PREPARED FOR:
OFFICE OF
UNDERGROUND
STORAGE TANKS
US. ENVIRONMENTAL
PROTECTION AGENCY
CONTRACT NO: 68-01-6939
INTERIM REPORT
FATE AND TRANSPORT
OF SUBSTANCES LEAKING
FROM UNDERGROUND
STORAGE TANKS
VOLUME I-
TECHNICAL REPORT
1986
COM
CAMP DRESSER & McKEE INC.
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INTERIM REPORT
FATE AND TRANSPORT OF SUBSTANCES
LEAKING FROM UNDERGROUND STORAGE TANKS
VOLUME 1 TECHNICAL REPORT
U.S. EPA CONTRACT NO: 68-01-6939
DOCUMENT CONTROL NO: 998-TS6-RT-CDZN-1
Prepared For:
Office of Underground Storage Tanks
U.S. Environmental Protection Agency
401 M Street, S.W.
Washington, D.C. 20460
Prepared By:
Camp Dresser & McKee Inc.
One Center Plaza
Boston, MA 02108
January 31, 1986
This document has been prepared for the U.S. Environmental
Protection Agency under Contract No. 68-01-6939. The material
contained herein is not to be disclosed to, discussed with, or
made available to any person or persons for any reason without
prior expressed approval of a responsible official of the U.S.
Environmental Protection Agency.
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EXECUTIVE SUMMARY
BACKGROUND AND OBJECTIVES
As part of the development of a comprehensive regulatory program for
underground storage tanks (USTs) by the U.S. Environmental Protection
Agency (Office of Underground Storage Tanks), this work assignment aims to
investigate the fate and transport of regulated substances leaking from
USTs.
The specific objectives of the work assignment are:
• To summarize the current understanding of the physical, chemical,
biological, and toxicological processes relative to leaks of
regulated substances stored in underground tanks.
• To evaluate existing methodologies and data pertinent to fate and
transport processes with a potential for applicability to the UST
program.
• To establish priorities for further investigation, data collection,
and/or methodological development.
This report presents the results of the first phase of this project,
summarizing the current understanding of the mechanisms or processes that
govern fate and transport of substances leaking from underground tanks.
This information is provided both as background to the ongoing program
development and implementation efforts, and as a means of directing the
further efforts of this work assignment to those critical areas where
additional study would be most useful for current and future UST program
tasks.
In order to focus the technical findings of this first phase of the work
assignment, a list of relevant program activities was developed as a tie to
on-going UST program deliberations. Where possible, the technical findings
have been tied to these activities particulary in Section 7.0 of this
report. A briefing for UST program staff is planned during the review
period of this report, both to present and discuss the report's technical
findings, and to discuss their applicability to current policy/regulatory
issues.
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The report is organized as follows: Section 1.0 gives an overview of the
project and its objectives. We present in Section 2.0 an overview of the
substances to be regulated under the UST program, specifically the
petroleum and hazardous substances covered by the RCRA subtitle I. While
the fate and transport of a chemical in the subsurface are interwoven, they
are for ease of presentation, discussed as separate mechanisms in Sections
3.0 and 4.0. The transport of a substance can be described by the
mechanisms that govern its movement in the environment. The fate of a
substance includes all of the physical, chemical, and biological changes it
undergoes in the environment. The physical and chemical nature of the
substances stored in underground tanks, and particularly the physical,
chemical, and toxicological properties relevant to fate and transport
mechanisms are then presented in Section 5.0. The environmental factors
and/or settings which can be used to describe leaking UST scenarios are
discussed in Section 6.0. Finally, in Section 7.0, we summarize the
technical findings of this report in the context of their applicability to
the development of the UST regulatory program.
REGULATED SUBSTANCES (SECTION 2.0)
Regulated substances under this program include petroleum and hazardous
substances.
Among the petroleum substances, those of interest include the following
petroleum products:
• Power fuels - aviation gasoline, motor gasolines, diesel fuel oils,
jet fuels, and gas turbine fuel oils.
• Heating oils - fuel oils (Nos. 1, 2, 4, 5 and 6).
• Solvents - Stoddard solvent, petroleum spirits, petroleum extender
oils, and aromatic solvents.
• Lubricants - automotive and industrial lubricants.
These substances were selected for detailed study based on three criteria:
meets statutory definition of a liquid, is commonly stored in underground
tanks, and is widely used in the United States.
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Of the 698 hazardous substances regulated by CERCLA and by extension
regulated under the LIST program, 477 have been determined to be likely to
be stored in underground tanks based on available inventories of existing
USTs and on knowledge of substance properties that make underground storage
desirable or necessary. A list of these chemicals, with their
corresponding CAS registry numbers, is included as Appendix A. More
research is needed to determine, in time, the likelihood that other
chemicals among the 221 remaining CERCLA-regulated substances are stored in
underground tanks.
At any rate, by far the greatest number of tanks storing regulated
substances in the United States contain petroleum products -- specifically,
power fuels and heating oils. This measure alone suggests that relatively
more attention be focused on the transport and fate of petroleum products
that may leak from USTs, than for other regulated substances.
TRANSPORT MECHANISMS (SECTION 3.0)
Liquid Phase Transport:
The transport (migration) of substances leaking from USTs depends on: the
quantity released, the physical properties of the contaminant, and the
structure of the subsurface soils and rock through which the contaminant is
moving.
Even though USTs leaks can be characterized as insidious spills, they may
be undetected for long periods of time, and thus, can release substantial
volumes of a substance into the subsurface. One reason the quantity of
released substance is important, is that it often determines the method,
and degree of groundwater contamination, i.e., by direct contact with the
groundwater or through solution in percolating rainwater.
The physical properties of the leaked substances that are important to
transport are: solubility, specific gravity, viscosity, surface tension,
evaporation rate, and vapor density. The influence of these properties on
transport mechanisms is discussed in detail in Section 3.0.
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Section 3.0 also discusses the transport mechanisms through various types
of subsurface media. Movement of contaminants is discussed for three
characteristic zones of the subsurface: the unsaturated zone lying below
the ground surface, the capillary zone that is a transition between the
unsaturated and saturated zones, and the saturated zone lying below the
water table.
Transport of a contaminant through the unsaturated zone is characterized by
vertical flow, driven by gravity, and lateral spreading associated with
capillary forces and soil heterogeneities.
In general, the contaminant plume will take a pear-shaped form as it moves
through the unsaturated zone. The plume will be wider (the pear "fatter")
in a less permeable soil than in a more permeable soil. The shape will be
irregular in a stratified soil with varying permeabilities.
The capillary zone has a significant impact on the movement only of
non-aqueous substances that are less dense than water. In such cases, the
primary direction of movement is lateral, with flow extending farthest in
the direction of the slope of the water table.
The transport of contaminants in the saturated zone can be characterized
for three classes of substances as follows: miscible or dissolved
substances, immiscible substances with specific gravity of less than 1.0,
and immiscible substance with specific gravity of more than 1.0. Dissolved
substances will enter the groundwater and will move in the general
direction of groundwater flow according to the mass transport laws of
advection and dispersion. Immiscible substances that are "lighter" than
water are typically only found in the shallow part of the saturated zone.
Immiscible substances that are denser than water will move down through the
saturated zone.
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Vapor Phase Transport:
A liquid contaminant leaking from an UST will enter the vapor phase
according to its vapor pressure (the higher the vapor pressure of the
substance, the more likely it is to evaporate). Once in the vapor phase,
the contaminant will move by advection -- being "blown along" by subsurface
air currents - and by diffusion. The movement of vapors is predominantly
in the horizontal direction.
Much less research attention has in the past been given to vapor phase
transport than to liquid transport; several research needs are identified.
FATE MECHANISMS (SECTION 4.0)
Solubility:
Contaminants can go into solution when: rainwater percolates through the
unsaturated and capillary zones bringing dissolved contaminant with it,
possibly including those that were in a vapor phase, nonaqueous phase
liquids migrate in the saturated zone or dissolve from the bottom of an
"oil pancake" in the capillary zone.
Irrespective of the density of the chemical, its rate of solution in
groundwater will depend on its solubility in water, area of contact, and
groundwater flow rate. In an inverse sense, these parameters determine how
long the nonaqueous phase liquid remains in the subsurface. As each of
these factors increases, the rate of solution increases.
Vaporization:
Vapors can be released by contaminants in soil or water. Vaporization
occurs from open bodies of water, such as lakes, from subsurface soils, and
from dissolved substances in groundwater, in that order of importance.
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The tendency of a chemical of limited solubility to evaporate is described
by Henry's Law constant, which expresses the driving force for transfer of
solute from aqueous to gaseous phase as the quotient of that chemical's
vapor pressure and solubility. The higher the Henry's Law coefficient for
a given substance, the greater its- tendency to evaporate. The constant is
temperature-dependent; evaporation increases with temperature.
Adsorption:
Adsorption of organic chemicals is most significant on organic matter or
clay minerals. When adsorption is due to organic matter in the aquifer,
the tendency for a chemical to adsorb is expressed in terms of the soil
adsorption coefficient, KQC.
Adsorption plays two significant roles in the fate of contaminants: it can
retard the forward progress of a contaminant plume, and it can act as a
residual source of contamination, extending the period of contamination.
Biotic Processes:
Some contaminants can be degraded by biological activity; both liquid and
gaseous compounds can be biodegraded. Degradation is most prevalent in the
unsaturated zone where conditions are more favorable for microbial
utilization of a contaminant.
There are three modes of biodegradation: aerobic degradation -- the
predominant mode taking place in the unsaturated zone of the subsurface
environment and in the saturated zone when dissolved oxygen is available;
anaerobic degradation -- is more important in the degradation of hazardous
substances as compared to petroleum hydrocarbons; and cometabolism --
contaminants that tend to be resistant to biodegradation can be attacked
more rapidly when found in the subsurface with other contaminants.
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The breakdown of a contaminant in the natural environment will depend on
the nature of the chemical, on the types of microorganisms present, and on
environmental factors. Degradation of contaminant constituents is
microorganism-specific and site-specific.
Abiotic Chemical Transformation Processes:
Two types of chemical processes can occur in the subsurface:
Oxidation/Reduction and Hydrolysis.
The tendency of a substance to donate or accept electrons in an abiotic
reaction may be given by two parameters: electrode potential and redox
potential. Only a relatively narrow portion of the range of electrode and
redox potentials are available in soils. Therefore, oxidation/reduction
can only be important when potent oxidizing or reducing agents are present.
Direct oxidation or reduction by such agents is unlikely to be a
significant fate of substances leaking from USTs, except for the most
reactive compounds.
The importance of hydrolysis from an environmental fate point of view is
that the resulting product is usually more susceptible to further attack by
processes of biodegradation, and the additon of the hydroxyl group that
makes the chemical more water-soluble. The kinetics of hydrolysis are
influenced by environmental factors. The rate of disappearance of a
chemical is directly proportional to the concentration of the compound.
The hydrolysis half-life is independent of the concentration.
PROPERTIES OF REGULATED SUBSTANCES (SECTION 5.0)
The properties of regulated substances in USTs have been identified and
three databases were developed: 1. properties of petroleum products stored
in underground tanks; 2. properties of hydrocarbons known to be
constituents of gasoline and fuel oil; and 3. the properties of
CERCLA-regulated hazardous substances.
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Petroleum products, exept in rare instances, have a specific gravity less
than 1.0 and will float on groundwater. Kinematic viscosity was related to
transport in the unsaturated zone. The lower the kinematic viscosity, the
faster a product will migrate through soil (when gravity is the predominant
driving force). High vapor pressure products, such as gasoline and jet
fuels, have greater tendency to release vapor phase contaminants than do
substances with low vapor pressures.
Relying on three parameters cited above, petroleum products were "grouped"
with respect to movement through the subsurface. Group I, or high vapor
pressure - low kinematic viscosity products, included automotive and
aviation gasoline, solvents, and No. 0-GT gas turbine fuel oil.
The hazardous substances were grouped by using a "cause and effect"
approach. Two-parameter "sortings of the database were used to assess
potential impact. Three impact scenarios were related to specific
parameters collected in the database, as follows: groundwater
contamination: solubility toxicity data, human exposure to toxic vapors:
vapor pressure-toxicity data, and explosion/fire: ignitibility-vapor
pressure data.
After assigning ranges which denote high, medium or low values of these
parameters, and after removing highly reactive chemicals from the analysis,
the sorts were performed resulting in lists of substances considered to
have "similar potential" for UST-related impact.
The results indicate that 32 chemicals with toxicity data (final or
proposed RQ) fall in the high toxicity/high solubility group and are
considered to have very high groundwater impact potential; only 5 chemicals
may be considered to have very high potential for human health impact due
to inhalation of vapors (high vapor pressure/high toxicity); and 9
substances are found in the highest ignitibility/high vapor pressure group.
A comparison was made of hazardous substances in the database with the
constituents of gasoline, fuel oil, and petroleum solvents. By comparing
the hazardous chemical lists resulting from the sorts to the petroleum
product constituents some valuable insight is achieved.
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The hazardous components of gasoline and fuel oil fall primarily in the
"mid-range" groups for potential to impact drinking water aquifers, the
potential to expose people to toxic vapor; and the potential for
fire/explosion. This mid-range finding reinforces what is felt
intuitively: certain hazardous substances have potential for greater
adverse .impact than the constituents of gasoline and other petroleum
products. This does not mean, however, that petroleum products do not
present environmental problems when released in the subsurface. The
hazardous components of gasoline include benzene, toluene, and xylene which
are fairly soluble, toxic, and produce explosive/toxic vapors.
ENVIRONMENTAL SETTINGS (SECTION 6.0)
In Section 6.0, the environmental factors which influence the fate and
transport of substances leaking from USTs were identified. Two types of
environmental factors were presented: natural and non-natural. The
natural environmental factors (e.g., climate and hydrogeology) determine
how vulnerable a site is to contamination, while the non-natural factors
(e.g., man-made conditions, population density) exacerbate a site's
vulnerability to contamination. As such, non-natural factors can increase
the hazards resulting from leaking contaminants from USTs.
The ultimate significance of these factors with respect to leaking USTs
varies from site to site, and also depends on the properties and quantity
of the leaked contaminant. For some of the natural environmental factors,
such as climatic variables or the use of groundwater for drinking water
supplies, it may be possible to assess the significant variations on a
nationwide geographic basis. For other environmental factors, particularly
the non-natural factors, it is possible to assess the influence of these
factors only on a site-specific basis. For example, in determining the
effect of vapor contaminants, a knowledge of the specific
micro-stratigraphy in the area of a leak and the presence of receptor
structures is very important. It is also important to recognize the
effects that an environmental factor has on fate and transport mechanisms
can be in similar or conflicting directions.
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Since within a regulatory frameworks the development of generic
environmental settings is useful to evaluate the potential consequences of
leaking UST incidents, three existing methodologies were examined to
determine their applicability. The methodologies are used in the
following: RCRA Risk and Cost Analysis Model (W-E-T), Liner Location Risk
and Cost Analysis Model, and DRASTIC and its revised version UST
DRASTIC/IMPACT.
DRASTIC is a simple and easy-to-use approach which when properly applied
may be a useful planning or screening tool. The NWWA assessed the
feasibility of revising DRASTIC to petroleum UST related problems. The
need to address site-specific factors was also recognized; these are
included in IMPACT. The revised DRASTIC and IMPACT make up the proposed
UST DRASTIC/IMPACT system. It appears that this system could be a useful
tool for comparative evaluation of environmental settings for hazards posed
by leaking UST incident for two reasons: 1. the methodology includes many
of the important environmental factors determined here to be relevant to
leaking UST occurrences, and 2. the scale at which DRASTIC develops an
.environmental setting is appropriate for UST related problems. Because the
UST DRASTIC/IMPACT system appears to be the most applicable of the
available methods to evaluate the groundwater pollution potential from a
leaking UST site, some notes on the use of the system are presented in
Section 6.0.
APPLICABILITY TO UST PROGRAM (SECTION 7.0)
Section 7.0 offers preliminary suggestions as to how scientific and
technical information — as presented in this report -- can be used in the
development and implementation of the UST regulatory program. Although
this report presents findings.of only the first phase of this work
assignments, there are several ways in which the findings may have
immediate and important use. Some of these are:
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• Section 2.0 through 6.0 of this report assemble, in one document,
up-to-date information on the relevant scientific topics. This
information can prove timely as the program begins to build its
regulatory framework. Also, the information can be "re-packaged" to
be distributed to state and local government agencies, or industrial
organizations, that are faced with real-world LIST incidents.
• The three databases developed under this work assignment thus far
(Volume II) can be of value in an immediate way, as a readily
available reference for properties of UST-regulated substances.
Beyond the general value of such a reference, the data provided in
these computerized information systems can help to prioritize
program efforts.
• From a global regulatory perspective (i.e., without focusing on
individual incidents for which specific chemical and setting
information is required), the technical information offered in this
report generally indicates the following:
- From the point of view of environmental settings, the use (or
potential use) of the underlying aquifer, the presence or absence
of man-made conduits, and the proximity to population, are
important features relevant to UST incidents. However, specific
hydrogeologic details cannot be easily incorporated in a
regulatory program without reference to specific and very
localized situations.
- As far as the contents of UST-regulated tanks are concerned, it
may be possible to state certain findings more defensibly in the
"negative" as opposed to the "positive." For example, one can
state with a high degree of certainty that some petroleum
products are unlikely to cause problems because they are largely
immobile if released in the subsurface environment.
Unfortunately, issues such as acceptable levels of toxicity and
exposure make positive statements about "very hazardous"
chemicals a more subjective matter. Also, information on the
distribution of tanks containing various substances may be
important to decide whether and where to draw "hazard" standards.
• On a site-specific basis, with information regarding tank contents
and localized hydrogeologic features in hand, findings of this
report can be used to prioritize actions in the areas of compliance
monitoring, corrective action requirements, and enforcement. For
example, state level personnel charged with implementing UST
programs may do the following:
- Having received notification of a leak and some degree of
information regarding the site and the contents of the tank,
ranking systems such as UST DRASTIC/IMPACT can be useful in
prioritizing or allocating available resources.
- Use the databases developed herein to assess the degree of hazard
associated with an UST incident. In particular, if such
databases were to be merged with "knowledge-based" systems for
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guiding UST incident response activity, state and local level
personnel may be better able to make proper use of resources and
take actions scaled to the level of hazard posed in a particular
situation.
Finally, the technical and scientific findings offered in this
report indicated generally where and how research activities may be
useful to the UST program -- for example, to study the movement of
contaminants in the unsaturated zone, fate mechanisms in general,
and vapor-phase movement.
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ACKNOWLEDGEMENTS
This report was prepared by Camp Dresser & McKee Inc. under Contract No.
68-01-6939 with the U.S. Environmental Protection Agency. It was prepared
under the direction of Ms. Pamela Harris, the USEPA project monitor from
the Office of Underground Storage Tanks.
The project was managed by Dr. Guillermo J. Vicens. Major portions of the
work were undertaken and reported by Bernadette H. Kolb, Dr. Myron S.
Rosenberg, Vicki L. Gerwert, Sarah R. Fisher, Walter R. Niessen, Anthony M.
Lo Re, and Dr. Jonathan A. French. Technical reviews of the report were
performed by: Donald G. Muldoon, Dr. Larry Partridge, Dr. Roger L. Olsen,
Prof. Lynn W. Gelhar (M.I.T.), Peter J. Riordan (independent consultant),
Dr. Brendan M. Harley, Robert A. Weimar, Patricia Billig, and Thomas F.
Cheyer. The technical editing of selected sections was performed by John
Bates.
COM acknowledges the assistance of Pamela Harris of USEPA in providing
overall guidance and coordination with other OUST program staff and others
throughout the Agency.
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TABLE OF CONTENTS
VOLUME I
Section Page
EXECUTIVE SUMMARY i
ACKNOWLEDGEMENTS xi i1
TABLE OF CONTENTS xiv
LIST OF FIGURES xix
LIST OF TABLES xxiii
1.0 INTRODUCTION 1-1
1.1 Project Background 1-1
1.2 Leaking UST Problems 1-1
1.2.1 Groundwater Contamination Incident 1-2
1.2.2 Vapor Incidents 1-5
.1.3 Objectives and Scope of Work 1-9
1.4 Relation to Other Studies 1-11
1.5 Relevant UST .Program Issues 1-13
1.6 Organization of Report 1-14
2.0 UST PROGRAM REGULATED SUBSTANCES. 2-1
2.1 Overview 2-1
2.2 Regulated Petroleum Substances in USTs 2-1
2.2.1 Overview 2-1
2.2.2 Petroleum Products 2-2
2.2.3 Petroleum Additives 2-4
2.3 Regulated Hazardous Substances in USTs 2-13
2.3.1 Overview 2-13
2.3.2 Data Sources 2-14
2.3.3 Data Resources Inc. and Quantum Analytics
Analysis of CA List 2-16
2.3.4 Fire Protection Guide on Hazardous Materials 2-16
2.3.5 Other Potential Sources of Data 2-20
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2.4 Remaining CERCLA Hazardous Substances 2-24
2.4.1 Likelihood of Storage in Underground Tanks 2-24
2.4.2 Additional Data Gathering and Analysis 2-24
2.5 Summary 2-25
3.0 TRANSPORT MECHANISMS , 3-1
3.1 Overview 3-1
3.1.1 Factors Affecting Liquid Transport 3-1
3.1.2 Transport Diagrams 3-12
3.2 Darcy's Law 3-15
3.2.1 Groundwater in the Saturated Zone 3-15
3.2.2 Dissolved Contaminants in the Saturated Zone 3-18
3.2.3 Immiscible Fluid Flow 3-22
3.3 Transport of Liquids in the Unsaturated Zone 3-23
3.3.1 Overview 3-23
3.3.2 The Physics of Unsaturated Zone
Contaminant Transport 3-25
3.3.3 Use of the Physics As It Applies To Leaking USTs .... 3-44
3.3.4 Estimating Relationships 3-54
3.4 Transport of Liquids in the Capillary Zone 3-57
3.4.1 Overview 3-57
3.4.2 Transport of Floating Substances 3-58
3.4.3 Estimating Relationships 3-66
3.5 Transport of Liquids in the Saturated Zone ;... 3-67
3.5.1 Overview 3-67
3.5.2 The Physics of Saturated Zone Transport 3-69
3.5.3 Applications 3-73
3.6 Vapor Transport in the Unsaturated Zone 3-80
3.6.1 Overview 3-80
3.6.2 Fundamentals of Transport 3-81
3.6.3 Summary of Vapor Phase Transport 3-90
3.7 Summary of Transport Mechanisms 3-90
4.0 FATE MECHANISMS 4-1
4.1 Overview 4-1
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4.2 Solubility 4-2
4.2.1 Overview 4-2
4.2.2 Environmental Factors That Influence Solubility 4-4
4.2.3 Current Understanding 4-6
4.3 Vaporization 4-8
4.3.1 Overview 4-8
4.3.2 Environmental Factors Affecting Vaporization 4-11
4.4 Adsorption 4-12
4.4.1 Overview 4-12
4.4.2 Environmental Factors That Influence Adsorption 4-18
4.4.3 Estimating Procedures 4-19
4.5 Biotic Processes 4-22
4.5.1 Overview 4-22
4.5.2 Factors That Influence Degradation 4-27
4.5.3 Types of Degradation 4-33
4.5.4 Impacts of Contaminants 4-36
4.6 Abiotic Chemical Transformation Processes 4-47
4.6.1 Overview 4-47
4.6.2 Hydrolysis 4-48
4.6.3 Oxidation/Reduction 4-57
4.7 Summary of Fate Mechanisms 4-59
5.0 PROPERTIES OF REGULATED SUBSTANCES IN USTs 5-1
5.1 Overview 5-2
5.2 Properties of Petroleum Products 5-2
5.2.1 Overview 5-2
5.2.2 Bulk Properties Database 5-2
5.2.3 Grouping by Bulk Properties 5-8
5.2.4 Constituents of Petroleum Products 5-14
5.3 Hazardous Substances in USTs 5-22
5.3.1 Overview 5-22
5.3.2 Development of The Database 5-22
5.3.3 Uses of The Data Base 5-28
5.3.4 Groups of Hazardous Substances 5-37
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5.4 Comparison of Hazardous Substances and
Petroleum Products 5-62
5.4.1 Gasoline 5-64
5.4.2 Fuel Oil 5-65
5.4.3 Additives and Trace Components 5-67
5.4.4 Petroleum Solvents 5-68
5.4.5 Key Findings 5-71
5.5 Summary 5-72
6.0 ENVIRONMENTAL SETTINGS 6-1
6.1 Overview 6-1
6.2 Environmental Factors Affecting Liquid
Fate and Transport 6-2
6.2.1 Natural Environmental Factors 6-2
6.2.2 Non-Natural Environmental Factors 6-7
6.3 Environmental Factors Affecting Vapor Transport
and Transport 6-9
6.3.1 Natural Environmental Factors 6-10
6.3.2 Non-Natural Environmental Factors 6-10
6.4 Environmental Setting Methodologies 6-18
6.4.1 RCRA Risk and Cost Analysis Model (W-E-T) 6-18
6.4.2 Liner Location Risk and Cost Analysis Model 6-21
6.4.3 DRASTIC 6-26
6.5 Summary 6-35
7.0 APPLICABILITY OF TECHNICAL FINDINGS TO THE LIST PROGRAM 7-1
7.1 Introduction 7-1
7.2 Problem Definition and Assessment 7-5
7.2.1 Properties of Regulated Substances 7-5
7.2.2 Manifestation of Problems 7-6
7.2.3 Release Settings 7-7
7.3 Regulation/Guidance Development 7-7
7.3.1 Technical Standards 7-8
7.3.2 Corrective Action 7-10
7.3.3 Notification and Reporting 7-12
7.4 Compliance and Enforcement 7-14
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7.5 Information Transfer 7-15
7 = 5.1 Research and Development ..,,,,.. 7-15
7.5.2 Training and Technical Assistance 7-17
7.5.3 Demonstration Projects 7-18
7.5.4 Outreach Programs 7-19
7.6 Summary 7-20
8.0 REFERENCES 8-1
VOLUME 2
Appendix A - Properties of CERCLA-Regulated Hazardous Substances
Stored in Underground Tanks A-l
Appendix B - Properties of Petroleum Products Stored in
Underground Tanks B-l
Appendix C - Properties of Hydrocarbons Known to be Constituents of
Gasoline and Fuel Oil C-l
Appendix D - Resulting Groups From Parametric Sort as of Hazardous
Substance Database 0-1
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LIST OF FIGURES
Figure Page
1-1 Location Plan for Site Treatment Containment Wells and South
Hollow Well Field - Provincetown, MA 1-3
1-2 Vapor Incident - North Babylon, NY 1-6
1-3 Vapor Incident - Livingston, MT 1-8
1-4 Task Flow Chart 1-10
2-1 Comparison of the Hazardous Substances on the CERCLA List with
the Hazardous Substances of the California List 2-17
3-1 Schematic Contaminant Plumes Showing Methods by Which
Groundwater can Become Contaminated 3-3
3-2 Schematic Contaminant Plumes Showing the Effects of Specific
Gravity on Immiscible Fluid Transport 3-6
3-3 Schematic of the Subsurface Environment 3-9
3-4 Schematic Plume Showing the Influence of Stratification of the
Unsaturated Zone on Contaminant Distribution 3-13
3-5 Migration Patterns in Porous Subsurface Media 3-14
3-6 Schematic Representation of Contaminant Migration from an UST
Through Fractured Porous Limestone 3-16
3-7 Macroscopic and Microscopic Concepts of Groundwater Flow 3-19
3-8 Determination of Wetting and Nonwetting Fluids 3-26
3-9 Fluid Saturation States (Regimes) of a Two-Phase System for a
Hydrophilic Porous Medium 3-28
3-10 Pressures on a Capillary Interface 3-30
3-11 Typical Retention Curves in Soil During Drainage 3-32
3-12 Effect of Moisture Content Hysteresis for a Coarse Material .... 3-34
3-13 Relative Permeability of Water in Unsaturated Sand 3-35
3-14 Variations of Relative Permeability of Wetting (K ) a'nd
Nonwetting (Krmw) Fluids With Saturation 3-37
3-15 Distribution of Fluid Flow for the Three Saturation Regimes .... 3-38
3-16 Effect of Hysteresis on Relative Permeability 3-40
-XIX-
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3-17 Schematic Distribtion of Air, Oil, and Water in a Porous
Med1 a 3-42
3-18 Relative Permeability as a Function of Saturation for
Ai r-Oi1-Water System 3-43
3-19 Schematic Diagrams of Different Contaminant Plume Shapes in
Soils of Varying Permeability in the Unsaturated Zone -3-47
3-20 Comparison of Primary Log Retention of Initially Dry and
Saturated Ottawa Sand by Column Drainage 3-53
3-21 Schematic of the Capillary Zone with a Typical Water
Saturation Curve 3-59
3-22 Variation in Water Content in the Capillary Zone with
Elevation 3-60
3-23 Contaminating Effect on Soil Caused by Fluctuating Water Table.. 3-65
3-24 Types of Dispersion 3-70
3-25 Schematic Diagram Showing the Contribution of Molecular
Diffusion and Mechanical Mixing to the Spread of a
Concentration Front 3-74
3-26 Effect of Layers and Lenses on Flow Paths in a Shallow
Steady-State Groundwater Flow System 3-75
3-27 Effect of Density on Migration of Contaminant Solution in
Uniform Flow Field 3-78
3-28 Growth, Steady-State Maturity, and Decay of a Vapor
Concentration Field, For a Point-Source Leak in a Deep
Homogeneous Mediurn 3-84
3-29 Steady-State Concentration Field for a Point-Source Leak Just
Above an Impermeable Lower Boundary (Ground Surface Permeable). 3-87
3-30 As in Figure 3-29, but now with Ground Surface Impermeable
(e.g. Frozen or Paved) 3-88
3-31 Steady-State Conditions as in Figure 3-28C, but with Vertical
Diffusion Coefficient 1/10 that of the Horizontal Diffustion
Coeff 1 ci ent 3-89
4-1 Langmuir and Freudlich Isotherms 4-15
4-2 Basic Activities and Requirements for a Cell to Utilize A
Contaminant 4-24
4-3 Contrasts in Electron and Carbon Flow 4-26
-xx-
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4-4 pH Dependence of KT for Hydrolysis by Acid-, Water-, and
Base-Promoted Processes 4-51
4-5 Examples of the Range of Hydrolysis Half-Lives for Various
Types of Organic Compounds in Water at pH 7 and 25 C 4-55
5-1 Common Hydrocarbons in Petroleum Products 5-6
5-2 Distillation Curves of Common Petroleum Products 5-9
5-3 Petroleum Products Grouped by Kinematic Viscosity 5-12
5-4 Solubility Data Distribution of Hydrocarbon Database 5-18
5-5 Percent Composition of Gasoline 5-19
5-6 Physical State Distribution of UST Hazardous Substances 5-30
5-7 Percent Inorganic and Organic UST Hazardous Substances 5-31
5-8 Solubility in Common Solvents of Solid Chemicals 5-34
5-9 Density Data Distribution of Liquid Chemicals 5-35
5-10 Vapor Data Density Distribution of Liquid Chemicals 5-36
5-11 Grouping Methodology 5-38
5-12 Toxicity Group Distribution 5-41
5-13 Solubility Data Distribution of Hazardous Substance Database ... 5-43
5-14 High, Medium, and Low Solubility Group Distribution 5-44
5-15 Vapor Pressure Data Distribution of Hazardous Substance
Database 5-46
5-15 High, Medium, and Low Vapor Pressure Group Distribution 5-47
5-17 High, Medium, and Low Ignitabil ity Group Distribution 5-49
5-18 Toxicity-Solubil ity Sort: High Solubility Results 5-55
5-19 Toxicity-Solubi 1 ity Sort: Medium Solubility Results 5-56
5-20 Toxicity-Solubility Sort: Low Solubility Results 5-57
5-21 Toxicity-Vapor Pressure Sort: High Vapor Pressure Results 5-58
5-22 Toxicity-Vapor Pressure Sort: Medium Vapor Pressure Results .... 5-59
5-23 Toxicity-Vapor Pressure Sort: Low Vapor Pressure Results 5-60
-xxi-
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5-24 Ignitability-Vapor Pressure Sort: High, Medium, and Low
Vapor Pressure Results 5-61
5-25 General Make-Up of Petroleum Products 5-63
6-1 Contaminant Transport Through Homogeneous & Heterogeneous
Soi 1 s 6-6
6-2 Climatic Effects on Subsurface Vapor Transport 6-14
6-3 Three-Digit Zip Code Map Florida 6-22
6-4 Liner-Nine Generic Groundwater Flow Fluids 6-24
6-5 Groundwater Regions of the United States 6-27
6-6 DRASTIC Hydrogeologic Setting 6-28
7-1 Chain of Events Linking UST Incidents with Potential Impacts ... 7-2
-xxn-
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LIST OF TABLES
Table Page
2-1 Major Products of Petroleum 2-3
2-2 U.S. 1983 Demand for Petroleum Products 2-5
2-3 U.S. Sales of Fuel Oils, By Uses 2-6
2-4 Evaluation of Petroleum Products Stored in Underground Tanks .... 2-7
2-5 Additive Type by Product 2-10
2-6 High Volume Chemicals Found in California USTs 2-18
2-7 Industry Classifications Not Included or Partially Included
on California List 2-22
2-8 Standard Industrial Classification Major Groups Not Included or
Partially Included on California List 2-23
3-1 Typical Values of Hydraulic Conductivity and Permeability 3-21
3-2 Primary Residual Saturation of Mineral Oil 3-51
3-3 Thickness of the Funicular Zone 3-64
4-1 Predominant Electron Acceptors Used in Electron-Transport Systems 4-25
4-2 Variables Potentially Affecting Biodegradation 4-32
4-3 Degradability of Hydrocarbons 4-39
4-4 Hazardous Substances Recognized as Readily Degraded 4-43
4-5 Resistance to Degradation of Pesticides 4-45
4-6 Persistance of Pesticides Likely to be Found in USTs 4-46
4-7 Types of Organic Functional Groups That Are Generally Resistant
to Hydrolysis 4-53
4-8 Types of Organic Functional Groups That Are Potentially
Susceptible to Hydrolysis 4-54
4-9 Hazardous Substances for Which Hydrolysis is a Significant
Fate Mechanism 4-56
5-1 Commercial Grade Classification of Petroleum Products 5-3
5-2 Kinematic Viscosity of Petroleum Products 5-10
-xxiii-
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5-3 Percent Aromatic Carbon Compounds 5-15
5-4 Percent Composition and Solubility of The Main
Constituents of Gasoline 5-20
5-5 Properties Related to Environmental Behavior 5-24
5-6 Chemical Classes of Hazardous Substances 5-32
5-7 Definitions Solubility in Water 5-43
5-8 Ignitability Scales for Reportable Quantity Adjustments 5-48
5-9 Pyrophoric, Explosive and High Water Reactive
Hazardous Substances 5-51
5-10 Chemicals Requiring Stabilization 5-52
5-11 Gas Phase Toxicity, Ignitibility, Solubility Data
Reportable Quantity Information 5-53
5-12 Hazardous Constituents of Gasoline and No. 2 Fuel Oil
Parametric Comparison 5-66
5-13 Average Heavy Metal Composition of Fuel Oil 5-69
5-14 Chemical Groups of Petroleum Solvents and Other
Industrial Solvents 5-70
6-1 Standard Environments for Human Health Risk Evaluation • 6-20
6-2 Four Mobility Classes 6-25
6-3 Assigned -Weights for DRASTIC Features 6-30
6-4 Ranges and Ratings for Depth to Water 6-31
6-5 Assigned Weights for UST DRASTIC Features 6-33
6-6 Ranges and Ratings for IMPACT Factors 6-34
6-7 Environmental Factors Affecting Liquid Fate and Transport 6-36
6-8 Environmental Factors Affecting Vapor Fate and Transport 6-37
7-1 Typical Program Activities 7-4
-XXIV-
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1.0 INTRODUCTION
1.1 PROJECT BACKGROUND
Substances leaking from underground storage tanks (USTs) can have adverse
impacts on public health and the environment. These impacts may include
the contamination of subsurface soils, the loss of an aquifer as a potable
drinking water supply, and the toxic or explosive buildup of vapors in
basements and other underground structures. To address this problem, the
1984 Resource Conservation and Recovery Act (RCRA) amendments call for the
development of a comprehensive regulatory program for underground tanks
storing petroleum and hazardous substances (subtitle I). This program, the
responsibility of EPA's Office of Underground Storage Tanks (OUST) of the
Office of Solid Waste and Emergency Response (OSWER), is designed to remedy
-- and ultimately to prevent -- threats to public health and the
environment from leaking underground storage tanks (USTs).
As part of the development of this program, this work assignment aims to
investigate the current understanding of the fate and transport of
regulated substances leaking from underground storage tanks. The overall
goal is to provide assistance to EPA in establishing the scientific and
engineering basis for the UST program regulatory efforts.
1.2 LEAKING UST PROBLEMS
Leaks of petroleum or hazardous substances from underground storage tanks
pose a threat to the environment and to public health. EPA has identified
spills or tank leaks of organic chemicals, especially hydrocarbons such as
gasoline, to be the most frequent contaminants of groundwater (USEPA,
1977). Four recent case studies well illustrate the types of problems
caused by leaking USTs.
1-1
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1.2.1 GROUNDWATER CONTAMINATION INCIDENTS
Provincetown, Massachusetts
In December 1977, the discovery of a leak in a gasoline storage tank
at a North Truro, Massachusetts automotive service station forced
the shutdown of Provincetown's nearby municipal well field. On
excavating the tank, it was estimated that as much as 2,000 or 3,000
gallons of high-test unleaded gasoline had been discharged into the
ground, less than 600 feet from the nearest well in the municipal
well field (Figure 1-1).
The gasoline percolated downward through the underlying sands until
it reached the groundwater table, about 45-50 feet below ground
surface, where it formed a pool of pure product that overlaid and
depressed the water table. Dissolved gasoline concentrations of up
to 20-80 parts per million (ppm) formed a plume of contaminated
water extending from the ends of the gasoline pool in the direction
of groundwater flow toward the well field.
To solve the immediate water supply problem, the Town installed or
gained temporary access to alternative sources of supply and
implemented a strict water conservation program. Cleanup of the
gasoline spill proceeded in two phases. The first phase involved
removing as much pure gasoline from the aquifer as possible. In
North Truro, however, the situation was complicated by the fact that
the water table was betweeen 45 and 50 feet below ground surface,
well below suction lift limits. Consequently, trenching,
excavation, and standard skimming techniques were ruled out.
Because of the limited extent of the gasoline lens, a two-well
system was judged sufficient for gasoline collection.
The second phase of the cleanup involved pumping and treating the
groundwater affected by the spill and recharging the treated
groundwater to the aquifer. The basic concept of this well field
cleanup program is to contain and remove the gasoline using a
36,000 gallon-per-day (gpd) groundwater recirculation cell. One
inner, shallow recirculation cell, consisting of four recovery wells
and a shallow recharge bed, contains and removes the contaminated
groundwater. An outer, deeper recirculation cell, with one
production well and one recharge chamber, can be operated at about
108,000 gpd. It provides additional containment in the event of
contaminated groundwater movement from the inner cell.
To meet stringent discharge criteria set by the State, the water is
treated by air stripping, filtration, and granular activated carbon
adsorption before being pumped to the recharge bed. In operation
since May 1985, the treatment facility will be operating for three
to five years before the'spill cleanup is completed and the system
is dismantled.
1-2
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OUTER CELL
CONTROL WELL
APPROXIMATE
LIMITS OF
OUTER HYDRAULIC
CONTAINMENT CELL
SITE
TREATMENT
PLANT
INNNER HYDRAULIC
CONTAINMENT CELL
^CONTAMINATED
,*oM
&,<'
t'4
^>
,,
-------
The sources and amounts for funding the various phases of the
project are: the Town of Provincetown, $1.1 million; Massachusetts
Department of Environmental Quality Engineering (DEQE), $1.2
million; Massachusetts Executive Office of Community Development
(EOCD), $750,000; and the U.S. Department of Housing and Urban
Development, $250,000. Although the well field is owned and
operated by Provincetown, it is located in neighboring Truro. The
towns jointly applied for the state EOCD grant towards construction
of the site treatment system.
Hal pole, Massachusetts Oil Contamination
During September and October, 1983, a series of events led to the
discovery of possible oil contamination of two important wells in
the School Meadow Brook Well field in Walpole.
The well field consists of a 150-acre parcel of property in
southeastern Walpole. The site is generally wetland, with areas
filled to construct the Town's public works facilities, including
well supplies and municipal garages. The well field's five
production wells at the site have a combined pumping capacity of
approximately 2.0 million gallons per day, producing about 60
percent of the Town's supply.
Routine backhoe excavations by Walpole DPW personnel to locate and
repair an existing drain in one area of the site indicated the
presence of subsurface oil contamination. Test holes were excavated
to determine the extent of the problem; soil and water samples were
collected from the backhoe test pits and analyzed for oil products.
Oil contamination was then confirmed in the backhoe test pits, the
test holes, and two nearby production wells.
Subsequent subsurface studies identified several potential sources
of contamination in the area: the soil and fill present over much of
an old DPW garage area, soil in an area previously used for storage
of oil, and the groundwater in the area immediately surrounding the
gasoline pumps at the new DPW fuel storage depot.
The origin of the oil was likely several abandoned USTs, as well as
previous on-site oil piping systems. The principal contaminants in
the garage areas are petroleum hydrocarbons dissolved in the
groundwater and adhering to the area soils. Water samples show the
presence of several volatile organics indicative of gasoline
contamination (benzene, toluene, ethylbenzene, and xylenes).
Consequently, the groundwater around this fuel storage area may also
be somewhat contaminated due to spillage or leakage. However, the
information collected indicated that the problem was not widespread.
It is likely that naturally-occurring biodegradation and adsorption
will reduce much of the contamination to non-detectable levels.
The recommended plan for cleaning up the contaminated area involves
excavation of contaminated materials from the old DPW garage area
and disposal of the materials offsite. This will remove the major
source of contamination, and, coupled with implementation of a site
management plan, should provide adequate protection of the drinking
water supply.
1-4
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Removal of contaminated soils and groundwater and off-site disposal
was performed during May and June, 1985. A total of 4,380 cubic
yards of contaminated soil was removed from the site and replaced
with clean fill. The total construction cost was $240,500.
1.2.2 VAPOR INCIDENTS
These case studies serve to demonstrate the potential for vapor phase
impacts, as well as groundwater and soil contamination, from UST leak
incidents.
• North Babylon, New York
In May 1983, a 4,000 gallon underground storage tank at a service
station in North Babylon, New York was discovered to be leaking
after fumes were reported in a commercial building next door
(Peterec, 1985). An estimated 100,000 gallons of gasoline released
underground reached the groundwater table and migrated towards and
under a subdivision of homes located across the street. Clean-up
began immediately to minimize groundwater and soil contamination.
An initial investigation in June 1983 discovered gasoline as a pure
.product "floating" on the groundwater 4 feet below the basement slab
of a home in the subdivision, though indoor air test results did not
detect the presence of any hydrocarbons. By late November 1983,
fumes were reported in numerous homes in the subdivision. A new
testing program then discovered a considerable concentration of
benzene in the basements and living areas of several homes. As
shown in Figure 1-2, investigators have concluded that vapors from
the floating gasoline pool migrated vertically through several feet
of coarse sand and gravel into the basements of residences, driven
in part by rising groundwater and the barrier to loss to the
atmosphere of freezing conditions at the surface of the ground.
Fume abatement systems were installed around these severely impacted
residences; several vertical perforated PVC pipes were inserted into
the soil and connected to a fan which withdrew the vapors before
they could enter the house. Meanwhile, the basement areas were
positively pressurized to reduce the vapor movement inward.
Though these abatement systems were effective in lowering the vapor
concentration in these residences, no standards for "safe levels"
have been established by state or federal agencies. In order to
protect the health of the individuals in the affected area, the gas
station owner was forced, through a court agreement, to purchase
twenty-one homes in the subdivision as well as to continue clean-up
and monitoring efforts. Source control measures to remove the
floating product and associated vapors were still underway as of
October, 1985. The remedial action program at this site has
provided more information and data than on any other spill on Long
1-5
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cr>
SEASONAL FROST
UNSATURATED ZONE
(COARSE SAND & GRAVEL)
FREE FLOATING HYDROCARBONS
• NOT DRAWN TO SCALE
CAMP DRESSER & McKEE INC.
FIGURE 1-2
VAPOR INCIDENT —NORTH BABYLON, NY
-------
Island, an area highly dependent on groundwater. This has resulted
in the North Babylon incident being possibly one of the best
documented incidents in the country.
• Livingston, Montana
Beginning in December, 1978, intermittent gasoline odors were
detected in a restaurant and adjacent house (Reichmuth, 1984). The
odor intensity appeared to increase during the winter months and
during periods of windy weather. In January, 1981, intense odors
prompted notification of the local fire marshal 1 who immediately
ordered evacuation of both establishements. Air monitoring found
hydrocarbon concentrations up to 9,000 ppm. This is just below the
lower flammable limit of air-gasoline mixture (14,000 ppm).
A subsequent investigation of two nearby gasoline service stations
found the service station furthest away to have a leaking LIST. The
actual leak occurred in the pipe connecting the 6,000-gallon
underground tank to the surface pump. Poor inventory control
practices and the uncertainty over how long the distribution line
had been actually leaking prevented an accurate determination of the
total gasoline loss. Estimates, however, ranged from 2,500 to 5,000
gallons.
Further investigation revealed that because of the direction of the
groundwater gradient, it was impossible for the groundwater to carry
gasoline between the leak and the restaurant. Attention was then
directed toward the subsurface soil environment which, as shown in
Figure 1-3, consisted of 15-20 feet of silty clay material above a
gravel layer. Vapor migration was eventually attributed to low
water table levels, especially during the winter months, which
exposed the normally saturated gravel. The high porosity of
underlying gravel allowed the vapors to easily migrate laterally
toward the restaurant and house located approximately 350 feet away.
During periods of high groundwater, the gravel layer was totally
saturated, thereby preventing the lateral migration of vapors. The
soil profile beneath the restaurant, in turn, consisted of 7 feet of
silty clay separating the gravel beneath the restaurant floor and
the gravel layer. Discontinuities in the silty clay layer provided
paths for the vapors to migrate relatively easily upward into the
restaurant. A residence 100 feet south of the leaking tank did not
experience any vapor problems apparently because the gravel layer in
that area was normally saturated by water. Two other conditions
also contributed to the variable flux in vapors: frozen soil
conditions and windy weather. Frozen soil conditions impeded
vertical movement by restricting loss to the atmosphere, thereby
promoting greater lateral movement. Windy conditions were closely
tied to low barometric pressures. This resulted in a "breathing"
effect whereby gas was released from the subsurface as pressure
fluctuated.
1-7
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RESTAURANT
SOUTH STATION
RESIDENCE
GASOLINE
SATURATED
ZONE
WATER LEVEL APPROX. 4508.2
GASOLINE
VAPORS
HORIZONTAL
SCALE IN FEET
GRAVEL
SOURCE: Adapted from Reichmuth (1984)
CAMP DRESSER & McKEE INC.
FIGURE 1-3
VAPOR INCIDENT —LIVINGSTON, MT
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1.3 OBJECTIVES AND SCOPE OF WORK
The incidents described above clearly demonstrate the complexity of leaks
from underground storage tanks. To assist in the development of a program
to address these incidents, the specific objectives of this work assignment
are:
• to summarize the current understanding of the physical, chemical,
biological, and toxicological processes relative to leaks of
regulated substances from underground tanks,
• to evaluate existing methodologies and data pertinent to fate and
transport processes with a potential for applicability to the UST
program, and
• to establish priorities for further investigation, data collection,
and/or methodological development.
To achieve these broadly-stated objectives, a set of specific tasks was
developed, as presented schematically in Figure 1-4. Our work began with
the identification of the components of the UST program within OSWER for
which an understanding of fate and transport phenomena is a prerequisite
(Task 1). Concurrently, existing data was used to assess the likely
character of current (and potential) leaking UST incidents (Task 2).
Analogous to the problem definition efforts of Task 2, Task 12 collected
data on actual leaking UST incidents where vapor phase processes either
were demonstrably important or where one might speculate that they could
have been. Similarly, Task 13 focused on development of a list of
hazardous substances likely to be stored in USTs primarily based on the
California UST notification program.
Task 3 (a review of relevant chemical, toxicological, and physical
properties of substances of interest) and Task 4 (an examination of the
environmental settings applicable to leaking UST incidents) start the key
technical/scientific aspects of the work. Task 5 (an analysis of the
current state of understanding of the various fate and transport mechanisms
that appear to be most relevant to the substances and settings of interest
to the UST program) uses the information gathered in Tasks 3 and 4.
1-9
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IDENTIFY LUST PROGRAM
NEEDS W/R F&T PHENOMENA
• OSWER PROGRAM REQUIREMENTS
• FIELD LEVEL NEEDS
i SUMMARIZE CHARACTER
OF VAPOR INCIDENTS
FROM LEAKING UST
• INCIDENT DATA
• DECISION NEEDS
SUMMARIZE CHARACTER
OF LUST PROBLEMS
•TANK POPULATIONS
•QUANTITIES OF
STORED MATERIALS
•INCIDENTS
SURVEY CHEMICAL
AND PHYSICAL
PROPERTIES
• TOXICITY
• SOLUBILITY
•DENSITY
e
J ANALYZE/REVIEW
PRESENT INFORMATION
ON F&T PHENOMENA
• FATE
• TRANSPORT
IDENTIFY AND EVALUATE
EXISTING F&T METHODOLOGIES
• OTHER EPA PROGRAMS
• INSTITUTIONAL /PRIVATE
IDENTIFY HAZARDOUS
SUBSTANCES STORED
IN UST
CALIFORNIA LIST
LIKELY TO BE IN UST
ESTABLISH RELEVANT
ENVIRONMENTAL
SETTINGS
•CLinflTOLOGV
•HVDROGEOLOGV
•DEHOGRflPHICS
7 RECOMMEND APPLICABLE METHODOLOGIES
• FOR PROGRAM ISSUES
1 FOR QUALITATIVE SCREENING
IDENTIFY FUTURE NEEDS
•DATA COLLECTION
•METHODOLOGY DEVELOPMENT
• IMPLEMENTATION
NOTE: F&T = FATE AND TRANSPORT
CAMP DRESSER & McKEE INC.
FIGURE 1-4
TASK FLOW CHART
l-in
-------
This Interim Report is being submitted as part of Task 5. It conveys the
reasoning (based on Tasks 1-5 and 12-13) that underlies the selection of
the specific fate and transport phenomena we believe to be important for
leaking UST decision-making. For this selected set of fate and transport
phenomena, the report discusses the current state-of-the-art regarding the
technical/scientific understanding of (and data base supporting) each item.
The Interim Report is to be followed by Task 6, which involves a thorough
review of methodologies/techniques/procedures in professional use to
analyze those fate and transport phenomena judged to be important in Task
5. With knowledge of current methods in hand, Task 7 would develop
recommendations of the methods most applicable to the needs of the UST
program. At the completion of this task, a second interim report would be
prepared. This report would outline the specific aspects of the UST
program's needs for which we recommend and suggest appropriate
methodologies with which to analyze fate and transport issues. Task 8
would then outline the technical areas in need of future effort. These
needs will reflect the recommendations developed in Task 7.
The objective of this report is to present the results of the first phase
of this project, specifically to summarize the current understanding the
mechanisms or processes which govern fate and transport of substances
leaking from underground tanks. It is the intent of this report to provide
this information as background to the ongoing program development and
implementation efforts, and to seek comment to focus the further efforts of
this work assignment to those critical areas where additional study would
be most useful for current and future UST program tasks.
1.4 RELATION TO OTHER STUDIES
To summarize the current understanding of the mechanisms and processes that
govern fate and transport of substances leaking from underground storage
tanks, an extensive review of the literature was performed. Also,
researchers, engineering consultants, and industry representatives were
contacted. Through these contacts the project team gained an understanding
1-11
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of the research being conducted and shared information with several OSWER
contractors who are or have been performing other pertinent work.
The literature review was facilitated by several computerized literature
searches. The data bases that were queried for information through DIALOG
information services were:
• Chemical Abstracts 1972-1985
• NTIS 1964-1985
• Waternet 1971-1985
• Pollution Abstracts 1970-1985
The references obtained from these sources and through the contacts
described above that were relevant to this study are included in Section
8.0. Specific literature used in our study is cited throughout the report,
particularly in Sections 3.0 through 6.0 on the current state of the
science.
A sample of some of the contacts made and general topics discussed is worth
highlighting. These are:
• Through OUST, information was gathered from a database being
developed for the State Release Incident survey.
• Data on the hazardous substances likely to be found in USTs and the
consituents of gasoline and other petroleum products was shared with
ICF in conjunction with their on-going work with OSWER/OPPE.
• A meeting was held with several staff members of the EPA R.S. Kerr
Research Laboratory in Ada, Oklahoma to discuss their current
research projects related to this work assignment.
t Information on the analysis of the California LIST notificaton data
base was obtained from Data Resources, Inc.
• Studies done by the American Petroleum Institute relating to fate
and transport were obtained through Todd G. Schwendmann.
• A brief discussion was held with Linda Alien of the National Water
Well Association on UST DRASTIC/IMPACT.
1-12
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• A meeting was held with Jeff van Ee and other staff members from
EPA/Environmental Monitoring System Laboratory (Las Vegas) on the
research they are conduting on techniques for monitoring leaks
outside USTs.
• Dr. David Kraemer of Arizona State University was contacted
concerning vapor transport mechanisms.
• A meeting with Steve Ragone of the USGS was held to discuss
environmental factors that influence fate and transport of
substances from leaking USTs and to discuss the USGS Toxic Waste
Groundwater Contamination Program.
• Information on work being conducted by ICF, especially the chemical
composition of petroleum products was obtained; and a meeting was
held with James R. Janis and James Bunting to discuss the
applicability of our technical findings to UST activities.
• A meeting was held with David Berg of EPA/ORD to discuss UST related
research that is being conducted at the Las Vegas, Cincinnati,
Edison, New Jersey and the Ada, Oklahoma facilities.
1.5 RELEVANT UST PROGRAM ISSUES
In order to focus the findings of this first phase of this work assignment,
a list was developed of relevant UST program issues. Although the focus of
our efforts and nature of our findings are technical, the purpose of
developing this list of issues and reporting them herein is to provide a
tie to on-going UST program deliberations. This list is probably not
al'1-inclusive, and in fact, a briefing for UST program staff is planned
during the review period of this report to both present and discuss the
technical findings and to complete this list of relevant policy/regulatory
issues.
A sampling of the relevant UST program issues which are addressed in the
report is:
• Whether there is a technical basis for distinguishing between
petroleum products and hazardous substances. (Sections 5.2 and 5.4)
• Whether and how petroleum products and hazardous substances might be
aggregated into groups for regulatory and standards-setting purposes
(Sections 5.2.3 and 5.3.3).
1-13
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• Whether representative (indicator) substances or chemicals can be
associated with certain chemical groups (Sections 5.3.2 and 5.4).
t Whether leak incidents can be characterized and possibly prioritized
by the mobility and/or toxicity of tank contents. (Sections 5.0
and 7.0).
• Whether contamination of groundwater is a consequence of all UST
leaks and must be addressed in all corrective actions. (Section 3.0
and 7.0).
• Whether regulatory distinctions could be drawn based on
environmental settings (Sections 6.0 and 7.0).
• Whether and how distinctions in leak incidents can be attributed to
the factors given in Section 9003 of subtitle I, e.g., location of
tank, soil and climate conditions, uses of tanks, hydrogeology,
water table, size of tank, etc. (Section 3.0 and 6.0).
• Whether and how vapors resulting from UST leaks pose a threat to
human health and the environment. (Section 3.6 and 4.3).
• Whether and how corrective actions (in cases of vapor or liquid
incidents) may be organized or prioritized. (Section 7.0).
• Whether and how prioritizeation might be achieved by groups of
products or substances matched with environmental settings.
(Section 7.0).
We should note that although many of the references in the above bullets
are to Sections 5.0 and 7.0 of this report, it is the basic understanding
of fate and transport processes in Sections 3.0 and 4.0 that form the basis
for these findings.
1.6 ORGANIZATION OF REPORT
To evaluate the potential seriousness of a leaking UST incident, a thorough
understanding of its fate and transport in the subsurface environment must
be developed. The purpose of the next several sections is to present a
comprehensive picture of the current understanding of the fate and
transport of substances (regulated by the UST program) after they have
leaked from an underground tank.
1-14
-------
First, we present in Section 2.0 an overview of the substances to be
regulated under the LIST program, specifically the petroleum and hazardous
substances covered by the RCRA subtitle I. While the fate and transport of
a chemical in the subsurface are interwoven, they are, for ease of
presentation, discussed as separate mechanisms in Sections 3.0 and 4.0.
The transport of a substance can be described by the mechanisms that govern
its movement in the environment. The fate of a substance includes all of
the physical, chemical, and biological changes it undergoes in the
environment.
The objective of the transport sections is to answer the question: What
governs the movement of a chemical within each of the compartments? The
fate of a chemical in the subsurface can be best understood in relation to
the following questions: How will a chemical partition among the major
compartments and their constituent media, and where and to what extent does
a chemical degrade and/or transform? Five fate mechanisms were determined
to be most important for chemicals leaking from USTs: solubility,
vaporization, adsorption, biodegration, and abiotic chemical
transformations.
The physical and chemical nature of the substances stored in underground
tanks, and particularly the physical, chemical, and toxicological
properties relevant to fate and transport mechanisms are then presented in
Section 5.0. The environmental factors and/or settings which can be used
to describe leaking LIST scenarios are discussed in Section 6.0.
Finally, in Section 7.0, we summarize the technical findings of this report
in the context of their applicability to the development of the UST
regulatory program. All of the references used in the project are listed
in Section 8.0
1-15
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-------
2.0 UST PROGRAM REGULATED SUBSTANCES
2.1 OVERVIEW
As defined in RCRA subtitle I (the UST legislation), the substances to be
regulated under the UST program include hazardous substances and petroleum
products, as follows:
"(A) any substance defined in section 101(14) of the Comprehensive
Environmental Response, Compensation, and Liability Act of 1980
(but not including any substance regulated as a hazardous waste
under subtitle C), and
(B) petroleum, including crude oil or any fraction thereof which
is liquid at standard conditions of temperature and pressure (60
degrees Fahrenheit and 14.7 pounds per square inch absolute
pressure)."
The hazardous substances regulated under CERCLA include 698 substances,
which may be found in a variety of mixtures, tailored to a specific end
use. Petroleum is refined into many products, some of which, in turn, are
often used as ingredients for petroleum byproducts (e.g., petrochemicals,
plastics, or rubber). The purpose of this section is to describe the
regulated substances stored in underground tanks, and more specifically,
those to be considered in this study.
2.2 REGULATED PETROLEUM SUBSTANCES IN USTs
2.2.1 OVERVIEW
Petroleum substances can be considered in two major groups:
• Crude oil and its intermediate substances — Crude oil is petroleum
in the state it was extracted from the earth. Its intermediate
substances include all other chemical mixtures that may be present
during the refining process.
t Finished petroleum products — Petroleum products are primary stocks
from the refining process that may have been blended, compounded or
otherwise modified to become commercial products. (Note that this
definition does not include secondary products (byproducts) of
petroleum, such as products of the petrochemical industry).
2-1
-------
We have used three criteria to determine which petroleum substances should
be studied in this report:
• whether the petroleum substance meets the statutory definition of a
regulated petroleum substance (a liquid at 60 F and one atmosphere
pressure),
• whether the petroleum substance is commonly stored in underground
tanks, and
• whether the substance is widely used in the United States.
The first two criteria isolate those petroleum substances likely to be
regulated under the LIST program, while the third criterion helps to focus
study on the petroleum substances of greatest concern under this program.
While crude oil. and most of its intermediate substances meet the legisla-
tive definition of a regulated petroleum-substance, they are not likely to
be stored in underground tanks (even when piping volumes are considered).
The sheer volume of crude oil processed at refineries makes it impractical
to store crude oil or its intermediate products underground. Therefore,
crude oil and its intermediate products need not be considered further in
this report.
The finished petroleum products that meet the criteria stated above for
regulated substances under the LIST program will be the focus of this study.
2.2.2 PETROLEUM PRODUCTS
A wide range of petroleum products can be refined from crude oil --
blended, compounded, or otherwise modified into commercial products ranging
from highly refined gasolines, to heavy fuel oils, to asphalt. Table 2-1
presents a summary of the names of petroleum products that can be derived
from crude oil. It should be noted that these names are not necessarily
the refiner's names. Thus, different names may be used to describe the same
basic refiner's product. For example, mineral spirits and Stoddard solvent
2-2
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TABLE 2-1
MAJOR PRODUCTS OF PETROLEUM
Power Fuels
Aviation Gasoline, Motor Gasoline, Diesel Fuel Oils, Naphtha-type Jet
Fuel, Kerosene-type Jet'Fuel and Gas-Turbine Fuel Oils.
Heating Oils
Liquified Petroleum Gas and Fuel Oils (e.g., Nos. 1, 2, 4, 5, and 6).
Illuminating Oils
Kerosene and Mineral Seal Oil
Solvents
Stoddard Solvents, Petroleum Spirits, Petroleum Extender Oils, and
Aromatic Solvents
Lubricants
Automotive Lubricants and Industrial Lubricants
r
Still Gas
Petroleum Coke
Building Materials
Asphalt Cements, Liquid Asphalts, Joint Filler, Roofing Asphalt,
Emulsified Asphalts and Road Oils
Insulating and Waterproofing Materials
Transformer Oils, Cable Oils, Waterproofing Asphalt and Waxes
Other Products
Cutting Oils, Heat Treating Oils, Heat-Transfer Oils, Hydraulic Oils,
Tree-Spray Oils, Insecticide-Base Oils, Weed-Control Oils, Medicinal
White Oils, and Petroleum Jelly
Sources: Standen (1967) and U.S. Department of Energy (1984)
2-3
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are generally the same refined 300-400°F-boiling-range naphtha (Standen,
1967). Also, distillate fuel oils are blended to make a wide variety of
commonly-known fuel products, such as diesel fuel oil. We recognize that
by using these names some overlap in Table 2-1 occurs.
The major categories of petroleum products were assessed for the three
criteria defined in 2.2.1. In order to determine the third criterion
(i.e., whether a product is widely used), Table 2-2 gives the U.S. 1983
demand for petroleum products. (Note that the categories of products listed
in Table 2-2 do not necessarily coincide with the products listed in Table
2-1 because of differences between names in this table and refiner's
names). The uses of distillate and residual oils are presented in Table
2-3.
Table 2-4 presents results of the three-criteria assessment. The petroleum
products studied in this report include:
• Power fuels - aviation gasolines, motor gasolines, diesel fuel oils,
jet fuels, and gas turbine fuel oils.
• Heating oils - fuel oils (Nos. 1, 2, 4, 5, and 6).
• Solvents - Stoddard solvent, petroleum spirits, petroleum extender
oils, and aromatic solvents.
• Lubricants - automotive and industrial lubricants.
The commercial grade classifications of the above products, including The
American Society of Testing Materials (ASTM) specifications, are presented
in Table 5-1, in Section 5.2.2.
2.2.3 PETROLEUM ADDITIVES
Additives are widely used in petroleum products -- including many of those
of importance in this report — to modify the physical and/or chemical
characteristics of the pure product, both to ensure product quality
requirements and to meet performance criteria.
2-4
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TABLE 2-2
U.S. 1983 DEMAND FOR PETROLEUM PRODUCTS
(Million Barrels)
Motor Gasoline 2417
Distillate Fuel Oil 982
Residual Fuel Oil 519
Jet Fuel (Kerosene Type) 306
Jet Fuel (Naphtha Type) 76
Lubricants 53
Kerosene 46
Special Naphthas1 30
Aviation Gasoline 9
Source: U.S. Department of Energy (1983)
Special naphthas are all finished products within the gasoline range that
are used as paint thinners, cleaners or solvents, including all commercial
hexane and cleaning solvents.
2-5
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TABLE 2-3
U.S. SALE OF FUEL OILS, BY USES
(Thousand Barrels)
Use
Vessels Bunkering
Electric Utility Company
Rail roads
Oil Company Use
Oil Company Fuel
Industrial Use
Heating Oils
Military Use (3)
Diesel
Miscellaneous Uses
TOTALS
Residual Oil
136,290
204,238
140
32,820
(1)
89,573
46,743
5,411
N/A
3,389
518,604
Distillate Oil
33,785
11,744
91,681
(1)
29,957
69,332(2)
311,289
15,925
399,013
19,201
981,927
Source: American Petroleum Institute (1985)
N/A - not available
(1) - different headings were used in the source tables. It is uncertain
if these two categories define the same use.
(2) - Includes oil company use.
(3) - Includes imports by military
2-6
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TABLE 2-4
EVALUATION OF PETROLEUM PRODUCTS
STORED IN UNDERGROUND TANKS
PRODUCT*
I
Meets Statutory
Definition
Power Fuels 0
Heating Oi Is 0
Liquified Petroleum
Gas X
3
Illuminating Oils 0
Petroleum Solvents 0
4
Lubricants 0
Still Gas X
Petroleum Coke X
c
Building Materials X
Insulating and
Waterproofing
Materials S
Other Products7 S
0 = meets criteria X = does
CRITERIA
II
Tank
Storage
.
•
0
0
0
0
X
X
X
s
s
not meet criteria
III
Widel
Used
0
0
0
X
0
0
X
0
0
X
s
s
Included
y In This Study
0
0
X
X3
0
0
X
X
X
X
X7
= some products
criteria, see
on next page
meet
notes
Notes on next page.
2-7
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TABLE 2-4 Continued
See Table 2-1 for a more detailed list of these product groups.
o
Criteria are as follows: (1) meets the statutory definition of a petroleum
substance, .i.e., a liquid at 60°F (15.6 C) and one atmosphere pressure;
(2) is known to be stored in underground storage tanks; and (3) is widely
used.
3
While illuminating oils are liquids, they are not commonly stored in USTs
when used for this purpose. However, kerosene will be studied because it
is essentially a No. 1 fuel oil and is included in that category.
Note that underground storage of lubricants is probably only found with
respect to bulk sales which includes only a fraction of their use.
Building materials include both liquid and solids. Most liquid building
materials are highly viscous and are not stored in underground tanks.
Some building materials may meet all three criteria, such as road oils,
which are similar to the lighter distillate oils.
'These materials include both liquids and solids, having highly specialized
6
uses.
The other products, as defined on Table 2-1, are liquids except for
petroleum jelly. The frequency with which these liquids are stored in
underground tanks is unknown. These products have highly specialized uses
and may be found in a variety of forms, however, due to the large variety
of forms. Cutting oils and heat treating oils are used widely in
industry; however, due to the large variety but limited volume of oil
marketed, these were not studied.
2-8
-------
Additives are also used as supply extenders, particularly in motor
gasoline, to economically increase the stock without greatly sacrificing
product quality. Additives usually constitute only a small portion of the
product volume, at concentrations ranging from a few parts per million to
as much as 10 percent.
The principal petroleum products containing additives are power fuels, fuel
oils, and lubricants. These products — often stored in underground tanks
— are also the petroleum products in greatest demand (Table 2-2). Table
2-5 identifies the additive types commonly found in these products
(kerosene and special naphthas are excluded from Table 2-5 because the use
of additives in these products could not be confirmed). Although the
additive types indicated are typically used in the products listed, their
use is dependent on the manufacturer and on the product's end use and thus
they may not be found in all cases.
The following paragraphs discuss the additive types listed in Table 2-5
with respect to their uses and chemical composition.
t Anti-foam — Anti-foam additives are used principally in lubricating
oils to reduce foaming. These additives normally consist of sili-
cones or organic copolymers.
• Anti-knock — Anti-knock additives are used to increase the octane
number of aviation and motor gasoline in order to achieve more ef-
ficient combustion. The most widely used anti-knock agents have
been lead derivatives such as tetraethyl lead and tetramethyl lead.
Due to mandated reduction in the lead content of gasoline, agents
containing lead have been steadily replaced by other lead-free
anti-knock additives such as ethyl alcohol ("gasohol"), ether and
aromatic compounds.
• Anti-oxidant — Anti-oxidants are used in fuels and lubricating oils
to prevent the oxidation of unstable hydrocarbon components which
could lead to the formation of gum deposits or acidic bodies. Com-
mon anti-oxidant additives include amine and phenol compounds such
as phenylene diamine and butylphenol. Other anti-oxidants include
zinc, calcium, barium and magnesium salts, including thiophosphates,
salicylates, phenates, and sulphonates.
• Anti-corrosion -- Anti-rust agents inhibit corrosion by coating
metal surfaces with a very thin protective film that prevents water
from contacting the surfaces. This protection is especially
important in storage tanks, pipelines, tankers, and engine fuel
systems. Common anti-rust additives include various fatty acid
amines, sulfonates, alkyl phosphates, and amine phosphates.
2-9
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TABLE 2-5
ADDITIVE TYPE BY PRODUCT
PRODUCT
ADDITIVE TYPE
Anti-foam
Anti-knock •
Anti-oxidant •
Anti-corrosion
Ant i-wea r/Ext reme
Pressure
Color Dyes •
Deicers •
Detergents/
Dispersants
Friction
Modifiers
Metal
Deactivators
Pour Point
Depressants
Static Dissipator
Supply/Octane
Improver
Tackiness Agents
Viscosity Index
Improver
AVIATION
AVIATION TURBINE
GASOLINE FUEL
DISTILLATE
FUEL
OIL
LUBRICATING
OILS
MOTOR
GASOLINE
RESIDUAL
FUEL
OIL
2-10
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• Anti-wear/extreme pressure — Anti-wear/extreme pressure additives
are used to improve lubrication properties under high load and high
temperature conditions. Anti-wear/extreme pressure additives in use
include fats, oils, and waxes and their derivatives, and compounds
of sulfur, chlorine and phosphorus. Metal dialkyldithiophosphates
are common anti-wear/extreme pressure additives for lubricating
oils.
• Color dyes -- Organic dyes are added to gasolines and aviation fue.ls
principally for the identification of various brands or grades of
fuel. They are also used in gasolines to increase sales appeal.
The concentration of dye is relatively small.
• Deicers -- Anti-icing agents are used to ensure that dissolved water
in fuels, which will tend to come out of solution as the fuel temp-
erature drops, does not freeze in the fuel system. The deicer may
be either a freezing-point depressant or a surface-active agent.
Freezing point depressants act in the same manner as does anti-
freeze in an engine's cooling system and include a variety of alco-
hols and glycols. The most common freezing-point depressants are
dipropylene glycol, hexylene glycol, ethanol , and iso-propanol.
Surfactants prevent adherence of ice crystals to each other and on
metal surfaces. The surfactants used to provide this property
comprise amides, amines, and amine or ammonium salts of
organophosphates.
• Detergents/dispersants -- Detergents/dispersants are widely used in
gasolines and engine oils to prevent formation of deposits on inter-
ior engine surfaces and to remove deposits from dirty surfaces.
This additive is especially important in gasolines because of the
susceptibility of engine carburetors to fouling from deposits.
Commonly used detergents in gasoline include amides and
alkylammonium dialkyl phosphates. Detergents used in engine oil are
most frequently barium and calcium sulfonates and phenates.
t Friction modifiers — Friction modifiers are used principally in lu-
bricating oils to adjust the frictional properties of the oil. It
is often desirable to reduce the frictional resistance of oils in
order to facilitate more efficient operation. Friction modifiers
include organic acids, amines or natural fat, oils and waxes and
their derivatives.
• Metal deactivators -- Metal deactivators are used in fuel and lubri-
cating oils and act (as chelating agents) by binding trace amounts
of dissolved copper that may be present in the fuel. Free copper is
undesirable because it is a powerful oxidation catalyst. If copper
is present even the most effective antioxidant might not be able to
prevent oxidation and the resulting gum deposits without the help of
a metal deactivator. Metal deactivators are usually diamines,
thriazoles, or thiazoles.
• Pour point depressants -- Pour point depressants are used mainly in
lubricating and fuel oils to improve their fluidity at low tempera-
tures. The pour point is an indication of the lowest temperature at
2-11
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which a fuel or lubricating oil can be stored and still be capable
of flowing under gravity forces. Common pour point depressants are
aromatic paraffins and alky! methacrylate polymers.
• Static dissipator — Static dissipators, or electrical conductivity
additives, are used principally in aviation turbine fuels to
increase the conductivity of the fuel and thereby reducing the
possibility of an electrostatic discharge causing an explosion
during high-speed refueling. ASTM specifications for aviation
turbine fuel additives restrict the use of a static dissipator to
two proprietary additives, Shell ASA-3 and Stadis 450.
• Supply/octane improver — Supply/octane quality additives have been
used recently to meet gasoline demand and quality requirements as
economically as possible. These additives are blended with finished
motor gasoline (in quantities upwards of 10%) to increase the supply
at an economical price. Common additives include ethanol, t-butyl
alcohol, methyl-t-butyl ether, and methanol. In addition to serving
as supply improvers, these additives have also proven beneficial to
increasing the octane number, thus doubling as an effective anti-
knock agent.
• Tackiness agent — Tackiness agents are sometimes used in
lubricating oils to make the oil adhere more firmly to surfaces.
These agents consist of high molecular weight polymers such as
polybutenes.
• Viscosity index improver — Viscosity index improvers are used in
lubricants to modify the viscosity/temperaure characteristics of the
lubricant. Viscosities of liquids are sensitive to temperature and
decrease with increasing temperature. These additives serve to re-
duce the rate at which viscosity decrease as temperature increases.
Viscosity index improvers include oil-soluble olefin polymers, alkyl
styrene polymers, and methacrylate fatty alcohol/lower alcohol ester
copolymers.
It is virtually impossible to create a detailed, specific, and complete
picture of additives in petroleum products. Each type of additive
described above has within it many associated chemicals, and the lucrative
market for effective chemical additives generates new formulations daily.
Moreover, as most additives are patented, their chemical formulations and
uses in specific products are considered proprietary. There are both
single and multi-purpose additives, and different batches of the same
petroleum product may contain either. Because of these difficulties, it is
usually possible to specifically identify only the most common chemical
additives.
2-12
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Many petroleum additives are hazardous substances. Therefore, their
presence in a petroleum product is important when addressing the
environmental significance of leaks from underground storage tanks, the
majority of which contain petroleum products. Most of the additives are
only present in concentrations of several parts per million and may not
pose a severe contamination threat by themselves.
Identifiable petroleum additives which are also on the list of hazardous
substances stored in underground tanks include:
Tetraethyl lead
Ethylene dibromide
Ethyl ene di chloride
Dimethyl amine
Methanol
As stated previously, other hazardous substances regulated by CERCLA may
also be used as petroleum product additives.
2.3 REGULATED HAZARDOUS SUBSTANCES IN USTs
2.3.1 OVERVIEW
The hazardous substances regulated under CERCLA were published by EPA as a
final rule in the Federal Register (USEPA, 1985a). This list includes 693
hazardous substances (specific chemicals and waste streams).
The list of substances that are designated as hazardous substances under
CERCLA was taken from five other statutory sources. These sources are:
• Clean Water Act Section 311(b)(4);
• Clean Water Act Section 307(a);
0 Clean Water Act Section 112;
t Resource Conservation and Recovery Act Section 3001; and
• Comprehensive Environmental Response, Compensation, and Liability
Act Section 102.
2-13
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The first four statutory sources listed above were used under the LIST
program to designate the hazardous substances on the CERCLA list. Section
102 of CERCLA was used to designate the hazardous wastes under RCRA
hazardous substances under CERCLA. Therefore, all specific chemicals
listed on the CERCLA list are regulated under the LIST program. The waste
streams are included in the list of regulated hazardous substances under
the LIST program until the wastes are discarded or intended to be discarded
as wastes.
Recent estimates of the universe of USTs to be regulated by EPA set the
hazardous substance portion of the tank population at less than one-tenth,
with most regulatable tanks containing petroleum substances (e.g., Data
Resources, Inc. 1985). Although the degree of risk from leaking USTs may
not be directly related to the number of tanks, these estimates would
indicate that relatively more incidents may result from tanks containing
petroleum substances than from tanks containing hazardous substances.
2.3.2 DATA SOURCES
Although extensive information is currently available about hazardous sub-
stances, few sources of information specifically provide data about the
likelihood of storage of these substances in underground tanks. There is
also no single characteristic of hazardous substances which immediately
points to the suitability of a substance for underground storage. These
substances exist as liquids, gases, and solids at ambient temperatures and
pressures, and are produced and used in a variety of forms and volumes.
The following describes the available data sources, and the results of our
review of these in terms of a list of regulated hazardous substances likely
to be found in USTs.
2-14
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California List of Hazardous Substances
Description. The most important source of data for this task is the list
of hazardous substances in USTs which was developed by the State of
California. This list contains substances that were reported by owners of
USTs as part of a statewide registration program. The California (CA) list
of hazardous substances stored in underground tanks identifies 1778
substances (1248 chemicals and 530 generic names of mixtures, e.g., "acid
and caustic mix," "No. 3 fuel oil"). The program registered both product
and waste tanks. Only those substances previously defined as hazardous by
one of the following agencies were included on the CA list of hazardous
substances in underground tanks:
• California Department of Health Services
• The Director's List of Hazardous Substances, Director of the
Department of Industrial Relations, State of California
• Division of Water Quality, State Water Resources Control Board
• United States Environmental Protection Agency
• Registry of the Toxic Effects of Chemical Substances, National
Institute for Occupational Safety and Health, U.S. Department of
Health and Human Services
The substances on the California list are not all stored as pure product in
underground tanks. The substances listed include chemicals that comprise
mixtures and/or solutions, the "hazardous" chemical components of the
mixtures, as well as pure chemicals. The California data do not indicate
whether the chemicals are pure chemicals or chemical components of mix-
tures, or whether the chemicals/mixtures are wastes or products. Many of
the substances are solids or gases which will only appear in USTs in aque-
ous solution or, more commonly, dissolved in organic solvents/carriers.
As noted by Data Resources, Inc. (1985), of the 14,000 records in the
California data base coded as non-motor fuel tanks, approximately 5,500
have no entry in the field for type of product stored. This means that
only about 60% of the data could be analyzed for tank contents.
2-15
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Use in This Report. The 698 substances on the CERCLA list were compared to
the 1,248 chemicals on the California list. Cross-checking of both
chemical numbers and names was performed because chemical numbers other
than Chemical Abstracts Service Registry Numbers (CASRN) were sometimes
used to identify chemicals on the California list. The comparison allowed
the selection of the 462 hazardous substances from the California list
which are also on the CERCLA list. Figure 2-1 presents this comparison
graphically. These 462 substances, listed in Appendix A, are likely to be
found in USTs but may include chemicals both stored as products or as
wastes. Those found in USTs only as wastes may not be subject to
regulation under the UST program.
2.3.3 DATA RESOURCES INC. AND QUANTUM ANALYTICS ANALYSIS OF CA LIST
Data Resources, Inc. (DRI) and Quantum Analytics (QA) have analyzed the
California data and produced a final draft report (Data Resources, Inc,
1985). Using a variety of analyses, DRI identified the high volume CERCLA
chemicals found in USTs and ranked them by volume of product and percent of
regulated tank population. Pertinent information is presented in Table
2-6.
2.3.4 FIRE PROTECTION GUIDE ON HAZARDOUS MATERIALS
Description
The Fire Protection Guide on Hazardous Materials (1984), produced by the
National Fire Protection Association, provides data on the flammabil ity of
hazardous substances as well as useful information about their physical
states, physical properties, and methods of shipping. Two of the four
documents that make up the Fire Protection Guide on Hazardous Materials
(1984) - NFPA 325 M (1984) and NFPA 49 (1984) - have been used as data
sources in this study.
NFPA 325 M (1984) summarizes available .data on the fire hazard properties
of more than 1,500 substances. Listed with each substance are, if
applicable, its flash point, ignition temperature, flammable limits,
2-16
-------
1248
HAZARDOUS
CHEMICALS
ON
CALIFORNIA
LIST
462
HAZARDOUS
SUBSTANCES
ON
CALIFORNIA
AND CERCLA
LISTS
CAMP DRESSER & McKEE INC.
FIGURE 2-1
COMPARISON OF THE HAZARDOUS SUBSTANCES
ON THE CERCLA LIST WITH THE HAZARDOUS
SUBSTANCES ON THE CALIFORNIA LIST
-------
TABLE 2-6
HIGH VOLUME CHEMICALS FOUND IN CALIFORNIA USTs
CAPACITY
NO. OF TANKS
CHEMICAL
Sodium Hydroxide
Sul furic Acid
Toluene
Dimethyl Ketone (acetone)
Methyl Ethyl Ketone (MEK)
Chromium
Potassium Hydroxide
Nickel
Xylene, all monomers
Methyl Alcohol
PERCENT
9.3
6.4
5.4
4.6
4.4
4.3
3.6
2.8
2.6
2.5
RANK
1
2
3
4
5
6
7
8
9
10
PERCENT
6.1
4.4
5.9
6.5
5.4
2.6
0.7
1.8
3.8
3.4
RANK
2
5
3
1
4
11
33
17
6
7
Subtotal
45.9%
40.6
Source: Data Resources, Inc. (1985)
2-18
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specific gravity, vapor density, boiling point, and relative solubility in
water. In addition, there is a code listed for extinguishing methods, and
for three hazard identifications: health, flammability and reactivity. Of
most immediate interest are the following five flammability codes:
Flammability Code Description
4 Very flammable gases or very volatile flammable
liquids
3 Materials which can be ignited under almost all
normal temperature conditions.
2 Materials which must be moderately heated before
ignition will occur.
1 Materials which must be preheated before ignition
can occur.
0 Materials that will not burn.
NFPA 49 (1984) is a compilation of information on hazardous chemicals which
focuses on hazardous properties and fire fighting phases. It includes,
along with the hazard codes for each chemical, a description of the
chemical and data regarding the fire and explosion hazards, life hazard,
personal protection, fire fighting phases, usual shipping containers, and
storage.
Use in This Report
The remaining 236 hazardous substances on the CERCLA list (i.e., those not
already selected through comparison with the California list) were compared
to the chemicals listed in NFPA 325 M and NFPA 49. Liquids or gases which
have a NFPA flammability code of 2, 3 or 4, and are on the CERCLA list were
selected for further consideration.
The National Fire Protection Association recommends outside or detached
storage for these substances. One method of storage for flammable or ex-
plosive substances is in underground tanks where the substances are
protected from high ambient temperatures and contact with fire or other
flammable substances. In addition, the underground tanks are protected
2-19
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from physical damage. While the same substance may be stored above ground
in a tank, the fire codes require that flammable substances stored above
ground be kept at certain distances from other facilities. It follows that
a flammable substance will most likely be stored below ground either to
maximize the use of space at the site or to protect the tank and stored
substance from fire, heat, and physical damage. Fifteen substances which
are regulated by CERCLA but are not on the California list have been
selected on the basis of the criteria described above and are also included
in Appendix A.
Three additional substances which also have a flammability code of 2, 3 or
4 and which are regulated by CERCLA have been determined to be unlikely to
be stored in underground tanks on the basis of information found in the
Fire Protection Guide on Hazardous Materials. One of these is paraldehyde
(CASRN 123637), which has a freezing point of 54°F. It is unlikely that a
substance with such a high freezing point would be stored below ground.
Another substance is crotonaldehyde (CASRN 4170303), which is stored and
shipped in small containers. Nickel carbonyl (CASRN 13463393) is shipped
in gas cylinders, and is also unlikely to be stored in an underground tank.
These three chemicals were not included in Appendix A.
2.3.5 OTHER POTENTIAL SOURCES OF DATA
There remain 221 CERCLA substances which were not included in Appendix A .
following the screening steps described above. Some of these substances
may be found in USTs, but there is not at this time sufficient evidence to
justify adding them to Appendix A. Three of them have been determined to
be very unlikely to be found in USTs. Potential sources of information for
further refinement of the list in Appendix A are discussed below.
Industry SIC Code
Industries which use the specific chemicals of interest are potential
sources of data on hazardous substances. A table including Standard
Industrial Classification (SIC) codes for the hazardous substances found in
USTs in California is reported by Data Resources, Inc. and Quantum
2-20
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Analytics (Data Resources, Inc., 1985). The DRI list was compared to two
sources to try to confirm if all industry types are represented in the
California data. The industries represented in California were compared to
a list with industry classification of the top 100 chemical producers
(Chemical & Engineering News, 1985). Five types of industries were
determined to be missing from, or only partially represented in, the
California data (Table 2-7). Agrichemicals, for example, was listed as a
partially represented category because the California data had reported
only three tanks to contain agricultural chemicals.
In addition, the SIC codes for the California list were compared to the
Standard Industrial Classification Manual (U.S. Office of Management and
Budget, 1972). Twelve major groups of industries, listed in Table 2-8,
were determined to be either missing from the California data or partially
represented by it. Two groups were considered to be partially represented:
Major Group 20 - Food and Kindred Products - because meats and grains are
not represented in the California data; and Major Group 30 - Rubber and
Miscellaneous Plastics Products - because rubber is not represented.
Additional investigation of these industries and their UST practices may
reveal additional regulated hazardous substances in USTs that are not
included in Appendix A. This work was not performed because most of the
regulated .hazardous substances likely to be found in USTs are included in
Appendix A, and thus, the possible addition of a few substances to the list
(considering that USTs containing hazardous substances represent only a
small percentage of the total number of regulated tanks) was not judged
significant to the objectives of this study.
UST Notification Program
Another potential source of data will be the UST notification program
currently being implemented by EPA, which is due to be completed in 1986.
Under this program, notification forms will be distributed to owners of
USTs that may contain regulated substances. If in fact CERCLA-regulated
hazardous substances are stored in underground tanks on site, the owners
are required to complete the forms and return them to the responsible state
agency.
2-21
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TABLE 2-7
INDUSTRY CLASSIFICATIONS NOT INCLUDED
OR PARTIALLY INCLUDED ON CALIFORNIA LIST
1. Rubber Products - manufacturing
2. Glass Products - manufacturing
3. Alcoholic Beverages - distilling
4. Non-metallic minerals
5. Agrichemicals
Source: Chemical & Engineering News (1985),
2-22
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TABLE 2-8 STANDARD INDUSTRIAL CLASSIFICATION MAJOR GROUPS NOT INCLUDED
OR PARTIALLY INCLUDED+ ON CALIFORNIA LIST
GROUP NUMBER
INDUSTRY
1. Major group 08 Forestry
2. Major group 10 Metal Mining
3. Major group 11 Anthracite Mining
4.' Major group 12 Bituminous Coal and Lignite Mining
5. Major group 14 Mining and Quarrying of Non-metallic Minerals
6. Major group 20 Food and Kindred Products
7. Major group 21 Tobacco Manufacturing
8. Major group 22 Textile Mill Products
9. Major group 23 Apparel and Other Finished Products
10. Major group 30+ Rubber and Miscellaneous Plastics Products
11. Major group 32 Stone, Clay Glass and Concrete Products
12. Major group 39 Miscellaneous Manufacturing Industries
(includes jewelry, flatware, musical in-
struments, athletic goods, artists materials,
linoleum, fur & pelt curing, etc.)
Source: U.S. Office of Management and Budget (1985)
2-23
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The receipt and analysis of the data from the notification program will be
a major step toward providing a solid data base on the universe of tanks
containing hazardous substances. The notification program will provide
extensive data on the owners of USTs containing hazardous substances, and
useful, if limited, information on the hazardous contents of the USTs (if
there is a mixture of chemicals in a tank, owners are required to report
only the CERCLA hazardous substances of greatest quantity in the mixture).
2.4 REMAINING CERCLA HAZARDOUS SUBSTANCES
2.4.1 LIKELIHOOD OF STORAGE IN UNDERGROUND TANKS
Of the 698 hazardous substances regulated by CERCLA, 221 are not currently
included in Appendix A. Of the 477 chemicals that do appear in Appendix A,
some may be present in waste mixtures that would not be regulated under the
UST program. Further research is necessary to establish whether these
chemicals are also found in USTs as products.
2.4.2 ADDITIONAL DATA GATHERING AND ANALYSIS
Additional data about the remaining 221 CERCLA hazardous substances is
needed in order to determine which of them are likely to be found in USTs.
Two methods might be used to approach this task. One is to pursue
additional data directly from the industries that were not well-covered by
the California inventory. This might reveal the storage of certain
chemicals which are used by the industries inventoried. Lists of the
industries not covered or partially covered in the California inventory are
shown in Tables 2-7 and 2-8.
A second approach is to collect information on the manufacture, use, and
physical properties of each remaining substance. Some substances may be
eliminated because, for example, they are not produced in quantities large
enough to store in underground tanks. Other substances may be solids that
!-24
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are rarely dissolved prior to use. It is likely that a few of the
remaining chemicals will be added to the list of those found in USTs.
There is no need under this work assignment to attempt to extend the list
presented in Appendix A. Most CERCLA hazardous substances already appear
in the list, and hazardous substances as a class represent only a small
portion-of the total number of regulated USTs. It may be worthwhile in
future efforts to identify substances that should be deleted from the list
because they exist in USTs as wastes only, and so are not subject to UST
regulations.
2.5 SUMMARY
Based on three criteria (liquid, stored in underground tanks, and wide
use), the petroleum substances of interest for the remaining work include
the following:
• Power fuels - aviation gasolines, motor gasolines, diesel fuel oils,
jet fuels, and gas turbine oils.
• Heating oils - fuel oils (Nos. 1, 2, 4, 5, and 6).
• Solvents - Stoddard solvent, petroleum spirits, petroleum extender
oils, and aromatic solvents.
• Lubricants - automotive and industrial lubricants.
Of the petroleum products listed above, power fuels and heating oils are
produced in greatest quantities, and therefore, are probably found more
frequently in underground tanks. We will focus most of our efforts in this
area.
Of the 698 hazardous substances regulated by CERCLA, 477 have been
determined to be likely to be found as products in USTs. A list of these
chemicals, with their corresponding CASRN numbers, is included in Appendix
A.
2-25
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These substances have been selected based on two criteria. Of the
substances listed in Appendix A, 462 are also found in the California list
of hazardous substances, which was developed from an inventory of existing
USTs. Fifteen of the substances are liquids or gases which are flammable
enough to suggest storage outdoors or in separate facilities, and thus seem
likely to be stored in underground tanks, although they were not included
in the California list.
The remaining 221 CERCLA substances are not readily identifiable as being
either likely or unlikely to be stored in underground tanks. Some may not
be produced in large enough quantities to warrant underground storage.
Some of the solids will not be stored in underground tanks in the solid
phase, but might appear dissolved in another substance. Additional
research must be done to confirm or deny the likelihood of underground
storage of the remaining CERCLA chemicals.
A potential source of additional data will be the nation-wide notification
program for USTs. The completed notification forms are due to be returned
to the states by the tank owners on or before May 8, 1986. This survey may
not reveal every chemical stored in underground tanks, but will provide
extensive data on the users of USTs. Research using information from
industries and other facilities which use USTs could resolve any remaining
questions regarding the likelihood of CERCLA substances being stored in
underground tanks.
These efforts have several implications. First, the list of 477 hazardous
substances given in Appendix A, rather than the full CERCLA list of 698
substances, will be the focus of work in Sections 3.0 through 6.0,
particularly in the discussion of the properties of the substances. Since
the other 221 CERCLA substances are unlikely to be stored in underground
tanks, they will not receive further attention.
Second, because many of the 477 substances are solids or gases that appear
in aqueous solution, or more commonly, are dissoved in organic
solvents/carriers, the fate and transport of these solutions and/or
solvents/carriers needs to be studied.
2-26
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Finally, the relative emphasis given hazardous substances versus petroleum
substances should take into account current estimates that the latter
represent over 90 percent of the regulated UST universe. Many of the
studies in other sections of this report deal with generic fate and
transport mechanisms, but those that deal with specific substances should
have relatively higher emphasis on petroleum substances.
2-27
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3.0 TRANSPORT MECHANISMS
3.1 OVERVIEW
3.1.1 FACTORS AFFECTING TRANSPORT
Necessary to an evaluation of the potential seriousness of leaking USTs is
a thorough understanding of how substances are transported through the
environment. The transport of a substance can be described by the
mechanisms that govern its movement in the environment.
The objective of the text addressing transport phenomena, is to answer the
question: What governs the movement of a chemical in liquid and vapor
phases within each of the compartments of the subsurface? To answer this
question, Section 3.0 has been organized as follows: first, a general
introduction to the factors affecting transport is presented at Section 3.0
below. Readers who are not interested in the detailed technical discussion
of transport may wish to move on to Section 4.0 after this overview; prior
to Section 3.2.
Section 3.2 introduces Darcy1s Law which is the foundation of quantitative
theory of the flow of fluids through porous media.
In the general introduction, three zones of the subsurface which are
important to the transport of contaminants are defined: The unsaturated
zone, the capillary zone and the saturated zone. The transport of liquid
phase contaminants in each of these zones is presented in Sections 3.3
through 3.5, respectively. Section 3.6 discusses vapor transport in the
unsaturated zone.
For each case involving leaks from USTs, the extent and impact of
contamination is unique. The degree to which contamination migrates in the
subsurface depends on three major factors:
3-1
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0 the quantity released;
• the physical properties of the leaked substance; and
• the structure of the subsurface.
Some of the basic, and decisive parameters of each of these factors are
discussed below to provide an initial understanding of transport in the
subsurface.
Quantity Released
Spills can be grouped into two major types - catastrophic or "sudden"
spills and insidious spills. A catastrophic spill involves the rapid
release of large volumes of a substance. Because leaks from USTs are not
typically of this type (pipeline ruptures and highway or railroad accidents
are more common examples), catastrophic releases will not be considered
further. Insidious spills, which are characteristic of leaking USTs,
involve slow, but often continuous, release of relatively small volumes of
substances. However, because a tank often leaks over a long period of
time, substantial volumes of a substance can be released into the
subsurface.
One reason the quantity of released substance is important is that it often
determines the method, and degree, of groundwater contamination. Two
methods of groundwater contamination are depicted in Figure 3-1. If the
volume of a release is large enough (i.e., exceeds the retentive capacities
of the soil media in the unsaturated zone), the leaked substance will come
into direct contact with the groundwater (Figure 3-1 (A)). Consequently,
contamination of the groundwater can be rapid and extensive. Even though a
smaller release will most likely remain in the unsaturated zone,
groundwater below the spill site can still become contaminated by the
solution of contaminant into percolating rainwater (Figure 3-1 (B)). While
3-2
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RAINFALL
GROUND SURFACE
FREE PRODUCT
OR DISSOLVED PRODUCT
WATER TABLE
DISSOLVED PRODUCT
SATURATED ZONE
(A) CONTAMINANT IN DIRECT CONTACT WITH THE WATER TABLE
RAINFALL
GROUNDSURFACE
• FREE PRODUCT
UNSATURATED ZONE
WATER TABLE
SATURATED ZONE
(B) GROUNDWATER CONTAMINATION RESULTING FROM SOLUTION
OF CONTAMINANT IN PERCOLATING RECHARGE WATER
CAMP DRESSER & McKEE INC.
FIGURE 3-1 SCHEMATIC CONTAMINANT PLUMES SHOWING METHODS
BY WHICH GROUNDWATER CAN BECOME CONTAMINATED
3-3
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the contamination will take a longer time to develop, the impact of
contamination can still be extensive.
Physical Properties
The miscibility (or immiscibility) of a leaked substance with water or, in
other words, the solubility of the substance in water leads to one of two
very different transport processes. For the purpose of the transport
section, substances will only be considered to be miscible (readily
dissolved in water) or immiscible (two fluids that maintain a sharp
interface when in contact with one another). Immiscible substances include
all of the petroleum products and non-aqueous phase liquid hazardous
substances. Some of the hazardous substances stored in underground tanks
will be miscible. Actually, the solubility of UST regulated substances in
USTs range from freely soluble to slightly soluble to pratically insoluble.
The different degrees of solubility, particularly with respect to hazardous
substances, are discussed in detail in Section 4.2 and Section 5.0.
Fluids miscible with water are fully displaced by water, and thus, travel
as a single phase fluid in the subsurface. Immiscible fluids are
transported in the unsaturated zone under two- (or multi-) phase flow
conditions and are held back at residual saturation in the voids of the
soil matrix. If the retentive capacity of the unsaturated zone is not
exceeded, substances held at residual saturation can only be further
transported by water according to their solubility (Schwille, No Date).
Note that Figure 3-l(A) also illustrates schematically transport of
miscible fluids or immiscible fluids released in sufficient quantity to
reach the water table. On the other hand, the transport of the substance
depicted in Figure 3-l(B) is only relevant for immiscible fluids held at
residual saturation in the unsaturated zone.
If the leaked substance is immiscible with water, the specific gravity
(S.G.) (or density relative to water) of the substance is the next
3-4
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parameter which governs its migration pathway. When the quantity of the
leaked substance is sufficiently large to reach the water table, as a
nonaqueous phase liquid (free product), the specific gravity of the
substance will determine whether the free product "floats" on the capillary
fringe (S.G. < 1) or "sinks" through the saturated zone to an impermeable
boundary (S.G. > 1). Figure 3-2 presents schematics of these conditions.
Even small differences of a few tens of grams per kilogram groundwater can
be decisive (Schwille, No Date).
Other important physical properties of a leaked substance that govern its
movement in the subsurface are (Schwille, No Date):
• Viscosity - affects the rate of movement under saturated and
unsaturated conditions of a substance moving as a separate phase.
• Surface tension - is responsible for capillary effects and
determines the spreading of fluids on the capillary fringe at the
end of the spreading phase.
• Evaporation rate - (dependent on the vapor pressure and latent
heat of evaporation) determines the potential for a substance to
have an associated vapor phase.
0 Vapor density - determines whether the vapor phase in the
unsaturated zone rises or sinks to spread on the capillary fringe.
3.1.3 Structure of the Subsurface
The type and composition of the subsurface formation also affects the
transport of a leaked substance. Subsurface materials can be divided into
two basic media - porous media and fractured (or fissured) rock media.
Porous media (often called soil media) is comprised of unconsolidated
particulate materials, for example gravel, sand or silt. Fluids--air,
water or oi'l--are contained within the pore spaces of soils. The size and
degree of interconnection of the pores affect the permeability of these
materials. Rock is consolidated material which can transport fluids
through interconnected pores (primary porosity) or through fracture
(secondary porosity) or both. Rock usually underlies a few inches to
3-5
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FREE PRODUCT
DISSOLVED
PRODUCT
UNSATURATED ZONE
SATURATED ZONE
IMPERMEABLE BOUNDARY
(A) AN IMMISCIBLE SUBSTANCE WITH A SPECIFIC GRAVITY LESS THAN 1 WILL FLOAT ON
THE WATER TABLE
UNSATURATED ZONE
SATURATED ZONE
(B) AN IMMISCIBLE SUBSTANCE WITH A SPECIFIC GRAVITY GREATER THAN 1 WILL SINK TO AN
IMPERMEABLE BOUNDARY
CAMP DRESSER & McKEE INC.
FIGURE 3-2
SCHEMATIC CONTAMINANT PLUMES SHOWING
THE EFFECTS OF SPECIFIC GRAVITY
ON IMMISCIBLE FLUID TRANSPORT
3-6
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several hundred feet of soil. With respect to the transport of fluids,
there are three basic rock types - bedded sedimentary, carbonate, and
crystalline.
• Sedimentary rocks usually occur in distinct layers or beds. The
beds may be horizontal or may slope or dip in some direction.
Joints and fractures often follow the bedding planes. The movement
of fluids may be affected by the orientation of these planes.
These rocks may also have significant primary porosity.
• Carbonate rock, limestone in particular, is partially soluble in
water. These rocks can sometimes have extremely high permeability
and porosity resulting from the development of large openings and
fissues that range from less than an inch to many feet.
• Crystalline rocks, such as granite and gneiss, are usually not
porous. These rocks are often fractured and a certain amount of
fluid may exist in and move through these openings. Fractures near
the surface may parallel the surface (sheet jointing); while the
orientation of deeper fractures may vary as a result of techtonic
stresses.
Although contaminant transport in fractured media is goverend by the same
processes as porous media, the factors listed above can result in quite
different migration pathways. The standard technique used to predict flow
in fractured media is to treat the fractured medium as an equivalent porous
medium (Freeze and Cherry, 1979). This technique is reasonable to describe
single-phase (miscible) fluid flow, but does not sufficiently describe
immiscible flow. Birk and Vorreyer (1978) listed several prime reasons for
the dissimilarities between migration of oil (an immiscible fluid) in •
fractured aquifers as described by complex mathematical solutions and seen
in the field.
• Oil migration is not uniform as in unfissured porous media, but is
along preferred fissures.
• Joint planes can retain or deflect oil movement.
0 Water table flucuations can open up new fissures causing migration
in different directions.
• Oil contents of polluted groundwater vary more in fissured media
than in unfissured porous media.
3-7
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• Because of insignificant porosity and smaller capillary effects
terms such as 'the thickness of the oil layer is...1 are not
helpful in fissured rocks since such terms suggest a more or less
uniform distribution of oil underground.
For the reasons listed above, this report will not address transport of
immiscible substances in fractured rock media.
When considering hydrology (water content) and the placement of USTs,
several zones of the subsurface can be defined for porous media: the
weathered soil zone; the excavation zone; the vadose zone; the capillary
zone; and the saturated zone. Figure 3-3 presents a schematic of these
subsurface zones. With the exception of the weathered soil zone,
substances leaking from underground tanks will encounter these zones in
succession. Each of these zones is discussed in general below, and
transport mechanisms in the latter three zones is discussed in detail in
Sections 3.3 through 3.5.
Weathered Soil Zone. The weathered soil zone is the upper region of the
unsaturated zone and extends from the ground surface to the level impacted
by major vegetative roots. This zone will not exist where rock outcrops
are present. Because USTs are generally located at least several feet
below the ground surface, substances leaking from underground tanks will
not typically encounter this zone. If leaked substances are transported
through the weathered soil zone, downward migration may be relatively rapid
and strongly localized compared to the underlying media because of presence
of root channels, animal burrows or dessication cracks (Convery, 1979).
Leaked substances may also be highly adsorbed in this zone, because of the
organic-rich nature of the soil.
Excavation Zone. The excavation pit of USTs are filled either with
disturbed soil removed from the pit or artificial fill. In the first case,
the backfilled material may be more permeable particularly in the vertical,
than that removed because it has been disturbed. These excavation areas
offer a migration route of minimum vertical resistance and fluids may move
3-8
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GROUND SURFACE .
UNSATURATED
ZONE
TANK
WEATHERED SOIL ZONE
EXCAVATION ZONE
VADOSE ZONE
CAPILLARY ZONE
VWATER TABLE
SATURATED ZONE
(GROUNDWATER)
IMPERMEABLE BOUNDARY
CAMP DRESSER & McKEE INC.
FIGURE 3-3
SCHEMATIC OF THE SUBSURFACE ENVIRONMENT
3-9
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downward more rapidly in them than in natural soils. Artificial fill,
typically sand and gravel, is often used when tanks are installed in
clay-rich or rock media. In this case, the artificial fill is typically
much more permeable than the natural media. When a leak occurs, the
substance pools in the excavation pit, which then behaves as an areal
source of contamination. Otherwise., transport of a leaked substance in the
excavation pit is governed by the same mechanisms as vadose zone movement.
Vadose Zone. The vadose zone is the lower region of the unsaturated zone
and extends from the weathered soil zone to the capillary zone. The terms
vadose zone and unsaturated zone are used interchangably, except where
their distinctions are of significance. Under uncontaminated conditions,
the interstices (pore spaces or fractures) in the vadose zone, are filled
with air and water. Leaked substances move downward through the
unsaturated zone under the influence of gravity. Some lateral movement
occurs due to capillary forces. Lateral movement in the unsaturated zone
also results from heterogeneities in the media and variations in water
saturation of the unsaturated zone (as discussed in Section 3.3).
If the leaked substance is an immiscible fluid, a certain amount of the
substance is immobilized (by capillary or interfacial forces) in the
interstices of the soil. The immobilized substance is termed to be held at
residual saturation which is dependent on .the soil structure and moisture
content, and the leaked substance. Neglecting gradients that affect the
flow of the immiscible substances, if the quantity of released substance
does not exceed the residual saturation of the media, the substances will
not reach the capillary zone (Schwille, No Date).
As the leaked substance migrates downward through the unsaturated zone,
volatile substances will vaporize and form .a gas envelope around the
infiltration core. Vapors are also generated from immiscible substances
floating on the capillary fringe and to a lesser extent from dissolved
contamination. Vapor migration in the unsaturated zone is due to diffusion
and advection. Vapor transport is presented in Section 3.6.
3-10
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Capillary Zone. The capillary zone is the transition zone between the
unsaturated and saturated zones. The moisture content of this zone ranges
• •
from the residual saturation of water at its upper surface to complete
saturation at the water table. Water in the interstices of this zone are
at subatmospheric pressures. The capillary zone will be thicker in fine-
grained media than in coarse-grained media.
Immiscible substances that reach the capillary zone will either float on
the capillary fringe or sink to an impermeable boundary depending on the
specific gravity of the substance. Substances with a specific gravity less
than water may form a mound on the capillary fringe. This occurs when the
infiltration rate of the leaked substance exceeds the lateral spreading on
the capillary fringe (Figure 3-2(A)). Under a high infiltration rate, the
substance can be depressed below the capillary zone into the saturated
zone. Downward migration of substance with a specific gravity greater than
water is slowed down when the substance reaches the water table because
groundwater must be displaced. These substances, however, sink through the
saturated zone to form a mound on an impermeable boundary (provided the
specific retention of the media/substance is exceeded). Spreading of the
mound then takes place under the pressure head of the leaked substance and
follows the slope of the impermeable boundary (Figure 3-2(B)) (Schwille, No
Date).
Saturated Zone. In the saturated zone below the water table, the
interstices are completely saturated with water (termed groundwater) which
exists in excess of atmospheric pressure. Groundwater flows under the
prevailing hydraulic gradients. Both free product and dissolved substances
can migrate in the saturated zone. Dissolved substances include miscible
substances and immiscible substances which dissolve at saturation
concentrations. Dissolved substances can move with the groundwater flow
and can travel long distances. The mechanisms that govern transport of a
contaminant in the saturated zone are advection - the movement of
contaminant with the mean groundwater flow - and dispersion - a mixing
3-11
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process that occurs on molecular (due to diffusion), microscopic (due to
variations in velocity within individual pores), and macroscopic (due to
soil heterogenieties) levels. The transport of a dense (S.G. > i) free
product in the saturated zone will be downward due to gravity. This
downward movement stops when the product is exhausted or reaches an
impermeable boundary. A free product may also move laterally in the
saturated zone under the influence of horizontal hydraulic gradients. A
blob of immiscible fluid of lower viscosity than water will move more
rapidly than water. If the product reaches an impermeable boundary, it
forms a pool that flows along the slope of the boundary. As free product
moves through the saturated zone it can leave a trail of contaminated
soil. Movement of the free product in the saturated zone has many
similarities to this process in the unsaturated zone. The contaminant is
free to dissolve to its saturation concentration.
3.1.2 TRANSPORT DIAGRAMS
Following are several schematic diagrams showing the influence of many of
the variables discussed above on the migration patterns of substances
leaking from USTs. Figure 3-4 depicts the influence of stratification of
the unsaturated zone on the distribution of a contaminant. The excavation
zone around a tank is assumed to be more permeable than the surrounding
natural soils. This figure shows that the different in permeabilities
between the "disturbed" excavation zone and the undisturbed soil is often
large enough that the leaked substance pools (builds up hydraulic head) in
the excavation zone before moving into the undistrubed soil.
Figure 3-5 presents three schematics of contaminant with different physical
properties moving through porous media.
(A) Miscible substance - moves through the subsurface as a single-
phase fluid.
(B) Immiscible substance (S.G. < 1) - moves through the unsaturated
zone as a second phase. Note that the infiltration rate is high
3-12
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GROUND SURFACE
TANK
/UNSATURATED £.,•-•:•, -
ZONE f: .;;•-. ,-;• -. ..'.
_ EXCAVATION ZONE
-; (HIGH PERMEABILITY)
INTERMEDIATE PERMEABILITY i
SOURCE: Adapted from Convery (1979)
CAMP DRESSER & McKEE iNC.
FIGURE 3-4
SCHEMATIC PLUME SHOWING THE INFLUENCE OF
STRATIFICATION OF THE UNSATURATED
ZONE ON CONTAMINANT DISTRIBUTION
3-13
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(A)
miscible
(B)
Immiscible
SG<1
(C)
Immiscible
SG<1
Ground surface
. Unsaturated zone
Saturated •.':.'..
zone \'"i'
Ground surface
Second phase
Unsaturated zone
Capillary zone
dissolved phase -"""."""saturated •:..
zone
Second . i'::lv>\"
Phase ••.'••':'.':•':
Unsaturated zone /••'•'.•••'•'•
Capillary zone '•:'.'.;•:':/.
dissolved ' : ;'•'/: ••'/ -
products saturated v/g*
-_._„zone:'••'.'•'•'''''':.
Impermeable
SOURCE:B,C taken from Schwille (No Date)
K - hydraulic
conductivity
CAMP DRESSER & McKEE INC.
FIGURE 3-5
MIGRATION PATTERNS IN POROUS
SUBSURFACE MEDIA
3-14
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enough to cause the free product to be forced below the water
table under the infiltrating core.
(C) Immiscible substance (S.G. > 1) - moves through the unsaturated,
capillary and saturated zones as a second-phase fluid to form a
mound on the impermeable boundary. While lateral spreading of the
mound occurs in all directions, the predominant direction is with
the slope of the impermeable boundary. This figure assumes a
static groundwater condition and also shows the influence of
stratification of the saturated zone on the dissolved plume (k is
hydraulic conductivity or permeability of the strata).
Figure 3-6 presents the contaminant distribution of a miscible substance in
a porous fractured aquifer. This figure shows how the contaminant
primarily migrates along the fractures and also diffuses into the pores of
the rock. As time passes, the zone of contamination will diffuse farther
into the porous matrix (Freeze and Cherry, 1979).
A comprehensive and more detailed description of the transport processes in
each of the major subsurface zones follows in Sections 3.3 through 3.5.
Section 3.2 discusses Dairy's Law—the basic equation governing groundwater
flow. For the reader not interested in the detailed technical discussion
of transport mechanisms which follows, you may want to move on to the
discussion of fate mechanisms in Section 4.0.
3.2 DARCY'S LAW
3.2.1 GROUNDWATER FLOW IN THE SATURATED ZONE
Darcy's Law is the foundation of quantitative theory of the flow of fluids
through porous media. Darcy, who conducted experiments in sand filters,
concluded that the rate of flow of water, Q, through the filter bed was
proportional to the area, A, of the sand and to the difference Ah, between.
the fluid heads at the inlet and outlet faces of the bed, and inversely
proportional to the distance, L, between inlet and outlet, or expressed
analytically that:
3-15
-------
GROUND SURFACE
DIFFUSION
POROUS ROCK .
SECONDARY
. OPENINGS ALONG
TX'.C^r7:^ BEDDING PLANES
'""•"" '""' """" AND FRACTURES
SOURCE: Adapted From Freeze and Cherry (1979)
CAMP DRESSER & McKEE INC.
FIGURE 3-6
SCHEMATIC REPRESENTATION OF CONTAMINANT
MIGRATION FROM AN UST
THROUGH FRACTURED POROUS LIMESTONE
3-16
-------
Q = cA Ah
L
where c is a combined constant of proportionality characteristic of the
sand. There are many variations of this equation which takes into account
the properties of the media, the fluid and the type of driving force.
These equations and factors, which affect their values in the subsurface,
are the subject of this section.
The flow of groundwater in any direction in the saturated zone is described
by:
Q--KA*
where: h = the hydraulic head (L)
— " the hydraulic gradient (dimensionless)
dl
K = the hydraulic conductivity (L/T)
p
A = the cross-sectional area (L )
Q = the flow rate (L3/T)
The negative sign is the result of defining dh in the same direction as dl
This equation is often written as:
q == - K-
q A K dl
where q (L/T) is defined as specific discharge or Darcy velocity
3-17
-------
Specific discharge is a macroscopic concept; i.e., it refers to the average
velocity of water across a cross-sectional area of porous medium and must
be clearly differentiated from the microscopic velocities associated with
the actual paths of individual particles of water as they wind their way
through the grains of the medium (Freeze and Cherry, 1979). These
differences are illustrated in Figure 3-7.
While specific discharge has units of velocity, it does not define the
average travel time of water between two points. Because the grains of the
media occupy much of the volume in the saturated zone, water flows only in
the interconnected pore spaces of a medium. The fraction of the total
volume of soil media available for the flow of water is called the
effective porosity, n . The effective porosity is generally smaller than
the total porosity (the volume of voids/total volume) of a medium because
some of the voids are not interconnected. To calculate the average
velocity of water, the following expression can be used:
e ne
where V is known as the mean effective or seepage velocity (L/T)
3.2.2 DISSOLVED CONTAMINANTS IN THE SATURATED ZONE
In general, the advective movement of dissolved (in groundwater)
contaminants in the saturated zone can be described by Darcy's Law (as
described above). Dispersion -- a mechanical mixing process -- is also
important to describing to migration of dissolved contaminants. Dispersion
is discussed in Section 3.5.2. The simpler version of Darcy's Law
presented above holds only if the properties (density and viscosity) of the
groundwater with dissolved contaminant are not significantly different than
water. The hydraulic conductivity (K) includes both the properties of the
fluid and the media, or:
3-18
-------
Q
OJ
I
SOURCE: Freeze and Cherry (1979)
CAMP DRESSER & McKEE INC.
FIGURE 3-7
MACROSCOPIC AND MICROSCOPIC CONCEPTS OF
GROUNDWATER FLOW
-------
K = kpg//a
^
where k = the intrinsic permeability of the media (L~)
P = the density of the fluid (M/L3)
2
g = the gravitational constant (L/T )
M = the dynamic viscosity of the fluid (M/LT)
The relationship between intrinsic permeability and hydraulic conductivity
for various media is shown in Table 3-1. These terms are often shortened
to permeability (intrinsic permeability) and conductivity (hydraulic
conductivity) and these shortened terms are generally used herein. Darcy's
Law can thus be written as:
The discussion to this point has assumed that the flow of groundwater can
be calculated in any direction. In field situations, the permeability of
the medium varies. These variations are primarily caused by
heterogeneities and anisotrophy. In an aquifer 'that is heterogeneous, the
permeability is dependent on the position within a geologic formation.
Freeze and Cherry (1979) define three types of heterogeneities: layered
discontinuous, and trending. Examples of these are given below.
• Layered - A stratified soil common in consolidated or
unconsolidated sedimentary deposits.
• Discontinuous - Interlayered deposits of clay in sand, the presence
of faults, the overburden-bedrock contract.
• Trending - Sedimentation process that creates deltas or glacial
outwash plains.
When the permeability of a medium is dependent on the direction of
measurement, the medium is anisotrophic. The most common form of
a.nisotrophy is transverse isotrophy which means that the permeability in
the horizontal direction is different than the vertical. Typical
3-20
-------
. TABLE 3-1
TYPICAL VALUES OF HYDRAULIC CONDUCTIVITY AND PERMEABILITY
— log,,, • K' (em seel —
Permeability
Aquifer
Soils
Roeks
-Ing,,, -Kern2)
2 -
1
) 1
1
Pervious
Clean
gravel
t
M
( 4
1
S (
Semipervious
Good
Clean sand or
xand and gravel
.
'
<
6
1
5 -1
7 S 4 10 II
1
lincvervious
Poor
Very line sand,
loess, loam, soK
Peat
sill.
net/
Stratified clay
Oil roeks
).
3
) 1
Sandstone
0 1
(
1
None
tJnwcalhcred c ay
Good
,. Breccia,
limestone,
. . . granite
dolomite
: 1> 14 15 16
1
-: -.1 -4 -5
Source: Bear (1979)
3-21
-------
horizontal to vertical anisotrphy ratios are from 10:1 to 3:1 (Freeze and
Cherry, 1979).
3.2.3 IMMISCIBLE FLUID FLOW
The flow of two or more immiscible fluids (e.g., air and water or air, oil
and water) is more complex because of the need to consider the interactions
between the fluids and between the fluids and the soil matrix. These
concepts are discussed in detail in Section 3.3.1 but can be summarized as
follows. The flow of immiscible fluids depends on capillary pressures
which are the result of surface tensions between the fluids, and the
saturation levels of each of the fluids which determines the relative
permeability of the fluid in the,medium. Darcy's Law in this case is
expressed for each fluid as:
where: kr = the relative permeability of the fluid (it varies from 0 to 1
and is determined from relative permeability graphs (Section
3.3.2).
^£ = the pressure gradient
dx
d£ = the change in elevation over the distance travelled
dx
When the pressure gradient is 0, and flow is vertically downward, dz = -1,
dx
and the movement of these fluids is expressed by:
q = r-
M
3-22
-------
where v is the kinematic viscosity of the fluid. In Section 3.3.2, this
version of Darcy's Law is used to express the relative (to water) rate of
movement of immiscible substance in the unsaturated zone.
3.3 TRANSPORT OF LIQUIDS IN THE UNSATURATED ZONE
3.3.1 OVERVIEW
In the unsaturated zone, the transport of a substance leaking from an LIST
can occur as a solute in the water phase, a constituent in the immiscible
phase, or vapor in the air phase. Only transport mechanisms for the first
two conditions (liquids) are presented in this section; vapor transport is
presented in Section 3.6.
Transport of fluids in the unsaturated zone is a more complex phenomena
than in the saturated zone, which under uncontaminated conditions is
described by single-phase flow mechanics. This complexity results because
fluids in the unsaturated zone are immiscible, i.e., the fluids displace
each other without mixing, and there is a distinct fluid-fluid interface
within each pore (Freeze and Cherry, 1979). Thus, transport of liquids in
the unsaturated zone is described by multi-phase flow phenomena. Subsets
of multi-phase flow can be defined depending on the number of immiscible
fluids involved.
Under uncontaminated conditions in the unsaturated zone, the voids are
partly filled with water at less than atmospheric pressure, the remainder
of the pore space being taken up by air. Two-phase flow phenomena govern
the movement of these fluids, or more generally, any two immiscible fluids.
The introduction of a contaminant miscible with water (a distinct interface
does not exist between the water and the contaminant) will also be governed
by two-phase flow, neglecting possible density differences. When another
fluid that is immiscible with water and air (e.g., oil or pure solvent) is
introduced into a soil medium, three-phase flow phenomena describe their
3-23-
-------
movement. The terms immiscible substance or oil will be used for this
fluid.
Besides the parameters of the fluids and the soils, the physics of
multi-phase flow involves taking into account relative permeablities and
capillary pressures, which depend on the saturations of the various fluids
and their movements (Fried et. al., 1979). The physics of unsaturated zone
movement is presented in Section 3.2. Section 3.3 discusses the use of the
physics as it relates to leaking substances from USTs.
The unsaturated zone (also called the zone of aeration) is located above
the water table and above the capillary zone (Freeze and Cherry, 1979).
When it rains, water infiltrates the weathered soil zone and then
percolates through the vadose zone. Simply described, water percolates
through the unsaturated zone, traveling primarily through the large voids
in the soil matrix, and is retained in the smaller voids by capillary
forces. In general, as the pore size increases, the percolation rate
increases. Depending on the pore size and distribution, percolation rates
range from inches per hour to inches per year. The moisture content of
these zones is less than the porosity of the soil medium and is described
in more detail below.
The moisture content in the weathered soil zone is relatively dynamic and
is affected by conditions at the ground surface (seasonal and diurnal
fluctuations of precipitation, irrigation, air temperature and humidity),
and/or by the presence of a shallow water table (Bear, 1979). The vadose
zone (also called the intermediate zone) is generally envisioned as the
region of the unsaturated zone where the moisture content is not changed
rapidly by changing conditions at either the ground surface or the water
table (Stallman, 1964). The thickness of this zone is dependent on the
depth of the water table below the ground surface. The moisture content of
this zone, after an extended period of gravity drainage without the
introduction of additional water at the surface, is at the irreducible
saturation of water (also called field capacity). This water is held by
3-24
-------
hygroscopic and capillary forces (Bear, 1979). It is recognized that
under certain environmental conditions (e.g., desert regions where it has
not rained for a long period of time or possibly under a building or a
parking lot) the moisture content of the soil in this zone may be very low.
3.3.2 THE PHYSICS OF UNSATURATED ZONE CONTAMINANT TRANSPORT
Transport in the unsaturated zone is very difficult to predict
quantitatively under field conditions. The flow of one immiscible fluid is
impeded by the presence of the other fluids. This section presents a
description of two- and three-phase flow conditions.
Two-phase Flow
This section describes the movement of two fluid phases that are immiscible
with each other in the unsaturated zone. Although the concepts presented
here were largely developed by petroleum engineers for oil and water, they
can be extended to describe the transport of any two immiscible fluids in
the unsaturated zone. Two-phase flow also partly describes the transport
mechanisms govering movement of an immiscible substance below the water
table, but because other factors also affect transport, this condition is
considered in Section 3.5.
Two-phase flow is not completely quantitatively understood. Four factors
have been selected to provide a qualitative description of two-phase flow
mechanisms: wettability, capillary pressure, moisture retention, and
relative permeability.
Wettability. In the case of air-water movement, air is the nonwetting
fluid and water is the wetting fluid. Movement of two immiscible fluids in
the unsaturated zone is described in terms of this distinction. The
determination of which immiscible fluid wets a solid depends on the contact
angle between the solid surface and the liquid-gas or liquid-liquid
interface, measured through the denser fluid as shown in Figure 3-8. When
3-25
-------
NONWETTING FLUID
(LIQUID OR GAS)
WETTING FLUID
(LIQUID)
CONTACT
ANGLE
SOLID
EXAMPLES:
NONWETTING FLUID WETTING FLUID
AIR
AIR
OIL
PURE SOLVENT
WATER
OIL
WATER
WATER
SOURCE: Adapted From Bear (1979)
CAMP DRESSER & McKEE INC.
FIGURE 3-8
DETERMINATION OF WETTING AND
NONWETTING FLUIDS
3-26
-------
the contact angle is less than 90 degrees, the denser fluid wets the solid;
when the contact angle is greater than 90 degrees, the denser fluid is the
nonwetting fluid. The contact angle is a function of the interfacial
tension between the fluids. A more detailed discussion of wettability can
be found in Bear (1979).
Four stages (or regimes) of the saturation of water relative to air (or any
two immiscible fluids) can be distinguished and are depicted in Figure 3-9:
pendular, insular, funicular and saturated. The following description was
taken from Bear (1979) and Convery (1979).
At very low saturations, water exists as pendular rings around grain
contacts within the porous medium (Figure 3-9 (A)). These rings of fluid
are completely isolated from one another, and do not form a continuous
water phase, except for a very thin film of water of nearly molecular
thickness (that does not behave as ordinary liquid water) on solid
surfaces. Hydraulic pressures cannot be transmitted through water in the
pendular regime, since it is not continuous.
If the saturation of the wetting phase increases, the pendular rings expand
until a continuous water phase is formed. Flow of water is then possible.
There is a coincident decrease in the saturation of air. The saturation
regime is called funicular (Figure 3-9 (B)). The phase distribution and
flow behavior of fluids in the funicular regime are complex, and are
strongly a function of the saturation history of the porous medium.
With increasing water saturation, air eventually becomes discontinuous and
is commonly isolated in the larger pores of the medium. Air is in a
condition of insular saturation (Figure 3-9 (C)). The air globules will
become mobile only if a pressure discontinuity exists across them within
the water to force them through capillary restrictions. Otherwise, the
droplets are immobile and remain trapped within the pores. The insular
drops will impede flow of water to some extent.
3-27
-------
I
(A) PENDULAR
(C) INSULAR
(B) FUNICULAR
(D) SATURATED
WATER
AIR
SOURCE: Adapted From Convery (1979)
CAMP DRESSER & McKEE INC.
FIGURE 3-9
FLUID SATURATION STATES (REGIMES) OF
A TWO-PHASE SYSTEM FOR A HYDROPHILIC
POROUS MEDIUM
3-28
-------
Under saturated conditions (Figure 3-9 (D)), one fluid, in this case water,
completely occupies the interstices of the media.
Capillary Pressure. When two immiscible fluids are in contact in a soil
medium, there is a distinct fluid-fluid interface with each pore. Because
of the interfacial tension which exists between these fluids, a
discontinuity in pressure exists across the interface separating them. The
magnitude of the pressure difference (called capillary pressure) depends on
the curvature of the fluid-fluid interface inside the pore space. In a
general two-phase system, the capillary pressure (P ) is the difference
between the pressure on the nonwetting side of the interface and that on
the wetting side (Bear, 1979).
c ~ P nonwetting ~ P wetting
The capillary pressure is thus the tendency of a partially saturated porous
medium to suck in the wetting fluid or to repel the non-wetting fluid. The
capillary pressure has a negative value and this is often called suction or
tension (Bear, 1979).
These principles are illustrated below, for the case of a gasoline-water
system. The example was taken from Williams and Wilder (1971).
When gasoline is at an insular state of saturation, it is discontinuous (in
globules or bubbles) and is primarily located in the larger pore spaces
where it is surrounded by water. Figure 3-10 shows the pressures at such
an interface between two soil particles. For equilibrium to exist:
Pgas - Pw = Pc = 2ocos9
rc
3-29
-------
GASOLINE
;:•;•.: '•'•• • .'.';•'..•.'•;'•:'.'.•'••..•'. WATER ."•
' ''
NECK OR
PORE THROAT
SOURCE: Williams and Wilder (1971)
CAMP DRESSER & McKEE INC.
FIGURE 3-10
PRESSURES ON A CAPILLARY INTERFACE
3-30
-------
where: P = Pressure on the gasoline side of the interface
P = Pressure on the waterside of the interface
W
P = Capillary Pressure
c
a = Surface energy (a function of the fluids)
9= Angle of contact between the denser fluid and the soil.
rc = Radius of the capillary
Until the pressure drop across the interface is greater than the capillary
pressure (P_,_ - P.. > P_) , the globule of gasoline will not move through
^uS W L
the pore throat. If gasoline is present at this state of saturation in a
porous medium on a macroscopic scale, it is immobile. At the macroscopic
scale, the average capillary pressure is equal to the difference between
the average pressure in the water and that in the gasoline (Bear, 1979).
Moisture Retention. Moisture retention curves indicate how water is
retained in a soil medium by capillary forces against gravity. The
pore-size distirbution and pore shapes of a porous medium affect the shape
of the retention curve (Bear, 1979). Figure 3-11 shows typical moisture
retention curves for drainage of (A) a well graded and poorly graded soil
(schematically); and (B) of various soils (experimentally). The important
aspects of these curves are:
• Critical capillary head - Point A in Figure 3-11 (A) is the point at
which the larger pores of a saturated medium will start to drain.
• Irreducible water content - Point B in Figure 3-11 (A) is the water
content level at which a certain quantity of water remains at the
pendular saturation level even at very high capillary pressures
(Bear, 1979).
3-31
-------
0
<
UJ
i
UJ
DC
D
V)
(/)
U)
oc
a
£'
<
cu
<
u
!
\
\
\ /-Well-graded soil
V^
\
\
\
\ I
V ^v
\, || N.
^^^^^^ N.
i\
/ Vi
Poorly graded soil \U
0 B,, B, .
*n
WATER MOISTURE CONTENT
(A) SCHEMATIC CURVES
3
/'/•
2
n c-
md < 500 u
W a X« v Ramona sand
V2 \ \ H Placentia clay loam
|\\ o • A Hanford/ sandy. loam
v-^ \ \ a Yolo clay loam
TV +\ °v • • Chino silty clay loam
i\\\ \
K\ \ \
n \ \ov \
\\ \ \X \
Vj^x \ \ \ X-
^^H, \1 \ \
% ^\ ° \
r * V \
I I I I I I I
0 0 20 30 40 50 60 70
i i
80 90 1 00
WATER SATURATION (%)
(B) CURVES OBTAINED DURING SATURATION
SOURCE: Bear (1979)
FIGURE 3-11
CAMP DRESSER & MCKEE INC. TYPICAL RETENTION CURVES IN SOIL
DURING DRAINAGE
3-32
-------
• Irreducible water saturation - Analogous to irreducible water
content.Figure 3-11 (B) shows that values of irreducible water
saturation are approached at lower capillary pressures for more
permeable than less permeable soils.
Different moisture retention curves would be obtained for the wetting
(imbibition) than for drainage of a porous medium. This phenomena is
called hysteresis. As shown in Figure 3-12 a higher moisture content is
obtained when a soil is being drained rather than during imbibition (Bear,
1979). Hysteresis is more complicated than depicted in Figure 3-12 because
the drying or wetting process can start at any point on the boundary
curves. This leads to drying and wetting scanning curves which are drawn
between the boundary curves. The interested reader is referred to Chapter
6-1 in Bear (1979).
Relative Permeability. The concept of relative permeability is perhaps
most easily understood as follows. Because two immiscible fluids occupy
the voids of a medium, only a portion of the voids is available for flow of
each fluid. In effect, the permeability of the formation to one of these
fluids is dependent on the. saturation level of the other fluid.
Early researchers assumed that two immiscible fluids flow simultaneously
through a medium; each fluid established its own set of tortuous paths
through the medium, which were assumed to be relatively stable. This
would imply that the relationship between fluid saturation and relative
permeability would be linear.
However, as discussed above, the effects of capillary pressure must also be
considered. The relationship between permeability and saturation is then
better described as a cubic parabola. The relationship of the relative
permeability of water with saturation as established by experiments by
Wyckoff and Botset (1936) is shown in Figure 3-13. This figure indicates
a rather rapid reduction in the permeability as moisture content decreases
from saturation. Essentially, the large pores drain first so that the flow
takes place through the smaller pores. At point A on this figure, water is
at irreducible saturation or is held at a pendular state (Bear, 1979).
3-33
-------
UJ
UJ
CC
UJ
oc
0.
I
IMBIBITION (OR WETTING)
. BOUNDARY DRYING CURVE
BOUNDARY
WETTING CURVE
DRAINAGE (OR DRYING)
STARTING WITH A
SATURATED SAMPLE
MOISTURE CONTENT
ENTRAPPED AIR
(MAY BE REMOVED WITH TIME)
SOURCE: Adapted From Bear (1979)
CAMP DRESSER & McKEE INC.
FIGURE 3-12
EFFECT OF MOISTURE CONTENT
HYSTERESIS FOR A COARSE MATERIAL
3-34
-------
MOISTURE CONTENT
1.0
DC
UJ
1
m
<
UJ
5
oc
UJ
Q.
UJ
1
UJ
oc
0.75
0.50
0.25
oc
1
HI
_i
CD
O
Z>
Q
UJ
DC
CE
25
50
75
100
SATURATION (%)
SOURCE: Bear (1979)
CAMP DRESSER & McKEE INC.
FIGURE 3-13
RELATIVE PERMEABILITY OF WATER
IN UNSATURATED SAND
3-35
-------
Figure 3-14 depicts schematically the relationship between relative
permeability and saturation level for both a wetting fluid and nonwetting
fluid. The interesting features of this figure are (van Dam, 1967 and
Convery, 1979):
• The permeability of a given porous medium to one fluid in the
presence of another fluid is reduced with respect to single-phase
permeability.
• The reduction in permeability is dependent on the wetting of the
porous medium by one of the two fluids.
• A minimum saturation for each fluid is required before the medium
is permeable to that fluid, i.e., the fluid can flow. This
saturation is called the residual saturation of the fluid (or
primary retention value). For water, the residual saturation is
often termed the irreducible saturation. Residual saturation is
discussed in detail in Section 3.3.3.
• Throughout most of the saturation range, the sum of the relative
permeabilities is less than unity.
With respect to Figure 3-14, Stallman (1964) notes that these relationships
may not hold for the water-water vapor and air system or for other
relatively volatile fluids. Under these conditions, the interaction
between the liquid and its vapor state may preclude their separation and
treatment as fluids in the pore space. For a further discussion of vapor
movement, see Section 3.6.
Figure 3-14 also shows the degree of water saturation in terms of the fluid
saturation regimes defined previouisly. Using water (the wetting fluid)
and oil (the nonwetting fluid), Williams and Wilder (1971) described
schematically the movement of two immiscible fluids in a soil matrix.
These stages are shown in Figure 3-15.
In Figure 3-15 (A), water is at irreducible saturation or at the pendular
state. It occupies the small interstices in the soil matrix and is
3-36
-------
100% -
1.0
0.5
1.0
0.5
100%
SOURCE: Van Dam. 1967
CAMP DRESSER & McKEE INC.
FIGURE 3-14
VARIATIONS OF RELATIVE PERMEABILITY OF
WETTING (Krw) AND NON-WETTING
(Krnw) FLUIDS WITH SATURATION
3-37
-------
A) PENDULAR REGIME
OIL FLOW ':
—HYDRAULIC GRADIENT
B) FUNICULAR REGIME
OIL FLOW
WATER FLOW
•HYDRAULIC GRADIENT
C) INSULAR REGIME WATER FLOW
-HYDRAULIC GRADIENT
\^'0\ OIL PHASE
WATER PHASE
SOURCE: Williams and Wilder (1971)
CAMP DRESSER & McKEE INC.
FIGURE 3-15
DISTRIBUTION OF FLUID FLOW FOR
THE THREE SATURATION REGIMES
3-38
-------
immobile. Oil occupies most of the available void space, particularly the
larger pores and flows at its relative permeability.
Water held at the funicular state is at a higher saturation level than at
the pendular state. It now is present as a continuous fluid. As shown in
Figure 3-15 (B), both oil and water flow in the medium under these
conditions.
When oil is in the insular state, water occupies most of the void space.
The oil phase is discontinuous and does not contact the solid, which is
wetted with water. Oil under these conditions generally does not flow.
Figure 3-15 (C) shows the water is the flowing fluid.
The relative permeability to a fluid at a given saturation depends on
whether that saturation is obtained by approaching from a higher or lower
value (i.e., the medium is wetting or drying). This phenomena is called
hysteresis. Therefore, there is no unique value of relative permeability
for a given saturation. Figure 3-16 depicts the effects of hysteresis on
relative permeability.
Three-phase Flow
When a third immiscible is added to the two-phase flow system described
above, then three-phase flow phenomena describe the migration process.
Although three-phase flow is more complicated, due to complex boundary
conditions and the hysteresis phenomena, it follows the same principles of
two-phase flow. It should be noted that the transport mechanisms discussed
here are applicable to any immiscible contaminant substance, e.g.,
gasoline, fuel oil, and chlorinated solvents; however, crude oils and other
petroleum products that have to be heated to flow behave differently. When
these substances cool to a lower temperature, than at a which all
components are dissolved in each other, they do not behave as Newtonian
fluids (Schwille, No Date).
3-39
-------
100%
SOURCE: van Dam (1967)
CAMP DRESSER & McKEE INC.
FIGURE 3-16
EFFECT OF HYSTERESIS ON RELATIVE
PERMEABILITY
3-40
-------
In a three-phase flow system, water is the wetting fluid relative to air or
oil, and oil is the wetting fluid relative to air. This is shown more
clearly below.
Wetting Fluid Nonwetting Fluid
Water Air
Water Oil
Oi 1 Air
As with the concept of relative permeability in a two-phase flow system,
the pores of a soil medium are occupied by air, oil, and water; Figure
3-17 is a schematic of three immiscible fluids (air, water, and oil) in a
soil pore. Each of these fluids sees a relative permeability of the medium
depending on the saturation level of each fluid. The relationships between
relative permeability and saturation level are shown in the triangle
diagram (Figure 3-18).
The most important aspects of this diagram are illustrated in Figure 3-18
(B). They show:
• There is a very small region where all three fluids can flow
simultaneously, and thus, that are even larger areas where at least
one of the fluids is immobile.
• The zones where each fluid is immobile or at its residual
saturation.
As mentioned at the beginning of this section, transport in the unsaturated
zone is very difficult to characterize quantitatively under field
conditions. Numerical techniques are available to solve the system of
differential equations which govern unsaturated zone movement but
computations may not be practical because of the fine discretization
required. Other important complicating factors are: (1) the lack of data
for the soil hydraulic parameters in the system of equations; and (2)
possible irregularities of boundaries and or heterogenity of medium. Even
3-41
-------
SOURCE: van Dam (1967)
CAMP DRESSER & McKEE INC.
FIGURE 3-17
SCHEMATIC DISTRIBUTION OF AIR, OIL,
AND WATER IN A POROUS MEDIA
3-42
-------
(A)
100%
Air
— Oil
Water
(B)
WATER
100%
AIR Immobile Saturation
100%
OIL
100%
SOURCE: van Dam (1967)
CAMP DRESSER & McKEE INC.
FIGURE 3-18
RELATIVE PERMEABILITY AS A FUNCTION OF
SATURATION FOR AIR-OIL-WATER SYSTEM
3-43
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so many useful concepts and relationships using the parameters described in
this section have been developed to aid in the prediction of transport
mechanisms. These concepts and relationships are presented in the next two
sections.
3.3.3 USE OF THE PHYSICS AS IT APPLIES TO LEAKING USTS
When a substance leaks from an UST, the most important transport question
to be answered is: what is the maximum migration of a contaminant? In
relation to the unsaturated zone, this question can be divided into two
parts: (1) how much of the unsaturated zone media will become contaminated
and at what rate; and (2) will the contaminant reach the water table?
Because answers to these questions are not known in a quantitative sense,
following is a qualitative discussion of the important parameters. Section
3.3.4 presents some estimation techniques.
The mobility of a substance in the unsaturated zone depends on:
• viscosity, which affects the velocity of the percolation process;
• quantity released and infiltration process, which may have an effect
on the percolation process;
• permeability of the medium, which affects the spill geometry; and
• for immiscible substances, residual saturation levels, which is the
retentive capacity of the soil medium.
Viscosity
The viscosity affects the velocity of the flow process through the Darcy
equation which shows that velocity is inversely proportional to viscosity
(see Section 3.2). Therefore, heavy oils do not readily penetrate the
soil; whereas, lighter products, gasoline in particular, move-through the
soil more easily than water does (Clean Environment Commission, No Date).
Section 5.1 applies these concepts in grouping petroleum products by
kinematic viscosity.
3-44
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In dry soils (e.g., deserts or under buildings), essentially two-phase flow
phenomena (air and oil) govern mobility. The relative permeability of the
immiscible substance can be used in Darcy's Law, if saturated flow is
assumed. This means that enough oil has to be released to displace the air
to its residual saturation level. In this case, velocities of percolation
are inversely proportional to kinematic viscosity, all other parameters
being equal (Schwille, No Date).
Velocity of contaminant - -JCJIgffltjcjflscosUy of water (velocity of Water)
Kinematic viscosity of contaminant
TP. is relationship assumes that gravity forces are dominant, >.e., the
influence of pressure gradients on a flow is negligible. For example, a
o
light heating oil (kinematic viscosity = 4 mm /second) will move 4 times
slower than water, and trichloroethylene (kinematic viscosity =
o
0.4mm /second) will move 2.5 times faster than water. Schwille (No Date)
notes that modeling studies confirm these results.
The viscosity, and thus pecolation velocity, can also change over time due
to "weathering" of a leaked substance. With respect to the lighter
petroleum products (those with a larger volatile content), the loss of
volatile compounds to evaporation or solubilization will increase the
viscosity and slow migration. This phenomena is probably most important in
environments where the depth to the water table is large.
Quantity Released
Movement in the unsaturated zone occurs because of hydraulic gradients
which are the sum of gravitational forces and pressure-head gradients
(Freeze and Cherry, 1979). If the quantity released is large enough to
achieve saturated conditions and the infiltration rate exceeds that which
can be moved by gravity, the pressure-head gradient from the pooled
3-45
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substance will increase the the hydraulic head, and thus, the percolation
rate.
Spill Geometry
Transport in the unsaturated zone is primarily downward, due to
gravitational forces, but lateral spreading will also occur. This section
address how the extent of contamination (spill geometry or shape of the
contaminant plume) varies with different subsurface permeability and
structure.
The extent to which the unsaturated zone will be contaminated is affected
by different permeabilities of uniform media, stratification or
heterogeneities in this zone and variations in water content. Schematic
drawings of different plume shapes assuming the capillary zone is not
reached are shown in Figure 3-19.
In a uniform (homogeneous) media, the contaminant plume is pear-
shaped; percolation is primarly vertical and capillary forces act to
spread the plume laterally with increasing depth. Figures 3-19 (A) and (B)
show the differences in relative shape for homogeneous soils that are
highly permable and less permeable, respectively. In less permeable media,
capillary forces are more significant and the plume is broader at its base
(CONCAWE, 1979). Thus, a larger zone of contamination is expected in
tighter soils.
As shown in Figure 3-19 (C), the shape of a contaminant plume in a
stratified soil (assume these are sand layers) has an undulating
appearance. As the contaminant moves from a layer of high permeability to
lower permeability, it has to overcome more resistance to flow. The result
is that the contaminant spreads out laterally and mounds at the interface
between the layer until sufficient head is reached to move it into the next
strata. Conversely, with a change of strata from low to higher
permeability, the plume shape is laterally contracted because the
3-46
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(A)
(B)
(C)
HIGHLY PERMEABLE
HOMOGENEOUS SOIL
LESS PERMEABLE
HOMOGENEOUS SOIL
.. . • STRATIFIED SOIL WITH
VARYING PERMEABILITY
SOURCE: A, B, C FROM CONCAWE 1979
CAMP DRESSER & McKEE INC.
FIGURE 3-19
SCHEMATIC DIAGRAMS OF DIFFERENT CONTAMINANT
PLUME SHAPES IN SOILS OF VARYING PERMEABILITY
IN THE UNSATURATED ZONE
3-47
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resistance to flow is lower. Because of the significant lateral spreading
in a stratified soil, there is a considerable reduction in the depth of
penetration of an equal spill volume in a stratified soil than in a
homogeneous soil.
Heterogeneities of much lower permeability than the surrounding soil (e.g.,
a clay lens in sands) can add a significant lateral spreading component to
the shape of a contaminant plume (Figure 3-19 (D)). In this example, the
resistance to vertical flow through the heterogeneity is large enough to
cause the contaminant to migrate laterally across the surface of the
heterogeneity before continuing its downward migration. If the zone of
much lower permeability is continuous (a clay layer), the vertical movement
of a contaminant can be arrested if it reaches residual saturation as shown
in Figure 3-19 (E) or it travels to the surface at an outcrop (Figure 3-19
(F)).
As discussed in the previous section, the flow of the immiscible substance
is only moderately reduced, compared to full saturation, when the soil
moisture content is less than irreducible saturation. This is because the
water is held in the less pervious areas of the pore space (Schwille, No
Date). As water content increases, the relative permeability of the
immiscible substance is more highly reduced (see the curves for the
nonwetting fluid and oil in Figures 3-14 and 3-18 respectively). If the
percolating contaminant encounters an area of perched water (water at
saturation in the unsaturated zone), the movement of the contaminant will
be slowed as it tries to displace some of the water (plume shape similar to
stratified soils) or it will flow over the surface of the perched water as
if it were like a clay lens. Schwille (No Date) notes that since the water
content of a media in the field is rarely known, the relative decline in
permeability can only be estimated to an order of magnitude.
3-48
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Residual Saturation of Immiscible Substances
For immiscible substances, the residual saturation of the leaked substance
is a measure of the retentive capacity of a soil medium for the substance.
Residual saturation is defined as the minimum content a fluid must attain
in order to move in a porous medium, or alternatively, the threshold
content below which it is immobile (CONCAWE, 1979). Note that residual
saturation levels are always much less than complete (100 percent)
saturation and other terms used to describle this phenomena - retention
capacity or specifc retention - although they are less frequently found in
the literature, may better describe the phenomena.
While it is a relatively easy laboratory procedure to determine the
residual saturation of an immiscible substance, estimates extrapolated to
field conditions are less accurate unless a good understanding of the
variation of water content is developed. In the absence of water, the
residual saturation of oil will be greater than when water is present. As
water content increases, the residual saturation of oil becomes constant.
Some of the values of residual saturation reported in the literature are
listed below:
• Schwille (No Date) reported residual saturation for oil in the
unsaturated zone is typically 3-5 1/m for high permeability media
and 30-50 1/m for low permeability media.
• He also noted that values for low molecular weight chlorinated
hydrocarbons (solvents) had been determined to be on the same order
of magnitude as oil.
• Dietz (1971) assumes that immobile oil saturation ranged from 10
percent of the pore volume for light oils (gasoline), to 15 percent
for medium oils (diesel, light fuel oil) to 20 percent for heavy oils
(lube oil, heavy fuel oil).
Convery (1979) makes a distinction between primary specific retention and
ultimate specific retention. This distinction recognizes that the terms
"residual" and "immobile" are relative. Primary specific retention
determines the spread of the immiscible substance and is the volume of a
3-49
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nonwetting fluid retained after one passage of the fluid through the medium
followed by gravity drainage. As water (a wetting fluid) repeatedly passes
through the medium, some of the nonwetting fluid will be displaced. This is
the ultimate specific retention and represents the long-term retention of
the medium. Therefore, care must be used in selecting a value of residual
saturation because at the primary residual saturation the immiscible
substance may have some mobility left and the substance can be removed from
the pores of a medium by draining, solubilizing or vaporizing.
The residual saturation of a leaked substance is primarily a function of
pore size (grain size and packing), pore size distribution (sorting), pore
shape (grain size) of the soil medium and its water content. The volume of
a liquid retained by a medium is an inverse function of grain and pore size
of the medium (Pfannkuch, 1983). Because permeability of a medium varies
directly with grain size and sorting, an inverse relationship is also
expected between retention and permeability and as indicated in observations
by Convery (1979). Thus, in general, a tighter soil will retain more of a
leaked substance than a more permeable soil.
Column drainage experiments reported by Pfannkuch (1983) were conducted to
measure primary specific retention of mineral oil under the following
conditions:
• Natural soils representing geologic materials and conditions typical
of the glaciated regions- of the north central and northeastern United
States;
• Dry Ottawa sand; and
t Ottawa sand with water at its irreducible saturation.
The results for the specific geologic strata experiments are shown in Table
3-2. In general, a broad trend of increasing primary oil retention with
decreasing grain size was indicated. However, below silt-sized particles,
there was a dramatic drop in primary retention. This is believed to be
caused by mineralogical change. Sand and silt consist primarily of quartz
3-50
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TABLE 3-2
PRIMARY RESIDUAL SATURATION OF MINERAL OIL
Geologic Material
Alluvium, Outwash sand weathered
Old Gray Till
Sandy Till (Red Till)
Old Gray Till
Loess
Primary Residual Saturation
(% Pore Volume)
15.0 - 23.4%
25.0 - 27.3%
20 - 43%
41.2 - 51.5%
Source: Phannkuch (1983)
3-51
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and feldspar and have a low negative surface charge. Clay minerals, which
have a smaller mean grain size, are characterized by high negative surface
charges. Thus it was concluded that the mineral oil (non-polar and no
significant charge) might be replused by the clay minerals, reducing
retention.
The experiments with natural soils also attempted to define the role of
sorting on residual saturations. This is difficult to do because the
tighter soils (highest retention) also tend to be the most poorly sorted.
Thus, the conclusion was simply: higher primary retention occurs in finer,
more poorly sorted soils.
In the Ottawa sand experiments, similar relationships were seen for the dry
and "wet" sands between grain size and residual saturation of mineral oil,
although the primary retention of oil in the wet soil (with water at
irreducible saturation) was lower than the dry soil. As shown in Figure
3-20, these experiments also indicate that the primary residual saturation
of mineral oil will be lower for an initially water saturated system
(characteristic of the saturated zone) than initially oil saturated systems
(characteristic of the unsaturated zone).
Immiscible substances which travel as a second-phase in the saturated zone
also have to exceed residual saturation levels before they can flow. As
.shown in the experiments discussed above, the volume of nonwetting substance
retained in a porous medium will be smaller when the medium'is saturated
with water (characteristic of the the saturated zone) than when it is
partially saturated with water (characteristic of the unsaturated zone).
Figure 3-14, the two-phase flow relative permeability graph, also
illustrates this principle. The .residual saturation for water (the wetting
fluid at the pendular state) is less than the residual saturation of oil
(the nonwetting fluid at the insular state).
3-52
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UJ
N
cc
o
2.50 I
2.00 -
1.50 -
1.00 -
.50 -
.9 1.0 1.1
LOG RETENTION
• INITIALLY SATURATED
O INITIALLY DRY
1.2
i
1.3
1.4
SOURCE: Pfannkuch (1983)
CAMP DRESSER & McKEE INC.
FIGURE 3-20
COMPARISON OF PRIMARY LOG RETENTION
OF INITIALLY DRY AND SATURATED OTTAWA
SAND BY COLUMN DRAINAGE
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3.3.4 ESTIMATING RELATIONSHIPS
Two well-known estimation relationships have been developed which help
answer the questions stated at the beginning of Section 3.3.3 and repeated
below:
• How much of the unsaturated zone media will become contaminated and
at what rate?
• Will the contaminant reach the water table?
Both relationships were developed by or for the petroleum industry for use
with crude oil and petroleum products. Because residual saturation is the
key factor in both relationships, and Schwille (No Date) noted similar
values for residual saturation of chlorinated hydrocarbons as petroleum,
these equations will be assumed to apply to these immiscible substances as
well. It should be stated, at the outset that no situations are known
where these equations have been applied in field situations and thus their
validity remains to be demonstrated.
The first relationship was reported in Davis et. al . (1972), and is called
the saturation equation. It is an estimation technique to determine the
volume of soil required to immobilize a spill. This equation will give a
conservative estimate of the required volume of soil because it does not
take into account the heterogeneous nature of field conditions.
soil required to attain = 0.2V
immmobile saturation PSn
(cubic yards)
where: V = the volume of oil spilled (barrels)
P = the porosity of the soil (percent)-
Sp = residual saturation
use 0.10 for gasoline, 0.15 diesel and light fuel oil and 0.20
for lube or heavy fuel oil.
3-54
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With regard to the rate at which the contaminant reaches the water table,
Shepherd (No Date) notes that one common misunderstanding is that it can
take months or years for a contaminant to reach the water table. While
this can be true under certain environmental conditions, he states that
storage and petroleum handling facilities are typically located over
permeable sands or glacial sediments where the depth to the water table is
less than 25 feet. Under these conditions and with a sufficient quantity
of release, he states that a leaked substance, such as gasoline, can reach
the water table in a few hours or, at most, a few days.
The second relationship reported by CONCAWE (1979) estimates the maximum
depth of penetration of infiltrating oil. It is simply a more detailed
version of the previous equation which has been rearranged to determine the
depth of penetration. This relationship assumes that oil infiltrated at
the ground surface and requires an estimate of the infiltration area.
Because the leaks of concern herein are from USTs, the applicability of
this relationship to this problem is suspect. While it might be possible
to estimate the cross-sectional area of an infiltrating leak, this would
require accounting for the effects of the excavation zone around the tank.
D = 1000V
A R k
where: D = maximum depth of penetration, m
V = volume of infiltration oil, m
2
A = area of infiltration, m
R = retention capacity of soil (1/m ) (see table below)
k = approximate correction factor for various oil viscosities
k = 0.5 for low viscosity products, e.g., gasoline
k = 1.0 for kerosene, gas oil and other products with
similar viscosities.
k = 2.0 for more viscous oils, such as light fuel oil.
3-55
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Typical values of R, the oil retention capacity of porous soil with natural
moisture content are given below.
Soil .R (1/m3)
Stone, coarse gravel 5
Gravel, coarse sand 8
Coarse sand, medium sand 15
Medium sand, fine sand 25
Fine sand, silt 40
If the soil consists of layers with different retention capacities, an
intermediate value of R should be selected. Note, however, that the
heterogeneities in the soil, which can rarely be properly represented, will
result in a decrease in downward penetration in the field than indicated by
this calculation. This equation is probably most useful for estimating
whether or not a spill of known volume will reach the water table.
Because both these relationships are simple, their underlying assumptions
and interpretation of their possibly results need to be highlighted:
• Both relationships assume a homogeneous, or nearly so, soil media.
0 The relationships do not account for the lateral spreading of an
immiscible substance due to capillary forces.
9 If the soil media is stratified, a worst case for both conditions
can be calculated using the most porous media.
• The equations do not recognize variations in water content, nor the
hysteresis phenomena.
• These equations do not consider the flow process which creates the
contaminated zone; it is assumed that the oil is distributed
uniformly at residual saturations.
3-56
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3.4 TRANSPORT OF LIQUIDS IN THE CAPILLARY ZONE
3.4.1' OVERVIEW
The capillary zone is the transition zone between the unsaturated and
saturated zones. As in the unsaturated zone, transport in the capillary
zone is governed by three-phase (air, water, and oil) flow phenomena
determined by capillary and gravitational forces and by the relative
permeability of these fluids. Unlike the unsaturated zone where flow is
primarily in a vertical direction, flow in the capillary zone is mainly
horizontal (CONCAWE, 1979).
In terms of the substances leaking from USTs, transport of immiscible
substances with a specific gravity less than water ("floaters") is of the
most significance in this zone and these are discussed in Section 3.4.2.
Transport of miscible substances and immiscible substances with a specific
gravity greater than water will simply slow down as they encounter the
increasing degrees of moisture saturation in this zone. An estimation
relationship used for this zone is presented in Section 3.4.3.
In the capillary zone, moisture content increases with depth and pressure
heads are less than atmospheric. Looking at the moisture content
distribution from the opposite direction (moving up from the water table),
water held by capillary forces occurs in all, then smaller, and finally the
smallest, connected pore spaces.
To describe the transport of floating immiscible substances (e.g.,
petroleum products) in the capillary zone, the capillary zone is often
divided into two regions: the capillary fringe and the funicular zone.
The region where all the pores are filled with water held by capillary
forces is called the capillary fringe, and the region where the water
content transitions from complete saturation to irreducible saturation is
called the funicular zone. Recall that for two-phase flow conditions with
3-57
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water at funicular saturation (Figure 3-14), both immiscible fluids will
flow. Figure 3-21 presents a schematic of these zones with a typical
moisture content curve.
The thickness of the capillary zone depends on the size of the voids and on
the degree of homogeneity of the soil, mainly the pore size distribution
(Bear, 1979). The table presented below lists two different estimates of
capillary zone thickness for various soil types.
Capillary Rise (in cm)
CONCAWE,(1979) Bekchurin (1958)
from Bear (1979)
Coarse grain sands 12-15 2-5
Medium grain sands 40 - 50 12 - 35
Fine grain sands 60 - 110 35 - 70
Silts and clays 175 - 250
Silt 70 - 150
Clay 200 - 400
Chalk 120 - 900
While these estimates vary for a particular soil type, they clearly show
that capillary rise increases with finer pore sizes. The effect of pore
size distribution on capillary rise is depicted schematically in Figure
3-22. In Figure 3-22 (A), the thickness of the capillary rise is shown to
be relatively constant for a homogeneous soil; this is an idealized
condition not likely to be encountered in the field. Large variations in
capillary rise are shown in Figure 3-22 (B) for non-homogeneous soils.
3.4.2 TRANSPORT OF FLOATING SUBSTANCES
If an immiscible substance with a specfic gravity less than water (the term
oil is used here) is released in sufficient quantity to exceed the residual
saturation of the unsaturated zone soil medium, it will reach the capillary
zone. Transport in the capillary zone is typically describing in terms of
3-50
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CAPILLARY ZONE
SOURCE: Adapted from Dietz (1967)
UNSATURATED ZONE
Capillary Rise Above Phreatic Level
In Slimmest Continuous Pores
FUNICULAR ZONE
Capillary Rise Above Phreatic Level
In Widest Pores
CAPILLARY FRINGE
Phreatic Level, As Found In
Observation Wells
SATURATED ZONE
100%
WATER SATURATION
CAMP DRESSER & McKEE INC.
FIGURE 3-21
SCHEMATIC OF THE CAPILLARY ZONE WITH
A TYPICAL WATER SATURATION CURVE
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(A)
(B)
capillaries of '
like diameter
0 50 100
Water content (% )
capillaries of
unlike diameter
V1
0 50 100
Water content (%)
SOURCE: Schwille (1967)
CAMP DRESSER & McKEE INC.
FIGURE 3-22
VARIATION IN WATER CONTENT IN THE
CAPILLARY ZONE WITH ELEVATION
3-60
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mounding on the capillary fringe and lateral spreading in the funicular
zone. These phenomena are not well understood. A general description of
these phenomena is given below.
When free oil initially reaches the capillary zone, its vertical movement
is stopped at the top of the capillary fringe. As more oil reaches this
region a layer of increasing thickness, a mound, begins to form on the
capillary fringe under the influence of the infiltrating oil. Van Dam
(1967) has reported that a minimum thickness of oil must be established
before lateral migration can occur. The weight of the oil will result in
the depression of the capillary fringe and possibly the water table.
Buoyant forces act to restore the water to its initial level and if the
minimum thickness is exceeded, the lateral spreading begins. In general,
lateral spreading occurs in the funicular zone and the oil is described in
terms of its characteristic shape a pancake or lens. Lateral spreading
will occur in all directions, but the predominant movement will be with the
slope of the water table. Lateral migration ceases when oil is at its
residual saturation.
Shape of the Oil Pancake
The infiltration rate, local water table configuration and permeability
distribution will determine the shape of free oil in the capillary zone
before lateral spreading begins. Van Dam (1967) states that "...oil will
spread preferentialy within the top of the capillary fringe in the
direction of the groundwater gradient or towards parts of the porous medium
where the highest permeabilities occur. The latter effects may prevail in
the case of strong capillary pressures, which phenomenon may limit the
extent of oil migration along a sloping groundwater table."
The degree of mounding on the water table is affected by the infiltration
rate. In general, significant mounding occurs only when the infiltration
rate exceeds the rate of lateral spreading. Shepherd (No Date) states that
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only an extremely rapid product-loss rate will depress the water table
significantly under a spill site.
While lateral spreading occurs in all directions, the predominant direction
is with the slope (gradient) of the water table. If the water table
gradients are low, a more circular pancake will form than the narrow
elongated pancakes which are characteristic of steeper gradients.
The permeability of the soil affects the rate and thickness of lateral
spreading of free oil. In low permeability soils, resistance to flow is
high and a thicker free oil pancake will form. High permeability soils
will allow the formation of thin and faster moving pancakes.
Two qualitative descriptions of leaks showing the impact of these factors
are cited below (Shepherd, No Date):
• A slow product leak in a very permeable soil with a steep hydraulic
gradient could result in a long narrow plume with a
capillary-induced saturated (oil and water) soil zone of less than
two feet.
• A rapid product loss with a relatively flat hydraulic gradient
could result in a oval plume around the leak site with a saturated
soil zone several feet thick, before lateral movement takes place.
Rate of Movement
As decribed above, movement of free oil in the capillary zone is governed
by multi-phase flow principles described in Section 3.2.2. The rate of
lateral spreading varies considerably over time (CONCAWE, 1979). Even with
greatly increased gradients resulting from the height of the mound under a
leaking UST, the rate of lateral spreading is slow (Shephard, No Date).
Within the capillary zone, the rate of movement will be governed by the
degree of water saturation. At the bottom of this zone, water saturation
is high, and oil may displace some of the water but this oil will encounter
3-62
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more resistance to flow. Nevertheless, experimental and field evidence
indicates that considerable migration occurs within the capillary fringe at
or very near the water table (Freeze and Cherry, 1979). Shepard (No Date)
notes that the maximum rate of movement is at about two-thirds of the
height of the capillary zone. This happens because near the top of the
funicular zone, the water saturation is low and the relative permeability
of oil will be higher.
The effects of viscosity described in section 2.3.3 will also impact the
movement of free product in the capillary zone. Gasoline which is less
viscous than water will move faster than the water in this zone.
Thickness of the Pancake Away From the Percolating Core
Dietz (1971) states that oil will spread over water in a layer than is
roughly equivalent to the thickness of the funicular zone. Table 3-3 lists
average funicular zone thickness.
The ultimate spread of the free oil is also affected by flucuation in the
water table. The flucuations can be caused by seasonal precipitation,
tidal forces or the turning on and off of a pumping well. Unless the
flucuations are rapid, the free oil will stay on the top of the capillary
fringe and migrate with the moving water table. Under rapidly flucuating
conditions (e.g., an increase of several feet because of the flooding of an
irrigation ditch), oil may be temporarily trapped at higher than residual
saturation under the water table. Bouyant forces will then act to move the
oil to the top of the water table.
Fluctuating water tables expose soil that was previously uncontaminated to
oil, as shown in Figure 3-23. The oil will be retained in these areas at
residual saturation levels. Thus, the effect of flucuating water tables is
to disperse the oil over a larger region. Because some of the oil is
retained in previously uncontaminated area, there is an apparent reduction
in the volume of the free oil. In fractured rock media, flucations in the
3-63
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TABLE 3-3
THICKNESS OF THE FUNICULAR ZONE
Sand
Average
Grain
Diameter
(mm)
Thickness
of the
Funicular Zone
(cm)
Extremely coarse-very coarse
Very coarse-moderately coarse
Moderately coarse-moderately fine
Moderately fine-very fine
2 - 0.5
0.5 - 0.2
0.2 - 0.05
0.05 - 0.015
1.8 -
9.0 -
22.4 -
28.1 -
9.0
22.4
28.1
45.0
Source: Dietz (1971)
3-64
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Original
water table
Oil at residual
saturation
Free oil
New water table
SOURCE: Davis et. al. (1972)
CAMP DRESSER & McKEE INC.
FIGURE 3-23
CONTAMINATING EFFECT ON SOIL CAUSED
BY FLUCTUATING WATER TABLE
3-65
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water may increase the apparent volume of free oil due to "washing out" of
oil from fractures previously unconnected to the water table (CONCAWE,
1979).
3.4.3 ESTIMATING RELATIONSHIPS
CONCAWE (1979) reported an estimation relationship to determine the area!
spread of free oil in the capillary zone. As with the estimation
relationships reported in Section 3.3, this relationship assumes that oil
infiltrated at the ground surface and requires an estimate of the
infiltration area. Because the leaks of concern herein are from USTs, the
direct applicability of this relationship to our problem is difficult.
While it might be possible to estimate the cross-sectional area of an
infiltrating leak, this would require accounting for the effects of the
excavation zone around the tank.
S = 1000 V-AxRxdxk
F
2
where: S = Maximum spread of oil on the water table, m
V = Volume of infiltrationg oil, m3
2
A = Area of infiltration at surface, m
R = Retention capacity of soil above the water table (see table
below), 1/m
d =.Depth to the water table, m
F = Oil contained immediately above the capillary fringe (see table
2
below), 1/m
k = Correction factor for various oil viscosities
k = 0.5 for low viscosity petroleum products, e.g. gasoline
k = 1.0 for kerosine, gasoil and products with similar
viscostiy
k = 2 for more viscous oils such as light fuel oils
Values for R and F are given below for various porous soils.
3-66
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son on
Stones, coarse gravel
Gravel , coarse sand
Coarse sand, medium sand
Medium sand, fine sand
Fine sand, silt
R
Retention Capacity
1/nT
5
8
15
25
40
F 2
1/nr
5
8
12
20
40
Further, the application of this relationship to field conditions has not
been documented. Until such documentation is gathered, use of this
relationship should be used with great discretion. The Davis et. al.
(1972) states that the ultimate extent of the oil pancake can be
calculated, but doing so under field conditions involves so many
assumptions and estimates that the result can easily be inaccurate by more
than 100 percent.
3.5 TRANSPORT OF LIQUIDS IN THE SATURATED ZONE
3.5.1 OVERVIEW
The saturated zone is located below the water table and under
uncontaminated conditions the interstices are filled with water (called
groundwater). The water table is defined as the surface where hydraulic
pressures are at atomospheric pressure. Groundwater exists under pressures
in excess of atmospheric pressure. Groundwater flow is quantitatively
described by Darcy's Law (see Section 3.2).
Both dissolved and immiscible contaminants migrate in the saturated zone.
There are several ways contaminants from leaking USTs can reach the
saturated zone. They include:
3-67
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• Dissolved Substances
- A miscible contaminant leaked from an UST migrates through the
subsurface in the groundwater and/or dissolves in subsurface
water before reaching the water table.
- An immiscible substance can be exhausted at residual saturation
in the unsaturated zone and infiltrating water passing by this
substance can dissolve some of the components and carry them to
the saturated zone.
- At the contact between a pancake of immiscible contaminant and
the water table, solution of contaminant can take place.
- An immmiscible substance can enter the saturated zone as a
second phase and available for solution in groundwater.
- Percolating rainwter can dissolve the vapors of a contaminant
in the unsaturated zone and transport the redissolved
contaminant to the water table.
• Immiscible Substances
The location below the water table of an immiscible substance as a
second phase is a function of the specific gravity of. the substance.
- If the specific gravity is less than water, the immiscible
substance is usually found in the upper parts of the saturated
zone, just below the water table and is at residual saturation.
There are two general ways for substances to migrate below the
water table: (1) as a result of-the mounding process during
percolation; or (2) flucatuating water tables.
- Substances with a specific gravity greater than water will sink
through the saturated zone until they are immobilized at residual
saturation or reach an impermeable boundary.
The rate and direction of movement of a contaminant in the saturated zone
are functions of the groundwater flow region, the local geology and the
chemicals and physical properties of the contaminant. Section 3.5.2
presents the qualitative and quantitative aspects of advection and
dispersion for a homogeneous media. In Section 3.5.3, applications of
these principles to the more complex situations likely to be found in the
field will be presented.'
3-68
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3.5.2 THE PHYSICS OF SATURATED ZONE TRANSPORT
Dissolved substances are carried along in the groundwater as a single phase
system. Transport of dissolved substances is governed by advection
(Darcy's Law) and dispersion. Transport of immiscible substances in the
saturated zone are governed by Darcy's Law and the two-phase flow
principles discussed in Section 3.3. They do not flow according to the
laws of hydrodynamic dispersion (Guswa and Lyman, 1982).
The transport of dissolved substances in the saturated zone is governed by
advection and dispersion. Advection is the movement of a contaminant plume
with the mean groundwater flow which is described by Darcy's Law (Section
3.2). Dispersion describes how a contaminant spreads out and is diluted as
it to occupies more of the saturated zone than can be explained by
advection only. Dispersion occurs on three levels; these are shown in
Figure 3-24.
• Molecular diffusion - the movement of contaminant with concentration
gradients;
• Microscopic dispersion (or mechanical mixing) - the variations of
direction and magnitude of velocity within a single pore space and
between pore spaces of different sizes;
• macroscopic (or field-scale) dispersion - due to variations in the
permeability (heterogeneities).
Together, the first two levels of dispersion are often called hydrodynamic
dispersion. While all three types of dispersion can occur in the saturated
zone, the degree to which any one type governs dispersion depends on the
characteristics of the zone. Molecular diffusion may be the principle
dispersion phenomenon at low groundwater velocities, such as those found
in clay or shale (Guswa and Lyman, 1982). At high groundwater velocities
in relatively homogenous permeable media (e.g., a homogeneous sand in a lab
column), pore scale mechanical mixing may be prinicpally responsible for
dispersion. However, practically all natural geologic materials are
heterogeneous, and consequently, macroscopic dispersion will be the
principle phenomenon in most field situations.
3-69
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DIRECTOR OF AVERAGE
FLOW
VELOCITY
DISTRIBUTION
MICROSCOPIC DISPERSION
MOLECULAR DIFFUSION
SOURCE: Bear (1979)
DISPERSION AROUND A CLAY
LENS IN A SANDY AQUIFER
DISPERSION CAUSED BY A
SAND LENS IN A SILTY AQUIFER
MACROSCOPIC DISPERSION
SOURCE: Johnson & Dendrou (1984)
CAMP DRESSER & McKEE INC.
FIGURE 3-24
TYPES OF DISPERSION
3-70
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Molecular Diffusion
Diffusion is the process whereby ionic or molecular constituents move under
the influence of their kinetic activity in the direction of their
concentration gradient. It can occur in conjunction with mechanical
dispersion or it can occur in the absence of any bulk hydraulic movement of
the solution. Diffusion ceases only when concentration gradients are
nonexistent.
Pick's first law states that the mass of a diffusing substance passing
through a given cross-section per unit time is proportional to the
concentration gradient and is expressed as:
F = - D dC_
dX
where:
F is the mass flux per unit area per unit time,
D is the diffusion coefficient,
C is the solute concentration,
dC_ is the concentration gradient.
dX
In porous media, the diffusion coefficient, D, is replaced by an
empirically derived "apparent diffusion coefficient, D*' which is
represented by the relation:
D* = AD
where A is an empirical coefficient, less than unity, that takes into
account the effect of the solid phase of the porous medium on the
diffusion. In laboratory studies of diffusion of non adsorbed ions in
porous geologic materials, values of A between 0.5 and 0.01 are commonly
observed. (Freeze and Cherry, 1979).
3-71
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The Advection and Dispersion Equation
The advection dispersion equation is the fundamental differential equation
for contaminant transport. Neglecting the loss or gain of solute mass due
to reactions, the equation in one-dimension for a non-reactive substance
can be written as:
change in concentration = dispersion + advection
over time
_ dC
at ax 37
where:
C is the solute concentration
D is the dispersion coefficient
V is the average linear pore velocity
x is the distance
t is the time
As discussed earlier, the coefficient of hydrodynamic dispersion can be
expressed in terms of two components, mechanical dispersion, and molecular
diffusion:
D = aL V + D*
where:
D is the dispersion coefficient
a, is the longitudinal dispersivity of the medium
V is the average linear pore velocity
D* is the apparent diffusion coefficient
3-72
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Many laboratory experiments have found the longitudinal dispersivity
to be on the order of millimeters or centimeters.
The effect of the dispersion coefficient is to cause some of the
contaminant to a move faster than the average linear velocity of the
groundwater and some of the contaminant to move slower than the average
linear velocity. This results in a smearing out and dilution of an
originally sharp boundary between contaminated and uncontaminated water.
Figure 3-25 illustrates the combined effect of molecular diffusion and
mechanical mixing to the spread of a concentration front. The dispersion
process causes spreading of the contaminant species in directions
transverse to the flow path, as well as parallel to it.
3.5.3 APPLICATIONS
The examples that will be presented here are for dissolved and immiscible
substances in the subsurface. Concepts to be presented include: how
variations in geology affect contaminant plumes, the effect of density on a
dissolved contaminant plume and transport of nonaqueous phase liquid
substances in the saturated zone.
Effects of Heterogeneities on a Dissolved Contaminant Plume
Macroscopic dispersion is caused by variations in permeability in the
saturated zone. These variations can occur at a small scale (a clay lens
in a sand aquifer) or at a large scale (the stratification of the
subsurface). Changes in permeability result in changes in the direction
and rate of groundwater flow. Field experiments have shown that
macroscopic longitudinal dispersivities can be many orders of magnitude
larger than those from lab tests; values in the range of 1 to 100 meters
are common (Anderson, 1979). Field data also seem to indicate that
macroscopic dispersivities.are dependent on the scale of the contamination
problem.
3-75
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V POSITION OF INPUT
WATER AT TIME T
RELATIVE
CONCENTRATION 0.5
(C/Co)
TRACER FRONT IF
DIFFUSION ONLY
DISTANCE X
DISPERSED
TRACER FRONT
SOURCE: Freeze and Cherry (1979)
CAMP DRESSER & McKEE INC.
FIGURE 3-25
SCHEMATIC DIAGRAM SHOWING THE CONTRIBUTION
OF MOLECULAR DIFFUSION AND MECHANICAL
MIXING TO THE SPREAD OF A CONCENTRATION FRONT
3-74
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Freeze and Cherry (1979) present a series of contaminant plumes in
cross-section to show the effect of simple layered heterogeneities on
transport patterns. These cross-sections are shown in Figure 3-26.
Figure 3-26 (A) shows the groundwater flow domain. At steady-state
groundwater flow occurs through a cross-section which is assumed to be
isotrophic with respect to hydraulic conductivity. The contaminant used in
this figure is assumed to be conservative and the effect of dispersion on
the plume is neglected. In Figure 3-26 (B) the contaminant is shown
flowing through a homogeneous medium. Figure 3-26 (C) shows the effects of
a thin continuous high horizontal hydraulic conductivity layer. The
contaminant flows almost totally within this layer and the travel time is
one-fifth of that in the homogeneous medium. The effects of discontinuous
layers of low conductivity materials in shown in Figure 3-26 (D). The
contaminant plume moves over the first lens, under the second one and then
through the second one to reach the discharge point. Figure 3-26 (E) is a
variation on case C where the high conductivity layer is discontinuous.
The contaminant plume moves within the high conductivity layer until this
layer ends in the middle of the cross-section. In the central part of the
cross-section the contaminant plumes spreads out occupying almost the
complete depth of the aquifer.
This set of figures was presented to show in a simple fashion the effect of
heterogenities. In the field, large conductivity changes across sharp
discontinuities are common. Thus, it can be seen that determining where a
contaminant plume will move in the saturated zone can be complicated.
Density Effects on a Dissolved Contaminant Plume
The advection-dispersion equation presented in Section 3.5.2 assumes that
there is no significant density differences between the groundwater
containing dissolved contaminant and the ambient groundwater. Because the
equations that include density differences are complex, only a qualitative
example is presented here. Basically, as the density of the contaminated
3-75
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Contaminant source
Stream-
Steody water tnbIe
No flow Divide
(A) BOUNDARY CONDITIONS
(B) HOMOGENEOUS CASE
K2/K,=100
(C) SINGLE HIGHER-CONDUCTIVITY LAYER
K1/K2. =
(D) TWO LOWER-CONDUCTIVITY LENSES
K2/K,=100
(E) TWO HIGHER-CONDUCTIVITY LENSES
SOURCE: Freeze and Cherry (1979)
CAMP DRESSER & McKEE INC.
FIGURE 3-26
EFFECT OF LAYERS AND LENSES ON
FLOW PATHS IN A SHALLOW STEADY-STATE
GROUNDWATER FLOW SYSTEM
3-76
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groundwater increases relative to the ambient groundwater, the more a
contaminant plume is likely to sink in an aquifer. Figure 3-27 shows the
relative locations for three plumes of increasing density in an initially
uniform flow field. Figure 3-27 (A) shows that a contaminant plume at
about the same density of ambient groundwater spreads in the shallow part
of the saturated zone. In contrast, Figure 3-27 (C) shows how large
density differences result in a steeply downward sinking plume (Freeze and
Cherry, 1979).
The density of contaminated groundwater is a function of the molecular
weight and the concentration of the dissolved contaminant. The density of
ambient groundwater also can vary, although most likely to a lesser
degree within a local area, because of the presence of dissolved minerals.
The largest density differences would be expected for high molecular weight
contaminants at high concentrations. This is somewhat of an anomaly,
however, because as discussed in Section 5.0, the solubility (saturation
concentration) within a given chemical group (e.g., paraffins or aromatics)
generally increases with decreasing molecular weight. Therefore,
significant density differences are most likely to occur when (1) low
molecular weight contaminants are found in very high concentrations; or (2)
high molecular weight contaminants are found near their saturation
concentrations.,
Transport of Immiscible Substances
It is more difficult to predict the migration path of an immiscible
substance- in the saturated zone than dissolved substances. This is because
flow of an immiscible substance is controlled by its own flow potential,
which depends on pressure, gravity and surface forces, and is not
necessarily similar to the groundwater flow potential (Faust, 1985). The
migration path can be characterized in terms of density (with respect to
water) and viscosity. Less viscous fluids will move more rapidly than more
viscous fluids. If the leaked substance is less viscous than water, it
will move faster than the prevailing hydraulic gradient. The effects of
3-77
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U)
CO
CONTAMINANT
SOURCE
\
(A)
I
(C)
WATER TABLE
(B) (C)
LEGEND
Slightly More Dense Than Groundwater
Larger Density Contrasts
SOURCE: Freeze and Cherry (1979)
CAMP DRESSER & McKEE INC.
FIGURE 3-27
EFFECT OF DENSITY ON MIGRATION OF CONTAMINANT
SOLUTION IN UNIFORM FLOW FIELD
-------
density are such that a more dense immiscible substance can pose a greater
danger in terms of migration potential than less dense substances, i.e.,
less dense substances are typically only found in the shallow part of an
aquifer, while more dense substance can penetrate deep into the saturated
zone. Migration of a substance less dense than water was described in
Section 3.4; the discussion below focuses on substances more dense than
water.
The movement of a dense immiscible substance in the saturated zone is
affected by two-phase phenomena and the groundwater gradients. In simple
terms, the density of the fluid leads to vertical downward movement and
horizontal groundwater gradients will tend to move the substance laterally.
The actual migration route (analogous to a trajectory) will depend on the
magnitude of each of these factors. Other factors, such as anisotrophy of
the porous medium and the presence of heterogeneities, will also influence
the migration route. As the immiscible substance moves through the
saturated zone, some of it will be retained at residual saturation. As
discussed in section 3.3.3, the quantity of substance held at residual
saturation will be smaller for the same medium in the saturated zone than
unsaturated zone. If the quantity of release exceeds the retention
capacity of both the unsaturated and saturated zones, the substance will
move downward until it encounters an impermeable boundary. The substance
will pool on this boundary and the primary flow direction will be along the
slope of the boundary and the gradient in the groundwater will have small
effects.
Numerical techniques for predicting the movement of immiscible fluids were
developed for study of petroleum reservoirs. Only recently has these
methods been applied to the study of immiscible groundwater contaminants
(e.g., Abriola and Pinder, 1985a, b; Faust, 1985). Only one case study
(Hyde Park Landfill in Niagara Falls, N.Y.) has been reported where these
techniques were applied to a field site (Osborne and Sykes, 1986).
However, the scale of that problem (80,000 tons of waste were dumped) does
not lend itself to analogies with leaks from USTs. Even so, the prediction
3-79
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of the migration pathway at this site and all sites involving immiscible
fluid transport is limited by the lack of generic and site specific data
which include: laboratory measurements of relative permeabilities and
capillary pressures; field determinations of saturations of immiscible
substances; and estimates of the quantity released (Faust, 1985).
3.6 VAPOR TRANSPORT IN THE UNSATURATED ZONE
3.6.1 OVERVIEW
The preceding sub-sections have been focused on liquid phase transport.
The movement of vapor phase contaminants -- a significant issue regarding
leaking USTs -- is addressed in this section. However, far less is known
and understood about the physics and chemistry of subsurface movement of
vapors than is known about liquids. As a results, the nature and treatment
of information provided here is different than in previous sections. Here,
in a sense, basic (fundamental) science is molded to the need to understand
vapor phase movement in the subsurface environment. Specifically, the text
does the following:
• introduces and describes the basic physical relationships that
underlie current theories regarding vapor phase movement; and
• adapts the basic physics (albeit crudely) to illustrate
phenomena/concepts of importance to the UST Program -- e.g., travel
times, migration pathways, end-point concentrations.
In preparing this section, the most directly-relevant references were found
to stem from research addressing the movement of landfill-generated vapors.
Unfortunately, however, even this topic has not been studied extensively.
To be certain that relevant information was not overlooked, a thorough
literature search was conducted and attempts were made to personally
contact individuals engaged in relevant work. All of this leads to the
conclusion that scientific inquiry into the area of subsurface vapor
movement is scant. Thus, treatment of the topic from an UST program
perspective, relies largely on fundamental physical laws.
3-80
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For vapors to move in the subsurface environment, the soils must be
sufficiently dry to permit interconnection of air passages among the soil
pores. For simplicity, consider this to be the case and further consider
the subsurface to be made up of uniform, fine sand. Two fundamental
parameters will govern the movement of vapors in this setting: vapor
concentration and flux. The leaked substance will have its greatest vapor
concentration at the leak site, where the free liquid product is
evaporating at the liquid-vapor interface. The rate of vaporization is
dependent on the vapor pressure of the substance, the pore pressure in the
soil, moisture content in the soil, and the temperature. The flux or vapor
flow is away from areas of high concentration to regions of ever less
concentration, ultimately the atmosphere.
3.6.2 FUNDAMENTALS OF TRANSPORT
The principal modes of vapor transport in soils are diffusion and
advection. It is important to consider the molecular diffusion coefficient
and the permeability of a particular soil to any particular consituent. It
is also important to consider the adsorption of vapor onto'soil particles
and its subsequent release to the vapor state. Vapor flux patterns are
determined by the presence of impermeable boundaries (e.g., frozen or paved
land surfaces), permeable boundaries such as the ground surface where the
concentration of vapor is kept essentially at zero by wind action, and
liquid-vapor boundaries where the vapor concentration is a function of the
vapor pressure of the substance.
Diffusion is the mass transport that occurs via the random motion of vapor
molecules. Because it is probable that more molecules will move from a
region of high concentration to one of low concentration than in the
opposite direction, one can predict a net flux of matter away from the
region of high concentration towards that of low concentration.
With pure diffusion, pore pressure is assumed to be uniform throughout the
soil medium; it is only the presence of a concentration gradient, and the
3-81
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tendency of random molecular motion to erase a gradient, that results in
transport.
In particularly fine-grained soils such as tight clays, where the
characteristic pore space is of a size less than that of the mean free path
length of a vapor molecule the diffusion process may better be called
effusion. Transport is still driven by the concentration gradient, but at
a pace severely retarded by collisions of vapor molecules with soil
particles (Kraemer, 1986).
Advection is the mass transport that occurs when the vapor field is
subjected to a total pressure gradient resulting in a down-gradient flux of
the subject vapor and all other pore gases present. The gradient may be
imposed by any of several causes:
• Barometric Pumping - Pore air pressure deep in the unsaturated zone
will, on the whole, reflect the mean atmospheric pressure at the
ground surface. A rise or fall in atmospheric pressure with respect
to the pore pressure will result in a flux into or out of the soil.
This mechanism is most important where the depth to the contaminant
source is small compared to the depth of the unsatured zone. It can
increase the rate at which free product in the soil volatilizes.
• Imposed Pressure Gradient - This may result from many causes, among
them:
- A heated building basement. In cold weather, the density of the
column of air in a building will be less than that found outside
and in the ground, so that the.basement air pressure is lower
than that surrounding the basement. Gas and vapor will seep into
the basement from the soil pores, given a pathway, e.g., a crack
in the basement wall or floor or with unfinished floors or
crawl spaces.
- Generation of gas or vapor at a rate such that removal of gas or
vapor by diffusive flux is not rapid enough to prevent a buildup
of total pore pressure in the soil near the generation or
evaporation site. For example, methane and carbon dioxide are
generated in municipal landfills. In warm weather, these gases
readily escape by diffusion upward, and there is negligible
buildup of pore pressure; but when the ground surface is frozen,
the gases can only escape by lateral underground travel over
relatively long distances. In such instances, the increase in
pore pressure may be as much as 30 kilo-Pascals, or about one ft
3-82
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head of water, (Mohren et. al., 1980; McBean, 1982; Merz and
Stone, 1969).
- Atmospheric horizontal pressure gradients, which result in wind
above the ground surface, will generally result in a small
horizontal advection of subsurface pore gases. Turbulent air
pressure fluctuations may also induce flow in the soil.
t Density Difference - If a vapor has a density sufficiently different
from that of other gases (such as air) in the soil pores, and it is
present in sufficiently great concentration, there will be a
gravity-driven density current of the vapor. In particular, a
relatively heavy vapor will tend to "pour" down to the bottom of the
unsaturated zone and pool as a lens on top of the water-saturated
zone (or on top of a lens of yet-denser vapor). Included in this
category are thermal gradients in the top few meters of the soil ,
caused by diurnal, short-term and seasonal air temperature changes.
For example, in winter the presence of cold, denser air at the soil
surface and warm less dense air below the surface may lead to
convection currents.
The Concept of a Concentration Field: Diffusive Transport Only
The growth of a vapor envelope about a leak in an isotropic unsaturated
soil medium is shown in Figure 3-28. Early in the leak history (Figure
3-28 (A)), the vapor is close about the spilled product. The concentric
circles shown represent concentric spheres in the actual three-dimensional
case, and are surfaces of constant concentration of vapor. On the free
product surface at the leak site, the concentration is at its maximum
value. Surfaces where the concentration are various fractions of M/(DH)
are shown, where M is the mass vaporization rate, D is the vertical
molecular diffusion constant, and H is the depth of the leak below the
ground surface.
Somewhat later in the leak history, the vapor envelope has grown (Figure
3-28 (B)). If the leak persists at a constant rate for a sufficiently long
time and other environmental factors do not change, the vapor envelope, or
more precisely the vapor concentration field, will approach a steady state
and not appreciably grow or change further (Figure 3-28 (O). This
steady-state distribution of vapor concentration is of fundamental
importance in leaking UST vapor impact assessment, in that it tells us the
3-83
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A: VAPOR ENVELOPE INITIALLY
GROWING AROUND POINT-SOURCE LEAK.
y
H
H
LEGEND
CDH
M
<.03
1 <
CDH
M
C = Vapor concentration
D = Molecular diffusivity
H = Depth of leak
M = Leak rate or volatilization rate
B: APPROACHING STEADY-STATE CONDITIONS
CAMP DRESSER & McKEE INC.
FIGURE 3-28A
GROWTH, STEADY-STATE MATURITY, AND DECAY OF A VAPOR
CONCENTRATION FIELD, FOR A POINT-SOURCE LEAK IN A DEEP
HOMOGENEOUS MEDIUM.
3-84
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C: STEADY-STATE CONDITIONS
T
LEGEND
CDH
< .03
M
C = Vapor concentration
D = Molecular diffusivity
H = Depth of leak
M = Leak rate or volatilization rate
D: RELAXING TO BACKGROUND
CONCENTRATION FOLLOWING CURTAILMENT OF LEAK.
CAMP DRESSER & McKEE !NC.
FIGURE 3-28B
GROWTH, STEADY-STATE MATURITY, AND DECAY OF A VAPOR
CONCENTRATION FIELD, FOR A POINT-SOURCE LEAK IN A DEEP
HOMOGENEOUS MEDIUM.
3-85
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relative concentration of vapor ultimately to be expected at any location
below ground.
Suppose that a continuous leak, that has established a steady-state vapor
envelope about it, is terminated. The vapor concentration field, or
envelope, surrounding the leak, will gradually dissipate, and
concentrations will relax towards background levels (Figure 3-28 (D)). The
time required to reach essentially steady-state conditions following first
leakage, and the time to relax from a steady-state condition to a vapor
free state following stop of a leak, are both of interest.
I Boundary Conditions - The vapor concentration fields depicted in
Figure 3-28 were for the simple case of a "point-source" leak
beneath the surface of a deep, homogeneous unsaturated soil layer
with a permeable surface. The concentration field will be
significantly different if there are impermeable boundaries across
which the vapor cannot flow. For example, Figure 3-29 shows the
steady-state concentration field for a point-source leak just above
a horizontal lower boundary such as the surface of the saturated
zone. The ground surface is still permeable, as in Figure 3-28.
However, Figure 3-30 shows the case of a point-source leak over an
impermeable lower boundary and under an impermeable ground surface
of considerable extent, such as a paved parking lot.
• Anisotropic Soil Conditions - Very often in undisturbed sedimentary
soils, where coarse and fine grained soils are found in alternate
layers, diffusion and advection occur much more readily in the
horizontal direction along the layers than in the vertical
directions which requires transversing fine layers as well as coarse
layers. This favoring of horizontal flow over vertical means that
the surface of constant concentration will appear "stretched"
horizontally. For example, the spheres of Figure 3-28 will be
replaced by the ellipsoids of Figure 3-31.
Pipeline Trenches, and Other Channels of Opportunity
The advection and diffusion of gas and vapor through tight, undisturbed
soils may proceed relatively slowly, compared to the ease with which gas or
vapor can flow in a coarse-grained homogeneous soil -- such as the crushed
rock typically used for bedding and backfill of buried pipeline. In fact,
pipeline trenches are notorious as efficient conduits for transporting
3-86
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CO
CD
/'/ /'' /// /// m it' /// // /// /// 111 inn; in Will /n ][/ i// I// i// !'/ i/f
IMPERMEABLE LOWER BOUNDARY
CAMP DRESSER & McKEE INC.
LEGEND
CDH
M
< .03
CDH
CDH
CDH
M
C = Vapor concentration
D = Molecular diffusivity
H = Depth of leak
M = Leak rate or volatilization rate
FIGURE 3-29
STEADY-STATE CONCENTRATION FIELD FOR A POINT-SOURCE
LEAK JUST ABOVE AN IMPERMEABLE
LOWER BOUNDARY (GROUND SURFACE PERMEABLE.)
-------
IMPERMEABLE
GROUND SURFACE
CO
i
co
co
IMPERMEABLE LOWER BOUNDARY
LEGEND
CDH
< .03
M
C = Vapor concentration
D = Molecular diffusivity
H = Depth of leak
M = Leak rate or volatilization rate
CAMP DRESSER & McKEE INC.
FIGURE 3-30
AS IN FIGURE 3-29, BUT NOW WITH GROUND SURFACE
IMPERMEABLE (E.G. FROZEN OR PAVED).
-------
H
LEGEND
CDH
M
< .03
CDH
1 <
CDH
M
C = Vapor concentration
D = Molecular diffusivity
H = Depth of leak
M = Leak rate or volatilization rate
CAMP DRESSER & McKEE INC.
FIGURE 3-31
STEADY-STATE CONDITIONS AS IN FIGURE 3-28C,
BUT WITH VERTICAL DIFFUSION COEFFICIENT
1/10 THAT OF THE HORIZONTAL DIFFUSION COEFFICENT
-------
gases and vapors from the part of the trench nearest the gas source to
buildings hundreds of feet distant. Therefore, any analysis of the vapor-
concentration field surrounding a spill should include a careful, complete
mapping of all buried pipelines in the study area. Furthermore, the design
of any pipeline which conceivably could lead harmful or explosive vapors or
gases to a valuable or inhabited structure should include adequate venting
of its trench for winter (frozen ground) conditions, and possibly trench
interruption by bringing the pipeline above ground for a short distance.
Adsorption and Release
In general, some of the vapor molecules of concern will be adsorbed onto
the surface of the soil particles. The rate at which they are adsorbed is
dependent on the vapor concentration in the soil pores and on the fraction
of the potential attachment sites on the soil particles which are occupied
by absorbed vapor molecules. With the passing of a vapor plume and
reduction in vapor concentration, the vapor molecules are once more
released from the particle. The process can be considered in terms of
reversable storage - release process. It has the effect of retarding the
establishment of a steady-state concentration field, and of retarding its
relaxation to background conditions after plume passage.
3.6.3 Summary of Vapor Phase Transport
The material provided above has been organized and presented to provide an
understanding of the basic physics which underlies vapor transport in an
UST setting. The most salient of the findings are summarized below:
• Evaporation is the rate at which a contaminant will enter the vapor
phase from the liquid phase is governed primarily by the vapor
pressure of the chemical of interest.
• Advection is a flux (movement of mass) process wherein a contaminant
in vapor phase is entrained in a subsoil current of. air, and "blown
along" with the air, should the air in the soil pore spaces be
itself moving. The air vapor may be driven by fluctuations in
barometric pressure, by pressure gradients across building
3-SO
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foundations, by the density difference between air and vapor denser
or less dense than air, and by evaporation or chemical generation of
vapor at a rate much greater than can be removed by diffusion alone.
• Diffusion is a flux that in general is superimposed on the advection
process. Diffusion is predominantly a gradient-driven process, the
rate of which is governed by the molecular diffusion coefficient and
the structure of the porous medium.
• Adsorption/Release address the processes whereby, as vapor phase
contaminants move through the soil matrix, there is a tendency for
molecules of the gas to adhere to soil particles (adsorption) and,
when concentrations are reduced subsequent to the passage of a
"plume", the contaminant may release itself from the soil particles
again. An indicator of the tendency for adsorption and release is
the octanol water partition coefficient (K ) of the contaminant.
ow
• Varying Permeability Due to Structures and Trenches block or inhibit
vapor flux, but the backfill surrounding such structures is often
more permeable to vapor flow than the undisturbed natural soil. In
particular, vapors are known to tend to move readily along buried
pipelines through the surrounding backfill.
• Molecular Diffusion Coefficient and Permeability of the Undisturbed
Soil are often much less in the vertical direction than in the
horizontal direction. Thus diffusion and advection fluxes of vapor
both tend to be significantly greater in the horizontal than in the
vertical direction.
• Studies on the subject of vapor transport in unsaturated soils has a
much shorter history and a far less plentiful body of technical
literature than the subject of groundwater flow. Yet with the
profileration of hazardous volatile chemicals manufactured,
transported, stored, used, and discarded by our industrial society,
and with the increasing age of the thousands of automotive service
stations across the country and the world with poorly installed,
maintained or monitored storage tanks, continued research on a broad
front appears justified.
3.7 SUMMARY OF TRANSPORT MECHANISMS
The transport (migration) of contaminants through the subsurface depends on
the quantity released from a leaking LIST, on the physical properties of the
leaked substance, and on the structure of the subsurface soils and rock
through which the contaminant is moving.
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Even though UST's leaks can be characterized as insidious spills, they may
be undetected for long periods of time, and thus, can release substantial
volumes of a substance into the subsurface. One reason the quantity of
released substance is important is that it often determines the method, and
degree, of groundwater contamination, i.e., by direct contact with the
water table or through solution in percolating rainwater.
The physical properties of the leaked substance that are important to
transport are:
. • Solubility - determines if single or multi-phase flow phenomena
will govern transport.
• Specific gravity - determines whether a nonaqueous phase liquid
will "float" on the water table or "sink" through the saturated
zone to an impermeable boundary.
• Viscosity - affects the rate of movement under saturated and
unsaturated conditions of a substance moving as a separate phase.
0 Surface tension - is responsible for capillary effects and
determines the spreading of fluids on the capillary fringe at the
end of the spreading phase.
• Evaporation rate - (dependent on the vapor pressure and latent
heat of evaporation) determines the potential for a substance to
have an associated vapor phase.
• Vapor density - determines whether the vapor phase in the un-
saturated zone rises or sinks to spread on the capillary fringe.
This section discusses the transport mechanisms through various types of
subsurface media. Movement of contaminants is discussed for three
characteristic zones of the subsurface: the unsaturated zone lying below
the ground surface, the capillary zone that is a transition between the
unsaturated and saturated zones, and the saturated zone lying below the
water table.
Unsaturated Zone:
Transport of a contaminant through the unsaturated (or vadose) zone is
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characterized by vertical flow, driven by gravity, and lateral spreading
associated with capillary forces and soil heterogeneities.
The migration of contaminant through this zone is most commonly a three-
phase flow process, in which fluids which are immiscible in each other,
air, water, and the contaminant — say, gasoline — are the three phases.
Under certain conditions, as when a contaminant is dissolved in water or
when subsurface soils are dry, the migration is a two-phase flow process.
The flow of two or more immiscible fluids depends on capillary pressures,
which are the result of surface tensions between the fluids, and the
saturation levels of each of the fluids which determines the relative
permeability of the fluids in a medium.
The velocity of the immiscible fluid is inversely proportional to
viscosity. Therefore, heavy oils do not readily penetrate the soil;
whereas lighter petroleum products, gasoline in particular, move through
the soil more easily than water does. In dry soils, (gravity dominated
flow with no pressure gradient) the velocities of downward flow (or
percolation) will be inversely proportional to the kinematic viscosities;
p
for example, light heating oil (kinematic viscosity = 4mm /second) will
move four times more slowly than water.
The downward flow of the contaminant will be arrested when:
• The contaminant reaches the water table,
• The contaminant encounters an impermeable layer, or
• An immiscible contaminant (i.e., one that will not mix with
water) reaches residual saturation levels.
Until one of the above occurs, the shape that the contaminant plume takes
in its migration through the subsurface depends on the permeability of the
soil medium, the viscosity of the contaminant, the capillary effects, and
on the degree of heterogeneities. In general, less permeable soils cause
more lateral spreading than do more permeable soils. The presence of
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heterogeneities, or a stratified soil, also increases the lateral spreading
and reduces the contaminant's depth of penetration in the subsurface.
In general, the contaminant plume will take a pear-shaped form as it moves
through the unsaturated zone. The plume will be wider (the pear "fatter")
in a less permeable soil than in a more permeable soil. The shape will be
irregular in a stratified soil with varying permeabilities.
for immiscible substances, the residual saturation of the contaminant is an
-important factor in estimating its spread and is a measure of the retentive
capacity of the soil with respect to a particular fluid. In general,
there is an inverse relationship between permeability and retention: less
permeable soils retain more of an immiscible substance than do more
permeable soils. In dry soils, the residual saturation of a substance will
be higher than in wet soils (which means the soil will tend to retain the
substance until a higher volume is reached). As the water content of the
soil increases, the residual saturation of the substance tapers off to a
constant level.
A leaked substance such as gasoline can travel through 25 feet of permeable
alluvial or glacial sediments in a few hours or, at most, a few days,
depending on specific conditions at the site.
Capillary Zone:
The capillary zone has a significant impact on the movement only of non-
aqueous phase liquids that are less dense than water.
In such cases, the primary direction of movement is lateral, with flow
extending farthest in the direction of the slope of the water table.
When free oil initially reaches the capillary zone, its vertical movement
is stopped at the top of the capillary fringe. As more oil reaches this
region, a layer of increasing thickness, a mound, begins to form on the
capillary fringe under the influence of the infiltrating oil. The weight
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of the oil will result in the depression of the capillary fringe and
possibly the water table. Buoyant forces act to restore the water to its
initial level and if a critical minimum thickness (which varies with soil
and contaminant properties) is exceeded, and lateral spreading begins.
Lateral spreading will, occur in all directions, but the predominant
movement will be with the slope of the water table. Lateral migration
ceases when the oil is at its residual saturation. The flow results in a
characteristic shape called an "oil pancake."
The shape of the oil pancake depends on permeability of the soil, the
percolation rate, and the local water table configuration. In general, the
more permeable the soil, the more the contaminant will spread and the less
thick the recharge mound will be. The steeper the hydraulic gradient, the
narrower the plume will be, elongated in the direction of groundwater flow.
As the water table fluctuates, over time, the nonaqueous phase liquid
contaminant can be spread out somewhat in the vertical plane. In porous
media, this can lead to an apparent decrease in the volume of free product,
as previously uncontaminated soil will now retain some of the contaminant
at residual saturation. In fractured rock media, on the other hand, there
may be an apparent increase in the volume of free product as contaminant
trapped in dead-end fractures may be "washed out" previously unconnected by
water table.
Saturated Zone:
The transport of contaminants in the saturated zone can be characterized
for three classes of substances as follows:
• miscible or dissolved substances,
• immiscible substances with specific gravity of less than 1.0, and
• immiscible substances with specific gravity of more than 1.0.
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Dissolved substances will enter the groundwater and will move in the
general direction of groundwater flow according to the mass transport laws
of advection and dispersion. Advection is the movement of a contaminant
plume with the mean groundwater flow. Dispersion describes how a con-
taminant spreads out and is diluted as it occupies more of the saturated
zone than can be explained only by advection. Dispersion occurs on three
levels - molecular diffusion, microscopic dispersion and macroscopic
dispersion. In most field situations, macroscopic dispersion, which is due
to variations in permeability (heterogenities), will be the principal
phenomenon.
Immiscible substances that are "lighter" than water are typically only
found in the shallow part of the saturated zone. The substance typically
takes the form of an emulsion and the rate of transport will depend on the
local groundwater gradients and the viscosity of the substance.
Immiscible substances that are denser than water will move downward through
the saturated zone. A dense immiscible substance can pose a greater danger
in terms of migration potential than less dense substances because more
dense substances can penetrate deep into the saturated increasing the
potential for its solution and the subsequent migration of dissolve
components with the prevailing groundwater flow pattern. Although some
residual saturation (and therefore retention in the soil) occurs as the
substance moves downward, it is less than in the unsaturated zone. If the
quantity of release exceeds the retention capacity of the unsaturated and
saturated zones, the nonaqueous phase liquid reaches an impermeable
boundary, it forms a mound and spreads with the slope of that boundary,
pooling in depressions in the boundary.
Vapor Phase Transport:
A liquid contaminant leaking from an UST will enter the vapor phase
according to its specific vapor pressure (the higher the vapor pressure of
the substance, the more likely it is to evaporate).
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Once in the vapor phase, the contaminant will move by advection -- being
"blown along" by subsurface air currents (which themselves are the result
of several forces) — and by diffusion. The movement of vapors is
predominantly in the horizontal direction.
Molecules of gas may adhere to soil particles by adsorption and then be
released after passage of the plume. Movement of the gas can be blocked by
monolithic buried structures, but vapors will move readily through the
backfill surrounding such structures (such as along the route of a buried
pipeline).
Much less attention has been given to vapor phase transport than to liquid
transport; several research needs are identified.
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4
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4.0 FATE MECHANISMS
4.1 OVERVIEW
In the previous section on the transport of regulated substances through
the subsurface, the substances were assumed to be non-reactive (or
conservative); i.e., no change in the mass or phase of the substance
occured as it migrated through different zones or media of the subsurface.
This section discusses the fate of regulated substances which includes the
physical, chemical, and biological changes a substance may undergo in the
subsurface environment.
These fate mechanisms can be divided into two types of processes --
physical processes which include solubility, vaporization, and adsorption,
and kinetic processes which include biotic processes and abiotic chemical
transformations. Physical processes are natural processes which transfer
the substances across media/phase interfaces. Kinetic processes are
processes which decrease the concentration of a chemical by degrading it
into other products.
Five fate mechanisms were determined to be important for substances leaking
from USTs:
Solubility - the partitioning of a chemical between nonaqueous and
dissolved phases (chemical and soil-water).
Vapori zation - the partitioning of a chemical between its liquid and
vapor phases (chemical and soil-air).
Adsorption - the partitioning of a chemical between the soil and
soil water.
Biotic Processes - the degradation of a chemical by a
microorganisms.
Abiotic Chemical Transformations - the degradation of a chemical
through chemical reactions. Two specific reactions are examined:
hydrolysis and oxidation/reduction.
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Whether or not a substance is subject to a fate mechanism depends on a
number of chemical-specfic and environment-specific factors. In most
situations, not all of the fate processes may occur at the same time. One
fate mechanism may dominate or several fate mechanisms may "share
responsibility" for the removal of a chemical from a given media and/or
phase. This is especially true for a mixture of chemicals released in the
subsurface. The presence of two or more chemicals can inhibit or stimulate
the fate mechanisms to which any one substance is subject. These
interactions are highly complex and poorly understood.
In the simplest case, however, for any one fate mechanism a chromatographic
process takes place. This means that there is an order in which the
chemicals in a mixture may be, for instance, solubilized or biodegraded
which leads to a change in the relative concentration of the components of
mixtures with distance from the leak location. The changes in the relative
concentrations of the components of mixtures is called the weathering of
the mixture.
The next five sections describe the fate mechanisms which were determined
to be important for the regulated substances in USTs. In these sections,
the underlying theory, effects of environmental factors and, to the extent
possible, the degree to which petroleum products and their constituents and
hazardous substances are transfered or degraded in the subsurface
environment are presented.
4.2 SOLUBILITY
4.2.1 OVERVIEW
Solubility can be defined as the mass of a substance that will dissolve in
a unit volume of solute under specified chemical or physical conditions
(Freeze and Cherry, 1979). The solute of interest is the water in the soil
matrix (soil water) and the term solubility used throughout this section
means solubility in water. Solubility can be interpreted as the
partitioning of a contaminant between its nonaqueous and dissolved phases,
in particular between the chemical and the soil water. While some of the
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substances in USTs may be partly dissolved in water, many more are stored
in the nonaqueous liquid phase, especially the petroleum products.
The water solubility of a substance is one of its most important
environmental parameters. Not only does solubility determine the extent to
which a contaminant will dissolve, it also impacts other fate mechanisms.
For example, a highly soluble substance often has a relatively low
adsorption coefficent and also tend to be more readily biodegradable by
microorganisms. The other fate mechanisms — volatilization and chemical
transformations (hydrolysis and oxidation-reduction) — are also affected
by the degree of solubility (Lyman, 1982b).
Substances dissolve in water because of a favorable energy change
associated with the formation of bonds between the water molecules and the
molecules of the substance in comparison with the cohesive bonds between
the molecules of the substance itself. The process may involve strong
ionic interactions (as with the solution of metallic salts) or less
energetic hydrogen bonds (as between organic alcohols and water).
When considering the "solution" of one liquid in another, the term
"immiscible" is used to describe a situation of limited solubility such
that two distinct phases will exist. However, that immiscible liquids
always exhibit some degree of mutual solubility. Indeed, the immiscible
combination of gasoline and water involves sufficient solubility of toxic
gasoline fractions to contaminate a water supply. Aternatively, two
liquids mixing/dissolving completely in one another are termed "miscible in
all proportions." Such a circumstance characterizes, for example, mixtures
of methyl or ethyl alcohol and water.
The saturation concentration, the maximum amount of a chemical that will
dissolve in water at a specified temperature, is an important property
value in UST leak studies. This is the maximum concentration that will be
reached at the interface between the leaking substance and the soil water;
the maximum concentration which, via Henry's Law, sets the peak vapor
pressure of volative substance in the subsurface; the maximum concentration
defining molecular diffusion of "floating" substances into the depth of the
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saturated zone; and the maximum concentration of a substance that will be
found in the groundwater at a spill site. Thus, the saturation
concentration of a substance can be used to scale the environmental
significance of a leak.
A word of caution should be interjected about placing undue reliance on the
relative solubility terms in judging environmental significance. The
solubility of most substances studied herein are described in analytical
chemistry texts are being "very si ightly soluble" or "insoluble." In fact,
no substance is completely insoluble, it is just that in the practice of
analytical chemistry, these low solubilities are not important. In the
study of groundwater contamination, the slightly soluble compounds (the
organic species are often called hydrophobic compounds) may be the
compounds of greatest interest. They can travel long distances in aquifer
systems and can be toxic to a level of a few parts per billion (ppb).
Further comment on this topic is provided in Section 5.0.
4.2.2 ENVIRONMENTAL FACTORS THAT INFLUENCE SOLUBILITY
In the laboratory, the solubility of a chemical is determined by placing an
excess of chemical in very pure water and allowing it to equilibrate at a
constant temperature. The chemical composition of water in a soil medium,
however, is different than very pure water because of the solution of
minerals. Its composition along with other factors determine the
solubility of a substance in soil water. The factors that have been
determined to influence the solubility of a substance are: temperature,
pH, dissolved organic matter, dissolved salts, the purity of the chemical,
redox potential, and the relative concentrations of other substances in
solution. The interactions of these factors, make it difficult to predict
the solubility of a substance in soil water (A.D. Little, 1981). From this
list of factors, the effects of temperature, pH, dissolved salts and
dissolved organic matter are discussed below (Lyman et. al., 1982b).
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Temperature
The effect of temperature on the solubility of substances is usually
profound. For most chemicals an increase in temperature results in an
increase in solubility. A few chemicals, such as p-dichlorobenzene, show a
decrease in solubility with increasing temperature over certain temperature
ranges.
fill
The pH of soil water also affects the solubility of chemicals. Organic
acids may be expected to increase in solubility with increased pH. Organic
bases, on the other hand, behave in the opposite way. The solubility of
netural organic chemicals (e.g., alkanes and chlorinated hydrocarbons is
also reported to be affected by pH, although the direction of the effect is
not indicated.
Dissolved Salts
The presence of dissolved inorganic salts generally reduces the solubility
of both organic and (other) inorganic substances. This "salting out"
effect can be significant. Also, relatively soluble substances (e.g.,
sodium stearate) can be converted to relatively insoluble substances (e.g.,
calcium or magnesium stearate) by cation exchange with groundwater-borne
ions. Dissolved salts may also affect" the stability of emulsions and other
colloidal mixtures.
Dissolved Organic Matter
Several investigations have been reported to show an increase in solubility
in the presence of dissolved organic matter. Among the compounds that
exhibited this effect were: n-alkanes, DDT and phthalate esters. The
solubilities of aromatic hydrocarbons were reported to be unaffected by the
presence of dissolved organic matter.
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4.2.3 CURRENT UNDERSTANDING
Nonaqueous phase liquids in the subsurface represent a source of soluble
contaminaints for groundwater. Chemicals dissolve at the interface between
the chemical and the soil water. The location of the nonaqueous phase
liquid in the subsurface determines the way contaminants can be
solubilized. In the unsaturated and capillary zones, chemicals are
dissolved by percolating rainwater. Vapors can also be solubilized in
percolating rainwater. The interface between the capillary and saturated
zones (the water table) is the location of much of the solution of floating
nonaqueous phase liquids. In the saturated zone, chemicals are dissolved
in the groundwater. The density of a nonaqueous phase liquid determines
where the liquid will be distributed in the subsurface. In general,
nonaqueous phase liquids with a density less than water will be found in
the unsaturated zone, capillary zone and possibly in the shallow part of
the saturated zone. In additon to these zones, more dense nonaqueous
liquids can be found throughout the saturated zone. Regardless of the
density of the fluid, the rate of solution of a chemical in soil water will
primarly depend on its water solubility, the chemical/soil water contact
area, and the flow rate of the water (Guswa and Lyman, 1982).
The relationship between these variables and the extent of groundwater
contaminant ion can, in general, be simply stated: the higher the
solubility of the nonaqueous phase liquid, the chemical/soil water contact
area and the flow rate, the higher the potential for a greater extent of
groundwater contaminantion. In an inverse sense, these parameters
determine how long the nonaqueous phase liquid remains in the subsurface.
Another distinction that can be made is between continuous and
discontinuous sources of groundwater contamination. The nonaqueous phase
liquid in direct contact with groundwater represents a continuous source of
contaminants. The solution of contaminants in percolating rainwater is a
discontinuous source; available only when water is mobile in the
unsaturated zone (Convery, 1979).
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In general, the lighter the petroleum product (e.g., gasoline versus fuel
oil), the greater the content of soluble components (Davis et. al., 1972).
If these products reach the water table, the concentration of the dissolved
components depends on the surface area of the "oil"-water contacts and on
the flow rate of water. Experiments by Fried et. al. (1979) showed that at
flow rates higher than those typically encountered in sand and gravel
aquifers, an equilibrium concentration was achieved (for gas, oil, toluene
and iso-octane) in about 10 centimeters. At low flow rates, solubilization
of components of petroleum products may be limited by diffusion. The
surface area of oil-water contacts is difficult to estimate because the
nonaqueous phase liquid is unlikely to be a continuous fluid in the
subsurface. In the unsaturated and saturated zones, immiscible fluids
leave a trail of nonaqueous product in the soil matrix (residual
saturation). Schwille (1967) has also reported the presence of oil
emulsions in the shallow part of the aquifer, which further increase the
contact area for solubilization of contaminants to take place.
Shepherd (No Date) reported concentrations of dissolved hydrocarbons that
can be expected in various places in the subsurface: when agitated in a
well bore, dissolved concentrations may reach 200 ppm; in an undisturbed
aquifer, concentrations for light hydrocarbons are approximately 50 ppm;
leaching of product-saturated soil from the capillary zone, both above and
below a rising water table, provides soluble components at concentrations
of 1 to 10 ppm. These ranges reflect both saturation effects and the
formation of emulsions and other physical processes as well as differences
in dilution rates.
The concentrations reported above are for bulk petroleum products. The
concentrations of individual constituents of a petroleum product will vary
with their water solubility (the chromatographic separation process
mentioned previously). Certain hydrocarbon components of petroleum
products and additives have a relatively high solubility, and thus, higher
concentrations of these chemicals may be found than those reported for the
bulk petroleum product. Section 5.0 discusses these factors and the
solubilites of the hazardous substances in detail.
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4.3 VAPORIZATION
4.3.1 OVERVIEW
Vaporization is the change in state from a condensed phase (solid or
liquid) to the gaseous state. In this study of leaking UST problems, the
evaporation of liquids is of primary interest. Vaporization is accompanied
by heat absorption (the heat of vaporization); the quantity of heat
required is unique to the specific chemical involved. For substances
introduced into the subsurface environment by leakage from USTs, the rate
of vaporization is controlled by the rate of transfer of heat to the
evaporating substance and by the rate of reduction in the concentration of
the substance at the liquid-gas interface. The evaporating substance is
removed from the liquid surface by diffusion, by adsorption onto soil
particles, and/or by solution in water in the soil. In the subsurface,the
significance of vaporization as a fate of a chemical is limited mainly to
the unsaturated, capillary and shallow part of the saturated zone (Guswa
and Lyman, 1982).
For pure materials, the concentration of the substance in the gas at the
liquid-gas interface is directly related to the vapor pressure; essentially
a direct function of temperature only. For solutions, the concentration of
a substance in a gas phase [C.] in equilibrium with the solution is related
to the concentration in the liquid phase [X.]. The partition coefficient
which expresses the proportionality between the two is H , the Henry's Law
A
Constant. Henry's Law expresses the relationship:
CC,] - Hx [X,]
where H is tabulated (or capable of being estimated) for many organic
compounds in water solution. It can be seen, for example, that at the
liquid concentration corresponding to the limit of solubility (saturation
of water with the substance), the gas phase concentration corresponding to
the vapor pressure of the pure substance (p.) must be in equil ibrium both
I ' "' " '
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with the pure material and with the material in solution at the same
temperature. Noting that at an absolute temperature T and a total pressure
of one atmosphere, the concentration of an ideal gas is given by:
[C.] = P^RT
where R is the Universal Gas Constant and P. is the partial pressure of the
vapor.
Hx sat. = [P.]
CX^RT sat.
It should be noted that the Henry's Law "Constant" is not a true constant
but varies with the concentration of the substance. Indeed, the
determination of H at saturation conditions will almost certainly produce
A
a value quite different than that measured in highly dilute solutions.
Also, the Henry's Law Constant is affected by the presence of other
substances in the water such as inorganic ions and other organic compounds.
However, in dilute solutions, the effect of the concentration of other
contaminants on the partial pressure over the liquid is generally small.
Since for most chemicals, the vapor pressure is a much stronger function of
temperature than is the solubility, the Henry's Law constant will increase
with temperature in roughly the same proportion as increasing vapor
pressure (Guswa and Lyman,-1982).
For many petroleum products,the materials stored in USTs will be complex
hydrocarbon mixtures. For these mixtures, the concentration of the pure
compound over the liquid will not equal the vapor pressure of the pure
compound but will be defined by a complicated function of the specific
proportions and materials involved. Moreover, since the proportions of
each component in the gas phase are not usually matched to the proportions
in the liquid phase, it can be seen that, as evaporation proceeds, the
relative concentration of each component in both the liquid and gas phases
will be constantly changing.
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As an aid to understanding the various physical and chemical factors
influencing the generation, fate and transport of contaminants via the
gaseous state, the following simplified scenario, perhaps typical of a UST
leak of gasoline, is presented.
1. The UST (assume it is above the water table) is perforated and the
contents flow out of the tank. The soil retains some of the
leaking substance but the contaminated zone grows and sinks toward
the water table.
2. At the surface of the contaminated soil volume, the gasoline
vaporizes. Locally, the soil is cooled in proportion to the latent
heat of evaporation. Heat flows to the cooled region by
conduction. The hydrocarbon mixture at the liquid-gas surface is
enriched by the more volatile, lower molecular weight fraction of
the gasoline, such as the pentanes and hexanes added to improve
winter starting.
3. If the soils are sandy (porous), the migration of hydrocarbon
vapors is relatively rapid. Since the adsorptive capacity of sand
for hydrocarbons is quite limited, there is little adsorption. In
comparison to situations with soils of similar porosity but higher
adsorption capacity, the evaporation rate is lower since continued
evaporation is paced by the reduction of the vapor concentration at
the liquid interface by diffusion alone and is not augmented by the
"loss" mechanism of adsorption.
4. If it rains, the vapor will (partially) dissolve in the rainwater
percolating down through the subsurface. As the contaminated
recharge water moves further downward (ahead of the advancing
gasoline plume) it may encounter air free of hydrocarbon vapors
whereupon dissolved hydrocarbons begin to diffuse out of the water
into the "clean air."
5. Eventually, the gasoline plume reaches and spreads out on the
groundwater surface and begins to move in the direction of water
flow. Thus, the surface area for vaporization grows. Further, the
more soluble components of the gasoline hydrocarbons (e.g.,
benzene) dissolve in the groundwater. Downstream, these materials
also volatile in accord with a Henry's Law kind of relationship.
As the vapors from this and all other hydrocarbon-gas interfaces
move adsorption, differing in degree for each different
hydrocarbon, may retard certain species more than others: a
chromatographic-type of separation which fractionates the vapor
stream.
6. At some point, the vapor flux reaches a boundary. The boundary
could be the surface of the ground with subsequent direct loss to
the atmosphere. The boundary could also be an impermeable surface
such as a rock ledge or a tightly packed clay stratum. Then, the
gas phase concentration would build and the movement of contaminant
in other directions would be accelerated.
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7. A third kind of boundary would be a cellar wall or unfinished
cellar floor. Diffusion of the gasoline vapors will be slowed by
the relatively high resistance of concrete but, given the increase
in concentration which then occurs, vapor infiltration by diffusion
may be increased through cracks or sewer penetrations. An
unfinished floor would offer little resistance to vapor migration.
As the vapor concentration builds in the cellar, the impact can
fall into two categories. At the lower concentration end, one must
be concerned about inhalation toxicity (e.g., for carcinogenic
benzene vapor). At higher concentrations, the potential grows that
the lower explosive limit may be reached such that ignition by an
electrical spark, operation of a direct-fired water heater or
furnace could initiate a catastrophic explosion.
4.3.? ENVIRONMENTAL FACTORS AFFECTING VAPORIZATION
Environmental factors which affect vaporization of a chemical from the
subsurface include: soil water content, airflow rate over the ground
surface, humidity, temperature and bulk properties of the soil, e.g.,
organic matter content, porosity, density, clay content, and surface area.
(Thomas, 1982). To some extent many of these factors e.g., porosity,
density, and soil water content affect the movement of a vapor from the
spill site in that they determine the degree of interconnections in the
soil. These factors are discussed in relation to vapor transport in
Section 3.6. Other factors primarily determine the degree of adsorption of
a chemical; these include organic matter content, clay content, surface
area and indirectly humidity. The influence of these factors on adsoption
is described in Section 4.4. The interrelationship of adsorption to
vaporization is discussed below.
The effects of adsorption on the vaporization of pesticides is discussed in
Thomas (1982). He assumes that these concepts are relevant to other
organic chemicals. This discussion can be summarized as follows:
Adsorption competes with vaporization and solubility as fate mechanisms
affecting the distribution of chemicals in the subsurface. For weakly
adsorped chemicals, an increase in the moisture content of the unsaturated
zone can increase the vaporization of the chemical because water will
displace the adsorped chemical from the soil, and the chemical .is then free
to dissolve in the soil water or vaporize in the soil air.
4-11
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Vaporization can also be important for persistent chemicals. DDT and
polynuclear aromatics, for example, tend to be strongly adsorbed in the
soil and are resistant to degradation there. As their transport downward
is negligible, their vaporization -- however slow — may be the only
pathway for their gradual removal from the soil.
4.4 ADSORPTION
4.4.1 OVERVIEW
Adsorption, in the context of this report, is the mechanism that relates
the affinity between a chemical and the soil. The chemical can be in any
phase: a nonaqueous phase liquid, a solute in a water phase, or a vapor
phase. Adsorption can occur on almost all soil types, although for many
soils, adsorption is negligible. In general, adsorption is most
significant on organic matter or clay inorganic soil minerals.
Adsorption of vapors and non-aqueous phase liquids has not been as widely
studied as of solutes. The discussion of adsorption in this section is
limited to solutes. When viewed in the context of solutes, adsorption
describes the distribution of a solute between a liquid and soil.
Adsorption in this section is also limited to organic chemicals.
Adsorption of inorganic chemicals is complex; an introduction to this
process can be found in Freeze and Cherry (1979). As related to organic
chemicals, most of the research, and thus the understanding, of adsorption
has been done of low concentrations of organic compounds.
Adsorption has two effects on the movement of contaminants in the
subsurface: 1. it retards the forward progress of a contaminant; and 2.
the adsorped contaminant can act as a residual source, effectively
extending the contaminantion period.
Adsorption is generally considered to be either an affinity for the solid
or a lack of affinity for the liquid (Weber, 1972). In the latter case,
adsorption is called hydrophobic bonding and occurs because of the
lyophobic (solvent-disliking) character of the solute relative to a
4-12
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particular solvent. The solutes, thus, usually have a low solubility in
that liquid. If the solute has a high affinity for soils, the three
primary types of surface phenomena that may be involved are (Hounslow, No
Date, and Tinsley, 1979.):
o Physical adsorption - results from van der Waals forces, which is
caused by electrostatic interaction between atoms and molecules
arising from the fluctuations in their elecron distributions.
Adsorbed molecules are not fixed to a specific site but are free to
move within the interface.
o Chemisorption - involves the direct formation of a chemical bond
between the adsorped molecule and the soil.
o Exchange adsorption - occurs when ions of one substance concentrate
at the surface of the soil as a result of electrostatic attraction
to charge sites at the soil surface.
Tinsley (1979) notes that classification of different adsorptive forces is
somewhat arbitrary and that adsorption of an organic chemical to a soil
surface, may involve several types of bonding.
The technique used to determine adsorption of a specific chemical on a
specific soil is to equilibrate a known mass of uncontaminanted soil sample
with a known volume of solution of specific concentration and measure the
resultant equilibrium concentration. The quantity adsorped can be
determined by difference. This experimental information is usually
expressed as an adsorption isotherm, where the quantity of chemical
adsorbed per unit mass of soil is expressed as a function of the
equilibrium concentration of the chemcial (Tinsley, 1979). When the data
is plotted, the shape of the data may give some insight to the nature and
possibly the mechanisms that describe the interactions. Hounslow (No Date)
gives a thorough description of this. The graphical relationship between
these parameters and their equivalent mathematical relationships are known
as isotherms. This information, while certainly applicable to the chemical
and soil of interest, may be of limited value towards explaining adsorptive
behavior of other chemicals because the underlying mechanisms causing the
adsorption are not well understood.
4-13
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Two relationships have been developed that tend to be more generally
applicable, the langmuir and Freundlich isotherms. These two widely used
relationships are shown in Figure 4-1.
Langmuir Isotherm
Langmuir isotherms were developed to describe the adsorption of gases by
solids. The Langmuir isotherm is generally linear at low concentrations
and shows limiting adsorption and high concentrations. It is generally
expressed as:
S = (SmbC) / (1 + bC)
where S is the number of moles of solute adsorbed per gram of adsorbent;
S is the number of moles of solute adsorbed per gram of adsorbent in
forming a complete single layer of adsorbed solute molecules on the surface
of the adsorbent; C is the concentration of solute in the liquid phase; and
b is a constant.
In developing this relationship, the following assumptions were made:
- • The energy of adsorption is a constant and is independent of the
extent of surface coverage.
•• The adsorption is on localized sites and there is no interaction
between the adsorbed molecules.
• The maximum adsorption possible is that of a complete monolayer.
Tinsley (1979) states that because the heterogeneous nature of a soil
surface invalidates the first assumption, the equation is generally not
useful in the study of the adsorption of compounds from solution;
particularly onto soils. It is more widely used in the adsorption of gases
onto solids.
4-14
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I
I—'
01
CO
LANGMUIR
C
CO
FREUDLICH
n=1
S=
Smbc
1+bc
S=KCn
SOURCE: Hounslow (No Date)
CAMP DRESSER & McKEE INC.
FIGURE 4-1
LANGMUIR AND FREUDLICH ISOTHERMS
-------
Freundlich Isotherm
The Freundlich isotherm is an empirical relationship primarily used to
describe nonlinear relationships. It has wide used in the study of
adsorption of solutes and can be expressed as follows:
S = KdCn
where K., the distribution coefficient, and n are constants and S and C
have the same definition as in the Langmuir isotherm. Some physical
significance has been attributed to the constants: n is a measure of the
degree of nonlinearity; and K. is indicative of the strength of adsorption
(Tinsley, 1979). A linear form of this equation is used in experimental
analysis:
log S' = log Kd + (l/n)log C
If n = 1, the isotherm is linear and the change is the amount adsorbed, S,
per change in concentration of the solute, C, is equal to a distribution
coefficient.
The distribution coefficient is widely used in studies of groundwater
contamination and is a valid representation of the partitioning between
liquids and solids only if the reactions that cause the partitioning are
fast and reversible and only if the isotherm is linear (Freeze and Cherry,
1979).
Retardation
As stated above, one of the effects of adsorption" is to retard the apparent
movement of a contaminant plume relative to the movement of a contaminant
not subject to adsorption (a conservative contaminant). Under certain
conditions, the relative velocity of the adsorbed contaminant can'be
estimated. These conditions are described below.
4-16
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When the partitioning of a contaminant can be adequately described by the
distribution coefficient, the retardation of the contaminant relative to
the bulk mass of the water can be described by the retardation equation:
where v is the average Linear velocity of the groundwater; v is the
velocity of the c/co = 0.5 point on the concentration profile of the
retarded constituent; p. is the bulk density of the soil and 9 is the
moisture content of the soil. The term 1+ Kd(^b/9)is referred to as the
retardation factor. The reciprocal of the retardation factor is the
relative velocity of the contaminant as compared to the bulk flow of
groundwater. Some comments on the use of this equation as it applies to
substances leaking from USTs are necessary.
• Applicat ion - The retardation equation when expressed as above
applies generally to unsaturated and saturated flow. For strictly
saturated flow, the effective porosity of the porous media can be
substituted for the moisture content.
• Chemicals - Most of the availble data is for pesticides, and to a
lesser degree, aromatic and polycyclic aromatic ("energy-
related") compounds for adsorption onto organic matter (Lyman,
1982a). The available data is presented in Section 3.4.3.
• Equilibrium - Adsorption reactions for contaminants in groundwater
are normally viewed as being relatively rapid to the flow velocity.
Tinsley (1979) states that there is some evidence that suggests that
equilibrium is slowly reached and cites some studies where
equilibrium has not been reached after 23 days.
• Reversible - The desorption process is not as well studied as
adsorption. Tinsley (1979) states tht in some cases where
desorption has been studied, the rate of desorption is substantially
slower than adsorption. Also, some of the adsorbed substance may be
diffucult to remove, such that 100 percent recovery of the
contaminant cannot be reached.
• Linearity - When the Freundlich isotherm is linear, n=l. When the
relationship between the mass adsorbed and the concentration of the
solution is nonlinear, no method is available for estimating the
exponential fraction, and some value for 1/n must be assumed. Lyman
(1982a) presents a table showing the errors that can be expected
with use of the linear form of the Freundlich isotherm when the
relationship is nonlinear. The magnitude of the error depends on
the degree of nonlinearity and how far one must extrapolate from the
range at which K, was measured.
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4.4.2 ENVIRONMENTAL FACTORS THAT INFLUENCE ADSORPTION
The primary environmental factors that influence whether or not adsorption
occurs, and to what degree, are the properties of the soil, temperature,
and pH. The most important property of the soil is its surface area which
determines the availability of adsorption sites.
Soi 1 s
As stated above, adsorption primarly occurs on two types of soil--clays and
organics. Some of the properties of these soils relevant to adsorption are
listed below (Guswa and Lyman, 1982 and Tinsley, 1979):
• Clay mineral s - are layered structures of silicates and metal
hydroxides. This layering has a planar geometry and thus a very
large surface area with a very high residual negative charge. Clay
m inerals can be distributed throughout a soil, but when found in
cohesive lens or layers have a very low permeability.
• Organics - are somewhat hydrophobic and organophilic, which means
they have an affinity for organic substances. Organic matter also
provides a very large surface area and has a high cation exchange
capacity (i.e., it is negatively charged). Adsorption on organics
is typically reported in relation to its organic carbon content.
Lyman (1982a) reports that while the ratio of organic carbon to
organic matter varies somewhat from soil to soil, a value of 1.724
is often assumed. The organic carbon content of a soil generally
decreases with depth. In the weathered soil zone and peat, the
organic carbon content is high (up to about 40 percent). Below this
zone, organic carbon content is much lower and are typically less
than 0.1 percent below one to two meters depth. However, in some
locations significant organic carbon content is found at significant
depths in the subsurface.
Because leaks from USTs usually occur several feet below the ground
surface, the role of organic matter in adsorption may be diminished. Guswa
and Lyman (1982) state.that at these depths the relative importance of
adsorption on clay minerals, or the organic fraction, may be very uncertain
and can only be resolved by laboratory experiments with the chemicals and
soils in question.
4-18
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Temperature
Adsorption is an exothermic process. The heat released during bonding
(called the heat of adsorption) can usually give some index of the strength
of the bond. Small heats of adsorption (less than 10 kcal/mole) usually
denote physical adsorption, which explains the ability of physcially
adsorpted molecules to move freely within the interface. Chemisorption has
a large heat of adsorption (from 30 - 50 kcal/mole) and results in a
stronger bond. Because the heat of adsorption is exothermic, the effect
of temperature on adsorption is such that an increase in temperature will
usually result in a decrease in adsorption (Tinsley, 1979). Some examples
of the effect of temperature change on K^ are cited in Lyman (1982a).
pH
For netural molecules, the effect of pH on adsorption is very small. A
more pronounced effect is seen for weak acids and bases which can assume a
charged form depending on the pH of the soil. The charged species is
always more water soluble than the neutral species, and thus, will be
adsorbed to a smaller degree (Tinsley, 1979). The charge on surface
adsorption sites of the solid also changes with pH.
4.4.3 ESTIMATING PROCEDURES
Most of the early work in estimating adsorption in natural systems was done
for pesticides and in soil near the ground surface (the weathered soil
zone). For example, Karickhoff. (1979) reports that work done by Lambert
and his co-workers demonstrated that for a given soil type, the sorption of
neutral organic pesticides was well correlated with the organic matter
content of the soil. The soils in the weathered soil zone are organic-rich
and can have organic matter content up to 40 percent. The soils of
interest in relation to leaks from USTs lie below the weathered soil zone.
As stated above, the organic carbon content generally decreases with depth
and may reach negligible amounts (< 0.1 percent) below one to two meters
depth. The significance of adsorption for substances leaking from USTs,
therefore, depends on the amount of organic matter in the subsurface soil
of interest and on adsorption to clay minerals.
4-19
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Adsorption on clay or organic matter is most important for natural organic
chemicals, e.g., hydrocarbon fuels or chlorinated solvents (Guswa and
Lyman, 1982). The water solubility of the chemical and its concentration
are other considerations that affect adsorption.
Adsorption studies have primarily been done on hydrophobic (compounds
having a water solubility of less than a few parts per million) organic
compounds because the fate of these chemicals in natural systems is highly
dependent on their sorptive behavior (Karickhoff, 1979). In general, the
amount of contaminant adsorbed by soils is a function of the concentration
of the solute in solution. Karickhoff (1985) states that it has been
determined that for hydrophobic compounds that if the equilibrium aqueous
phase pollutant concentration is below 10" moles/liter or less than one
half of the solute water solubility (whichever is lower) sorption isotherms
to natural sediments are generally linear. Linear isotherms result when
the adsorption coefficient is independent of concentration.
McCarty et. al. (1981) stated that while the interactions of hydrophobic
solutes with organic matter have been shown to be reasonably predictable
over a wide range, interactions with inorganic surfaces do not generally
follow a simple pattern. Also, the role of the inorganic matrix is likely
to dominate not only the solute transport of polar and ionized species, but
also of npnionized, hydrophobic solutes if the organic content is below a
critical level. The theory and some experimental data to determine this
critical level of organic content are presented in this paper. They
conclude that if aquifer materials have a very low organic content, it is
possible that sand and clay may dominate the retardation effect. The data
are not presented here because the authors state that "some of the
assumptions made in arriving at these conclusions are tenuous." Some of
the other factors that they suggest that need to be investigated are
effects of multiple dissolved organic solutes on one another, effects of
metal-organic complexes, interactions of ionized organic molecules and the
specific role of various aquifer minerals, including clay and silica.
4-20
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When adsorption is due to organic matter, the tendency for a chemical to
adsorp is expressed in terms of K , the soil adsorption coefficient. K
(J C. DC*
is defined as the ratio of the amount of chemical adsorbed per unit weight
of organic carbon in the soil to the concentration of the chemical in
solution at equilibrium (Lyman, 1982a):
K = g adsorbed/g organic carbon
g/ml solution
Expressed in these units, K may range from 1 to 10 million. As with the
distribution coefficient, K,, this relationship assumes that adsorption is
fast and reversible and that the isotherm is linear. The relationship of
K to K. is as follows:
Koc • Kd/foc
where f is the fraction of organic carbon in the soil. Lyman (1982a)
reports that numerous studies have shown that values of K obtained in
this manner (for a specific chemical) are relatively constant and
reasonably independent of the soil used. If an experiment for a specific
soil/chemical combination is not done to determine K. several techniques
for estimating K are available.
The estimation techniques are empirically derived equations which relate
K to some other property of the chemical, most commonly the octanol-water
partitioning coefficient (K ), the water solubility (S) and others have
ow
been used. They are usually expressed in log-log form as follows:
log KQC = a log (S or KQW) + b
where a and b are constants. Lyman (1982a) and Hounslow (No Date)
summarize several of these estimation relationships.
4-21
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4.5 BIOTIC PROCESSES
4.5.1 OVERVIEW
In the soil environment two biotic processes occur: biodegradation and
biotransformation. These processes are oxidation/reduction reactions which
are mediated by microorganisms. Biodegradation is the decomposition of a
contaminant by microorganisms to ultimately produce energy, carbon dioxide,
and water. Biotransformation can be, or is for the purposes of this
report, assumed to be partial biodegradation. Contaminant compounds are
not degraded to carbon dioxide via the biotransformation process, but are
degraded to simpler compounds, termed degradative products. These
degradative products, however, may be more or less soluble and/or toxic
than the original compounds. The microorganisms primarily responsible for
biodegradation in the soil include bacteria, fungi, and yeasts. In this
section, when a specific biotic degradation process is not referred to, the
term degradation is used.
Biotic processes in the subsurface are complex. Biotic processes may occur
in three zones of the subsurface: the unsaturated, capillary, and saturated
zones. In these zones, microbes may degrade substances that are present in
various states. These states include substances which are:
• adsorbed to soil particles,
• dissolved in water, or
• held at residual saturation in the soil matrix.
Even though a contaminant may be present in all three states, degradation
may not occur in all states. Degradation by microbes in each state is
dependent on environmental, microbial, and contaminant factors. Each of
these factors must be appropriate for biotic processes to occur. It should
be noted that degradation of vapors does occur; vapors that are
resolubilized in the pore water may be readily biodegraded (Atlas, 1984).
4-2?
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Basic to biotic processes in the soil is the microbial utilization of a
contaminant as a source of carbon and energy. Carbon, the major 'elemental
requirement of compounds for cell construction, is released in either
organic form or as carbon dioxide (Brock and Brock, 1978). Energy, which
is used to drive biosynthetic reactions, is obtained from the degradative
oxidation/reduction reactions. Biosynthetic reactions require both energy
and a source of carbon in order for cell growth to occur. Basic steps
involved in cell utilization of contaminants are shown in Figure 4-2.
Microorganisms can be divided into two groups based on their carbon and
energy source: (1.) autotrophs and (2.) heterotrophs. Microorganisms may
degrade a contaminant in either an aerobic or anaerobic environment and may
be classified as facultative, possessing the ability to degrade both
aerobically and anaerobically, or obligate, possessing the ability to
degrade a contaminant in only one type of environment, aerobic or anaerobic
(Brock, 1979).
Requiring an electron-transport system, the release of energy from
contaminant degradation is primarily by respiration and may occur both
aerobically and anaerobically. Aerobic respiration utilizes oxygen as an
electron acceptor in the oxidative/reductive process. Anaerobic
respiration utilizes an electron acceptor other than oxygen. Accepting
electrons from the energy source, a contaminant, an electron acceptor
becomes reduced. Table 4-1 lists some of the common aerobic and anaerobic
electron acceptors with their respective reduced chemicals and/or
compounds. The contrasts in electron and carbon flow in the oxidation of
an organic energy source is shown in Figure 4-3. Oxidative and reductive
processes are always paired, hence, without the presence of an electron
acceptor microbial utilization of a contaminant will not occur.
4-23
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DEGRADATION:
BIODEGRADABLE
CONTAMINANT
MICROBES
ENERGY RELEASED + CO2 + H2O +
INTERMEDIATE END PRODUCTS
I
ro
BIOSYNTHESIS:
INTERMEDIATE
END PRODUCTS
+ ENERGY RELEASED
MICROBES
NEW MICROBIAL CELLS
CAMP DRESSER & McKEE INC.
FIGURE 4-2
BASIC ACTIVITIES AND REQUIREMENTS FOR A CELL
TO UTILIZE A CONTAMINANT
-------
TABLE 4-1
PREDOMINANT ELECTRON ACCEPTORS USED
IN THE ELECTRON-TRANSPORT SYSTEM
Aerobic
Respiration
Acceptor
Oxygen (02)
Reduced Product
Water
Anaerobic
Respi ration
Nitrate (NO,")
Ni
itrite (N02~)
Sulfate (S042~)
Carbon dioxide (CO,,)
Ferric (Fe )
Elemental Sulfur (S°)
2_
Tetrathionate (S/,0C
Nitrate (N02~), nitrous
oxide (NpOT, nitrogen (N2)
Nitrous oxide, nitrogen
Sulfide (H2S)
Methane (CH4)
2+
Ferrous (Fe )
Sulfide (H2<
Thiosulfate
2-,
Source: Brock (1979)
4-25
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AEROBIC RESPIRATION
ORGANIC COMPOUND
CARBON FLOW
ELECTRON
FLOW
ANAEROBIC RESPIRATION
ORGANIC COMPOUND.
CARBON FLOW
ELECTRON
FLOW
NO3
so-
SOURCE: Brock (1979)
CAMP DRESSER & McKEE INC.
FIGURE 4-3
CONTRASTS IN ELECTRON AND CARBON FLOW
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4.5.2 FACTORS THAT INFLUENCE DEGRADATION
Regardless of whether a chemical is readily degradable, the ultimate
breakdown of a chemical, in many cases, will be determined by:
• environmental factors,
• the type of microorganisms, and
• the nature of the chemicals.
Environmental Factors
Environmental factors control metabolic activity in general rather than
degradation specifically. The importance of particular parameters varies
with each ecosystem. In one environment a contaminant can persist
indefinitely, whereas under a different set of environmental conditions the
same contaminant can be completely degraded in a few hours or days.
Environmental variables which may affect biodegradation include:
• temperature,
• pH,.
• moisture,
• oxygen availability, and
• nutrients.
The following discussions are a condensation of a detailed review by Scow
(1982).
Temperature. The growth of microbes has been observed in temperatures
ranging from -12 to 100°C. Individual species are usually adapted to a
30-40 degree range within these extremes. Depending on the temperature in
which microorganisms have a competitive advantage over other species,
microbes may be classified in one of three groups: psychrophiles (< 25°C),
mesophiles (25°C - 40°C), and thermophiles (> 40°C). Temperatures outside
4-27
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a microbes range are not necessarily lethal; many microbes have a dormant
state that permits survival until conditions are supportive for growth. The
rates of biological reactions increase with increasing temperature, within
the range tolerated by a microorganism. This relationship may be
complicated, however, by microbial populations that are adapted to
temperature extremes, for example, psychrophil ic organisms may show
increased efficiency during the winter.
pH. As a group, microorganisms have adapted to the entire pH range
normally encountered in soil environments. Optimal growth for. fungi
primarily occurs under slightly acidic conditions, between pH 5 and 6.
Bacterial growth is favored by slightly alkaline conditions and is
inhibited when the pH decreases to approximately 5. Microbial oxidation is
most rapid between pH 6 and 8.
Moisture and Oxygen. In the unsaturated zone, air and moisture compete for
soil pore space. The amount of oxygen in the soil pore spaces is dependent
on relative saturation levels of water and air. The amount of available
oxygen affects the type and rate of biotic degradation reactions. Oxygen
levels are reduced by microbial utilization of non-replaceable oxygen
during aerobic metabolism and by encroachment of water into pore spaces
containing air. Encroachment of water can reduce oxygen diffusion rates by
as much as two thirds. It is important to note that the measure of soil
moisture that is most relevant to microorganisms is not moisture content
but "water pressure." This is the difference between the energy state of
the soil and the free energy of'water and represents the total
contributions of gravity, soil matrix, and osmotic pressure.
Nutrients. Microorganisms require substances other than substrates for
induction of enzymes and degradation. The presence or absence of such
substances influences the rate of biodegradation. The major elemental
requirements of a cell are carbon, nitrogen, phosphorous, sulfur,
potassium, magnesium, calcium, iron, and sodium (Brock and Brock, 1978).
The three elements in the soil environment that are most important in
determining growth are carbon, nitrogen, and phosphorous because of their
high concentration in a cell (Mitchell, 1974). If the elements are not
4-28
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present in sufficient concentration (termed "limiting nutrients"), the full
growth range of an organism, will be inhibited.
Nature of Chemical
Correlations between physical and chemical properties and the
degradability of organic compounds have been reported. These properties
include:
• compound structure,
• compound solubility, and
• chemical adsorption coefficients.
In general, highly branched and short chained compounds are less degradable
than unbranched and long chained compounds. For example, normal alkanes
from n-octane to n-licosane are more susceptible to biodegradation than
their lower molecular weight analogs, i.e., n-heptane through methane.
Additional compound structural factors affecting the degradability of a
compound are listed in Scow (1982).
Water-insoluble compounds are believed to persist longer than those that
are water soluble. Possible reasons for this behavior include:
1. inability of the compound to reach the reaction site in the
microbial eel 1,
2. a reduced rate of reaction when degradation is regulated by the
rate of solubilization, and
3. the inaccessability of insoluble compounds because of increased
adsorption.
Chemicals that are readily adsorbed may persist in the environment. In
addition, adsorption may be the primary factor preventing, or significantly
delaying, the degradation in soil of some usually metabolizable compounds.
Some compounds are physically trapped within the lattice structure of clay
pores too small for penetration by microorganisms. Alternately, by
combining with clay or other material, a compound may become unable to
penetrate cell membranes, i.e., undergo degradation.
4-29
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The concentration of substrate is also influential in- the microbial
process. If the concentration is too low, degradation may be limited,
possibly due to the lack of sufficient stimulus to initiate enzymatic
response. High concentrations of substrate may be toxic or inhibitory to
metabolism, depleting the supply of oxygen. Optimum concentration of a
substrate is chemical and species-specific.
Type of Microorganisms
Microbial influences include the species:
• distribution and concentration,
• previous history,
• inter-and intra-specific interactions, and
• ability to synthesize the enzyme systems required for the breakdown
of organic compounds.
The distribution and concentration of microbes in the soil varies. Both
environmental parameters or the presence of toxic substances may limit
microbial colonization. Soil microbial populations in the same vicinity
may differ in numbers of microorganisms because of the wide temporal and
spatial distribution of organic matter available for microbial utilization.
The number of microbes in the soil- is not as important as other factors
because microbes can rapidly increase their population once they have
acclimated to a new substrate. However, if short time periods are of
concern, the size of the microbial population may have a significant
effect. For example, complete degradation of glucose can vary from a few
hours to days depending on the number of organisms present (Scow, 1982).
The previous history of microbes in relation to the particular compound
undergoing degradation may influence the reaction rate. If a microbe has
not been recently exposed to the compound, the microbe may have to
acclimate to the compound. Acclimation periods vary from a few hours to a
few days or even longer. Four types of acclimation may take place (Wilson
and Hensen, 1985):
4-30
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1. Reproduction - Microbial cells capable of degrading the contaminant
are actively present and readily grow as degradation occurs.
2. Induction - Microbial cells are not readily capable of degrading
the contaminant and, hence, produce enzymes that aid in the
transformation of contaminant to a degradable condition. For
example, enzymes may induce solubilization of a compound that is
not readi 1 y soluble.
3. Coloni zation - Microbes recently exposed to a similar contaminant
can be transported to a new area of contamination.
4. Genetic - Microbial species in a genera will exchange plasma until
the presence of a particular plasma in a species is conducive to
degradation.
Inter- and intra-specific interactions among species indirectl y affect the
rate of degradation. Processes such as competition and predation can
inhibit degradation, whereas, the degradation of a substrate may be
cooperative, with successive species degrading the initial substrate in
sequential steps. For example, extracellular enzymes of one organism may
break down a compound such as pol ysaccharide sufficiently for uptake and
metabolism by another organism.
Microbial species utilization of a substrate is enzyme-specific. A
compound subjected to structural alteration may require a different enzyme
catalyst. The absence of appropriate enzymes for microorganisms in a
genera will inhibit degradation. For example, if a compound is degraded to
an alcohol, then to an acid, and then to inorganic matter by
enzyme-specific species I, II, and III of the Norcardia genera,
respectively, the compound will remain an acid if the required enzymes for
species III are not present.
Table 4-2 summarizes the variables potentially influencing degradation.
4-31
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TABLE 4-2
VARIABLES POTENTIALLY AFFECTING BIODEGRADATION
ENVIRONMENTAL FACTORS
• Temperature
• pH
• Oxygen
t Moisure
• Nutrients
NATURE OF CHEMICAL
0 Physical and Chemical Properties
• Concentration
NATURE OF MICROORGANISMS
• Spatial Distribution and Concentration
• Previous History
• Intra- and Inter-species Interactions
• Synthesis of Enzymes
4-32
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4.5.3 TYPES OF DEGRADATION
Degradation is the enzyme-catalyzed transformation of chemicals. In all
zones of the subsurface a contaminant may be degraded in three ways:
• aerobic degradation,
• anaerobic degradation, and
• cometabolically.
The degradation of a contaminant, however, may be hindered by sparing.
Sparing is the process in which the presence of a chemical will reduce or
inhibit the utilization of another chemical. Such diauxic phenomena, the
phenomenon in which, given two carbon sources, an organism preferentially
metabolizes one (completely) before commencing to metabolize the other,
does not alter the metabolic pathways of degradation, but rather determine
whether enzymes necessary for metabolic attack of a particular contaminant
are produced or active. A lag phase commonly separates the two phases of
growth. For example, the presence of acetate, an intermediate product in
'hydrocarbon degradation, has been found to reduce utilization of hexadecane
(Atlas, 1981). The effects of such sparing processes have a marked
influence on the persistance of particular contaminants.
Aerobic and Anaerobic Degradation
Aerobic and anaerobic degradation occur in all zones of the soil. Aerobic
conditions are more common in the unsaturated zone where conditions are
more favorable for oxygen and nutrient exchange. Aerobic degradation in
the saturated zone is controlled by the amount of dissolved oxygen in the
groundwater. Anaerobic conditions may result from aerobic conditions in
which oxygen levels are reduced by microbial depletion of non-replaceable
oxygen during metabolism or by filling of soil pore spaces with
oxygen-depleted water. Certain organic compounds have been found to be
degraded only under anaerobic conditions, e.g., tetrachloroethylene. The
4-33
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energy yield from oxidation/reduction in anaerobic degradation is lower
than aerobic degradation (Mitchell, 1974). This results in:
• much lower cell yields,
• a much slower rate of decomposition, and
• incomplete degradation of the substrate to organic acids.
Bouwer and McCarty (1984) reported experimental results that indicated that
overall decomposition rates under methanogenic (anaerobic) conditions
appeared to be five to ten times slower than under aerobic conditions.
Cometabolism
Cometabolism is an important transformation process of contaminants by
microbes. Cometabolism is the process in which microbes are able to
transform a secondary substrate because of the presence of another
substrate, the primary substrate. Unable to provide growth support to a
microbe, a secondary substrate is utilized due to the microbial growth
support on a primary substrate. Because of the transformation process of
Cometabolism, less degradable contaminants may be attacked more rapidly
when found in the subsurface with other contaminants that can function as a
primary substrate (VanLooke et. al., 1975).
Kinetics of Degradation
The rate at which a compound is degraded is dependent on the structure of
the compound and the metabolic capacities of the microbes in the ecosystem
receiving the compound. The metabolic capacities of the microbes are
controlled by the environmental factors presented in Section 4.5.2.
The factors that affect which of the basic rate equations are applicable in
deriving a biodegradation rate constant include: whether a chemical is
degraded cometabol ical ly or metabolical ly, strongly adsorbed, and/or
subject to competing reactions (Scow, 1982). For example, one rate law may
not adequately describe a chemical over its total degradative curve because
4-34
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of changes in the chemical's concentration-dependency and availability over
time. In most cases, however, one rate order is assumed to be in effect
over the entire degradation curve. The rate of growth of a microbial
population, reflecting the rate of biodegradation, is expressed in three
equations representing:
• Degradation in a non-limiting environment,
• Degradation in a limiting environment, and
• Degradation by cometabolism.
These equations are discussed in detail in Scow (1982) and Bouwer and
McCarty (1984).
VanLooke et. al. (1975) reports in biodegradation trials, betweeen 10 and
90 percent of the hydrocarbons are converted to microbial biomass. Field
studies by Raymond et al. (1976a) indicated the average reduction of oil
concentration due to biodegradation ranged from 48.5 to 90 percent
depending upon the type of oil and soil. However, due to the large number
of different hydrocarbons and the many different soil types, no general
rules may be proposed with regard to petroleum elimination in the
subsurface (VanLooke et. al., 1975). Any attempt to integrate information
on rates is inhibited by the diversity in measurement techniques, soils,
environmental conditions, and the quality and quantity of petroleum
products.
The extent of microbial degradation of hydrocarbons in the saturated zone,
groundwater, is not as well understood as in the unsaturated zone. Rates
of degradation are usually low because nitrogen and phosphorous
concentrations are commonly too low, and oxygen is often limited (Convery,
1979). Under anaerobic conditions, sometimes resulting from microbial
utilization of limited oxygen in aerobic conditions, rates may be only 10
percent of aerobic values. Bouwer and McCarty (1984) reported experimental
results which indicated that overall decomposition rates under methanogenic
(anaerobic) conditions appeared to be five to ten times slower than under
aerobic conditions.
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4.5.4 IMPACT OF CONTAMINANTS
The microbial degradation of contaminants has received considerable
attention over the past decade. The factors that influence degradation,
the metabolic pathways of compounds, the rate of degradation, and the
applicability of degradation as a method of corrective action include some
of the areas studied to date. The current understanding of microbial
degradation of petroleum products and hazardous substances in the
subsurface is discussed below.
Petroleum Products
The majority of research on microbial degradation of hydrocarbons has been
conducted in laboratory studies under controlled conditions, and the
difficulty in estimating degradation in the natural soil environment from
such laboratory results is stressed by several researchers (Brookman et.
al., 1985). Factors such as oxygen, nutrient content, pH, soil
composition, temperature, mi.crobial populations, moisture content, and
composition and concentration of substrate influence the microbial
degradation in the soil environment.
ZoBell (1946) reviewed the action of microbes on hydrocarbons and
recognized that many microorganisms have the ability to utilize
hydrocarbons as sole sources of energy and carbon and that such
microorganisms are widely distributed in nature. He further recognized
that the microbial utilization of hydrocarbons is highly dependent on the
nature of the chemical composition of the petroleum product and on
environmental factors.
Twenty-one years after ZoBell's classic review, the attention of the
scientific community was dramatically focused on the problems of oil
pollution due to the sinking of the supertanker Torrey Canyon in the
English Channel. After this incident, several studies were initiated on
the fate of petroleum in various ecosystems.
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In 1968, the first detailed case study of petroleum product soil and
groundwater contamination was conducted by McKee et. al. (1972) at a
gasoline leakage site in Los Angeles County, California. Experimental
results indicated that gasoline at residual saturation would not be
mobilized by fluctuating water tables, several bacterial species of the
Pseudomonas and Arthrobacter genera utilize gasoline as a source of energy
for growth, and oxidation of gasoline occurred rapidly in the zone of
aeration and more slowly in the saturated zone.
These initial observations of hydrocarbon degradation, provide the base for
current research. To date, researchers have identified:
• microorganisms responsible for degradation of hydrocarbons,
• the degradabil ity of hydrocarbon constituents,
• the processes responsible for hydrocarbon constituent degradation,
and
• areas in the contaminated soil where degradation is more prevalent.
Microorganisms Responsible for Degradation. The microorganisms primarily
responsible for the aerobic degradation of hydrocarbons are well
documented. Atlas (1984) reports bacteria and fungi are the microbes
mainly responsible for the degradation of hydrocarbons. In decreasing
order, Pseudomonas, Arthobacter, Alcaligenes, Corynebacterium,
Flavobacterium, Achromobacter, Microccus, Nocardia, Mycobacterium, and
Acinetobacter are identified to be the most consistently isolated
hydrocarbon-degrading bacteria in the soil and Trichoderma, Penicillium,
Aspergillus and Mortierella to be the hydrocarbon degrading fungi isolated
repeatedly in the soil. Four of the most consistently isolated bacteria,
Arthobacter, Pseudomonas, Alcaligenes, and Flavobacterium, are the most
dominant bacterial genera found in soils indicating most soils may possses
some of the bacteria responsible for hydrocarbon degradation.
The relative contribution of bacteria and fungi to hydrocarbon aerobic
degradation is not clear. Bacteria tend to respond more rapidly to
hydrocarbon contamination of soil, whereas, fungi may be inhibited
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initially. The activity of fungi, however, tends to persist after
bacterial activity has tapered off. Atlas (1984) reports fungi utilize
long chain alkanes, e.g., C~0, and bacteria prefer to metabolize straight
chain aliphatics but also attack branched and aromatic hydrocarbons.
Specifically, Raymond et. al. (1976b) indicate Pseudomonas genera are
responsible for aromatic hydrocarbon degradation and Nocardia are probably
responsible for the major paraffinic hydrocarbon component degradation.
Several microorganisms have been isolated that anaerobically degrade
hydrocarbons. The genera of Pseudomonas bacteria are facultative, i.e.,
capable of aerobically and anaerobically degrading hydrocarbons. The
Moraxella genera of bacteria have been shown to metabolize aromatic
hydrocarbons anaerobically (Davis, 1967).
Degradability of Hydrocarbons. Researchers have found normal (straight
chain) paraffinic hydrocarbons are the most susceptible to degradation and
aromatics are the least degradable. Varying degradabil ity of hydrocarbons
reported by researchers are shown in Table 4-3. Atlas (1981), however,
reports the qualitative hydrocarbon content of the petroleum product
influences the degradability of individual hydrocarbon components.
Examination of the susceptibility to microbial degradation of hydrocarbons
in weathered heavy No. 6 fuel oil (Bunker C) and light No. 2 fuel oil
revealed far less degradation of heavy No. 6 fuel oil. Major differences
in the susceptibility to degradation of each of the identical compounds
within the context of the different hydrocarbon mixture, i.e., No. 6 and
No. 2 fuel oil, were reported.
Processes Responsible for Hydrocarbons Degradation. Knowledge of the
metabolic pathways of hydrocarbon degradation provides insight to
identifying the degradative processes utilized by hydrocarbons. The
anaerobic degradation of hydrocarbons has been regarded as negligible to
nonexistent. This is based on the existance of petroleum in the
subsurface, in an anaerobic environment, for millions of years. Wilson and
Rees (1985), however, have reported laboratory experiments indicate
anaerobic degradation of benzene, toluene, ethylbenzene and o-xylene. The
disappearance of toluene was rapid, whereas, the other compounds required
considerable lag time before degradation occurred.
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Class I
TABLE 4-3
DEGRADABILITY OF HYDROCARBONS
DECREASING BIODEGRADABILITY
Class II
Class III
Class IV
Reference
n-Paraffins
r tn T
V> Q U U "^ 1 O
Paraffinic
Med. M.W.-459
Branched
alkanes
Paraffinic
MW. 407
Aromatic
MW. 347
n-Paraffins
C5 to C8
Aromati c-
naphthenic
MW. 740
Cyclic HCs,
aliphatics,
aromatics
Naphthenic
MW. 1194
VanLooke
et.al. (1975)
Davis (1967)
HCs C1Q-C16
Davis (1967)
n-alkanes,
n-alkylaro-
matic,
aromatic HCs
(CIQ - c22)
n-alkanes,
a Ikyl aromatic
aromatic HCs
C5 - Cg
Branched
C10 " C22
n-ankanes,
Hi ghly
Atlas (1984)
alkyl aromatic, condensed
aromatic HCs aromatic,
(> C2o) cycloparaffinic
w/ ^ 4 rings
Normal
(straight
chain)
paraffinic
HCs
Branched-
chain
paraffins
and cyclo-
paraffins
Aromatic HCs
Davis et.al
(1972)
Content:
High
paraffinic
HCs
Content:
High
naphthenic
HCs
Content:
High aromatic
HCs
Davis (1967)
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The importance of oxygen for hydrocarbon degradation is indicated by the
fact that the major degradative pathways for both saturated and aromatic
hydrocarbons involve oxygenases and molecular oxygen (Atlas, 1981).
Saturated (alkanes) and aromatic hydrocarbon degradation utilize molecular
oxygen for the subsequent formation of fatty acids and catechols,
repectively, with eventual oxidation and release of carbon dioxide (Brock,
1979). Atlas (1981) reports fatty acids, some which are toxic to
microorganisms, have been found to accumulate during hydrocarbon
degradation, indicating incomplete oxidation of saturated hydrocarbons.
Particularly caprilic and laurinic fatty acids may be inhibitory towards
hydrocarbon tnetabol i zing microorganisms (VanLooke et. al., 1975).
Dissimilar to alkanes which are usually attacked at the end methyl by
microbes, unsaturated hydrocarbons (alkenes) are attacked by microbes at
various locations producing a variety of end products. Watkinson (1978)
reports alkene degradative end products include, but are not limited to:
• 1,2 diols,
• w-unsaturated acids,
• w-unsaturated 1° and 2° alcohols, and
• 1,2 epoxide.
The degradation of some petroleum hydrocarbons by cometabolism has been
reported. Bouwer and McCarty (1984) report experimental results indicate
ethyl benzene and napthalene are degraded by cometabolism in the presence of
acetate. VanLooke et. al. (1975) states researchers have found
alkyl benzenes with a methyl- or ethyl-side chain can only be metabolized by
cometabolism, congruent to Bouwer's findings. There have been several
reports of the cometabolic degradation of alicyclic compounds, which are
particularly resistant to microbial attack (Dart, 1980). The degradation
of cyclohexane after a cometabolic transformation by microbes who are
unable to metabolize cyclohexane independently has been reported (VanLooke
et. al., 1975). Cooxidation involving gaseous hydrocarbons has been
reported by researchers. Atlas (1984) reports, researchers have found
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ethane, butane, and propane cooxidized by Pseudomonas methanica in the
presence of methane. In addition, the utilization of cyclohexane by
propane-grown cells has been reported by researchers.
Areas of Prevalent Degradation. The areas in the contaminated soil
environment where degradation is most prevalent and rapid is dependent on
the environmental factors and nature of microorganisms discussed in Section
4.5.2. Soil microorganisms degrade petroleum contaminants rapidly under
optimum conditions, however, these conditions hardly ever exist in natural
soils.
McKee et. al. (1972) were the first to report the areas of varying degree
of degradation. Bacterial degradation of gasoline at residual saturation
in test columns was observed to be rapid in the zone of aeration above the
water table but much slower and less effective in the water-saturated zone.
Davis et. al. (1972) reported similar findings: the rate of degradation
will be highest near the surface of the contaminated soil and lowest in the
zone where the oil pancake spreads over the water table. The rate is low
in this area primarily for three reasons: reduced aeration, reduced
nutrient supply, and markedly increased saturation of the soil by oil.
Borden et. al. (No Date) developed a conceptually simple model to predict
the degradation of organic contaminants under oxygen limiting conditions.
Model simulations indicate that an anoxic (oxygen depleted) zone will
develop between the advancing contaminant plume and the oxygenated
formation water. The rate of degradation will be most rapid at the sides
of the plume near the source where the anoxic zone is narrowest rather than
the downstream edge where the anoxic zone is widest.
Hazardous Substances
Like petroleum products, the degradation of hazardous substances has
recently received considerable attention. Encompassing a broad range of
chemical properties, the study of the degradation of hazardous substances
has focused in the area of determination of those hazardous substances that
are degradable. Some hazardous substances are not naturally found in the
subsurface, influencing their persistance in the subsurface.
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As discussed in the previous subsections, degradation of a chemical is
site-specific, i.e., dependent on environmental factors, the nature of
microorganisms. For this reason it is difficult to "label" those
substances that are degradable. In this section the following topics of
the degradability of hazardous substances will be discussed:
• Hazardous substances recognized by researchers as readily degradable,
• Susceptibility of pesticides to degradation,
• Hazardous substances susceptable to biotransformation; and
• The influence of concentration on the degradation of hazardous
substances.
Hazardous Substances Readily Degraded. Some hazardous substances have been
widely recognized as substances that are readily degraded in various
environments based on experimental and field observations. These hazardous
substances are listed in Table 4-4. The 47 substances comprising Table 4-4
were identified from the following sources:
• Water-Related Environmental Fate of 129 Priority Pollutants (Versar,
1979), and
• Technical Background Document to Support Rulemaking Pursuant to
CERCLA Section 102 (Environmental Monitoring & Service, 1985)- RQ
requirements of a hazardous substance were increased if a chemical
possessed a predominant fate process which influenced the
elimination of the chemical in the environment.
Pesticides. Due to the recalcitrant nature of pesticides, the degradation
of pesticides has received considerable attention. The recalcitrance of a
pesticide can be predicted on the basis of molecular structure. An
approximation of comparable resistance to degradation as a function of
structure is listed in Table 4-5. The persistence of common pesticides
likely to be found in USTs is shown in Table 4-6. As indicated in the
tables, the chlorinated hydrocarbons are the most resistant to degradation.
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TABLE 4-4
HAZARDOUS SUBSTANCES RECOGNIZED
AS READILY DEGRADED
FROM TECHNICAL BACKGROUND DOCUMENT FOR RQ ADJUSTMENTS1
Acetaldehyde
Acetic Acid
Acetic Anhydride
Acetone
Acetonitrile
Ammonia
Amyl Acetate
Aniline
Ben zon it rile
1 - Butanol
Butyl acetate
Cresol
Cumene
Dipropylamine
Dimethyl amine
Ethyl acetate
Ethylenediamine
Furfural
Isobutyl Alcohol
Methanol
Methyl Ethyl Ketone
Methyl Isobuytl Ketone
Phenol
n - Propylamine
Pyridine
Quinoline
Resorcinol
Tri ethyl ami ne
Vinyl Acetate
FROM 129 PRIORITY POLLUTANTS2
METALS AND INORGANICS
Antimony
Arsenic
Cyanide
Lead
Mercury
Selenium
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TABLE 4-4
(continued)
PESTICIDES, PCBs, AND RELATED COMPOUNDS
Alorin
Endosulfan and Endosulfan Sulfate
2-Chloronaphtalene
MONOCYCLIC AROMATICS
Phenol
2,4 - Dichlorophenol
Pentachlorophenol
PHTHALATE ESTERS AND POLYCYCLIC AROMATIC HYDROCARBONS
Acenaphthene
Napthalene
Anthracene
Phenanthrene
Benzo [b] Fluoranthene
Pyrene
Sources: 1. Environmental Monitoring & Services (1979)
2. Versar (1979)
4.44
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TABLE 4-5
RESISTANCE TO DEGRADATION OF PESTICIDES
Decreasing
Biodegradability
Aliphatic Acids
Organophosphates
Long chain Phenoxyaliphatic Acids
Short chain Phenoxyaliphatic Acids
Monosubstituted Phenoxyaliphatic Acids
Dissubstituted Phenoxyaliphatic Acids
Trisubstituted Phenoxyaliphatic Acids
Dinitrobenzene
Chlorinated Hydrocarbons (DDT)
Source: Mitchell (1974)
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TABLE 4-6
PERSISTANCE OF PESTICIDES LIKELY
TO BE FOUND IN USTs
Pesticide
Degradation Time in Soil or Water
Chlordane - chlorinated hydrocarbon
DDT - chlorinated hydrocarbon
Dieldrin - chlorinated hydrocarbon
Parathion - organophosphorus
Malathion - organophosphorus
11 years
3 years
3 years
3 years
1 week
Source: Mitchell (1974)
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Biotransformation. Researchers have identified chemicals that are
biotransformed. Bouwer et. al. (1981) found that chloroform and several
other halogenated methanes were transformed readily in anaerobic water and
tri- and tetrachloroethylene disappeared at a slower rate. Parsons (1983)
found carbon tetrachloride was transformed to chloroform in anaerobic
subsurface materials. Similarily, tetrachloroethylene was transformed to
trichloroethylene, then to all three dichloroethylenes, and to vinyl
chloride. Such a reaction is significant for the resulting substances may
be more toxic than the parent compound, such as the transformation of
tetrachloroethylene to vinyl chloride.
Influence of Concentration. Wilson and McNabb (1983) have studied the
effect of concentration on the prospect of degradation of a hazardous
substance. The relationship between the concentration of a substrate and
its fate is complex. At reasonably high concentrations (>100 ug/1)
utilization of a pollutant may provide an ecological advantage resulting in
an increase of microbial population that metabolize the organic pollutant.
However, at low concentrations (<10 ug/1) the use of the pollutant does not
provide enough ecological advantage to lead to enrichment of microbes. As
a result, compounds that usually are considered degradable may not be
transformed by subsurface microbes. At high concentrations (>1,000-10,000
ug/1) metabolism of a pollutant can entirely deplete the oxygen or other
metabolic requirements in groundwater resulting in partially degraded
compounds that may only be degraded further after dispersion or other
physical processes that mix contaminated water with oxygenated water.
4.6. ABIOTIC CHEMICAL TRANSFORMATION PROCESSES
4.6.1 OVERVIEW
Chemical transformations decrease contamination concentration by degrading
the chemicals into other products. These chemical mechanisms collectively
define the persistence of a contaminant in the subsurface. Significant
subsurface abiotic chemical transformation mechanisms include: hydrolysis,
and oxidation and reduction.
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4.6.2 HYDROLYSIS
Overview
Hydrolysis is the reaction of a chemical with water, usually resulting in
the introduction of a hydroxyl function (H+, OH-) into a molecule and loss
of a leaving group (X). The importance of hydrolysis from an environmental
fate point of view is the resulting product usually is more susceptible to
further attack through the process of bidegradation and the addition of a
hydroxyl group may make the chemical more water soluble (Neely, 1985).
Although hydrolysis of organic and inorganic compounds may occur, for the
purposes of this discussion, only organic compounds will be addressed.
This section also does not address chemicals that are "water reactive,"
i.e., when in contact with water the chemical violently reacts (e.g.,
chlorosulfuric acid).
Hydrolysis belongs to the general class of nucleophilic displacement
reactions where the nucleophile (N) displaces a leaving group (X):
R - X - N —> RN + X
When an organic compound undergoes hydrolysis, a "nucleophile" (N) e.g.,
water or hydroxide ion, attacks an "electrophile" (R) e.g., carbon atom', or
phosphorous atom, and displaces a "leaving group" (X) e.g., chloride or
phenoxide. These displacement reactions are usually represented by two
distinct Substitutions Nucleophilic kinetic patterns: S^, (Unimolecular)
and SN£ (Bimolecular).
The "unimolecular" SNI process is a two-step process characterized by a
rate independent of the concentration and nature of the .nucleophile, (HJD+,
OH). The S^, .process is dependent on the concentration of the organic
compound (R-X), which undergoes a relatively rapid nucleophilic attack
following the formation of chemical constituents products from a compound
and is favored by:
4-48
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• R-systems that form stable carbonium ions, e.g., tributyl and
triphenyl methyl systems;
• X-systems that are good leaving groups, e.g., halide ions,
p-toluene-sulfonate ions; and
• high dielectric-constant solvents such as water.
In an S^.p process the rate depends on the concentration and nature of the
nucleophile. The S^ process is characterized as a one-step bimolecular
process (Harris, 1982) and is favored by:
• R-systems with low carbonium stability, e.g., methyl and other
primary alkyl systems;
• X-systems that are poor leaving groups, e.g., NH2" or CH^CH^O"; and
• Organic solvents such as acetone.
Hydrolysis of compounds in water has been assumed to follow a first-order
decay in the concentration of the organic species (Harris, 1982). The rate
of disappearance of RX, is directly proportional to the concentration of
the compound. This first-order dependence is important, because it implies
that the hydrolysis half-life of RX is independent of the RX concentration.
The half-life can be expressed as:
t1/2 = 0.693/KT
The total hydrolysis rate constant, K-r, is generally assumed to depend on
the rate constants for neutral and specific acid- and base-catalyzed
hydrolysis, hydrogen ion concentration and hydroxyl ion concentration. It
should be noted that Harris (1982) states, "estimation of hydrolysis rates
is only feasible when the hydrolysis pathway is reasonably simple and
straight-forward and the product(s) can be predicted."
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Environmental Factors
Environmental factors control the kinetics of hydrolysis. The factors that
have the potential for impacting the chemical transformation process of
hydrolysis include:
• temperature,
• pH, and
0 reaction medium - freshwater and salt water.
Based on the likelihood of leaking substances from USTs interfacing with
salt water, the influence of this reaction medium will not be discussed.
Temperature. The rate of hydrolysis of chemicals increases with increases
in temperature. The quantitative relationship between the rate constant
and temperature is often expressed by the Arrhenius equation. The
temperature dependence of the hydrolysis rate constant is complex because
constants in the Arrhenius equation are themselves temperature dependent.
Despite these complexities, Harris (1982) states some useful "rules of
thumb" for temperatures in the vicinity of 0-50°C:
t a 1° change in temperature causes a 10% change in k (the rate
constant);
• a 10° change in temperature causes a factor of 4.5 change in k; and
• a 25° change in temperature causes a factor of 10 change in k.
pH. The pH dependence of the hydrolysis rate is summarized in Figure 4-4.
Representing a compound, the shape of the pH-rate profiles for hydrolysis
depend on the magnitude of the natural hydrolysis rate constant, K ,
compared to those of the specific acid/base-catalyzed processes. The three
transition points, Iw, I.g, INB, in the figure represent the values of pH
at which the acid- or base-catalyzed processes begin to make significant /
contributions to the hydrolysis rate constant, K,.. If these transition
points are between the typical aquatic environmental pH range of 5-8, acid
or base catalysis must be considered in predicting rates of hydrolysis.
4-50
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log k
o
log ky = log kH — pH
log kT = log kQH KW + pH
\ log kT = log kQ /'
K. X. ' I
AN
S 'NB
'AB
PH
SOURCE: Harris (1982)
CAMP DRESSER & McKEE INC.
FIGURE 4-4
pH DEPENDENCE OF Kj FOR HYDROLYSIS BY
ACID-, WATER-, AND BASE-PROMOTED PROCESSES
-------
Current Understanding and Gaps
The complex aspects of the mechanisms of hydrolysis has received
considerable attention. For the purposes of this project the major
emphasis is to identify those classes of compounds that are likely to be
found in underground tanks that may undergo significant hydrolysis.
Compounds Resistant to Hydrolysis. Involving the reaction of a chemical
with water, hydrolysis of compounds primarily takes place i'n the saturated
zone of the soil. In this zone, a compound's potential to react with water
is increased.
Some organic compounds are generally resistant to hydrolysis. Table 4-7
lists organic functional groups that are relatively resistant to
hydrolysis. Functional groups that are potentially susceptible to
hydrolysis are listed in Table 4-8. The multi-functional groups of the
categories listed in Table 4-7 may be hydrol ytically reactive if they
contain a hydrolyzable functional group in addition to the organic groups
in the table, i.e., alcohols, acids (Harris, 1982). Examples of the range
of hydrolysis half-lives for various types of organic compounds that are
susceptible to hydrolysis are illustrated in Figure 4-5.
Comparison of the functional groups in Tables 4-7 and 4-8 to the petroleum
products and hazardous substances in Section 5.0 indicates that petroleum
products are primarily comprised of organic compounds that are generally
resistant to hydrolysis. Based on this comparison, it will be assumed that
hydrolysis is not a major fate of petroleum products.
The determination of which hazardous substances hydrol yze is complex and
chemical specific. The classification of a specific chemical within a
functional group that is susceptible to hydrolysis is dependent on factors
such as molecular structure and electronegativity of the compound and the
other compounds interacting in the hydrolysis reaction. For some hazardous
substances hydrolysis has been identified to be a major process for the
substance's removal in the subsurface. These hazardous substances are
listed in Table 4-9'. The hazardous substances were identified from
tabulated data from the following sources:
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TABLE 4-7
TYPES OF ORGANIC FUNCTIONAL GROUPS THAT ARE
GENERALLY RESISTANT TO HYDROLYSIS
Alkanes
Alkenes
Alkynes
Benzenes/biphenyl s
Polycyclic aromatic hydrocarbons
Heterocyclic polycyclic
aromatic hydrocarbons
Halogenated aromatics/PCBs
Dieldrin/aldrin and related
halogenated hydrocarbon pesticides
Aromatic nitro compounds
Aromatic amines
Alcohols
Phenols
Gl ycols
Ethers
Aide! ydes
Ketones
Carboxylic acids
Sulfonic acids
Source: Harris (1982)
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TABLE 4-8
TYPES OF ORGANIC FUNCTIONAL GROUPS THAT ARE
POTENTIALLY SUSCEPTIBLE TO HYDROLYSIS
Alkyl halides
Amides
Amines
Carbamates
Carboxylic acid esters
Epoxides
Nitriles
Phosphonic acid esters
Phosphoric acid esters
Sulfonic acid esters
Sulfuric acid esters
Source: Harris (1982)
4-54
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Alkyl
Hal ides
n = t3
ANyl and
Benzyl Halides
n=9
Polyhalo
Methanes
n= 10
E pox ides
n= 14
Aliphatic
Acid Esters
n = 18
Aromatic
Acid Esters
n=2l
Amides
n « 12
Carbamates
n = 18
Phosphonic Acid Esters,
Oialkylphosphonates
n = 6
Phosphoric Acid,
Thiophosphoric Acid
Esters n = 6
Phosphoric Acid Halides,
Dialkylphosphonohatidates,
Oialkylphosphorohalides
n« 13
Pesticides and Misc.
Compounds
n " 13
t—
[>•—
-
0 •
— 1
X = F
'
X =CI
Key:
• Average
[> Median
n No. of Compounds Represented
2.2x10* 2.2x10" 2.2x10' 2.2 yi
yr yr yr
8 days 1.9hr I.ISmin 0.69s
Half-Life
SOURCE: Harris (1982)
CAMP DRESSER & McKEE INC.
FIGURE 4-5
EXAMPLES OF THE RANGE OF HYDROLYSIS
HALF-LIVES FOR VARIOUS TYPES OF ORGANIC
COMPOUNDS IN WATER AT pH 7 AND 25°C
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TABLE 4-9
HAZARDOUS SUBSTANCES FOR WHICH HYDROLYSIS
IS A SIGNIFICANT FATE MECHANISM
From RCRA Cost Analysis Method
Chloroform
1,2 - Dichloroethane
1,3 - Dichloropropene
1,3 - Dinitrobenzene
Ethylene oxide
Methyl chloride
Methyl methacrylate
Parathion
Phthalic anhydride
Vinyl chloride
Cyanide (hydrogen cyanide)
9
From 129 Priority Pollutants"
Pesticides, PCBs, and Related Compounds
Endosulfan and Endosulfan Sulfate Heptachlor
Halogenated Aliphatic Hydrocarbons
Hexachlorocyclopentadiene
Halogenated Ethers
Bis(chloromethyl)ether
3
From RQ Adjustments
Acetic anhydride
Acetyl chloride
Ethyl, acetate
Malathion
Mevinphos
Phorate
Source: 1. ICF (1984)
2. Versar (1979)
3. Environmental Monitoring & Services (1985)
4-56
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• RCRA Cost-Analysis Model (ICF, 1984) - predominant fate processes of
hazardous substances were identified,
• Water-Related Environmental Fate of 129 Priority Pollutants
(Versar, 1979), and
• Technical Background Document to Support Rulemaking Pursuant to
CERCLA Section 102 (Environmental Monitoring & Services, 1985) -- RQ
requirements of a hazardous substance were increased if a chemical
possessed a predominant fate process which influenced the
elimination of the chemical in the environment.
4.6.3 OXIDATION/REDUCTION
Overview
Oxidation/reduction reactions may be biotic or abiotic. Biotic
oxidation/reduction (biodegradation) is discussed in Section 4.2.
Oxidation is the process in which an atom losses electrons. Reduction is
the process in which an atom accepts electrons. If an atom losses an
electron, another atom must gain an electron, for this reason
oxidation/reduction reactions (redox reactions) are always paired.
Each ion-electron equation includes both the oxidized form and the reduced
form of the same species.
Cu2+ + Zn > Cu + Zn2+
oxidized reduced reduced oxidized
Therefore, a complete redox reaction contains at least two oxidants •
(electron acceptors) and two reductants (electron donors). A redox
reaction can occur only if the two oxidants differ in oxidizing strengths.
The tendency of a substance to donate or accept electrons in abiotic
reactions may be given by two parameters: electrode potential and redox
potential.
4-57
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Environmental Factors
Two environmental factors influence the potential for redox reactions to
take place in the subsurface: oxygen availability, and pH.
Oxygen Availability. The role of soil in oxidation-reduction reactions is
to provide electron acceptors for the oxidation of organic compounds.
Oxygen is the strongest common electron acceptor in aerobic conditions and
generally yields less toxic products and the most energy from oxidation.
Oxygen is desirable as an electron-acceptor for these reasons. When oxygen
is unavailable (anaerobic conditions) the prominent electron acceptors in
soils are: Fe (III), N03", Mn2+, S042", N20, and H. The decreasing order
of utilization of principle electron acceptors in soils is 0?
2+ ?+
disappearance, NO-j- disappearance, M formation, Fe formation, HS~
formation, H~ formation, and CH. formation (Bohn et. al., 1979). Secondary
electron acceptors yield less energy and products may be often more toxic.
For example, hydrogen sulfide is more toxic than sulfate, and ammonia and
nitrite are more toxic than nitrate.
pH. The pH of the subsurface influences the availability of electrons,
i.e., redox potential. In the pH range of 6-8, the lower the pH (more
acidic soil), the greater the redox potential. Low pH is characteristic of
compounds "electron rich" or reduced. These compounds in soil 'may readily
provide electron acceptors for redox reactions.
Current Understanding and Gaps
Only a relatively narrow portion of the total range of electrode and redox
potentials are available in soils (Bohn et. al., 1979). Therefore,
oxidation/reduction can only be important when potent oxidizing or reducing
agents are present. Such agents do not exist in natural soil groundwater
systems. In natural systems, near-surface soils and soil waters will
contain oxygen, a very mild oxidizing agent, and in deeper soils such mild
reducing agents as H^ and CH. may be present 'in small amounts. Direct
oxidation or reduction by such agents is unlikely to be a significant fate
of substances leaking from USTs except for the most reactive compounds.
4-58
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4.7 SUMMARY OF FATE MECHANISMS
Solubility
Contaminants can go into solution when:
• Rainwater percolates through the unsaturated and capillary zones
bringing dissolved contaminant with- it, possibly including those
that were in a vapor phase.
• Nonaqueous phase liquids dissolve from a spreading mound on the
water table.
• Contaminant dissolves from the bottom of an "oil pancake" in the
caillary zone.
As a general rule, the lighter the petroleum product, the greater its
content of soluble components. Gasoline, one of the most soluble petroleum
products has a solubility in water ranging from 20 to 80 mg/1. Certain
hydrocarbon components of petroleum products and additives have a
relatively high solubility, and thus, high concentrations of these
chemicals may be found than those reported for bulk petroleum products.
Oils typically form an oil pancake atop the water table. Contaminants
dissolve and enter the groundwater at the interface of the oil pancake and
the water. The potential solution of contaminants which occurs in this
interface is proportional to the area of interface (a thinner, more widely
spread pancake offers more area of interface and so potentially.a faster
rate of solution). Lab experiments have shown that at velocities
typically found in sand and gravel aquifers, saturation concentration
is typically reached in about 10 centimeters.
A diffusion corona of dissolved substance forms in the saturated zone below
the oil pancake. There may also develop a transition zone between the oil
pancake and the diffusion zone, consisting of oil emulsions, increasing the
area of contact between water and oil and so increasing solubility.
4-59
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Irrespective of the density of the chemical, its rate of solution in
groundwater will depend on its solubility in water, area of contact, and
groundwater flow rate. In an inverse sense, these parameters determine how
long the nonaqueous phase liquid remains in the subsurface. As each of
these factors increases, the rate of solution increases. Note that if
groundwater movement is very slow, molecular diffusion may limit the rate
of solution.
Vaporization
Vapors can be released by contaminants in soil or water. Vaporization
occurs from open bodies of water, such as lakes, from subsurface soils, and
from dissolved substances in groundwater, in that order of importance.
The tendency of a chemical of limited solubility to evaporate is described
in Henry's Law constant, which expresses the driving force for transfer of
solute from aqueous to gaseous phase as the quotient of that chemical's
vapor pressure and solubility. The higher the Henry's Law coefficient for
a given substance, the greater its tendency to evaporate. The constant is
temperature-dependent; evaporation increases with temperature.
Vaporization can also be important for persistent chemicals. DDT and
polynuclear aromatics, for example, tend to be strongly adsorbed in the
soil and are resistant to degradation there. As their transport downward
is negligible, their vaporization — however slow -- may be the only
pathway for their gradual movement out of the soil.
Adsoprtion
Adsorption of organic chemicals is most significant on organic matter or
clay minerals. When adsorption is due to organic matter in the aquifer,
the tendency for a chemical to adsorb is expressed in terms of the K the
U \* '
soil adsorption coefficient.
4-60
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Where organic carbon content in an aquifer is low (less than 0.1%), certain
mineral solids such as clay minerals may exert a substantial adsorption
effect. The relative importance of adsorption on clay minerals, or the
organic fraction, may be very uncertain and can only be resolved by
laboratory experiments with the chemicals and soils in question.
Adsorption plays two significant roles in the fate of contaminants:
o It can retard the forward progress of a contaminant plume.
o It can act as a residual source of contamination, extending the
period of contamination.
Biotic Processes
Some contaminants can be degraded by microbial activity; both liquid and
gaseous compounds can be biodegraded. Degradation is most prevalent in the
unsaturated zone where conditions are more favorable for microbial
utilization of a contaminant.
There are three modes of degradation:
• Aerobic degradation: This is the predominant mode of degradation,
taking place in the unsaturated zone of the subsurface environment
and in the saturated zone when dissolved oxygen is available.
• Anaerobic degradation: Some substances, such as
tetrachloroethylene, can be biologically broken down only in an
anaerobic environment. Anaerobic conditions may result from aerobic
conditions in which oxygen levels are reduced by microbial depletion
of non-replaceable oxygen.
• Cometabo1 ism: Contaminants that tend to be resistant to
biodegradation can be attacked more rapidly when found in the
subsurface with other contaminants.
The breakdown of a chemical in the natural environment will depend on the
nature of the chemical itself, on the types of microorganisms present, and
on environmental factors.
4-61
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Degradation of contaminant constituents is microorganism-specific and
site-specific. In general, larger molecules and/or branch-chained
chemicals are less degradable than small molecules and/or straight-chained
chemicals. Among the hydrocarbons, the normal paraffins are the most
susceptible to degradation, and the aromatics the least susceptible.
Encompassing a broad range of chemical properties, hazardous substances are
difficult to "label" as degradable. However, some hazardous substances
have been widely recognized as substances that are readily degraded in
various environments.
The rate at which a compound is degraded is dependent on the structure of
the compound and the metabolic capacities of the microbes in the ecosystem
receiving the compound. The rate of degradation is expressed in three
equations representing degradation by commetabolism and degradation in a
non-limiting and limiting environment. No general rules may be proposed
with regard to contaminant elimination in the subsurface because of the
diversity in soils, environmental conditions, and the quality and quantity
of contaminant.
Abiotic Chemical Transformation Processes
Two types of chemical processes can occur in the subsurface:
• Oxidation and Reduction
0 Hydrolysis.
Oxidation and reduction is the process in which an atom losses an electron
and an atom gains an electron, respectively. Oxidation/reduction reactions
are always paired. The tendency of a substance to donate or accept
electrons in a biotic reaction may be given by two parameters: electrode
potential and redox potential. Only a relatively narrow portion of the
range of electrode and redox potentials are available in soils. Therefore,
oxidation/reduction can only be important when potent oxidizing or reducing
agents are present. Direct oxidation or reduction by such agents is
unlikely to be a significant fate of substances leaking from USTs.except
for the most reactive compounds.
4-62
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The importance of hydrolysis from an environmental fate point of view is
that the resulting product is usually more susceptible to further attack by
processes of biodegradation, and the presence of the hydroxyl group makes
the chemical more water-soluble.
Organic chemicals can be usefully divided into functional groups that
represent their susceptibility to hydrolysis. Hydrocarbons, as a class,
are generally resistant to hydrolysis. For some hazardous substances,
hydrolysis has been identified to be a predominant process for the
substance removal.
The kinetics of hydrolysis are influenced by environmental factors. The
rate of disappearance of a chemical is directly proportional to the
concentration of the compound. The hydrolysis half-life is independent of
the concentration.
4-63
-------
-------
5.0 PROPERTIES OF REGULATED SUBSTANCES IN USTs
5.1 OVERVIEW
Section 5.0 addresses the physical and chemical nature of regulated
substances stored in underground tanks. The section is organized as
follows:
• Section 5.2 provides information on "bulk" properties of
petroleum products, as well as on the specific chemical
constituents present in the bulk products.
• Section 5.3 concerns UST-regulated hazardous substances, and
uses physical, chemical, and toxicological data to group the
substances according to their likely behavior after leaking
from USTs.
• Section 5.4 develops salient points-of-comparison between
petroleum products (gasoline and fuel oil) and hazardous
substances, as well as information on the specific constituents
present in the bulk products.
• -Section 5.5 summarizes the preceding work.
A major contribution of this effort is the development of computerized
databases on the properties of substances regulated under the UST
program. Three such data bases were developed and are contained in
their entirety in three appendices as follows:
• Appendix A - Properties of CERCLA-regulated hazardous
substances stored in USTs,
• Appendix B - Properties of petroleum products stored in USTs,
and
• Appendix C - Properties of hydrocarbons known to be
constituents of gasoline and fuel oil.
5-1
-------
5.2 PROPERTIES OF PETROLEUM PRODUCTS
5.2.1 OVERVIEW
This section of the report addresses petroleum products and their basic
properties. The following topics are presented, in order:
• bulk product properties relevant to UST leak incidents,
• grouping of petroleum products with respect to these properties, and
• discussion of the chemical composition of gasoline and fuel oil.
5.2.2 BULK PROPERTIES DATABASE
Physical and chemical property information was collected for the four
groups of petroleum products addressed in this study - power fuels, heating
oils, solvents, and lubricants (Section 2.0 described the selection of
these products). In Table 5-1 these groups are broken down into commercial
grade classifications, established by the American Society of Testing
Materials (ASTM), used in industry for both retail and wholesale commerce.
Information relevant to the mobility, toxicity, or human exposure
characteristics of a product was collected for the products listed on Table
5-1. Appendix B presents a complete summary of petroleum product data.
Two primary sources of information were identified for petroleum property
data. First, the 1984 Annual Book of ASTM standards for Petroleum Products
and Lubricants (ASTM, 1984) details product specifications for the commer-
cial grade petroleum products. For the most part, property values are
specified as a minimum/maximum range rather than absolute value, and
therefore the ASTM data are of a general nature. This reflects the fact
that the end uses which characterize the petroleum products can be
satisfied by a relatively broad range of pure substances and mixtures.
Second, the National Institute for Petroleum and Energy Research (NIPER),
in work performed for the American Petroleum Institute (API), compiles
annual (or semi-annual) product data from a nationwide sampling program for
gasoline (winter and summer), jet fuel, heating oils, and diesel fuel
(Shelton and Dickson, 1985). These data, unlike the ASTM specifications,
characterize the "real" universe of products as used in commerce.
5-2
-------
TABLE 5-1
COMMERCIAL GRADE CLASSIFICATION OF PETROLEUM PRODUCTS
I. POWER FUELS - CIVIL USE
A. Aviation gasoline and additives
Grade 80
Grade 100
Grade 100LL (Low Lead)
B. Jet fuel and additives
Jet A (kerosene type)
Jet A-l (kerosene type)
Jet B (wide cut or naphtha)
C. Automotive (motor gasoline) and additives
Leaded
Unleaded
D. Diesel fuel oil and additives
No. 1-D, 2-D and 4-D
E. Gas turbine fuel oils
No. 0-GT, 1-GT, 2-GT, 3-GT, 4-GT
II. HEATING AND ILLUMINATING OILS
A. Fuel Oils
No. 1, 2, 4, 5 and 6
B. Kerosene
III. SOLVENTS
A. Petroleum Spirits, Type 2, 3 and 4 and
commercial hexane
B. Mineral Spirits or Stoddard Solvent (Type 1,
Petroleum Spirit)
C. High flash aromatic naphthas, Type I and II
D. VM&P Naphthas - moderately volatile hydrocarbon
solvents, Type I, II, III
E. Petroleum Extender Oils
Types 101, 102, 103 and 104
ASTM
Designation
D910
D1655
D439
D975
D2880
D396
D3699
D235,
D1836
D235
03734
D3735
D2226
1
Source: ASTM (1984)
5-3
-------
Other information sources used to compile the petroleum product data base
included textbooks in petroleum refining engineering. Information on
petroleum products meeting Great Britain's product specifications was also
used in this work when necessary to augment US-based data and to fill data
gaps.
Certain information was unavailable for inclusion in our database.
Generally, information was available for performance-related properties
specific to particular products. Few information sources were identified
that contained data specifically developed for the evaluation of the bulk
product properties relevant to subsurface fate or transport. Although
petroleum products are routinely tested to demonstrate compliance with
product performance standards or specifications, most of the ASTM-specified
tests are not directly related to the mobility, toxicity, or human exposure
properties of the products.
After a review of this available information, five properties -- specific
gravity, kinematic viscosity, vapor pressure, percent composition, and
distillation temperatures, -- were chosen for inclusion in the database
because they relate to the fate and transport of the product, and/or
provide a basis for distinguishing between products.
Each of these five properties is discussed in more detail below:
• Specific gravity (s.g.) -- The dimensionless ratio of the density of
a petroleum product with respect to the density of water. Specific
gravity is a dimensionless parameter (no units are used). The
petroleum industry typically reports specific gravity in degrees API
(°API 60/60) measured at 60°F for both water and product. °API can
be converted to specific gravity (with respect to water) by:
s.g. = .(°API + 131.5)/141.5
The specific gravity of water equals 1.0. Petroleum products,
except in some rare cases, have a specific gravity less than 1.0,
typically between 0.6 and 0.9. Thus, most petroleum products will
float on groundwater. Density and specific gravity have the same
numerical values at 4°C where the density of water exactly equals
1.0.
5-4
-------
• Kinematic viscosity -- Calculated by dividing the viscosity by the
density is a measure of a product's resistance to gravity flow, the
pressure head of the fluid being directly proportional to the
density. The time of flow of a fixed volume of fluid is directly
proportional to its kinematic viscosity.
UNITS
c.g.s. system 1 cm2/sec = l.Stoke (ST)
S.I. system 1 m /sec = 10 Stoke
p
The centistoke (cST = 10 Stokes) is the dimension most often used
in reporting the kinematic viscosity of petroleum products by ASTM.
Kinematic viscosity is a useful parameter for comparing the relative
rate of movement of products through the unsaturated (vadose) zone.
The lower the kinematic viscosity, the faster a product will migrate
through soil driven by gravity forces. Petroleum products are
grouped by kinematic viscosity in Section 5.2.3.
• Vapor pressure — The (gas phase) pressure that a material will
develop within a closed container. ASTM specifications require the
Reid Vapor Pressure Test be employed for gasoline and other volatile
non-viscous products. The tests are conducted at 100°F using a
special sampling and analysis procedure; the "true" vapor pressure
is higher than the Reid vapor pressure by about 5 to 9 percent but
this relationship varies widely (Nelson, 1969).
UNITS OF PRESSURE
1 Kilopascals (kPa) = 7.5 mm Hg. = 0.01 atmosphere
High vapor pressure products, such as gasoline and jet fuels, have
the potential for creating vapor phase problems in other subsurface
structures near leaking tanks. On the other hand, when products
with low vapor pressure leak, vapor-related impacts can usually be
expected to be of less concern. It should be noted, however, that
the concentration threshold significant for fire/explosion is
usually in the "percent range" where chronic inhalation toxicity may
be important for concentrations measured in parts per million. In
Section 5.2.3 the relative vapor pressure of specific products is
discussed.
• Percent composition -- Petroleum products are mixtures of pure
hydrocarbons (molecules consisting only of carbon (C) and hydrogen
(H)) and, are divided into four generic types of hydrocarbon
molecules, as shown on Figure 5-1 and discussed below.
%P - Paraffins (alkanes) -- Straight (n or normal) or
branched-chain single-bonded (saturated) carbon chains.
%0 - Olefins (alkenes) -- Straight or branched-chain carbon
chains with at least one double bond.
5-5
-------
COMMON HYDROCARBONS
IN PETROLEUM PRODUCTS
ALIPHATICS
AROMATICS
l~n
I
ON
4
PARAFFINS
(ALKANES)
1
|
STRAIGHT BRANCHED
CHAIN
C-C-C-C
C-C-C-C
-c c
Isopentane
Pentane
or
1-Methylbutane
OLEFINS
(ALKENES)
I
1 J
STRAIGHT ^
CHAIN BRAN(
C = C-C-C-C C=C
MONO
BENZENE
NAPHTHAENO
NAPHTHALENE INDANE
NAPHTHENES
(CYCLOALKANES)
1
CYCLIC
C
cAc
c' — Ic
Cyclopentane
^ BRANCHED
C CYCLIC
CL_J r*
\^
2-Methylcyclopentane
CAMP DRESSER & McKEE INC.
FIGURE 5-1
COMMON HYDROCARBONS
IN
PETROLEUM PRODUCTS
-------
%N - Naphthenes (cycloalkanes) -- Single bonded carbon chains
allied in ring form; may have branched chains.
%A - Aromatics -- Those molecules with one or more benzene rings;
may have branched chains. Polynuclear aromatic hydrocarbons
are often abbreviated as PAH's, when discussed as a group.
The percent composition of petroleum products directly affects the
various performance properties of power fuels and heating oils,
solvents, and lubricates. The differences in properties, both
physical and chemical, among these four hydrocarbon types, (and
among structural variations within a type) characterize individual
petroleum products.
Also, the typical composition can give a general indication of
relative toxicity of the product. Of the four groups, the aromatics
are known to be the most toxic to humans with respect to both
inhalation and ingestion effects. In Section 5.2.3, this concept is
developed in detail for specific products, and in Section 5.2.4,
percent composition is discussed with respect to specific
constituents of products. Limited data were available for this
property.
• Distillation temperatures (volume percent evaporated) -- A
standardized test involving distillation over a programmed
temperature span to establish the range of boiling point of the
product mixture. The ASTM distillation tests vary between products
and are not indicative of the actual boiling range of the
hydrocarbon constituents of the product. Little fractionation
occurs in these distillation tests, particularly for oil and heavier
products, and the hydrocarbons do not distill one-by-one in the
order of their boiling points, but as successively higher boiling
mixtures (Nelson, 1969). The initial boiling point, end point, and
intermediate temperatures have little fundamental significance, but
have been found useful in showing relative behavior corresponding to.
points from other similar ASTM distillation tests (Nelson, 1969).
The data can give an indication of the relative boiling points (and,
indirectly, molecular weight distribution) of product constituents,
e.g., the fact that gasoline has a lower boiling range than kerosene
implies lower molecular weight constituents, typically lower
viscosity, higher vapor pressure, etc. of gasoline (Section 5.2.3).
The database for the above-listed parameters, for each petroleum product in
bulk form, is included in Appendix B. The data collected can be used with
some careful qualitative assumptions, to group the petroleum products with
respect to their transport, fate, and subsequent potential for human
exposure as a result of a leaking UST incident.
5-7
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5.2.3 GROUPINGS BY BULK PROPERTIES
Petroleum products can be divided into groups based on to their fate and
transport properties. For an overall picture of the range of properties of
petroleum products stored in USTs, Figure 5-2 provides distillation curves
for a variety of products. The data used to develop this figure are from
Sheldon and Dickson (1985) and represent the average distillation
temperatures observed in a nationwide sampling study. As stated earlier,
boiling range is a general indicator of other physical and chemical
properties including viscosity, density, and vapor pressure.
Figure 5-2 illustrates the lower distillation temperatures of motor gaso-
line, the higher distillation range of fuel oils, and the relatively narrow
distillation range of aviation turbine fuel. It also shows the similar
distillation temperatures of No. 2 Fuel Oil and No. 2D Diesel Fuel. This
is indicative of the fact that a number of oil products with different end
uses (and specifications), have very similar distillation ranges.
Petroleum products may be divided into groups of similar potential for
mobility and toxicity in the environment using available data for kinematic
viscosity, vapor pressure, and percent aromatic composition. Because
property data are not available for all products, however, some qualitative
judgment was necessary to generate the groupings. Although other
properties (e.g., solubility and toxicity information) could also be used
to group the products, insufficient data preclude the use of these other
relevant properties.
Kinematic Viscosity
The kinematic viscosity of a petroleum product affects the rate at which a
product will leak from a tank and the velocity with which the product will
move through the vadose zone. Simply stated, the lower a product's
viscosity, the faster it will move through the vadose zone away from the
tank. Table 5-2 summarizes available kinematic viscosity data from
Appendix B. Although the data are from various sources, they can be
assumed to be, for our purposes, comparable. The kinematic viscosity
values range from less than than 1 cST for gasoline to a (maximum) 638 cST
for No. 4GT gas turbine fuel oil.
5-8
-------
400
300
o
o
LU
o:
DC
LJJ
Q.
2
111
•200
100
DIESEL FUEL OIL 2-D
FUEL OIL #2
I
20
MOTOR GASOLINE
WINTER UNLEADED
OCTANE < 90
I
40
\
60
I
80
100
PERCENT EVAPORATED
SOURCE: Adapted From NIPER (1984)
CAMP DRESSER & McKEE INC.
FIGURE 5-2
DISTILLATION CURVES OF COMMON
PETROLEUM PRODUCTS
5-9
-------
TABLE 5-2
KINEMATIC VISCOSITY OF PETROLEUM PRODUCTS
Product Kinematic Viscosity (cST @38°C or 40°C)
Minimum Maximum Typical
AUTOMOTIVE GASOLINE 0.5 0.65<1 NA2
FUEL OIL No. 1 1.4 2.2 1.65
No. 2 2.0 3.6 2.97
No. 4-light 2.0 5.8 NA
No. 4-heavy 5.8 26.4 NA
No. 5-light >26.4 65 NA
No. 5-heavy >65 194 NA
DIESEL FUEL OIL
No. ID 1.3 2.4 1.64
No. 2D 1.9 4.1 1.97
No. 4D 5.5 24.0 NA
GAS TURBINE FUEL OIL
No. 1-GT 1.3 2.4 NA
No. 2-GT 1.9 4.1 NA
No. 3-GT 5.5 638 NA
No. 4-GT 5.5 638 NA
KEROSENE 1.0 1.9
Data is in centistokes (IcST = 1 cm/sec). While kinematic viscosity is
dependent on temperature, no significant kinematic viscosity change is to
be expected between 38°C and 40 C.
2
Not available.
5-10
-------
Although data are not available for all products, viscosity typically in-
creases with boiling point (and molecular weight) for pure hydrocarbons.
Consequently, the boiling range of the products, provide a good indication
of relative kinematic viscosity, and for the purposes of this study, where
some data were unavailable, products with similar boiling ranges were
grouped together. Two products — petroleum extender oils and lubricating
oils -- could not be grouped because kinematic viscosity will vary among
the four groups used below depending on a specific industrial use.
Figure 5-3 identifies four kinematic viscosity groupings of petroleum
products. Products in the lowest kinematic viscosity category, Group I,
include gasolines and solvents. These -products will move at the fastest
rate away from a tank leak. Groups II and III also can be expected to move
by gravity through porous soils (but relatively slowly compared to those in
Group I). The heavy oils in Group IV will most likely remain localized in
the tank area.
Kinematic viscosity is a temperature-dependent property. Fuel products
with high kinematic viscosity need to be heated if fuel is to move freely.
For free flow in the storage tank, the viscosity must not exceed 30 cST (25
poises at a fuel density = 0.8533 (Hobson, 1984)). Group IV products most
likely will be preheated in the tank prior to removal. With respect to
mobility in the subsurface, Group IV (and at lower temperatures Group III
products) will most likely be immobile after a leak.
Vapor Pressure
Vapor pressure describes the potential for product loss through the gas
phase, i.e., its tendency to evaporate. The higher the vapor pressure, the
faster or greater the rate of vapor formation. Specifically, the vapor
pressure scales the peak concentration of a substance over the liquid and,
thus, the diffusion rate at which vapors enter the soil pore space.
5-11
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GROUP
GROUP II
GROUP III
GROUP IV
I
I—1
ho
AUTOMOTIVE GASOLINE
AVIATION GASOLINE
VM&P NAPHTHAS (ALL TYPES)
AROMATIC NAPHTHAS (TYPE I & II)
GAS TURBINE FUEL OIL #0-GT
PETROLEUM SPIRITS (ALL TYPES)
JET FUEL A, A-1, B
KEROSENE
FUEL OIL #1
DIESEL FUEL #1D
GAS TURBINE FUEL OIL #1-GT
FUEL OIL #2, #4
DIESEL FUEL #2D #4D
GAS TURBINE FUEL OIL #2-GT
FUEL OIL #5, #6
GAS TURBINE FUEL OIL 03-GT, #4-GT
LUBRICATING OILS
LOW KINEMATIC VISCOSITY
(FASTEST MOVERS)
HIGH KINEMATIC VISCOSITY
(SLOWEST MOVERS)
HIGH VAPOR PRESSURE
(GREATEST VAPOR RELEASED)
LOW VAPOR PRESSURE
(LEAST VAPOR RELEASED)
CAMP DRESSER & McKEE INC.
FIGURE 5-3
PETROLEUM PRODUCTS GROUPED BY
KINEMATIC VISCOSITY
-------
As mentioned previously, the true vapor pressure is not generally measured
for mixtures such as petroleum products. The petroleum industry uses the
Reid vapor pressure test and other methods. The Reid test is only required
for the most volatile products such as aviation gasoline and motor
gasoline. Loss of product due to vapor release is high for these products
as indicated by their relatively high vapor pressure.
Lower molecular weight hydrocarbons, i.e., those with lower boiling points
and viscosities will have higher vapor pressures. It follows that the
higher-boiling range, more-viscous products will have lower vapor
pressures. Thus, the groups developed for kinematic viscosity, shown in
Figure 5-3, may also be viewed as vapor pressure groups.
Percent Composition
No measure of toxicity has been specifically developed for many of the bulk
petroleum products of interest here. Some efforts have been made toward a
toxicological understanding or quantification of the significance of
automotive gasoline (ICF, 1985), but no similar or related data were found
for other products. In the absence of aquatic or mammalian toxicity data
for the products, an approximation of relative toxicity of most products is
illustrated here based on hydrocarbon percent composition.
Four major types of hydrocarbons are found in petroleum products as discus-
sed previously and illustrated in Figure 5-1. Of these four types, the
aromatic carbon compounds are generally known to be the most toxic to
human beings exposed through ingestion or inhalation.
For the purpose of ranking the relative toxicity of petroleum products, the
percent aromatic composition was chosen as a useful parameter. The
assumption is that the higher aromatic compounds in a petroleum product,
result in relatively greater toxicity of the product. The lack of other
relevant data leaves the percent aromatic content as the only readily
available indicator.
5-13
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For some of the highly mobile (low kinematic viscosity and high vapor
pressure) products shown in Group I and II in Figure 5-3, Table 5-3 lists
the percent aromatic content typically found in the bulk petroleum
products. Thus, the products listed in Table 5-3 are all highly mobile and
may be considered to vary in relative toxicity as indicated by percent
aromatic composition.
5.2.4 CONSTITUENTS OF PETROLEUM PRODUCTS
To understand the environmental behavior of petroleum products leaked from
USTs, a conceptual understanding of the chemical nature of petroleum is
necessary. Petroleum products are mixtures of hydrocarbon compounds with a
broad range of physical, chemical, and toxicological properties. In the
previous section, four groups of hydrocarbons were defined as being the
primary constituents of petroleums: paraffins, olefins, naphthenes, and
aromatics. Figure 5-1 illustrated the chemical structure of these four
groups. It is the difference in properties, both physical and chemical,
among these hydrocarbon types that characterizes individual petroleum
products and their respective environmental behavior. The members of each
group act similarly with respect to environmental behavior, but to
different degrees.
This section addresses petroleum products with respect to their hydrocarbon
constituents. By looking at the products' individual constituents, we can
illustrate the division of products into environmental compartments. Any
petroleum product acts as a mixture or "bulk" product only in the
unsaturated zone of the subsurface (See Section 3.0). When, however, the
product "evaporates" or "dissolves," it is not the bulk product which is
acting, but rather the independent constituents behaving in many ways that
reflect their unique physical and chemical properties.
Development Of The Data Base
A computerized database of the constituents of motor gasoline and other
petroleum products, including their important physical and chemical
properties, was developed for this study. This database is presented in
Appendix C.
5-14
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TABLE 5-3
PERCENT AROMATIC CARBON COMPOUNDS
(Weight Percent)
1
Petroleum Product
Naphthas - High Flash Aromatic
Types I and II
VM and P Naphthas
Types I and II
Motor Gasoline
Aviation Gasoline
Jet Fuel (A, A-l and B)
Diesel Fuel Oil
Percent Aromatic
90-minimum
33 maximum
20-30 typical
10 typical
20 maximum
20-40 typical
1
Data sources are cited in Appendix B.
5-15
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Little information is readily available concerning the specific composition
of most products, with the exception of gasoline and No. 2 fuel oil. Five
sources of information were identified which detailed gasoline and/or No. 2
fuel oil composition. Appendix C lists these constituents and the
references from which the data were assembled.
The following physical and chemical properties are included in the
database:
• Chemical formula
• Molecular weight
• Hydrocarbon class
• Density
• Boiling point
• Melting point
• Solubility in water
• Vapor pressure
The chemical constituents are grouped into the four main hydrocarbon
groups plus six other chemical groups, also known to be present in petrol-
eum products to a lesser extent. These six groups are: aromatic amines,
alcohols, carboxylic acids, conjugated dienes, and phenols and cresols.
The chemicals in these groups may contain nitrogen or oxygen as well as
carbon and hydrogen. Other chemical groups may be present in a specific
petroleum product depending on the source of the crude oil and the refining
process. Additives to petroleum products could include many types of
chemicals (Section 2.2).
Initially, when developing this database, the objective was to compile con-
stituent information not only on gasoline and No. 2 fuel oil, but for other
products as well. Although many constituents listed in Appendix C may be
present in other petroleum products, it must be understood that the heavier
oils will contain higher molecular weight hydrocarbons beyond those
included i'n our database. Many hydrocarbons found in petroleum products
5-16
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are not listed in Appendix C. Also, the analyses used to characterize
samples summarized, but undoubtedly did not identify every constituent
present. Instead the analysis indicated targeted specific "typical
compounds. In general, it may be safe to assume that almost any stable
hydrocarbon within a specific product's boiling range may be present in
that product.
Uses of The Hydrocarbon Database
The database may be used to illustrate the nature of petroleum products
related to environmental behavior as discussed in the following paragraphs.
The solubility in water of a chemical is related to the potential for
groundwater related impacts of UST leaks. Some hydrocarbon groups are
found to be more soluble than others. The solubility data available in the
hydrocarbon database is plotted as a cumulative distribution function in
Figure 5-4. The highly soluble groups include the phenolics, alcohols, and
carboxylic acids followed by the aromatics. Very insoluble compounds
include, typically, the paraffins and the olefins. Most gasoline
constituents have a solubility less than
100 mg/1 in water as evidenced by the shape of the curve in Figure 5-4.
Although the alcohols and phenolic compounds are highly soluble, they are
not among the most prevalent chemical groups in petroleum products.
Figure 5-5 summarizes percent hydrocarbon composition of an API reference
sample of unleaded motor gasoline (Reference Fuel PS-6) used in an
inhalation study with laboratory animals sponsored by the American
Petroleum Institute (Domask, 1983). A total of 151 compounds were
identified in the PS-6 fuel sample. Only 42 compounds, however, made up
about 75 percent of the sample volume. These 42 compounds are the major
constituents shown in Figure 5-5. Table 5-4 lists these 42 main compounds
by chemical group and carbon number, with their respective percent
composition. The table also lists the solubility of the various
constituents.
5-17
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FIGURE 5-4
SOLUBILITY DATA DISTRIBUTION OF HYDROCARBON DATABASE
-------
100
lit
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100
API/EPA REFERENCE FUEL PS-6
90 -
80
70
60
50
40
30
20
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LEGEND 0
F\| TOTAL %
I771 WAIN CONSTITUENTS %
74.2
TOTAL
57.95
26.08
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8.96
3.29
PARAFINS AROMATICS OLEFINS
HYDROCARBON TYPE
4.68
1.89
NAPHTHENES
CAMP DRESSER & McKEE INC.
FIGURE 5-5
PERCENT COMPOSITION OF GASOLINE
-------
TABLE 5-4
Percent Composition and Solubility
of the Main Constituents of Gasoline
Hydrocarbon
Type
n-paraffins
C3-C10
isoparaffins
C4-C13
% Volume
in Fuel
11.40
46.55
Cycloparaffins
(naphthenes)
C5-C13
mono-olefins
C2-C12
Major Constituents
C4 n-butane
C5 n-pentane
C5 n-hexane
% Accounted
for by Major
Constituents
10.19
C4 isobutane
C5 isopentane
C6 2-methylpentane
3-methylpentane
2,3-dimethyl butane
C7 2-methylhexane
3-methylhexane
2,3-dimethylpentane
2,4-dimethylpentane
C8 2,2,4-trimethylpentane
2,3,4-trimethylpentane
2,3,3-trimethyl pentane
2,2,3-trimethylpentane
2,2,5-trimethylpentane
C9 2-methyloctane
3-methyloctane
4-methyloctane
4.68 C5 cyclopentane
C6 methylcyclopentane
C7 methylcyclohexane
1,cis,3-dimethyl-
cyclopentane
1,trans,3-dimethyl-
cyclopentane
8.96 C3 propylene
C4 trans butene-2
cis butene-2
C5 pentene-1
trans pentene-2
cis pentene-2
C6 2-methylpentene-1
2-methylpentene-2
10.19
1.14
10.26
8.81
4.54
11.75
1.51
3OT
0.15
0.97
0.77
T78?
0.03
0.75
1.22
1.26
Solubility
(mg/1)
6-1.4
38.5
9.5
48.9
13.8
12.8
22.5
1
54
64
25
4.06
,14
,36
,59
2.59
1.42
1.42
0.0115
160.
14.
7.07
7.07
430
430
148
203
203
78
5-20
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Hydrocarbon
Type
Aromatics
Alkybenzenes
C6-C12
% Volume
in Fuel
26.08
Indans/
tetralins
C9-C13
Napthalenes
C10-C12
1.54
0.74
TABLE 5-4
(Continued)
Major Constituents
% Accounted
for by Major
Constituents
C6 benzene
C7 toluene
C8 ethylbenzene
0-xylene
M-xylene
P-xylene
C9 1-methyl, 3-ethylbenzene
1-methyl, 4-ethylbenzene
1,2,4-trimethyl benzene
1,
3,
9,
69
99
83
5.33
20.84
Solubility
(mg/L)
1780.
515.
152.
175.
162.
198.
40.
40.
57.
1
TOTAL
100
74.2
1
All data reported at
20°C.
5-21
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As evidenced by Table 5-4, those chemicals of relatively high solubility
found in a large amount in gasoline are the aromatic compounds in the Cg to
Cg carbon range. These include benzene, toluene, and xylene.
The aromatic compounds are known to be the most toxic of the gasoline
components and also appear to be somewhat soluble. The main constituents
of gasoline are further discussed in Section 5.4 with respect to their
hazardous nature, solubility, ignitability, and vapor pressure in
comparison to regulated hazardous substances.
5.3 HAZARDOUS SUBSTANCES IN USTs
5.3.1 OVERVIEW
Not all hazardous substances regulated by the UST program are likely to be
stored in USTs as pure products. (A list of those substances known or
likely to be stored in USTs was developed for this study as described in
Section 2.0). A computerized chemical and physical property database for
these substances was created to investigate the diversity of UST substances
and to assess their potential for causing adverse impacts when leaked in
the subsurface. Also, in Section 5.4 the database is used to compare the
constituents of petroleum products with hazardous substances.
5.3.2 DEVELOPMENT OF THE DATABASE
Physical and chemical property data were collected for the 477 hazardous
substances identified as being known, or likely, to be stored in USTs. The
database can be found in its entirety in Appendix A; it was developed so
that the following kinds of work could be undertaken with the aid of
computer-based information:
• Grouping regulated substances based on their anticipated vapor and/
or liquid fate and transport properties, as well as on their poten-
tial to impact on human health.
• Assessing technical similarities and differences between petroleum
and hazardous substances.
5-22
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Properties of Interest
In order to simplify data collection, an initial screening was made of
relevant physical and chemical properties. A qualitative evaluation of the
relevance of different properties plus an assessment of data availability
with respect to these properties was performed. The objective was to
collect data on those properties which can be used to group as many
chemicals as possible according to their relative potential for causing
environmental impacts.
Those properties which identify or physically describe a chemical include,
Chemical Abstract Service Registry Number (CASRN), chemical formula,
molecular weight, density, boiling point and melting point. To determine
physical state (solid, liquid, or gas) the boiling point and melting point
must be known. Chemical formula and molecular weight are important to
identify chemical groups and to make comparisons to similar chemicals.
These important properties are usually available and are included in the
database.
Many physical and chemical properties have been identified in the
literature as important in environmental behavior (Lyman et.al., 1981).
Properties related to subsurface fate and transport mechanisms include, but
are not limited to those listed in Table 5-5.
Of the properties listed in Table 5-5, the vapor pressure and water sol-
ubility may be the most important and widely studied parameters with res-
pect to leaking UST impacts. Vapor pressure is a property which our
research showed to be available for a large number of substances. It is a
useful parameter to scale the production and transport rate of hazardous
vapors following a chemical leak. Likewise, water solubility data can
indicate the potential for a chemical to dissolve and migrate in
groundwater or percolating rainwater.
5-23
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TABLE 5-5
PROPERTIES RELATED TO ENVIRONMENTAL BEHAVIOR'
Solubility in water and other solvents
Vapor Pressure
Octanol/Water Partitioning Coefficient
Adsorption Coefficient for Soils and Sediments
Rate of Biodegradation
Dynamic and Kinematic Viscosity
Rate of Hydrolysis
Volatilization from Water (Henry's Law Coefficient)
Volatilization from Soil
Diffusion Coefficients in Air and Water
Heat of Vaporization
Vapor Density
Flash Points
Explosive Limits - Lower and Upper
*Not listed in order of importance.
5-24
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Many other properties on TabT'e 5-5 are considered very important to fate
and/or transport mechanisms, but data on these properties are limited
compared to the data available on vapor pressure and solubility. With
respect to vapor phase impacts, for example, diffusion coefficients,
volatilization from water and soil, vapor density, flash point, lower
explosive limit, and heat of vaporization are all considered relatively
important. Diffusivity and vapor pressure are the primary factors
controlling vapor transport, but diffusion coefficients are available only
for the more common chemicals. Volatilization rates are measured by
several different methods; volatilization rates with respect to substances
in solution with water may be estimated using Henry's Law coefficients.
The "lower explosive limit" of a substance indicates the gas phase
concentration where an explosion may occur. Flash point, another common
measure of flammability, is a measure related to the temperature at which
vapors will ignite.
Drinking water impacts occur primarily when dissolved chemicals are pumped
out of wells and/or when discharges to surface water occur. Many of the
properties of Table 5*-5 are related to primary fate mechanisms which
inhibit transport of a chemical: rate of biodegradation, adsorption
coefficients for soils and sediments, and rate of hydrolysis. Due to the
lack of good data on the mechanisms, however, only the octanol/water
partitioning coefficient was included in the database. Data are available
for the other parameters mentioned above to a lesser extent than for the
primary properties discussed above. The octanol/water coefficient was
selected because of the general availability of data for priority
pollutants and other chemicals and the high reliability/reproducibility of
data. The other fate mechanisms above (except hydrolysis) are tested in a
variety of ways, and data are not always comparable under different test
conditions.
Another important set of relevant properties comprises those which
determine the toxicity of a chemical. Various measures of toxicity have
been established in the fields of industrial hygien.e and safety, cancer
research, and wildlife and fisheries conservation. The toxicity scales
established are not necessarily equivalent to each other, although each
5-25
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scale may measure potential danger to human health and/or the environment,
The hazardous substances regulated under the LIST program are those in the
CERCLA list. Much effort has been made by EPA in assessing the relative
toxicity of these chemicals for Reportable Quantity (RQ) rulemaking
pursuant to CERCLA Section 102.
The Reportable Quantity Rulemaking Technical Background Document
(Environmental Monitoring and Services, Inc., 1985), serves as a
comprehensive database and specifies currently available toxicological and
other data pertaining to hazardous substances. The RQ methodology assigns
the letters X, A, B, C, and D to recognize a reportable quantity of 1, 10,
100, 1000 and 5000 pounds, respectively. A 1-pound (X) chemical presents
the highest level of hazard. The RQ is the minimum amount released to the
environment that must be reported to the Agency. The RQ assignment is
based not only on toxicity, but also on ignitabi lity, reactivity with
water, self-reactivity, biodegradation, hydrolysis and photolysis. Each of
these categories was also assigned an X, A, B, C, or D level as will be
discussed further in Section 5.3.3.
In summary, the following properties are included in the database given in
Appendix A:
Chemical formula
Boiling point
Density
Vapor pressure
Percent lower explosive limit
Molecular weight
Melting point
Solubility in water
Vapor density
Octanol/water parti-
tioning coefficient
• RQ (for toxicity) • RQ (for ignitability)
Data Management System
The data collected were organized using the database management program
"SYMPHONY" marketed by Lotus Development, Inc. While fundamentally being a
spreadsheet application program, data entry and editing is done on a "form"
which transmits information .directly into the spreadsheet on a
one-chemical-at-a-time basis. The SYMPHONY program has applications which
allow graphing and sorting of the data.
5-26
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Data Sources
Six primary sources were used to collect chemical and physical property
data. They are listed in Appendix A and were used in the following manner.
The data collection effort began with the CRC and Merck Index as primary
sources. Basic information such as formula, molecular weight, specific
gravity, melting point and boiling point were sought for each chemical.
Also, solubility in water and other solvents was noted. Not all chemicals
were found in the CRC or Merck Index. Environmental properties such as
octanol/water partitioning coefficient were not available in these primary
sources. The first phase of data collection ended by a search of the
Handbook of Environmental Data on Organic Chemicals to identify basic
property data for chemicals not found in the primary sources.
The second phase of data collection focused on environmental and vapor
phase property data. It began by utilizing the Hazardline Computer access
database developed by the Occupational Health Services (a private or-
ganization located in Seacacus, New Jersey). About 300 chemicals for which
limited information had been collected in the first phase were queried
•through the Hazardline system. The Hazardline database not only
supplemented data collected in the initial phase, but was used for data
verification (discussed below). The NFPA guide to Hazardous Materials was
used to collect data on the lower explosive limit of flammable chemicals.
The Handbook of Environmental Data for Organic Chemicals was utilized to
supplement environmental property data and vapor phase property data
collected from Hazardline. The water related environmental fate of 129
Priority Pollutants, data available through EPA, was used to verify
information regarding these relatively well-studied chemicals.
5-27
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Data Verification
Standard procedures for data verification were to check data input to the
computer on the previous day, against the computer-stored data and hand-
written data worksheets. All identified errors were corrected and any data
for which questions arose were marked and revised as necessary.
Database Limitations
Readily accessible and relevant data sources have been referenced in this
effort. Of the 477 substances investigated, limited or no data were found
for about 7 percent. Most of these chemicals are either phenoxy herbicides
or complicated nitroso compounds and sulfonates. These chemicals are not
included in the following discussions of the database. Also, a few of the
477 chemicals in the database are cited more than once such as sulfuric
acid which appears under two CASK numbers. In addition, several chemicals
addressed are simple mixtures (e.g., dichloropropane and dichloropropene
mixture) or general chemicals (e.g., cresol(s)) for which the components of
the mixture are known and are themselves included among the 477 (e.g.,
ortho-, meta-, and para-cresol). In all data presentations in the text the
"population" of substances being addressed is defined.
5.3.3 USES OF THE DATABASE
The universe of hazardous substances stored in underground tanks includes a
diverse group of chemicals with a broad range of physical and chemical
properties. The development of the database, described in the previous
section, allows the extraction of information pertinent to the UST program.
Information is presented addressing three major topic areas:
• the nature and diversity of UST substances,
• grouping of hazardous substances in relation to potential environ-
mental impact,
• a comparison of hazardous substances with petroleum products
(Section 5.4).
5-28
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Diversity of Hazardous Substances
To appreciate the diversity among the various substances regulated under
the UST program, the database was utilized to divide the total group of
substances into subgroups based on various physical and chemical features.
Of interest specifically were the following: the phase normally found
(i.e., solid, liquid), the broad chemical categories in which the various
substances fall, the solubility of the chemicals in various solvents, and
the liquid and vapor density of chemicals.
• Physical states — As indicated on Figure 5-6, the hazardous
substances are found at normal conditions to be 5% in the vapor
phase, 42% in the liquid phase, and 53% as solids. Data collected
for melting point and boiling point were used to assign the
reference state. In the absence of data, the phase was assigned
based on information in the cited references (Section 5.3.1).
Normal conditions in this study have been assumed to be 15.6 °C
(60 F) and one atmosphere pressure (760 torr). Most other data
included in the database is referenced at standard temperatures of
20 or 25 C. But because the UST regulation defines liquid petroleum
products as inclusive if liquids at 15.6°C, rather than the more
conventional metric use of 20 or 25°C, this lower temperature
standard was selected.
The fact that the majority (53%) of the substances in the database
are solids shows the importance of industrial solvents used in
storing hazardous substances. Many solvents are also hazardous and
fall in the (42%) liquid category. As for the solids, gases may be
stored either dissolved in a solvent.or stored under pressure in
liquid form.
• General chemical composition -- As shown in Figure 5-7, the database
indicates that roughly 20% of the substances of interest are
inorganic compounds; the remaining 80% are organic. Most hazardous
man-made chemicals, e.g., pesticides and herbicides, are organic.
The hazardous characteristic of the majority of the inorganic
compounds arise from the cation (metallic) species.
The organic and inorganic chemicals can be divided into basic types
of chemicals which are associated by chemical structure. These
classes of chemicals behave differently and have different degrees
of environmental impacts. Although generalizations can be made
about classes of chemicals, variation of properties within a group
can be quite large. Table 5-6 lists some class names commonly
associated with chemicals found to be hazardous substances.
5-29
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GAS
5%
PHYSICAL STATE
AT 15.6°C (60°F) AND 1 ATM.
CAMP DRESSER & McKEE INC.
FIGURE 5-6
PHYSICAL STATE DISTRIBUTION OF UST
HAZARDOUS SUBSTANCES
5-30
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CAMP DRESSER & McKEE INC.
FIGURE 5-7
PERCENT INORGANIC AND ORGANIC
UST HAZARDOUS SUBSTANCES
5-31
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TABLE 5-6
CHEMICAL CLASSES OF uST HAZARDOUS SUBSTANCES
Alcohols and Glycols
Aldehydes
Aliphatics
Alkyl Amines
Alkyl Hal ides
Alkylene Oxides
Amides, Anilides, Imides
Aromatics
Aryl Amines
Azo Compounds
Chromates
Cresols
Cyanates
Cyanides and Nitriles
Epoxides
Esters
Ethers
Halogenated Aliphatics
Halogenated Aromatics
Halogens
Heavy Metals
Hydrazines and Hydrazides
Inorganic Hal ides
Ketones
Nitrates, Nitrites
Nitro Compounds
Olefins •
Organic Acids
Organic Ammonium Compounds
Organics
Organo Metal lies
Organophosphates
Oxides
Phenols and Creosols
Phosphates and Phosphonates
Phosphorous and Compounds
Sulfates
Sulfides
Sulfites
Sulfones, Sulfoxides, Sulfonates
Ureas
5-32
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.; Many chemicals in the database fall into more than one chemical
class such as diethylarsine, an organic chemical associated with1 .a
toxic element. Also, some chemicals in the database are very
complicated, and do not fit easily into simple chemical classes.
Because of the complications which arise in assigning specific
chemical classes to each chemical, the database was not used to
generate data of this nature.
• Solubility of solids — About 53% of the hazardous substances are
found as solids as shown on Figure 5-6. An important aspect of the
fate and transport of these chemicals is the type of solvent in
which they are stored. Most often, these solids will be transported
through the environment with the solvent until precipitation of the
solid, vaporization of the solvent, or dissolution into water. To
address this issue, the solubility of chemicals in four solvents was
noted. The bar chart shown on Figure 5-8 illustrates the number of
chemicals "highly" soluble in water, ether, ethyl alcohol and
benzene. As expected, some substances are soluble in more than one
solvent -- especially alcohol and water. Also, solubility data were
not available for many chemicals in solvents other than water.
Highly soluble was defined as solubility greater than 10,000 mg/1 or
1 percent. Solubilities reported as "very soluble", "freely
soluble," and "miscible" were included in this bar chart.
Hundreds of industrial solvents are used in the United States. Most
of them are organic chemicals similar to benzene or ether and will
readily dissolve solid organic chemicals. Inorganics tend to be
solubilized most easily by water. Solvents may serve as an
indicator of the presence of other hazardous substances which
originally had been associated with the solvent -- in the soil
environment.
• Density data — The distribution based on this parameter is
presented in Figure 5-9. Density data were found only for 318
hazardous substances. The results show 29% of liquid chemicals and
46% of solids are heavier than water which has a density of 1.0.
Groundwater contamination occurs not only by dissolved substances
but also by those which are insoluble or slightly soluble and found
floating on the water surface or sinking to the aquifer bottom (see
Section 3.0). These chemicals will not travel with the groundwater.
Some insoluble chemicals that are less viscous and dense than water
may move along the groundwater surface at a rate greater than water.
• Vapor density -- Figure 5-10 shows the distribution of available
vapor density data (119 of 477) for substances in the database,
which is related to gravitational effects of vapor movement. Many
chemicals produce vapors heavier than air (density = 1.0) and will
tend to sink in the subsurface or remain near to the groundwater
surface.
5-33
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00
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CAMP DRESSER & McKEE INC.
FIGURE 5-8
SOLUBILITY IN COMMON SOLVENTS OF SOLID CHEMICALS
-------
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70
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on nn a
0.00 0.40 0.80 1.20 1.60 2.00
DENSITY (grams/cc)
2.40
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CAMP DRESSER & McKEE INC.
FIGURE 5-9
DENSITY DATA DISTRIBUTION OF LIQUID CHEMICALS
-------
110 -
100 -
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VAPOR DENSITY DATA DISTRIBUTION OF LIQUID CHEMICALS
-------
The above illustrations of data show that chemicals of many types are found
in USTs, many of which will cause potential adverse environmental impacts
when leaked in certain environments. All of the chemicals are hazardous
whether due to their toxicity or highly reactive nature. The following
section presents groups of chemicals which may have similar impacts on
humans and the environment.
5.3.4 GROUPS OF HAZARDOUS SUBSTANCES
While the information provided above indicates the range and diversity of
substances regulated under the UST program, it does not directly relate to
potential leaking UST impacts on human health and the environment. To
provide useful and directly relevant information, presented below are
database analyses focused on potential impacts.
To group the substances, for illustration, a "cause and effect" approach
has been used. The methodology relies on two-parameter "sortings" of the
database to assess potential impact as shown on Figure 5-11. There are two
contamination pathways of concern: groundwater and vapor migration leading
to human exposure. Figure 5-11 cites the basic concepts utilized in
deciding what chemical, physical, and/or toxicological parameters might be
combined to yield useful information.
• Solubility and toxicity (Sort One) -- As discussed in Sections 3.0
and 4.0, the solubility of a contaminant reflects its likelihood to
become dissolved in the saturated zone of the subsurface or to be
solubilized by percolating rainwater. Toxicity measures potential
impact on human health. Thus the combination of these parameters
yields information on which of the regulated substances may have
relatively greater or lesser potential to affect human health
through ingestion or skin exposure to contaminated groundwater.
• Vapor pressure and toxicity (Sort 2) -- Vapor pressure is a measure
of the extent to which a substance will release mass as a vapor.
When combined with the toxicity parameter, vapor pressure and
toxicity together represent a measure of the extent that significant
vapor phase exposure might cause human health impacts.
• Ignitability and vapor pressure (Sort 3) -- Ignitability as used
here, represents a measure of how likely a sustance is to cause fire
or explosion. Combining ignitability with vapor pressure yields
insight regarding whether particular substances might be considered
to pose high risk of fire or explosion.
5-37
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IMPACT SCENARIO
GROUNDWATER CONTAMINATION
AND SUBSEQUENT HUMAN
EXPOSURE
HUMAN EXPOSURE TO
TOXIC VAPORS
EXPLOSION OR
FIRE
PARAMETERS SELECTED & REASON
• Solubility in water—related to pollutant
concentration in the saturated zone.
• Toxicity —related to adverse health
impacts.
• Vapor Pressure—related to vapor
phase concentration.
• Toxicity—related to adverse health
impacts.
Ignitability—related to ignition of
concentrated vapors.
Vapor Pressure—related to vapor
phase concentration.
CAMP DRESSER & McKEE INC.
FIGURE 5-11
GROUPING METHODOLOGY
-------
In order to perform the three, dual-parameter sortings specified above, it
was first necessary to decide how to assign meaningful ranges of the
parameters which denote high (H), medium (M), or low (L) values of
solubility, toxicity, vapor pressure, and flammability.
Hazardous substances exist as solids, liquids, or gases at normal
conditions. Solids obviously are not stored in tanks, and will be first
dissolved in an appropriate solvent. Thus, phase is important in assessing
the relative impact of a chemical. Solids were therefore not included in
the analysis concerning vapor impacts. The 19 gases in the database are
also not included in the dual-parameter sortings, but they are evaluated
with respect to the same parameters in this section. Two chemicals which
are found as gases are known to be always marketed and stored in solution:
formaldehyde and hydrochloric acid. These two chemicals have been assigned
to be liquid solutions in this analysis.
Some chemicals included in the database are highly reactive with water,
often immediately producing toxic gases. Other substances will chemically
transform (oxidation, hydrolysis) immediately upon exposure to ambient
conditions. Of these chemicals, those with the potential for catastrophic
effects were also removed from the analysis and evaluated individually.
The following addresses the four parameters used to perform the sortings.
Toxicity
As discussed in Section 5.3.1, data used for toxicity are from the CERCLA
Reportable Quantity (RQ) Background Document (Environmental Monitoring &
Services, Inc., 1985). In that study each chemical was evaluated based on
available data for aquatic, mammalian, carcinogenic and chronic toxicity.
Final rulemaking established RQs for 340 of the 698 CERCLA substances;
those for which it was felt sufficient data/information existed (USEPA,
1985a). After further efforts, RQs for 105 more chemicals were established
and reported as a Notice of Proposed Rulemaking (NPRM). The remaining 253
chemicals are being further assessed for carcinogenic and/or chronic
5-39
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toxicity. These 253 chemicals are "assigned RQs" equal to either the RQ
assigned for the Clean Water Act (CWA 311) Reportable Quantity Rules, or an
RQ equal to 1 pound (X) - the lowest RQ.
Figure 5-12 summarizes the RQ-toxicity assignments made for the chemicals
in the database. The letter code X, A, B, C and D are used for 1, 10, 100,
1000, and 5000 pound RQ's, respectively. So called "statutory chemicals"
are those for which explicit toxicity data are insufficient and which are,
therefore, subject to change. These are identified by the upper portions
of the bars on Figure 5-12. Many of the statutory chemicals are human
carcinogens and will most likely remain in the X, A, and B categories (and
are herein assumed to be relatively high in toxicity).
The strength of using the RQ toxicity information lies in the fact that all
477 chemicals of issue in this study can be assigned a relative toxicity
value. Thus, for illustrative purposes and completeness, the RQ data are
used to group toxicity information.
Solubility
As shown on Figure 5-13, numerical data available in the database for water
solubility were plotted on a logarithmic scale as a cumulative distribution
function. Figure 5-13 indicates where the high, medium and low
classifications were set. Of the 477 substances in the database,
solubility data existed for 374 substances. For about 40% of the
chemicals, only written descriptions of solubility were available, i.e.,
"soluble," "slightly soluble," etc. To assign high, medium and low
solubility for written descriptions, a review of suggested definitions for
solubility was made as shown in Table 5-7. Note the 1000 - 10,000 mg/1
category may be described as "highly soluble" or "slightly soluble."
With all of this in mind, relative high, medium, and low solubility groups
were assigned as shown on Figure 5-13. Figure 5-14 shows the number of
substances in each group; the 374 substances shown represent about 80% of
the total population of 477.
5-40
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FIGURE 5-13
SOLUBILITY DATA DISTRIBUTION OF HAZARDOUS SUBSTANCE DATABASE
-------
TABLE 5-7
DEFINITIONS
SOLUBILITY IN WATER
(mg/1)
Solubility Class
Verschuessen (1977)
Bennett (1974)
Extremely Soluble
Freely Soluble
> 10,000
> 10,000
Highly Soluble
Slightly Soluble
1,000-10,000
1,000-10,000
Moderately Soluble 200-1,000
Very Slighly Soluble
100-1,000
Slighly Soluble 20-200
Practically Insoluble < 20
< 100
5-43
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CAMP DRESSER & McKEE INC.
FIGURE 5-14
HIGH, MEDIUM AND LOW SOLUBILITY GROUP DISTRIBUTION
-------
Those substances which decompose in water (11 total) are not included in
this analysis. Also, numeric data not reported between 15°C and 25°C were
individually placed in the appropriate high, medium, or low group.
Vapor Pressure
A cummulative distribution function for vapor pressure data (for liquid
substances) was developed and is shown in Figure 5-15. High, medium, and
low categories as indicated on this figure are:
High (H) range > 100 torr
Medium (M) range 10-100 torr
Low (L) range > 10 torr
Note: 760 torr = 1 atmosphere
Vapor pressure were available for 117 of the 188 liquid chemicals in the
database. However, based on chemical structure, 34 additional chemicals
were assigned, "H", "M" or "L" values.
Figure 5-16 indicates the number of substances falling in the H, M and L
groupings. The largest fraction has low vapor pressure and therefore can
be considered not to pose significant vapor problems.
Ignitability
In assigning RQs to the CERCLA substances, EPA reviewed ignitability as a
special characteristic. Relying primarily on these data, the substances
are grouped in terms of high, medium, and low potential for
ignition/explosion. The X category of RQ assignment was not used by EPA
with respect to ignitability. The A, B, C and D assignments as defined by
EPA are shown on Table 5-8 and the results for 111 chemicals presented on
Figure 5-17. Data for 77 liquid chemicals were not available. As stated
previously, solids were not assessed for explosion/fire impact and gases
were evaluated separately.
5-45
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VAPOR PRESSURE DATA DISTRIBUTION OF HAZARDOUS SUBSTANCE DATABASE
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FIGURE 5-16
HIGH, MEDIUM AND LOW VAPOR PRESSURE GROUP DISTRIBUTION
-------
TABLE 5-8
IGNITABILITY SCALES FOR REPORTABLE QUANTITY ADJUSTMENTS
Category
D
C
B
A
X
Ingitability
(Fire)'
FP(cc) 100-140°F
(37.8-60°C)
FP < 100°F (37.8°C)
BP _> 100°F (37.8°C)
FP(cc) < 100°F (37.8°C)
BP < 100°F (37.8°C)
Pyrophoric or
self-ingitable
(Not used)
RQ (Pounds)
5000
1000
100
10
1
Notes: FP(cc) = Flash point, closed cup
BP = Boiling Point
Source: Environmental Monitoring and Services Inc. (1985)
5-48
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CAMP DRESSER & McKEE INC.
FIGURE 5-17
HIGH, MEDIUM AND LOW IGNITABILITY GROUP DISTRIBUTION
-------
With respect to using the RQ data in this report, the following points
should be kept in mind: class A chemicals are pyrophoric and if large
quantities of these materials are released into the environment,
catastrophic results are to be expected. Six class A chemicals have been
removed from the dual parameter sorts as discussed below. The ignitability
of a chemical can be related, in general terms, to the flash point and
boiling point of a substance. The RQ measure of ignitability is associated
(in the dual-parameter sorts) with vapor pressure, though these are not
totally independent variables.
Reactivity
Some substances in the database are very highly reactive with water.
Others are unstable at normal conditions and may polymerize or
spontaneously burn. The RQ assessment work (Environmental Monitoring &
Services Inc., 1985) reviewed water-reaction and self-reaction (stability)
as a separate chemical feature. Highly reactive chemicals, with potential
catastrophic impact, have been removed from the dual parameter sorts and
are listed in Table 5-9. More chemicals than those listed in Table 5-9 may
be highly reactive. Further investigation is necessary in this regard,
particularly with respect to toxic gas production at a water interface.
Self-reactive compounds, which may polymerize without stabilizers, are
listed on Table 5-10. These are a .special group of chemicals, which may
easily transform and solidify due to polymerization when leaked from a
tank.
Results of Gas Phase Analysis
Hazardous substances found as gases under standard conditions are treated
as special cases herein. Table 5-11 summarizes relevant data for the 19
gases in the database. Some of the gases are highly soluble in water, such
as ammonia and trimethylamine. Most of the gases are in the highly
flammable "B" group for ignitability.
5-50
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TABLE 5-9
PYROPHORIC, EXPLOSIVE AND HIGH WATER REACTIVE
HAZARDOUS SUBSTANCES
CHEMICAL NAME
Aluminum phosphide
Ammonium bi fluoride
Ammonium sulfide
Calcium carbide
Calcium hypochlorite
Chromic acid
Diamine
Di ethyl arsine
Dimethyl hydrazine
Furandione, 2-
Hydrocyanic acid
Methyl hydrazine
Methyl isocyanate
Nitroglycerine
Phosphorus pentasulfide
Potassium cyanide
Sodium
Sodium cyanide
Zinc cyanide
Zinc phosphide
REACTIVITY
CLASS
B
B
B
A
A
A
A
A
A
A
A
A
-
A
B
A
A
A
A
B
REASON FOR
EXCLUSION FROM ANALYSIS
PH3: Water Reaction
HF: Water Reaction
H2S: Water Reaction
Inflames (solid): Water
Reaction
Strong oxidizer (solid):
Causes fires
Strong oxidizer (solid)
Spontaneously ignites
Pyrophoric
Pyrophoric
Reacts with water
CN": Water Reaction
Spontaneous ignition
Water Reactive
Explosive
H2$: Water Reaction
CN": Water Reaction
Inflames: Water Reaction
CN~: Water Reaction
CN": Water Reaction
PH: Water Reaction
Class:
A = Inflames, pyrophoric
B = Extreme reaction
Source: Environmental Monitoring & Services Inc. (1985)
5-51
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TABLE 5-10
CHEMICALS REQUIRING STABILIZATION
Chemical Reactivity Class
Aziridine B
Benzoylchloride B
Ethanal B
Propylene oxide B
Styrene C_
B - High self reaction; may polymerize; requires stabilizer
C - moderate; contamination may cause polymerization; no inhibitor
required.
Source: Environmental Monitoring & Services Inc. (1985)
5-52
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TABLE 5-11
GAS PHASE TOXICITY, IGNITABILITY, SOLUBILITY DATA
REPORTABLE QUANTITY INFORMATION
RQ-TOX
CHEMICAL NAME (XABCD)
Ammonia
Carbonyl chloride
Chlorine
Chlorine cyanide
Chloroethane
Cyanogen
Dichlorodi fluoromethane
Dimethyl ami ne
Ethyl ene oxide
Flourine
Hydrogen phosphide
Hydrogen sulfide
Methanethiol
Methyl bromide
Methylchloride '
Monomethylamine
Nitric oxide
Trimethylamine
Vinylchloride
A
A
A
A
D
C
D
C
X
C
B
B
B
C
C
C
C
C
X
RQ-IGN
(ABCD)
B
B
B
B
B
B
B
B
B
B
A
B
B
RQ-STATUS
(FPS)1
P
F
F
F
P
F
F
P
S
F
F
P
F
F
P
F
F
P
S
RELATIVE SOLUBILITY
GROUPING
High-
Medium
Medium
High
Medium
Medium
Low
High
High
Low
Medium
High
Medium
Medium
High
High
Low
P = Proposed
S = Statutory
5-53
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Toxic and explosive gases are a special case of hazardous substances. All
should be considered to have a high potential for causing vapor incidents.
Results of Dual Parameter Sorts
Having established the relative solubility, vapor pressure, ignitability
and toxicity categories for as many of the substances in the database as
possible, the procedure in performing the dual-parameter sorts was
straightforward. For example, with respect to solubility and toxicity, a
chemical assigned both high solubility and high toxicity combined to form a
substance with a high potential for groundwater contamination. The sorting
procedure indicates which regulated substances have this HIGH-HIGH
characteristic; these clearly have greater potential for adverse impacts
than those with LOW-LOW characteristics. In between, there are many
substances which exhibit either HIGH/LOW or LOW/HIGH or MEDIUM values of
the dual parameters. Similar concepts apply to each of the dual parameter
sorts performed here.
Figure 5-18 through 5-24 present the results of the three dual parameter
sorting procedures. (Note that ordinate scales vary from figure to
figure). For all results presented below, complete lists of chemicals
falling in each group can be found in Appendix D.
t Figures 5-18, 5-19, and 5-20 correspond to Sort One (toxicity and
solubility) and show the distribution of toxicity (RQ assignments)
for highly soluble, medium soluble, and low soluble substances,
respectively. Note that the figures differentiate between
substances for which adequate toxicity data exists (those with
final/proposed RQs) and those assigned RQs based on statutory
directives (insufficient data).
• Figures 5-21, 5-22, and 5-23 correspond to Sort Two (toxicity and
vapor pressure) and again show the toxicity RQ assignments for
substances with high, medium, and low vapor pressures, respectively.
Distinctions between whether RQs are final/proposed final or
statutory are shown in these figures.
• Figure 5-24 corresponds to Sort Three (ignitability and vapor
pressure) and shows the low, medium, and high vapor pressure
substances in their appropriate ignitability class. Ignitability
RQs are all based on actual data so no differentiation between
assignments is shown here as on earlier figures.
5-54
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-------
Some salient observations on the nature and degree of impacts associated
with UST-regulated hazardous substances follow from these results.
Appendix D provides lists — by name -- of which substances fall into which
categories. More generally, however, the results suggest that it is
possible to group a high number of substances based on various attributes
related to fate and transport as well as to impact.
For example, the highly soluble substances in toxic groups X, A, and B
(Figure 5-18) clearly pose a greater threat of groundwater contamination
(and, potentially to human health) than do the low solubility substances in
toxicity groups C or D (Figure 5-20). Similarly, the substances with high
vapor pressure and in toxic groups X, A, or B (Figure 5-21) pose greater
threats of vapor-related health impacts than the low vapor pressure
substances in toxic groups C and D (Figure 5-23).
The degree of fire/explosion potential is self-evident from Figure 5-24;
the high vapor pressure substances in the B ignitability class are
generally more pisky than the low vapor pressure substances in the C
ignitability class. The results reflect the fact that vapor pressure and
ignitability are not completely independent variables. Future research on
vapor phase migration in the subsurface may support a different approach to
grouping the substances by fire/explosion potential.
5.4 COMPARISON OF HAZARDOUS SUBSTANCES AND PETROLEUM PRODUCTS
RCRA subtitle I explicity allows EPA to establish different regulatory
requirements for petroleum products and hazardous substances. Recognizing
this, the following discussion presents a comparison of petroleum product
constituents with those substances in the hazardous substances database.
The discussion is limited by what is explicity known about the specific
chemical composition of petroleum products.
In simplified form, Figure 5-25 illustrates the general make-up of
petroleum products. While all petroleum products do not necessarily have
every component shown on Figure 5-25, the majority of widely used products
5-62
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PETROLEUM PRODUCTS
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FIGURE 5-25
GENERAL MAKE-UP OF PETROLEUM PRODUCTS
-------
do. The hazardous nature of petroleum products is due primarily to the
aromatic fraction of pure hydrocarbons, and secondarily, to additives and
trace contaminants.
5.4.1 GASOLINE
Gasoline is the most ubiquitous petroleum product stored in underground
tanks. Also, of all petroleum products, gasoline is the most widely
studied. Motor gasoline is considered one of the most mobile petroleum
products with high potential for vapor and aqueous phase contamination
(Section 5.2) and is grouped in the lowest kinematic viscosity group (Group
I) in Figure 5-3. Available information on gasoline composition (Section
5.2.3) allows a direct comparison of gasoline components to hazardous
substances.
Figure 5-5, given in Section 5.2.3, illustrates the hydrocarbon composition
of a reference sample of gasoline (Reference Fuel PS-6), used in an
inhalation study with laboratory animals sponsored by the American Petroleum
Institute (Domask, 1983). As discussed previously, 42 of 151 identified
compounds accounted for about 75 percent of the gasoline volume analyzed.
Of these 42 main consitiuents, 5 are hazardous substances regulated under
CERCLA, and are found in the following volume percent concentrations:
Chemical
Benzene
Toluene
Xylene and Ethyl benzene
TOTAL
Percent of Gasoline (by Volume)
1.69
3.99
9.83
15.51
Therefore, approximately 16 percent of the composition of gasoline may be
considered to be made up of regulated hazardous substances. The substances
listed above are aromatic hydrocarbons. Some of the remaining aromatic
hydrocarbons (roughly 10% based on Figure 5-5) may be toxic as well.
Additional gasoline constituents listed in the petroleum hydrocarbon
database (Appendix C) which are regulated hazardous substances include,
cyclohexane, naphthalene, and phenol. All the above listed substances may
produce toxic and explosive vapors as well as contaminate water supplies.
5-64
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Some other constituents of gasoline may be hazardous to humans and/or the
environment but, if they are not regulated under the LIST program, they are
not cited here.
In another study by Maynard and Sanders (1969), toluene was found to be the
constituent with the highest percent concentration (by mass, not volume)
gasoline at 12.20 percent. Percent concentrations of specific constituents
vary from sample to sample, but for the purposes of this discussion, it is
fair to assume that the aromatic fraction in gasoline will be on the order
of 25 to 35 percent. Moreover, Morris (1985) states that as a result of the
restriction of tetraethyl lead as an anti-knock additive of gasoline, the
aromatic hydrocarbon content of gasoline has increased and can be expected
to approach 35 percent for unleaded gasoline instead of the current average
of between 28 and 31 percent for both leaded and unleaded product. This is
noteworthy because aromatics pose the greatest potential for impact on human
health and the environment.
The hazardous gasoline components listed above are included in the dual
parameter sorts/groupings of the database presented in the previous section.
The resulting groups of these chemicals are for the most part in the
mid-range of potential impact as summarized on Table 5-12.
5.4.2 . FUEL OIL
Like gasoline, fuel oil is extensively used in the U.S. and is produced in
great quantity. The composition of fuel oil includes a wide range of
hydrocarbons and trace contaminants because it is a less refined distillate
of crude oil with much less stringent specification than motor or aviation
fuels.
The data sources for the composition of No. 2 fuel oil used in this study
did not provide information on constituents as comprehensively as the data
sources used for gasoline. Thus, the hazardous substance database was used
to assemble/compare properties based on the chemical names of identified
fuel oil constituents. The following chemicals were found to be hazardous
constituents of No. 2 fuel oil:
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TABLE 5-12
HAZARDOUS CONSTITUENTS OF GASOLINE AND
NO. 2 FUEL OIL
PARAMETRIC COMPARISON
Toxicity
Relative Parameters1
Vapor
Solubility Pressure
GASOLINE AND FUEL
OIL CONSTITUENTS
#2 FUEL OIL
CONSTITUENTS (ONLY):
Ignitibi1ity
Benzene
Cyclohexane
Ethyl benzene
Naphthalene
Phenol
Toluene
Xylene (0-.M-.P-)
B(s)1
C
C
B
B
C
C
M
L
M
no data
H
M
M
M
M
L
!_**
!_**
M
M
C
C
no data
D
D
C
C
Anthracene
Benzole Acid
Cresol (o-, p-)
Phenanthrene
Quinoline
Methanol
D
D
B
D
C
D
L
M
L
L'
H
H
!_**
!_**
[_**
[_**
L
H
D
D
D
D
D
C
1See Section 5.3.3, (s): Statutory
**Solid at normal conditions but may be dissolved in liquid fuel
5-66
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Anthracene Phenanthrene
Benzole Acid Quinoline
Benzene Methanol
Cresol (o-,m-, & p-) Toluene
Ethyl benzene Xylene
Naphthalene
Of these chemicals, six were also found to be hazardous constituents of
gasoline as discussed above. Table 5-12 cites these substances.
Similar to gasoline, these substances fall in the mid-range of the
parametric groupings based on relative impact. Constituents include
polynuclear aromatic hydrocarbons (PAH's) which are important carcinogens.
Typically these environmentally significant substances are found in
groundwater contaminated by oils.
5.4.3 ADDITIVES AND TRACE COMPONENTS
*
In addition to examining gasoline and fuel oil from the point of view of the
basic hydrocarbon composition, additives and trace contaminants can be
present in gasoline and other petroleum products in significant
concentrations.
Because most additives are complex mixtures and are used in small quantities
and/or their chemical specific formulation is proprietary information, very
few specific additives have been identified as regulated hazardous
substances (Section 2.0). The following, however are known to be hazardous
substances and are used widely in petroleum product property modification:
• Ethylene dibromide
• Ethylene dichloride
• Methanol
• Tetraethyl lead
• Dimethyl amine
Of the above, methanol is probably the most widely used as as a deicer and
in "gasohol."
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Trace contaminants include heavy metals often found in heavier fuel oils and
lubricating oils, as well as hydrocarbon-derived chemicals such as the
halogenated aromatics (e.g. dichlorobenzene). A sampling study of New
Jersey refineries and fuel oil bulk storage terminals showed that only trace
amounts of heavy metals are found in No. 2 fuel oil, but that higher, though
not necessarily significant, amounts are found in No. 4 and No. 6 fuel oil.
(Turgeon, 1985; GCA, 1983), as shown on Table 5-13.
5.4.4 PETROLEUM SOLVENTS
The principal finished petroleum products used as solvents (covered under
ASTM specification) are mineral and/or petroleum spirits, Varnish Maker's
and Painter's (VM&P) naphtha, and high flash aromatic naphthas. Typical
physical properties of these products are included in Appendix B. Based on
their properties, these solvents fall in Group I in Figure 5-3; like
gasoline, solvents are considered to have high potential for vapor and
aqueous phase impact on the environment. The aromatic portion of these
products most likely includes benzene, xylene, toluene, and other alkylated
aromatics known to be hazardous. (This can be inferred because the boiling
points of these chemicals falls in the range specified for the solvents).
Petroleum solvents are widely used in industry; however there are many other
organic industrial solvents on the market. Many of these industrial
solvents are on the list of hazardous substances (about 50 hazardous
substances are easily identified as solvents). Table 5-14 summarizes
chemical types and uses of organic industrial solvents and differentiates
between those categorized as "petroleum products." Examples are given of
various industrial solvents included in the database. Some organic solvents
may be manufactured by the processing (e.g., chlorination) of hydrocarbon
solvents.
The importance of solvents is three-fold:
• A significant number of organic industrial solvents may be considered
as hazardous petroleum products and/or as hazardous substances,
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TABLE 5-13
AVERAGE HEAVY METAL COMPOSITION OF FUEL OIL
(mg/1)
Fuel Type # Samples As Cd_ £r _H£ Ni_ Ste R> _Zn_
No. 2 8 2.6 <1 <1.2 <0.05 <1.5 2.7 1.3
No. 4 12 2.1 <1 0.58 <0.05 11.8 1.1 3.3 10.3
No. 6 18 2.75 <1 0.9 <0.05 28.4 4.3 5.2 1.1
Source: 6CA (1983)
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Table 5-14
CHEMICAL GROUPS OF PETROLEUM SOLVENTS
AND OTHER INDUSTRIAL SOLVENTS
Major Chemical Class
Petroleum Solvents
Paraffinic
Aromatic
Naphthenic
Other Industrial Solvents
Alcohols
Esters
Ethers and glycolethers
Halogenated Hydrocarbons
Ketones
Applications
Example of Solvent
or Hazardous
Substance List
adhesives, coatings,
drycleaning, inks,
metal decreasing,
Pharmaceuticals
adhesives, coatings,
inks, fuel additives,
Pharmaceuticals
adhesives, coatings,
Pharmaceuticals,
metal degreasing
NAJ
benzene
methylcyclohexane
adhesives, coatings, methanol
inks, fuel additives,
Pharmaceuticals,
photographic film
adhesives, coatings, amylacetate
inks, Pharmaceuticals
coatings, inks, fuel
additives, pharma-
ceuticals, lubricating
oil refining
tetrahydrofuran
metal degreasing, fuel trichloroethylene
additives dry cleaning
adhesives, coatings, acetone
textile manufacturing,
lubricating oil
refining, inks,
photographic film,
Pharmaceuticals
No paraffinic solvents are on the hazardous substance list; however, these
can be very volatile.
Source: Adapted from Standen (1969)
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• Solvents are used in large quantities and are often stored in USTs,
• Solvents are used to dissolve solid chemicals or to dilute liquid
chemicals such that they are in a usable form ; The solute, or
dissolved substance, may be a hazardous substance.
Groundwater contamination by solvents is relatively common compared to
contamination by other substances. Solvents are highly mobile, often quite
volatile, widely used, chemicals.
5.4.5 KEY FINDINGS
Motor gasoline, fuel oil, and petroleum solvents all have hazardous
constituents as shown in the preceding discussion. Primarily, the aromatic
hydrocarbons are the hazardous constituents present to the greatest extent
in these products, and the conclusion can be made that the aromatics pose
the greatest threat from UST-related incidents. Other components of concern
in petroleum products include heavy metals and hydrocarbon derivatives, but
these components are typically found in the part per million range, whereas,
aromatics are found in the parts per hundred (percent) range.
The dual parameter sorts discussed in the previous section lend themselves
very well to a comparison of hazardous substances and petroleum products.
As presented above the hazardous components of gasoline and fuel oil fall
primarily in the mid-range groups for potential to impact drinking water
aquifers, (toxicity-solubility) and the potential to expose people to toxic
vapor (toxicity-vapor pressure). Ignitability-vapor pressure data also
fall in the mid-range group, however the high ignitability group is reserved
for highly explosive chemicals, with immediate catastrophic effect. A
mid-range potential for adverse impact, based solely on this admittedly
simplified sorting procedure, obviously does not preclude the occurrence of
problems, it indicates that some hazardous substances have greater potential
for adverse impact than gasoline and other petroleum products.
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5.5 SUMMARY
The properties of regulated substances in USTs have been identified and
three databases were developed: 1. properties of petroleum products stored
in underground tanks; 2. properties of hydrocarbons known to be
constituents of gasoline and fuel oil; and, 3. the properties of
CERCLA-regulated hazardous substances.
Petroleum products were examined with respect to five parameters judged to
be relevant to subsurface fate and transport behavior. The properties
included:
• specific gravity,
• kinematic viscosity,
0 vapor pressure, and
• percent composition, of its constituents.
Petroleum products, except in rare instances, have a specific gravity less
than 1.0 and will float on groundwater.
Kinematic viscosity is related to transport by gravity in the vadose
(unsaturated) zone. The lower the kinematic viscosity the faster a product
will migrate through soil (driven by gravity forces).
High vapor pressure products, such as gasoline and jet fuels, have greater
tendency to release vapor phase contaminant than do substances with low
vapor pressures.
Relying on three parameters cited above, petroleum products were "grouped"
with respect to movement through the subsurface. Group I, or high vapor
pressure, low viscosity products, included motor and aviation gasoline,
solvents and No. 0-GT gas turbine fuel oil.
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Because little toxicity data has been explicitly developed for most bulk
petroleum products, the percent aromatic carbon composition was used here to
illustrate relative toxicity of petroleum products. Petroleum solvents,
automotive and aviation gasoline were found to have a high aromatic carbon
content and to be highly mobile (as judged by vapor pressure and kinematic
viscosity).
To appreciate the diversity among the various regulated substances in the
UST program, the database was used to generate information regarding phase,
chemical classes, solubility, and liquid (and vapor) density of chemicals.
The hazardous substances are found (at standard conditions) to be 5 percent
gases, 42 percent liquids and 53 percent solids. Roughly 20 percent of the
substances of interest are inorganic compounds; the remaining 80 percent are
organic. Density data revealed that 29 percent of liquid chemicals and 46
precent of solids are heavier than water. Groundwater contamination occurs
not only by dissolved substances but also by those which are insoluble or
slightly soluble, and which float on the water surface or sink to the
aquifer bottom.
The hazardous substances were grouped by using a "cause and effect"
approach. Two-parameter "sortings" of the database were used to assess
potential impact. Three impact scenarios were related to specific
parameters collected in the database, as follows:
t Groundwater contamination: solubility-toxicity data
• Human exposure to toxic vapors: vapor pressure-toxicity data
• Explosion/Fire: Ignitibility-vapor pressure data
After assigning ranges which denote high, medium or low values of these
parameters, and after removing highly reactive chemicals from the analysis,
the sorts were performed resulting in lists of substances considered to have
"similar potential" for UST-related impact.
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The results indicate that 32 chemicals with toxicity data (final or proposed
RQ) fall in the high toxicity/high solubility group and are considered to
have very high groundwater impact potential; only 5 chemicals may be
considered to have very high potential for human health impact due to
inhalation of vapors (high vapor pressure high toxicity); and 9 substances
are found in the highest ignitibility high vapor pressure group.
A comparison was made of hazardous substances in the database with the
constituents of gasoline, fuel oil, and petroleum solvents. By comparing
the hazardous chemical lists resulting from the sorts to the petroleum
product constituents some valuable insight is achieved.
The hazardous components of gasoline and fuel oil fall primarily in the
"mid-range" groups for potential to impact drinking water aquifers, the
potential to expose people to toxic vapor; and the potential for
fire/explosion. This mid-range finding reinforces what is felt intuitively:
certain hazardous substances have greater potential for greater adverse
impact than the constituents of gasoline and other petroleum products. This
does' not mean, however, that petroleum products do not present environmental
problems when released in the subsurface.
The hazardous components of gasoline include benzene, toluene, and xylene
which are fairly soluble, toxic and produce explosive/toxic vapors. These
chemicals make useful indicators of gasoline spills and the degree and
extent of contamination. With respect to oil spills, the above chemicals as
well as the polynuclear aromatic hydrocarbons (PAHs) are useful indicators.
The organic industrial solvents are important for they may be considered as
hazardous petroleum products and/or as hazardous substances. Solvents are
used in large quantities and are often found in underground tanks. These
solvents are used to dissolve solid chemicals or dilute liquid chemicals
such that they are in a useable form, and the solute, or dissolved
substance, may be a hazardous substance. Groundwater contamination by
solvents is relatively common compared to contamination by other substances.
Solvents are highly mobile, often quite volatile, widely used chemicals.
5-74
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6.0 ENVIRONMENTAL SETTINGS
6.1 OVERVIEW
When a leaking UST incident occurs, two types of environmental factors are
of concern: natural and non-natural. The natural environmental factors
determine how vulnerable the site is to contamination; climate and local
hydrogeology are two important examples of such factors. Non-natural
environmental factors exacerbate the site's vulnerability to contamination.
As such, non-natural factors can increase the hazards resulting from
leaking USTs.
To completely characterize an UST incident, it is necessary to integrate
these two types of environmental factors. The potential hazards resulting
from leaking UST incidents vary from incident to incident based on
site-specific environmental factors and hence are highly dependent on
site-specific conditions. The objective of this section is to describe the
current understanding of the environmental factors that influence the fate
and transport of a contaminant from a leaking UST.
The environmental factors for liquids and vapors are presented in 6.2 and
6.3, respectively. In these sections, the natural environmental factors
are grouped into two categorires: climate and hydrogeology. Cultural and
physical variables reflect the non-natural environmental factors that
influence the potential hazards posed by leaking UST incidents.
For regulatory purposes, it is useful to establish generic environmental
settings to evaluate the potential consequences of leaking UST incidents.
These generic environmental settings are not intended to describe all
variations encountered at individual sites, but rather to describe types of
settings and the types of problems each can present. To address this
regulatory need, a study was made of some existing methodologies used to
develop environmental settings were investigated to determine their general
applicability to evaluating potential hazards resulting from leaking UST
incidents. These methodologies are discussed in 6.4.
6-1
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6.2 ENVIRONMENTAL FACTORS AFFECTING LIQUID FATE AND TRANSPORT
6.2.1 NATURAL ENVIRONMENTAL FACTORS
The environmental factors that influence the potential hazards posed by
liquids leaking from USTs were selected from the analysis of the fate and
transport mechanisms presented in Sections 3.0 and 4.0. As these
environmental factors are addressed in the discussion of each fate and
transport process in Sections 3.0 and 4.0, this section presents a summary
overview of these factors.
Climate
The important climatic environmental factors are net recharge and
temperature. With respect to the potential hazards posed by leaking UST
site(s) within an ecosystem, climate is unlikely to vary significantly
(Kendeigh, 1974). However, within the ecosystem, climatic factors may vary
seasonally. For example, during rainy seasons when there is high net
recharge, there can be increased movement of liquid contaminants in the
unsaturated zone.
Net Recharge. The primary source of recharge to groundwater is
precipitation, which infiltrates the surface soil and percolates through
the subsurface. Not all precipitation infiltrates and percolates through
soil. Precipitation may "pool" in surface depressions, evaporate, or
transpire, reducing the amount of precipitation that recharges the
groundwater. It is the amount of precipitation that percolates through the
soil that is important, and not the amount of precipitation. The amount of
water per unit area of land which penetrates the ground surface and reaches
the water table is net recharge. Some examples of how net recharge
influences both the transport and fate of a contaminant are discussed
below.
Recharge water is the principal vehicle for leaching and transporting
liquid contaminants to the water table. Percolating water may dissolve
soluble components of a contaminant retained in the soil and transport the
6-2
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dissolved components through the soil, possibly reaching the water table.
In the saturated zone, the quantity of water available for dispersion and
dilution is also controlled by net recharge. In areas where an aquifer is
unconfined, recharge to the aquifer usually occurs more readily and the
pollution potential is generally greater than for confined aquifers (Alien
et.al., 1985).
With respect to leaking USTs, the pores (or voids) of the unsaturated zone
matrix are typically filled with air, water, and the leaked contaminant.
As recharge water percolates through the soil, the water fills some of the
pore space, displacing the air. The amount of moisture in the soil
influences the fate of a contaminant. For example, the influence of
recharge water on biodegradation can be summarized as follows: as recharge
water replaces air in soil pore spaces, the amount of oxygen available for
biotic degradation is reduced. The amount of available oxygen influences
the type and rate of biodegradative reactions.
Temperature. Temperature at the site can affect both transport and fate
mechanisms in the subsurface, but it has the more important effect on fate
mechanisms. For example, since adsorption is an exothermic process,
adsorption may decrease with increasing temperatures. Biodegradation,
however, generally increases with increasing temperatures (Scow, 1982).
These effects primarily manifest themselves in the unsaturated zone,
because at a given site the temperature of. groundwater remains fairly
constant.
Hydrogeology
The hydrogeologic factors that are relevant to evaluating the potential
hazards from a leaking UST are:
• Unsaturated zone media;
• Depth to groundwater; and
0 Saturated zone characteristics.
6-3
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Unsaturated Zone Media. The nature of the soil media of the unsaturated
zone is one of the most important environmental factors in the assessment
of potential hazards from a leaking UST incident. The composition and
characteristics of the soil -- such as the particle size and porosity and
the relative percentage of organic and inorganic matter -- determine the
attenuation characteristics at a site. The media also control the path
length and routing, affecting the time available for attenuation and the
quantity of contaminant that can be transported.
The ability of a soil to transport contaminants is the soil's transport
capacity (Wood, 1984). The greater the transport capacity, the greater the
migration of contaminants. Variables of the soil media that affect its
transport capacity include porosity, heterogeneity, and permeability.
Porosity and permeability affect the ability of a soil media to transport a
liquid, with permeability as the most important variable in its influence
on transport capacity. The lower the permeability of the soil, the more
slowly a liquid will be transported. The influence of media properties
such as distribution of grain sizes, the sphericity and roundness of the
grains, and the nature of their packing are reflected in the permeability
of a soil (Freeze and Cherry, 1979). Porosity is the ratio of the volume
of voids (open pores spaces) in the soil to the total volume of the soil;
it determines the amount of liquid that can be stored in soil. Generally,
within deposits of well-sorted sand or in fractured rock formations, high
porosities are associated with high permeabilities. This relationship,
however, does not hold for all soil types. For example, clay-rich soils
usually have high porosities and low permeabilities because of many
factors, including the distribution of grains and nature of packing
(Freeze and Cherry, 1979).
In the subsurface, variations in soil types may be present, e.g., clay
lenses or stratification of soil layers, termed heterogeneities. The
presence of heterogeneities influences contaminant transport patterns in
the subsurface. To illustrate the effect of heterogeneities on the
transport pattern, a schematic of transport patterns for homogeneous and
6-4
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heterogeneous soil media are shown in Figure 6-1. If it were not for the
effects of heterogeneities in natural geologic materials, the problems of
prediction and detection of contaminant behavior would be greatly
simplified (Freeze and Cherry, 1979).
Biodegradation, adsorption, chemical transformations, solubility and
vaporization are affected by the soil media. The composition and
characteristics of a soil media that are important to the fate mechanisms
are: pH, the percent of organic matter, and particle size. The degree to
which these fate mechanisms are influenced by these factors varies. For
example, solubility is primarily influenced by the pH and organic matter of
the soil media. Adsorption of organic chemicals is primarily a function of
the fraction of organic matter.
Depth to Groundwater. The depth to groundwater is important because it
determines the depth of unsaturated zone media which a contaminant must
travel before reaching the water table. In general, as the depth to water
increases there is a greater chance for the attenuation of a contaminant to
occur because deeper water levels result in longer travel times. For
example, biodegradation of a contaminant may be increased because of
prolonged exposure to microbial populations in the saturated zone.
Saturated Zone Characteristics. Characteristics of the saturated zone that
are important to assessing the potential hazards from leaking UST incidents
are the saturated zone media, hydraulic gradient, and hydraulic
conductivity.
Saturated zone media refers to the consolidated or unconsolidated soil
particles whose pores are filled with water. Media in the saturated zone
is termed "aquifer media" when its permeability is sufficient for it to
transmit significant quantities of water for use (Freeze and Cherry, 1979).
The saturated zone media affects the rate and path length over which a
contaminant is transported. The path length determines the amount of time
available for attenuation processes to occur such as adsorption,
dispersion, and degradation. In rock media, the path which a contaminant
follows is strongly influenced by fracturing and/or solution channels.
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LAND SURFACE
HIGHLY PERMEABLE
HOMOGENEOUS SOIL
LESS PERMEABLE
HOMOGENEOUS SOIL
STRATIFIED SOIL WITH
VARYING PERMEABILITY
SOURCE: CONCAWE (1979)
CAMP DRESSER & McKEE INC.
FIGURE 6-1
CONTAMINANT TRANSPORT THROUGH
HOMOGENEOUS AND HETEROGENEOUS SOILS
6-6
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The hydraulic gradient and conductivity of the saturated zone influence the
rate at which groundwater flows. The travel time of a contaminant is
determined by the quotient of the hydraulic conductivity and effective
porosity (the fraction of the porosity available for flow). Thus, the
higher the hydraulic conductivity of a medium, the faster a contaminant
will travel. The hydraulic conductivity of a media is a function, not only
of the porous medium, but also of the fluid (Freeze and Cherry, 1979). If
the fluid has a density or viscosity significantly different than water,
the intrinsic permeability of the media must be used along with these fluid
properties to determine the flow rate of a contaminant (Section 3.2
discusses this concept in detail). Transport of a contaminant in the
saturated zone is described by Darcy's Law and dispersion (Section 3.5.2).
The hydraulic gradient is the absolute value of the change in groundwater
elevation per unit of length. The direction which a contaminant will be
transported away from the point at which it entered the saturated zone is
influenced by the hydraulic gradient. High hydraulic gradients and
conductivities increase transport away from the source of entry and reduce
the attenuation time, thus increasing the potential hazards resulting from
a leaking LIST incident.
6.2.2 NON-NATURAL ENVIRONMENTAL FACTORS
Non-natural factors (cultural and physical variables) are also important
for the assessment of the significance of an UST leak. For example, a
leaking UST may have a low hazard potential because of natural factors, but
because it is located near a highly populated area, it may be of greater
concern than a leaking UST located in a remote area where natural
environmental factors would otherwise indicate high hazard potential. The
cultural and physical variables are therefore more highly site-specific
than natural factors and thus their effects are not as easy to
characterize. The cultural and physical variables that are important to
leaking UST incidents include:
6-7
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• Population density near the leak site;
• Horizontal distance to the point of concern; and
• Quantity and rate of the release.
Population Density
Population density refers to the number of individuals who would be exposed
to a contamination incident. As long as the leaking contaminant remains in
the unsaturated zone, the population exposed may be limited because of the
localized nature of liquid contaminant from a leaking UST (vapors, however,
may travel significant distances in the unsaturated zone (Section 6.3)).
When contaminants reach the saturated zone and contaminants enter the
groundwater, a much larger population may be exposed to risk.
Horizontal Distance
Horizontal distance represents the distance between an UST and the point of
concern. For liquids leaking from USTs, the point of interest is generally
a well which supplies drinking water. Obviously, the greater the
horizontal distance, the lower the potential for the water supply to be
contaminated. However, the hydraulic gradient and conductivity of the
saturated zone may influence the significance of the horizontal distance.
For example, an aquifer having high hydraulic conductivity and gradient
will transport a contaminant further and faster than an aquifer with low
hydraulic conductivity and gradient.
Quantity and Rate of Release
The quantity and rate of release affect the distribution of a contaminant
in the subsurface. As discussed in Section 3.1, insidious spills are
characteristic of UST leaks. Insidious spills may be either continuous or
intermittent. Generally, UST leaks are continuous. Intermittent leaks,
however, can be associated with overfilling of an UST. In general, a
continuous leak will have a greater potential hazard than intermittent
leaks. Factors that influence the rate of a continuous leak are
6-8
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controlled by the size and location of the tank's point of leakage. The
location of the tank's leakage point is important because the volume of
contaminant available for leakage varies with the fluctuating level of the
tank contents. If the leakage point is located near the top of the tank,
the contaminant can leak only when the tank is full.
In evaluating the significance of potential hazards posed by a leaking LIST,
the quantity of contaminant released affects the method, and often degree,
of groundwater contamination. If the volume of the release exceeds the
retentive capacity of the soil, the leaked contaminant will reach the water
table, and groundwater contamination may result.
6.3 ENVIRONMENTAL FACTORS AFFECTING VAPOR FATE AND TRANSPORT
The generation of vapors from substances leaking from USTs is a common
occurrence. The vapor concentration is determined by the physical and
chemical properties of the substance (especial ly the vapor pressure) with
the greatest vapor release associated with more volatile substances. Vapor
flows are principally generated from liquid substances as they migrate
downward through the unsaturated zone, and from the exposed surface of
immiscible substances floating on the water table. Vapors can also
originate from a plume containing dissolved volatile contaminants, though •
to a much lesser extent.
The underground release of a substance with a high vapor pressure does not
by itself constitute a problem, since the surrounding environment must be
vulnerable also. The following section identifies conditions that render
the environment vulnerable to vapor contaminants. Many of these factors,
either individually or in combination, create paths of lowered resistance
to vapor movement and in regions where accumulation occurs, create
conditions favorable to vapor movement. Other factors are important
because they inhibit vapor movement, particularly the restriction of loss
to the atmosphere. When venting to the atmosphere is reduced, an increase
in vapor concentration can result in increased lateral infiltration into
subsurfce structures. The principal environmental factors relevant to
vapor transport were introduced in the analysis presented in Section 3.6.
In this summary overview, as with the liquid phase discussion, these
factors are divided into natural and non-natural environmental factors.
6-9
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A major basis for this summary is a recent study conducted by COM (1985) -
aimed at summarizing the vapor phase problems from UST leak incidents.
This qualitative assessment was based on information collected on incidents
involving the subsurface movement and infiltration of vapors, i.e., vapor
incidents.
Investigation by COM (1985) of the vapor incident problem has identified a
number of variables that are suspected of influencing the probability and
degree of vapor incidents. A vapor incident is most likely to occur when
any of the following site-specific conditions are met:
• A substantial quantity of a substance with relatively high vapor
pressure, low heat of vaporization, low solubility in water, and
specific gravity less than that of water
• A favorable environment with restriction of escape from the ground
surface
t A susceptible underground structure
• Points/sources of entry into the structure
The following discussion describes the environmental characteristics
associated with vapor incidents.
6.3.1 NATURAL ENVIRONMENTAL FACTORS
Climate
Climatic environmental factors that are important to vapor phase fate and
transport processes are:
• Precipitation
0 Groundfrost
• Barometric Pressure
• Wind
• Temperature
6-10
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Precipitation. Precipitation is important to vapor transport because heavy
rain tends to saturate the upper portion of the unsaturated zone. As
precipitation enters the ground surface, the air that normally occupies
much of the surficial soil pore space is displaced with water. This may
result in a buildup of pressure in the soil gas and a counterflow that
vents soil gas vertically to the surface. If the precipitation rate is
high and the permeability of the soil is low, the resulting "saturated
layer" can act as an impermeable upper boundary by eliminating the
available open pore space, thereby impeding upward movement of vapors. The
end result is that instead of being dispersed to the atmosphere, vapors
move laterally, increasing the possibility that they might enter an
underground structure. Vapors move toward underground structures because,
like the atmosphere, such structures present relatively low concentration
zones, or "sinks," towards which the concentration gradient (driving force
for diffusion) is established. The saturation of the upper part of the
unsaturated zone, though temporary, will persist until the moisture
saturating this zone evaporates, transpires, and/or percolates through the
subsurface.
Precipitation also plays a role in the influence of groundwater on vapor
transport. Net recharge, which is the precipitation that reaches the water
table, is the primary source of recharge to groundwater. As such,
fluctuations in the elevation of the water table result due to seasonal
changes in precipitation and thus net recharge. Because the saturated zone
represents the lower boundary to vapor movement, an increase in the
groundwater level will force any free product floating on the surface and
the associated vapors upward. In effect, this shortens the distance to the
atmosphere and/or to a susceptible area and provides a pressure driving
force or gradient in that direction.
As water percolates through the unsaturated zone, it may dissolve vapors
which were generated from a leaking substance. These dissolved vapors can
be a significant source of groundwater contamination. The resolution of
vapors can be responsible for much of the groundwater contamination found
up the groundwater gradient from the spill site. On the other hand, once
in solution, vapor contaminants may be susceptible to fate processes such
as biodegradation.
6-11
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Groundfrost. Like precipitation, the existence of groundfrost results in
the formation of a relatively impermeable zone at the ground surface. In
this case, moisture present in the upper level of the soil freezes, thereby
essentially "plugging" the open pore space and restricting vapor transport
through this zone and preventing the release of vapors to the atmosphere.
The restricted upward vapor movement again leads to greater lateral
movement. Unlike precipitation, however, which generally results in only
temporary saturation of the ground surface, groundfrost generally exists
for an extended period. This presents the potential for sustained lateral
vapor migration, and in turn a greater potential for vapor infiltration
into an underground structure.
Barometric Pressure. Fluctuating barometeric pressure creates a pumping or
"breathing" effect within the unsaturated zone. Changes in barometric
pressure are transmitted into the subsurface. The transmission of air
pressure through the soil results in alternating compression and expansion
of the soil air, which in turn creates a pumping effect. Vapors present in
the soil pores are entrained in the resulting flow of air and are more
readily dispersed to the atmosphere. Because the depth of penetration is
limited, this process is most important where the depth to the
contamination area is much less than the full depth of the unsaturated
zone.
Wind. Wind also plays a role in subsurface vapor phase fate and transport.
Windy conditions increase the dispersion rate of vapors into the
atmosphere. The distinct concentration gradient across the boundary
between the vapor-laden soil and the relatively concentration-free
atmosphere is thereby maintained. Research by Fukuda (1955) and others
also suggests that wind eddies create a slight fluctuation (high frequency,
low amplitude) of atmospheric pressure over the soil surface. As with a
change in barometric pressure, air pressure transmitted into the soil
results in fluctuating vertical air movement through the soil. The
movement of soil air entrains vapors present in the soil pore spaces, thus
increasing the effect of advection on vapor phase transport. However,
investigation by Fukuda (1955) has shown that the amount of vapor
transported from the soil as a result of windy conditions is often small
compared to the effects of other processes.
6-12
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Temperature. Increases in temperature result in higher rates of
vaporization of a leaked contaminant. Changes in temperature also affect
several of the vapor transport mechanisms, such as diffusion. Section 3.6
addresses this topic in more detail.
Figure 6-2 illustrates the climatic effects on subsurface vapor transport.
Figure 6-2 (A) shows the influence of precipitation or groundfrost on
vertical and lateral migration of vapors. Figure 6-2 (B) depicts the
entrainment of subsurface vapors by soil air movement created by
fluctuations in atmospheric pressure. As vapors reach the soil-air
interface, they are readily dispersed by the wind.
Hydrogeology
The hydrogeologic factors that are important to vapor phase fate and
transport are:
• Unsaturated zone media
• Groundwater characteristics
Unsaturated Zone Media. The nature of the soil media of the unsaturated
zone is one of the most significant environmental factors in contro.lling
vapor movement. The most important soil parameters are:
• Porosity
• Type and extent
0 Moisture content
• Adsorption capacity
The porosity of a soil is the percentage of the total volume of the
material that is occupied by open pores. Generally, the pore spaces are
interconnected, permitting the movement and storage of liquids and/or
gases. In the unsaturated zone the pore spaces are filled with water and
air. The open, air-filled pore spaces provide pathways along which vapor
movement occurs. Vapor transport is governed largely by diffusion and
6-13
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GROUNDFROST OR SATURATED GROUNDSURFACE
PRODUCT VAPORS
FREE FLOATING PRODUCT
(A)
INFLUENCE OF GROUNDFROST AND PRECIPITATION ON VAPOR MOVEMENT
WIND
PRODUCT VAPORS
ENTRAINED IN
SOIL AIR MOVEMENT
SOIL AIR MOVEMENT
ENHANCED BY FLUCTUATION
x IN ATMOSPHERIC
/ PRESSURE
GROUNDSURFACE
FREE FLOATING PRODUCT
(B)
INFLUENCE OF BAROMETRIC PRESSURE AND WIND ON VAPOR MOVEMENT
CAMP DRESSER & McKEE INC.
FIGURE 6-2
CLIMATIC EFFECTS ON SUBSURFACE
VAPOR TRANSPORT
C-14
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advection. Soils with a greater porosity are therefore more conducive to
vapor transport.
The type and extent of soil also affects vapor movement. Coarse-grained
soils such as gravel and sand generally have large porous spaces and
therefore are very permeable* allowing relatively easy vapor transport by
advection. Fine-grained soils such as silts and clay have exceedingly
small pore sizes and therefore are often quite impermeable. In the
subsurface, variations in soil types are common. The extent of a permeable
soil type will influence the degree of vapor movement. The presence of an
impermeable zone such as a clay lens may restrict vapor migration almost
completely. On the other hand, the availability of loosely packed soil and
bedrock fractures or other channels provide extremely permeable pathways.
Because vapor transport in the unsaturated zone occurs through the
interconnected soil pore spaces, the moisture content of the soil has an
important influence. In a soil with a high moisture content, a large
percentage of the pore space is occupied by water. Thus^ the available
air-filled pore spaces through which vapors migrate are limited,
restricting the extent of vapor transport. In the areas where the entire
pore space becomes saturated with water, no vapor movement occurs. This
condition periodically arises after a heavy period of precipitation as was
described under climatic environmental factors. Increases in the soil
moisture content can result in the resolution of vapors or in an increase
in the vaporization of a contaminant that was weakly adsorbed to the soil
(Thomas, 1982).
As vapors move through the soil there is a tendency for contaminant
molecules to adhere to the soil particles (adsorption). The adsorptive
capacity of the soil is, therefore, important in assessing the extent of
vapor movement that is restricted by adsorption. Although adsorption slows
down the rate of progress of the vapor front, it does not otherwise alter
its form or peak concentration. With the passing of a vapor plume and
reduction in vapor concentration, the vapor molecules are released from the
soil particles again.
6-15
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Groundwater Characteristics. Groundwater characteristics in the vicinity
of the contaminated area also influence vapor phase transport. Groundwater
is particularly important because:
• it transports liquid contaminants away from the original spill area
thereby increasing the zone of liquid contamination from which
vapors are generated;
• immiscible substances with a density less than water float on its
upper surface, the water table, increasing the potential for
vaporization of a leaked contaminant;
• fluctuations of the water table result in a "pumping of vapors and
soil gas"; and
0 fluctuations of the water table result in a greater vertical spread
of a floating substance increasing the potential for vaporization.
6.3.2 NON-NATURAL ENVIRONMENTAL FACTORS
Non-natural factors (cultural and physical variables) can exacerbate the
potential hazard from vapors. The non-natural environmental factors
important to vapor phase release from leaking LIST incidents include:
• Man-made conditions;
• Population density;
• Proximity to susceptible structure; and
• Quantity released.
Man-made Conditions. Man-made conditions have been observed to greatly
influence the movement of subsurface vapors. The construction of
underground structures such as sewers and utility conduits provide pathways
of reduced resistance to diffusion through which vapors can be transported
considerable distances. In addition, the presence of underground
structures such as basements act as "target" areas to which vapors can
infiltrate. The construction of these structures also disturbs the
adjacent soil media, generally rendering the soil surrounding the structure
more permeable and therefore more conducive to vapor transport. The
importance of these conditions is evident in that a number of vapor
incidents have involved man-made conditions (COM, 1985).
6-16
-------
Like groundfrost or precipitation, paved surfaces restrict the upward
movement of vapors. The increase in contaminant concentration in the
subsurface because of reduced atmospheric venting can lead to an increase
in the lateral diffusive flux. Under these circumstances, there is often
an increased vapor infiltration of subsurface structures.
Population Density. Population density defines the number of individuals
who could be exposed to vapor concentration following a leaking UST
incident. This factor does not directly influence any vapor fate and
transport processes, but it is extremely important to scaling the
significance of the potential hazards from vapors. Commercial and
residential areas tend to be susceptible to vapor infiltration and
subsequent detection because of high population density coupled with the
large number of USTs (especially for petroleum products) and underground
strutures in these areas. Industrial locations maybe somewhat isolated
from residential and commercial areas, and because vapor impacts generally
occur in relatively close proximity to the leak or fill product, there is
considerably less potential for vapor exposure or detection in industrial
settings.
Proximity to a Susceptible Structure. The vertical and lateral distance to
a susceptible structure is of interest because vapor transport generally
has a localized effect. Vapor movement is principally restricted to the
area in close proximity to the actual leak or resulting free product,
unless a conveyance mechanism such as a gravel lens or other favorable
subsurface stratum or man-made conduit exists. Structures in close
proximity to the leak are therefore usually more vulnerable to vapor
infiltration.
Quantity Released. The volume of product released is important in
characterizing the potential scope of the area that could be affected.
Large spills can create a greater sphere of influence than small spills.
In addition, large liquid spills may tend to migrate on the groundwater
table posing the potential for vapor incidents at some considerable
distance from the leak. The size of the leak will also
6-17
-------
influence the duration of the vapor incident. Substantial spills may cause
vapor problems well after most of the free product has been recovered.
6.4 ENVIRONMENTAL SETTING METHODOLOGIES
To address a regulatory need for generic environmental settings, some of
the methodologies used to describe generic settings were studied. The
methodologies were evaluated for fulfilling the requirement of representing
the environmental, physical, and cultural factors determined important to
potential hazards posed by leaking UST incidents in Sections 6.2 and 6.3.
The methodologies to be discussed were utilized in the following:
• RCRA Risk and Cost Analysis Model (W-E-T);
• Liner Location Risk and Cost Analysis Model; and
• DRASTIC
The discussion is broken down into an overview of each methodology and its
applicability to the UST program.
6.4.1 RCRA RISK AND COST ANALYSIS MODEL (W-E-T)
The RCRA Risk and Cost Analysis Model (ICF, 1984) determines the risks and
costs of management methods for hazardous wastes. Hazardous waste
management practices are represented by W-E-T cells. These W-E-T cells are
combinations of Waste streams, an Environment, and Technologies. For
example, the combination of landfilling (T), spent stripping and clean bath
solutions from electroplating operations (W), in a high population density
area underlain by a productive aquifer (E) comprises a W-E-T cell in the
model. The methodology for defining environmental settings in a W-E-T cell
is the item of interest here.
Methodology
The environmental settings of a W-E-T cell are defined in terms of three
factors: population density, surface water environment, and groundwater.
6-18
-------
These factors were assumed to be the most significant ones for evaluating
the potential for human and ecosystem exposure to hazardous chemicals.
Population density is defined in three ranges varying by one order of
magnitude each:
2
• High - 250 people/km and over
• Medium - 25-249 people/km2
2
• Low - fewer than 25 people/km
The surface water environment is classified by two schemes. Human health
risk classification is based on surface water assimilative capacity. This
classification is divided into two categories: low (e.g., stream flows of
co 73
10 m /day), and high (e.g., stream flows of 10 m /day and lakes greater
than 3x10 m ). "Ecorisk" classification is based on ecosystem type,
using seven groups, such as large rivers, small streams, and sea coasts.
Two types of groundwater systems were defined: areas underlain by
productive groundwater formations, and all other areas. These two types
were assigned primarily on the basis of hydrodynamic characteristics of the
water-bearing formations. Productive formations are those generally
capable of yielding 50 gpm of water and containing less than 2,000 ppm of
dissolved solids.
Combining the various environmental factors, a total of 12 standard
environmental settings is available for evaluating human health risks as
shown on Table 6.1. Each three-digit zip code region was assigned one of
these settings.
6-19
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TABLE 6-1
STANDARD ENVIRONMENTS FOR HUMAN HEALTH RISK EVALUATION
Population
Density
High
High
High
High
Medium
Medium
Medium
Medium
Low
Low
Low
Low
Assimilative
Capacity
Low
Low
High
High
Low
Low
High
High
Low
Low
High
High
Groundwater
System
Productive
Non-Productive
Productive
Non-Productive
Productive
Non-Productive
Productive
Non-Producti ve
Productive
Non-Productive
Productive
Non-Productive
Surface Water
Number of
Zip-Code Areas
10
22
25
19
135
141
49
17
39
94
6
2
Total 559
Source: ICF (1984)
6-20
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Applicability to Leaking USTs
The W-E-T model methodology does not fulfill the requirements for the
development of environmental settings for leaking USTs for two primary
reasons: 1. the settings do not include the important environmental
factors in leaking UST incidents described in Sections 6.2 and 6.3; and 2.
the settings are distributed geographically over large areas.
To properly assess the consequences of leaking UST incidents utilizing
environmental settings, the influence of the environmental factors in
Sections 6.2 and 6.3 must be reflected in the generic settings. The RCRA
model of environmental settings uses two of the environmental factors from
these sections—population exposed and groundwater characteristics. It
does not include the influence of fate mechanisms on leaking chemicals.
As stated in the introduction to this section, leaking UST incidents are
site-specific. The distribution of generic settings by three-digit zip
code characterizes areas too large for the evaluation of hazards resulting
from leaking USTs. For example, the three-digit zip code distribution for
the state of Florida is shown in Figure 6-3; an environmental setting
representing the three-digit zip code 326, Gainesville, would include two
types of aquifer media: coastal plain sand underlain by carbonate rocks,
and carbonate rocks. These, aquifer medias may not have the same influence
on the potential hazards from a leaking UST incident, but would be
characterized in the same environmental setting utilizing three-digit zip
codes. The assignment of generic settings by four-or five-digit zip codes
might be adequate for the requirements of leaking UST environmental
settings. If, for example, five-digit zip codes were used to represent an
environmental setting, the area of Gainesville would be divided into eight
zip-code areas.
6.4.2 LINER LOCATION RISK AND COST ANALYSIS MODEL
The Liner Location Risk and Cost Analysis Model (EPA, 1985b) estimates:
1. the chronic risk to human health from land disposal of hazardous wastes
6-21
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MIAMI
330*
• 331
MILITARY
340
LEGEND
• Serves as associate post offices within that 3-digit ZIP Code Area
• Is a city which has been assigned its own 3-digit ZIP Code but
which is not a sectional center.
SOURCE: U.S. Postal Service — Retail Operations Division (1981)
CAMP DRESSER & McKEE INC.
FIGURE 6-3
THREE-DIGIT ZIP CODE MAP
FLORIDA
G-22
-------
under different scenarios of technology, location, and waste stream; and
2. the cost of different designs and sizes of landfills and surface
impoundments. The locations of land disposal sites are characterized by
environmental settings.
Methodology
The environmental settings for the Liner Location Risk and Cost Analysis Model
characterize the saturated zone at the location of the land disposal site.
The model uses the generic well distance, mobility class of a chemical, and
flow field combinations that most closely resemble conditions at a site.
These nine generic flow fields are shown in Figure 6-4.
Three generic well distances were used: 60, 600, and 1,500 meters. These
distances were determined from the review of distances to wells
downgradient of existing facilities,. the density of wells in areas where
private wells are used, and the predictive capabilities of the model.
The contaminants of concern were grouped into four broad mobility classes
based on their retardation value. The model uses the median retardation
value for each class as the value describing, the mobility of contaminants
in the generic flow field scenarios. These classes are presented in Table
6-2. The model generally ignores chemical interactions which could serve
to increase mobility for contaminants except those in Class 4.
The development of the generic flow field was based on a three-step
process:
1. determination of representative ranges for parameters describing
aquifer configurations, aquifer sizes, and groundwater flows;
2. construction of eight generic hydrogeologic settings using aquifer
configuration data ranges; and
3. matching these eight settings with representative aquifer size and
groundwater flow data to create the nine generic flow fields.
6-23
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B
C .
30M
1 M/Y
01 M/Y
30M
10 M/Y
1 M/Y
30M
100 M/Y •
IOM/Y
30M
1000 M/Y-
100 M/Y
30M
10,000 M/Y
1000 M/Y
o
i
15M
30M
0.05 M/Y
0.5 M/Y
100M/Y-*-
2 M/Y
30M
60M
100 M/Y -»-
10 M/Y
10 M/Y-*-
0.1 M/Y
EXPLANATION
WATER TABLE BOUNDARY
OUTFLOW .
BOUNDARY
30M
10 M/Y-
1 M/Y
INFLOW
BOUNDARY
NO-FLOW
BOUNDARY
H
15M
15M
10 M/Y -*-
.1 M/Y
100 M/Y •*-
I
1 M/Y
30M
30M
30M
10 M/Y «
nn1^ ««/v"^—
mn M/Y «
0.5 M/Y
I
0.5 M/Y
1 M/Y
AVERAGE LINEAR GROUNDWATER VELOCITY VECTORS
(METERS/YEAR) THROUGH EACH LAYER OF SATURATED
MATERIAL WITH CONSTANT THICKNESS (METERS).
PI INDICATE LAYER IS A NON-AQUIFER.
SOURCE: U.S. EPA (1985b)
CAMP DRESSER & McKEE INC.
FIGURE 6-4
LINER-NINE GENERIC GROUNDWATER
FLOW FIELDS
-------
TABLE 6-2
FOUR MOBILITY CLASSES
Class Retardation Range Retardation
1 _< 10 1.3
2 10 and <_ 100 32
3 >100 and <_ 10,000 360
4 > 10,000 160,000
Source: EPA (1985b)
6-25
-------
The nine generic flow fields, three exposure distances, and four mobility
classes can he combined to describe 108 scenarios of contaminant transport.
Applicability to Leaking USTs
The methodology util ized in the .Liner Model represents only the saturated
zone, so, the methodology is not applicable to the development of generic
settings for leaking UST incidents that affect two primary zones of the
subsurface, the unsaturated and saturated zones and may involve vapor phase
concern. (In fact, in some leaking UST incidents, only the unsaturated
zone will be affected). The methodology may be useful, however, to the
development of environmental settings for leaking UST incidents for the
saturated zone portion of the generic settings.
6.4.3 DRASTIC
DRASTIC (Alien et. al., 1985) is a standardized system for evaluating
groundwater pollution potential using hydrogeologic settings developed
by the National Water Well Association (NWWA). The system has two major
portions: the designation of mappable units, termed hydrogeologic settings;
and the application of a scheme for relative ranking of hydrogeologic
parameters which helps the user evaluate the relative groundwater pollution
potential of any hydrogeologic setting.
Methodology
The hydrogeologic settings developed were based on the 15 groundwater
regions described by Heath (1984) as shown in Figure 6-5. Because
pollution potential cannot be determined on a regional scale, smaller
"hydrogeologic settings" were developed within these regions. A typical
hydrogeologic setting is presented in Figure 6-6.
Evaluation of the relative groundwater contamination potential of any
hydrogeologic setting was determined to be controlled by the following
(most important) mappable factors: . jDepth to water, Recharge (Net), Aquifer
Media, Soil media, Topography, Jjnpact of the vadose zone and Conductivity
6-26
-------
2. Alluvial Basins
i- »•——__
ft.
Glaciated
Central
Region
'6. Nonglaciated
Central
Region
j>\ ur\ «/ f.
\\6- Nonglaciated Central "% y
=r*^r |Re9io"~ \
I
800 kilometers
SOURCE: Heath (1984)
CAMP DRESSER & McKEE INC.
FIGURE 6-5
GROUNDWATER REGIONS OF THE UNITED STATES
-------
Northeast and Superior Uplands
(9Ga) River Alluvium With Ovarbank
This hydrogeologic setting is characterized by low
topography and thin to moderately thick deposits of
alluvium along parts of river valleys. The alluvium is
underlain by fractured bedrock of sedimentary,
metamorphic, or igneous origin. Water is obtained
from sand and gravel layers which are interbedded
with finer-grained alluvial deposits. The flood plain is
covered by varying thicknesses of fine-grained silt
and clay, called overbank deposits. The overbank
thickness is usually greater along major streams (as
much as 40 feet) and thinner along minor streams.
Precipitation is abundant, but recharge is somewhat
reduced because of the silty overbank deposits and
subsequent clayey loam soils which typically cover
the surface. Water levels are typically moderately
shallow and may be hydraulically connected to the
stream or river. The alluvium may serve as a signifi-
cant source of water and is also usually in direct
hydraulic connection with the underlying bedrock.
SOURCE: Alleret. al. (1985)
Setting 9 Q« Rivar Alluvium With Ovarbank
Feature
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
Range
15-30
7-10
Sand and Gravel
Clay Loam
0-2%
Silt/Clay
1000-2000
Weight
5
4
3
2
1
5
•
3
General
Rating
7
8
8
3
10
1
8
Number
35
32
24
6
10
5
24
DRASTIC Index 136
Setting 9 Oa River Alluvium With Ovarbank
Agricultural
Feature
Range
Weight Rating Number
Depth to Water
Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact Vadose
Zone
Hydraulic
Conductivity
15-30
7-10
Sand and Gravel
Clay Loam
0-2%
Silt/Clay
1000-2000
5
4
3
5
3
7
8
8
3
10
35
32
24
15
30
16
Agricultural
DRASTIC Index
156
CAMP DRESSER & McKEE INC.
FIGURE 6-6
DRASTIC HYDROGEOLOGIC SETTING
-------
(Hydraulic) of the aquifer. These factors form the acronym, DRASTIC. The
numerical ranking system used to assess groundwater contamination potential
in hydrogeologic settings is calculated using the DRASTIC factors. The
system contains three parts: weights, ranges, and ratings. Each DRASTIC
factor was evaluated with respect to the others and assigned a relative
weight ranging from 1 to 5. These weights are shown in Table 6-3; the most
significant factors having a weight of 5 and the least significant a weight
of 1. These weights are constant in the application of DRASTIC.
For each DRASTIC factor, there have been defined either ranges or media
types that have an impact on pollution potential. These ranges have been
evaluated with respect to each other within each DRASTIC factor to
determine the relative significance to pollution potential, and have been
assigned a rating varying between 1 and 10 (the higher the rating, the
greater the pollution potential). For example, the ranges and ratings for
depth to water are shown in Table 6-4.
The numerical value of pollution potential for any hydrogeologic setting is
calculated by using an additive model. The equation for determining the
pollution potential is:
DRDW + RRRW + ARAW + SRSW + TRTW + IRIN + CRCW = DRASTIC index
where:
R = Rating
W = Weight
The higher the DRASTIC index, the greater the groundwater pollution
potential. From the DRASTIC index it is possible to identify and compare
the relative susceptibility of different areas to groundwater
contamination.
6-29
-------
TABLE 6-3
ASSIGNED WEIGHTS FOR DRASTIC FEATURES
Feature Weight
Depth to Water Table 5
Net Recharge 4
Aquifer Media 3
Soil Media 2
Topography 1
Impact of the Vadose Zone 5
Hydraulic Conductivity 3
of the Aquifer
Source: Alien et.al. (1985)
6-30
-------
TABLE 6-4
RANGES AND RATINGS FOR DEPTH TO WATER
Depth to Water
(feet)
Range
0-5
5-15
15-30
30-50
50-75
75-100
100+
Rating
10
9
7
5
3
2
1
Source: Alleret.al. (1985)
6-31
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Applicability to Leaking USTs
The feasibility of applying DRASTIC to petroleum USTs has been studied in
detail by the National Water Well Association (NWWA). A revised version of
DRASTIC called UST DRASTIC/IMPACT has been developed (NWWA, No Date). In
LIST DRASTIC, the weights and ratings of the DRASTIC factors have been
revised to reflect groundwater pollution potential for any hydrogeologic
setting as related to petroleum USTs. The UST DRASTIC weights are shown in
Table 6-5. A hydrogeologic setting with a UST DRASTIC value of 140 or
above has been defined to have a potentially high vulnerability to
pollution from leaking petroleum USTs.
IMPACT incorporates site-specific factors into the UST DRASTIC/IMPACT
system. The important site-specific factors were determined to be:
inclination of the water table, Measured horizontal distance, population
exposed, Application rate, Concentration, and Trapping/migration potential
of non-aqueous liquid wastes; forming the acronym, IMPACT. The numerical
ranking system is similar to DRASTIC although the factors were assigned
equal weights. Table 6-6 shows the ranges and rating for each of the
IMPACT factors. The numerical IMPACT index is determined by the following
equation:
IMP x ACT = IMPACT
3 3
The combined UST DRASTIC/IMPACT value that is assigned to a specific petroleum
UST site is calculated by multipl ying the DRASTIC index by the IMPACT
index. This resultant value is capable of "changing" one of the most
harmless UST sites (as calculated by UST DRASTIC only) into an area that
might warrant concern because of, for example, its relative value to the
local community and other factors.
6-32
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TABLE 6-5
ASSIGNED WEIGHTS FOR UST DRASTIC FEATURES
Feature
Depth to Water Table
Net Recharge
Aquifer Media
Soil Media
Topography
Impact of the Vadose Zone
Hydraulic Conductivity of the Aquifer
UST
DRASTIC
5
3
3
4
1
5
2
DRASTIC
5
4
3
2
1
5
3
Source: Alien et.al. (1985) and NWWA (No Date)
6-33
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TABLE 6-6
RANGES AND RATINGS FOR IMPACT FACTORS
Factor
Range
Rating
Inclination of Water Table
0.1 ft/ft
0.01 ft/ft
.001 ft/ft
1.5
1.25
1.0
Measured Horizontal Distance
Population Exposed
Application Rate
Concentration
Trapping/Migration Potential
< 1000 ft 1.5
1000-3000 ft 1.25
> 3000 ft 1.0
> 100 people 3.0
10-100 people 2
> 10 people 1.0
Continuous 1.5
Intermittent 1.25
Slug 1.0
High 1.5
Medium 1.25
Low 1.0
High 1.5
Moderate 1.25
None 1.0
Source: NWWA (No Date)
6-34
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The UST DRASTIC/IMPACT approach is of interest because it focuses on the
environmental factors that are recognized to be important in assessing the
hazard of leaking petroleum USTs. It must be recognized, however, that the
individual numerical ratings are rather arbitrary and are not specifically
related to the physical, chemical, and biological characteristics of a
site/contaminant that determine the actual hazard. Also, it needs to be
recognized that, because of the handbook nature of the UST DRASTIC/IMPACT
approach, the method is susceptible to misuse by individuals who lack the
professional expertise to judge the nature of the setting and the relative
importance of various factors. For these reasons, the method should be
subjected to independent field testing before it can be considered as a
reliable tool for generic evaluation of the hazards of leaking petroleum
USTs. Such testing could be done by using information from several
specific sites in different environments where leaks have occurred, and
having several individuals independently evaluate the site characteristics
within the UST DRASTIC/IMPACT framework. (These individuals would not have
access to detailed information on the contamination situation at the
sites.) The UST DRASTIC/IMPACT numerical ratings could then be compared
with the actual contamination consequences at the specific sites' in order
to assess the reliability of the generic approach.
6.5 SUMMARY
In this section, the environmental factors which influence the fate and
transport of substances leaking from USTs were identified. Two types of
environmental factors were presented: natural and non-natural. The
natural environmental factors determine how vulnerable a site is to
contamination, while the non-natural factors exacerbate a site's
vulnerability to contamination. As such, non-natural factors can increase
the hazards resulting from leaking contaminants from USTs.
The environmental factors important in the assessment of the potential
hazards resulting from leaking USTs are listed for liquids and vapors in
Tables 6-7 and 6-8, respectively. The ultimate significance of these
factors with respect to leaking USTs varies from site to site, and also
depends on the properties and quantity of the leaked contaminant. For some
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TABLE 6-7
ENVIRONMENTAL FACTORS AFFECTING
LIQUID FATE AND TRANSPORT
NATURAL FACTORS
• Climate
Net 'Recharge
Temperature
• Hydrogeology
Unsaturated Zone Media
Depth to Groundwater
Saturated Zone Charateristics
NON-NATURAL FACTORS
• Population Density
• Horizontal Distance to Point of Concern
• Quantity and Rate of Release
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TABLE 6-8
ENVIRONMENTAL FACTORS
AFFECTING VAPOR FATE AND TRANSPORT
NATURAL FACTORS
• Climate
Precipitation
Groundfrost
Barometric Pressure
Wind
Temperature
• Hydrogeology
Unsaturated Zone Media
Groundwater Characteristics
NON-NATURAL FACTORS
• Man-Made Conditions
• Population Density
• Proximity To A Susceptible Structure
• Quantity Released
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of the natural environmental factors, such as climatic variables or the use
of groundwater for drinking wat£r supplies, "it may be possible to assess
the significant variations on a nationwide geographic basis. For other
environmental factors, particularly the non-natural factors, it is possible
to assess the influence of these factors only on a site-specific basis.
For example, in determining the effect of vapor contaminants, a knowledge
of the specific micro-stratigraphy in the area of a leak and the presence
of receptor structures is very important.
It is also important to recognize the effects that an environmental factor
has on fate and transport mechanisms can be in similar or conflicting
directions. Some examples of these effects are:
• Areas with high net recharge increase the potential for a
contaminant to enter solution but can decrease the importance of
biodegradation and adsorption mechanisms.
t High permeability soil media general ly increases the migration of
liquids and vapors.
• Increases in temperature in the subsurface generally increase
vaporization and solubility of a contaminant, but decrease
adsorption.
• Adsorption of a contaminant increases in soils with a high organic
content.
• Dissolved contaminants will migrate further and faster in an aquifer
with high hydraulic conductivity and gradients.
• In areas with a shallow water table, the potential for groundwater
contamination is high.
t In northern climates where the upper soil zone freezes in the
winter, the reduction in venting of vapor contaminants to the
atmosphere can result in increased lateral movement of vapors, which
in turn creates the potential for greater vapor infiltration into
subsurface structures.
These examples illustrate the virtual impossibility of determining the
potential hazards of leaking USTs, except on a site-specific basis.
However, since within a regulatory framework the development of generic
environmental settings is useful to evaluate the potential consequences of
leaking UST incidents, three existing methodologies were examined to
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determine their applicability. The methodologies are used in the
following:
• RCRA Risk and Cost Analysis Model (W-E-T)
• Liner Location Risk and Cost Analysis Model
• DRASTIC and its revised version UST DRASTIC/IMPACT
The methodology for the RCRA model was determined to be unsatisfactory for
evaluating the potential hazards from leaking USTs because it does not
consider fate mechanisms, and the scale of application is too large
(three-digit zip code regions).
Because the methodology used to develop generic environmental settings in
the Liner model only characterized the saturated zone, use of this
methodology in assessing the potential hazards from leaking USTs is
relevant only if the hazard of concern is contamination of groundwater
resources. While this methodology is not presented in a form which can be
used to evaluate the chronic risk to human health from drinking groundwater
contaminated with substances leaking from USTs, it may be possible to
modify the methodology to do this.
When assessing which environmental factors should be included in DRASTIC (a
numerical system for the ranking of groundwater pollution potential), the
NWWA selected factors that required information which is available from a
variety of sources. As such, DRASTIC is a simple and easy-to-use approach
which when properly applied may be a useful planning or screening tool.
The NWWA assessed the feasibility of applying DRASTIC to petroleum UST
related problems; consequently, several of the weights and ratings were
revised. The need to address site-specific factors was also recognized;
these are included in IMPACT. The revised DRASTIC and IMPACT make up the
proposed UST DRASTIC/IMPACT system. It appears that this system could be a
useful tool for comparative evaluation of environmental settings for
hazards posed by leaking UST incidents for two reasons: 1. the
methodology includes many of the important liquid and vapor environmental
factors determined relevant to leaking UST occurrences, and 2. the scale at.
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which DRASTIC develops an environmental setting is appropriate for LIST
incidents.
Because the LIST DRASTIC/IMPACT system appears to be the most applicable of
the available methods to evaulate the groundwater pollution potential from
a leaking UST site, some notes on the use of the system are presented here.
One possible use of this system would be to rank leaking UST sites for
prioritizing corrective action and/or enforcement action. This system,
however, has not been developed to evaluate the possible hazards of two
nearby sites, e.g., two service stations on opposite corners of an
intersection. Also, as discussed in Section 6.4.3, because of the handbook
nature of the UST DRASTIC/IMPACT approach, the method is susceptible to
misuse by individuals who lack the professional expertise to judge the
nature of the setting and the importance of various factors. A testing
system to compare the numerical ratings with actual leaking UST sites is
proposed in this section.
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7.0 APPLICABILITY OF TECHNICAL FINDINGS TO THE UST PROGRAM
7.1 INTRODUCTION
In the preceding four sections of this report, we have presented a compre-
hensive description of the current understanding of the fate and transport
of substances leaking from USTs. The objective of these preceding sections
was to incorporate all of the relevant technical and scientific information
into a form and format useful for UST program activities. This has been
accomplished by organizing the preceding text as follows:
• A description of the transport and fate of regulated substances in
the subsurface environment (Sections 3.0 and 4.0);
t The chemical and physical properties of petroleum products and
hazardous substances as they relate to fate and transport processes
(Section 5.0); and
• The environmental context (or setting) and its relationship to fate
. and transport phenomena (Section 6.0).
In order to place these scientific/technical efforts in proper perspective,
it is useful to consider the chain-of-events that relates the source of a
contaminant to its potential environmental effect. The diagram in Figure
7-1 illustrates this chain-of-events, and relates each step in the sequence
to studies of underground storage tanks. For the most part, the work
reported here concerns the portions of Figure 7-1 shown within the dashed
lines, although relevant information regarding the source of releases and
causation of impacts is incorporated in the report as appropriate.
Figure 7-1 suggests that there are various points of intervention in the
process of moving from the source or release of a contaminant to its effect
on the environment. The Agency — the UST Program Office -- has the
responsibility to decide where and how the regulatory process intervenes.
Making such choices in an informed manner necessitates an understanding of
the science presented previously. However, because the number and
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GENERIC DESCRIPTION
UST • RELATED DETAILS
SOURCE
r
TRANSPORT
EMITS
POLLUTANT(S,
TRANSMITTED
MEDIA
TOWARDS
1
RECEPTOR(S)
FATE
CAUSING -
IMPACT(S)
* TANKS
• LEAKS
• RUPTURES
• IMPROPER OPERATIONS
PETROLEUM PRODUCTS
HAZARDOUS SUBSTANCES
• FREE PRODUCT
• SOLUTIONS
• VAPORS
• SOIL
• AIR
• WATER
• MAN-MADE STRUCTURES
• SOURCES OF
DRINKING WATER
• HABITABLE SPACES
HUMANS
SENSITIVE ECOSYSTEMS
CHRONIC EXPOSURE
ACUTE EXPOSURE
MORTALITY
MORBIDITY
PROPERTY DAMAGE
CAMP DRESSER & McKEE INC.
FIGURE 7-1
CHAIN OF EVENTS LINKING UST
INCIDENTS WITH POTENTIAL IMPACTS
7-2
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complexity of relevant scientific topics is large, a summary is presented.
here to place technical matters in the context of the UST program and,
therefore, to help guide and focus future work assignment efforts. The
model of the "cause-effect" process given in Figure 7-1 provides a
reference point that helps keep various topics in perspective to the total
picture.
The Agency has available many possible activities that might be used to
prevent and remedy the effect of leaking USTs. Some activities, such as
development of regulations, are the result of congressional requirements.
Others -- for example, priority-setting or outreach programs -- are dis-
cretionary within the Agency as to extent and focus. A list of the kinds
of activities the UST program may eventually utilize is found in Table 7-1.
This list was developed -- by COM and only for the purpose of illustration
-- based upon our understanding of recent Agency experiences. Scientific
and technical information is, of course, relevant to many of these
activities.
In an attempt to identify the most likely and relevant of the possible ;_
Agency endeavors, the early stages of this work assignment included
contact/conversation with numerous personnel involved in UST matters --
both within and outside the EPA. As a result of these discussions, we have
presented and organized our findings in this section as follows:
• Section 7.2: Problem Definition — Attempts to put technical
information in the context of helping to consistently and correctly
understand the overall problem of leaking USTs.
• Section 7.3: Regulation/Guidance Development -- Suggests how and why
technical and scientific matters can relate to the development of
priorities for technical standards, corrective action, and reporting
requirements.
• Section 7.4: Compliance and Enforcement -- Focuses on how the EPA
may use scientific/technical findings to implement and target its
activities; i.e., to set priorities.
• Section 7.5: Information Transfer -- Discusses how the need for
research and development, demonstration projects, training and
technical assistance, and outreach programs may be tailored and
adjusted to achieve the greatest environmental benefit based on
scientific and technical information regarding leaks from USTs.
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TABLE 7-1
TYPICAL PROGRAM ACTIVITIES
0 PROBLEM DEFINITION AND ASSESSMENT
• REGULATION AND GUIDANCE DEVELOPMENT
• PRIORITY SETTING AND CONFLICT RESOLUTION
• BUDGETING AND PROGRAM EVALUATION
• DATA COLLECTION AND MANAGEMENT
• STATE AND LOCAL PROGRAM DEVELOPMENT
• COMPLIANCE MONITORING AND ENFORCEMENT
• TRAINING AND TECHNICAL ASSISTANCE
• RESEARCH, DEVELOPMENT, AND DEMONSTRATION PROJECTS
• COMMUNICATIONS AND OUTREACH
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7.2 PROBLEM DEFINITION AND ASSESSMENT
Since the enactment of subtitle I in the RCRA amendments of 1984, EPA has
been moving ahead to develop and implement a comprehensive regulatory
program. While this process continues, it is useful to extract from the
technical information presented earlier in this report those items that
provide a similar and technically sound view of the problem at hand. In
the text to follow, we attempt to establish a foundation for this similar-
ity of viewpoint by providing a synopsis of key facts and highlighting
those topics which warrant careful attention to technical and scientific
matters.
7.2.1 PROPERTIES OF REGULATED SUBSTANCES
Subtitle I draws a distinction between petroleum products and other
hazardous substances. It is worthwhile to point out certain facts
concerning these regulated substances.
• Petroleum products, as a category, are generally not distinguishable
from the list of regulated hazardous substances. Gasoline, for
example, is known to contain — as pure hydrocarbons, additives,
and/or trace contaminants -- over 15% of substances which appear as
regulatable hazardous substances (i.e., are on the CERCLA list).
Moreover, of these substances, a large fraction are aromatic
hydrocarbons and therefore, are likely to be relatively toxic and
move readily in the environment (Section 5.4).
• The bulk properties of petroleum products are such that various
sub-groupings related to the potential for movement in the environ-
ment can be assembled. In general, the lighter fractions of petrol-
eum products with low kinematic viscosity and high vapor pressure
can be expected to be more mobile than the heavier (tar-like)
petroleum products such as No. 6 fuel oil (Figure 5-3 in Section
5.2.3).
• The hazardous substances subject to UST program regulation span a
range of physical, chemical, and toxicological properties. As with
petroleum products, the entire list of hazardous substances may be
divided into subsets that represent varying levels of potential for
movement and/or impact on the environment. (Figures 5-18 through
5-23 in Section 5.3.2).
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7.2.2 MANIFESTATION OF PROBLEMS
The cause-effect model cited in Figure 7-1 identifies various receptors and
impacts that are relevant to the regulation of substances in USTs. Some of
the technical and scientific facts presented in the previous sections are
relevant to a thorough understanding of how releases of regulated
substances may cause actual problems. The following findings emerge:
• The contamination of groundwater aquifers is one possible result of
a leaking UST incident. Where such aquifers are used directly for
water supply or communicate with surface water bodies used for
potable water, the potential exists for human exposure.
• Depending on a combination of factors, including the hydrogeological
conditions at a site and the chemical and physical properties of a
substance released from an UST, not all releases have the potential
to reach the saturated zone. (Sections 3.3, 3.4, and 3.5).
0 Scaled by the vapor pressure of a substance released into the
subsurface environment, there is the potential for vapor phase
contaminants to migrate and impact human beings and/or the
environment. Hazards resulting from vapor phase migration can be
acute or chronic. For example, migration can lead to an increase in
the vapor phase concentration of certain contaminants such that
there exists a high potential for explosion. On the other hand,
even low concentration levels of a vapor phase contaminant can enter
habitable space undetected such that chronic human exposure (and
resulting health "impact) is possible. (Section 3.6).
• Due to the underlying physics of the fate and transport process,
particularly"in altered settings where most petroleum product tanks
are located, vapor phase movement can occur much more rapidly than
liquid contaminant transport in solution in the saturated zone.
Movement of vapor phase contaminant in altered settings (conduits)
can be as high as "tens of feet per hour" as opposed to contaminants
in solution which move at the rate of the ambient groundwater, on
the order of "feet per day."
• Vapor release can be increased by corrective action measures
involving excavation. This is because, as soil is removed, the
distance to the atmosphere (to near-zero concentration) is decreased
and therefore the gradient-driven diffusion process is accelerated.
• As an immiscible substance moves through the subsurface, the soil it
encounters becomes contaminated. The amount of residual con-
tamination that remains in the soil matrix is a function of the
physical and chemical, properties of both the contaminant and the
soil media. Contaminated soil may produce a slow (aperiodic or.:
chronic) release of contamination over a long period of time, the
residual contaminant in the soil may remain in place almost
indefinitely, or be biodegraded and/or transformed over time (see
Section 3.0 and 4.0).
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7.2.3 RELEASE SETTING
When Congress enacted RCRA subtitle I, -it explicitly allowed fcff the
implementation of the UST regulatory program to reflect differences in
hydrogeological settings. As reported in Section 6.0, the setting in which
a release of contaminant occurs can greatly influence whether and how
problems may result. Some observations are as follows:
• Hydrogeological features such as depth-to-groundwater, soil type,
the localized slope of the water table, etc., all play a role in the
transport of regulated substances in the subsurface environment.
However, only in site-specific terms is it possible to combine the
chemical and physical properties of a contaminant with the
hydrogeological properties of a site to quantify information
concerning environmental or human health impacts. (Section 6.2).
• In addition to the transport-related aspects of the hydrogeological
setting, many issues related to the fate of particular chemicals
depend on the setting in which they are released. For example,
soils that have a high carbon content can be expected to adsorb
contaminants, "delaying" their dispersion into the environment. The
properties of certain regulated substances, in combination with
certain soils, may limit the severity or extent of present or future
impacts from a release, as discussed in parts of Sections 3.0, 4.0,
and 6.0.
• Because most USTs are located in or near areas of human activity,
human influence on the overall impact chain-of-events (Figure 7-1)
must be taken into account. Obviously, whether or not an aquifer is
"vulnerable" to contamination is linked to whether or not it is (or
may be) used as a source of drinking water, or recharges surface
waters used for drinking water. Also human-related is the presence
of disturbed soil or utility conduits (e.g., sewer lines, electric
and telephone lines,) which can provide for the rapid and widespread
migration of subsurface contaminants, particularly in the vapor
phase.
7.3 REGULATION/GUIDANCE DEVELOPMENT
The foregoing section highlights the technical/scientific investigations
undertaken in this work assignment. The objective has been to ensure that
the full complexity and scope of the problem is understood. Here, the
objective is to provide the details of our technical and scientific
findings that may prove illustrative to the Agency'.s development of
regulations and/or .guidance. As such, technical findings are applied to
the three areas in which the Agency is currently pursuing regulatory and
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guidance development: technical standards, corrective action, and
notification/reporting requirements. Each of these areas is discussed
separately in the material that follows.
7.3.1 TECHNICAL STANDARDS
Technical standards -- as part of the UST program — could specify per-
formance-related details of tank design, installation, monitoring, and/or
construction. To consider levels of performance in various settings may
require the use of the types of scientific information presented in the
earlier sections of this report. Thus, in an effort to support the
Agency's development of technical standards, the following observations are
offered:
• With reference to Figure 7-1 provided earlier, a range of receptor
effects is possible as a result of an UST incident:
- If one is concerned with contamination of groundwater resources
(i.e., pollution of the saturated zone), toxicity and solubility
characteristics of the leaked substance are of major concern. A
screening of the regulated hazardous substances based on these
two parameters is provided in Section 5.3.3. About 32 hazardous
substances -- when sorted according to high toxicity and high
solubility criteria -- are (or may be) considered to have a very
high potential for groundwater contamination out of the 374
liquids and solids with available data.
If one is concerned about potential vapor phase impacts of
human exposure to a contaminant, toxicity as well as vapor
pressure properties are significant. Liquid substances were
screened with respect to these two parameters to illustrate the
concept. The results, as reported in Section 5.3.2, indicate
that only 5 substances for which final RQ data exist and another
2 which await final RQs would be considered high risk from this
dual-parameter standpoint.
If one is concerned about the potential for explosive hazards
from release of regulated substance, the vapor pressure and
ignitability parameters are significant. Again, the screening in
Section 5.3 indicates that only 9 substances can be considered
high risk from this perspective.
- There are no chemicals that appear as "high risk" from all three
sorts. On the other hand, if one adds the screening results from
the above dual-parameter sorts, excluding duplication, the
results indicate that roughly 50 substances may be considered --
for one reason or another -- high risk. In addition, 19 vapor
7-8
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phase substances and 20 highly reactive substances must also be
considered high risk.
- If one is concerned about the fate and transport of petroleum
products in their bulk form, then two parameters are of
significance. The kinematic viscosity affects the rate of
movement of the substance, and the vapor pressure indicates the
potential for the substance to have an associated vapor phase.
Starting with 17 individually identified petroleum products as
listed in Figure 5-3, the results of sorting petroleum products
according to kinematic viscosity and vapor pressure indicate that
6 of these products would be considered high risk when viewed in
terms of the potential to contaminate groundwater and/or the
potential to release vapor to the environment. On the other
hand, the 3 petroleum products shown in Group IV (Figure 5-3) may
be regarded as largely immobile.
• The media in which pollutants move has direct bearing on the impacts
that will or may be manifest.
- When considering the potential for human exposure to a
contaminant, particularly in the vapor phase, the presence or
absence of man-made structures is an important feature of the
environmental setting. Most important is the presence of sub-
surface structures (basements, for example) where gas-phase
contaminants can collect and, potentially, lead to toxic exposure
or explosion/fire hazards. The existence of conduits (e.g.,
i sewer lines) to act .as channels for the migration of vapors (and
in some cases liquids) is obviously a site-specific feature of an
UST incident. However, because such conduits often lead to
basements and/or can cause migration well away from the area in
which the tank is located, the presence or absence of conduits in
the tank's vicinity may prove to be useful in establishing
regulations and standards. A distinction between tank standards
in "developed" and "undeveloped" settings, for example, might be
a worthwhile differentiation.
- The natural hydrologic features at the site of an UST leak have
an obvious role in the extent of migration (and consequent
environmental impacts). Systems for evaluating environmental
parameters, such as the UST DRASTIC/IMPACT model, have been
discussed in Section 6.0. From a technical standards viewpoint,
such evaluation systems offer the theoretical potential to
incorporate environmental settings in the regulatory process.
However, application of these systems is obviously site-specific.
In addition, these parameter-based systems are inherently
somewhat subjective and may require expertise to be used
properly.
- Regulations and standards should be directed toward avoidance of
the aquifer contamination that results when a toxic contaminant
enters an environment that permits access to groundwater. One
may envision a system of technical standards wherein the required
level of performance is tailored to whether or not the underlying
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aquifer is (or could be) used as a source of potable drinking ••
water, interconnects with areas of valuable/unique environmental
habitat, or is used for agricultural purposes. The Agency's
groundwater protection strategy may prove useful in this regard.
Such broad-based attributes of a setting (as contrasted with
DRASTIC-type parameters) would be easier to determine than
site-specific hydrogeological parameters.
- As is clear from the above discussion, environmental settings
cannot be separated from the chemical and physical properties of
the substances at issue. To illustrate the point, as the
discussion in Section 4.0 indicates, some soils, when combined
with some regulated substances, yield a situation wherein
biotransformation can result in the formation of innocuous
products. In contrast, certain hydrogeological characteristics
may combine with physical/chemical properties to result in by-
products more toxic than the original contaminant, or situations
of rapid and extensive spread of contaminant plumes.
7.3.2 CORRECTIVE ACTION
While the scientific/technical information developed in this report has
potential utility with respect to tank technical standards, there is,
perhaps, an even greater utility with respect to corrective action. The
following discussion illustrates the usefulness of the scientific and
technical information presented in Sections 3.0 through 6.0. with respect
to several distinct topics relevant to corrective action:
• Extent of Contamination
- Contamination from an UST incident has the potential to occur in
both the unsaturated and saturated zones.
- Given the location of USTs, virtually all leaks result in
contaminants entering the unsaturated zone; these then have the
potential to exhibit vapor phase movement.
- Some contaminants (e.g., Group IV in Figure 5-3) may remain
"trapped" in the unsaturated zone. These contaminants can remain
bound up in the soil matrix substantially immobilized, migrate
away from the leak as a vapor, or be solubilized in percolating
rainwater. Other contaminants may be biodegraded and/or
transformed into new products.
- Some contaminants reach the saturated zone and produce a "plume"
of contaminant in solution with (or as part of).the overall
•• groundwater flow field. Depending on its relative solubility,
the proportions of the spill and the groundwater flow, the
contaminant may float as a lens on top of the water table,
dissolve in the groundwater flow, or sink to the impermeable
surfaces which confine the aquifer.
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• Limited Actions
- One key technical finding of this work assignment, is the
realization that under some circumstances, regulated products do
not and will not migrate very far. If these same substances are
not very toxic or in other ways threatening, detailed study
and/or sophisticated corrective action may not be warranted.
Information and findings presented earlier (e.g., sorting results
and settling information) may be useful in deciding when and
where "limited" corrective actions may be justified.
• Clean Up Level
- Among the difficult issues that have been raised in all of EPA's
corrective action programs is "how clean is clean." In
establishing this standard for LIST regulations, the physics and
chemistry involved in the fate and transport processes must be
taken into account. The technical decision as to where, when,
and how to monitor for the presence of contaminants is influenced
by numerous and complex (sometimes uncertain) scientific
principles. For some chemicals in some settings the probability
of large-scale vertical migration is minimal, negating the need
to monitor in the saturated zone. Vapor phase monitoring in the
vadose (unsaturated) zone may be appropriate only for certain
chemicals with high vapor pressure. The fact that for UST
situations, solid hazardous substances are most often stored in
solution (in particular solvents) suggests that monitoring for
the presence of the solvent may be an appropriate indicator of
the presence of the chemical of interest. Thus, scientific
insight is useful in deciding on how monitoring can play a role
in the overall UST program.
• Effectiveness of Actions
- The "no action" alternative for corrective action might be
appropriate in some situations. Obviously, the source of the
release should be remedied in all instances. However, when the
amount or nature of the released contaminant(s) in conjunction
with the environmental setting appears to present no acute threat
to human health and the environment (and substantial dilution,
biodegradation and/or chemical transformation is likely), the no
action alternative may be appropriate.
- When large quantities of contaminants are released, large
portions of the unsaturated zone are often contaminated. Again,
as a function of the long-term fate possibilities and immediate
exposure potential at a given site, soil removal activity may be
warranted. In such instances 1. the remediation work is usually
costly, and 2. if free product is present on the surface of the
groundwater table during- excavation, the rate of release of
vapor-phase contaminant will be accelerated by the soil removal
process itself. The latter effect could result in local acute
air quality impacts where, before soil removal, slow release of
the subsurface contaminant may have been negligible.
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- Where extensive saturated zone contamination occurs and the
aquifer is the source of present or future water supply or
agricultural use, there may be no alternative to extraction of
groundwater and subsequent treatment. This is an expensive
proposition. Bioreclamation and other novel methods of in-situ
remediation hold some promise, but they are relatively
experimental and may .engender public fear of "biological
meddling."
• Reponsible Party Designation
- The majority of discovered LIST incidents occur in developed as
opposed to undeveloped settings. This reflects both the large
numbers of tanks located in developed areas as well as the
presence of receptors to "sense" that a leak has occurred.
However, this fact may complicate the process of identifying the
particular tank responsible for a given leak. For motor fuels in
particular, it can be difficult to differentiate one gasoline
from another and there are often many gasoline stations in an
area where a leak has occurred. Man-made utility conduits (which
provide channels for rapid and broad migration of contaminants)
also complicate the process of identifying and locating a
particular leaking tank.
7.3.3 NOTIFICATION AND REPORTING
There are three areas of information exchange applicable to the LIST program
where the scientific and technical information developed in this document
may prove pertinent. Understanding the science that underlies the fate and
transport of regulated substances gives an indication of what information
or data are pertinent to collect and/or to analyze. The three information-
related topics include: 1. the notification requirements for all owners and
operators of USTs, 2. requirements associated with the reporting of a
release, and 3. the reporting of corrective actions taken.
The notification program currently underway requires owners of regulated
USTs to inform the state wherein they are located of certain features
regarding the tank and its contents. This effort is clearly worthwhile for
the Agency, as well as the states, to define the universe of tanks subject
to regulation. The manner in which these data are organized and used can
be directly tied to the kinds of "screening options," if any, the Agency
selects for technical standards and corrective action. At present, the
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level of information forthcoming in the first round of notification
compliance may not provide all of the information necessary for very
sophisticated sorting. Nonetheless, a great deal of useful information can
be obtained by combining the expected information with knowledge of
physical and chemical properties of constituents to obtain an estimate on
the extent and severity of potential problems.
With respect to the reporting of a release and the corrective actions
taken, there is a direct link between the reporting requirements and
whether or not a policy of limited response — in some cases -- is adopted.
Information to be reported at the time of discovery of a leak can be
significant in terms of prioritization and targeting of EPA and/or state
agency resources towards corrective actions at that site. The information
reported and the level of detail required can be used in specifying the
extent of corrective action requirements in site-specific situations.
Certainly, adequate information regarding the magnitude and type of
contaminant released, as well as the natural and man-made environment in
which the release occurred, will be useful. As already discussed, in some
instances this* infjprmation would lead to a conclusion that no short- or
long-term problem would exist, and therefore, corrective action need not be
rigorous nor timely. In other instances just the opposite would be true.
Reporting requirements associated with the completion of corrective action
would obviously relate to the "how clean is clean" issue. Here again, an
understanding of the physics and chemistry that underlie the fate and
transport processes would be necessary in establishing that a given
standard of cleanup has been achieved. This information would also relate
to where, when, and how monitoring needs to be done to demonstrate
regulatory requirements have been satisfied.
In considering options for the development of corrective action require-
ments, it may be worthwhile for the Agency to retain some flexibility to
respond to leaks. That is, it is conceivable for the Agency to take the
position that corrective action requirements be tailored towards the
potential for migration from or exposure at a given site. Thus, if cer-
tified hydrogeologists or engineers analyze and investigate a particular
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situation and can demonstrate that minimal impacts are likely, then re-
quirements for rigorous (and costly) corrective action might be diminished.
An analogy to the Agency's implementation of Section 301(h) of the Clean
Water Act might be drawn.
7.4 COMPLIANCE AND ENFORCEMENT
The discussion above has focused on how and why technical and scientific
information might prove relevant to technical standards, corrective action,
and various reporting requirements. All of these necessary LIST program
activities are explicitly required under the provisions of subtitle I. In
the area of compliance and enforcement, however, the Agency may have
somewhat more latitude in organizing and implementing the LIST program.
This is especially important in light of the fact that the regulated
community consists of very large numbers of small scale enterprises.
Scientific and technical findings of the sort presented herein can be
useful in helping EPA (and the states) to allocate and target limited
resources towards the greatest environmental benefits or safeguards. For
example, compliance and enforcement of technical standards attempts to
prevent the release of contaminants into the environment. Because the
chemical and physical properties of regulated substances combined with
their environmental settings cause certain situations to be more
significant than others, these release prevention efforts ought to target
such situations as high-priority. Information regarding the location and
contents of USTs can be used to identify either geographic areas and/or
particular industrial or commercial sectors which seem to exhibit high risk
potential.
As opposed to technical standards and their objective of problem "preven-
tion," corrective action may be thought of as focusing on "remediation."
Again, scientific and technical information can be most fruitfully applied
in directing limited resources towards the "high risk" problem areas. In
deciding where corrective action reporting requirements (both pre-action
and post-action) must be most closely monitored with respect to compliance
and enforcement, for example, the Agency's efforts might be guided by a
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combination of the physical and chemical properties of the contaminant
combined with information regarding the setting wherein the UST incident
occurs. In implementing and enforcing corrective action measures at a
given site, necessary levels of cleanup can, of course, also be guided by
the risk potential inherent in the situation.
In a sense, the Agency's ability to target compliance and enforcement
activities towards sites or situations with high-risk characteristics might
conceivably be more effective in achieving the objectives of subtitle I
(i.e., reducing the risks of public health or environmental damage from
leaking underground tanks) than simply implementation of regulatory and
guidance documents as directed by Congress. Programs designed to "reach
out" to high-risk portions of the regulated community, as will be discussed
in Section 7.5 below, can demonstrate why and how compliance with technical
standards, for example, is in the best interest of the tank owners.
Scientific and technical information can be used to identify these
potential high-risk groups.
7.5 INFORMATION TRANSFER
In the remaining portions of this section, we offer some suggestions and
insights regarding research and development activities, demonstration
projects, training and technical assistance activities, and outreach
programs. In all cases the objective will be to point out how and why
scientific and technical findings developed here have a place in such
efforts.
7.5.1 RESEARCH AND DEVELOPMENT
As evidenced in earlier discussions in Sections 3.0 through 6.0, a great
deal of research and development activity has been already (and continues
to be) performed in relevant topic areas. The impetus for much of this
work goes beyond simply the UST program and encompasses related regulatory
programs under TSCA, CERCLA, RCRA, Clean Water Act, Safe Drinking Water
Act, etc. The work itself is being performed at universities, by private
enterprise, and by EPA's own ORD organization.
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Although the LIST program cannot directly influence research activities at
all relevant facilities, many of these on-going research efforts will be
useful in current UST efforts. The urgency of need for various research
activities must reflect the kinds of regulatory options the Agency selects.
With reference to Figure 7-1, for example, if technical standards are
promulgated (and complied with) such that the possibility of future
releases is reduced, then perhaps research efforts aimed at pollutant
migration may not be a high priority.
Based on our investigations of the current state of science regarding
UST-related phenomena, and noting that UST releases have already occurred
and are likely to continue to occur for some time into the future, we
present here a list of some of the topic areas we believe to merit
consideration for future research and development.
• Data Regarding Regulated Substances -- As indicated in Section 5.0,
it is possible to group regulated substances into subsets based on
physical, chemical, and toxicological properties. Unfortunately, a
complete set of the requisite data regarding each regulated sub-
stance is not available. Worthwhile research could, for example,
concentrate on developing toxicological parameters for chronic and
acute exposure via water borne and, perhaps, vapor phase pathways
for all regulated substances. Viscosity is another parameter that
. is currently quantified for only a small portion of the substances
at issue in the UST program yet is a property of considerable
significance to subsurface transport rate (Section 5.0).
• Vapor Migration Modeling -- As pointed out previously, vapor phase
contamination resulting from UST releases is of demonstratable and
serious concern in some settings. To date, the processes governing
release and migration of vapor phase contaminants in the subsurface
environment have not been thoroughly investigated nor quantified.
The development of an analytical model aimed at evaluating vapor
phase release and migration would be a useful addition to currently
available techniques. Such a model would enable the Agency (and/or
the states) to quickly judge the extent of vapor phase migration and
subsequently draw judgements as to whether or not a potentially
serious impact is likely.
• Unsaturated Zone and Multi-Phase Transport -- As Section 3.0
indicates, details of fate and transport in the unsaturated zone as
well as fate and transport of multi-phase contaminants are presently
not well understood. Because such phenomena may have serious
environmental consequences, however, the need for research in these
areas is important. The Ground Water Research Review Committee of
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EPA's Science Advisory Board (USEPA, 1985c) makes the following
point: "The Committee recommends that EPA increase research in the
basic process that govern the transport and fate of contaminants in
groundwater ..."
• Make-Up of the Regulated Community -- Data collection and management
activities aimed at fully understanding the nature and extent of the
LIST problem is crucial to effective program development. Through
the notification program currently underway, as well as through the
reporting activities associated with corrective action, the Agency
has the opportunity to collect useful information that will help
identify and characterize the regulated community in detail.
Unfortunately, the states are thus far only charged with collecting
required information, although EPA has developed a basic system
available to the states to organize/manage this data and work
continues on improving the package. The processing and summarizing
of these data on a nationwide basis would be helpful in the future
development of the program in conjunction with the information
presented in this report on, for example, the solubility or mobility
of some chemicals.
7.5.2 TRAINING AND TECHNICAL ASSISTANCE
One of the fundamental precepts of the UST program is that implementation
will be accomplished largely at the state level. With this in mind, it
seems reasonable to expect that the kinds of technical and scientific
information presented here in Sections 2.0 through 6.0 could be packaged
for distribution in the form of a guidance documents. This would assist
state personnel as well as EPA regional personnel. In addition, such an
exposition of technical information,'properly focused, would be useful to
local levels of government as well as to portions of the regulated
community. Also, depending on the form and substance of technical
standards and corrective action regulations, background documents
explaining why or how various groupings or sub-groupings of substances
and/or settings are to be regulated differently would be necessary.
Guidance or background documents such as these may serve as the outline for
training workshops to be conducted by the Agency.
In addition to an overview of the basic science involved in regulating
USTs, more explicit guidance may be necessary in the implementation of
state .(and perhaps local) regulatory programs. Perhaps one of the most
valuable contributions the Agency might make towards effective
7-17
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implementation would be the development of "models" or protocols for
actions to be taken when one discovers an actual UST incident. Technical
assistance in the areas of prioritizing situations and guiding corrective
action will be necessary. For CERCLA, for example, these technical
assistance efforts take the form of computer-aided systems to provide
guidance on the complex task of establishing appropriate corrective action
in a site-specific situation. In the jargon of today, such an approach may
be considered an "expert system." On the other hand, as a first step, the
same objective can be achieved with more traditional guidance checklists or
manuals. There are advantages and disadvantages to both approaches.
7.5.3 DEMONSTRATION PROJECTS
Because there is only limited documented experience to date, the
effectiveness of various corrective actions for remedying UST-related
problems remains an area of uncertainty. In order to better understand
this issue, there is a need for demonstration projects. Not only would
such demonstration projects help to determine the technical methods best
suited to particular situations, they would help to quantify the real costs
involved in various types of remedial activities. Moreover, demonstration
projects may prove useful in dispelling false notions of clean-up related
risks such as uncovered in the Provincetown situation. As briefly
described in Section 1.2, when Provincetown, Massachusetts sought to remedy
groundwater contamination caused by an underground storage tank leak, a
system of groundwater extraction wells and subsequent treatment was the
clean-up technique implemented at a cost of over $3 million. However,
in-situ biodegradation may have been a less costly and equally effective
remedy. The public's perception was that the biodegradation option
involved "risks" due to the use of microorganisms, although the
microorganisms involved are naturally found in the environment.
Demonstration projects may be able to provide objective data and
consequently lead to more cost-effective corrective action options.
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7.5.4 OUTREACH PROGRAMS
The potential utility of outreach programs was briefly touched upon in the
previous discussion of compliance and enforcement. We return to the topic
now because it represents an area of activity that can prove to be useful
for the Agency to achieve LIST program objectives. Assuming that the data
collection efforts of the Agency, combined with the kinds of scientific and
technical information provided herein, can be used to target high risk UST
populations (by geography, industrial sector, etc.), programs can be
designed to point out to these high-risk groups the advantages of complying
with both technical standards and corrective action requirements set by the
Agency.
To illustrate the point, assume high-risk populations can be defined, as
1. all tanks over sole source aquifers; 2. all tanks containing specific
chemicals regardless of location, etc. Once high risk groups are
identified, programs can be developed to commmunicate to these audiences.
Issues such as third party liability, responsible party designation, etc.,
can be explained to demonstrate why and how it is in their best interest to
meet (or exceed) regulatory requirements.
Even if the Agency itself does not wish to target high-risk audiences based
on the sorts of scientific/technical principles illustrated herein, those
who own or buy tanks might find it useful to understand the scientific and
technical issues involved to appreciate the risks they face from their
ownership and operation of USTs. This risk management information might be
distributed by the Agency through tank manufacturers, local fire marshals,
and others. The result might be that owners of tanks with extremely
dangerous substances or in sensitive settings understand and appreciate
their situation, and take prudent precautionary measures in the
installation and operation of tanks. One possible extension of this con-
cept would be to use the insurance industry as a vehicle to help achieve
compliance with UST program objectives. That is, it is possible to
envision that the insurance industry might set premiums as a function of
risk categories developed through the scientific and technical groupings
suggested here. Should this occur, the insurance premium rate structure
may help achieve satisfaction of UST program objectives.
7-19
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7.6 SUMMARY
Several preliminary suggestions are offered as to how scientific and
technical information — as presented in this report -- can be used in the
development and implementation of the UST regulatory program. Obviously,
precise recommendations must follow from the choices the Agency makes in
developing and implementing the regulatory program. In turn, although
scientific and technical information is pertinent, these Agency choices may
reflect, in large measure information regarding how many tanks, containing
what substances, are distributed throughout the United States.
Although this report presents findings of only the first phase of the work
assignment, there are several ways in which the findings may have immediate
and important use. These are summarized below:
• Sections 2.0 through 6.0 of this report assemble, in one document,
up-to-date information on the relevant scientific topics to the UST
program. This information can prove quite timely as the program
begins to build its regulatory framework. Also, the information can
be "re-packaged" to be distributed to state and local government
agencies, industrial organizations, etc., that are faced with
real-world UST incidents.
• The three databases developed under this work assignment thus far
(Volume II) can be of value in an immediate way, as a readily
available reference for properties of UST-regulated substances.
Beyond the general value of such a reference, the data provided in
these computerized information systems can help to prioritize
program efforts.
• From a global regulatory perspective (i.e., without focusing on
individual incidents for which specific chemical and setting
information is known), the technical information offered in this
report generally indicates the following:
- From the point of view of environmental settings, the use (or
potential use) of the underlying aquifer, the presence or absence
of man-made conduits, the proximity to population, etc. are the
important features relevant to UST incidents. However, specific
hydrogeological details cannot be easily incorporated in the
regulatory program without reference to specific and very
localized situations.
- As far as the contents of UST-regulated tanks are concerned, it
may be possible to state certain findings more defensibly.in the
"negative" as opposed to the "positive." For example, orife can
7-20
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state with a high degree of certainty that some petroleum
. products are unlikely to cause problems because they are largely
immobile if released in the subsurface environment.
Unfortunately, issues such as acceptable levels of toxicity and
exposure make positive statements about "very hazardous"
chemicals a more subjective matter. Also, information on the
distribution of tanks containing various substances may be
important.to decide whether and where to draw "hazard" standards.
On a site-specific basis, with information regarding tank contents
and localized hydrogeologic features in hand, findings of this
report can be used to prioritize actions in the areas of compliance
monitoring, corrective action requirements, and enforcement. For
example, state level personnel charged with implementing UST
programs may do the following:
- Having received notification of a leak and some degree of infor-
mation regarding the site and the contents of the tank, ranking
systems such as UST DRASTIC/IMPACT can be useful in prioritizing
(ranking) where to invest what levels of available resources.
- Use the databases developed herein to assess the degree of hazard
associated with an UST "incident. In particular, if such
databases were to be merged with "knowledge-based" systems for
guiding UST incident response activity, state and local level
personnel may be better able to make proper use of resources and
take actions scaled to the level of hazard posed in a particular
situation.
Finally, the technical and scientific findings offered earlier
indicate generally where and how research activities may be useful
to the UST program -- for example, to study the movement of
contaminants in the unsaturated zone, fate mechanisms in general,
and vapor-phase movement in general.
7-21
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•
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8-6
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-------
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8-9
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8-10
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8-11
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8-12
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HOW DOES THE UST MODEL WORK.?
SOME KEY QUESTIONS PERTAINING TO REGULATING L'ST'S
HOW DOES THE UST MO DE L_. WORK? ...--.
THE OBJECTIVE AND BASIC METHOD OF THE UST MODEL
Difficulties in tank/piping/detection analyses
Breaking the analysis into subsystems and component parts
The role of randomized outcomes and repetition
. THE COMPONENT PARTS OF THE UST MODEL
Environmental Setting, Tank-Pipe System, Tank Monitoring
Failure
Re lease
Detection
Transport
Repair/Replacement
Corrective Action/ Remedial Action
WHY USE'THE.-UST ;MODEL?
ROLE OF THE MODEL ..IN. THE' RIA
SOME PRELIMINARY R'ESULTS '
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HOW DOES THE UST MODEL WORK?
THE OBJECTIVE AND BASIC METHOD
5 = = = = = = = = = = = = = — = = = = = = — =S = .= = = == = = = = = = = = = = = S=S = = = = S = = = -= = = = = =: = = = =:
PROBLEM FACED BY ANALYSTS OF TANK/PIP I NG/DETECT10N SYSTEM
Complexity of Che systems
Unexpected interactions
Unpredictability
APPROACH USED BY UST SIMULATION TO OVERCOME THIS PROBLEM
Build a simulated system realistic enough to mimic
real systems, but simple enough to understand and use
To build a simulated system that acts like a typical real
system, the model uses:
Data on the performance of individual elements
Engineering judgment on their interaction
Randomizing factors to simulate the effects of chance
USE OF REPEATED TRIALS TO INCREASE CONFIDENCE IN THE RESULTS
WAYS TO USE THE SIMULATION
INCREMENTAL IMPROVEMENTS IN THE ANALYSIS
THE COMPONENT PARTS OF THE UST MODEL
-------
HOW DOES THE UST MODEL WORK?
THE OBJECTIVE AND BASIC METHOD
:== = = = = = = = = = = = = =: = = = = =: = = = = = = ==^=2 = = = = — = = = = = = = = =: = = = = = = = = === = = = = = :
PROBLEM FACED BY ANALYSTS OF TANK/PIPING/DETECT ION SYSTEM
APPROACH USED BY UST SIMULATION TO OVERCOME THIS PROBLEM
USE OF REPEATED TRIALS TO INCREASE CONFIDENCE IN THE RESULTS
On any one trial, the simulated tank system could cause
a little or a lot of trouble
But by repeating the trial many times the combination of
events that would be seen in the real world is approximated
We can then see not only the average performance but how
often particularly bad or good outcomes will occur
WAYS TO USE THE SIMULATION
We can also compare different tank and detection options in
virtually identical circumstances to see which performs
better according to our criteria
We can compare the outcomes generated by the program to
data from the real world
STRUCTURE OF ANALYSIS FACILITATES.INCREMENTAL IMPROVEMENTS
Those interested in assessing the model need not have
expertise in every aspect of tank systems to make useful
contributions.
THE COMPONENT PARTS OF THE UST MODEL
-------
- 13 -
HOW THE' MODEL WORKS
(Tank by Tank, Month by Month)
no
MODEL INPUTS
SYSTEM FAILURES
•f Release ? J
yes
I
RELEASE CHARACTERISTICS
TRANSPORT / PLUME CHARACTERISTICS
LEAK DETECTION
G
Release
Detected
•ye s-
REPAIRS,
REPLACEMENTS,
&
CORRECTIVE
ACTION
-. no
-------
- 19 -
ANALYTICAL PROBLEMS AND THE MODEL APPROACH
HOW THE MODEL WORKS
MODEL INPUTS
Tank (System) Characteristics:
Tank Material
Piping Material
Primary or Secondary Containment
Failure Probabilities:
Structural Deficiencies
Rup t ures
Corrosion
Detection and Monitoring Methods;
Type of Equipment Installed
Frequency of Testing or Monitoring
Sensitivity of Detection Threshold
Hydrogeological Setting:
Soil Characteristics
Depth to Aquifer
Ground Water Velocity
SYSTEM FAILURES
RELEASE CHARACTERISTICS
TRANSPORT / PLUME CHARACTERISTICS
LEAK DETECTION
REPAIRS, REPLACEMENTS & CORRECTIVE ACTION
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- 20 -
HOW THE MODEL WORKS
MODEL INPUTS
SYSTEM FAILURES
Using the model inputs and a routine that
simulates random events, the model:
Calculates Time of Failure
Determines Type and Location of Failure
Tank or Pipe Deficiency
Tank Rupture
Tank or Pipe Corrosion -- Initial Hole Size
RELEASE CHARACTERISTICS
TRANSPORT / PLUME CHARACTERISTICS
LEAK DETECTION
REPAIRS, REPLACEMENTS, & CORRECTIVE ACTION
-------
HOW THE 'MODEL WORKS
MODEL INPUTS
SYSTEM FAILURES
RELEASE CHARACTERISTICS
TRANSPORT / PLUME CHARACTERISTICS
Using inputs from RELEASE CHARACTERISTICS module on
release volume and release duration:
Calculates the Volume of Product in Unsaturated Zone
Calculates Area of Floating Plume
Floating Plume Areas are Linked to
ground water transport model
LEAK DETECTION
REPAIRS, REPLACEMENTS & CORRECTIVE ACTION
-------
- 23 -
HOW THE MODEL WORKS
MODEL INPUTS
SYSTEM FAILURES
RELEASE CHARACTERISTICS
TRANSPORT / PLUME CHARACTERISTICS'
LEAK DETECTION
Using Inputs from RELEASE and TRANSPORT / PLUME modules
concerning leak rates, leak durations, release volumes,
and transport, determines if release is detected:
Is Total Volume Released > Volume Threshold of Detection
Method?
Is the Total Concentration of Product in Water >
Concentration (PPM) Threshold of Detection Method?
Is the Total Concentration of Product in Vapors >
Concentration Threshold of Detection Method?
If Release is NOT Detected, Continue Transport Routine
If Release IS Detected, Calculate Time to Detection
REPAIRS, REPLACEMENTS & CORRECTIVE ACTION
-------
HOW THE MODEL WORKS
MODEL INPUTS
SYSTEM FAILURES
RELEASE CHARACTERISTICS
TRANSPORT / PLUME CHARACTERISTICS
LEAK DETECTION
REPAIRS, REPLACEMENTS & CORRECTIVE ACTION
Using inputs from previous modules on type and
location of failure, volume of release in
unsaturated zone, and area of floating pluae:
Calculates Cost of Any Repairs to System
Calculates Cost of Tank Excavation and Retirement
Calculates Cost of Replacing New Tanks
Calculates Area to be Excavated for Corrective Action
Calculates Cost of Corrective Action
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- 25 -
WHY USE THE UST MODEL?
SOME KEY QUESTIONS PERTAINING TO REGULATING USTS
HOW DOES THE UST MODEL WORK?
WHY USE THE UST MODEL?
The model addresses many of the key questions.
What Is the extent of the current problem?
What are the most cost-effective ways to reduce damages?
How will better leak detection reduce corrective action costs?
Need a tool to aid in understanding the entire system.
Tank performance / failures
Release Characteristics
Transport
Corrective Action
Costs
The interactions among these elements are complex
UST information is limited.
For example, can never actually know how many plumes from USTs
Can only simulate results
Model can handle probabilistic events
Model allows sensitivity analysis, allowing us to understand
the effects of uncertainty in key parameters
We have no real world experience with many policy options
of interest.
Cannot investigate actual instances of use of most options
Again, results must be simulated
I
ROLE OF THE UST MODEL IN THE RIA
PRELIMINARY RESULTS
-------
- 26 -
ROLE OF UST MODEL IN THE RIA
SOME KEY QUESTIONS PERTAINING TO REGULATING USTS
HOW DOES THE UST MODEL WORK?
WHY USE THE UST MODEL?
ROLE OF THE UST MODEL IN THE RIA
MODEL'S PRIMARY INPUTS TO RIA:
Costs
Effect iveness
MODEL RESULTS ARE INPUTS TO OTHER ANALYSES:
Economic Impacts
Implement at ion
Risks, Benefits
OTHER INPUTS TO RIA:
Versar Damage Case Study
OTS National Survey of Underground Storage Tanks
Other Studies
SOME PRELIMINARY RESULTS
-------
SOME PRELIMINARY RESULTS
SOME KEY QUESTIONS PERTAINING TO REGULATING USTS
HOW DOES THE UST MODEL WORK?
WHY USE THE UST MODEL?
ROLE OF THE MODEL IN THE RIA
SOME PRELIMINARY RESULTS
A LARGE NUMBER OF BARE STEEL TANKS ARE PROBABLY LEAKING
BIG IMPROVEMENTS WITH CORROSION PROTECTION:
Leak Prevent ion
Release Volume
Number of Plumes
Risk
IMPROVEMENTS WITH LEAK DETECTION:
Minimize Number of Leaks
Minimize Damages
BASELINE HEALTH EFFECTS ARE NOT ALARMING:
One cancer case per one million tanks per year
IMPORTANCE 0,F HYDROGEOLOGICAL SETTING:
Number of Plumes
Risk
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