xvEPA
United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
P.O. Box 15027
Las Vegas NV 89114-5027
June 1987
600S87005
Research and Development
Processes Affecting
Subsurface Transport of
Leaking Underground
Tank Fluids
-------
June 1987
PROCESSES AFFECTING
SUBSURFACE TRANSPORT OF
LEAKING UNDERGROUND TANK FLUIDS
by
Scott W. Tyler
Michael R. Whitbeck
Marcia W. Kirk
John W. Hess
Water Resources Center
Desert Research Institute
Reno, Nevada 89506
Lome G. Everett
Kaman Tempo
Santa Barbara, California 93102
David K. Kreamer
Department of Civil Engineering
Arizona State University
Tempe, Arizona 85287
Barbara H. Wilson
University of Oklahoma
R.S. Kerr Laboratory
Ada, Oklahoma 74820
CR 810052
Project Officer
Jeff van Ee
Advanced Monitoring Systems Division
Environmental Monitoring Systems Laboratory
Las Vegas, Nevada 89114
ENVIRONMENTAL MONITORING SYSTEMS LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
LAS VEGAS, NEVADA 89114
-------
PREFACE
This report documents the primary fate and transport
processes affecting fluids leaking from underground
tanks. Since these studies span a broad range of
scientific disciplines, the Desert Research Institute,
under its cooperative agreement with EPA/EMSL-LV,
has solicited input from its own staff as well as from
experts throughout the country. Each of the sections
describing the processes has been separately written
by individual experts in the fields of fluid flow, vapor
transport, surface chemistry, and the microbiology of
subsurface environments. Each author was given the
task of describing the state of knowledge in his or
her field and how this knowledge is applicable to the
detection of leaks from underground storage tanks.
Since this document had to be completed in a very
short time period, literary freedom was given to
each author as to section organization and con-
clusions. It is suggested that the reader consider this
report, therefore, as an edited collection of treatises
whose sole purpose is to describe the fate and trans-
port processes of fluids leaking from underground
storage tanks.
Scott W. Tyler
John W. Hess
-------
ABSTRACT
As a result of increased public awareness concerning
the ground-water contamination potential of leaking
underground storage tanks, attention is being given to
developing monitoring strategies for these facilities.
The most common strategies include tank material
inventories, tank integrity testing, and monitoring of
the soils and water adjacent to the tank. Research
sponsored by the U.S. Environmental Protection
Agency is presently being conducted to determine the
feasibility of these strategies.
This document focuses solely on the processes
affecting migration of fluids from a leaking tank and
their effects on monitoring methodologies. Based
upon the reviews presented, soil heterogeneities and
the potential for multiphase flow will lead to high
monitoring uncertainties if leak detection systems rely
on liquid sampling alone. Vapor transport is also
affected by these properties although to a lesser
degree. More research is needed, however, to better
understand the physics of vapor transport. Vapor
transport of contaminants to the monitoring sensors
will also be strongly affected by the volatility of the
fluid. Difficulties in detection and monitoring systems
may also be generated from fluid interactions with the
soil and microbes. The processes of adsorption,
partitioning, and microbial alteration of fluids in the
subsurface may have strong effects on the uncertainty
of monitoring systems. These fate processes have
received less attention than liquid and vapor transport
processes and will require significantly more research
before their effects are fully understood.
Present research indicates that high uncertainties in
monitoring reliability will be associated with systems
placed within the native, heterogeneous subsoil.
Monitoring systems placed in engineered (and,
therefore, more controlled and homogeneous) near-
tank environments may have much higher certainty of
detecting a leaking tank or leaking distribution system.
ACKNOWLEDGMENTS
The editors of this document wish to thank Dr. Lome
Everett, Dr. David Kreamer, Dr. Michael Whitbeck,
and Ms. Barbara Wilson for their significant
contributions to this report. Without their timely efforts
and expertise, this report could not have been
completed. We would also like to thank Ms. Marcia
Kirk for her background research and review
comments. We also thank Glenn Broughton and
Barry Hibbs of Arizona State University for their
assistance. The editors also wish to thank Jeff van Ee
and Leslie McMillion of the US EPA Environmental
Monitoring Systems Laboratory - Las Vegas for their
support and helpful review comments. Special
appreciation is also given to Carolyn Hagopian,
Barbara Salmon, and Deborah Wilson because
without their expert clerical and word processing
support, this report would not have been possible.
IV
-------
CONTENTS
Preface '"
Abstract |v
Acknowledgments iv
Figures vii
Tables viii
SECTION 1
Introduction (S.W. Tyler and J.W. Hess) 1-1
Scope of Report 1-1
Underground Tank Environments 1-2
Tank Failure Mechanisms 1-4
References 1-5
SECTION 2
Liquid Transport From Underground Storage Tanks (L.G. Everett)' 2-1
Introduction 2-1
Significant Physical-Chemical Properties 2-1
Vadose Description 2-2
Saturated Zone 2-6
Flow Regimes 2-6
Liquid Monitoring Difficulties and Future Research 2-12
References 2-17
SECTION 3
Vapor Transport and Its Implications to Underground Tanks (O.K. Kreamer) 3-1
Introduction 3-1
Sources of Gases in the Unsaturated Zone 3-1
Factors Which Effect Movement of Gases Near Underground Storage Tanks 3-3
Importance of Vapor Transport Surrounding Underground Storage Tanks 3-5
Vapor Transport Processes 3-6
Vapor Transport Proceses in the Underground Storage Tank Environment 3-8
Existing Gaseous Measurement Methodologies 3-10
Limits of Present Knowledge and Future Directions 3-14
References 3-15
SECTION 4
Soil Surface and Interfacial Effects in the Underground Storage Tank
Environment (M.R. Whitbeck and M.W. Kirk) 4-1
Introduction 4-1
Partitioning and Adsorption of Chemicals in the Underground Storage
Tank Environment 4-1
Leak Detection 4-3
Differences Between Laboratory and Underground Storage Tank Environment 4-4
Theoretical Deficiencies 4-4
Data Needs 4-5
Future Directions and Strategies 4-5
References 4-6
-------
SECTION 5
Implications of Subsurface Biological Activity for Monitoring Underground
Storage Tanks (B.H. Wilson) 5-1
Organic Pollutants in Ground Water 5-1
Conclusions 5-3
References 5-4
SECTION 6
Conclusions and Recommendations 6-1
Process Parameters: A Synopsis 6-1
Process Impacts on Monitoring 6-1
Indirect Techniques 6-4
Process Matrices 6-4
Recommendations 6-4
VI
-------
FIGURES
Number Page
1.1 Elements of an Underground Storage Tank Installation (New York State, 1983). 1-3
2.1 Migration of Leaked Material through the Soil Zone. 2-3
2.2 Effect of Clay Lens in Soil on Hydrocarbon Migration Path (API, 1980). 2-4
2.3 Seepage of Oil through the Soil Zone (after Schwille, 1967). 2-4
2.4 Trapped Product Droplets (API, 1980). 2-5
2.5 Hydrocarbon Leakage Flow Paths (API, 1972). 2-5
2.6 Hydrocarbon Migration Pattern (Schwille, 1984). 2-7
2.7 Composition of a Region of Macropore-Mesopore Media. 2-7
2.7 Variation of Porosity, Specific Yield, and Specific Retention with Grain Size (after Bear, 1972). 2-10
2.9 Two-phase Flow Relative Permeability (and Dam, 1967). 2-10
2.10 A. Relative Permeabilities Three-phase Flow (after van Dam, 1967). 2-11
B. Three-phase Relative Permeability (after van Dam, 1967).
2.11 Spreading of Spill on Water Table Surface (after Schwille, 1967). 2-12
2.12 Model Experiment: Influence of Changing the Water Level on the Oil Distribution 2-14
(after Schwille, 1967).
2.13 A. Impregnation Body (Petroleum Product) having Reached Ground Water 2-15
(Zillion and Nutzer, 1975).
B. Thickness of Layer of Oil in the Ground and in a Strainer Tube (Influence of Capillary
Pressures in the Case of a Continuous Layer of Oil).
2.14 Displacement of Oil Envelope by Water - Considered Microscopically (Schwille, 1967). 2-16
3.1 Volatized Product Vapor Migration Opposite Ground-water Flow. 3-2
3.2 False Positives due to Background Vapor Concentrations. 3-4
3.3 Three-dimensional Diffusion or Advection of Product Vapor. 3-11
VII
-------
TABLES
Number Page
6.1 Fluid Transport Parameters 6-2
6.2 Vapor Transport Parameters 6-2
6.3 Surface Chemistry Parameters 6-2
6.4 Microbiological Parameters 6-3
6.5 Advantage Matrix for Liquid Monitoring Technologies 6-5
6.6 Complications Matrix for Liquid Monitoring Technologies 6-6
6.7 Advantage Matrix for Vapor Monitoring Technologies 6-6
6.8 Complications Matrix for Vapor Monitoring Technologies 6-7
6.9 Advantage Matrix for Geophysical Technologies 6-7
6.10 Complications Matrix for Geophysical Technologies 6-8
viii
-------
SECTION 1
Introduction
As a result of an increased awareness towards the
sensitivity of the ground-water reserves of our nation
to anthropogenic sources of pollution, research has
begun to mitigate these effects. These sources of
pollution are a result of waste disposal practices,
withdrawal of mineral and petroleum resources, land
management practices, agriculture, construction, and
leaks and spills.
Although initial Federal and State water protection
legislation focused on waste treatment and disposal
practices (Water Pollution Control Act, Toxic Sub-
stances Control Act, Resource Conservation and
Recovery Act), recent legislation has focused on
control and monitoring of accidental releases of con-
taminants from underground storage tanks. The
impetus for this legislation lies in the magnitude of the
potential source of pollution.
It has been estimated that there are between 3 to 5
million underground storage facilities containing
potentially hazardous substances in the United States.
Even under ideal installation and operating con-
ditions, a percentage of tanks will fail within their
expected service life. It is estimated that 100,000
tanks are presently leaking while another 350,000 are
expected to leak within the next five years (U.S. EPA,
1985). Since these tanks are located below ground,
immediate visual detection of tank failure is unlikely.
Minute amounts of many of the stored materials can
pollute large amounts of ground water, and simple
tank inventory monitoring may be insensitive to the
detection of a potentially serious leak.
Based on these scenarios, amendments to the Re-
source Conservation and Recovery Act (RCRA) of
1984 authorized a federal underground tank program
for regulated products. The U.S. Environmental
Protection Agency (EPA) has defined an underground
tank as a tank with 10 percent or more of its volume
(including piping) underground. The term regulated
products includes petroleum products and hazardous
substances addressed in the Comprehensive Envi-
ronmental Response, Compensation, and Liability Act
(CERCLA).
The tank regulations mandated by RCRA are of two
kinds. Interim standards have been identified for new
tanks. New tanks must be cathodically protected,
constructed of noncorrosive material, clad in non-
corrosive material or designed to prevent leakage in
the event of corrosion or structural failure, if a new
tank is placed in soil having a resistivity of greater than
12,000 ohm/cm, then it is exempt from the above
standard. The second set of regulations will be the
final standards for new and existing facilities.
EPA has indicated that it plans to release final rules
for all the above standards at the same time in
February 1987. In addition, many states have passed
or are in the planning stages of legislation regulating
underground tanks (Askenaizer et al., 1985).
Scope of Report
The RCRA regulations have mandated prompt action
by EPA for all phases of underground tank man-
agement. To develop the standards required by
RCRA, clear input from a wide range of scientific
disciplines will be needed. The complexities of tank
and piping construction and the reliable monitoring
systems needed to insure rapid leak detection span
many fields of research. The purpose of this doc-
ument is to describe the processes and scenarios
pertinent to one phase of underground tank man-
agement: leak detection monitoring in the subsoil. To
accomplish this goal, experts in the fields of liquid
flow, vapor flow, adsorption, and microbiology have
each written a section on these subjects. The
discussions concentrate on organic contaminants
because of their greater potential for contamination
and because of the sheer numbers of the tanks;
however, the processes described are also applicable
to many inorganic fluids. Other studies sponsored by
EPA, but not covered in this report, include in-tank
monitoring, sensor development, and clean-up
technologies.
To design leak detection monitoring systems, it is
critical that the factors affecting the fate and transport
of leaking fluids be well understood. For example,
many sensors require vapors to migrate from the leak
to the sensor; however, if vapors are quickly adsorbed
by soil materials, the monitoring system is improperly
designed. It is the goal of this document to provide a
sound background for the EPA to build its research
program for leak detection monitoring systems. By
documenting the deficiencies in our understanding of
1-1
-------
each of the fate and transport processes, research
programs can be developed to address these issues.
Concurrently, those processes which may enhance
sensor efficiencies may be explored in research
programs in much greater detail and may provide
adequate monitoring systems immediately.
Each of the next four sections discusses the present
state of understanding of processes controlling mi-
gration of leaking fluids and assesses the current state
of subsurface monitoring of those processes. The
final section describes in matrix form the effects of
each process on existing monitoring techniques. Such
techniques include liquid monitoring, vapor sampling,
and borehole and surface geophysical methods.
These generic classifications of monitoring
technologies cover most of the presently available
technology. Because of the recent growth of the
monitoring industry, these categories may need to be
expanded in the near future.
This document is not intended as a "how to" manual.
As the reader will see, the current understanding of
the processes is well based, but the complexities of
tank environments indicate that much more research
is needed before reliable monitoring systems become
the industry standard. Since each section was written
by respected experts in their fields, the conclusions
reached may not always agree. This diversity in
opinion, however, can lead to a healthy and viable
research program. It is hoped that this document will
provide valuable input towards tank legislation and will
guide future research.
Underground Tank Environments
Since each of the sections deals with one physical,
chemical, or biological process in the subsoil sur-
rounding underground tanks, a brief description of the
overall tank environment is necessary. Not only does
this set the stage for the reader, but it also points out
many of the complexities involved.
Figure 1.1 shows one installation of underground
storage tanks and a hypothetical associated leak
detection scheme. It is obvious from the figure that a
wide range of both constructed and geologic con-
ditions are possible in the underground tank envi-
ronment.
Underground tanks are utilized in a very wide spec-
trum of environments ranging from drinking water
storage to radioactive waste disposal. The majority of
tanks, however, contain petroleum products. Tank
sizes range from tens of liters as in the case of
gasoline station waste oil storage to millions of liters of
highly radioactive waste. Tanks are constructed of a
myriad of materials; however, most fall into the
category of steel, fiberglass, concrete, or a composite
of these materials.
Steel tanks are used quite extensively for storing a
wide range of products. Corrosion of steel tanks in an
underground environment can drastically reduce the
effective life of a storage tank. A protective coating,
cathodic protection, or electrical isolation are gen-
erally used to extend tank life.
Fiberglass reinforced plastic (FRP) tanks are con-
structed of a plastic resin reinforced with fiberglass.
The plastic resin provides chemical resistance while
fiberglass gives the tank its structural strength. It is
essential that the stored product be compatible with
the tank material. Several resins and glass materials
may be used in the fabrication of FRP tanks.
Relatively new and effective designs include com-
posite tanks and double wall tanks. Composite tanks
incorporate steel tanks that are clad with a coating of
fiberglass reinforced plastic. These tanks combine the
corrosion resistance of FRP with the strength of steel.
Tanks may be single or double wall. Double walled
tanks are essentially a tank within a tank. This type of
storage tank may be fabricated from coated steel or
fiberglass. The space between the tanks may be
pressurized or evacuated, and leaks due to internal or
external corrosion may be detected by a loss of
pressure or vacuum. The internal space may also be
sampled.
Improvements in piping and piping design for product
handling have been initiated because of increased
concern for leak prevention. Fiberglass-reinforced
plastic piping is often used for handling petroleum
products. Flexibility and corrosion resistance are the
principal reasons for its wide acceptance. Expansion
and swing joints are also used to reduce stresses
which are due to thermal expansion or misalignment
or both of piping systems. Misalignment of product
handling systems may be caused by several factors,
for example, improper installation or differential
settlement of the storage tank and piping. Double
walled piping is also used when very hazardous or
toxic materials are to be handled.
Prior to installation of a storage facility, an evaluation
of the site conditions must be made. Clays, wet soils,
cinders, and acid soils tend to be highly corrosive.
Abandoned piping and tanks in the area may
accelerate corrosion of unprotected steel tanks.
Improperly abandoned storage tanks may collapse
and may cause excessive stress on recently installed
tanks.
1-2
-------
TANK TRUCK
©OVERFILL PREVENTION DEVICE (A
* ^Sr
.VAPOR RECOVERY LINE
-FILL LINE
VENT UME
PRODUCT
DISPENSER
CORROSION-RESISTANT
STORAGE TANK
EXCAVATION
CAP
OBSERVATION
WELL
PFA GRAVEL
OR BAND FILL
EXCAVATION WALLS
AND FLOORS OF
IMPERVIOUS MATERIAL
AUTOMATIC
SHUTOFF
VALVE
PRODUCT DELIVERY LINE
LEAK DETECTOR
JBMEROED PUMP ASSEMBLY
Well designed underground storage systems usually contain the following: (1) corrosion resistant tank; (2)
striker plate under tank fill line; (3) submerged pump with leak detector on product delivery line; (4) float vent
valve in tank vent line; (5) excavation walls and floor of impervious material; (6) asphalt or concrete excavation
cap; (7) automatic shutoff valve on delivery line at pump island; (8) overfill prevention device at fill line on tank
truck; (9) vapor recovery in tank truck during filling operation; (10) observation wells located inside excavation
boundaries; (11) pea gravel or sand fill for excavation.
Figure 1.1
Elements of an Underground Storage Tank Installation (New York State, 1983)
Depth to the local ground-water table must also be
investigated. In areas of high water table or in a flood
plain, a method of anchoring the tank must be
devised, since tanks that are low in stored product
may float to the surface in an area of high water table.
The resulting accident could have a significant en-
vironmental effect.
Once the site investigation is accomplished,
excavation can begin. A secondary containment
system may be installed after the excavation is
complete. In addition to any secondary containment,
fill material may be placed and compacted on the
bottom of the excavation to isolate the tank from the
surrounding soil. Steel tanks require sand fill material
while pea gravel (9.5 mm) is recommended by
manufacturers for fiberglass tanks. In an area where
tank buoyancy may be a problem, a 0.3 meter
reinforced concrete pad is typically placed at the
bottom of the excavation, and the tank is anchored to
prevent its displacement.
The tank is placed in the excavation and is backfilled
to about 3/4 of the tank height. Compaction of the
backfill material is critical, providing most of the lateral
support for the storage tank. Excessive void space
may cause stresses beyond the design limits of the
tank. This is especially true for fiberglass tanks since
approximately 90 percent of the lateral support is
given by the surrounding soil. Product lines and fill
pipe are then installed. When galvanized pipe is used,
swing joints are required at all locations where the
direction of flow changes. Standard screw con-
nections are used with fiberglass piping due to the
flexible nature of the material. Fill material is placed
above the top of tank and compacted. Coarse gravel
is then placed to grade, even with the land surface. In
many facilities, such as those used for retail gasoline
distribution, an asphalt or concrete cover is also
added. If no structures or cover are placed over the
tanks, the surface can be mounded to discourage
ponding of surface water and resultant infiltration.
1-3
-------
Tank Failure Mechanisms
In general, there are three primary causes of leaks
from underground storage tanks: corrosion, improper
installation, and poor operating practices. Corrosion
results from an interaction between tank or piping
materials and the surrounding environment, both
internal and external. Galvanic and electrolytic
corrosion are the two principal forms of electrical
corrosion. Electrolytic corrosion is the result of stray
electric currents from outside sources entering and
leaving by way of an electrolyte. Soil takes the form of
the electrolyte in the case of underground structures.
Galvanic corrosion is self-generated from electrical
potential differences between dissimilar metals
immersed in an electrolyte (soil). A current is gen-
erated when two dissimilar metals are connected and
placed in an electrolyte. Corrosion will occur in one of
the metals. Current from the corroding metal (anode)
will flow into the electrolyte to the other noncorroding
metal (cathode) to complete the circuit.
Methods currently employed to protect against
corrosion are cathodic protection, electrical isolation,
protective coatings, or a combination of these.
Cathodic protection works by reversing the elec-
trochemical action of corrosion. The current is forced
to flow into the tank thereby protecting the structure.
There are two basic types of cathodic protection:
sacrificial anode (galvanic) protection and an
impressed current method. Galvanic protection uses
a sacrificial metal (zinc or magnesium) in contact with
the structure to be protected. The impressed current
cathodic protection relies on an outside direct current
source to cause current flow in the proper direction.
Electrical isolation involves the use of dielectric fittings
and bushings that electrically isolate metal com-
ponents of the system. This minimizes the generation
of currents which are due to contact of dissimilar
metals.
Improper installation practices for underground stor-
age tanks and components are a common source of
leaks. Structural damage to piping and tank coatings
may cause localized corrosion. This can lead to
leakage once the tank is in place. Failure to use
proper bedding or backfill-compaction procedures or
both may result in a leaking storage tank. Another
source of product leaks results from piping joints that
fail because of improper fittings or joints that loosen or
crack over time.
Operating practices can also contribute to a loss of
product. Over-filling tanks and spilling during transfer
operations are probably the two most common
problems. Also, the puncturing of a tank during
inventory measurement is possible.
Many tank operation scenarios add to the difficulties of
subsurface leak detection monitoring. Spills or leaks
above the tanks during filling or dispensing may move
downward through the subsoil either as a result of the
spill or through infiltration of precipitation. This source
of contamination may allow for false positive signals
from subsurface monitors designed to monitor tank
performance. Tank and piping monitoring systems,
therefore, must be designed to be insensitive to non-
tank sources of contamination. Tanks may be located
in single or multiple units. In the case of multiple units,
pinpointing a leaking tank using subsurface methods
may be difficult if several tanks contain the same
material.
As can be seen, the subsoil environment surrounding
underground tanks is complicated by many sources.
Methods to monitor tank performance must be chosen
carefully for proper performance. Additionally, results
of monitoring must be carefully analyzed and
performance limits must be understood if monitoring
systems are to be successful. The following sections
outline the current understanding of the processes
controlling contaminant migration in the subsurface
and how these processes may influence leak detec-
tion monitoring strategy.
References
Askenaizer, D.J., W. Barcikowski, K.V.B. Jennings,
J.E. Sarna. 1985. Development of a Compliance
Program for Underground Tanks Containing
Hazardous Substances. University of California at
Los Angeles, Report #85-81.
U.S. EPA. 1985. Leaking Underground Storage
Tanks, EPA/530-SW-85-009.
1-4
-------
SECTION 2
Liquid Transport From Underground
Storage Tanks
Introduction
Although the Underground Storage Tank Permit
System of the EPA covers liquid inorganic chemicals,
natural mineral organics, and synthesized chemical
organic liquids stored in hazardous waste tanks, the
largest number of tanks by far are fuel storage tanks.
It is estimated that California alone has over 220,000
underground storage tanks in place today. By
January 1986, each of these tanks in California, as
mandated by the Sher Act, will require some
combination of soil-core monitoring, soil pore-liquid
monitoring, soil-gas monitoring, and ground-water
monitoring in addition to tank testing and tank
monitoring programs. When viewed across the nation,
the magnitude of the monitoring requirements is
considerable. In response to this need, this section is
directed towards a descriptive understanding of
organic and inorganic liquid flow related to monitoring
in the subsurface. The alternative to early alert
monitoring, which is ex post facto liquid hydrocarbon
removal by hydraulic sweeping, biodegradation, or
excavation, can still result in enormous environmental
liability.
The discussion of liquid hazardous waste migration,
which occurs as a continuous multiphase flow under
the influence of capillary, viscous, and gravity forces,
will include an appreciation of both unsatu rated and
saturated flow regimes. A natural subset of these flow
regimes includes both uniform Darcian flow and
fracture flow. The hazardous wastes discussed will
include both inorganic chemicals, such as from plating
factories, and organic chemicals, which include both
natural mineral products such as natural crude oils
and synthetic chemical products such as liquid
pesticides.
The movement of a separate liquid hydrocarbon
phase in a water and sometimes in air-filled porous
soil and the movement of organically active dissolved
hydrocarbon components that are subject to bio-
degradation, absorption onto soil particles, and
volatilization are very complex. Neither of these major
transport mechanisms is well understood today. The
dissolved component mechanism is the subject of
intense study at Stanford University, MIT, University of
Illinois, EPA, and several other research institutions
and universities. The liquid phase transport mech-
anism has been virtually ignored by the research
community in the United States, although it has been
the subject of empirical studies in Europe (e.g.,
Schwille, 1967, 1981, 1985). Recently, however,
American researchers have been developing some
laboratory results (Convery, 1979; Eames, 1981) and
some simple numerical simulations of multiphase
transport which focus on immiscible transport of
continuous phases. A few researchers, notably at
Delaware (Baehr and Corapciaglu, 1984), Princeton,
and the New Mexico Institute of Mining and
Technology, are looking into interphase transfer,
including the volatilization and solution of hydrocarbon
components.
Significant Physical-
Chemical Properties
Superimposed on the unknowns associated with
porous media and fracture flow under unsaturated and
saturated conditions are the physical-chemical prop-
erties of concern in establishing a monitoring program.
The fundamental question is whether the fluid is
miscible or immiscible in water. Fluids which are mis-
cible with water are fully dissolved by water and as
such can be handled through existing hydrodynamic
equations and principles. Fluids which are immiscible
in water, however, are held in the soil matrix at
residual saturation. Residual saturation refers to that
volume of fluid which is immobile with soil matrix.
Density of the fluid is important because this
parameter determines the level to which migration will
take place. The density factor is further complicated
by the fact that at certain temperatures, crude oil
components are dissolved within one another. For
example, if crude oils are cooled, substances such as
paraffins and others can crystallize out and form
agglomerates (micro-crystalline precipitation) which
can interfere with fluid flow through the fine pores. In
addition, fresh crude oils with volatile components
become increasingly viscous as they evaporate.
Viscosity is a major factor controlling the velocity of
the flow process. For example, certain chlorinated
hydrocarbons having relatively high vapor pressures
have been shown to evaporate 50 percent in less than
0.5 hours. This substantial loss in the volatile phase
results in significant changes in density and viscosity
of the bulk fluid. Surface tension is a property that is
responsible for capillary effects and for spreading on
2-1
-------
top of the water table. Although typical hazardous
waste hydrocarbons in soils are treated as a non-
wetting fluid, some of the hazardous waste may have
wetting properties. Although we recognize that typical
hazardous waste is wetting relative to air, our ap-
preciation for field capacity and residual saturation
may change if hazardous waste hydrocarbons exhibit
some wetting properties relative to soil. Since hy-
drocarbons can exist as an immiscible phase, as part
of the water phase, and as part of the soil-gas phase,
a determination of their various permeabilities is
extremely complex.
Vadose Zone Description
Leakage from underground storage tanks typically
occurs into the vadose zone (Figure 2.1) which is the
geological profile extending from ground surface to the
upper surface of the principal water-bearing formation.
As pointed out by Bouwer (1978), the term "vadose
zone" is preferable to the often-used term "unsat-
urated zone" because saturated regions are frequently
present in the vadose zone. Davis and DeWiest
(1966) subdivided the unsaturated zone into three
regions designated as the soil zone, the intermediate
unsaturated zone, and the capillary fringe. Hydro-
carbon movement is different in each of these zones.
Soil Zone
The surface soil zone is generally recognized as that
region that manifests the effects of weathering of
native geological material. The movement of water
and contaminants in the soil zone occurs mainly as
unsaturated flow caused by infiltration, percolation,
redistribution, and evaporation (Klute, 1965). In some
soils, primarily those containing horizons of low
permeability, saturated regions may develop during
infiltration and may create shallow perched water
tables (Everett, 1980) and free product zones. The
effects of soil layering on fluid migration are shown in
Figure 2.2. Loss of volatile organics and micro-
biological activity are highest in the soil zone.
The physics of unsaturated soil-water movement has
been intensively studied by soil physicists, agricultural
engineers, and microclimatologists. In fact, copious
literature is available on the subject in periodicals
(Journal of the Soil Science Society of America, Soil
Science) and books (Kirkham and Powers, 1972;
Hillel, 1971). Similarly, a number of published
references on the theory of flow in shallow perched
water tables are available (Luthin, 1957; van
Schilfgaarde, 1970). Soil chemists and soil micro-
biologists have also attempted to quantify chemical-
microbiological transformations during soil-water
movement (Bonn et al., 1979; Rhoades and Bernstein,
1971; Dunlap and McNabb, 1973). Leakage from
underground tanks, however, typically occurs below
the soil zone.
As shown in Figure 2.3, during the seepage period
through the soil zone and into the intermediate un-
saturated zone, the oil moves under the influence not
only of gravity but also of capillary forces in all
directions. Therefore, a zone develops around the
core of the infiltration body which can be compared
with the capillary fringe of the aquifer. Schwille (1967)
calls it the "oil wetting zone." In it, as in the capillary
fringe, the oil saturation decreases in an outward
direction. In the "oil percolation zone," gravitational
forces are dominant. Below a certain degree of
saturation, the oil is retained in an immobile state by
capillary forces. The oil saturation corresponding to
this state is called "residual oil saturation," and the oil
present in the pore structure under these conditions is
called "residual oil." If the saturation of a non-wetting
oil is reduced any further, the available flow channels
become discontinuous and leave behind only isolated
islands of the non-wetting fluid (Figure 2.4). For all
pressure gradients which occur in laminar flow, these
islands are largely stable.
Intermediate Unsaturated Zone
Weathered materials of the soil zone may gradually
merge with underlying deposits which are generally
unweathered and which comprise the intermediate
unsaturated zone. In some regions, this zone may be
practically nonexistent as the soil zone merges directly
with bedrock. In alluvial deposits of western valleys,
however, this zone may be hundreds of meters thick.
If the hydraulic properties of the intermediate unsat-
urated zone are uniform with depth, the migration
(both vertically and horizontally) will be controlled by
the leak rate and the soil texture. If the materials are
layered (as is often the case), horizonal spreading of
the contaminant at larger interfaces may dominate.
These two processes are graphically shown in Figure
2.5. Water in the intermediate unsaturated zone may
exist primarily in the unsaturated state, and in regions
receiving little inflow from above, flow velocities may
be negligible. Perched ground water and free product
lenses, however, may develop in the interfacial de-
posits of regions containing varying textures. Alter-
natively, saturated conditions may develop as a result
of deep percolation of water and oil from the soil zone
during prolonged leakage. Studies by McWhorter and
Brookman (1972) and Wilson (1971) have shown that
perching layers intercepting downward-moving water
may transmit the water laterally at substantial rates.
Thus, these layers serve as underground spreading
regions transmitting water and oil laterally away from
the overlying source area. Water and oil may move
downward from these layers and intercept a sub-
2-2
-------
Proper Location of a Sampling Point
Siting Considerations
Organic
Liquid
Monitoring
Point
Chemical
Adsorption Biological \ Dilution and
Degradation \ Dispersion Cultural and
Chemical °ther Sitin9
Transformation Considerations
Figure 2.1
Migration of Leaked Material Through the Soil Zone
-------
Figure 2.2
Effect of Clay Lens In Soil on Hydrocarbon Migration Path (API, 1980)
free groundwater level
wetting zone
percolation zone
SEEPAGE
Figure 2.3
Seepage of OH Through the Soil Zone (after Schwille, 1967)
2-4
-------
WATER
FLOW
8AND
GRAINS
TRAPPED
OIL
FLUSHING WILL NOT REMOVE ALL OF THE TRAPPED
PRODUCT BECAUSE OF CAPILLARY ATTRACTION
Figure 2.4
Trapped Product Droplets (API, 1980)
LAND SURFACE
SLOW SEEPAGE
INTO
PERMEABLE
SOIL
HIGH VOLUME
SEEPAGE
INTO
PERMEABLE SOIL
SEEPAGE INTO
STRATIFIED SOIL
WITH VARYING
PERMEABILITY
Figure 2.5
Hydrocarbon Leakage Flow Paths (API, 1972)
2-5
-------
stantial area of the water table. Because of dilution
and mixing below the water table, the effects of leak-
age may not be noticeable until a large volume of the
aquifer has been affected.
The number of studies on water movement in the soil
zone greatly exceeds the studies in the intermediate
zone. Reasoning from Carey's equation, Hall (1955)
developed a number of equations to characterize
mound (perched ground water) development in the
intermediate zone. Hall also discusses the hydraulic
energy relationships during lateral flow in perched
ground water. Bear et al. (1968) described the req-
uisite conditions for perched ground-water formation
when a region of higher permeability overlies a region
of lower permeability in the unsaturated zone.
Capillary Fringe
At the base of the intermediate unsaturated zone is
the capillary fringe. The capillary fringe merges with
underlying saturated deposits of the principal water-
bearing formation. This zone is not characterized as
much by the nature of geological materials as by the
presence of water or oil or both under conditions of
saturation or near saturation. Studies by Luthin and
Day (1955) and Kraijenhoff Van DeLeur (1962) have
shown that both the hydraulic conductivity and flux
may remain high for some vertical distance in the
capillary fringe, depending on the nature of the
materials. In general, the thickness of the capillary
fringe is greater in fine materials than in coarse
deposits. Apparently, few studies have been con-
ducted on flow and chemical transformations in this
zone. Taylor and Luthin (1969) reported on a com-
puter model to characterize transient flow in this zone
and compared results with data from a sand tank
model. Freeze and Cherry (1979) indicated that oil
reaching the water table following leakage from a
surface source flows in a lateral direction within the
capillary fringe in close proximity to the water table.
Because oil and water are immiscible, the oil phase
does not penetrate below the water table. Although
many components of hydrocarbons are only very
slightly soluble, the solubility levels greatly exceed the
concentrations deemed safe for consumption. This
can result in a large amount of contaminated ground
water.
Saturated Zone
Two processes control the rate of spreading of im-
miscible hydrocarbons in the saturated zone. The
fluid density, if less than that of water, stabilizes the
plume on the capillary fringe. Lateral spreading of the
plume ceases when the residual oil saturation is
reached behind the spreading plume. If the spilled
material is equal or more dense than the ground
water, the plume will migrate vertically through the
aquifer. This vertical migration may have some
horizontal component which is due to the lateral
motion of the ground-water flow.
The second and potentially more serious component
of spreading is a result of the slight solubility of many
hydrocarbons in water. Although slight, the equi-
librium concentration levels frequently exceed those
deemed safe for human consumption. These dis-
solved constituents move with the ground water and
are affected by the processes of diffusion, mechanical
dispersion, adsorption and retardation, and microbial
degradation. Each of these processes tend to de-
crease the concentration of the original contaminant.
Flowing ground water can dissolve certain com-
ponents from the oil and move them away from the
spill site. Around the oil infiltration zone proper, a
diffusion corona is formed if the flow direction of the
ground water changes often, and a diffusion trail is
formed if the direction of the flow is mostly the same,
leeward of the oil body. This downgradient transport
is shown in Figure 2.6 (the substances are no longer
bound to the capillary fringe but can migrate to lower
parts of the aquifer). Presumably, the boundary
between the oil zone and the diffusion zone is blurred
by a transition zone of minute droplets of oil which are
pulled out of the oil core by escaping surface active
substances and which move ahead of the oil front.
Flow Regimes
Recent studies have demonstrated that soil water
movement in the unsaturated zone is considerably
more complex than the classical concept and that
rapid infiltration to soil depths not predicted by Darcian
flow can occur in soils with continuous or dis-
continuous, or both, structural macropores. Macro-
pores are large channels or fractures through which
fluids may rapidly flow downward with little resistance
to flow. From these macropores, the diffusion of fluid
into the surrounding rock or soil matrix may be very
stow, allowing water or hydrocarbons to travel great
distances downward. Figure 2.7 shows a series of
discontinuous macropores in an otherwise uniform
soil.
The macropore flow phenomena, which is primarily
restricted to highly structured soils, or fractured rocks,
involves the rapid transmission of free water through
large, continuous pores or channels to depths greater
than predicted by Darcian flow. The observation that
a significant amount of water movement can occur in
soil macropores was first reported by Lawes et al.
(1882). Reviews of subsequent work are provided by
2-6
-------
\ Rain \ Ground surface \
Unsaturated zone
Capillary fringe
Water flow
Oil dissolved In water
Saturated zone
Impermeable
Figure 2.6
Hydrocarbon Migration Pattern (Schwille, 1984)
DISCRETE MACROPORE
O SOLID MATERIAL
MESOPORES
Figure 2.7
Composition of a Region of Macropore-mesapore Media
2-7
-------
Whipkey (1967) and Thomas and Phillips (1979).
Macropore flow can occur in soils at moisture contents
less than field capacity (Thomas et al., 1978). The
depth of macropore flow penetration is a function of
initial water content, the intensity and duration of the
precipitation event, and the nature of the macropores
(Aubertin, 1971; Quisenberry and Phillips, 1976).
Macropores need not extend to the soil surface for
flow to occur, nor need they be very large or cylindrical
(Thomas and Phillips, 1979). Exemplifying the role of
macropores, Bouma et al. (1979) reported that planar
pores with an effective width of 90 m occupying a
volume of 2.4 percent were primarily responsible for a
relatively high hydraulic conductivity of 60 cm/day in a
clay soil. Aubertin (1971) found that water can move
through macropores very quickly to depths of 10 m or
more in sloping forested soils. Some researchers feel
that liquid moving in the macropore flow regime may
bypass the soil solution in entrapped or matrix pores
surrounding the macropores and may result in only
partial displacement or dispersion of dissolved con-
stituents (Quisenberry and Phillips, 1978; Wild, 1972;
Shuford et al., 1977; Kissel et al., 1973; Bouma and
Wosten, 1979; Anderson and Bouma, 1977).
The current concept of infiltration in well-structured
soils combines both classical wetting front movement
and macropore flow. Aubertin (1971) found that the
bulk of the soil surrounding the macropores was
wetted by radial movement from the macropores
sometime after macropore flow occurred. A number of
researchers have presented mathematical models in
an attempt to explain the macropore flow phenomena
(Seven and Germann, 1981; Edwards et al., 1979;
Hoogmoed and Bouma, 1980; Skopp et al., 1981).
The occurrence of macropore flow poses implications
for unsaturated zone monitoring and the protection of
ground water. The first implication is that leakage may
flow more rapidly through structured soils or fractured
rock than would be predicted by porous media theory.
Under this short circuit scenario ground-water con-
tamination is probable when a shallow, well-structured
soil is underlain by creviced bedrock (e.g., limestone
solution channels, Shaffer et al., 1979) or a high water
table or both (Anderson and Bouma, 1977). The
second implication is that hazardous constituents
moving with the rapid macropore flow may not be
detected by using traditional soil-monitoring tech-
nology.
Current literature on soil-water movement in the un-
saturated zone describes two flow regimes, the
classical wetting front infiltration of Bodman and
Colman (1944) and macropore flow. The classical
concept of infiltration depicts a distinct, somewhat
uniform, wetting front slowly advancing in a Darcian
flow regime after a precipitation event. Contemporary
models of water flow have this classical concept
combined with the macropore flow phenomena.
Darcian Flow
The fundamental principle of unsaturated and sat-
urated flow is contained in Darcy's law. In 1856,
Henry Darcy, in a treatise on water supply, reported
on experiments of the flow of water through sands.
He found that the flow of water was proportional to the
head loss and inversely proportional to the thickness
of sand traversed by the water. Considering a
generalized sand column with a flow rate, Q, through a
cylinder of cross-sectional area, A, Darcy's law can be
expressed as:
Ah
Q = KA
AL
(2.1)
More generally, the velocity is given by:
Q
A
dh
K
(2.2)
where dh/dL is the hydraulic gradient. The quantity, K,
is a proportionality constant known as the hydraulic
conductivity. The velocity in Equation (2.2) is an
apparent one defined in terms of the discharge and
the gross cross-sectional area of the porous medium.
The actual velocity varies from point to point
throughout the column.
Darcy's law is applicable only within the laminar range
of flow where resistive forces govern flow. As ve-
locities increase, turbulent flows occur and cause
deviations from the linear relation of Equation (2.2) to
become dominant. Fortunately, for most natural
ground-water motion, Darcy's law can be applied.
Unsaturated Flow
When a porous medium becomes less than fully sat-
urated, the hydraulic conductivity sharply decreases.
This is caused by two phenomena. When the medium
begins to desaturate, capillary theory suggests that
the large diameter pores will be the first to drain.
Since these pores can most easily conduct a large
percentage of flow when saturated, their desaturation
significantly reduces the fluid-transmitting properties of
the medium. The second cause for the loss in con-
ductivity is the increased tortuous path length that the
fluid must flow through as a result of the dewatering or
drainage of large pores.
2-8
-------
Another very important concept of multiphase flow is
specific retention. Specific retention stems from the
agricultural literature and loosely refers to the vol-
umetric fluid content retained in a porous media that
has been allowed to drain by gravity. The term is
closely related to the unsaturated hydraulic con-
ductivity. When the media drains, the conductivity is
reduced, and drainage slows. At some point, the
continued drainage is insignificant (from an agricultural
perspective) when compared with the initial drainage.
The specific yield is defined as the volume of water
drained in this process and is equal to the difference
between the saturated porosity and the specific
retention. Figure 2.8 shows the values of porosity,
specific yield, and specific retention for various soils.
From these simple concepts of specific retention, the
theory of multiphase flow may be developed. Given
that little or no flow occurs when the soil moisture is
below a critical level, it can be assumed that the
conductivity or permeability is equal to zero below this
water content. If the second phase in the medium is
oil, it too will have a specific retention or residual
saturation below which the permeability is also zero.
Figure 2.9 graphically shows these results in the form
of a relative permeability plot. The horizontal axis
represents the percent saturation of each fluid while
the vertical axis represents the relative permeability
(Kr) of the medium. The relative permeability is cal-
culated as the ratio of the permeability at a given sat-
uration to the permeability of the medium at 100
percent saturation.
Two-Phase Flow
The flow of two immiscible fluids simultaneously in a
porous medium has been described by van Dam
(1967). van Dam's experimental results showed that:
1. The permeability of a given porous medium to
one fluid in the presence of another fluid is
reduced with respect to single-phase
permeability.
2. The reduction in permeability is dependent on
the wetting of the porous medium by one of the
two fluids.
3. There must be a minimum saturation for each
fluid before the medium is permeable to the
fluid.
Figure 2.9 shows that the flow of oil (non-wetting fluid)
is not possible before an oil saturation of approx-
imately 10 percent is obtained. For water, a minimum
saturation of approximately 20 percent is necessary
before the flow can take place, van Dam's equations
are equally applicable to a water-air system and an oil-
air system. The latter system will apply after the
introduction of a quantity of oil into a permeable
substratum before the oil has penetrated as far as the
ground-water level. In this case, a minimum oil
saturation of approximately 10 to 20 percent (de-
pending on the porous medium) must be achieved
before migration of oil in the substratum can take
place. This minimum saturation is called the residual
oil saturation and represents the quantity of oil which
is held permanently by the porous medium. This
phenomenon limits the migration distance of a given
quantity of oil introduced into a permeable soil. Where
the infiltrated volume of oil is large enough to reach
the ground water, an analogous situation could arise
in the aquifer. The contaminated zone could be re-
stricted to a porous volume large enough to contain
the infiltrated oil at a saturation equal to the residual
saturation.
Three-Phase Flow
The mathematical description of three-phase flow,
although far more difficult than two-phase flow, has
been developed by van Dam (1967). The three-phase
relative permeabilities for air, oil, and water systems
are shown schematically in Figure 2.10-A. Each point
within the triangle corresponds to a different degree of
saturation for air, oil, and water as indicated on the
scales along the sides of the triangle. Equal values for
the relative permeabilities for each of the three phases
are indicated by "isoperms" drawn inside the triangle.
From the diagram it appears that there are large
regions where at least one of the three fluids present
is immobile and that only in a very limited "saturation
region" is simultaneous flow of all three phases
possible. This is more clearly indicated in Figure
2.10-B. Another significant observation is that
residual water saturation is almost the same irre-
spective of the magnitude of oil and air saturations,
while the residual air saturation is largest in the region
where oil and water saturation are the same order of
magnitude. Finally, the residual oil saturation is larger
if no water saturation is present but becomes nearly
constant with increasing water saturations approach-
ing the residual value.
The capillary forces prevailing on introduction of oil
into a partially water-filled porous medium are
governed by very complicated conditions. For the
sake of simplicity, it will be assumed that everywhere
in the porous medium there will be at least a residual
water saturation and that the grains are always
enclosed by a thin water film such as found in wet
soils with a shallow ground-water system. When oil or
any other third phase is introduced into the soil, three-
phased conditions will exist. In this case, water is the
wetting fluid with respect to oil, but oil will, by pref-
2-9
-------
Figure 2.8
Variation of Porosity, Specific Yield, and Specific Retention with Grain Size
(after Bear, 1972)
0%---*-WATER SATURATION 100%
100% OIL SATURATION * 0%
Figure 2.9
Two-phase Flow Relative Permeability (van Dam, 1967)
2-10
-------
too% ao AO 40 30
WATER
Figure 2.10 A
Relative Permeablilities Three-phase Flow (after van Dam, 1967)
WATER
100%
Figure 2.10 B
Three-phase Relative Permeability (after van Dam, 1967)
2-11
-------
erence, wet the water surface exposed to air. Con-
ditions may arise where the ground-water level is
depressed and where oil comes in contact with water
without the presence of air. In this case, two-phased
conditions would prevail.
The following conclusions may be drawn from van
Dam's work concerning the migration of hydrocarbons
in isotropic homogeneous media. Oil will first migrate
downwards in a vertical direction towards the ground-
water table and will then spread in a horizontal
direction parallel to the ground-water table (Figure
2.11). The latter phenomenon is caused by capillary
forces in the air-water capillary zone which prevents
oil from entering the aquifer proper. The presence of
air and water in residual saturations in the porous
medium will reduce the migration velocity of the oil.
The volume of the porous medium which is invaded by
a limited quantity of oil will be restricted for these two
reasons: (a) in all regions through which the oil
passes, a minimum residual saturation must be es-
tablished before the oil can continue to flow, (b) and if
the oil spreads on top of the capillary zone above the
water table, a minimum thickness must be established
before lateral migration can occur. It follows that large
quantities of oil must be introduced into a permeable
substratum before the presence of oil can be observed
at some distance from the source of infiltration. In the
case of an inclined ground-water table, the oil infil-
trated zone will move in the direction of the ground-
water gradient. Oil migration in a direction opposite to
the direction of the ground-water gradient will be
limited to relatively short distances. In a hetero-
geneous porous medium, the migration of oil in the
presence of ground water will, by preference, follow
the most permeable layers. Strong capillary forces in
a porous medium with a low permeability will prevent
the entry of oil into these zones. This is particularly so
in clay environments. The existence of oil infiltrations
in the subsurface can effectively be detected by ob-
servation wells that are perforated throughout the
section covering the capillary zone above the ground-
water table.
Liquid Monitoring Difficulties
and Future Research
The fundamental problem of leaking underground
tanks is that there are many accidental surface spills
over time which create a complex background
condition of spatially and temporarily variable contam-
ination. The residual hydrocarbons in the soil can
occupy from 15 percent to 40 percent or more of the
pore space. The residual hydrocarbons act as a
continual source of contaminants as water comes into
contact with the trapped immiscible phase and
leaches soluble components. Consequently, back-
ground conditions and spillage can result in significant
soil and water contamination.
Qround surface
Oil phase
Unsaturated zone
Capilary fringe
Oil components
dissolved in water
Saturated
zone"
Figure 2.11
Spreading of Oil Spill on Water Table Surface (after Schwille, 1967)
2-12
-------
For the case of a miscible inorganic hazardous waste
moving in the unsaturated zone as two-phase flow or
in the saturated zone as single-phase flow, the
problems are seemingly straightforward. From a
predictive standpoint, one can feel relatively com-
fortable with Darcy's Law and existing ground-water
flow models. The area of concern is that unsaturated
zone models to date have not been able to adequately
predict flow conditions to the satisfaction of most
hydrologists. The reason principally is the site specific
variability of the hydrogeology both vertically and
horizontally in these upper materials. In addition, one
must recognize that the presence of discontinuous
macropores, fractures or fissures, wormholes, and
deep roots, etc., offer pathways for transporting con-
siderable volumes of water.
Various techniques such as neutron probes and
tensiometers (Everett and McMillion, 1985), can be
used to monitor the vertical and horizontal distribution
of soil moisture. One of the fundamental problems of
sampling, however, is related to the sphere of
influence of each of the sampling devices. The sphere
of influence of various unsaturated zone sampling
devices has not been determined, and, consequently,
the number and depths of sampling devices beneath
underground tanks is dependent upon professional
judgment.
When dealing with an increase in soil moisture which
results in a decrease in soil suction, one can monitor
leakage of immiscible and dissolved fractions of
immiscible solution with tensiometers, neutron probes,
or other vadose zone monitoring devices (Everett and
McMillion, 1985). The halo of dissolved hydrocarbon
components which precedes the immiscible phase
offers the opportunity for monitoring in the subsurface.
A critical milestone in unsaturated zone monitoring is
to determine when significant unsaturated flow is
expected to begin. When dealing with residual oil
saturation, however, devices have not been developed
which will monitor the build-up of residual oil in the
unsaturated zone.
High success in collecting soil pore liquid sample
containing volatile hydrocarbons has been obtained by
using ceramic suction lysimeters. The ceramic has a
pore size of approximately 2.3 microns and, as such,
does not allow air to enter. The suction lysimeters are
installed wet (Everett and Wilson, 1984), and, as a
result, all the pore spaces in the ceramic are initially
filled with water. The miscible components of
hydrocarbons, including 1, 1-dichloroethane, trichlo-
roethene, benzene, ethylbenzene, toluene, and meta,
ortho, and para zylenes, have been successfully
sampled by using suction lysimeters. As a result, a
strong case can be made for using suction lysimeters
to detect the dissolved fractions within hydrocarbons
as they migrate in the unsaturated zone. Unfor-
tunately, direct sampling of the immiscible phase is
hampered by the limited oil wettability of the ceramic
material.
From the analysis of van Dam (1967), the water table
acts as a barrier for the downward migration of
immiscible low density organic hazardous wastes.
Based on the specific densities, the hydrocarbons are
pushed up by the water below the oil core, and this
causes the hydrocarbon to spread along the surface of
the water table and ultimately the capillary fringe. One
problem associated with ground-water monitoring
occurs when obtaining a pumped sample. If the cone
of depression lowers the water table in the area of the
leakage (see Figure 2.12), oil is spread over a greater
thickness of aquifer, and, hence the zone of residual
saturation is increased in thickness. A second
problem associated with monitoring hydrocarbons in
the saturated zone is related to the interpretation of
the physical thickness of the hydrocarbon lense. As
demonstrated in Figure 2.12, the hydrocarbon lense
may ride on top of the capillary fringe downgradient
from the leakage area. Although one can perforate an
observation well through the capillary fringe into the
intermediate zone to pick up a hydrocarbon lense, it is
difficult to interpret the thickness of the lense itself. A
mathematical interpretation of this phenomenon is
presented by Zillion and Nutzer (1975) and is repre-
sented in Figure 2.13. A rule-of-thumb has emerged
from their interpretation which indicates that one
typically observes a four-to-one ratio of oil observed in
an observation well to what is actually present as a
hydrocarbon lense.
While the problem of two-phase flow through sat-
urated porous media is complex, several case
histories are available to provide insights into mi-
gration rates. The problem of immiscible two-phase
flow in fractured media is considerably more complex
but has been recently defined by Schwille (1984). The
problem of three-phase flow in the unsaturated zone is
technically, mathematically, and conceptually ex-
tremely challenging. If one superimposes precipitation
events on top of a hydrocarbon leak, the residual
saturation concepts (see Figure 2.14) become even
more complex as the water phase displaces the oil
envelopes. In isolated instances involving dense
hydrocarbons, the oil moves downward through the
water table under gravity forces and offers a more
complex situation.
Physical models including the capillary fringe should
be developed (Schwille, 1984) to achieve a basic
representation of the migration process for chosen
representatives of each hazardous waste group of
substances. Using these models, one could stand-
ardize the determination of retention capacity including
2-13
-------
oil
K = 3.0x10 ~3m/»/. '.J
J = 0.01
10O cm
Figure 2.12
Model Experiment: Influence of Changing the Water Level on the Oil Distribution
(after Schwille, 1967)
instrumentation, soil type, water content, and
methods. In addition, the need exists to develop
typical diagrams for relative permeability for air, water,
and immiscible fluid phases.
It is very clear that the basic understanding of hy-
drology, including the phenomenology and associated
mathematics, needs to be expanded to include liquid
hydrocarbon transport. There is a need for geo-
chemists, petroleum engineers, microbiologists, and
hydrogeologists to work in concert in expanding
hydrogeologic principles to hydrocarbon transport and
monitoring.
2-14
-------
... WATER FLOW....
Figure 2.13 A
Impregnation Body (Petroleum Product) Having Reached Ground Water
(Zillion And Nutzer, 1975)
Figure 2.13 B
Thickness of Layer of Oil in the Ground and in a Strainer Tube (Influence of Capillary Pressures in
the Case of a Continuous Layer of Oil)
2-15
-------
Figure 2.14
Displacement of Oil Envelope by Water - Considered Microscopically
(Schwllle, 1967)
2-16
-------
References
American Petroleum Institute. 1972. The Migration of
Petroleum Products in Soil and Ground Water.
Washington, D.C.
American Petroleum Institute. 1980. Underground,
Spill Clean-Up Manual. Washington, D.C.
Anderson, J.L., and J. Bouma. 1977. Water Movement
Through Pedal Soils: I. Saturated Flow. Soil Sci.
Soc. Am. J. 41:413-418.
Aubertin, G.M. 1971. Nature and Extent of
Macropores in Forest Soils and Their Influence on
Subsurface Water Movement. USDA Forest Serv.
Res. Paper NE-912. Northeast Forest Exp. Stn.,
Upper Darby, PA.
Baehr, A., and M.Y. Corapciaglu. 1984. A Predictive
Model for Pollution from Gasoline in Soils and
Ground Water. In: Petroleum Hydrocarbons and
Organic Chemicals in Ground Water, NWWA,
Houston, TX, November.
Bear, J. 1972. Dynamics of Fluids in Porous Media.
American Elsevier, New York, New York.
Bear, J., D. Zaslavsky, and S. Irmay. 1968. Physical
Principles of Water Percolation and Seepage. UN
Educational, Scientific and Cultural Organization.
Seven, K., and P. Germann. 1981. Water Flow in Soil
Macropores. 2. A Combined Flow Model. J. Soil
Sci. 32:15-29.
Bodman, G.B., and E.A. Coleman. 1944. Moisture and
Energy Conditions During Downward Entry of
Water into Soils. Soil Sci. Soc. Amer. Proc., 8:
116-122.
Bohn, H.L, B.L. McNeal, and G.A. O'Connor. 1979.
Soil Chemistry. Wiley Interscience, New York.
Bouma, J., A. Jongerius, and D. Schoondebeek. 1979.
Calculation of Hydraulic Conductivity of Some
Saturated Clay Soils Using Micromorphometric
Data. Soil Sci. Soc. Am. J. 43:261-265.
Bouma, J., and J.H.M. Wosten. 1979. Flow Patterns
During Extended Saturated Flow in Two
Undisturbed Swelling Clay Soils with Different
Macrostructures. Soil Sci. Soc. Am. J. 43:16-22.
Bouwer, H. 1978. Groundwater Hydrology. McGraw-
Hill, New York.
Convery, M.P. 1979. The Behavior and Movement of
Petroleum Products in Unconsolidated Surficial
Deposits. M.S. Thesis, U. of Minn.
Davis, S.N., and R.J.M. De Wiest. 1966.
Hydrogeology. John Wiley and Sons, New York.
Dunlap, W.J., and J.F. McNabb. 1973. Subsurface
Biological Activity in Relation to Ground Water
Pollution. EPA-660/2-73-014. U.S. Environmental
Protection Agency, Corvallis, Oregon.
Eames, V. 1981. Influence of Water Saturation on Oil
Retention Under Field and Laboratory Conditions.
M.S. Thesis, U. of Minn.
Edwards, W.M., R.R. Van Der Ploeg, and W. Ehlers.
1979. A Numerical Study of the Effects of Non-
Capillary Sized Pores Upon Infiltration. Soil Sci.
Soc. Am. J. 43:851-856.
Everett, L.G. 1980. Groundwater Monitoring. General
Electric Co. Technology Marketing Operations,
Schenectady, New York.
Everett, L.G., and L.G. McMillion. 1985. Operational
Ranges for Suction Lysimeters. Groundwater
Monitoring Review 5(3): 51-60.
Everett, L.G., and L.G. Wilson. 1984. Unsaturated
Zone Monitoring for Hazardous Waste Land
Treatment Units. EPA/530-SW-84-016.
Environmental Monitoring Systems Lab., Office of
Research and Development, U.S. Environmental
Protection Agency, Las Vegas, NV.
Freeze, R.A., and J.A. Cherry. 1979. Groundwater.
Prentice-Hall, Englewood Cliffs, New Jersey.
Hall, W.A. 1955. Theoretical Aspects of Water
Spreading. Amer. Soc. Agric. Eng. 36(6): 394-397.
Hillel, D. 1971. Soil and Water Physical Principles and
Processes. Academic Press, New York.
Hoogmoed, W.B., and J. Bouma. 1980. A Simulation
of Model for Predicting Infiltration Into Cracked
Clay Soil. Soil Sci. Soc. Am. J. 44:485-462.
Kirkham, D., and W.L. Powers. 1972. Advanced Soil
Physics. Wiley Interscience, New York.
Kissel, D.E., J.T. Ritchie, and E. Burnett. 1973.
Chloride Movement in Undisturbed Swelling Clay
Soil. Soil Sci. Soc. Am. Proc. 37:21-24.
2-17
-------
Klute, A. 1965. The Movement of Water in
Unsaturated Soils. In: The Progress of Hydrology,
Proc. First Int. Seminar for the Hydrology Prof.,
National Science Foundation Science Seminar,
Dept. of Civil Eng., U. of Illinois, July 13-25.
Kraijenhoff van deLeur, D.A. 1962. Some Effects of
the Unsaturated Zone on Nonsteady Free-Surface
Groundwater Flow as Studied in a Scaled
Granular Model. J. Geophys. Res., 67(11): 4347-
4362.
Lawes, J.B., J.H. Gilbert, and R. Warrington. 1882. On
the Amount and Composition of the Rain and
Drainage Waters Collected at Rothamsted.
William Clowes and Sons, Ltd, London.
Luthin, J.M. (ed). 1957. Drainage of Agricultural
Lands. Amer. Soc. Agron., Madison, Wisconsin.
Luthin, J.M., and P.R. Day. 1955. Lateral Flow Above
a Sloping Water Table. Soil Sci. Soc. Amer. Proc.,
19:406-410.
McWhorter, D.B., and J.A. Brookman. 1972. Pit
Recharge Influenced by Subsurface Spreading.
Ground Water 10(5): 6-11.
Quisenberry, V.L., and R.E. Phillips. 1976. Percolation
of Surface Applied Water in the Field. Soil Sci.
Soc. Am. J. 40:484-489.
Rhoades, J.D., and L. Bernstein. 1971. Chemical,
Physical and Biological Characteristics of Irrigation
and Soil Water. In: Water and Water Pollution
Handbook, Vol. 1, L.L. Ciaccio, (ed)., pp. 141-222.
Marcel Dekker, Inc., New York.
Schwille, F. 1967. Petroleum Contamination of the
Subsoil - A Hydrogeological Problem. In: The Joint
Problems of the Oil and Water Industries.
Proceedings of a symposium held at the Hotel
Metropole, Brighton, 18-20 January.
Schwille, F. 1981. Groundwater Pollution in Porous
Media by Fluids Immiscible with Water. In: Quality
of Groundwater. Proceedings of a Symposia,
Noordwijkerhout, the Netherlands. Elsevier,
Amsterdam.
Schwille, F. 1984. Migration of Organic Fluids
Immiscible with Water in the Unsaturated Zone. In
Pollutants in Porous Media. Ecological Studies
47, Springer -Verlag.
Shaffer, K.A., D.D. Fritton, and D.E. Baker. 1979.
Drainage Water Sampling in a Wet, Dual-Pore Soil
System. J. Environ. Qual. 8:241-246.
Shuford, J.W., D.D. Fritton, and D.E. Baker. 1977.
Nitrate-Nitrogen and Chloride Movement Through
Undisturbed Field Soil. J. Environ. Qual. 6:736-
739.
Skopp, J., W.R. Gardner, and E.J. Tyler. 1981. Solute
Movement in Structured Soils: Two-Region Model
with Small Interaction. Soil Sci. Soc. Am. J.
45:837-842.
Taylor, G.S., and J.N. Luthin. 1969. The Use of
Electronic Computers to Solve Subsurface
Drainage Problems. Water Resources Research
5(1): 144-152.
Thomas, G.W., and R.E. Phillips. 1979. Consequence
of Water Movement in Macropores. J. Environ.
Qual. 8:149-152.
van Dam, J. 1967. The Migration of Hydrocarbons in
a Water Bearing Stratum. In: The Joint Problems
of the Oil and Water Industries. Proceedings of
the symposium held at the Hotel Metropole,
Brighton, 18-20 January.
van Schilfgaarde, J. 1970. Theory of Flow to Drains.
Advances in Hydroscience. 6:43-106.
Whipkey, R.Z. 1967. Theory and Mechanics of
Subsurface Storm Flow. pp. 155-260. In: W.E.
Sopper and H.W. Lull (ed). Int. Symp. on For.
Hydrol., Natl. Sci. Found., 29 August - 10
September 1965, Penn. State U., University Park,
PA. Pergamon Press, NY.
Wild, A. 1972. Nitrate Leaching Under Bare Fallow at
a Site in Northern Nigeria. J. Soil Sci. 23:315-324.
Wilson, L.G. 1971. Observations on Water Content
Changes in Stratified Sediments During Pit
Recharge. Ground Water 9(3): 29-40.
Zillon, F., and J. Nutzer. 1985. Protection of
Groundwater from Oil Pollution. Prepared by
CONCAWE, Special Task Force No. 11, April
1979.
2-18
-------
SECTION 3
Vapor Transport and Its Implications to
Underground Tanks
Introduction
Up until recent times, there had been little scientific
interest in vapor movement around buried storage
tanks. Most studies of gaseous migration just below
ground surface were carried out by agronomists or
agricultural scientists interested primarily in properties
related to soil aeration, fumigation, and denitrification.
However, recently there has been a great increase in
the attention directed to unsaturated zone vapor flux
around buried tanks; this increased attention has been
fueled by a growing realization of the importance and
utility of the unsaturated zone flux in detection of leaks
and contaminant plumes.
In the underground storage tank environment, gas-
eous movement can be a significant component of
overall migration of leaked product, particularly if that
product is highly volatile. Movement of compounds in
the vapor phase through advection or diffusion or both
processes can and often does occur in all directions
from a leak source. Therefore, a portion of volatile,
leaked product vapor could migrate in a direction
opposite of the underlying ground-water flow (Figure
3.1). The vapor could then enter the ground-water
system by redissolving across the capillary fringe and
water table and could appear to be liquid pollutant
moving up the hydraulic gradient.
This outward flux of gases in the unsaturated zone
has important implications to leak detection. A
gaseous phase detector would be less likely to "miss"
a gas moving radially outward, than a monitoring well
attempting to detect a finite plume moving down the
hydraulic gradient in the aqueous phase. Because
vapor phase monitors are normally emplaced at
shallower depths than aqueous phase samplers, there
is some emplacement cost saving with vapor detection
systems in the unsaturated zone. There are some
situations where vapor detection devices would be
more effective than their liquid detection counterparts
in the saturated zone, particularly where the saturated
zone is at great depth below the ground surface and
leak source. Several factors, such as background
contamination and geologic stratification, can have a
negative effect on leak detection in the vapor phase,
but monitoring of gases in unsaturated porous media,
while not perfect, has been shown to be a useful tool if
interpreted correctly. Recent legislation in the State of
California has recognized the value of gaseous moni-
toring and has opened the way for its use in situations
of contamination.
The following section will describe vapor transport
surrounding underground storage tanks, and will
discuss naturally occurring and man-made gases in
the subsurface, mechanisms of transport with par-
ticular attention to transport in the underground tank
environment, existing types of vapor monitoring meth-
odologies, requirements and shortcomings of present
vapor detection systems and theory, and future direc-
tions of vapor-leak monitoring.
Sources of Gases in the
Unsaturated Zone
The most prevalent gas exchange process governing
the composition of the soil atmosphere is respiration;
the biological consumption of O2 and the concurrent
production of CO2. Primary mechanisms of respira-
tion include:
a. The oxidation of organic materials used as an
energy source by aerobic bacteria and the
production of CO2 during their metabolic
activity and
b. The oxidation of synthesized organic
compounds by plant roots and the
corresponding production and liberation of CO2.
An important ramification of respiration involves a
gradual increase in the CO2 concentration in the
unsaturated zone from about 0.03 percent at the soil
surface to 1-5 percent beneath the plant rooting zone
(Bolt and Bruggenwert, 1976). With depth, oxygen
exhibits a decrease in concentration correlating with
progressive increase in CO2 levels.
Because of the formation of the CO2 /O2 gradient, the
ambient soil gases continually move to equilibrate with
the outer atmosphere (21 percent O2, 0.03 percent
CO2). This produces replenishment of O2 into the
subsurface with simultaneous emission of CO2 at the
soil-air interface. Because the ability of CO2 and O2 to
3-1
-------
MONITORING WELL
IMPERVIOUS CONFINING LAYER
co
to
GASEOUS MIGRATION
OPPOSITE GROUNDWATER FLOW
WATER TABLE \ / VOLATIZATION
t t i
DIRECTION OF GROUNDWATER FLOW
REDISSOLUTION
Figure 3.1
Volatlzed Product Vapor Migration Opposite Ground-water Flow
-------
move out of and into the unsaturated zone is largely a
function of continuous porosity, soil media of low
porosity (e.g., well graded soils) will seriously impede
the aeration process, and oxygen concentrations will
drop to near zero values, while there will be a cor-
responding buildup in CO2 concentrations.
In such oxygen deficient environments, processes of
anaerobic reduction and gaseous formation can occur.
Anaerobic bacteria use oxygen previously bound to
nitrogen, carbon, or other elements to sustain life
processes. Through a reductive process to support
their metabolic activity, these bacteria strip oxygen
from compounds such as carbon dioxide (CO2) and
nitrate (NO3), and liberate gaseous end products such
as methane and ammonia.
While the soil atmosphere consists primarily of
naturally occurring gases, the use of fumigants and
pesticides has contributed significantly to the
concentrations of subsurface gases where applied.
Chloropicrin, ethylene dibromide, methyl bromide,
carbon disulfide, and numerous other fumigants are
frequently added to the soil in an effort to control
parasitic fungi, nematodes, and weeds. These fumi-
gants vary considerably in their diffusivity and inter-
action with the soil environment and, therefore, have
variable influences on the soil atmosphere where
applied. Pesticides are usually applied to the soil in
the solid or aqueous phase. The rate at which pesti-
cides volatilize is affected by their chemical properties,
soil characteristics, and environmental conditions.
The vapor density of an applied pesticide therefore
depends on the combination of these factors in the soil
environment.
Spills and dumping of chemical products onto the
ground surface or into dry wells can also have a major
effect on soil vapor concentrations. This problem is of
particular importance in underground tank leak de-
tection because tank product is often spilled on the
ground surface during many filling and withdrawal
operations, and a background of product vapors will
exist in the vadose zone without a leak being present.
Surface spills of fluids identical to those contained in
an underlying tank constitute one of the most serious
threats to accurate gaseous leak detection. Back-
ground vapor concentrations in the unsaturated zone
surrounding a tank could cause false positives and
could reduce leak detectability. This is shown graph-
ically in Figure 3.2.
Bacterial degradation of pesticides and leak products
should be mentioned as a secondary source of vapors
in the unsaturated zone. One important process
involves the decomposition of synthetic organic com-
pounds by soil microorganisms which liberate simple
gases such as NH3 and CO2. For example, Mckee et
al. (1972) describe the utilization of gasoline and other
petroleum products as a food source and consequent
release of CO2 by the bacterial genera Pseudomonas
and Arthrobacter.
Finally, downward percolating rainfall can bring dis-
solved gases, picked up in the atmosphere, into the
unsaturated zone. For example, the fluorocarbons F-
11 and F-12 which are used as refrigerants or spray-
can propellents are ubiquitous in most shallow vadose
zone air that has been measured, probably as a result
of coming out of solution from rainwater that has
infiltrated. Thus, gases in the unsaturated zone can
have many different sources and concentrations.
Vapor from leaking tanks enters this variable en-
vironment.
Factors Which Effect Movement
of Gases Near Underground
Storage Tanks
The mobilities of gases near underground storage
tanks are affected by a great number of complex,
sometimes interrelated parameters. Aspects of the
local geology, such as the geometry, homogeneity,
and bulk composition of the local stratigraphy, and
tank design and tank installation are the most
important factors that impact gaseous behavior, next
to leak size and shape. (Leak source strength and
configuration will be discussed later in this section.)
Rate of gas exchange between the soil air and outer
atmosphere depends greatly on the resistance local
stratigraphy imposes on gas mobility. Stratigraphic
factors influencing gaseous transport include: (a)
differences in soil types, (b) soil and rock heteroge-
neities, and (c) geological discontinuities.
Differences in soil types can have significant impact
on gas mobilities. A clean gravel, for instance, would
act as a conduit for soil gases because of lesser
resistance to vapor diffusion and advection than tightly
packed silts, clays, or poorly sorted material. An air
impermeable clay lens, on the other hand, will act as a
vapor barrier and would therefore deflect gases into
surrounding mediums. If water is removed from the
same clay lens, however, shrinkage might occur, and
the resultant microfissures would then act as a sec-
ondary conduit for gases.
Heterogeneities in soil and rock also influence gas
mobilities. A gas slowly diffusing through a mod-
erately permeable loam soil might, for example,
encounter a buried stream channel of well-sorted
sands and gravels. The porous sand and gravel
material would then act as a vapor sink, and the gas
would rapidly diffuse into the more porous medium. In
3-3
-------
VAPOR MONITOR
a
\\ FLUID SPILLS
r-?!! //&/&/*//*
VOLATIZED PRODUCT
UNDERGROUND STORAGE TANK
FALSE POSITIVE POSSIBLE
WATER TABLE
MONITORING WELL
DIRECTION OF GROUNDWATER FLOW
Figure 3.2
False Positives due to Background Vapor Concentrations
-------
response to the diffusion increase, a concentration
gradient would be established and would result in the
movement of additional gas from the source toward
the buried stream channel. A preferred orientation of
gaseous flow would be established from gas source to
vapor sink.
Subsurface discontinuities such as faults can increase
vapor mobilities by creating a conduit for gases along
the fault face, fissure, or other conduit, or can de-
crease potential movement of gases by the dis-
placement of a gas impermeable layer adjacent to a
zone where gases could move more easily. There-
fore, vapor transport either could be enhanced or
diminished in areas which have experienced sig-
nificant rock displacement events.
Clearly the potential for gaseous movement in an
underground tank environment hinges significantly on
the composition, geometrical nature, and physical
properties of the stratigraphic medium. Another
important factor influencing gaseous behavior in the
subsurface is related to the design and installation of
the underground tank.
The following is a list of subsurface tank installation
specifications outlined by the American Petroleum
Institute which directly influence gas mobilities:
1. At least 15 cm, and preferably 30 cm of clean,
well sorted sand or gravel should be placed
under the tank.
2. Tanks should be surrounded with at least 15
cm of noncorrosive, inert, and uniform material
such as clean, uniform sand or gravel.
3. In areas of little traffic, the tank should be
covered by a minimum of 60 cm or not less than
30 cm of well tamped, uniform material plus a
10 cm reinforced concrete slab.
4. In areas of heavy traffic, the tank should be
covered by at least 90 cm, or not less than 45
cm of well tamped, uniform material plus 15 cm
of reinforced concrete or 20 cm of asphalt
concrete.
The use of uniform, well sorted sands and gravels
around underground storage tanks, emplaced for
proper structural support and corrosive resistance,
creates a medium into which both naturally occurring
and leaking product gases are diffusible. The
movement of these gases, whether into the outer
atmosphere or into adjacent soil or rock mediums,
critically depends on the interrelationship between the
amount of gas present, the air permeability or dif-
fusivity of the adjacent soil medium, and the presence
or absence of asphalt or reinforced concrete at the
ground surface. For example, volatilized product
leaking from a buried tank overlain by an extensive
concrete cover would tend to have greater transport
laterally and downward into the surrounding fill
material than would a similar leaking tank without a
concrete ground surface cover. In the latter case, the
uncovered ground surface would act as a math-
ematical sink, that is, volatilized product would move
preferentially, though not exclusively, toward ground
surface. If the soil material adjacent to the tank is
essentially impermeable to air, the leaking gases
would move along any existing fractures or other open
conduits, particularly to ground surface if such frac-
tures existed. The movements of gases are therefore
partly dependent on the combination of surface and
stratigraphic variables at a given site.
Tank liners may also influence gaseous transport in
the subsurface. Liners are commonly installed be-
neath underground storage tanks to help prevent
ground-water contamination if a leak occurs. The liner
material used, such as synthetic membranes or clay,
will restrict gaseous movements into adjacent soils if
its integrity remains intact. Any volatile product would
therefore preferentially move toward ground surface at
uncovered buried tank sites where properly sealed
liners are used.
Importance of Vapor
Transport Surrounding
Underground Storage Tanks
Pollutant Migration
Because many of the substances in underground
storage tanks are moderately to highly volatile, a
leaking tank often involves the evaporation of some
liquid into the gaseous phase, and resulting vapor
transport in all directions into the soil. In some field
studies, this vapor movement has been interpreted to
comprise a significant component of overall con-
taminant migration. Therefore, an understanding of
the movement of leaking tank contents in the vapor
phase may be required, in addition to understanding
the liquid phase, to correctly predict the migration of
potential contaminants.
The gaseous behavior of tank products, such as
petroleum hydrocarbons, is largely a function of the
diffusivity and air permeability of the vapor in the
porous media of concern, and its distribution co-
efficients describing interactions with soil particles and
moisture. The potential for contamination of soils and
ground water in a particular tank location depends on
3-5
-------
the combination of these parameters at that site.
Even so, the following general statements may be
made outlining the contribution of the vapor phase to
contamination near a underground tank.
1. Vaporization may remove some of the potential
subsurface contamination as volatilization
allows a portion of the leaked compound to be
released into the outer atmosphere. This also
implies that an above-ground surface problem
could be created if dangerous gases were
allowed to build up (in a building or other
surface structure) by vapor flux upward across
the ground surface.
2. Despite statement 1, additional contact with
waters percolating in the unsaturated zone is
made possible through vapor transport, in
which larger areas of ground water and soil
surfaces may be contaminated.
3. The areal extent of possible vapor phase
transport and contamination is largely
dependent upon several factors including: the
gaseous transport properties of the soil, the
volatile properties of the tank product, and
the interaction of the resultant vapor with
soil particles and water contained in the
porous medium.
Leak Detection
Although the movement of vapors from leaking tanks
in the unsaturated zone presents an additional threat
of soil and ground-water contamination, beneficial
aspects of vapor transport may be realized from
economical leak detection utilizing vapor sensing
methods. Vapor detection sensors adjacent to leaking
underground tanks have some advantages over
conventional static line and volumetric leak detection
methodologies, e.g., vapor detection methods do not
require the tanks to be topped off with product and
may proceed without the disruption of operations
associated with static line and volumetric testing
methods.
Models may be formulated to correlate the concen-
tration of vapor detected with the amount of product
leak by defining and calibrating a set of parameters
(e.g., effective diffusion coefficient, air permeability,
the vapor/liquid partitioning ratio of the particular tank
components). Unfortunately, not only is precise
calibration of these parameters difficult to achieve, but
also the effect of many environmental factors is
usually unknown. Important variables, such as the
sorption of gases on the soil matrix, their solubility in
water, and amount of vapors attributable to accidental
spillage during tank filling would greatly affect not only
quantitative correlation, but also impact qualitative
assessment of leaks. For example, a tank leak of
acetone would likely release appreciable vapor,
because vapor pressure (184.5 Torr at 20°C) of ace-
tone is high relative to many other solvents. However,
acetone is miscible with water in all proportions, and if
there is a high moisture content in the environment
outside the leaking tank, the gaseous migration and
subsequent sensing of this solvent could be impaired
relative to other compounds less soluble in water.
Therefore, although attempts can be made to estimate
leak location and amount, meaningful interpretation of
subsurface vapor concentration is often impossible.
Vapor Transport Processes
The following is a description of gaseous diffusion,
advection (also called transpiration, forced diffusion,
laminar viscous flow, convective flow, or bulk flow),
and effusion (also called free-molecule or Knudsen
flow) which are mechanisms of vapor transport
originally discussed by Thomas Graham in the 19th
Century (Mason and Evans, I969).
Gaseous Diffusion
Diffusion is a process whereby elements in a single
phase equilibrate. This process arises because of
random molecular motions; each molecule is con-
stantly undergoing collision with other molecules, and
the result is constant motion with many changes in
direction and no preferred direction of motion. Al-
though this motion is random, there is a net transfer of
molecules of one compound from regions of high
concentration to low concentration. The net transfer of
molecules is predictable, whereas the direction of any
individual molecule at any moment is not. Gaseous
diffusion is a spreading out or scattering of gases and
may be divided into the categories of ordinary gas-
eous diffusion, thermal gaseous diffusion, and particle
diffusion.
Ordinary gaseous diffusion is a process in which the
components of any gas-filled space will eventually
become thoroughly mixed. Taylor and Ashcroft (1972)
outline the rate potential of ordinary gaseous diffusion
in the soil atmosphere, stating that no ordinary
diffusion will take place if the density (concentration) of
the diffusing substance is the same throughout a given
region. Further, if the density of the diffusing sub-
stance is different at different points, diffusion will take
place from points of greater to lesser density, and will
not cease until the density at all places is the same.
Therefore it follows that ordinary gaseous diffusion of
each gas component in the subsurface will occur
3-6
-------
whenever there is a difference in its concentration
between (a) the soil and outer atmosphere, or (b) at
points within the soil because of irregularities in the
consumption and release of gases.
The rate at which ordinary gaseous diffusion occurs is
governed, in part, by the magnitude of the con-
centration gradient at a point. In turn, the magnitude
of the concentration gradient is dependent upon soil
type, soil homogeneity, and amount of gas generated
by a source. The interrelationships between these
factors will govern the extent in which ordinary
gaseous diffusion becomes a major source of unsat-
urated zone vapor transport.
Ordinary gaseous diffusion is normally described by
an equation written by Pick in 1855 after the principles
described by Graham and in analogy with Fourier's
Law for heat conduction. Pick's Law, as the equation
has become known, relates diffusive flux to a
concentration gradient over distance multiplied by an
empirically derived coefficient called the effective
diffusion coefficient. The magnitude of the effective
diffusion coefficient is dependent of the properties of
the porous media (tortuosity, porosity, moisture
content) and properties of the diffusing gas.
Thermal gaseous diffusion occurs when a temperature
gradient is established in a gaseous mixture. The
non-uniformity of temperature results in a con-
centration gradient as denser molecules move down
temperature gradient while less dense molecules
move in the direction of increasing temperature (Grew
and Ibbs, 1952). Causes of thermal gradients near
subsurface tanks may vary from heating of indoor
facilities, to radiant energy absorbed by surface
concrete, to proximity to a geothermal source. The
greater the temperature gradient, the more effectively
a gaseous mixture may be separated into its denser
components. Thermal diffusion is a slow process,
however, and is not usually considered to contribute
significantly to vapor transport in an underground tank
environment.
Particle diffusion refers to diffusion of ions to or from
exchange sites on soil particles. Hellfereich (1962)
states the chemical potential for interactions between
the gas molecules and a sorptive matrix depends on:
(a) the generation of diffusion induced electric forces,
(b) the adsorbent selectivity or preference for a
particular gas, and (c) specific interactions such as
Coulombic forces, Van der Waals forces, and hydro-
gen bonding.
Particle diffusion is significant when interactions
between the exchange medium and diffusing gas
molecules are great. Often an equilibrium condition is
attained, however, in which the contribution of ions to
the particle site are matched by those returning to the
gas phase. Particle diffusion, in such instances,
makes insignificant contributions to the concentration
gradient and, thus, has little influence on vapor
densities.
Advectlon
Advection (also called bulk flow, convective flow,
laminar viscous flow, transpiration, and forced dif-
fusion) is a transport process in which a gas moves in
response to a pressure gradient. Unlike diffusion,
advection occurs as a bulk flow, i.e., the mixture of
gases has no tendency to migrate according to the
concentrations of its separate gaseous components; it
behaves as one gas. This is because any diffusive
effects that would vary the movements of individual
gases according to their concentration are minor in
comparison to the overall pressure gradient flow. In
advection, the gas acts as a fluid continuum driven by
the pressure gradient.
Movement of gases in the unsaturated zone through
advection is governed partly by the air permeability of
the porous medium and partly by the pressure created
by the gaseous source. For example, if a gas were to
form rapidly in a slightly permeable medium, a
pressure gradient would be established forcing the
gas to move through the medium. Other factors which
could cause pressure gradients influencing advection
include barometric pressure changes, rise and fall in
water table, wind fluctuation, and rainfall percolation.
Laminar viscous flow of a fluid through porous media
is usually described by Darcy's Law (analogous to
Fick's and Fourier's Laws) which is an empirical
equation relating fluid flux to a pressure gradient over
distance times a constant of proportionality called
permeability or, in the case of water flow, hydraulic
conductivity. Permeability, in turn, is dependent on
properties of the flowing fluid and the porous media
through which it flows.
Klinkenberg (1941) pointed out that gases do not stick
to pore walls as required in Darcy's Law and that a
phenomenon known as "slip" occurs. This gives rise
to an apparent dependence of permeability on
pressure, referred to commonly as the Klinkenberg
effect, and often must be considered in mathematical
descriptions of advective flow.
Thermal advection is another type of flow and is
caused by a thermally induced pressure gradient. As
temperature increases in unsaturated porous media,
the resultant increase in molecular energies results in
increased pressure exerted by the gaseous con-
stituents. These heated gases then flow in an effort to
reestablish a pressure equilibrium.
3-7
-------
The effectiveness of thermal advection depends
somewhat on the thermal regime of the soil. A soil
medium with low thermal conductivity can create a
greater thermal gradient over a given distance
because heat is contained. Air is a poor thermal
conductor, therefore thermal advection is enhanced
(because of high temperature gradient development)
in soils with low bulk density (high air content). But
the overall effect of thermal advection as a transport
mechanism is considered to be limited. Currie (1971)
cites Keen on the question of thermal advection as a
soil aeration mechanism (Keen, 1931):
Keen pointed out that because there is a phase
lag in temperature at depth (it is 12 hours at 30
or 40 cm), there are periods during summer
days when the surface is colder than the soil at
depth. He calculated the amount of convective
flow, and hence exchange, that might occur in
those circumstances and decided that, in the
absence of any suitable air entry point at depth,
it is extremely unlikely that this form of
ventilation will occur to any useful degree.
However, if a heat generating source below ground
surface exists, thermal advection could become
significant.
Effusion
In porous media where pore spaces are extremely
small, collisions between gas molecules can be
ignored compared to collisions of molecules to the
surrounding walls. If molecule-wall collisions dom-
inate, the flux of molecules through any shape pore
space is equal to the number of molecules entering
the pore space times the probability that any one
molecule will pass through and not be deflected back
the way it entered. This forward flux is called effusion,
free-molecule, or Knudsen flow and its mathematical
description is different from diffusion or advection
processes. Because underground storage tank ex-
cavations are normally backfilled with coarse sand or
gravel, effusion is unlikely to be a dominant driving
force in the immediate tank surroundings.
Combinations of Transport Processes
The previously described vapor transport processes
can and often do occur in combination. Effusion and
advection in combination leads to the phenomenon,
discovered experimentally in 1875 by Kundt and
Warburg, known as "viscous slip." Advection and
diffusion processes occurring in concert were called
"diffusive slip" when first discussed by Kramers and
Kistemaker in 1943. A third combination, that of
effusion and diffusion, was researched extensively in
the early to mid-1940's in support of work on isotope
separation. And all three processes of diffusion,
advection, and effusion could be significant in a given
field situation and could be described by yet another
combination. This variability in types of transport
processes points out the need to clearly understand
gaseous migration in any given field situation before
accepting a mathematical representation of its mi-
gration in that situation.
Vapor Transport Processes
in the Underground Storage
Tank Environment
The primary mechanisms of vapor transport in the
underground tank environment are ordinary gaseous
diffusion and isothermal advection. If a leak is small
and there is no great pressure buildup, vapor transport
of the volatile contents away from the tank will occur,
in large part, through ordinary gaseous diffusion. The
concentration gradient is established between the
leaked volatile product and uncontaminated soil air
results in the movement of the leaked vapor usually in
all directions into the soil. Diffusion of the gaseous
form of a leaked product will continue indefinitely into
the soil atmosphere and outer atmosphere until the
concentration gradient is eliminated. For cases of a
high pressure gas leak or a quickly forming gas,
sufficient pressure may build up in the soil gases to
create advective flow.
While the effective diffusion coefficient and air per-
meability are substantively different coefficients of
proportionality, both are dependent on properties of
the porous media. Both coefficients are affected by
differing media tortuosity, porosity, moisture content,
texture, and other aspects of geometry. Both coef-
ficients characterize the ease with which a fluid can be
made to move (by diffusion or flow) through porous
material and, as defined in Pick's or Darcy's Law, are
macroscopic properties of the soil.
Most underground storage tanks are surrounded with
a uniform, coarse-grained backfill to reduce tank
corrosion. In this situation, rapid diffusion and ad-
vection will be promoted.
There are other sources of pressure gradients which
can influence advective motion in the unsaturated
zone around tanks. These influences include both
man-made and environmental factors. Some man-
made factors are tank installation and air rotary/air
percussion drilling, while environmental influences
include barometric pressure changes, water table
fluctuations, rainwater percolation, and the effects of
wind. There are also several factors that modify vapor
3-8
-------
migration by changing the direction, speed, or nature
of a moving gas. These include media heterog-
eneities, sorption, and microbiological transformation.
Most underground tank environments will experience
significant changes in barometric pressure. Surface
atmospheric pressure fluctuations are not immediately
transmitted below ground surface, but arrive lagged in
time. There is currently some debate over the extent
to which barometric pressure variations augment
gaseous exchange across the ground surface. It is
thought by some that an increase in barometric
pressure would compress the volume of the soil
atmosphere and permit a volume of atmospheric air to
penetrate the soil. A decrease in barometric pressure
would have the opposite effect, causing some of the
soil air to enter the outer atmosphere. This effect
would permit a flux of naturally occurring and leaking
product vapors to be aerated out of the storage tank
environment.
The significance of barometric pressure as an ad-
vective flow mechanism is in question. Buckingham
(1904) determined that barometric changes causing
the penetration of atmospheric air will amount to only
3 to 5.6 mm of mercury in a 3 meter permeable soil
column. From this estimate it would appear that
barometric effects as an advective mechanism are
minimal at best. However, experimentation in regions
with deep water tables indicates that in these sit-
uations, barometric fluctuations may have a significant
effects on the near surface gaseous regime.
Advective movements can be caused by fluctuations
in the water table in an underground storage tank
environment. An increase in water table height would
result in the displacement of a similar volume of soil
air once equilibrium is attained. The opposite effect
accompanies a falling water table. Therefore, a leak
product may move into the outer atmosphere or
deeper into the soil, accompanying water table fluc-
tuations.
Surface rainfall can have an impact on vapor move-
ments by creating an advancing wetting front which
traps soil air between the front and the water table as
it percolates downward. This creates greater pressure
on the soil air. A large portion of the soil air remains
entrapped in soil pores and is not affected by the
advancing wetting front. Rommel (1922) showed that
rainfall produces about 1/12 to 1/16 of the normal soil
aeration. Therefore, rainwater percolation could
become significant as an aeration mechanism in
regions of high precipitation, given the right sub-
surface conditions.
The pressure and venting effects of wind as an
advection mechanism exert little influence over the
exchange of soil air with the outer atmosphere.
Rommel (1922) concluded that wind action is not
responsible for more than 1/1000 of the normal
aeration of a vegetated soil. A more porous soil might
experience significantly greater venting effects. The
effects of wind over a leaking underground tank
environment will depend on the areal extent of con-
crete groundcover, if one exists, and the porosity of
the tank backfill.
Environmentally induced advection mechanisms gen-
erally have only a minor effect on vapor transport.
Given the right conditions, however, these mech-
anisms may exert a significant influence on vapor
transport in a leaking underground storage tank
environment. More information is needed to accur-
ately quantify these effects.
Tank installation will completely upset the "normal"
subsurface gaseous regime by allowing air from the
atmosphere to mix with backfill material. If a steady-
state vapor condition can possibly be achieved after
excavation in a given area, the amount of time nec-
essary to establish this equilibrium would be difficult to
mathematically predict.
Air drilling, whether rotary or air percussion, also
perturbs the surrounding subsurface environment by
large losses of circulating air from the borehole into
the formation. Loss of circulating air from a drilled
borehole can be at least 10 to 30 percent. If air is
circulated at average rates of 20 m3/min, massive
quantities of air would be lost, much into the formation,
during the normal time it would take to drill. Again,
recovery to any preexisting steady-state condition
would be almost impossible to accurately predict
mathematically.
Probably the most important consideration in any
process of mass transport through porous media is the
effect of heterogeneities on movement. Although
many tanks are backfilled uniformly after em-
placement, unequal tamping or settling could modify
the normally uniform material or perhaps its uniform
moisture distribution. Many heterogeneities commonly
exist in geologic materials, and areas outside of the
region excavated for tank emplacement would very
often be expected to contain heterogeneities that
would affect vapor migration. Layers of fine materials,
such as silt or clay, would act as barriers to move-
ment, particularly if an accompanying high moisture
content were present and pore spaces were
occluded. Secondary permeabilities created by fis-
sures, plant roots, animal burrows, or other agents
would increase and direct the transport of vapors
around underground tanks. Despite their importance,
a full understanding of the distribution of heter-
ogeneities at any given site is difficult to achieve.
3-9
-------
Sorptive effects are complex phenomena which
depend on numerous interacting variables. Taylor and
Ashcroft (1972) define adsorption as the concentration
of a substance at the surface or interface of another
material. Adsorption can result from chemical ad-
sorption (Coulombic forces), physical adsorption (Van
der Waals forces), or hydrogen bonding (Lyman et al,
1982). If a volume of a gas is adsorbed near the
source of a leak, its vapor density in the soil air will be
reduced. Consequently, its transport potential will be
reduced because of a decrease in concentration
gradient. Given a continuous input of vapor from any
source, however, equilibrium may be attained between
ions being adsorbed and those leaving adsorbent
sites. In this situation, vapor transport would not be
greatly affected. The potential for sorption processes
to effectively reduce the vapor density of a gaseous
component is largely determined by the volume input
of the gaseous source in the leaking tank envi-
ronment.
The adsorption of any vapor product liberated from a
leaking underground storage tank is primarily
governed by the interactions between the chemical
structure and properties of the gas; the chemical,
biological, and physical characteristics of the soil; and
general environmental conditions. The following
general principles are often important in these sub-
surface vapor-filled porous media:
1. The forces of adsorption increase with
increasing polarity of a gaseous molecule.
2. The charge on the adsorbent may contrast
with the polarity of a gaseous molecule and add
an additional adsorptive force.
3. The efficiency of an adsorbent depends largely
on its specific surface (area per unit mass).
4. The adsorption of gases generally increases
with increasing organic content.
5. The volumetric water content may displace
ions by competing for adsorbent sites.
6. Temperature increases in the soil will result in
adsorption losses, due to higher kinetic
energies of molecules at sorted sites.
Microbes in the subsurface environment can entirely
transform moving vapors. The effect of this microbial
action will be discussed in Section 5 of this report.
Existing Gaseous
Measurements Methodologies
Sampling gaseous vapors in shallow soils can provide
an effective method of detecting underground tank
leaks and a resultant contaminant plume. As stated
earlier, a major advantage of gaseous leak detection
is the radially outward movement of gases which
usually fills the porous media more completely than
liquid migration (Figure 3.3). According to Scheinfeld
and Schwendeman (1985), characteristics that a leak-
detection system should exhibit include the ability to:
detect an unconfined fluid of concern, be main-
tenance-free, screen out false alarms, be cost-
effective, be tamper proof and secure, be applicable to
existing tank systems, and be simple, safe, and
reliable. They further state that, optionally, in
environmentally sensitive areas, leak detection sys-
tems might be desired to monitor continuously and
automatically. The science of vapor measurement is,
in some ways, in its infancy. New methods are
appearing with great frequency. Therefore, the
following review is meant to be a summarization of the
different categories of measurement, not a detailed
listing of each existing method. This review is in no
way intended to constitute an endorsement of any of
the methodologies.
The methods of gaseous detection of leaks can be
broken into the categories of quantitative meas-
urement and non-quantitative or "red flag" methods.
More accurately, at this writing, no method of out-of-
tank vapor detection is quantitative to the point of
allowing an accurate estimation of a volume of tank
leakage; rather, what is called "quantitative" herein is
meant to designate methods by which varied
subsurface vapor concentrations can be measured.
The term "non-quantitative" method is used to de-
scribe a measurement technique which signals the
presence of a given vapor at a certain level but which
cannot distinguish varying concentration magnitudes.
So-called quantitative measurement methods can be
further grouped into: (a) analysis of withdrawn vapor
samples (instantaneous collection), (b) surface sam-
pling, (c) longer term, static emplacement of absorbent
material which is subsequently removed and ana-
lyzed, (d) in situ quantitative measurement, and (e)
tracer techniques.
One group of quantitative methods is based on the
analysis of a vapor sample obtained by inserting a
probe into the soil and by withdrawing subsurface gas
with a metal bellows pump or similar device. Analysis
of the sample is then performed on-site or at a
3-10
-------
PRODUCT VAPOR MIGRATION
' ':.'"**''>.
. >»
. ./;. v
.'.;. ^
LIQUID PRODUCT LEAK
WATER TABLE
Figure 3.3
Three-dimensional Diffusion or Advectlon of Product Vapor
-------
laboratory. Samples can be withdrawn from a driven
probe or from a permanent installation of air pie-
zometer-type probes.
A driven probe is a small diameter pipe with a drive
point below a screened region that allows sampling of
vapors in an undisturbed part of the vadose zone.
Driven probes can be shallow (a depth of 1 to 3 m), or
can be driven in the center of a hollow stem auger in
advance of the bit during breaks in drilling. As drilling
proceeds, it is possible to obtain samples at various
depths giving an indication of the vertical profile of
vapor concentration at a given site. Soil gas is
withdrawn from shallow drive point probes to clear
other gases out of the evacuation line; the pumped
volume is usually several times the internal volume of
the evacuation line. A smaller sample is then ana-
lyzed. Sample size depends on the composition and
concentration of the suspected contaminant.
There are many pitfalls for the inexperienced in-
vestigator in this methodology. Great care must be
exercised when driving a probe around buried tanks
and associated piping, particularly because many tank
owners are not certain of the exact position of all
subsurface utilities, pipes, tanks, and other features
below their property. In addition, steps must be taken
to ensure that with each sample no atmospheric air is
sucked downward from the ground surface (or
borehole), around the outside of the driven probe.
The material used for probes and evacuation lines
must be free of contamination and must not adsorb
critical compounds of interest. (Almost surprisingly,
PTFE Teflon-type tubing can be a poor material for
gaseous sampling because many subsurface con-
taminant gases can move into and through its walls.
This same property is very valuable in the use of the
material as a permeation device - a controlled emis-
sion source for gaseous tracer tests [Kreamer, 1982]).
Probes placed in predrilled boreholes can be used as
long-term sampling locations for monitoring a storage
facility or an existing contamination plume. There are
several different designs for long-term probes which
can be resampled. Most are installed and backfilled
with sand or gravel, and a seal is placed between
vertically displaced sampling stations to prevent
vertical vapor movement in the borehole. Air drilling,
moisture changes in the borehole (particularly if
cement is used as grout between stations), and
advective perturbation of the subsurface gaseous
regime by overpumping are but a few of the potential
problems with long-term sampling. The latter potential
problem can be reduced by design of a very low
volume evacuation line (this will also reduce the
absorptive surface area) and by pumping only a few
internal line volumes before sampling.
Surface sampling is carried out in a variety of ways.
One of the most quantitative is a method that places a
chamber, open at the bottom, on the ground surface.
The chamber is allowed to fill with gases moving
upward, and periodically analyzed for gases of
interest. Quantitative aspects of this surface flux
measurement suffer when concentrations or pressures
build up in the chamber which inhibit further upward
movement. The use of a flushing or carrier gas
through the chamber reduces most of these effects.
Once a vapor sample is brought to the surface, there
are many ways of measuring its composition, each
with varying degrees of sensitivity, precision, ac-
curacy, size, and expense. For example, illustrating
the wide variety of field methods of measurement
available, a partial list could include: (a) portable gas
chromatographs with direct or syringe injection and
with any of a number of detectors including electron
capture (ECD), flame ionization (FID), and photo-
ionization (PID), (b) small, portable devices with just a
detector, PID or FID, which gives an indication of bulk
gases such as total hydrocarbons, (c) combustible gas
indicators (CGI) used for general assessment of
explosive potential by aspirating vapor over a heated
filament which changes temperature and electrical
resistance, (d) catalytic detectors which are similar to
but have more sensitive response than CGIs, (e)
metal oxide semiconductors and adsorption sensors
which vary their internal resistance in response to
certain vaporized compounds, and (f) many other
types of devices and instrumentation. Collection and
transportation of samples to the laboratory can have
the benefit of the use of more powerful analytical
chemistry techniques, such as gas chromato-
graphy/mass spectrometry (GC/MS), but has the
potential to suffer if there are changes in the sample
during transportation. Again, it should be emphasized
that some of the methods presented here are quan-
titative only in the loosest sense; that is, they can only
indicate relative increases or decreases in bulk
concentrations.
Static collection methods of sampling vapors in the
vadose zone have the potential to produce quan-
titative results. In these static methods, volatile pro-
ducts within the soil are collected over time by an
absorbent material placed below land surface near the
tank. The absorbent material is removed after some
time and is then analyzed for contaminants. Collec-
tion of a sample occurs over a period of one to two
weeks, and therefore is known as a static, as opposed
to an instantaneous, collection method.
Because static collection of vapor samples occurs
over a longer period of time, short-term variations of
gaseous concentration at a given site are less im-
portant than in instantaneous collection methods.
3-12
-------
Fluctuations in barometric pressure, ground-water
elevation, and wind can all influence an instantaneous
collection method which is repeated in the same
location over time. Potential problems with static
collection include the premature desorption of col-
lection material before analyses are conducted and
inefficiency of the sorbent collection material to sorb
particular compounds.
A variety of in situ measurement techniques exist, and
new methods are being developed and tested pres-
ently. Some examples of in-place subsurface meas-
urement technology, to illustrate this variety, include:
metal oxide semiconductors, adsorption sensors, and
remote fiber spectroscopy.
Metal oxide semiconductors and adsorption sensors
are devices which change their internal resistance in
reaction to a number of gases. As mentioned earlier,
they can be used to measure samples pumped to the
surface, but they can also be placed in a monitor or
vapor monitor well. The resistance change can be
calibrated to ascertain the concentration of the gases.
These sensors have a number of different config-
urations for the detection of vapors.
Remote fiber spectroscopy (RFS) utilizes two emer-
ging technologies: development of intense light
sources and efficient optical fibers in the UV part of
the electromagnetic spectrum. RFS employs a laser
light source, fiber optics for signal transmission, and
an optrode for vapor detection. A laser light source
focuses an input beam into the optical fiber which is
coupled to an optrode in contact with the sample.
Light that has interacted with the optrode returns on
the same optical fiber and the resultant signal is then
sorted by an optical spectrometer for analysis. Each
optrode can measure a specific chemical or physical
property. The intensity of the signal in a specific
frequency is related to the chemical reaction or
physical change. One limitation of RFS detection is
optrode development, which has just begun and
cannot yet provide the wide range of monitoring
capabilities it is projected to have in the future. Any
given sample site may vary in concentration of
contaminants, and the optrode must be able to
monitor specific organic or inorganic or both com-
pounds consistently when there has been no sample
preparation.
Tracers have been used in leak detection technology,
and several different uses have been found for
gaseous tracers. One use is calculation of diffusive or
permeable aspects of the porous media by tracer
release in the vadose zone at a known, controlled rate
and configuration. The arrival of the released tracer is
measured at distance, and, subsequently, media
properties can be calculated. In another method,
tracer is introduced into a product storage tank at a
concentration such that properties of the product
remain unchanged. The most volatile component of a
product leak is the tracer. As a leak occurs the tracer
quickly diffuses into the surrounding soil. Gaseous
samples can be taken and analyzed for the tracer.
Still another method is geared toward distinguishing
between a liquid product leak and vapor leakage from
a tank. Recalling that vapor leaks of a product can
occur around fittings at the top of a tank, it can be
difficult if not impossible to distinguish between a
product leak and vapor leak with vapor phase
monitoring. In this situation two volatile tracers can be
used to differentiate vapor and product leaks. One
tracer (No. 1) is injected into the product while the
other (No. 2) is introduced into the air space above the
product. Sampling of the soil vapors is then per-
formed. If only tracer No. 1 is discovered, then a pro-
duct leak is occurring provided that the tracer
introduced into the tank air space does not readily
diffuse into the tank liquid product or is not pref-
erentially detained in the soil medium. Sampling is
completed before a significant amount of tracer No. 2
enters into the liquid phase in the tank. Detection of
both tracers indicates a vapor leak or possibly a
product and vapor leak occurring simultaneously. This
procedure is relatively new, and supporting research is
ongoing.
All of the above "quantitative" measurement methods
are most commonly used as non-quantitative or "red-
flag" indicators of leaking tanks. Occasionally, these
methods give helpful quantitative information, but their
primary value is in locating leaks and in giving an early
warning which is more of a qualitative site assess-
ment. It is often very useful to use one or several of
these out-of-tank vapor monitoring methods in concert
with other techniques.
There are many strictly non-quantitative vapor
measurement methods. For example, one group of
devices involves the degradation or dissolution of a
weighted cord hooked to an alarm. These product-
soluble compounds, such as styrene-butadiene
copolymer, are primarily used for liquid phase warning
but could be adapted to vapor warning as well. These
same product-soluble materials can be used to
insulate conductors of an electrical resistivity-type
warning device. Another group of warning systems
are product permeable devices which utilize materials
that are hydrophobic (water repellent) but are
oleophillic (permeable to selected organic material).
Vapors from product leaks can penetrate these
materials and set off an alarm. Several other types of
non-quantitative measurement methods exist.
If a leak has already occurred, gaseous monitoring
methods can be extremely useful in contaminant
3-13
-------
plume delineation. Determining the areal extent of a
contaminant is possible by analyzing overburden soil
for chemical vapors. In an area of suspected
contamination, it is possible to transect an area until
boundaries of a plume are defined. Routine meas-
urement of methane, benzene, toluene, and hydro-
carbons provides information about ground-water
plume movement. A comparison of contaminant
concentration in the vadose zone and ground water is
used to locate sources within the plume.
Several final generalized statements can be made
regarding present vapor detection technology in
addition to what is presented above. First, gaseous
sampling is not optimal for all compounds. Contam-
inants that are most easily detected in gaseous
sampling are low molecular weight petroleum and
halogenated hydrocarbons. These products have a
high vapor pressure, high liquid-gas partitioning
coefficient, and low aqueous solubility. Second, con-
cerning sampling techniques, precautions should be
taken so that samples are not contaminated and so
that false positives are not obtained. Third, calibration
of field and lab equipment is required on a regular
basis to provide consistent results. Fourth, very little
information exists on the operable life and reliability of
many devices, in part because many are so new. This
fact will make evaluation of these leak detection
systems difficult. Because the technology is new and
because great pressure presently promotes the rapid
marketing of new devices, quality control is essential.
Finally, vapor leak detection cannot determine leak
rate from an underground storage tank in most
circumstances. The utility of vapor leak detection is in
identifying the presence and nature of leaks, and in
contaminant plume identification once major leaks
have occurred.
Limits of Present Knowledge
and Future Directions
The direction of future efforts in the arena of vapor
transport and measurement surrounding underground
storage tanks should be directed toward answering
the shortcomings of our present knowledge. In
particular, there is a need to construct workable, cal-
ibratible models of contaminant flow based on theory;
to develop usable, reliable vapor detection technology
which will stand the test of time; and to establish better
design criteria for the installation and operation of
underground tanks.
One of the greatest problems with any attempt to
apply transport theory to actual field situations is the
lack of field verification to this point in time. There has
been relatively little on-site investigation to identify or
determine the magnitude of field parameters, bound-
ary conditions, or environmental effects on vapor
transport. As a result, application of transport theory
is done based on hypothetical conditions, unverified
assumptions, or is carried out merely as a heuristic
exercise. Careful field and laboratory measurement of
the factors that affect transport is crucial to our
understanding and interpretation of field results and
presents a great challenge for the future.
There is presently a great rush to develop leak de-
tection technology based on gaseous movement in the
unsaturated zone. The impetus comes from an in-
dustry need to monitor their underground storage
tanks inexpensively and reliably, and to assess the
extent of existing pollution problems quickly. There is
concern that rapid marketing of new products might
leave aspects of device performance untested for
years. Large-scale leak detection device failures
could destroy the underpinnings of a regulatory struc-
ture built on leak detection safeguards and could
eventually impair the credibility of such a program.
Development of uniform performance standards and
testing criteria for leak detection devices is an area
that needs much consideration.
In the future, successful vapor leak detection might be
able to reliably quantify rates of tank leakage. The
optimal design or designs to accomplish this
monitoring could eventually be included as one of
several alternatives for overall new tank construction.
In addition, new information on vapor migration and
the factors that affect it in the subsurface tank envi-
ronment could aid future decisions on tank installation,
design, and liner or barrier placement.
3-14
-------
References
Buckingham, E. 1904. Contributions to Our Knowl-
edge of the Aeration of Soils. U.S. Dept. Agri.,
Bureau of Soil Bulletin 25.
Currie, J.A. 1971. Movement of Gases in Soil Res-
piration. SCI Monograph No. 37, pp. 153-159.
Grew, K.E., and T.L. Ibbs. 1952. Thermal Diffusion in
Gases. Cambridge University Press.
Helfferich, F. 1962. Ion Exchange. McGraw-Hill, New
York.
Keen, B.A. 1931. The Physical Properties of the Soil.
Longmans, Green & Co., London.
Klinkenburg, L.J. 1941. The Permeability of Porous
Media to Liquids and Gases. American Petroleum
Institute Drilling Products Practices, pp. 200-213.
Kreamer, D.K. 1982. In Situ Measurement of Gas
Diffusion Characteristics in Unsaturated Porous
Media by Means of Tracer Experiments. Ph.D.
Dissertation. Department of Hydrology and Water
Resources, University of Arizona, Tucson,
Arizona.
Lyman, W.J., F.W. Reehl, and D.H. Rosenblatt. 1982.
Handbook of Chemical Property Estimation
Methods. McGraw-Hill, New York.
Mason, E.A., and R.B. Evans. 1969. Graham's Laws:
Simple Demonstrations of Gases in Motion. Part
1, Theory. J. Chem. Ed. 6(6): 358-364.
McKee, J.E., F.B. Laverty, and R.M. Hertel. 1972.
Gasoline in Groundwater. J. WPCF 44(2): 293-
302.
Rommel, L.G. 1922. Luftvaxlingen Marken Som
Ekologisk Faktor. Medd. Statens Skogsfarcoks-
anstalt, 19, no. 210.
Scheinfeld, R.A., and T.G. Schwendemann. 1985. The
Monitoring of Underground Storage Tanks. In:
Proceedings of Petroleum Hydrocarbons and
Organic Chemicals in Ground Water, NWWA,
November 13-15, Houston, Texas.
Taylor, A.T., and G.L. Ashcroft. 1972. Physical
Edaphology. W.H. Freeman and Co., San
Francisco.
3-15
-------
-------
SECTION 4
Soil Surface and Interfacial Effects in the
Underground Storage Tank Environment
Introduction
The detection of contaminants leaking from
underground storage tanks or monitoring such tanks
for leaks in the underground environment is
dependent on the physical and chemical properties of
the contaminants that might be sensed by chemical
instrumentation. Sensible properties of the leaking
contaminant or its degradation products that may be of
use include:
refractive index,
thermal conductivity,
acoustic conductivity,
electrical conductivity,
ultraviolet, visible, or infrared absorbance spectra,
fluorescence spectra, or
electrochemical oxidation/reduction potentials.
These properties may be profoundly affected by the
physical chemistry of the contaminants at interfaces.
Adsorption and partitioning between available phases
are the more notable of these interfacial phenomena.
The adsorption-desorption of contaminants with soil
particles, distribution between immiscible phases, and
emulsification constitute important parameters in:
pollutant transport kinetics,
bioavailability,
chemical degradation, and the
sensible properties given above.
Additionally, the leaking chemicals may greatly modify
the physical and chemical properties of the sur-
rounding fill material. Adsorption of aromatic hydro-
carbons, for example, can distort the interatomic
distances in clays and, subsequently, can greatly
affect the porosity and permeability.
The evaluation of the myriad of possible effects in the
environment of leaking underground storage tanks
may seem at first to be a desirable undertaking.
Evaluation of their individual and collective impact on
those properties, such as transport, are important in
leak detection strategies. The use of numerical
models permits a convenient means to estimate the
impact of these phenomena collectively and
individually and also to estimate their relative
importance under specified conditions.
Partitioning and Adsorption
of Chemicals in the Underground
Storage Tank Environment
The partitioning of chemicals among different available
phases represents the most significant surface/
interfacial phenomena and is crucial to the under-
standing and modeling of the transport and reactions
of contaminants. In the underground storage tank
environment, the partitioning of contaminants can be
among many different phases:
liquid to liquid
liquid to gas
liquid to solid
gas to solid.
The first two processes are usually referred to as
partitioning while the latter two are referred to as
adsorption.
Adsorption processes may be physical or chemical.
Chemisorption usually involves the formation of new
bonds and is associated with large energy changes
(»10 kcal/mole). This typically results in formation of
monolayer coverage of reactive molecules at active
sites on the adsorption surface. Physical adsorption,
on the other hand, usually involves only relatively
weak Van der Waals forces, rather small energy
changes (<10 kcal/mole), and adsorption of many
molecular layers (Jaycock and Parfitt, 1981; Oudar,
1975).
Adsorbed molecules, particularly chemisorbed
molecules, will have different physical properties than
their free counterparts. They would not necessarily
possess the same ultraviolet, visible, infrared, or
fluorescent spectra and would typically have different
oxidation-reduction potentials and different bio-
availability and chemical reactivities.
There are many factors in the adsorption and de-
sorption processes. The moisture content or presence
of other organics can either compete for adsorption
4-1
-------
sites or provide a second solute phase. The ad-
sorption and desorption processes involve activation
energies and are thus temperature dependent. The
forces involved may be either electrostatic or dipole-
dipole depending on the particular circumstances.
The molecular polarizability (polarizability refers to the
induration of a dipole moment by an external electric
field because of the displacement of the average
positions of the electrons relative to the nuclei of the
molecule), dipole moment, charge distribution, size,
and shape can affect the adsorption process
(Browman and Chesters, 1975). The same
considerations must also be given to the solid phase
properties and the solvent phase as well (the solvent
phase might be water or contaminant). Generally, the
solubility of the contaminant is quite important in the
adsorption equilibria. There is usually an inverse
relation between the solubility and the extent of
adsorption. This is Lundelius' rule; the stronger the
solute-solvent bond, then the smaller the extent of
adsorption (Jaycock and Parfitt, 1981).
The equilibrium relationship between the contaminants
concentration in solution and that adsorbed to soil
particles can be described from a theoretical basis
using the Langmuir adsorption isotherm:
0 =
b[C]
(4.1)
where 6 is a fractional surface coverage, [C] is
concentration in solution, and b is a parameter related
to the energetics of the process (Oudar, 1975). This
equation is derived assuming a constant temperature,
monolayer coverage of contaminant molecules at
active sites, and a homogeneous distribution of active
adsorption sites. Actually b should also be a function
of 6, and there typically would not be a homogeneous
distribution of adsorption sites. The Langmuir
adsorption isotherm may perform well when the con-
taminant is strongly adsorbed but generally is not
applicable to the underground storage tank environ-
ment where sorption effects may be weak.
The Freundlich isotherm is most commonly used to
describe adsorption to solid surfaces in the soil en-
vironment:
X/M = K [C]n
(4.2)
where X is the mass of contaminant adsorbed to a
mass M of soil, [C] again is the concentration of
contaminant in solution at equilibrium, and K and n are
fitting parameters. This is purely an empirical relation
(Jaycock and Parfitt, 1981), but it can be derived from
the Langmuir adsorption isotherm by assuming a
heterogeneous surface with adsorption at each class
of sites following Langmuir behavior.
The principal drawback of the Freundlich isotherm is
that it predicts that the amount of material adsorbed
increases indefinitely as the concentration (or partial
pressure) increases and is thus unsatisfactory for
application at high contaminant concentrations
(Oudar, 1975), the same conditions that might exist
with a leaking storage tank.
There are many other adsorption isotherms of in-
creasing complexity (Oudar, 1975; Jaycock and
Parfitt, 1981). However, because of the large number
of parameters and general complexity of the soil
environment, it may not be useful to apply more
theoretical models to this system.
Liquid-liquid partitioning can be described in terms of
the solubility of a solute in the different immiscible
phases. The interfacial tension between the two
phases can be strongly affected by trace con-
centrations of other dissolved species; the "salting out"
of emulsions being a familiar example. Dissolved
salts reduce the solubility of non-electrolytes in water
and reduce the surface tension of electrolytes
(Bikerman, 1958).
The partitioning of contaminant between its bulk phase
and aqueous phase or between aqueous phase and
some other organic phase, such as humic material,
can usually be presented as a simple linear
distribution coefficient or equilibrium expression.
Knowledge of these relationships is critical to the
understanding of retardation of contaminants in soils
and ground water. The partitioning might also be
expected to obey a simple linear free-energy rela-
tionship (if the effect of a substituent on a molecule is
to only cause a small perturbation in the free energy
change for a process or reaction, there will be a linear
relationship between the substituents and the parent
molecule for that process or reaction). Both aqueous
solubility and partitioning between aqueous phase and
a soil organic (humic) phase should correlate on a log
scale (Chiou et a!., 1983; Miller et al., 1985). The
octanol-water partition has been correlated with
solubility and the soil organics - water partitioning of
non-ionic organic contaminants. As with other linear
free-energy relationships, this follows qualitatively
from the similarity of the processes. A more rigorous
attempt to justify the correlation has been offered by
Chiou (1983) using the Flory-Huggins polymer solution
theory. The Flory-Huggins theory is a direct gen-
eralization of the Bragg-William approximation in the
lattice model of binary solutions and provides for the
marked differences in size for the solute and solvent
when one is a polymer. This theory is useful in pre-
4-2
-------
dieting both solubility and the miscibility of the polymer
with another chemical. While the Flory-Huggins
treatment is only approximate and not strictly suited
for very dilute solutions (Hill, 1960) (low humic
content), it nevertheless serves rather well. Humic
material is chemically quite complex and contains an
abundance of polar substituents such as -COOH, -
C=O, aliphatic-, phenolic-, and enolic-OH, -NH2, and -
SH groups as well as aromatic rings (Browman and
Chesters, 1975). These substituent groups provide
the adsorption sites on the polymer. Both lattice
statistical models of adsorption to polymers and the
Flory-Huggins theory are derived with the assumptions
that all the sites on the polymer are equivalent, which
clearly is not the case for humic material.
Comparatively little attention has been given the
liquid-gas partitioning or gas transport in the soil
environment. However, the forces involved are similar
to those of liquid-liquid partitioning. Accordingly, there
should be linear free-energy relationships for the
liquid-gas partitioning. For a homologous series, the
activity at the liquid-vapor surface for an aqueous
solution of organic solutes should increase strongly
and regularly as the series is ascended; this is
Traube's rule. A similar relationship for a solid
interface might be expected but is not realized; other
factors such as pore diffusion may dominate (Jaycock
and Parfitt, 1981).
It should be noted that the retarded transport of an
organic contaminant in the soil/ground-water en-
vironment need not necessarily involve adsorption to
active sites on particles but could also conceivably
involve distribution between the bulk solvent and a
second solvent, typically water or humic material,
adsorbed on the particles. Similarly the bulk phase
could be either water or the leaked contaminant
depending on the particular soil conditions.
Many of these processes have been studied indi-
vidually under controlled laboratory conditions for
idealized systems but few if any laboratory studies
have quantitatively dealt with these processes as a
system. Since all of these processes may occur
simultaneously, numerical model calculations could be
most advantageous in evaluating their effects. Such
model calculations can be useful in utilizing the data
from the diverse laboratory studies to estimate the
importance of the various processes in concert with
one another.
All possible chemical processes and reactions in a
subsurface regime are represented by the reaction
term in the advection-dispersfon equation. Processes
which can be described by a distribution coefficient,
such as adsorption or ion-exchange, are represented
by a retardation factor which is the ratio of the ground-
water average linear velocity to the rate of the
advancing contaminant front. Rate controlled
reactions such as hydrolysis, oxidation/reduction, or
biodegradation can be represented by a decay
function.
Movement and reaction of emulsions of organic chem-
icals in water have not yet been addressed but may be
important in biodegradation of chemicals. Non-linear
adsorption can be accounted for by modifying the
conceptual model to include kinetic processes. Pol-
lutant concentrations in the soil-air are generally not
computed in the commercially available, traditional
differential equation type of models (Bonazountas,
1983). Bonazountas claims that compartmental type
models can handle more complex geochemical pro-
cesses and can include the estimation of pollutant
concentration in the soil-air. Baehr and Corapcioghi
(1984) have recently developed an unsaturated zone
model which predicts hydrocarbon concentrations in
all phases (water, air, immiscible, absorbed) in time
and space. This type of model has the potential to
assess the long-term impact of residual hydrocarbons
trapped in pore spaces and to access the impacts to
underlying ground water. The difficulty in simulating
the trans-formations and reactions in a contaminant
plume arises from a deficiency in being able to
quantify the individual processes and mathematical
complexity of computing the resultant solute distri-
butions by using differential equations. Generally, all
the processes are lumped together into one reaction
parameter and are then calibrated with actual field
data to match the observed results. The science of
contaminant transport prediction via computer models
is, therefore, only as accurate as the understanding of
processes which govern their movement and data
which are input into the model.
Leak Detection
The time interval in which a leak detection monitoring
device located outside the tank should be able to
detect a leak depends upon a number of variables
which can be addressed by solute transport modeling.
The velocity of a leaking organic chemical is a function
of multiple variables including the properties of the
porous media adjacent to the tank (including the tank
backfill material), properties of the leaking chemical,
vertical and area! position of the monitoring devices,
distance to the water table, water table gradient, and
soil moisture content.
The consequences of the surface chemical effects are
immediately apparent with respect to the transport of
the pollutant to a sensor location. It should also be
apparent that since, in general, chemical instrumen-
tation does not detect the total amount of contaminant
4-3
-------
present but rather usually only the contaminant in a
particular phase (such as gas or aqueous), these
properties are important in selecting the most judicious
form of the contaminant to sample.
Perhaps less apparent is the potential application of
the surface chemical properties to the actual instru-
mentation itself. The adsorption of an organic
substance to a metal electrode can be used to precon-
centrate it for later sampling. This can be cyclic
voltametry, simple amperometry, or in the form of a
CHEM-FET (chemically sensitive field-effect tran-
sistors which are sensitive analytical chemical tech-
niques that can measure trace concentrations of
contaminants by the voltage-current changes that
occur at exposed electrodes when the analyte(s) are
oxidized or reduced). The accumulation of the
pollutant on a surface need not be restricted to metal
electrodes. Polynuclear aromatic compounds may
deposit on glass surfaces such as fiber optic probes
where they may be detected by their spectroscopic
properties such as laser-induced fluorescence.
Chemical and biological degradation of contaminants
may depend on the partitioning between phases or
adsorption to soil particles. This may lead to the
formation of new contaminants not being sensed by a
particular instrument. For example, degradation of
tetralkyl leads may produce lead(ll) halides, car-
bonates, di- and tri-alkyl lead salts. A sensor specific
for tetraethyl lead might, in general, respond equally or
not at ail to mono-, di-, or tri- ethyl lead chloride or to
lead(ll) chloride.
In conclusion, there is a considerable uncertainty
regarding the detailed chemical kinetics and reaction
mechanisms of many toxic pollutants in the soil/
ground-water environment (Callahan et al., 1979).
Differences Between Laboratory
and Underground Storage
Tank Environment
In general, results from laboratory tests cannot be
directly correlated to field situations. Knowledge of the
assumptions and limitations under which laboratory
experiments are conducted should be used to deter-
mine initially the applicability of data to a natural
hydrogeologic regime.
Measurement of retardation parameters in the labora-
tory by batch equilibrium tests or column breakthrough
curve experiments is common but does not account
for the heterogeneities or anisotropic conditions
encountered in the hydrogeologic regime surrounding
an underground storage tank. The properties of the
porous media may be altered upon transfer of the
sample to the laboratory. Disturbance of the original
orientation or compaction of the geologic sample can
cause a change in permeability or porosity of the
sample and can have a significant affect on the speed
and directional movement of the leaking chemical.
The geochemical properties of the sample can also be
altered by degassing of CO2 or invasion of O2. A
small difference in mineral or organic content of the
geologic medium can also dramatically affect the
resultant contaminant dis-tribution. These subsurface
parameters are difficult to accurately quantify in the
laboratory or by field methods.
Laboratory tests are usually run with low solute
concentrations which may not be the case around a
leaking tank. Non-linear adsorption may occur when
large quantities of free product exist near the tank or
when the adsorptive or permeable properties of the
porous media may be altered. A permeability ex-
periment demonstrated an average 10,000 percent
increase in permeability of four clays to xylenes over
that measured with water (Anderson, 1982). A
possible explanation of this large increase in per-
meability may be due to a structural change of the
clays caused by the adsorption of xylenes.
Theoretical Deficiencies
There is a basic inconsistency between theoretical
frame-works of numerical ground-water models,
laboratory experiments, and field research
(Bonazountas, 1983). A need exists to efficiently and
simplistically model the transport of these chemicals
from leaking underground storage tanks. The relative
importance or sensitivity of those parameters
controlling the transport processes of organic chem-
icals can be determined by a numerical modeling
sensitivity analysis. The most sensitive variables are
those which need to be most accurately measured or
calculated in the laboratory and field. Once these
parameters are identified, future research can be
directed towards more accurately determining these
variables instead of those properties or coefficients
which can simply be estimated.
A better understanding of the adsorption and dis-
tribution processes would significantly improve the
predictive capability of models (Cherry et al., 1985).
Prediction of the dilute yet often significant edges of a
plume can be important in the early detection of leaks
from leaking underground storage tanks.
To meet these needs, it is necessary to have better
models which describe adsorption and phase par-
titioning of contaminants including non-equilibrium
effects (Valocchi, 1985). The limitations of the
Freundlich isotherm and the applicability of the Flory-
4-4
-------
Muggins solution theory to the transition region from
low to high humic content need to be addressed.
There is a need for theoretical or semi-empirical
relationships describing adsorption of multicomponent
systems. The possible occurrence and behavior of
emulsions in the environment needs to be addressed
as well.
Data Needs
1. Most laboratory data have been collected over
narrow temperature ranges or at a single
(ambient) temperature. Both adsorption and
solubility are temperature dependent, and,
thus, more data at a range of temperatures
need to be acquired.
2. The partitioning of contaminant mixtures and
separate components onto various mineral
and organic soil types needs to be defined so
that a relatively quick estimate of
contaminant migration from leaking tanks can
be determined.
3. The role of inorganic mineral surfaces in
hydrophobic adsorption should be further
explored since many aquifers are low in organic
matter.
4. Solute competition for adsorption sites at
higher solute concentrations, such as those
encountered near leaking underground tanks,
should be investigated (Cherry et al., 1985).
5. The role of dissolved organic matter in the
transport of trace level organics in ground
water is another area of concern. This could
be an important process in the migration of
organic contaminants away from leaking
underground tanks. A greater degree of
characterization of naturally occurring organic
material is necessary to establish the range of
values for partition coefficients that may
occur and to relate the properties of the
dissolved organic carbon (DOC) to the
partitioning processes.
6. The potential for oxidative or hydrolytic
degradations in the underground storage tank
environment should be investigated for specific
contaminants which are susceptible to these
types of reactions.
Future Directions and Strategies
A greater amount of laboratory data is needed both to
more closely approximate realistic field conditions and
to accommodate the development of better theories or
models for the transport and reactions of organic
contaminants in the underground storage tank en-
vironment. Particular attention should be paid to tem-
perature dependence and the role of dissolved organic
matter.
The improvement to such models would provide a
valuable asset in evaluating leak detection and moni-
toring strategies. Such models may prove invaluable
in selecting the optimum sensor locations and in the
selection of the preferred contaminant for multicom-
ponent storage tanks.
4-5
-------
References
Anderson, D. 1982. Does Landfill Leachate Make Clay
Liners More Permeable? ASCE-Civil Eng., Sept.,
pp. 66-69.
Bikerman, J.J. 1958. Surface Chemistry. Academic
Press Inc., New York.
Bonazountas, M. 1983. Soil and Groundwater Fate
Modeling. In: Fate of Chemicals in the Environ-
ment. R.L. Swann and A. Eschenroeder, (eds).
ACS Symposium Series 225. pp. 41-66, ACS,
Washington, D.C.
Browman, M.G., and G. Chesters. 1975. Fate of Pol-
lutants in the Air and Water Environments, Part 1.
I.H. Suffet (ed). John Wiley and Sons, New York.
Callahan, M.J., M.W. Slimak, N.W. Gabel, I.P. May,
C.F. Fowler, J.R. Freed, P. Jennings, R.L. Durfee,
F.C. Whitmore, B. Maestri, W.R. Mabey, B.R. Holt,
and C. Gould. 1979. Water Related Environmental
Fate of 129 Priority Pollutants, Vol. II. EPA-440/4-
79-029b.
Cherry, J.A., R.W. Gillman, and J.F. Baker. 1985.
Contaminants in Groundwater: Chemical Pro-
cesses. In: Groundwater Contamination - Studies
in Geophysics, pp. 46-64. National Academy
Press, Washington, D.C., 1984.
Chiou, C.T., P.E. Porter, and D.W. Schmedding. 1983.
Partition Equilibria of Nonionic Organic Com-
pounds between Soil Organic Matter and Water.
Env. Sci. and Tech., 17:227-231.
Hill, T.L. 1960. Statistical Thermodynamics, Addison-
Wesley Pub. Co., Inc., Reading, Massachusetts.
Jaycock, M.J., and G.D. Parfitt. 1981. Chemistry of
Interfaces. Halstead Press, John Wiley and Sons,
New York.
Miller, M.M., S.P. Waslik, G.L. Huang, W.Y. Shiu, and
D. Mackay. 1985. Relationships between Octanol-
Water Partition Coefficient and Aqueous Solubility.
Env. Sci. and Tech. 19:522-529.
Oudar, J. 1975. Physics and Chemistry of Surfaces.
Blackie and Son Ltd., Glasgow.
Valocchi, A.J. 1985. Validity of the Local Equilibrium
Assumption for Modeling Sorting Solute Transport
through Homogeneous Soils. Water Resources
Res. 21 (6):808-820.
4-6
-------
SECTION 5
Implications of Subsurface Biological Activity for
Monitoring Underground Storage Tanks
Predicting the behavior of organic compounds
released into the subsurface environment is a chal-
lenging task. Microorganisms in the subsurface can
transform many of the organic contaminants that
typically escape underground storage tanks. However,
the rate and extent of transformation is controlled by
the geochemical and hydrological properties of the
subsurface. As a result, biological activity at a
particular site must be described within a clearly
defined chemical and physical context. In laboratory
studies, microbiologists usually study rapidly growing
cultures of organisms that are only limited by the
capacity of their internal machinery to process
nutrients. In the subsurface this situation is rare. More
commonly the populations of metabolically capable
organisms increase until they are limited by some
requisite for metabolism. Once this point is reached,
the rate of transformation of an organic material is
controlled by transport processes that supply the
limiting nutrient.
The vast majority of microbes in the subsurface are
firmly attached to soil particles. As a result, nutrients
must be brought by advection or diffusion through the
mobile phases, water and soil gas. In the most
common case, the organic compound to be consumed
for energy and cell synthesis is brought in aqueous
solution in infiltrating water. At the same time oxygen,
the electron acceptor used to oxidize the carbon
source, is brought by diffusion through the soil gas. In
the unsaturated zone, volatile organic compounds can
also move readily as vapors in the soil gas. Below the
water table all transport must be through liquid
phases, and as a result the prospect for aerobic
metabolism is severely limited by the very low
solubility of oxygen in water. In the final analysis,
predictions of biological activity encompass:
1. The stoichiometry of the metabolic process
2. The concentration of the required nutrients in
the mobile phases
3. The advective flow of the mobile phases or the
steepness of concentration gradients within
the phases
4. The opportunity for colonization of the
subsurface by metabolically capable organisms
5. The toxicity exhibited by the waste or a
co-occurring material.
Biological processes have two obvious implications for
monitoring releases from underground storage tanks.
Soil microbes can consume a small leak and can
prevent the spread of organic contaminants; in this
way they effectively mask the presence of the leak. In
this role biological activity protects the quality of
associated ground water, but it greatly complicates the
task of monitoring for a leak by chemical analysis for
compounds originally present in the tank. The second
implication follows from the first. If release of organic
materials elicits biological activity, it may be more
convenient to monitor for some other consequence of
metabolism, particularly if the concentration of the
released material is reduced below the analytical
detection limit. The most promising candidates are (1)
reduction in oxygen concentration, (2) increase in sol-
uble iron, (3) methane production, and (4) a reduction
in electrode potential associated with changes in the
redox status of the subsurface environment.
Organic Pollutants in Ground Water
Ground water pumped from aquifers is an important
source of water in much of the industrialized world.
Unfortunately, many ground-water supplies have been
polluted with organic chemicals used in industry,
agriculture, and the home. These chemicals find their
way into aquifers from accidental spills, leaking
underground storage tanks, landfills of industrial or
municipal wastes, septic tanks, leaking industrial
impoundments, agricultural pest control, and even
through pollutants dissolved in rain water. Micro-
organisms in aquifers can transform many organic
pollutants into harmless substances. Occasionally the
organic contaminants are transformed into new
substances that are even greater threats. The biode-
gradation of a particular class of organic contaminants
depends on the physiological capabilities of the
organisms present in the aquifer. These capabilities
depend in turn on the geochemical environment of the
organisms. As a result, the behavior of organic com-
pounds released from underground storage tanks
depends on the geochemical properties of the ground
waters that receive them.
5-1
-------
Ground Water Containing Oxygen
Many water-table aquifers contain oxygen; these
aquifers can support aerobic microorganisms that can
degrade a wide variety of organic contaminants.
Examples include benzene, toluene, the xylenes, and
other alkylbenzenes that leak into ground water from
gasoline spills or solvent spills (Wilson et al., 1986;
Lee et al., 1984); naphthalene, the methylnaph-
thalenes, fluorene, acenaphthene, dibenzofuran, and
a variety of other polynuclear aromatic hydrocarbons
released from spilled diesel oil or heating oil (Wilson et
al., 1985); acetone, isopropanol, methanol, ethanol,
and t-butanol from solvent spills and gasoline (Novak
et al., 1984; Lokke, 1984; Jhaveri and Mazzacca,
1983); and many methylated phenols and heterocyclic
organic compounds seen in certain industrial waste
waters. Many synthetic organic compounds can also
be degraded. Examples include dichlorobenzenes
(Kuhn et al., 1985), the mono-, di-, and trichloro-
phenols (Sulflita and Miller, 1985), the detergent-
builder nitrilotriacetic acid (NTA) (Ward, 1985), and
some of the simpler chlorinated compounds such as
methylene chloride (dichloromethane) (Jhaveri and
Mazzacca, 1983).
The extent of biodegradation of these compounds in
ground water will depend on the concentration of
oxygen. For the compounds discussed above, roughly
two parts of oxygen are required to completely
metabolize one part of the organic. For example,
microorganisms in a well-oxygenated ground water
containing 4 milligrams/liter (mg/L) of molecular
oxygen can degrade only 2 mg/L of benzene. The
solubility of benzene, 1780 mg/L, is much greater than
the capacity for its aerobic degradation in ground
water. Obviously the prospects for aerobic metabolism
of these compounds will depend on their concentration
as well as on the concentration of other degradable
organic materials in the aquifer. Concentrated plumes
of organic contaminants cannot be degraded aero-
bically until dispersion or other processes dilute the
plume with oxygenated water.
Many of the commonly encountered organic pollutants
in aquifers are synthetic organic solvents that do not
ordinarily degrade in oxygenated waters. Examples
include tetrachloroethylene (PCE), trichloroethylene
(TCE), cis and trans 1,2-dichloroethylene, ethylene
dichloride (1,2-dichloroethane), 1,1,1-trichloroethane
(TCA), 1,1,2-trichloroethane, carbon tetrachloride, and
chloroform.
Ground Waters Producing Methane
Organisms that produce methane, called methano-
gens, are only active in highly reduced environments.
Molecular oxygen is very toxic to them. Methane can
be produced by fermentation of a few simple organic
compounds such as acetate, formate, methanol, or
methylamines. Molecular hydrogen can also support
a form of respiration in which the hydrogen is used to
reduce inorganic carbonate to methane. Although the
microorganisms that actually produce the methane
can use a very limited set of organic compounds, they
can act in consort with other microorganisms which
break more complex organic compounds down to
substances that the methanogenic organisms can use.
These partnerships or consortia can totally degrade a
surprising variety of natural and synthetic organic
compounds.
The rates of reaction are usually slow and often
require long lag periods before active transformation
begins (Wilson, 1985). Microbiologists are accus-
tomed to microorganisms that grow to high densities in
only a few days, and rarely conduct experiments that
last longer than a few weeks. However, the residence
time of organic pollutants in aquifers is at least months
or years and is frequently decades to centuries. As a
result, much of what was learned in earlier laboratory
studies cannot be applied to the subsurface environ-
ment. Currently, microbiologists are re-examining the
potential for biodegradation of organic contamination
in ground waters that actively produce methane and
are finding many unexpected reactions.
It was previously thought that the metabolism of
benzene, toluene, the xylenes, and other alkylben-
zenes required molecular oxygen as a co-substrate for
the enzyme that began the metabolism of this class of
compounds (Young, 1984). Thus, their metabolism
would not be expected in methanogenic environments.
Recently the metabolism of these compounds was
demonstrated in methanogenic river alluvium that has
been contaminated with landfill leachate (Wilson and
Rees, 1985). When radioactive toluene was added to
this material at least half the carbon was metabolized
completely to carbon dioxide. The same material also
metabolized several methyl- and chlorophenols
(Sulflita and Miller, 1985).
The halogenated solvents that are persistent in
oxygenated ground water can be transformed in
methanogenic ground water. Examples include tri-
chloroethylene, tetrachloroethylene, the dichloro-
ethylenes, 1,1,1 dichloroethane, carbon tetrachloride,
and chloroform (Parsons et al., 1984, 1985; Wood et
al., 1985). The chlorinated ethylenes undergo a
sequential reductive dehalogenation from tetrachloro-
ethylene to trichloroethylene, then to the dichloro-
ethylenes (primarily the cis isomer), and finally to vinyl
chloride (Wood et al., 1985). In some materials
appreciable quantities of vinyl chloride accumulate,
and the accumulation is unfortunate because this
compound is considerably more toxic and carcin-
5-2
-------
ogenic than its parent compound. In other materials
the vinyl chloride is further metabolized. The factors
that control the fate of vinyl chloride are at present
entirely unknown (Wilson, 1985). The chloroalkanes
follow a similar pattern (Wood et al., 1985); carbon
tetrachloride is converted to chloroform, then to meth-
ylene chloride, while 1,1,1-dichloroethane is converted
to 1,1-dichloroethane which in turn goes to ethyl
chloride.
These reductive dehalogenations resemble respi-
rations. In aerobic respiration, molecular oxygen
accepts an electron and is reduced to the hydrogen-
ated compound, water. The chlorinated compounds
accept electrons and are reduced to the corre-
sponding hydrogenated compound while the chlorine
is released as a chloride ion. It is unknown whether
these reductive dehalogenations benefit the micro-
organisms that carry them out. However, the active
microorganisms must have a source of hydrogen or
some other organic compound to provide the electrons
for the reduction of the chlorinated compounds. The
source of electrons can be a co-occurring contam-
inant, such as volatile fatty acids in landfill leachate, or
it can be a geological material. Reductive dechlori-
nation of trichloroethylene has been associated with
flooded surface soil, buried soils in glaciated areas,
buried layers of peat, and coal seams (J.T. Wilson,
personal communication).
Ground Waters Reducing Sulfate or Nitrate
Once oxygen is depleted, certain classes of organic
compounds can be degraded by bacteria that respire
nitrate or sulfate. Ground waters recharged through
soils that support intensive agriculture often have high
concentrations of nitrate, and ground waters with
appreciable concentrations of sulfate are widespread,
particularly in arid regions. Microorganisms respiring
nitrate can degrade a number of phenols and cresols
(methylphenols). Recently, it has been shown that
nitrate-respiring organisms in river alluvium could
degrade all three xylenes (dimethylbenzenes) (Kuhn
et al., 1985). However, the microorganisms could not
degrade para-dichlorobenzene. Nitrate-respiring
micro-organisms can also degrade carbon tetra-
chloride and a variety of brominated methanes.
However, they have not been shown to degrade
chloroform or those chlorinated ethylenes or ethanes
which are also stable in oxygenated ground water
(Bouwer and McCarty, 1983).
Like the methanogens, the sulfate-respiring bacteria
can participate in consortia that degrade a wide variety
of natural organic compounds. In contrast to the
behavior of methanogenic subsurface material,
chlorinated derivatives of naturally occurring aromatic
compounds were not degraded in river alluvium
containing appreciable sulfate concentrations (200
mg/L) and exhibiting active sulfate respiration (Sulflita
and Miller, 1985; Sulflita and Gibson, 1985). As they
did in methanogenic material, tetrachloroethylene and
trichloroethylene underwent reductive dehalogena-
tions.
Conclusions
Biological activity can complicate monitoring for
materials released from underground storage tanks.
Materials in solution in ground water as well as vapors
in the unsaturated zone can be completely degraded,
or they can be transformed to new compounds. The
behavior of the released materials is controlled by the
availability of oxygen or other electron acceptors
required for microbial metabolism. It may be more
convenient, therefore, to monitor for the conse-
quences of biological activity, such as oxygen sag,
methane production, or reduced electrode potential,
than for the presence of the material reduced from the
tank.
5-3
-------
References
Bouwer, E.J.. and P.L. McCarty. 1983. Trans-
formations of Halogenated Organic Compounds
Under Denitrification Conditions. Applied and
Environmental Microbiology 45(4):1295-1299.
Jhaveri, V., and A.J. Mazzacca. 1983. Bioreclamation
of Ground and Groundwater Case History.
Presented at the 4th National Conference on Man-
agement of Uncontrolled Hazardous Waste Sites,
Washington, D.C., 31 October-2 November 1983.
Kuhn, E.P., P.J. Colberg, J.L. Schnoor, O. Wannen, A.
Zehnder, and R.P. Schwarzenbach. 1985. Micro-
bial Transformation of Substituted Benzenes
during Infiltration of River Water to Groundwater:
Laboratory Column Studies. Environmental
Science and Technology 19(10):961-968.
Lee, M.D., J.T. Wilson, and C.H. Ward. 1984.
Microbial Degradation of Selected Aromatics in a
Hazardous Waste Site. Developments in Industrial
Microbiology 25:557-565.
Lokke, H. Leaching of Ethylene Glycol and Ethanol in
Subsoils. 1984. Water, Air, and Soil Pollution
22:373-387.
Novak, J.T., C.D. Goldsmith, R.E. Benoit, and J.H.
O'Brien. Biodegradation of Alcohols in Subsurface
Systems. 1984. In: Degradation, Retention, and
Dispersion of Pollutants in Groundwater. Spe-
cialized Seminar. Copenhagen, Denmark, 12-14
September, 1984, pp. 61-75.
Parsons, F., G.B. Lage, and R. Rfce. 1985. Biotrans-
formation of Chlorinated Organic Solvents in Static
Microcosms. Environmental Toxicology and
Chemistry 4:739-742.
Parsons, F., P.R. Wood, and J. DeMarco. 1984.
Transformations of Tetrachlorethene and Tri-
chlorothene in Microcosms and Groundwater.
Journal of the American Water Works Association
76(2):56-59.
Sulflita, J.M., and S.A. Gibson. 1985. Biodegradation
of Haloaromatic Substrates in a Shallow Anoxic
Groundwater Aquifer. Proceedings of Second
International Conference on Ground Water Quality
Research, March 26-29,1984, Tulsa, Oklahoma.
Sulflita, J.M., and G.D. Miller. 1985. Microbial Me-
tabolism of Chlorophenolic Compounds in Ground
Water Aquifers. Environmental Toxi-cology and
Chemistry 4:751-758.
Ward, T. 1985. Characterizing the Aerobic and
Anaerobic Microbial Activities in Surface and
Subsurface Soils. Environmental Toxicology and
Chemistry 4:727-737.
Wilson, B. 1985. Behavior of Trichloroethylene, 1,1-
Dichloroethylene, cis-1,2-Dichloroethylene, and
trans-1,2-Dichloroethylene in Anoxic Subsurface
Environments. M.S. Thesis, University of
Oklahoma.
Wilson, B.H., and J.F. Rees. 1985. Biotransformation
of Gasoline Hydrocarbons in Methanogenic
Aquifer Material. In: Proceedings of the NWWA/
API Conference on Petroleum Hydro-carbons and
Organic Chemicals in Groundwater, Houston,
Texas, 13-15 November 1985.
Wilson, J.T., J.F. McNabb, J.W. Cochran, T.H. Wang,
M.B. Tomson, P.B. Bedient. 1985. Influence of
Microbial Adaption on the Fate of Organic Pol-
lutants in Ground Water. Environmental Toxi-
cology and Chemistry 4:721-726.
Wilson, J.T., G.D. Miller, W.C. Ghiorsc, and F.R.
Leach. 1986. Relationship Between ATP Content
of Subsurface Material and the Rate of Biode-
gradation of Alkybenzenes and Chlorobenzene.
Journal of Contaminant Hydrology, in press.
Wood, P.R., R.F. Lang, and I.L. Payan. 1985.
Anaerobic Transformation, Transport, and Re-
moval of Volatile Chlorinated Organics in Ground
Water. In: Ground Water Quality. C.H. Ward, W.
Giger, and P.L. McCarty (eds). John Wiley &
Sons, New York, pp. 493-511.
Young, L.Y. 1984. Anaerobic Degradation of Aro-matic
Compounds. In: Microbial Degradation of Aromatic
Compounds. D.T. Gibson (ed). Marcel Dekker,
New York, pp. 487-523.
5-4
-------
SECTION 6
Conclusions and Recommendations
As the preceding sections have shown, there exist
many gaps in our understanding of transport and fate
processes of fluids from leaking underground storage
tanks. The difficulties and uncertainties associated
with each of the fate and transport processes
described indicate that continued research is needed if
monitoring is to be successful. To bridge some of
these gaps, this section will restate the important
parameters relating to each process, describe the
monitoring approaches, and provide recommendations
for future research.
Process Parameters: A Synopsis
In each of the preceding sections, detailed de-
scriptions of two transport processes -- liquid and
vapor migration - and two fate processes - surface
effects and microbial degradation -- have been
presented. This section briefly summarizes the
important physical parameters of each process. A
knowledge and understanding of these parameters will
be necessary if monitoring systems are to be properly
used.
Table 6.1 shows the key parameters of the soil,
contaminant, and the environment which affect fluid
transport. Although some of the characteristics are
easily measured, i.e., fluid density, others are difficult
or impossible to measure and will require considerable
research. Table 6.2 shows some of the important
parameters controlling vapor transport of a volatile
substance. Although the list may be slightly shorter
than that for fluid transport, many of the parameters
have received very limited research. Table 6.3
describes the parameters controlling surface chemical
effects. Since this is not a transport process, those
parameters mentioned in Tables 6.1 and 6.2 must also
be examined to determine if the pollutant will reach the
soil particle surfaces. Finally, Table 6.4 summarizes
the key parameters controlling bioactivity in the
subsurface.
In summary, a great deal of data and research is
required to adequately describe the fate and transport
processes in the subsurface. Further research will be
needed to quantify the most important parameters.
Process Impacts on Monitoring
At this time, no one monitoring strategy is applicable
or standardized for monitoring subsurface contam-
ination. This apparent lack of standardization is easily
explained by the relative infancy of the science. It is
only in the last ten years that significant national
emphasis has been placed on monitoring ground-
water quality.
Subsurface monitoring approaches can be broken
down into two major categories: direct and indirect
measurements. Direct measurement includes
methods such as analysis of water samples, vapor
samples, and waste teachability as indicators of
contamination. Indirect measurements do not meas-
ure the contaminant per se but instead measure a
property which can be related to the effects of con-
tamination. Such indirect methods include surface
geophysics and borehole geophysics.
With regard to leak detection monitoring approaches,
both methods may be used. Direct measurement
techniques can be broken down into the following four
categories:
Active Liquid Sampling
Passive Liquid Sampling
Active Vapor Sampling
Passive Vapor Sampling.
Active liquid sampling implies that driving gradients
are artificially induced to increase the zone of effective
sampling. In the case of saturated zone monitoring,
well(s) are pumped to generate a cone of depression,
and fluid within the cone of depression flows by gravity
(or fluid pressure in the case of a confined aquifer) into
the well. Fluid samples from the well are analyzed for
dissolved contaminants while immiscible contaminants
may be monitored on the top or the bottom of the
water level in the well. The range of active sampling is
a function of the hydraulic parameters of the aquifer.
Active liquid samplers in the vadose zone include soil
suction lysimeters. These devices, acting similarly to
pumped wells, draw surrounding soil water (and pre-
sumably dissolved contaminants) into the sampler
through capillary gradients. The range of influence of
these devices, however, is limited to less than a
meter.
6-1
-------
Soil
Multiphase permeability
Residual saturation
Pore size distribution
Fracture density
Wettability
Soil texture
Porosity
Variability of soil properties
Table 6.1
Fluid Transport Parameters
Contaminant
Density
Viscosity
Solubility
Surface tension
Environmental
Temperature
Precipitation
Depth to water table
Water table gradient
Soil
Porosity
Water content
Soil structure and
variability
Permeability to air
Soil texture
Table 6.2
Vapor Transport Parameters
Contaminant
Volatility
Vapor diffusivity
Distribution coefficients
Environmental
Biological activity
Recharge
Temperature
Barometric changes
Water table fluctuations
Soil
Table 6.3
Surface Chemistry Parameters
Contaminant
Environmental
Moisture content
Organic content
Clay content
Soil surface area
Pore water chemistry
Solubility
Concentration
Temperature
Pressure
6-2
-------
Soil
Soil gas diffusion
Colonization potential
Oxygen concentration
Methane concentration
Contaminant velocity
Table 6.4
Microbiological Parameters
Contaminant
Environmental
Nutrient loading
Toxicity
Solubility
PH
Temperature
Recharge and
ground-water transport
The advantage of active liquid samplers is to sig-
nificantly increase the volume of the subsurface that is
sampled by a monitoring device. These techniques
also allow for the tentative quantification of the
magnitude of the contaminant. In addition, such
techniques tend to return to background levels quickly,
which allows for the delineation of small leaks and
spills versus massive contamination. On the other
hand, operational costs of active methods tend to be
rather high.
Passive liquid samplers operate in the same areas as
do active samplers. Instead of drawing potential
contaminant towards the sampler, however, passive
samplers assume that the contamination plume will be
large enough that it will pass through the samplers.
Passive liquid samplers include monitor-well bailing or
infrequent pumping, soil core water extraction, or
buried contaminant adsorption devices.
These techniques have low operating costs; however,
many samplers or samples are needed because of the
very limited range of effective monitoring. Since
convective transport occurs under slow, natural
gradients in passive samplers, long time periods may
be needed to return a sensor to a background con-
dition after a small spill or leak.
Active vapor samplers are useful for volatile contam-
inant monitoring. As in active liquid sampling, a zone
of lower air pressure is produced at a well point in the
unsaturated zone. Because of the much higher con-
ductivity of soils to air than water (except in nearly
saturated soils), soil air containing vapors from volatile
contaminants move rapidly by convection and dif-
fusion to the well point. The soil air is then analyzed
by various techniques for the dissolved contaminants
of interest.
This technique offers several advantages over liquid
sampling. The first major advantage is the speed at
which volatile contaminants may be detected. Vapor
movement through partially saturated soils is con-
trolled by concentration gradients, pressure gradients
and the soil water content. Since the conductivity of
soil to air is much higher than that of water, the vapor
migration rates may be on the order of hours to days
while the liquid migration rate may be on the order of
months to years. This time factor may be very critical
to prevent significant damage to existing water
supplies.
Both active liquid and active vapor sampling will
produce similar results with respect to sample inte-
gration, i.e., transient small spills and leaks should be
distinguished from larger, continuous leaks by the
concentration levels in the pumped sampler. Short
duration "spikes" would therefore not be indicative of a
continuous vapor source. Vapor samplers should
have considerably lower operating costs than would
active liquid samplers.
Passive vapor samplers have much of the same
advantages as do active vapor samplers and one
major disadvantage. Because of the rapid convection
and diffusion of vapors in soils, a passive vapor
sampler will have a large radius of sample influence,
and, therefore, few samplers would be needed. The
technique may be seriously deficient, however, in its
ability to discriminate between transient spills and
leaks from more serious long-term leaks. If a spilled
material is held up in the vadose zone, its removal by
volatilization may be very slow. Passive vapor sam-
plers near this source will continue to measure high
vapor content even though the liquid source is small.
A more major leak may never be detected by this
sampler since there may be no increase in vapor
density.
6-3
-------
Indirect Techniques
Two major classes of indirect measurement tech-
niques are currently available: surface geophysics and
borehole geophysics.
As the nomenclature implies, surface geophysical
techniques involve measurement of subsurface
properties from the surface. Within this category
numerous techniques are available; each having a
common thread. Each technique is designed to
measure variations of some physical property relating
to subsurface contamination. Variations in the hori-
zontal aspect (profiling) or vertical aspect (sounding)
are mapped, and anomalies can be used to delineate
contamination.
Surface geophysical methods may show promise for
the detection of some types of migrating contamin-
ants, particularly conductive materials. The methods
are economical, easy to repeat, and non-destructive.
On the negative side, techniques are not yet available
to delineate areas of low level contamination deamed
unsafe for drinking. Further clouding of the signal is
derived from the subsurface environment adjacent to
the tank which may be highly variable due to tank con-
struction.
Borehole geophysical methods allow for a detailed
view of the subsurface in the vertical dimension. The
techniques are quite similar to surface techniques;
however, the technique is limited to boreholes. Since
the technique is non-destructive, repeatable, and
comes in contact with the formation, it may be quite
useful in determining hydraulic properties and con-
taminant locations as a function of depth. It is limited,
however, to point location (wells) and has some of the
same drawbacks as passive liquid sampling.
Process Matrices
To accurately assess the applicability of monitoring
technologies to leak detection monitoring of under-
ground storage tanks, it is necessary to understand
both the advantages and limitations of each tech-
nology. In the previous sections, data have been
presented describing the effects of physical processes
on leaking fluids. This section presents these effects
versus the various monitoring technologies in a matrix
format. In the first matrix, the advantages produced
by each of the processes toward the adequacy and
reliability of the monitoring technology are presented.
For example, an advantageous process would be
microbial production of a reduced dissolved oxygen
"halo" in the saturated zone which would allow for
early detection of potential contamination by either
active or passive liquid sampling. The second matrix
shows the disadvantages or complications produced
by the process with respect to the monitoring tech-
nology. An example of a disadvantage would be the
impacts of adsorbtion and partitioning of the contam-
inant on soil surfaces retarding or decreasing the
contaminant to below detectable limits before it
reaches a passive liquid sampler. Leaks would there-
fore go undetected for a significant length of time, and
a large part of the soil/aquifer system would be con-
taminated.
The matrices are presented in the following order:
Table
6.5 Active and Passive Liquid Sampling -
Advantages
6.6 Active and Passive Liquid Sampling -
Disadvantages
6.7 Active and Passive Vapor Sampling -
Advantages
6.8 Active and Passive Vapor Sampling -
Disadvantages
6.9 Surface and Borehole Geophysics -
Advantages
6.10 Surface and Borehole Geophysics -
Disadvantages
Recommendations for Future Work
Based upon the preliminary discussions presented in
this document, the following conclusions may be
made:
1. There are many complicating factors for
detection of leaks from underground tanks. No
one monitoring approach will be applicable for
all applications.
2. Monitoring in the near-field environment of the
tank(s) holds less uncertainty than does
far-field monitoring.
3. Active samplers appear to be less affected by
transient spills; however, further research is
needed to develop sensor criteria.
In each of the processes described, the hetero-
geneous nature of geologic materials indicated the
need for a detailed sampling array. Uncertainty in flow
directions because of variations in conductivity, water
content, texture, etc., produced similar uncertainties in
terms of whether or not a sensor would detect a leak.
6-4
-------
Since an increase in the distance from the leak to the
sensor also increases the number of heterogeneities
encountered and, hence, the increase in the un-
certainty in flow direction, it is important to locate the
sensor close to the leak source. To take this point a
step further, tank installations may be engineered to
reduce further the heterogeneity near the tank, and
this would make the sensor location less sensitive.
For example, in an engineered installation, i.e., gravel
backfill, with sensors located within the backfill, the
uncertainty in leak detection would be much less than
if the sensors were located in the heterogeneous soils
outside of the immediate tank site.
Although additional research and field experience is
required, it appears that criteria for monitoring at
engineered installations could be developed fairly
easily. Developing criteria for preexisting installations
where engineering controls were not used will be
much more difficult, and much higher uncertainties will
be associated with these leak detection systems.
In conclusion, the study of monitoring systems for
underground storage tanks is in its infancy. This
document presents the fundamental theories and an
understanding of the processes controlling con-
taminant migration and fate in the subsurface. It is
hoped that future work will be based upon these
foundations.
Fate and Transport
Process
Table 6.5
Advantage Matrix for Liquid Monitoring Technologies
Monitoring Technology
Active Liquid Sampling
Passive Liquid Sampling
Liquid Flow
Vapor Flow
Physico/Chemical
Transformations
Microbiological
Activity
Control of gradient possible.
Quantification of leak possible.
Reduces effects of heterogeneity.
Easy clean-up possible.
Immiscible fluids easily monitored.
Vapor migration may contaminate
a large volume of aquifer because of
resolubilization aiding in rapid detection.
Short residence times may reduce
sorption in kinetic-controlled reactions.
Short residence times may reduce sorption
in kinetic-controlled bio-reactions.
Depressed D.O. halo may proceed actual
contaminant plume.
Increased soluble iron may occur because
of biotransformations, which is easily
monitored.
Quantification of leak possible.
Immiscible fluids easily monitored.
Low operating cost.
Vapor migration may contaminate
a large volume of aquifer because
of resolubilization aiding in rapid
detection.
If water levels do not fluctuate, a low
density contaminant will float on the
water surface and will only adsorb
on a small volume of aquifer material.
Depressed D.O. halo may proceed
actual contaminant plume.
Increased soluble iron may occur
because of biotransformations.
6-5
-------
Fate and Transport
Process
Table 6.6
Complications Matrix for Liquid Monitoring Technologies
Monitoring Technology
Active Liquid Sampling
Passive Liquid Sampling
Liquid Flow
Vapor Flow
Physico/Chemical
Transformations
Microbiological
Activity
Uncertainty in flow direction.
Large amount of water may be produced.
If contaminant is immiscible, spreading
of material may occur.
Dilution at pumped well may reduce
contaminant level to below detectable limits.
Non-leak sources may lead to false positives.
Large, explosive leaks may be present long
before detection.
Sorption may reduce contaminant levels below
detectable limits.
Selective microbial degradation may reduce
monitored parameter.
Pumping may stimulate activity adjacent to
well, masking actual contamination.
- Uncertainties in flow direction.
- Difficulty to collect representative
sample from bailing or intermittent
pumping.
- Non-leak sources may lead to false
positives.
- Large, explosive leaks may be
long before detection.
- Sorption may reduce contaminant
levels below detectable limits.
- Selective microbial degradation may
reduce monitored parameter.
Fate and Transport
Process
Table 6.7
Advantage Matrix for Vapor Monitoring Technologies
Monitoring Technology
Active Vapor Sampling
Passive Vapor Sampling
Liquid Flow
Vapor Flow
Physico/Chemical
Transformations
Microbiological
Activity
- Areas of low moisture content will enhance
flow.
- Rapid migration leading to early detection.
- Active pumping will lessen impacts of
transient spills.
- Concentration gradients may be discernible.
Sorption effects may be of secondary
importance.
Degradation effects may be of secondary
importance.
If degradation is occurring, monitoring for
depressed O2 or methane may be possible.
- Areas of bw moisture content will
enhance flow.
- Rapid migration leading to early
detection.
- Large, explosive leaks may be present
long before detection.
- Concentration gradients may be
discernible.
- Sorption effects may be of secondary
importance.
- Degradation effects may be of
secondary importance.
- If degradation is occurring, monitoring
for depressed O2 or methane may be
possible.
6-6
-------
Fate and Transport
Process
Table 6.8
Complications Matrix for Vapor Monitoring Technologies
Monitoring Technology
Active Vapor Sampling
Passive Vapor Sampling
Liquid Flow
Vapor Flow
Physico/Chemical
Transformations
Microbiological
Activity
- Variations in soil-water content affect vapor
migration.
- High water table precludes techniques.
- Requires volatile contaminant.
- Non-leak sources may lead to false positives.
- Variation in temp, pressure, water content
affect transport.
- Vapor migration paths variable.
- Leaking fluid must be available.
- Selective sorption may reduce vapor levels.
High O2 levels may enhance biological
activity, and hence reduce contaminant levels
below detectable limits.
- Variations insoil-water content affect
vapor migration.
- High water table precludes techniques.
- Requires volatile contaminant.
- Non-leak sources may lead to false
positives.
- Variation in temp, pressure, water
content affect transport.
- Vapor migration paths variable.
- Leaking fluid must be available.
- Selective sorption
may reduce vapor levels.
- High O2 levels may enhance biological
activity, and hence reduce contaminant
levels below detectable limits.
Fate and Transport
Process
Table 6.9
Advantage Matrix for Geophysical Technologies
Monitoring Technology
Surface Geophysics
Borehole Geophysics
Liquid Flow
Vapor Flow
Physico/Chemical
Transformations
Microbiological
Activity
Conductive plumes may be discerned easily.
Spreading of plume may be advantageous to
detection.
- No advantages.
- Transformation or sorption may produce more
"visible" plumes by alteration of aquifer
properties.
- Transformation or sorption may produce more
"visible" plumes by alteration of aquifer
properties.
- Vertical delineation possible.
- Spreading of plume advantageous as
it increases the range of detection
of the tools.
- No advantages.
- Transformation or sorption may
produce more "visible" plumes by
alteration of aquifer.
- Transformation or sorption may
produce more "visible" plumes by
alteration of aquifer properties.
6-7
-------
Table 6.10
Complications Matrix for Geophysical Technologies
Monitoring Technology
Fate and Transport
Process Surface Geophysics Borehole Geophysics
Liquid Flow - Techniques may be insensitive to leaking fluid. - Heterogeneities will require
- Variations in water content and aquifer large sampling network.
properties may obscure results. - Technique may be insensitive to
leaking fluid.
Vapor Flow - Present technologies not applicable to vapor - Present technologies not applicable
flow. to vapor flow.
Physico/Chemical - Adsorption may reduce sensitivity. - Adsorption may reduce sensitivity.
Transformations
Microbiological - Biodegradation may reduce sensitivity. - Biodegradation may reduce sensitivity.
Activity
6-8
*U.S. Government Printing Office : 1988 - 516-002/80048
------- |