510R04002
4>EPA How To Evaluate
Alternative Cleanup
Technologies For
Underground Storage
Tank Sites
A Guide For Corrective Action
Plan Reviewers
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Solid Waste And
United States Emergency Response L-P^ 510-f< 04-002
Environmental Protection 5401G IVay 2.004
Agency *(W» eos y :v,'r,ust/pubs
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United States Solid Waste and EPA510-R-04-002
Environmental Protection Emergency Response May 2004
Agency 5401G www.epa.govloustlpubsltums.htm
-&EPA How to Evaluate
Alternative Cleanup
Technologies for
Underground Storage
Tank Sites
A Guide for Corrective Action
Plan Reviewers
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Table Of Contents
I. Introduction
II. Soil Vapor Extraction
III. Bioventing
IV. Biopiles
V. Landfarming
VI. Low-Temperature Thermal Desorption
VII. Air Sparging
VIII. Biosparging
IX. Monitored Natural Attenuation
X. In-Situ Groundwater Bioremediation
XI. Dual-Phase Extraction
XII. Enhance Aerobic Bioremediation
XIII. Chemical Oxidation
Appendix Abbreviations & Definitions
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Chapter I
Introduction
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Chapter I
Introduction
Background
As of September 30, 2003, more than 439,000 releases from leaking
underground storage tanks (LUSTs) have been reported nationwide.
Cleanups have been initiated at more than 403,000 of these sites, and
more than 303,000 sites have been cleaned up. The backlog of sites still to
be cleaned up is more than 136,000. In many cases, the workload for state
regulators (who must oversee 50 to 400 cleanups at a time) is burdensome.
To compound the problem, cleanups are expensive. The costs of
remediating sites with soil contamination vary between $10,000 and
$125,000. Costs for remediating sites with groundwater contamination
can range from $100,000 to over $1 million depending on the extent of
contamination.
A primary factor in the high cost cleanups is the use of cleanup methods
that are either inappropriately selected or not optimally designed and
operated given the specific conditions of the site. Pump-and-treat, the
most commonly used method for remediating groundwater, is often
unsuccessful because either the source of contamination is not adequately
addressed, or the systems are not optimized. Even when properly
operated, pump-and-treat systems have inherent limitations1: they may
not work well in complex geologic settings or heterogeneous aquifers; they
often stop reducing contamination long before reaching intended cleanup
levels; and in some situations they can make sites more difficult to
remediate by smearing contamination across the subsurface. Landfilling,
the most frequently used method for addressing contaminated soils, does
not remediate soils; this method simply movess the problem from one
location to another. In addition to being costly, transporting
contaminated soil off-site increases the risk of harming human health and
the environment.
With so many sites requiring remediation at such an enormous cost, the
Environmental Protection Agency (EPA) actively promotes faster, more
effective, and less costly alternatives to traditional cleanup methods. EPA's
Office of Underground Storage Tanks (OUST) continues to work with
'In spite of its limitations, there may be some situations where pump-and-treat is the most
appropriate remediation alternative available (e.g., to remediate a small, dissolved phase plume or
to contain the plume in order to prevent migration into uncontarninated areas).
May 2004 M
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state and local governments to encourage the use of the most appropriate
cleanup technology for each and every site. When this manual was first
published in 1994, it covered the first eight technologies listed in the table
of contents (Chapters II through IX). The manual was updated in 1995 to
add two additional technologies (Chapters X and XI). Back then, these
<--n technologies were referred to as "alternative technologies" because
although they had the ability to make cleanups faster, more effective, and
less costly than traditional options, they were not widely used (although
they certainly are today). The current update (May 2004) adds two new
technologies (chemical oxidation-Chapter XI, and enhanced aerobic
bioremediation-Chapter XII) to the suite of "alternative technologies".
Purpose Of This Manual
The purpose of this manual is to provide you—state and local
regulators—with guidance that will help you review corrective action plans
(CAPs) that propose alternative cleanup technologies. The manual does
not advocate the use of one technology over another; rather it focuses on
appropriate technology use, taking into consideration sitespecific
conditions and the nature and extent of contamination. While the manual
focuses on the remediation of leaking underground storage tank sites, some
of its basic concepts can be applied at hazardous substance and hazardous
waste sites as well. .
The manual is designed to enable you to answer two basic questions
when reviewing a CAP:
• Has an appropriate cleanup technology been proposed?
• Does the CAP provide a technically sound approach to the
cleanup?
Scope And Limitations
This manual is intended to provide technical guidance to state
regulators who oversee cleanups and evaluate CAPs. The document does
not represent the issuance of formal policy or in any way affect the
interpretation of the regulations.
The text focuses on engineering-related considerations for evaluating
each technology. It does not provide instruction on the design and
construction of remedial systems and should not be used for designing
May 2004 1-2
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GAPs. Nor should it be used to provide guidance on regulator)' issues such
as securing permits and establishing cleanup standards, health and safety
issues, state-specific requirements, or cleanup costs.
This document is not intended to be used as the sole reference for CAP
review. Rather, it is intended to be used along with published references,
guidance from others more experienced with alternative technologies,
information from training courses, and current journals. The material
presented is based on available technical data and information and the
knowledge and experience of the authors and the peer reviewers.
How To Use This Manual
We encourage you to use this manual at your desk as you review CAPs.
We have designed the manual so that you can tailor it to meet your state's
or your own needs. The three-ring binder allows you to insert additional
material and remove certain tools (e.g., flow charts, checklists) for
photocopying. Also, you can add your own notes in the margins provided.
The manual contains discussions of eight different alternative cleanup
technologies. Tabs signal the beginning of each chapter (including the
Introduction and Abbreviations And Definitions) so you can flip quickly to
the appropriate section. We have included a table of contents in each
chapter to help you locate the information you need.
Each technology chapter contains the following tools which can help
expedite and/or improve the review process:
• An evaluation process flow chart, generally the third exhibit in each
chapter, can help you understand the overall review process for
each technology. This flow chart serves as a "road map" for the
chapter and for the decisions you will make during the evaluation
process.
• A checklist(s), located at the end of each chapter, can help you
determine whether or not the CAP contains all of the necessary
information. The checklist lists the most important factors to
evaluate for the successful implementation of each technology.
• A list of current references, located near the end of each chapter,
provides sources of additional information.
• Advantages and disadvantages of each technology, initial screening
criteria, and other data specific to each technology.
May 2004 1-3
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How to Obtain Additional Copies of the Manual
A limited number of single copies are available directly from OUST.
Contact OUST by telephone at 703-603-9900 and ask for "publications
outreach". The entire document is also available in electronic format
(PDF) from the "Publications" section of OUST's web site at
http://www. cpa.gov/oust/pubs/tums. htm.
May 2004 1-4
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Chapter IX
Monitored Natural Attenuation
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Contents
Overview IX-1
Natural Attenuation Processes IX-2
Corrective Action Plan (CAP) IX-3
Initial Screening Of Monitored Natural Attenuation Effectiveness IX-6
Contaminant Transport and Fate IX-6
Contaminated Soil IX-8
Source Mass IX-10
Source Longevity IX-12
Potential for Receptor Impacts IX-16
Contaminated Groundwater IX-17
Plume Persistence IX-17
Plume Migration IX-19
Detailed Evaluation Of Monitored Natural Attenuation Effectiveness IX-22
Natural Attenuation Mechanisms IX-22
Biological Processes IX-24
Physical Processes IX-24
Site Characterization IX-25
Contaminated Soil IX-29
Permeability IX-29
Soil Structure and Layering IX-30
Sorption Potential IX-30
Soil Saturation Limit IX-31
Soil Gas Composition IX-33
Soil Moisture IX-34
pH IX-34
Temperature IX-34
Microbial Community IX-34
Rate Constants and Degradation Rates IX-35
Time To Achieve Remediation Objectives IX-35
Contaminated Groundwater IX-36
Effective Solubility IX-36
Henry's Law Constant IX-38
Permeability IX-38
Groundwater Seepage Velocity IX-40
Sorption and Retardation IX-41
Retarded Contaminant Transport Velocity IX-42
Precipitation/Recharge IX-43
Geochemical Parameters IX-43
Rate Constants and Degradation Rates IX-46
Plume Migration IX-49
Time Frame To Achieve Remediation Objectives IX-49
May 2004 IX-iii
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Long-Term Performance Monitoring IX-54
Contaminated Soil IX-55
Contaminated Groundwater IX-57
Contingency Plan IX-61
Contaminated Soil IX-62
Contaminated Groundwater IX-62
References IX-63
Checklists: Evaluating CAP Completeness and Potential Effectiveness of MNA . . IX-65
Initial Screening-Soil Contamination
Initial Screening-Groundwater Contamination
Detailed Evaluation-Soil Contamination
Detailed Evaluation-Groundwater Contamination
Long-Term Performance Monitoring-Soil Contamination
Long-Term Performance Monitoring-Groundwater Contamination
IX-iv May 2004
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List Of Exhibits
Number Title Page
IX-1 Advantages And Disadvantages Of Monitored Natural Attenuation .... IX-4
IX-2 Conceptualization Of Electron Acceptor Zones In The Subsurface .... IX-5
IX-3 Initial Screening Of Monitored Natural Attenuation Applicability IX-7
IX-4 Processes Governing The Partitioning Of LNAPL Into The Soil,
Water, And Air In The Subsurface Environment IX-9
IX-5 Maximum Hydrocarbon Concentrations For Soil-Only Contamination IX-11
IX-6 Graph Of Hyperbolic Rate Law For Aerobic
Biodegradation of Gasoline IX-14
IX-7 Rate of Aerobic Biodegradation Of Hydrocarbons (mg/kg/d) That Can Be
Sustained Diffusion Of Oxygen Through The Vadose Zone (Calculated
For A Smear Zone That Is One Meter Thick) IX-15
IX-8 Time Required (Years) To Consume Hydrocarbons Present At
Residual Saturation IX-16
IX-9 Benzene Attenuation Rates Reported By Peargin (2000) IX-19
IX-10 Initial Dissolved Concentrations (mg/L) Of Benzene And MTBE That
Can Be Biodegraded To Target Levels Within Various Time Periods . IX-20
IX-11 Detailed Evaluation Of Monitored Natural Attenuation Effectiveness . IX-23
IX-12 Primary Monitored Natural Attenuation Mechanisms IX-25
IX-13 Site Characterization Data Used To Evaluate Effectiveness Of
Monitored Natural Attenuation in Groundwater IX-27
IX-14 Factors Affecting MNA Effectiveness: Contaminated Soil IX-29
IX-15 Koc Values For Common Petroleum Fuel Constituents IX-32
IX-16 Factors Affecting MNA Effectiveness: Contaminated Groundwater .. IX-37
IX-17 Solubilities of Common Petroleum Fuel Constituents IX-39
IX-18 Henry's Law Constants For Petroleum Fuel Constituents IX-39
May 2004 IX-v
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List Of Exhibits (cont'd)
Number Title Page
IX-19 Retardation Coefficients For Different Organic Compounds And
Different Organic Carbon Content IX-42
IX-20 Redox Potentials For Various Electron Acceptors IX-45
IX-21 MTBE Concentration Measured In Monitoring Wells Over Time .... IX-50
IX-22 MTBE Concentration Measured In Monitoring Wells Over Time .... IX-51
IX-23 Rates Of Attenuation Of MTBE In Monitoring Wells And The
Projected Time To Reach A Cleanup Goal Of 20 ug/L As
Calculated From Data Presented In Exhibits IX-21 And IX-22 IX-51
IX-24 Evaluation Of Long-Term Performance Monitoring Plan IX-56
IX-25 Performance Monitoring Frequency, Analytes, and Sampling
Locations IX-57
IX-26 Example of Optimal Groundwater Sampling Network Design for
Performance Monitoring IX-61
IX-vi May 2004
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Chapter IX
Monitored Natural Attenuation
Overview
The term "monitored natural attenuation" (MNA) refers to the reliance on
natural attenuation processes (within the context of a carefully controlled and
monitored site cleanup approach) to achieve site-specific remediation objectives
within a time frame that is reasonable compared to that offered by other more
active methods (EPA, 1999). Long-term performance monitoring is a
fundamental component of a MNA remedy, hence the emphasis on "monitoring"
in the term "monitored natural attenuation". Other terms associated with natural
attenuation in the literature include "intrinsic remediation", "intrinsic
bioremediation", "passive bioremediation", "natural recovery", and "natural
assimilation". Note, however, that none of these are necessarily equivalent to
MNA.
MNA is often dubbed "passive" remediation because natural attenuation
processes occur without human intervention to a varying degree at all sites. It
should be understood, however, that this does not imply that these processes
necessarily will be effective at all sites in meeting remediation objectives within a
reasonable time frame. This chapter describes the various chemical and
environmental factors that influence the rate of natural attenuation processes.
Because of complex interrelationships and the variability of cleanup standards
from state-to-state and site-to-site, this chapter does not provide specific
numerical thresholds to determine whether MNA will be effective.
The fact that some natural attenuation processes are occurring does not
preclude the use of "active" remediation or the application of enhancers of
biological activity (e.g., electron acceptors, nutrients, and electron donors)1. In
fact, MNA will typically be used in conjunction with, or as a follow-up to, active
remediation measures, and typically only after source control measures have been
implemented. For example, following source control measures2, natural
attenuation may be sufficiently effective to achieve remediation objectives
without the aid of other (active) remedial measures, although this must be
conclusively demonstrated by long-term performance monitoring. More typically,
active remedial measures (e.g., SVE, air-sparging) will be applied in areas with
high concentrations of contaminants (i.e., source areas) while MNA is employed
1 However, by definition, a remedy that includes the introduction of an enhancer
of any type is no longer considered to be "natural" attenuation.
2 Note that MNA may be an appropriate remediation option only after separate
phase product has been removed to the maximum extent practicable from the subsurface
**•**" as required under 40 CFR 280.64.
May 2004 IX-1
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for the dilute contaminant plume. In any case, MNA should be used very
cautiously as the sole remedy at any given site since there is no immediate backup
(although there should be contingency plans in place) should MNA fail to meet
remediation objectives.
EPA does not consider MNA to be a "presumptive" or "default" remedy - it is
merely one option that should be evaluated with other applicable remedies (EPA,
1999). EPA does not view MNA to be a "no action" or "walk away" approach,
but rather considers it to be an alternative means of achieving remediation
objectives that may be appropriate for specific, well-documented site
circumstances where its use meets the applicable statutory and regulatory
requirements (EPA, 1999). As there is often a variety of methods available for
achieving remediation objectives at any given site, MNA may be evaluated and
compared to other viable remediation methods (including innovative
technologies) during the study phases leading to the selection of a remedy. As
with any other remedial alternative, MNA should be selected only where it meets
all relevant remedy selection criteria, and where it will meet site remediation
objectives within a time frame that is reasonable compared to that offered by other
methods (EPA, 1999). Exhibit IX- 1 provides a summary of the advantages and
disadvantages of using monitored natural attenuation as a remedial option for
petroleum-contaminated soils and groundwater.
Natural Attenuation Processes
Natural attenuation processes include a variety of physical, chemical, and
biological processes that, under favorable conditions, reduce the mass, toxicity,
mobility, volume, and/or concentration of contaminants in soil and/or
groundwater. Processes that result only in reducing the concentration of a
contaminant are termed "nondestructive" and include hydrodynamic dispersion,
sorption and volatilization. Other processes, such as biodegradation and abiotic
degradation (e.g., hydrolysis), result in an actual reduction in the mass of
contaminants and are termed "destructive" (Weidemeier, et. al, 1999). For
petroleum hydrocarbons, biodegradation is the most important (and preferred)
attenuation mechanism since it is the only natural process that results in actual
reduction in the mass of petroleum hydrocarbon contamination. Aerobic
biodegradation consumes available oxygen resulting in anaerobic conditions in the
core of the plume and a zone of oxygen depletion along the outer margins. As
illustrated by Exhibit IX-2, the anaerobic zone is typically more extensive than the
aerobic zone due to the rapid depletion of oxygen, the low rate of oxygen
replacement, and the abundance of anaerobic electron acceptors3 relative to
dissolved oxygen (Weidemeier, et. al., 1999). For this reason, anaerobic
biodegradation is typically the dominant process . For both aerobic and anaerobic
3 Anaerobic electron acceptors include nitrate, sulfate, ferric iron, manganese,
and carbon dioxide. For aerobic respiration the electron acceptor is oxygen.
IX-2 May 2004
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processes, the rate of contaminant degradation is limited by the rate of supply of
the electron acceptor not the rate of utilization of the electron acceptor by the
microorganisms. As long as there is a sufficient supply of the electron acceptor,
the rate of metabolism does not make any practical difference in the length of time
required to achieve remediation objectives.
Corrective Action Plan (CAP)
The key components of a corrective action plan (CAP) that proposes MNA as
a remediation alternative are:
• documentation of adequate source control,
• comprehensive site characterization (as reflected in a detailed conceptual site
model),
• evaluation of time frame for meeting remediation objectives,
• long-term performance monitoring, and
• a contingency plan(s).
This chapter is intended to be an aide in evaluating a CAP that proposes MNA
as a remedial option for petroleum-contaminated soil and groundwater. Note that
a state may have specific requirements that are not addressed in this chapter.
The evaluation process is presented in the four steps described below. A series of
checklists have also been provided at the end of this chapter. They can be used as
tools to evaluate the completeness of the CAP and to help focus attention on areas
where additional information may be needed.
Step 1: An initial screening of monitored natural attenuation applicability.
This initial step is comprised of several relatively easily answered questions
which should allow for a quick decision on whether or not MNA is even
potentially applicable.
Step 2: A detailed evaluation of monitored natural attenuation
effectiveness. This step provides further criteria to confirm whether
monitored natural attenuation is likely to be effective. To complete this
evaluation, you will need to review monitoring data, chemical and physical
parameters of the petroleum constituents, and site conditions. You will then
need to determine whether site and constituent characteristics are such that
monitored natural attenuation will likely result in adequate reductions of
contaminant concentrations.
Step 3: An evaluation of monitoring plan. Once it has been determined that
MNA has the potential to be effective, the adequacy of the proposed long-term
performance monitoring schedule must be evaluated.
May 2004 IX-3
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Exhibit IX-1
Advantages And Disadvantages Of Monitored Natural Attenuation
Advantages
Overall costs may be lower.
Minimal disturbance to the site
operations.
Potential use below buildings and other
areas that cannot be excavated.
Does not generate remediation wastes.
However, be aware of risks from methane
produced during natural biodegradation
of petroleum hydrocarbons.
Reduced potential for cross-media
transfer of contaminants commonly
associated with ex-situ treatment.
Reduced risk of human exposure to
contaminants near the source area.
Natural biodegradation may result in the
complete destruction of contaminants in-
situ.
May be used in conjunction with, or as
follow-up to, active remedial measures.
Disadvantages
• Much less effective where TPH
concentrations in soil are high (> 20,000
to 25,000 mg/kg). Not suitable in the
presence of free product.
• Not suitable when contamination has
impacted a receptor (e.g., impacted
ground water supply well, vapors in a
building).
• Despite predictions that the contaminants
are stationary, some migration of
contaminants may occur. Not suitable if
receptors might be affected.
• Longer periods of time may be required
to mitigate contamination (especially true
for heavier petroleum products).
• May fail to achieve the desired cleanup
levels within a reasonable length of time
(and an engineered remedy should instead
be selected).
• Site characterization will necessarily be
more detailed, and may include additional
parameters. Site characterization will be
more costly.
• Institutional controls may be necessary to
ensure long term protectiveness.
• Performance monitoring will generally
require more monitoring locations.
Monitoring will extend over a longer
period of time.
• It may be necessary to implement
contingency measures. If so, this may
increase overall cost of remediation.
• May be accompanied by changes in
groundwater geochemistry that can
mobilize other contaminants.
IX-4
May 2004
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Exhibit IX-2
Conceptualization of Electron Acceptor Zones In the Subsurface
Plume of Dissolved
Fuel Hydrocarbons
(Adapted from Wiedemeier et al., 1999. NOTE: Due to the presence of the mobile NAPL
pool-"free product"-the site depicted in Exhibit IX-2 above would not be an appropriate candidate
for MNA. After the free product has been removed from the subsurface to the maximum extent
practicable, then the site may be evaluated as to whether or not it would be an appropriate candiate
for MNA.)
Step 4: An evaluation of the contingency plan. In the event that monitoring
indicates that MNA does not appear to be effective in meeting remediation
objectives in a reasonable time frame, a more aggressive remediation
technology will need to be implemented. Several potential alternative
technologies are presented in other chapters in this manual, and the applicable
chapter should be consulted to evaluate the appropriateness of the contingency
remedy.
May 2004
IX-5
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Initial Screening Of Monitored Natural
Attenuation Applicability
The policies and regulations of your state determine whether MNA will be
allowed as a treatment option. As the first step in the screening process,
determine if your state allows the use of MNA as a remedial option. For example,
MNA may not be allowed if the contaminant mass is large enough that
groundwater impacts are likely (or have already occurred), or if sampling indicates
the presence of free product, or an existing contaminant plume isn't shrinking, or
if there are potential receptors located nearby. Also be aware that it is possible
that while allowing MNA as a remedial option, your state may have requirements
that are more stringent than those described in this chapter.
Although the specific screening criteria for both contaminated soil and
groundwater might be expected to be very different due to the characteristics of
the impacted media, they are actually quite similar. For both media the criteria
focus on two elements: (1) source longevity and (2) potential receptor impacts.
Source longevity influences not only the time to achieve remediation objectives
but also the potential for groundwater contamination and plume migration.
Receptors may be impacted through direct contact with source materials (such as
residual soil contamination or free product), or through ingestion of dissolved-
phase contaminants or inhalation of vapor-phase contaminants. The objective of
the initial screening is to determine how long the source is likely to persist, and
whether or not there are likely to be impacts to receptors during this time. The
following section will provide guidance on how these criteria should be evaluated
for either contaminated soil or contaminated groundwater. Exhibit IX-3 is a flow
chart that can serve as a roadmap for the initial screening evaluation process. If
results of the initial screening indicate that MNA is not likely to be effective, then
other more aggressive measures (for example excavation of contaminated soil, or
pump-and-treat for groundwater) should be employed.
Contaminant Transport and Fate
The most commonly encountered petroleum products from UST releases are
gasoline, diesel fuel, kerosene, heating oils, and lubricating oils. Each of these
petroleum products is a complex mixture often containing hundreds of
compounds. Transport and fate characteristics of individual contaminants are a
function of their chemical and physical properties.
Each fuel constituent will migrate via multiple pathways depending on its
chemical and physical characteristics. Consequently, different chemicals will have
different migration pathways. For example, a portion of the benzene in the fuel
will partition out of the pure ("free product") phase and into the vapor phase, the
sorbed phase, and the dissolved phase. Although the majority of the benzene mass
will stay in the free product phase, a significant portion will either volatilize or
dissolve into either soil moisture in the vadose zone or groundwater in the
saturated zone.
IX-6 May 2004
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Exhibit IX-3
Initial Screening of Monitored Natural Attenuation Applicability
Soil
Does
your state allow
MNAas a remediation alternative
for soil?
Has
free product beers
recovered tatne maximum
extant practicable'
cantartnant mass be
degraded within a reasonable
period of firne?
Ar*
adequatscantroteiii
pfece hi ensure that no receptors
come into contact wttti
contaminated so!!?
initial screening for
UN A Applicability
Natural i
ft note i
option at the site
Cofwder other
Air Sparging
thjgi-Phsse
Extraction
m-«itii
Qrourutwater
is not a remedial
option a! 1he site.
tachnoicgiss
•Free Product
• Btosparging
Air SparBing
• Duaf-PhsM
iUHlitU
GrourKfwater
VES
I.
MNA has the potential to be
affective at ttis site,
Procsed to Detailed Evaluation
Groundwater
Doss
your state alsw MWA
as a remsdiatien attamafeB
for groundwater?
YES
Has
free product been
jecovfif sd to the maximum extent
practicable?
Y6S
Me plume
shnnking such mat
remediation objectives will
be achieved within a
.reasonable time,
frame'?
YES
Should
"the plume unexpectedly"
^migrate, are Ihere any receptors,
wrthin a 2-year tra/et
time?
NO
May 2004
IX-7
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Only a relatively small percentage will sorb onto soil particles. If the soil
contains a higher percentage of organic carbon, a higher percentage of benzene
will potentially be sorbed. In contrast to benzene's behavior, ethylbenzene will
more likely sorb onto soil particles and would not be as soluble in water. Exhibit
IX-4 is a schematic illustration of the interrelationships among the attenuation
processes that govern the partitioning of free product into the soil, water and air in
the subsurface environment.
Contaminated Soil
Often the primary concern associated with contaminated soil is that it can
result in contamination of groundwater resources. Secondary concerns are direct
exposure to the contaminated soil itself and vapors originating in the source area.
However, given the particular conditions at a site, the relative order of these
concerns may change. The potential for receptor impacts depends upon a number
of site-specific conditions of which two of the most important are source mass and
source longevity.
Despite the relatively low solubility of the hydrocarbons in petroleum fuels,
they can be leached downward from the soil in the source area into the underlying
groundwater. For the more soluble gasoline additives (for example MTBE and
ethanol) this is especially true. Contaminated soil in the vadose zone can also be
the source of vapors which migrate through the more permeable pathways in the
soil and can accumulate in subsurface areas such as basements, parking garages,
sewers and utility vaults. Where these vapors collect in sufficient quantity they
can present an immediate safety threat from explosion, fire, or asphyxiation.
Inhalation of lower concentrations of vapors over the long-term can lead to
adverse health effects. All of these problems are magnified with increasing mass
of contaminants and increasing amount of time that they are allowed to remain in
the subsurface. The best way to reduce the likelihood of groundwater
contamination and shorten the time required to achieve remediation objectives is
to quickly and completely eliminate the mass of contamination in the subsurface.
Contaminated soils may be remediated by a variety of in situ and ex situ
technologies described in other chapters of this document. These include
bioventing (Chapter III), soil vapor extraction (Chapter II), enhanced aerobic
biodegradation (Chapter XTI), chemical oxidation (Chapter XIII), low temperature
thermal desorption (Chapter VI), biopiles (Chapter IV) and landfarming (Chapter
V).
In several of the following sections on evaluation of MNA for soil-only sites
(both in the initial and detailed evaluation sections) examples will be presented to
illustrate the evaluation methodology. For consistency, three representative soils
types are used with parameter values derived from the literature. Also, a
hydrocarbon density of 730 kg/m3 (typical of gasoline) was used and assumed to
be representative of gasoline. Though it is possible that some of these examples
may be representative of some actual sites, these exhibits are intended only to
illustrate a methodology that could be used; in all cases site-specific data should
be used to develop screening values.
IX-8 May 2004
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Exhibit IX-4
Processes Governing the Partitioning of LNAPL Into the Soil,
Water, and Air in the Subsurface Environment
DISSOLUTION
Effective Solubility
C: -XS
VOLATILIZATION
Raouh's Law
Pr> - P
WATER
Henry's Law
i/ — ft i f
1%, - W, I l^0
where: Kd = the distribution coefficient
Koc = organic carbon normalized soil/water partition coefficient
f,,,. = fraction of organic carbon in soil
CL = effective solubility of a given solute
X = mole fraction of a given solute in a mixture
S = pure phase solubility of a given solute
Pp = partial pressure of a given gas
Pv = vapor pressure of a given gas
X^ = mole fraction of a given gas in a mixture
KH = Henry's law constant for a given solute
Ca = concentration of a given solute in vapor phase
Cw = concentration of a given solute in aqueous phase
Cs = concentration of a given solute in soil phase
May 2004
IX-9
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If there is a possibility that groundwater will be impacted, or if protection of a
particular groundwater resource is of vital importance, then a more detailed
analysis (including the collection and analysis of groundwater samples) should be
conducted and the appropriateness of MNA as a remedial alternative should be
based on groundwater criteria instead of soil criteria.
Source Mass
Regardless of how biodegradable a contaminant may be, the larger the
contaminant mass to be degraded, the longer it will take to do so. Obviously, the
more biodegradable a contaminant is, the faster it will be degraded relative to a
more recalcitrant (nondegradable) contaminant. The larger the source and the
longer it resides in the subsurface, the greater the likelihood that groundwater
contamination will occur. This is especially true when the depth to groundwater
is relatively shallow, the amount of annual rainfall (and hence groundwater
recharge) is high, and the soil is relatively permeable (and the soil surface is not
covered with an impervious material such as asphalt or concrete).
Although an accurate estimate of the mass of the fuel release usually is not
known, a legitimate attempt should be made to quantify the release volume. In
the absence of reliable inventory data, the volume of fuel in the subsurface can be
estimated by first determining the extent of contaminated soil and then integrating
saturation data from soil samples over the volume of the contaminated soil mass.
(For more information, see EPA, 1996b, Chapter IV.) The objective is to
sufficiently characterize the extent and level of contamination with a minimum
number of samples, although the accuracy of the volume estimate generally
increases with an increasing number of samples. At a minimum, samples should
be collected from locations where contamination is known to be greatest (e.g.,
beneath the leaking UST or piping). Soil samples should be collected from the
source area in the unsaturated zone and in the smear zone (if any) to define the
three-dimensional extent of contamination.
These samples should be analyzed for the BTEX contaminants, TPH, and any
other contaminants of co m at the site. If the primary contaminants of concern
at the site are volatile or, j chemicals (VOCs), monitoring of soil gas should
supplement direct soil nr_ ^urements at some locations. In addition, soil gas
samples should be analyzed for oxygen, carbon dioxide, and methane (and
sometimes hydrogen) to determine the microbial activity in the soils. As described
above, reduced oxygen concentrations in the plume area (relative to background)
and elevated carbon dioxide concentrations are a good indication that
biodegradation is occurring.
Different soil types have different capacities for "holding" or "retaining"
quantities of hydrocarbons released into the subsurface. The capacity for any
particular soil type depends upon properties of both the soil and the type(s) of
hydrocarbons released. In general, residual hydrocarbon saturation (sr) increases
with decreasing grain size. If it is assumed that a given volume of soil is initially
hydrocarbcr -free, the volume of hydrocarbon that the soil can retain is given by:
where: Vr - volume of hydrocarbon retained [L3]
IX-10 May 2004
-------
A-,. = residual hydrocarbon saturation [volume hydrocarbon/volume
soil]
nt, - effective porosity [volume pore space/volume soil]
Vmii = volume of soil [L3]
The above equation is simplistic and does not address factors such as
spreading of the hydrocarbon, the rate at which the soil absorbs the liquid, or mass
loss due to volatilization. However, it can be used as a screening criterion to
determine whether a given UST release is likely to result in free product
accumulation at the water table.
Exhibit IX-5 presents typical ranges for the concentration of hydrocarbons
(e.g., TPH) that each of three representative soil types could retain in the
unsarurated zone. Values in the second column under "Concentration" are in
terms of mass per square meter (kg/m2). To obtain these values, first multiply the
concentration in mg/kg by the bulk density of the soil (in kg/m3) then divide by 1
million (to convert from mg to kg). Next, multiply the result by the thickness (in
meters) of the contaminated soil. These concentrations can then be used to
develop a rough "rule of thumb" to predict whether a spill will reach the water
table. The volume of the material receiving the spill is estimated by multiplying
the depth to ground water (in meters) by the "surface" area of the spill-this is the
assumed thickness (in meters) of the contaminated soil. If no other information
is available, assume the surface area is 1 m2 (necessary to yield a volume). If the
known (or suspected) volume of release (in gallons) divided by the volume (in
cubic meters) to the water table exceeds the number of gallons per cubic meter
(last column), then it is likely that free product will be present.
Exhibit IX-5
Maximum Hydrocarbon Concentrations For Soil-Only Contamination
Soil
Type
silty
clay
sandy
silt
coarse
sand
Residual
Hydrocarbon
Saturation
0.05 to 0.25
0.03 to 0.20
0.01 to 0.10
Bulk
Density8
(kg/m3)
1,350
1,650
1,850
Porosity"
0.36
0.41
0.43
Concentration
mg/kg
10,000 to
49,000
5,000 to
36,000
2,000 to
17,000
kg/m2
13 to 66
9 to 60
3 to 31
gal/m3
5 to 24
3 to 22
1 toll
Sources: aBoulding(1994),p.3-37.
b Carsell and Parrish (1988)
Another use for the data in Exhibit IX-5 would be to compare measured
hydrocarbon concentrations in soil samples with those in the table (second to last
and next to last columns)—if measured concentrations are close to or exceed
those in the table for a given soil type, then it could be expected that free product
might accumulate at the water table. In situations where free product is present,
monitored natural attenuation is not an appropriate remedial alternative because
natural processes will not reduce concentrations to acceptable levels within a
reasonable time period (i.e., a few years). At all sites where investigations
May 2004
IX-11
-------
indicate that free product is present, Federal regulations (40 CFR 280.64) require
that it be recovered to the maximum extent practicable. Free product recovery,
and other engineered source control measures, are the most effective means of
ensuring the timely attainment of remediation objectives. For more guidance on
free product recovery, see U.S. EPA, 1996a.
From Exhibit IX-5 we see that one cubic meter of silty clay could potentially
retain 5 to 24 gallons of gasoline assuming that it was spread evenly through the
soil. For a LUST site where the depth to groundwater below the point of the
release was, for example, 5 meters (15 feet), there is no information on the surface
area of the spill, and the soil type is silty clay, then a release of up to 120 gallons
(24 gallons per meter times five meters depth) might be retained within the
unsaturated zone and free product would not be expected to accumulate on the
water table. In contrast, a coarse sand might potentially retain a release of only 55
gallons. In either or both of these cases even if the release volume was small
enough so that free product did not collect at the water table there could still be a
groundwater impact through leaching of soluble hydrocarbons by infiltration of
precipitation and groundwater recharge. In such an instance, release volumes
much smaller than theoretically retained could result in significant and
unacceptable groundwater impact.
Source Longevity
Once it has been determined that the entire release volume will remain trapped
within the vadose zone and there is no likelihood of groundwater contamination,
the next step is to estimate the lifetime of the residual contamination. The two
primary factors that control source longevity are: (1) mass of contaminants present
in the source area, and (2) availability of electron acceptors, of which oxygen is
the most important.
As previously discussed, the larger the contaminant mass, the longer the
period of time required for it to be completely degraded. Across a wide range of
concentrations, the rate of biodegradation of petroleum hydrocarbons follows a
hyperbolic rate law:
where: V = the achieved rate of biodegradation (mg/liter in groundwater
or mg/kg in soil)
Vmax = the maximum possible rate of biodegradation at high
concentrations of hydrocarbon
C = the concentration of hydrocarbon (mg/liter or mg/kg)
K = half-saturation constant (the concentration of hydrocarbon
that produces one-half of the maximum possible rate of
biodegradation; mg/liter in water or ppm [volume/volume in
soil gas] or mg/kg in sediment)
When hydrocarbon concentrations (Q are significantly lower than the half-
saturation constant (K), the sum of (K+C) is approximately equivalent to K.
Because F"max and K are constants, the rate of biodegradation (V) is proportional to
IX-12 May 2004
-------
the concentration of hydrocarbon (C). As the concentration of hydrocarbon
decreases through biodegradation, the rate of biodegradation declines as well (i.e.,
biodegradation follows a first-order rate law). When hydrocarbon concentrations
are significantly higher than the half saturation constant, the sum of (K+C) is
approximately equivalent to C and the value of C/(K+C) approaches 1.0. Thus,
the achieved rate of biodegradation (F) approaches the maximum rate (Vmax).
When C is more than ten times the value of K, the rate of biodegradation will be
more than 90% of the maximum rate (Vmax). These relationships are illustrated in
Exhibit IX-6.
In Exhibit IX-6, Vmax has been set at a value of 0.4 mg TPH per kg sediment
per day. This corresponds to the Vmax published for aerobic degradation of
aviation gasoline vapors by Ostendorf and Kampbell (1991). The concentration
of hydrocarbon vapors was calculated from the concentration of TPH, assuming
that the air-filled porosity was 10%, the water-filled porosity was 10%, the
sediment bulk density was 1.8 kg/liter, and the partition coefficient of dissolved
hydrocarbon between water and air was 0.24. The rate of biodegradation was
calculated from the concentration of hydrocarbons vapors, using a half saturation
constant for aerobic biodegradation of aviation gasoline vapors of 260 ppm
(Ostendorf and Kampbell, 1991).
The point of the preceding discussion is that at the high hydrocarbon
concentrations typical of source areas in the unsaturated zone, the amount of
hydrocarbons degraded per unit time is approximately constant, regardless of the
actual concentration of hydrocarbons (i.e., biodegradation follows a zero-order
rate law). And, because the rate of degradation is constant with time, the time
required for complete biodegradation is directly proportional to the initial
concentration of hydrocarbons to be degraded. The difference between such an
approximate rate (zero-order) and the true rate (first-order) is less than the usual
statistical variation in the measurements.
The applicability of the above equation has been demonstrated in the field by
Moyer et al. (1996). Thier work demonstrates that a zero-order rate law is the
appropriate law to describe the biodegradation of hydrocarbons in the unsaturated
zone. They found that the half saturation constant for biodegradation of
hydrocarbon vapors in a sandy soil varied from 0.2 mg/kg to 1.6 nag/kg. As
explained in the preceding paragraphs, when hydrocarbon concentrations are more
than ten times the half saturation constant (i.e., 2 mg/kg to 16 mg/kg for this
example), the rate of biodegradation will approach the maximum rate. Note that
these concentrations are already near or below cleanup (or action) levels for
hydrocarbons in soil at many sites. Consequently, it can be assumed that
biodegradation of hydrocarbons, at least in the relatively shallow unsaturated
zone, should follow a zero-order rate law all the way down to cleanup levels. Be
aware that this approximation applies only to petroleum hydrocarbons in the
unsaturated zone: a first-order rate law must be used to determine the rate of
biodegradation of hydrocarbons dissolved in groundwater.
May 2004 EX-13
-------
Exhibit IX-6.
Graph of hyperbolic rate law for aerobic biodegradation of gasoline
0.45 1000000
0.4
Rate of Biodegradation /" / 100000
0.35
_§
c
_o
To
T3
5
S1
E
0.01 0.1 1 10 100 1000 10000
Concentration of TPH (mg/kg)
Generally, petroleum hydrocarbons will be degraded most rapidly by
microorganisms that require oxygen to sustain their metabolism. In situations
where there is an abundance of oxygen and an excess of hydrocarbons for them to
metabolize, aerobic microorganisms should degrade hydrocarbons at or near the
theoretical maximum rate. But, this rarely occurs in the field for a variety of
reasons. Oxygen is rapidly depleted in source areas in particular. Oxygen
diffusion from the atmosphere through the soil in the soil gas to the smear zone
containing hydrocarbons is a slow process, and when subsurface oxygen is
depleted, it takes a relatively long time to replenish. As a consequence, the rate of
aerobic biodegradation is limited by the rate that oxygen is supplied to the
microorganisms by diffusion through the vadose zone.
Aerobic biodegradation is most effective in soils that are relatively permeable
(with a hydraulic conductivity of about 1 ft/day or greater) to allow transfer of
oxygen to subsurface soils where the microorganisms are degrading the petroleum
constituents. Not surprisingly, the length of time required for oxygen to diffuse
into the soil increases as the depth increases. The diffusion rate is also
proportional to the air-filled porosity of the soil and the steepness of the diffusion
gradient. Finer textured materials have more water-filled porosity and less air-
filled porosity at field capacity. Soils with a low oxygen diffusion capacity can
hinder aerobic biodegradation. Exhibit IX-7 presents calculations of the rate that
hydrocarbons that could be mineralized if oxygen diffusion was the limiting
factor.
IX-14 May 2004
-------
Exhibit IX-7
Rate of Aerobic Biodegradation of Hydrocarbons (mg/kg/d)that can be
Sustained by Diffusion of Oxygen through the Vadose Zone (Calculated
for a Smear Zone that is One Meter Thick)
Depth to Top of
Contaminated Soil
(meters) Silty Clay Sandy Silt Coarse Sand
1 5
2 2
3 2
4 1
12
6
4
3
22
11
7
6
Comparing Exhibit IX-5 and Exhibit IX-7, it is readily apparent that aerobic
degradation of hydrocarbons under natural conditions won't expeditiously cleanup
contamination, especially in tight soils. Using the biodegradation-capacity data in
Exhibit IX-7 and applying it to the range of contamination levels in Exhibit IX-5
for each of the three representative soil types, projections can be made on the
length of time (in years) that would be required for aerobic biodegradation to
completely mineralize residual gasoline in the unsaturated zone. As a rough
approximation, the time required to degrade hydrocarbons in the vadose zone can
be estimated by dividing the highest concentration of hydrocarbon (TPH in
mg/kg) by the rate of biodegradation of hydrocarbon (mg/kg per day). For
example, a silty clay is able to retain 10,000 mg/kg to 49,000 mg/kg of
hydrocarbon at residual saturation, but will support aerobic degradation of only 5
mg/kg/day at a depth of only 1 meter below land surface. Even for this relatively
shallow contamination, it is projected that complete degradation would require
from 6 to 28 years. With each meter of increased depth, the length of time
increases by a multiple of approximately this same amount. Thus, for a depth of 3
meters, the projected length of time ranges from 17 to 84 years (approximately 3
times the range of 6 to 28 years).
These calculations of the rate of biodegradation allowed by diffusion of
oxygen put an upper boundary on the rate of biodegradation, and a lower
boundary on the time required to clean up a spill of gasoline. For comparison,
results are also presented (last column of Exhibit IX-8) of the calculated time
required for clean up when the maximum rate of biodegradation (Vmax) is
relatively slow. The time required was calculated using the VmtK (0.41 mg/kg per
day) reported by Ostendorf and Kampbell (1991) in the well-oxygenated
unsaturated zone above the residually-saturated capillary fringe at an aviation
gasoline release site in Michigan. The fertility of the sediment at this site is low,
and as a consequence, the rate of biodegradation is slow compared to rates at other
sites. When the rate of biodegradation is slow, the time required to clean up the
gasoline may be longer than would be expected if the supply of oxygen supplied
through diffusion was the limiting criteria.
May 2004 IX-15
-------
Time
Soil
Type
silty
clay
sandy
silt
coarse
sand
Exhibit IX-8
Required (Years) To Consume Hydrocarbons Present
Saturation
TPHat
Residual
Saturation
(mg/kg)
10,000 to
49,000
5,000 to
36,000
2,000 to
17,000
Oxygen Diffusion-Limited
Depth (meters) to top of contaminated soil in
the vadose zone
1234
6 to 28 11 to 56 17 to 84 23 to 113
Ito9 2 to 17 4 to 26 5 to 34
<1 to 2 <1 to 4 1 to 6 1 to 8
At Residual
Bio-
degradation
-Limited
0.41 mg/kg per
day
67 to 326
33 to 240
13 to 113
These Exhibits (IX-5 through IX-8) demonstrate several important points.
First, and most importantly, there is no substitute for field-measured rates of
biodegradation. Estimates based on theory, microcosm studies, literature values, or
modeling results should not be relied on as the sole basis for regulatory decision-
making. Second, even for permeable material (e.g., coarse sand) the concentration
of hydrocarbon that can be biodegraded within a reasonable time frame (e.g., 1 to 5
years) is relatively low. Third, although oxygen won't be the limiting criteria at
many sites, the rate of aerobic biodegradation may still result in time frames
measured in decades to achieve remediation objectives. And fourth, given the long
projected times to achieve remediation objectives through reliance on natural
processes alone, it will often be more effective and efficient to use an active
remediation technology (e.g., bioventing, soil excavation, SVE) to mitigate the
contaminant source even in the rare case where groundwater impacts are not
anticipated.
Potential For Receptor Impacts
For contamination which remains in the soil in the vadose zone, the primary
potential impacts to receptors are from direct contact with (or ingestion of)
contaminated soil, safety threats due to fire and explosion hazards from
accumulations of vapors, and health effects cause by inhalation of vapors. Each of
these potential impacts should be fully evaluated. It is important to determine
whether there are receptors that could come into contact with contaminated soil.
Because soils associated with UST contamination are generally below the surface
of the ground, there will usually be limited opportunity for receptors to come into
contact with them. However, if the contaminated soils might be excavated (e.g.,
for construction) before contaminant concentrations have been adequately reduced,
receptor contact with contaminated subsurface soil could occur unless appropriate
controls are implemented. If direct contact with contaminated soils is likely,
controls to prevent such contact (or alternative remedial methods) should be
IX-16 May 2004
-------
implemented. The CAP should address these potential concerns and means of
control.
Vapor generation and migration are generally of greater concern with the more
volatile and flammable petroleum fuels (e.g., gasoline). However, even with less
volatile, combustible fuels (e.g., heating oil) sufficient accumulations of vapors
may occur. Like liquids, vapors move faster through the soil in zones of higher
permeability than in zones of low permeability. Common vapor migration routes
are in the coarse backfill around utility lines and conduits, in open conduits such as
sewers, and through naturally permeable zones in the soil (e.g., gravel stringers,
fractures). Basements tend to draw in vapors in response to differential pressure
gradients. In any of these situations, accumulations of vapors can present a safety
threat from fire or explosion, as well as adverse long-term health effects. The
potential for vapor generation and migration, and means to mitigate these hazards,
should be addressed in the CAP.
Contaminated Groundwater
\
The two most common sources of groundwater contamination are from
contaminated soil and free product. If left unaddressed, contaminated soil and/or
free product can be a source of groundwater contamination that may persist for
decades to centuries. Under certain conditions vapors, which are released directly
into the soil, can also result in groundwater contamination. While some states may
have in place resource nondegradation policies that could drive cleanup decisions,
more often than not these decisions are made based on health-related impacts to
human receptors followed by consideration of potential impacts to third parties.
The two primary questions to consider when evaluating the potential impacts of
contaminated groundwater are: "How long will the contaminant plume persist?"
and "Will the contaminant plume migrate from the source area and reach current or
future receptors?"
Plume Persistence
There are two key factors which control the persistence of a contaminant
plume: (1) source mass, and (2) contaminant biodegradability. As one would
expect, the larger the source mass the longer the persistence of the source and the
greater the likelihood that a significant groundwater plume will form. If the
volume of the release is sufficient such that free product is present on the water
table, then MNA is not an appropriate remediation alternative. In fact, Federal
regulations under 40 CFR 280.65 require that free product be recovered to the
maximum extent practicable. For more information on free product recovery, see
U.S. EPA, 1996a.
The longevity of the source is controlled by the rate of weathering of the
residual fuel in the source area. If a portion the residual fuel is above the water
table, volatilization also can remove contaminant mass. As groundwater flows past
residual fuel, the water soluble constituents such as benzene, toluene,
ethylbenzene, and three isomers of xylene (BTEX) plus oxygenates such as MTBE
and ethanol will partition from the residual fuel mass into the groundwater and be
transported downgradient. The concentration of any particular fuel constituent in
groundwater is proportional to its mole fraction in the residual fuel. Over time, the
mass of water soluble components remaining in the residual fuel is depleted and
the groundwater concentrations of these components decrease. Conversely, as the
May 2004 IX-17
-------
mole fraction of less soluble components increases, their concentrations in the
plume actually increase. Once the soluble components have dissolved into the
groundwater, they can also be removed by biodegradation. The rate at which all
these processes remove these components from residual fuel is roughly
proportional to the fraction of the components that remain the residual fuel. As a
consequence, the rate of overall weathering will typically follow a first order rate
law with time.
To estimate the achieved rate of attenuation of benzene and MTBE in
groundwater in contact with residual gasoline, Peargin (2000) examined the long-
term trends in the concentration of benzene and MTBE in monitoring wells that
were screened in the LNAPL smear zone at 23 UST release sites. Source
remediation had been completed at 8 of these sites; no remediation had been
attempted at the remaining 15 sites. The first order rate of attenuation of benzene
and MTBE was calculated from monitoring data from 79 wells for which
statistically significant rates of attenuation could be derived. Exhibit IX-9 is a plot
of the calculated attenuation rate versus initial benzene concentration for both
remediated and non-remediated sites.
Although the rates of natural attenuation of benzene in the smear zone varied
widely, there is a clear difference between rates at sites where active remediation
had been completed, and sites with no active remediation. At sites with active
remediation, the rate of attenuation of benzene in the source is near to or greater
than 0.0022 per day, equivalent to a half-life of just under one year. At sites
without remediation, the mean rate of attenuation of benzene is 0.00037 per day,
equivalent to a half-life of more than five years. For benzene, the attenuation rate
at remediated sites is about 6 times faster than that for the non-remediated sites.
Peargin (2000) also presented data on the persistence of MTBE in wells in the
smear zone. These data indicate the mean rate of attenuation at sites without
remediation is 0.00011 per day, equivalent to a half life of seventeen years. For
sites with active remediation the rate of attenuation of MTBE is 0.0035 per day,
equivalent to a half-life of about 6 months. For MTBE, the attenuation rate at
remediated sites is about 30 times faster than that for the non-remediated sites.
Note that for several of the non-remediated sites contaminant concentrations
are increasing over time. It is also apparent that slower rates of attenuation of the
source are associated with higher initial contaminant concentrations, thus, a longer
period of time is required to achieve adequate reductions in concentration. For the
case of both benzene and MTBE, significant reductions in the amount of time
required to achieve cleanup goals can be realized if the source is adequately
remediated. This is especially true with larger and more recent releases.
If the source contains sufficient mass of contaminants such that natural
degradation will require longer than a decade (or other reasonable period of time),
then MNA is generally not an appropriate remedial alternative. For a time frame of
this duration, performance monitoring is going to be costly, and it is highly
uncertain that the remedy will be protective. There is simply too much mass in the
system and more aggressive measures should be implemented to reduce the mass
in order for MNA to be able to achieve remediation objectives within a time frame
that is reasonable.
IX-18 May 2004
-------
Exhibit IX-9
Benzene Attenuation Rates Reported By Peargin (2000)
0.01 i
0.008 ~
:0.19
'0.24
•o
1-
&>
gl.
g 0.006
•*•*
cs
A
A A
^L
A remediated
S 1
5 iAA ll! not remediated
^ 0.004 -
o
o£
•I 0.002 -
U
o
^ * A ^
***; A ^^
A
A» 'il.
•B «
^ gj *~
." „ 'W
0.32
0.47
0.95
J
f»«
"«
-0.002
5 10 15 20 25
Initial Concentration Benzene (nig/liter)
30
35
Plume Migration
Because monitored natural attenuation relies on natural processes to prevent
contaminants from migrating, it is important to determine the status of the
contaminant plume (that is whether it is "stable"4, shrinking, or expanding) and
4 By definition, a "stable" plume is one that forms where there is a continuous (infinite) source of
contaminants such that concentrations within the plume never change (i.e., neither increase nor
decrease and, thus, "stable"). Only when the flux of contaminants into the plume is exactly equal
to the mass of contaminants that are degraded is the plume truly "stable". If the mass into the
plume exceeds the mass that is biodegraded, then the plume expands; if the mass into the plume is
less than the mass degraded, then the plume contracts. In practice, it may be difficult (or
impossible) to determine whether the plume is expanding, contracting or stable. And unless there
is a continuous release, a source isn't truly infinite. But, the source mass may be so large and the
flux of contaminants into the plume so great that for practical purposes it behaves as an infinite
source and the plume expands (though maybe very slowly) for a very long period of time. The
implications of an expanding or stable plume is that remediation objectives can never be achieved
in a "reasonable" time frame because infinity is not a reasonable length of time. Only after the
contaminant source has been eliminated and the plume has been demonstrated to be contracting
should MNA be evaluated as a potential remedial alternative.
May 2004
IX-19
-------
Exhibit IX-10
Initial Dissolved Concentrations (Jlg/L) Of Benzene And MTBE That Can
Be Biodegraded To Target Levels Within Various Time Periods
Remediated Source
(k= 0.0022/d)
Non-Remediated
Source
(k= 0.0003 7/d)
Remediated Source
(k= 0.0035/d)
Non-Remediated
Source
(k= 0.0001 1/d)
BEIsfflfef^-tai^etJiig/LafrendofiiiterTaft-/;; *„.
1 year
11
6
2 years
25
7
5 years
280
10
10 years
15,000
20
< •;• , ''-. ?jfthaKwtmget ill f$fL*tmfr VfftlfrlJai - "v -
1 year
72
21
2 years
260
22
5 years
12,000
24
10 years
7,000,000
30
whether receptors might be impacted by the release. These impacts could include
ingestion of groundwater, direct contact with contaminated groundwater at
discharge points (e.g., streams or marshes), or inhalation of contaminant vapors,
especially in a basement or other confined space. As a safety measure, sentinel
wells may be installed between the leading downgradient edge of the dissolved
plume and a receptor (e.g., a drinking water supply well). A contaminated sentinel
well provides an early warning that the plume is migrating. As such, sentinel
well(s) should be located far enough up gradient of any receptor to allow enough
time before the contamination arrives at the receptor to initiate other measures to
prevent contamination from reaching the receptor, or in the case of a supply well,
provide for an alternative water source. For those responsible for site remediation,
this is a signal that MNA is not occurring at an acceptable rate, or that site
conditions have changed (i.e., transience) and the contingency remedy should be
implemented. Sentinel wells should be monitored on a regular basis to ensure that
the plume has not unexpectedly migrated.
Exhibit IX-10 compares maximum dissolved concentrations of benzene and
MTBE that can be degraded over various time periods at sites where sources have
been remediated and where sources have not been remediated. Note that for sites
where the sources have not been remediated, the maximum concentrations of
benzene or MTBE that can be biodegraded within a decade are not too much
higher than the target concentrations.
The CAP should contain information regarding the location of potential
receptors, the quality of groundwater, depth to groundwater, rate and direction of
IX-20
May 2004
-------
groundwatcr flow and its variability, groundwater discharge points, and use of
groundwater in the vicinity of the site. If potential receptors are located near the
site, the CAP should also contain monitoring results that demonstrate that receptors
are not likely to be exposed to contaminants. Determination of whether a receptor
is in close proximity to a site may be considered in terms of either contaminant
travel time from the toe of the plume to the receptor or the distance separating the
toe of the plume from the receptor. Both of these will vary from site to site
depending upon site specific factors. The length of time necessary for
contaminants to travel from the source to a downgradient receptor can be estimated
only from site-specific data, which are the highest measured hydraulic
conductivity, the hydraulic gradient, (effective) porosity, distance between the
source and the nearest receptor, and the bulk density of the soil and its organic
carbon content. The last two of these parameters, coupled with the contaminant's
soil sorption constant (Koc, which is discussed later), are necessary to determine if
movement of the contaminant will be retarded by sorption to soil organic matter, or
whether it will move at close to the velocity of the groundwater (i.e., not be
retarded, hence "conservative"). It is important to realize that conservative
contaminants (although initially at low concentrations) actually arrive at receptors
before the time estimated based on average groundwater seepage velocity. The
consequence is that estimated travel times based on average parameter values are
longer than in actual fact, and receptors may be at risk sooner than anticipated.
The subsurface migration of dissolved contaminants through porous media is as a
dispersed plume rather than a concentrated, discrete slug. Whereas a slug that
enters a well instantaneously raises the concentration of the extracted water to that
of the slug, the leading edge of a contaminant plume is typically very dilute and
concentrations in the well increase gradually with time. When contaminants first
arrive at the well the concentration is very low, typically below even taste and odor
thresholds. Continued exposure to such low, but gradually increasing,
concentrations can cause receptors to become desensitized over time to the extent
that they are unaware that their water is contaminated even though concentrations
may be several hundreds of times greater than recognized taste and odor
thresholds.
For biodegradable contaminants, a minimum travel time of 2 years or more
should allow for an evaluation of the potential effectiveness of monitored natural
attenuation and provide sufficient time to implement contingency measures should
monitored natural attenuation prove to be ineffective in meeting remediation
objectives. Therefore, if the maximum expected contaminant transport velocity
(whether for a retard or conservative contaminant) at a site is 2 feet per day, it
would require 2 years for such a contaminant to travel 1,500 feet (approximately 1A
mile). Therefore, at this site, all downgradient receptors within % mile of the
source should be identified and all wells be sampled and included in the regular
monitoring program. It should be noted that the presence of layers of high
permeability soil or rock, fractures or faults, karst, or utility conduits may
accelerate the migration of contaminants. It is also possible that contaminants
could be migrating along pathways that were undetected during characterization of
the site. If less biodegradable and more mobile contaminants (such as MTBE) are
of concern, then the travel time criteria should be reduced.
If the groundwater is potable and future land use is expected to be residential,
potential future receptors should also be considered. If this information is not
provided in the CAP, you will need to request the missing data. If contaminants
May 2004 IX-21
-------
are expected to reach receptors, an active remedial technology should be used
instead of MNA.
Only under some rare circumstances might MNA be considered a remedial
option even when there is potential for lingering groundwater contamination. For
instance, active remediation to protect a groundwater resource may not be required
if the affected groundwater is not potable (e.g., because of high salinity or other
chemical or biological contamination) nor will it be used as a potential source of
drinking water within the time frame anticipated for natural attenuation processes
to reduce contaminant concentrations to below established regulatory levels.
Exposure to petroleum contaminant vapors may also be a concern at some
sites. Hazardous contaminants can volatilize from the dissolved-phase from a
contaminated groundwater plume. Vapors tend to collect in underground vaults,
basements, or other subsurface confined spaces, posing exposure risks from
inhalation and creating the possibility of explosions. Inhalation and dermal
exposure to volatile contaminants can also be significant if groundwater is used for
bathing (even if it is not used for drinking), or even lawn irrigation and car
washing. If vapor migration and associated health and safety risks are not
addressed in the CAP, request additional information.
Detailed Evaluation Of Monitored Natural
Attenuation Effectiveness
Once the initial screen has been completed, and is has been determined that
monitored natural attenuation could potentially be effective at a site, it is necessary
to conduct a more detailed evaluation of the CAP to determine whether or not
MNA is likely to be effective. Exhibit IX-11 is a flow chart that can serve as a
guide through the detailed evaluation process. A thorough understanding of
natural attenuation processes coupled with knowledge of the site conditions and the
contaminants present will be necessary to make this determination. This section
begins with a general overview of natural attenuation mechanisms and site
characterization and before getting into the specific parameters that should be
evaluated for an MNA remedy for contaminated soil and contaminated
groundwater.
Natural Attenuation Mechanisms
In order to assess site conditions to determine whether MNA is an acceptable
alternative to active treatment, it is important to understand the mechanisms that
degrade petroleum fuel components in soil and groundwater. Although it is not
likely that all environmental conditions will be within optimal ranges under natural
field conditions, natural attenuation processes will, to some degree, still be
occurring. Mechanisms may be classified as either destructive (i.e., result in a net
decrease in contaminant mass) or non-destructive (i.e., result in decrease in
concentrations but no net decrease in mass). Mechanisms that result in destruction
of petroleum hydrocarbons (and other fuel components) are primarily biological.
The primary non-destructive mechanisms are abiotic, physical phenomena,
although some abiotic processes are destructive. However, because most of these
processes are relatively insignificant for hydrocarbon fuel components they will not
be presented in the following discussion. The primary biological mechanisms of
IX-22 May 2004
-------
Exhibit IX-11
Detailed Evaluation of Monitored Natural Attenuation Effectiveness
Detailed Evaluation of
MNA. Effectiveness
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May 2004
IX-23
-------
MNA are aerobic and anaerobic metabolism. The primary physical mechanisms
are volatilization, sorption, and dispersion. Characteristics of these mechanisms
are summarized in Exhibit IX-12.
Biological Processes
The driving force for the biodegradation of petroleum hydrocarbons is the
transfer of electrons from an electron donor (petroleum hydrocarbon) to an electron
acceptor. To derive energy for cell maintenance and production from petroleum
hydrocarbons, the microorganisms must couple electron donor oxidation with the
reduction of an electron acceptor. As each electron acceptor being utilized for
biodegradation becomes depleted, the biodegradation process shifts to utilize the
electron acceptor that provides the next greatest amount of energy. This is why
aerobic respiration occurs first, followed by the characteristic sequence of
anaerobic processes: nitrate reduction, manganese-reduction, iron-reduction,
sulfate-reduction, and finally methanogenesis.
Aerobic biodegradation of petroleum fuel contaminants by naturally occurring
microorganisms is more rapid than anaerobic biodegradation when there is an
abundant supply of both electron acceptors and electron donors. Aerobic
biodegradation occurs even at low concentrations of dissolved oxygen.
Heterotrophic bacteria (i.e., those that derive carbon for production of cell mass
from organic matter) are capable of carrying out aerobic metabolism at oxygen
concentrations that are below the detection limit of most conventional methods for
measuring oxygen content. The rate of oxygen depletion due to microbial
metabolism typically exceeds the rate at which oxygen is naturally replenished to
the subsurface. This results in the core region of the hydrocarbon plume being
anaerobic (see Exhibit IX-2). Once the oxygen in the contaminated zone has been
depleted (below about 0.5 mg/L), there is generally ample time for anaerobic
reactions to proceed because the lifespan of contaminant sources and plumes is
measured in years, even after most of the source material has been removed. In
anaerobic biodegradation, an alternative electron acceptor (e.g., NO3", SO42", Fe3^,
Mn4+, and CO2) is used. Within only the past few years it has been realized that
because there is a potentially much larger pool of anaerobic electron acceptors in
groundwater systems, the vast majority of the contaminant mass removed from the
subsurface is actually accomplished by anaerobes.
Physical Processes
Physical processes such as volatilization, dispersion, and sorption also
contribute to natural attenuation. Volatilization removes contaminants from the
groundwater or soil by transfer to the gaseous phase. In general, volatilization
accounts for about 5 to 10 percent of the total mass loss of benzene at a typical site,
with most of the remaining mass loss due to biodegradation (McAllister, 1994).
For less volatile contaminants, the expected mass loss due to volatilization is even
lower. Dispersion ("spreading out" of contaminants through the soil profile or
groundwater unit) results in lower concentrations of contaminants, but no reduction
in contaminant mass. In soil, hydrocarbons disperse due to the effects of gravity
and capillary forces (suction). In groundwater, hydrocarbons disperse by advection
and hydrodynamic dispersion. Advection is the movement of dissolved
components in flowing groundwater. Hydrodynamic dispersion is the result of
mechanical mixing and molecular diffusion. If groundwater velocities are relatively
high, mechanical mixing is the dominant process and diffusion is insignificant. At
low velocity, these effects are reversed. Sorption (the process by which particles
IX-24 May 2004
-------
Exhibit IX-12
Primary Monitored Natural Attenuation Mechanisms
Mechanism
Description
Potential For BTEX Attenuation
Biological >»;,-
Aerobic Respiration
Anaerobic Respiration
• Denitrification
• Sulfate reduction
• Iron reduction
• Manganese
reduction
• Methanogenesis
Physical _,; ':-^fie
Volatilization
Dispersion
Sorption
Microbes utilize oxygen as an
electron acceptor to convert
contaminants to CO2, water,
and biomass.
Alternative electron acceptors
(t>.g.,N03-,S042-,Fe3+,Mn4+,
CO,) are utilized by microbes
to degrade contaminants.
Most significant attenuation
mechanism if sufficient oxygen is
present. Soil air (O2) > 2 percent.
Groundwater D.O. = measurable
Rates are typically much slower than
for aerobic biodegradation but
represent the major biodegradation
mechanisms
Contaminants are removed
from groundwater by
volatilization to the vapor
phase in the unsaturated zone.
Mechanical mixing and
molecular diffusion processes
reduce concentrations.
Contaminants partition
between the aqueous phase
and the soil matrix. Sorption
is controlled by the organic
carbon content of the soil, soil
mineralogy and grain size.
Normally minor contribution relative
to biodegradation. More significant for
shallow or highly fluctuating water
table. No net loss of mass.
Decreases concentrations, but does not
result in a net loss of mass.
Sorption retards plume migration, but
does not permanently remove BTEX
from soil or groundwater as desorption
may occur. Nonet loss of mass.
Source: Adapted from McAllister and Chiang, 1994.
such as clay and organic matter "hold onto" liquids or solids) retards migration of
some hydrocarbon constituents (thereby allowing more time for biodegradation
before the contaminants reach a receptor).
Site Characterization
Site characterization (and monitoring) data are typically used for estimating
attenuation rates, which are in turn used to estimate the length of time that will be
required to achieve remediation objectives. Exhibit IX-13 lists the data that
should be collected during site characterization activities and summarizes the
relevance of these data. In general, the level of site characterization necessary to
support a comprehensive evaluation of MNA is more detailed than that needed to
support active remediation. This is not to say, however, that a "conventional" site
characterization (typically consisting of 1 up gradient well and 2-3 wells
downgradient with long screened intervals that intersect the water table) is
adequate even for active remediation technologies. The primary reason why active
remediation technologies often fail to meet remediation objectives is not so much
that the technologies don't work, as it is that they are inappropriately designed and
May 2004
IX-25
-------
implemented based on information from inadequate site characterization. Many of
these systems (especially pump-and-treat) are merely active containment measures,
and while they often don't result in expeditious cleanup, they may at least serve to
minimize the spread of contamination. Because an MNA remedy lacks an active
backup system, it is even more important that site characterization be as accurate
and comprehensive as possible.
Soil borings should be conducted such that continuous lithologic logs are
generated that cover the interval from ground surface to significantly below the
seasonal low water level. Care should be exercised to ensure that contaminants are
not introduced into previously uncontaminated areas and that conduits for cross-
contamination are not created—wells with long screened intervals that could
interconnect different water-bearing strata should not be installed. Use of direct
push technology is ideally suited for this purpose (see U.S. EPA, 1997, for more
information). With increasing distance from the source area, delineation of
preferential contaminant transport pathways is especially important because these
pathways, which are often relatively small in scale, control contaminant migration.
Monitoring wells should be "nested" and arrayed in transects that are perpendicular
to the long axis of the plume. Several transects should be established to fully
characterize both the subsurface stratigraphy and the contaminant plume in three-
dimensions. In order to determine rates of biodegradation, several wells along the
centerline of the plume are required. If an insufficient number of "cross-gradient"
are installed, it will be impossible to determine where the centerline of the plume is
located. Data from wells that are located off the centerline (in either the lateral or
vertical direction) are erroneous, and lead to an overestimate of the rate of
biodegradation. If the rate of biodegradation is overestimated, then the length of
time required to reach remediation objectives will be underestimated. It is also
especially important that all monitoring wells be sampled on a regular basis to
ensure that seasonal variations in both water levels and contaminant concentrations
are identified.
Data collected during site characterization should be incorporated into a
conceptual site model. A conceptual site model is a three-dimensional
representation that conveys what is known or suspected about contamination
sources, release mechanisms, and the transport and fate of those contaminants.
The conceptual site model should not be static-it should be continually refined as
additional data are acquired. In some cases, new data may require a complete
overhaul of the conceptual site model. The conceptual model serves as an aide in;
directing investigative activities, evaluating the applicability of potential remedial
technologies, understanding potential risks to receptors, and developing an
appropriate computer model of the site.
"Conceptual site model" is not synonymous with "computer model," although
a calibrated computer model may be helpful for understanding and visualizing
current site conditions or for predicting likely future conditions. However,
computer modelers should be cautious and collect sufficient field data to test
conceptual hypotheses and not "force-fit" site data into a pre-conceived, and
possible inaccurate, conceptual representation. After the site conceptual model has
been developed, it is possible to evaluate the applicability of using a computer
model for simulating the site.
Computer models will not be applicable at all sites for a variety of reasons. All
models are based on a set of simplifying assumptions. These assumptions reduce
the enormous complexity of a real-world site to a manageable scale, but at the price
of increased uncertainty. Model developers identify significant processes that form
IX-26 May 2004
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the theoretical basis of the model. Mathematical relationships are then derived for
these processes and solved for contaminant concentrations, mass balances, fluxes,
velocities, etc. Many different approaches have been used. The simplest models
typically have the most restrictive assumptions: one-dimensional steady-state flow
of water and transport of contaminants, homogeneous soil properties, well-defined
source terms, infinite aquifer extent, among others. These formulations lead to
analytical solutions that are easy to use and require only a few input parameters.
Although outwardly simple, these models may not be adequate to represent
contaminant transport at a certain site. Proper use, however, requires that the site
conceptual model match the assumptions of the theoretical model. However,
evaluation of whether or not the assumptions of the model are met requires that
sufficient data have been collected in order to develop a site conceptual model,
because it cannot be assumed a priori that a simplified model is adequate to
represent complex site conditions. When model assumptions are not met then
other approaches must be pursued.
Exhibit IX-13
Site Characterization Data Used To Evaluate Effectiveness Of
Monitored Natural Attenuation In Groundwater
Site Characterization Data
Application
Direction and gradient of groundwater
flow
Hydraulic conductivity
Definition of lithology
Aquifer thickness
Depth to groundwater
Range of water table fluctuations
Delineation of contaminant source and
soluble plume
Date of contaminant release
Historical concentrations along the
primary flow path from the source to the
leading edge
Background electron acceptor levels up
gradient of the source and plume
Geochemical indicators of MNA:
Alkalinity, hardness, pH, and soluble Fe
and Mn, sulfate, nitrate, carbon dioxide,
methane, (sometimes hydrogen) and
redox potential both inside and outside
the contaminant plume
Locations of nearest groundwater
recharge areas (e.g., canals, retention
ponds, catch basins, and ditches)
Estimate expected rate of plume migration.
Estimate expected rate of plume migration.
Understand preferential flow paths.
Estimate volatilization rates and model
groundwater flow.
Estimate volatilization rates.
Evaluate potential source smearing, influence
of fluctuations on groundwater
concentrations, and variation in flow
direction.
Compare expected extent without MNA to
actual extent.
Estimate expected extent of plume migration.
Evaluate status of plume (i.e., steady state,
decreasing, migrating).
Determine assimilative capacity of aquifer.
Evaluate the mechanisms and effectiveness
of MNA processes.
Identify areas of natural groundwater
aeration.
Source: Adapted from McAllister and Chiang, 1994.
May 2004
IX-27
-------
One type of model that might be used instead of an analytical solution is a
numerical model. Numerical models allow for complex geology, variable
boundary conditions, transient flow and transport conditions, among other features.
The features of the site that commonly lead to selection of a numerical model are
heterogeneous transport properties (e.g., hydraulic conductivity, porosity, etc.),
complex stratigraphy, and irregular flow boundaries. In general, as the complexity
of the model increases, so does the amount and quality of data required as input.
The complexity of some sites may preclude modeling because of the investment in
data collection and analysis that would be required. Prime examples are karst and
fractured rock sites where the cost of determining the location of preferential
pathways that control contaminant migration is likely to be prohibitive. It cannot
be assumed that site complexity and size are proportional—it may be just as
prohibitively expensive to adequately model a small site as a large site.
Determining the values of input parameters to the model is a major concern
(and usually a major expense). Subsurface properties may be difficult to measure
and vary tremendously even over small distances. Some parameters required by
the model may not be measured, but rather estimated from the scientific literature,
rules-of-thumb, or "guesstimation". Some required parameters may be
theoretically ill-founded (e.g., dispersivity) or based upon assumptions that may be
only imperfectly met (i.e., degradation by first order rate processes). Model results
are only as good as the data that goes into them, assuming that the model being
used is appropriate under the given conditions at the site. Where the input
parameter sets are constructed from such a set of estimates and imperfect
measurements, a large amount of uncertainty will exist in the model results.
Without comparison to measured concentrations, fluxes and/or other model
outputs, the ability of the model to reproduce observed field conditions will be
unknown.
"Calibration" has been developed as the process for minimizing the differences
between model results and field observations. Through model calibration a
parameter set is selected that results in model output that best fits the observed
data. But, because of the number of parameters that must be identified, calibration
is known to produce non-unique results. This is particularly the case in
heterogeneous environments where every parameter of the model can vary from
point-to-point. Confidence in the model, however, is increased by using the
calibrated model to predict the response to some additional concentration or flux
data (i.e., that were not previously used in calibration). At each step in this process
additional site investigation data improves knowledge of the behavior of the
system. Projecting future contaminant levels from observed current levels requires
proper use of a simulation model. This process is uncertain for many reasons.
Some of the simple reasons are related to inability to predict future land and water
use, future weather patterns, uncharacterized subsurface variability, and others.
Where confidence in the data is uncertain, the most conservative (i.e., protective)
assumptions and parameters should be used. As such, prediction can best be
thought of as an extrapolation from existing conditions. Often, with each new set
of field data, model input parameters are adjusted so that model output matches
this most recent data, but earlier field conditions would not be accurately simulated
using these newer input values. What this means is that model simulations of
future behavior may be as inaccurate as are earlier simulations of present
conditions. Under no circumstances should predictive modeling be used as the
sole justification for selecting an MNA remedy, nor for terminating long-term
performance monitoring.
IX-28 May 2004
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Contaminated Soil
A detailed analysis of whether MNA is likely to be effective in meeting
remediation objectives is understandably more involved than the simple screening
procedure outlined earlier. Exhibit IX-14 lists the factors which influence the
effectiveness of MNA for contaminated soil. The CAP should be closely
examined to ensure that each of these factors has been addressed. The significance
of each of these factors is discussed in the following sections.
Exhibit IX-14
Factors Affecting MNA Effectiveness: Contaminated Soil
Factor
Effect On Monitored Natural Attenuation
Permeability
Soil Structure and
Layering
Sorption Potential
Soil Gas Composition
Soil Moisture
pH
Temperature
Microbial Community
Coarse-grained soils provide the greatest drainage and
aeration, but may also promote contaminant leaching and
migration.
Layered soils inhibit vertical migration and dispersion of
contaminants, but may promote lateral spreading.
Higher organic carbon content and smaller grain size in soil
results in greater sorption of contaminants and retarded
migration.
Presence of oxygen necessary for aerobic biodegradation.
Measurement of other parameters provides information on
biodegradation processes.
Required for microbial activity. Optimal moisture is
between 12 and 30% by weight (75-90% of field capacity).
Generally not a limiting factor within a wide range (4-9).
Biodegradation activity is greatest between soil pH values of
6 to 8.
Generally not a limiting factor within a wide range (0-45°C).
Generally present in almost all subsurface environments.
Permeability
Soil "permeability" controls the rate at which fluids (gases and liquids) move
through the unsaturated zone. This directly influences the rate at which
contaminants are leached from the source area to the water table, as well as the rate
of vapor movement through the soil. While there are a number of ways to measure
the permeability of soil, arguably the most familiar measure is hydraulic
conductivity, which is a function of the properties of both the porous medium and
the fluid. Another common measure of permeability is intrinsic permeability,
which is a function of the properties of only the porous medium. Intrinsic
permeability and hydraulic conductivity are related through this equation:
K = k
PS
May 2004
IX-29
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where: K = hydraulic conductivity
k - intrinsic permeability
p = density of the fluid (in this case, water)
g = acceleration due to gravity
fj. = viscosity (dynamic) of the fluid
Fine-grained soils (e.g., clays and silts), have lower hydraulic conductivity than
coarse-grained soils (e.g., sand and gravel). Thus, sandy soils (which have a
hydraulic conductivity of about 2 ft/day or greater) promote drainage and aeration,
which is favorable to both *'ie dispersion and biodegradation of contaminants.
However, high permeabilm also promotes faster migration of contaminants, which
could result in more rapid and severe groundwater impacts. Clays and silts on the
other hand, which due to their high sorptive capacities (owing to both small
particle size and higher organic matter content), typically result in slower migration
(i.e., retardation) of contaminants and less degradation than that observed in more
permeable soils. Thus, even though biodegradation may take longer, there may be
little or no impact to underlying groundwater resources.
So/7 Structure and Layering
Soil structure refers to the arrangement of soil particles into groups. Soil
structure can enhance or inhibit contaminant migration. Layered soils tend to
hinder the vertical migration of contaminants while enhancing lateral spreadin
Soil macropores (naturally occurring fissures, cracks, root holes, or animal
burrows), however, can facilitate the vertical interchange of contaminants fron; ;ae
ground surface through the soil to groundwater, as well as in the reverse direction.
Low-permeability layers can also reduce aeration of the soils, slowing aerobic
biodegradation. The soil types and structures may be identified by reviewing soil
boring logs. Impervious soil covers (e.g., concrete, asphalt) restrict the infiltration
of water and air downward through the unsarurated zone, which can reduce the
leaching rate of contaminants, in addition to the rate of oxygen replenishment.
While both of these effects can lead to reduced rate of biodegradation, in some
situations the benefit afforded by reduction in leaching of contaminants to the
groundwater may offset the decrease in rate of biodegradation of contaminants.
Sorption Potential
Sorption is the general term for the interaction between contaminants and
paniculate surfaces. There are two types of sorptive processes: adsorption, where
an excess of contaminant molecules accumulate on the surface of the particle, and
absorption, where there is relatively uniform penetration by contaminant molecules
into the surface of the particle. Because the nature of the contaminant-solid
interaction is difficult to measure even under laboratory conditions, and thus it is
essentially wholly unknown in the field, the generic term "sorption" is used to
describe the phenomena without regard to the exact mechanism. The solid, or
sorbing material, is referred to as the sorbent; a contaminant, which sorbs to the
solid sorbent, is referred to as a sorbate. Partitioning is the term used to describe
the process by which the contaminant (usually from the liquid, gas, or dissolved
phase) is sorbed onto the particle surface.
Sorption potential is closely associated with soil type and soil organic matter
content. Finer-grained soils typically have a higher organic carbon content than
coarser-grained soils, and the higher the organic content, the greater the tendency
IX-30 May 2004
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to sorb organic compounds. The range of organic carbon typically found in soil is
from 1 to 3%. The organic matter content in subsurface soils is typically an order
of magnitude or more lower than in surface soils because most organic residues are
incorporated or deposited on the surface. Fine-grained soils have more binding
sites that can immobilize hydrocarbon compounds in the soil matrix, and soils with
a high organic carbon content (i.e., > 2 percent) also have greater capacities for
holding fluids, which retards downward migration and facilitates biodegradation.
Sorption is important because it slows down (or retards) the rate of advance of
the contamination front in the subsurface. Contaminants that sorb tightly to soil
particles may be less subject to transport in the gaseous phase or in solution,
whereas contaminants that are not tightly sorbed can be transported through soils,
aquatic systems, and the atmosphere. Sorption is usually reversible for petroleum
fuel constituents, but the rates of sorption and desorption may not be the same.
With respect to the impact on MNA, the higher the sorption potential, the greater
the retardation of contaminant migration. Increased sorption will increase the time
required for contaminants to reach receptors, allowing greater time for
biodegradation to occur. Conversely, sorbed contaminants may not be available to
microorganisms as a food source. In this case, the contamination may linger
undegraded for exceedingly long periods of time during which they can act as a
slow, steady source of contamination. This can be particularly troublesome where
groundwater resources are impacted. If this is (or is likely to be) the situation, then
more aggressive source mitigation efforts should be undertaken prior to selecting
MNA as a remediation alternative.
Partitioning between the contaminant phase and the solid (soil) phase is
described by the distribution (or sorption) coefficient (Kd), which is a function of
the organic matter in the soil (foc) and the organic carbon partition coefficient (KM):
Kd ~ Koc ' foe
where: K, = distribution coefficient
Koc = organic carbon partition coefficient
foc = fraction of organic carbon in the soil
Koc values can range from 10° to 107. Compounds that have higher Koc and Kd
values tend to remain sorbed on soil and not migrate and dissipate as readily as
those with lower Koc and Kd values. The Koc values of BTEX contaminants are all
low, indicating relatively weak sorption potential, as shown in Exhibit IX-15. None
of the BTEX contaminants will remain strongly sorbed to soils; rather, other
factors such as volatilization and solubility will be more important to their
degradation because these factors increase the likelihood that contaminants will
dissipate. Heavier petroleum constituents tend to have greater Koc values and will
thus sorb more strongly to soils, retarding contaminant migration. MTBE and
ethanol have even lower Koc values than the BTEX components; therefore MTBE
and ethanol will sorb poorly onto organic matter in the soil.
So/7 Saturation Limit
Two of the primary concerns associated with contaminated soil are the
potential for (1) generation of volatile emissions and (2) leaching of contaminants
May 2004 IX-31
-------
into groundwater. Each of these potentials is compound-specific and must be
determined for each contaminant of concern.
Koc Values For
Contaminant
Benzene
Toluene
Ethylbenzene
m-Xylene
o-Xylene
p-Xylene
MTBE
Ethanol
Naphthalene
Exhibit IX-15
Common Petroleum Fuel Constituents
Soil Sorption Constant
49
95
250
190
129
260
11
16
1,300
Koc (L/kg)
Source: Suggested values from CHEMFATEDatabase, Syracuse Research Corp..
http://esc.syrres.com/efdb/chemfate.htm
The soil saturation concentration (Csat) corresponds to the contaminant
concentration in soil at which the sorptive limits of soil particles, the solubility
limits of soil pore water, and saturation of soil pore gas have been reached. Above
this concentration, the soil contaminant may be present in free phase (i.e.,
nonaqueous phase liquids for common petroleum hydrocarbons and other fuel
additives). Csa, is a function of the amount of contaminant hi the vapor phase in the
pore spaces of the soil in addition to the amount dissolved in the soil's pore water
and the amount sorbed to soil particles. The equation for Csat is:
where: Csat = soil saturation concentration (mg/kg)
S = solubility in water (mg/L)
Pb = dry soil bulk density (kg/L)
Kd = distribution coefficient
6w = water-filled soil porosity (vol/vol)
KH = Henry's Law constant (dimensionless)
6a = air-filled soil porosity (vol/vol)
At Csat for a given contaminant, the emission flux from soil to air reaches a
plateau and emissions will not increase above this level no matter how much more
chemical is added to the soil. Therefore, the inhalation route of exposure is not
IX-32 May 2004
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likely to be of concern for those contaminants with regulatory threshold
concentrations (e.g., site-specific screening levels, or SSLs) above Csat. However,
if the concentration of a contaminant is above CM/, there is a potential for free
phase liquid to be present and accumulations of NAPL may occur at the water
table. In such cases further investigation of potential groundwater impacts is
necessary.
The equation above may be modified so that it may be used to determine
whether contaminant concentrations in soil are likely to result in groundwater
impacts. The modified equation is:
r -
<- -
where: C, = screening level in soil (mg/kg)
Cw = target leachate concentration (mg/L)
Koc = organic carbon partition coefficient
fM. = fraction of organic carbon in the soil
0M. = water-filled soil porosity (vol/vol)
6a = air-filled soil porosity (vol/vol)
KH = Henry's Law constant (dimensionless)
ph = dry soil bulk density (kg/L)
In the above equation, CH. is set at the regulatory concentration limit for a
specific contaminant. After plugging in site-specific values for the remainder of
the parameters, C, yields the maximum allowable soil concentration for that
contaminant. If this value is less than measured concentrations in the soil, then
groundwater contamination is likely and MNA is not an acceptable remediation
alternative on the basis of soil contamination. To determine if MNA may be
appropriate for the site, a detailed evaluation of the potential groundwater impacts
must be conducted. For more information on the Soil Saturation Limit, see U.S.
EPA, 1996b.
So/7 Gas Composition
It is important to measure the concentration of oxygen, carbon dioxide,
methane, and volatile organics in soil gas in the source area. This will yield
information on the progress of biodegradation of petroleum contaminants. The
oxygen concentration will yield information on the effectiveness of oxygen
replenishment, which is essential for aerobic biodegradation. Carbon dioxide is an
indicator of aerobic respiration as well. Methane production is the result of
anaerobic metabolism. The concentration of volatile organics will indicate
whether or not vapor migration could be a potential problem at the site. The
presence of volatile organics is also an indicator of the distribution of
contamination in the subsurface.
The vapor pressure of a contaminant is a measure of its tendency to evaporate,
or to move from the product phase to air. Contaminants with higher vapor
pressures (i.e., those contaminants that readily evaporate at room temperature)
more readily disperse, as they have a greater tendency to partition into the vapor
***"" phase and are, therefore, more mobile in soil vapor. Alternatively, contaminants
with relatively low vapor pressures are less likely to vaporize and become airborne.
May 2004 IX-33
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Volatilization from soil or groundwater is highest for contaminants with higher
vapor pressures.
So/7 Moisture
Soil moisture is largely a function of precipitation in the region and the
retention capacity of the soil. Infiltrating precipitation transports oxygen and
nutrients as it percolates downward through the subsurface soils. In addition, water
facilitates the movement of bacteria to other parts of the soil, where they can
continue to degrade petroleum contaminants. However, especially in areas
covered by pavement, replenishment of soil moisture is limited, and the amount of
average annual rainfall may overestimate the amount of moisture replenishment
that actually occurs. This is important because a moderate level of soil moisture is
necessary to support the growth of microbial populations. Also, microbes can only
utilize petroleum hydrocarbons when the hydrocarbons are in the dissolved phase.
In the unsaturated zone, soil moisture content of 75 to 90 percent of field capacity,
is considered optimal for aerobic microbial activity. High precipitation and highly
permeable soils lead to increased leaching rates to groundwater.
pH
Soils that have a pH of 6 to 8 generally promote optimal bacterial growth.
However, the range under which significant biodegradation has been observed to
occur is from 4 to 9 (Wilson, 2001). The significance of this is that biodegradation
is not all that sensitive to pH, and minor variances from the optimal range usually
will have no significant detrimental effect.
Temperature
As with pH, the temperature range under which biodegradation occurs is quite
broad; significant biological activity has been observed under near freezing
conditions to almost boiling. This is not to say that the rate of biodegradation will
be the same all year long. Especially in colder climates, biodegradation rates
measured during the summer season should not be assumed to continue all year
'round. Temperature measurements are also important because certain parameters
(e.g., pH, concentration of dissolved gases) are temperature dependent.
Microbial Community
Microbes capable of degrading petroleum products are present in almost all
subsurface environments. Therefore, the exercise of collecting soil samples and
conducting laboratory microcosm studies is generally not necessary. However, in
some situations, it may be important to analyze soil samples with the intent of
confirming the presence of hydrocarbon degrading microorganisms, and the
absence of toxic levels of contaminants (e.g., heavy metals, corrosive materials,
and pesticides) that could inhibit the effectiveness of the microbial community. If
microcosm studies are conducted, the collection of soil material, the procedures
used to set up, monitor, and analyze the study, and the interpretation of the results
should be based on established procedures, such as those described in Section
C.3.4, "Design, Implementation, and Interpretation of Microcosms Studies", in
EPA's Technical Protocol for Evaluating Natural Attenuation of Chlorinated
Solvents in Ground Water (U.S. EPA, 1998) and/or Section 2, "Laboratory
Studies", in EPA's report on Natural Attenuation of MTBE in the Subsurface
under Methanogenic Conditions (U.S. EPA, 2000b).
IX-34 May 2004
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Rate Constants and Degradation Rates
The selection of MNA as an appropriate remedy for a given site should be
based on a comparison of the rate of remediation that is expected using natural
processes to the rate that is expected from active remediation. For most LUST
sites, natural biodegradation will be the most important component of natural
attenuation. Biodegradation reactions involving organic chemicals occur at rates
which are a function of various site-specific environmental conditions. Projections
of natural biodegradation should be extracted from site-specific data, and not from
rates published in the literature for other sites. Degradation rate constants
determined in the laboratory are generally higher than rates that occur under field
conditions. This is particularly true when the rate in laboratory is limited by the
activity of the microorganisms and the rate in the field is limited by the supply of
oxygen. Wherever possible, field-determined rates should be used to estimate the
time required to achieve remediation objectives. A site-specific rate may not be
constant over time, in both the short-term (i.e., seasonally) and the long-term.
Under no circumstances should such estimates be used as justification to close a
site. Site closure decisions should be based on monitoring data, not predictions.
Time To Achieve Remediation Objectives
As with any remediation method, one of the fundamental questions that arises
is "How much time will be required before remediation objectives are achieved?"
Suitable methodology has been presented in the earlier "Screening" section. This
same methodology should be employed here, but with site-specific parameters
instead of the generic parameters we used to illustrate the methodology.
After estimating a time to achieve remediation objectives, it is necessary to
evaluate whether or not this time is "reasonable" for a given site. As this is a site-
specific decision, no single generic number can be presented in this chapter. In
general, a "reasonable" time frame is one that is comparable to that which could
be achieved through active remediation (U.S. EPA, 1999). Since there are
typically a variety of potential remediation options for a given site, there is likely to
be more than one estimate of time necessary to achieve remediation options.
Evaluation of the most appropriate time frame must be determined through an
analysis of the various remedy alternatives. Some of the factors that should be
considered in making a determination as to which time frame (and remediation
alternative) is most appropriate include:
• Subsurface conditions which can change over an extended time frame required
to achieve remediation objectives;
Whether the contamination, either by itself or as an accumulation with other
nearby sources (on-site or off-site), will exert a long-term detrimental impact
on available water supplies or other environmental resources;
• Uncertainties regarding the mass of contaminants in the subsurface and
predictive analyses (e.g., remediation time frame, timing of future demand, and
likelihood of receptors coming in contact with contaminants);
• Reliability of monitoring (and, if implemented, institutional controls) over the
entire length of the time period required to achieve remediation objectives;
May 2004 IX-35
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• Public acceptance of the time frame required to reach remediation objectives;
and
• Provisions by the responsible party for adequate funding of monitoring,
performance evaluation, and regulatory oversight over the time period required
to achieve remediation objectives.
In general the time frame required for MNA remedies is often longer than that
required for more active remedies. As a consequence, the uncertainty associated
with the above factors increases significantly. Adequate performance monitoring
and contingency remedies should be utilized because of this higher level of
uncertainty. When determining reasonable time frames, the uncertainty in
estimated time frames should be considered, as well as the ability to establish
performance monitoring programs capable of verifying the performance expected
from natural attenuation in a timely manner. Statistical confidence intervals should
be estimated for calculated attenuation rate constants (including those based on
methods such as historical trend data and microcosm studies). When predicting
remedial time frames, sensitivity analyses should also be performed to indicate the
dependence of the calculated remedial time frames on uncertainties in rate
constants and other factors. A statistical evaluation of the rate constants estimated
from site characterization studies of natural attenuation of groundwater
contamination often reveals that the estimated rate constants contain considerable
uncertainty. As an example, analysis of natural attenuation rates from many sites
indicates that a measured decrease in contaminant concentrations of at least one
order of magnitude is necessary to determine the appropriate rate law to describe
the rate of attenuation, and to demonstrate that the estimated rate is statistically
different from zero at a 95% level of confidence (Wilson, 2001). Due to variability
resulting from sampling and analysis, as well as plume variability over time,
smaller apparent reductions are often insufficient to demonstrate (with 95% level
of confidence) that attenuation has in fact occurred at all (U.S. EPA, 1999). When
these conditions cannot be met using MNA, a remedial alternative that more likely
would meet these expectations should be selected.
Contaminated Groundwater
A detailed analysis of whether MNA is likely to be effective in meeting
remediation objectives is understandably more involved than the simple screening
procedure outlined earlier. Exhibit IX-16 lists the factors which influence the
effectiveness of MNA for contaminated groundwater. The CAP should be closely
examined to ensure that these factors have been addressed. The significance of
each of these factors is discussed in the following sections.
Effective Solubility
Solubility is the amount of a substance that will dissolve in a given amount of
another substance (e.g., water). Therefore, a contaminant's solubility provides
insight to its fate and transport in the aqueous phase. Contaminants that are highly
soluble (e.g. MTBE, ethanol) have a tendency to dissolve into the groundwater and
are not likely to remain in the sorbed phase. They are also less likely to volatilize
from groundwater into soil vapor. Conversely, chemicals that have low water
solubilities tend to remain either in the sorbed phase or are likely to volatilize into
soil vapor. In general, lower molecular weight contaminants tend to be more
soluble and, therefore, migrate and disperse much more readily in groundwater or
soil moisture than do heavier contaminants.
IX-36 May 2004
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Exhibit IX-16
Factors Affecting MNA Effectiveness: Contaminated Groundwater
Factor
Effect On Monitored Natural Attenuation
Effective Solubility
Henry's Law Constant
The greater the contaminant's solubility, the greater the
dispersion in groundwater. However, in a mixture, the
solubility of each component is reduced-effective solubility
is less than pure phase solubility.
A measure of a contaminant's tendency to partition between
the aqueous phase and gaseous phase. The higher the Henry's
law constant, the greater the tendency to volatilize from the
dissolved phase
Groundwater Seepage
Velocity
Higher velocity increases migration of dissolved
contaminants, also promotes reoxygenation and
replenishment of electron acceptors.
Sorption and Retardation Higher organic carbon content and smaller grain size in soil
results in greater sorption of contaminants and retarded
migration.
Due to effects of sorption, contaminant transport velocity is
lower than groundwater seepage velocity.
Primary benefit is in transport of dissolved oxygen into
subsurface. Recharge can also cause plumes to dive and
evade monitoring system.
Provide information on assimilative capacity of aquifer and
the nature and effectiveness of biodegradation processes.
Retarded Contaminant
Transport Velocity
Precipitation/Recharge
Geochemical Parameters
When contaminants are released into the environment from a mixture such as a
petroleum hydrocarbon fuel, the water solubility of each individual compounds is
typically lower than its pure phase solubility. This reduced solubility is referred to
as effective solubility and is a function of the mole fraction (or proportion) of a
given component in the whole mixture. The effective solubility equation can be
written as:
CL = X-S
where:
s =
~ effective solubility
mole fraction of component in mixture (e.g., NAPL)
pure phase solubility in water
For complex mixtures it is necessary to estimate the weight percent and an
average molecular weight of the unidentified fraction of the NAPL before the
calculation can be completed. The effective solubility relationship indicates that
for groundwater in contact with NAPL, the total concentration of the contaminant
in the plume remains constant, even if the total concentration of the NAPL in the
soil increases. Stated another way, aqueous-phase concentrations in leachate will
increase together with soil concentrations only while the soil contaminants are
sorbed (there is no NAPL present on the groundwater). Once the soil
concentration reaches a point where NAPL is present, the concentration in the
May 2004
IX-37
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plume reaches a maximum concentration determined by the mole fraction of the
contaminant in the NAPL and it's aqueous solubility. Exhibit IX-17 lists the
solubility of the BTEX contaminants, MTBE, and ethanol. The higher the
solubility, the more likely it is that the contaminant will be transported with
flowing groundwater. Less soluble components may also be transported, although
the aqueous concentration will be lower. More soluble gasoline additives (e.g.,
MTBE, other ethers) are transported farther and faster than hydrocarbons. Often
these additives can be detected in distant wells long before hydrocarbons would
arrive (if they weren't first biodegraded to below detection limits).
Henry's Law Constant
Partitioning of a contaminant between the dissolved phase and the vapor phase
is governed by Henry's law, and the Henry's law constant is a measure of a
contaminant's tendency to volatilize from groundwater into soil gas. Henry's law
states that the concentration of a contaminant in the gas phase is directly
proportional to the compound's concentration in the dissolved phase.
The equation for Henry's law is:
Cg = KH Q
where: C = contaminant concentration in gas phase (arm)
K?H = Henry's law constant (atm Dnr/mol)
Cw = contaminant concentration in dissolved phase (mol/m3)
As shown in Exhibit DC-18, the Henry's law constants for the BTEX
compounds are relatively low, and those for MTBE and ethanol are even lower.
This means that there will be relatively little volatilization from the dissolved
phase to the gas phase, and there is even less tendency for this to occur as the
plume dives below the top of the water table. The consequence of this is that
volatilization can be neglected entirely when using models to simulate
biodegradation. However, volatilization may be of concern with regard to the
accumulation of vapors at unsafe or unhealthy levels in basements, parking
garages, utility conduits, sewers, etc.
Permeability
Aquifer "permeability" controls the rate at which liquids move through the
saturated zone. This directly influences the rate at which contaminants are
transported from source areas to receptors. While there are a number of ways to
measure the permeability of aquifer media, arguably the most familiar measure is
hydraulic conductivity, which is a function of the properties of both the porous
medium and the fluid. Another common measure of permeability is intrinsic
permeability, which is a function of the properties of only the porous medium
Intrinsic permeability (k) and hydraulic conductivity (K) are related through this
equation:
IX-38 May 2004
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Exhibit IX-17
Solubilities of Common Petroleum Fuel Constituents
Constituent
Benzene
Toluene
Ethylbenzene
m-Xylene
o-Xylene
p-Xylene
MTBE
Ethanol
Typical Percentage
in
Gasoline"
1 to 4
2tolO
5 to 20
T
2 to 8
(all 3 isomers)
OtolS
OtolO
Pure Compound
Solubility in Water"
(mg/L)(25°C)
1,780
515
152
160
220
215
51,000C
infinite0
Effective
Solubility in Water0
(mg/L) (25°C)
24 to 95
12 to 60
8 to 33
3 to 13
3 to 14
4 to 16
5,600 to 8,760
57,000d
Sources:
2 A Guide to the Assessment and Remediation of Underground Petroleum Releases, API Publication 162, 3rd
Edition, 1996.
b Selection of Representative TPH Fractions Based on Fate and Transport Considerations, Volume 3, Total
Petroleum Hydrocarbon Criteria Working Group Series, 1997.
http://www.aehs.com/publications/catalog/contents/Volume3.pdf
c Recommended values from CHEMFATE Database, Syracuse Research Corp.,
http://esc.syrres.com/efdb/chemfate.htm
d "Achieving Clean Air and Clean Water: the Report of the Blue Ribbon Panel on Oxygenates in Gasoline",
September, 1999, http://www.epa.gov/otaq/consumer/fuels/oxypanel/r99021.pdf
Exhibit IX-18
Henry's Law Constants For Petroleum Fuel Constituents
Henry's Law Constant
(@20-25° C)
Contaminant
Benzene
Toluene
Ethylbenzene
m-Xylene
o-Xylene
p-Xylene
MTBE
Ethanol
(atm • nrVmol)
5.55E-03
6.64E-03
7.88E-03
7.43E-03
5.19E-03
7.66E-03
5.87E-04
5.20E-06
(cone/cone)
0.227
0.272
0.322
0.304
0.212
0.313
0.024
0.0002
(atm)
308
369
438
413
288
426
32.6
0.29
Source: Recommended values from CHEMFATE Database, Syracuse Research Corp.,
http://esc.SYrres.com/efdb/chemfate.htrn
May 2004
IX-39
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where: K = hydraulic conductivity
k = intrinsic permeability
p = density of the fluid (in this case, water)
g = acceleration due to gravity
/J. = viscosity (dynamic) of the fluid
Fine-grained media (e.g., clays and silts), have lower hydraulic conductivity
than coarse-grained soils (e.g., sand and gravel). Thus, sandy media (which have a
hydraulic conductivity of about 2 ft/day or greater) promotes groundwater
reaeration, which is favorable to both the dispersion and biodegradation of
contaminants. However, high permeability also promotes faster migration of
contaminants, which could result in more rapid and severe groundwater impacts.
Clays and silts on the other hand, which due to their high sorptive capacities
(owing to both small particle size and higher organic matter content), typically
result in slower migration (i.e., retardation) of contaminants and less degradation
than that observed in more permeable soils.
Groundwater Seepage Velocity
Dispersion and migration of contaminants increases with increasing
groundwater flow rate. True groundwater velocity is referred to as the seepage
velocity. Seepage velocity can be calculated from:
KI
where: q = seepage velocity [L/T]
K = hydraulic conductivity [L/T]
7 = hydraulic gradient [unitless]
ne = effective porosity [unitless]
For a given hydraulic gradient, the higher the hydraulic conductivity the higher
the seepage velocity. Transport of dilute dissolved contaminants is a function of
advection, dispersion, and chemical and physical reactions. Advection refers to the
movement imparted by flowing groundwater, and the rate of transport is usually
taken to be equal to the average linear groundwater velocity. Hydrodynamic
dispersion occurs as a result of molecular diffusion and mechanical mixing and
causes the dissolved contaminant plume to spread out with distance from the
source. Molecular diffusion is generally only significant when groundwater
movement is very slow. Mechanical mixing occurs as groundwater flows through
the aquifer matrix twisting around individual grains and through interconnected
pore spaces at differing velocities. The movement of some dissolved contaminants
may also be affected by chemical and physical reactions, such as sorption and
biodegradation, which act to reduce the transport velocity and decrease
concentrations in the plume.
Classical tracer studies devised to study advection-dispersion phenomena
typically employ a cylindrical column filled with a porous media. A continuous
supply of tracer at a specified concentration is introduced at one end of the column
under steady flow conditions and outflow concentrations are measured at various
times after the tracer is injected. A graph of the outflow concentration with time is
IX-40 May 2004
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known as a breakthrough curve. Such a graph shows concentrations gradually
increasing with time. The inflection point of this curve represents the arrival time
of an undiluted slug of contaminant moving at the average linear groundwater
velocity. There are two problems with the comparison of true contaminant
transport and an undiluted slug. First, due to the presence of the porous media,
slug (or plug) flow is impossible. Even at a relatively small scale (such as these
cylindrical columns) the "plume" of tracer would be dispersed with distance in the
column due to molecular diffusion and mechanical. Second, some of the tracer
molecules are moving faster than the average linear groundwater velocity, and
some are moving slower. This is also true for the water molecules although the
velocity of individual water molecules is never measured. A common
misconception is, thus, that due to dispersion, contaminants may move faster than
groundwater. A correct statement is that some contaminants may move faster than
the average linear velocity of the groundwater. This distinction is very important.
It also leads to another important realization, which is that if some contaminant
molecules are traveling faster than the average linear groundwater velocity, then
the maximum linear groundwater velocity rather than the average linear
groundwater velocity should be used to calculate how long (or short) a time it will
take contaminants to first reach a receptor.
Sorption and Retardation
As previously discussed in the soil contamination section, the organic carbon
partition coefficient (Koc) is an approximation of the propensity of a compound to
sorb to organic matter found in the soil. The sorption coefficient (Kd) value is an
expression of the tendency of a contaminant to remain sorbed on soil and is the
product of Koc. and the fraction organic carbon (fol) in the soil. Sorption tends to
slow the transport velocity of contaminants dissolved in groundwater. When the
average velocity of a dissolved contaminant is less than the average seepage
velocity of the groundwater, the contaminant is said to be retarded. The coefficient
of retardation, R, is used to "correct" the contaminant transport velocity. Under
conditions where sorption is adequately described by Kd, (which is when the
fraction of organic carbon is greater than 0.001), the retardation coefficient can be
determined from:
where: R = coefficient of retardation [dimensionless]
p, = bulk density of soil in the aquifer [M/L3]
Kd = distribution coefficient [L3/M]
n = porosity [dimensionless]
Typical retardation coefficients for various organic compounds and different
organic carbon content are given in Exhibit IX-19.
May 2004 IX-41
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Exhibit
Retardation Coefficients
Compounds And Different
Contamin
ant log (K0 c)
MTBE 1.08
Benzene 1.58
Ethylbenze 1.98
ne
Toluene 2.13
Xylene 2.38
(mixed)
Fraction
0.0001
(low for
aquifers)
1.0
1.0
1.0
1.1
1.1
IX-19
For Different Organic
Organic Carbon Content
of Total Organic
0.001
(median for
aquifers)
1.1
1.2
1.5
1.7
2.2
Carbon (foc)
0.01
(high for
aquifers)
1.6
2.9
5.7
7.6
13
in Soil
0.1
(typical of
soils)
7
20
48
68
120
Source: Wiedeimeier, et al, 1999, Table 3-4, p. 145.
Retarded Contaminant Transport Velocity
As mentioned in the preceding section, sorption tends to slow the velocity of
contaminants in a plume, but not the seepage velocity of the groundwater itself. To
"correct" for the effect of sorption, the coefficient of retardation is used to adjust
the groundwater seepage velocity:
where:
R
= contaminant velocity [L/T]
= groundwater seepage velocity [L/T]
= coefficient of retardation
From the retardation equation in the preceding section, when the distribution
coefficient (Kd) is equal to zero (which means there is no sorption effect), then the
coefficient of retardation is equal to unity and the contaminant velocity (qc) is equal
to the seepage velocity (qs). As the value of Kd increases, R also increases, and the
contaminant velocity becomes more retarded (i.e., decreases).
Another method that is commonly used to determine retarded contaminant
transport velocity is to divide the measured length of the contaminant plume by it's
known age. The advantage to this method is that the transport velocity is basec -m
actual field data, and is therefore, site-specific. The danger inherent in this meihod
is underestimation of the true transport velocity which leads to overestimation of
the rate of biodegradation. This can occur if the measured length of the plume is
shorter than the actual length of the plume. Such an underestimation of plume
length is a common consequence of relying on "conventional" monitoring wells
IX-42
May 2004
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(rather than nested wells arrayed in transects) for delineation of the leading edge
(or "toe") of the plume.
PrecipitationlRecharge
Recharge from precipitation can also cause contaminant plumes (even those
comprised of contaminants that are less dense than water) to "dive" below the level
of the water table. The plume migrates deeper and deeper with increasing distance
from the source. As a consequence, the plume may migrate undetected below the
screened intervals of shallow monitoring wells. Note that this phenomenon does
not require a downward vertical gradient. It is a consequence of a layer of fresh
water accumulating on top of the contaminant plume so gently that significant
mixing does not occur (there will be some diffusion from the plume into the
overlying clean water, but this is a very slow process). This is one of the primary
reasons why nested, or multi-level, wells are absolutely required for an adequate
site characterization. Even for typical less-dense than water contaminants such as
BTEX, plume diving is a common phenomenon. In areas where much of the
ground surface is covered with an impervious layer such as concrete or asphalt,
actual recharge (especially in the source area) may be only a fraction of the total
amount of annual rainfall. This may slow down the process of leaching
contaminants from the source mass causing it to linger as slow, but relatively
steady, source of groundwater contaminants for an extended period of time.
Geochemical Parameters
Biodegradation of organic compounds results in measurable changes in the
chemistry of the groundwater in the affected area. By measuring the temporal and
spatial distribution of these chemical changes, it is possible to document and
evaluate the extent to which natural attenuation processes are occurring. Isopleth
(or isoconcentration) maps should be prepared for all contaminants of concern as
well as each of the geochemical parameters discussed in this section. These maps
will aide in the qualitative interpretation of data on the distribution and relative
transport and degradation rates of the contaminants of concern. There are three
general groups of chemical changes: electron acceptors, metabolic byproducts, and
daughter products.
Electron acceptors are elements or compounds that occur in relatively oxidized
states and include dissolved oxygen, nitrate, ferric iron, manganic manganese,
hydroxide, sulfate, and carbon dioxide. These compounds are reduced through
coupled oxidation and reduction reactions during microbial respiration to yield
energy to the microorganisms for growth and activity.
Dissolved oxygen is typically the first electron acceptor to be utilized during
the biodegradation of many organic compounds, including constituents of
petroleum hydrocarbon fuels. As a consequence, the concentration decreases and
dissolved oxygen concentrations below background levels indicate aerobic
biodegradation is occurring. After dissolved oxygen concentrations in the aquifer
fall below about 0.5 mg/L, anaerobic processes (initially denitrification) will begin
if sufficient anaerobic electron acceptors are present. It is extremely difficult to get
an accurate measurement of dissolved oxygen concentration. Several factors
influence the aqueous solubility of dissolved oxygen including temperature. Other
factors that can influence a reading include the instrument itself (the design,
calibration, maintenance, and operation) and the sample collection technique (it is
very easy to oxygenate a sample, yielding a falsely high level of dissolved oxygen).
In spite of these difficulties, it is extremely important to collect groundwater
May 2004 IX-43
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samples for dissolved oxygen measurements as the difference between background
concentrations and concentrations within the contaminant plume can be used to
estimate the mass of contaminants that are aerobically biodegraded.
After dissolved oxygen has been depleted, biodegradation shifts from aerobic
to anaerobic. The first anaerobic electron acceptor that may be utilized is nitrate by
the process of denitrification. In the zone where denitrification is occurring, nitrate
levels are lower than background. As with dissolved oxygen, the difference
between levels within and outside the plume can be used to estimate the mass of
contaminants being degraded by denitrification. The next electron acceptors to be
oxidized under anaerobic conditions are manganic manganese, ferric iron, and
sulfate. The final step in the anaerobic biodegradation series is methanogenesis,
which utilizes carbon dioxide as the electron acceptor. As with nitrate (and
dissolved oxygen before it), the difference between concentrations of these electron
acceptors within and outside the plume can be used to estimate the mass of
contaminants that are being degraded by each of these processes.
The sum of the estimated mass of degraded contaminants from all processes
(both aerobic and anaerobic) can be used to provide an estimate of the
biodegradative capacity of the subsurface system. Note that it is important to go
through the exercise each time that samples are collected because natural processes
are dynamic and even subtle changes can affect the rate and completeness of
biodegradation. Such changes, if caught in time, will allow for contingency
measures to be implemented should MNA prove not to be protective over the long
period of time required to meet remediation objectives.
The second group of indicators of biodegradation are the metabolic byproducts.
Each of the biodegradation processes mentioned above reduces an oxidized
electron acceptor resulting in generation of measurable reduced species. The
oxidation/reduction (redox) potential of groundwater is a measure of electron
activity and is an indicator of the relative tendency of a solution to accept or
transfer electrons. Because redox reactions in groundwater are biologically
mediated, the rates of biodegradation both influence and depend on redox
potential. Many biological processes operate only within a prescribed range of
redox conditions. The oxidation-reduction (redox) potential of the groundwater
changes, with conditions becoming more reducing, through the sequence oxygen,
nitrate, iron, manganese, sulfate, and carbonate. The redox potential of
groundwater generally ranges from 800 millivolts to about -400 millivolts (Exhibit
IX-20). The lower the redox potential, the more reducing and anaerobic the
environment. Although the redox potential cannot be used for quantitative
interpretation, the approximate location of the fuel hydrocarbon plume can be
identified in the field through measurement of redox potential if background
organic carbon concentrations are low. NOTE: field measurements will likely not
be in the same units as indicated in Exhibit IX-20.
Each biodegradation process is also associated with a characteristic hydrogen
concentration. By carefully measuring dissolved hydrogen concentrations, it is
possible to distinguish among the various anaerobic zones. This level of detail is
especially important at sites with chlorinated solvents, and less important for
petroleum fuel hydrocarbon sites. Aerobic respiration, denitrification, iron and
manganese reduction, and sulfate reduction result in generation of carbon dioxide.
Though it is difficult to obtain an accurate measure of dissolved carbon dioxide
because of carbonate in the groundwater, elevated levels of carbon dioxide relative
IX-44 May 2004
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Exhibit IX-20
Redox Potentials For Various Electron Acceptors
Redox Potential (En°) in Millivolts @ pH = T = 25 C
c o
§ ®
O LJJ
_§ o>
< c
'
CO T3
0) 0)
' W
CD
O
or
1000
Aerobic
Anaerobic
500
0-
-500
- O. + 4H* + 4e 2H,O (E° = 820)
- 2NO, + 12H' + 10e" N2 + 6H2O (E° = +740)
s) + HCO, + 3H* + 2e MnCO,(s) + 2H2O
(E° = +520)
. FeOOH(s) + HCOf + 2H* + e~
SO4Z + 9H' + 8e" HS + 4H2O
' CO2 + 8H" + Be" CH. + 2H2O
FeCO3 + 2H2O
<^° = -50)
(Er° = -220)
(Ef = -240)
Source: Modified from Norn's et a/., (1994)
to background may be observed and it is possible to estimate the degree of
microbiological activity. Another consequence of carbon dioxide production is an
increase in alkalinity. Alkalinity is important in the maintenance of groundwater
pH because it buffers the groundwater system against acids produced during
aerobic and anaerobic biodegradation. Measurement of dissolved inorganic carbon
provides sufficient information to calculate alkalinity and CO2. The reduction of
oxidized forms of iron and manganese (Fe3+ and Mn4+, respectively) results in the
production of reduced species which are water soluble. Elevated levels of these
reduced metals (Fe2+ and Mn2+, respectively) in the plume relative to background is
indicative of anaerobic biodegradation. Hydrogen sulfide is produced during sulfate
reduction. Methane is produced by methanogenesis, which occurs only under
strongly reducing conditions.
The third group of chemical indicators is daughter products. For most
petroleum hydrocarbons daughter products are not significant. For MTBE,
however, one of the intermediate degradation products is tertiary-butyl alcohol
(TEA) which is more difficult to remediate than MTBE itself, and more toxic.
However, TBA is also used as a fuel oxygenate in its own right, as well as an
impurity in MTBE. Some conventional analytical techniques actually degrade
MTBE and form TBA during sample analysis. When this occurs, obviously the
analytical results are not representative of what's occurring in the subsurface. So,
while the presence of TBA is of concern (and should be appropriately remediated)
it does not necessarily indicate the biodegradation of MTBE and concentration data
should not be used to establish biodegradation rates for MTBE-the estimated rate
May 2004 IX-45
-------
will likely he higher than what is actually occurring. Some of the daughter
products of chlorinated solvents (particularly vinyl chloride) are of significant
concern because of their toxicity.
Rate Constants and Degradation Rates
Rate constants for biodegradation or for the rate of bulk attenuation of
contaminants in groundwater can be used to estimate how far a plume may extend.
In some cases these rates can be incorporated into computer models, and the
models can be compared to the existing distribution of contamination to determine
if a plume is expanding or receding. However, they can not be used to estimate
how long a plume will persist in the absence of source control. For most plumes,
the rate of attenuation in ground water is faster than the rate of attenuation of the
source. As a consequence, the persistence of the plume is controlled by the rate of
attenuation of the source, and the rate of attenuation of the source must be
understood to be able predict the time required to achieve remediation objectives.
A decision on whether or not MNA is an appropriate remedy for a given site is
usually based on estimates of the rates of natural attenuation processes, and
biodegradation rates in particular, for most LUST sites. Biodegradation reactions
involving organic chemicals occur at rates which are a function of various site-
specific environmental conditions. Quantifying the rate of biodegradation is
important for biologically-mediated remediation alternatives, and especially MNA,
since this rate is used to estimate the time required to achieve remediation
objectives. It is important to note, however, that there are different types of rate
calculations and it is imperative to use the constant that is appropriate for the given
situation or the resultant "answer" will be incorrect. Biodegradation rate constants
generally fall into three categories:
• concentration vs. time attenuation rate constant: the rate constant, in units of
inverse time (e.g., per day, time"1), is equal to the slope of the line plotted as
natural log of concentration vs. time measured at a selected monitoring
location. This constant represents the change in source strength over time and
can be used to estimate the time required to reach a remediation goal.
Concentration vs. time constants provide information regarding potential
source persistence at a single location only-they cannot be used to evaluate
distribution of contaminant mass within the source area.
• concentration vs. distance attenuation rate constant: the rate constant, in units
of inverse time (e.g., per day, time"1), is derived by plotting the natural log of
concentration vs. distance, and (only if the data follow a first-order decay
pattern) calculating the rate as the product of the slope of the line and the
groundwater seepage velocity. Plots of concentration vs. distance serve to
characterize the distribution of contaminant mass within space at a given point
in time, but a single plot yields no information about the variation in
concentration over time. These constants cannot be used to estimate the time
required to meet a remediation goal. They indicate how quickly contaminants
are attenuated (e.g., accounting for sorption, dispersion, and biodegradation)
once they leave the source area, but provide no information on how quickly a
residual source zone is being attenuated. Because most LUST sites will, to
some degree, have a lingering residual source (despite best efforts to
completely recover free product), these constants are inappropriate for
estimating plume longevity for most sites.
IX-46 May 2004
-------
biodegradation rate constant: the rate constant is denoted by the Greek lambda
(D) and is in units of inverse time (e.g., per day, time"1). It can be derived in a
variety of ways, including field tests and computer model simulations. The
biodegradation rate constant is NOTthz same as the concentration vs. distance
attenuation rate constant since the latter reflects the combined effects of
sorption, dispersion, and biodegradation. The biodegradation rate constant can
be used to provide information on plume stability using models, but it cannot
be used for estimating remediation time frames.
There are three commonly used models which describe the biodegradation of
organic compounds in groundwater: (1) first-order decay, (2) Monod kinetics, and
(3) "instantaneous reaction". Perhaps the most commonly used approach is to
make the assumption that the biodegradation rate can be approximated using a
first-order decay equation of the form:
C/-r -kt
— C • f^
where:
C = biodegraded contaminant concentration
Q) = initial contaminant concentration
k = rate of decrease of contaminant (time"1)
t = time of interest
To estimate the time required to achieve a specific clean up goal, the above
equation is rearranged to solve for t as follows:
ln(C/C0)
-k
In this configuration, C is the clean up goal concentration (or regulatory
maximum allowable concentration), and C0 is the most recent measured
concentration. Note that if k is in units of "per day" (d"1), then / will also be in
days.
The first order decay model assumes that the solute degradation rate is
proportional to the solute concentration. The higher the concentration, the higher
the degradation rate. The primary advantage of this approach is that for many
organic chemicals, k has been determined from laboratory experiments. The
weaknesses of the model are that it does not account for site-specific information
such as the availability of electron acceptors, and there is often considerable
uncertainty in extrapolating laboratory constants to the field environment. In fact,
there is substantial evidence that the first-order model may overestimate the
amount of aerobic biodegradation of petroleum hydrocarbons. Under no
circumstances laboratory-derived attenuation rates be used as the sole justification
for selecting an MNA remedy, evaluating the length of time required to meet
remedial objectives, or in deciding to terminate long-term performance monitoring.
One final advantage of using the first-order model is that first-order rate
constants may easily be converted to half-lives (tK) since they are inversely related
to one another:
_ 0.693
May 2004 IX-47
-------
A more complex, and more accurate, model is the Monod kinetic model which
is also referred to as the Michaelis-Menten kinetic model. This model is the
hyperbolic saturation function and, for calculating the reduction in contaminant
concentration, has the form:
where: C = contaminant concentration
Mt = total microbial concentration
U = maximum contaminant utilization rate per unit mass
"max . . r
microorganisms
Kc = half-saturation constant
A? = time interval of interest
This model is actually quite complex; the graph of this rate equation has
regions that are zero-order, first-order, and mixed-order. The rate constant
accounts for both the activity of the degrading population and the dependence of
the reaction on the substrate concentration. Although this model may be the most
accurate of the three models, the difficulty in estimating //max and Kc generally
preclude its use under field conditions.
The "instantaneous reaction model" is also known as the electron-acceptor-
limited model, and is used for simulating the aerobic biodegradation of petroleum
hydrocarbons. The basis for this model is the observation that microbial
biodegradation kinetics are fast in comparison with the transport of oxygen. The
model assumes that the rate of utilization of the contaminant and oxygen by the
microorganisms is very high, and that the time required to biodegrade the
contaminant is very short (almost instantaneous) relative to the seepage velocity of
the groundwater. The equation for the instantaneous reaction model using oxygen
as the electron acceptor is:
where: A CR = change in contaminant concentration due to biodegradation
O = concentration of oxygen in groundwater
F = utilization factor, the ratio of oxygen to contaminant
consumed
The primary advantages of the instantaneous reaction model is that kinetic dak-
are not required, because reactions are not limited by microbial kinetics. The
model is, however, not applicable in all circumstances. Its applicability is limited
to situations in which microbial biodegradation kinetics are fast relative to the rate
of the groundwater flow that mixes electron acceptors with dissolved
contaminants. There is increasing evidence that anaerobic biodegradation of
petroleum hydrocarbons can be simulated using the assumption of instantaneous
reactions (Wiedemeier, et al, 1999).
IX-48 May 2004
-------
Plume Migration
In determining whether a plume is shrinking, "stable" or migrating, the
uncertainty associated with defining the limits of contaminant plumes should be
considered. For example, a plume is typically delineated for each contaminant of
concern as a 2- or 3-dimensional feature. Plumes are commonly drawn either by
hand or computer contouring programs which estimate concentrations between
actual data points. In reality, a plume boundary is defined by a zone rather than a li
ne. Fluctuations within this zone are likely to occur due to a number of factors
(e.g., analytical, seasonal, spatial, etc.) which may or may not be indicative of a
trend in plume migration. Therefore, site characterization activities and
performance monitoring should focus on collection of data of sufficient quality and
quantity to enable decisions to be made with a high degree of confidence. The only
appropriate sites for a MNA remedy, therefore, are those where the plume can be
statistically demonstrated to be shrinking. (See footnote #4, p.IX-19.)
Time Frame to Achieve Remediation Objectives
As with any remediation method, one of the fundamental questions that arises
is "How much time will be required before remediation objectives are achieved?"
At the current state of practice, the only practical approach available uses a
statistical analysis of long term monitoring data from wells in the source area of the
contaminant plume.
As an example of this approach, we'll use data presented by Kolhatkar et al.
(2000). They collected long-term groundwater monitoring data from three wells at
a gasoline release site in New Jersey. Their original data displayed extreme
oscillations bouncing up and down from less than 1 ug/L to a high value and back
over a single sampling interval. Although the scatter in the data set is typical of the
variation seen at many other sites, the influence of these outliers on the statistical
estimate of the rate of attenuation was removed by editing the data set to remove
those points where the concentration of MTBE was less than 1 ug/L. These edited
data are tabulated as Exhibit IX-21 and presented graphically as Exhibit IX-22.
The first order rate constant for attenuation was extracted from the data by
taking the natural logarithm of the concentrations of MTBE in each well at each
date and then, for each well, performing a linear regression of the natural logarithm
of concentration on the time when the sample was collected. The slope of the
regression for each well is the instantaneous rate of change of concentration of
MTBE with time. The slope is the negative of the first order rate constant for
attenuation. The rates calculated from the data in Exhibits IX-21 and XI-22 are
presented hi Exhibit IX-23. For purposes of illustration, the concentration at the
last time of sampling and the rate constants were used to forecast the time required
to reach a cleanup goal of 20 (ig/liter.
Because there is natural scatter in the long-term monitoring data, there is
uncertainty in the estimate of the rate of natural attenuation, in the projected time
frame to achieve clean up. To account for this uncertainty, a confidence interval
was calculated for each estimate of the rate of attenuation at a pre-determined level
of confidence of 90% and 95% (Exhibit IX-23). The level of confidence is simply
the probability that the true rate is contained within the calculated confidence
May 2004 IX-49
-------
MTBE Concentration
Date
9/17/93
9/23/94
5/17/96
8/10/96
11/7/96
12/8/97
3/27/98
7/23/98
9/18/98
12/16/98
3/1/99
6/21/99
9/7/99
9/7/99
12/30/99
3/20/00
6/22/00
MW-5
Concentration
(Ppb)
1,900
1,800
1,300
980
620
500
635
470
1,210
379
700
574
792
1,050
525
501
420
Exhibit
Measured
IX-21
In Monitoring
MW-6
Concentration
Date (ppb)
9/17/93
9/23/94
5/17/96
8/10/96
11/7/96
3/27/98
9/18/98
3/1/99
9/7/99
3/20/00
6/22/00
270
200
120
120
66
71.2
44
42.2
43.2
36
51.2
Wells Over
Time
MW-ll
Date
9/23/94
5/17/96
1 1/7/96
12/8/97
3/27/98
7/23/98
12/16/98
3/1/99
6/21/99
9/7/99
9/7/99
12/30/99
3/20/00
6/22/00
Concentrati
on
(ppb)
2200
880
660
339
426
419
144
123
464
195
155
220
173
146
interval. Given the need to protect human health and the environment, and the
absence of an active remediation system to serve as a fail-safe, a 90% confidence
level is a reasonable level of confidence for many sites. At other sites a more
stringent confidence level (e.g. 95%) may be more appropriate, depending the level
of risk that is acceptable.
In most applications of regression the user wishes to calculate both an upper
boundary and lower boundary on the confidence interval that will contain the true
rate at the pre-determined level of confidence. This is termed a "two tailed"
confidence interval because the possibility of error (the tail of the probability
frequency distribution) is distributed between rates above the upper boundary and
IX-50
May 2004
-------
Exhibit IX-22
MTBE Concentration Measured In Monitoring Wells Over Time
2500
2000 i
B 1500
1
pa
1000
500
0
Jan-93 Jan-94 Jan-95 Jan-96 Jan-97 Jan-98 Jan-99 Jan-00 Jan-01
Date
Exhibit IX-23
Rates Of attenuation Of MTBE In Monitoring Wells And The Projected Time
Required To Reach A Clean Up Goal Of 20 ng/L As Calculated From The Data
Presented In Exhibits IX-21 And IX-22
MTBE (ug/L)
First Last
Sample Sample
Well 1993 2000
MW-5 1900 420
MW-11 2200 146
MW-6 270 51.2
Estimated rate and
time required
Rate
(per Time
year) (years)
0.188 16
0.453 4.4
0.290 3.2
Rate and time
significant at 90%
confidence
Rate
(per Time
year) (years)
0.127 24
0.365 5.4
0.246 3.8
Rate and time
significant at 95%
confidence
Rate
(per Time
year) (years)
0.109 28
0.337 5.9
0.231 3.8
May 2004
IX-51
-------
below the lower boundary of the confidence interval. As a consequence, tables of
critical values in statistical reference books and computer applications provide a
"two-tailed" confidence interval. At a 80% level of confidence, the estimate will be
in error 20% of the time. The true rate will be contained within the calculated
confidence interval 80% of the time, 10% of the time the true rate will be faster
than the upper boundary of the confidence interval, and 10% of the time the true
rate will be slower than the lower boundary of the confidence interval. Using the
data in Exhibit IX-21 for MW-5, the slope of a regression of the natural logarithm
of concentration of MTBE on time is -0.188 per year. The first order rate of
change of concentration of MTBE on time is -0.188 per year, corresponding to a
rate of attenuation of+0.188 per year. The boundaries of the "two tailed"
confidence interval on the rate at 80% confidence are 0.248 per year and 0.127 per
year. This means that 80% of the time the true rate will be between 0.248 and
0.127 per year, that 10% of the time the true rate is greater than 0.248 per year, and
10% of the time the true rate is less than 0.127 per year. The true rate will be
greater than 0.127 per year 90% of the time.
Long-term monitoring data at many sites typically exhibits a great deal of
variation. These variations are not necessarily errors in sampling and analysis of
groundwater samples. In many cases they reflect real changes in the plume caused
by seasonal variations in precipitation and groundwater elevations. These
variations are a natural property of the plume. Where long-term monitoring data
define a statistically significant trend of increasing contaminant concentrations,
such sites are not appropriate candidates for MNA. Where the long-term
monitoring data exhibit a statistically significant trend of decreasing
concentrations, such sites may be appropriate for MNA. If no trend is discernible,
then additional data should be collected over time. If the variation is large enough,
one boundary of the "two tailed" confidence interval will be a positive number and
the other boundary will be a negative number. When zero is included in the
confidence interval on the rate, there is no evidence in the data that the true rate is
different from zero. If this is the case it is possible that attenuation is occurring in
that particular well over time, but the monitoring data do not present evidence that
attenuation is occurring at the predetermined level of confidence. The variation in
the monitoring data is too great to determine the trend over time one way or the
other. Again, there is no appropriate role for MNA at these sites, because it is
impossible to predict how long it will take to reach the clean-up goals.
There is little value in estimating the shortest possible time that would be
required to reach the goals for clean up; remedial options are compared and
evaluated based on the greatest time required to reach goals. At the selected level
of confidence, all the possibility of error should be assigned to rates that are slower
that the lower boundary of the confidence interval. This is a "one-tailed"
confidence level; it includes all true rates that are faster than the lower boundary of
the confidence interval. A "one tailed" confidence interval can be calculated as
the slower of the two confidence intervals from a "two-tailed" test that has twice
the uncertainty. In the example above, where "two tailed" confidence intervals
were calculated for a confidence level of 80%, the true rate will be greater than a
rate of 0.127 per year 90% of the time. The "one tailed" confidence intervals
reported in Exhibit IX-23 were calculated in this fashion.
Note that for a given number of observations, as the level of confidence is
increased, the interval that is expected to contain the real value for the rate constant
increases as well. As the level of confidence increases, the lower boundary on the
rate constant decreases, and the projected time required to meet the clean up goal
IX-52 May 2004
-------
increases. In the examples presented in Exhibit IX-23, the estimated rate of natural
attenuation of MTBE in MW-5 is 0.188 per year, which requires 16 years to attain
a concentration of 20 ug/L. At a 90% confidence level, the lower boundary of the
confidence interval is 0.127 per year, which requires 24 years to meet the goal. At
a 95% confidence level, the lower boundary is 0.109 per year, which requires 28
years to reach the goal. At the 95% confidence level the upper bound of the time
expected to reach the clean up goal has increased by a factor of almost two (from
16 years to 28 years). This does not necessarily mean that the actual time to
achieve cleanup will be 28 years; it simply means that the length of time that will
actually be required is estimated to be no more than 28 years at a 95% level of
confidence.
The ability to extract a rate of attenuation from long term monitoring data is
related to the number of measurements, and the time interval over which they are
collected. As an example, the rate of attenuation extracted from the last three years
of monitoring data for well MW-5 (3/27/98 to 6/22/2000 in Exhibit IX-21 and IX-
22) is 0.106 per year, but the "one tailed" 90% confidence interval is all rates
greater than -0.125 per year. The confidence interval includes zero. If only these
three years of data were available, there would be no evidence of natural
attenuation of MTBE in well MW-5 at 90% confidence. The rate extracted from
the last four years of data (5/17/1996 to 6/22/2000) is 0.130 per year. The 90%
confidence interval on the rate (0.0302 per year) would reach the clean-up goal in
100 years. As presented in Exhibit IX-23, the rate extracted using all the seven
years of monitoring data is 0.188 per year. The 90% confidence interval on the
rate would reach cleanup in 24 years. A few extra years of monitoring data have a
strong influence on the ability to extract useful rate constants.
Rate constants for natural attenuation can be used to project the time required
to reach a clean-up goal once the source has been adequately addressed. However,
there are a number of key points to keep in mind. First, an appreciable record of
long term monitoring data must be available to make a statistically valid projection
of the rate of natural attenuation. As a practical matter it is difficult to extract rate
constants that are statistically significant with fewer than six sampling dates, or
with a sampling interval of less than three years. Second, it is unrealistic to expect
just a few years of monitoring data to accurately predict plume behavior several
decades into the future. Third, it is important to realize that these estimates are
merely estimates and that the true rate is likely to change over time. Fourth, under
no circumstances should such estimates be used as justification to close a site. Site
closure decisions should be based on actual long term monitoring data, not
predictions. Fifth, monitoring should continue at any given site for a specified
period of time (typically 1 to 2 years or more) after cleanup goals have been
achieved to ensure that contaminant levels do not rebound and exceed the required
cleanup level due to long-term fluctuations in groundwater table elevation or
changes in flux from lingering vadose zone contamination.
After estimating a time to achieve remediation objectives, it is necessary to
evaluate whether or not this time is "reasonable" for a given site. As this is a site-
specific decision, no single generic number can be presented in this chapter. In
general, a "reasonable" time frame is one that is comparable to that which could
be achieved through active remediation (U.S. EPA, 1999). Since there are
typically a variety of potential remediation options for a given site, there is likely to
be more than one estimate of time necessary to achieve remediation options.
Evaluation of the most appropriate time frame must be determined through an
analysis of the various remedy alternatives. Some of the factors that should be
May 2004 IX-53
-------
considered in making a determination as to which time frame (and remediation
alternative) is most appropriate include:
• Classification of the affected resource (e.g., drinking water source, agricultural
water source) and value of the resource;
• Relative time frame in which the affected portions of the aquifer might be
needed for future water supply (including the availability of alternate supplies);
• The stability of ground water flow in the aquifer. How might the plume change
over the extended time frame necessary to achieve remediation objectives;
• Reliability of monitoring and of institutional controls over long time periods;
• Public acceptance of the time frame required to reach remediation objectives;
and
• Provisions by the responsible party for adequate funding of monitoring and
performance evaluation over the time period required to achieve remediation
objectives.
Long-Term Performance Monitoring
The two fundamental objectives of performance monitoring are to verify that:
(1) contaminant levels are decreasing, and (2) contamination is not spreading (i.e.,
the plume is not migrating, but rather is shrinking). Due to the potentially longer
remediation time frames, potential for ongoing contaminant migration, and other
uncertainties associated with using MNA, performance monitoring is of even
greater importance for MNA than for other types of remedies. The monitoring
program developed for each site should specify the location, number, frequency,
and type of samples and measurements necessary to evaluate whether the remedy is
performing as expected and is capable of attaining remediation objectives. The
objectives for all monitoring programs should include the following:
• Demonstrate that natural attenuation is occurring according to expectations;
• Detect changes in environmental conditions (e.g., hydrogeologic, geochemical,
microbiological, or other changes) that may reduce the efficacy of any of the
natural attenuation processes;
• Identify any potentially toxic and/or mobile transformation products;
• Verify that the plume(s) is shrinking;
• Verify no unacceptable impact to downgradient receptors;
• Detect new releases of contaminants to the environment that could impact the
effectiveness of the MNA remedy;
• Verify attainment of remediation objectives.
IX-54 May 2004
-------
The frequency of monitoring should be adequate to detect, in a timely manner,
the potential changes in site conditions listed above. At a minimum, the monitoring
program should be sufficient to enable a determination of the rate(s) of attenuation
and how that rate is changing with time. When determining attenuation rates, the
uncertainty in these estimates and the associated implications should be evaluated
(see McNab and Dooher, 1998). Flexibility for adjusting the monitoring frequency
over the life of the remedy can be included in the monitoring plan. For example, it
may be appropriate to decrease the monitoring frequency at some point in time,
once it has been determined that natural attenuation is progressing as expected or
very little change is observed from one sampling round to the next. In contrast, the
monitoring frequency may need to be increased if unexpected conditions (e.g.,
plume migration) are observed. Exhibit IX-24 is a flowchart that can serve as a
roadmap to guide you in evaluating the long-term performance monitoring plan. A
table summarizing the contaminants to monitor and the suggested monitoring
frequency is presented as Exhibit IX-25, while more specific details are discussed
in the sections that follow.
Performance monitoring should continue until remediation objectives have
been achieved, and generally for a period of 1 to 2 years longer to ensure that
contaminant levels remain below target levels. Under no circumstances should the
results of predictive modeling (including statistical extrapolation) be used to justify
a decision to terminate performance monitoring. This decision should be based
only on adequate field data that convincingly demonstrates that contaminant levels
have met remediation objectives. The institutional and financial mechanisms for
maintaining the performance monitoring program should be clearly established in
the remedy decision or other site documents, as appropriate.
As with the active remediation technologies also described in this manual, if
MNA does not appear to be effective in remediating the contamination at the site
within a reasonable time frame, then an alternative active remedial-technology
(specified in the contingency plan section of the CAP) will be required.
Contaminated Soil
For a given volume of contaminated soil, the objective of sampling is to collect
a minimum number of samples such that, with a satisfactory degree of confidence,
the spatial distribution of contamination is accurately defined. Because this
process will be repeated multiple times in the future, the methodology for selecting
sampling locations and physically collecting the samples must be robust.
MNA is assumed to be effective if both the volume and the mass of
contaminants are lower with each successive sampling event, and that after some
reasonable period of time, contaminant levels fall below (and remain below)
remediation objectives. One of the challenges of routine soil sampling is collecting
sequential samples that can be compared with earlier samples in the series. Soil
sampling is by its nature destructive, so once a discrete sample is collected, another
one cannot be collected from exactly that same point in space. There is an implicit
assumption that a future sample, collected in close proximity to a past sample, will
be close enough so that the analytical results can be compared to determine if
concentrations are decreasing at that location. At a minimum, samples should be
collected from locations where contamination is known to be greatest (i.e., source
area) from previous sampling events. Generally, eight samples per sampling event
should be sufficient to demonstrate whether or not concentrations are decreasing.
May 2004 IX-55
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Exhibit IX-24
Evaluation of Long-Term
Performance Monitoring Plan
Soil
Groundwater
Are
samples to be
collected 1 to 2 years past
when remediation objectives are
anticipated to be
achieved?
Are
samples to be
collected 1 to 2 years
past when remediation objectives
are anticipated to
be achieved?
sample collection
frequency quarterly for
the first two years and then
at least annually
thereafter?
Is sample
collection frequency
at least bi-annually?
Are
a minimum of
3 transverse plume
transects, 1 upgradient transect
and 1 plume centerline transect
plus all sentinel wells,
to be sampled?
Area
sufficient
number of
locations scheduled
to be sampled?
The performance
monitoring plan
is inadequate and
must be revised
prior to approving
MNAasa
remediation
alternative at
this site.
Are
samples to be
analyzed for TPH, BTEX, other
contaminants of concern
and any other relevant
parameters?
Are
samples to be
analyzed for TPH, BTEX,
other contaminants of concern
and any other relevant
parameters?
Are
samples to be
analyzed for dissolved oxygen;
NCV, Fe2*, Mn2*, SO,2', CH,, pH,
redox potential, and dissolved
inorganic carbon?
Are
soil gas samples
to be analyzed for
02, C02, CH4, and
VOCs?
The performance monitoring plan I
is of sufficient scope and frequency!
Vand can be considered complete.^
Evaluation of Long-Term Performance Monitoring Plan
IX-56
May 2004
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Exhibit IX-25
Performance Monitoring Frequency, Analytes, And Sampling Locations
Medium
Soil
Groundwater
Monitoring
Frequency
at least bi-annually
quarterly for the
first two years,
then at least
annually thereafter.
What To Monitor
BTEX; TPH; any
other contaminants of
concern; Soil gas O2
,CO2 and CH4.
BTEX; TPH; any
other contaminants of
concern; D.O., Fe2+,
SO42-,CH4, NO3',Mn2+
pH, and dissolved
inorganic carbon.
Where/Number Of
Samples To Monitor
a statistically significant
number of continuous soil
cores located throughout the
area of contamination.
a minimum of 3
perpendicular transects
through the plume, 1
perpendicular transect up
gradient of the plume, with
multiple depth-discrete
samples collected from each
location, plus all sentinel
wells (if any)
Sampling events should occur at least bi-annually (i.e., every two years) to
demonstrate reductions in contaminant concentrations.
Soil samples should be analyzed for the BTEX contaminants, TPH, and any
other contaminants of concern at the site. If the primary contaminants of concern
at the site are volatile organic chemicals (VOCs), monitoring of soil gas should
supplement direct soil measurements at some locations. In addition, soil gas
samples should be analyzed for oxygen, carbon dioxide, and methane (and
sometimes hydrogen) to determine the microbial activity in the soils. As described
above, reduced oxygen concentrations and elevated carbon dioxide concentrations
(relative to background) in both the source area, and soils overlying the dissolved
plume, are a good indication that biodegradation is occurring.
Contaminated Groundwater
Typically, groundwater monitoring wells are installed during site
characterization activities (and often during active remediation), and, being
permanent fixtures (relative to soil sampling locations) there is not as much
uncertainty about the locations from which to collect groundwater samples (i.e.,
wells) as there is about soil sample collection. The fundamental objectives,
however, are the same: define the extent of contamination in three-dimensions,
and identify trends in concentration levels.
Groundwater monitoring should be designed to ensure that the vertical and
lateral extent of contaminants in groundwater is evaluated. Each distinct flow zone
and geochemical regime should be monitored to assess remediation status. In
general, for each distinct flow zone at the site, the following locations should be
monitored: background, source area, main body of the plume, and the distal
portions and boundaries.
May 2004
IX-57
-------
Typical target zones for monitoring a contaminant plume include:
• Source areas, and within and immediately downgradient of potential source
areas. The monitoring objective is to estimate a source mass which is critical
for determining potential source longevity. These sampling points will also
enable determination of future contaminant releases to the environment.
• Flow zones with highest contaminant concentrations or hydraulic conductivity.
These are the zones where maintenance of a steady state or shrinking plume is
a primary concern. A change in conditions in these zones may lead to a
relatively rapid impact to a down-gradient receptor.
• Distal or fringe portions of the plume. These are areas where reductions of
contaminants to levels required by remedial action objectives (e.g., site-specific
cleanup targets) may be attained most rapidly and where increases in
concentrations that indicate impending plume expansion may be observed.
• Plume boundaries. Multi-level monitoring points should be placed at the side
gradient, downgradient, and vertical plume boundaries, and between these
boundaries and potential receptors. Results from these monitoring locations
may directly demonstrate any unacceptable plume expansion.
• Zones in which contaminant reduction appears to be recalcitrant. These are
the areas where attaining cleanup targets within reasonable time frames may be
impeded due to site conditions (e.g., presence of residual source materials, low
flux of electron receptors). Such areas, if present, will be determined through
data obtained throughout the performance monitoring period. These areas may
require additional characterization and remedial actions to reduce contaminant
concentrations to desired levels.
• Background locations. Background locations include monitoring points that
are hydraulically up gradient and side gradient with respect to the plume.
Multiple monitoring points should be used to determine the variability of
background conditions. Data concerning the movement of electron receptors,
donors, and any contaminants into the plume are required to interpret data from
the plume. Background geochemical data is used to determine whether the
observed differences in geochemical parameter concentrations within the
plume are due to contaminant transformation processes rather than natural
variations. Changes in geochemistry within the plume may not be directly
related to attenuation of the contaminants, so geochemical changes outside the
plume should be assessed and compared to geochemical changes taking place
within the plume. If up gradient and lateral monitoring points show
geochemical changes similar to changes in the plume, such changes may not be
attributable solely to contaminant-related processes (i.e., degradation), and
therefore may not serve as supporting evidence for degradation processes.
Another type of well that should be monitored on a regular basis is a sentinel
well. This is a well that is located between the leading downgradient edge of the
dissolved plume and a receptor (e.g., a drinking water supply well). A sentinel
well(s) should be located far enough up gradient of any receptor to allow enough
time before the contamination arrives at the receptor to initiate other measures to
prevent contamination from reaching the receptor, or in the case of a supply well,
provide for an alternative water source. A contaminated sentinel well provides an
early warning that the plume is migrating. For those responsible for site
IX-58 May 2004
-------
remediation, this is a signal that MNA is not occurring at an acceptable rate and the
contingency remedy should be implemented. For the downgradient well users, an
alternate supply of water may be required.
In order to demonstrate that MNA is occurring, a sufficient number of
monitoring wells that are appropriately located (both horizontally and vertically)
are necessary. The density of sampling points will depend on site geology and
hydrology, the overall size of the contaminant plume and the spatial scales at
which contamination distribution varies horizontally, vertically, and temporally,
and the desired level of confidence in the evaluation. Plumes vary significantly in
concentration laterally and in vertical cross-section, making evaluation of
contamination distribution and remedy performance difficult. Therefore, a dense
network of multi-level monitoring points is required.
The recommended approach is to construct monitoring points that are
positioned in transects both in the direction of groundwater flow as well as
perpendicular to it (see Exhibit IX-26 for an optimal network design). The
horizontal and vertical spacing of the monitoring clusters in each transect is
determined by the scale of the hydrogeological heterogeneities that control
contaminant transport and the dimension and spatial heterogeneity of the resulting
contaminant distribution. The horizontal distance between transects is generally
based on changes in contaminant concentration along the plume, and the location
of the source and distal portions of the plume. The use of a transect-based
approach to monitoring will greatly reduce the uncertainty in performance
monitoring evaluations at sites by improving the definition of contaminant
distribution and variability in three-dimensions. Transects also provide a better
definition of contaminant distribution under conditions of changing hydraulic
gradients. With reference to Exhibit IX-26, recommended transects would be as
follows:
• source zone: B1 through B3
• mid-plume (transverse to flow): either Cl through C5, or Dl through D5
• plume toe: El through E4
• up gradient: Al and A2
• plume centerline: B2-C3-D3-E3
Groundwater monitoring should be conducted no less than quarterly during the
first two years to allow for determination of seasonal variation. Some sites may
require quarterly (or more frequent) sampling for more than two years in order to
establish a statistically significant trend. Thereafter, sampling frequency might
then be reduced depending upon contaminant travel times and other site-specific
factors (e.g., travel time to nearest receptor). At a minimum, groundwater sampling
should be conducted on an annual basis after the first two years.
Groundwater samples should be analyzed for VOCs and other contaminants of
concern, TPH (near the source area), dissolved oxygen, pH, temperature, redox
potential, alkalinity, hardness, and other geochemical indicators as indicated in
Exhibit IX-25. Isopleth (or isoconcentration) maps should be prepared for all
contaminants of concern as well as each geochemical parameter. These maps will
aide in the qualitative interpretation of data on the distribution and relative
transport and degradation rates of the contaminants of concern.
May 2004 EX-59
-------
Exhibit IX-26
Example of Optimal Groundwater Sampling Network Design
for Performance Monitoring
Note: Figure not to scale.
Source Area
e
E4
(A) Plan view of Optimal Groundwater Monitoring Network
B
Flow
F3
F1
(B) Longitudinal Cross-Section of Optimal Groundwater Monitoring Network
IX-60
May 2004
-------
Exhibit IX-26
(continued)
(C) Transverse Cross-section of Optimal Groundwater Monitoring Network at Transect "C1
D
(D) Transverse Cross-section of Optimal Groundwaler Monitoring Network at Transect '!>"
Note: Figure not to scale.
Contingency Plan
A contingency remedy is a cleanup technology or approach specified in the site
remedy decision document that functions as a "backup" remedy in the event that
the selected remedy (in this case MNA) fails to perform as anticipated. A
contingency remedy may specify a technology (or technologies) that is (are)
different from the selected remedy, or it may simply call for modification of the
selected technology, if needed. Contingency remedies should generally be
flexible—allowing for the incorporation of new information about site risks and
technologies. It is also recommended that one or more criteria ("triggers") be
established, as appropriate, in the remedy decision document that will signal
unacceptable performance of the selected remedy and indicate when to implement
May 2004
IX-61
-------
contingency remedies. In establishing triggers or contingency remedies, however,
care is needed to ensure that sampling variability or seasonal fluctuations do not
unnecessarily trigger a contingency.
Contaminated Soil
Trigger criteria for contaminated soil should generally include, but not be
limited to, the following:
• Contaminant concentrations in soil that are not decreasing as originally
predicted during remedy selection;
• Migration of vapors into nearby structures (e.g., sewers, basements);
• Near-source samples show large concentration increases indicative of a new or
renewed release; and
• Changes in land use that might result in exposure.
Potential contingency remedies which are documented in other chapters of this
guidance manual are: Thermal Desorption (Chapter VI), Land Farming (Chapter
V), Biopiles (Chapter IV), SVE (Chapter II), Bioventing (Chapter III), Enhanced
Aerobic Bioremediation (Chapter XII), and Chemical Oxidation (Chapter XIII).
Contaminated Groundwater
Trigger criteria for contaminated groundwater should generally include, but not
be limited to, the following:
• Increasing contaminant concentrations in groundwater or the appearance of free
product in monitoring wells;
• Near-source wells exhibit large concentration increases indicative of a new or
renewed release;
• Contaminants are identified in monitoring wells located outside of the original
plume boundary;
• Impacts to nearby receptors (especially wells) indicating that MNA is not
protective;
• Contaminant concentrations are not decreasing at a sufficiently rapid rate to
meet the remediation objectives;
• Concentrations of geochemical parameters are changing such that they indicate
a declining capacity to support biodegradation of contaminants; and
• Changes in land and/or groundwater use will adversely affect the
protectiveness of the MNA remedy.
Potential contingency remedies which are documented in other chapters of this
guidance manual are: Air Sparging (Chapter VII), Biosparging (Chapter VIII), In-
Situ Groundwater Bioremediation (Chapter X), Dual-Phase Extraction (Chapter
IX-62 May 2004
-------
XI), Enhanced Aerobic Bioremediation (Chapter XII), and Chemical Oxidation
(Chapter XIII).
References
American Petroleum Institute. 1996. A Guide to the Assessment and Remediation
of Under ground Petroleum Releases, API Publication 162, 3rd Edition.
Bedient, P.B., H.S. Rifai, and C.J. Newell. 1994. Groundwater Contamination:
Transport and Remediation. Englewood Cliffs, NJ: PTR Prentice Hall.
Boulding, J.R. 1994. Description and Sampling of Contaminated Soils: A Field
Guide. 2nd, Ed., Boca Raton, FL: Lewis Publishers, by CRC Press.
Hinchee, R.E. and S.K. Ong. 1992. A rapid in situ respiration test for measuring
aerobic biodegradation rates of hydrocarbons in soil. Journal of Air & Waste
Management. Vol. 42, no.10, pp.1305-1312.
Kolhatkar, R., J. Wilson, and L.E. Dunlap. 2000. Evaluating Natural
Biodegradation of MTBE at Multiple UST Sites. Proceedings of the Petroleum
Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection,
and Remediation. Conference and Exposition, Anaheim, California, November
15-17, pages 32-49.
McAllister, P.M. and C.Y. Chiang. 1994. "A Practical Approach to Evaluating
Natural Attenuation of Contaminants in Groundwater." Groundwater
Monitoring Review, Vol. 14, no. 2, Spring, pp. 161-173.
McNab, W.W. and B.P. Dooher. 1998. "A Critique of a Steady-State Analytical
Method for Estimating Contaminant Degradation Rates." Ground Water, Vol.
36, no. 6, pp. 983-987.
Mercer, J.W. and R.M. Cohen. 1990. "A Review of Immiscible Fluids in the
Subsurface: Properties, Models, Characterization and Remediation." Journal
of Contaminant Hydrology, Vol. 6, no. 2, pp. 107-163.
Moyer, E.E., D.W. Ostendorf, R.J. Richards, and S. Goodwin. 1996. "Petroleum
Hydrocarbon Bioventing Kinetics Determined in Soil Core, Microcosm, and
Tubing Cluster Studies." Ground Water Monitoring and Remediation, Winter
1996, pp. 141-153.
Nales, M., B. J. Butler, and E. Edwards. 1998. Anaerobic Benzene
Biodegradation: A Microcosm Survey. Bioremediation Journal, Vol. 2, no. 2,
pp. 125-144.
National Research Council (NRC). 2000. Natural Attenuation for Groundwater
Remediation. Washington, DC: National Academy Press.
Norris, R.D., Hinchee, R.E., Brown, R.A., McCarty, P.L., Semprini, L., Wilson,
J.T., Kampbell, D.H., Reinhard, M., Bower, E.J., Borden, R.C., Vogel, T.M.,
Thomas, J.M., and C.H. Ward. 1994. Handbook of Bioremediation. Boca
Raton, FL: CRC Press.
May 2004 IX-63
-------
Ostendorf, D.W. and D.H. Kampbell. 1991. "Biodegradation of Hydrocarbon
Vapors in the Unsaturated Zone." Water Resources Research, Vol. 27, no.4,
pp. 453-462.
Peargin, T.R. 2000. Relative Depletion Rates of MTBE, Benzene, and Xylenes
from Smear Zone NAPL. Proceedings of the Petroleum Hydrocarbons and
Organic Chemical in Ground Water: Prevention, Detection, and Remediation
Conference Special Focus: Natural Attenuation and Gasoline Oxygenates.
November 14-17, Anaheim, CA, pp. 207-212.
Peargin, T.R., D.C. Wickland, and GD. Beckett. 1999. Evaluation of Short Term
Multi-phase Extraction Effectiveness for Removal of Non-Aqueous Phase
Liquids from Groundwater Monitoring Wells. Proceedings of the Petroleum
Hydrocarbons and Organic Chemical in Ground Water: Prevention, Detection,
and Remediation. November 17-19, Houston, TX, pp. 16-25.
Suarez, M.P. and H.S. Rifai. 1999. Biodegradation Rates for Fuel Hydrocarbons
and Chlorinated Solvents in Groundwater, Bioremediation Journal, Vol. 3,
no.4, pp.337-362.
U.S. EPA. 2002. "Calculation and Use of First-Order Rate Constants for
Monitored Natural Attenuation Studies" EPA/540/S-02/500, Charles J. Newell,
Hanadi S. Rifai, John T. Wilson, John A. Connor, Julia A. Aziz, and Monica P.
Suarez. Office of Research and Development.
(http://www.epa.gov/ahaazvuc/download/issue/540S02500.pdf)
U.S. EPA, 2003 (in press). Performance Monitoring for Natural Attenuation
Remedies in Groundwater. Publication 9355.4-25. Office of Solid Waste and
Emergency Response and Office of Research and Development, National Risk
Management Research Laboratory.
U.S. EPA. 2000a. "Data Quality Objectives Process for Hazardous Waste Site
Investigations" EPA/600/R-00/007, January.
(http://www.epa.gov/QUALITY/qs-docs/g4hw-Jinal.pdf)
U.S. EPA. 2000b. "Natural Attenuation of MTBE in the Subsurface under
Methanogenic Conditions", EPA/600/R-00/006, Office of Research and
Development, January.
(http://www.epa.gov/ahaazvuc/download/reports/mtbereport.pdf)
U.S. EPA. 1999. "Use of Monitored Natural Attenuation at Superfund, RCRA
Corrective Action, and Underground Storage Tank Sites." OSWER Directive
9200.4-17P, April 21. (http://www.epa.gov/oust/directiv/d9200417.pdf)
U.S. EPA. 1998. Technical Protocol for Evaluating Natural Attenuation of
Chlorinated Solvents in Ground Water, EPA/600/R-98/128, Office of Research
and Development, September.
(http://www.epa.gov/ahaazvuc/download/reports/protocol.pdf)
U.S. EPA. 1997. "Expedited Site Assessment Tools For Underground Storage
Tank Sites: A Guide for Regulators." EPA 510-B-97-001, March, Office of
Underground Storage Tanks, (http://www.epa.gov/swerustl/pubs/sam.htm)
IX-64 May 2004
-------
U.S. EPA. 1996a. "How to Effectively Recover Free Product at Leaking
Underground Storage Tank Sites: A Guide for State Regulators." EPA 510-R-
96-001, September, Office of Underground Storage Tanks.
(http://www. epa.gov/oust/pubs/fprg.htm)
U.S. EPA. I996b. "Soil Screening Guidance: Technical Background Document"
EPA/540/R95/128, May, Office of Solid Waste and Emergency Response.
(http://www.epa.gov/superfund/resources/soil/tocO.pdf)
U.S. EPA. 1989. "Methods for Evaluating the Attainment of Cleanup Standards"
EPA 230/02-89-042, February, (http://www.epa.gov/tio/stats/vollsoils.pdf)
U.S. EPA. 1986. "Sampling Plan", Chapter Nine in "Test Methods for Evaluating
Solid Waste, Physical/Chemical Methods". (http://www.epa.gov/SW-
846fchap9.pdf)
Weiner, J.M, T.S., Lauck, and D.R. Lovely. 1998. Enhanced Anaerobic Benzene
Degradation with the Addition of Sulfate. Bioremediation Journal, Vol. 2,
nos. 3&4,pp.l59-173.
Wiedemeier, T.H., J.T. Wilson, D.H. Kampbell, and R.N. Miller. 1994.
"Proposed Air Force Guidelines for Successfully Supporting the Intrinsic
Remediation (Natural Attenuation) Option at Fuel Hydrocarbon-Contaminated
Sites." Proceedings, 1994 Outdoor Action Conference, May 23-25,
Minneapolis, MN.
Wiedemeier, T.H., H.S. Rifai, C.J. Newell, and J.T. Wilson. 1999. Natural
Attenuation of Fuels and Chlorinated Solvents. New York: John Wiley & Sons.
Wilson, J.T. 2001. Personal communication.
Checklists: Evaluating CAP Completeness
andPotential Effectiveness of MNA
These checklists can help you to evaluate the completeness of the CAP and to
identify areas that require closer scrutiny. As you go through the CAP, complete
the appropriate checklists which follow. They can be attached to the CAP for
quick future reference. If the answer to any of the questions below is no, then the
CAP is incomplete and you will need to request additional information to
determine if MNA will achieve remediation objectives at the site.
May 2004 IX-65
-------
Initial Screening-Soil Contamination ONLY
Site Name: Date
Addressl: Initials.
Address2:
Project/Case Number:
Recommendation:
Yes No
o o Has source mass been estimated?.
o o Is the source mass likely to remain trapped within the soil?_
o o Has source longevity been estimated?.
o Is the estimate of the length of time required to meet remediation objectives ^
reasonable?
V, i
o Is there no threat of potential receptors coming in contact with contaminated
soil?
o Is there no threat to potential receptors from vapor migration?
IX-66 May 2004
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Initial Screening-Groundwater Contamination
Site Name: Date
Addressl: Initials,
Address2:
Project/Case Number:
Recommendation:
Yes No
o o Has free product (if present initially) been recovered to the maximum extent
practicable? .
o o Has source mass been estimated?
o o Has the plume lifespan been estimated?.
o o Is the estimate of the length of time required to meet remediation objectives
reasonable?
o o Based on evaluation of field data, is the plume shrinking?
o o Are all potential receptors located at a distance represented by a minimum 2-
year travel time?
May 2004 IX-67
-------
Detailed Evaluation-Soil Contamination
Site Name: Date
Addressl: Initials,
Address!:
Project/Case Number:
Recommendation:
Yes No
o o Has comprehensive, 3-dimensional site characterization been
completed?
o o Has soil permeability been measured?
o o Is soil structure and layering conducive to natural attenuation
processes?
Has soil organic carbon content (f^.) been
measured?
o o Have soil saturation limits been calculated for all contaminants of
concern?
o o Are all soil saturation limits for all contaminants of concern below levels
expected to cause unacceptable groundwater impacts?
o o Have soil gas samples been collected and analyzed?
o o Have soil geochemical parameters been measured and are they likely to support
long-term biodegradation?
o o Have rate constants or biodegradation rates been
calculated?
o Is the estimated time to achieve remediation objectives
reasonable?
o Is there no current or future threat to potential receptors?.
IX-68 May 2004
-------
Detailed Evaluation-Groundwater Contamination
Site Name: Date
Addressl: Initials,
Address!:
Project/Case Number:
Recommendation:
Yes No
o o Has comprehensive, 3-dimensional site characterization been
completed?
o o Has the hydraulic conductivity of the most permeable transport zone been
measured?
o o Has the retarded contaminant transport velocity been estimated?
o o Has the propensity for plume diving been determined?
o o Have contaminants of concern been measured for all monitoring
points?
o o Have geochemical parameters been measured for all monitoring
points?
o o Have isopleth maps been prepared for each parameter?
o o Have rate constants or biodegradation rates been calculated?
o o Is the estimated time to achieve remediation objectives reasonable?_
o o Is there no current or future threat to potential receptors?_
May 2004 IX-69
-------
Long-Term Performance Monitoring-Soil Contamination
Site Name: Date
Addressl: Initials.
Address2:
Project/Case Number:
Recommendation:
Yes No
o o Does the monitoring schedule extend for 1-2 years past when remediation
objectives are expected to be achieved?
o o Is sample collection frequency at least bi-
annually?
o o Are a sufficient number of locations to be sampled?
o o Are samples to be analyzed for BTEX, TPH, and other contaminants of concern
(if any)?
o o Are supplemental soil gas samples to be collected and analyzed?
IX-70 May 2004
-------
Long-Term Performance Monitoring-Groundwater
Contamination
Site Name: Date.
Addressl: Initials,
Address2:
Project/Case Number:
Recommendation:
Yes No
o o Does the monitoring schedule extend for 1-2 years past when remediation
objectives are expected to be achieved?
o o Is sample collection frequency at least quarterly for the first two
years?
o o Is sample collection frequency after the first two years at most annually?
o o Are a minimum of 3 transverse plume transects, 1 up gradient transect, and 1
plume centerline transect scheduled to be sampled every sampling
event?
o o Are all sentinel wells (if any) scheduled to be sampled every sampling event?
o o Are samples to be analyzed for BTEX, TPH, and other contaminants of concern
(if any)?
o o Are samples to be analyzed for geochemical indicators and degradation
products?
May 2004 IX-71
-------
-------
Chapter XII
Enhanced Aerobic Bioremediation
-------
-------
Contents
Overview XII-1
Oxygen Releasing Compounds XII-2
Pure Oxygen Injection XII-6
Hydrogen Peroxide Infiltration XII-8
Ozone Injection XII-11
Enhanced Aerobic Bioremediation Technology Effectiveness
Screening Approach XII-13
Step 1 - Initial Screening of Enhanced Aerobic Bioremediation
Effectiveness XII-15
Overall Viability XII-15
Step 2 - Detailed Evaluation of Enhanced Aerobic Bioremediation
Effectiveness XII-17
Potential Effectiveness of Enhanced Aerobic Bioremediation .... XII-17
Site Characteristics Affecting Enhanced Aerobic
Remediation XII-20
Constituent Characteristics Affecting Enhanced
Aerobic Bioremediation XII-31
Step 3 - Evaluation of Enhanced Aerobic Bioremediation Design XII-41
Design Basis XII-43
Cleanup Goals XII-43
Enhanced Aerobic Bioremediation Technology Selection XII-47
Design Components XII-49
Components of Enhanced Aerobic Bioremediation
Systems XII-53
Step 4 - An Evaluation of the Operation and Monitoring Plan XII-58
Remedial Progress Monitoring XII-58
Evaluation Sampling XII-59
Evaluation Criteria XII-63
References XII-66
Checklist: Can Enhanced Aerobic Bioremediation Be Used At This Site? XII-68
June 2003 Xll-ii
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List of Exhibits
Number Title Page
XII-1 Enhanced Aerobic Bioremediation Primary
Advantages and Disadvantages XII-3
XII-2 Enhanced Aerobic Bioremediation Technologies
Comparative Matrix XII-4
XII-3 Typical Enhanced Aerobic Bioremediation Using
Oxygen Releasing Compounds XII-7
XII-4 Typical Enhanced Aerobic Bioremediation Using
Pure Oxygen Injection XII-9
XII-5 Typical Enhanced Aerobic Bioremediation Using
Hydrogen Peroxide XII-10
XII-6 Initial Screening for Potential Effectiveness of
Enhanced Aerobic Bioremediation XII-14
XII-7 Detailed Screening for Potential Effectiveness of
Enhanced Aerobic Bioremediation XII-19
XII-8 Key Parameters Used To Evaluate Enhanced
Aerobic Bioremediation Applicability XII-20
XII-9 Organic Compound Oxidation Stoichiometry XII-22
XII-10 Relationship Between Heterotrophic Bacterial
Counts And Likely Enhanced Aerobic Bioremediation
Effectiveness XII-23
XII-11 Inorganic Oxidation Processes That Consume
Dissolved Oxygen in Groundwater XII-24
XII-12 Intrinsic Permeability And Enhanced Aerobic
Bioremediation Effectiveness XII-27
XII-13 Relationship Between Dissolved Iron And
Enhanced Aerobic Bioremediation Effectiveness XII-30
XII-14 Composition And Relative Biodegradability Of
Petroleum Products XII-32
XII-15 Constituent Concentration And Enhanced Aerobic Bioremediation
Effectiveness XII-35
May 2004 Xll-iii
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List Of Exhibits (continued)
XII-16 Solubility Values And Organic Carbon Partition
Coefficients For Select Petroleum Hydrocarbon
Constituents XII-35
XII-17 MTBE Considerations for Applying Enhanced
Aerobic bioremediation XII-39
XII-18 Detailed Effectiveness Evaluation of Enhanced Aerobic
Remediation Effectiveness for MTBE - Key Site
Considerations XII-41
XII-19 Enhanced Aerobic Bioremediation Design Basis
Factors XII-43
XII-20 Clean Up Concentrations Potentially Achieved
By Enhanced Aerobic Bioremediation XII-45
XII-21 Basic Stoichiometry Oxygen Production From
Chemical Decomposition XII-47
XII-22 Relative Oxygen Delivery Efficiencies For
Various Enhanced Aerobic Bioremediation
Technologies XII-49
XII-23 Common Enhanced Aerobic Bioremediation
Design Elements XII-51
XII-24 Major Components of Enhanced Aerobic
Remediation Systems XII-53
XII-25 Common Performance Monitoring Parameters
And Sampling Frequencies XII-62
May 2004 Xll-iv
-------
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Chapter XII
Enhanced Aerobic Bioremediation
Overview
Enhanced aerobic bioremediation technologies are used to accelerate naturally
occurring in-situ bioremediation of petroleum hydrocarbons, and some fuel
oxygenates such as methyl tertiary-butyl ether (MTBE), by indigenous
microorganisms in the subsurface. Enhanced aerobic bioremediation technologies
include biosparging; bioventing1; use of oxygen releasing compounds; pure oxygen
injection; hydrogen peroxide infiltration; and ozone injection2. These technologies
work by providing a supplemental supply of oxygen to the subsurface, which
becomes available to aerobic, hydrocarbon-degrading bacteria. The stoichiometric
ratio of oxygen per hydrocarbon is 3 M O2 per 1 mole of hydrocarbons. Oxygen is
considered by many to be the primary growth-limiting factor for hydrocarbon-
degrading bacteria, but it is normally depleted in zones that have been
contaminated with hydrocarbons. By using these technologies, rates of
biodegradation of petroleum hydrocarbons can be increased at least one, and
sometimes several, orders of magnitude over naturally-occurring, non-stimulated
rates.
Enhanced aerobic bioremediation technologies can be used to address
contaminants in the unsaturated zone, the saturated zone, or both. Bioventing, for
example, specifically targets petroleum hydrocarbon contaminants in the
unsaturated zones and does not address contaminants in the capillary fringe or
saturated zone. Most, but not all, enhanced aerobic bioremediation technologies
primarily address petroleum hydrocarbons and some oxygenates that are dissolved
in groundwater or are sorbed to soil particles in the saturated zone. The
technologies are typically employed outside heavily contaminated source areas
which will usually be addressed by more aggressive remedial approaches.
When used appropriately, enhanced aerobic bioremediation technologies are
effective in reducing levels of petroleum contamination at leaking underground
storage tank sites. Gasoline constituents dissolved in water are a likely target of
enhanced aerobic bioremediations. Enhanced aerobic bioremediation technologies
are most often used at sites with mid-weight petroleum products (e.g., diesel fuel,
1 For more information on Biosparging and Bioventing, see How to Evaluate Alternative
Cleanup Technologies for Underground Storage Tank Sites: A Guide for Corrective Action Plan
Reviewers (US EPA 510-R-04-002), Chapter III ("Bioventing") and Chapter VIII
("Biosparging").
Other enhanced aerobic bioremediation technologies, including surfactant enhanced
microbubble injection and permeable polymeric tubing oxygen diffusion, are not discussed in this
chapter because of their limited use and experimental status.
May 2004 XII-1
-------
readily and can be removed more rapidly using other technologies (e.g., air
sparging or soil vapor extraction). However, if these lighter products are present,
enhanced aerobic bioremediation technologies can also effectively reduce
contaminant concentrations. Heavier petroleum products such as lubricating oils
generally take longer to biodegrade than the lighter products, but enhanced aerobic
bioremediation technologies may still be effective at sites contaminated with these
products.
It is generally not practical to use enhanced aerobic bioremediation
technologies to address free mobile product or petroleum contamination in low
permeability soil (e.g., clay). Because enhanced aerobic bioremediation is a
relatively slow cleanup approach, it is not recommended to address current or
imminent excessive human health or environmental risks.
Exhibit XII-1 summarizes -tie general advantages and disadvantages of
enhanced aerobic bioremediation technologies. Discussions of bioventing and
biosparging, two other enhanced aerobic bioremediation technologies, are
provided in How to Evaluate Alternative Cleanup Technologies for Underground
Storage Tank Sites: A Guide for Corrective Action Plan Reviewers (US EPA 510-
R-04-002, 2004), Chapter III ("Bioventing"), Chapter VIII ("Biosparging"), and
Chapter X ("In-Situ Groundwater Bioremediation").
A brief description of several of the technologies is provided below.
Oxygen Releasing Compounds
Various enhanced aerobic bioremediation approaches rely on oxygen releasing
compounds to remediate petroleum contamination. More commonly used oxygen
releasing compounds include calcium and magnesium peroxides that are
introduced to the saturated zone in solid or slurry phases. These peroxides release
oxygen to the aquifer when hydrated by groundwater as the peroxides are
ultimately converted to their respective hydroxides. Magnesium peroxide has bee
more commonly applied in field applications than calcium peroxide because of
magnesium peroxide's lower solubility and, consequently, prolonged release of
oxygen. Magnesium peroxide formulations placed in the saturated zone during a
short-term injection event can release oxygen to groundwater over a four- to
eight- month period. Significant quantities of magnesium peroxide are required
based on stoichiometry and the fact that 90% of the weight of the compound is not
oxygen. Oxygen amounting to approximately 10% of the weight of magnesium
peroxide placed in the saturated zone is released to the aquifer over the active
period.
May 2004 XII-2
-------
Exhibit XII-1
Enhanced Aerobic Bioremediation
Primary Advantages and Disadvantages
Advantages
Works with and enhances natural
in-situ processes already at play
(typically uses natural
groundwater gradient, naturally
occurring biodegradation)
Destroys the petroleum
contamination in place
Produces no significant wastes
(off-gases or fluid discharges)
Can be a low-energy approach
Is relatively inexpensive
Complements more aggressive
technologies (e.g., groundwater
extraction) and less aggressive
approaches (e.g., intrinsic
remediation) that can be
integrated into site remediation
Causes minimal disturbance to
site operations
Has simple operation and
monitoring requirements
Is potentially more reliable than
other, more active remedial
technologies (e.g., groundwater
extraction and treatment)
Can be used in tandem with other
remedial technologies that
address small amounts of
residual soil and groundwater
contamination
Disadvantages
May have longer remedial time
frames than more aggressive
approaches
May not be able to reduce
contaminants to background or
very low concentrations
Typically requires long-term
monitoring of residual
contamination in soil and
groundwater
May require permits for
nutrient/oxygen injection
May not be fully effective on all
petroleum hydrocarbons and
product additives (e.g., MTBE)
Often must be accompanied by
other technologies (e.g., product
recovery) to address source
areas
May significantly alter aquifer
geochemistry
Can be misapplied to remediation
at some sites if the conditions for
use are not fully understood
Oxygen supplied by enhanced
aerobic bioremediation may be
lost to chemical reactions in the
subsurface which do not promote
hydrocarbon contaminant
oxidation and degradation.
May 2004
XII-3
-------
Exhibit XII-2 compares the relative advantages and disadvantages of several
different enhanced aerobic bioremediation technologies currently in use.
Exhibit XII-2
Enhanced Aerobic Bioremediation Technologies Comparative
Matrix
No mechanical components
required
Minimal engineering design
requirements
Relatively low capital and
operating costs
Abiotic oxidation of
contaminants contacting
reagents
Remediates contamination in
unsaturated soils
Locally saturates groundwater
with oxygen to further enhance
biodegradation and oxygen
distribution
Can efficiently sustain
widespread ambient (up to
8 mg/L) oxygen concentrations
in groundwater
Can efficiently sustain
widespread ambient (up to
~2*\%) oxygen concentrations in
unsaturated s^;'~
Generally cor. ed safe
Electricity/powe source
generally not required
Oxygen
Releasing
Compounds
Hydrogen
Peroxide
Infiltration
Pure Oxygen
Injection
sr
• ', . ' •>
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Biosparging
X
X
Bioventing
!*-">''• •
X
X
X
May 2004
XII-4
-------
Exhibit XII-2
Enhanced Aerobic Bioremediation Technologies Comparative
Matrix (continued)
Disadvantages ' ":<< , . ^ <
Heavy reliance on groundwater
advection, dispersion, and
diffusion to distribute oxygen
can limit treatment coverage
and prolong remediation
Increased risk of fugitive vapors
entering building structures and
utility conduits, particularly in
absence of vapor recovery
technology (e.g., soil vapor
extraction)
Does not target or treat
saturated zone
On-site reactive chemical
handling and storage required
On-site gas production and
delivery equipment (e.g., ozone
generator) typically required
Relatively few petroleum
remediation projects completed
using this technology
May require reinjection permits
Radius of influence limited if
using "socks"
Zone of influence may be
limited with compounds that
are suspended in a well.
Oxygen
Releasing
Compounds
\t - ^N* '
X
X
X
X
Hydrogen
Peroxide
Infiltration
'*
X
X
X
X
Pure Oxygen
Injection
si
s|
0 =
Biosparging
Bioventing
„•!> " - ' -
V- ^ ' ^ ' ' '
X
X
X
X
X
X
X
X
X
X
X
Oxygen releasing compounds may be introduced into the saturated zone in
several ways. The most common approaches include:
• Placing the compounds into drilled boreholes or other excavations
(e.g., tank fields)
• Injecting a compound slurry into direct-push borings (e.g., Geoprobe)
May 2004
XII-5
-------
• Mixing oxygen-releasing compounds directly with contaminated soil
and then using the mixture as backfill or hauling it to a disposal site
• Suspending oxygen releasing compounds contained in "socks" in
groundwater monitoring wells
• A combination of the above
Oxygen-releasing compounds may also be used to address source areas, entire
plumes or plume tails (e.g., a treatment curtain aligned perpendicular to
contaminant flow direction). Exhibit XII-3 provides a conceptual depiction of the
deployment of oxygen releasing compounds to address a petroleum hydrocarbon
plume. Many site-specific conditions must be considered before a remedial
approach using this technology can be devised and implemented. One such site-
specific concern is the proximity of drinking water supply wells to the treatment
area and how the injected oxygen or other nutrients may affect these wells.
Another concern is the limited zone of influence of oxygen releasing compounds
when deployed in a well, which often provide increased oxygen levels only up to
twice the diameter of the well. While the scope of this document does not allow a
more in-depth discussion of this or other site-specific implementation, it is
important to carefully consider site-specific issues (e.g., contaminant composition
and behavior, site geology and hydrology) along with the conceptual information
provided in this chapter.
The following sections describe the use of pure oxygen injection, hydrogen
peroxide infiltration, and ozone injection.
Pure Oxygen Injection
Injecting pure oxygen into groundwater can be a relatively efficient means of
increasing dissolved oxygen levels in groundwater to promote aerobic
biodegradation of petroleum hydrocarbons. In contrast to other enhanced aerobic
bioremediation technologies, there is no carrier (e.g., amended groundwater) or
delivery media (e.g., oxygen releasing compounds slurry) associated with pure
oxygen injection. Approximately one gram of oxygen is delivered to the
subsurface for every gram of oxygen directed to the subsurface. Oxygen is several
times more soluble in groundwater when it is introduced in pure form than if the
dissolved oxygen is derived by forcing groundwater to come into contact with
atmospheric air, such as occurs with biosparging. Dissolved oxygen
concentrations of up to 40-50 parts per million(ppm) can be achieved through pure
oxygen injection, which contrasts to dissolved oxygen concentration limits of
approximately 8-10 ppm when the saturated zone is aerated using atmospheric air, which
contains approximately 21% oxygen.
Pure oxygen is most commonly introduced into the subsurface via vapor-phase
injection. Vapor-phase oxygen (approximately 95% oxygen) is injected into the
saturated zone near the base of the dissolved petroleum hydrocarbon
contamination using a network of sparge wells. Oxygen sparge rates lower than
May 2004 XII-6
-------
Exhibit XII-3
Typical Enhanced Aerobic Remediation Using
Oxygen Releasing Compounds
• Application of Oxygen Releasing
Compound to Base of LIST Excavation (OPT)
Injected Oxygen Releasing
Compounds (e.g., using
Geoprobe) Along Treatment
Curtain
LIST Excavation
(Excavated/Backfilled)
Monitoring
Wells (TYP)
jpr- Filter Socks
17 Containing Oxygen
]/ Releasing
/ Compounds (TYP)
Groundwater and Plume Flow
Goundwater
Flow
Dissolved
Petroleum
Hydrocarbons
May 2004
XII-7
-------
air sparge flow rates are used in order to maximize contact time between the
oxygen and contaminated groundwater before the injected oxygen rises through >M^
the contaminated zone to the water table. Trapping of sparged oxygen in the soil
matrix (e.g., in soil pore spaces or semi-confining laminates) beneficially prolongs
contact between the pure oxygen and the oxygen-depleted groundwater. Series of
vertical oxygen injection wells are often alternately sparged in order to increase
dissolved oxygen levels more efficiently over larger areas.
The spacing of injection wells is typically site-specific and based on the
thickness of contaminated material, geology, hydrogeology, and other factors
affecting the delivery and distribution of dissolved oxygen. Volatile organic vapor
production and migration concerns are reduced with oxygen sparging relative to
air sparging because of the significantly lower oxygen sparge air flow rates.
However, vapor production and migration can be a concern and should be
evaluated on a site-specific basis. A conceptual schematic of a pure oxygen
injection system is depicted in Exhibit XII-4.
Hydrogen Peroxide Infiltration
Extracted and treated groundwater is amended and mixed with hydrogen
peroxide prior to re-infiltration or re-injection. The hydrogen peroxide-amended
groundwater is pumped into infiltration galleries or injection wells located in or
near suspected source areas. Generally, the infiltration/injection and groundwater
extraction scheme is designed to promote the circulation and distribution of ****>
hydrogen peroxide and dissolved oxygen through the treatment area. "****"
Exhibit XII-5 provides a conceptual illustration of a hydrogen peroxide
enhanced aerobic bioremediation system. The precipitation of chemical oxidants
(e.g., iron oxides) can present potentially significant equipment fouling problems in
this type of system, depending on the concentrations of naturally occurring levels
of inorganic compounds, such as iron, in the subsurface.
Introducing hydrogen peroxide, which is a chemical oxidant, to the saturated
zone can significantly augment existing oxygen levels because it naturally
decomposes rapidly, generating oxygen. For each part (e.g., mole) of hydrogen
peroxide introduced to groundwater, one-half part of oxygen can be produced.
Hydrogen peroxide has the potential of providing some of the highest levels of
available oxygen to contaminated groundwater relative to other enhanced aerobic
bioremediation technologies because it is infinitely soluble in water. In theory,
10% hydrogen peroxide could provide 50,000 ppm of available oxygen.
However, when introduced to groundwater, hydrogen peroxide is unstable and
can decompose to oxygen and water within four hours. This limits the extent to
which the hydrogen peroxide may be distributed in the subsurface before it is
transformed. Introducing concentrations of hydrogen peroxide as low as 100 ppm ^m^^
can cause oxygen concentrations in groundwater to exceed the solubility limit of
May 2004 XII-8
-------
Exhibit XII-4
Typical Enhanced Aerobic Remediation Using
Pure Oxygen Injection
Oxygen Generator
Surge Tank
UST
O2 Injection
Sparge Well
(TYP)
Air Compressor
Alternating Sets of
Injection Wells
V
/— - \J£ •
-. u
-lift,
Legend
Adsorbed Phase
Dissolved Phase
--2- Vapor Phase
May 2004
XII-9
-------
Exhibit XII-5
Typical Enhanced Aerobic Remediation Using
Hydrogen Peroxide
Hydrogen Peroxide/Bionutrient
-Former USTs Amendment and Transfer System"
.Treated Groundwater
Reinfiltration Split Stream
Treated Off-Gas
(if applicable)
Former UST Field
(Excavated/Backfilled)
HzOz/Bionutrient
Injection Gallery
(TYP)
Groundwater
Extraction Well
(TYP,
HiOz/Bionutrient
Injection Well
(TYP)
Contaminated'
Groundwater
Groundwater/Plume Flow
Groundwater
Treatment
System
Treated
Water
Discharge
Legend
Dissolved
Hydrocarbons
May 2004
XII-10
-------
oxygen in groundwater (40-50 ppm). When this occurs, oxygen gas is formed,
which can be lost in the form of bubbles that rise through the saturated zone to the
water table and into the unsaturated zone.
For enhanced aerobic bioremediation purposes, hydrogen peroxide is used at
concentrations that maximize dissolved oxygen delivery to the petroleum-
contaminated area while minimizing losses of oxygen through volatilization.
Hydrogen peroxide is cytotoxic to microorganisms at concentrations greater than
100-200 ppm. This toxicity to aerobic petroleum degrading microbes can be
amplified if carbon sources and nutrients are depleted in the contaminated media.
Concentrations and application rates are typically determined on a site-specific
basis, depending on site conditions, contaminant levels, and cleanup goals.
Hydrogen peroxide in a more concentrated form and in the presence of an iron
catalyst can also be used to chemically oxidize site contaminants. This application
of peroxide is not discussed in this chapter. When used in this manner, hydrogen
peroxide's reaction with ferrous iron produces Fenton's reagent. Fenton's reagent
chemical oxidation requires a comprehensive three-dimensional site
characterization to locate preferential pathways for migration. It is important that
any hydrogen peroxide remediation system contain an adequate number of soil
vapor extraction wells to completely capture vapors. For more information on the
use of hydrogen peroxide as an oxidant, see How to Evaluate Alternative Cleanup
Technologies for Underground Storage Tank Sites: A Guide for Corrective Action
Plan Reviewers (US EPA 510-R-04-002), Chapter XIII, "Chemical Oxidation".
The potential dangers of working with hydrogen peroxide should not be
overlooked when considering the technology and determining how it should be
applied. Hydrogen peroxide is an oxidant that can cause chemical bums. When
introduced into a petroleum-contaminated area at high concentrations, hydrogen
peroxide can produce heat and elevated oxygen levels that may lead to fire or
explosions. Use of concentrated peroxide should be avoided to help reduce these
hazards.
Ozone Injection
Ozone injection is both a chemical oxidation technology and an enhanced
aerobic bioremediation technology. Oxidation of organic matter and contaminants
occurs in the immediate ozone application and decomposition area. Outside the
decomposition area, increased levels of dissolved oxygen can enhance aerobic
bioremediation. Ozone is a strong oxidant with an oxidation potential greater than
that of hydrogen peroxide. It is also effective in delivering oxygen to enhance
subsurface bioremediation of petroleum-impacted areas. Ozone is 10 times more
soluble in water than is pure oxygen.
Consequently, groundwater becomes increasingly saturated with dissolved
oxygen as unstable ozone molecules decompose into oxygen molecules. About
May 2004 XII-11
-------
one-half of dissolved ozone introduced into the subsurface degrades to oxygen
within approximately 20 minutes. The dissolved oxygen can then be used as a
source of energy by indigenous aerobic hydrocarbon-degrading bacteria.
Because of its oxidization potential, injected ozone can also be toxic to
indigenous aerobic bacteria and can actually suppress subsurface biological
activity. However, this suppression is temporary, and a sufficient number of
bacteria survive in-situ ozonation to resume biodegradation after ozone has been
applied.
Ozone may be injected into the subsurface in a dissolved phase or in gaseous
phases. Groundwater is often extracted and treated, then used to transport
(through re-injection or re-infiltration) the dissolved phase ozone and oxygen into
the subsurface contaminated area. More commonly, however, gaseous ozone is
injected or sparged directly into the contaminated groundwater. Because of its
instability, ozone is generated on-site and in relatively close proximity to the target
contaminated area. Typically, air containing up to 5% ozone is injected into
strategically placed sparge wells. Ozone then dissolves in the groundwater, reacts
with subsurface organics, and decomposes to oxygen. Vapor control equipment
(e.g., an soil vapor extraction and treatment system) may be warranted when
ozone injection rates are high enough to emit excess ozone to the unsarurated
zone, which may slow deployment timetables in some states. In many states,
vapor control equipment requires a permit for off-gas treatment.
Special Considerations for MTBE. The gasoline additive methyl tertiary
butyl ether (MTBE) is often found in the subsurface when gasoline has been
released. In addition, MTBE is sometimes discovered at spill sites of middle
distillate petroleum products like diesel, jet fuel, kerosene, and fuel oil. As such,
whenever a petroleum hydrocarbon spill is investigated and remediated, the
presence/absence of MTBE in the soil and ground water should be verified.
Several crucial characteristics of MTBE affect the movement and remediation
of MTBE, including:
• MTBE is more soluble in water than most C6-C10 gasoline-range
hydrocarbons. For example, MTBE is 28 times more soluble in water
than is benzene.
• MTBE is less volatile from water (i.e., has a lower Henry's Constant)
than most C6-C10 hydrocarbons. For example, MTBE is 11 times less
volatile from water than is benzene.
• MTBE adheres less to soil organic matter than most C6-C10
hydrocarbons. This means that it has lower retardation and more rapid
transport in groundwater than most gasoline-range compounds.
May 2004 XII-12
-------
• At most sites, MTBE is less biodegradable in the subsurface than other
gasoline compounds.
Because of these characteristics, some MTBE from a gasoline spill will be
found with the BTEX compounds in the soil and groundwater near the site of
petroleum release. But it is also quite common to find a dissolved-phase MTBE-
only plume downgradient of the BTEX/TPH plume. Thus, when considering using
enhanced aerobic bioremediation techniques for gasoline plumes that include
MTBE, recognize that the MTBE may exist in two distinct regions:
• A near-source area where MTBE co-occurs with more readily
biodegradable BTEX/TPH compounds
• A distal area where the only compound of concern is MTBE
Any petroleum impact remediation plan that addresses MTBE should account
for the probable MTBE-only plume downgradient of the MTBE & hydrocarbon
plume. The MTBE-only plume often has decreased levels of dissolved oxygen,
due to its occurrence in the "oxygen shadow" region downgradient from the
spilled petroleum source area where natural biodegradation is typically occurring
(Davidson, 1995).
Enhanced Aerobic Bioremediation Technology Effectiveness
Screening Approach
The descriptions of the various enhanced aerobic bioremediation technologies
in the overview provide the basic information needed to evaluate a corrective
action plan that proposes enhanced aerobic bioremediation. To assist with
evaluation of the enhanced aerobic bioremediation corrective action plan, a step-
by-step technology effectiveness screening approach is provided in a flow diagram
in Exhibit XII-6. This exhibit summarizes this evaluation process and serves as a
roadmap for the decisions to make during evaluation of the corrective action plan.
A checklist has also been provided at the end of this chapter, which can be used to
evaluate the completeness of the enhanced aerobic bioremediation corrective
action plan and to focus attention on areas where additional information may be
needed. The evaluation process can be divided into the four steps described
below.
• Step 1: An initial screening of enhanced aerobic bioremediation
effectiveness allows quick determination of whether enhanced aerobic
bioremediation should be considered as a remedial approach for the site.
• Step 2: A detailed evaluation of enhanced aerobic bioremediation
effectiveness provides further screening criteria to confirm whether enhanced
aerobic bioremediation is likely to be effective. First, certain site-specific data
on the nature/extent of contamination, potential risk to human health/the
environment, subsurface geology and hydrogeology, and other relevant site
May 2004 XII-13
-------
Exhibit XII-6
Initial Screening for Potential Effectiveness of
Enhanced Aerobic Bioremediation
Is free
floating product
present?
Remove free product
Are
nearby
basements, sewers,
or other subsurface
confined spaces
present?
Will
SVEbe
used to control
migration of
vapors?
Is the
target contaminant
zone comprised of, or does it
contain significant amounts
of, unstratified dense
clay lenses?
Enhanced aerobic
remediation has the potential
to be effective at the site.
Proceed to detailed evaluation.
Enhanced aerobic
remediation is not
likely the most
effective technology
for use at this site.
Consider other
technologies.
• Vacuum-enhanced
pump-and-treat
• Irvsitu groundwater
bioremediation
May 2004
Xli-14
-------
characteristics need to be evaluated. Next, the site-specific data must be compared
to the criteria provided in the Exhibit to assess whether enhanced aerobic
bioremediation is likely to be effective.
» Step 3: An evaluation of the enhanced aerobic bioremediation system
design in the corrective action plan allows a reviewer to determine whether
basic design information has been defined, necessary design components have
been specified, the construction process flow designs are consistent with
standard practice, and adequate feasibility testing has been performed.
• Step 4: An evaluation of the operation and monitoring plans allows a
reviewer to determine whether baseline, start-up and long-term system
operation and monitoring are of sufficient scope and frequency and whether
remedial progress monitoring and contingency plans are appropriate.
Step 1 - Initial Screening of Enhanced Aerobic Bioremediation
Effectiveness
This section reviews the initial screening tool to examine whether enhanced
aerobic bioremediation is likely to be an effective approach to remediate the
petroleum-impacted areas at a site. Before accepting enhanced aerobic
bioremediation as the preferred remedial approach, determine whether the
corrective action plan has taken into account key site-specific conditions. In
addition, evaluate several "bright lines" that define the limits of enhanced aerobic
bioremediation overall viability as a remedial technology. These bright lines will
assist with evaluating the corrective action plan and determining whether enhanced
aerobic bioremediation is appropriate as an appropriate solution. After
establishing the overall viability of an enhanced aerobic bioremediation approach,
look at basic site and petroleum contaminant information in order to further
determine the expected effectiveness of enhanced aerobic bioremediation at the
site.
Overall Viability
The following site conditions are considered to be the "bright lines" that define
the general limits of enhanced aerobic bioremediation viability at a site. If review
of the corrective action plan indicates that any of the following conditions exist,
enhanced aerobic bioremediation is not likely to be a feasible or appropriate
remedial solution for the site.
May 2004 Xll-15
-------
» Free mobile product is present and the corrective action plan does not ,*M*:
include plans for its recovery. Enhanced aerobic bioremediation will not SMS^-
effectively address free product that will serve as an on-going source of
dissolved phase contamination. Biodegradation of the petroleum
hydrocarbons occurs predominantly in the dissolved-phase because the
compounds must be able to be transported across the microbial cell
boundary along with water, nutrients, and metabolic waste products.
Therefore, in the presence of free product, rates of hydrocarbon mass
destruction using enhanced aerobic bioremediation will be limited by the
rate at which the free product is dissolved into groundwater. The relatively
low solubilities of petroleum hydrocarbon constituents will likely extend
remediation for several years, and could allow further expansion of the
contaminated area if free product is not removed. Additionally, some
enhanced aerobic bioremediation technologies could actually spread the
free product. For free product recovery approaches see How to
Effectively Recover Free Product At Leaking Underground Storage Tank
Sites: A Guide for State Regulators, US EPA 510-R-96-001, September
1996.
• Potentially excessive risks to human health or the environment have been
identified and the corrective action plan does not include a supplemental
mitigation plan. While enhanced aerobic bioremediation can reduce
petroleum hydrocarbon concentrations in the subsurface, site conditions 'J^
may limit the level of such reductions and can significantly extend remedial ****"
timeframes. Close proximity of the petroleum contamination to basements,
utilities, water supply wells, surface water bodies, or other potential
receptorsthat could pose excessive risks should be mitigated using
technologies that complement enhanced aerobic bioremediation (e.g., soil
vapor extraction (SVE), hydraulic controls to protect water supply wells).
Without the use of other remedial approaches, enhanced aerobic
bioremediation may not be able to reduce concentrations of petroleum
contaminants to sufficiently low concentrations to protect receptors in the
predicted timeframes.
• The target contaminant zone includes unstratified dense clay. For
remedial success, enhanced aerobic bioremediation technologies must
effectively introduce and distribute oxygen to indigenous microorganisms
present in the treatment zone, allowing microbial populations to expand
and metabolize the petroleum contaminants. With the relatively low
permeabilities inherent to clay or clay-rich soils, oxygen and oxygen carrier
media (e.g., air) cannot be easily introduced or distributed. Any
distribution of oxygen that could be delivered to such soils (e.g., placement
of oxygen releasing compounds in borings or excavations) would largely be
controlled by molecular diffusion, a very slow and ineffective process. >***,
Treatment zone oxygen levels, therefore, would not be uniformly
May 2004 XI I-16
-------
increased, and biodegradation of the petroleum hydrocarbons could not be
effectively enhanced.
While these bright lines offer general guidance on the applicability of enhanced
aerobic bioremediation technologies, there may be site-specific application-specific
exceptions to the rule. It may be appropriate, for example, for enhanced aerobic
bioremediation technologies to be used to address contamination on the periphery
of contamination while a different technology is employed to treat the source zone.
Step 2 - Detailed Evaluation of Enhanced Aerobic Bioremediation
Effectiveness
Potential Effectiveness of Enhanced Aerobic Bioremediation
Before performing a more detailed evaluation of enhanced aerobic
bioremediation's potential saturated zone remedial effectiveness and future success
at a site, it is useful to review several key indicators. Two factors influence the
effectiveness of enhanced aerobic bioremediation at a site: saturated zone
permeability, and biodegradability of the petroleum constituents.
• Saturated soil permeability. Soil permeability can strongly affect the
rate at which oxygen is supplied and uniformly distributed to the
hydrocarbon-degrading bacteria in the subsurface. Enhanced aerobic
bioremediation of groundwater, contaminants in fine-grained soils, or in
clays and silts with low permeabilities, is likely to be less effective than
in coarse-grained soils (e.g., sand and gravels) because it is more
difficult to effectively deliver oxygen in low-permeability materials. In
coarse-grained soils, oxygen can be more easily delivered to bacteria,
and beneficial populations of hydrocarbon-degrading bacteria may
come into contact with more of the petroleum, which enhances
biodegradation.
• Biodegradability. Biodegradability is a measure of a contaminant's
propensity to be metabolized by hydrocarbon-degrading
microorganisms. Petroleum products are generally biodegradable, as
long as indigenous microorganisms have an adequate supply of oxygen
and nutrients. However, the rate and degree to which petroleum
products can be degraded by the microorganisms present in the
subsurface is largely determined by the relative biodegradability of the
petroleum products. For example, heavy petroleum products (e.g.,
lubricating oils, fuel oils) generally contain a higher proportion of less
soluble, higher molecular weight petroleum constituents that are
biodegraded at a slower rate than more soluble, lighter fraction
petroleum compounds (e.g., gasoline). As a general rule, these
characteristics of petroleum compounds can limit biodegradation rates.
May 2004 XI1-17
-------
Less soluble compounds are generally less available in the aqueous
phase for microorganisms to metabolize. Larger petroleum molecules
can slow or preclude the transport of some of these molecules into
microbial cells for degradation, and larger or longer chain length
structural properties may hinder the ability of the micro-organisms'
enzyme systems to effectively attack the compounds. Therefore, even
under identical site conditions, bioremediation of a lubricating oil spill
will generally proceed more slowly than at a gasoline release.
However, cleanup goals are frequently tied to specific petroleum
compounds rather than the range of organic constituents that may
comprise a petroleum product. Therefore, when considering enhanced
aerobic bioremediation, the biodegradability of specific petroleum
compounds common to the petroleum product and cleanup goals are of
greatest relevance. Even though bioremediation of lubricating oil
contamination may occur relatively slowly, cleanup of a lubricating oil
spill site via bioremediation may be achieved more quickly than
bioremediation of a gasoline spill site because fewer compounds in
lubricating oil dissolve in groundwater, reducing the number of target
species to clean up.
Some chemical species present in gasoline, such as methyl tertiary-butyl ether
(MTBE), are more recalcitrant to bioremediation than are some of the heaviest and
most chemically complex petroleum compounds. The detailed enhanced aerobic
bioremediation effectiveness evaluation section of this chapter consider the
biodegradability of specific petroleum hydrocarbon constituents, such as the
benzene, toluene, ethylbenzene, and xylene (BTEX) compounds, as well as that of
fuel oxygenates, such as MTBE.
The following section provides information needed to make a more thorough
evaluation of enhanced aerobic bioremediation effectiveness and help to identify
areas that may require special design considerations. Exhibit XII-7 provides a
stepwise process that reviewers should use to further evaluate whether enhanced
aerobic bioremediation is an appropriate technology for a contaminated UST site.
To use this tool, determine the type of soil present and the type of petroleum
product released at the site.
To help with this more detailed evaluation, this section covers a number of
important site-specific characteristics influencing the potential effectiveness of
enhanced aerobic bioremediation that were not considered or fully explored in the
initial screening of the remedial approach. Additionally, this section provides a
more detailed discussion of key contaminant characteristics that influence the
potential effectiveness of enhanced aerobic bioremediation. Key site and
contaminant factors that should be explored in the detailed evaluation of enhanced
aerobic bioremediation are listed in Exhibit XII-8. The remainder of this section
details each of the parameters described in Exhibit XII-8. After reviewing and
comparing the information provided in this section with the corresponding
May 2004 XII-18
-------
Exhibit XII-7
Detailed Screening for Potential Effectiveness
of Enhanced Aerobic Bioremediation
Identify site characteristics important
to enhanced aerobic bioremediation
effectiveness.
identify constituent characteristics
important to enhanced aerobic
bioremediation effectiveness.
YES
Is
the dissolved
oxygen level at least
2 mg/L?
Is
intrinsic
permeability > 10' cm2?
Is
soil free
of impermeable
layers or other conditions
that would disrupt
air flow?
Are
groundwater
gradient and conductivity
high enough to ensure distribution
oxygen throughout targeted
contamination
zone?
YES
"is
groundwater
shallow enough to
allow introduction of atmospheric
oxygen through infiltration
of rain, snow,
etc.?
Is the
dissolved iron
concentration at the site
< 10 mg/L?
YES
NO
Enhanced aerobic
bioremediation is
not likely the most
effective technology
for use at this site.
Consider other
technologies.
• Vacuum-enhanced
pump-and-treat
• In-situ groundwater
, bioremediation ,
Are
all targeted
constituents sufficiently
biodegradable?
Are
heavy metals
< 2,500 ppm?
Does
theKocofthe
target contaminant show
that it is likely to be treatable
with enhanced aerobic
bioremediation?
Enhanced aerobic bioremediation |
is likely to be effective at the site.
YES
May 2004
XII-19
-------
information in the corrective action plan, it should be possible to evaluate whether
enhanced aerobic bioremediation is likely to be effective at the site.
Exhibit XII-8
Key Parameters Used to Evaluate Enhanced Aerobic
Bioremediation Applicability
Site Characteristics
Constituent Characteristics
Oxygen Demand Factors
• Five-Day Biological Oxygen
Demand (BOD5)
• Contaminant theoretical oxygen
demand
• Naturally occurring organic
material (humic substances)
- Microbial population
density/activity
- Nutrient concentrations
- Temperature
- PH
Advective and Dispersive Transport
Factors
Intrinsic permeability
Soil structure and stratification
Hydraulic gradient
Depth to groundwater
Dissolved iron content
Chemical Class and Susceptibility to
Bioremediation
Contaminant Phase Distribution
Concentration and Toxicity
Bioa vailability Characteristics
• Solubility
• Organic carbon partition
coefficient (K^-J/sorption potential
Site Characteristics Affecting Enhanced Aerobic Bioremediation
The effectiveness of enhanced aerobic bioremediation depends largely on the
ability to deliver oxygen to naturally occurring hydrocarbon-degrading
microorganisms in the target treatment area. Oxygen can be introduced and
removed from a contaminated groundwater zone in many different ways.
Dissolved oxygen may enter the contaminated zone from any of the following
sources:
• Flow of groundwater into the contaminated zone from
background (upgradient) areas
• Precipitation infiltration
• Other enhanced aerobic bioremediation sources
Losses of oxygen from the contaminated zone may occur through:
May 2004
XII-20
-------
• Biodegradation of organic contaminants
• Oxidation of naturally occurring organic and inorganic material in the soil
• Volatilization of dissolved oxygen
• Flow of groundwater containing depleted levels of dissolved oxygen
leaving the contaminated zone
The success of enhanced aerobic bioremediation, therefore, hinges on the
balance between oxygen sources, oxygen uptake, and the degree to which the
transport of dissolved oxygen in groundwater is limited. To support aerobic
biodegradation of petroleum contaminants, the most favorable dissolved oxygen
(DO) level is 2 mg/L or higher. Anaerobic biodegradation processes in the
anaerobic shadow become limited once dissolved oxygen levels approach or fall
below 2 mg/L. Site characteristics affecting the delivery and distribution of
oxygen in the subsurface and the effectiveness of enhanced aerobic bioremediation
technology are discussed in the following sections.
Oxygen Demand Factors. Groundwater in petroleum spill source area and
downgradient of the spill area is usually depleted of oxygen. This zone of oxygen-
depleted groundwater, commonly referred to as the anaerobic shadow, results
from the use of oxygen by naturally occurring microorganisms during aerobic
metabolism of the spilled petroleum organic compounds. The oxygen is used in
the microbiologically mediated oxidation of the petroleum contaminants. Aerobic
biodegradation processes in the anaerobic shadow become limited once dissolved
oxygen levels approach or fall below 2 mg/L. Enhanced aerobic
bioremediation technologies can boost oxygen levels in the source area and in the
anaerobic shadow to assist naturally occurring aerobic biodegradation processes
but there are other oxygen demands that need to be considered before attempting
to oxygenate the anaerobic shadow.
Each enhanced aerobic bioremediation technology has a particular way of
delivering oxygen to the saturated zone. Once delivered to the saturated zone,
dissolved oxygen can be further distributed in the treatment zone by groundwater
advection and dispersion. However, from the point where it is introduced into the
aquifer, dissolved oxygen concentration decreases along the groundwater flow
path not only through mixing with the oxygen-depleted groundwater, but also
because of biologically mediated and abiotic oxidation processes. The rate and
degree to which oxygen concentrations decrease along the groundwater flow path
and the degree to which the anaerobic shadow may be oxygenated depends, in
part, on the degree to which oxygen is lost to microbiological and abiotic
consumption in the saturated zone.
Demand for oxygen in the subsurface environment may stem from organic or
inorganic sources. Microbial biodegradation of released petroleum hydrocarbons
or naturally occurring organics (e.g., humic substances) as a carbon source by
aerobic microorganisms will generate demand for oxygen.
May 2004 XII-21
-------
Oxygen Demand From Biodegradation of Organic Compounds. Oxygen
levels are generally depleted in the subsurface, but are particularly depleted at
petroleum UST spill sites. This oxygen shortage results from the relative isolation
of the subsurface from the oxygen-replenishing atmosphere, as well as the oxygen
demands of naturally occurring organic and inorganic compounds and petroleum
hydrocarbon releases. Because of these oxygen-depleted conditions, the most
basic requirement for enhanced aerobic bioremediation is to deliver sufficient levels
of oxygen to maintain an aerobic subsurface environment.
Exhibit XII-9 outlines the stoichiometric reactions for the complete oxidation
or biodegradation of some common components of gasoline and other petroleum
products. In theory, oxygen levels of at least 3 to 3.5 times the amount of
subsurface petroleum mass that needs to be removed to meet cleanup goals must
be delivered to the groundwater and distributed over the planned remedial period.
Given typical oxygen solubility limits and the mass of contaminants that are often
found at leaking underground storage tanks sites, delivering the required amount
of oxygen can be a significant challenge. In practice, to convert one pound of
hydrocarbon material into carbon dioxide and water requires between 3 and 5
pounds of available oxygen. This is valuable for evaluating the potential
effectiveness of enhanced aerobic bioremediation.
Exhibit XII-9
Organic Compound Oxidation Stoichiometry
Petroleum
Hydrocarbon
Benzene
Toluene
Ethylbenzene
Xylenes
Cumene
Naphthalene
Fluorene
Phenanthrene
Hexane
Oxidation Reaction
C6H6+ 7.5 O2 •*• 6CO2+3H2O
C6H5CH3 + 9 O2 •*> 7CO2 + 4H2O
C2H5C6H5 + 10.5 02 •** 8C02 + 5H2O
C6H4(CH3)2 + 10.5 O2 •*• 8CO2 + 5H2O
C6H5C3H7 + 12O2 -*- 9O2 + 6H2O
C10H8 + 12O2 -* 10CO2 + 4H2O
C13H10 + 1 5.5O2 -M 3CO2 + 5H2O
C14H10 + 16.5O2 -+ 14CO2 + 5H2O
C6H14+ 9.5 O2 -**6CO2+7H2O
Oxygen
Requirement
(gram O2 per
gram
Contaminant)
3.1
3.1
3.2
3.2
3.2
3.0
3.0
3.0
3.5
Because the solubility of O2 by natural oxygen replenishment is limited and
relatively low (9 mg/L at 25°C), only a small amount of organic or inorganic
May 2004
XII-22
-------
matter in the subsurface can consume all the naturally present dissolved O2 in
groundwater. For example, using the above stochiometric equation for the
complete oxidation of benzene, oxidation of 2.9 mg/L of benzene would
theoretically consume about 9 mg/L of O2, leaving no residual oxygen in the water.
It can be readily understood how external sources of oxygen enhanced aerobic
bioremediation technologies can help aerobic bacteria by providing a source of
energy so they may consume the petroleum as a source of carbon.
Microbial Population. Oxygen demand is also a function of the vitality of the
microbial population. The larger and more active the population of aerobic
microorganisms, the larger the biological oxygen demand. However, subsurface
conditions may not be conducive to producing large numbers of microbial
populations. Exhibit XII-10 shows the likely effectiveness of enhanced aerobic
bioremediation as a function of the presence of heterotrophic bacteria in the
subsurface.
Exhibit XII-10
Relationship Between Heterotrophic Bacterial Counts And Likely
Enhanced Aerobic Bioremediation Effectiveness
Background Heterotrophic Enhanced Aerobic Bioremediation
Bacteria Levels Effectiveness
>1,000 CFU/gram dry soil Generally effective
<1,000 CFU/gram dry soil May be effective; further evaluation
needed to determine if toxic
conditions are present
Nutrients. The activity of the microbial population and the corresponding
biological oxygen demand also depend on the availability of inorganic nutrients
such as nitrogen and phosphate to support cell growth and sustain biodegradation
processes. Nutrients may be initially available in sufficient quantities in the aquifer,
but with time, they may need to be supplemented with additional nutrient loading
to maintain adequate bacterial populations. Excessive amounts of certain nutrients
(e.g., phosphate or sulfate) can repress bio-metabolism. The
carbon:nitrogen:phosphorus ratios necessary to enhance biodegradation fall in the
range of 100:10:1 to 100:1:0.5, depending on the constituents and bacteria
involved in the biodegradation process.
However, to avoid over-application of nitrogen and phosphorus, which can
unnecessarily incur added costs, plug wells, and even contaminate ground water
with nitrate, it is important to understand how much carbon can be metabolized
based on oxygen-limiting conditions. Nitrogen and phosphorus should be added to
reach the proportions identified in the previous paragraph, based on the amount of
carbon that can be metabolized at any given time compared to the total average
concentration of carbon (i.e., petroleum contamination) in the subsurface. For
example, if during full-scale operation a net 0.6 pound per hour of pure oxygen is
May 2004 XI1-23
-------
introduced to the treatment area and is assumed to be completely consumed by
aerobic microbial activity, approximately 0.17 pound per hour (4 pounds per day) >•%
of hydrocarbon is theoretically microbiologically oxidized (using a 3.5:1 ^^
oxygen:hydrocarbon stoichiometric ratio). Then, using the 100:10:1 to 100:1:0.5
C:N:P theoretically optimal ratio range for this example, between 0.4 and 0.04
pounds per day of nitrogen and 0.04 to 0.02 pounds per day of phosphorus may
need to be added to the treatment area to keep up with the estimated carbon
metabolism rate.
Alternatively, it would be reasonable for a practitioner to suggest monitoring
oxygen demand during full-scale system operation before considering adding any
nitrogen or phosphorus. If oxygen demand were to fall below about 10 mg/L in
the petroleum contaminated area, the subsurface could be tested for nitrogen or
phosphorus to determine whether insufficient concentrations of these
micronutrients is limiting microbial activity. Only after this determination is made
should nitrogen or phosphorus be added. Generally, nitrogen should not limit
aerobic degradation processes unless concentrations fall significantly below 1
mg/L. This alternative may be particularly attractive at sites located near areas
where aquifers already have nitrogen problems because it may be difficult to secure
permits for the injection of these micronutrients. If nitrogen addition is necessary,
slow-release sources should be used. Nitrogen addition can lower pH, depending
on the amount and type of nitrogen added.
^Mk
pH. Although the optimum pH for bacterial growth is approximately 7, '*
enhanced aerobic bioremediation can be effective over a pH range of 5 to 9 pH "*****'
units. Adjustment of pH conditions outside this range is generally not considered
to be viable because it is difficult to overcome the natural soil buffering capacity,
and because of the potential for rapid changes in pH to adversely affect bacterial
populations. Oxygen releasing compounds may raise the pH even higher than the
5-9 range, which can be fatal to microbes.
Temperature. Oxygen uptake and bacterial growth rate are directly affected
by temperature. From 10°C to 45°C, the rate of microbial activity typically
doubles for every 10°C rise in temperature. Below 5°C, microbial activity
becomes insignificant. In most areas of the United States, the average
groundwater temperature is about 13°C. Groundwater temperatures may be
somewhat lower or higher in the extreme northern and southern states. While
individual microorganism growth rates decrease with temperature, a higher steady
state biomass of active organisms (each one working more slowly, but more of
them working) can result from lower temperatures. Because of this and the
increased solubility of oxygen at lower temperatures, biodegradation can
sometimes be as fast or faster at lo er temperatures than at more moderate
temperatures.
Inorganic Oxygen Demand. Oxygen demand arises from a depletion of '•*""*-.
subsurface oxygen from biological or inorganic processes coupled with poor
May 2004 XII-24
-------
oxygen replenishment, in contrast to surface water bodies, groundwater systems
are typically isolated from the atmosphere, limiting the opportunity for natural
oxygen to be replenished. This atmospheric isolation allows dissolved oxygen
levels to become depleted and subsurface conditions to become geochemically
reduced. Introducing and distributing oxygen under these reduced conditions are
challenging for the application of enhanced aerobic bioremediation, because
introduced oxygen may react with and become lost to organic or inorganic
chemical constituents that would otherwise be relatively inconsequential to the
environmental cleanup.
Exhibit XII-11 presents a sample of some common inorganic processes that
consume oxygen in groundwater.3 Corrective action plan data should be reviewed
to identify what is already known about aquifer conditions in the area around the
site to determine whether signficant reduced inorganic species exist in the
subsurface that could remove oxygen from groundwater. If so, these species can
limit the ability of biodegrading bacteria to effectively implement enhanced aerobic
bioremediation. In such cases, soil core samples may need to be collected and
analyzed for reduced iron, sulfide or other inorganic constituents. These samples
can help to determine the potential loss of oxygen to the aquifer and to verify that
enhanced aerobic bioremediation will be able to effectively deliver sufficient
oxygen to overcome these limiting factors. This assessment cannot be made from
analyses of groundwater samples, because the reduced inorganic complexes are
primarily precipitated in the aquifer material.
Exhibit XII-11
Inorganic Oxidation Processes That Consume Dissolved Oxygen In
Groundwater
Process Reaction
Sulfide Oxidation O2 + 1/2HS- •*• YzSO2' + 1/2H+
Iron Oxidation Y*O2 + Fe+2 + H+ •*• Fe+3 + 1/2H2O
Nitrification O2 + 1/2NH4+ •*• 1/2NO3- + H+ + 1/2H2O
Manganese Oxidation O2 + 2Mn2+ + 2H2O -»• 2MnO2 (s) +4H+
Iron Sulfide Oxidation 15/4O2 + FeS2 (s) + 7/2H2O •*• Fe(OH)3 (s) +2SO42' +
4H+
Many inorganic oxygen-consuming reactions produce solid precipitates that
can accumulate in soil pore spaces. As discussed below, these precipitates can
restrict soil permeabilities and thus further affect the ability of enhanced aerobic
bioremediation technologies to deliver and distribute oxygen to hydrocarbon-
degrading microorganisms.
3
From Freeze R.A. and John A. Chem', 1979. Groundwater. Prentice Hall.
May 2004 XII-25
-------
Advective and Dispersive Transport Factors. The site conditions affecting
advection and dispersion of dissolved oxygen are outlined below. These
conditions are:
• Intrinsic permeability
• Soil structure and stratification
• Hydraulic gradient
• Depth to groundwater
• Iron and other reduced inorganic compounds dissolved in groundwater
Each of these factors is described in more detail below.
Intrinsic Permeability. Intrinsic permeability is a measure of the ability of
soil to transmit fluids. Intrinsic permeability is the single most important soil
characteristic in determining the effectiveness of enhanced aerobic bioremediation,
because intrinsic permeability controls how well oxygen can be delivered and
dispersed to subsurface microorganisms. Hydraulic conductivity is a measure of
the resistance of aquifer material to groundwater flow. This unit of measure is
particularly relevant to understanding the ability to move oxygen dissolved in
groundwater through the saturated treatment zone. Hydraulic conductivity is
related to intrinsic permeability by the following equation.
where: K - hydraulic conductivity (L/T)
k — intrinsic permeability (L2)
7 = weight density of water (F/L3)
H = dynamic viscosity of water (F» T/L2)
L = mean grain diameter
T = transmissivity
F = fluid density
Intrinsic permeability often decreases near injection wells or infiltration
galleries. This also commonly results from precipitation of carbonates, or
precipitates of other minerals derived from fertilizer solutions. In general, oxygen
is more easily distributed in soils with higher soil permeabilities (e.g., coarse-
grained soils such as sands) than in soils with lower permeabilities (e.g., fine-
grained clayey or silty soils).
Calculation of intrinsic permeability can be derived from hydraulic conductivity
measurements taken from on-site pump testing. Pump test or slug test-derived
permeability ranges are typically representative of average hydraulic permeability
conditions for heterogeneous conditions. Alternatively, intrinsic permeability can
be estimated from soil boring logs.
May 2004 XII-26
-------
Permeabilities derived from pump or slug test analyses or estimated from
boring logs are only approximations of actual subsurface conditions and should be
regarded as such in the evaluation of enhanced aerobic bioremediation potential
effectiveness.
Intrinsic permeability can vary over 13 orders of magnitude (from 10~15 to 10~3
cm2) for the wide range of earth materials. Exhibit XII-12 provides general
guidelines on the range of intrinsic permeability values over which enhanced
aerobic bioremediation is likely to be effective.
The intrinsic permeability of a soil is likely to decrease as enhanced aerobic
bioremediation progresses. If the soil intrinsic permeability indicates borderline
potential effectiveness (e.g., 10* < k < 10~7), the geochemistry should be further
evaluated.
Exhibit XII-12
Intrinsic Permeability And Enhanced Aerobic Bioremediation
Effectiveness
Hydraulic
Conductivity (K)
(in ft/s)
Intrinsic
Permeability (k)
(in ft2)
Enhanced Aerobic Bioremediation
Effectiveness
K>10-6
10'6< K <10'7
K<10'7
k>10-12
10-12
-------
However, the petroleum mass in the silt and clay horizons will likely not
biodegrade, and will also likely diffuse into the sand zone, causing a rebound in *+^
dissolved hydrocarbon concentrations at the site. .^
Unless site soils are homogeneous, average soil intrinsic permeability may not
adequately determine the viability of enhanced aerobic bioremediation approaches
because discrete low permeability soil horizons may exist, and these horizons
might contain a large fraction of the subsurface petroleum mass. In most cases, it
is prudent to evaluate petroleum mass distribution across all soil types to determine
whether enhanced aerobic bioremediation is likely to be effective and will achieve
cleanup objectives. If select soil horizons containing hydrocarbon mass are not
expected to be effectively treated using enhanced aerobic bioremediation ,
enhanced aerobic bioremediation may not be viable for the site. For example, if
50% of the contaminant mass is contained and isolated in low permeability soil
horizons and the site cleanup goals is a 95% reduction in petroleum contaminant
concentrations, then it is reasonable to conclude that the goal cannot be achieved
using enhanced aerobic bioremediation. However, in such circumstances,
combining enhanced aerobic bioremediation with other technologies that enhance
the permeability of low permeability horizons in the contaminated zone (e.g., soil
fracturing) could be considered. Soil fracturing could allow dissolved oxygen and
other microbial nutrients to be effectively delivered through the engineered
fractures in low permeability soil. However caution should be observed when
considering this option because the same fractures produced to enhance
permeability for nutrient delivery could also be a potential preferential flow path *****
for contaminant plume migration. "*****
Hydraulic Gradient. Enhanced aerobic bioremediation technologies
ultimately rely on groundwater advection and dispersion (i.e., flow) to distribute
dissolved oxygen to the subsurface. Distribution of introduced dissolved oxygen is
most effective under hydrogeologic conditions conducive to higher groundwater
flow rates. These conditions exist when the combined values of hydraulic gradient
and hydraulic conductivity are relatively high.
Note that state regulations may either require permits for nutrient injection or
prohibit them entirely. Depending on the specific enhanced aerobic bioremediation
technology and the state in which the site is located, permits that may be required
include underground injection, treated groundwater discharge (to sanitary or storm
sewer, or air (soil vapor) discharge. Several federal, state and local programs exist
that either directly manage or regulate Class V aquifer remediation wells, and
many of these require permits for underground injection of oxygen or bionutrients.
As the hydraulic gradient increases, the groundwater velocity increases
proportionately. This same relationship exists between groundwater velocity and
soil permeability. Groundwater velocity is inversely proportional to soil porosity.
As porosity increases, groundwater velocity decreases. For purposes of evaluating >«^
the feasibility of using an enhanced aerobic bioremediation technology, keep in
May 2004 XII-28
-------
mind that the principal direction of groundwater flow and oxygen transport is
along the line of maximum hydraulic gradient.
To maximize the distribution of dissolved oxygen through and biodegradation
rates in the contaminated zone, enhanced aerobic bioremediation technologies
often introduce dissolved oxygen at levels that exceed the solubility limit of oxygen
in groundwater under atmospheric conditions. However, when the oxygen is not
rapidly dissipated or used (e.g., as an electron acceptor during microbial
respiration), the oxygen can partition out of the dissolved-phase and be lost to the
unsaturated zone as a gas.
Depth to Groundwater. The depth to groundwater at a site can also affect
the availability and transport of dissolved oxygen to the subsurface. Infiltrating
precipitation, such as rainfall or snow, is a source of dissolved oxygen to the
saturated zone. When groundwater is relatively deep or confined, less
precipitation infiltrates, minimizing the amount of atmospheric dissolved oxygen
that reaches the groundwater. Also, pavement prevents infiltration of rainfall or
snowmelt At sites where the water table is close to the surface, more mixing of
groundwater with air-saturated precipitation occurs, resulting in more opportunity
for groundwater to be oxygenated. When this occurs, dissolved oxygen levels in
groundwater can even approach those found in streams and other surface water
bodies.
Iron and Other Reduced Inorganic Compounds Dissolved in
Groundwater. In addition to being a significant oxygen sink, the effective
intrinsic permeability of the saturated zone can be significantly reduced if the
enhanced aerobic bioremediation treatment zone contains naturally elevated levels
of reduced iron (e.g., ferrous iron, or Fe+2) or other mineral species. The net
impact of elevated levels of reduced species can therefore be a loss of delivered
oxygen and a decreased ability to distribute any excess oxygen to the aerobic
microorganisms involved with the degradation of the petroleum hydrocarbons.
Precipitation of oxidized inorganic complexes and biological mass can foul
monitoring and injection well screens and potentially aquifer pore space where
oxygen is delivered to the subsurface.
Exhibit XII-13 can be used as a guide to help determine whether the corrective
action plan has considered site levels of dissolved iron and if dissolved iron levels
at the site could have an adverse effect on the enhanced aerobic bioremediation
approach.
In some situations, hydraulic gradients can be enhanced to help increase
groundwater flow and oxygen delivery rates and flush dissolved oxygen through
the contaminated zone. One common approach is to create an artificial gradient by
removing groundwater downgradient of the source area, treating it, and re-
introducing it in the upgradient source area. For example, hydrogen peroxide
^^ enhanced aerobic bioremediation applications often require extracting
May 2004 XII-29
-------
Exhibit XII-13
Relationship Between Dissolved Iron And Enhanced Aerobic
Bioremediation Effectiveness
Dissolved Potential Effectiveness of
Iron Concentration Enhanced Aerobic Bioremediation
(mg/L)
Fe+2 < 10 Enhanced aerobic bioremediation will likely be
effective.
10 > Fe+2 > 20 Enhanced aerobic bioremediation injection wells
and delivery systems will require periodic testing
and may need periodic replacement.
Fe+2 > 2 Enhanced aerobic bioremediation may not be cost
effective due to loss of dissolved oxygen to the
formation and equipment maintenance problems
associated with inorganic precipitation. This would
especially be the case where groundwater is
extracted, treated, amended with oxygen (e.g.,
hydrogen peroxide) and reinjected.
contaminated groundwater from the downgradient port; on of the dissolved
hydrocarbon plume, treating the extracted groundwater for hydrocarbons, and re-
injecting the treated groundwater amended with hydrogen peroxide into one or
more upgradient locations.
This lowers the groundwater level in the downgradient extraction locations and
raises it in upgradient injection locations, which provides an artificially increased
gradient. This, in turn, increases the rate of groundwater and oxygen flow across
the contaminated zone.
Even with preferential hydrogeologic conditions, distributing dissolved oxygen
throughout the subsurface is difficult because of the inherent limits of groundwater
flow and the number of oxygen "sinks," or uptakes, that can exist, particularly in
areas contaminated with petroleum hydrocarbons. These limitations frequently
require that the corrective action plan call for placement of a large number of
oxygen delivery points in the treatment area to decrease enhanced aerobic
bioremediation technology's reliance on groundwater flow as the principal source
of distributed oxygen.
In addition to being a parameter considered in evaluating the potential
effectiveness of enhanced aerobic bioremediation, hydraulic gradient is an
engineering design issue. If the gradient is not steep enough to provide adequate
flow and oxygen transport through the contaminated zone, then certain
engineering provisions (e.g., spacing application points more closely, creating
May 2004 XII-30
-------
artificial hydraulic gradients) can be added to the design to enhance oxygen
distribution. However, economic considerations limit the extent to which design
changes can be made in an enhanced aerobic bioremediation delivery system to
ensure adequate oxygen distribution.
Constituent Characteristic Affecting Enhanced Aerobic Bioremediation
It is important to evaluate the potential impacts of site contaminants on the
performance of the proposed enhanced aerobic bioremediation approach. In
particular, it is important to review how the chemical structure, chemical
properties, concentrations and toxicities of the petroleum contaminants can
influence remedial performance.
Chemical Class and Susceptibility to Bioremediation. Petroleum products
are complex mixtures of hundreds or even thousands of hydrocarbon chemical
constituents, other chemical constituents and additives. Each of these constituents
has a different atomic structure that determines, in part, its relative
biodegradability. Although nearly all constituents in petroleum products found at
leaking underground storage tank sites are biodegradable to some extent,
constituents with more complex molecular structures are generally less readily
biodegraded than those with simpler structures. On the other hand, most low-
molecular weight (nine carbon atoms or less) aliphatic and monoaromatic
constituents are more easily biodegraded than higher molecular weight aliphatic or
polyaromatic organic constituents.
Exhibit XII-14 lists the relative biodegradability of various petroleum products
and constituents. The exhibit shows that hydrocarbon molecules containing a
higher number of carbon atoms (e.g., lubricants with 26- to 38-carbon chains)
degrade more slowly, and perhaps less completely, than those with shorter carbon
chains (e.g., gasoline). However, cleanup goals are frequently tied to a small
subset of chemical compound components of the various petroleum products in
Exhibit XII-9 rather than a total petroleum hydrocarbon concentration. Often
chemical compounds in petroleum products identified in Exhibit XII-14 as being
less readily biodegradable are not present at contaminated sites at levels
significantly above cleanup standards because of the low solubility characteristic
that these compounds can have. Consequently, cleanup standards for
contaminants in less readily biodegradable petroleum formulations may be reached
through enhanced aerobic bioremediation more quickly than those for more soluble
compounds in more biodegradable formulations.
Certain petroleum constituents are more recalcitrant than most other
constituents. For example, MTBE, a gasoline additive, is frequently found at
leaking UST sites because of its environmental persistence and its apparent
resistance to bioremediation. Some researchers have estimated that the half-life of
MTBE in the environment is at least two years, whereas the typical half-life for
BTEX compounds in the environment is approximately two to three months.
May 2004 XII-31
-------
Exhibit XII-1 4
Composition id Relative Biodegradability Of Petroleum Products
Product
Natural Gas
Gasoline
Kerosene,
Diesel
Light Gas
Oils (e.g., No
2 Fuel Oil)
Heavy Gas
Oils and Light
Lubricating
Oils
Lubricants
Asphalts
Relative Product
Major Components Biodegradability
Normal and branched-chain alkanes. Higher
One to five carbons in length. Examples:
ethane, propane.
Normal and branched hydrocarbons >
between 6 and 10 carbons in length.
Examples: n-butane, n-pentane, n-
octane, isopentane, methylpentanes,
benzene, toluene, xylenes,
ethylbenzene.
Primarily 11 to 12 carbon hydrocarbons,
although the range of carbons extends
well above and below this range.
Generally contains low to non-detectable
levels of benzene and polyaromatic
hydrocarbons. Jet fuel oils have a
similar composition. Examples: n-
nonane, n-decane, n-dodecane,
naphthalene, n-propylbenzene.
Twelve to 1 8 carbon hydrocarbons.
Lower percentage of normal alkanes
than kerosene. These products include
diesel and furnace fuel oils (e.g., No. 2
fuel oil). Examples: fluorene,
naphthalene, phenanthrene,
isopropylbenzene.
Hydrocarbons between 18 and 25
carbons long.
Hydrocarbons between 26 and 38
carbons long. ^
k
Heavy polycyclic compounds. Lower
May 2004
XII-32
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Therefore, one should carefully consider the biodegradability of the target
contaminants when forecasting the potential effectiveness and usefulness of an
enhanced aerobic bioremediation technology. The enhanced aerobic
bioremediation design and implementation should focus on the most recalcitrant
compounds within the released petroleum product, unless another remedial
technology is being proposed to address those compounds.
It is not necessarily the most recalcitrant or most difficult compound to
bioremediate that determines the duration of a remediation project. For example,
the baseline concentration of the most recalcitrant site compound may be much
closer to its respective cleanup goal or an acceptable risk-based concentration than
a more readily biodegradable petroleum constituent at a baseline level much
greater than its cleanup goal. In this case, the more biodegradable constituent may
initially be the focus of the enhanced aerobic bioremediation design and cleanup.
As remediation progresses, the mix of petroleum products remaining should
periodically be compared to the site's proposed cleanup level to determine whether
the remedial approach needs to be enhanced to address the remaining target
compounds.
Researchers have estimated and published biodegradation rate constants for
various petroleum hydrocarbons. These rate constants can indicate the relative
biodegradability of petroleum hydrocarbon constituents under field conditions.
However, actual degradation rates for target contaminants may depend on
constituent-, site-, and enhanced aerobic bioremediation implementation-specific
conditions. For example, the mixture and concentrations of the different
petroleum constituents in the site soil and groundwater may play an important role
in determining relative degradation rates. The amount of natural organic matter in
the soil and the degree to which the petroleum constituents attach themselves to it
will affect the relative rates of biodegradation. These issues, especially as they
relate to contaminant characteristics that affect aerobic bioremediation, are
discussed below.
Contaminant Phase Distribution. Spilled petroleum products may be
partitioned into one or more phases and zones in the subsurface including:
• Unsaturated soils (sorbed phase)
• Saturated soil (sorbed phase)
• Dissolved in groundwater (aqueous phase)
• Unsaturated soil pore space (vapor phase)
• Free mobile product (liquid phase)
• Free residual product smeared onto soil above and below the water
table
Understanding how the petroleum contaminant mass is distributed in the
subsurface can be important to both evaluating the applicability of enhanced
aerobic bioremediation and identifying a particular enhanced aerobic
May 2004 XII-33
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bioremediation technoloi hat will be effective. Depending on site-specific
cleanup goals and conta ,nt levels, a disproportionate amount of contaminant <*•*»
mass in one medium or; ,ner could preclude the use of enhanced aerobic >,„*
bioremediation technologies. For example, if a relatively large portion of the mass
of a site target compound (e.g., benzene) is held in residual free product that is
vertically smeared above and below the water table, enhanced aerobic
bioremediation may not be able to achieve the site cleanup goals within a
reasonable period of time. However, in such a case, enhanced aerobic
bioremediation could still potentially be used at the fringes of the contaminated
area while a more aggressive technology is employed in the residual-free product
zone.
Information on the distribution of target compounds in the subsurface can also
be used to help identify the most appropriate enhanced aerobic bioremediation
technology for a site. Depending on where most of the target contaminant mass is
located, one or more of the enhanced aerobic bioremediation technologies may be
viable. For example, a disproportionate amount of target contaminant mass in the
unsaturated soil would logically lead to the selection of an unsaturated zone
enhanced aerobic bioremediation approach (e.g., bioventing). On the other hand,
if a disproportionate amount of target contaminant mass is in the saturated zone,
one of the enhanced aerobic bioremediation technologies that introduces high
concentrations of dissolved oxygen to the subsurface may be a reasonable
approach.
Concentration and Toxicity. High concentrations of petroleum organics or * "
heavy metals in site soils and groundwater have traditionally been thought to be
potentially toxic to, or inhibit growth and reproduction of, biodegrading bacteria.
Soil containing petroleum hydrocarbons in amounts greater than 50,000 ppm, or
heavy metals in excess of 2,500 ppm, was thought to be inhibitory and/or toxic to
many aerobic bacteria. However, it is becoming increasingly evident that many
microorganisms are able to tolerate and adapt to petroleum concentrations well
above 50,000 ppm. Some researchers have even reported being able to isolate
living bacteria directly from gasoline product.
While it appears that bacteria may be more adaptable than initially believed, to
the extent that these higher levels of petroleum hydrocarbons represent a large
mass of contamination in unsaturated or saturated soil in contact with
groundwater, the adapted populations of bacteria may not be able to address the
contaminant mass in a reasonable timeframe. When considering the feasibility of
enhanced aerobic bioremediation, it is important to evaluate the mass of the target
contaminants of concern relative to potential biodegradation rates and the cleanup
timeframe objective.
It is possible that the effects of elevated contaminant levels can include partial
biodegradation of only a fraction of the hydrocarbons at reduced rates, or reduced --•"••*.
bacterial reproduction rates or metabolism, resulting in minimal or no appreciable
May 2004 XII-34
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soil treatment. The guidance threshold values summarized in Exhibit XII-15 can
be compared to average site concentrations provided in the corrective action plan
as another way of forecasting the potential effectiveness of enhanced aerobic
bioremediation. Again, it is important to recognize that the values shown in
Exhibit XII-15 are guidance values only.
As outlined in Exhibit XII-15, the threshold petroleum concentrations above
which biodegradation is inhibited could also indicate the presence of free or
residual product in the subsurface. In the initial effectiveness screening of
enhanced aerobic bioremediation (Step 1), one of the feasibility bright lines
discussed was the absence of free mobile product. If threshold soil petroleum
levels exist, then free or residual petroleum product most likely exists in the soil,
and enhanced aerobic bioremediation will not be effective without first removing
the product through other remedial measures.
Exhibit XII-15
Constituent Concentration and Enhanced Aerobic Bioremediation
Effectiveness
Contaminant Levels (ppm) R. Danced Aerobic
Bioremediation Effectiveness
Petroleum constituents < 50,000 Possibly effective
Heavy metals < 2,500
Petroleum constituents > 50,000 Not likely to be effective either due
or to toxic or inhibitory conditions to
Heavy metals > 2,500 bacteria, or difficulty in reaching
cleanup goal within reasonable
period of time
Bioavailability Characteristics. The extent to which and the rate at which a
particular petroleum hydrocarbon compound can be biodegraded by
microorganisms depends not only on the compound's inherent biodegradability, but
also on the availability of the compound to hydrocarbon-degrading bacteria
("bioavailability"). Several contaminant properties contribute to bioavailability in
the subsurface. In particular, the compound-specific properties of solubility and
the organic carbon partition coefficient (K^) help establish the relative
bioavailability of contaminants. These properties can be used to help determine the
susceptibility of the contaminant mass to enhanced microbial degradation and,
ultimately, the potential effectiveness of enhanced aerobic bioremediation. Note
that some compounds (e.g., MTBE) may be relatively bioavailable, but are difficult
to biodegrade. Special considerations for MTBE are discussed beginning on page
XII-39. This section continues with a discussion of the parameters of solubility
and K^. and their influence on enhanced aerobic bioremediation effectiveness.
May 2004 XII-35
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Solubility. Solubility is the maximum concentration of a chemical that can be
dissolved in water at a given temperature without forming a separate chemical
phase on the water (i.e., free product). Most petroleum compounds have relatively
low solubility values, thus limiting the concentrations of contamination that can be
dissolved in groundwater and limiting their bioavailability in the aqueous phase.
This is because less contaminant mass is able to reside in groundwater for
biodegradation relative to contaminants with higher solubility limits. However, the
solubility values for petroleum hydrocarbons range significantly - over four orders
of magnitude - as shown in Exhibit XII-16. The solubility values in Exhibit XII-
16 represent those of pure phase chemicals. For example, benzene dissolved in
water by itself (with no other compounds present) can reach a maximum
concentration in water of about 1.79 g/L before a separate phase develops. When
multiple compounds are present such as at a petroleum release site, effective
solubility values can be expected to be lower. While not representing effective
solubility concentrations that may exist at particular petroleum release sites, the
values present in Exhibit XII-16 provide a sense for the relative solubility
concentrations for a range of fuel components. It is beyond the scope of this
document to describe the chemistry involved and how effective solubility might be
estimated.
Exhibit XII-1 6
Solubility Values And Organic Partition Coefficients For Select
Petroleum Hydrocarbon Constituents
Compound
MTBE
Benzene
Toluene
Ethylbenzene
Xylenes (total)
Cumene
Naphthalene
Acenaphthene
Molecular
Weight (g/mol)
88.15
78
92.15
106.17
106
120.19
128
154
Solubility in
Water (g/L)
51
1.79
0.53
0.21
0.175
50
0.031
.0035
Organic
Carbon
Coefficient
(Koc in mL/g)
12
58
130
220
350
2,800
950
4,900
Compounds with higher solubility values are generally smaller, lower molecular
weight molecules (e.g., benzene). When spilled, these compounds exist in
groundwater at higher relative concentrations and move more quickly through the
aquifer than do compounds of higher molecular weights. These compounds are
May 2004
XII-36
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generally more biodegradable because of both their relatively smaller size and
bioavailability in the aqueous phase, because proportionately more contaminant
mass is in the groundwater where it may be mineralized by aerobic bacteria.
Larger and higher molecular weight hydrocarbon molecules are generally less
soluble in water; therefore, their dissolved concentrations in groundwater tend to
be limited (e.g., acenaphthene ). This property not only reduces the availability of
these hydrocarbons to biodegradation, it also limits the mass of these contaminants
that can migrate with groundwater over time. For bioremediation of higher
molecular weight compounds at a particular site, these two factors may offset one
another. In simpler terms, bioremediation of the larger hydrocarbons may take
longer, but there is more time to complete the biodegradation because the
contamination is not moving away from the treatment area as quickly. The most
appropriate remediation for sites that are contaminated mostly with heavy
petroleum constituents might be excavation and application of an off-site remedial
technology, such as thermal desorption, or proper disposal of the contaminated
soil.
Solubility is also an indicator of likely contaminant sorption onto soil. When
contaminants are sorbed onto soil particles, they are less available for
bioremediation. A compound with a relatively high solubility has a reduced
tendency to sorb to soil contacting contaminated groundwater. Conversely,
contaminants with relatively low solubility values will generally have an increased
tendency to sorb to soil contacting contaminated groundwater. This concept is
described in more detail below.
Koc Factor. When groundwater is contaminated by a release from a
petroleum underground storage tank, the proportion of hydrocarbon mass in the
soil is often far greater than that dissolved in groundwater. This is due in part to
the relatively low solubility thresholds for petroleum contaminants. However,
another factor is the relatively strong tendency for most petroleum hydrocarbons
to sorb to naturally occurring organic carbon material in the soils. This tendency,
along with the sheer mass of soil relative to groundwater in a contaminated area,
can lead to hydrocarbon mass distributions that are so lopsided they can make the
mass in the dissolved-phase appear insignificant. However, because
bioremediation occurs in the dissolved phase, that portion of a petroleum mass is
always significant in a bioremediation project. It is important to also know how
the target organic petroleum compounds are partitioned between the dissolved and
unsaturated and saturated sorbed phases.
is a compound-specific property that helps define the equilibrium condition
between organic carbon and the contaminant concentrations in an aqueous
solution. Using site-specific soil organic carbon content data (i.e., fraction of
organic content or foe), K^. can be used to determine the equilibrium contaminant
concentrations between groundwater and soil below the water table. The typical
organic carbon content in surface soils ranges from 1 to 3.5 percent. In subsurface
May 2004 XII-37
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soils, organic carbon content is an order of magnitude lower because most organic
residues are either incorporated or deposited on the surface.
The equation below shows how Koc is defined and used with site-specific
fraction of organic carbon (f^) data to determine the soil-to-groundwater
concentration equilibrium ratio, Kd. Knowing the contaminant concentration in
one media (e.g., groundwater), the contaminant concentration in the other media
(e.g., soil) can be predicted using the site- and constituent-specific Kd sorption
constant.
f\i = foe X KOC
where:
Kd = grams contaminant sorbed/grams organic carbon
= grams contaminant/mL solution
K,,,. = compound-specific sorption constant and
fx = fraction of organic carbon in site soil
Higher Koc and Kd values indicate more contaminant mass is likely to be
retained in soil and therefore less readily bioavailable. Conversely, lower K^. and
KJ, values indicate lower contaminant concentrations will exist in equilibrium in soil
for given concentrations in groundwater. Exhibit XII- 16 provides petroleum
constituent K,,,, values for a list of common petroleum hydrocarbon. A comparison
of the solubility and K^. values for the sample group of petroleum hydrocarbons
reveals the inverse relationship between the two parameters. For example,
compounds with higher solubility values have lower K^. constants.
The relative proportions of contaminants in the sorbed and dissolved phases is
important to establish when evaluating the likely effectiveness of enhanced aerobic
bioremediation. A disproportionate amount of target hydrocarbon contaminant
mass sorbed to the soil, and therefore less bioavailable, may signal that enhanced
aerobic bioremediation by itself may not be an effective method of reducing
subsurface contaminant mass. In this case, it may be necessary to combine
enhanced aerobic bioremediation with other technologies that can help bring more
contaminant mass out of the sorbed phase and into the dissolved phase so it can be
biodegraded. This highlights the importance of establishing a cleanup goal up
front.
In the absence of site-specific data that reveal the distribution of contaminant
mass, solubility and K^ data can be used to obtain a general understanding of the
likelihood that enhanced aerobic bioremediation is applicable at the site.
Petroleum contaminants with generally high solubility limits and low K^. values
tend to be more bioavailable in groundwater, and the contaminant mass can often
be destroyed by enhanced aerobic bioremediation technologies. When
contaminant solubility constants are generally low and K,,,, values are high,
enhanced aerobic bioremediation will be limited in its effectiveness.
May 2004 XII-38
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Special Considerations for MTBE. Not all sites have indigenous microbial
suites capable of degrading MTBE. The MTBE chemical bonds are strong and not
easily cleaved through chemical or biological means. As such, when enhanced
aerobic bioremediation is to be utilized for addressing MTBE, it may be prudent to
verify that native MTBE-degraders exist at a site, before implementing a costly and
complex enhanced aerobic bioremediation plan. This can be done with standard
microcosm tests. Such laboratory test can be also used to optimize the Enhanced
aerobic bioremediation procedures for the site so as to insure enhanced
biodegradation of both petroleum compounds and MTBE. If the microcosm tests
indicate that insufficient MTBE-degrading microbes exist at a site, then it may be
necessary to bioaugment the site by increasing the numbers of microbes. Caution
is necessary when bioaugmenting with a cultured microbial suite as the technical
effectiveness, cost-effectiveness, and longevity of microbes need to be well
understood. Due to the vagaries of geochemistry and microbiology in the
subsurface, site-specific microcosms and/or pilot tests may be advisable before full-
scale implementation of a bioaugmentation system.
When MTBE biodegrades, it often produces an intermediary product called
tertiary butyl alcohol (TEA). The subsurface creation of TBA has been noted at
some enhanced aerobic bioremediation field sites that contain MTBE. Therefore,
any enhanced aerobic bioremediation application at a site containing MTBE has
the potential to create TBA. This constituent of concern has been noted to rapidly
disappear from the subsurface at some biodegradation sites, while at other sites,
the TBA seems to be recalcitrant. Field workers need to be aware of the possible
subsurface creation of TBA, and seek to avoid creating a undesirable, recalcitrant
TBA plume.
The presence of TBA in the subsurface at an MTBE-impacted site is not
definitive proof of MTBE biodegradation. TBA is a gasoline additive that can be
present in concentrations of up to 9.5% by volume, and it is often found in
commercial-grade MTBE at 1-2% by volume. Therefore, it is possible to detect
subsurface TBA at an MTBE site, even if no MTBE biodegradation is occurring.
Careful study of TBA/MTBE ratios, as well as their plume patterns relative to each
other and relative to the enhanced aerobic bioremediation activities can help to
determine if the TBA was in the original gasoline spill or if it is present due to
biodegradation of TBA. It is also important to note that as an alcohol, TBA can
be difficult to detect at low levels in water samples; detection limits from
laboratory analyses can vary widely, and many analyses will not find TBA when it
is present in low concentrations.
When considering enhanced aerobic bioremediation for a site that also contains
the gasoline additive methyl tertiary butyl ether, the presence of MTBE mandates
that several issues be considered. Exhibit XII-17 provides a list of the questions
that should be asked before enhanced aerobic bioremediation is considered for
treating MTBE at a petroleum UST site.
May 2004 XII-39
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EXHIBIT XII-17
MTBE Considerations For Applying Enhanced Aerobic
Bioremediation
• Does the presence of MTBE require treating a larger region of the aquifer?
• Does the presence of MTBE require treating a deeper portion of the aquifer,
especially in the downgradient area of the plume where MTBE plumes
sometimes "dive" ?
• Does either of these mandates require installing more oxygen application
points?
• Are native MTBE-degrading microbes known to exist at that specific site?
Are they sufficient in number to be effective? Are they located where the
MTBE presently is? Are they located where the MTBE will be in the future?
• Is the addition of an MTBE-degrading microbial suite needed?
• Has the greater mobility of the MTBE been accounted for in the plan?
• Does the presence of more readily biodegradable compounds (example:
BTEX) indicate a delay before MTBE is consumed by microbial
populations? If so, what are the implications of this?
• Is the same remediation method being used for the hydrocarbons also
sufficient to address the MTBE? Does the site contain a sufficient oxygen
load and appropriate microbial suite (native or bioaugmented)?
• Has the corrective action plan accounted for the possible biological
formation of the intermediary product tertial butyl alcohol (TBA), including
the possibility of creating an undesirable TBA plume?
• Has the corrective action plan accounted for the possible biological
formation of the intermediary product tertial butyl alcohol (TBA), including
the possibility of creating an undesirable TBA plume?
The various technical issues raised in Exhibit XII-17 demonstrate that while
enhanced aerobic bioremediation for MTBE and other similar oxygenates can be
promising, a number of special factors should be considered before moving
forward with application of an enhanced aerobic bioremediation project for
MTBE. Although the addition of supplemental microbial suites (bioaugmentation)
is beyond the scope of this chapter, it can be considered for such sites. For more
information on the use of bioaugmentation, see How to Evaluate Alternative
Cleanup Technologies for Underground Storage Tank Sites: A Guide for
Corrective Action Plan Reviewers (US EPA 510-R-04-002), Chapter X ("In-Situ
Groundwater Bioremediation").
May 2004 XII-40
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As discussed earlier, assessing the applicability of an enhanced aerobic
bioremediation plan for MTBE is more complex than a similar assessment for
other gasoline compounds. While typical gasoline compounds like BTEX have
been found to be nearly ubiquitously biodegradable under a wide variety of
subsurface conditions, the same cannot be said for MTBE. Studies of MTBE
biodegradability have produced highly variable results.. Therefore, it is not yet
possible to make universal statements about enhanced aerobic bioremediation
effectiveness for MTBE. Instead, the reviewer is advised to carefully consider
site-specific conditions before committing to enhanced aerobic bioremediation for
MTBE. Exhibit XII-18 on the next page provides some guidance.
Because MTBE biodegradability still appears to be site-specific and because
the state of knowledge is still developing, it may be advisable to conduct site-
specific microcosm studies using the intended enhanced aerobic bioremediation
method before committing to a full-scale remediation plan for MTBE. Such
microcosm studies may investigate: MTBE biodegradation under varying
conditions, the need for bioaugmentation, the production of TEA, etc.
Step 3 - Evaluation of Enhanced Aerobic Bioremediation Design
This section provides guidance on reviewing and evaluating the enhanced
aerobic bioremediation design. It focuses on prompting reviewers to identify and
review key elements of corrective action plans to help ensure they demonstrate a
coherent understanding of the basis for the enhanced aerobic bioremediation
system design. In addition, this section provides information on typical enhanced
aerobic bioremediation technology components to help verify that the corrective
action plan has included the basic equipment requirements for the remedial system.
It is assumed that the detailed technology screening process (described in Steps
1 and 2) has verified that enhanced aerobic bioremediation appears to be
appropriate and is expected to be an effective cleanup approach, given site-specific
conditions. If the enhanced aerobic bioremediation effectiveness evaluation has
not been completed, it is strongly recommended that this be done before the design
is evaluated.
May 2004 XII-41
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Exhibit XII-18
Detailed Evaluation of Enhanced Aerobic Bioremediation
Effectiveness for MTBE
REVIEW SITE-SPECIFIC NATURE OF MTBE PLUME
• Is full lateral extent of MTBE plume defined?
• Is full vertical extent of MTBE plume defined?
• Is transport rate of dissolved-phase MTBE defined?
I
Conduct studies (e.g.,
Microcosms) to evaluate the
site-specific potential for
MTBE biodegradation.
Does the enhanced aerobic bioremediation
plan account for BTEX/TPH compounds that|
can co-occur with MTBE?
NO
UNKNOWN
Enhanced aerobic bioremediation
effectiveness may be reduced for
MTBE due to preferential
biodegradation of BTEX.
YES
Conduct studies (e.g.,
Microcosms) to evaluate the
site-specific potential for
MTBE biodegradation.
Are native MTBE-degrading microbes
present on-site in sufficient numbers and
proper locations?
NO
Bioaugmentation may be
necessary.
UNKNOWN
YES
Conduct studies (e.g.,
Microcosms) to evaluate the
site-specific potential for
MTBE biodegradation.
Are oxygen and nutrients present at levels
high enough to biodegrade MTBE,
BTEX/TPH?
NO
Add nutrients or oxygen as
required.
UNKNOWN
YES
Conduct studies (e.g.,
Microcosms) to evaluate the
site-specific potential for
MTBE biodegradation.
Is there sufficient MTBE present to
sustain long-term microbial activity?
NO
UNKNOWN
Low concentration and/or low
mass of MTBE may be
inadequate to sustain microbial
activity, making enhanced
aerobic bioremediation
inadequate for a dispersed, low-
concentration MTBE plume.
YES
Conduct studies (e.g.,
Microcosms) to evaluate the
site-specific potential for
MTBE biodegradation.
Will the site microbiology and geochemistry
result in the formation of TEA due to partial
biotransformatJon of MTBE?
YES
Consider confirming by
monitoring for TEA.
UNKNOWN
NO
ENHANCED AEROBIC BIOREMEDIATION
HAS THE POTENTIAL TO BE EFFECTIVE
FOR MTBE AT THE SITE. PROCEED TO
•EVALUATE THE DESIGN"
May 2004
XII-42
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Design Basis
Review of the corrective action plan should find consistency between site
characterization work and information that is presented as the basis for the
enhanced aerobic bioremediation design in the corrective action plan. To conduct
the enhanced aerobic bioremediation effectiveness evaluation, the reviewer should
have a solid understanding of the nature and extent of the site-specific petroleum
constituents of concern, including an understanding of the contaminant phases
present and the relevant site chemical, physical, and biological properties. When
preparing and reviewing the corrective action plan design, the reviewer should also
understand the site geology and hydrogeology, and the risks associated with the
contamination. These data, which should have been developed and interpreted as
part of the site characterization effort, serve as the foundation for the remedial
system design.
While the site characterization data provide the core raw materials for the
design, further refinement is often needed and useful. For example, while the site
characterization work may identify potential human or ecological receptors that
may be exposed to the contamination, specific cleanup goals may not have been
established. In such cases, the specific remedial goals would need to be developed
and identified in the corrective action plan through one or more established
approaches, such as adopting state-published cleanup standards, developing site-
specific risk-based standards acceptable to the state, or employing other state-
specific and approved methods.
The corrective action plan may also include the results and interpretation of
follow-up studies completed after the original site characterization. The need for
such studies is often identified after a review of the site characterization shows that
additional information is needed to complete the remedial system design. For
example, the site characterization may suggest that one or more of the constituents
of concern is believed to be marginally biodegradable, and the level of expected
biodegradation is difficult to predict from the existing data.
Examples of typical information expected to be developed during the site
characterization, or as a result of follow-up studies that are completed to support
the basis for the technology selection and design of the corrective action plan, are
summarized in Exhibit XII-19. Each of the items listed in Exhibit XII-19 is
described in more detail below.
Cleanup Goals
The evaluation of alternative remedial approaches and the subsequent design of
the selected approach are strongly influenced by the cleanup goals that the
remediation program must achieve. Often, preliminary goals identified during the
site characterization work evolve as a better understanding of site conditions and
potential receptors is attained. However, owing to their importance for
May 2004 XI1-43
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remediation planning and design, the cleanup goals should be fully evolved and
solidified in the corrective action plan.
Exhibit XII-19
Enhanced Aerobic Bioremediation Design Basis Factors
Design Basis Factor
Cleanup Goals
• Target contaminant levels (soil- and
groundwater)
• Remediation timeframe
Geology
Uniformity
Stratigraphy
Geochemistry
Bedrock
Soil permeabilities
Hydrogeology
• Depth to groundwater
• Groundwater elevation and gradient
• Aquifer/water bearing unit class (e.g,
confined, unconfined, perched,
bedrock)
• Hydraulic parameters (e.g.,
conductivity, transmissivity,
storativity, effective porosity)
• Modeling results
Design Basis Factor
Petroleum Contamination
• Target chemical constituents
• Target contaminant and total
hydrocarbon mass estimates (sorbed,
dissolved, liquid and vapor phases)
• Extent (vertical and lateral)
• Bioavailability
• Biodegradability
• Fate and transport characteristics
Source(s) of Design Information
Receptor survey, pre-design exposure or
risk assessment analyses (potentially
including numerical modeling), or state
requirements
Site characterization soil borings, well
installations, sampling/analysis, and site
observations. Local geologic studies.
Site characterization well gauging,
aquifer pump testing, data analyses, and
local hydrogeologic studies.
Source(s) of Design Information
Soil, groundwater and other media
sampling/laboratory analysis, review of
published data on contaminants and data
interpolation and analysis.
Cleanup goals usually provide the end-point concentrations for petroleum
constituents in soil and groundwater that are acceptable to state or other
regulatory agencies. These cleanup thresholds could be goals that represent any of
the following:
• Health-based numeric values for petroleum chemical constituents
published by the respective regulatory agency
• Cleanup goals developed and proposed by the contractor specifically
May 2004
XII-44
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for the contaminated site that are acceptable to the Implementing
Agency
• Goals derived from site-specific risk assessment involving contaminant
fate and transport modeling coupled with ecological and human-health
risk assessment
• Generic state cleanup goals
Additional project goals that may or may not be regulatory requirements
include hydraulic control of the contamination, a cleanup timeframe, or other
performance goals established in the corrective action plan. Regardless of what
the cleanup goals are and how they are established, the state-sanctioned goals
should noted in the corrective action plan and recognized as a fundamental basis
for the technology selection and design.
The cleanup goals presented in the corrective action plan answer important
questions relevant to the viability of the selected remedial approach and the
adequacy of the remedial design. These two critical questions are:
• Can the cleanup concentration goals be met by the designed enhanced
aerobic bioremediation system?
• Can sufficient oxygen be delivered to the contaminated area to enable
contaminants to be biodegraded to meet cleanup goals within a
reasonable period of time?
Each of these questions is discussed in more detail in the paragraphs that follow.
» Can the cleanup concentration goals be met by the designed enhanced
aerobic bioremediation system?
Below a certain "threshold" petroleum constituent concentration, bacteria may
not be able to derive sufficient carbon from petroleum biodegradation to sustain
vigorous levels of biological activity. As concentrations of petroleum
contaminants fall below the threshold, further biodegradation of the petroleum
hydrocarbons can become relatively insignificant. The level of diminishing returns
is site-specific and representative of petroleum contamination that has been
reduced in concentration to the technological limit of the specific enhanced aerobic
bioremediation.
Although the threshold limit of enhanced aerobic bioremediation approaches
can vary greatly, depending on bacteria-, petroleum constituent- and site-specific
factors, it is generally observed that petroleum constituent soil concentrations
cannot be reduced below 0.1 ppm without using supplemental technologies. In
addition, reductions in total petroleum hydrocarbons (TPH) of greater than 95
percent can be very difficult to achieve because of petroleum products often
contain "recalcitrant" or non-degradable petroleum hydrocarbons.
May 2004 XII-45
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While further bioremediation of petroleum contaminant levels in the subsurface
may become limited at some point due to the limited availability of a useable
carbon source, it is quite possible that the target chemical constituents that may
exist in soil and groundwater at that time may meet the cleanup standards. Even
though total hydrocarbon levels may remain elevated in subsurface soil, the
chemical constituents comprising the hydrocarbon mass may be those that are less
soluble and of reduced environmental concern.
Exhibit XII-20
Cleanup Concentrations Potentially Achieved By Enhanced
Aerobic Bioremediation
Cleanup Requirement
Feasibility of Meeting Cleanup
Levels
Petroleum constituent concentration Feasible
in soil >0.1 ppm (each contaminant
with corresponding dissolved levels in
groundwater) and TPH reduction
< 95%
Constituent concentration in soil Potentially infeasible to remediate in
< 0.1 ppm (each contaminant with reasonable timeframe; laboratory or
corresponding dissolved levels in field trials may be needed to
groundwater) or TPH reduction demonstrate petroleum concentration
> 951% reduction potential
If comparing existing levels of site petroleum contamination to the cleanup
goals indicates that either of these guidance criteria summarized in Exhibit XII-20
is exceeded, the proposed enhanced aerobic bioremediation. The system design
may not achieve the expected remedial objectives in a reasonable time frame.
• Can sufficient oxygen be delivered to the contaminated area to enable
contaminants to be biodegraded to meet cleanup goals within a
reasonable period of time?
Cleanup goals establish the concentrations and allowable residual mass of
petroleum constituents that can acceptably remain in the subsurface soil and
groundwater subsequent to remediation. The difference between the current level
of petroleum mass in the soil and groundwater and the allowable residual mass left
in the subsurface is the mass that needs to be biodegraded using enhanced aerobic
bioremediation. Using the theoretical 3 to 3.5 pounds of O2 to degrade roughly 1
pound of petroleum hydrocarbon ratio discussed earlier, it is possible to estimate
the minimum mass of O, needed to achieve the required petroleum mass
biodegradation. This value assumes that there are no significant oxygen "sinks" in
the subsurface (e.g., mineral species that oxidize such as iron) that would increase
the total demand for oxygen.
May 2004 XI1-46
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For example, if the corrective action plan data indicate that approximately
5,000 pounds of petroleum hydrocarbons are in the site subsurface but the cleanup
goals allow only 500 pounds to remain after remediation (based on allowable soil
and groundwater constituent concentration limits), then 4,500 pounds of
hydrocarbons require bioremediation. Assuming anaerobic biodegradation and
abiotic degradation of site contamination are negligible, and that there are no other
sources of oxygen or significant oxygen losses or sinks, and 3.5 pounds of O2 are
needed to aerobically biodegrade each pound of petroleum, then it can be
estimated that a minimum of 15,750 pounds of oxygen would need to be provided
by the enhanced aerobic bioremediation technology during remedial program
implementation. During review of the corrective action plan, therefore, estimate
the oxygen mass required to bioremediate the contamination and determine how
the demand will be met by the proposed enhanced aerobic bioremediation system.
Furthermore, if pure oxygen injection is the proposed enhanced aerobic
bioremediation technology, and the remediation timeframe is 3 years, the
corrective action plan design should show how the pure oxygen injection system
will be able to deliver and distribute a minimum of 15,750 pounds of oxygen over
the 3-year period. In other words, the corrective action plan should demonstrate
that an average of at least 0.6 pounds of pure oxygen per hour can be delivered
over the 3-year period.
The example discussed above assumes that losses of oxygen to the aquifer are
negligible, hi reality, as discussed earlier in this chapter, significant losses of
oxygen can occur from the application of the enhanced aerobic technology itself
and from abiotic and microbiologically mediated reactions with the aquifer
material. An attempt should be made to estimate what these potential oxygen
losses could be in order to factor those losses into the oxygen delivery plan and
cleanup schedule.
If the corrective action plan does not estimate the oxygen and bio-nutrient
delivery requirements or does not demonstrate how the oxygen and bio-nutrient
delivery requirements will be met by the enhanced aerobic bioremediation system,
the corrective action plan may be incomplete. Under such circumstances, it may
be prudent to request that this information be provided before approving the plan.
Similarly, if site-specific cleanup goals have not been clearly established in the
corrective action plan or previously, it may be appropriate to refrain from
completing the review of the design until this critical information is provided.
Enhanced Aerobic Bioremediation Technology Selection
With the design basis established in the corrective action plan, the corrective
action plan can be reviewed to confirm that enhanced aerobic bioremediation is a
reasonable site-specific choice of remediation technology. Depending on project-
specific circumstances, there can be only one or a few enhanced aerobic
bioremediation technologies equally viable and appropriate for a site.
May 2004 XII-47
-------
Alternatively, site-specific or project-specific circumstances may suggest that one
of the enhanced aerobic bioremediation would address the on-site contamination
better than any other technology.
Exhibit XII-2 presents the key advantages and disadvantages of each of the
enhanced aerobic bioremediation technologies. Use these factors to evaluate the
feasibility of using an enhanced aerobic bioremediation approach. Other
differences between and among alternative enhanced aerobic bioremediation
technologies can help to distinguish their most appropriate application(s). A key
characteristic useful for evaluating the feasibility and appropriateness of a
proposed enhanced aerobic bioremediation technology is oxygen delivery
efficiency. More information on how this characteristic can be used is provided in
the next paragraphs.
Oxygen Delivery Efficiency. All enhanced aerobic bioremediation
technologies need to deliver oxygen to the subsurface to encourage aerobic
biodegradation of petroleum contamination to occur. The effectiveness of each
enhanced aerobic bioremediation technology is directly related to the amount of
oxygen it can deliver and uniformly distribute in the contaminated area. Because
of this commonality, it makes sense to explore the relative efficiency with which
each technology is able to deliver oxygen to the treatment area as a distinguishing
feature.
Oxygen produced from the decomposition of compounds used in enhanced
aerobic bioremediation approaches follows the stoichiometric relationships shown
in Exhibit XII-21. For instance, for every two parts of hydrogen peroxide injected,
only one part of oxygen is produced. In contrast, one part ozone yields 1.5 parts
of oxygen, a seemingly more efficient means of generating oxygen.
Exhibit XII-21
Basic Stoichiometry Oxygen Production From Chemical
Decomposition
Enhanced Aerobic Bioremediation
Technology
Basic Oxygen-Producing
Stoichiometry
Hydrogen Peroxide
Ozone
2H,O, -^
O3 -»-1.5O2
Magnesium Peroxide
Sodium Peroxide
MgO2 + H2O -*• Mg(OH)2 + 1/2O2
Na,O, + H,O -»• NaOH + H,O,
•"*«•*»'
May 2004
XII-48
-------
A more practical way of measuring oxygen delivery efficiency is to determine
the total amount of mass of carrier material (e.g., groundwater containing
hydrogen peroxide) that needs to be delivered to the subsurface in order to deliver
1 gram of oxygen. In essence, this is a measure of the amount of effort, energy,
and perhaps, time required to deliver oxygen using the different enhanced aerobic
bioremediation technologies. Exhibit XII-22 compares seven alternative methods
of delivering oxygen to the subsurface using this measure of delivery efficiency. It
compares:
• Three approaches that use groundwater as the oxygen carrier
~ Re-injection of groundwater fully aerated with ambient air
-- Re-injection of groundwater fully aerated with pure oxygen
~ Re-injection of groundwater containing 100 ppm of hydrogen peroxide
• One method that delivers oxygen in the solid phase (oxygen releasing
compounds)
• Three approaches that deliver oxygen in the vapor phase
~ Ozone injection
~ Biosparging/bioventing
— Pure oxygen injection
While the re-infiltration of hydrogen peroxide-amended groundwater may be
the least efficient method of oxygen delivery to the contaminated area, the
hydraulic gradients induced by this activity may enhance the distribution of oxygen
in the subsurface. For more information on factors affecting the distribution of
oxygen in the subsurface, refer to discussions presented earlier as part of the
detailed enhanced aerobic bioremediation effectiveness evaluation.
Each of the major headings in the table above is discussed in more detail below.
Design Components
Although the design elements of alternative enhanced aerobic bioremediation
technologies can vary significantly, Exhibit XII-23 describes the most common
design elements. Several of the more important elements are discussed below to
assist with evaluation of the corrective action plan.
Oxygen and Bio-nutrient Delivery Design should be based primarily on
petroleum mass reduction requirements, site characteristics and cleanup goals.
Oxygen will generally need to be applied at a minimum 3:1 ratio relative to the
petroleum hydrocarbon mass targeted for remediation. Bio-nutrient formulation
and delivery rate (if needed) will be based on soil sampling. Common nutrient
additions include nitrogen (in an aqueous solution containing ammonium ions) and
phosphorus (in an aqueous solution containing phosphate ions). Note that state
regulations may either require permits for nutrient and/or air injection or prohibit
,^-r them entirely.
May 2004 XII-49
-------
Exhibit XII-22
Relative Oxygen Delivery Efficiencies For Various Enhanced
Aerobic Bioremediation Technologies
Oxygen
Delivery
Approach
Description
Oxygen
Concentration
in Delivery
Material (mg/L)
Mass of
Oxygen
Carrier
per Unit
Mass of
Oxygen
Delivered
(g/g)
,* • ..,..- !J\?Y'. f, • , -,*-, ... -
.=Aqueou^f%aii»Oj^eci|ffiBlWr«r^t- •-* ". :0a'^i •,•• -4;'?: • • >
Re-injection of
Aerated/ treated
Groundwater
Re-injection of
Pure Oxygen-
Amended
Groundwater
Re-injection of
H2O2-Amended
Groundwater
Ambient Air
Saturated
Pure O2
Saturated
100mg/Lof
H202
9
45
50
110,000
22,000
20,000
ft
Injection of
Oxygen-
Releasing
Compounds
Mg-peroxide
N/A
10
Injection of
Ozone
Biosparging with
Air or Oxygen,
or
Bioventing
Injection of Pure
Oxygen
5% Ozone
(Converted
toO2)
21% Oxygen
(Ambient)
95% Oxygen
98
275
1,250
12
4
1
Relative
Oxygen
Delivery
Efficiency
Lo
t
\
Hie
west
i
1
jhest
"<«•»*'
May 2004
XII-50
-------
Exhibit XII-23
Common Enhanced Aeration Remediation Design Elements
• Oxygen and Bio-nutrient Delivery Design
- Theoretical oxygen mass requirement
- Bio-nutrient needs (e.g., N, P )
- Application delivery rate
- Number and depth of application points/position
- Equipment
• Permit Requirements and Thresholds
- Underground injection/well installation
- Air injection into subsurface
- Groundwater (wastewater) discharge
- Air (soil vapor) discharge
• Performance Monitoring Plan
— Ongoing distribution of oxygen and bio-nutrients
- Expansion of microbial population
- Reduction in contaminants (sorbed and dissolved phases)
• Contingency Plan
- Inadequate oxygen distribution
- Stagnation or die-off of microbial population
- Lower-than-expected petroleum mass reduction rates
- Excessive contaminant migration
- Build-up of excessive recalcitrant petroleum constituents
- Fugitive (soil vapor) emissions
- Difficult-to-treat/fouling of treated wastewater discharge
- Clogging of equipment or injection areas with iron oxide or biomass
- Other contingencies
Permit Requirements and Thresholds should be identified in the design so
that the system can be constructed to comply with permit requirements and
constraints. Depending on the specific enhanced aerobic bioremediation
technology and the state in which the site is located, permits that may be required
include underground injection, treated groundwater discharge (to sanitary or storm
sewer, or air (soil vapor) discharge.
Several federal, state, and local programs regulate Class V aquifer remediation
wells, and many require permits for underground injection of oxygen or bio-
nutrients. On the federal level, management and regulation of these wells fall
primarily under the underground injection control program authorized by the Safe
Drinking Water Act (SDWA). Some states and localities have used these
authorities, as well as their own authorities, to extend the controls in their areas to
May 2004
XII-51
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address concerns associated with aquifer remediation wells. Aquifer remediation
injection wells are potentially subject to at least three categories of regulation.
First, a state's underground injection control (UIC) program, operating with
approval from the federal program, may have jurisdiction over such wells.
Second, in some states without UIC programs, the state's program for
groundwater protection or pollution elimination program requirements may apply
to remediation wells. Third, remediation wells may be regulated by federal and
state authorities, through Superfund programs, corrective action programs under
RCRA (including the UST program), or other environmental remediation
programs. In the case of remediation programs, the regulatory requirements
typically address the selection of aquifer remediation as a cleanup alternative and
establish the degree of required cleanup in soil and groundwater, while deferring
regulation of the injection wells used in the remediation to other programs. In the
case of voluntary cleanup programs, some concern exists because they may not be
approved or completed according to standards typical of cleanups overseen by a
state or federal agency.4
Performance Monitoring should be accounted for in the design hi the form of
a written plan that can be used to objectively evaluate enhanced aerobic
bioremediation system performance. The plan should clearly describe the
approaches and methods that will be used to evaluate enhanced aerobic
bioremediation system effectiveness in each of the following:
Delivering oxygen (and bio-nutrients) to the subsurface
Distributing oxygen and bio-nutrients through the contaminated area
Increasing microbial population density
Reducing sorbed and dissolved phase petroleum concentrations
Achieving other performance requirements consistent with site-
specific cleanup goals
Contingency Plans should also be accounted for and prepared as part of the
design. The design should anticipate low-likelihood problems and potentially
changing environmental conditions, as well as outline specific response actions that
may be taken. Examples include response actions to take if any performance
monitoring data indicate the following:
• Inadequate oxygen distribution
• Stagnation or die-off of microbial populations
• Low petroleum mass reduction rates
• Excessive contaminant migration
4 US EPA, Ofice of Solid Waste memo dated 12/27/00 on the Applicability
of RCRA Seciton 3020 to In-Situ Treatment of Ground Water.
May 2004 XII-52
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• Recalcitrance of constituents
• Fugitive emissions
• Any other reasonably plausible scenario that can arise under site-
specific conditions and project-specific circumstances.
Components of Enhanced Aerobic Bioremediation Systems
After review of factors that affect the selection and design of a particular
enhanced aerobic bioremediation technology and the critical elements that should
be included in the corrective action plan for enhanced aerobic bioremediation,, it is
now appropriate to discuss major components of various enhanced aerobic
bioremediation systems.
Exhibit XII-24 summarizes some of the major equipment components
associated with each of the more common enhanced aerobic bioremediation
technologies. Depending on which enhanced aerobic bioremediation technology
has been selected in the corrective action plan, a subset of these major system
components should be presented and discussed and schematically depicted (e.g.,
process flow diagram) in the corrective action plan. The design should relate
capacities of these equipment components to design requirements (e.g., required
oxygen production/delivery rates).
As shown in Exhibit XII-24, enhanced aerobic bioremediation systems
employing oxygen-releasing compounds appear to require the least equipment in
part because there is no need for any mechanical equipment once the oxygen-
releasing compounds are deployed. By contrast, re-injection of hydrogen
peroxide-amended groundwater requires the most equipment and a large number
of mechanical components (e.g., pumps, blowers, etc.).
While the sets of major equipment components used by the enhanced aerobic
bioremediation technologies differ significantly, the use of wells by each different
approach warrants recognition and further discussion. In particular, the
orientation, placement, number and construction of this common design element is
worthy of a brief review.
Injection, Extraction and Re-infiltration Wells. Three important
considerations for these wells are orientation, placement and number, and
construction.
• Well Orientation. Both horizontal and vertical wells can be used to treat
subsurface petroleum releases with any of the various enhanced aerobic
bioremediation systems. Hydrogen peroxide-amended groundwater can be
re-infiltrated using either vertical or horizontal wells. Although vertical
wells are more common for ozone or pure oxygen injection, horizontal
wells can be used.
May 2004 XII-53
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Exhibit XII-24
Major Components of Enhanced Aerobic Bioremediation Systems
Component
Function
Oxygen Releasing Compound Systems
Borings and
Excavations
Application Wells
Monitoring Wells
Used to inject or place a slurry of oxygen releasing compounds
so that oxygen may be slowly imparted to the water bearing
zone.
Often used to suspend a solid form of oxygen releasing
compounds to provide oxygen to groundwater.
Used to evaluate effectiveness of remedial approach.
Comparative analyses over time of groundwater samples from
these wells for dissolved oxygen and petroleum contamination
generally indicate how effectively oxygen is being
delivered/dispersed and contaminants are being reduced.
Hydrogen Peroxide Injection Systems
Extraction Wells
Injection Wells or
Infiltration
Galleries
Extraction,
Injection,
Transfer, and
Metering Pumps
and Tanks
Groundwater
Treatment
Equipment
Instrumentation
and Controls
Often used to extract contaminated groundwater downgradient of
the contaminated area for treatment and re-injection in the
upgradient source area for plume containment and/or
accelerated groundwater flow through the contaminated area.
Injection wells, infiltration galleries or a combination of these are
typically used to re-inject treated and hydrogen peroxide-
amended groundwater so that dissolved oxygen may be flushed
through the treatment zone.
Extraction, injection, transfer, and metering pumps are used for
various purposes including: transferring groundwater from and
back into the ground; transferring extracted groundwater
between different components of the treatment system; and
metering hydrogen peroxide and bio-nutrients into the infiltration
system to maintain design concentrations.
Extracted groundwater may be treated to remove petroleum
hydrocarbons by various means such as: oil/water separation; air
stripping; or granular activated carbon sorption or others.
Used to integrate and activate/deactivate system components.
Help maintain the balance of flows consistent with the design
and to safeguard against inadequate treatment or inappropriate
discharges.
May 2004
Xll-54
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Exhibit XII-24
Major Components of Enhanced Aerobic Bioremediation Systems
(continued)
Component
Function
Hydrogen Peroxide injection Systems (continued)
Monitoring Wells
Used to collect environmental samples
analyzed in laboratories and field to
evaluate on-going effectiveness of
remediation. Groundwater well samples
tested for dissolved oxygen and
contamination to evaluate overall
effectiveness of oxygen
delivery/dispersal and the contaminant
reductions over time.
Pure Oxygen Injection Systems
Sparging Wells
Used as conduits to bubble pure oxygen
into contaminated groundwater. The
oxygen is delivered to the base of the soil
and groundwater petroleum
contamination so that it will rise through
the contaminated material providing
oxygen to the hydrocarbon degrading
bacteria.
Air Compressing Equipment
Used to pressurize ambient air to:
prepare it for subsequent treatment to
increase Oxygen levels/purity; and to
provide pressure needed to inject oxygen
and ambient air beneath the water table.
Oxygen Generating Equipment
Used to generate nearly-pure oxygen gas
(- 95%) from ambient air. Synthetic
zeolite sorbers are frequently employed
to simply remove nitrogen from ambient
air to produce high-purity oxygen.
Instrumentation and Controls
Used to integrate and activate/deactivate
system components to maintain the
balance of flows consistent with design
and to safeguard against inadequate
treatment or inappropriate discharges.
Monitoring Wells
Used to collect environmental samples
tested in laboratories and the field to
evaluate on-going effectiveness of
remediation. Comparative analyses over
time of groundwater samples from these
wells for dissolved oxygen and petroleum
contamination generally indicate how
effectively oxygen is being delivered or
dispersed and contaminant reductions
are occurring.
May 2004
XII-55
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Exhibit XII-24
Major Components of Enhanced Aerobic Bioremediation Systems
(continued)
Component
Function
Ozone Injection Systems
Sparging Wells
Used as a conduit to inject ozone into
contaminated groundwater. The ozone is
sparged near the base of the soil and
groundwater petroleum contamination so
that it may contact the contaminants and
provide oxygen to the hydrocarbon
degrading bacteria.
Air Compressing Equipment
Used to pressurize ambient air needed to
generate ozone and to provide the
pressure needed to inject the ozone
beneath the water table. Air compressor
equipment must supply oil and
contaminant free air to minimize in-line
reactions with and premature
decomposition of ozone.
Ozone Generating Equipment
Used to generate ozone gas on-site,
typically at concentrations of about 5%.
Soil Vapor Extraction/
Treatment Equipment (Optional)
Used, if necessary, to control fugitive soil
vapor ozone and volatilize organic
compounds emissions in the unsaturated
zone. May consist of low vacuum/flow
blower to generate vacuum conditions in
unsaturated zone and collect the vapors.
Vapor treatment may consist of granular
activated carbon or biofilters for low
contaminant concentration air stream.
Instrumentation and Controls
Used to integrate and activate/deactivate
system components to maintain the
balance of flows consistent with the
design and to safeguard against
inadequate treatment or inappropriate
discharges.
Monitoring Wells
Used to collect environmental samples
tested in laboratories and the field to
evaluate ongoing effectiveness of
remediation. Comparative analyses over
time of groundwater samples from these
wells for dissolved oxygen and petroleum
contamination generally indicate how
effectively oxygen is being delivered or
dispersed and contaminant reductions
are occurring.
May 2004
XII-56
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Well orientation should be based on site-specific needs and conditions. For
example, horizontal systems should be considered when evaluating sites
that require re-infiltration of amended groundwater into shallow
groundwater at relatively high flow rates. They are also readily applicable
if the affected area is located under a surface structure (e.g., a building), or
if the thickness of the saturated zone is less than 10 feet.
• Well Placement and Number of Wells. The number and location of wells
are determined during the design to accomplish the basic goals of: (1)
optimizing reliable oxygen and bio-nutrient delivery to the contaminated
area; and (2) providing conduits to measure enhanced aerobic
bioremediation system performance. For hydrogen peroxide re-infiltration
systems this typically means placing re-injection wells in the source area(s)
while extracting groundwater from downgradient locations aimed at
simultaneously providing enhanced hydraulic gradient and accelerated
oxygen distribution across the impacted area. The number, location, and
design of the extraction wells will largely be determined from site-specific
hydrogeology, the depth(s) and thickness(es) of the contaminated area(s),
and the results of field-scale pilot testing and hydraulic modeling.
Determining the number and spacing of the wells for ozone or pure oxygen
injection may also be determined through field-scale pilot testing. However, the
following general points should be considered.
• Closer well spacing is often appropriate in areas of high contaminant
concentrations to enhance contaminant contact and oxygen
delivery/distribution where the oxygen demand is the greatest.
• Direct delivery of oxygen into the contaminated material using closer
well spacings can deliver and disperse more quickly than oxygen
delivery through groundwater advection/dispersion and could
significantly decrease the treatment timeframe.
• At sites with stratified soils, wells screened in strata with low
permeabilities often require closer well spacing than wells screened in
strata with higher permeabilities.
• Well Construction. Enhanced aerobic bioremediation system wells are
generally constructed of one- to six-inch diameter PVC, galvanized steel,
or stainless steel pipe. Oxygen or ozone injection sparge wells have
screened intervals that are normally one to three feet in length and situated
at or below the deepest extent of sorbed contaminants. Injection sparge
points must be properly grouted to prevent the injected oxygen from
moving directly up the well annulus to the unsaturated zone rather than
being forced into the contaminated aquifer ("short circuiting" of the
May 2004 XII-57
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injected oxygen). When horizontal injection wells are used, they should be
designed and installed carefully to ensure that the injected oxygen exits ****.
along the entire screen length. .^
Re-infiltration wells typically have screen lengths that extend from the base of
the wells into the unsaturated zone. Groundwater extraction wells should ideally
be screened in the saturated interval containing the greatest mass of hydrocarbons.
Field-scale pilot studies and subsequent data analysis and hydraulic modeling can
help to determine the configuration and construction design of groundwater
extraction and injection wells.
Step 4 - An Evaluation of the Operation and Monitoring Plan
Remedial Progress Monitoring
Significant uncertainties associated with site conditions can remain even as
remedial designs are completed and implemented. In the post-remedial startup
period, these unknowns frequently can result in operations that vary from the
design. These variances can be small or large and often require adjustments to
account for unforeseen conditions and optimize system performance.
Unfortunately, in many cases, the need for these adjustments can go unrecognized
for a long time.
In some cases, the delay in recognizing that remedial system adjustments are "^^
necessary may be attributed to relatively slow responses in subsurface conditions "****'
to the applied technology (e.g., increases in microbial population and
biodegradation of contaminants). Because these subsurface responses to the
applied remedial technology can be delayed, there is often the tendency to give the
remedial program more time to work (sometimes up to years) before making
system modifications or adjustments. In other cases, the delay may stem from
misuse or misinterpretation of site data leading to a belief that the remedial system
is performing well when it is not. An example of this misuse is the practice of
using groundwater analytical data from oxygen delivery wells as an indicator of
remedial progress. In this case, an assessment is biased by the localized effects of
bioremediation in the immediate vicinity of the oxygen delivery wells, but does not
provide an objective measure of the enhanced aerobic bioremediation system's
ability to distribute oxygen and promote biodegradation throughout the treatment
area.
Wells that are used to carry out remedial actions should not be used as
monitoring wells. Monitoring wells should be separate wells used only for that
purpose. If remediation involves injection of gases, the monitoring wells should be
tightly capped until used. If they are not capped, the monitoring wells can provide
a path of least resistance for the injected air to return to the surface. Air can
channel to a monitoring well, then bubble up through the standing water in the well
preferentially removing contaminants from the area in and immediately around the •*"*»>.
well while the rest of the aquifer is short circuited.
May 2004 XII-58
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However, at many sites remedial system operational efficiencies are not
optimized simply because an adequate performance monitoring plan has either not
been developed or has not been fully implemented. In such cases, the designed
remedial system may be installed, started up, and allowed to run its course with
insufficient numbers or types of samples collected to determine whether the
remedial system is performing in accordance with design expectations. The result
of such monitoring approaches can be the discovery of a sub-standard or failed
remediation program years after its implementation.
The previous section discussed the importance of developing a comprehensive
remedial progress monitoring plan. Because of its importance, this section covers
the topics that should be addressed in such a plan to ensure objective gauging of
remedial system performance and necessary optimization adjustments can be made
early on and throughout the duration of enhanced aerobic bioremediation. In
particular, a focused discussion on performance sampling and enhanced aerobic
bioremediation system evaluation criteria is provided to assist with the corrective
action plan review.
Evaluation Sampling
Evaluation sampling is performed to gauge the effectiveness of the enhanced
aerobic bioremediation system relevant to design expectations. Based on a
comparison of the actual field sampling data to design and operational
expectations, timely modifications to the system or operating procedures (if any)
can be made to optimize system performance early in the remediation program.
Projects with regular performance reviews guided by the results of such
sampling/monitoring programs have a greater chance of achieving the design
remedial goals within desired time frames, potentially at lower cost.
Various environmental media are sampled to evaluate system performance.
Groundwater, soil, and soil vapors from the treatment area and vicinity are
commonly sampled to determine the degree to which the enhanced aerobic
bioremediation system is meeting the basic objectives of the approach, including:
• Delivering oxygen to the saturated zone at required design rates
• Distributing dissolved oxygen across the target contaminated area to
restore and maintain aerobic conditions
• Reducing concentrations of petroleum hydrocarbons in soil and
groundwater at design rates through biodegradation of the petroleum
compounds
Exhibit XII-25 identifies those parameters that are commonly measured in
groundwater, soil, and soil vapor samples to help evaluate enhanced aerobic
May 2004 XII-59
-------
Exhibit XII-25
Common Performance Monitoring Parameters
and Sampling Frequencies
Analytical
Parameter
Sampling Frequency
Startup
Phase
(7-10
days)
Daily
Remediation/Post-
Application
Long-Term
Monitoring Phase
(on-going)
Weekly
to
Monthly
Quarterly
to
Annually
Purpose
Groundwater
Dissolved
Oxygen
Redox
Potential
PH
H2O2 or Ozone
Bio-nutrients
Petroleum
COCs
X
X
X
X
X
X
X
X
X
X
Determines system's
effectiveness in distributing
oxygen and ability to
maintain aerobic conditions
(i.e., dissolved oxygen > 2
ppm) in treatment area.
Provides data to optimize
system performance.
Yields data on system's
ability to increase the extent
of aerobic subsurface
environment.
Confirms pH conditions are
stable and suitable for
microbial bioremediation or
identifies trends of concern.
Provides information on
distances these oxygen-
producing compounds can be
transmitted by the remedial
system before decomposing
Determines if bio-nutrients
injected into the groundwater
are being consumed during
bioremediation or
accumulating and potentially
degrading groundwater
quality
Indicates remedial progress
May 2004
XII-60
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Exhibit XII-25
Common Performance Monitoring Parameters and Sampling
Frequencies (continued)
Analytical
Parameter
Sampling Frequency
Startup
Phase
(7-10
days)
Daily
Remediation/Post-
Application
Long-Term
Monitoring Phase
(on-going)
Weekly
to
Monthly
Quarterly
to
Annually
Purpose
Groundwater (continued)
Degradation
Daughter
Constituents
(e.g., TBA)
Water Table
Elevations
Carbon dioxide
Oxygen
Volatile
Petroleum
COCs
Fugitive Ozone
or Hydrogen
Peroxide
X
X
X
X
X
Petroleum
COCs
X
X
Offer direct evidence of
contaminant bioremediation
and enhanced aerobic
bioremediation effectiveness
Determines if hydraulic
conditions (groundwater flow)
are consistent with design
intent or if enhanced aerobic
bioremediation technology
application has had an
unanticipated affect on these
conditions
Soil Vapor
X
X
X
X
Provides evidence of
biodegradation
Indicates potential losses of
introduced oxygen through
the unsaturated zone
Suggests residual sources in
soil or fugitive emissions
associated with the remedial
effort
Determines losses of
oxygen-yielding reagents
delivered to the subsurface
Soil
X
Provide a measure of
remedial progress and the
extent to which
biodegradation of sorbed
contaminants is limited by
May 2004
XII-61
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bioremediation progress and system performance. A brief description of the
respective sampling frequencies and the relevance and significance of each >•%
parameter to the performance evaluation are also provided in the exhibit. A key ••*..
element is the location(s) where performance evaluation sampling takes place
relative to subsurface oxygen delivery points. As stated in the exhibit,
performance evaluation samples should not normally be collected from oxygen
delivery locations.
The performance of the enhanced aerobic bioremediation system should be
determined by the chemistry and microbiology of soil and groundwater located
between, around, and downgradient of oxygen delivery locations rather than inside
or in the immediate vicinity of the oxygen delivery points. Conditions inside or in
the immediate vicinity of oxygen injection locations have been preferentially altered
by enhanced aerobic bioremediation to enhance biodegradation of the petroleum
contaminants. Therefore, data from these locations are not representative of the
subsurface conditions that exist beneath most of the site. To understand the effect
the enhanced aerobic bioremediation system is having on the subsurface conditions
as a measure of its performance, samples of soil, groundwater and soil gas should
be collected from alternate locations.
In reviewing of the performance monitoring plan in the corrective action plan,
a reviewer should verify that a sufficient number of sampling locations exist
between oxygen application points to provide the necessary performance sampling ^^
data. A description of how these data may be used to evaluate the enhanced
aerobic bioremediation system performance is provided below.
Particular attention should be taken with respect to sampling groundwater, soil
vapor, and soil. In reviewing a sampling plan, pay attention to the proposed
sampling frequencies and methods. Some factors to look for include:
Groundwater sampling. Samples should be collected from monitoring wells
located in and around the treatment area and from extraction wells (if used).
Samples should not be collected from oxygen delivery wells for evaluating system
performance because they would only be representative of highly localized effects
of the remediation program.
Soil vapor sampling. Samples should be collected from monitoring wells
located in and around the tre. ->nent area that are screened in the unsaturated zone
and from soil vapor extraction wells (if used). Samples should not be collected
from oxygen delivery wells for evaluating system performance because they would
only be representative of highly localized effects of the remediation program.
Soil sampling. Samples should be collected from borings or using Geoprobe
sampling equipment in and around the treatment area. Soil samples should
May 2004 XII-62
-------
consistently be collected from same contaminated sections of stratigraphic interval
for comparison to earlier samples from same locations and depths.
Evaluation Criteria
The evaluation sampling described above provides evidence needed to assess
the enhanced aerobic bioremediation system performance. This evidence requires
examination and interpretation to confirm enhanced aerobic bioremediation system
effectiveness and whether system modifications may be warranted. A discussion of
these data and how system performance can be interpreted is provided below. In
particular, an evaluation of performance is examined from the following two broad
enhanced aerobic bioremediation system requirements:
• Oxygen delivery and distribution
• Aerobic biodegradation
Each of these is described in more detail in the following paragraphs.
Oxygen Delivery and Distribution. Performance sampling may indicate that
the enhanced aerobic bioremediation system is meeting design specifications for
oxygen delivery and distribution if the data show the following:
• Vadose zone air sampling suggests that there are negligible losses
of supplied oxygen to the atmosphere
• Oxygen is being delivered to the subsurface at the mass delivery
rate required by the design
• Dissolved oxygen levels in groundwater samples collected across
the target treatment area have been elevated to concentrations of 2
mg/L or more and reduction/oxidation conditions are uniformly in
the aerobic range ( greater than or equal to 750 mV)
If the performance monitoring data suggest that one or more of these
conditions is not met, the system may not be meeting the requirements of the
design and system adjustments or modifications may need to be made. As
previously discussed, the remedial system design should include contingency
planning that explores performance deficiency scenarios and identifies possible
solutions.
Oxygen delivery deficiencies can normally be overcome by adjusting system
flow rates or upgrading equipment capacities. However, occasionally, oxygen
delivery rates may be limited by the capacity of the subsurface to absorb and/or
transport the delivered oxygen mass. This may occur if an infiltration system
component becomes hydraulically overloaded by the infiltration rates needed to
meet the design oxygen delivery objectives. Also, groundwater could become
May 2004 XII-63
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over-saturated with dissolved oxygen at injection points requiring oxygen delivery
rates to be reduced to avoid off-gassing losses of oxygen to the atmosphere. In ,*•
both cases, additional infiltration or injection points could readily be added to the •<«««>
system to expand the oxygen delivery capacity to design-specified levels.
Loss of oxygen to the unsaturated zone and ultimately the atmosphere removes
this supply of oxygen available to biodegrading microorganisms. One way to limit
oxygen losses without decreasing application rates is to add application points with
proportionally less oxygen delivered to each location. Another approach is to
alternate the supply of oxygen to various locations in the contaminated zone,
allowing existing levels of oxygen to dissipate before introducing oxygen again.
Perhaps the most challenging performance problem occurs when an enhanced
aerobic bioremediation system is unable to restore and maintain aerobic conditions
in a portion or multiple portions of a contaminated area. Oxygen distributed from
delivery points can fail to reach target contaminated areas for many reasons:
• High biological oxygen demand in the delivery point vicinity
• Elevated soil organic content
• Low permeability heterogeneous soils
• Low hydraulic gradient and groundwater flow
Possible remedies to the performance problem include adding additional
oxygen delivery points, increasing oxygen delivery rates, or enhancing hydraulic J-**
gradients and groundwater flow. ****"'
Aerobic Biodegradation. Successful oxygen delivery and distribution is
probably the most important performance measure for an enhanced aerobic
bioremediation system. However, this is only part of the performance. The
second part requires confirmation that enhanced in-situ biodegradation of the
petroleum contaminants is occurring as a result of, and at rates anticipated by, the
enhanced aerobic bioremediation design. Performance monitoring that suggests
that an enhanced aerobic bioremediation system is operating effectively includes
the following.
• Decreasing dissolved and sorbed petroleum contaminant concentrations
(i.e., gradual reduction of subsurface petroleum mass consistent with
design expectations).
» Production of carbon dioxide in the subsurface, as evidenced by
baseline and subsequent vadose zone sampling and field analyses.
Carbon dioxide production in the saturated zone may also be evaluated
by sampling groundwater and analyzing the groundwater for total
inorganic carbon.
May 2004 XII-64
-------
• Significantly increased microbial activity in the contaminated area as
suggested by comparison of baseline and subsequent microbial
population plate counts.
If only one or two of these conditions exist, there may not be enough evidence
to conclude that bioremediation is a significant contributor to contaminant
reduction or to conclude that the enhanced aerobic bioremediation system is
effective. For example, apparent contaminant reductions in dissolved and sorbed
phases could occur as a result of groundwater advection and dispersion or simply
because of natural fluctuations in water levels. Or, if hydraulic manipulation
(engineered hydraulic gradients) of the groundwater is part of the enhanced
aerobic bioremediation system, apparent contaminant reductions could result from
dilution or separation of the groundwater from the contaminated soil (e.g., if the
water table is depressed below the contamination). In this case, contamination
levels in groundwater could rebound to near preexisting concentrations if the
hydraulic controls are turned off and groundwater re-contacts the contaminated
soil.
The appearance of significant levels of carbon dioxide subsequent to enhanced
aerobic bioremediation system activation is a good indicator of enhanced biological
activity. However, if elevated carbon dioxide levels in the unsaturated zone are
unable to be detected, this does not necessarily mean that microbial activity has not
been enhanced. Carbon dioxide entering the vadose zone may be diluted by pore
space air exchanges with the atmosphere, operation of vapor control systems, and
other means, making it difficult to distinguish small differences in concentrations.
Possibly the most direct indication that enhanced aerobic bioremediation has
increased the number of hydrocarbon degrading bacteria is observation of
significantly increased populations of heterotrophic bacteria in the target treatment
area. While larger populations of heterotrophic bacteria may not always translate
to increased levels of petroleum hydrocarbon biodegradation, the increased
number of bacteria over the baseline levels would serve as a strong indicator of
biodegradation. If performance sample analyses detect intermediate degradation
daughter products, this may be further evidence of contaminant biodegradation
that has been enhanced.
May 2004 XII-65
-------
References
Baker, Katherine H. and D.S. Herson. Bioremediation. McGraw-Hill, Inc. 1994.
Brown, Richard, Chris Nelson, M. Leahy. Combining Oxidation and
Bioremediation for the Treatment of Recalcitrant Organics. In Situ and On
Site Bioremediation. Battelle Press. 1997.
Brown, R. A., R.D. Norris, R.L Raymond. Oxygen Transport in Contaminated
Aquifers With Hydrogen Peroxide. API Conference "Petroleum Hydrocarbon
and Organic Chemicals in Groudnwater: Prevention, Detection and
Restoration". Houston, TX. 1984.
Brown, R.A. Bioremediation of Petroleum Hydrocarbons: A Flexible, Variable
Speed Technology. Remediation. Summer/1996.
Brown, R. A. and R. D. Norris. The Evolution of a Technology: Hydrogen
Peroxide in In-Situ Bioremediation. In Situ Hydrocarbon Bioremediation.
Hinchee, R.E. Ed. CRC Press, Boca Raton. 1994.
Carter, Sean. Enhanced Biodegradation of MTBE and BTEX Using Pure Oxygen
Injection. In Situ and On Site Bioremediation. Battelle Press. 1997.
Chapman, Steven W. Semi-Passive Oxygen Release Barrier for Enhancement of
Intrinsic Bioremediation. In Situ and On site Bioremediation. Vol 4. 1997.
Chapelle, Francis H. Bioremediation of Petroleum Hydrocarbon-contaminated
Ground Water: The Perspectives of History and Hydrogeology. Ground
Water, 37, 1. Jan-Feb 1999.
Chien Chin. Assessment of the Applicability of Chemical Oxidation Technologies
for the Treatment of Contaminants at LUST Sites, Chemical Oxidation,
Technologies for the Nineties. Technomic. 1993.
Christian, B.J, L.B. Pugh and B.H. Clarke. Aromatic Hydrocarbon Degradation in
Hydrogen Peroxide- and Nitrate-Amended Microcosms. In Situ and On Site
Bioremediation Symposium. Battelle Press. 1995.
Cole, G. Mattney. Assessment and Remediation of Petroleum Contaminated Sites.
CRC Press. 1994.
Dupont, Ryan R. and Robert E. Hinchee. Assessment of In-Situ Bioremediation
Potential and the Application of Bioventing at a Fuel-Contaminated Site, and
Brown, R. A. and R. D. Norris.. Oxygen Transport in Contaminated Aquifers.
Presented at NWWA and API Conference "Petroleum Hydrocarbon and
Organic Chemicals in Groundwater: Prevention, Detection and Restoration".
Houston, TX. 1984.
Fry, Virginia A., J. D. Istok, K. T. O'Reilly. Effect of Trapped Gas on Dissolved
Oxygen Transport - Implications for In Situ Bioremediation. Ground Water,
V34, 2. Mar-Apr 1996.
Hinchee, R.E., D.C. Downey, P.K. Aggarwal. Use of Hydrogen Peroxide as an
Oxygen Source of In-Situ Biodegradation: Part I. Field Studies. Journal of
Hazardous Materials. 1991.
May 2004 XII-66
-------
Johnson, R, J. Pankow, D. Bender, C. Price and J. Zogorski. MTBE, To What
Extent Will Past Releases Contaminate Community Water Supply Wells?
Environmental Science and Technology/News. 2000.
Nelson, C.H. and R. A. Brown. Adapting Ozonation for Soil and Groundwater
Cleanup. Chemical Engineering. November 1994.
Norris, R. D., K. Dowd and C. Maudlin. The Use of Multiple Oxygen Sources
and Nutrient Delivery Systems to Effect In-situ Bioremediation of Saturated
and Unsaturated Soils. In Situ Hydrocarbon Bioremediation. Hinchee, R.E.
Ed. CRC Press, Boca Raton. 1994.
Prosen, B. J., W.M. Korreck and J. M. Armstrong. Design and Preliminary
Performance Results of a Full-scale Bioremediation System Utilizing on On-
site Oxygen Generation System. In-Situ Bioreclamation: Applications and
Investigations for Hydrocarbon and Contaminated Site Remediation. R. E.
Hinchee, Ed. Butterworth-Heinemann. 1991.
Riser-Roberts, Eve. Remediation of Petroleum Contaminated Soils. Lewis
Publishers, 1998.
USEPA. Cost and Performance Report, Enhanced Bioremediation of
Contaminated Groundwater. 1998.
USEPA. The Class V Underground Injection Control Study, Volume 16, Aquifer
Remediation Wells, EPA/816-R-99-014. 1999.
May 2004 XII-67
-------
o o Is the density and configuration of oxygen delivery points
adequate to uniformly disperse dissolved oxygen through the
target treatment zone, given site geology and hydrologic
conditions?
3. Written Performance Monitoring Plan
Yes No
o o Will a comprehensive set of baseline sampling be performed
prior to enhanced aerobic bioremediation system start-up?
o o Does the plan specifically exclude sampling from oxygen
delivery wells when collecting data to evaluate enhanced
aerobic bioremediation system performance?
o o Are monitoring wells adequately distributed between oxygen
delivery locations to collect groundwater and soil vapor samples
to evaluate the performance of the enhanced aerobic
bioremediation system?
o o Does the written plan include periodically collecting soil
samples from the contaminated interval(s) at locations between
oxygen delivery locations?
o o Will the soil, soil vapor and groundwater samples be analyzed
for the majority of the recommended performance monitoring
parameters?
o o Will frequencies of performance monitoring generally
correspond to those identified in Exhibit XII - 25?
May 2004 XII-69
-------
Chapter XIII
Chemical Oxidation
-------
-------
Contents
Overview XIII-1
Hydrogen Peroxide/Fenton's Reagent XIII-4
Permanganate XIII-10
Ozone Xni-11
Special Considerations for MTBE XIII-12
Chemical Oxidation Technology Effectiveness Screening Approach XIII-13
Step 1: Initial Screening of Chemical Oxidation Effectiveness XIII-15
Overall Viability XIII-15
Potential Effectiveness of Chemical Oxidation XIII-16
Step 2: Detailed Evaluation of Chemical Oxidation Effectiveness XIII-18
Site Characteristics That Affect Chemical Oxidation XIII-18
Oxidant Demand Factors XIII-19
Advective Dispersive Transport Factors XIII-20
Constituent Characteristics That Affect Chemical
Oxidation XIII-24
Step 3: Evaluation of Chemical Oxidation Design XIII-26
Design Basis XIII-27
Cleanup Goals XIII-27
Chemical Oxidation Technology Selection XIII-30
Design Components XIII-31
Oxidation Application Design XIII-32
Permit Requirements and Thresholds XIII-32
Performance Monitoring XIII-33
Contingency Plans XIII-33
Components of Chemical Oxidation Systems XIII-34
Injection, Extraction and Re-Infiltration Wells XIII-37
Well Construction XIII-38
Step 4: An Evaluation of the Operation and Monitoring Plan XIII-38
Remedial Progress Monitoring XIII-38
Evaluation Sampling XIII-39
Evaluation Criteria XIII-43
Oxidant Delivery and Distribution XIII-43
Permanent Contaminant Mass Reduction and
Attainment of Cleanup Goal XIII-44
References XIII-46
Checklist: Can Chemical Oxidation Be Used At This Site? XIII-48
May 2004 Xlll-ii
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List of Exhibits
Number Title Page
XIII-1 Chemic., Oxidation Primary Advantages and Disadvantages XIII-3
XIII-2 Chemical Oxidation Technologies Comparative Matrix XIII-5
XIII-3 Initial Screening For Chemical Oxidation Remediation
Potential Effectiveness XIII-14
XIII-4 Detailed Screening For Chemical Oxidation Remediation Potential
Effectiveness XIII-17
XIII-5 Key Parameters Used To Evaluate Chemical
Oxidation Applicability XIII-18
XIII-6 Inorganic Oxidation Processes That Consume
Dissolved Oxygen in Groundwater XIII-19
XIII-7 Organic Compound Oxidation Stoichiometry XIII-20
XIII-8 Intrinsic Permeability And Chemical Oxidation Effect XIII-21
XIII-9 Solubility Values and Organic Carbon Partition Coefficients
for Select Petroleum Hydrocarbon Constituents XIII-25
XIII-10 Chemical Oxidation Design Basis Factors XIII-29
XIII-11 Relative Power of Chemical Oxidants XIII-31
XIII-12 Common Chemical Oxidation Remediation Design Elements XIII-32
XIII-13 Major Components of Chemical Oxidation Systems XIII-34
XIII-14 Common Performance Monitoring Parameters and
Sampling Frequencies XIII-40
May 2004
Xlll-iii
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Chapter XIII
Chemical Oxidation
Overview
Petroleum contaminant decomposition and in-situ destruction may be
accomplished using chemical oxidation technologies. In contrast to other remedial
technologies, contaminant reduction can be seen in short time frames (e.g., weeks
or months). As discussed in this chapter, a variety of chemical oxidants and
application techniques can be used to bring oxidizing materials into contact with
subsurface contaminants to remediate the contamination. With sufficient contact
time with the organic contaminants, chemical oxidants may be capable of
converting the petroleum hydrocarbon mass to carbon dioxide and water and
ultimately irreversibly reduce concentrations of petroleum hydrocarbons in soil and
groundwater. While many of the chemical oxidants have been used in wastewater
treatment for decades, only recently have they been used to treat hydrocarbon-
contaminated groundwater and soil in-situ.
Chemical oxidation technologies are predominantly used to address
contaminants in the source area saturated zone and capillary fringe. Cost concerns
can preclude the use of chemical oxidation technologies to address large and dilute
petroleum contaminant plumes. More frequently, chemical oxidation technologies
are employed to treat smaller source areas where the petroleum mass is more
concentrated. However, where excessive petroleum contaminant mass exists in
the source area and where there is a significant thickness of mobile non-aqueous
phase liquids (NAPLs), other remedial technologies (e.g., free product recovery)
may need to precede chemical oxidation for the remediation to be safe and cost-
effective.
Concurrent treatment of source area saturated and unsaturated zones usually
requires the integration of chemical oxidation with other remedial technologies that
target unsaturated zone contamination (e.g., soil vapor extraction). Frequently,
soil vapor extraction, which is used to treat the unsaturated zone, is included as a
component of chemical oxidation remedial solutions even if there is no specific
need to treat unsaturated soils in the source area. Use of soil vapor extraction in
conjunction with chemical oxidation can help alleviate safety issues associated with
controlling and recovering off-gas containing volatile organic carbons (VOCs),
oxygen, oxidants and other reaction byproducts that can be generated by various
chemical oxidants.
As discussed in greater detail below, each chemical oxidant and application
technology has advantages and disadvantages. Some oxidants are stronger than
others, yet some weaker oxidants may persist in the subsurface, allowing longer
contact times with the contaminants. Careful evaluation of the contaminants of
concern is needed before selecting a chemical oxidation technology. Certain
May 2004 XIII-1
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contaminants (e.g., benzene) that are frequently remedial drivers at petroleum UST
release sites are unable to be readily chemically oxidized in-situ using some
chemical oxidants (e.g., permanganate).
Understanding the site hydrogeologic conditions is important when considering
chemical oxidation technologies because these conditions often determine the
extent to which the chemical oxidants may come into contact with the petroleum
contaminants. Chemical oxidants may not be able to penetrate low permeability
homogenous soils or horizons in heterogeneous soils that contain the bulk of
petroleum contaminant mass.
Soil reactivity with chemical oxidants is also important when considering the
costs of the use of chemical oxidation. Excessive loss of a chemical oxidant that is
reacting with organics in soil, instead of reacting with the contaminants, may
preclude the use of the technology as an economically viable approach to site
remediation. Different chemical oxidation technologies are most appropriate for
particular hydrogeologic conditions. For example, Fenton's Reagant may not be
ideal for groundwater with high concentrations of carbonate. The carbonate ion
preferentially scavenges the hydroxyl radicals created by Fenton's Reagant
reactions before they have a chance to react with the petroleum contaminants. By
contrast, the presence of carbonate minerals in the geologic matrix has generally
positive effects on permanganate oxidation.
Remedial strategies for petroleum UST sites that include a combination of
active source zone treatment with enhanced natural attenuation outside the
contaminant plume core may consider chemical oxidation technologies. Many
chemical oxidation techniques also provide residual dissolved oxygen that is used
by aerobic microorganisms to biodegrade contaminants. In addition, these
technologies may also oxidize reduced electron acceptors (e.g., nitrogen to nitrate,
sulfides to sulfate), which are then used by anaerobic microorganisms to
biodegrade contaminants. For more information on enhanced aerobic remediation
technologies, see "How to Evaluate Alternative Cleanup Technologies for
Underground Storage Tank Sites: A Guide for Corrective Action Plan Reviewers"
(EPA-510-B-95-007, 1995). For specific information on aerobic remediation
technologies, see Chapter III, Bioventing, Chapter VIII, Biosparging, and Chapter
X, In-situ Groundwater Bioremediation.
Exhibit XIII-1 summarizes the general advantages and disadvantages of
chemical oxidation technologies.
Several chemical oxidants have been used to remediate petroleum
contaminated UST sites. The most commonly used (and most effective) are
Hydrogen Peroxide/Fenton's Reagent and Ozone. Sodium or Potassium
Permanganate have been used, but experience with these compounds is more
limited, although some recent bench-scale and field studies are showing promise.
May 2004 XIII-2
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EXHIBIT XIII-1
Chemical Oxidation
Primary Advantages and Disadvantages
Advantages
Disadvantages
Contaminant mass can be
destroyed in-situ.
Rapid destruction/degradation of
contaminants (measurable
reductions in weeks or months).
Produces no significant wastes
(VOC off-gas is minimal), except
Fenton's.
Some oxidants (not Fenton's) are
capable of completely oxidizing
MTBE (but production of
degradation products may be
problematic).
Reduced operation and
monitoring costs.
Compatible with post treatment
monitored natural attenuation and
can even enhance aerobic and
anaerobic biodegradation of
residual hydrocarbons.
Some oxidation technologies
cause only minimal disturbance to
site operations.
Potentially higher initial and
overall costs relative to other
source area solutions.
Contamination in low permeability
soils may not be readily contacted
and destroyed by chemical
oxidants.
Fenton's Reagant can produce
significant quantity of explosive
off-gas. Special precautions (i.e.,
SVE system) are required for
appropriate implementation of
remedial action involving Fenton's
Reagent/hydrogen peroxide.
Dissolved contaminant
concentrations may rebound
weeks or months following
chemical oxidation treatment.
Dissolved contaminant plume
configuration may be altered by
chemical oxidation application.
Significant health and safety
concerns are associated with
applying oxidants.
May not be technically or
economically able to reduce
contaminants to background or
very low concentrations.
Significant losses of chemical
oxidants may occur as they react
with soil/bedrock material rather
than contaminants.
May significantly alter aquifer
geochemistry; can cause clogging
of aquifer through precipitation of
minerals in pore spaces.
May 2004
XIII-3
-------
There has also been recent interest in and some field applications using sodium
persulfate (Na2S2O8) to oxidize organic contaminants or to reduce the oxidant
demand of native soils before other oxidants are applied to the contamination.
Some research has demonstrated that when mixed with ferrous iron as a catalyst,
the sulfate free radical (SO4~) can be produced, which has an oxidation potential
only slightly less than Fenton's Reagent. Field testing of this oxidant to date has
primarily involved the destruction of chlorinated organics rather than petroleum
hydrocarbons. Given the experimental status of this oxidant, it is not further
described or discussed in this chapter.
A brief description of the three main petroleum hydrocarbon oxidants and
associated application technologies is provided below. Exhibit XIII-2 compares
the relative advantages and disadvantages of these chemical oxidation
technologies.
Hydrogen Peroxide and Fenton's Reagent
Hydrogen peroxide is a strong oxidant that can be injected into a contaminated
zone to destroy petroleum contaminants. When injected to groundwater,
hydrogen peroxide is unstable, and reacts with organic contaminants and
subsurface materials. It decomposes to oxygen and water within hours of its
introduction into groundwater generating heat in the process. Peroxide is typically
shipped to a remediation site in liquid form at dose concentrations ranging from
five percent to 50 percent by weight.
The reactivity of hydrogen peroxide can limit the extent to which it may be
distributed in the subsurface before it decomposes. Injecting concentrations of
hydrogen peroxide as low as 100 ppm (a small fraction of one percent) can cause
oxygen concentrations in groundwater to exceed the solubility limit of oxygen in
groundwater (typically 9-10 mg/L). When this occurs, oxygen gas is formed, and
is lost in the form of bubbles that rise through the saturated zone to the water table
and into the unsaturated zone.
Hydrogen peroxide is particularly effective when it reacts with ferrous iron
(Fe2+) to produce Fenton's Reagent. Ferrous iron may be naturally present in the
subsurface soils and/or groundwater, or it can be added as a catalyst solution
together with the hydrogen peroxide to produce this aggressive chemical reaction.
Hydrogen peroxide in the presence of ferrous iron (Fe2+) reacts to form
hydroxyl radicals (OH»), ferric iron (Fe3+), and hydroxyl ions (OH"). The hydroxyl
ions are very powerful oxidizers, and react particularly with organic compounds.
The hydroxyl radicals break the petroleum hydrocarbon bonds of common
petroleum constituents such as benzene, toluene, ethylbenzene, and xylene, as well
as petroleum aromatic hydrocarbons (PAHs) and methyl tertiary butyl ether
(MTBE), a common gasoline additive.
May 2004 XIII-4
-------
Fenton's Reagent requires soluble Fe2 to form OHv This optimal reaction
occurs under relatively low pH conditions (e.g., pH of 2 to 4). pH adjustment in
the treatment area is often necessary to enable the oxidation process to proceed
efficiently. This can be accomplished by either acidifying the hydrogen peroxide or
by adding a chelating acid. Using a ferrous sulfate solution "simultaneously adjusts
aquifer pH and adds the iron catalyst needed for Fenton's Reagent. Because of the
low pH requirement, Fenton's Reagent treatment may not be efficient or effective
in limestone geology or sediments with elevated pH levels or with significant
capacity to buffer these reactions. In addition, reaction between hydrogen
peroxide and ferric iron can consume hydrogen peroxide, reducing the
effectiveness of the oxidant dose. The same effect may also occur in soils with
high ferric iron content.
Exhibit XIII-2
Chemical Oxidation Technologies Comparative Matrix
lUVMNflH,:,.. ''***, -^
Potential to complete
remediation in shortest time
Capacity to oxidize MTBE and
benzene
No significant VOC off-gas
produced by heat of reaction
Oxidizes over extended period,
increasing possibility of contact
with contaminants
Increases dissolved oxygen
levels for potentially enhanced
aerobic bioremediation
Reduced health and safety
concerns during application
Can be applied using automated
system
Hydrogen
Peroxide/
Fenton's
Reagant
;^
X
X
X1
X
Permanganate
'&&*/&, *** v'l ''/};
• '*".'"'
X
X
X2
Ozone
X
X
X
1 If solid peroxide is injected below 10% strength, the heat of dilution is mitigated and VOC
generation typically avoided.
2 Note that sodium permanganate is often applied as a liquid at 40% strength, which poses a
significant handling and explosion risk.
May 2004
XIII-5
-------
Exhibit XIII-2
Chemical Oxidation Technologies Comparative Matrix (continued)
Disadvantages
Inability to effectively oxidize
benzene or MTBE
Increased risk of fugitive vapors
entering building structures, utility
conduits, particularly in absence
of adequate vapor recovery
technology (e.g., soil vapor
extraction)
Increased risk of plume
reconfiguration
Low permeability soil horizons
less likely to be penetrated by
oxidant over short injection
period
On-site reactive chemical
handling and storage required
On-site gas production and
delivery equipment (e.g., ozone
generator) required
Few petroleum remediation
projects completed using this
technology due to limited
effectiveness
Possible production of unwanted
compounds or by-products in the
subsurface3
Potential to precipitate solids and
clog aquifer pores
Hydrogen
Peroxide/
Fenton's
Reagant
r,' ''^4^ 'rA'Stft"*'
X
X
X
X
X
X
Permanganate
'•>'<.' r Cx' ',
X
X
X
X
X
X
Ozone
^ ' . /«
~* > '
"t i_ - -. *5 ^
X
X
X
X
X
X
X
Fenton-like reactions produce the hydroxyl radical (OH») which is one of the
strongest oxidants, but the reaction proceeds so quickly that the radicals may not
have sufficient time to come into contact with contaminant molecules so that they
can be destroyed before the hydrogen peroxide decomposes. Also, some
3 Chemical oxidation may cause some may create some toxic or highly mobile secondary
products. Ensure that analyses for potential secondary products are included in any
corrective action plan that proposes the use of chemical oxidants.
May 2004 XIII-6
-------
contaminants may sorb so tightly to organic material in the soil that they are
effectively protected from destruction.. This may be particularly true for sites with
significant layers or lenses of low permeability that results from high clay content.
In such cases, the oxidant may successfully address contaminants in more
permeable layers or lenses of soil while leaving the bulk of the contamination that
resides in the low permeability soils.
Difficulty in addressing contamination in low permeability soils may be
alleviated to some degree by controlled pneumatic or hydraulic fracturing of the
soil. However, engineered hydraulic fractures generally cannot be spaced more
closely than about 5 feet, which means that chemical oxidants must still penetrate a
substantial thickness of low permeability soil to come into contact with the
contamination. Deep soil mixing with large diameter drill augers is the most
effective method currently available to increase contact between adsorbed
contaminants and the oxidants. In any case, long term post-injection monitoring of
contaminant levels in groundwater is critical to evaluating the success of putting
Fenton's Reagent into contact with adsorbed contaminants. If inadequate contact
occurs, contaminant levels in groundwater samples will rebound as the adsorbed
contaminant mass gradually (typically over months) bleeds back into groundwater.
Controlled oxidation is increasingly being practiced using solid peroxides, pH
modifiers, and catalysts that promote the generation of free radicals. This new
approach moderates the rate of dissolution and peroxide generation, which in turn
controls that rate of reaction between peroxide and the petroleum contaminants.
The use of slurried peroxides creates the opportunity to release oxidants and
oxygen over a longer period, which can promote subsequent aerobic remediation.
"Modified" Fenton-type systems use pH-neutral and even higher pH conditions
along with slurried solid peroxides and metallic or organo-metallic catalysts. The
reaction of the oxidants with the catalysts generate hydroxyl radicals, which react
with the organic contaminants within the subsurface. The advantage to this
approach is the ability to use Fenton's Reagant under neutral pH conditions,
requiring no acidification of the aquifer. It leads to a mix of reducing and
oxidizing reactions in the subsurface, which moderates the rate of dissolution and
peroxide generation, which moderates the rate of reaction between the peroxide
and the petroleum contaminants. This releases oxidants and oxygen over a longer
period, and may promote subsequent aerobic remediation.
Fenton-like reactions are exothermic and can raise the temperature of
groundwater, produce steam and generate significant pressures in the application
area, particularly when the Fenton's is added at strengths approaching 10-12%.
Especially in deep vadose zones and in monitoring or injection wells where
pressures may be elevated, Fenton's-like reactions can lead to explosive conditions
and present safety concerns that need to be promptly and effectively managed. In
addition, migration of explosive vapors along preferential pathways may pose an
explosion hazard.
May 2004 XIII-7
-------
Several incidents resulting in spontaneous explosions of subsurface vapors have
occurred during Fenton's Reagent treatment of petroleum contaminated sites.
Other incidences have involved VOC vapor migration and intrusion into buildings
and contaminant plume expansion. To manage these risks, at a minimum, it is
important before a chemical oxidation strategy is selected and implemented to:
• Locate pockets of high levels of petroleum contamination in the treatment
area.
• Identify and evaluate preferential flow paths.
• Clear the area of subsurface utilities, basements or other enclosed spaces
that could accumulate and transmit vapors.
• Ensure that no petroleum storage tanks or lines are in the treatment area.
During application of an oxidation technology, consider the following to manage
risks:
• Use a field photo-ionization or flame ionization detector (PID/FID) and
explosimeter to monitor for explosive conditions.
• Install and operate a soil vapor collection system during Fenton's Reagent
treatment until such time it can be demonstrated that there is no significant
threat.
• Use a heat probe to monitor subsurface temperatures. Hydrogen peroxide,
for example, decomposes at temperatures above 65°C, so as reactions
progress in the subsurface, it is important to control the temperature to
ensure maximum efficacy of the oxidation process.
• Closely monitor hydrogen peroxide and catalyst injection into the treatment
area and adjust levels based on field analyses of soil gas and groundwater
samples.
• Consider hydraulically containing groundwater during the treatment
process to minimize the possibility of the chemical reaction pressures
expanding the contaminant plume. Note, however, that dissolved gases in
groundwater often prevent this approach from being as effective as
predicted.
Other safety concerns include those associated with storing and using
concentrated hydrogen peroxide on site. Many applications of the technology
have involved the storage and use of thousands of gallons of fifty-percent
hydrogen peroxide. Skin burns and blindness can result from contact with this
chemical at this concentration. Safety precautions include the use of skin
protection and safety glasses during application of these chemicals. A shower and
eye wash facility may need to be constructed for the duration of the application.
May 2004 XIII-8
-------
Hydrogen peroxide and catalyst solutions needed for Fenton's Reagent are
usually added to the treatment area by pressure injection into one or more
designated chemical oxidation injection wells, or gravity injection into one or more
monitoring or other wells.
In pressure injection, compressed air is used to sparge the ferrous iron catalyst
and relatively large volumes of peroxide solution (e.g., hundreds to thousands of
gallons) into the contaminated soil and groundwater over a short period of time
(e.g., days). The sparged air forces the chemical reactants down the injection well
point(s) and out into the impacted saturated soil. This is an aggressive approach
that poses inherent increased risks of VOC vapor production and migration as well
as plume re-configuration. Plume re-configuration may occur because the zone of
influence during injection is limited, and permeability decreases with application of
the technology, which may create preferential flowpaths with continued injection.
Operation of a soil vapor extraction system concurrently with oxidant injection is a
sensible precaution.
In gravity injection, small volumes of reagents are gravity-fed into injection
well(s) over a longer application period. The distribution and dissipation of the
reagents in the saturated zone is largely controlled by the site hydrogeologic
conditions. The gravity injection approach may reduce some of risks associated
with chemical oxidation technologies. Additionally, given its prolonged
application period, the oxidants may be able to penetrate into more of the lower
permeability soils to address contaminants in these areas.
In both cases, multiple injection events, separated by extended periods of
groundwater monitoring, may be required in order to approach cleanup objectives.
Establishing which injection or application approach is likely to be most efficient or
cost effective for a given site is challenging, given the recent emergence of this
technology and the limited volume of scientifically defensible information that is
currently available for the two basic application methods. Site-specific safety
concerns may be a key determining factor of the most appropriate injection
approach.
An additional benefit of hydrogen peroxide and Fenton's Reagent is the
temporary increase of oxygen levels in and around the treatment area. The
increased oxygen levels at the fringes of the treatment area can enhance naturally
occurring aerobic biodegradation processes that reduce contaminant mass. While
there may be concerns about oxidizing hydrocarbon-degrading bacteria in the
chemical oxidation treatment area, many studies have shown that soil cannot be
readily sterilized by Fenton's Reagent and that microbial populations rapidly
rebound following chemical oxidation treatment. In addition to enhancing aerobic
biodegradation, reduce nitrogen and sulfur are oxidized to nitrate and sulfate,
which can be used by anaerobic microbes.
May 2004 XIII-9
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Permanganate
Permanganate is emerging as a chemical oxidant that can be used to destroy
petroleum and other organic compounds in soil and groundwater, and has
successfully treated MTBE in recent laboratory and bench-scale studies. This
oxidant is weaker than hydrogen peroxide. Its inability to oxidize benzene can lead
to the early elimination of permanganate as a candidate for oxidation technology at
petroleum cleanup sites.
However, permanganate has several advantages over other oxidants. It:
• Oxidizes organics over a wider pH range.
• Reacts over a prolonged period in the subsurface allowing the oxidant to
more effectively permeate soil and contact adsorbed contaminants.
• Does not normally produce heat, steam and vapors or associated health and
safety concerns.
Permanganate may be applied to sites as either potassium permanganate
(KMnO4) or sodium permanganate (NaMnO4). Where cost dominates over
engineering factors at a site, potassium permanganate is the preferred chemical
form because it is more widely available, less costly, and is available in solid form,
which facilitates transport and handling. Where other factors are aore important,
the liquid form of sodium permangante is preferable.
When choosing potassium permanganate for application at a site, be aware of
three properties that can cause concern to owner, operators or state regulators.
First, potassium permanganate is derived from mined potassium ores which, by
their nature, typically contain salt and metal impurities (e.g., arsenic, chromium,
lead). Depending on water quality criteria in the state in which the site occur and
the quality and concentration of potassium permanganate used to oxidize the site
contaminants, these impurities may generate concern. (This is also true of sodium
permanganate, which is mined and processed in similar fashion.)
Second, potassium permanganate is used to produce Pharmaceuticals and
should be used and monitored carefully.
Third, potassium permanganate in flowable form contains silica, which can
accumulate in wells and plug the screen.
As with other chemical oxidation technologies, the success of the use of
permanganate relies heavily on its ability to come into contact with the site
contaminants. The delivery mechanism must be capable of dispersing the oxidant
throughout the treatment zone. To accomplish this, permanganate may be
delivered in solid or liquid form in a continuous or cyclic application schedule
using injection probes, soil fracturing, soil mixing, groundwater re-circulation or
treatment fences.
May 2004 XI11-10
-------
Dissolved permanganate has been delivered to injection or re-circulation wells
at concentrations ranging from 100 to 40,000 milligrams per liter (mg/L).
Contaminated soils have been successfully oxidized through slurry injection, deep
soil mixing or hydraulic fracturing using concentrated permanganate solutions
ranging from 5,000 to 40,000 mg/L or up to 50 percent by weight solid
permanganate.
In-situ permanganate reactions can yield low pH (e.g., pH 3) and high Eh
conditions (e.g., +800 mV), which can temporarily mobilize naturally-occurring
metals and metal contaminants that may also be present in the treatment area. The
release of these metals from the aquifer formation, however, may be offset by
sorption of the metals onto strongly sorbent MnO2 solids that are precipitated as a
byproduct of permanganate oxidation. In addition, high sodium permanganate
concentrations can create sodium problems with clay permeability at the edges of
the injection zone due to swelling clays and potential aquifer clogging. Cr(OH)3 in
soils may be oxidized to hexavalent chromium, which may persist for some time.
This may generate concern if the aquifer is being used for drinking water.
Questions remain about the mass of MnO4 that is generated, and the effect, if any,
the mass may have on subsurface permeability and remediation performance.
Ozone
Ozone (O3) is a strong oxidant with an oxidation potential about 1.2 times
greater than hydrogen peroxide. It can be used to destroy petroleum
contamination in-situ. Ozone, a gas, is typically generated on-site using a
membrane filtration system and typically delivered to the subsurface through
sparge wells. Delivery concentrations and rates vary, however, because of the high
reactivity of ozone and associated free radicals. Ozone needs to be generated in
close proximity to the treatment area, and sparge wells generally need to be spaced
closely in the target remedial zone.
Ozone can also be injected into the subsurface in a dissolved phase. The gas
may be transferred to the dissolved phase on-site by sparging upgradient water
with ozone. Groundwater that is extracted upgradient from the area to be treated
may be amended with ozone, then re-injected or re-infiltrated into the subsurface,
where it transports the dissolved phase ozone and oxygen into the contaminated
area. (Check with appropriate state groundwater authorities to learn whether
groundwater re-injection is allowed in the state.) More commonly, gaseous ozone
is injected or sparged directly into contaminated groundwater.
Typically, air containing up to five percent ozone is injected into strategically
placed sparge wells. Ozone then dissolves in the groundwater, reacts with
subsurface organics, and ultimately decomposes to oxygen. Ozone can oxidize site
contaminants directly or through formation of hydroxyl radicals (OH«), strong
nonspecific oxidants with an oxidation potential that is about 1.4 times that of
May 2004 XIII-11
-------
ozone. It is capable of oxidizing BTEX constituents, PAHs, and MTBE (with
limited effectiveness).
Heat and VOC vapors may be generated as a result of ozone sparging and the
oxidation reactions when ozone concentrations are high. As a result, vapor
control equipment (e.g., a soil vapor extraction and treatment system) is often
needed to operate in conjunction with the ozone sparging system to capture and
prevent the vapors from migrating to, entering and impacting subsurface utilities or
nearby structures.
Ozone is also effective in delivering oxygen to enhance subsurface
bioremediation of petroleum-impacted areas. Ozone is 10 times more soluble in
water than is pure oxygen. Consequently, groundwater becomes increasingly
saturated with dissolved oxygen as the unstable ozone molecule decomposes into
oxygen molecules. About one-half of dissolved ozone introduced into the
subsurface degrades to oxygen within approximately 20 minutes. The dissolved
oxygen can then be used by indigenous aerobic hydrocarbon-degrading bacteria.
The oxidizing properties of injected ozone can temporarily suppress subsurface
biological activity in the immediate injection area. However, this suppression has
been found to be temporary, and sufficient bacteria survive in-situ ozonation to
resume biodegradation once ozone has been applied. Additionally, aerobic
bacteria along the fringes of the treatment area may thrive under the oxygen rich
conditions produced during ozone treatment. Biodegradation enhancement is a
primary benefit of this oxidation technology.
Special Considerations for MTBE
As mentioned above, any of the three oxidation approaches may be applicable
for remediating MTBE, either in the presence or absence of other gasoline
hydrocarbons. Hydrogen peroxide and ozone addition have both been used on a
number of MTBE-impacted field sites, with successes reported at many of them.
The success of these techniques may be attributable to the combined effects of the
oxidation, increased dissolved oxygen levels in the groundwater, and generated
heat.
The available field data on these chemical oxidation projects for MTBE
treatment is somewhat sparse. Some literature reports do not contain enough
time-series sampling data on groundwater concentrations to ensure that the
beneficial reductions of MTBE are not short-lived and that groundwater
concentrations do not later rebound.
Very little published data exists on using permanganate on MTBE-impacted
field sites, but recent laboratory batch testing looks promising. The method's
ability to oxidize MTBE, but not benzene, may have application where an active
remediation technology is desired for treating the MTBE, but the benzene is to be
May 2004 XI11-12
-------
addressed by monitored natural attenuation. Further development and field
confirmation of potassium permanganate's effectiveness for MTBE is needed.
With any oxidation method, the potential for creating unwanted intermediary
products or other unwanted by-products always needs to be considered. In studies
of aboveground oxidation of MTBE-impacted groundwater, the primary
byproducts of concern were found to be acetone, tertiary butyl alcohol (TEA) and
tertiary butyl formate (TBF), and bromate (for ozone-based oxidation). The
possible in-situ formation of these by-products, as well as their potential fate and
possible impacts, should be considered as part of any plan to conduct subsurface
chemical oxidation of MTBE. Several laboratory studies that addressed the
oxidation of MTBE-impacted water have indicated that combining ultraviolet light
with hydrogen peroxide may oxidize MTBE more effectively, with fewer
byproducts. Although the UV light requirement may render this application
infeasible for in-situ chemical oxidation projects, the effectiveness of ex-situ
treatment technologies may be enhanced.
Another consideration for MTBE is whether chemical oxidation technologies
can be cost effective for a highly soluble compound like MTBE and that is often
found to exist in laterally extensive, mobile groundwater plumes. Chemical
oxidation can be quite effective on the high hydrocarbon concentrations typically
seen in groundwater and soils in source areas, but may not be applicable to the
expansive, lower-concentration, dissolved-phase plumes often associated with
MTBE-impacted sites.
Chemical Oxidation Technology Effectiveness Screening
Approach
The descriptions of the various chemical oxidation technologies in the
overview should provide the basic understanding needed to move forward with
evaluation of a corrective action plan that proposes to use chemical oxidation. To
assist with evaluation of the chemical oxidation corrective action plan, a step-by-
step technology effectiveness screening approach is provided in a flow diagram in
Exhibit XIII-3. This exhibit summarizes the evaluation process and serves as a
roadmap for the decisions to be made during evaluation of a corrective action plan
that proposes chemical oxidation technologies. A checklist has been provided at
the end of this chapter for use as a tool to both evaluate the completeness of the
chemical oxidation corrective action plan and to focus attention on areas where
additional information may be needed.
Note that the first step in this screening includes information that can only be
gleaned from a thorough assessment of the site, such as soil permeabilities and the
nature of the aquifer geology, including heterogeneity, the presence of preferred
pathways, and other characteristics. Before embarking on the selection of a
chemical oxidation technology, be sure that a complete, and preferably three-
dimensional, delineation of the subsurface and contaminant plume has been
conducted.
May 2004 XIII-13
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Exhibit XIII-3
Initial Screening for Potential Effectiveness
of Chemical Oxidation
REVIEW GENERAL SITE INFORMATION
• Fr*« Product/LNAPLs
* Soil P*mi«at»Wty
* urns Use/Proximity or Utilities
Man IP.CIUO* other
FR£E PRODUCT
Are recoverable volumes of mobile
free residua) product (LNAPLs) »n
the proposed treatment
area?
technologies (e g.. physical
extractor!) to reduce subsurface volume
f mob*e free residua!
before cnerwcai oxidation
Does
ihe plan Mate
soil fracturing, physical nixing
~tacrmof09«s to enhance so»
or to PU£ tie wfWent yt awct
contact with the
contamnaBon?
Is tatgst contarninatiofi located
in unstratified d^nse
dayort*?
UTIOTIES
Is target contamination
located in and around active
buned u6Jtie
-------
The evaluation process can be divided into the four steps described below.
• Step 1: An initial screening of chemical oxidation effectiveness allows
quick determination of whether chemical oxidation should be considered as a
remedial approach for the site.
• Step 2: A detailed evaluation of chemical oxidation effectiveness provides
further screening criteria to confirm whether chemical oxidation is likely to be
effective. First, extract from the corrective action plan certain site-specific
data on the nature/extent of contamination, potential risk to human health/the
environment, subsurface geology and hydrogeology, and other relevant site
characteristics. Then, compare the site-specific data to the criteria provided in
the Exhibit to assess whether chemical oxidation is likely to be effective.
• Step 3: An evaluation of the chemical oxidation system design in the
corrective action plan allows determination of whether basic design
information has been defined, necessary design components have been
specified, the construction process flow designs are consistent with standard
practice, and adequate feasibility testing has been performed.
• Step 4: An evaluation of the operation and monitoring plans allows
determination of whether baseline, start-up and long-term system operation
and monitoring are of sufficient scope and frequency and whether remedial
progress monitoring and contingency plans are appropriate.
Step 1: Initial Screening of Chemical Oxidation Effectiveness
This section allows you to perform an initial screening of whether chemical
oxidation is likely to be an effective approach to remediate the petroleum-impacted
areas at a site. Before selecting chemical oxidation as the preferred remedial
approach, determine whether the corrective action plan has taken into account key
site-specific conditions. In addition, evaluate several "bright lines" defining the
limits of chemical oxidation overall viability as a remedial technology. These
bright lines will assist in evaluating the corrective action plan and in determining
the appropriateness of chemical oxidation as the site remedial solution. After
establishing the overall viability of an chemical oxidation approach, basic site and
petroleum contaminant information can be examined to further determine the
expected effectiveness of chemical oxidation as the remedial choice.
Overall Viability
The following site conditions are considered to be the "bright lines" defining
the general limits of chemical oxidation viability at a site. If review of the
corrective action plan indicates that any of the following conditions exist, chemical
oxidation is not likely to be a feasible or appropriate remedial solution for the site.
•««•*»- • Free mobile product is present and the corrective action plan does not
include other means for its recovery. Chemical oxidation is not likely to
May 2004 XI11-15
-------
cost-effectively address free product. Significant thickness and volumes of
free product may need to be recovered using conventional approaches >•*.
before oxidizing the residual hydrocarbons. For some chemical oxidation ,,„„,.
technologies, free product poses a safety issue, increasing chances of an
explosion.
• Utilities (active gas mains, petroleum USTs/piping, sewers, etc.) lie in
the immediate vicinity of the treatment area. Concerns associated with
the heat, VOC vapors, elevated oxygen levels and potential corrosion that
can occur from the induced chemical reactions during application of
oxidants may preclude the use of this technology until the utilities can be
removed or relocated. Potential risks associated with the use of chemical
oxidation in the presence of buried utilities include explosion, combustion,
and vapor intrusion into buildings.
• The target contaminant zone is comprised of or includes unstratified
dense clay. With the low permeabilities inherent to clay or clay-rich soils,
chemical oxidants cannot easily come into contact with the adsorbed
contaminants. Without adequate contact, the petroleum contaminants will
remain adsorbed to the low permeability soil, which often contains most of
the contaminant mass, rendering remediation unsuccessful. Soil fracturing,
use of slow reaction oxidants (e.g., permanganate) or multiple oxidant
applications may be used to help bring the oxidants into contact with the
contaminants, but technical and cost considerations may lead to ^^
consideration of other remedial approaches or technologies. '***'
Potential Effectiveness of Chemical Oxidation
Before performing a more detailed evaluation of chemical oxidation's potential
remedial effectiveness and future success at a site, it is useful to review several key
indicators. One key factor that influences the effectiveness of chemical oxidation
at a site is sou permeability.
Chemical oxidation of contaminants in fine-grained soils, or in clays and silts
with low permeabilities, is likely to be less effective than chemical oxidation of
contaminants in coarse-grained soils (e.g., sand and gravels) because it is more
difficult to effectively contact chemical oxidants with organic contaminants in low-
permeability materials.
It is also important to determine whether the chemical oxidant that may be
used to address site contaminants is able to readily oxidize the chemical
constituents of concern. For example, permanganate cannot readily oxidize
benzene or MTBE, which may be target contaminants. The detailed chemical
oxidation effectiveness evaluation section of this chapter considers the oxidizing
strength of various oxidants and the resistance of specific petroleum hydrocarbon ^*"l%1
constituents to oxidation. The flowchart in Exhibit XIII-4 outlines the factors that
May 2004 XI11-16
-------
Exhibit XIII-4
Detailed Screening for Potential Effectiveness of
Chemical Oxidation
REVIEW KEY SITE INFORMATION
* Caufconate Geology «e.g~ Limestone'!
; organic raaUtnai' feWl *:«;da?vi
Conduct pifot study to verify
that soil fracturing mixing cr
an aiie* corttftiwnante in Icwei
contact and
destroy adsorbed
be ef teitsritiy
oxittzed'
Conduct pilot study to
ostidant can
be delivered so and is
efftdw* on cortammants in
lew f»frweasiWy tenses asd
teyers.
stnsted trart
centaronants in senses car"vsJ*$
be targeted and efficiently
wieteed''
Qxtdants Only
Coneuet pilot study to
cfwDonstrate effective
oontarrwant destracfeon tsraier
etevatsd pH ste conations,.
CoiTipi«fe economic
evalu»t»on to p?o>ect
chemical oxidaRl costs
and vsfriy cost
01 solution
osidam derriariii
B not»than ewitamirwrst
CO is tocery to be effective
at the ste. Proceed to
evaluate the dss»gn.
May 2004
XIII-17
-------
should be evaluated in the detailed screening for the use of chemical oxidation
technologies.
Step 2: Detailed Evaluation cr Chemical Oxidation Effectiveness
If initial screening of the corrective action plan indicates that chemical
oxidation may be feasible and potentially effective for the site, then a more detailed
evaluation of the proposed chemical oxidation remedy should be performed to
confirm this assessment. To help with this more detailed evaluation, this section
covers a number of important site-specific characteristics influencing the potential
effectiveness of chemical oxidation that were not considered or fully explored in
your initial screening of the remedial approach. Additionally, this section provides
a more detailed discussion of key contaminant characteristics influencing the
potential effectiveness of chemical oxidation.
Key site and contaminant factors that should be explored in the detailed
evaluation of chemical oxidation are listed in Exhibit XIII-5. The remainder of this
section details each of the parameters described in Exhibit XIII-5. After reviewing
and comparing the information provided in this section with the corresponding
information in the corrective action plan, it should be possible able to evaluat
whether chemical oxidation is likely to be effective at the site.
Exhibit XIII-5
Key Parameters Used to Evaluate Chemical Oxidation Applicability
Site Characteristics
Oxidant Demand Factors
Advective and Dispersive
Transport Factors
- Intrinsic Permeability
- Soil Structure and Stratification
- Hydraulic Gradient
- Iron and Other Reduced
Inorganic Compounds
Dissolved in Groundwater
Constituent Characteristics
• Chemical Class and Susceptibility
to Chemical Oxidation
• Solubility Characteristics
- Solubility
- K Factor
Site Characteristics That Affect Chemical Oxidation
This section addresses three factors at a site that can affect the ability of
chemical oxidants to treat petroleum-contaminated groundwater at a site:
• Oxidant Demand Factors
• Advective and Dispersive Transport Factors
• Constituent Characteristics Factors
May 2004
XIII-18
-------
Each of these factors is described in detail below.
Oxidant Demand Factors. Once introduced into the saturated zone, chemical
oxidants and catalysts may be distributed by advection and dispersion to address
the target treatment zone. Ideally, the oxidant concentrations are sustained from
the point of application until the oxidants contact the contaminants. However, the
concentrations of oxidant more typically decrease by dilution through mixing with
subsurface pore water and through consumption via chemical reactions that are not
related to the degradation of the target constituents of concern. The loss of
oxidant due to subsurface reactions unrelated to contamination oxidation, often
referred to as the natural oxidant demand (NOD), is a significant consideration in
determining the economic viability of chemical oxidation and in engineering the
appropriate oxidation application dose and approach.
NOD stems from reaction with organic and inorganic chemical species
naturally present in the subsurface. Oxidants that react with the natural organic
material (NOM) are lost and are, therefore, subsequently unable to react with the
target contaminants. In certain soil types (e.g., peat), the NOM and therefore the
NOD can be extremely high. Inorganic oxidant demand may exist if naturally-
occurring reduced mineral species (e.g., ferrous iron) are present in the
groundwater or saturated soils. These reduced compounds can also react with the
oxidants to remove oxygen available for reaction with the target contaminants.
Exhibit XIII-6 presents a sample of some common inorganic processes that
consume oxygen and oxidants in groundwater.
NOD almost always exceeds contaminant oxidant demand. If insufficient
doses of oxidants are not provided to satisfy both demands, the target
contaminants may not be degraded to the desired level. Bench testing should be
used to determine the NOD for the saturated zone.
Exhibit XIII-6
Inorganic Oxidation Processes That Consume Dissolved Oxygen
in Groundwater
Process
Sulfide Oxidation
Iron Oxidation
Nitrification
Manganese Oxidation
Iron Sulfide Oxidation
Reaction
O2 + 1/2HS •* 1/2 SO2' + 1/2H+
%O2 + Fe2+ + H+ •*• Fe3+ + 1/2H2O
O2 + 1/2NH4+ -» 1/2NO3- + H+ + 1/2H2O
O2 + 2Mn2+ + 2H2O -»• 2MnO2 (s) +4H+
15/4O2 + FeS2 (s) + 7/2H2O •* Fe(OH)3 (s)
+2SO42- +4H+
May 2004 XI11-19
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Exhibit XIII-7 explores the theoretical oxygen demand of a number of
petroleum hydrocarbon constituents common to petroleum UST cleanup sites.
The exhibit outlines the stoichiometric reactions for the complete oxidation of the
typical target hydrocarbons. In theory, oxygen levels of at least 3 to 3.5 times the
amount of subsurface petroleum mass that needs to be removed to meet cleanup
goals must be delivered to the groundwater and distributed over the planned
remedial period.
Exhibit XIII-7
Organic Compound Oxidation Stoichiometry
Petroleum
Hydrocarbon
MTBE
Benzene
Toluene
Ethylbenzene
Xylenes
Cumene
Naphthalene
Fluorene
Phenanthrene
Hexane
Oxidation Reaction
C5H12O+ 7.5 O2 -» 5CO2+6H2O
C6H6 + 7.5 O2 -* CO2 +3H2O
C6H5CH3 + 9 O2 -*• 7CO2 + 4H2O
C2H5C6H5 + 10.5 O2-* 8CO2 + 5H2O
C6H4(CH3)2 + 10.5 O2 -* 8CO2 + 5H2O
C6H5C3H7 + 12O2 -+ 9CO2 + 6H2O
C10H8 + 1 2O2 -M 0CO2 + 4H2O
C13H10 + 15.5O2 -* 13CO2+ 5H2O
C14H10+16.5O2 -* 14CO2+ 5H2O
C6H14+ 9.5 O2 -* 6CO2 + 7H2O
Oxygen
Requirement
(g O2 per
g Contaminant)
2.7
3.1
3.1
3.2
3.2
3.2
3.0
3.0
3.0
3.5
A number of experiments and field tests have determined that site NOD is
highly variable and not easily predicted. For example, NOD associated with
permanganate application has been found to vary from two to over 100 mg MnO4"
per mg of total organic carbon (TOC) in the treatment area soil, and equal to or
greater than the contaminant oxygen demand.
Oxidizing reactions associated with the NOD can produce solid precipitates
that can accumulate in soil pore spaces. Particles may be produced by shearing off
fragments of natural soil or by yielding reaction products (e.g., iron or
manganese oxides). Permanganate oxidation results in the production of MnO2
solids as a reaction product. These precipitates can potentially decrease soil
permeability and remediation system function and performance; however, their
effects in this regard have not been fully examined and are not well understood.
Advective and Dispersive Transport Factors. The site conditions affecting
advection and dispersion of dissolved oxygen are:
May 2004
XIII-20
-------
• Intrinsic Permeability
• Soil Structure and Stratification
• Groundwater Velocity
• Iron and Other Reduced Inorganic Compounds Dissolved in Groundwater
Each of these conditions is described in detail below.
Intrinsic Permeability. Intrinsic permeability is a measure of the ability of soil to
transmit fluids. Intrinsic permeability often decreases near injection wells or
infiltration galleries. This is also commonly a result of the precipitation of
carbonate species, such as calcite. In general, oxygen is more easily distributed in
soils with higher soil permeabilities (e.g., sands and gravels) than in soils with
lower permeabilities (e.g., clays or silts). Intrinsic permeability can be calculated
from hydraulic conductivity measurements taken from on-site pump testing. Pump
test or slug test-derived permeability ranges are representative of average hydraulic
permeability conditions for heterogeneous conditions. Alternatively, intrinsic
permeability can be estimated from soil boring logs and laboratory tests. Intrinsic
permeability values obtained through empirical means are less accurate and result
in a wider range of permeability estimates. In any case, derived permeabilities are
only approximations of actual subsurface conditions and should be regarded as
such in the evaluation of chemical oxidation as a remediation technology. Intrinsic
permeability can vary over 13 orders of magnitude (from
10"16 to 10"3 cm2) for the wide range of earth materials. Exhibit XIII-8 provides
general guidelines on the range of intrinsic permeability values over which
chemical oxidation is likely to be effective.
Exhibit XIII-8
Intrinsic Permeability and Chemical Oxidation Effect
Hydraulic Intrinsic
Conductivity (K) Permeability (k) Chemical Oxidation Effectiveness
(in ft/sec) (in ft2)
K> 1Q-6 k> 10'12 Effective to generally effective
10'6< K < 1C)-7 10-12 < k < 10-13 Possibly effective; needs further
evaluation
K < 10'7 k < 10'13 Marginally effective to ineffective
It is important to note that the intrinsic permeability of a soil can decrease as
chemical oxidation progresses. The most likely cause of reduced intrinsic
permeability while implementing chemical oxidation is the precipitation of
inorganic complexes that form during oxidation of reduced, naturally occurring
mineral species such as ferrous iron. If the soil intrinsic permeability indicates
borderline potential effectiveness (i.e., 10-9 < k < 10-10), the geochemistry
should be further evaluated. It may be necessary to determine the concentration of
May 2004 XIII-21
-------
reduced inorganic species, primarily iron, in the soil to assess whether subsurface
flow pathways could become constricted by precipitation of inorganic compounds,
such as ferric oxides.
Soil Structure and Stratification. Soils in a target treatment area are not
uniformly permeable (i.e., heterogeneous), but rather have large-scale and
small-scale variations in permeability (i.e., heterogeneous). Heterogeneity controls
movement of fluids in the subsurface. Soil heterogeneity plays a very important
role in chemical oxidation technologies because oxidants and catalyst reagents
introduced to the subsurface are distributed preferentially along higher permeability
layers in the saturated soil. For example, in a heterogeneous soil comprised of
sand, silt and clay layers, oxidants may be effectively distributed through the sand
layer to successfully reduce petroleum hydrocarbons there, but will be ineffectively
delivered and distributed to the silt and clay layers. If the silt and clay layers are
thick relative to the sand horizon and contain significant petroleum hydrocarbon
mass, chemical oxidation technologies may be inefficient or ineffective. In
addition, the tendency for the development or enhancement of preferential flow
paths may be increased by the addition of Fenton's reagant or the use of ozone
sparging.
Unless site soils are homogeneous, average soil intrinsic permeability may not
adequately determine the viability of chemical oxidation approaches because
discrete low permeability soil horizons may exist, and these horizons might contain
a large fraction of the subsurface petroleum mass. In most cases, it is prudent to
evaluate petroleum mass distribution across all soil types to determine whether
chemical oxidation is likely to be effective and will achieve cleanup objectives. If
select soil horizons containing hydrocarbon mass are not expected to be effectively
treated using chemical oxidation, chemical oxidation may not be viable for the site.
For example, if 50 percent of the contaminant mass is contained and isolated in
low permeability soil horizons, and the site cleanup goals is a 95 percent reduction
in petroleum contaminant concentrations, then it is reasonable to conclude that the
goal cannot be achieved using chemical oxidation. However, in such
circumstances, combining chemical oxidation with other technologies that enhance
the permeability of low permeability horizons in the contaminated zone (e.g., soil
fracturing) could be considered. Or, alternatively, following source removal
addition of peroxides could be employed to increase the rate of aerobic
biodegradation to achieve remediation objectives. For more information about
enhanced aerobic bioremediation, refer to Chapter XII in this manual.
Groundwater Velocity. Chemical oxidation technologies may rely on
groundwater advection and dispersion to distribute oxidants and catalyst reagents
in the subsurface. Distribution of oxidants and reagents can be most readily
accomplished under hydrogeologic conditions conducive to higher groundwater
flow rates. True groundwater velocity is referred to as the seepage velocity (q^)
and can be calculated from the equation at the top of the next page:
May 2004 XIII-22
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K(dhldl}
where: dh/dl = aquifer hydraulic gradient (maximum difference in water
table elevation or potentiometric surface (L)/distance
between upgradient and downgradient measurement
points (L)
K = hydraulic conductivity (L/T)
nc = soil effective porosity (dimensionless)
As the hydraulic gradient increases, groundwater velocity increases
proportionately. This same relationship exists between groundwater velocity and
soil permeability. Groundwater velocity is inversely proportional to soil porosity.
As porosity increases, groundwater velocity decreases. When a significant
hydraulic gradient exists, targeted delivery of oxidant to the contaminant zones
may require injection and extraction wells.
In addition, transport of dilute dissolved contaminants is a function of
advection, dispersion, and chemical and physical reactions. Advection refers to the
movement imparted by flowing groundwater, and the rate of transport is usually
taken to be equal to the average linear groundwater velocity. Hydrodynamic
dispersion occurs as a result of molecular diffusion and mechanical mixing and
causes the dissolved contaminant plume to spread out with distance from the
source. Molecular diffusion is generally only significant when groundwater
movement is very slow. Mechanical mixing occurs as groundwater flows through
the aquifer matrix twisting around individual grains and through interconnected
pore spaces at differing velocities. The movement of some dissolved contaminants
may also be affected by chemical and physical reactions, such as sorption and
biodegradation, which act to reduce the transport velocity and decrease
concentrations in the plume.
Iron and Other Reduced Inorganic Compounds Dissolved in Groundwater.
The effective intrinsic permeability of the saturated zone can be significantly
reduced if the chemical oxidation treatment zone contains naturally elevated levels
of reduced iron (e.g., ferrous iron, or Fe2+) or other mineral species. For example,
when dissolved iron is exposed to chemical oxidants, it may be oxidized to ferric
iron (Fe3+) oxide that can precipitate within the saturated zone and occlude soil
pore space. On a large scale, this could reduce effective soil porosity, and oxidant
delivery efficiency and availability. In such cases, decreases in soil porosity can be
expected to occur closest to the oxidant delivery locations (i.e., near oxidant
injection wells). Bench-scale tests may need to be performed to evaluate the
inorganic NOD of the aquifer material and determine the feasibility of the remedial
approach.
In addition to being considered in evaluating the potential effectiveness of
chemical oxidation, hydraulic gradient can be an engineering design issue. If the
May 2004 XIII-23
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gradient is not steep enough to provide adequate flow and oxidant transport
through the contaminated zone, then certain engineering provisions (e.g., spacing
application points more closely, creating artificial hydraulic gradients) can be
added to the design to enhance oxidant distribution.
Constituent Characteristics That Affect Chemical Oxidation. It is
important to evaluate the potential impacts of site contaminants on the
performance of the proposed chemical oxidation approach. In particular, it is
important to review how the chemical structure, chemical properties,
concentrations and toxicities of the petroleum contaminants can influence remedial
performance.
Petroleum products are complex mixtures of hundreds or even thousands of
hydrocarbon chemical constituents, other chemical constituents and additives.
Each of these constituents has a different atomic structure that determines, in part,
how easily it may be chemically oxidized.
With the notable exception of benzene, most petroleum hydrocarbons have
been demonstrated to be oxidized by all three primary chemical oxidants. Benzene
is not readily oxidized by permanganate, and oxidation of MTBE has only been
demonstrated to be oxidizable by permanganate at bench scale.
The two factors related to chemical classes, and their susceptibilities to
chemical oxidation, are their solubility characteristics and their K,,,. values. Each is
discussed in more detail below. "*****
Solubility Characteristics. Solubility is the maximum concentration of a
chemical that can be dissolved in water at a given temperature without forming a
separate chemical phase on the water (i.e., free product). Most petroleum
compounds have low solubility values, thus limiting the concentrations of
contamination that can be dissolved in groundwater. The solubility values for
petroleum hydrocarbons range over four orders of magnitude, as shown in Exhibit
XIII-9.
Compounds with higher solubility values are generally smaller, lower molecular
weight molecules (e.g., benzene). When spilled, these compounds exist in
groundwater at higher relative concentrations and move more quickly through the
aquifer than do compounds of higher molecular weights. Larger and higher
molecular weight hydrocarbon molecules are generally less soluble in water;
therefore, their dissolved concentrations in groundwater tend to be limited (e.g.,
naphthalene). Long-chain hydrocarbons are often saponified by chemical
oxidation, making them more soluble, particularly in the presence of any free
product.
May 2004 Xiil-24
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Exhibit XIII-9
Solubility Values and Organic Carbon Partition Coefficients For
Select Petroleum Hydrocarbon Constituents
Compound
MTBE
Benzene
Toluene
Ethylbenzene
Xylenes (total)
Cumene
Naphthalene
Acenaphthene
Molecular
Weight (g/mol)
88
78
92
106
106
120
128
154
Solubility in
Water (g/L)
51
1.79
0.53
0.21
0.175
50
0.031
.0035
Organic Carbon
Coefficient
(Koc - ml/g)
12
58
130
220
350
454
950
4,900
Solubility is also an indicator of likely contaminant sorption onto soil. There is
an inverse relationship between a chemical compound's solubility and its organic
carbon partition coefficient (Koe). A compound with a high solubility has a
reduced tendency to adsorb to soil that is in contact with contaminated
groundwater and may be more readily contacted by chemical oxidants.
Conversely, contaminants with low solubility values will likely have an increased
tendency to adsorb to soil that is in contact with contaminated groundwater and
may be less readily chemically oxidized. Note that some compounds are less
predictable in this relationship, such as cumene. Cumene has a strong ability to
sorb to soils, despite its very high solubility. If cumene is a key target
contaminant, chemical oxidation may not be the most appropriate technology for
removing it from groundwater. The relationship between solubility and K^. is
described in more detail below.
Koc Factor. When groundwater is contaminated by a petroleum UST release, the
proportion of hydrocarbon mass in the soil is often far greater than that dissolved
in groundwater. This is due in part to the low solubility thresholds for petroleum
contaminants. However, another factor is the strong tendency for most petroleum
hydrocarbons to adsorb to naturally occurring organic carbon material in the soils.
This tendency along with the sheer mass of soil relative to groundwater in a
contaminated area can lead to hydrocarbon mass distributions that are so unevenly
distributed that they can make the mass in the dissolved-phase appear insignificant.
Because of the high proportionate amount of contaminant mass in the adsorbed
phase, it is important to understand the ability of the chemical oxidant to come into
contact with the soil contamination.
May 2004
XIII-25
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. is a compound-specific property that helps define the equilibrium condition ^n^
between organic carbon and the contaminant concentrations in an aqueous x^-
solution. Using site-specific soil organic carbon content data (i.e., fraction of
organic content or foe), K^ can be used to determine the equilibrium contaminant
concentrations between groundwater and soil below the water table. The typical
organic carbon content in surface soils ranges from 1 to 3.5 percent. In aquifer
soils, organic carbon content is an order of magnitude lower - from 0.1 and 0.01
percent - because most organic residues are either incorporated into the soil matrix
or deposited on the surface.
Higher Koc and Kd values indicate that more contaminant mass is likely to be
retained in soil, and therefore potentially less readily contacted by chemical
oxidants. Conversely, lower Koc and Kd values indicate that lower contaminant
concentrations will exist in equilibrium in soil for given concentrations in
groundwater. A comparison of the solubility and Koc values for the sample group
of petroleum hydrocarbons reveals the inverse relationship between the two
parameters (i.e., compounds with higher solubility values have lower K^.
constants).
In the absence of site-specific data that reveal the distribution of contaminant
mass, solubility and K^ data can be used to obtain a general understanding of the
likelihood that chemical oxidation is applicable at the site. Petroleum contaminants
with high solubility limits and low K^ values are more likely to come in contact '^^
with chemical oxidants and to be destroyed by chemical oxidation technologies. "**"***
When contaminant solubility constants are low and K^ values are high, chemical
oxidants may not have adequate contact with the contaminants to effectively
destroy contaminant mass, particularly in low permeability soils.
Step 3: Evaluation of Chemical Oxidation Design
This section provides guidance on reviewing and evaluating a chemical
oxidation remediation system's design. This section focuses on identifying and
reviewing key elements of corrective action plans to help ensure they demonstrate
a coherent understanding of the basis for the chemical oxidation system design.
This section provides information on typical chemical oxidation technology
components to help verify that the corrective action plan has included the basic
equipment requirements for the remedial system.
It is assumed that it has already been verified, through the detailed technology
screening process described in Steps 1 and 2, that chemical oxidation appears
appropriate and is expected to be an effective cleanup approach, given site-specific
conditions. If chemical oxidation effectiveness evaluation has not been completed,
it is strongly recommended that this be done prior to evaluating the design.
May 2004 XIII-26
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Two important factors that need to be considered in evaluating the design of
chemical oxidation treatment are (1) the design basis and (2) the site cleanup goals.
Each of these factors is discussed in more detail below.
Design Basis
A review of the corrective action plan should find consistency between site
characterization work and information that is presented as the basis for the
chemical oxidation design in the corrective action plan. It is important that during
the chemical oxidation effectiveness evaluation a reviewer has a solid
understanding of the nature and extent of the site-specific petroleum constituents
of concern, including an understanding of the contaminant phases present and the
relevant site chemical, physical, and biological properties. When preparing and
reviewing the corrective action plan design, it is important to understand the site
geology and hydrogeology, and the risks associated with the contamination. These
data, which should have been developed and interpreted as part of the site
characterization effort, serve as the foundation for the remedial system design.
While site characterization data provide the core raw materials for the design,
further refinement is often needed and useful. For example, while the site
characterization work may identify potential human or ecological receptors that
may be exposed to contamination, specific cleanup goals may not have been
established. In such cases, the specific remedial goals would need to be developed
and identified in the corrective action plan through one or more established
approaches. These approaches may include adopting state-published cleanup
standards, developing site-specific risk-based standards acceptable to the state, or
employing other state-specific and approved methods.
A corrective action plan may also include the results and interpretation of
follow-up studies completed after the original site characterization. The need for
such studies is often identified after a review of the site characterization shows that
additional information is needed to complete the remedial system design. For
example, the site characterization may suggest that one or more of the constituents
of concern is believed to be marginally degradable, either chemically or
biologically, and the level of expected degradation is difficult to predict from the
existing data.
Examples of typical information expected to be developed during the site
characterization, or as a result of follow-up studies that should be completed to
support the basis for the technology selection and design of the corrective action
plan, are summarized in Exhibit XIII-10.
Cleanup Goals
The evaluation of alternative remedial approaches and the subsequent design of
the selected approach are strongly influenced by the cleanup goals that the
remediation program must achieve. Often, preliminary goals identified during the
May 2004 XIII-27
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site characterization evolve as a better understanding of site conditions and
potential receptors is attained. However, owing to their importance to remediation
planning and design, the cleanup goals should be fully evolved and solidified in the
corrective action plan.
These goals usually provide the end-point concentrations for petroleum
constituents in soil and groundwater that are acceptable to state or other
regulatory agencies. These cleanup thresholds could be goals that represent any of
the following:
• Health-based numeric values for petroleum chemical constituents published
by the respective regulatory agency.
• Cleanup goals developed and proposed by the contractor specifically for
the contaminated site.
• Goals derived from site-specific risk assessment involving contaminant fate
and transport modeling coupled with ecological and human-health risk
assessment.
Additional project goals that may be regulatory requirements include hydraulic
control of the contamination, a cleanup time frame, or other performance goals
established in the corrective action plan. Regardless of the cleanup goals and how
they are established, the state-sanctioned goals should be noted in the corrective
action plan and recognized as a fundamental basis for the technology selection and
design.
The cleanup goals presented in the corrective action plan answer important
questions about the viability of the selected remedial approach and the adequacy of
the remedial design. The critical question is, Can the cleanup concentration goals
be economically met by the designed chemical oxidation approach? It is important
to understand how much oxidant will be consumed by NOD reaction, and how
much will be lost attempting to permeate low permeability soils, in order to weigh
the economics and technical feasibility of the approach. Multiple applications of
the chemical oxidants may be required in order to accomplish the site objectives.
Many logistical, political, risk-related, and cost issues are associated with
successive attempts to oxidize the site contamination, and should be considered
when such a proposal is put forth in a corrective action plan. Verification that the
target petroleum constituents of concern can be chemically oxidized by the oxidant
of choice should be completed.
May 2004 XIII-28
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Exhibit XII1-10
Chemical Oxidation Design Basis Factors
Design Basis Factor
Source(s) of Design Information
Cleanup Goals
• Target contaminant levels (soil and
groundwater)
« Remediation timeframe
• Plume control
• Others
Receptor survey, pre-design exposure or
risk assessment analyses (potentially
including numerical modeling), or state
requirements.
Geology
• Uniformity (homogeneity, heterogeneity)
» Stratigraphy (vertical profile of sand, silt,
clay, etc.)
• Geochemistry (reduced mineral content,
organic content, mineral demand for
ferrous iron, sulfite, nitrite, dissolved
oxygen, etc.)
« Bedrock (description, depth, strike, dip,
fracturing, etc.)
• Soil permeabilities
Site characterization, soil borings, well
installations, sampling and analysis, and
site observations.
Local geologic studies.
Hydrogeology
• Depth to groundwater
• Groundwater elevation and gradient
• Aquifer/water bearing unit class
(confined, unconfined, perched, bedrock,
etc.)
• Hydraulic parameters (conductivity,
transmissivity, storativity, effective
porosity, etc.)
• Geochemistry (aqueous demand for
ferrous iron, sulfite, nitrite, dissolved
oxygen, etc.)
• Modeling (simulation of groundwater flow
and effects of manipulation of hydraulic
head)
Site characterization, well gauging,
aquifer pump testing, data analyses, and
local hydrogeologic studies.
Petroleum Contamination
• Target chemical constituents
• Concentrations of other contaminants
that can consume oxygen
• Mass estimates (adsorbed, dissolved,
liquid and vapor phases)
• Extent (vertical and lateral)
• Fate and transport characteristics
(solubility, partition coefficients)
• Vapor pressure and Henry's law constant
for contaminants, especially if these
contaminants are driven into the vapor
phase by the remediation process
• Modeling (simulation of contaminant
transport under various scenarios)
Soil, groundwater and other media
sampling/laboratory analysis, review of
published data on contaminants and data
interpolation and analysis.
Materials Safety Data Sheets can provide
this information.
May 2004
XIII-29
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Chemical Oxidation Technology Selection
With the design basis established in the corrective action plan, it is now
possible to review the corrective action plan to confirm that the proposed
candidate chemical oxidation technology is a reasonable site-specific choice.
Depending on project-specific circumstances, there may be a few chemical
oxidation technologies equally viable and appropriate for a site. Alternatively, site-
specific or project-specific circumstances may suggest that one of the chemical
oxidation technologies would address the on-site contamination far better than any
of the others.
Exhibit XIII-2 presented a comparative summary of each of the chemical
oxidation technologies. These factors can be used to help evaluate the
appropriateness and feasibility of the chemical oxidation approach outlined in the
corrective action plan. Other differences among alternative chemical oxidation
technologies can also help to distinguish their most appropriate application(s).
Two characteristics that can be useful in evaluating the feasibility and
appropriateness of a proposed chemical oxidation technology are (1) oxygen
production for enhancement of aerobic biodegradation, and (2) chemical oxidation
potential. Each of these is described in more detail below.
Oxygen Enhanced Biodegradation and Chemical Oxidation Potential
Another distinguishing characteristic of some chemical oxidation technologies is
their ability to impart oxygen to the groundwater, which enhances aerobic
biodegradation of contaminants while chemically oxidizing petroleum
contaminants. In particular, both ozone and hydrogen peroxide are strong
oxidizers. During their decomposition, these oxidizers may also generate the
hydroxyl radical, an even more powerful chemical oxidizer of organic compounds.
As these chemical oxidants react in the subsurface, oxygen is produced which may
help enhance aerobic biodegradation processes occurring along the fringes of the
treatment area. These chemical oxidation technologies not only chemically oxidize
the contaminants in the treatment area but also provide oxygen to promote
biodegradation of petroleum contamination. In addition, chemical oxidants can
oxidized ferrous iron minerals to ferric iron, and transform other reduced forms to
oxidized forms that anaerobic microbes can use.
Ozone and hydrogen peroxide can help to fully or partially chemically oxidize
the recalcitrant subsurface petroleum contamination while providing oxygei for in-
situ bioremediation of the contamination. Either of these technologies may be
applied to sequentially treat the contamination via oxidation, followed by
bioremediation, or configured for concurrent treatment relying on oxidation for
core treatment with bioremediation as the treatment approach in the peripheral
reaches of the plume.
May 2004 XIII-30
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Exhibit XIII-11
Relative Power of Chemical Oxidants4
Compound
Hydroxyl Radical
Sulfate Radical
Ozone
Hydrogen Peroxide
Permanganate
Chlorine Dioxide
Chlorine
Oxygen
Bromine
Iodine
Oxidation
Potential (volts)
2.8
2.6
2.1
1.8
1.7
1.5
1.4
1.2
1.1
0.76
Relative Oxidizing Power
(CI2 = 1.0)
2.1
1.9
1.5
1.3
1.2
1.1
1.0
0.90
0.80
0.54
However, both ozone and hydrogen peroxide are non-selective with respect to
reaction with subsurface organic material. If naturally occurring organic materials
(e.g., humic substances) are present in the site subsurface, injected ozone or
infiltrated hydrogen peroxide may be lost through the oxidation of these organics,
leaving fewer of the oxidants available to react with (and oxidize) the petroleum
contaminants. The relative oxidizing power of the chemical oxidants may also be
helpful in determining the most appropriate chemical oxidant for site conditions.
Exhibit XIII-11 shows that the hydroxyl radical (Fenton's Reagent), ozone,
hydrogen peroxide and permanganate, in order of decreasing oxidation strength,
are among the strongest chemical oxidizers.
Design Components
Although the design elements of alternative chemical oxidation technologies
can vary, Exhibit XIII-12 describes common ones. Several of the more important
elements are discussed below to assist with evaluation of the corrective action
plan. Each of the major headings in the exhibit above is discussed in more detail
below.
4 Note that these compounds are provided for comparative purposes only. Many of these
compounds are not typically used for in-situ chemical oxidation.
May 2004
XIII-31
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Exhibit XIII-12
Common Chemical Oxidation Remediation Design Elements
Oxidant and Catalyst Delivery Design
- Theoretical oxidant mass requirement
- Natural oxidant demand estimates
- Application delivery rate
- Number and depth of application points/position
- Equipment
Permit Requirements and Thresholds
— Underground injection/well installation
- Groundwater (wastewater) discharge
- Air (soil vapor) discharge
Performance Monitoring Plan
— On-going distribution of oxidants
- Reduction in contaminants (adsorbed and dissolved phases)
Contingency Plan
— Inadequate oxidant distribution
- Lower-than-expected petroleum mass reduction rates
- Excessive contaminant migration
- Build-up of excessive recalcitrant petroleum constituents
- Fugitive (soil vapor) emissions
- Difficult-to-treat/fouling of treated wastewater discharge
- Aquifer clogging with precipitates or biomass
Oxidant Application Design should be based primarily on contaminant mass
reduction requirements, site characteristics and cleanup goals. Oxidants need to be
applied at concentrations and total mass levels that satisfies both the NOD and the
oxidant demand of the petroleum hydrocarbons. Note that state regulations may
either require permits for oxidant or catalyst injection or prohibit them entirely.
Permit Requirements and Thresholds should be identified in the design so
that the system can be constructed to comply with permit requirements and
constraints. Depending on the specific chemical oxidation technology and the state
in which the site is located, permits may be required for underground injection,
treated groundwater discharge (to sanitary or storm sewer, or air (soil vapor)
discharge. Several federal, state and local programs either directly manage or
regulate aquifer remediation wells (ARWs). Many of these programs require
permits for underground injection of oxygen. On the federal level, management
and regulation of these wells fall primarily under the underground injection control
program authorized by the Safe Drinking Water Act (SOWA). Some states and
localities have used these authorities, as well as their own, to extend the controls in
their areas to address concerns associated with ARWs.
Aquifer remediation injection wells are potentially subject to at least three
categories of regulation. First, a state's underground injection control program (or
May 2004 XIII-32
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in direct implementation states, the federal UIC program) may have jurisdiction
over such wells. Second, in some states without UIC programs, the state's
program for groundwater protection or national pollution discharge elimination
system (NPDES) requirements may apply to remediation wells. Third, remediation
wells may be regulated by federal and state authorities through Superfund
programs, corrective action programs under RCRA, the UST program, or other
environmental remediation programs. In the case of remediation programs, the
regulatory requirements typically address the selection of aquifer remediation as a
cleanup alternative and establish the degree of required cleanup in soil and
groundwater, while deferring regulation of the injection wells used in the
remediation to other programs.
Performance Monitoring should be accounted for in the form of a written
data quality objective plan that can be used to objectively evaluate chemical
oxidation system performance. The monitoring plan should outline a data quality
objective process that defined the criteria that the data collection should satisfy,
including when to collect samples, where to collect samples, the tolerable level of
decision error for the study, and how many samples to collect, balancing risk and
cost in an acceptable manner. It should describe the approaches and methods that
will be used to evaluate chemical oxidation system effectiveness in each of the
following:
• Delivering the oxidant and catalyst to the subsurface.
• Distributing the oxidant throughout the contaminated area.
• Reducing adsorbed and dissolved phase petroleum concentrations.
• Achieving other performance requirements consistent with site-specific
cleanup goals.
• Confirming chemical oxidation effectiveness through long-term
monitoring.
Contingency Plans should also be prepared as part of the remedial design.
The design should anticipate low-likelihood problems and potentially changing
environmental conditions, as well as outline specific response actions that may be
taken. Examples include response actions to take if performance monitoring data
indicate any of the following:
» Inadequate oxidant distribution
• Inadequate permeation of low permeability soil zones
• Low petroleum mass reduction rates
• Excessive contaminant migration
• Recalcitrance of constituents
• Production of excessive fugitive emissions
• Rebound in contaminant levels measured during long term post-
application monitoring
• Evidence of oxidant moving in wrong direction
May 2004 XIII-33
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Components of Chemical Oxidation Systems
Having briefly covered factors that affect the selection and design of a
particular chemical oxidation technology and the critical elements that should be
included in the corrective action plan chemical oxidation design, it is now
appropriate to discuss major components of various chemical oxidation systems.
This discussion should help in the evaluation of the corrective action plan chemical
oxidation design.
Exhibit XIII-13 summarizes some of the major equipment components
associated with each of the more common chemical oxidation technologies. Note
that this exhibit continues across three pages. Depending on which chemical
oxidation technology has been selected in the corrective action plan, a subset of
these major system components should be presented and discussed and
schematically depicted (e.g., process flow diagram) in the corrective action plan.
The design should relate capacities of these equipment components to design
requirements (e.g., required oxidant production and delivery rates).
Exhibit XIII-13
Major Components of Chemical Oxidation Systems
Component
Function
:enton's Reagent Injection Systems
Extraction Wells
Injection Wells or
Infiltration
Galleries
Extraction,
Injection, Transfer,
and Metering
Pumps and Tanks
Blowers
Wells may be used to capture soil vapor generated by the
oxidation process that may be heated and contain elevated levels
of VOCs and oxygen (i.e., soil vapor extraction). Can also be
used to help control groundwater flow during oxidant and catalyst
delivery (i.e., groundwater extraction).
Injection wells, infiltration galleries or a combination of these may
be used to inject hydrogen peroxide catalyst solution, and
compressed air for reagent contact with the treatment zone
contaminants. Diluted peroxide and peroxide slurries can be
injected via lance points.
Extraction, injection, transfer, and metering pumps may be used
for various purposes including: transferring groundwater from and
back into the ground; transferring extracted groundwater between
different components of the treatment system; and metering
hydrogen peroxide and catalyst into the infiltration system to
maintain design concentrations. Note that pumps can be
damaged by hydrogen peroxide and may need frequent
replacement.
Extraction blower(s) may be used to draw soil vapor from
extraction wells to capture fugitive VOC vapors and oxygen.
May 2004
XIII-34
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Exhibit XIII-1 3
Major Components of Chemical Oxidation Systems (continued)
Component
Hydrogen Pero)
Groundwater and
Vapor Treatment
Equipment
Instrumentation
and Controls
Monitoring Wells
yx -~x " ' -\ -<• -' '.'i
Extraction Wells
Injection Wells or
Infiltration
Galleries
Extraction,
Injection, Transfer,
and Metering
Pumps and Tanks
Groundwater
Treatment
Equipment
Instrumentation
and Controls
Function
ude/Fenton's Reagent Injection Systems (continued)
Extracted groundwater or soil vapor may be treated to remove
petroleum hydrocarbons by various means such as: granular
activated carbon adsorption, air stripping or others.
Used to integrate and activate/deactivate system components.
Help maintain the balance of flows consistent with the design and
to safeguard against inadequate treatment or inappropriate
discharges.
Used to collect environmental samples analyzed in laboratories
and field to evaluate on-going effectiveness of remediation.
Groundwater well samples tested for peroxide and contamination
to evaluate overall effectiveness of oxidant delivery/dispersal and
the contaminant reductions over time. Long term monitoring of
contaminant concentrations is essential to evaluating the
effectiveness of the technology.
* Perniari^i^ Injectlow%slem8 •"• '-^. ^-sjf*^
Wells may be used to enhance hydraulic gradient across the
treatment area so that permanganate can be more rapidly
delivered to and put in contact with site contaminants.
Injection wells, infiltration galleries or a combination of these may
be used to inject permanganate or permanganate amended
groundwater into the treatment zone. Upgradient injections of
amended groundwater with downgradient extraction of
groundwater may enhance the hydraulic gradient across the
treatment zone, thereby accelerating permanganate delivery to
the contamination.
Extraction, injection, transfer, and metering pumps may be used
for various purposes including: transferring groundwater from and
back into the ground; transferring extracted groundwater between
different components of the treatment system; and metering
permanganate into the infiltration system to maintain design
concentrations.
Extracted groundwater may be treated to remove petroleum
hydrocarbons by various means such as: granular activated
carbon adsorption, chemical oxidation, air stripping or others.
Used to integrate and activate/deactivate system components to
maintain the balance of flows consistent with design and to
safeguard against inadequate treatment or inappropriate
discharges.
May 2004
XIII-35
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Exhibit XIII-1 3
Major Components of Chemical Oxidation Systems (continued)
Component
'•'••*$&>••.-:•'•#$*
Lance Injection
Points
Large Diameter
Auger Deep Soil
Mixing
Monitoring Wells
'' "', ?^
Sparging Wells
Air Compressing
Equipment
Ozone Generating
Equipment
Soil Vapor
Extraction/
Treatment
Equipment
(optional)
Instrumentation
and Controls
Monitoring Wells
Function
' • •'"'' •*»*' >l 'liL*s";V J&'^iL ' ^%''"- •'•»•« '''•"'' A\ ^'*'-- '•' * '•'''"•
langanate injection Systems (continued) • r**«v . •• - f*?
Permanganate in slurry form may be injected into the subsurface
over a grid using push-point technologies.
Permanganate may be mixed deeply into the contaminated soil
and groundwater using large diameter augers in patterned drilling
over contaminated areas.
Used to collect environmental samples tested in laboratories and
the field to evaluate on-going effectiveness of remediation.
Comparative analyses over time of groundwater samples from
these wells for dissolved oxygen and petroleum contamination
indicates how effectively oxygen is being delivered/dispersed and
contaminant reductions are occurring.
- Ozone Injection Systems :, '.. ' , '*, '"'• •'••
Used as a conduit to inject ozone into contaminated groundwater.
The ozone is sparged near the base of the soil and groundwater
petroleum contamination so that it may contact the contaminants
and provide oxygen to the hydrocarbon degrading bacteria.
Used to pressurize ambient air needed to generate ozone - " to
provide the pressure needed to inject the ozone beneath t:
water table. Oil-less compressors are preferred, because a ,
compressor equipment must supply oil- and contaminant-free air
to minimize in-line reactions with and pre-mature decomposition
of ozone.
Used to generate ozone gas on-site, typically at concentrations of
about 5%.
Used, if necessary, to control fugitive soil vapor ozone and
volatilize organic compounds emissions in the unsaturated zone.
May consist of low vacuum/flow blower to generate vacuum
conditions in unsaturated zone and collect the vapors. Off-gas
treatment may be necessary and may be accomplished using
granular activate carbon, biofilters or other technologies.
Used to integrate and activate/deactivate system components to
maintain the balance of flows consistent with the design and to
safeguard against inadequate treatment or inappropriate
discharges.
Used to collect environmental samples tested in laboratories and
the field to evaluate on-going effectiveness of remediation.
Comparative analyses over time of groundwater samples from
these wells for dissolved oxygen and petroleum contamination
indicates how effectively oxygen is being delivered/dispersed and
contaminant reductions are occurring.
May 2004
XIII-36
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While the sets of major equipment components used by the chemical
oxidation technologies differ significantly, the use of wells by each different
approach warrants recognition and further discussion. In particular, the
orientation, placement, number and construction of this common design element is
worthy of a brief review, wells or gravity-fed into vertical delivery wells.
Additionally, hydrogen peroxide-amended groundwater can be re-infiltrated using
either vertical or horizontal wells. Although vertical sparge wells are more
common for ozone injection, horizontal sparge wells can be used. Permanganate
amended groundwater can similarly be re-infiltrated using vertical wells, horizontal
wells, infiltration trenches or combined approaches. Well orientation should be
based on site-specific needs and conditions. For example, horizontal systems
should be considered when evaluating sites that require re-infiltration of amended
groundwater into shallow groundwater at high flow rates. They are also readily
applicable if the affected area is located under a surface structure (e.g., a building),
or if the thickness of the saturated zone is less than 10 feet.
Injection, Extraction and Re-infiltration Wells. Three important
considerations are well orientation, well placement and number, and well
construction.
• Well Orientation. Both horizontal and vertical wells can be used to treat
subsurface petroleum releases with any of the various chemical oxidation
systems. However, hydrogen peroxide and a catalyst (Fenton's Reagent) is
most commonly injected into vertical sparge wells.
• Well Placement and Number of Wells. The number and location of wells
are determined during the design to accomplish the basic goals of (1)
optimizing reliable oxidant and catalyst delivery to the contaminated area,
and (2) providing conduits to measure chemical oxidation system
performance. For permanganate re-infiltration systems this typically means
placing re-injection wells in the upgradient portion of the source area(s)
while extracting groundwater from downgradient locations. This approach
simultaneously provides an enhanced hydraulic gradient, which can
accelerate oxidant distribution across the impacted area. The number,
location and design of the extraction wells will largely be determined from
site-specific hydrogeology, the depth(s) and thickness(es) of the
contaminated area(s), and the results of field-scale pilot testing and
hydraulic modeling. Note that well placement may need to be changed as
remediation progresses, as wells often generate preferential flow paths over
time.
Determining the number and spacing of the wells for ozone injection may also
be determined through field-scale pilot testing. However, the following general
points should be considered.
May 2004 XIII-37
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• Closer well spacing is often appropriate in areas of high contaminant
concentrations to enhance contaminant contact and oxidant
delivery/distribution where the oxidant demand is the greatest.
• Direct delivery of oxidant into the contaminated material using closer
well spacings can deliver, disperse, and significantly decrease the
treatment timeframe through groundwater advection/dispersion more
quickly than oxidant delivery.
At sites with stratified soils, wells screened in strata with low permeabilities
often require closer well spacing than wells screened in strata with higher
permeabilities.
Well Construction. Chemical oxidation system wells are constructed of 1- to
6-inch diameter PVC, galvanized steel, or stainless steel pipe, although caution
should be exercised in the use of stainless steel pipe in low-pH conditions. Ozone
injection sparge wells have screened intervals that are normally 1-3 feet in length
and situated within the contaminated zone. Injection sparge points must be
properly grouted to prevent the oxidants from moving directly up the well annulus
to the unsaturated zone rather than being forced into the contaminated aquifer
("short circuiting" of the injected oxygen) when horizontal injection oxidant exits
along the entire screen length. Exhibit XIII-14 shows a cross-section typical
ozone or hydrogen peroxide (Fenton's Reagent) sparge well.
Re-infiltration wells typically have screen lengths that extend from the base of
the wells into the unsaturated zone. Groundwater extraction wells should ideally
be screened in the saturated interval containing the greatest mass of hydrocarbons.
Field-scale pilot studies and subsequent data analysis and hydraulic modeling can
greatly assist one in determining the configuration and construction design of
groundwater extraction and injection wells.
Step 4: An Evaluation of the Operation and Monitoring Plan
Remedial Progress Monitoring
Significant uncertainties associated with site conditions can remain even as
remedial designs are completed and implemented. In the start-up period, these
unknowns frequently can result in operations that vary from the original design.
These variances often require adjustments to account for unforeseen conditions
and to optimize system performance. Unfortunately, in many cases, the need for
these adjustments can go unrecognized for a long time.
In some cases, the delay in recognizing that remedial system adjustments are
necessary may be attributed to slow responses in subsurface conditions to the
applied technology. Because these subsurface responses to the applied remedial
technology can be delayed, there is often the tendency to give the remedial
program more time to work (sometimes years) before making system
May 2004 XIII-38
-------
modifications or adjustments. In other cases, the delay may stem from misuse or
misinterpretation of site data, which can lead to the conclusion that the remedial
system is performing well when it is, in fact, not. An example of this misuse is the
practice of using groundwater analytical data from chemical oxidant delivery wells
as an indicator of remedial progress. In this case, an assessment is biased by the
localized effects of concentrated chemical oxidation in the immediate vicinity of
the oxidant delivery wells, but does not provide an objective measure of the
chemical oxidation system's ability to distribute the oxidant and contact the
adsorbed contaminants throughout the treatment area.
However, at many sites remedial system or application operational efficiencies
are not optimized simply because an adequate performance monitoring plan has
either not been developed or has not been fully implemented. In such cases, the
designed remedial system may be installed, implemented, and allowed to run its
course with insufficient numbers or types of samples to determine whether the
remedial system is performing in accordance with design expectations. The result
of such monitoring approaches can be the discovery of a sub-standard or failed
remediation program years after its implementation.
The previous section discussed the importance of developing a comprehensive
remedial progress monitoring plan. This covers the topics that should be
addressed in such a plan to ensure objective gauging of remedial system
performance. Necessary optimization adjustments can be made early in the
remediation program as well as throughout the duration of a chemical oxidation
remedial program. The following section provides a focused discussion on
evaluation sampling and chemical oxidation evaluation criteria that should be
examined during review of a operations and monitoring plan that proposes to use
chemical oxidation.
Evaluation Sampling
Evaluation sampling is performed to gauge the effectiveness of the chemical
oxidation program relevant to design expectations. Based on a comparison of the
actual field sampling data to design and operational expectations, timely
modifications to the system or operating procedures can be made to optimize the
application of chemical oxidants early in the remediation program. Projects with
regular performance reviews guided by the results of such sampling and
monitoring programs have a greater chance of achieving the design remedial goals
within desired timeframes and, potentially, at a lower cost.
Various environmental media are sampled to evaluate system performance.
Groundwater, soil, and soil vapors from the treatment area and vicinity are
commonly sampled to determine the degree to which the chemical oxidation
program is meeting the basic objectives of the approach, including:
• Delivering oxidants to the treatment zone at required design rates.
May 2004 XIII-39
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Distributing the oxidants across the target contaminated area to contact
the contaminants.
• Reducing concentrations of petroleum hydrocarbons in soil and
groundwater at design rates through chemical oxidation of the
petroleum compounds.
Exhibit XIII-14 identifies those parameters that are commonly measured in
groundwater, soil and soil vapor samples to help evaluate chemical oxidation
progress and system performance. A brief description of the respective sampling
frequencies and the relevance and significance of each parameter to the
performance evaluation are also provided in the exhibit. A key element is the
location(s) where performance evaluation sampling takes place relative to
subsurface oxidant delivery points. As stated in the exhibit, performance
evaluation samples should not normally be collected from oxidant delivery
locations.
Exhibit XIII-1 4
Common Performance Monitoring Parameters
and Sampling Frequencies
Sampling Frequency
Analytical
Parameter
Start-
up
Phase
(7-10
days)
Daily
Remediation/
Post-Application Long-
Term Monitoring Phase
Weekly to
Monthly
Quarterly to
Annually
Purpose
GROUNDWATER
Samples should be collected from monitoring wells located in and around the treatment area
and from extraction wells (If used). Samples should not be collected from oxidant delivery wells
for evaluating system performance because they represent highly localized effects of the
remediation program.
Dissolved
Oxygen
Redox
Potential
X
X
X
X
Determines the effect of
the oxidants on dissolved
oxygen levels and
potential to boost aerobic
biodegradation as a
secondary benefit.
Yields data on system's
ability to increase the
extent of aerobic
subsurface environment.
May 2004
XIII-40
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Exhibit XIII-14
Common Performance Monitoring Parameters
and Sampling Frequencies (continued)
Sampling Frequency
Analytical
Parameter
Start-
up
Phase
(7-10
days)
Daily
Remediation/
Post-Application Long-
Term Monitoring Phase
Weekly to
Monthly
Quarterly to
Annually
Purpose
GROUNDWATER (continued)
Samples should be collected from monitoring wells located in and around the treatment area
and from extraction wells (if used). Samples should not be collected from oxidant delivery wells
for evaluating system performance because they represent highly localized effects of the
remediation program.
PH
H2O2, Ozone,
or Perman-
ganate
Petroleum
COCs
Degradation
Daughter
Constituents
(e.g., TBA)
Water Table
Elevations
X
X
X
X
Confirms pH conditions
are stable and suitable for
Fenton's Reagent, or
identifies trends of
concern.
Provides information on
distances the oxidizing
compounds are able to be
transmitted by the
remedial system before
decomposing.
Indicates remedial
progress.
Could indicate incomplete
oxidation process.
Determines if hydraulic
conditions (groundwater
flow) are consistent with
design intent or if
chemical oxidation has
had an unanticipated
affect on these conditions.
May 2004
XIII-41
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Exhibit XIII-14
Common Performance Monitoring Parameters
and Sampling Frequencies (continued)
Sampling Frequency
Analytical
Parameter
Start-
up
Phase
(7-10
days)
Daily
Remediation/
Post-Application Long-
Term Monitoring Phase
Weekly to
Monthly
Quarterly to
Annually
Purpose
SOIL VAPOR
Samples should be collected from monitoring wells located in and around the treatment area
that are screened In the unsaturated zone and from soil vapor extraction wells (if used).
Samples should not be collected from oxidant delivery wells for evaluating system performance
because they represent highly localized effects of the remediation program.
Carbon
Dioxide
X
Provides evidence of
chemical oxidation.
Oxygen
Indicates potential losses
of introduced oxygen
through the unsaturated
zone.
Volatile
Petroleum
Contaminants
(Constituents)
of Concern
(COCs)
X
Suggests residual sources
in soil or fugitive
emissions associated with
the remedial effort.
Fugitive
Ozone or
Hydrogen
Peroxide
X
Determines losses of
oxygen-yielding reagents
delivered to the
subsurface.
SOIL
Samples should be collected from borings or using push point or drill rig sampling equipment in
and around the treatment area. Soil samples should consistently be collected from same
contaminated sections of stratigraphic interval for comparison to earlier samples from same
locations and depths.
Petroleum
COCs
Provide a measure of
remedial progress,
contaminant mass
reducions and the extent
to which chemical
oxidation of adsorbed
contaminants is limited.
May 2004
XIII-42
-------
The performance of the chemical oxidation system should be determined by the
chemistry of soil and groundwater located between, around and downgradient of
oxidant delivery locations rather than inside or in the immediate vicinity of the
oxidant delivery points. Conditions inside or in the immediate vicinity of oxidant
injection locations have been preferentially altered by chemical oxidation to destroy
the petroleum contaminants. Therefore, data from these locations are not
representative of the subsurface conditions that exist beneath most of the site. To
understand the effect the chemical oxidation system is having on the subsurface
conditions as a measure of its performance, samples of soil, groundwater and soil
gas should be collected from alternate locations. In review of the performance
monitoring plan in the corrective action plan, it should be verified that a sufficient
number of sampling locations exist between oxidant application points to provide
the necessary performance sampling data. A description of how these data may be
used to evaluate the chemical oxidation system performance is provided below.
Evaluation Criteria
The evaluation sampling described above provides the evidence needed to
assess the chemical oxidation system performance. This evidence requires
examination and interpretation to confirm chemical oxidation system effectiveness
and whether system or application modifications may be warranted. A discussion
of these data and how system performance can be interpreted is provided below.
In particular, an evaluation of performance is examined from the following two
broad chemical oxidation system requirements:
• Oxidant Delivery and Distribution
• Permanent Contaminant Mass Reduction and Attainment of Cleanup Goal
Each of these is discussed in more detail below.
Oxidant Delivery and Distribution. Performance sampling may indicate that
the chemical oxidation system is meeting design specifications for oxidant delivery
and distribution if the data show the following:
• Oxidant and catalyst are being delivered to the subsurface at the design
mass delivery rate or design adjusted rate based on analysis of field
monitoring data; and
• The oxidant and catalyst are detected in samples from the treatment area at
the design concentrations.
If the performance monitoring data suggest that one or more of these
conditions is not met, the system may not be meeting the requirements of the
design, and system adjustments or modifications may need to be made. As
previously discussed, the remedial system design should include contingency
planning that explores performance deficiency scenarios and identifies possible
solutions.
May 2004 XIII-43
-------
Oxidant delivery deficiencies may be overcome by simply adjusting system
flow rates, upgrading equipment capacities or increasing oxidant dose
concentrations. However, occasionally, oxidant delivery rates may be limited by
the capacity of the subsurface to transport the delivered oxidant mass.
Perhaps the most challenging performance problem is when a chemical
oxidation system or program is unable to deliver oxidants to a portion or multiple
portions of a contaminated area. There are many ways that oxidants distributed
from delivery points could fail to reach target contaminated area. These may
include:
• Low permeability heterogeneous soils.
• Low hydraulic gradient and groundwater flow.
Possible remedies to the performance problem include adding additional
oxidant delivery points; increasing oxidant delivery rates; increasing dose
concentrations; or enhancing hydraulic gradients and groundwater flow.
Permanent Contaminant Mass Reduction And Attainment of Cleanup
Goal. The effectiveness of a chemical oxidation program can only be determined
after examining the reduction in contaminant mass, and after identifying whether
the contaminant mass reduction is sufficient for the soil and groundwater to
permanently meet cleanup standards.
It is not sufficient to simply review groundwater monitoring data collected
during and weeks or even months after completing a chemical oxidation program.
These data are often biased, reflecting the successful oxidation of the most readily
contacted contaminants, predominantly contaminants in the most permeable soil
zones. False positive evaluations of chemical oxidation program performance can
result from reliance on short-term post-chemical oxidation application
groundwater monitoring data. These false positive evaluations may become
evident during long-term groundwater monitoring when contaminant levels
rebound as untreated contaminant mass in the less permeable soil bleeds back out
and re-contaminates the more permeable zones. Long term (e.g., months to
years), post-chemical oxidation groundwater monitoring is needed to evaluate the
effectiveness of a chemical oxidation program.
Program effectiveness may also be evaluated by estimating the mass of
contaminants destroyed, which can be accomplished using sample analytical data.
Provided that a sufficient number of soil samples are collected and analyzed for the
treatment area, soil sampling using identical methods before and after
implementation of a chemical oxidation program may indicate the volume of
contaminant mass destroyed by the oxidants. Comparing the estimated actual
mass destruction with the projected mass destruction (as predicted in the
corrective action plan) will reveal the relative effectiveness the oxidant application
program. If the contaminant mass destroyed is roughly the amount predicted
during the design, the chemical oxidation program can be considered a success.
May 2004 XIII-44
-------
Should significantly more contaminant mass be destroyed than predicted, the
program might be characterized as highly successful, but if significantly less
contaminant mass is destroyed than predicted, it may be more accurately
characterized as a failure. As the remediation program progresses, it may be
necessary to review the project goals, particularly if the source has been effectively
reduced (e.g., 70-90%), but significant contaminant mass remains in the associated
plume. It may be necessary to perform a second phase of remediation (e.g., apply
a different oxidant, move to monitored natural attenuation) to determine whether
site cleanup has been achieved or is feasible.
The most direct measurement of the success of a chemical oxidation program
is to determine whether the groundwater and soil remedial objectives have been
met and can be sustained indefinitely following chemical oxidation treatment.
Post-application monitoring should be conducted for a minimum of one year
following chemical oxidation treatment to confirm that short-term reductions can
be sustained, indicating that contaminant levels have been adequately reduced
throughout the contaminated soil and groundwater.
May 2004 XIII-45
-------
References ^^
^
Baker, Katherine H. and D.S. Herson. Bioremediation. McGraw-Hill, Inc. 1994. N***"
Brown, Richard, Chris Nelson, M. Leahy. Combining Oxidation and
Bioremediation for the Treatment of Recalcitrant Organics. In Situ and On
Site Bioremediation. Battelle Press. 1997.
Brown, R. A., R.D. Norris, R.L Raymond. Oxygen Transport in Contaminated
Aquifers With Hydrogen Peroxide. API Conference "Petroleum Hydrocarbon
and Organic Chemicals in Groundwater: Prevention, Detection and
Restoration", Houston, TX. 1984.
Brown, R.A. Bioremediation of Petroleum Hydrocarbons: A Flexible, Variable
Speed Technology. Remediation. Summer/1996.
Brown, R. A. and R. D. Norris. The Evolution of a Technology: Hydrogen
Peroxide in In-situ Bioremediation. In Situ Hydrocarbon Bioremediation.
Hinchee, R.E. Ed., CRC Press, Boca Raton. 1994.
Carter, Sean. Enhanced Biodegradation of MTBE and BTEX Using Pure Oxygen
Injection. In Situ and On Site Bioremediation. Battelle Press. 1997.
Chapman, Steven W. Semi-Passive Oxygen Release Barrier for Enhancement of
Intrinsic Bioremediation. In Situ and On Site Bioremediation. Vol.4. 1997.
California MTBE Research Partnership. Treatment Technologies for Removal of ^^
Methyl Tertiary Butyl Ether (MTBE) from Drinking Water, National Water
Research Institute, Fountain Valley, California, February 2000. "****
Chapelle, Francis H. Bioremediation of Petroleum Hydrocarbon-contaminated
Ground Water: The Perspectives of History and Hydrogeology. Ground
Water, Vol. 37, No. 1. Jan.-Feb. 1999.
Chien Chin. Assessment of the Applicability of Chemical Oxidation Technologies
for the Treatment of Contaminants at LUST Sites, Chemical Oxidation,
Technologies for the Nineties. Technomic. 1993.
Christian, B.J, L.B. Pugh and B.H. Clarke. Aromatic Hydrocarbon Degradation in
Hydrogen Peroxide- and Nitrate-Amended Microcosms. In Situ and On Site
Bioremediation Symposium. Battelle Press. 1995.
Cole, G. Mattney. Assessment and Remediation of Petroleum Contaminated Sites.
CRC Press. 1994.
Droste, Edward. Observed Enhanced Reductive Dechlorination After In-situ
Chemcial Oxidation Pilot Test. Proceedings, Remediation of Chlorinated and
Recalcitrant Compounds. Battelle Press. 2002.
Dupont, Ryan R. and Robert E. Hinchee. Assessment ofln-Situ Bioremediation
Potential and the Application ofBioventing at a Fuel-Contaminated Site, and
Brown, R. A. and R. D. Norris. Oxygen Transport in Contaminated Aquifers. '—^
Presented a NWWA and API Conference "Petroleum Hydrocarbon and
May 2004 XIII-46
-------
Organic Chemicals in Groundwater: Prevention, Detection and Restoration".
Houston, TX. 1984.
Fry, Virginia A., J. D. Istok, K. T. O'Reilly. Effect of Trapped Gas on Dissolved
Oxygen Transport - Implications for In Situ Bioremediation. Ground Water,
Vol. 34, No. 2. Mar-Apr 1996.
Groundwater Remediation Technologies Analysis Center. Trends in Applying
Innovative In-situ Chlorinated Solvents Remediation Technologies - Poster.
Battelle Conference on Remediation of Chlorinated and Recalcitrant
Compounds, Monterey CA, 2002.
Hinchee, R.E., D.C. Downey, P.K. Aggarwal. Use of Hydrogen Peroxide as an
Oxygen Source of In-situ Biodegradation: Part I. Field Studies. Journal of
Hazardous Materials. 1991.
Johnson, R, J. Pankow, D. Bender, C. Price and J. Zogorski. MTBE, To What
Extent Will Past Releases Contaminate Community Water Supply Wells?
Environmental Science and Technology/News. 2000.
Nelson, C.H. and R. A. Brown. Adapting Ozonation for Soil and Groundwater
Cleanup. Chemical Engineering. November 1994.
Norris, R. D., K. Dowd and C. Maudlin. The Use of Multiple Oxygen Sources
and Nutrient Delivery Systems to Effect In-situ Bioremediation of Saturated
and Unsaturated Soils. In Situ Hydrocarbon Bioremediation. Hinchee, R.E.
Ed. CRC Press, Boca Raton. 1994.
Prosen, B. J., W.M. Korreck and J. M. Armstrong. Design and Preliminary
Performance Results of a Full-scale Bioremediation System Utilizing on On-
site Oxygen Generation System. In-situ Bioreclamation: Applications and
Investigations for Hydrocarbon and Contaminated Site Remediation. Eds., R.
E. Hinchee. Butterworth-Heinemann. 1991.
Riser-Roberts, Eve. Remediation of Petroleum Contaminated Soils. Lewis
Publishers. 1998.
Siegrist, Roberts L., et al. Principles and Practices of In-situ Chemical Oxidation
Using Permanganate. Battelle Press. 2001.
USEPA. In Situ Remediation Technology: In Situ Chemical Oxidation. EPA
542-R-98-008. 1998.
USEPA. Cost and Performance Report, Enhanced Bioremediation of
Contaminated Groundwater. 1998.
USEPA. The Class V Underground Injection Control Study, Volume 16, Aquifer
Remediation Wells, EPA/816-R-99-014,1999.
May 2004 XIII-47
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Checklist: Can Chemical Oxidation Be Used At This Site?
This checklist can help you to evaluate the completeness of the corrective
action plan and to identify areas that require closer scrutiny. As you go through
the corrective action plan, answer the following questions. If the answer to several
questions is "no", you will most likely want to request additional information to
determine if the proposed chemical oxidation technology and approach will
accomplish the site cleanup goals.
1. Site Factors
Yes No
o o Is the soil intrinsic permeability greater than 10~9 cm2?
o o Is the soil generally free of impermeable or low permeability
layers that could retain significant petroleum contaminant mass
and limit the bioavailability of this mass?
o o Is the soil profile determined from geologic boring logs
generally free of natural organic material (e.g., layers of peat or
humic material)?
o o Is the soil temperature expected to be 10°C or higher during
remediation?
o o Is the pH of site groundwater between 5 and 9?
o o Is the dissolved iron concentration in the site groundwater
<10mg/L?
o o Have imminent likely excessive risks to human health or the
environment (if any, associated with the petroleum
contamination) been eliminated?
o o Does the state have specific permitting requirements?
2. Chemical Oxidation Design
Yes No
o o Has the mass of petroleum hydrocarbons requiring
biodegradation been estimated?
o o Has the mass of dissolved oxygen required to biodegrade the
petroleum contaminants been estimated?
o o Can the proposed chemical oxidation approach deliver the
necessary oxygen mass to the treatment area within the
estimated cleanup time?
o o Is the capacity of the chemical oxidation treatment system
sufficient to generate and deliver oxygen at the required design
rate?
o o Is the density and configuration of oxygen delivery points
adequate to uniformly disperse dissolved oxygen through the
target treatment zone, given site geology and hydrologic
conditions?
May 2004 XIII-48
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3. Permitting Issues
Yes No
o o Does the state have specific permitting requirements? If so, are
they addressed in the plan?
4. Written Performance Monitoring Plan
Yes No
o o Will a comprehensive set of baseline sampling be performed
prior to chemical oxidation system start-up?
o o Does the plan specifically exclude sampling from oxygen
delivery wells when collecting data to evaluate chemical
oxidation system performance?
o o Are monitoring wells adequately distributed between oxygen
delivery locations to collect groundwater and soil vapor samples
to evaluate the performance of the chemical oxidation system?
o o Does the written plan include periodically collecting soil
samples from the contaminated interval(s) at locations between
oxygen delivery locations?
o o Will the soil, soil vapor and groundwater samples be analyzed
for the majority of the recommended performance monitoring
parameters?
o o Will frequencies of performance monitoring correspond to
those identified in Exhibit XIII-14?
May 2004 XIII-49
-------
-------
APPENDIX
Abbreviations and Definitions
-------
-------
Appendix
Abbreviations and Definitions
Abbreviations
AS Air Sparging
ASTM American Society of Testing and Materials
atm atmosphere (pressure)
BTEX Benzene, Toluene, Ethylbenzene, Xylenes
Btu British thermal unit
CAP Corrective Action Plan
CPU Colony Forming Units
DNAPL Dense Non-Aqueous Phase Liquid
DO Dissolved Oxygen
DPE Dual-Phase Extraction
FID Flame lonization Detector
GAC Granular Activated Carbon
GC Gas Chromatograph
HOPE High Density Polyethylene
Hg Mercury, elemental
LEL Lower Explosive Limit
LNAPL I jght Non-Aqueous Phase Liquid
LTTD Low Temperature Thermal Desorption
LUST leaking Underground Storage Tank
MS Mass Spectrometer
NAPL Non-Aqueous Phase Liquid
NPDES National Pollutant Discharge Elimination System
OUST Office of Underground Storage Tanks (USEPA,
Washington, DC)
PAH Polyaromatic Hydrocarbon
PID Photoionization Detector
PNA Polynuclear Aromatic Hydrocarbon
ppb parts per billion
ppm parts per million
psi pounds per square inch (pressure)
PVC Polyvinyl Chloride
QA Quality Assurance
QC Quality Control
ROI Radius of Influence
SVE Soil Vapor Extraction
TCLP Toxicity Characteristic Leaching Procedure
(EPA Method 1311)
TEA Terminal Electron Acceptor
TPH Total Petroleum Hydrocarbons
TRPH Total Recoverable Petroleum Hydrocarbons
(EPA Method 418.1)
UEL Upper Explosive Limit
UST Underground Storage Tank
VOC Volatile Organic Compound
May 2004
Appendix-1
-------
Definitions
abiotic: not biotic; not formed by biologic processes.
absorption: the penetration of atoms, ions, or molecules into the bulk
mass of a substance.
Actinomycetes: any of numerous, generally filamentous, and often
pathogenic, microorganisms resembling both bacteria and fungi.
adsorption: the retention of atoms, ions, or molecules onto the surface of
another substance.
advection: the process of transfer of fluids (vapors or liquid) through a
geologic formation in response to a pressure gradient that may be caused
by changes in barometric pressure, water table levels, wind fluctuations, or
infiltration.
aeration: the process of bringing air into contact with a liquid (typically
water), usually by bubbling air through the liquid, spraying the liquid into
the air, allowing the liquid to cascade down a waterfall, or by mechanical
agitation. Aeration serves to (1) strip dissolved gases from solution, and/or
(2) oxygenate the liquid. The rate at which a gas transfers into solution can
be described by Pick's First Law.
aerobic: in the presence of oxygen.
afterburner: an off-gas posttreatment unit for control of organic
compounds by thermal oxidation. A typical afterburner is a refractory-
lined shell providing enough residence time at a sufficiently high
temperature to destroy organic compounds in the off-gas stream.
aggregate: coarse mineral material (e.g., sand, gravel) that is mixed with
either cement to form concrete or tarry hydrocarbons to form asphalt.
algae: chiefly aquatic, eucaryotic one-celled or multicellular plants without
true stems, roots and leaves, that are typically autotrophic, photosynthetic,
and contain chlorophyll. Algae are not typically found in groundwater.
aliphatic: of or pertaining to a broad category of carbon compounds
distinguished by a straight, or branched, open chain arrangement of the
constituent carbon atoms. The carbon-carbon bonds may be either
saturated or unsaturated. Alkanes, alkenes, and alkynes are aliphatic
hydrocarbons.
alkanes: the homologous group of linear saturated aliphatic hydrocarbons
having the general formula CnH2n+2- Alkanes can be straight chains,
branched chains, or ring structures. Also referred to as paraffins.
Appendix-2 May 2004
-------
alkenes: the group of unsaturated hydrocarbons having the general
formula CnH2n and characterized by being highly chemically reactive. Also
referred to as olefms.
alkynes: the group of unsaturated hydrocarbons with a triple Carbon-
Carbon bond having the general formula CnH2n.2.
ambient: surrounding.
anaerobic: in the absence of oxygen.
anisotropic: the condition in which hydraulic properties of an aquifer are
not equal when measured in all directions.
aqueous solubility: the extent to which a compound will dissolve in
water. The log of solubility is generally inversely related to molecular
weight.
aquifer: a geologic formation capable of transmitting significant quantities
of groundwater under normal hydraulic gradients.
aquitard: a geologic formation that may contain groundwater but is not
capable of transmitting significant quantities of groundwater under normal
hydraulic gradients. In some situations aquitards may function as
confining beds.
aromatic: of or relating to organic compounds that resemble benzene in
chemical behavior. These compounds are unsaturated and characterized
by containing at least one 6-carbon benzene ring.
asymptote: a line that is considered to be the limit to a curve. As the
curve approaches the asymptote, the distance separating the curve and the
asymptote continues to decrease, but the curve never actually intersects the
asymptote.
attenuation: the reduction or lessening in amount (e.g., a reduction in the
amount of contaminants in a plume as it migrates away from the source).
Atterberg limits: the moisture contents which define a soil's liquid limit,
plastic limit, and sticky limit.
auger: a tool for drilling/boring into unconsolidated earth materials (soil)
consisting of a spiral blade wound around a central stem or shaft that is
commonly hollow (hollow-stem auger). Augers commonly are available in
flights (sections) that are connected together to advance the depth of the
borehole.
May 2004 Appendix-3
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autoignition temperature: the temperature at which a substance will
spontaneously ignite. Autoignition temperature is an indicator of thermal
stability for petroleum hydrocarbons.
auto trophic: designating or typical of organisms that derive carbon for the
manufacture of cell mass from inorganic carbon (carbon dioxide).
bacteria: unicellular microorganisms that exist either as free-living
organisms or as parasites and have a broad range of biochemical, and
often pathogenic, properties. Bacteria can be grouped by form into five
general categories: cocci (spherical), bacilli (rod-shaped), vibrio (curved
rod-shaped), spirilla (spiral), and filamentous (thread-like).
baghouse: a dust-collection chamber containing numerous permeable
fabric filters through which the exhaust gases pass. Finer particulates
entrained in the exhaust gas stream are collected in the filters for
subsequent treatment/disposal.
ball valve: a valve regulated by the position of a free-floating ball that
moves in response to fluid or mechanical pressure.
Bentonite: a colloidal clay, largely made up of the mineral sodium
montmorillonite, a hydrated aluminum silicate. Because of its expansive
property, bentonite is commonly used to provide a tight seal around a well
casing.
berm: a sloped wall or embankment (typically constructed of earth, hay
bales, or timber framing) used to prevent inflow or outflow of material
into/from an area.
bioassay: a method used to determine the toxicity of specific chemical
contaminants. A number of individuals of a sensitive species are placed in
water containing specific concentrations of the contaminant for a specified
period of time.
biodegradability (or biodegradation potential): the relative ease with
which petroleum hydrocarbons will degrade as the result of biological
metabolism. Although virtually all petroleum hydrocarbons are
biodegradable, biodegradability is highly variable and dependent
somewhat on the type of hydrocarbon. In general, biodegradability
increases with increasing solubility; solubility is inversely proportional to
molecular weight.
biodegradation: a process by which microbial organisms transform or
alter (through metabolic or enzymatic action) the structure of chemicals
introduced into the environment.
biomass: the amount of living matter in a given area or volume.
Appendix-4 May 2004
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boiling point: the temperature at which a component's vapor pressure
equals atmospheric pressure. Boiling point is a relative indicator of
volatility and generally increases with increasing molecular weight.
Btu: the quantity of heat required to raise the temperature of one pound of
water one degree Fahrenheit at 39°F; used as the standard for the
comparison of heating values of fuels.
bubble radius: the maximum radial distance away from a biosparging
well where the effects of sparging are observable. Analogous to radius of
influence of an air sparging well.
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 (voids)
between the soil grains.
butterfly valve: a shut-off valve usually found in larger pipe sizes (4 inches
or greater). This type of valve can be used for non-critical flow control.
capillary fringe: the zone of a porous medium above the water table
within which the porous medium is saturated by water under pressure that
is less than atmospheric pressure.
capillary suction: the process whereby water rises above the water table
into the void spaces of a soil due to tension between the water and soil
particles.
catalytic oxidizer: an off-gas posttreatment unit for control of organic
compounds. Gas enters the unit and passes over a support material coated
with a catalyst (commonly a noble metal such as platinum or rhodium)
that promotes oxidation of the organics. Catalytic oxidizers can also be
very effective in controlling odors. High moisture content and the presence
of chlorine or sulfur compounds can adversely affect the performance of
the catalytic oxidizer.
chemotrophs: organisms that obtain energy from oxidation or reduction
of inorganic or organic matter.
coefficient of permeability: see hydraulic conductivity.
condensate: the liquid that separates from a vapor during condensation.
conductivity: a coefficient of proportionality describing the rate at which a
fluid (e.g., water or gas) can move through a permeable medium.
Conductivity is a function of both the intrinsic permeability of the porous
medium and the kinematic viscosity of the fluid which flows through it.
May 2004 Appendix-5
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cone of depression: the area around a discharging well where the
hydraulic head (potentiometric surface) in the aquifer has been lowered by
pumping. In an unconfined aquifer, the cone of depression is a cone-
shaped depression in the water table where the media has actually been
dewatered.
confined aquifer: a fully saturated aquifer overlain by a confining layer.
The potentiometric surface (hydraulic head) of the water in a confined
aquifer is at an elevation that is equal to or higher than the base of the
overlying confining layer. Discharging wells in a confined aquifer lower
the potentiometric surface which forms a cone of depression, but the
saturated media is not dewatered.
confining layer: a geologic formation characterized by low permeability
that inhibits the flow of water (see also aquitard).
conservative: (a) in the case of a contaminant, one that does not degrade
and the movement of which is not retarded; is unreactive. (b) in the case of
an assumption, one that leads to a worst-case scenario, one that is most
protective of human health and the environment.
constituent: an essential part or component of a system or group (e.g., an
ingredient of a chemical mixture). For instance, benzene is one constituent
of gasoline.
cyclone: a type of separator for removal of larger particles from an
exhaust gas stream. Gas laden with particulates enters the cyclone and is
directed to flow in a spiral causing the entrained particulates to fall out and
collect at the bottom. The gas exits near the top of the cyclone.
Darcy's Law: an empirical relationship between hydraulic gradient and
the viscous flow of water in the saturated zone of a porous medium under
conditions of laminar flow. The flux of vapors through the voids of the
vadose zone can be related to a pressure gradient through the air
permeability by Darcy's Law.
degradation potential: the degree to which a substance is likely to be
reduced to a simpler form by bacterial activity.
denitrification: bacterial reduction of nitrite to gaseous nitrogen under
anaerobic conditions.
density: the amount of mass per unit volume.
diffusion: the process by which molecules in a single phase equilibrate to
a zero concentration gradient by random molecular motion (Brownian
motion). The flux of molecules is from regions of high concentration to low
concentration and is governed by Pick's Second Law.
Appendix-6 May 2004
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dispersion: the process by which a substance or chemical spreads and
dilutes in flowing groundwater or soil gas.
dissolution: dissolving of a substance in a liquid solvent (e.g., water).
downgradient: in the direction of decreasing static head (potential).
drawdown: lowering the water table due to withdrawal of groundwater as
from a well.
dynamic viscosity: a measure of a fluid's resistance to tangential or shear
stress.
effective porosity: the amount of interconnected pore space in a soil or
rock through which fluids can pass, expressed as a percent of bulk volume.
Some of the voids and pores in a rock or soil will be filled with static fluid
or other material, so that effective porosity is always less than total
porosity.
effluent: something that flows out, especially a liquid or gaseous waste
stream.
empirical: relying upon or gained from experiment or observation.
entrained: particulates or vapor transported along with flowing gas or
liquid.
enzyme: any of numerous proteins or conjugated proteins produced by
living organisms and functioning as biochemical catalysts.
eucaryotes: an organism having one or more cells with well-defined
nuclei.
evaporation: the process by which a liquid enters the vapor (gas) phase.
ex situ: moved from its original place; excavated; removed or recovered
from the subsurface.
facultative anaerobes: microorganisms that can grow in either the
presence or the absence of molecular oxygen. In the absence of oxygen
these microorganism can utilize another compound (e.g., sulfate or nitrate)
as a terminal electron acceptor.
Pick's First Law: an equation describing the rate at which a gas transfers
into solution. The change in concentration of gas in solution is
proportional to the product of an overall mass transfer coefficient and the
concentration gradient.
May 2004 Appendix-7
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Pick's Second Law: an equation relating the change of concentration with
time due to diffusion to the change in concentration gradient with distance
from the source of concentration.
field capacity: the maximum amount of water that a soil can retain after
excess water from saturated conditions has been drained by the force of
gravity.
flow tube: a calibrated flow measuring device made for a specific range of
flow velocities and fluids.
flux: the rate of movement of mass through a unit cross-sectional area per
unit time in response to a concentration gradient or some advective force.
free product: a petroleum hydrocarbon in the liquid ("free" or non-
aqueous) phase (see also non-aqueous phase liquid, NAPL).
friable: easily crumbled, not cohesive or sticky.
fungi: aerobic, multicellular, nonphotosynthetic, heterotrophic
microorganisms. The fungi include mushrooms, yeast, molds, and smuts.
Most fungi are saprophytes, obtaining their nourishment from dead
organic matter. Along with bacteria, fungi are the principal organisms
responsible for the decomposition of carbon in the biosphere. Fungi have
two ecological advantages over bacteria: (1) they can gr.r> •• in low moisture
areas, and (2) they can grow in low pH environments.
gate valve: a valve regulated by the position of a circular plate.
globe valve: a type of stemmed valve that is used for flow control. The
valve has a globe shaped plug that rises or falls vertically when the stem
handwheel is rotated.
groundwater: the water contained in the pore spaces of saturated geologic
media.
grout: a watery mixture of cement (and commonly bentonite) without
aggregate that is used to seal the annular space around well casings to
prevent infiltration of water or short-circuiting of vapor flow.
heat capacity: the quantity of energy that must be supplied to raise the
temperature of a substance. For contaminated soils heat capacity is the
quantity of energy that must be added to the soil to volatilize organic
components. The typical range of heat capacity of soils is relatively
narrow, therefore variations are not likely to have a major impact on
application of a thermal desorption process.
Appendix-8 May 2004
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Henry's law: the relationship between the partial pressure of a compound
and the equilibrium concentration in the liquid through a proportionality
constant known as the Henry's law constant.
Henry's law constant: the ratio of the concentration of a compound in air
(or vapor) to the concentration of the compound in water under
equilibrium conditions.
heterogeneous: varying in structure or composition at different locations
in space.
heterotrophic: designating or typical of organisms that derive carbon for
the manufacture of cell mass from organic matter.
homogeneous: uniform in structure or composition at all locations in
space.
hose barb: a twist-type connector used for connecting a small diameter
hose to a valve or faucet.
hydraulic conductivity: a coefficient of proportionality describing the rate
at which water can move through a permeable medium. Hydraulic
conductivity is a function of both the intrinsic permeability of the porous
medium and the kinematic viscosity of the water which flows through it.
Also referred to as the coefficient of permeability.
hydraulic gradient: the change in total potentiometric (or piezometric)
head between two points divided by the horizontal distance separating the
two points.
hydrocarbon: chemical compounds composed only of carbon and
hydrogen.
hydrophilic: having an affinity for water, or capable of dissolving in
water; soluble or miscible in water.
hydrophobic: tending not to combine with water, or incapable of
dissolving in water; insoluble or immiscible in water. A property exhibited
by non-polar organic compounds, including the petroleum hydrocarbons.
hypoxic: a condition of low oxygen concentration, below that considered
aerobic.
in-line rotameter: a flow measurement device for liquids and gases that
uses a flow tube and specialized float. The float device is supported by the
flowing fluid in the clear glass or plastic flow tube. The vertical scaled flow
tube is calibrated for the desired flow volumes/time.
May 2004 Appendix-9
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in situ: in its original place; unmoved; unexcavated; remaining in the
subsurface.
indigenous: living or occurring naturally in a specific area or
environment; native.
infiltration: the downward movement of water through a soil in response
to gravity and capillary suction.
injection well: a well used to inject under pressure a fluid (liquid or gas)
into the subsurface.
inlet well: a well through which a fluid (liquid or gas) is allowed to enter
the subsurface under natural pressure.
inoculate: to implant microorganisms onto or into a culture medium.
inter granular: between the individual grains in a rock or sediment.
intrinsic permeability: a measure of the relative ease with which a
permeable medium can transmit a fluid (liquid or gas). Intrinsic
permeability is a property only of the medium and is independent of the
nature of the fluid.
isotropic: the condition in which hydraulic properties of an aquifer are
equal when measured in any direction.
kinematic viscosity: the ratio of dynamic viscosity to mass density.
Kinematic viscosity is a measure of a fluid's resistance to gravity flow: the
lower the kinematic viscosity, the easier and faster the fluid will flow.
liquid limit (LL): the lower limit for viscous flow of a soil.
liquidity index (LI): quantitative value used to assess whether a soil will
behave as a brittle solid, semisolid, plastic, or liquid. LI is equal to the
difference between the natural moisture content of the soil and the plastic
limit (PL) divided by the plasticity index (PI).
lithology: the gross physical character of a rock or rock types in a
stratigraphic section.
lower explosive limit (LEL): the concentration of a gas below which the
concentration of vapors is insufficient to support an explosion. LELs for
most organics are generally 1 to 5 percent by volume.
magnehelic gauge: a sensitive differential pressure or vacuum gauge
manufactured by Dwyer Instrument Co. that uses a precision diaphragm
to measure pressure differences. This gauge is manufactured in specific
Appendix-10 May 2004
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pressure or vacuum ranges such as 0 to 2 inches of water column.
Magnehelic gauges are typically used to measure SVE system vacuums.
manifold: a pipe with several apertures for making multiple connections.
manometer: an instrument for measuring fluid pressure. Typically a U-
shaped tube in which opposing fluid pressures reach an equilibrium. The
pressure is equal to the differences in the levels of the fluid on either side of
the tube.
methanogenic: referring to the formation of methane by certain
anaerobic bacteria during the process of anaerobic fermentation.
microaerophilic: obligate aerobes that function best under conditions of
low oxygen concentration.
microcosm: a diminutive, representative system analogous to a larger
system in composition, development, or configuration. As used in
biodegradation treatability studies, microcosms are typically constructed in
glass bottles or jars.
microorganisms: microscopic organisms including bacteria, protozoans,
yeast, fungi, mold, viruses, and algae.
moisture content: the amount of water lost from a 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.
molecular weight: the amount of mass in one mole of molecules of a
substance as determined by summing the masses of the individual atoms
which make up the molecule.
molecular diffusion: process whereby molecules of various gases tend to
intermingle and eventually become uniformly dispersed.
monoaromatic: aromatic hydrocarbons containing a single benzene ring.
non-aqueous phase liquid (NAPL): contaminants that remain as the
original bulk liquid in the subsurface (see also free product).
nutrients: major elements (e.g., nitrogen and phosphorus) and trace
elements (including sulfur, potassium, calcium, and magnesium) that are
essential for the growth of organisms.
obligate anaerobes: organisms for which the presence of molecular
oxygen is toxic. These organisms derive the oxygen needed for cell
synthesis from chemical compounds.
May 2004 Appendix-11
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obligate aerobes: organisms that require the presence of molecular
oxygen (O2) for their metabolism.
occlude: to cause to become obstructed or closed and thus prevent
passage either into or from.
octanol/water partition coefficient (!£„„,): a coefficient representing the
ratio of the solubility of a compound in octanol (a non-polar solvent) to its
solubility in water (a polar solvent). The higher the Kow, the more non-
polar the compound. Log Kow is generally used as a relative indicator of
the tendency of an organic compound to adsorb to soil. Log Kmv values are
generally inversely related to aqueous solubility and directly proportional
to molecular weight.
off-gas treatment system: refers to the unit operations used to treat (i.e.
condense, collect, or destroy) contaminants in the purge gas from the
thermal desorber.
olefins: see alkenes.
orifice plate: a flow measurement device for liquids or gases that uses a
restrictive orifice plate consisting of a machined hole that produces a jet
effect. Typically the orifice meter consists of a thin plate with a square
edged, concentric, and circular orifice. The pressure drop of the jet effect
across the orifice is proportional to the flow rate. The pressure drop can be
measured with a manometer or differential pressure gauge.
oxidation-reduction (redox): a chemical reaction consisting of an
oxidation reaction in which a substance loses or donates electrons, and a
reduction reaction in which a substance gains or accepts electrons. Redox
reactions are always coupled because free electrons cannot exist in solution
and electrons must be conserved.
paraffins: see alkanes.
partial pressure: the portion of total vapor pressure in a system due to
one or more constituents in the vapor mixture.
permeability: a qualitative description of the relative ease with which
rock, soil, or sediment will transmit a fluid (liquid or gas). Often used as a
synonym for hydraulic conductivity or coefficient of permeability.
pH: a measure of the acidity of a solution. pH is equal to the negative
logarithm of the concentration of hydrogen ions in a solution. A pH of 7 is
neutral. Values less than 7 are acidic, and values greater than 7 are basic.
phototrophs: organisms that use light to generate energy (by
photosynthesis) for cellular activity, growth, and reproduction.
Appendix-12 May 2004
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pilot test: operation of a small-scale version of a larger system to gain
information relating to the anticipated performance of the larger system.
Pilot test results are typically used to design and optimize the larger
system.
pitot tube: a device used to measure the total pressure of a fluid stream
that is essentially a tube attached to a manometer at one end and pointed
upstream at the other.
plastic limit (PL): the lower limit of the plastic state of a soil.
plastic soil: one that will deform without shearing (typically silts or clays).
Plasticity characteristics are measured using a set of parameters known as
Atterberg Limits.
plasticity index (PI): the range of water content in which soil is in a
plastic state. PI is calculated as the difference between the percent liquid
limit and percent plastic limit.
polyaromatic hydrocarbon: aromatic hydrocarbons containing more
than one fused benzene ring. Polyaromatic hydrocarbons are commonly
designated PAH.
polynuclear aromatic hydrocarbon: synonymous with polyaromatic
hydrocarbon. Designated PNA.
pore volume: the total volume of pore space in a given volume of rock or
sediment. Pore volume usually relates to the volume of air or water that
must be moved through contaminated material in order to flush the
contaminants.
porosity: the volume fraction of a rock or unconsolidated sediment not
occupied by solid material but usually occupied by water and/or air.
pressure gradient: a pressure differential in a given medium (e.g., water
or air) which tends to induce movement from areas of higher pressure to
areas of lower pressure.
procaryotes: a cellular organism in which the nucleus has no limiting
membrane.
protozoa: single-celled, eucaryotic microorganisms without cell walls.
Most protozoa are free-living although many are parasitic. The majority of
protozoa are aerobic or facultatively anaerobic heterotrophs.
psi (pounds per square inch): a unit of pressure or pressure drop across a
flow resistance. One psi is equivalent to the pressure exerted by 2.31 feet of
water column.
May 2004 Appendix-13
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psig (pounds per square inch (gauge)): 0 psig = 14.696 psia (psi
absolute) =1.0 atmosphere.
pugmill: a chamber in which water and soil are mixed together. Typically
mixing is aided by an internal mechanical stirring/kneading device.
radius of influence: the maximum distance away from an air injection or
extraction source that is significantly affected by a change in pressure and
induced movement of air.
recalcitrant: unreactive, nondegradable; refractory.
redox: short for oxidation-reduction.
refractory index: a measure of the ability of a substance to be
biodegraded by bacterial activity. The lower the refractor)' index, the
greater the biodegradability.
retardation: preferential retention of contaminant movement in the
subsurface resulting from adsorptive processes or solubility differences.
saturated zone: the zone in which all the voids in the rock or soil are filled
with water at greater than atmospheric pressure. The water table is the
top of the saturated zone in an unconfined aquifer.
septa fitting: a special fitting used to seal vials (a liner for a threaded cap)
or gas chromatographs (GCs) to provide closure. Septas can be
manufactured in single, double, or triple layers of silicone rubber and
other plastic materials. A syringe with a measured quantity of contaminant
can be injected through a septa closure and into a GC column for
separation analysis.
sentinel well: a groundwater monitoring well situated between a sensitive
receptor downgradient and the source of a contaminant plume upgradient.
Contamination should be first detected in the sentinel well which serves as
a warning that contamination may be moving closer to the receptor. The
sentinel well should be located far enough upgradient of the receptor to
allow enough time before the contamination arrives at the receptor to
initiate other measures to prevent contamination from reaching the
receptor, or in the case of a supply well, provide for an alternative water
source.
SESOIL: a one-dimensional model for estimating pollutant distribution in
an unsaturated soil column. SESOIL results are commonly used to
estimate the source term for groundwater transport modeling of the
saturated zone.
short circuiting: as it applies to SVE and bioventing, the entry of ambient
air into the extraction well without first passing through the contaminated
Appendix-14 May 2004
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zone. Short circuiting may occur through utility trenches, incoherent well
or surface seals, or layers of high permeability geologic materials.
soil moisture: the water contained in the pore spaces in the unsaturated
zone.
solubility: the amount of mass of a compound that will dissolve in a unit
volume of solution.
sorbent canisters: gas-tight canisters typically filled with activated carbon
(charcoal) for collection and transport of vapor samples. In the laboratory
the vapors are desorbed and analyzed to identify the organic compounds
and quantify their concentration.
sorbent tubes: glass tubes filled with a sorbent material that reacts
chemically with specific organic compounds. Based on the nature of the
sorbent and the extent of the chemical reaction, organic compounds can
be identified and their concentration quantified.
sorption: a general term used to encompass the processes of absorption,
adsorption, ion exchange, and chemisorption.
sparge: injection of air below the water table to strip dissolved volatile
organic compounds and/or oxygenate the groundwater to facilitate
aerobic biodegradation of organic compounds.
specific gravity: the dimensionless ratio of the density of a .substance with
respect to the density of water. The specific gravity of water is equal to 1.0
by definition. Most petroleum products have a specific gravity less than
1.0, generally between 0.6 and 0.9. As such, they will float on water—these
are also referred to as LNAPLs, or light non-aqueous phase liquids.
Substances with a specific gravity greater than 1.0 will sink through water-
-these are referred to as DNAPLs, or dense non-aqueous phase liquids.
sticky limit: the limit at which a soil loses its ability to adhere to a metal
blade.
stratum: a horizontal layer of geologic material of similar composition,
especially one of several parallel layers arranged one on top of another.
stratification: layering or bedding of geologic materials (e.g., rock or
sediments).
sump: a pit or depression where liquids drain, collect, or are stored.
Tedlar bags: gas-tight bags constructed of non-reactive material (Tedlar)
for the collection and transport of gas/vapor samples.
May 2004 Appendix-15
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terminal electron acceptor (TEA): a compound or molecule that accepts
an electron (is reduced) during metabolism (oxidation) of a carbon source.
Under aerobic conditions molecular oxygen is the terminal electron
acceptor. Under anaerobic conditions a variety of terminal electron
acceptors may be used. In order of decreasing redox potential, these TEAs
include nitrate, manganic manganese, ferric iron, sulfate, and carbon
dioxide. Microorganisms preferentially utilize electron acceptors that
provide the maximum free energy during respiration. Of the common
terminal electron acceptors listed above, oxygen has the highest redox
potential and provides the most free energy during electron transfer.
thermal desorption system: refers to a thermal desorber and associated
systems for handling materials and treated soils and treating offgases and
residuals.
thermal desorber: describes the primary treatment unit that heats
petroleum-contaminated materials and desorbs the organic materials into
a purge gas or off-gas.
total petroleum hydrocarbons (TPH): a measure of the concentration or
mass of petroleum hydrocarbon constituents present in a given amount of
air, soil, or water. The term total is a misnomer, in that few, if any, of the
procedures for quantifying hydrocarbons are capable of measuring all
fractions of petroleum hydrocarbons present in the sample. Volatile
hydrocarbons are usually lost in the process and not quantified.
Additionally, some non-petroleum hydrocarbons may be included in the
analysis.
total recoverable petroleum hydrocarbons (TRPH): an EPA method
(418.1) for measuring total petroleum hydrocarbons in samples of soil or
water. Hydrocarbons are extracted from the sample using a
chlorofluorocarbon solvent (typically Freon-113) and quantified by
infrared spectrophotometry. The method specifies that the extract be
passed through silica gel to remove the non-petroleum fraction of the
hydrocarbons.
travel time: the time it takes a contaminant to travel from the source to a
particular point downgradient.
turbine wheel: a rotor designed to convert fluid energy into rotational
energy. Hydraulic turbines are used to extract energy from water as the
water velocity increases due to a change in head or kinetic energy at the
expense of the potential energy as the water flows from a higher elevation
to a lower elevation. The fluid velocity tangential component contributes
to the rotation of the rotor in a turbomachine.
unconfined aquifer: an aquifer in which there are no confining beds
between the capillary fringe and land surface, and where the top of the
saturated zone (the water table) is at atmospheric pressure.
Appendix-16 May 2004
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unsaturated zone: the zone between land surface and the capillary fringe
within which the moisture content is less than saturation and pressure is
less than atmospheric. Soil pore spaces also typically contain air or other
gases. The capillary fringe is not included in the unsaturated zone.
unsaturated: the characteristic of a carbon atom in a hydrocarbon
molecule that shares a double bond with another carbon atom.
upgradient: it the direction of increasing potentiometric (piezometric)
head.
vadose zone: the zone between land surface and the water table within
which the moisture content is less than saturation (except in the capillary
fringe) and pressure is less than atmospheric. Soil pore spaces also typically
contain air or other gases. The capillary fringe is included in the vadose
zone.
vapor density: the amount of mass of a vapor per unit volume of the
vapor.
vapor pressure: the force per unit area exerted by a vapor in an
equilibrium state with its pure solid, liquid, or solution at a given
temperature. Vapor pressure is a measure of a substance's propensity to
evaporate. Vapor pressure increases exponentially with an increase in
temperature.
venturi: a short tube with a constricted throat for determining fluid
pressures and velocities by measuring differential pressures generated at
the throat as a fluid traverses the tube.
viscosity: a measure of the internal friction of a fluid that provides
resistance to shear within the fluid. The greater the forces of internal
friction (i.e. the greater the viscosity), the less easily the fluid will flow.
volatilization: the process of transfer of a chemical from the aqueous or
liquid phase to the gas phase. Solubility, molecular weight, and vapor
pressure of the liquid and the nature of the gas-liquid interface affect the
rate of volatilization.
•water table: the water surface in an unconfmed aquifer at which the fluid
pressure in the pore spaces is at atmospheric pressure.
weathering: the process during which a complex compound is reduced to
its simpler component parts, transported via physical processes, or
biodegraded over time.
wellhead: the area immediately surrounding the top of a well, or the top
of the well casing.
May 2004 Appendix-17
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windrow: a low, elongated row of material left uncovered to dry.
Windrows are typically arranged in parallel.
Appendix-18 May 2004
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