOOD ^
Small
(<100)
o
Slow Release
O
Long
(>12)
O
Shallow
(10"ป)
O
High
(>0.1)
O
Cool
(<10)
O
Low
(<0.11)
O
Medium
(100-1000)
O
0
Medium
(1-12)
O
Medium
(1-5)
0
Medium
(10'6-10-4)
O
Medium
(.005-0.1)
O
Medium
(10-20)
O
Medium
(0.1-1)
O
Large
(>1000)
O
Instantaneous
Release
0
Short
(<1)
O
Deep
(>5)
O
Low
(<10'6)
O
Low
(<0.05)
O
Warm
(>20)
O
High
(>1)
O
High
(>20)
O
High
(>2)
O
Low
(<10)
O
High
(>1000)
O
Medium
(2-20)
O
Medium
(1-2)
O
Medium
(10-100)
O
Medium
(100-1000)
O
Low
(<2)
O
Low
(<1)
O
High
(>100)
O
Low
(<100)
O
27
-------
TABLE 13 LIKELIHOOD OF CONTAMINANTS DISSOLVED IN PORE
WATER BEING PRESENT IN THE UNSATURATED ZONE
FACTOR
RELEASE- RELATED
Amount Released
Rate Of Release
Time Since Release
SITE - RELATED
Depth To Groundwater
Moisture Content
Soil Porosity
Rainfall Infiltration Rate
Soil Sorption Capacity
(Surface Area)
CONTAMINANT- RELATED
Liquid Viscosity
Liquid Density
Vapor Pressure
Water Solubility
UNITS
gallons
months
meters
% volume
% volume
cm/day
m2/g
cP
g/cm3
mm Hg
mg/L
SITE OF
INTEREST
/JJMiHi
JMJLJMS
(nLJJSLTSJLS
rxJrjvJJ^J
iijtS&v!
^UflLgM*
INCREASING LIKELIHOOD ^
Small
(100)
o
Instantaneous
Release
O
Long
(>12)
O
Medium
(100-1000)
O
o
Medium
(1-12)
O
Large
(>1000)
O
Slow Release
O
Short
(<1)
0
Shallow
(<1)
O
Low
(<10)
O
Low
(<20)
O
Low
(<0.05)
O
Low
(<0-1)
O
Medium
(1-5)
O
Medium
(10-30)
O
Medium
(20-40)
O
Medium
(0.05-0.1)
O
Medium
(0.1-1)
O
Deep
(>5)
O
High
(>30)
O
High
(>40)
O
High
(>0.1)
O
High
(>1)
0
High
(>20)
O
High
(>2)
0
High
(<100)
O
Low
(<100)
O
Medium
(2-20)
O
Medium
(1-2)
O
Medium
(10-100)
O
Medium
(100-1000)
O
Low
(<2)
O
Low
(<1)
O
Low
(>10)
O
High
(>1000)
o
28
-------
The soil temperature also affects contaminants mobility. A
contaminant's vapor pressure, and therefore the ease with which
contaminants move into air spaces in soil, increases with increasing
temperature. A greater percentage of the total amount of released
contaminants is likely to be in the vapor phase in warmers regions or
seasons. High soil temperatures also tend to reduce liquid viscosity,
increasing ease of movement down through soil to the water table and out
of the unsaturated zone. Other site-specific parameters relating to
contaminant sorption on soils are described below.
Contaminant-Specific Parameters
A contaminant's vapor pressure and water solubility are indicators
as to how easily liquid contaminants will move into air and water.
Contaminants with high vapor pressures partition readily into the vapor
phase; highly soluble contaminants dissolve readily. In each case, the
amount of product remaining as residual liquid is lessened.
Viscosity, which decreases with increasing temperature, influences
the mobility and phase partitioning of a contaminant. A highly viscous
constituent is likely to remain in the liquid phase (perhaps sorbed to
soil) rather than volatilize or dissolve. It will also remain in the
unsaturated zone longer than a constituent with lower viscosity. On the
other hand, a contaminant with a high liquid density may be more mobile
and eventually move to the saturated zone, thus reducing concentration in
the unsaturated zone.
The extent to which a contaminant will adhere or sorb to soil
particles depends upon the phase it is in (liquid, vapor, or aqueous
solution) and a variety of properties inherent to the contaminant. For
liquid contaminants, interfacial tension (with water as well as with the
soil matter) in conjunction with the pore size openings determines the
capillary suction or holding force. For contaminant vapors, vapor
pressure (which is primarily a function of the contaminants molecular
weight and the soil temperature) is the important factor. The higher the
partial pressure of a contaminant in the vapor phase, the greater will be
the amount sorbed to the soil. For contaminants in aqueous solution, the
soil sorption constant is used as the measure of the equilibrium
distribution between sorbed and dissolved fractions of a contaminant.
Understandably, the soil sorption constant is inversely proportional to
the contaminants water solubility (i.e., the higher the solubility, the
lower the degree of sorption).For soils with an organic carbon content
(oc) greater than approximately 0.1 percent by weight, sorption from
aqueous solution is almost completely controlled by the organic carbon,
and the soil sorption constant is found to be proportional to the
contaminant's octanol-water partition coefficient (Kow) (i.e., higher
values of organic carbon or KQW lead to higher degrees of sorption). For
soils with less than 0.1 percent organic carbon, the surface area of the
soil often becomes the primary soil property affecting the degree of
sorption from solution.
Tables 11, 12, and 13 are most useful when all the parameters are
known; however, it is not necessary to know the value of every parameter
to use the tables. In some cases, two parameters address similar
29
-------
physical properties and can serve as substitutes for one another. For
example, knowing that the rainfall infiltration rate is extremely low
would make the water solubility less important in determining which phase
predominates.
EVALUATING CONTAMINANT MOBILITY
The final step in a site assessment focuses on the mobility of the
contaminants, both through the unsaturated zone and between phases. Many
in-situ corrective actions depend on mobilizing contaminants (i.e.,
moving contaminants from one phase to another that can be more directly
removed with a given treatment technology). For example, soil venting
works to both transport vapor phase contaminants through the air spaces
in the soil and transfer contaminants from the residual liquid and
dissolved phases to the vapor phase. As clean (non-contaminated) air
replaces the contaminant-saturated vapors that are removed, contaminants
remaining as residual liquid will volatilize into the fresh air, seeking
to re-establish equilibrium. As the process continues, more and more
contaminant is mobilized from the residual liquid or dissolved phases
into the vapor phase, where it can be captured by the soil venting
system.
Different soil treatment technologies have different time frames for
mobilization and for treatment, but all four in-situ technologies
described in this report depend to some degree on the ability to mobilize
the contaminants: venting will be successful if substantial amounts of
vapor can be drawn through the soil; flushing and bioremediation both
must move liquids through the unsaturated zone to reach the contaminants
to be effective; and hydraulic methods are successful only if the
contaminants can flow to the drain. Contaminant mobility is not
important for excavation.
It is necessary to discuss the mobility of contaminants in the
subsurface separately for each phase. The factors that control transport
differ for each phase, as discussed below.
Residual Liquid Contaminant
The movement of bulk liquids in the unsaturated zone is dominated by
three forces: gravity; pressure gradients; and capillary suction.
Gravity exerts a direct, downward force, the magnitude of which depends
only on the density of the contaminant. Pressure gradients generally
result from infiltrating liquid (precipitation and contaminants), and
most often act in the same direction as gravity. Capillary suction
depends on the soil characteristics and the forces it generates act in
all direction, although not equally.
In addition to these three major forces, other physical, chemical,
and environmental factors can influence a liquid's mobility in the
unsaturated zone (see Table 14). In Table 14, items are listed with both
qualitative descriptors (high, medium and low), and corresponding
quantitative ranges of values. Although the quantified ranges are
somewhat subjective, they have been arranged so that the right-hand
column indicates "high mobility" and the left-hand column "low mobility."
30
-------
TABLE 14. FACTORS TO EVALUATE THE MOBILITY OF fl3\
LIQUID CONTAMINANTS VW
FACTOR
UNITS
SITE OF
INTEREST
RELEASE RELATED
Time Since Release
Months
SITE- RELATED
Hydraulic Conductivity
Soil Porosity
Soil Surface Area
Soil Temperature
Rock Fractures
Moisture Content
cm/sec
% Soil
Volume
m2/g
ฐC
% Volume
INCREASING MOBILITY >^
Long
(>12)
o
Medium
(1-12)
O
Short
(<1)
O
Low
(<10'5)
O
Low
(<10)
0
High
(>1)
O
Low
(<10)
O
Absent
0
High
(>30)
O
Medium
(10'5-110-3)
0
High
(>30)
O
Low
(<0.1)
0
High
(>20)
O
Present
O
Low
(<10)
O
CONTAMINANT- RELATED
Liquid Viscosity
Liquid Density
cPoise
g/cm3
High
(>20)
O
Low
(<1)
0
Medium
(2-20)
O
Medium
(1-2)
O
Low
(<2)
O
High
(>2)
O
31
-------
The site-related factors in Table 14 can vary with depth below the
ground surface, particularly for hydraulic conductivity, soil porosity,
and soil surface area. Often subsurface materials are deposited in
layers with each layer having different characteristics. For a
preliminary site assessment, accurate data on the vertical soil profile
may not be available and a best estimate of typical conditions throughout
the unsaturated zone must suffice. If profile data are available, Table
14 can be completed for each distinct soil layer in the unsaturated zone
using the appropriate soil-related data.
Using Table 14 to evaluate conditions at a site of interest, it is
possible to get an understanding of the relative mobility of liquid
contaminants at that site. If the preponderance of factors at a site
fall in the right-hand column of Table 14, liquid contaminants would
likely be more mobile and likely to migrate than if most factors matched
those in the left-hand column. Where site-specific values are
unavailable, default values can be used from the tables previously
provided in this section.
One factor not listed in Table 14, yet likely to have an effect on
liquid mobility, is whether the soil is "water-wet" or "oil-wet." This
factor was not included because data are hard to find. Nonetheless, the
reader should be aware of the following issues: "water-wet" soil
describes the conditions where water preferentially coats the soil
particles, effectively forming a barrier between the petroleum product
and the soil particles. In this case, the contaminant viscosity and the
interfacial tensions (IFTs) between the contaminant and water will
control mobility. In "oil-wet" soils, where oil directly coats the soil
particles, transport depends on the viscosity and the IFTs between the
hydrocarbon and the soil. Because soil-hydrocarbon IFTs are stronger
than soil-water IFTs, "oil-wet" scenarios often result in lower
contaminant mobility.
Contaminant Vapors
Vapors are generally mobile in the unsaturated zone. The degree of
mobility greatly depends on the air-filled porosity of the soil (the
total porosity less that portion filled by water or liquid contaminants).
Several other factors also influence vapor transport in the unsaturated
zone as listed in Table 15.
Contaminants vapors may be mobilized (and subsequently removed) by
several natural or induced processes or forces. These include:
Bulk transport due to pressure gradients (e.g., gradients
induced by vacuum extraction wells or by natural changes in
barometric pressure);
Bulk transport due to vapor density gradients (which could
result, for example, if the contaminant vapor has a
significantly different density than the soil air due to
differences in molecular weight or temperature);
32
-------
TABLE 15. FACTORS TO EVALUATE THE MOBILITY OF
CONTAMINANT VAPORS
FACTOR
UNITS
SITE OF
INTEREST
INCREASING MOBILITY
SITE - RELATED
Air Filled Porosity
% Volume
Low
0
Medium
(10-30)
o
High
(>30)
O
Total Porosity *
% Volume
Low
(<10)
O
Medium
(10-30)
O
High
(>30)
O
Water Content
% Volume
High
(>30)
Medium
(10-30)
O
Low
(<10)
O
Depth Below Surface
meters
Deep
(>10)
O
Medium
(2-10)
O
Shallow
(<2)
O
CONTAMINANT- RELATED
Vapor Density
g/m
Low
(<50)
O
Medium
(50-500)
O
High
(>500)
O
* the total porosity less that fraction filled with water equals the air filled porosity
33
-------
in-situ generation of gases or vapors (e.g., vapors volatilizing
from liquid contaminants or gases generated by microbial
biodegradation of contaminants that force movement of vapors
through the soil);
ซ Molecular diffusion due to concentration gradients.
The current scientific understanding of these process spans the
range from well-documented to rudimentary and hypothetical. Molecular
diffusion is perhaps, the best understood and the easiest to address
experimentally and, hence, empirically. However, in some circumstances
it may not be the most important process governing vapor mobility.
A soil with medium to high water-filled porosity will tend to retard
vapor movement because of reduced air-filled porosity (less flow area)
and because of absorption (dissolution) of vapors into the pore water.
However, if one applies a vacuum extraction system inducing soil gas
flow the soil will tend to dry out and this will increase vapor
mobility over the treatment period.
Vapor density differences will be important only when liquid
contaminant of sufficient volatility is present. For example, the
density of air saturated with gasoline vapors (i.e, in contact with
liquid gasoline) is about 1,950 g/m3 at 20ฐC; moist air has a density of
about 1,200 g/m3 at 20ฐC. In the absence of other driving forces,
heavier (contaminant containing) vapors should tend to migrate downward
in the unsaturated zone. The driving force (density difference) will
diminish as the vapors become more diluted (i.e., less dense).
Contaminants Dissolved in Pore Water
Table 16 lists factors that can help determine the relative mobility
of contaminants dissolved in pore water in the unsaturated zone. In
general, dissolved contaminants move with the pore water in which they
are dissolved. The mobility of the pore water itself is dominated by the
same factors that govern mobility of residual liquid contaminants:
gradients, gravity, and capillary suction. As noted previously,
contaminants dissolved in flowing pore water may be retarded (relative to
the average water velocity) due to sorption on soil particles. The
extent of sorption will increase with decreasing water solubility of the
contaminant, and with increasing organic carbon content and/or surface
area of the soil.
As with Table 14, the site-related factors in Table 16 may vary with
depth. If information about the soil profile is available, Table 16 can
be completed for each distinct soil group. Otherwise, a best estimate of
actual conditions must be made to complete the preliminary assessment.
34
-------
TABLE 1 6, FACTORS TO EVALUATE THE MOBILITY
OF CONTAMINANTS IN PORE WATER
FACTOR
UNITS
SITE OF
INTEREST
xJJGJHv
SK-wH
*WKM?
KKK?
KJ&&H
<&gufluy
INCREASING MOBILITY >^
SITE RELATED
Hydraulic Conductivity
Moisture Content
Rainfall Infiltration Rate
Soil Porosity
Rock Fractures
Depth Below Surface
cm/sec.
% Volume
cm/day
% Volume
meters
Low
(10-5)
o
Low
(<10)
O
Low
(< 0,05)
O
Low
(<10)
O
Absent
0
Shallow
(<2)
0
Medium
(10-5- 10'3)
O
Medium
(10-30)
O
Medium
(0,05-0.1)
O
Medium
(10-30)
O
0
Medium
(2-10)
O
High
(>io-3)
O
High
(>30)
O
High
(>0,1)
O
High
(>30)
O
Present
0
Deep
(>10)
0
CONTAMINANT- RELATED
Water Solubility
mg/L
Low
(<100)
O
Medium
(100 to 1000)
O
High
(>1000)
O
35
-------
SECTION 3
TECHNOLOGY SELECTION
INTRODUCTION
This section describes five technologies used to remediate UST sites:
soil venting (including vacuum extraction); biorestoration; soil flushing;
hydraulic barriers; and excavation. The first four are in situ treatment
methodsf performed with the contaminated soil remaining Th place.
Alternatively, contaminated soil can be excavated. Excavated soil may be
treated above ground, either on-site or at another location, or it may be
landfilled without treatment. Above ground soil treatments include
incineration, thermal stripping and other enhanced volatilization methods,
soil washing, and biorestoration.
From an environmental viewpoint, in situ treatment is often preferred
over excavation, but excavation is commonly employed because it can be
implemented quickly, has been used widely, and also can present to a
concerned public a timely response to the situation. Excavation, however,
may not be desirable where large volumes of soil are contaminated or where
excavation of the contaminated soil could undermine the foundations of
existing structures. Bringing excavated soils to the surface is often
accompanied by uncontrolled release of contaminant vapors. In situ methods
afford a greater degree of control of these vapors.
Each of the five technologies is described herein and factors
critical to the success of each are presented. These critical success
factors (CSFs) are parameters that influence the likelihood of success of a
particular method. For example, high contaminant volatility is a critical
success factor for soil venting. These CSFs can be used to assess the
likely effectiveness of each technology at a particular site. Finally,
other advantages and disadvantages of each technology are discussed.
A worksheet has been provided for each soil treatment technology to
help the user assess the likely success of the technology at a particular
site. The CSFs important for that technology are listed. When completed,
these worksheets provide insight as to which technologies are likely to be
effective, and more importantly, which are not. The three right-hand
columns of each worksheet provide ranges of values for the CSFs that
suggest whether a technology is "less likely," "somewhat likely," or "more
likely" to be effective. A column is provided for the user to write down
the values for the CSFs at the site of interest. For each CSF, the user
determines into which of the three "likelihood" ranges the site falls. If
most of the CSFs fall into the "less likely" column, another technology may
be better suited to cleaning up the site. If the CSFs are fairly evenly
distributed among the three columns, other factors can be considered that
can help in the assessment (i.e., can a parameter be enhanced to increase
the likelihood of success?). A more detailed example of how these
worksheets are used is provided in the sample problem in the Appendix.
36
-------
SOIL VENTING
Soil venting is a general term that refers to any technique that
removes contaminant vapors from the unsaturated zone. Venting may be
passive (with no energy input) or active. Passive venting, which is often
used at sanitary landfills for methane gas removal, consists of perforated
pipes sunk into the contaminated area that provide an easy path to the
atmosphere. These vents sometimes have a wind-driven turbine at the outlet
to provide a slight draft.
Active venting uses an induced pressure gradient to move vapors
through the soil and is more effective than passive venting. Most common
is vacuum extraction technology, where extraction wells are placed near the
release site and a vacuum applied to the wells. Figure 5 shows such a
system. The soil gas is drawn through the soil to the extraction well and
brought to the surface. Positive-pressure injection wells may be employed
to increase the removal rate. The use of plastic sheeting or other similar
impervious layers at the ground surface may be required to avoid short
circuiting of air flow.
The contaminant-saturated vapors that are removed from the
unsaturated zone may require treatment prior to discharge to the
atmosphere, depending on local air-discharge restrictions. Granular
activated carbon (GAC) can be used to capture contaminants prior to
discharge; catalytic combustion is also used. One recent proposal involves
passing the vapors through an acclimated biofilter bed, where the
contaminant vapors are biodegraded (COM, 1988).
Theoretical Factors
Venting removes vapor-phase contaminants from the unsaturated zone.
As discussed in Section 2, contaminants exist primarily in three phases in
the unsaturated zone: vapor; liquid; or dissolved in pore water. Venting
is most effective on contaminants that exist predominantly in the vapor
phase, or are easily volatilized.
Venting removes contaminant vapors trapped in soil air spaces, but
also affects, to a limited extent, residual liquid contaminant and
dissolved contaminants. Hydrocarbons typically are found in all three
phases, and an equilibrium is established, with a certain fraction existing
in each phase. The portion found in each phase depends on both the
particular compound and the local conditions. If conditions change, the
equilibrium will shift, and contaminants will transfer between phases until
equilibrium is re-established.
For example, the removal of contaminant-saturated air by vacuum
extraction causes an equilibrium shift. As clean (non-contaminated) air
replaces the contaminant-saturated vapor that is removed, contaminants
remaining as a residual liquid contaminant and those dissolved in pore
water will volatilize into the fresh air, seeking to re-establish
equilibrium. As this process continues, more and more of the liquid and
dissolved contaminants will volatilize and be removed, eventually leading
to a decrease in the overall in situ contaminant concentration. Vacuum
37
-------
INJECTION
MANIFOLD
EXTRACTION
MANIFOLD
(OPTIONAL)
ELECTRIC
AIR FLOW
HEATER
(OPTIONAL)
FORCED
DRAFT INJECTION
FAN
(IF REQUIRED)
VAPOR
TREATMENT
UNIT
INDUCED
DRAFT EXTRACTION
FAN
n
U)
CO
VERTICAL EXTRACTION
VENT PIPE (TYP)
SLOTTED
VERTICAL INJECTION
VENT PIPE (TYP)
SOIL CONTAMINATION
Figure 5. Schematic Diagram of Vacuum Extraction System.
-------
extraction is sometimes known as "in situ stripping" due to the similarity
of the governing processes of vacuum extraction to those that govern air
stripping.
The success of a vacuum extraction program depends both on the
properties of the contaminants and the properties of the soil. Of
particular importance are three equilibrium relationships: 1) the
contaminant-air equilibrium, described by the contaminant's partial vapor
pressure; 2) the equilibrium between contaminant dissolved in pore water
and the soil vapor, described by the contaminant's Henry's law constant;
and 3) the equilibrium between the contaminant dissolved in pore water and
contaminant adsorbed to soil particles, described by the soil-sorption
constant (K ).
o c
Table 17 lists CSFs for soil venting. Among the most important are
contaminant vapor pressure and soil-sorption capacity. As the table shows,
compounds with high vapor pressures are "more likely" to be removed by
vacuum extraction than those with low vapor pressure. Course materials,
such as sand and gravel, which have low soil sorption coefficients (i.e.
surface area), are also "more likely" to be amenable to vacuum extraction
than fine-grained materials like clay or silt. Each of the remaining CSFs
can be evaluated similarly to provide a preliminary screening of the
suitability of soil venting at the site of interest.
The water solubility of each contaminant will also affect the success
of venting, although this factor is relatively less important than those
listed above. Highly soluble compounds may tend to exist predominantly
dissolved in pore water, with less in the vapor phase. Vacuum extraction
tends to dry out the soil, however, and over time dissolved contaminants
will likely volatilize and be removed.
Soil properties also greatly influence the success of soil venting.
Air conductivity is an important parameter to consider in soil venting.
Soils with low air conductivity, such as clay, restrict the movement of
vapors through the soil and towards wells. Contaminants can still be
removed from soils with low air conductivity by soil venting, but the
process requires more closely spaced wells or a greater vacuum. Increasing
the vacuum creates a greater pressure gradient through the soil formation,
increasing the rate of movement of contaminated vapors through the air
spaces in the soil. By increasing the number of wells per unit area, the
average distance contaminant vapors must travel to be captured by the
system is decreased, thereby increasing the productivity of the system.
Most soils have preferential flow paths that are responsible for much
of the soil's permeability. These flow paths result from things such as
root intrusions, shrinking/swelling, wetting/drying, and uneven settling of
the formation. They can prevent the vapors from coming into intimate
contact with all of the contaminated soil, thus decreasing the
effectiveness of the technique.
Other important properties include soil temperature and moisture
content. The ambient temperature of the soil has a strong effect on the
volatility of the contaminant. As temperature rises, vapor pressure and
Henry's constant rise dramatically; Munz and Roberts (1987) report that for
39
-------
TABLE 1 7 WORKSHEET FOR EVALUATING
THE FEASIBILITY OF SOIL VENTING
BEING EFFECTIVE AT YOUR SITE
CRITICAL SUCCESS
FACTOR
SITE RELATED
Dominant
Contaminant Phase
Soil Temperature
Soil Air Conductivity
Moisture Content
Geological
Conditions
o Soil Sorptlon Capacity
- Surface Area
Depth to Ground Water
UNITS
Phase
ฐC
cm/ssc.
% volume
2
m /g
meters
CONTAMINANT- RELATED
' Vapor Pressure
Water Solubility
mm Hg
mg/L
SITE OF
INTEREST
V
SUCCESS
LESS
LIKELY
Sorbed to soil
O
Low
(<10)
O
Low
(<10-6)
0
Moist
(>30)
O
Heterogeneous
0
High
(>1)
O
Low
(<1)
0
Low
(<10)
O
High
(> 1000)
0
SUCCESS
SOMEWHAT
LIKELY
Liquid
0
Medium
(10-20)
O
Medium
(10-6-10~4)
O
Moderate
(10-30)
O
0
o
Medium
(1-5)
O
^
SUCCESS
MORE
LIKELY
Vapor
0
High
(>20)
o
High
(>10-4)
O
Dry
(<10)
0
Homogeneous
O
Low
(<0.1)
O
High
(>5)
O
Medium
(10to100)
O
Medium
(100-1000)
O
High
(> 100)
O
Low
(< 100)
O
OTHER CONSIDERATIONS
Treatment can be done on-site
Cost Is from $1 5 to $60 per cubic yard. . Care must be taken to avoid explosions because vapors
Efflctlveness decreases after several months of treatment. are concentrated
Capable of removing thousands of gallons. Cleanup takes time so that this technology Is not
Air emissions will likely need to be treated with GAC. appropriate when emergency response Is needed
40
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each rise of 10ฐC in temperature, Henry's constants increase by a factor of
1.6. While higher temperatures increase contaminant volatility, higher
temperatures lower the air conductivity of the soil. However, the effects
of temperature on volatility far outweigh its effect on air conductivity,
and soil venting would be expected to be more successful in areas where
soil temperature is high. In some cases, air is heated prior to injection
to raise subsurface temperatures and increase volatilization. Figure 3
shows groundwater temperatures for the continental United States. Values
from this table can be used as surrogate values for soil temperature.
The moisture content has two effects on the soil. First, because
soil with a high water content has relatively less air-filled porosity,
higher water content leads to a lower air conductivity and therefore a
lower removal rate. Second, pore water can absorb (dissolve) contaminants
from the vapor phase, which serves to retard the removal of contaminant
vapors. This is especially true of contaminants with low vapor pressures
and low Henry's constants. Dry soil is thus better suited to in situ
stripping than wet soil. The vacuum extraction process tends to dry out
the soil; over time the air conductivity will increase and the dissolved
contaminants will volatilize, both of which tend to increase the degree of
removal.
Other Factors
Cost. In general, soil venting is a relatively inexpensive technique
compared to other alternatives, especially when large volumes of soil must
be treated. The capital costs of venting consist basically of the
extraction and monitoring well construction, one or more centrifugal
blowers and housing, pipes, valves, fittings, and other hardware, and
electrical instrumentation. Operations and maintenance costs consist of
labor, power, maintenance, and monitoring. Venting wells constructed of
two-inch diameter slotted PVC pipe cost approximately $20 per linear foot
(0.3 m) for a 20-foot depth. Vacuum pump sizing depends on local soil
conditions and the volume to be treated. A pump capable of removing 40-60
cubic feet (1.2 to 1.8 cubic meters) per minute (CFM) at 1.5 inches (3.8
cm) of water ranges in price from $500-$2000; larger pumps may cost up to
$4,000. Operating costs vary depending on time operation and local utility
rates. One EPA study (1987d) reported the actual costs of soil venting at
a Florida site to be $106,000 (capital) and $68,000 (annual O&M). Air
treatment, if necessary, would have more than doubled costs. These figures
correspond to roughly $20 to $60/yd3 ($25 to $78/m3). Other costs reported
in the literature range from $15-20/yd ($20 to 26/m3), exclusive of air
emission control (Anastos et al., 1986). Air emission control via GAG is
usually assumed to double the total capital cost of the cleanup.
Time Scale. Soil venting programs are relatively easy to implement,
and may be installed and started in two to four weeks. This time is
devoted to determining the extent of contamination, designing the system,
acquiring pumps and piping, and installing the equipment.
A venting program is typically operated for six to twelve months.
The removal rate is usually highest at the beginning of the program (once
the vacuum is established) and falls off after the most volatile
contaminants are removed (see Figure 6). Volatilization from dissolved and
sorbed contaminants then becomes rate-limiting, and the system's
effectiveness may decline dramatically.
41
-------
Gasoline Component Concentration
c
o
c
o
a
a
II
o o
IS
-------
commonly found in subsurface soils. Natural breakdown of petroleum
hydrocarbons is likely to occur whenever they are introduced to the
subsurface, but without the addition of nutrients and oxygen biodegradation
occurs very slowly.
Many Superfund cleanups have used biorestoration for cleanup, and this
technique is now being used at UST sites. The effectiveness in removing
hydrocarbons to low levels is site-specific, and some situations are not
suitable for this technique. When applicable, biorestoration is often a
cost-effective alternative to other treatment schemes and has the added
advantage of transforming organic pollutants to less toxic end products
rather than transferring them to another phase.
Biodegradation of hydrocarbons can occur aerobically (in the presence
of oxygen) or anaerobically (without oxygen). Experimental data indicate
that aerobic processes are far more effective than anaerobic processes, but
recent a recent study (Major et al, 1988) suggests that the effectiveness
of anaerobic biodegradation may approach that of aerobic processes under
the right conditions. However, the focus of this manual is on the more
well documented aerobic processes.
For a successful biorestoration program, an adequate supply of oxygen
and nutrients must be available to the bacteria throughout the zone of
contamination. This is usually accomplished by adding nutrient-enhanced
water to the unsaturated zone through infiltration basins at the ground
surface or through recharge wells. The water used to transport the
nutrients can also work to dissolve sorbed contaminants and transport them
to the water table. If the groundwater below the site has already
undergone treatment, the contaminants and nutrients could recontaminate the
groundwater if not properly contained. A pumping well can be installed to
control local groundwater flow and recirculate water to the unsaturated
zone. To increase efficiency, biorestoration of the unsaturated zone can
be implemented in conjunction with treatment of groundwater.
Biorestoration and air stripping are two groundwater treatment methods
compatible with biorestoration of the unsaturated zone.
Although the biorestoration method described here is in situ above-
ground equipment is required. At a minimum, an above-ground mixing tank is
required to add nutrients and oxygen to the water being introduced to the
unsaturated zone. The tank is also used as temporary storage of
recirculated water, where additional bacteria can be grown and the organic
content of the water can be reduced. Care must be taken when introducing
nutrient laden water to the subsurface so that none of the treated water
escapes the capture zone of the recovery well. Figure 7 shows a typical
biorestoration system.
Theoretical Considerations
The careful addition of nutrients like oxygen, nitrogen and phosphorous
can improve conditions for the biological breakdown of petroleum
constituents to simple, non-toxic end products. What to add and how much
is difficult to predict because of the many factors that influence the
biodegradation process. To increase the likelihood of a successful
biorestoration program at a particular site, pilot studies using samples
collected at the site should be conducted to determine the appropriate
nutrient additives and the required application rate.
43
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Nutrients Oxygenation
WFILTRATIQN
TRENCHS
/i\ /I
CONTAMINATED
SOIL
Figure 7. Schematic Diagram of a Biorestoration System for the Unsaturated Zone
44
-------
Three general mechanisms are used by microbes to catabolize (break down
to simpler substances) hydrocarbons: 1) aerobic respiration; 2) anaerobic
respiration; and 3) fermentation (EPA, 1986). Aerobic respiration is
typically the most rapid and most complete degradation process, and avoids
the problematic end products (e.g., hydrogen sulfide) that result from
anaerobic respiration. Petroleum hydrocarbons are composed of hydrogen
and carbon and, under ideal conditions, the end products of (aerobic)
biodegradation are carbon dioxide, water, and biomass. Under less than
ideal conditions (i.e., inadequate oxygen supply, lack of nutrients, etc.),
less complete degradation may take place resulting in only partial
breakdown of the hydrocarbons. If conditions can be enhanced, more
complete biodegradation can take place. The fact that biodegradation can
transform a contaminant to a non-toxic or less-toxic end product is a major
advantage this technique holds over other cleanup methods. The other
techniques serve only to concentrate, disperse, or relocate the
contaminants, and further treatment or handling is usually required.
Naturally occurring soil microorganisms are capable of degrading many
chemicals compounds, including many petroleum constituents. More than 200
different soil microbes have been identified as being capable of
assimilating petroleum (EPA, 1988c). Although it is not possible to
predict with certainty whether a certain compound will be degraded at a
specific site, the refractory index (RI) indicates a compound's
susceptibility to degradation. The RI, a ratio of the BOD5 (5-day
biochemical oxygen demand) to the COD (chemical oxygen demand), predicts
the likelihood of biodegradation for a compound (Lyman et al., 1982). For
gasoline constituents, phenol, benzene, and toluene are all ranked as
"relatively degradable" (see Table 9). In general, petroleum hydrocarbons
are degradable in most natural settings.
The factors affecting the appropriateness and potential effectiveness
of biorestoration for a particular site may be divided into three broad
classes: 1) the susceptibility of the contaminants to biodegradation; 2)
the various environmental factors at the site; and 3) the site
hydrogeology. The biodegradability of the contaminants, discussed above,
is the primary factor influencing the choice of biorestoration; situations
where the contaminants of concern are relatively undegradable would not be
appropriate for this treatment. Fortunately, many petroleum products are
relatively degradable.
There are many site-specific environmental factors that affect the
feasibility of biorestoration as a treatment alternative (EPA, 1985).
These factors include, among others:
available oxygen concentration
appropriate levels of macronutrients, and micronutrients
redox potential
soil pH
degree of water saturation
soil temperature
competition, predators, presence of toxins
concentration of contaminants
hydraulic conductivity of soil
45
-------
The oxygen concentration is the single most important ingredient_in a
successful biorestoration program. Although biodegradation may continue to
occur anaerobically, the lack of oxygen severely limits the rate of
cleanup. Oxygen may be introduced to the subsurface in several ways. Air
sparging pumps air (and oxygen) through slotted wells into the unsaturated
zone. Hydrogen peroxide can be introduced through recharge wells or
infiltration trenches and can result in available oxygen levels of up to
250-400 mg/L in the groundwater, compared to about 10 mg/L of oxygen by air
sparging (Raymond, 1987). Conner (1988) reported that a vacuum extraction
system resulted in increased microbial activity. In addition to oxygen,
soil microbes also require macronutrients (nitrogen and phosphorous) and
micronutrients to survive and prosper. These nutrients are typically added
in order to facilitate biodegradation.
Soil moisture content and contaminant solubility in water are also
important factors because the most rapid biodegradation occurs in the
dissolved contaminant phase. Biodegradation is usually limited by the
solubility of a compound in water, as most microorganisms either inhabit
soil moisture or need moisture to acquire nutrients and avoid desiccation
(EPRI, 1988). Moisture content between 50 and 80 percent of the water
holding capacity is considered optimal (Bossert and Bertha, 1984). Soils
with moisture content in this range promote adequate dissolution of
contaminants and contain sufficient air voids to supply oxygen. However,
the addition of aerated water to the soils reduces the need to maintain air
voids in the soil.
Although most biodegradation takes place in the dissolved phase, sorbed
contaminants are also biodegraded but at a much slower rate. Microbes
inhabiting pore water can serve to partially breakdown sorbed compounds,
increasing their likelihood of dissolution. Once dissolved, the compounds
can be more readily biodegraded.
The concentration of contaminant dissolved in water affects
biodegradation. If too low, bacteria may favor another competing food
source; if too high, the contaminant may be toxic to the bacterial
population. Aerobic bacteria are typically used for organic concentrations
between 50 and 4,000 mg/L (Nyer, 1985). For in situ treatment, organic
concentrations as low as 10 mg/L may be sufficient. Dissolved petroleum
hydrocarbons are unlikely to be found at concentrations as high as 4,000
mg/L.
Slightly alkaline soil pH is optimal for biodegradation, but anything
in the range of 6.0-8.0 is considered acceptable. Most soils are slightly
acidic and neutralization may be required at some sites. The temperature of
the soil environment will also affect the rate of degradation. Warmer
temperatures generally result in higher rates of degradation. While
biodegradation has been shown to occur over a wide temperature range, the
range of 20ฐ-35ฐC seems optimal but is typically above the normal
subsurface temperature range. Also, microbes generally have a low
tolerance for severe temperature changes as are experienced in northern
regions.
46
-------
A knowledge of the hydraulic conductivity of the soil, and the site
hydrogeology in general, is also important in assessing the feasibility of
biorestoration for a particular site. Even when all other factors are
positive, in situ biorestoration will not be successful if a low hydraulic
conductivity prevents the added nutrients and oxygen from contacting the
zone of contamination. The residence time should be short enough so that
the oxygen concentration is sufficient throughout the site for microbes to
degrade all of the organic compounds. Also, the geochemistry of the
subsurface could inhibit adequate mixing if reactions (such as metal oxide
precipitation) clog the soil. The soil microbes themselves may clog the
soil and decrease the hydraulic conductivity.
Table 18 lists the critical success factors for biorestoration. By
comparing the parameters at the site of interest to those in this table, a
general understanding can be obtained of the suitability of biorestoration
at that site. A preponderance of CSFs that match the rightmost column
would indicate that biorestoration is likely to be effective at that site.
Other Factors
Cost. The costs of biorestoration for the unsaturated zone vary widely
and are difficult to quantify and compare. Also, most reported costs refer
to cleaning up groundwater rather than the unsaturated zone. One estimate
(Olsen et al., 1986) gave costs of $60 to $123 per cubic yard ($78 to $160
per cubic meter). Unit costs for larger volumes are generally lower due to
economies of scale.
Time Scale. A biorestoration program can be set up relatively quickly,
but it may take several weeks to several months for the microbes to become
adjusted and start significant degradation if the contaminant release is
recent, or if non-indigenous bacteria are used. The system may need to be
"fine-tuned" (i.e., varying the levels of oxygen and nutrients added) to
operate efficiently. The start of a biorestoration program may be delayed
due to the drilling of injection and extraction wells, the design and
procurement of construction of the oxygenation equipment, and the need for
injection permits.
It is difficult to estimate the length of time it will take to clean
up a particular site with biorestoration or even obtain accurate
measurements to determine the degree of contaminant removal once
remediation has begun. Studies show that the length of cleanup is
generally six months to two years, although some sites may take more or
less time. As with other methods, the removal rate typically will decrease
with time, assuming the contaminant concentration decreases over time.
Other Advantages/Disadvantages. This method has several other,
non-scientific factors that may influence the use of this technology at UST
release sites:
Of the technologies discussed in the manual, in situ biorestoration
is the least-understood. Although much research has been performed
in recent years, the specific mechanisms, kinetics, and pathways by
which compounds are degraded are not we11-understood. The data
base on many contaminants, such as gasoline constituents, remains
47
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TABLE 18. WORKSHEET FOR EVALUATING
THE FEASIBILITY OF BIORESTORATION
BEING EFFECTIVE AT YOUR SITE
CRITICAL SUCCESS
FACTOR
RELEASE -RELATED
Time SI nee Release
STTE RELATED
Dominant
Contaminant Phase
Soil Temperature
Soil Hydraulic
Conductivity
Soil pH
Moisture Content
UNITS
Months
Phase
ฐC
cm/sec.
pH Units
% Volume
SITE OF
INTEREST
CONTAMINANT- RELATED
Solubility
Biodegradabllity
- Refractory Index
Fuel Type
mg/L
Dimensionless
SUCCESS
LESS
LIKELY
Short
(<1)
o
Liquid
O
Low
(<5)
O
Low
( 8)
0
Dry
(<10)
0
SUCCESS
SOMEWHAT
LIKELY
Medium
(1-12)
0
Vapor
O
Medium
(5-10)
O
Medium
(10-5-10'3)
O
o
Moderate
(10 to 30)
O
(D
SUCCESS
MORE
LIKELY
Long
(>12)
O
Dissolved
O
High
(>10)
O
High
(> 10'3)
O
(6-8)
O
Moist
(>30)
O
Low
(< 100)
O
Low
(<0.01)
0
No. 6 Fuel Oil
(Heavy)
O
Medium
(100 to 1000)
O
Medium
(0.01 to 0.1)
O
No. 2 Fuel Oil
(Medium)
O
High
(> 1000)
O
High
(>0.1)
0
Gasoline/ Diesel
(Light)
O
OTHER CONSIDERATIONS
Cost is from $60 to $1 25 per cubic yard.
Completely destroys contaminants under optimal conditions
Effectiveness varies depending on subsurface conditions
Biologic systems subject to upset
Public opinion sometimes against putting more chemicals in ground
Difficult to monitor effectiveness
Minimizes health risk by keeping contaminants in ground and on site
Takes long time to work not for emergency response
48
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weak. One problem is that soil hydraulic conductivity may be
reduced at high growth rates of bacteria (microorganisms),
inhibiting further progress. Research continues on many of these
topics.
The difficulty in measuring the degree of removal reinforces fears
that contamination is not being removed. When soil venting is
used, for example, it is possible to quantify pounds of contaminant
extracted very easily; judging biorestoration's effectiveness, on
the other hand, is more difficult.
These disadvantages have sometimes led to reluctance among
regulators to approve biorestoration as a cleanup method. As the
technology becomes better understood, however, it is expected that
this treatment technique will receive more favorable attention from
those selecting and approving corrective action plans.
49
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SOIL FLUSHING
Soil flushing refers to the in situ process where the zone of
contamination is flooded with water or a water-surfactant mixture in order
to dissolve the contaminants into the water or otherwise mobilize the
residual contaminant to the water table. The contaminants are then brought
to the surface for treatment by strategically placed extraction wells. The
wells must be located such that the groundwater is completely controlled
hydraulically, to ensure that the leached or mobilized contaminants do not
escape once they reach the groundwater. This process may also be referred
to as "ground leaching," "solvent flushing," or "extraction." Figure 8
shows a schematic diagram of a soil flushing system.
One soil flushing method removes contaminants by using water to
dissolve the liquid, sorbed, or vapor contaminant. These processes are
controlled by the contaminants' solubilities and Henry's law constants.
Contaminants that are highly soluble in water, such as methanol, acetone,
or phenol, are easily removed after only a few flushes with water. Other
compounds, such as the gasoline additive tetraethyl lead or many of the
major constituents in #6 fuel oil, are very insoluble and would not be
solubilized to a high degree even after many flushes. The Henry's Law
constant describes a contaminant's partitioning behavior between the vapor
phase and liquid solution (how much contaminant exists in air versus how
much exists in water). The introduction of large quantities of flushing
solution will change the equilibrium of the vapor/liquid partitioning, and
can result in some contaminant vapors being solubilized. A low Henry's
constant is indicative of a tendency to exist in solution.
A second soil flushing method mobilizes contaminants existing as free
product in the soil pores, and adsorbed to the soil. Contaminants found in
these phases can be mobilized by the pressure gradient of the infiltrating
flushing water. The viscosity and density of contaminants control the
extent to which a compound may be mobilized as free product. This type of
soil flushing would be expected to remove a greater portion of gasoline
than heating oil or #6 fuel oil, because gasoline is less viscous then
either of the other two petroleum products. Many of the constituents of
gasoline are also more soluble than those of #6 fuel oil or heating oil and
would be more readily mobilized in the dissolved phase as well. Tables 7
and 8 list the solubility, viscosity, density, and many other properties
for many chemicals.
Similarly, compounds that are strongly sorbed to soil particles will
not be as easily removed by flushing with water only. A compound's
soil/water partitioning coefficient, KO , may be used to identify the ease
with which a compound will leave the soil. A compound's octanol/water
coefficient, KOW, is more widely available and is often used as a surrogate
for its soil/water coefficient. Organics with KOW values less than 10,
such as low-molecular weight alcohols and phenols" are very soluble, and
can be leached from the soil by natural processes. Compounds with KQW
values from 10 to 1000, such as low-to-medium molecular weight ketones,
aldehydes, and aromatics, are somewhat amenable to flushing. Organics with
Kow greater than 1000 typically would require a surfactant (EPA, 1987a).
50
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ATMOSPHERE
SPRAY
RECHARGE
SYSTEM
STORAGE LAGOON
ADDITIONAL
SURFACTANT
DISCHARGE
: LAGOON
WITHDRAWAL WELLS
Figure 8. Schematic Diagram of Soil Flushing System.
-------
A surfactant is often added to the flushing water to help mobilize
subsurface contaminants. Surfactants are natural or synthetic chemicals
that have the ability to promote the wetting, solubilization, or
emulsification of various organic chemicals. Many different types of
surfactants are available, with varying properties; EPA (1986) provides an
extensive summary of surfactant types and properties.
Surfactants can increase the "detergency" of an aqueous solution in
three ways (EPA, 1986):
Preferential wetting - Surfactants can decrease the interfacial
tension between the aqueous and solid phases (soil), allowing the
water to preferentially wet the soil (resulting in a "water-wet"
condition), thus displacing or partially displacing the
contaminant.
Solubilization - Surfactants may enhance the solubility of certain
contaminants.
Emulsification - Surfactants may enhance the emulsification or
the dispersion of an insoluble organic phase within the aqueous
phase of a contaminant.
In addition, organics that have been mobilized by surfactants are
more accessible to biodegradation.
Local soil conditions also affect the likely effectiveness of soil
flushing. Flushing is best suited to soils with a high hydraulic
conductivity, such as gravel or sand. Soils with high silt and clay
content impede movement of the flushing solution through the soils,
resulting in less removal. Soils with high organic carbon content
(especially those with more than 1% organic matter) and high clay content
would tend to have stronger sorption characteristics and thus be less
amenable to flushing. As with soil venting, the effectiveness of soil
flushing depends on the degree to which all the contaminant is exposed to
the flushing solution. Soils that are well-compacted may hold contaminants
that are not reached by the leaching fluid. Preferential flow paths in the
soil may also decrease the degree of intimate contact between the flushing
solution and the contaminated soil, leading to decreased removal
efficiencies.
A variation of the soil flushing process, known as "soil washing,"
takes place above ground in a reactor. Experience has shown that soil
washing produces greater removal and better overall results than an in situ
flushing system. By performing the washing ex situ (above ground), the
leaching solution can more completely remove contaminants because the two
major constraints of in situ flushing low hydraulic conductivity and
non-uniform contaminant contact due to preferred flow paths are
overcome. Although this discussion focuses on the in situ flushing
process, many of the critical success factors apply to both soil flushing
and soil washing. The site geology, however, will obviously be less
important for above ground soil washing.
52
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Table 19 lists the critical success factors for soil flushing. It
can be used to evaluate whether soil flushing or washing will be effective
at a particular site.
Other Factors
Experience. Actual field experience with in situ soil flushing for
petroleum remediation is very limited, and thus data on effectiveness,
costs and limitations are generally unavailable. The petroleum industry
has experimented for several years with enhanced oil recovery, which uses
surfactants to increase the production of an oil deposit. While similar,
this experience is not directly applicable to soil cleanups.
Cost. Cost data are sparse. The Superfund site at Palmetto Wood, SC
cited costs of $3,710,000 (capital) and $300,000 (annual O&M). These
figures, on a unit basis, equal $185/yd3 ($240/m3) for capital costs, and
$15/yd ($20/m ) annually for O&M. At Palmetto Wood, SC, soil flushing
will be used to clean 20,000 yd (15,400 m3) of soil contaminated with
metals (EPA, 1988b).
Other Advantages/Disadvantages. One principal drawback of soil
flushing is the generation of large quantities of contaminated elutriate
that requires treatment. Elutriate is the mixture of water, surfactants,
and contaminants that is recovered in the soil flushing process. In some
cases, the elutriate may be discharged to a local POTW, but often an
on-site treatment must be devised. As with many other treatment methods,
soil flushing requires that the groundwater flow pattern be well-defined to
ensure complete recovery of the elutriate. If this is not the case,
physical barriers such as1 slurry walls may be required. This technique
also requires access to a source of water for flushing. Typically,
groundwater is extracted, treated, and recycled as the flushing solution.
Site-specific factors that may limit the effectiveness of flushing
include soils with pockets of low hydraulic conductivity. This limits the
ability to pass large quantities of water through the contaminated soil.
Many UST sites, especially those in urban settings, do not lend themselves
to flushing due to nearby pipes and underground utilities. Soil flushing
will be less effective at sites where the contaminants are relatively
insoluble or tightly bound to the soil. The lack of an existing water
supply may also be limiting.
The use of surfactants involves several considerations. The
interactions of the surfactant with the biological, physical, and chemical
properties of the unsaturated zone are typically uncertain, and must be
determined at each site. For example, the addition of a surfactant
containing sodium may lower soil permeability due to its reactive effect on
the soil/sodium adsorption ratio (EPA, 1987b), which with time would
decrease the effectiveness of this technique. The groundwater geochemistry
also should be assessed for troublesome, naturally-occurring constituents
prior to the addition of any surfactant. For example, hard water may
render a surfactant ineffective. Soil type may also reduce a surfactant's
effect. High clay content can cause chemical adsorption of the surfactant
to the soil, thereby reducing available surfactant concentrations and
limiting its effectiveness. Biological effects on the surfactant may also
53
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TABLE 19. WORKSHEET FOR EVALUATING ff^\
THE FEASIBILITY OF SOIL FLUSHING LnmiJ
BEING EFFECTIVE AT YOUR SITE
CRITICAL SUCCESS
FACTOR
UNITS
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
SUCCESS
SOMEWHAT
LIKELY
IIIIITIB
mini iw
^^^^J^F
SUCCESS
MORE
LIKELY
SITE RELATED
Dominant
Contaminant Phase
Soil Hydraulic
Conductivity
- Soil Surface Area
- Carbon Content
Fractures in Rock
Phase
cm/sec.
m2/g
% Weight
Vapor
O
Low
(<10~5)
O
High
(>1)
O
High
O
Present
O
Liquid
O
Medium
(10-5-10~3)
0
Medium
(0.1-1)
O
Medium
(1 - 10%)
0
0
Dissolved
0
High
O
Small
O
Low
O
Absent
O
CONTAMINANT- RELATED
Water Solubility
Sorptlon Characteristics
- Soil Sorptlon Constant
Vapor Pressure
Liquid Viscosity
Liquid Density
mg/L
L/kg
mm Hg
cPoise
g/cm 3
Low
(< 100)
O
High
(> 10,000)
0
High
(> 100)
O
High
(>20)
O
Low
O
Medium
(100 to 1000)
O
Medium
(100-10,000)
O
Medium
(10-100)
O
Medium
(2 -20)
O
Medium
(1-2)
O
High
(> 1,000)
O
Low
(< 100)
O
Low
0
Low
O
High
0
OTHER CONSIDERATIONS
Cost is from $150 to $200 per cubic yard.
Using surfactants may increase effectiveness
Effluent requires separation techniques such as distillation, evaporation, centrifugation
Most effective when used ex-situ (above ground)
54
-------
be important. In some cases, a surfactant may biodegrade too quickly,
reducing its exposure time to the contaminated soil. On the other hand,
the surfactant should be degradable by the soil microbes at a slow rate so
that surfactant buildup does not occur.
A general limitation of soil flushing is the inability at the present
time to develop a method to separate the surfactant from the water, so that
the surfactant can be recycled. Until the surfactant can be separated from
the water, the high rates of surfactant consumption will limit the
cost-effectiveness of soil flushing.
HYDRAULIC METHODS
Hydraulic methods include sumps, French drains, and other equipment
and designs that allow for passive removal of accumulated free product from
the unsaturated zone. This section does not address caps, slurry walls, or
other physical barriers to flow, or technologies such as freezing, which
although used in certain circumstances, typically are impractical at UST
sites.
Hydraulic barriers are simple and relatively inexpensive, but do not remove
contaminants to low levels. They are unlikely to be selected as a primary
clean up option and might best serve as a means to easily collect some of
the residual liquid in the unsaturated zone before implementation of
another technology. For example, when a leaking UST is removed, the
resulting excavated area might begin to accumulate residual liquid. The
heavy machinery already on site could be used to enlarge or deepen the
excavated area to promote further accumulation of product with little added
effort.
Theoretical Considerations
Hydraulic methods create an area of high permeability in the
unsaturated zone, allowing residual liquid contaminants to flow readily
toward that permeable zone. Typically, a trench is dug into the
contaminated soil and residual liquid will begin to seep into the trench.
An impervious layer can be placed at the base of the trench to prevent
re-infiltration of the product. As the product accumulates, it can be
pumped out or removed manually, maintaining a gradient which facilitates
further seepage into the trench. Figure 9 shows a typical drain system.
Hydraulic methods collect and remove only the mobile liquid and
dissolved contaminants (and may serve as a passive venting system).
Contaminants sorbed to soil particles or held as residual saturation are
little affected by hydraulic methods and typically must be removed by other
means. The method is best used in situations where the mobile phase
residual liquid and pore water content is relatively high. Hydraulic
methods are most effective for recent releases of significant quantities of
contaminants at shallow depths.
Table 20 lists several CSFs for hydraulic methods. The single most
important critical success factor is the amount of contaminant in the
liquid phase. This method will not be effective unless significant
quantities of liquid contaminant are in the soil. The CSFs from the site
55
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Ul
(Ti
Pump or Manually
Remove NAPL
\
Contaminated Soil
NAPL
Figure 9. Schematic Diagram of a Typical Hydraulic Barrier System
-------
TABLE 20. WORKSHEET FOR EVALUATING f N
THE FEASIBILITY OF HYDRAULIC BARRIERS \iS3S5SSsA
WORKING AT YOUR SITE
CRITICAL SUCCESS
FACTOR
RELEASE - RELATED
Time Since Release
Volume of Spill
SITE RELATED
Dominant
Contaminant Phase
Soil Hydraulic
Conductivity
Soil Sorption Capacity
- Surface Area
Carbon Content
Temperature
Depth to Ground water
UNITS
months
gallons
phase
cm/sec.
m2/g
% weight
ฐC
meters
SITE OF
INTEREST
^^
SUCCESS
LESS
LIKELY
Long
(> 12 months)
0
Small
O
Vapor
O
High
(> 10'3)
O
High
O
High
(> 10%)
O
Low
O
High
O
SUCCESS
SOMEWHAT
LIKELY
Medium
(1 12 months)
O
Medium
(100-1000)
O
O
Medium
(10'5-10"3)
O
Medium
(0.1 - 1)
O
Medium
(1 10)
O
Medium
(5-10)
O
Medium
(1-5)
0
yssssssg
T&SsSssr
SUCCESS
MORE
LIKELY
Short
O
Large
(> 1000)
O
Liquid
O
Low
O
Low
O
Low
O
High
O
Low
O
CONTAMINANT- RELATED
Liquid Viscosity
c Poise
High
(>20)
O
Medium
(2 to 20)
O
Low
O
OTHER CONSIDERATIONS
Cost is from $10 to $90 per cubic meter.
Only affects liquid portion of release not portion sorbed to soil.
- Typically limited to shallow (<3 meters) depths.
Not effective in removing contaminants to low levels.
Most effective when contamination is confined to small areas.
Not effective for #6 fuel oil and other viscous fluids.
Not effective if contamination Is greater than 15 meters deep.
57
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of interest can be entered on the worksheet for easy comparison to the
preferred conditions to provide preliminary screening as to the likely
effectiveness.
Other conditions may enhance or impede the effectiveness of hydraulic
methods. Releases that are concentrated in small areas are more likely
candidates for hydraulic barriers than releases that have spread over a
large area. Releases of relatively immobile hydrocarbons such as #6 fuel
oil are not amenable to this method. Also, releases that have moved a
great distance below the surface are less amenable to this method because
of limitations with excavation equipment.
Other Factors
Cost. Perhaps the most important variable is the depth of the
drains. Two examples (EPA, 1987b) ranged in price from $7.00/sq. ft.
($75/sq. m.) to $67-88/sq. ft. ($720 - $950/sq. m.), an order of magnitude
difference. The first case involved a three-foot deep interceptor trench;
in the second case, the trench was excavated to a depth of 12 - 17 feet
(3.6 - 5.2 m).
EXCAVATION
Excavation is an alternative to the four in situ treatment methods.
Excavated soil may be treated on site, treated off site, or disposed of
(landfilled) without treatment. Treated soil is sometimes placed back in
the excavation at the site.
At present, excavating contaminated soil is more common than in situ
treatment. Excavation, however, has many drawbacks not faced with in situ
methods:
Excavating contaminated soils allows uncontrolled release of
contaminant vapors to the atmosphere, increasing exposure risks
(EPA, 1989);
* Above- and below-ground structutes buried utility lines, sewers
and water mains, and buildings can pose real problems if
contamination extends near or below the structures;
Above-ground treatment methods tend to be more expensive than in
situ methods;
Disposal of contaminated soil is becoming increasingly difficult,
and in some regulatory regions the soil is considered a hazardous
waste;
A source of backfill is required to fill the excavation.
Still, excavation is performed quite regularly. It is easy to
undertake and may be done quickly; it is a well known technique; and it has
the ability to remove most or all of the contamination from the site.
58
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This report does not discuss in detail methods for treating excavated
soil contaminated with hydrocarbons. A wide variety of methods may be
used, including incineration, using soil as an aggregate in asphalt
production, soil washing, enhanced volatilization, microbial degradation,
and others. Short summaries of four above-ground treatment methods are
presented later in this manual.
Table 21 lists the CSFs for excavation. The parameters from the site
of interest can be compared with the CSFs on this table for a preliminary
screening of the likely effectiveness of excavation.
Other Factors
Cost. The cost of excavation varies with the volume of soil to be
removed, the type of equipment used, the cost of backfill, and the cost of
treatment or disposal. EPA (1987b) quotes the costs on a volume and
equipment basis:3 removal by backhoes costs between $1.75 and $5.00/yd3
($2.30 - $6.50/m ), depending on the backhoe capacity; while dozers and
loaders range from $1.20 to $4.50/yd3 ($1.60 - $5.90/m3). Backfill
materials cost $10-20/yd3 ($13 - $26/m ), depending on the distance 3
transported. Transportation to disposal site may cost from $0.50-$1.00/yd
($0.63 - $1.30/m ) per mile transported. Landfill tipping fees are
estimated at $125-240/yd3 ($163 - $312/m3); these costs are expected to
increase as landfill capacity becomes more limited. Total disposal costs
can range from $50 to $300 or more per cubic yard ($65 - $390/m3).
Time Scale. This technique can be undertaken immediately, and is
often used where urgent and immediate action is needed. The ability to
respond quickly to a release is one of the main advantages that excavation
enjoys over in situ methods. The soil and the associated contamination can
be removed in only a few hours, rather than the several months it takes for
the other methods described previously.
Disposal. Disposal of contaminated soils at landfills is becoming
increasingly restricted by regulation, as well as being more expensive.
Many states and localities now require disposal in secure landfills (which
costs far more than if disposed at sanitary landfills). Also, landfill
capacity is shrinking and sometimes poses an unexpected restriction.
ABOVE-GROUND TREATMENT METHODS
Because disposal of excavated soil is becoming increasingly
difficult, treating excavated soil is becoming more attractive. Excavated
soil may be treated on-site or off-site. This subsection briefly discusses
various above-ground soil treatment methods that may be used for excavated
soils. EPA (1988c) discusses in detail above-ground soil treatment
technologies.
Incineration
Contaminated soil may be incinerated in rotary kilns, fluidized beds,
or other systems. The systems may be either mobile units or fixed
facilities. Both rotary kilns and fluidized beds are capable of destroying
or removing 99 percent or more of the contaminants.
59
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TABLE 21. WORKSHEET FOR EVALUATING
THE FEASIBILITY OF EXCAVATION
BEING EFFECTIVE AT YOUR SITE
CRITICAL SUCCESS
FACTOR
UNITS
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
SUCCESS
SOMEWHAT
LIKELY
SUCCESS
MORE
LIKELY
SITE RELATED
Proximity of Above and
Below Ground Structures
Buildings nearby
Buried pipes
and cables
o
No nearby
structures
O
Volume of Soil
Contaminated
Cubic
Meters
Large
(> 1,000)
O
Medium
(100-1,000)
O
Small
(< 100)
O
> Depth of Contamination
Meters From
Surface
Deep
O
Medium
(1-5)
O
Shallow
(<1)
O
Proximity of Site to
- Traffic
Near
O
Far
O
- Businesses
Near
O
Far
O
- Disposal Site
Far
O
Near
O
- Backfill Source
Far
O
Near
O
OTHER CONSIDERATIONS
Cost is from $50 to $300 per cubic yard.
Appropriate when urgent response is necessary
Brings contaminants to surface, thereby increasing exposure risks
Significant amounts of surface area disturbed relative to depth excavated
Requires suitable means of disposal. This is becoming increasingly difficult
because some landfill operators consider petroleum-laden soil to be a hazardous waste.
60
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Incineration capacity in the United States is rapidly becoming
outpaced by demand. Additions to capacity typically face delays due to
public opposition and the permitting process.
Incineration costs vary significantly with soil type and the
contaminant characteristics. One estimate (EPA, 1988c) cited costs of $200
to $640/yd ($260 - $832/m3) for 20,000 yd3 (15,400 m3) of soil. Smaller
quantities of soil would likely have higher unit incineration costs.
Soil Washing
Soil washing is the term used herein to refer to the aboveground
process whereby contaminants are removed from soil via a leaching medium,
typically water. Soil washing and soil flushing differ only in that soil
flushing is an in situ process while soil washing is ex situ.
Above-ground washing has many advantages over soil flushing. The
'most important limitation of flushing a soil's low hydraulic
conductivity does not constrain washing. The mixing of the leaching
solution and contaminated soil in a countercurrent reactor also results in
a much higher degree of contact between the soil and leaching material,
which results in greater removal rates. Leaching soil aboveground also
removes the possibility of contaminating underlying aquifers, which could
result if the soil flushing was operated improperly.
The effectiveness of this process depends on several factors, but the
most important are sorption of the contaminants to the soil, the solubility
of the contaminant's constituents, and the presence of clay and silt in the
soil.
Contaminants such as creosote coal-tars have very high sorptive
characteristics. Such tightly-bound contaminants are difficult to remove
by flushing or washing. Highly soluble contaminants are more easily
removed by the leaching liquid than are insoluble contaminants. Finally,
soil with high clay or silt content will impede contaminant-soil separation
and thus decrease the effectiveness of this method.
The cost of soil washing has been reported (EPA, 1988b) to range from
$150-200/yd3 ($195 - $260/m3) for the processing costs alone (excluding
excavation and disposal costs).
Enhanced Volatilization
Enhanced volatilization refers to any process or technique that
removes contaminants from soil by increasing the volatilization rate,
either by heating the contaminants or increasing the exposure to
uncontaminated air. EPA (1988c) discussed four specific enhanced
volatilization techniques: mechanical volatilization; enclosed mechanical
aeration; low temperature thermal stripping; and pneumatic conveyer
systems. Of these four types of systems, thermal stripping was reported to
be most effective.
61
-------
Rototilling disturbs the surface of the soil to a depth of about one
foot, thereby increasing the access of fresh air to the soil. Soil may
then be excavated and the underlying soil rototilled, so that successive
iterations result in volatilization of contaminants from soils from greater
depths. Enclosed mechanical aeration systems employ pug mills or rotary
drums to increase the turbulence in the reactor, thereby increasing
air/contaminant contact and, thus, removal. Low temperature thermal
stripping systems are similar to mechanical aeration systems but, in
addition, heat is added to increase the volatilization rate. Pneumatic
conveyers use both increased temperature and high velocity air flow to
remove contaminants. All of the systems capture the volatilized
contaminants.
Enhanced volatilization methods have not been widely used in
full-scale applications. Pilot-scale studies have predicted removal
efficiencies of up to 99.99 percent based on post-aeration soil sampling.
Of course, removal efficiencies would be higher for more volatile
contaminants.
Cost data for these systems are scarce. EPA (1988c) estimated that
actual processing costs for less than 10,000 yd3 (7,700 m ) of soil would
exceed $275/yd3 ($210/m3).
SUMMARY
Table 22 presents a summary of the critical success factors for the
four in situ treatment methods and excavation. This table is comprehensive
in that it includes all the CSFs from Tables 17 through 21. Table 22
provides the preferred conditions for each technology.
Table 22 includes only the objective (scientific) factors that could
affect the choice of technologies at a specific site. Economic, political,
regulatory, and other potentially controversial factors, not listed on
Tables 17 through 21, often are as important as these objective factors.
Therefore, this summary table is useful for direct comparison of the
technologies on the grounds of technical feasibility only. When potential
treatment technologies have been narrowed to those that appear technically
feasible, other considerations (cost, public perception, etc.) will likely
affect the final selection.
62
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CRITICAL SUCCESS
FACTORS
RELEASE - RELATED
TIME SINCE RELEASE
VOLUME OF SPILL
VOLUME SOIL CONTAMINATED
.* Srre^ RELATED
HYDRAULIC CONDUCTIVITY
SOIL TEMPERATURE
SOIL SORPTION CAPACITY
SOIL MOISTURE CONTENT
SUFFICENT OXYGEN/NUTRIENTS
SOILpH
DOMINANT CONTAMINANT PHASE
SOIL CARBON CONTENT
DEPTH TO GROUNDWATER
PROXIMITY TO STRUCTURES
PROXIMITY TO LANDFILL
COWTAMSNANT RELATED
CONTAMINANT SOLUBILITY
VAPOR PRESSURE
BIODEGRADABILITY
VISCOSITY
DENSITY
FUEL TYPE'
UNITS
months
gallons
3
m
cm/sec
c
L/kg
%
pH units
Vapor Dte*olved
liquid
%
meters
SOIL
VENTING
Large (>1 000)
High(>10'3)
High (>20)
Low(<100)
Low(<10)
Vapor and/ or Liquid
Low(<1)
Deep(>15)
mg/L
mm Hg
Refsctory
Index
cPoise
3
g/cm
Low (< 100)
High (>1 00)
Low(<1)
Light
BIO-
RESTORATION
Long (12)
High(>10'3)
High (>20)
High (>30)
Sufficient
6.0 - B.O
Dissolved
High (>1 000)
High(>0.1)
Light
SOIL
FLUSHING
High(>103)
Low(<100)
Dissolved and/ cr Liquid
Low(<1)
Shallow (< 3)
HYDRAULIC
BARRIERS
Short (< 1 )
Large (>1 000)
Low(<10'5)
Liquid
Low(<1)
Shallow (< 3)
High (>1 000)
Low(<10)
Low (< 2)
High (> 2)
_
Low (< 2)
EXCAVATION
Small (< 100)
Small (<100)
Shallow (< 3)
Far
Near
e.g., TJyhf . Gasoline. 'Haavy- . W Fuel Oil
TABLE 22. SUMMARY OF CRITICAL SUCCESS FACTORS FOR OPTIMUM PERFORMANCE I
-------
SECTION 4
MONITORING AND FOLLOW-UP MEASUREMENTS
INTRODUCTION
Once the most feasible soil treatment technologies have been
screened, field samples should be collected before final selection of a
corrective action technology and final design. Field samples are used to
verify soil and site conditions and to confirm assumptions that have been
made regarding the subsurface. It is always prudent to collect soil
samples and corings prior to making a large investment in a particular
technology.
The important parameters identified in this manual provide targets
for field sampling. For example, air conductivities greater than 10
cm/sec suggest a better chance that soil venting would likely be effective.
It is wise to measure this important parameter in the field before
proceeding with final design. After site conditions are confirmed with the
field data, or at least better understood, final selection and design can
proceed.
Specifying Cleanup Goals
Cleanup goals are established based on the extent of the site
specific health threat posed by the release. Once established, cleanup
goals will determine what treatment technology is ultimately needed, how
quickly the release needs to be cleaned up, and what removal efficiencies
are required. If an uncontrolled petroleum release poses a significant
health threat, then more urgent action will be needed. Hydraulic barriers
and/or soil venting may be installed to contain the release and control
vapors if immediate action is warranted. If the health threat is not
immediate, then soil treatment technologies such as biorestoration that
require more time, but tend to destroy rather than relocate hydrocarbons,
may be preferred.
Selecting Design Criteria
Once the cleanup goals have been set and the treatment technologies
selected, the next step is to develop the design criteria for the treatment
technology. Treatment technologies remove contaminants at varying rates,
and some contaminants may not be removed at all. A treatment technology is
usually designed to remove one or more specific constituents to a specified
level that is often set to conform to regulatory standards. Performance is
then evaluated by measuring the concentrations of each contaminant of
concern, and comparing those levels to cleanup goals.
Relative contaminant concentrations will change over time depending
on their chemical properties, so it is possible that different contaminants
will be used for design and for tracking performance at different stages of
the cleanup.
64
-------
Benzene, toluene, ethylbenzene and xylene (BTEX) are often used for
design criteria and performance monitoring, especially for gasoline
releases. These chemicals are aromatics and make up a significant portion
of petroleum products (as much as 20% by weight in gasoline). Based on
present knowledge, the toxicity of the BTEX compounds is orders of
magnitude greater than the other natural petroleum constituents. They are
also typically more soluble and mobile than other constituents. Of the
aromatics, benzene is of greatest concern since it is a known human
carcinogen. Benzene is also one of the more soluble and volatile
aromatics. It is almost always detected in the subsurface when a gasoline
UST release occurs. When measuring vapors, it is common to monitor BTEX
and the alkanes, total hydrocarbons (which includes all the aromatics) or
total volatiles.
With time, aromatics dissolve in water (solubilize), evaporate into
air (volatilize), and biodegrade until they cannot be detected. The
contaminants likely to remain are more complex, less soluble, and less
volatile than the aromatics. "More complex" constituents are constituents
that have greater molecular weights and more complex molecular structures.
They may eventually break down into simpler molecules. Constituents that
are more complex than the simple aromatics (like BTEX) include certain
additives like tetraethyl lead.
There are many different additives found in petroleum products, some
of which can pose significant health risks. It may be desirable to include
an additive in the design criteria or performance monitoring, but only on a
site by site basis after specific additives have been identified. Often
there will be little evidence to suggest the additive does or does not
present a health risk.
MONITORING PERFORMANCE AND PROGRESS
Once a technology has been selected, designed and installed, it is
essential that the performance of the treatment system be continually
evaluated to ensure that it is operating effectively. If a technology is
performing poorly, it may be due to an improper design arising from an
incomplete site assessment.
It may be necessary to re-examine the data that were collected for
the site assessment (see Figure 1) and collect more data to enhance the
site assessment. Design modifications may be warranted if the
understanding of site conditions changes appreciably.
Monitoring the treatment performance not only helps to evaluate how
effectively the technology is working, but also how the overall cleanup is
progressing. The results of the performance monitoring can be combined
with other information to track the overall progress of the cleanup.
Soil Venting
Typically, gas concentrations at the wells are measured in the form
of total hydrocarbons or total volatiles. Gauges can be placed on each of
the extraction wells to monitor the vacuum pressure and extraction rate.
Knowing the gas (or air) extraction rate and the concentration of the
65
-------
contaminants in the gas, the total pounds of contaminants removed can be
estimated. This can be compared with the estimate of total petroleum
product released to the environment to estimate how much product remains in
the subsurface.
Once soil gas concentrations begin to taper off, it should not be
assumed that the cleanup is over. Soil gas concentrations take time to
build up as shown in Figure 10. Once the vacuum is shut off, it is likely
that soil gas concentrations will increase again as equilibrium between the
soil vapor and liquid contaminants and dissolved contaminants is
re-established. Concentrations can often reach their original levels after
venting is stopped. Long-term monitoring is needed to ensure that as much
petroleum as possible has been recovered.
Biorestoration
In most cases, soil samples are collected and analyzed for the
constituents of concern. Carbon dioxide concentrations can be measured to
determine how much biological activity is occurring (aerobic biodegradation
produces carbon dioxide as one of its end products). Because
biorestoration can fail, it is important to collect additional samples and
analyze the availability of oxygen, nutrients, pH, redox potential, and
microbial populations to make sure that suitable conditions are maintained.
Soil Flushing
Leachate is analyzed for the specific contaminants of concern or for
groups of constituents like total hydrocarbons, with results reported as
weight of contaminant per volume of fluid. Knowing the volume of fluid
that has been processed, it is possible to estimate the total amount of
petroleum recovered.
Hydraulic Barriers
Contaminants are primarily recovered as residual liquid in this
method. The volume or mass of fuel is typically measured directly and
compared with estimates of total product released. However, the hydraulic
barrier method is not effective as a complete solution and is unlikely to
be the primary clean up technology. Product recovery by this method can
probably be discontinued when the rate of additional product recovered by
the system decreases to the point where long time periods are required to
allow significant amounts of product to be recovered.
Excavation
The purpose of excavation is to remove the contaminated soil until
concentrations in the remaining soil are below the cleanup criteria, until
excavation is no longer feasible, or until a stage is reached where other
remediation technologies are more appropriate. There are several
approaches to determine when a criteria level has been reached. Soil gas
concentrations in the excavation can be measured to determine whether they
exceed the cleanup criteria.
66
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o
<
Q.
V)
UJ
DC
O
o.
z
y>
DC
O
Q.
<
Z
o
m
oc
<
o
o
DC
Q
r
u.
O
g
<
cc
t-
UJ
o
o
o
VENTING
STARTS
VENTING
STARTS
VENTING
STARTS
VENTING
STOPS
TIME
Figure 10. How Concentrations of Hydrocarbon
Vapors Change During Soil Venting.
-------
An alternative approach, and a more accurate one, would be to measure
hydrocarbon concentrations in the soil both vertically and spatially in the
soil matrix (see Figure 11). The area needed to be excavated can be
interpolated from the vertical profiles (a cleanup level of 100 ppm of
total hydrocarbons was assumed in Figure 11).
68
-------
AREA TO BE
EXCAVATED *
* A CLEANUP LEVEL OF 100 PPM OF TOTAL HYDROCARBONS HAS BEEN ASSUMED
Figure 11. Determining Area to be Excavated Using Hydrocarbon Concentrations
-------
REFERENCES
Anastos, G., M.H. Corbin and M.F. Coia. In-situ Air Stripping: A New
Technique For Removing Volatile Organic Chemicals From Soils. In:
Superfund 86. 1986.
API. Enhancing the Microbial Degradation of Underground Gasoline by
Increasing Available Oxygen. Prepared by Texas Research Institute. 1982.
Bossert, I., R. Bartha The Fate of Petroleum in Soil Ecosystems. Petroleum
Microbiology. New York: Macmillan. 1984.
Camp, Dresser & McKee Inc. CDMnews, 1988, Vol. 22, pp. 8.
Conner, R.J., Case Study on Soil Venting. Pollution Engineering, 1988,
Vol. 20, No. 7, pp. 74-78.
Electric Power Research Institute (EPRI). Remedial Technologies For Leaking
Underground Storage Tanks. Chelsea, Michigan: Lewis Publishers. 1988.
Ellis, W.D., J.R. Payne, A,N. Tafuri, and F.J. Freestone. The Development
of Chemical Counter-Measures for Hazardous Waste Contaminated Soil.
EPA-600/D-84-039. Municipal Environmental Research Laboratory, U.S. EPA,
Cincinnati, OH. 1984.
EPA. Estimation of the Public Health Risk from Exposure to Gasoline Vapor
via the Gasoline Marketing System (draft), Office of Health and
Environmental Assessment, Office of Air Quality Planning and Standards,
Washington, DC. 1984.
EPA. Handbook, Remedial Action at Waste Disposal Sites (Revised).
EPA/625/6-85/006. Hazardous Waste Engineering Research Laboratory, Office
of Research and Development, Cincinnati, OH. 1985.
EPA. Systems to Accelerate In Situ Stabilization of Waste Deposits.
EPA/540/2-86/002. Hazardous Waste Engineering Research Laboratory,
Office of Research and Development, Cincinnati, OH. 1986.
EPA. A Compendium of Superfund Field Operations Methods. Washington, D.C.
1987a.
70
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EPA. Underground Storage Tank Corrective Action Technologies.
EPA/625/6-8/015. Prepared by PEI Associates for HWERL, ORD, Cincinnati,
OH. 1987b.
EPA. Field Evaluation of Terra Vac Corrective Action Technology at a
Florida UST Site (Draft). Prepared by Camp Dresser & McKee for HWERL,
ORD, Edison, NJ. 1987c.
EPA. Draft Seminar Proceedings on the Underground Environment of an UST
Motor Fuel Release. Prepared by Camp Dresser & McKee and PEI Associates
for HWERL, ORD, Edison, NJ. 1987d.
EPA. Draft Internal Working Document Loci Research Report on Mobilization,
Immobilization, and Transformation of Motor Fuel Constituents and Organic
Chemicals in the Subsurface Environment. Prepared by PEI Associates Inc.
for HWERL, ORD, Edison, NJ. 1988a.
EPA. Superfund Record of Decision Update. USEPA, Hazardous Site Control
Division, 4:2 (March 30, 1988). 1988b.
EPA. Cleanup of Releases from Petroleum USTs: Selected Technologies.
EPA/530/UST-88/001. USEPA. Office of Underground Storage Tanks.
Prepared by Camp Dresser & McKee Inc. April 1988c.
EPA. Estimating Air Emissions From Petroleum UST Cleanups. U.S. EPA,
Office of Underground Storage Tanks, Washington, D.C. 1989.
Ghassemi, M., A. Phanahloo, S. Quinlivan. Physical and Chemical
Characteristics of Some Widely Used Petroleum Fuels: a Reference Data
Base for Assessing Similarities and Differences between Synfuel and
Petrofuel Products, Energy Sources, 1984, 7, pp. 377-401.
Heath, R.C. Basic Ground-Water Hydrology. Water Supply Paper 2220. United
States Geological Survey. 1984. pp. 71.
Hillel, Daniel. Fundamentals of Soil Physics. Harcourt Brace and
Jovanovich. New York. 1980.
Hoag, G. E., C.J. Bruell, M.C. Marley. A study of the Mechanisms
Controlling Gasoline Hydrocarbon Partitioning and Transport in
Groundwater Systems, University of Connecticut, Prepared for the United
States Geological Survey, Reston, VA, 1984.
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ICF, Inc. Petroleum Data Collection and Assessment of Risk Approach.
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Krishnayya, A.V., M.J. O'Connor, J.G. Agar and R.D. King. Vapour-
Extraction Systems: Factors Affecting Their Design and Performance.
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Groundwater: Prevention, Detection and Restoration, 1988.
Lyman, W.J., W.F. Reehl, and D.H. Rosenblatt. Handbook of Chemical Property
Estimation Methods. McGraw-Hill Book Co. New York. 1982.
Major, D.W., C.I. Mayfield, J.F.Barker. Biotransformation of Benzene by
Denitrification in Aquifer Sand. Ground Water.26(1): 8-14. 1988.
Munz, C. and P.V. Roberts. Air-Water Phase Equilibrium of Volatile Organic
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69. 1987.
Nyer, E.K. Groundwater Treatment Technology. New York: Van Nostrand
Rheinhold. 1985.
Olsen, R.L., p.R. Fuller, E.J. Hinzel, and P. Smith. Demonstration of Land
Treatment for Hazardous Waste. Superfund 86. 1986.
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APPENDIX A
HOW TO USE THIS MANUAL: A SAMPLE PROBLEM
This manual discusses the selection of an appropriate corrective
action for contamination in the unsaturated zone. The purpose of this
appendix is to help the user apply this information to actual situations.
Although the case study that is presented is hypothetical, it represents
events and conditions likely to occur during an actual contaminant release.
The user is guided through a step-by-step process to help narrow the range
of treatment technologies. As stated earlier, this manual does not address
emergency measures required in response to a leaking UST. It is assumed
that these measures have been completed prior to the selection process.
There are many, sometimes complex, factors to consider during the
selection process. To accurately assess the conditions at a site, and
therefore, select the best corrective action, field sampling and analysis
is required. In addition, a degree of engineering judgment and scientific
interpretation of the collected information is needed. Therefore, it is
important to understand that a corrective action should not be selected
based solely on information provided in this maniiaF!Preliminary
conclusions from this manual should be reviewed by experienced engineers
and hydrogeologists and validated with actual field data.
This manual is set up to aid in the selection of a corrective action;
it can be used as a tool to assess the feasibility of a proposed corrective
action or provide a quick preliminary assessment of the various
alternatives. For these purposes, it is not necessary to conduct extensive
field studies or perform detailed investigations to use this manual.
Information provided in the text of this report along with information
readily available through resources such as local EPA, USGS or state
environmental offices can be used to assess the relative merits of
different corrective actions in a comparatively short time and at minimal
cost.
The case study presented is hypothetical. Many assumptions were
made concerning various factors or parameters associated with the selection
process. Because of this, it is more a quick preliminary screening than an
actual detailed selection process. Where possible, estimates for the
different parameters were obtained from information in the text. For other
parameters, it was assumed that some simple field tests were conducted to
estimate their value. The source of information for all parameters is
identified.
An important consideration in attempting to select a corrective
action is whether to base the analysis on the characteristics of the
contaminant mixture or one or more "targeted" constituents. In this
sample problem and in most real-world cases, the analysis will be based on
the hydrocarbon mixture, particularly if the release is recent. However,
it is unlikely than one corrective action will be best suited for removing
every constituent. After a preliminary assessment has identified the most
promising technologies, it may be prudent to check their effectiveness at
73
-------
removing selected contaminants of concern. This can be fairly easily
accomplished using the worksheets provided since most of the input to the
worksheets would be the same as for the initial assessment, the only new
input being contaminant-specific parameters.
DESCRIPTION OF THE CASE
The setting of the case study is a northeastern, suburban community
with a population of about 50,000. A rapid release of 500 gallons of
gasoline occurred due to rupture of an UST in June. Authorities were
immediately notified. The release occurred at a gasoline station near a
relatively busy intersection. There are several homes and small commercial
businesses in the proximity of the gasoline station.
DESCRIPTION OF THE METHODOLOGY
Step 1: Collect Information About the Release
The first step after emergency response action is to conduct a site
investigation. The release-related information necessary to initiate the
investigation is listed in Table 1. For this example, the following is
known:
l(a) The contaminant is automotive gasoline.
l(b) The volume of contaminant released was estimated to be 500
gallons. The release was due to rupture of an UST by heavy
machinery.
l(c) The nature of release was a quick spill.
l(d) The spill was immediately detected by workers at the gasoline
station.
l(e) The spill occurred one month ago.
In addition to the above information, it was determined that none of
the contaminant was recovered during the initial emergency response to the
spill. The gasoline quickly infiltrated into the soil at the site.
Step 2; Collect Information About the Site
Table 3 (p.14) lists parameters necessary to characterize the site.
Table 4 (p.15) presents estimates of various physical and geological
parameters related to the unsaturated zone. For this example, all of the
site-related parameters were estimated from readily available information.
To estimate many of the site-specific parameters, soil type must be
known. USGS and Soil Conservation Service geologic maps of the area show
the subsurface to be predominantly sand and gravel to a depth of 6 meters
(20 feet). A single shallow well was installed and the depth to
groundwater was estimated to be 5 meters (16 feet). Once the soil type is
known, the remaining site-specific parameters may be estimated:
74
-------
2(a) Porosity = 35 percent (Average for sand and gravel from
Table 3, p.14)
2(b) Particle Density = 2.65 g/cm3 (Average for sand and gravel from
Table 3)
2(c) Bulk Density = 1.75 g/cm3 (Average for sand and gravel from
Table 3)
2(d) Hydraulic Conductivity =0.01 cm/sec (Average for sand and
gravel from Table 3)
2(e) Permeability = 10"4 cm2 (Average for sand and gravel from
Table 3)
2(f) Depth to Groundwater Table = 5 m (16 feet). This value was
assumed to be measured. Often depth to groundwater can be
obtained from local sources such as USGS records.
2(g) Soil Moisture Content = 10 percent. This value was obtained
from Figure 4 (p. 17) for the sand soil type using field
capacity as an estimate of moisture content.
2(h) Soil Surface Area = 0.005 m2/g. This value was obtained from
Table 4 (p. 15) for the medium to coarse sand soil type.
2(i) Rainfall Infiltration Rate =0.14 cm/day. Infiltration rates
can usually be obtained from local sources such as the USGS.
In this case, an approximation of 50 percent of total daily
rainfall for the northeastern U.S. (0.28 cm/day) was assumed
for the infiltration rate. It is important to identify actual
rainfall occurrences within the last month or two, particularly
for recent spills. This will show whether historic data
reflects what has actually happened at the site. In the
northeast, rainfall patterns are quite consistent throughout
the year and historic data is assumed adequate for the
assessment. Also, paved areas should be noted because
infiltration may be reduced significantly.
2(j) Soil temperature = 8ฐC. Soil temperature was estimated from
Figure 3 (p.16) for the northeastern U.S. and could also be
easily measured.
2(k) Organic Content = 0.2%. The organic content of a soil is
difficult to estimate without collecting and analyzing soil
samples. Because the organic content of a sand and gravel soil
type is usually relatively low, a value of 0.2 percent (near
the low end of the scale) was assumed for this example.
Ordinarily, the organic content of a soil must be determined
from field samples.
2(1) Soil pH = 5.5 This parameter can easily be measured through a
simple field analysis. For this example, a pH value of 5.5 was
assumed.
75
-------
2(m) The presence of rock fractures in the subsurface can be
estimated from readily-available local geographic information.
It is assumed that there are no rock fractures in this case
study.
Step 3: Collect Information About the Contaminants
Table 5 (p. 18) lists contaminant-specific parameters. The specific
table or tables from which a particular parameter value may be estimated is
also presented in Table 5 in the default source column.
In step l(a), the released contaminant was determined to be
automotive gasoline. For a preliminary site assessment, properties of the
bulk product rather than individual constituents are used. Estimates for
the parameters found in Table 5 for gasoline are as follows:
3(a) Vapor pressure = 469 mm Hg (Average from Table 7, p.21)
3(b) Water Solubility =158 mg/L (Average from Table 7)
3(c) Viscosity =0.45 cPoise (Average from Table 7)
3(d) Liquid Density =0.73 g/cm3 (from Table 7)
3(e) Vapor Density = 1,950 g/m3 (Average from Table 7)
3(f) *Soil Sorption Constant =38.1 L/kg (from Table 8, p.22)
3(g) Refractory Index =0.02 (From Table 9, p.23)
Values for the parameters in Steps 3(a) through 3(e) could as easily
been obtained for a constituent of gasoline by using Table 6 (p.20) and
Table 8 (p.22). Table 6 lists major constituents of various petroleum
products and Table 8 lists parameter values for many of the constituents.
The remainder of the assessment would be carried out as will be done for
the bulk product. A technology that looked promising at removing the bulk
product could be checked to see if it was equally effective in removing a
contaminant of concern.
NOTE: *Information on Soil Sorption Constant for a gasoline mixture is not
readily available. Sorption characteristics of a substance are related to
its molecular weight. In order to provide an estimate of the Soil Sorption
Constant for gasoline, a constituent of gasoline for which the constant was
known and which is similar to gasoline in molecular weight was used. The
molecular weight of benzene (78 g/mole) is similar to the molecular weight
of gasoline (approximately 100 g/mole). Also, benzene is a known human
carcinogen. Therefore, by substituting the Soil Sorption Constant of
benzene for that of gasoline, a reasonable approximation is made and the
analysis is better focused on a known health threat.
76
-------
All the information collected in Steps l(a) through 3(g) can be used
to complete the worksheets provided in this manual. To illustrate, Tables
A-l through A-ll are presented here with the appropriate information filled
in. Additional copies of all blank worksheets are provided at the back of
this manual.
Step 4: Evaluate Contaminant Phase Distribution (Tables A-l to A-3)
Once a site investigation has been conducted and site-specific and
contaminant-specific information has been assembled, an evaluation of the
contaminant phase distribution should be made.
A qualitative evaluation of phase distribution can be accomplished by
determining the likelihood of the contaminant being in one of the three
phases: residual liquid, vapor, or dissolved in pore water. Table 11
(p.26), Table 12 (p.27) and Table 13 (p.28) can be used to demonstrate the
likelihood of each phase being present as discussed in Section 2 of the
manual.
For this example, Tables 11, 12, and 13 have been used as worksheets
to determine the likelihood of the contaminant being present in any one of
the three phases and are included as Tables A-l, A-2, and A-3 in the case
study. As shown in the worksheets, there is no dominant pattern indicating
that a majority of the contaminant would be present in any one phase. It
is likely that some portion of the contaminant exists in each phase.
Because the spill was recent, it is likely most of the contaminant is in
the residual liquid phase, and this is assumed in the assessment.
Step 5: Determine Contaminant Mobility (Tables A-4 to A-6)
The mobility of contaminants in each of the phases is influenced by
different factors. For this reason, each phase is addressed separately.
Residual Liquid Mobility. The relative degree of mobility can be
estimated through use of Table 14 (p. 31). The table is set up the same as
Tables 11, 12, and 13. Table A-4 shows values from this case study. As
can be seen, the factors are distributed throughout the three right-most
columns, indicating likely migration from the residual liquid phase.
Vaoor Mobility. Table 15 (p. 33) can be used to estimate vapor phase
mobility in the same manner that Table 14 demonstrated residual liquid
phase mobility. Table A-5 presents the results from the case study. It
shows that three of the five factors fall within the middle range of mobility
and two fall within the high range. The worksheet indicates the vapor
phase of the contaminant is relatively mobile.
Dissolved Mobility. Mobility of the contaminant dissolved in pore
water can be estimated through the use of Table 16 (p. 35). Table A-6
presents the factors determined from the case study. The majority of the
factors indicate that there is a potential for a high degree of dissolved
contaminant mobility.
77
-------
TABLE A-1. LIKELIHOOD OF LIQUID CONTAMINANTS BEING ^M\
PRESENT IN THE UNSATURATED ZONE WjF
FACTOR
RELEASE- RELATED
Amount Released
Rate Of Release
Time Since Release
SITE - RELATED
Depth To Groundwater
Hydraulic Conductivity
Rainfall Infiltration Rate
Soil Temperature
Soil Sorption Capacity
(Surface Area)
CONTAMINANT- RELATED
Liquid Viscosity
Liquid Density
Vapor Pressure
Water Solubility
UNITS
gallons
months
meters
cm/sec
cm/day
ฐC
m2/g
cP
3
g/cm
mm Hg
mg/L
SITE OF
INTEREST
5oo
jrtSTv\\Aot*t>!s
or^e.
5"
lo'2-
o.tf
B
0.005-
o.i5
0.12,
4^
158
INCREASING LIKELIHOOD ^
Small
(<100)
O
Slow Release
O
Long
(>12)
O
Medium
(100-1000)
0
Medium
(1-12)
Large
(>1000)
O
Instantaneous
Release
Short
(<1)
O
Shallow
(<1)
O
High
(>10-3)
High
(>0.1)
Warm
(>20)
O
Low
(<0.1)
Medium
(1-5)
Medium
(10-5-10'3)
O
Medium
(0.05 -0.1)
0
Medium
(10-20)
O
Medium
(0.1 1)
O
Deep
(>5)
O
Low
(<10-5 )
0
Low
(<0.05)
O
Cool
(<10)
High
(>1)
O
Low
(<2)
High
(>2)
O
High
(>100)
High
(>1000)
O
Medium
(2-20)
O
Medium
(1-2)
O
Medium
(10-100)
O
Medium
(100-1000)
High
(>20)
O
Low
(<1)
Low
(<10)
O
Low
(<100)
O
78
-------
TABLE A-2. LIKELIHOOD OF CONTAMINANT VAPORS BEING f59&
PRESENT IN THE UNSATURATED ZONE RKJl
FACTOR
RELEASE- RELATED
Amount Released
Rate Of Release
Time Since Release
SITE -RELATED
Depth To Groundwater
Air Conductivity
Rainfall Infiltration Rate
Soil Temperature
Soil Sorptlon Capacity
(Surface Area)
CONTAMINANT- RELATED
Liquid Viscosity
Liquid Density
Vapor Pressure
Water Solubility
UNITS
gallons
months
meters
cm/sec
cm/day
ฐC
m /g
cP
3
g/cm
mm Hg
mg/L
SITE OF
INTEREST
500
Jrfyk'N'TWovi.S
OflC
s
lo-3
OJ4
8
0,ooฃ
OAS
0.73
-f12)
O
Shallow
(<1)
O
High
(>104)
High
(>0.1)
Cool
(<10)
Low
(<0.11)
Medium
(100-1000)
O
Medium
(1-12)
Medium
(1-5)
Medium
(10-6-10"ป)
O
Medium
(.005-0.1)
O
Medium
(10-20)
O
Medium
(0.1-1)
O
Large
(>1000)
O
Instantaneous
Release
Short
(<1)
0
Deep
<>5)
O
Low
(<10-6)
O
Low
(<0.05)
0
Warm
(>20)
O
High
(>1)
0
High
(>20)
O
High
(>2)
0
Low
(<10)
O
High
(>1000)
Medium
(2-20)
O
Medium
(1-2)
O
Medium
(10-100)
O
Medium
(100-1000)
O
Low
(<2)
Low
(<1)
High
(>100)
Low
(<100)
O
79
-------
TABLE A-3. LIKELIHOOD OF CONTAMINANTS DISSOLVED IN PORE
WATER BEING PRESENT IN THE UNSATURATED ZONE
FACTOR
RELEASE- RELATED
Amount Released
Rate Of Release
Time Since Release
SITE - RELATED
Depth To Groundwater
Moisture Content
Soil Porosity
Rainfall Infiltration Rate
Soil Sorptlon Capacity
(Surface Area)
CONTAMINANT- RELATED
Liquid Viscosity
Liquid Density
Vapor Pressure
Water Solubility
UNITS
gallons
months
meters
% volume
% volume
cm/day
ma/g
cP
g/cm3
mm Hg
mg/L
SITE OF
INTEREST
5oo
&\5\<(*'\tW\tO<4$
0*C
s
10
35
0.14
0.005
0.45
0,73
^^
158
XKMK4.
fMMM!
JMMHJ
SHEHS
JJ/VjMj
^LffJgJt*
INCREASING LIKELIHOOD >^
Small
(100)
0
Instantaneous
Release
Long
(>12)
0
Medium
(100-1000)
O
Medium
(1-12)
Large
(>1000)
O
Slow Release
0
Short
(<1)
0
Shallow
(<1)
o
Low
(<10)
O
Low
(<20)
O
Low
(<0.05)
O
Low
(<0.1)
Medium
(1-5)
Medium
(10-30)
Medium
(20-40)
Medium
(0.05-0.1)
O
Medium
(0.1-1)
O
Deep
(>5)
O
High
(>30)
O
High
(>40)
O
High
(>0.1)
High
(>1)
O
High
(>20)
O
High
(>2)
O
High
(<100)
Low
(<100)
0
Medium
(2-20)
O
Medium
(1-2)
O
Medium
(10-100)
O
Medium
(100-1000)
Low
(<2)
O
Low
(<1)
Low
(>10)
O
High
(>1000)
O
80
-------
TABLE A-4. FACTORS TO EVALUATE THE MOBILITY OF ^J^
LIQUID CONTAMINANTS W
FACTOR
UNITS
SITE OF
INTEREST
RELEASE RELATED
Time Since Release
Months
OAZ.
SITE- RELATED
Hydraulic Conductivity
Soil Porosity
Soil Surface Area
Soil Temperature
Rock Fractures
Moisture Content
cm/sec
% Soil
Volume
m2/g
ฐC
% Volume
/o-2
3>$
o.oo 5
8
At-se^
\o
CONTAMINANT- RELATED
Liquid Viscosity
Liquid Density
cPoise
g/cm
0.1 S
O.~73
INCREASING MOBILITY ^
Long
(>12)
O
Medium
(1-12)
Short
(<1)
O
Low
(<10-5)
O
Low
(<10)
O
High
(>1)
O
Low
(<10)
Absent
High
(>30)
O
Medium
(1CT5-10'3)
0
Medium
(10-30)
O
Medium
(0.1-1)
O
Medium
(10-20)
0
o
Medium
(10-30)
O
High
(>10-3)
High
(>30)
Low
(<0.1)
High
(>20)
O
Present
O
Low
(<10)
O
High
(>20)
O
Low
(<1)
Medium
(2-20)
0
Medium
(1-2)
O
Low
(<2)
High
(>2)
O
81
-------
TABLE A-5. FACTORS TO EVALUATE THE MOBILITY OF
CONTAMINANT VAPORS
FACTOR
UNITS
SITE OF
INTEREST
ffi
lUsf
INCREASING MOBILITY ^
SITE - RELATED
Air Filled Porosity
- Total Porosity *
Water Content *
Depth Below Surface
% Volume
% Volume
% Volume
meters
25
35
10
5
Low Mec
(<10) (10-
0 1
Low Mec
(<10) (10-
o c
High Mec
(>30) (10-
0 I
Deep Mec
(>10) (2-
0 4
CONTAMINANT- RELATED
Vapor Density
3
g/m
ijso
Low Mec
(< 50) (50-
o c
" the total porosity less that fraction filled with water equals the air filled porosity
iium High
30) (> 30 )
1 0
Jium High
30) (> 30 )
)
!ium Low
30) (<10)
ป O
ium Shallow
10) (<2)
> O
ium High
500) (>500)
)
82
-------
TABLE A-6. FACTORS TO EVALUATE THE MOBILITY
OF CONTAMINANTS IN PORE WATER
FACTOR
UNITS
SITE OF
INTEREST
6&t&!&&,
JSSHS
jsssa
SESsg
SHMM?
'*u&guy
INCREASING MOBILITY ^
SITE RELATED
Hydraulic Conductivity
Moisture Content
Rainfall Infiltration Rate
Soil Porosity
Rock Fractures
Depth Below Surface
cm/sec.
% Volume
cm/day
% Volume
meters
\o~*
lo
O.j-t
3ฃ
Abseil
ฃ
Low
(10-5)
o
Low
(<10)
O
Low
(<0.05)
O
Low
(<10)
O
Absent
ฎ
Shallow
(<2)
O
Medium
(10-5- 10-3)
O
Medium
(10-30)
ฎ
Medium
(0.05-0.1)
O
Medium
(10-30)
O
o
Medium
(2-10)
6
High
(>10"3)
ซ
High
(>30)
O
High
(>0.1)
ฎ
High
(>30)
ฎ
Present
O
Deep
(>10)
O
CONTAMINANT- RELATED
Water Solubility
mg/L
\S8
Low
(< 100)
O
Medium
(100 to 1000)
High
(>1000)
O
83
-------
Step 5 results suggest that conditions are conducive for the
contaminants to move into and out of each phase. Because it is a
relatively new release, most of the product is likely to still be in the
liquid contaminant phase. Site conditions are such that liquid
contaminants can move easily into both the vapor and dissolved phases.
These results have implications for cleanup, as discussed below.
Step 6; Evaluate Soil Treatment Technologies
The information gained from the previous steps which can be useful in
selecting possible corrective actions. Section 3 presents a detailed
description of the five types of corrective actions which are discussed in
this manual. Included in the description of the various corrective actions
are some of the unique factors that may need to be addressed before final
selection can be made.
In this step, each of the corrective actions will be evaluated for
their likelihood of success in removing the contaminant of concern, in this
case gasoline. Tables 17 through 21 list a unique set of critical success
factors (CSFs) for each corrective action. These worksheets can help
determine the likely effectiveness of each technology of a particular site.
Copies of the worksheets are provided here as Tables A-7 through A-ll for
use in the evaluation process. Additional copies of all worksheets are
provided at the back of the manual.
Because of the unique character of each site analysis, the worksheets
will not always indicate whether a particular technology will likely
succeed or fail. Typically, some CSFs will indicate a high degree of
success while others will indicate the opposite. For this reason, it is
better to complete each worksheet and conduct a comparative analysis
between technologies, identifying one or more which are more likely to
succeed.
The information required to complete the worksheets is available from
the steps outlined earlier. Information for determining the CSFs not
already introduced is presented below.
Proximity of Above and Below Ground Structures - as was described
initially in the case study, there are several businesses
proximate to the release site. Also, because the site is close to
a relatively busy intersection, buried cables and pipes are likely
to exist in the area.
Volume of Contaminated Soil - a rough estimate of this CSF is
obtained by multiplying the depth to groundwater by an
approximation of the surface area over which the release occurred.
From Step 2(f), the estimated depth of contaminated soil is about
5 yd (4.6m). The ground surface area exposed to contamination was
estimated to be approximately 25 yd2 (21 m2). Volume = 5 yd (4.6
m) x 25 yd2 (21 m ) = 125 yd* (97 m3)
84
-------
TABLE A-7. WORKSHEET FOR EVALUATING
THE FEASIBILITY OF SOIL VENTING
BEING EFFECTIVE AT YOUR SITE
CRITICAL SUCCESS
FACTOR
SITE RELATED
Dominant
Contaminant Phase
Soil Temperature
Soil Air Conductivity
Moisture Content
Geological
Conditions
Soil Sorptlon Capacity
- Surface Area
Depth to Ground Water
UNITS
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
Phase
ฐC
cm/sec.
% volume
2
m /g
meters
1-1 Q (^ , c\
8
)o-3
JO
/4ssu./vJ
nฃTฃro 53*1 e30)
O
Heterogeneous
High
(>D
0
Low
(<1)
O
SUCCESS
SOMEWHAT
LIKELY
Liquid
Medium
(10-20)
0
Medium
(lO^-IO"4)
O
Moderate
(10-30)
9
O
o
Medium
(1-5)
(H
Igg;
SUCCESS
MORE
LIKELY
Vapor
o
High
(>20)
O
High
(> 10'4)
Dry
(<10)
O
Homogeneous
0
Low
(<0.1)
High
(>5)
O
CONTAMINANT- RELATED
Vapor Pressure
Water Solubility
mm Hg
mg/L
+tf
158
Low
(<10)
O
High
(>1000)
O
Medium
(10 to 100)
O
Medium
(100-1000)
High
(> 100)
Low
(< 100)
O
OTHER CONSIDERATIONS .TrealTOrtcanb9do0n,ซ9
Cost Is from $15 to $60 per cubic yard. Care must be taken to avoid explosions because vapors
Efflcttveness decreases after several months of treatment. are concentrated
Capable of removing thousands of gallons. Cleanup takes time so that this technology Is not
Air emissions will likely need to be treated with GAC. appropriate when emergency response Is needed
85
-------
TABLE A-8. WORKSHEET FOR EVALUATING fST^
THE FEASIBILITY OF BIORESTORATION jm
BEING EFFECTIVE AT YOUR SITE \S/
CRITICAL SUCCESS
FACTOR
RELEASE - RELATED
Time Since Release
SITE RELATED
Dominant
Contaminant Phase
Soil Temperature
Soil Hydraulic
Conductivity
Soil pH
Moisture Content
UNITS
Months
Phase
ฐC
cm/sec.
pH Units
% Volume
SITE OF
INTEREST
Qr\0
Lr\A
8
-2.
) 0
^.5
|o
SUCCESS
LESS
LIKELY
Short
(< 1)
O
Liquid
*
Low
(<5)
O
Low
(<10~5)
O
(< 6 or > 8)
Dry
(< 10)
0
SUCCESS
SOMEWHAT
LIKELY
Medium
(1 -12)
Vapor
O
Medium
(5-10)
Medium
(10'5-10'3)
O
o
Moderate
(10 to 30)
SUCCESS
MORE
LIKELY
Long
(> 12)
O
Dissolved
O
High
(> 10)
0
High
MO"3)
9
(6-8)
O
Moist
(>30)
0
CONTAMINANT- RELATED
Solubility
Blodegradabillty
- Refractory Index
Fuel Type
mg/L
3imensionless
I ฃ6
o.oz.
j
Low
(<100)
0
Low
O
No. 6 Fuel Oil
(Heavy)
O
Medium
(100 to 1000)
Medium
(0.01 to 0.1)
No. 2 Fuel Oil
(Medium)
0
High
(> 1000)
O
High
0
Gasoline/ Diesel
(Light)
OTHER CONSIDERATIONS
Cost is from $60 to $1 25 per cubic yard.
Completely destroys contaminants under optimal conditions
Effectiveness varies depending on subsurface conditions
Biologic systems subject to upset
Public opinion sometimes against putting more chemicals in ground
Difficult to monitor effectiveness
Minimizes health risk by keeping contaminants in ground and on site
- Takes long time to work not for emergency response
86
-------
TABLE A-9. WORKSHEET FOR EVALUATING
THE FEASIBILITY OF SOIL FLUSHING
BEING EFFECTIVE AT YOUR SITE
CRITICAL SUCCESS
FACTOR
UNITS
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
SUCCESS
SOMEWHAT
LIKELY
(B
miilljj
SUCCESS
MORE
LIKELY
SITE RELATED :
Dominant
Contaminant Phase
Soil Hydraulic
Conductivity
- Soil Surface Area
Carbon Content
Fractures In Rock
Phase
cm/sec.
m2/g
% Weight
2-(<^ivd
/o-2
O.005
0.2-
Akse^t
Vapor
O
Low
(<10's)
O
High
(>1)
O
High
(>10%)
0
Present
O
Liquid
Medium
(10-5-10'3)
O
Medium
(0.1-1)
O
Medium
(1 - 10%)
O
o
Dissolved
O
High
MO-3)
ซ
Small
(<0.1)
Low
(<1%)
%
Absent
ซ
CONTAMINANT- RELATED
Water Solubility
Sorptlon Characteristics
- Soil Sorptlon Constant
Vapor Pressure
Liquid Viscosity
Uquid Density
mg/L
L/kg
mm Hg
cPoise
g/cm3
1 58
38
Itf
0.^5
onz
Low
(<100)
O
High
(>1 0,000)
O
High
(> 100)
High
(>20)
O
Low
(<1)
Medium
(100 to 1000)
Medium
(100-10,000)
O
Medium
(10-100)
O
Medium
(2 -20)
O
Medium
(1-2)
O
High
(> 1,000)
O
Low
(< 100)
Low
(<10)
0
Low
(<2)
ซ
High
(>2)
0
OTHER CONSIDERATIONS
Cost is from $150 to $200 per cubic yard.
* Using surfactants may increase effectiveness
Effluent requires separation techniques such as distillation, evaporation, centrifugation
Most effective when used ex-situ (above ground)
87
-------
TABLE A-10. WORKSHEET FOR EVALUATING
THE FEASIBILITY OF HYDRAULIC BARRIERS
WORKING AT YOUR SITE
CRITICAL SUCCESS
FACTOR
RELEASE - RELATED
Time Since Release
Volume of Spill
SITE RELATED
Dominant
Contaminant Phase
Soil Hydraulic
Conductivity
Soil Sorptlon Capacity
- Surface Area
Carbon Content
Temperature
Depth to Groundwater
UNITS
months
gallons
phase
cm/sec.
m2/g
% weight
ฐC
meters
SITE OF
INTEREST
one.
Soo
Ll^
|o-z
0.0 OS
0.2.
8
s
SUCCESS
LESS
LIKELY
Long
(> 12 months)
O
Small
O
Vapor
O
High
(> 10'3)
High
"o
High
(> 10%)
0
Low
O
High
O
SUCCESS
SOMEWHAT
LIKELY
Medium
(1 - 12 months)
Medium
(100 - 1000)
ft
O
Medium
(10-5-10~3)
O
Medium
(0.1-1)
O
Medium
(1-10)
O
Medium
(5-10)
Medium
(1-5)
90ฎffi0
SUCCESS
MORE
LIKELY
Short
O
Large
(> 1000)
O
Liquid
Low
O
Low
Low
High
"o
Low
0
CONTAMINANT- RELATED
Liquid Viscosity
c Poise
0.45
High
(>60)
Medium
(2 to 20)
O
Low
OTHER CONSIDERATIONS
Cost is from $10 to $90 per cubic meter.
Only affects liquid portion of release not portion sorbed to soil.
Typically limited to shallow (<3 meters) depths.
Not effective in removing contaminants to low levels.
Most effective when contamination is confined to small areas.
Not effective for #6 fuel oil and other viscous fluids.
Not effective if contamination is greater than 15 meters deep.
-------
TABLE A-11. WORKSHEET FOR EVALUATING
THE FEASIBILITY OF EXCAVATION
BEING EFFECTIVE AT YOUR SITE
CRITICAL SUCCESS
FACTOR
UNITS
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
SUCCESS
SOMEWHAT
LIKELY
SUCCESS
MORE
LIKELY
SITE RELATED
Buildings nearby
Buried pipes
and cables
Proximity of Above and
Below Ground Structures
o
No nearby
structures
O
Volume of Soil
Contaminated
Cubic
Meters
Large
(> 1,000)
O
Medium
(100 -1,000)
O
Small
Depth of Contamination
Meters From
Surface
Deep
(>5)
O
Medium
(1-5)
Shallow
(<1)
O
Proximity of Site to
- Traffic
Near
Far
O
- Businesses
Near
Far
O
Disposal Site
Far
Near
O
- Backfill Source
Far
Near
O
OTHER CONSIDERATIONS
Cost is from $50 to $300 per cubic yard.
Appropriate when urgent response is necessary
Brings contaminants to surface, thereby increasing exposure risks
Significant amounts of surface area disturbed relative to depth excavated
Requires suitable means of disposal. This is becoming increasingly difficult
because some landfill operators consider petroleum-laden soil to be a hazardous waste.
89
-------
Proximity of Site to Disposal Site - this is important in that
disposal costs of hazardous substances are expensive and are
likely to become more so. For this case study, the assumption was
made that an adequate disposal site was at a great enough distance
to significantly increase the cost of the corrective action.
A review of the completed worksheets shows that soil venting and soil
flushing are likely to be the most effective soil treatment technologies
given the present understanding of the site. The worksheets for these
technologies show numerous factors where success is "somewhat" to "more"
likely. Site conditions may also be amenable to using a hydraulic barrier
method to recover some residual liquid. The worksheet for biorestoration
is inconclusive; none of the three "likelihood" columns definitively
indicate success or failure of the method. Excavation at the site appears
unlikely since most of the CSFs indicate success to be less likely. A
brief analysis of the results of the worksheets for each technology is
presented below.
Soil Venting - Site conditions are conducive for soil venting to be
effective at this site. The worksheet suggests that effectiveness could be
improved if soil temperature could be increased somewhat. Further, it was
assumed that heterogeneous soil conditions exist. It would be prudent to
collect soil borings to better define the substrata. If clay layers or
lens exist at the site of interest, then the effectiveness of soil venting
could be "short-circuited." The less permeable clay would tend to retard
and restrict the movement of air and limit the distribution of nutrients
throughout the contamination zone.
Biorestoration - Conditions at the site suggest that biorestoration
might be effective if soil temperature and pH could be made more favorable.
The relatively high hydraulic conductivity shows that oxygen and nutrients
can be readily distributed throughout the contaminated zone to promote
microbial activity. This option may be more desirable if used in
conjunction with biorestoration of groundwater in the saturated zone, where
addition of nutrient and oxygen laden water to the unsaturated zone could
serve to flush contaminants down to the saturated zone as well as
biodegegrade them.
Soil Flushing - Site conditions are such that soil flushing is likely
to be highly effective at this site, although the density of the released
petroleum is somewhat low. The solubility, on the other hand, is
sufficiently high so that much of the contamination could be leached out of
the soil.
Hydraulic Barriers - The worksheet suggests that site conditions are
SU(Ph that hydraulic' barriers might be effective at this site. Because the
spill was recent, much of the contaminant is likely to be in the residual
liquid_phase. If further analysis showed that a collection trench could be
dug quickly and easily, some of the residual liquid could be recovered with
minimal cost and effort.
90
-------
Excavation - The worksheet suggests that excavation is not a suitable
alternative at this site. Buildings and buried cables are nearby, and a
disposal site for the excavated material is far away. Excavation could be
undertaken at the site, but it would have to be done painstakingly. Other
treatment technologies would be easier to undertake. However, if speed of
clean-up is an overriding concern, excavation could prove to be a viable
option.
91
-------
GLOSSARY OF TERMS
Aerobic - in the presence of oxygen.
Anaerobic - in the absence of oxygen.
Biodegradation - a process by which microbial organisms transform or alter
through enzymatic action the structure of chemicals introduced
into the environment.
Biomass - The amount of living matter in a given area or volume.
Bulk Density - the amount of mass of a soil per unit volume of soil; where
mass is measured after all water has been extracted and total
volume includes the volume of the soil itself and the volume of
air space between the soil grains.
Capillary Suction - process where water rises above the water table into
the void spaces of a soil due to tension forces between the water
and soil particles.
Capillary Fringe - The zone of a porous medium above the water table within
which the porous medium 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.
Consolidated Soil - when a soil is subjected to an increase in pressure due
to loading at the ground surface, a re-adjustment in the soil
structure occurs. The volume of space between the soil particles
decreases and the soil tends to settle or consolidate over time.
Degradation Potential - the degree to which a substance is likely to be
reduced to a simpler form by bacterial activity.
Dissolution - Dissolving of a material in a liquid solvent (e.g., water).
Elutriate - the mixture of water, surfactants, and contaminants that is
recovered during the soil flushing process.
Fermentation - the breakdown of complex molecules in an organic compound.
92
-------
Free Product - a contaminant in the unweathered phase, where no dissolution
or biodegradation has occurred.
Field Capacity - the percentage of water remaining in the soil 2 or 3 days
after gravity drainage has ceased from saturated conditions.
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.
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).
Interfacial tension - phenomena occurring at the interface of a liquid and
gas where the liquid behaves as it if were covered by an elastic
membrane in a constant state of tension. The tension is due to
unbalanced attractive forces between the liquid molecules at the
liquid surface.
Liquid Density - the amount of mass of a liquid per unit volume of the
liquid.
Mobility - the ability of a substance to move into or out of a phase due to
physical or chemical processes.
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.
Molecular Diffusion - process where molecules of various gases tend to
intermingle and eventually become evenly dispersed.
Molecular Weight - the amount of mass in a mole of molecules of a substance
determined by summing the masses of the individual atoms
comprising the molecule. One mole is equivalent to 6.02 x 10
molecules.
93
-------
Partial Pressure - the portion of total vapor pressure in a system due to
one or more constituents in the vapor mixture.
Particle Density - the amount of mass of a substance per unit volume of the
substance.
Permeability - a measure of a soils resistance to fluid flow.
Permeability, along with fluid viscosity and density are used to
determine fluid conductivity.
Phase - the physical form in which a substance is found. As discussed in
this manual, the three major phases are liquid, vapor and
dissolved in pore water.
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.
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.
Refractory Index - a measure of the ability of a substance to be
biodegraded by bacterial activity.
Residual Saturation - the amount of water or oil remaining in the voids of
a porous medium and held in an immobile state by capillary 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.
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 as organic carbon.
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.
94
-------
Surfactant - natural or synthetic chemicals that have the ability to
promote the wetting, solubilization, or emulsification of various
organic chemicals.
Unconsolidated Soil - soil which has not been subjected to pressure due to
loading at the ground surface.
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.
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 usually less than saturation. The vadose zone
includes the capillary fringe.
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.
95
-------
Blank Worksheets
96
-------
TABLE A-1. LIKELIHOOD OF LIQUID CONTAMINANTS BEING ^^
PRESENT IN THE UNSATURATED ZONE W
FACTOR
RELEASE- RELATED
Amount Released
Rate Of Release
- Time Since Release
SITE - RELATED
Depth To Groundwater
Hydraulic Conductivity
Rainfall Infiltration Rate
Soli Temperature
Soil Sorptlon Capacity
(Surface Area)
CONTAMINANT- RELATED
Liquid Viscosity
Liquid Density
Vapor Pressure
. Water Solubility
UNITS
gallons
months
meters
cm/sec
cm/day
ฐC
m2/g
cP
3
g/cm
mm Hg
mg/L
SITE OF
INTEREST
INCREASING LIKELIHOOD ^
Small
(<100)
o
Slow Release
O
Long
(>12)
O
Medium
(100-1000)
O
o
Medium
(1-12)
O
Large
(>1000)
O
Instantaneous
Release
O
Short
(<1)
O
Shallow
(<1)
O
High
(>10-3)
O
High
(>0.1)
0
Warm
(>20)
O
Low
(<0.1)
O
Medium
(1-5)
O
Medium
(10-6-10'3)
O
Medium
(0.05-0.1)
O
Medium
(10-20)
0
Medium
(0.1-1)
O
Deep
(>5)
0
Low
(<10-5 )
O
Low
(<0.05)
0
Cool
(<10)
O
High
(>1)
O
Low
(<2)
O
High
(>2)
0
High
(>100)
0
High
(>1000)
0
Medium
(2-20)
O
Medium
(1-2)
O
Medium
(10-100)
O
Medium
(100-1000)
O
High
(>20)
O
Low
(<1)
O
Low
(<10)
O
Low
(<100)
0
97
-------
TABLE A-2. LIKELIHOOD OF CONTAMINANT VAPORS BEING ffiฃ&\
PRESENT IN THE UNSATURATED ZONE Km]
FACTOR
RELEASE- RELATED
Amount Released
Rate Of Release
Time Since Release
SITE - RELATED
Depth To Groundwater
Air Conductivity
Rainfall Infiltration Rate
Soil Temperature
Soil Sorptlon Capacity
(Surface Area)
CONTAMINANT- RELATED
Liquid Viscosity
Liquid Density
Vapor Pressure
Water Solubility
UNITS
gallons
months
meters
cm/sec
cm/day
ฐC
m /g
cP
3
g/cm
mm Hg
mg/L
SITE OF
INTEREST
INCREASING LIKELIHOOD ^
Small
(<100)
o
Slow Release
O
Long
(>12)
O
Shallow
(<1)
O
High
(>1&*)
O
High
(>0.1)
O
Cool
(<10)
O
Low
(<0.11)
O
Medium
(100-1000)
O
o
Medium
(1-12)
O
Medium
(1-5)
O
Medium
(10'6-10-4)
O
Medium
(.005-0.1)
O
Medium
(10-20)
O
Medium
(0.1-1)
O
Large
(>1000)
O
Instantaneous
Release
O
Short
(<1)
O
Deep
(>5)
O
Low
(20)
O
High
(>1)
O
High
(>20)
0
High
(>2)
O
Low
(<10)
O
High
(>1000)
O
Medium
(2-20)
O
Medium
(1-2)
O
Medium
(10-100)
O
Medium
(100-1000)
O
Low
(<2)
O
Low
(<1)
O
High
(>100)
O
Low
(<100)
O
98
-------
TABLE A-3. LIKELIHOOD OF CONTAMINANTS DISSOLVED IN PORE
WATER BEING PRESENT IN THE UNSATURATED ZONE
FACTOR
RELEASE- RELATED
Amount Released
Rate Of Release
Time Since Release
SITE - RELATED
Depth To Groundwater
Moisture Content
Soil Porosity
Rainfall Infiltration Rate
Soil Sorptlon Capacity
(Surface Area)
CONTAMINANT- RELATED
Liquid Viscosity
Liquid Density
Vapor Pressure
Water Solubility
UNITS
gallons
months
meters
% volume
% volume
cm/day
m2/g
cP
g/cm3
mm Hg
mg/L
SITE OF
INTEREST
4ฃWS>y
Kฃ$&
fMMKJ
Jra-K-H
5J-JMWS
ifjyjQup
INCREASING LIKELIHOOD ^
Small
(100)
o
Instantaneous
Release
O
Long
(>12)
0
Medium
(100-1000)
O
o
Medium
(1-12)
O
Large
(>1000)
O
Slow Release
O
Short
(<1)
O
Shallow
(<1)
O
Low
(<10)
O
Low
(<20)
O
Low
(<0.05)
O
Low
(<0.1)
O
Medium
(1-5)
O
Medium
(10-30)
0
Medium
(20-40)
O
Medium
(0.05-0.1)
0
Medium
(0.1-1)
0
Deep
(>5)
O
High
(>30)
O
High
(>40)
O
High
(>0.1)
O
High
(>1)
0
High
(>20)
0
High
(>2)
O
High
(<100)
O
Low
(<100)
O
Medium
(2-20)
O
Medium
(1-2)
O
Medium
(10-100)
0
Medium
(100-1000)
O
Low
(<2)
O
Low
(<1)
O
Low
(>10)
O
High
(>1000)
O
99
-------
FACTOR
UNITS
SITE OF
INTEREST
RELEASE RELATED
Time Since Release
Months
SITE- RELATED
Hydraulic Conductivity
Soil Porosity
Soil Surface Area
Soli Temperature
Rock Fractures
Moisture Content
cm/sec
% Soil
Volume
m2/g
ฐC
% Volume
CONTAMINANT- RELATED
Liquid Viscosity
Liquid Density
cPoise
g/cm
INCREASING MOBILITY ^
Long
(>12)
o
Medium
(1-12)
O
Short
(<1 )
0
Low
(<10-5)
O
Low
(<10)
O
High
(>1)
O
Low
(<10)
O
Absent
0
High
(>30)
O
Medium
(10-5-10"3)
O
Medium
(10-30)
0
Medium
(0.1-1)
O
Medium
(10-20)
0
o
Medium
(10-30)
0
High
(>10-3)
O
High
(>30)
0
Low
(<0.1)
O
High
(>20)
0
Present
O
Low
(<10)
O
High
(>20)
O
Low
(<1)
0
Medium
(2-20)
O
Medium
(1-2)
O
Low
(<2)
O
High
(>2)
O
100
-------
TABLE A-5. FACTORS TO EVALUATE THE MOBILITY OF f^^\
CONTAMINANT VAPORS งงงง3
FACTOR
UNITS
SITE OF
INTEREST
INCREASING MOBILITY ^
SITE - RELATED
Air Filled Porosity
- Total Porosity *
Water Content *
Depth Below Surface
% Volume
% Volume
% Volume
meters
Low Met
(<10) (10-
o c
Low Mec
(<10) (10-
o c
High Mec
(> 30) (10-
0 C
Deep Mec
(>10) (2-
o c
CONTAMINANT- RELATED
Vapor Density
3
g/m
Low Mec
(< 50) (50-
o c
lium High
30) (> 30 )
) O
iium High
30) (> 30 )
) O
ium Low
30) (<10)
) O
iium Shallow
10) (<2)
) O
ium High
500) (>500)
) O
* the total porosity less that fraction filled with water equals the air filled porosity
101
-------
TABLE A-6. FACTORS TO EVALUATE THE MOBILITY
OF CONTAMINANTS IN PORE WATER
FACTOR
UNITS
SITE OF
INTEREST
xjSSJRJy
5KKK!
ESSES
jssss
ฃฃฃฃ
^S-S=ฃ
INCREASING MOBILITY ^
SITE RELATED
Hydraulic Conductivity
Moisture Content
Rainfall Infiltration Rate
Soil Porosity
Rock Fractures
Depth Below Surface
cm /sec.
% Volume
cm/day
% Volume
meters
Low
(10-5)
O
Low
(<10)
O
Low
(<0.05)
O
Low
(<10)
O
Absent
0
Shallow
(<2)
0
Medium
(10-5-10-3)
O
Medium
(10-30)
O
Medium
(0.05-0.1)
O
Medium
(10-30)
O
0
Medium
(2 - 10)
0
High
(>10'3)
O
High
(>30)
O
High
(>0.1)
O
High
(>30)
O
Present
0
Deep
(>10)
O
i CONTAMINANT- RELATED
Water Solubility
mg/L
Low
(< 100)
O
Medium
(100 to 1000)
O
High
(>1000)
O
102
-------
TABLE A-7. WORKSHEET FOR EVALUATING
THE FEASIBILITY OF SOIL VENTING
BEING EFFECTIVE AT YOUR SITE
CRITICAL SUCCESS
FACTOR
UNITS
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
SUCCESS
SOMEWHAT
LIKELY
*n
tH?
SUCCESS
MORE
LIKELY
SITE RELATED
Dominant
Contaminant Phase
Soil Temperature
Soil Air Conductivity
Moisture Content
Geological
Conditions
Soil Sorptlon Capacity
- Surface Area
Depth to Ground Water
Phase
ฐC
cm/sec.
% volume
2
m /g
meters
Sorbed to soil
o
Low
(<10)
O
Low
(<10-6)
O
Moist
(>30)
0
Heterogeneous
O
High
(>1)
O
Low
(<1)
O
Liquid
O
Medium
(10-20)
O
Medium
(lO^-IO"4)
O
Moderate
(10-30)
O
o
0
Medium
(1-5)
0
Vapor
O
High
(>20)
O
High
(>10-4)
O
Dry
(<10)
O
Homogeneous
O
Low
(<0.1)
0
High
(>5)
0
CONTAMINANT- RELATED i^ & /n^'^M- ^ . ' . '- . . '/V: ; '"' . >"* - , '-^
Vapor Pressure
Water Solubility
mm Hg
mg/L
Low
(<10)
O
High
(> 1000)
0
Medium
(10 to 100)
0
Medium
(100-1000)
O
High
(> 100)
O
Low
(< 100)
0
OTHER CONSIDERATIONS .Treatrrartcanbedorwo,.e
Cost Is from $1 5 to $60 per cubic yard. . Care must be taken to avoid explosions because vapors
Efflctlveness decreases after several months of treatment. are concentrated
Capable of removing thousands of gallons. . cleanup takes time so thai thla technology Is not
Air emissions will likely need to be treated with GAG. appropriate when emergency response is needed
103
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TABLE A-8. WORKSHEET FOR EVALUATING fclL\
THE FEASIBILITY OF BIORESTORATION J*
BEING EFFECTIVE AT YOUR SITE V^*V
CRITICAL SUCCESS
FACTOR
RELEASE -RELATED
- Time Since Release
SITE RELATED
Dominant
Contaminant Phase
Soil Temperature
Soil Hydraulic
Conductivity
Soil pH
Moisture Content
UNITS
Months
Phase
ฐC
cm/sec.
pH Units
% Volume
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
Short
(< 1)
O
Liquid
O
Low
(<5)
O
Low
0
(< 6 or > 8)
O
Dry
(< 10)
0
SUCCESS
SOMEWHAT
LIKELY
Medium
(1-12)
O
Vapor
O
Medium
(5-10)
O
Medium
(1Q-5-10'3)
O
o
Moderate
(10 to 30)
O
SUCCESS
MORE
LIKELY
Long
(> 12)
O
Dissolved
O
High
(> 10)
O
High
>o
(6-8)
O
Moist
(>30)
0
CONTAMINANT- RELATED
Solubility
Blodegradablllty
Refractory Index
ซ Fuel Type
mg/L
Jlmensionless
Low
(< 100)
O
Low
(<0.01)
O
No. 6 Fuel Oil
(Heavy)
0
Medium
(100 to 1000)
0
Medium
(0.01 to 0.1)
O
No. 2 Fuel Oil
(Medium)
O
High
(> 1000)
O
High
(>0.1)
O
Gasoline/ Diesel
(Light)
O
OTHER CONSIDERATIONS
Cost Is from $60 to $125 per cubic yard.
Completely destroys contaminants under optimal conditions
Effectiveness varies depending on subsurface conditions
Biologic systems subject to upset
Public opinion sometimes aga nst putting more chemicals in ground
Difficult to monitor effectiveness
Minimizes health risk by keep ng contaminants in ground and on site
Takes long time to work not for emergency response
104
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I _
TABLE A-9. WORKSHEET FOR EVALUATING
THE FEASIBILITY OF SOIL FLUSHING
BEING EFFECTIVE AT YOUR SITE
CRITICAL SUCCESS
FACTOR
UNITS
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
SITE RELATED
* Dominant
Contaminant Phase
Soil Hydraulic
Conductivity
- Soil Surface Area
- Carbon Content
Fractures In Rock
Phase
cm/sec.
m2/g
% Weight
Vapor
o
Low
(<10-5)
O
High
(>1)
O
High
(>10%)
O
Present
0
SUCCESS
SOMEWHAT
LIKELY
Liquid
O
Medium
(10'5-10'3)
O
Medium
(0.1-1)
0
Medium
(1 -10%)
O
0
SUCCESS
MORE
LIKELY
Dissolved
O
High
^lO-3)
O
Small
<0.1)
O
Low
(<1%)
O
Absent
O
CONTAMINANT- RELATED
Water Solubility
Sorption Characteristics
- Soil Sorption Constant
Vapor Pressure
Liquid Viscosity
Liquid Density
mg/L
L/kg
mm Hg
cPoise
g/cm 3
Low
(<100)
O
High
(>1 0,000)
O
High
(> 100)
0
High
(>20)
O
Low
(<1)
O
Medium
(100 to 1000)
O
Medium
(100-10,000)
0
Medium
(10-100)
O
Medium
(2 -20)
O
Medium
(1-2)
0
High
(> 1,000)
O
Low
(< 100)
O
Low
(<10)
O
Low
<2)
O
High
(>2)
O
OTHER CONSIDERATIONS
Cost is from $150 to $200 per cubic yard.
Using surfactants may increase effectiveness
Effluent requires separation techniques such as distillation, evaporation, centrifugation
Most effective when used ex-situ (above ground)
105
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TABLE A-10. WORKSHEET FOR EVALUATING ( \
THE FEASIBILITY OF HYDRAULIC BARRIERS
WORKING AT YOUR SITE
CRITICAL SUCCESS
FACTOR
RELEASE - RELATED
Time Since Release
Volume of Spill
SITE RELATED
Dominant
Contaminant Phase
Soil Hydraulic
Conductivity
Soil Sorptlon Capacity
- Surface Area
Carbon Content
Temperature
Depth to Groundwater
UNITS
months
gallons
phase
cm/sec.
m2/g
% weight
ฐC
meters
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
Long
(> 12 months)
O
Small
O
Vapor
O
High
(> 10'3)
0
High
"0
High
(> 10%)
O
Low
O
High
O
SUCCESS
SOMEWHAT
LIKELY
Medium
(1 12 months)
O
Medium
(100-1000)
O
o
Medium
(10'5-10"3)
O
Medium
(0.1-1)
0
Medium
(1 10)
O
Medium
(5-10)
0
Medium
(1-5)
O
nmOC*Hsl
reงง^
SUCCESS
MORE
LIKELY
Short
(< 1)
0
Large
(>1000)
O
Liquid
O
Low
O
Low
"6
Low
O
High
O
Low
O
CONTAMINANT- RELATED
Liquid Viscosity
c Poise
High
O
Medium
(2 to 20)
O
Low
(<2)
O
OTHER CONSIDERATIONS
Cost is from $10 to $90 per cubic meter.
Only affects liquid portion of release not portion sorbed to soil.
- Typically limited to shallow (<3 meters) depths.
Not effective in removing contaminants to low levels.
Most effective when contamination is confined to small areas.
Not effective for #6 fuel oil and other viscous fluids.
Not effective if contamination is greater than 15 meters deep.
106
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TABLE A-11. WORKSHEET FOR EVALUATING
THE FEASIBILITY OF EXCAVATION
BEING EFFECTIVE AT YOUR SITE
CRITICAL SUCCESS
FACTOR
UNITS
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
SUCCESS
SOMEWHAT
LIKELY
SUCCESS
MORE
LIKELY
SITE RELATED
Proximity of Above and
Below Ground Structures
Buildings nearby
Buried pipes
and cables
o
No nearby
structures
O
> Volume of Soil
Contaminated
Cubic
Meters
Large
(> 1,000)
O
Medium
(100-1,000)
O
Small
(< 100)
O
Depth of Contamination
Meters From
Surface
Deep
Medium
(1-5)
O
Shallow
(<1)
O
Proximity of Site to
- Traffic
Near
O
Far
O
- Businesses
Near
O
Far
O
- Disposal Site
Far
O
Near
O
- Backfill Source
Far
O
Near
O
OTHER CONSIDERATIONS
Cost is from $50 to $300 per cubic yard.
Appropriate when urgent response is necessary
Brings contaminants to surface, thereby increasing exposure risks
Significant amounts of surface area disturbed relative to depth excavated
Requires suitable means of disposal. This is becoming increasingly difficult
because some landfill operators consider petroleum-laden soil to be a hazardous waste.
AU.S. GOVERNMENT PRINTING OFFICE:
1 a 9 0 7 18 159 00*23
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