&EPA
United States
Environmental Protection
Agency
Risk Reduction Engineering
Laboratory
Cincinnati OH 45268
EPA/600/2-90/027
June 1990
Research and Development
Assessing UST Corrective
Action Technologies:
Early Screening of Clean-
up Technologies for the
Saturated zone
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DISCLAIMER
The information in this document has been funded by the U.S. Environmental
Protection Agency under Contract 68-03-3409 to CDM Federal Programs
Corporation. It has been subjected to the Agency's peer and administrative
review, and has been approved for publication. Mention of trade names or
commercial products does not constitute endorsement or recommendation for
use.
11
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FOREWORD
Today's rapidly developing and changing technologies and industrial
products and practices frequently carry with them the increased generation
of materials that, if improperly dealt with, can threaten both public
health and the environment. The U.S. Environmental Protection Agency is
charged by Congress with protecting the Nation's land, air, and water
resources. Under a mandate of national environmental laws, the agency
strives to formulate and implement actions leading to a compatible balance
between human activities and the ability of natural resources to support
and nurture life. These laws direct the EPA to perform research to define
our environmental problems, measure the impacts, and search for solutions.
The Risk Reduction Engineering Laboratory is responsible for
planning, implementing and managing research, development, and
demonstration programs to provide an authoritative, defensible engineering
basis in support of the policies, programs and regulations of the EPA with
respect to drinking water, wastewater, pesticides, toxic substances, solid
and hazardous wastes, and Superfund-related activities. This publication
is one of the products of that research and provides a vital communication
link between the researcher and the user community.
An area of major concern is the health impacts associated with
uncontrolled releases of petroleum hydrocarbons from underground storage
tanks. This document focuses on remediation of petroleum product
contamination of the saturated zone.
E. Timothy Oppelt, Director
Risk Reduction Engineering Laboratory
111
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ABSTRACT
This manual assists the user in making a preliminary evaluation of
the likely effectiveness of various remediation technologies in the event
of a release of petroleum products into the saturated zone. The manual:
1) helps the user develop a conceptual understanding of site conditions
before extensive field studies are completed; 2) discusses remediation
goals; 3) identifies and helps the user evaluate technologies capable of
meeting remediation goals; and 4) discusses follow-up measures.
To help the user develop a conceptual understanding of the site
(i.e., conduct a preliminary site assessment) the manual shows how to
collect basic information about the subsurface environment and the released
product. The manual also provides default values for some parameters if
field data are not available or have not yet been collected. The user is
then assisted in making a preliminary determination of the likely phase
partitioning and mobility of petroleum products in the saturated zone. The
three "phases" considered in this manual are: 1) non-aqueous phase liquid
(NAPL); 2) dissolved in groundwater; and 3) sorbed to soil particles.
The overall focus of the manual is on making a preliminary screening
of what saturated zone treatment technologies would likely be effective at
a given release site. To facilitate the screening process, worksheets are
provided to aid in evaluation of the alternative technologies. For each
worksheet, factors critical to the successful implementation of a given
technology are presented. For each factor, the site conditions which favor
success as well as inhibit success are presented. The worksheets can be
compared to screen those technologies most likely to be effective at a
given site.
IV
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CONTENTS
Foreword iii
Abstract iv
Figures vi
Tables vi i
Acknowledgement viii
Abbreviations and Symbols ix
1. Introduction 1
Background 1
Purpose 3
Limitations 5
Scope 6
Approach and Organization •• 6
2. How to Conduct a Site Assessment 10
Introduction 10
Gathering Release Information 11
Gathering Site-Specific Information 13
Gathering Contaminant-Specific Information 21
Evaluating Contaminant Phases 28
Evaluating Contaminant Mobility 39
Setting Remediation Goals . 48
3. Technology Selection 53
Introduction 53
Containing NAPL and/or Dissolved Contaminant 55
Recovery of Floating NAPL 65
Treatment of Contaminants Dissolved in Groundwater ... 72
Above-ground Treatment 72
In-situ Treatment 82
New Technologies 87
Technology Compatibility 88
4. Follow-up Measures 91
Re-Evaluate Remediation Goals 91
Monitoring Technical Performance 91
References 96
Appendices
A How to Use the Worksheets 99
Glossary Ill
Blank Worksheets 116
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FIGURES
Number Page
1 Three Phases In Which Petroleum Products Can be Found
In Saturated Zone 2
2 Progression of a Typical Release Through the Subsurface 4
3 An Overview of the Approach 8
4 Groundwater Temperatures in the United States 18
5 Potential Exposure Pathways of Contaminants in the
Saturated Zone 20
6 Apparent Versus True Thickness of Floating NAPL 31
7 Ratio of Apparent to True NAPL Thickness 33
8 The Effect of Density on Contaminant Plume Migration 41
9 Examples of Poorly Designed Monitoring and Recovery Wells ... 42
10 Seasonal Variations in Groundwater Flow 44
11 Variability of Subsurface Stratigraphy 46
12 Inter-Aquifer Contaminant Transport 47
13 Density-to-Viscosity Ratios for Water and Hydrocarbons 49
14 Estimating Groundwater Travel Time 50
15 Schematic Diagram of Trench Excavation 56
16 Typical Pumping Well Installation for Containment of
NAPL and/or Dissolved Contaminant 60
17 Effect of Hydraulic Conductivity Differences on the Area of
Influence of Pumping Wells 61
18 Single and Dual Pump Recovery Systems 67
19 Schematic Diagram of a Vacuum Extraction System 69
20 Retention of Dissolved Contaminants in Less Permeable
Formations 74
21 Schematic Diagram of a Packed Tower Air Stripper 77
22 Typical In Situ Biorestoration System 84
VI
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TABLES
Number Page
1 Basic Release Information Assumed to be Known 12
2 Site-Specific Parameters 15
3 Physicochemical Properties of Rocks and Soil 16
4 Relationship Between Diameter of Particles and
Surface Area 17
5 Contaminant-Specific Parameters 23
6 Unweathered Composition of Three Common
Hydrocarbon Mixtures 24
7 Physicochemical Properties of Five Common Hydrocarbon
Mixtures 26
8 Physicochemical Properties of Individual Hydrocarbon
Constituents 27
9 Refractory Index for Common Hydrocarbon Compounds 29
10 Likelihood of Liquid Contaminants Being Present on the
Water Table 35
11 Likelihood of Dissolved Contaminants Being Present
in Groundwater 36
12 Worksheet to Evaluate the Feasibility of Trench Excavation .. 58
13 Worksheet to Evaluate the Feasibility of Pumping Wells 63
14 Worksheet To Evaluate the Feasibility of Vacuum Extraction
for Floating NAPL Recovery 71
15 Worksheet for Evaluating the Feasibility of Air Stripping
Being Effective at Your Site 78
16 Worksheet for Evaluating the Feasibility of Carbon Adsorption
Being Effective at Your Site 81
17 Worksheet for Evaluating the Feasibility of Biorestoration
Being Effective at Your Site „ 85
18 Compatibility of Saturated Zone and Unsaturated Zone
Technologies 90
vn
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ACKNOWLEDGEMENTS
This manual was produced by Camp Dresser & McKee Inc. (COM) under the
supervision of Dr. Myron S. Rosenberg as a work product associated with
Work Assignment No. 107 under Contract 68-03-3409 with EPA's Office of
Research and Development (ORD).
COM would like to acknowledge the guidance and assistance provided by
Mr. Anthony N. Tafuri, ORD's Project Officer, and Mr. Chi-Yuan Fan, ORD's
Technical Project Monitor for this Work Assignment. COM also appreciates
the help provided by Ms. Iris Goodman of EPA's Office of Underground
Storage Tanks in developing the overall focus of the manual.
Dr. Warren Lyman, Mr. David Noonan and Mr. Patrick Reidy of CDM
prepared this manual. Research and scientific evaluations that form the
basis of this manual were developed by Dr. Lyman and Mr. William Thompson
of PEI Associates, Inc., and were initiated and guided by Mr. John Farlow
of ORD. Mr. Roland Robinson, assisted by Ms. Catherine Hooper, prepared
the manuscript, and Mr. A. Russell Briggs prepared the graphics.
Vlll
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LIST OF ABBREVIATIONS AND SYMBOLS
ABBREVIATIONS
API
CDM
CSF
EPA
ORD
RREL
TOC
USGS
UST
WSF
ZOC
on/sec
cP
on/day
in/yr
g/cm
g/m3
gpm
mVg
mg/L
ram Hg
ppm
ppb
SYMBOLS
°C
9
cm
mm
L
gal
ft
mo
Hg
C
RI
American Petroleum Institute
Camp Dresser & McKee Inc.
critical success factor
Environmental Protection Agency
EPA's Office of Research & Development
EPA's Risk Reduction Engineering Laboratory
total organic carbon
United States Geological Survey
underground storage tank
water soluble fraction
zone of contribution
centimeters per second
centipoise
centimeters per day
inches per year
grams per cubic centimeter
grams per cubic meter
gallons per minute
meters square per gram
milligrams per liter
millimeters of mercury
liters per kilogram
parts per million
parts per billion
degrees Celsius
gram
centimeter
millimeter
liter
gallon
feet
month
year
percent
mercury
concentration of contaminant in ground water [mg/L]
distribution coefficient
soil/water partitioning coefficient
octanol/water partitioning coefficient
indicates alkaline or acid conditions in log units
refractory index
IX
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SECTION 1
INTRODUCTION
BACKGROUND
The release of petroleum products into the environment has become a
growing concern over the years. Of particular concern is contaminant
movement into the subsurface, where valuable groundwater and connected
surface water resources are at risk. Awareness of the problem has resulted
in the development and use of a variety of remediation techniques to remove
petroleum products from the subsurface.
An effective response to a release of petroleum products requires
understanding site conditions and defining appropriate remediation goals.
These vary from site to site and can range from no immediate action to
removal of all petroleum product contamination from the subsurface.
Selecting the technology or technologies that best meets these goals
involves identifying potential impacts to the surrounding environment,
recognizing the regulatory restrictions that may govern clean-up criteria,
and evaluating the likely effectiveness of alternative technologies.
To adequately address the problems associated with a release, those
responsible for remediation efforts must be well informed. This manual:
1) shows the user how to develop a conceptual understanding of site condi-
tions; 2) discusses various remediation goals that may be best suited to a
given site; 3) identifies the alternative technologies that address the
problem; and 4) provides a means to evaluate the likely effectiveness of
the technologies.
The focus of this manual is on technologies designed specifically for
clean up of the saturated zone and covers only remediation of contaminants
known to have reached the water table. In this manual, the saturated zone
is defined as the zone below the ground surface in which all pore spaces
are filled with water. Contamination reaching the saturated zone is
typically only a fraction of the total mass released, but the associated
costs are often greater than for treatment of the unsaturated zone.
Petroleum products reaching the saturated zone are subject to greatly
increased mobility of the contaminant, particularly in the horizontal
direction. Mobilization of the contaminants increases the magnitude of
clean-up efforts and the opportunity for liability.
Research done for EPA's RREL Office in Edison, New Jersey identified
13 different conditions under which contaminants released to the subsurface
as bulk petroleum product could be found (EPA, 1988a). However, most of
the released product will exist in only a few of these phases. For
simplicity, this manual assumes that petroleum products found in the
saturated zone exist almost exclusively in three phases: 1) non-aqueous
phase liquid (NAPL); 2) dissolved in groundwater (dissolved phase); and 3)
sorbed to soil particles (solid phase). Figure 1 is a schematic diagram
showing each of the three saturated zone phases covered in the manual.
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DISSOLVED
CONTAMINANT
CAPILLARY
ZONE
f
WATER
TABLE
SATURATED
ZONE
DIRECTIONOF
GROUNDWATER FLOW
SORBED
CONTAMINANT
SOIL
PARTICLE
Figure 1. Three Contaminant Phases in the Saturated Zone
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When petroleum products are released into the subsurface, they move
primarily by gravity down through the unsaturated zone, with only minor
horizontal movement (away from the release source) due to dispersion and
capillary forces. As the plume encounters the capillary zone above the
water table, the weight of the plume depresses first the capillary zone and
then the water table. The upward buoyancy forces of the groundwater slow
the downward movement of the leading edge of the plume, forcing the
contaminant to move horizontally in all directions. The liquid product
forms a pancake shape on top of the water table as the remainder of the
plume is drawn downward by gravity. If the release is terminated, the
trailing edge of the plume will reach the water table and eventually a
quasi-equilibrium is reached. The floating plume will then be confined to
horizontal flow, usually in the same direction as local groundwater.
Figure 2 shows a typical sequence of events in the first stages of a
release as they relate to the saturated zone.
After the bulk product has reached the water table, its liquid
density determines for the most part how the NAPL will move vertically in
the saturated zone. If heavier (more dense) than water, the bulk product
will sink through the saturated zone until it reaches a barrier. Most
petroleum products are lighter than water, and bulk product will tend to
float on the surface of the water table. However, some lighter than water
product can still find its way below the water table. As the force exerted
by the contaminant plume depresses the water table, groundwater around the
plume tends to rise or mound and equilibrium is lost. (A larger mass of
free product will cause a greater mounding effect than a smaller one.) The
groundwater then seeks to re-establish its equilibrium and may enclose part
of the free product plume, trapping NAPL below the water table that might
normally be expected to float. NAPL can also end up below the groundwater
table in areas with fluctuating groundwater levels (e.g., due to
infiltration events or tides), or due to burial by freshly infiltrating
water. NAPL trapped below the water table can present recovery problems.
Described herein are several technologies which are currently widely
used or show potential for success in removing contaminants from the
saturated zone. In addition, the manual assists the user in determining
when a technology is likely to be effective and, perhaps more importantly,
when it is not.
PURPOSE
The intended users of the manual are state and local officials
responsible for overseeing remediation or monitoring of releases of
petroleum products in the saturated zone, including leaking underground
storage tanks (USTs). These officials can use the manual to screen
alternatives prior to collection of extensive field data. The knowledge
gained will help the regulators work with the responsible party to develop
a well-focused field program and better guide the early stages of
remediation.
By using this manual, screening of technologies can be accomplished
at relatively low cost and within a short time (approximately one to two
weeks). The savings in time and cost are realized by estimating many of
the parameters needed to characterize a site. The tables, figures and
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* Vv^K^^
Unsaturated Zone Jjjik'.I—*- NAPL
i
Capillary Zone
Water
Table
Saturated Zone
Confining Layer
4ZZ5ZZZ4t&^^
Beginning of Release
Unsaturated Zone
L .
Capillary Zone
| Capillary;
Saturated Zone
A
B
NAPL
Dissolved Contaminant
Table
Confining Layer
Release Continues
"•'•'•' 'cbritamiriated Soil ••••••••••••••••••••••••••••••
CortTlnlriQ Layer
>^/^/^X/^XX>^
Source of Release Is Eliminated
Figure 2. Progression of a Typical Petroleum Product Release
from an Underground Storage Tank
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explanatory text in this manual help the user estimate these parameters and
complete a site assessment. As used in this manual, a site assessment is
analogous to developing a conceptual understanding of a site so that a
working hypothesis can be made of how the petroleum products will behave
in the subsurface and the impact of the release on the environment. The
site assessment will typically be based on readily available data, but can
and should be enhanced with field measurements.
It is important to distinguish between the estimated characterization
of a site under a preliminary site assessment and an actual characteriza-
tion based on extensive field studies. A site assessment uses estimates of
many site-specific parameters, and the more parameters that are estimated,
the less accurate will be the understanding of actual site conditions.
Nonetheless, the manual can be used to help produce a reasonably accurate
characterization of the site, define reasonable clean-up goals, and
identify those technologies most likely to be effective in achieving the
clean-up goals.
LIMITATIONS
Users of the manual should be aware of the following limitations of the
document.
• This is Not a Design Manual - This document is not intended to be
a design manual for engineers whose primary goal is final
technology selection and design. Rather, it is intended as a
pre-design or pre-selection manual more suited to a preliminary
screening of alternatives likely to be effective.
• Saturated Zone Coverage Only - This document addresses contaminant
releases that are known or assumed to have reached the saturated
zone. It does not cover the unique problems associated with
remediation of the unsaturated zone. Technical guidance in
evaluating other stages of remediation can be found in the
companion Unsaturated Zone document, among other sources.
• Not for Emergency Response - The comprehensive clean-up of an UST
involves several stages of remediation. First among these are
emergency measures involving tank removal and response to any
potentially explosive conditions resulting from the leak. These
measures are assumed to have been addressed prior to initiation of
saturated zone assessment.
• Evaluation is Scientifically Based - Evaluation of the various
clean-up technologies presented in this manual is based primarily
on scientific and engineering related factors. Other considera-
tions, such as cost or equipment availability, are often very
important in assessing and selecting a technology. The user
should be aware of such considerations when assessing a site.
• Focus on Petroleum Hydrocarbons as Contaminants - Because
petroleum products are the most common materials stored in USTs,
petroleum is targeted as a contaminant in this manual. However,
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this does not preclude the use of the approach of the manual in
evaluating other types of contaminants.
Clean-up Goals Can Only be Addressed Qualitatively - Technology
selection is usually based on what clean-up goals must be
achieved. It is beyond the scope of this manual to define what
clean-up goals should be set at each site. Rather, a qualitative
discussion is provided that raises the issues that are important
in setting up clean-up goals and defining "how clean is clean."
SCOPE
The following items provide an overview of the principal components
of the manual.
• For each treatment technology a unique set of factors (critical
success factors or CSFs) plays a major role in the success or
failure of the technology at a given site. For each technology
covered in this manual, important CSFs are identified that should
be considered when evaluating a site.
• The user is aided in developing a conceptual understanding of site
conditions and in evaluating the contaminant in the three chemical
phases discussed in this manual: 1) non-aqueous phase liquid
(NAPL); 2) dissolved in groundwater, and 3) sorbed to the soil.
• Several technologies are presented that have proved successful, or
shown promise of being successful, and worksheets are provided to
assist in evaluating the technologies.
• Procedures to quickly assess the advantages and disadvantages of
the technologies without conducting extensive and costly field
studies are presented.
APPROACH AND ORGANIZATION
The manual is designed to be understandable to a user without an
extensive technical background. Therefore, a simplified, yet scientifi-
cally-based, approach to assessing petroleum releases and evaluating
clean-up technologies is used. The approach emphasizes posing and
answering the types of questions that should be asked when confronted with
removing contaminants from the saturated zone.
Some aspects of a site assessment require a rather complex evaluation
process, making it difficult to obtain quantifiable results. These are
dealt with in a more qualitative manner without the need of complicated
equations or professional expertise. Much of the information required to
assess a site can be obtained through the explanatory text, tables,
figures, tabulated default values, and worksheets presented in the manual.
Examples are provided in the Appendix to demonstrate how to use the
worksheets.
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Figure 3 shows an overview of the basic components of the approach
used in the manual: 1) site assessment; 2) technology screening; and
3) follow-up measures. Each is briefly discussed below.
Site Assessment
Section 2 helps the user develop a conceptual understanding of
overall site conditions by:
• showing the user what information is required and ways it can be
obtained;
• helping the user estimate the mobility of the contaminant in the
saturated zone (how quickly the contaminant moves through the
subsurface);
• helping the user estimate the degree of partitioning of
contaminant in the saturated zone (how much of the contaminant is
likely to be in each of the three phases); and
• discussing the various types of remediation goals that might be
appropriate at a given site.
Technology Screening
After a conceptual understanding of site conditions has been
developed, but prior to screening of alternative technologies, the user
should decide on the most appropriate first course of action (i.e., set
remediation goals). This decision will guide the beginning stages of
remediation. Section 3 presents various technologies that are applicable
to remediation of the saturated zone. As Figure 3 shows, the section is
organized so that technologies best suited to a particular remediation goal
are grouped together and can be easily compared. Factors that influence
the overall effectiveness of each technology are identified in worksheets,
which can be used to aid in screening the technologies.
More than one technology may be required at a site to improve overall
remediation. However, not all technologies are compatible when used in
combination. Included at the end of Section 3 is a brief discussion of
which technologies are more or less compatible.
Follow-up Measures
Follow-up measures are an important aspect of overall remediation,
particularly since remediation often involves several different stages or
phases that are implemented over an extended time period. This manual
offers a method of quickly pre-screening alternatives as a first response
to a petroleum product release. Further stages of remediation can also be
examined using this manual once secondary goals have been defined. This is
illustrated in Figure 3. For example, the most immediate goal of
remediation may be to prevent off-site migration of dissolved contaminants.
Urgency may dictate that this phase begin prior to finalizing all stages of
remediation. At a later time additional goals may be defined that can be
examined the same way the initial goal was.
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SECTION 2
E ASSESS
SIT
Consider Available
Hydrogeologic Data
Develop a Conceptual Understanding of the Site
o How mobile are the contaminants
o What phase(s) are they in
o Locate vulnerable receptors
Define Remediation Goals
What Is the Primary or Immediate Goal of Remediation?
Remediation
Coals
3
REENI
ECTI
OGY SC
TEC
fao Immediate] |
[ Action J \Q(
Prevent Further
ontammantMiqration
Treatment of
Contaminanta
May Be
Required
Recover or Remove
Contaminants
]
J
o Pumping
Wells
o Trench
Excavation
o Vacuum
. Extraction
V>
%
Re-evaluate Remediation Goals
No Action
Should active
remediation be initiated?
Containment
la removal / treatment
of contamlnanta required?
Recovery/ Removal
Should exiating recovery
operalona be expanded?
NO
Are initial remediation goals being met?
YES
YES
Continue to monitor until goals are
met and/or until regulatory agency
approves discontinuance
Figure 3. An Overview of the Approach
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Another important function of site monitoring is to evaluate the
success of the technology being used to remediate the site. A given
technology may not be successful at a site for a variety of reasons,
including a poor understanding of site conditions. Section 4 discusses how
to monitor the performance of a technology so that poor performance can be
recognized and corrected.
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SECTION 2
HOW TO CONDUCT A SITE ASSESSMENT
INTRODUCTION
After emergency measures have been addressed at a site, the first
step in a remediation plan should be to gain a general understanding of
conditions at the site. The more that is known about the site, the easier
it will be to set clean-up goals and select a plan that will effectively
achieve these goals. The framework for a typical site assessment is
outlined in this section and can be followed to help develop a preliminary
"working hypothesis" of site conditions. The assessment should, at a
minimum, answer the following questions:
• what contaminants were released? - The type of bulk petroleum
product, the constituents comprising the bulk product, and the
physical and chemical properties of both the bulk product and its
constituents must be known.
• How much of the contaminant remains as free product and how much has
dissolved into the groundwater? - Estimating the volume of
contaminant aids in understanding subsurface behavior and helps to
formulate clean-up goals and assess removal effectiveness during
follow-up monitoring.
• How far, how quickly, and in what directions) has the contaminant
traveled? - The areal and vertical extent and movement
characteristics of the contaminant are very important in deciding
what course of action is to be taken.
• What receptors are likely to be impacted by the release? - Knowledge
of possible impact to receptors (e.g., nearby wells or streams) will
determine to a great extent clean-up priorities and short and long
term goals.
• What regulatory requirements apply to the site? - Regulatory
standards are instrumental in determining the degree of clean-up
required and the quality of effluent discharged from treatment
processes. These in turn play a large role in developing the goals
of remediation and selecting the technologies that will effectively
meet those goals.
Answering these questions will give the user a conceptual under-
standing of site conditions and help guide the initial stages of the
remedial clean-up plan. More information can improve the evaluation, but
is not essential for a preliminary assessment.
A distinction is made here between bulk product and the individual
constituents that make up the bulk product. Examples of bulk product
include automotive gasoline, No. 2 fuel oil, and jet fuel. These bulk
products, in turn, are composed of hundreds of individual constituents,
each with its own set of chemical properties. The individual constituents
10
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dissolve in water and evaporate at rates that are somewhat different than
the rate of the bulk product. (The chemical properties of the bulk product
tend to be a composite of the chemical properties of its individual
constituents.) The distinction between bulk product and individual
constituents is important because the bulk product will "weather" (break
down into its simpler component parts) over time. Further discussion of
the distinction between weathered and unweathered product is provided in
the subsection on obtaining contaminant-specific information.
An important first step in developing a good conceptual understanding
of a site involves collecting data unique to the site in question. Data
collection can be grouped into three general categories: 1) release-
related; 2) site-related; and 3) contaminant-related. Each of these
categories is described in greater detail on the following pages. For a
site assessment, these data can, for the most part, be found in tables and
figures from various literature sources or through local or regional
agencies or offices. The data are used to evaluate contaminant behavior in
the subsurface (i.e. where is it, what phase is it in, and where is it
going?) and how the contaminants will impact the environment.
The following subsections are structured to help the user collect and
evaluate data useful in a site assessment. Tables, figures and relatively
simple calculations are presented along with the text to aid in a con-
ceptual understanding of site conditions and in formulating clean-up goals.
GATHERING RELEASE INFORMATION
The initial step in any investigation is to become familiar with the
circumstances of the release. Answering the questions in Table 1 provides
a familiarity with the site and a foundation for conducting the remainder
of the assessment.
What was released? - Knowing what type of petroleum product was
released allows a determination of the physical and chemical properties and
composition of the contaminant. Often the owner/operator will know the
source of the release.
How much was released? - Knowing the volume of contaminant released
is important.A very small release may not even reach the saturated zone
before attenuation prevents further downward migration, particularly in
areas where the depth to groundwater is great. Indications of a very large
release should alert responsible parties to engage a clean-up team with the
appropriate experience and capabilities. Knowing the size of the release
also helps to define clean-up goals and provide a means to measure
performance.
For a leaking UST, the time period over which the leak occurred and
the flow rate must be estimated, making volume determinations difficult,
particularly since the leak rate can vary over time. Often, the best way
to estimate the volume of a release is to examine unaccounted-for losses in
the records kept by the owner/operator of the tank. These can provide
clues as to the volume of the release and when the leak started. These
records are not always accurate and may not reflect the actual history of
the leak, especially if the problem was known but concealed. But there is
11
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TABLE 1. BASIC RELEASE INFORMATION
ASSUMED TO BE KNOWN
INFORMATION NEEDED
WHY INFORMATION IS IMPORTANT
What contaminants were released?
Physical and chemical properties differ for each
contaminant, leading to varying phase
partitioning, mobility, and degradation
characteristics for each contaminant.
Corrective action selection is tied to these
characteristics.
How much was released?
The amount released directly affects the
phases in which the contaminant may be found.
What was the nature of the
release (quick spill/slow leak)?
Phase partitioning and mobility of the released
contaminant are both affected by the nature of
the release. As a result, selection of
appropriate corrective actions may differ for
quick spills versus leaks over extended period
of time.
How long since the release?
Contaminants "weather" over time, that is,
change in composition due to processes such
as degradation, volatilization, and natural
flushing from infiltrating rainfall. This change in
composition directly affects the physical and
chemical properties of the bulk contaminant.
How was the release detected?
May provide insight into above questions and
areal extent and distribution of contamination in
the subsurface.
12
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often little else to go on and estimates based on this type of data must
suffice. Additionally, the owner/operator may have results of tank
tightness tests for the leaking UST that may provide clues as to the likely
rate of the release.
What was the nature of the release? - The nature of the release
refers to how and when the contaminant was released to the environment.
Was it a large release over a relatively short time span (a quick spill) or
did small amounts of product escape through a crack in an UST over a long
time period (a slow leak)? Answering this question provides information
about the likely phase distribution and mobility of the contaminant and
some idea of the extent of weathering. A slow leak that began long ago is
likely to have spread farther and undergone considerably more weathering
than a quick release. There is also likely to be less free product in a
slow leak than in a quick spill (assuming equal spill volumes).
GATHERING SITE-SPECIFIC INFORMATION
Site-specific information pertains to the hydrologic and geologic
characteristics of the site. Historic hydrologic data is typically
available through state or local climatology offices or through the local
USGS office. These records can provide a reliable and accurate source of
typical rainfall patterns in the region. Geologic characteristics on the
other hand vary greatly, even over short horizontal or vertical distances.
Most states have extensive resources which show typical regional geologic
features and conditions. Accurately estimating soil parameters at an
individual site is difficult. Regional geologic data, such as can be found
at the local USGS office, can be used to estimate the type(s) of soil
formation likely to underlie a given site.
A great deal of useful information pertaining to the site is likely
to have already been compiled and available through a variety of sources.
A review of these data can prove invaluable to a preliminary assessment.
The following list can help the user collect site-specific data:
• Local public works departments or similar agencies can provide
plans of roadways, water pipelines, and sanitary and storm sewers.
These drawings may contain boring logs that can help characterize
site stratigraphy. Department employees are usually knowledgeable
about the types of materials likely to be encountered during
excavation and depth of the local water table. They are also
likely to have information on the location of groundwater wells
and surface water bodies used for drinking water in the area.
• Utility companies should be advised of the project and usually can
provide plans of gas or oil pipelines and power and telephone
cables.
• State UST programs may have a variety of useful information.
• The USGS can provide topographic, geologic and hydrologic maps and
data. Local sources of water supply may also be found at the USGS
office along with pumping rates of groundwater wells.
13
-------
• U.S. Department of Agriculture Soil Conservation Service (SCS) has
extensive data bases related to the subsurface parameters. County
extension offices are common and can provide maps showing the
types of soil formations and various hydrogeologic parameters
likely to be encountered at the site.
• State or local climatology offices can provide typical annual
rainfall rates and actual rainfall records for the period since
the release occurred.
• The U.S. Census Bureau has a variety of water use data available
to the public.
• Engineering firms that designed or built nearby buildings may have
drawings and other data useful in the assessment. Local surveyors
may also be able to provide useful information.
• Ask questions! This is only a starting point for collecting
historic data. The people you talk to may not have what you are
looking for, but they might know where to get it.
Estimating the type of soil formation (geologic conditions)
underlying a site is probably the single most difficult aspect of a site
assessment. It is also a very important parameter because many other
site-specific parameters are estimated based on the understanding of site
geology. Therefore, spending a significant time to develop an
understanding of site geology is justified.
Table 2 presents site-specific data useful in a site assessment.
This table, in conjunction with others in this subsection, can be used to
estimate many site-specific parameters. Included in Table 2 are:
1) site-specific parameters; 2) a default source for obtaining the data;
and 3) the importance of the parameter in the assessment. The default
source column has been provided to facilitate a quick, preliminary site
assessment. Field measurements are indicated as the default source where
the measurements are easily obtained or where estimates are inappropriate
due to inadequate or highly variable data. For the remaining parameters,
values can easily be found in tables and figures within the manual or
through the sources listed above. An example of how to use Table 2 in
conjunction with other tables and figures in the manual is presented later
in this subsection.
Descriptions of the field measurements required in Table 2 are not
provided in this manual. If a description of these or other field
measurements is desired, there are many publications that discuss in detail
how to conduct field tests and collect and analyze samples.
Table 3 lists values for physicochemical properties of various types
of soils and rock. These properties are important in determining how
liquids (both groundwater and bulk product) move through the subsurface.
Table 4 shows typical soil grain sizes and unit surface area values for
various soil types. Soil surface area per unit volume of soil can be used
to estimate the degree to which contaminants become sorbed to soil
particles. Soil grain size can be used to estimate the capillary forces
14
-------
TABLE 2. SITE-SPECIFIC PARAMETERS
PARAMETER (UNITS^
Soil Porosity (%)
Particle Density (g/cm3)
Bulk Density (g/cm3)
Hydraulic Conductivity (cm/sec)
Permeability (cm2)
Air Conductivity
Depth to Groundwater (m)
Groundwater Temperature (°C)
Soil pH
Rainfall Infiltration Rate
(cm/day)
Soil Surface Area (m2/g)
Organic Content
of Soil (%)
DEFAULT SOURCE
Table 3
Table 3
Table 3
Table 3
Table 3
Table 3
Field Measure or through
Site or Local records
Figure 4
Measure
Local precipitation and
evaporation records
Table 4
Measure
IMPORTANT FOR DETERMINING:
Mobility, Phase
Mobility, Phase
Mobility, Phase
Mobility, Phase
Mobility, Phase
Mobility, Phase
Phase
Mobility, Phase
Bacterial Activity
Mobility, Phase,
Mobility, Phase
Mobility, Phase
15
-------
TABLE 3. PHYSICOCHEMICAL PROPERTIES OF ROCKS AND SOIL
Rock/Soil Type
UNCONSOLIDATED
Gravel
Sand
Loam
Silt
Clay
CONSOLIDATED
Sandstone
Shale
Granite
Granite (fractured)
Limestone
Limestone (Karstic)
Basalt (permeable)
Soil
Porosity
(%)
25-40
25-50
42-50
35-50
40-70
5-30
0-10
0-5
0-10
0-20
5-50
5-50
Particle
Density
(g/cm3)
2.65
2.65
2.65
2.65
2.25
2.65
2.25
2.70
2.70
2.87
2.71
2.96
Bulk
Density
(g/cm3)
1.59-1.99
1.33-1.99
1.33-1.54
1.33-1.72
0.68-1.35
1.86-2.52
1.98-2.25
2.57-2.70
2.43-2.70
2.30-2.87
1.36-2.57
1.48-2.81
Hydraulic
Conductivity
(cm/sec)
10-1 - 102
10'4-10
10-5-10'1
10-7-10'3
10-10 -ID'7
10'8 -ID"4
10' H -10'7
10'1 1-10'8
10-6-10'2
10-7-10'4
10'4-1
10'5 - 1
Permeability
(cm2)
10'6-10-1
10-9-10-5
10-1° -10'6
10-12-10'8
10-15-10-12
10-13-10'9
10'16-10-12
10-16 -10'13
10"11 -1Q-7
10-12-10-9
10"9 -10"5
10-10.10-5
Air
Conductivity
(cm/sec)
10'2-10
10'5-10-1
10-6-10-2
ID'8 -10'4
10-H-10-8
10-9-10'5
10'12-10"8
10-12.10-9
ID"7- 10"3
10'8-10"5
10"5. 1Q"1
10-6-10-1
* Air conductivity values at 10°C. Values are based on the ratio of hydraulic conductivity to air conductivity (Kr); where K^ (viscosity of air/viscosity of water)
(density of water/density of air)
Source: Adapted from Freeze and Cherry, 1979, and Krishnayya etal. 1988.
-------
TABLE 4. RELATIONSHIP BETWEEN DIAMETER OF PARTICLES
AND SURFACE AREA
Diameter of
Particles (mm)
1.0-2.0
0.5-1.0
0.25 - 0.5
0.1 -0.25
0.05-0.1
0.002 - 0.05
<0.002
Description
Very Coarse Sand
Coarse Sand
Medium Sand
Fine Sand
Very Fine Sand
Silt
Clay
Approximate Surface
Area (rr^/g)
0.001 - 0.003
0.003 - 0.005
0.005 - 0.01
0.01 - 0.03
0.03 - 0.1
0.1- 1
>1
Source: Adapted from Hillel, 1980
17
-------
likely to be exerted on groundwater or liquid contaminant. Figure 4 is a
contour map of the United States showing typical groundwater temperatures.
Groundwater temperature is useful in determining whether biorestoration
will be effective at a site. An estimate of the groundwater temperature at
a site of interest can be made by interpolating between the two contour
lines on either side of the site location.
After geologic conditions have been estimated, Table 2, along with
other tables and figures of this subsection, can be used to provide default
values for some site-specific parameters. To illustrate, assume that the
subsurface is found to be predominantly sand and gravel with several
discontinuous clay layers or lenses at various intervals. If an estimate
of the hydraulic conductivity at the site is needed two values will be
required because the clay lenses are much less permeable and restrict
groundwater movement relative to the sand and gravel. In Table 2, the user
can locate the hydraulic conductivity parameter in the left-most column and
identify Table 3 as the default source. Turning to Table 3, the user can
locate values in the hydraulic conductivity column corresponding to sand,
gravel and clay. Notice that the values are presented as ranges, with no
other information, it is reasonable to assign values in the middle of the
range. For clay, an assumed value of 10 cm/sec is reasonable. For a
sand and gravel formation, a value of 10~ cm/sec is near the middle of the
two ranges and can serve as a preliminary estimate.
Other Important Site-Related Information - An important component of
a good remediation plan is a prioritization of clean-up activities. To
decide the best course of action at a site all exposure pathways (e.g.
groundwater, vapors, etc.) must be identified. This will help those
implementing a remediation plan address the most urgent aspects of
remediation first. Figure 5 is a schematic diagram of a typical site where
contamination of the saturated zone has occurred. The diagram includes
possible receptors and shows some of the exposure pathways whereby
contaminants could impact the receptors.
If done early in the remediation program, priority planning can save
unnecessary effort. For example, if a groundwater supply were located
1,000 feet downgradient of the release site, a first priority would be to
prevent the contaminant plume from reaching the supply. If this condition
is not identified early in the assessment, containment of the contaminant
plume might not be implemented quickly enough to prevent contamination of
the supply. As shown in Figure 3, this manual can help the user plan and
guide site activities. It identifies alternative technologies appropriate
for a given activity (i.e., remediation goal) and helps the user select the
best alternative.
Below are some of the factors which should be considered.
• Location of water resources - This includes surface water bodies
(lakes, rivers and wetlands) and aquifers. It is important to
note whether the resource is used for drinking water, but most
water resources will fall under regulatory protection regardless
of use. It is also important to note seasonal fluctuations in
water levels, which influence regional groundwater flow. In the
18
-------
(From Heath, 1983)
Figure 4. Groundwater Temperature in the United States (°C) at Depths of 10 to 25 M
-------
Human Inhalation due
to Vapor Accumulation
In Basements
Human and/or
Aquatic Wildlife
Exposure
Water
Supply
Well
NJ
O
Dissolved
Product
Plume
Confining Layer
Human Ingestlon from
Dissolved Containments
in Drinking Water
Figure 5. Potential Exposure Pathways Due to a Petroleum Product Release
-------
dry summer months, groundwater may flow into a surface water body
but during the spring, flow may be from the surface water to the
groundwater. The pumping rates of all groundwater wells in the
region should be determined, including seasonal changes in the
pumping rate. Pumping wells alter local groundwater flow
direction and may draw a contaminant plume when other data would
suggest the plume would travel elsewhere. The pumping rates can
also be used to determine capture zones within which groundwater
will travel to the well, but this is usually time consuming and
costly.
• Location of homes and buildings - Many petroleum products are very
volatile and vapors from the contaminant plume can accumulate in
the basements of homes and buildings. These vapors can be inhaled
and can also pose an explosive threat.
• Location of underground utilities in the site area - Backfill
materials used in utility trenches are often far more permeable
than the surrounding natural formations. Contaminants may travel
preferentially along these pathways, resulting in unexpected plume
migration patterns. Utilities may also interfere with
construction of some technologies.
• Clean-up criteria and disposal standards - Local or regional
regulatory agencies should be contacted at the start of the site
assessment. They often require that clean-up efforts continue
until groundwater quality reaches state or federal legal require-
ments and that wastewater or solid waste disposal regulations are
met. They will also likely want to be informed of remediation
plans and kept abreast of clean-up progress. Knowing what the
clean-up criteria are is important in technology selection because
not all technologies are equally effective in removing
contaminants.
GATHERING CONTAMINANT-SPECIFIC INFORMATION
Petroleum products are a mixture of many compounds. The physical and
chemical properties of the constituents are different than those of the
mixture, leading to the question of what to target during site assessment.
In general, remediation efforts are usually tied to one or more individual
constituents, but some aspects of the assessment warrant targeting the
properties of the mixture.
Properties of individual constituents should be targeted:
• When the phase of contamination being evaluated is other than the
original NAPL. For example, evaluation of the dissolved phase
should focus on the more soluble constituents of the mixture since
they are likely to comprise the greatest portion of dissolved
contaminant mass. For most petroleum products, the BTEX compounds
(benzene, toluene, ethylbenzene and xylene) comprise a significant
portion of all dissolved constituents and can serve as the focus
of dissolved contaminant evaluation.
21
-------
• If the mixture has "weathered" considerably. Weathering is the
process where a complex compound is reduced to its simpler
component parts or biodegraded over time. If a petroleum product
has been in the subsurface for several years, concentrations of
certain contaminants will be significantly different than when
originally released.
• To focus the assessment on a compound that is thought to present
the greatest potential threat. For instance, it may be desirable
to select a remedial technology that is likely to be effective in
removing benzene, a known human carcinogen and common constituent
of gasoline. An understanding of how benzene behaves in the
subsurface would be developed rather than an understanding of the
behavior of the mixture.
• To design a treatment system. The design of many technologies are
based on theories and equations that are sensitive to the
properties of the compound being treated. Specific compounds will
generally be targeted based on their impact to the site.
Properties of the NAPL mixture should be targeted:
• When the phase of contamination being evaluated is the original
NAPL.
• To evaluate the physical movement of NAPL through the subsurface.
Table 5 lists the contaminant properties useful in evaluating
contamination in the saturated zone and can be used in a manner similar to
Table 2. Included in Table 5 are: 1) contaminant-specific parameters
useful in assessing a site; 2) default sources where parameter values can
be located within the text; and 3) what each parameter is used for in the
assessment. To facilitate a preliminary site assessment, listings of
parameter values have been compiled from various literature sources. These
can be used to estimate contaminant-specific parameter values.
Looking at Table 5, the user can identify from the default source
column where in this manual literature values for each parameter may be
found. To illustrate, assume an estimate of the liquid density of
automotive gasoline is needed. The user can locate liquid density in Table
5 and find that the default source is Table 7. Moving to Table 7, the
typical value for automotive gasoline in the liquid density column is 0.73
g/cm .
Table 6 presents the composition of three common hydrocarbon products
in bulk. The table can be used to determine which constituents are likely
to be in a given petroleum product and to estimate what percentage of the
total product they represent.
Tables 7 and 8 list default values for various physical and chemical
properties of common hydrocarbon mixtures and individual hydrocarbon
constituents respectively. Table 7 would be consulted if the assessment
targeted removal of the bulk product. For example, if removal of floating
22
-------
TABLES. CONTAMINANT SPECIFIC PARAMETERS
PARAMETER (UNITS) DEFAULT SOURCE IMPORTANT FOR DETERMINING:
Vapor Pressure (mm Hg) Tables 7,8 Mobility, Phase
Water Solubility (mg/L) Tables 7,8 Mobility, Phase
Liquid Viscosity (cP) Tables 7, 8 Mobility
Liquid Density (g/cm3) Tables 7,8 Mobility
Soil Sorption Coefficient (L/kg) Table 8 Mobility, Phase
Refractory Index (ratio) Table 9 Degradation
Unweathered Composition (-)* Table 6 Mobility, Phase, Degradation
* Composition will change over time due to "weathering." See discussion in text.
23
-------
TABLE 6. UNWEATHERED COMPOSITION OF THREE COMMON HYDROCARBON PRODUCTS
Selected Representative Concentrations (%w/w]
Hydrocarbon
Group
n-Alkanes
C4
C5
C6
C7
C8
C9
C10-C14
Branched Alkanes
C4
C5
C6
C7
C8
C9
C10-C14
Cycloalkanes
C5
C6
C7
C8
Others
Olefins
C4
C5
C6
Others
Mono-aromatics
Benzene
Toluene
Xylenes
Ethylbenzene
C3-benzenes
C4-benzenes
Others
Representative
Hydrocarbon
n-Butane
n-Pentane
n-Hexane
n-Heptane
n-Octane
n-Nonane
n-Decane
Isobutane
Isopentane
2-Methylpentane
2-Methylhexane
2,4-Dimethylhexane
2,2,4-Trimethylhexane
2,2,5, 5-Tetramethylhexane
Cyclohexane
Methylcyclohexane
1 ,2,4-Trimethylcyclopentane
1 ,1 ,3-Trimethylcyclohexane
1-Butene
1-Pentene
1-Hexene
Benzene
Toluene
m-Xylene
Ethylbenzene
1 ,3,5-Trimethylbenzene
1,4-Diethylbenzene
1 2
Automotive #2 Fuel
Gasoline Oil
10.8 -
4.8 -
1.9 -
2.0 -
0.2 -
1.3
0.4 -
0.2
18.8 -
0.7 -
8.6 -
4.6 -
1.4 -
1.8 -
1.2 -
0.5 -
3.2 -
0.2
1.0 -
0.2 -
0.2 -
1.6 -
5.5 -
0.9
1.3
0.8
2.5 -
19.3 -
0.9 -
4.0
5.6 -
1.2 -
3.2
2.1 -
1.6
29.6
7.0
4.5
12.9
2.3
0.8
0.8
59.5
2.2
17.3
9.7
8.3
16.7
2.7
2.6
13.7
3.9
1.4
0.7
7.5
13.5
3.3
1.8
7.5
40.9
4.4
6.5
8.8 0.07
1.4 0.03
11.3 0.67
2.6 0.88
5.2
3
Jet Fuel
JP-4
0.12
1.06
2.21
3.67
3.80
2.25
8.73
0.66
2.27
5.48
8.82
3.36
1.35
2.40
3.77
1.35
3.21
0.50
1.33
2.32
0.37
3.59
3.98
(Continued)
24
-------
TABLE 6 (continued)
Hydrocarbon
Group
Phenols
Phenol
C1 -phenols
C2-phenols
C3-phenols
C4-phenols
Indanol
1
Representative Automotive
Hydrocarbon Gasoline
Phenol
o-Cresol
2,4-Dimethylphenol
2,4,6-Trimethylphenol
m-Ethylphenol
Indanol
2
#2 Fuel
Oil
0.001
0.01
0.02
0.02
0.01
0.001
3
Jet Fuel
JP-4
Poly-aromatics Fluorene
0.57
Nitro-aromatics
C1 -Anilines
C2-Anilines
Complex Anilines
Di-aromatics
Saturated hydro-
carbons
C8
C9
C10
C11
C12
C13
C14
C15
C16
C17
C18
C19
C20
C21
C22
C23
C24
Pristane
Phytane
Unknowns
Quinoline
Naphthalene
n-Octane
n-Nonane
n-Oecane
n-Undecane
n-Dodecane
n-Tridecane
n-Tetradecane
n-Pentadecane
n-Hexadecane
n-Heptadecane
n-Octadecane
n-Nonadecane
n-Eicosane
n-Heneicosane
n-Docosane
n-Tricosane
n-Tetracosane
0.003
0.004
0.002
0.7 3.43 1.59
0.05
0.20
0.58
0.98
1.14
1.20
1.31
1.42
1.53
1.51
1.31
1.16
0.99
0.51
0.29
0.15
0.05
0.52
0.46
6.6-13.8
NOTE: Blanks indicate the unavailability of data and do not indicate the absence of a particular
compound from the hydrocarbon product.
SOURCES:
Column 1: Hoag et al, 1984; EPA, 1984; Ghassemi et al, 1984.
Column 2: ICF, 1984
Column 3: Smith et al, 1981.
25
-------
TABLE 7. PHYSICOCHEMICAL PROPERTIES OF FIVE COMMON HYDROCARBON MIXTURES
PRODUCT
Automotive Gasoline
#2 Fuel Oil
#6 Fuel Oil
Jet Fuel (JP-4)
Mineral Base
Crankcase Oil
Air
Saturated Aqueous
Vapor
LIQUID
DENSITY
(g/cm3)
(0.73)
0.72-0.76 [15.6]
(0.91)
0.87-0.95
(0-91)
0.87-0.95
0.75
0.84-0.96 [15]
1
LIQUID
VISCOSITY
(cPoise)
(0.45)
0.36-0.49 [15.6]
(1.56)
1.15-1.97 [21]
(254)
14.5-493.5 [38]
0.829 [21]
275 [38]
1
WATER VAPOR
SOLUBILITY PRESSURE
(mg/L) (mm Hg)
(158) (469)
131-185 [13-25] 263-675
3-10 [20-23] (14.3)
2.12-26.4
~5 (14.3)
2.12-26.4
10-20 91
insoluble N/A
760
17 5
I / . ij
[38]
[21]
[21]
N/A = Not Available
Notes: All values for 20°C unless noted in brackets [ ].
Values in parentheses are typical of the parameter ().
Values for air and saturated aqueous vapor are included, where applicable, as a means of comparison.
Source: Compiled from various published and unpublished sources.
-------
Table 8 . Chemical Properties of Hydrocarbon Constituents
Chemical Class
n-Alkanes
C4
C5
C6
C7
C8
C9
C10
Mono-aromatics
C6
C7
C8
C8
C9
C10
Phenols
Phenol
C1 -phenols
C2-phenote
C3-phenols
C4-phenols
Indanol
Di-aromatics
Representative Liquid Density
Chemical (o/cm.3)
@20°C
n-Butane
n-Pentane
n-Hexane
n-Heptane
n-Octane
n-Nonane
n-Decane
Benzene
Toluene
m-Xytene
Ethylbenzene
1 ,3,5-Trimethylbenzene
1,4-Diethylbenzene
Phenol
m-Cresol
2,4-Dimethylphenol
2,4,6-Trimethylphenol
m-Ethylphend
Indanol
Naphthalene
0.579
0.626
0.659
0.684
0.703
0.718
0.730
0.885
0.867
0.864
0.867
0.865
0.862
1.058
1.027
0.965
NA
1.037
NA
1.025
Liquid
viscosity
(cPoise)
@20°C
0.177
0.224
0.306
0.409
0.542
0.620
0.740
0.638
0.580
0.608
0.666
0.727
0.700
12.7
20.8
NA
NA
NA
NA
NA
Water
solubility
(mg/L)
@25°C
61.1
41.2
12.5
2.68
0.66
0.122
0.022
1780
537
162
167
72.6
15
438
26175
NA
NA
NA
NA
31.7
Pure Vapor
Pressure
(mmHg)
@20°C
1560
424
121
35.6
10.5
3.2
0.95
75.2
21.8
6.16
7.08
1.73
0.697
0.529
0.15
0.058
0.012
0.08
0.014
0.053
Soil Sotption
Constant (Koc)
(L/kg)
@25°C
250
320
600
1300
2600
5800
13000
38
90
220
210
390
1100
110
8.4
NA
NA
NA
NA
690
NOTE: NA - Not availabe
SOURCE: Compiled from various published and unpublished sources.
27
-------
NAPL is being considered, the mobility of the floating NAPL plume would be
evaluated using the properties of the bulk liquid.
Table 8 would be used for individual constituents. For example, if
the user wished to evaluate the effectiveness of various technologies at
removing or treating dissolved contaminants, the more soluble components of
the bulk liquid, such as the BTEX compounds, would be targeted. It should
be noted that the contaminant-specific properties of petroleum product
constituents in Table 8 are for pure compounds. These values may be
different than would be found when a given compound is mixed with other
organic compounds. For example, the water solubility of pure benzene is
1,780 mg/L, but in a gasoline/water mixture the dissolution of benzene is
more likely to be in the 20 to 80 mg/L range.
Table 9 lists selected refractory index values for both hydrocarbon
mixtures and individual constituents. The refractory index is a measure of
the relative biodegradability of a compound. It is useful in evaluating
the likely effectiveness of biodegradation as a treatment technology and
can also be used to help determine the impact of weathering on a very old
release. For example, if a release occurred many years ago, natural
biodegradation has likely occurred. Less biodegradable compounds such as
xylene or ethylbenzene are more likely to persist in the groundwater than
the more degradable benzene. Values for some bulk product mixtures, such
as gasoline and kerosene, as well as common hydrocarbon constituents are
presented.
EVALUATING CONTAMINANT PHASES IN THE SATURATED ZONE
Petroleum products reaching the saturated zone will primarily be
found in one of three phases: 1) non-aqueous phase liquid (NAPL); 2)
dissolved in groundwater; and 3) sorbed to soil particles. Initially, for
relatively large releases, almost all of the petroleum product will exist
as NAPL, but over time, a portion of the NAPL will transfer to the
dissolved and sorbed phases. If the release is relatively small, all the
NAPL may be trapped in the unsaturated zone as residual liquid and
dissolved product will be the only contaminants reaching the saturated zone
(via infiltrating rainwater).
Estimating how much contaminant is in each of the three phases is
important in staging an effective remediation of the saturated zone. If
most of the contaminant is in the dissolved phase and/or if dissolved
contaminant is considered most likely to impact local receptors,
remediation efforts should recognize this. For a large, recent release,
most of the contaminant that reaches the water table likely remains as bulk
product on the water table and a remediation plan should likely be focused
towards NAPL recovery. In either case, the user could refer to Figure 3 to
identify technologies that are appropriate for a given condition.
Background
Quantitative estimates of the mass or volume of contaminant in any
one of the three phases requires field sampling and analysis. An overview
of commonly used procedures and some of the problems encountered in
28
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TABLE 9. REFRACTORY INDEX FOR COMMON HYDROCARBONS
RELATIVELY UNDEGRADABLE
COMPOUND RATIO
Butane ~0
Ethane ~0
Heptane ~0
Hexane ~0
Isobutane ~0
o-Xylene <0.008
m-Xylene <0.008
Ethylbenzene <0.009
MODERATELY DEGRADABLE
Gas oil (cracked) -0.02
Gasolines (various) -0.02
Mineral spirits -0.02
Nananol >0.033
Undecanol <0.04
1-Hexene <0.044
Dodecanol 0.097
RELATIVELY DEGRADABLE
p-Xylene <0.11
Toluene <0.12
Jet fuels (various) ^0.15
Kerosene ~0.15
Range oil HD.15
Naphthalene <0.20
Hexanol 2.0.20
Octanol 0.37
Benzene <0.39
Phenol 0.81
*The refractory index is the ratio of BODS to COD
Source: Adapted from Lyman et al, 1982
29
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estimating the amount of contaminant in each phase is presented below.
This is followed by worksheets that can help the user qualitatively
estimate phase partitioning with a minimum of field data.
Volume of Floating NAPL
Volume determinations of floating hydrocarbons are typically obtained
by the following equation:
where: A^ = areal extent of the NAPL plume;
TN = average thickness of the NAPL plume; and
n = effective porosity of the soil formation.
Several methods for determining areal extent of the plume are
described in the contaminant mobility subsection immediately following this
subsection.
The effective porosity of the soil formation reflects the actual
volume available (of the total pore volume) that can be occupied by NAPL.
Since some water will be retained in the pore spaces as residual
saturation, the volume available for NAPL will be less than the total pore
volume reflected in porosity values.
NAPL thickness is fairly difficult to estimate accurately because the
apparent thickness of the plume, as measured in a monitoring well, is
typically greater than the true thickness. NAPL thickness will also vary
throughout the plume, particularly if pumping wells are installed to
contain the plume. Measuring plume thickness at several locations allows
a better estimate of the average thickness.
Figure 6 is a schematic diagram of a typical monitoring well within a
free floating contaminant plume. The apparent thickness is generally
greater than the true thickness because the actual free product plume out-
side the well floats on the capillary zone above the water table rather
than on the water table itself. Capillary forces within the well are
virtually non-existent and the product within the well floats on the water
table. Additional free product will flow into the well due to the gradient
difference between product in the well and product in the adjacent forma-
tion. As additional product migrates to the well, the weight of the
column of bulk product in the well tends to depress the water level in the
well below that outside the well, increasing the difference between
apparent and true thickness.
Among the factors that influence the ratio of apparent to true free
product thickness are the capillary forces of the soil formation,
interfacial tension forces (ITFs), and the volume of product released.
Fine-grained materials such as clay or silt will have thicker capillary
zones and greater apparent to true thickness ratios than coarse materials.
If the ITFs between the contaminant and the soil particles are great, less
product is likely to migrate to the well, decreasing the ratio. The ratio
is also generally greater for smaller releases than for larger releases,
30
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Monitoring
Well
Ground Surface
i
Capillary Zone
f
Water Table
i
True NAPL
Thickness
T
Figure 6. Apparent Versus True Floating NAPL Thickness in a Groundwater Monitoring Well
T
Apparent
NAPL
Thickness
i
-------
because the capillary fringe is compressed more for large releases. The
apparent to true thickness ratio in clay can be as high as 10:1 while for a
coarse gravel this ratio may approach 1:1.
Figure 7 provides rough estimates of typical apparent to true NAPL
thickness ratios for various soil types. The equations used to develop the
figure assume a uniform pore size in the formation (Farr et al., 1990).
Most soils do not have uniform pore sizes, particularly tHe silts and clays
to the right side of the figure. However, these ratios provide a
reasonable estimate for a preliminary assessment. The shaded portion of
the figure represents possible ratios for a given soil type over the range
of typical porosity and residual saturation values for the soil. The curve
in the middle of the shaded area is the average ratio.
Several authors have proposed other methods to estimate free product
thickness. An empirical method (Hall et al., 1984) relates true to apparent
thickness with a formation factor. The formation factor varies with pore
size and hydrocarbon surface tension. Bail-down testing is a widely used
field method to evaluate the true thickness of petroleum hydrocarbon product
in a monitoring well (Testa and Paczkowski, 1989). These sources, among
others, can be consulted to estimate true thickness of the NAPL plume.
Mass of Dissolved and Sorbed Contaminants in Groundwater
To quantitatively estimate the mass of dissolved contaminants, the
dissolved plume volume and average concentration must be known. This
requires numerous samples from wells throughout the plume, both
horizontally and vertically. The accuracy of such an estimate will depend
on how well the horizontal and vertical extent of plume have been
delineated (plume volume) and how well the collected groundwater samples
characterize the plume (average concentration).
Contaminants are primarily introduced to groundwater as dissolved
product where they migrate through the subsurface with the groundwater. In
the subsurface, some fraction of the contaminants in solution will become
attached or sorb to the surrounding soil particles. This partitioning
removes a portion of the solute from solution, thus decreasing the
concentration and total amount of compound available for transport further
along the flow path (Button and Barker, 1985). Sorption of dissolved
contaminants slows the center of mass of the plume relative to groundwater
movement. This does not mean that the dissolved contaminant velocity is
less than that of groundwater. Rather, the dissolved concentrations at the
front of the plume are lessened due to adsorption, causing the plume center
of mass to move slower than groundwater flow.
Thurman (1985) found that the degree of partitioning between the
dissolved and sorbed phases depends primarily upon the percent organic
content on the fine sediments, the amount of fine sediments and the water
solubility of the organic solute. The greater the concentration of solute
the more contaminant will be adsorbed. The organic content of the
formation is important in sorption, but most groundwaters have relatively
low organic content, typically less than 0.1 percent. Button and Barker
(1985) found that the center of mass of four different contaminant plumes
were not significantly retarded in a formation with low organic content
32
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10 _,
CO
01
*
o
Q.
2
UJ
cc
HI
oc
o
g
i
8 -
6 _
4 -
2 -
0 -I
Figure 7. Ratio of Apparent to True NAPL Thickness for Various Soil Types
33
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(less than 0.1 percent). Schwarzenbach and Westall (1981) noted that only
soils less than 0.125 mm in diameter were important in sorption of organic
solutes. This would limit most sorption processes in groundwater aquifers
to silts, clays, and fine sands.
One method of estimating contaminant partitioning in the saturated
zone is based on a retardation equation. The retardation factor (R) is the
ratio of average linear velocity of groundwater to the velocity of the
center of mass of the plume and is defined by the following equation:
R = 1 + (pB/n)kd (2)
where: pB = bulk density of the soil;
nB = soil porosity; and
k = distribution coefficient.
a
The retardation factor can be used to relate the amount of contami-
nant in the dissolved phase to the amount in the sorbed phase. Bulk
density and porosity are soil properties that can be found in Table 3. The
distribution coefficient (kd) is the ratio of mass of sorbed contaminant
per unit mass of soil to concentration of dissolved contaminant and varies
for a given compound. Literature values of kd for various compounds can
also be found in various sources.
The mass of dissolved contaminants (Md) present in the saturated zone can
be expressed as:
Md = C n V (3)
where: C = the average concentration of dissolved contaminant;
n = soil porosity; and
V = the volume of the dissolved plume.
The mass of sorbed contaminant (M ) can then be estimated in the following
way:
H = (R-l) M. (4)
5 Q
where: R = retardation factor.
Estimate of Partitioning
It is possible to approximate the phase partitioning in the saturated
zone using Tables 10 and 11. These tables present some of the important
factors to consider in evaluating phase partitioning. They also provide a
way to estimate the likelihood of contamination being present in various
phases by identifying the range of values over which conditions will favor
one or another of the phases. For example, depth to groundwater plays an
important role in determining how much NAPL reaches the water table. Table
34
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Table 10. Likelihood of NAPL Being Present on the Water Table
FACTOR
RELEASE- RELATED
*
• Amount Released
*
• Rate Of Release
• Time Since Release
SITE- RELATED
*
• Preferential Flow Pathways
• Depth To Qroundwater
• Hydraulic Conductivity
(of the Unsaturated Zone)
• Rainfall Infiltration Rate
• Soil Temperature
• Soil SorpHon Capacity
(Surface Area)
CONTAMINANT- RELATED
• Liquid Viscosity
• Vapor Pressure
• Water Solubility
UNITS
gal
months
-
meters
cm/sec
cm/day
°C
arrf/g
cP
mm Hg
mg/L
SITE OF
INTEREST
INCREASING LIKELIHOOD ^
Small
(< 1,000)
0
Slow Release
o
Long
(>12)
O
Medium
(1000-10,000)
0
o
Medium
(1-12)
O
Large
(> 10,000)
o
Instantaneous
Release
0
Short
(<1)
O
Not Present
O
Deep
(>15)
O
Low
(<1ff5)
O
High
(0.2)
O
Cool
(<10)
O
High
(>1)
O
Unknown
O
Medium
(2-15)
O
Medium,
(10'5-101
O
Medium
(0.05-0.2)
0
Medium
(10-20)
0
Medium
(0.1-1)
O
Present
O
Shallow
(<2)
0
High
MO"3)
0
Low
(<0.05)
O
Warm
(>20)
O
Low
(<0.1)
O
High
(>20)
O
High
(>100)
0
High
(>1000)
O
Medium
(2-20)
O
Medium
(10-100)
O
Medium
(100-1000)
O
Low
(<2)
O
Low
(<10)
O
Low
(<100)
0
* CSFs denoted with an asterisk are typically more important than other CSFs
35
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Table 11. Likelihood of Dissolved Contaminants
Being Present In the Saturated Zone
FACTOR
RELEASE. RELATED
• Amount Released
*• Rate Of Release
* • Time Sine* Release
SITE* RELATED
• Depth To Ground water
• Fluctuating Water Table
• Hydraulic Conductivity
(of the Uneaturated Zone)
* • Rainfall Infiltration Rate
• Soil Temperature
• Soil Sorptton Capacity
(Surface Area)
CONTAMINANT- RELATED
• Liquid Viscosity
• Vapor Pressure
*• Water Solubility
UNITS
gal
-
months
meters
-
cm/sec
cm/day
°C
m2/g
cP
mmHg
mg/L
SITE OF
INTEREST
INCREASING LIKELIHOOD ^
Small
(100)
o
Instantaneous
Release
O
Short
(<1)
O
Medium
(100-1000)
O
--
Medium
(1-12)
O
Large
(>1000)
O
Slow Release
0
Long
(>12)
O
Deep
(>15)
O
Steady
0
Low
(<1cVS)
Low
(<0.05)
O
Cool
(<10)
O
High
(>1>
0
Medium
(2-15)
O
Moderately
Fluctuating
O
Medium
(10-S-10-3)
Medium
(0.05-0.2)
O
Medium
(10-20)
O
Medium
(0.1-1)
O
Shallow
(<2)
O
Highly
Fluctuating
0
High
(>10-<>)
O
High
<>o*>
Warm
(>20)
O
Low
(XJ.1)
O
High
(>20)
O
High
(>100)
O
Low
(<100)
O
Medium
(2-20)
O
Medium
(10-100)
O
Medium
(100-1000)
O
Low
(<2)
0
Low
(<10)
O
High
(>1000)
O
CSFs denoted with an asterisk are typically more important than other CSFs
36
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11 shows that the likelihood of NAPL reaching the water table is signi-
ficantly greater if depth to groundwater is less than 2 meters than if it
is greater than 15 meters. The factors are categorized as release-related,
site-related, and contaminant-related. Examples are provided in the
Appendix of this manual that demonstrate how the worksheets can be used.
The factors of Tables 10 and 11 are not weighted to reflect their
relative importance, but some do play a greater role than others in
determining phase distribution. However, because the relative importance
of the factors will vary from site to site, it is impossible to design an
absolute ranking scheme for the factors. To help address this issue, the
importance of each of the factors is discussed below, followed by a brief
discussion of the relative importance of the factors.
Release Information
The amount and rate of release will affect the probability of all
three phases being found in the saturated zone. Some releases are quick
spills with high rates of release while others are leaks where small
amounts of product are released over a period of time. Hydrocarbons will
persist as bulk liquid longer for larger releases than for smaller
releases, raising the likelihood of NAPL reaching the water table for large
release incidents. Also, the greater the amount of NAPL reaching the
saturated zone, the greater the amount of dissolved and sorbed contaminant
there will be. A slow rate of release results in conditions more suitable
to transfer from NAPL to other phases. Less NAPL is likely for a slow leak
than for a quick spill given that the same total amount of product was
released.
The time since the release may also be important. More recent
releases of petroleum would more likely result in a higher proportion of
NAPL relative to dissolved contaminants being present in the saturated
zone. The dissolved and sorbed phases contain incrementally more of the
contaminant mass for older releases because there is more opportunity for
contaminants to dissolve in groundwater and sorb to soil particles over
time.
Site-Related Information
The depth to groundwater and the soil sorption capacity (or surface
area) of the unsaturated zone soils influence how much of the total release
reaches the water table. The greater the depth to groundwater and the more
surface area available for adsorption, the more likely contaminants will
attenuate in the unsaturated zone before reaching the water table. This
will result in less total contaminant in all three phases in the saturated
zone. The surface area of soils below the water table plays an important
role in partitioning between the dissolved and sorbed phases. Clays and
silts, which have a relatively large surface area, will retain more
contaminants in the sorbed phase than sands and gravels.
The relative ease with which contaminants can move down through the
unsaturated zone to the water table is related to the hydraulic
conductivity of soils above the water table and the existence of
preferential flow pathways in the soils. Sands and gravels with high
-------
hydraulic conductivities allow more rapid transport of NAPL downward
through the unsaturated zone. Preferential flow pathways are fractures,
fissures or other openings in the soil, both natural and man-made, which
allow more rapid movement of liquids than would normally be expected. Some
examples are tree roots, well casings, and fractures due to shrinking and
swelling of certain types of clay.
As rainfall infiltrates the unsaturated zone, the soluble components
of the residual NAPL will dissolve and migrate towards the saturated zone.
Therefore, the greater the rainfall infiltration rate, the greater will be
the percentage of dissolved contaminants.If the release was fairly
recent, actual rainfall records since the release provide better data than
annual averages.
A fluctuating water table will cause increased transfer from NAPL to
the dissolved phase.As the water table falls, the floating NAPL plume
also falls. When the water table rises again, some NAPL gets trapped
between the soil particles. A greater contact area between NAPL and
groundwater is created and dissolution is increased.
Temperature affects partitioning of petroleum products in the
subsurface primarily because of its effect on other parameters. As
temperature increases; viscosity decreases, vapor pressure increases and
water solubility increases. Warmer temperatures would likely result in
increased amounts of NAPL and dissolved product in the saturated zone.
Contaminant-Related Parameters
A contaminant's water solubility is a measure of how easily it will
dissolve in water. As solubility increases, more of the contaminant is
likely to transfer from NAPL to the dissolved phase. Solubility is also
temperature dependent; warmer groundwater will usually have more dissolved
contaminant than cooler groundwater. Once in the dissolved phase,
compounds with lower solubilities are more likely to sorb to soil particles
below the water table than more soluble compounds.
A !°w liquid viscosity material, such as gasoline, will move more
easily down through the unsaturated zone than a more viscous fuel oil.
Therefore, lower viscosity suggests a greater likelihood of petroleum
products reaching the saturated zone.
The vapor pressure of a material indicates the relative degree to
which it will transform from the liquid or dissolved phase to the vapor
phase. As vapor pressure increases, more of the contaminant is likely to
be in the vapor phase, and therefore, less is likely to reach the saturated
zone.
Relative Importance of Factors
The extent to which contaminants reach the saturated zone as NAPL
plays a large role in the overall phase distribution of the contaminant in
the saturated zone. If large amounts of NAPL reach the water table, then
large amounts of dissolved and sorbed contaminants can also be expected.
Therefore, factors that are important in evaluating NAPL are also important
38
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in evaluating other phases. The most important factors for each phase are
discussed below.
NAPL - The amount released is probably the most important factor. If
the amount released is very large, a significant amount of NAPL is likely
to reach the water table regardless of other factors. For smaller
releases, the rate of release, depth to groundwater, presence of
preferential flow pathways and liquid viscosity play a larger role than
other factors.
Dissolved Phase - As stated above, the factors that play an important
role in estimating the presence of NAPL in the saturated zone are also
important in determining the total mass of dissolved contaminant. For
large releases, where significant amounts of NAPL reach the water table,
the water solubility of the contaminant is probably most important. For
small releases, the rainfall infiltration rate and soil porosity play
increasingly important roles.
Sorbed - In the saturated zone, the amount of sorbed contaminant is
closely related to the amount of dissolved contaminant. For a given
contaminant and a given soil formation, the percentage of dissolved phase
contamination that becomes sorbed to soil particles is roughly constant and
can be significant. It is not uncommon for more than one-half of all
contaminants below the water table to be in the sorbed phase. The most
important factors in estimating the ratio of dissolved to sorbed contami-
nants are the water solubility of the contaminant and the type of soil.
EVALUATING CONTAMINANT MOBILITY
Implementing an effective remedial technology for the saturated zone
requires an understanding of how far the contaminants have traveled
(vertically as well as horizontally) and the direction and velocity of
plume movement. Delineating the extent of contamination is an essential
part of designing a containment system and also reveals what receptors may
already be affected. Plume flow direction and velocity can also be used to
predict what receptors are likely to be impacted and when the contaminant
plume would reach the receptor if containment is not implemented.
The liquid and dissolved contaminant phases are highly mobile while
sorbed contaminants are relatively immobile. The focus of this subsection
is on the two mobile contaminant phases.
Extent of Contaminant Plume
Delineating the contaminant plume aids in locating monitoring and
recovery wells and in assessing the impact to nearby receptors. Since the
NAPL and dissolved contaminant can move at different rates and in different
directions in the subsurface, the two phases may need to be delineated
separately. Delineation of the down-gradient and lateral extent of the
plume is generally more important than of the up-gradient extent because
movement in the up-gradient direction is generally limited. However, the
presence of pumping wells or surface waters up-gradient of the release
should be noted. Seasonal changes in surface water elevations or changes
in pumping rates can cause changes in local groundwater flow directions.
39
-------
Monitoring wells, soil gas sampling, ground penetrating radar and
resistivity surveys are among the techniques that can be used to determine
the areal extent of contamination in the saturated zone. Use of monitoring
wells is an accurate but expensive way to determine the extent of
contamination. Other methods are less accurate but are often used as a
preliminary means of delineating and determining where to best locate
monitoring wells. Soil gas sampling is relatively inexpensive, can be done
quickly and is probably the most commonly used of these methods.
Most petroleum products are less dense than water and will float on
the water table in the bulk liquid phase. However, if denser than water,
the contaminant will sink through the aquifer until it reaches an
impermeable barrier. The areal extent of a denser than water contaminant
plume is more difficult to estimate. Floating product will move in the
same general direction as groundwater, but movement of dense contaminants
at the bottom of the aquifer is controlled by the grade of the impermeable
barrier and may be different than groundwater movement. Figure 8 shows how
the effects of density can influence contaminant migration. Notice that
the flow direction of the denser than water bulk liquid is opposite that of
the groundwater, but the dissolved component travels with the groundwater.
Estimation of the areal extent of denser than water petroleum products is
likely to require monitoring wells, especially if the saturated thickness
of the aquifer is great.
The vertical extent of dissolved contaminant is important for several
reasons:
• to calculate the volume of contaminant in the dissolved phase;
• to determine the depth of screening for monitoring wells; and
• to determine pumping rates required in recovery or containment
wells.
If all the monitoring wells at a site are screened at or near the
water table, they might not intercept a dissolved plume flowing deeper in
the aquifer. Also, the entire dissolved plume may not be contained if the
pumping rate is not adequate or recovery wells are not installed deep
enough. A dissolved plume may exist relatively deep in the aquifer if the
vertical component of groundwater flow is large. This is most prevalent at
sites where the water table is relatively flat and the recharge rate is
high. Infiltrating rain water acts to "push" the plume down below the
water table. Figure 9 shows some of the ways poorly planned well
installation can hinder the effectiveness of a remediation system.
An accurate estimate of the vertical extent of contamination is very
difficult to make without monitoring wells, but a reasonably accurate
preliminary estimate is possible before wells are installed. The most
important factor in estimating vertical depth is contaminant density. For
contaminants lighter than water, the dissolved product will likely be
nearer the water table. Dense contaminants migrate to the bottom of the
aquifer. If dissolved constituents are similar in density to water, they
will likely be found dispersed throughout the aquifer. The time since the
release can also influence the vertical location, because some natural
40
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SOURCE OF PRODUCT
(GREATER DENSITY THAN WATER) SQURCE Qp pRODUCT
(LESSER DENSITY THAN WATER)
UNSATURATED
ZONE
WATER TABLE
FLOW OF DISSOLVED
PRODUCT
DIRECTION OF
GROUNDWATER FLOW
(Adapted from Geraghty and Miller, 1985)
Figure 8. The Effect of Density on Contaminant Plume Migration
-------
Monitoring Wells
Ground Surface
Water Table
Dissolved Plume
Confining Layer
Recovery
Well
Ground Surface
Dissolved Plume
Confining Layer
Figure 9. Poorly Designed Monitoring and Recovery Wells
42
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dispersion of the dissolved plume will occur over time. For a very recent
release of lighter than water contaminant, any dissolved product is likely
to be close to the water table. As the dissolved product flows with the
groundwater, dispersion forces will cause some of the contaminants to move
lower in the aquifer. Another factor is site stratigraphy. Low permeabi-
lity layers (aquitards) below the water table may effectively prevent
vertical dispersion of dissolved contaminants.
Mobility of Contamination in the Saturated Zone
Many factors combine to influence how NAPL and dissolved product move
through the subsurface. The following are some of the important factors:
• subsurface stratigraphy, aquifer saturated thickness;
• local topography, location of nearby water bodies;
• location, depth, and pumping rates of nearby wells;
• regional and local groundwater flow direction(s), water table
gradients (potentiometric head differences);
• hydraulic conductivity of formation(s); and
« density and viscosity of the bulk product or its constituents.
These data can be used to estimate predict how quickly, and in what
directions contaminant plumes will travel. They are also essential in
designing an effective remedial system. The importance of these parameters
is discussed below.
Groundwater flow direction is important in predicting where a
contaminant is likely to migrate and what receptors may potentially be
impacted. Horizontal groundwater flow typically follows the contours of
the overlying land surface and will tend to be directed from areas of high
relief toward streams, lakes and wetlands. Topographic maps can give a
very rough approximation of regional flow patterns in an area, where the
direction of groundwater flow is considered to be perpendicular to
elevation contour lines. However, data from monitoring wells provide a
much better estimate of flow direction because groundwater elevations can
be measured.
Predicting flow direction would be relatively easy in a homogeneous
aquifer with no nearby surface waters or pumping wells that influence the
local water table levels. This condition is rare however and most often
the site geology will be heterogeneous horizontally and vertically and
natural or man-made fluctuations in the water table will occur. Local
variability in flow patterns can be caused by seasonal surface water
changes, site stratigraphy, and nearby pumping wells. Most surface water
bodies interface with groundwater and the flow across the interface can be
in either direction. In some cases the flow is always in the same
direction (i.e. from the surface water to the groundwater or vice versa).
But often, the direction will change seasonally. Figure 10 shows how
43
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Ground Surface
Surface Water to Groundwater
Flow During Wet Weather Periods
Ground Surface
x;
Groundwater to Surface Water
Flow During Dry Weather Periods
Figure 10. Seasonal Variations in Flow Between Groundwater and Surface Water Bodies
-------
seasonal changes in water levels can affect flow patterns. It is important
to understand this relationship at a site when assessing the impact of the
release.
Site stratigraphy is another important factor in estimating
groundwater flow direction. Subsurface formations serve either as
confining units or as aquifers. A confining unit or aquitard is
characterized by low permeability that does not readily permit water to
pass through it despite the fact that it stores large quantities of water
(EPA, 1987). A formation serves as an aquifer if its permeability is
sufficient to provide a usable quantity of water. Water moves readily
through a highly permeable formation such as sands and gravels, which are
often productive aquifers. The less permeable silts and clays are common
examples of aquitards.
Subsurface formations are typically deposited in horizontal layers
and may become folded or faulted or otherwise structurally changed over
geologic time. As a result, the thickness and depth below ground surface
of formations can vary significantly. However, groundwater movement will
generally remain in specific formations (i.e., the more permeable ones).
Figure 11 illustrates common geologic conditions.
Subsurface formations can also change character in the horizontal
direction. Groundwater follows the path of least resistance and will tend
to change direction when the type of formation changes, flowing around a
less permeable area and being drawn towards a more permeable one.
Pumping wells change the water table in an aquifer and therefore
influence groundwater flow in their vicinity. The greater the pumping
rate, the larger the influence the well will have.
Vertical as well as horizontal flow should be considered at a site,
including flow between aquifers. The relatively permeable aquitard
separating two aquifers can leak, allowing interaquifer groundwater flow.
Interaquifer flow can occur from the upper aquifer to the lower, or if the
lower confined aquifer has sufficient hydraulic pressure, from lower to
upper aquifer. Groundwater, and contaminants, can also migrate between
aquifers where wells have been drilled through the aquitard. Figure 12
shows vertical groundwater flow pathways.
The velocity of a contaminant in the saturated zone is a useful
parameter to estimate travel times. Dissolved product will generally
travel at the same rate as groundwater, although the rate of migration of
some dissolved contaminants may be retarded (relative to the average water
velocity) due to sorption on soil particles. The extent of sorption will
increase with decreasing water solubility of the contaminant, and with
increasing organic carbon content and/or surface area of the soil.
Floating NAPL will generally flow at a rate different than the underlying
aquifer. The difference in velocity between water and petroleum products
is due to viscosity and density differences.
45
-------
en
SAND AND GRAVEL
Figure 11. Variability of Subsurface Stratigraphy
-------
Recharge Area
Impermeable
Layer
Water Supply Well
Confining Layer
'^^^^^^f^tmtSUHH^^ff^^^^^^^^^^^
'WWV / / f'S' S^S*/' /-yy-v /V";
^/SS//Js
-------
The velocity of a fluid through a porous medium such as soil can be
represented by the equation:
v = ki/n (5)
where: v = velocity of groundwater (cm/sec);
k = fluid conductivity (cm/sec);
n = effective porosity of the formation (dimensionless); and
i = gradient of the water table or floating NAPL (cm/cm).
Porosity is the ratio between the total volume of air space and the
total soil volume, however not all of the air volume in a soil is available
for transport of fluid. Materials such as clay can have a high porosity
but a fairly low effective porosity because many of the pore spaces are not
interconnected. The effective porosity of the formation should be used in
velocity calculations to reflect this. The hydraulic gradient is the slope
of the water table, where the slope is defined as the change in water level
per unit distance in the direction of flow. For estimating the velocity of
floating NAPL, using the hydraulic gradient is not likely to introduce
significant errors.
The conductivity of a fluid is a measure of the ease with which it
will flow through a porous medium and varies for different fluids (e.g.
water flows through a given soil more easily than a viscous oil). The
conductivity of a fluid is directly proportional to its density and
inversely proportional to its viscosity. Conductivity is also affected by
temperature, because viscosity decreases as temperature increases. Figure
13 compares the density to viscosity ratios for several petroleum products
and water as they vary with temperature. As the density to viscosity ratio
increases, fluid mobility increases. Note that the mobility of gasoline in
the subsurface is similar to water but lubricating oil is relatively
immobile.
Figure 14 shows an example of how the travel time of groundwater can
be estimated. When estimating velocity or travel time for dissolved
contaminant, the hydraulic conductivity (i.e., water conductivity) is
appropriate to use in Equation (5). But if the velocity of floating NAPL
is needed, conductivity of the particular contaminant should be used. This
is an important point to consider when developing an understanding of the
potential impacts of a release. For example, if a relatively immobile
product such as No. 2 fuel oil is released, the resulting dissolved
contaminants in the groundwater will move at roughly the same velocity as
the groundwater. However, the bulk fuel oil on the water table will move
much more slowly than the groundwater. If the release were gasoline,
dissolved contamination and the bulk liquid would both move at about the
same velocity, and the bulk liquid may travel somewhat more quickly. The
immediate goals of remediation in these two scenarios may be quite
different. As shown in Figure 3, when goals are different, the type of
remedial response will likely be different as well.
SETTING REMEDIATION GOALS
An effective remediation plan must adequately address all phases of a
clean-up. Complete restoration at a release site is desirable, but not all
48
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CM
O
OT
o
140
100
50
20
GASOLINE
KEROSENE^
10
'(0
g
Q
5
5
4|
3
0.6'
3°
10°
13°
16°
19°
22°
25°
Temperature (°C)
Source: Adapted from CH2MHHI, 1989
Figure 13. Density-to-Viscosity Ratios for Water and Hydrocarbon
Products as a Function of Temperature
49
-------
Gasoline Spill
Elev. = 150'
Elev. = 100'
Water Table
Sand and
Gravel
kr 50 ft/day
n = 0.25
Gradient = i = (150 -100) / 5,280 = 0.009 ft/ft
Velocity = K i / n = 50 (0.009) / 0.25 = 1.8 ft/day
Travel time = 5,280 /1.8 = 2933.3 days = 8.0 years
Figure 14. Estimating Groundwater Travel Time
50
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sites require a remediation program designed to restore the site to a
pristine condition. The unique conditions at each release site must be
considered to set realistic remediation goals. This subsection discusses
various remediation goals and some of the types of the site conditions that
may influence goal selection.
If the goal of remediation is to return the site to pre-release
conditions, there are several typical clean-up phases that are likely to be
implemented. Emergency responses are a first priority to ensure that no
immediate health or safety threat exists. Containment of the dissolved and
NAPL contaminants limits the extent of contamination and facilitates
restoration efforts. Removal and/or treatment of subsurface contamination
must be accomplished and finally, a site monitoring program should be set
up to detect any changes that would suggest restoration was incomplete.
The time-frame over which the components of remediation are
implemented will vary from site to site. Short- and long-term objectives
should be outlined that define what order the various stages should be
implemented. Emergency measures should always be addressed quickly, but
implementation of other aspects of remediation may not be as urgent if
contaminant mobility is limited or the impact to receptors is determined to
be minimal. A prioritization scheme should also be evaluated so that the
most urgent or important tasks are accomplished first. For example,
containment of the dissolved plume may take precedence over containment of
NAPL because it may represent a more immediate threat to a drinking water
supply. Initial design of a containment system would focus on preventing
dissolved contaminant migration.
Because various site activities will be implemented at different
times, it is important to consider how initial activities may affect future
ones. For instance, saturated zone clean up may be implemented before
unsaturated zone clean up and the technology used to treat the saturated
zone may influence what technologies will be successful in the unsaturated
zone. Also, contamination from the unsaturated zone can migrate to the
saturated zone increasing the total length and cost of treatment. Such
problems can be avoided with effective planning.
The objective or approach to a remediation plan can be put into three
broad categories: 1) no active remediation; 2) containment of subsurface
contaminants; and 3) treatment and/or removal of contaminants at the site.
The approach taken at a particular site will depend on many different
factors, including the pre-existing groundwater quality, the potential
impact to vulnerable receptors, the feasibility of removing contaminants,
and regulatory requirements. An approach other than complete restoration
of the site should be preceded by a careful evaluation of site conditions
and be made in conjunction with regulatory officials. Each of the three
categories is described below and some of the site conditions which might
lead to their selection is discussed.
No-Action Alternative - Site conditions may lead to the decision to
take no immediate action at a site. The no-action alternative may be a
short term condition, in effect only until remediation efforts are fully
evaluated. This might apply if local drinking water supplies or other
vulnerable receptors were far enough removed from the site to warrant no
51
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immediate action. And in some cases, well-head treatment of contaminated
drinking supplies has been found to be a more cost-effective alternative to
containment and/or treatment of the release. On the other hand, it may be
decided that no action is required at the site for the foreseeable future.
If the site is in an industrial area where groundwater quality was poor
before the release occurred, an extensive remediation effort may not be
warranted.
Containment - Containment involves preventing further migration of
subsurface contaminants so that additional areas are not impacted by the
release. Containment may be the primary objective of remediation if
treatment or removal of the contaminants is not considered feasible or
there is difficulty in selecting or designing an effective treatment
method. Containment may also be a short term objective if permitting
problems delay implementing an above-ground treatment scheme. The long
term objective in this case would be contaminant removal once the proper
permitting was obtained.
Treatment and/or Removal of Contaminants - Implementation of a
remedial plan that will return the environment to pristine conditions (or
at least to pre-release conditions) is a lofty goal. This level of clean
up is very difficult to achieve, especially for sites with large releases
and complex stratigraphy. Regulatory officials may require that some
percentage (e.g. 99%) of all contaminants must be removed or state
legislation may mandate minimum groundwater quality levels. Such
requirements will dictate complete restoration as a goal at the site,
typically at tremendous cost.
A study of Superfund sites (CH2M Hill, 1989) where groundwater
extraction techniques were used found that contaminant removal to
health-based standards is possible under favorable site conditions, but the
time-frame for completion is often underestimated. The total clean up time
for sites where large releases have occurred is likely to be measured in
decades^Favorable site conditions are not common, and contaminant
removal to health-based standards may be difficult at many sites. The
difficulty in meeting the stringent clean-up requirements results in
remediation continuing long after targeted clean-up finish dates have
passed. The main causes for delays in completing remediation are poor
delineation of the contaminant plumes and the presence of residual liquid
contaminants in the groundwater which are not readily removed from the
subsurface.
Summary
The site assessment outlined in this section should enable the user
to develop a conceptual understanding of the site and to reasonably predict
how a given contaminant is likely to behave in the subsurface. In
addition, the user should be familiar with some of the important issues in
deciding what level of clean up is required and the time frame over which
remediation efforts should take place. The next section presents several
technologies that can be used to accomplish remediation goals and helps in
evaluating which are likely to be most effective at a given site.
52
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SECTION 3
TECHNOLOGY SELECTION
INTRODUCTION
There are a variety of ways to address a release of petroleum product
into the environment. These range from no immediate action to a full scale
remediation effort designed to restore the environment to pre-release
conditions. With the exception of the "no-action" alternative, each
approach will involve using one or more of the available clean-up
technologies. The approach best suited to a particular site will depend on
the goals of remediation and unique site conditions.
A typical remediation plan that has restoration to pre-release
conditions as the goal would likely have three main components: 1)
containment of NAPL and/or dissolved product; 2) NAPL removal; and 3)
dissolved product removal. If the goal of remediation is only to prevent
the contaminants from moving away from the site, then the last two
components would be unnecessary. As shown in Figure 3, this section
identifies and describes technologies that are effective in addressing the
various goals of remediation. Presented at the end of the section is a
discussion of how the various technologies can be used in combination to
enhance the overall remediation effort.
This manual focuses on technologies that are commonly used and/or
well understood. Promising new technologies are discussed briefly at the
end of this section. Outlined below are the technologies covered herein.
• Containment Methods - Preventing migration of contaminants can
involve NAPL and/or dissolved product. Trench excavation is
effective only for containment of NAPL. Pumping wells"can be
installed to contain both NAPL and dissolved product.
• Recovery of NAPL - NAPL recovery is often accomplished in
conjunction with either trench excavation or pumping well
installation. Various types of removal equipment can be used once
the NAPL has been controlled. NAPL recovery can also be
accomplished with or without implementing a containment system by
using vacuum extraction.
• Dissolved Product Removal - Groundwater treatment methods can be
either above-ground or in situ. The above-ground methods covered
in the manual are air stripping and carbon adsorption. In-situ
treatment is where contaminants are treated or removed without
disturbing the subsurface or bringing groundwater above ground.
The only in-situ groundwater treatment method covered in detail in
this manual is biorestoration. Air sparging, another in-situ
groundwater technology, is only briefly discussed.
53
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For each technology described herein, factors are presented that
should be considered when evaluating the technology at a given site. These
critical success factors (CSFs) are parameters that influence the likeli-
hood of success of a particular method. For example, high contaminant
volatility is a CSF for air stripping. These CSFs can be used to assess
the likely effectiveness of each technology at a particular site. Finally,
other advantages and disadvantages that can influence technology selection
are discussed.
A worksheet has been provided for each technology to help the user
evaluate the likely success of the technology at a particular site. The
CSFs important for that technology are listed. Completed worksheets
provide insight as to which technologies are likely to be effective, and
more importantly, which are not. The three right hand columns of each
worksheet provide ranges of values for the CSFs that suggest whether a
technology is "less likely," "somewhat likely," or "more likely" to be
effective. A column is provided for the user to write down the values for
the CSFs at the site of interest. For each CSF, the user determines which
of the three "likelihood" ranges the site falls. If most of the CSFs fall
into the "less likely" column, another technology may be better suited to
cleaning up the site. If the CSFs are fairly evenly distributed among the
three columns, other factors can be considered that can help in the
assessment (i.e. can a parameter be enhanced to increase the likelihood of
success?).
The importance of each CSF in evaluating the effectiveness of a given
technology is not the same"! Some CSFs will play a greater role than others
and in some cases, a single CSF may outweigh all others in importance at a
given site. For example, if the depth to the water table is very large,
trench excavation will not be feasible even if all other CSFs are
favorable. However, it is impossible to attach a quantitative weighting
system to the CSFs because the relative importance of the CSFs varies from
site to site (i.e., what is relatively important at one site might be less
so at another). To address this issue, a discussion of the CSFs
accompanies each worksheet followed by a brief discussion of the relative
importance of the CSFs.
Both cost-effectiveness and technical feasibility are considered in
the worksheets. For example, the "depth to groundwater" CSF for trench
excavation (See Table 12) shows that if the water table is greater than 5
meters below ground surface, success is less likely. This is based on the
fact that the cost of trench excavation increases significantly at depths
greater than 5 meters. Trench excavation is technically feasible under
deep groundwater conditions, but the high costs make other methods
comparatively more attractive. The CSFs in the worksheets may target
either the technical feasibility or the cost-effectiveness of the given
technology, and in some cases, both.
The Appendix of this manual provides examples of how to use the
worksheets. Values for all applicable CSFs are assumed for a hypothetical
release situation, and four different worksheets are completed using the
CSF data. Along with each completed worksheet is a brief interpretation of
the results. Included at the back of this manual are copies of each
54
-------
worksheet in the manual. These can be detached and copied, with the copies
being used for any number of release sites.
CONTAINING NAPL AND/OR DISSOLVED CONTAMINANT
Preventing further migration of subsurface contaminants is often a
major part of a remediation effort and may be the primary goal of the
project. Containment lessens the potential threat to nearby surface water
and groundwater supplies. It can also serve to consolidate widely spread
contaminants into a smaller area, facilitating their removal. The two
containment methods presented in this manual, trench excavation and pumping
well installation, are described below.
Trench Excavation
A very simple way to prevent migration of floating NAPL is to dig a
trench downgradient of the plume. As the NAPL reaches the trench, it can
be intercepted, removed, and disposed of, thereby preventing migration
beyond the trench. Besides its simplicity, trench excavation is a useful
containment method because it can be implemented quickly. However, it is
not feasible in regions such as the Southwest where depth to the water
table is often great. It is also not as effective in containing dissolved
contaminants or denser than water NAPL, which may pass below the trench.
At a minimum, implementing a trench excavation containment system
requires the following information:
• direction of plume flow;
• depth to water table;
• down-gradient and lateral extent of the NAPL plume; and
• location of underground utilities that may interfere with the
excavation.
Figure 15 shows schematic plan and section views of a typical trench
excavation operation. The trench is dug to a depth several feet below the
water table and should be long enough to intercept the full lateral extent
of the plume. The trench must also be perpendicular to local groundwater
flow direction. As the plume moves toward the trench, free product
accumulates in the trench and can be recovered manually or with pumps. To
prevent the free product from moving past the trench, an impermeable liner
can be placed on the downstream side of the trench. To increase the rate
of recovery of free product, pumps may be used to lower the water table.
As the water table is lowered, the hydraulic gradient toward the trench is
increased and free product is drawn more quickly to the trench. However,
care must be taken to ensure that the water table is not lowered below the
bottom of the trench, allowing NAPL to pass by.
When trenches are used to contain and remove free product, removal
efforts must be continuous. Otherwise the product will migrate to the ends
of the trench and possibly pass the impermeable baffle.
55
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PLAN VIEW
RELEASE AREA
TRENCH
CROSS SECTION
NAPL
WATER
TABLE
JS7..
Figure 15. Schematic Diagram of Trench Excavation
56
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Evaluating Trench Excavation as a Containment System
Table 12 lists factors to consider in evaluating the likely
effectiveness of trench excavation as a containment system at a particular
site. Each of the factors in Table 12 are discussed below.
Knowing the time since release and the size of the release can give
an idea of how far the contaminant has traveled in the subsurface. A
widely dispersed plume will require a longer trench, increasing the cost of
the system. The greater the size of the release and the more time that has
elapsed since the release, the greater the areal extent of the plume will
be. Also, the longer ago the release occurred, the more likely significant
amounts of contaminant have dissolved into the groundwater. Dissolved
contaminants generally are not contained using this method.
The depth to groundwater should be known prior to selection. Trench-
excavation more than 5 meters below the ground surface becomes increasingly
expensive because special equipment must be used. This equipment is not
always readily available and may limit the usual quick response time of
excavation. In addition, deeper trenches are less stable and usually must
be supported. Trenches that must be maintained for long time periods may
require protection such as rip rap or wood sheeting along the slopes to
prevent erosion.
The soil formation at the site should be relatively cohesive and
self-supporting.Otherwise the trench walls may be difficult to maintain.
In densely populated or commercially developed areas, above- and below-
ground structures may interfere with excavation or placement of the trench.
Trench excavation is only effective at containing NAPL floating on
the water table. Therefore, the type of contaminant released must be
known. If denser than water, the contaminant will sink to the bottom of
the aquifer where it cannot be recovered using this method.
Relative Importance of CSFs
From a cost perspective, depth to groundwater and the presence of
interfering structures are the CSFs most likely to prevent or prohibit use
of this method. From an engineering perspective, the liquid density of the
contaminant is very important, since a "denser than water" compound is not
likely to be contained effectively with a trench.
Pumping Well Installation
Pumping wells are an effective and commonly used method for
containing a contaminant in the saturated zone. Unlike trenches, pumping
wells are effective in containing dissolved contaminants and denser than
water NAPL as well as floating NAPL. The basic concept behind this method
is to artificially lower the water table at the site, drawing local
groundwater and contaminants to the well. This not only prevents the
contaminant plume from migrating but also can concentrate the floating
NAPL, if present, in a single area, facilitating recovery.
57
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Table 1 2. Worksheet for Evaluating the Feasibility of Using
Trench Excavation to Contain Floating NAPL
CRITICAL SUCCESS
FACTOR
RELEASE -RELATED
• Amount Released
• Time Since Release
SITE -RELATED
*
• Depth to Groundwater
• Stability of Soil Formation
*
• Presence of Interfering
Structures
UNITS
gallons
months
meters
CONTAMINANT- RELATED
• Uquld Viscosity
it
• Liquid Density
cP
g/cm3
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
Large
(>500,000)
o
Long
(>12)
O
Deep
(>5)
O
Unstable
O
Present
O
SUCCESS
SOMEWHAT
LIKELY
Medium
(50,000-500,000)
O
Medium
(1-12)
O
Medium
(1-5)
O
SUCCESS
MORE
LIKELY
Small
(<50,000)
O
Short
(<1)
O
Shallow
(<1)
O
Stable
O
Not Present
0
High
(>2)
O
High
(>D
O
Medium
(1-2)
O
Low
(<1)
0
Low
(<1)
O
OTHER CONSIDERATIONS
• Trench excavation can generally be implemented quickly.
• Costs for this method are typically lower than for other methods
• Denser than water contaminants cannot typically be contained with trench excavation
* CSFs denoted with an asterisk are typically more important than other CSFs
58
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The two most important considerations in this method are well
location and pumping rate. To determine location and pumping rate, site
hydrogeology and the vertical and areal extent of contamination must be
known. Each pumping well has an zone of contribution (ZOC), within which
local groundwater will flow towards the well. For a given soil formation,
the size of the ZOC increases as the pumping rate increases. To prevent
contaminant migration, the well configuration and pumping rate must be such
that the contaminant plume is completely contained within the well's ZOC.
If the contaminant plume is very large or soil conditions prevent adequate
pumping rates, several wells may be required to fully contain the plume.
Figure 16 shows a typical pumping well used to prevent contaminant
migration.
Difficulties with this method arise at sites with complex
stratigraphy because hydraulic conductivity varies for different types of
formations. Estimating the average hydraulic conductivity of such a site
is difficult, and the extent of the ZOC will not be equal in all
directions. This could result in poor location of wells. Figure 17 shows
how stratigraphy can influence pumping well containment.
Local variations in the groundwater table also make it difficult to
accurately delineate the ZOC of the containment wells. As described in
Section 2, flow direction between surface water bodies and the adjacent
groundwater can change seasonally. This would cause the ZOC of a pumping
well to change also. Nearby drinking water wells also influence the
natural water table gradient and must be taken into account to accurately
delineate the pumping well ZOC.
The time-frame for implementing a pumping well system should be
considered"Often a pump test, lasting days to weeks, must be conducted in
order to accurately determine hydraulic conductivity. Difficulties may
arise in accurately delineating the ZOC causing additional delays in
implementing the system. At some sites, it may be crucial that a
containment system be deployed quickly before the contaminant plume can
reach a vulnerable receptor.
It is desirable to maintain a constant pumping rate for this method.
If the pumping rate is not kept constant, fluctuations in the water table
can cause NAPL to be trapped below the water table which can present
recovery problems. Also, if the pumping rate is decreased, the water table
may rise sufficiently to allow the outer edge of the plume to resume
migration from the site.
Determining the ZOC of a Well
To determine whether a well is properly located and an adequate
pumping rate has been applied, the ZOC of the well must be estimated. At
some sites, this can prove to be very difficult and the accuracy of the
estimate will depend on the accuracy of the site assessment. Javandel and
Tsang (1986) present a method for determining the ZOC of single wells or
groups of wells based on the following parameters:
59
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Regional Groundwater
Flow Direction
Boundary of
Contaminant
Plume
Zone of
Contribution
of Well
PLAN VIEW
Discharge from
SECTION VIEW
Figure 16. Schematic Diagram of a Pumping Well Used to Contain Petroleum
Products in the Saturated Zone
60
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Highly
Permeable
Formation
Zone of
Contribution
Pumping
Well
O
c
O
7
O
0)
CO
c
O
O
O
oc
Homogeneous Formation
o
CO
•o
c
3
O
O
Zone of
•Contributlo
Pumping
Well
Heterogeneous Formation
Figure 17. Distortion of the Zone of Contribution of a Pumping Well Due to a Heterogeneous Formation
-------
• pumping rate;
• hydraulic conductivity;
• aquifer thickness; and
• hydraulic gradient.
The hydraulic conductivity, aquifer thickness, and hydraulic gradient
are identified during the site assessment. An appropriate pumping rate can
then be determined that will induce a ZOC containing the entire contaminant
plume. The equations used in this method are fairly complex and will
usually require the use of a computer. For a more detailed coverage of
this method, the reader can consult Todd (1980) or Freeze and Cherry (1979)
as well as the previously mentioned source.
Alternatively, a simplification of the above method approximates the
elliptical shape of the actual ZOC with a rectangular shape. The
up-gradient, down-gradient, and lateral extent of the ZOC is estimated
based partially on the boundary conditions of the equations used in the
preceding method and partially on steady state theory. Under steady state
theory, the area of a ZOC of a well with a constant pumping rate is
proportional to the rate of recharge available through infiltrating
rainfall. Recharge rates can be estimated fairly easily. Once the total
area of the ZOC is known, the approximate shape of the ZOC can be
delineated. This method can prove useful for a preliminary assessment, but
caution should be exercised and a factor of safety applied to the
calculations commensurate with the confidence the user has in the accuracy
of the site assessment. EPA (1987c) discusses this method in greater
detail.
Evaluating Pumping Wells as a Containment System
Table 13 lists factors to consider in evaluating the likelihood of
using pumping wells as a containment method. The importance of each of the
factors is discussed below.
The amount of petroleum product released and the time since release
may play roles in choosing a containment system. For larger and/or older
releases, the areal extent of the resulting NAPL plume will be greater, and
a larger trench will be required to effectively contain the plume. Pumping
wells may be more cost-effective than trenches, if very long trenches are
required.
An important consideration in designing an effective containment
system using pumping wells is site stratigraphy. For aquifers in
homogeneous formations and with flat, non-fluctuating water tables, it is
fairly easy to estimate the ZOC of a well. However, it is difficult to
accurately estimate the effect of a pumping well on the surrounding
groundwater at sites with complex stratigraphy. Under such conditions, it
is necessary to conduct extensive field studies to better understand the
site. These field studies take time, and implementation of the containment
system will be delayed allowing the plume to continue to migrate.
62
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Worksheet for Evaluating the Feasibility of
TABLE 13. Using Pumping Wells to Contain NAPL and/or
Dissolved Contaminant in the Saturated Zone
CRITICAL SUCCESS
FACTOR
RELEASE - RELATED
• Amount Released
• Time Since Release
SITE -RELATED
it
• Site Stratigraphy
• Depth to Groundwater
UNITS
gallons
months
,
meters
SITE OF
INTEREST
^r
SUCCESS
LESS
LIKELY
Small
(<50,000)
O
Short
\* ')
O
Complex
0
Shallow
(<5)
0
SUCCESS
SOMEWHAT
LIKELY
Medium
(50,000-500,000)
O
Medium
(1 -12)
O
o
SUCCESS
MORE
LIKELY
Large
(>500,000)
O
Long
(>12)
O
Simple
O
Deep
(>5)
O
CONTAMINANT- RELATED
• Liquid Density
• Liquid Viscosity
g/cm
cP
Low
(<1)
O
High
(>2)
0
Medium
(1-2)
0
High
O
Low
(<1)
0
OTHER CONSIDERATIONS
• Installation of pumping wells can be delayed due to difficulties in delineating well ZOCs.
• Extracted groundwater must often be disposed of or treated.
- This may be the only effective containment method at sites where depth to water table is great.
* CSFs denoted with an asterisk are typically more important than other CSFs
63
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The depth to groundwater at a site is important from a cost
viewpoint.Pumping wells can be used at most sites regardless of depth to
groundwater, but they generally become much more cost-effective than
trenches if the depth to groundwater is greater than 5 meters.
Petroleum products that are heavier than water will sink below the
water table and migrate towards the bottom of the aquifer. Those products
can be contained with pumping wells, but generally not with trenches.
Therefore, the liquid density of the petroleum products should be
considered when evaluating containment systems. The liquid viscosity of
the bulk liquid is somewhat less important. High viscosity products flow
less easily than lower viscosity products, and it is generally more
difficult to control the movement of products with higher viscosity.
However, a highly viscous product will typically be less mobile in the
subsurface, and containment may not be as urgent.
Relative Importance of CSFs
The ultimate success of this method depends on how accurately
groundwater flow patterns can be identified and controlled. This requires
a good understanding of site stratigraphy, which is the most important CSF
in Table 13 in terms of technical performance. Although not listed as a
CSF, another important consideration is the degree of immediacy associated
with implementing a containment system. Because pumping wells often take
longer to install, trenches may be preferable when containment is more
urgent.
Choosing a Containment System
Tables 12 and 13 can be used to compare trench excavation versus
pumping wells at a given site. Some of the major advantages and
disadvantages of the two methods are discussed below.
• Time-Frame - Trenches can generally be excavated quickly. A
pumping well can sometimes be installed quickly as well, but
certain site conditions can delay implementation. If the site
stratigraphy is complex, the proper placement of the well and the
optimal pumping rate may require additional time to determine.
• Depth to Water Table - Trench excavation is typically not
effective at sites where the depth to water table is greater than
5 meters (15 feet). Pumping wells can be effective at at a wide
variety of depths.
• Size of Contaminant Plume - Very large releases and widely spread
plumes are often better contained with pumping wells than
trenches. However, if the soil formations have a low permeability
(e.g., clays), the ZOCs for recovery wells may be very small.
Under such conditions, the number of wells required to contain the
release could improve the attractiveness of trenches.
• Location of Contaminants in the Saturated Zone - Trenches are used
primarily for floating NAPL plumes.Dissolved contaminants or
denser than water contaminants will generally pass underneath the
64
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trench. Pumping wells are more flexible in that they can be used
to contain "denser than water" contaminants as well as "floaters."
• Proximity of Above- or Below-Ground Structures - It may be
difficult to dig a trench in a highly developed area, where there
are many homes or buildings. Pumping wells require much less
space and are often better suited to these areas. The presence of
underground utilities is also more likely to present a problem for
trenches than for pumping wells.
• Increased Soil Contamination - One drawback to pumping wells is
that as the water table is depressed, more contaminants become
sorbed to soil particles as the floating NAPL moves downward with
the water table. Trenches avoid this problem.
RECOVERY OF FLOATING NAPL
Recovery of NAPL floating on the water table is a common part of site
remediation when a release of petroleum products reaches the saturated
zone. This is particularly true of large releases. The timely removal of
floating NAPL limits the movement of contaminants to the dissolved and
sorbed phases, which may be more costly to remove than floating NAPL. In
addition, recovered NAPL may be of commercial value and proceeds from the
recovered product can help defray clean-up costs.
There are several methods used to recover floating NAPL. The most
common of these use a pumping or oil/water separation scheme, depending on
the containment method implemented at the site. Some type of containment
system is usually needed to aid in these types of NAPL recovery to maintain
an adequate supply of recoverable product. Where the floating NAPL layer
is very thick (there have been documented cases of NAPL over 15 feet thick)
recovery can continue for extended periods without a containment system
being installed. Another recovery method that is not as common but has
been used effectively is vacuum extraction. Each of these methods is
discussed below. Discussion of NAPL recovery associated with the two
containment systems of the previous subsection is limited to the types of
recovery equipment commonly used with each. A more detailed discussion of
vacuum extraction is presented.
Hazards Associated with NAPL Recovery
The release of petroleum products, which are often highly volatile,
creates the potential for fire and explosion hazards. At a minimum, the
following steps should be taken to reduce the potential for fire or
explosion at a site:
• assess the potential hazards;
• isolate the danger areas;
• eliminate sources of ignition, particularly where pumps or other
recovery equipment is concerned; and
• ventilate confined areas.
65
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A remediation program should always be preceded by a health and
safety plan that considers in detail fire and explosion hazards that might
be encountered.
NAPL Recovery with Trench Excavation
The two most common types of equipment used to recover NAPL from
trenches are skimmers and filter separators. Skimmers are designed to
float and automatically pump NAPL off the water surface. Some are equipped
with sensors that can detect petroleum products and only operate when a
sufficient thickness of NAPL is present. Filter separators work much like
skimmers except that the filter, which allows only petroleum product to
pass through it. This device floats in the trench while the pump is
maintained above-ground. For either type of equipment, the recovered NAPL
must be stored in recovery drums for treatment or disposal.
NAPL Recovery with Pumping Well Installation
The two main types of recovery systems involving pumping wells are
single pump and dual pump systems. In single pump systems, one pump is
used to both contain the plume and recover NAPL. For dual systems, one
pump is used to create a depression in the water table and another to
remove NAPL floating on water table. Figure 18 shows typical single pump
and dual pump recovery systems.
Single Pump Method
In this method, both the floating NAPL and groundwater are recovered
through a single pipeline and stored or treated at the, surface. NAPL can
be removed from the groundwater/NAPL mixture by way of oil/water separation
equipment.
Single pump systems are best suited for small volume spills and sites
with shallow groundwater tables. A major drawback to this method is that
groundwater and NAPL are mixed together during the recovery process,
resulting in two obstacles to treatment. First, the groundwater/NAPL
mixing that occurs during recovery makes it more difficult to separate the
two above ground. Second, because the groundwater and NAPL are not kept
separate during recovery, large volumes of groundwater must be treated that
may have previously been unpolluted.
Dual Pump Method
In this method, two pumps are installed in a recovery well. The
lower pump is used to depress the local groundwater table, and serves to
both prevent further migration of NAPL and concentrate the NAPL in a
smaller area. The upper pump is positioned so that floating NAPL is
recovered while the pump is operating. Each of the pumps is typically
equipped with sensors to ensure that each of the pumps operates as desired.
The dual system is more commonly used than the single pump method,
primarily because mixing of groundwater and NAPL is avoided. Also, the
NAPL recovery pump need only be operating when a significant thickness of
66
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SINGLE PUMP SYSTEM
OIL-WATER SEPARATOR
Ol
CONCENTRATED
FREE PRODUCT
DUAL PUMP SYSTEM
CONCENTRATED
FREE PRODUCT
WATERTABLE
DEPRESSION PUMP
AQUIFER
PRODUCT RECOVERY PUMP
Figure 18. Single Pump and Dual Pump Gasoline Recovery Systems
67
-------
NAPL is present. The costs associated with dual pumps are higher, however,
particularly if multiple recovery wells are required.
Another important consideration when using dual pump recovery is that
lowering of the water table causes increased soil contamination because the
floating NAPL will fall with the water table. Removal of these
contaminants after the water table has been restored to its normal level
can be very difficult. A remediation plan for the soils above the water
table will not address these contaminants unless the water table remains
lowered.
Vacuum Extraction of Floating NAPL
Vacuum extraction, more commonly associated with treatment of the
unsaturated zone, has been shown to be successful in removing floating NAPL
as well. Data from the site of a large gasoline spill show that 50 gallons
per day of product was recovered from a single well pumping 75 cfm of
contaminated air (CDM, 1990). The floating product was located in a medium
to coarse sand with a less permeable silty layer approximately 10 feet
above the product. The silt layer acted to confine contaminant vapors and
enhance the effectiveness of the system. One advantage to using this
method is that the system can be readily designed to treat both floating
NAPL and contaminants in the unsaturated zone if needed.
Vacuum extraction to recover floating NAPL is a specialized form of
soil venting where natural volatilization of liquid contaminants is
enhanced by inducing pressure gradients in the soil above the NAPL. As
the NAPL floats on the water table some of the contaminant will transfer
from the liquid phase to the vapor phase (volatilize). The rate of natural
volatilization depends primarily on the vapor pressure of the contaminants
and the volume of air spaces in the soils above the NAPL plume. Vacuum
extraction enhances natural volatilization by removing the vapors from the
soil and bringing them to the surface. This creates a disequilibrium
between the liquid and vapor phases and allows volatilization to continue
at a greater rate.
Figure 19 shows a schematic diagram of a typical vacuum extraction
system designed to remove floating NAPL. The contaminant vapors are
removed by way of extraction wells that are screened just above the
floating NAPL plume. The number and placement of the network of wells
depends on the areal extent of the NAPL plume and the type of formation at
the site. A vacuum is applied to the wells and the vapors are drawn
through the soil air spaces to the wells, where they are captured and
brought to the surface. To further enhance recovery, positive-pressure air
injection wells may be employed. These wells force uncontaminated air into
the subsurface and serve both to direct the contaminated vapors to the
extraction wells and to provide a renewed source of air which promotes
further transfer from the liquid to the vapor phase. If properly placed,
positive injection wells can act as a barrier to off-site migration of
contaminant vapors. For example, if injection wells are placed between the
plume and nearby homes, vapors can be prevented from migrating towards the
homes.
68
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VACUUM
EXTRACTION UNIT
CONTAMINANT
VAPORS
CONTAMINANT
VAPORS
CONFINING LAYER
%v WS.'WS/X/\\X/'S.<'WW^/^V^v t t f f f t t't t t t f f \/\'\'\'\'\i\'\'r.
. V-0 '.s 0'. f'''ssssss^ y,X •',. -\ '\'..'-.'-. '•/-. '•/-.'-S •/•/•/•/•/-S •/•/-S •/•/•/•
Figure 19. Schematic Diagram of a Vacuum Extraction System for Recovering Floating NAPL
-------
Vacuum extraction can be enhanced by: 1) placing an impermeable
barrier (e.g., plastic sheets or clay soil) over the ground surface in the
area being treated; and 2) depressing the groundwater table under the NAPL.
The first action prevents short circuiting of the air flow; the second
exposes more NAPL to the effects of the vacuum extraction system. A
drawback to depressing the water table during vacuum extraction is that
more soils get exposed to contamination. The contaminants that sorb to the
newly exposed soils can be difficult and costly to remove.
Once brought to the surface, the contaminant-saturated vapors can bs
discharged to the atmosphere or treated to prevent air pollution. Vapors
may require treatment prior to discharge to the atmosphere, depending on
local air-discharge restrictions. If so, there are several treatment
methods that can be used prior to discharge, including carbon adsorption
and catalytic combustion.
Evaluating Vacuum Extraction
Table 14 lists CSFs for vacuum extraction and can be used as a
worksheet to help evaluate this method at a particular site. Among the
most important factors is the air conductivity of the soil. Coarse
materials, such as sand and gravel, which tend to have greater air
conductivity, are "more likely" to be amenable to vacuum extraction than
fine-grained materials like clay or silt. Each of the remaining CSFs can
be evaluated similarly to provide a preliminary screening of the
suitability of vacuum extraction at the site of interest.
Soil temperature has two opposing effects on vacuum extraction.
Contaminant volatility increases significantly with temperature, as
evidenced by increases in contaminant vapor pressure with temperature. The
air conductivity of the soil formation, however, will decrease with
temperature due to decreases in air density and increases in air viscosity
with temperature. The increased contaminant volatility will generally far
outweigh the negative impacts of decreased air conductivity, and vacuum
extraction can be expected to be more successful in areas where soil
temperature is high. In some cases, air is heated prior to injection to
raise the temperature of the soil and increase volatilization.
The water content of the formation also influences the effectiveness
of vacuum extraction in two ways. First, a formation with a high water
content will generally have a lower air conductivity than the same
formation with a lower water content because it will have proportionately
less air space. Second, pore water can absorb (dissolve) contaminants from
the vapor phase, which serves to retard the removal of contaminant vapors.
Dry soil is thus better suited to in-situ stripping than wet soil.
However, the vacuum extraction process tends to dry out the soil over time,
increasing the rate of removal.
The soil surface area and the organic content of the formation both
influence the degree to which contaminants will be in the sorbed phase as
opposed to the vapor phase. The greater the unit surface of a soil, the
more likely contaminants will become sorbed to soil particles and the less
effective vacuum extraction will be. Organic matter in the subsurface also
serves to increase sorption of contaminants.
70
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Table 14. Worksheet for Evaluating the Feasibility of Using
Vacuum Extraction to Remove Floating NAPL at Your Site
CRITICAL SUCCESS
FACTOR
SITE -RELATED
• Soil Air Conductivity
• Soil Temperature
• Moisture Content
• Soil Surface Area
• Carbon Content
UNITS
cm/sec
°C
% volume
2
m la
% weight
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
Low
O
Low
(<10)
0
High
(>30)
O
High
(>1)
O
High
(>10)
O
SUCCESS
SOMEWHAT
LIKELY
Medium
O
Medium
(10-20)
0
Medium
(10-30)
O
Medium
(0.1-1)
O
Medium
(1-10)
0
SUCCESS
MORE
LIKELY
Higtj
0
High
(>20)
O
Low
(<10)
O
Low
(<0.1)
0
Low
(<1)
O
CONTAMINANT- RELATED
• Vapor Pressure
• Water solubility
mm Hg
mg/l
Low
(<10)
O
High
(>1000)
O
Medium
(10-100)
O
Medium
(100-1000)
O
High
(>100)
0
Low
(<100)
0
OTHER CONSIDERATIONS
• Treatment of contaminant vapors may be required before discharge to atmosphere.
* CSFs denoted with an asterisk are typically more Important than other CSPs
71
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Two important contaminant-related CSFs are vapor pressure and water
solubility. Compounds with higher vapor pressures will transform more
readily from NAPL to the vapor phase and therefore are more easily removed
by vacuum extraction. Compounds with low solubility generally have greater
capacity to sorb to soil particles, and relatively insoluble compounds such
as heavy fuel oils will not be as readily removed as more soluble products
like gasoline.
Other considerations are less scientific in nature but can play a
significant role in determining the applicability of vacuum extraction for
removing floating NAPL. One advantage of this method over pumping wells is
that a recovery system can be implemented without bringing groundwater to
the surface. Extracted groundwater must generally be treated and/or
disposed of. This will usually require special permitting which can delay
implementation of the recovery system.
Relative Importance of CSFs
The two most important CSFs for vacuum extraction are the air
conductivity of the soil and the vapor pressure of the contaminants.
Together these two play the biggest role in determining the rate at which
contaminants can be removed from the subsurface. Formations with low air
conductivity require more extraction wells and/or increased suction at each
well to attain the same removal rate as more highly conductive formations.
Products such as heavy fuel oils that have low vapor pressures do not
transfer from NAPL to the vapor phase as readily as lighter products such
as gasoline, and will take longer to remove by vacuum extraction.
TREATMENT OF CONTAMINANTS DISSOLVED IN GROUNDWATER
Above-Ground Treatment
Above-ground treatment of groundwater is generally accomplished by
installing extraction or recovery wells that bring the groundwater to the
surface where it can be treated (i.e., pump and treat). The groundwater is
then either disposed of or recharged to the subsurface. The two most
commonly used above-ground treatment technologies are air stripping and
carbon adsorption. These two are covered in detail in this section.
Biological treatment of extracted groundwater shows promise of being
effective, but has yet to be widely used or proven. Above-ground
biological treatment is discussed in somewhat less detail in the New
Technologies subsection.
Extraction of Groundwater
Each of the above-ground methods requires a groundwater extraction
system to bring contaminated groundwater to the surface. Recovery of
contaminants from the saturated zone using extraction methods can be
difficult. Some of the difficulties inherent in an extraction system are
presented prior to detailing each of the above-ground technologies.
As discussed in Section 2, contaminants can exist below the water
table in three different phases: 1) dissolved; 2) sorbed; or 3) as NAPL.
Dissolved contaminants can usually be removed fairly easily, particularly
72
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relative to the other two phases, which tend to persist in the saturated
zone. However, the dissolved portion of total contaminant mass below the
water table may be small. The effectiveness of a groundwater extraction
system will often depend on the amount of sorbed or NAPL product below the
water table and how readily these phases can be recovered.
As groundwater is extracted from the aquifer through a recovery well,
the dissolved contaminants in permeable formations are readily mobilized.
Dissolved contaminants in less permeable formations take longer to recover.
Sorbed contaminants are not directly affected by pumping wells and can only
be removed after going back into solution. As pumping continues, the
dissolved concentrations of contaminant in the surrounding groundwater
decrease, promoting dissolution of the sorbed contaminant, which can then
be brought to the surface. NAPL trapped below the water table can also be
difficult to remove. The interfacial tension forces between the soil
particles and the NAPL are often large enough to inhibit mobilization, even
in the presence of large hydraulic gradients. In such cases, dissolution
of the NAPL to the dissolved phase over time will govern the extraction
rate of the NAPL. Figure 20 shows dissolved contaminants being extracted
by a recovery well over time. At time Tl, pumping has just recently been
started and the dissolved contaminants are fairly evenly dispersed
throughout the aquifer. At time T2, dissolved contaminants in the more
permeable sand and gravel have been largely removed, but the low
permeability clay lenses continue to slowly release dissolved contaminants.
If significant amounts of NAPL and sorbed product exist in the
saturated zone, the time-frame for remediation can be very long using
groundwater extraction techniques, often spanning decades. The volume of
groundwater withdrawn over this period can become very large. Unless
injection wells are used to supply the clean water, which is rarely the
case, this approach may utilize a considerable volume of uncontaminated
groundwater surrounding the site to flush the contaminants from the
polluted area (Mackay and Cherry, 1989). All the water recovered in the
system must be treated, including the previously uncontaminated
groundwater.
Figure 20 also illustrates the importance of follow-up monitoring at
a site. Over time, the contaminant levels throughout much of the site may
be very low and monitoring wells will probably reflect this. If treatment
is discontinued, however, sorbed contaminants and immobilized NAPL will
continue to dissolve into the groundwater. Unless monitoring continues
well after remediation appears complete, the remaining pollutants will go
undetected.
Air Stripping
Air stripping is the generic term used to describe several similar
above-ground methods for treating contaminated groundwater. It is
one of the most common methods used to remove volatile organic compounds
from groundwater in the United States. The widespread use of air stripping
is due to its proven success, its cost-effectiveness, and the fact that the
concepts behind air stripping are well understood and design of the system
is fairly straightforward.
73
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Recovery
Wei!
Recovery
Well
Ground Surface
Ground Surface
Original
Water Table
T1
LEGEND
CZZD
Dissolved Contamination
in Permeable Sand and Gravel
Dissolved Contamination
in Less Permeable Clay Layer
Figure 20. Retention of Dissolved Contaminants in Less Permeable
Formations During Groundwater Extraction Treatment
-------
Air stripping removes contaminants from groundwater through a
controlled disequilibrium designed to promote the transfer of contaminants
from the dissolved phase to the vapor phase. This is accomplished for all
air stripping methods in two basic ways: 1) the surface area of the
contaminated groundwater is increased, thus increasing the opportunity for
volatilization; and 2) disequilibrium between the liquid and gaseous phases
is maintained by replenishing the supply of "clean" air to the system
through either natural or induced means.
There are four common air stripping methods: 1) diffused aeration,
2) tray aeration, 3) spray aeration, and 4) packed towers. Of these,
packed towers are most commonly used to remove petroleum hydrocarbons from
groundwater because of their great cost-effectiveness. The main components
of each are described below.
Diffused Aeration - In diffused aeration, the contaminated
groundwater is channeled through a large holding tank or pond with one or
more diffuser pipes along the bottom. Air is pumped through the diffuser
pipes and bubbles up through the tank, providing a medium into which the
dissolved contaminant can volatilize. Once the air reaches the water
surface it can be vented to the atmosphere or collected and treated.
The efficiency of diffused aeration can be increased by decreasing
the bubble size, increasing the air to water ratio, agitating the water, or
increasing the depth of water in the tank if the air bubbles are not
saturated with contaminants when exiting the tank. Removal rates for
diffused aeration range from 70 to 90 percent for many VOCs, which is often
not an adequate removal efficiency at petroleum release sites.
Tray Aeration - Tray aeration is not as efficient in removing
volatile organics as other air stripping methods presented in this section,
but because of its simplicity and low maintenance, it is sometimes used as
a pre-treatment for other methods. A typical tray aeration tank contains
staggered slat trays. Water is fed into the top of the tank and flows by
gravity through a distribution network onto the the trays. The trays
disperse the water, increasing the water surface area exposed to the
atmosphere. A blower can be installed at the base of the unit to increase
the rate at which "clean" air is introduced to the system. As the air
passes up through the unit, contaminant vapors are accumulated and
eventually vented to the atmosphere or collected and treated.
Spray Aeration - In this method, contaminated groundwater is sprayed
through nozzles over a pond or basin, greatly increasing the surface area
of the water. As the water falls back to the pond, VOCs are transferred to
the atmosphere. Because the contaminated groundwater system is exposed to
the atmosphere, there is a constant source of "clean" air for stripping.
However, it is difficult to contain and treat the contaminated air if this
is desired or required. Other drawbacks include the relatively large land
areas needed for the spray ponds and possible transport of contaminants to
neighboring areas from mists created during spraying.
In addition to being used as a primary means of removing VOCs from
groundwater, spray aeration has also been used to recharge the aquifer
while at the same time providing additional removal of contaminants. This
75
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is done by spraying the effluent water from the treatment unit over the
recharge area of the aquifer. The effluent not only percolates through the
soil to the underlying aquifer but also undergoes additional contaminant
removal.
Packed Towers - Of the air stripping methods, packed towers work best
at removing volatile organic compounds (VOCs) from groundwater and they are
often the most cost-effective method as well. For these reasons, packed
towers are the most widely used air stripping method.
The tower, which can be more than 100 feet high, consists of a
cylindrical column containing a packing material specially designed and
arranged to increase the surface area of the groundwater being passed
through it. Figure 21 shows a typical packed tower. Contaminated
groundwater is pumped to the top of the column and allowed to flow by
gravity down through the packing material while uncontaminated air is
pumped up through the column. This produces a high air to water ratio and
results in high removal efficiency. Packed towers can be designed to
remove 90 to 99 percent of many VOCs from groundwater.
The design of packed towers is well understood relative to other
treatment technology designs. Various design parameters can be adjusted to
meet the requirements at a given site. The air to water ratio is one of
the important design parameter and is chosen on the basis of
cost-effectiveness and the Henry's law constant of the design contaminant
(EPA, 1988b). A higher ratio will increase operating costs but will allow
a decrease in the size of the tower, which can be quite large. Typical air
to water ratios range from 20:1 to 100:1.
Evaluating Air Stripping
The worksheet of Table 15 can be used to help evaluate the likely
effectiveness of air stripping at a particular site. The importance of
each of the CSFs is discussed below.
Air stripping becomes more cost-effective relative to other
technologies as the volume of groundwater to be treated increases. The
amount released and the time since release both influence how much
dissolved contamination there will be at a given site. A greater volume of
contaminated groundwater is more likely for larger, older releases than
smaller, more recent releases. The fact that air strippers can effectively
treat large volumes of groundwater makes this technology especially
attractive when large pumping rates are required to contain the dissolved
plume.
Groundwater temperature affects air stripping performance, primarily
because vapor pressure and temperature are related. Vapor pressure
increases with temperature, and higher temperatures allow better removi
rates. The impact of temperature on air stripping is most relevant in
colder regions where winter removal rates may be significantly lower than
summer rates unless system modifications are made. The presence of high
concentrations of iron and manganese in groundwater retards hydrocarbon
removal because they can precipitate onto the packing materials in the
stripper. High total suspended solids concentrations are often an
76
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OFF-GASES
INFLUENT
INCOMING
AIR
I
BLOWER
DEMISTOR
innnnn
WHTWELL
DISTRIBUTOR
•PACKING MATERIAL
^SUPPORT PLATE
TO STORAGE TANK,
DISTRIBUTION
SYSTEM
OR ADDITIONAL
TREATMENT
EFFLUENT
BOOSTER
PUMP
Figure 21. Typical Packed Tower Air Stripper
77
-------
Table 15. Worksheet for Evaluating the Feasibility of Using Air
Stripping to Treat Extracted Groundwater at Your Site
CRITICAL SUCCESS
FACTOR
RELEASE - RELATED
*
• Amount Released
*
• Time Since Release
SITE - RELATED
• Groundwater Temperature
• Total Suspended
Solids Content of GW
• Total Dissolved Iron and
Manganese Content of GW
UNITS
gallons
months
°C
mg/L
mg/L
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
High
(<1,000)
o
Short
(20)
O
High
(>5)
O
SUCCESS
SOMEWHAT
LIKELY
Medium
(1,000-50,000)
0
Medium
(1-12)
O
Medium
(10-20)
0
Medium
(5-20)
O
Medium
(0.2-5)
O
SUCCESS
MORE
LIKELY
Large
(>50,000)
0
Long
(>12)
O
High
(>20)
O
Low
(<5)
O
Low
(<0.2)
O
CONTAMINANT- RELATED
• Vapor Pressure
*
• Water Solubility
*
• Dissolved Contaminant
Concentration
mm Hg
mg/L
mg/L
Low
(<10)
0
High
(>1000)
O
Low
(100)
O
Low
(<100)
O
High
(>100)
O
78
-------
indication of the presence of iron and manganese in groundwater.
Pre-treatment to remove suspended solids can enhance air stripping
effectiveness.
The vapor pressure and water solubility of the contaminants affect
how well an air stripping system works.As vapor pressure increases, the
ease with which the contaminants are removed from the groundwater
increases. Solubility has the opposite effect. Highly soluble contami-
nants tend to remain in solution longer as they pass through the air
stripper and are not as effectively removed as less soluble ones. The
influent concentration of the contaminants also is an important considera-
tion.Air strippers can effectively remove high concentrations of volatile
hydrocarbons.
Other factors to consider are regulatory in nature. These include
effluent standards and permitting requirements. State or local
requirements for groundwater quality may dictate the degree of treatment
required at a site. Air strippers are not as effective at removing
contaminants to very low levels as some other treatment methods. Special
permits may be required to dispose of treated groundwater or even to set up
the treatment system. Many states also have air discharge requirements
that will necessitate treating contaminant vapors.This is usually
accomplished by collecting the vapors and passing them through a carbon
filter.
Relative Importance of CSFs
Among the most important CSFs in Table 15 are the contaminant-related
factors (vapor pressure, water solubility, and dissolved contaminant
concentration). Each of these factors is important in determining the
technical effectiveness of an air stripping facility. The release-related
factors (amount released and time since release) are also important for
estimating the relative cost-effectiveness of this technology. The
site-related CSFs also influence air stripping, but the treatment process
can be modified to reduce any negative impact these factors might have.
Activated Carbon Adsorption
Like air stripping, activated carbon is commonly used in the United
States to remove VOCs from contaminated groundwater. One of the most
desirable aspects of activated carbon adsorption as a treatment process is
its ability to remove contaminants to very low levels. Quality of effluent
from a properly designed carbon adsorption unit can be at or below drinking
water standards.
The effectiveness of carbon as a treatment process is due to its
ability to function as a adsorbent for molecules dissolved in water. This
ability is largely due to carbon's large internal surface area. The actual
surface area of commercially available activated carbon depends on the
source of carbon and the manufacturer, but an average value is about 1,000
mVg.
Two forms of carbon are used to treat water; powdered activated
carbon (PAC) and granular activated carbon (GAC). Powdered carbon is
79
-------
dispersed freely into the water and subsequently filtered out. It is
commonly used as a polishing step in the treatment of drinking water but
rarely for treatment of contaminated groundwater because it is not easily
reused. Granular carbon is coarser than powdered carbon and is typically
contained in a cylindrical unit. Contaminated water is passed through the
unit until the adsorption capacity of the carbon is spent. The granular
carbon can then be regenerated for future use. Because only the granular
form of carbon is commonly used in treatment of contaminated groundwater,
only GAG treatment is discussed in this manual.
Carbon adsorption is less cost-effective at treating large volumes of
groundwater than some other technologies (e.g. air stripping) because of
the relatively high cost of carbon. Typical surface loading rates through
GAC cylinders are from 5 to 7 gpm/ft . The cylinders holding the GAC
usually range from 4 to 12 feet in diameter. A 10-foot diameter tank could
treat about 400 to 500 gpm of contaminated groundwater, but the volume of
carbon required to maintain this flow rate could be prohibitively
expensive.
Design of a GAC system is not as straightforward as for air stripping
because of the complex interaction of the factors influencing removal
efficiency. Since the effectiveness of a particular design cannot be
accurately predicted, pilot studies, should be undertaken before installing
a GAC system.
Evaluating Carbon Adsorption
Many factors contribute to the successful operation of a GAC system.
The worksheet of Table 16 lists some of these factors and can help the user
evaluate carbon adsorption as a groundwater treatment technology. Each of
the factors of Table 16 are discussed below.
The amount released and the time since release can give a rough idea
of the volume of contaminated groundwater that will be encountered at the
site. The effectiveness of carbon adsorption decreases as the volume of
water requiring treatment increases. A larger amount of dissolved
contamination can be expected for larger releases than for small ones,
given other conditions are the same. The more time that has elapsed since
the released first occurred, the more dissolved contamination there will be
and the more widely dispersed the dissolved plume will be. The total
volume of contaminated groundwater is, therefore, likely to be greater for
larger, older releases.
The influent contaminant concentration should be considered when
evaluating this method. GAC is extremely effective at removing very low
levels of contamination, but if the total organic carbon (TOC) content of
the contaminated groundwater is high, frequent carbon changes will be
required. This can add significantly to treatment costs, and sites with
high TOC concentrations may be better suited to another technology.
Carbon adsorption works better when the suspended solids and
dissolved iron and manganese content of the groundwater are low. High
concentrations (5 mg/L or more) of total iron and manganese may precipitate
onto the carbon surface inhibiting the adsorption process. Excessive
80
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Table 16. Worksheet for Evaluating the Feasibility of Using Carbon
Adsorption to Treat Extracted Groundwater at Your Site
CRITICAL SUCCESS
FACTOR
UNITS
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
SUCCESS
SOMEWHAT
LIKELY
SUCCESS
MORE
LIKELY
RELEASE - RELATED . ., ' , . . , • ' ^ : . .,-- .;^ ^^ /y^^
• Amount Released
• Time Since Release
SITE* RELATED
• TOC Content of GW
• Suspended Solids
Content of GW
- Total Dissolved Iron and
Manganese Content of GW
gallons
months
mg/L
mg/L
mg/L
Large
(>50,000)
o
Long
(>12)
O
High
(>5)
0
High
(>20)
O
High
(>5)
O
Medium
(1,000-50,000)
O
Medium
(1-12)
O
Medium
(1-5)
O
Medium
(5-20)
O
Medium
(0.2-5)
O
Small
(<1,000)
O
Short
(<1)
0
Low
< ; :' ' • "' ;:: I-
*
> Water Solubility
*• Molecular Weight
mg/L
g/mole
High
(>1000)
O
Low
(<100)
O
Medium
(100-1000)
0
Medium
(100-200)
O
Low
(<100)
0
High
(>200)
O
OTHER CONSIDERATIONS
• Can remove contaminants to drinking water standards.
• Costs increase significantly if large volumes or highly contaminated groundwater is encountered.
* CSFs denoted with an asterisk are typically more important than other CSFs
81
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suspended solids (greater than 20 mg/L) may have to be filtered out to
prevent clogging of the carbon.
Highly soluble compounds are not as readily removed by GAC as less
soluble ones.The more soluble compounds are more likely to stay in
solution as the groundwater is passed through the GAG unit. Contaminants
with high molecular weight are more readily adsorbed than lighter
contaminants.
Relative Importance of CSFs
The most important factors are the TOC content of the groundwater,
the water solubility of the contaminants, and the molecular weight of the
contaminants. High TOC content would make carbon adsorption prohibitively
expensive as the sole treatment process. Used in conjunction with a
pre-treatment process, however, carbon adsorption can be used at virtually
any site if very low effluent standards are required. Water solubility and
molecular weight of the contaminants are important in evaluating the
technical performance of carbon adsorption. The dissolved and suspended
solids content of groundwater can usually be made more amenable to carbon
adsorption through pre-treatment and are less important than other CSFs.
In-situ Treatment
Biorestoration
Biorestoration relies on naturally occurring or genetically altered
microorganisms to transform contaminants to less hazardous compounds.
Biodegradation of contaminants is enhanced by the addition of oxygen and
nutrients. Bacteria capable of biodegrading petroleum hydrocarbons are
commonly found in subsurface soils and some natural breakdown of
hydrocarbons released to the subsurface is likely to occur at all sites.
However, without the addition of nutrients and oxygen, biodegradation
usually occurs very slowly. This treatment method is not as widely used as
air stripping or carbon adsorption to treat groundwater, but under the
right conditions, biorestoration can be very cost-effective and its use is
likely to grow in the future.
In-situ biorestoration differs from air stripping and carbon
adsorption in two main ways: 1) the contaminated groundwater remains below
ground during remediation; and 2) the contaminants are not merely separated
from the groundwater but are transformed to less toxic or even harmless
compounds. In-situ biorestoration can lessen the risk of exposure incurred
when contaminants are brought to the surface. It may also lessen or
eliminate the need to treat or otherwise dispose of the contaminants as may
be required in separation techniques.
Biodegradation of hydrocarbons can occur aerobically (in the presence
of oxygen) or anaerobically (without oxygen). It is widely accepted that
aerobic processes are far more effective in biodegrading petroleum
hydrocarbons than anaerobic processes, and therefore, the focus of
discussion is on aerobic processes.
82
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Under complete aerobic degradation, hydrocarbons are biotransformed
to carbon dioxide, water and biomass rather than physically separated, as
with other technologies. Under less than ideal conditions (i.e.,
inadequate oxygen supply, lack of nutrients, etc.), less complete
degradation may take place resulting in only partial breakdown of the
hydrocarbons. If conditions can be enhanced, more complete biodegradation
can take place. The fact that biodegradation can transform a contaminant
to a non-toxic or less-toxic end product is a major advantage this
technique holds over other cleanup methods. The other techniques serve
only to concentrate, disperse, or relocate the contaminants, and further
treatment or handling is usually required.
Although the biorestoration method described here is in situ, above
ground equipment is required. At a minimum, an above ground mixing tank is
required to add nutrients and oxygen to the water being introduced to the
unsaturated zone. In some cases, the nutrient and oxygen rich treatment
solution is also seeded with microorganisms known to be able to degrade
contaminants at the site. However, some scientists believe that such
seeding is unnecessary because the required organisms are ubiquitous in the
environment. The mixing tank is also used as temporary storage of
recirculated water, where additional bacteria can be grown and the organic
content of the water can be reduced. Oxygen and nutrients are usually
supplied by adding nutrient-enhanced water through infiltration basins at
the ground surface or through recharge wells. A pumping well can be
installed to control local groundwater flow and recirculate treated
groundwater to the subsurface. Figure 22 shows a typical biorestoration
system.
Conventional methods introduce oxygen to the subsurface by direct
aeration or by the addition of hydrogen peroxide which, upon decomposition,
generates oxygen. A recent study (Major et al., 1988) suggests that
natural biodegradation may also be enhanced through the addition of a
nitrate solution to the subsurface, but this method remains unproven and is
not detailed here. A brief discussion of enhanced biodegradation through
nitrate addition is presented in the New Technologies subsection.
It is difficult to predict the amounts of oxygen and nutrients that
must be added to obtain optimal biodegradation at a given site. If too few
nutrients are added, remediation efforts may be incomplete. Excessive
nutrient addition can result in biological growth that clogs the soil,
decreasing hydraulic conductivity and obstructing further treatment. To
increase the likelihood of a successful biorestoration program at a
particular site, pilot studies using samples collected at the site should
be conducted to determine the appropriate nutrient additives and the
required application rate.
Evaluating Biorestoration
Table 17 lists the critical success factors for biorestoration. By
comparing the parameters at the site of interest to those in this table, a
general understanding can be obtained of the suitability of biorestoration
at that site. A preponderance of CSFs that match the rightmost column
would indicate that biorestoration is likely to be effective at that site.
83
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RECOVERY
WELL
NUTRIENTS OXYGENATION
MIXING TANK
INJECTION
WELL
LI-
WATER
TABLE
.V..—-
GROUNDWATER
FLOW DIRECTION
ZONE OF PETROLEUM LADEN SOILS
Figure 22. Typical In-Situ Biorestoration System
84
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Table 17. Worksheet for Evaluating The Feasibility of Using
Biorestoration to Treat Groundwater at Your Site
CRITICAL SUCCESS
FACTOR
RELEASE- RELATED
• Time since release
SITE- RELATED
*
• Hydraulic Conductivity
• Site Stratigraphy
• Groundwater temperature
• pH
UNITS
months
cm/sec
°C
pH units
CONTAMINANT- RELATED
• Blodegradablllty
Refractory Index
• Total Organic Carbon
Content of GW
mg/L
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
Short
(<1)
O
Low
(<10 "5 )
O
Complex
O
Low
(<5)
0
(< 6 or >8)
O
SUCCESS
SOMEWHAT
LIKELY
Medium
(1-12)
O
Medium
(lO^-IO"5 )
0
Medium
(5-10)
O
Low
(<0.01)
O
(1000)
0
Medium
(0.01-0.1)
0
SUCCESS
MORE
LIKELY
Long
(>12)
O
High
(>10"3 )
0
Simple
0
High
(>10)
O
(6-8)
O
High
(>0.1)
O
(10-1000)
O
OTHER CONSIDERATIONS
• It is difficult to monitor the effectiveness of the system
• Minimizes health risks by keeping contaminants below the ground surface.
* CSFs denoted with an asterisk are typically more important than other CSFs
85
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Biorestoration is perhaps the only remedial technology for which a
longer time since release can be an advantage. Because naturally-occurring
microorganisms usually require time to acclimate, a biorestoration program
initiated well after the time of release will often meet with more initial
success than are started shortly after the release. This is particularly
relevant for releases that are only discovered well after the release
occurred.
A knowledge of the hydraulic conductivity of the soil, and the
site hydrogeology in general, is extremely important in assessing the
feasibility of biorestoration for a particular site. Even when all other
factors are positive, in-situ biorestoration will not be successful if a
low hydraulic conductivity prevents the added nutrients and oxygen from
contacting the zone of contamination.
If the site stratigraphy is simple and fairly homogeneous, exposing
the dissolved contaminants to oxygen and nutrients will be relatively easy.
At more complex sites where groundwater flow patterns are difficult to
predict, this technology will likely meet with less success.
Slightly alkaline soil pH is optimal for biodegradation, but anything
in the range of 6.0-8.0 is considered acceptable. Most soils are slightly
acidic and neutralization may be required at some sites. The temperature
of the soil environment will also affect the rate of degradation. Warmer
temperatures generally result in higher rates of degradation. While
biodegradation has been shown to occur over a wide temperature range, the
range of 20-35°C seems optimal but is typically above the normal subsurface
temperature range. Also, microbes generally have a low tolerance for
severe temperature changes as are experienced in northern regions.
The susceptibility of a given contaminant to biodegradation is an
important consideration. Some compounds are much more readily biodegraded
than others. Although it is not possible to predict with certainty whether
a certain compound will be degraded at a specific site, the refractory
index (Ri) indicates a compound's susceptibility to degradation. The Rl, a
ratio of the BODS (biochemical oxygen demand) to the COD (chemical oxygen
demand), predicts the likelihood of biodegradation for a compound (Lvman et
al., 1982). —
The concentration of contaminant dissolved in water affects
biodegradation^if too low, bacteria may favor another competing food
source; if too high, the contaminant may be toxic to the bacterial
population. Aerobic bacteria are classically used for organic
concentrations between 50 and 4,000 mg/L (Nyer, 1985). For in~situ
treatment, organic concentrations as low as 10 mg/L may be sufficient.
Dissolved petroleum hydrocarbons are unlikely to be found at concentrations
as high as 4,000 mg/L.
Relative Importance of CSFs
Of the CSFs listed in Table 17, hydraulic conductivity and site
stratigraphy are probably the most important. The success of this method
depends on delivering nutrients and oxygen to the contaminants. Complex
stratigraphy and/or poorly conductive formations prohibit this transport.
86
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The biodegradability of the contaminants is also important, but the water
soluble fraction of most petroleum products is comprised mostly of aromatic
hydrocarbons, which have been shown to be fairly degradable in natural
settings.
NEW TECHNOLOGIES
The treatment technologies previously discussed in this manual are
only a portion of all available technologies, but they represent the most
well understood and commonly used technologies. Other technologies show
signs of being successful alternatives to the above-mentioned technologies.
A few of these are described briefly below.
Biorestoration with Nitrate Addition
A recent study (Major et al., 1988) indicates that biodegradation of
petroleum hydrocarbons may be effectively promoted by the addition of
nitrate rather than oxygen. The laboratory study suggests that
denitrification by subsurface microorganisms, where nitrate is reduced to
nitrogen and oxygen, creates an environment suitable for biodegradation of
the BTEX compounds (benzene, toluene, ethylbenzene and the xylenes). This
process may be as effective as more conventional in-situ approaches that
rely on the addition of nutrients and oxygen. However, it is difficult to
equate laboratory success with field success due to the unique conditions
found at each site.
If successful, nitrate addition could have significant advantages
over conventional biodegradation techniques. Because nitrate is highly
soluble, distribution of nitrate throughout the contaminated aquifer is
easier than distribution of oxygen through either air sparging or hydrogen
peroxide addition. Also, because the dosages can be relatively
concentrated, fewer applications may be required, reducing the length and
cost of remediation.
Air Sparging
Air sparging is a relatively inexpensive treatment technology
developed, and primarily used, in Europe. There is no indication that this
technology is currently being used in the United States and very little
literature on air sparging is available in this country. Air sparging
differs from the other saturated zone technologies presented in this manual
in that it must be used in combination with treatment of soils in the
unsaturated zone. The method involves using a vacuum extraction system to
remove contaminant vapors from the unsaturated zone (soil venting) and
pumping compressed air into the saturated zone to volatilize and mobilize
contaminants dissolved in the groundwater.
Air sparging to remove contaminants from the saturated zone can be
likened to in-situ air stripping. As in the more common above-ground air
stripping techniques, sparging involves exposing contaminated groundwater
to "clean" air which facilitates the volatilization of the contaminants
from the groundwater. An injection well is installed in the region of
greatest contamination, with the well screen located below the contaminant
plume. Compressed air is pumped into the aquifer through the injection
87
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wells. The resultant air bubbles percolate up through the groundwater and
are exposed to contaminants. Because no free air initially exists in the
saturated zone, disequilibrium exists between the vapor and the dissolved
phases of the contaminants. Contaminants in the dissolved phase will
volatilize into the air bubbles until equilibrium is reached. Eventually
the contaminant laden air bubbles reach the unsaturated zone and are
captured through vacuum extraction.
Above-Ground Biological Treatment
Biological treatment of wastewater has proven quite cost-effective
and it is the most widely used method of treatment for industrial and
municipal wastewaters (Nyer, 1985). Above-ground biological treatment of
contaminated groundwater is not as common, but could prove to be a
successful alternative to other above-ground treatment technologies. The
main restrictions to its use are: 1) the fact that the system must be run
24 hours a day; 2) start up of the system can take weeks to months; and 3)
conventional biological reactors are not designed for the relatively low
total organics concentrations found in contaminated groundwater. Research
into applying above-ground biological methods to treat groundwater is
ongoing, and may provide an effective means of remediation under the right
conditions.
TECHNOLOGY COMPATIBILITY
Saturated Zone Technologies
The worksheets of this section can be used to evaluate the likely
effectiveness of a single technology at a given site. However, clean up of
the saturated zone can often be enhanced by using more than one technology.
If the user finds that none of the technologies in the manual adequately
address remediation goals, combining technologies may prove helpful. Below
are examples of promising technology combinations.
• Removal of Floating NAPL - An increasingly common concern when
using pumping wells to contain and recover floating NAPL is
increased soil contamination due to lowering of the water table.
Pumping wells are often required at a site, particularly if
dissolved contamination is significant. Still, smearing of
uncontaminated soil by floating NAPL can be minimized if trenches
or vacuum extraction is initially used to recover as much NAPL as
is practical before pumping wells are installed and the water
table lowered.
• Treating Contaminated Groundwater - Air stripping and carbon
adsorption are very compatible at sites where groundwater
contamination is extensive and a high level of treatment is
required. In such a case, neither of the technologies would
likely be adequate by themselves. Air stripping can effectively
treat large volumes of groundwater, but is not usually capable of
removal to the low ppb range. Carbon adsorption is best suited
for removing low-level contamination to near the drinking water
standards. A combination of air stripping as a pre-treatment and
carbon adsorption as a finishing process can be very effective.
88
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Saturated and Unsaturated Zone Technologies
This manual focuses on remediation of the saturated zone (and ex-
cludes the unsaturated zone), but clean up of the saturated and unsaturated
zones should not be considered as distinct activities. To have an
effective overall remediation plan, the potential impact that clean up in
one zone has on another should be considered. Some technologies used to
treat these two zones are more compatible than others. For example,
biorestoration of the saturated and unsaturated zones may be quite
feasible, but vacuum extraction of floating NAPL would be inhibited by soil
flushing in the unsaturated zone.
Table 18 shows the degree of compatibility between the more well
known unsaturated zone and saturated zone technologies. Each of the
saturated zone technologies in Table 18 are covered in this manual. The
unsaturated zone technologies are not covered here but can be found in
other literature including the companion document to this manual (EPA,
1989). Each of the boxes corresponding to an unsaturated zone/saturated
zone technology combination shows the degree of compatibility between the
two. The "very compatible" descriptor is attributed to two technologies
that tend to enhance remediation of one or the other, while "not
compatible" technologies retard the effectiveness of one or both. This
table can be used to help in planning an effective remediation program
that has short term goals and activities that are compatible with planned
future activities.
89
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TABLE 18. COMPATIBILITY OF SATURATED ZONE
TECHNOLOGIES WITH UNSATURATED TECHNOLOGIES
SATURATED ZONE TECHNOLOGIES
UNSATURATED
ZONE
TECHNOLOGIES
Vacuum
Extraction
Groundwater
Extraction Methods
Vacuum Extraction
of Floating NAPL
In-Situ
Biorestoration
Soil
Flushing
In-Situ
Biorestoration
Excavation
COMPATIBLE
+ Lowering of groundwater
with recovery wells can
increase influence of
vacuum extraction
Lowered water table may
cause increased sorbed
phase.
VERY COMPATIBLE
+ Extracted GW can be used
as flushing medium after
treatment.
COMPATIBLE
+ After treatment, extracted
groundwater may be
enhanced for biorestoration
and re-introduced to
saturated zone.
SOMEWHAT COMPATIBLE
+ Both can be implemented
without impeding the
performance of either.
VERY COMPATIBLE
+ One system can be designed
for both unsaturated and
saturated zone treatment.
- Little or no affect on
dissolved contaminant in
saturated zone.
SOMEWHAT COMPATIBLE
- Vacuum extraction of float-
ing NAPL must likely be
completed before soil
washing begins.
COMPATIBLE
+ Vacuum extraction may
induce increased oxygen
levels, promoting natural
biodegradation in the
unsaturated zone.
SOMEWHAT COMPATIBLE
Excavation activities could
interfere with vacuum
extraction.
SOMEWHAT COMPATIBLE
- Introduction of nutrient
rich water to subsurface
could interfere with
vacuum extraction.
COMPATIBLE
GW extracted from
containment wells can be
used to distribute nutrients as
well as flush contaminants
Surfactants may not be
useful if they inhibit
biodegradation
VERY COMPATIBLE
+ One system can be designed
to add nutrients to both the
saturated and unsaturated
zones.
COMPATIBLE
+ Both can be implemented
without impeding the per-
formance of either.
90
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SECTION 4
FOLLOW-UP MEASURES
RE-EVALUATE REMEDIATION GOALS
Remediation of petroleum product releases is often a long process.
Initially, immediate goals will be set and addressed. Over time, the
initial goals may be altered as new information becomes available and new
goals added as different stages of the project are reached. For each new
activity, alternative solutions should be evaluated. The basic method
outlined in this manual for quickly evaluating alternative technologies can
also be used in these later stages of remediation (as shown in Figure 3).
For example, the initial goal of remediation may be to prevent dissolved
contaminants from moving off site. A containment system may be installed
which recirculates (as opposed to treats) the groundwater. A later goal
may be to treat the groundwater before recirculation or disposal. The user
could use the manual to quickly screen the technologies applicable to
recovery or removal of contaminants dissolved in groundwater.
MONITORING TECHNICAL PERFORMANCE
Remediation is generally both lengthy and costly, and monitoring is
the only way to determine whether the program is working. Proper
monitoring can detect problems in the design or implementation of the
selected technology that may be correctable with little added effort. The
earlier such problems are detected, the more time and money will be saved.
For example, a containment system should always be accompanied by a
well-designed monitoring program to ensure that the entire plume is
contained. If monitoring wells are not well placed, further migration of
the plume could go undetected. This could result in the need for increased
pumping rates or additional containment wells once the continued migration
is discovered.
The performance of the treatment system should be continually
evaluated to ensure that it is operating effectively. If a technology is
performing poorly, it may be due to an improper design arising from an
incomplete site assessment. It may be necessary to re-examine the data
that were collected for the site assessment (see Figure 3) and collect more
data to enhance the site assessment. Design modifications may be warranted
if the understanding of site conditions changes appreciably.
Monitoring Well Networks
Regardless of the type of remediation program chosen, monitoring
contamination in the saturated zone will always require monitoring wells to
track performance or detect contaminant movement. The following are some
of the points to consider in setting up a monitoring well network at a
site.
91
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• At least one well should be placed up-gradient of the site to
detect background contamination levels. It is important to make
sure the well is out of the zone of contamination produced by the
release so that accurate background levels are obtained. It is
also important that comparisons are made between background and
contaminated samples taken from the same depth in the aquifer. If
more than one aquifer is being sampled, up-gradient background
levels should be taken for each aquifer.
• There should be enough down-gradient wells to adequately monitor
the horizontal and vertical extent of contamination. In general,
more wells are needed when complex stratigraphy or fluctuating
water tables are encountered.
• All pathways for potential migration of contaminants should be
monitored. For example, if multiple aquifers exist at a site,
every aquifer that might potentially be contaminated through
interaquifer exchange should be monitored.
• Wells should be screened at the appropriate depths. Wells should
be screened at or near the water table for lighter than water
contaminants and near the bottom of the aquifer for denser than
water contaminants.
• The length of well screens should be considered. Longer screens
are more likely to intercept the contaminant plume, but may result
in diluted samples which do not reflect actual contaminant
concentrations. Shorter screens give better concentration
estimates but must be accurately located to ensure the plume is
intercepted. Fluctuating water tables must also be considered and
longer screens used if necessary to ensure the water table does
not drop below the screen.
• Samples from pumping wells are generally diluted. They can
provide useful information about the overall level of clean up
achieved. However, monitoring well samples should also be taken
to determine actual contaminant concentrations at specific
locations in the subsurface.
• When determining water table contours, comparison of data should
be made only between wells screened at the same vertical location
in the same formation.
Measuring the Effectiveness of a Containment System
Delineating a pumping well's ZOC and taking water level measurements
at the site are helpful in predicting whether floating NAPL or dissolved
contaminants have been contained. However, sampling of wells is required
to detect contaminants that may be escaping the capture zone.
The distribution of monitoring wells and the vertical location of
screens on the wells is very important. Some of the wells must be outside
the known limit of contamination in order to verify that migration has been
prevented. The wells should also be screened based on the contaminant
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properties. To detect floating NAPL, well screens should be at the water
table. If the contaminant is very dense, well screens should be near the
bottom of the aquifer.
Criteria Used to Measure Removal Progress
Measuring the progress of floating NAPL removal is usually
accomplished simply by comparing the volume of NAPL removed to an estimate
of the total floating NAPL. Measuring removal progress for dissolved or
sorbed contaminant is more difficult. Design of a treatment technology to
remove dissolved or sorbed contamination is generally based on one or more
constituents of concern rather than the petroleum product mixture. The
constituent used in design is chosen based on its potential threat to the
surrounding environment. Performance is then evaluated by measuring the
concentrations of each contaminant of concern, and comparing those levels
to clean-up goals.
Benzene, toluene, ethylbenzene and xylene (BTEX) are typically used
for design criteria and performance monitoring of dissolved and sorbed
contaminants in the saturated zone. These chemicals are aromatics and make
up a significant portion of petroleum products (as much as 20% by weight in
gasoline). Based on present knowledge, the toxicity of the BTEX compounds
is orders of magnitude greater than the other natural petroleum
constituents. They are also typically more soluble and mobile than other
constituents. Of the aromatics, benzene is of greatest concern since it is
a known human carcinogen. Benzene is also one of the more soluble and
volatile aromatics. It is almost always detected in the subsurface when an
UST release occurs. When measuring vapors, it is common to monitor BTEX
and the alkanes, total hydrocarbons (which includes all the aromatics) or
total volatiles.
In addition to the BTEX compounds, there are many different additives
found in petroleum products, some of which can pose significant health
risks. It may be desirable to use an additive for design criteria or
performance monitoring, but only on a site by site basis after specific
additives have been identified. Often there will be little evidence to
suggest the additive does or does not present a health risk.
Relative contaminant concentrations change over time depending on
their chemical properties, so it is possible that different contaminants
will be used for design and for tracking performance at different stages of
the clean-up. With time, aromatics dissolve in water (solubilize),
evaporate into air (volatilize), and biodegrade until they cannot be
detected. The contaminants likely to remain are more complex, less
soluble, and less volatile than the aromatics. "More complex" constituents
are constituents that have greater molecular weights and more complex
molecular structures. These more complex molecules may break down into
simpler molecules eventually. Constituents that are more complex than the
simple aromatics (like BTEX) include certain additives like tetraethyl
lead.
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How to Measure Clean Up Progress
There are two basic measures of remediation progress in the saturated
zone: 1) observed concentration levels in the subsurface; and 2) the
percentage of contaminants removed to date from the subsurface. Clean up
criteria are often set based on restoring groundwater quality to a some
minimum level or removing some percentage of the total amount of
contaminants released. These types of clean-up criteria are well suited to
the above-mentioned progress measures.
Sampling and analyzing groundwater is the best way to determine
whether significant amounts of contamination exist in the saturated zone.
If clean-up criteria are based on a minimum contamination level,
determining actual contamination levels in the groundwater is the only way
to determine if the criteria have been met. A valuable tool to help
evaluate remediation progress is to periodically sample at all monitoring
wells. The data from these analyses can be used to create contour maps of
contamination levels at the site. Over time, comparison of the contour
maps can provide a time history of groundwater quality at the site.
Estimating the mass or volume of contaminants that have been removed
from the subsurface is also a useful way to measure clean up progress at a
site. This is especially true of recovery of NAPL, where concentration
levels provide no useful information. The accuracy of this type of
progress measure depends on the accuracy of the initial estimate of release
volume and the accuracy of removal measurements. If the initial amount of
released contaminants is unknown, the percentage of total clean up
completed can not be determined. However, estimates of the amount of
contaminants removed can still be useful as a means to monitor the
performance of the technology.
Estimating the volume of contaminants released to the subsurface can
be difficult. It is even more difficult to estimate how much contaminant
has reached the saturated zone relative to the amount that attenuates in
the unsaturated zone. Estimating the amount of contaminants removed during
treatment is less difficult but often requires frequent and regular
sampling. The common monitoring procedures used for measuring the mass of
dissolved contaminants removed from groundwater for each of the treatment
technologies is discussed briefly below.
• Air Stripping - In packed tower design, the tower is sized to
handle a pre-determined flow rate and the contaminant removal
efficiency can be fairly accurately determined by altering the
design criteria. Influent and effluent concentrations can be
taken periodically to ensure the desired removal rate is being
maintained. To estimate the amount of contaminants removed from
the subsurface, the average influent concentration can be
multiplied by the removal rate to give the total contaminant mass
removed.
• Carbon Adsorption - In this method, the effluent contaminant
concentrations are generally monitored to determine when the
carbon has used up its adsorption capacity and to ensure that
effluent concentrations are meeting clean-up criteria. After
94
-------
"breakthrough" (the point when contaminant concentrations are
detected in the effluent) the carbon is replaced.
Carbon adsorption removes virtually all targeted contamination
from the groundwater if properly designed and maintained. To
measure the mass of contaminant removed, the influent
concentration must be monitored and the flow rate through the GAC
unit known. Multiplying the two gives the mass removal rate of
the system; and knowing the length of time the system has operated
allows the total mass removed to date to be determined.
• Biorestoration - It is more difficult to determine how well this
method is progressing through direct measurements than any of the
other technologies. Indirect measures of biological activity are
often substituted for direct measurements by determining carbon
dioxide concentrations in the saturated zone (aerobic
biodegradation produces carbon dioxide as one of its end
products). A time-history of contaminant concentrations can also
be made to help monitor treatment. It is important to regularly
analyze the availability of oxygen, nutrients, pH, and microbial
populations to make sure that conditions are suitable for
biodegradation.
When is Remediation Complete?
Remediation of a site is complete when the clean-up goals and
criteria are met and maintained. Just meeting the clean-up criteria is not
sufficient reason to suspend remediation. Monitoring of the site should
continue after the criteria appear to have been met because contamination
levels can increase after treatment stops. The following are some of the
causes of increased contamination levels at a site.
• Sorbed contaminants or contaminants in low permeability zones can
persist in the subsurface, but may not be detected at monitoring
wells while the system is operating. However, after shutdown
these contaminants will tend to disperse causing future increases
in contamination levels.
• Groundwater flow patterns created by recovery wells can dilute
samples. After pumping stops, normal flow patterns return and
concentration levels may increase.
• Cessation of pumping at a recovery well causes a rise in the water
table. Contaminants sorbed to the soil may dissolve when contact
with water is resumed, recontaminating the groundwater.
• Natural flushing by rainfall can cause contaminants trapped in the
unsaturated zone to migrate to the saturated zone.
The decision to discontinue monitoring should be made jointly with
regulatory officials and experienced professionals to ensure that
remediation is complete.
95
-------
REFERENCES
CH2M Hill. Evaluation of Ground-Water Extraction Remedies. Draft Project
Summary Report 1989.
EPA. Estimation of the Public Health Risk from Exposure to Gasoline Vapor
via the Gasoline Marketing System (draft). Office of Health and
Environmental Assessment, Office of Air Quality Planning and Standards,
Washington, DC. 1984.
EPA. A Compendium of Superfund Field Operations Methods. Washington, D.C.
1987a.
EPA. Ground Water. EPA/625/6-87/016. Cincinnati, OH. 1987b.
EPA. Strategies for Development of an UST Program Targeting Methodology.
EPA Office of Underground Storage Tanks. Washington, D.C. 1987c.
EPA. Draft Internal Working Document Loci Research Report on Mobilization,
Immobilization, and Transformation of Motor Fuel Constituents and Organic
Chemicals in the Subsurface Environment. Prepared by PEI Associates Inc.
for HWERL, ORD, Edison, NJ. 1988a.
EPA. Cleanup of Releases from Petroleum USTs: Selected Technologies.
EPA/530/UST-88/001. USEPA. Office of Underground Storage Tanks.
Prepared by Camp Dresser & McKee Inc. April 1988b.
EPA.; Estimating Air Emissions From Petroleum UST Cleanups. U.S. EPA,
Office of Underground Storage Tanks, Washington, D.C. 1989a.
EPA. Assessing UST Corrective Action Technologies. U.S. EPA, Washington,
D.C. 1989b.
Farr, A.M., R.J. Houghtalen, and D.B. McWhorter. Volume Estimation of Light
Nonaqueous Phase Liquids In Porous Media. Ground Water. Vol.28, No.l 1990
Freeze, R.A. and J.A. Cherry. Groundwater. Prentice-Hall, Inc. Englewood
Cliffs, NJ. 1979.
Geraghty, J.J. and D.W. Miller. Fundamentals of Ground Water
Contamination, Short Course Notes. Geraghty and Miller Inc. 1985.
96
-------
Ghassemi, M., A. Phanahloo, S. Quinlivan. Physical and Chemical
Characteristics of Some Widely Used Petroleum Fuels: a Reference Data
Base for Assessing Similarities and Differences between Synfuel and
Petrofuel Products, Energy Sources, 1984, 7, pp. 377-401.
Hall, R., s.B. Blake, S.C. Champlin, Jr. Determination of Hydrocarbon
Thickness in Sediments Using Borehole Data. Fourth National Symposium on
Aquifer Restoration and Ground Water Monitoring, National Water Well
Association, Columbus, OH. 1984.
Heath, R.C. Basic Ground-Water Hydrology. Water Supply Paper 2220. United
States Geological Survey. 1983. pp. 71.
Hillel, Daniel. Fundamentals of Soil Physics. Harcourt Brace and
Jovanovich, Publishers. New York. 1980.
Hoag, G. E., C.J. Bruell, M.C. Marley. A study of the Mechanisms
Controlling Gasoline Hydrocarbon Partitioning and Transport in
Groundwater Systems, University of Connecticut, Prepared for the United
States Geological Survey, Reston, VA, 1984.
ICF, Inc. Petroleum Data Collection and Assessment of Risk Approach.
Draft Report, 1984.
Javandel and Tsang. Capture Zone Type Curves: A Tool for Aquifer Cleanup.
Ground Water, Vol.24, No.5. 1986.
Krishnayya, A.V., M.J. O'Connor, J.G. Agar and R.D. King. Vapour
Extraction Systems: Factors Affecting Their Design and Performance.
Proceedings of Petroleum Hydrocarbons and Organic Chemicals in Ground
Water: Prevention, Detection and Restoration. 1988.
Lyman, W.J., W.F. Reehl, and D.H. Rosenblatt. Handbook of Chemical Property
Estimation Methods. McGraw-Hill Book Co. New York. 1982.
Mackay, D.M. and J.A. Cherry. Groundwater Contamination: Pump-and-treat
remediation. Environmental Science and Technology. Vol.23, No.6. 1989.
Major, D.W., C.I. Mayfield, J.F. Barker. Biotransformation of Benzene by
Denitrification in Aquifer Sand. Groundwater. 26(1): 8-14. 1988.
97
-------
Nyer, E.K. Groundwater Treatment Technology. New York: Van Nostrand
Rheinhold. 1985.
i
Schwarzenbach, R.P- and J. Westall. Transport of Nonpolar Organic
Compounds from Surface Water to Groundwater. Laboratory Sorption
Studies. Environmental Science and Technology. V 35. 1981
Smith, J.H., J.C. Harper, H. Jaber. Analysis and Environmental Fate of Air
Force Distillate and High Density Fuels, Tyndall Air Force Base,
Engineering and Services Laboratory. Florida. 1981.
Sutton, P.A. and J.F. Barker. Migration and Attenuation of Selected
Organics in a Sandy Aquifer - A Natural Gradient Experiment. Ground
Water. Vol.23, No.l. pp. 10-16. 1985
Testa, S.M. and M.T. Paczkowski. Volume Determination and Recoverability of
Free Hydrocarbon. Ground Water Monitoring Review. Vol. 9, No. 1. 1989.
Thurman, E.M. Organic Geochemistry of Natural Waters. Kluwer Academic
Publishers. 1985.
Todd, O.K. Ground Water Hydrology. Wiley & Sons, New York. 1980.
98
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APPENDIX A
HOW TO USE THE WORKSHEETS
The examples provided herein demonstrate how to use the worksheets that are
presented throughout the manual. Included with each example is a brief
discussion of the completed worksheet and how it can be interpreted. A
copy of each blank worksheet is provided at the back of the manual. These
can be detached and copied for use in evaluating any number of release
sites.
Site Setting - Each of the examples assumes that the release occurred
in the same setting and under the same circumstances. Automotive gasoline
is assumed to have leaked over an eight month period from an UST in the
upper Midwest region of the United States. The site is in a rural area
with no major developments in the immediate vicinity. The UST is in a
silty sand formation about 200 feet thick and overlies sedimentary bedrock.
The typical depth to water is about 10 meters (33 feet).
Listed in Table A-l are all the CSFs required to complete the sample
worksheets along with their assumed values and sources (e.g., tables or
figures in the text, simple field tests, or other sources such as a local
USGS office). Note that the contaminant-related CSFs have values for both
the bulk liquid (gasoline) and a constituent of the bulk liquid (toluene).
This is because the bulk liquid properties are appropriate for some of the
worksheets while the constituent properties are appropriate for others.
Toluene is used because it is commonly found in a variety of petroleum
products and is relatively soluble (see discussion in Section 2). Which to
target, bulk liquid or individual constituents, is discussed for each
sample problem.
99
-------
Table A-l. Assumed Data for Sample Problems
CSF
VALUE
SOURCE
12,000 gal * estimated from Owner records
Slow Leak * estimated from Owner records
8 months * estimated from Owner records
RELEASE-RELATED
Amount Released
Rate of Release
Time Since Release
SITE-RELATED
Depth to Groundwater
Preferential Flow Pathways
Hydraulic Conductivity
Rainfall Infiltration Rate
Soil Temperature
Soil Surface Area
Interfering Structures
TOC Content of GW
Suspended Solids Content
Iron and Manganese Content
CONTAMINANT-RELATED
Liquid Viscosity (gasoline)
Vapor Pressure (gasoline)
Vapor Pressure (toluene)
Water Solubility (gasoline)
Water Solubility (toluene)
Molecular Weight (toluene) 92.1 g/mole many sources
10 meters
Unknown
10~4 cm/sec
0.1 cm/day
7°C
0.05 m2/g
Not Present
3 mg/L
25 mg/L
<0.2 mg/L
0.45 cP
469 mm Hg
21.8 mm Hg
158 rag/L
537 mg/L
* from USGS records
* site walkover
Table 3 (pg. 16)
* from USGS records
Figure 4 (pg. 19)
Table 4 (pg. 17)
* local public works dept.
* groundwater sample
* groundwater sample
* groundwater sample
Table 7 (pg. 26)
Table 7 (pg. 26)
Table 8 (pg. 27)
Table 7 (pg. 26)
Table 8 (pg. 27)
* CSFs that have sources marked with an asterisk have been given assumed
values which could have been obtained from the source shown.
100
-------
Example 1 - Table 10 (pg. 35)
"Likelihood of NAPL being Present on the Water Table"
Table 10 from the manual has been completed using the data in Table
A-l and is included on the following page. Note that the properties of the
bulk liquid rather than individual constituents should be used to determine
values for CSFs in Table 10 (see Section 2 of the manual). A summary of
the data in the completed worksheet is shown in Table A-2. The middle
column shows the number of times, or frequency, that CSFs fall into each of
the three likelihood categories. The right most column shows the frequency
that CSFs identified as more important than others in the worksheet fall
into each of the likelihood categories.
Table A-2
No. of Relatively
Likelihood Frequency Important CSFs
Higher
Medium
Lower
3
6
3
2
2
1
The distribution of CSFs in the completed worksheet do not clearly
indicate that NAPL would or would not be present on the water table. This
is true of the more important CSFs as well as the CSFs in general, and
further study of the CSFs is required to better interpret the results.
The "amount released" and "liquid viscosity" CSFs are in the high
range and more important relative to some other CSFs. This suggests that a
fairly large volume of relatively mobile petroleum product was released.
The "depth to groundwater", "hydraulic conductivity", and "soil surface
area" CSFs are in the medium range and suggest that bulk gasoline could
move somewhat easily down through the unsaturated zone to the water table.
This would be especially true if "preferential flow pathways" were
discovered at the site during a more thorough investigation. However, the
"rate of release" was slow, and NAPL will have had time to transform to
other phases.
The worksheet indicates that, presently, some NAPL likely remains on
the water table. Prior to this evaluation, a significant amount of the
release probably reached the water table as NAPL. But, over time, a
portion of the floating NAPL is likely to have dissolved, sorbed, or
volatilized.
101
-------
Table 10. Likelihood of NAPL Being Present on the Water Table
FACTOR
RELEASE- RELATED
*
• Amount Released
• Rate Of Release
• Time Since Release
SITE -RELAYED
*
• Preferential Flow Pathways
• Depth To Qroundwater
• Hydraulic Conductivity
(of the Unsaturated Zone)
• Rainfall Infiltration Rate
• Soil Temperature
• Soil Sorptton Capacity
(Surface Area)
CONTAMINANT- RELATED
*
• Liquid Viscosity
• Vapor Pressure
• Water Solubility
UNITS
gal
months
-
meters
cm/sec
cm/day
°C
cnf/g
cP
mm Hg
mg/L
SITE OF
INTEREST
(2)000
S/OUJ
8
(/ntaouio
lo
1C''
0. I
7
0.05
o.^S
*t>°\
IS*
INCREASING LIKELIHOOD ^
Small
(<1,000)
o
Slow Release
•
Long
(>12)
O
Medium
(1000-10,000)
O
o
Medium
(1-12)
Large
(>10,000)
•
Instantaneous
Release
O
Short
(<1)
O
Not Present
O
Deep
(>15)
O
Low
(<1Cf5)
0
High
(0.2)
O
Cool
(<10)
•
High
(>1)
O
Unknown
•
Medium
(2-15)
•
Medium,
(10's-10^
•
Medium
(0.05-0.2)
•
Medium
(10-20)
0
Medium
(0.1-1)
O
Present
O
Shallow
(<2)
O
*3>)
0
Low
(<0.05)
O
Warm
(>20)
O
Low
(<0.1)
•
High
(>20)
O
High
(>100)
•
High
(>1000)
O
Medium
(2-20)
O
Medium
(10-100)
O
Medium
(100-1000)
•
Low
<<2)
•
Low
(<10)
O
Low
(<100)
O
* CSFs denoted with an asterisk are typically more important than other CSFs
102
-------
Example 2 - Table 11 (pg. 36)
"Likelihood of Dissolved Contaminants Being Present
in the Saturated Zone"
Table 11 has been completed using the data in Table A-l and is
included on the following page. Bulk liquid properties are used in this
example and are more appropriate than individual constituents for two main
reasons: 1) the amount of dissolved contamination in the saturated zone
depends to a large degree on the amount of bulk liquid that reaches the
water table, which in turn is partially dependent on properties of the bulk
liquid, and 2) the total amount of contaminant that will transform from a
given amount of bulk liquid to the dissolved phase is related to the water
solubility of the bulk liquid, not the solubilities of the individual
constituents. A summary of the data in the completed worksheet is shown in
Table A-3.
Table A-3
No. of Relatively
Likelihood Frequency Important CSFs
Higher
Medium
Lower
4
6
2
2
3
0
Based on the worksheet results, it is likely that there is a
significant amount of dissolved contamination in the saturated zone:
• only two of the 12 CSFs (soil temperature and vapor pressure) suggest a
"lower likelihood" of dissolved contamination, and these are not among
the more important of the CSFs in Table 11;
• the water solubility of gasoline is in the moderate range, but is high
relative to other bulk petroleum products;
• the low liquid viscosity of the bulk liquid, the relatively large
amount of product released, and the moderate depth to groundwater at
the site suggest that a significant amount of NAPL could have reached
the water table, where dissolution of the product is facilitated;
• a fairly long time has elapsed since the release first occurred, which
would allow spreading of the NAPL plume and increased dissolution of
product;
• infiltrating rain water has likely acted to flush contaminants trapped
in the unsaturated zone down to the saturated zone in the dissolved
phase; and
• the moderate fluctuations in the water table will increase contact area
between NAPL and groundwater, furthering dissolution of NAPL.
103
-------
Table 11. Likelihood of Dissolved Contaminants
Being Present In the Saturated Zone
FACTOR
RELEASE- RELATED
• Amount Released
* • Rate Of Release
* - Time Since Release
SITE -RELATED
• Depth To Groundweter
• Fluctuating Water Table
• Hydraulic Conductivity
(of the Unsaturated Zone)
* • Rainfall Infiltration Rate
• Soil Temperature
• Soil Sorption Capacity
(Surface Area)
CONTAMINANT- RELATED
• Liquid Viscosity
• Vapor Pressure
*• Water Solubility
UNITS
gal
months
meters
cm/sec
cm/day
°C
m /g
cP
mm Hg
mg/L
SITE OF
INTEREST
INCREASING LIKELIHOOD ^
11)000
•S'/Ou)
8
Small
(100)
0
Instantaneous
Release
o
Short
(<1)
O
Medium
(100-1000)
O
--
Medium
(1-12)
•
Large
(>1000)
•
Slow Release
•
Long
(>12)
O
10
MocUyeJe
I*'*
0. /
7
o.os
Deep
(>15)
0
Steady
0
Low
(<18'5)
Low
(<0.05)
O
Cool
(<10)
•
High
(>1)
0
Medium
(2-15)
•
Moderately
Fluctuating
•
Medium
(10 -'-lO"3)
Medium
(0.05-0.2)
Medium
(10-20)
0
Medium
(0.1-1)
0
Shallow
(<2)
O
Highly
Fluctuating
O
High
(>10-a)
O
High
(>0.2)
O
Warm
(>20)
O
Low
(>0.1)
•
a 45
4*1
IS8
High
(>20)
0
High
(>100)
•
Low
(<100)
O
Medium
(2-20)
0
Medium
(10-100)
O
Medium
(100-1000)
•
Low
(<2)
•
Low
(<10)
O
High
(>1000)
O
* CSFs denoted with an asterisk are typically more important than other CSFs
104
-------
The overall conditions at the site are favorable for the presence of
dissolved contamination, and a fairly large dissolved plume can be
expected. As discussed in Section 2 of the manual, the dissolved plume
will likely be comprised mostly of aromatic hydrocarbons, which make up a
significant portion of the water soluble fraction of most petroleum
products. Of the aromatic hydrocarbons, the BTEX compounds (benzene,
toluene, ethylbenzene and the xylenes) are often a significant contributor
to the water soluble fraction. One or more of the BTEX compounds are often
targeted in groundwater monitoring programs and design of treatment
systems.
105
-------
Example 3 - Table 12 (pg. 58)
"Worksheet for Evaluating the Feasibility of Using
Trench Excavation to Contain Floating NAPL"
Table 12 of the main text has been completed using the data in Table
A-l and is included on the following page. The properties of the bulk
gasoline rather than individual constituents of gasoline are appropriate
for evaluating containment of NAPL and are used in the completed worksheet.
A summary of the results is shown in Table A-4.
Table A-4
No. of Relatively
Feasibility Frequency Important CSFs
More Feasible 5 2
Somewhat Feasible 1 0
Less Feasible 1 1
At first glance, the distribution of CSFs in Table A-4 might suggest
that trench excavation looked promising at this site. Five of the seven
CSFs are in the "more feasible" category, including two identified in the
worksheet as more important relative to some other CSFs. One CSF is in the
"somewhat feasible" and one in the "less feasible" category. However, the
depth to groundwater of 10 meters, the only "less feasible" parameter, is
very important in evaluating this technology from a cost perspective.
Digging and maintaining trenches to depths greater than about 5 meters
below ground surface can be very costly. This factor could outweigh all
other CSFs in the worksheet, even when they appear to be quite favorable.
A first pass evaluation of the completed worksheet indicates that
another alternative (e.g. pumping wells) may be better suited to this site.
But this does not rule out trench excavation altogether. A first pass
evaluation of pumping wells at the site may show that technology to be no
more feasible than trench excavation. If so, further information would be
required before screening of containment technologies could be
accomplished.
106
-------
Table 12. Worksheet for Evaluating the Feasibility of Using
Trench Excavation to Contain Floating NAPL
CRITICAL SUCCESS
FACTOR
RELEASE ^RELATED
• Amount Released
- Time Since Release
SITE- RELATED
*
• Depth to Groundwater
• Stability of Soil Formation
*
• Presence of Interfering
Structures
UNITS
gallons
months
meters
SITE OF
INTEREST
IZjOOO
JO
£t«Ue
AJ
/vOf\fc.
SUCCESS
LESS
LIKELY
Large
(>500,000)
0
Long
(>12)
O
Deep
(>5)
•
Unstable
O
Present
0
SUCCESS
SOMEWHAT
LIKELY
Medium
(50,000-500,000)
O
Medium
(1-12)
•
Medium
(1-5)
O
SUCCESS
MORE
LIKELY
Small
(<50,000)
•
Short
O
Shallow
(<1)
O
Stable
Not Present
•
CONTAMINANT* RELATED
• Liquid Viscosity
• Liquid Density
cP
g/cm3
0.45"
O.ll
High
(>2)
O
High
O
Medium
(1-2)
O
Low
(<1)
e
Low
'•
OTHER CONSIDERATIONS
• Trench excavation can generally be Implemented quickly.
• Costs for this method are typically lower than for other methods
• Denser than water contaminants cannot typically be contained with trench excavation
* CSFs denoted with an asterisk are typically more important than other CSFs
107
-------
Example 4 - Table 16 (pg. 81)
"Worksheet for Evaluating the Feasibility of Using
Carbon Adsorption to Treat Extracted Groundwater"
Table 16 has been completed using the data in Table A-l and is
included on the following page. The contaminants in the dissolved phase
are of primary importance in evaluating carbon adsorption. Therefore,
values for CSFs used in the completed worksheet are based on the properties
of a constituent of gasoline (toluene) rather than properties of bulk
gasoline (see Section 2). Other of the more soluble compounds in gasoline
(e.g. benzene) could also be evaluated in the same manner as toluene. A
summary of the results is shown in Table A-5.
Table A-5
No. of Relatively
Feasibility Frequency Important CSFs
More Feasible 1 0
Somewhat Feasible 4 2
Less Feasible 2 1
The results of the worksheet suggest that carbon adsorption is not
likely to be feasible as the primary treatment process at this site (at
least for toluene):
• the amount of product released and the time elapsed since the release
first occurred indicate that a moderately large volume of groundwater
would require treatment. Treatment by carbon adsorption is generally
more costly than other technologies and not suited to large volume
clean ups;
• the water solubility and molecular weight CSFs indicate that carbon
adsorption can remove toluene from the groundwater, but not as
efficiently as heavier, less soluble compounds. This would result in
more carbon use and/or a slower treatment rate;
• the total organic carbon (TOC) content of the groundwater is within a
range treatable by carbon adsorption.
• the iron and manganese in the groundwater is favorable to carbon
adsorption but the suspended solids content is high. However,
filtering the groundwater prior to treatment should solve this
problem.
A first pass evaluation of carbon adsorption at this site indicates
that other technologies might be better suited to removing toluene fre .
groundwater at the site. However, other factors not included on the
108
-------
Table 16. Worksheet for Evaluating the Feasibility of Using Carbon
Adsorption to Treat Extracted Groundwater at Your Site
CRITICAL SUCCESS
FACTOR
UNITS
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
SUCCESS
SOMEWHAT
LIKELY
SUCCESS
MORE
LIKELY
RELEASE- RELATED
• Amount Released
• Time Since Release
SITE -RELATED
• TOC Content of GW
• Suspended Solids
Content of GW
- Total Dissolved Iron and
Manganese Content of GW
gallons
months
mg/L
mg/L
mg/L
12)000
8
3
2S
4Q.Z.
Large
(>50,000)
0
Long
(>12)
o
High
(>5)
0
High
(>20)
•
High
(>5)
O
Medium
(1,000-50,000)
•
Medium
(1-12)
O
Medium
(1-5)
•
Medium
(5-20)
O
Medium
(0.2-5)
O
Small
(<1 ,000)
O
Short
(1000)
O
Low
(<100)
•
Medium
(100-1000)
•
Medium
(100-200)
0
Low
(<100)
0
High
(>200)
O
OTHER CONSIDERATIONS
• Can remove contaminants to drinking water standards.
• Costs increase significantly if large volumes or highly contaminated groundwater is encountered.
* CSFs denoted with an asterisk are typically more important than other CSFs
109
-------
worksheet, such as the level of treatment required, may make this
alternative more attractive. For example, if the groundwater must be
treated to drinking water standards, there are no proven alternatives to
carbon adsorption. In such a case, the best course of action might be to
pre-treat the groundwater using another technology and use carbon
adsorption as a finishing process.
110
-------
GLOSSARY OF TERMS
Absorption - the penetration of atoms, ions or molecules into the bulk mass
of a substance.
Adsorption - the retention of atoms, ions or molecules onto the surface of
another substance.
Aerobic - in the presence of oxygen.
Anaerobic - in the absence of oxygen.
Aquifer - a geologic formation capable of transmitting significant
quantities of groundwater under normal hydraulic gradients.
Aquitard - a geologic formation that may contain groundwater but not
capable of transmitting significant quantities of groundwater
under normal hydraulic gradients.
Aromatic - of or relating to organic compounds that resemble benzene in
chemical behavior.
Attenuation - to reduce or lessen in amount (e.g., a reduction in the
amount of contaminants in a plume as it migrates from the source).
Biodegradation - a process by which microbial organisms transform or alter
through enzymatic action the structure of chemicals introduced
into the environment.
Biomass - The amount of living matter in a given area or volume.
Bulk Density - the amount of mass of a soil per unit volume of soil; where
mass is measured after all water has been extracted and total
volume includes the volume of the soil itself and the volume of
air space between the soil grains.
Capillary Suction - process where water rises above the water table into
the void spaces of a soil due to tension forces between the water
and soil particles.
Capillary Fringe - The zone of a porous medium above the water table within
which the porous medium is saturated but is at less than
atmospheric pressure. The capillary fringe is considered to be
part of the vadose zone, but not of the unsaturated zone.
Confined Aquifer - an aquifer under greater than atmospheric pressure,
bounded above and below by relatively impermeable formations.
Confining Layer - a geologic formation exhibiting low permeability that
inhibits the flow of water.
Ill
-------
Consolidated Soil - when a soil is subjected to an increase in pressure due
to loading at the ground surface, a re-adjustment in the soil
structure occurs. The volume of space between the soil particles
decreases and the soil tends to settle or consolidate over time.
Constituent - an essential part or component of a system or group:
examples are an ingredient of a chemical system, or a component of
an alloy.
Degradation Potential - the degree to which a substance is likely to be
reduced to a simpler form by bacterial activity.
Dissolution - Dissolving of a material in a liquid solvent (e.g., water).
Downgradient - in the direction of decreasing static head.
Drawdown - lowering of the water table due to withdrawal of groundwater
from a well.
Fluid Conductivity - the constant of proportionality in Darcy's Law
relating the rate of flow of a fluid through a cross-section of
porous medium in response to a hydraulic gradient. Fluid
conductivity is a function of the intrinsic permeability of a
porous medium and the kinematic viscosity of the fluid which flows
through it. Fluid conductivity has units of length per time
(cm/sec).
Free Product - a contaminant in the unweathered phase, where no dissolution
or biodegradation has occurred.
Field Capacity - the percentage of water remaining in the soil 2 or 3 days
after gravity drainage has ceased from saturated conditions.
Henry's Law - the relationship between the partial pressure of a compound
and the equilibrium concentration in the liquid through a constant
of proportionality known as Henry's Law Constant. See partial
pressure.
Heterogeneous - varying in structure or composition and having different
properties in different locations or directions.
Homogeneous - similar in structure or composition throughout and having
similar properties in all locations and directions.
Hydraulic Conductivity - the constant of proportionality in Darcy's Law
relating the rate of flow of water through a cross-section of
porous medium in response to a hydraulic gradient. Also known as
the coefficient of permeability, hydraulic conductivity is a
function of the intrinsic permeability of a porous medium and the
kinematic viscosity of the water which flows through it.
Hydraulic conductivity has units of length per time (cm/sec).
112
-------
Hydraulic Gradient - the change in piezoraetric head between two points
divided by the horizontal distance between the two points, having
dimensions of length per length (cm/cm). See piezometric head.
Hydrocarbon - one of a very large group of chemical compounds composed only
of carbon and hydrogen; the largest source of hydrocarbons is from
petroleum crude oil.
Infiltration - the downward movement of water through a soil from rainfall
or from the application of artificial recharge in response to
gravity and capillarity.
Interfacial tension - phenomena occurring at the interface of a liquid and
gas where the liquid behaves as it if were covered by an elastic
membrane in a constant state of tension. The tension is due to
unbalanced attractive forces between the liquid molecules at the
liquid surface.
Liquid Density - the amount of mass of a liquid per unit volume of the
liquid.
Mobility - the ability of a substance to move into or out of a phase due to
physical or chemical processes.
Moisture Content - the amount of water lost from the soil upon drying to a
constant weight, expressed as the weight per unit weight of dry
soil or as the volume of water per unit bulk volume of the soil.
For a fully saturated medium, moisture content equals the
porosity; in the vadose zone, moisture content ranges between zero
and the porosity value for the medium. See porosity, vadose zone,
saturated zone.
Molecular Diffusion - process where molecules of various gases tend to
intermingle and eventually become evenly dispersed.
Molecular Weight - the amount of mass in a mole of molecules of a substance
determined by summing the masses of the individual atoms
comprising the molecule. One mole is equivalent to 6.02 x 102
molecules.
Non-Aqueous Phase Liquid (NAPL) - contaminants that remain as the original
bulk liquid in the subsurface.
Octanol-Water Partition Coefficient (Kow) - a coefficient representing the
ratio of solubility of a compound in octanol to its solubility in
water. As KOW increases, water solubility decreases.
Partial Pressure - the portion of total vapor pressure in a system due to
one or more constituents in the vapor mixture.
Particle Density - the amount of mass of a substance per unit volume of the
substance.
113
-------
Permeability - a measure of a soils resistance to fluid flow.
Permeability, along with fluid viscosity and density are used to
determine fluid conductivity.
Phase - the physical form in which a substance is found. As discussed in
this manual, the three major phases are liquid, vapor and
dissolved in pore water.
Piezometric Head - the level to which water from a given aquifer will rise
by hydrostatic pressure. For the uppermost unconfined aquifer,
the piezometric head is identical to the water table elevation.
For confined aquifers, the piezometric head can be above or below
the water table.
Porosity - the volume fraction of a rock or unconsolidated sediment not
occupied by solid material but usually occupied by water and/or
air. Porosity is a dimensionless quantity.
Pressure Gradient - a pressure differential in a given medium, such as
water or air, which tends to induce movement from areas of higher
pressure to areas of lower pressure.
Refractory Index - a measure of the ability of a substance to be
biodegraded by bacterial activity.
Residual Saturation - the amount of water or oil remaining in the voids of
a porous medium and held in an immobile state by capillary and
dead-end pores.
Retardation - preferential retention of contaminant movement in the
subsurface resulting from adsorption processes or solubility
differences.
Rip Rap - protective covering, often stone or coarse gravel, for earthen
slopes to prevent erosion.
Saturated Zone - as defined in this manual, the zone of the soil in which
all space between the soil particles is occupied by water,
including the capillary zone.
Soil Sorption Coefficient - a measure of the preference of an organic
chemical to leave the dissolved aqueous phase in the soil and
become attached or adsorbed to soil particles as organic carbon.
Solubility - the amount of mass of a compound that will dissolve into a
unit volume of solution.
Sorption - a general term used to encompass the process of absorption,
adsorption, ion exchange, and chemisorption.
Stratigraphy - the study of original succession and age of subsurface
layers; dealing with their form, distribution, composition, and
physical and chemical properties.
114
-------
Surfactant - natural or synthetic chemicals that have the ability to
promote the wetting, solubilization, or emulsification of various
organic chemicals.
Unconfined Aquifer - an aquifer that is under atmospheric pressure. It is
usually the uppermost aquifer in the subsurface with its upper
limit being the water table.
Unsaturated Zone - the portion of a porous medium, usually above the water
table in an unconfined aquifer, within which the moisture content
is less than saturation and the capillary pressure is less than
atmospheric pressure. The unsaturated zone does not include the
capillary fringe.
Upgradient - in the direction of increasing static head.
Vadose Zone - the portion of a porous medium above the water table within
which the capillary pressure is less than atmospheric and the
moisture content is usually less than saturation. The vadose zone
includes the capillary fringe.
Vapor Density - the amount of mass of a vapor per unit volume of the vapor.
Vapor Pressure - the equilibrium pressure exerted on the atmosphere by a
liquid or solid at a given temperature. Also a measure of a
substance's propensity to evaporate or give off flammable vapors.
The higher the vapor pressure, the more volatile the substance.
Viscosity - a measure of the internal friction of a fluid that resists
shear within the fluid. The greater the internal friction forces
of a fluid (i.e., the greater the viscosity), the less easily the
fluid will flow.
Volatilization - the process of transfer of a chemical from the water or
liquid phase to the air phase. Solubility, molecular weight, and
vapor pressure of the liquid and the nature of the air-liquid/
water interface affect the rate of volatilization. See
solubility, vapor pressure.
Water Content - see moisture content
Water Table - the water surface in an unconfined aquifer at which the fluid
pressure in the voids is at atmospheric pressure.
Weathering - the process where a complex compound is reduced to its simpler
component parts, transported through physical processes, or
biodegraded over time.
115
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BLANK WORKSHEETS
116
-------
Table 10. Likelihood
FACTOR
RELEASE RElATEb
*
• Amount Released
*
- Rate Of Release
• Time Since Release
SITE- RELATED ••,,%.;/^-:..::
*
• Preferential Flow Pathways
• Depth To Groundwater
• Hydraulic Conductivity
(of the Unsaturated Zone)
• Rainfall Infiltration Rate
• Soil Temperature
• Soil Sorption Capacity
(Surface Area)
CONTAMINANT- RELATED
• Liquid Viscosity
• Vapor Pressure
• Water Solubility
UNITS
gal
months
-
meters
cm/sec
cm/day
°C
crrf/g
cP
mm Hg
mg/L
of NAPL Being Present on the Water Table
SITE OF
INTEREST
INCREASING LIKELIHOOD ^
Small
(< 1,000)
0
Slow Release
0
Long
(>12)
0
Not Present
o
Deep
(>15)
0
Low
(<10'5)
0
High
(02)
O
Cool
(<10)
0
High
(>1)
O
High
(>20)
O
High
(>100)
O
High
(>1000)
O
Medium
(1000-10,000)
O
O
Medium
(1-12)
O
Unknown
O
Medium
(2-15)
O
Medium
(IQ-'-IO^
O
Medium
(0.05-0.2)
0
Medium
(10-20)
0
Medium
(0.1-1)
O
Medium
(2-20)
0
Medium
(10-100)
0
Medium
(100-1000)
O
Large
(>1 0,000)
O
Instantaneous
Release
O
Short
(<1>
0
Present
O
Shallow
(<2)
O
High
(>W* )
O
Low
(<0.05)
0
Warm
(>20)
O
Low
(<0.1)
O
Low
(<2)
O
Low
(<10)
O
Low
(<100)
O
* CSFs denoted with an asterisk are typically more important than other CSFs
117
-------
Table 11. Likelihood of Dissolved Contaminants
Being Present In the Saturated Zone
FACTOR
RELEASE- RELATED
• Amount Releaced
* • Rate Of RelMM
* • Time Since Release
SITE 'RELATED
• Depth To Groundwater
. Fluctuating Water Table
• Hydraulic Conductivity
(of the Uneaturated Zone)
* • Rainfall Infiltration Rate
• Soil Temperature
• Soil Serptlon Capacity
(Surface Area)
CONTAMINANT- RELATED
> Liquid Viscosity
> Vapor Preaaure
*• Water Solubility
UNITS
gal
months
meters
cm/sec
cm/day
°C
ma/g
OP
mm Hg
mg/L
SITE OF
INTEREST
INCREASING LIKELIHOOD ^
Small
(100)
0
Instantaneous
Release
o
Short
(<1)
O
Medium
(100-1000)
O
--
Medium
(1-12)
O
Large
(>1000)
O
Slow Release
O
Long
(>12)
O
Deep
(>15)
O
Steady
O
Low
«£«,
Low
(D
O
High
(>80)
O
High
(>100)
O
Low
(<100)
O
Medium
(2-15)
O
Moderately
Fluctuating
O
Medium
(10's-10-3)
Medium
(0.05-0.2)
O
Medium
(10-20)
O
Medium
(0.1-1)
0
Medium
(2-20)
O
Medium
(10-100)
O
Medium
(100-1000)
O
Shallow
(<2)
O
Highly
Fluctuating
0
High
(>10-3)
O
High
(>0.2)
O
Warm
(>20)
0
Low
(>0.1)
0
Low
(<2)
O
Low
(<10)
O
High
(>1000)
0
* CSFs denoted with an asterisk are typically more important than other CSFs
118
-------
Table 1 2. Worksheet for Evaluating the Feasibility of Using
Trench Excavation to Contain Floating NAPL
CRITICAL SUCCESS
FACTOR
RELEASE - RELATED
• Amount Released
• Time Since Release
SITE -RELATED
*
• Depth to Groundwater
• Stability of Soil Formation
*
• Presence of Interfering
Structures
UNITS
gallons
months
meters
CONTAMINANT- RELATED
• Liquid Viscosity
*
• Liquid Density
cP
g/cm3
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
Large
(>500,000)
o
Long
(>12)
0
Deep
(>5)
0
Unstable
O
Present
O
SUCCESS
SOMEWHAT
LIKELY
Medium
(50,000-500,000)
O
Medium
(1-12)
O
Medium
(1-5)
O
-
High
(>2)
0
High
<>D
O
Medium
(1-2)
O
-
SUCCESS
MORE
LIKELY
Small
(<50,000)
0
Short
<
-------
Worksheet for Evaluating the Feasibility of
TABLE 1 3. Using Pumping Wells to Contain NAPL and/or
Dissolved Contaminant in the Saturated Zone
CRITICAL SUCCESS
FACTOR
RELEASE -RELATED
- Amount Released
• Time Since Release
SITE -RELATED
*
• Site Stratigraphy
• Depth to Qroundwater
UNITS
gallons
months
-
meters
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
-
Small
(<50,000)
o
Short
(500,000)
O
Long
(>12)
0
Simple
O
Deep
(>5)
O
CONTAMINANT- RELATED
• Liquid Density
• Liquid Viscosity
g/cm
cP
Low
(2)
O
-
Medium
(1-2)
0
High
(>1)
O
Low
(
-------
Table 14. Worksheet for Evaluating the Feasibility of Using
Vacuum Extraction to Remove Floating NAPL at Your Site
CRITICAL SUCCESS
FACTOR
SITE -RELATED
• Soil Air Conductivity
• Soil Temperature
• Moisture Content
• Soil Surface Area
• Carbon Content
UNITS
cm/sec
°C
% volume
2
m /g
% weight
SITE OF
INTEREST
^^
SUCCESS
LESS
LIKELY
Low
(<10 * )
O
Low
(<10)
O
High
(>30)
O
High
(>D
O
High
(>10)
O
SUCCESS
SOMEWHAT
UKELY
Medium
(lO^-IO"6)
0
Medium
(10-20)
O
Medium
(10-30)
O
Medium
(0.1-1)
O
Medium
(1-10)
O
SUCCESS
MORE
UKELY
High
MO"4)
0
High
(>20)
O
Low
(<10)
O
Low
(<0.1)
0
Low
(1000)
O
Medium
(10-100)
O
Medium
(100-1000)
O
High
(>100)
O
Low
(<100)
O
OTHER CONSIDERATIONS
• Treatment of contaminant vapors may be required before discharge to atmosphere.
* CSFs denoted with an asterisk are typically more important than other CSFs
121
-------
Table 15. Worksheet for Evaluating the Feasibility of Using Air
Stripping to Treat Extracted Groundwater at Your Site
CRITICAL SUCCESS
FACTOR
RELEASE -RELATED
• Amount Released
*
• Tim* Sine* Release
SITE -RELATED
• Groundwater Temperature
• Total Suspended
Solid* Content of GW
• Total Dissolved Iron and
Manganese Content of GW
UNITS
gallons
months
°C
mg/L
mg/L
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
High
(<1,000)
o
Short
(<1)
O
Low
(<10)
0
High
(>20)
O
High
(>5)
O
SUCCESS
SOMEWHAT
UKELY
Medium
(1,000-50,000)
O
Medium
(1-12)
O
Medium
(10-20)
0
Medium
(5-20)
O
Medium
(0.2-5)
O
SUCCESS
MORE
UKELY
Large
(>50,000)
O
Long
(>12)
O
High
(>20)
O
Low
(<5)
0
Low
(<0.2)
O
CONTAMINANT- RELATED
it
• Vapor Pressure
*
• Water Solubility
*
• Dissolved Contaminant
Concentration
mm Hg
mg/L
mg/L
Low
(<10)
O
High
(>1000)
O
Low
(<1)
O
Medium
(10-100)
0
Medium
(100-1000)
0
Medium
(1-100)
0
High
(>100)
O
Low
(<100)
O
High
(>100)
O
OTHER CONSIDERATIONS
• Air stripping is not effective in removing contaminants to drinking water standards.
• Permitting requirements can delay implementation.
• Can cost effectively treat large volumes of groundwater.
• Treatment of contaminant vapors may be required before discharge to atmosphere.
* CSFs denoted with an asterisk are typically more important than other CSFs
122
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Table 16. Worksheet for Evaluating the Feasibility of Using Carbon
Adsorption to Treat Extracted Groundwater at Your Site
CRITICAL SUCCESS
FACTOR
UNITS
SITE OF
INTEREST
RELEASE- RELATED
• Amount Released
• Time Since Release
SITE* RELATED
*-TOC Content of GW
• Suspended Solids
Content of GW
• Total Dissolved Iron and
Manganese Content of GW
CONTAMINANT- RELATED
• Water Solubility
*• Molecular Weight
gallons
months
mg/L
mg/L
mg/L
mg/L
g/mole
SUCCESS
LESS
LIKELY
SUCCESS
SOMEWHAT
LIKELY
SUCCESS
MORE
LIKELY
Large
(>50,000)
O
Long
(>12)
O
High
(>5)
0
High
(>20)
O
High
(>5)
O
Medium
(1,000-50,000)
O
Medium
(1-12)
0
Medium
(1-5)
O
Medium
(5-20)
0
Medium
(0.2-5)
O
High
(>1000)
0
Low
(<100)
O
Medium
(100-1000)
0
Medium
(100-200)
0
Small
(<1,000)
O
Short
(<1)
O
Low
(<1)
O
Low
(<5)
O
Low
(<0.2)
O
Low
(<100)
O
High
(>200)
O
OTHER CONSIDERATIONS
• Can remove contaminants to drinking water standards.
• Costs increase significantly if large volumes or highly contaminated groundwater is encountered.
* CSFs denoted with an asterisk are typically more important than other CSFs
123
-------
Table 17. Worksheet for Evaluating The Feasibility of Using
Biorestoration to Treat Groundwater at Your Site
CRITICAL SUCCESS
FACTOR
RELEASE- RELATED
• Tim* sine* release
SITE- RELATED
*
• Hydraulic Conductivity
• Site Stratigraphy
• Groundwater temperature
• pH
UNITS
months
cm/sec
°C
pH units
CONTAMINANT- RELATED
• Btodegradabillty
Refractory Index
- Total Organic Carbon
Content of GW
-
mg/L
SITE OF
INTEREST
SUCCESS
LESS
LIKELY
Short
(<1)
o
Low
(8)
O
Low
(<0.01)
O
(<10or>1000)
O
SUCCESS
SOMEWHAT
UKELY
Medium
(1-12)
O
Medium
(lO^-KT9)
O
.
Medium
(5-10)
O
Medium
(0.01-0.1)
O
-
SUCCESS
MORE
UKELY
Long
(>12)
O
High
(>10-*)
O
Simple
O
High
(>10)
O
(6-8)
0
High
(>0.1)
O
(10-1000)
O
OTHER CONSIDERATIONS
• It Is difficult to monitor the effectiveness of the system
• Minimizes health risks by keeping contaminants below the ground surface.
* CSFs denoted with an asterisk are typically more important than other CSFs
124
* U.S. GOVERNMENT PRINTING OFFICE: 1 9 9 o - 7 <« 8 -1 5 . 0 it
------- |