United States
Environmental Protection
Agency
Solid Waste And
Emergency Response
5403W
EPA510-B-95-007
May 1995
s/EPA
How to Evaluate
Alternative Cleanup
Technologies for
Underground Storage
Tank Sites
A Guide for Corrective Action
Plan Reviewers
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ISBN 0-16-042641-3
90000
For sale by the U.S. Government Printing Office
Superintendent of Documents, Mail Stop: SSOP, Washington, DC 20402-9328
ISBN 0-16-042641-3
9 "780160" 426414'
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United States Solid Waste And EPA 510-B-95-007
Environmental Protection Emergency Response May 1995
Agency 5403W
vvEPA How to Evaluate
Alternative Cleanup
Technologies for
Underground Storage
Tank Sites
A Guide for Corrective Action
Plan Reviewers
-------
Acknowledgements
The Environmental Protection Agency's (EPA's) Office of Underground
Storage Tanks (OUST) would like to express its gratitude to several
people who reviewed and commented on this document during its draft
stages. Members of the formal peer review group include: George
Mickelson (Wisconsin); Jarm Norman (Maryland); Patricia Ellis
(Delaware); Kevin Sullivan (Nevada); Chris Chandler (Texas); Susan
Booher (Arkansas); Lynda Gresham (Arkansas); Joan Coyle (Region 1);
Gilberto Alvarez (Region 5); Evan Fan, James Yezzi, Katrina Varner, and
Charlita Rosal (Office of Research and Development); Bruce Bauman
(American Petroleum Institute); Don Adams (North American Thermal
Soil Recycling Association); and Dana Tulis, Hal White, Kate Becker, and
Gregory Waldrip (OUST). The feedback and input provided by these
individuals and others too numerous to mention enabled OUST to
produce a customer-oriented document which we hope will truly meet
state regulators' needs.
Deborah L. Tremblay
Project Manager
October, 1994
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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. Natural Attenuation
X. In-Situ Bioremediation
XI. Dual-Phase Extraction
XII. Abbreviations And Definitions
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Chapter I
Introduction
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Chapter I
Introduction
Background
As of June 1994, state and local environmental agencies across the
nation have reported more than 260,000 releases from leaking
underground storage tanks. Still the number of confirmed releases
continues to grow, with over 1,000 new releases reported each week.
This burgeoning number of releases has created a growing and, in many
cases, unmanageable workload for state regulators who often must
oversee 50 to 400 cleanups at a time.
To compound the problem, these cleanups are expensive. Costs of
remediating sites with soil contamination generally vary between
$10,000 and $125,000. Depending on the extent of contamination, costs
for remediating sites with groundwater contamination can range from
$100,000 to over $1 million.
A primary factor in the high cost of cleanups is the use of ineffective
cleanup methods. Pump-and-treat, the most commonly used method for
remediating groundwater, often results in unsuccessful cleanups. Even
when properly operated, pump-and-treat systems have inherent
limitations: They do not work well in complex geologic settings or
heterogeneous aquifers; they often stop reducing contamination long
before reaching intended cleanup levels; and they often 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 moves
the problem from one location to another. In addition to being costly in
many states, 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) is promoting faster, more
effective, and less costly alternatives to traditional cleanup methods.
EPA's Office of Underground Storage Tanks (OUST) is working with state
and local governments to encourage the use of cleanup technologies that
are proven but are not yet widely used. These "alternative technologies"
have the ability to make cleanups faster, more effective, and less costly
than traditional options such as pump-and-treat or excavation and
disposal in a landfill. The U.S. EPA encourages state regulators to
consider alternative cleanup technologies for remedial actions at all
leaking underground storage tank sites.
October 1994 1-1
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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 site-
specific 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:
O Has an appropriate cleanup technology been proposed?
O 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
CAPs. Nor should it be used to provide guidance on regulatory 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.
1-2 October 1994
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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. Add your own notes or information to 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:
O An evaluation process flow chart, 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.
O A checklist, 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.
O A list of current references, located near the end of each chapter,
provides sources of additional information.
O In addition, each chapter has a number of charts that display
advantages and disadvantages of each technology, initial screening
criteria, and other data specific to each technology.
Please note the evaluation form located at the end of the manual. We
are very interested in your comments on the usefulness of this
document. OUST relies on your feedback to improve our products.
Please fill out the form and return it to us.
How to Obtain Additional Copies of the Manual
OUST plans to make this manual available through the Government
Printing Office. To obtain the information you will need to order copies,
please call EPA's RCRA/Superfund Hotline. The Hotline is open Monday
through Friday from 8:30 to 7:30 p.m. EST. The toll-free number is 800
424-9346; for the hearing impaired, the number is TDD 800 553-7672.
October 1994 1-3
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Chapter II
Soil Vapor Extraction
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Contents
Overview II-l
Initial Screening Of SVE Effectiveness II-4
Detailed Evaluation Of SVE Effectiveness 11-7
Factors That Contribute To Permeability Of Soil II-8
Intrinsic Permeability II-8
Soil Structure And Stratification II-9
Depth To Groundwater 11-10
Moisture Content 11-10
Factors That Contribute To Constituent Volatility 11-11
Vapor Pressure II-11
Product Composition And Boiling Point 11-12
Henry's Law Constant 11-13
Other Considerations 11-13
Pilot Scale Studies 11-14
Evaluation Of The SVE System Design 11-15
Rationale For The Design 11-15
Components Of An SVE System 11-17
Extraction Wells 11-18
Manifold Piping 11-22
Vapor Pretreatment 11-22
Blower Selection 11-23
Monitoring And Controls 11-24
Optional SVE Components 11-24
Evaluation Of Operation And Monitoring Plans 11-27
Start-Up Operations 11-27
Long-Term Operations 11-27
Remedial Progress Monitoring 11-28
References 11-30
Checklist: Can SVE Be Used At This Site? 11-31
October 11994 H-iii
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List Of Exhibits
Number Title Page
ll-l Typical SVE System II-2
11-2 Advantages And Disadvantages Of SVE . . II-3
II-3 SVE Evaluation Process Flow Chart II-5
II-4 Initial Screening For SVE Effectiveness II-7
II-5 Key Parameters Used To Evaluate Permeability
Of Soil And Constituent Volatility II-8
II-6 Intrinsic Permeability And SVE Effectiveness II-9
II-7 Depth To Groundwater And SVE Effectiveness 11-11
II-8 Vapor Pressures Of Common Petroleum
Constituents 11-12
II-9 Petroleum Product Boiling Point Ranges 11-12
11-10 Henry's Law Constant Of Common
Petroleum Constituents 11-13
11-11 Schematic Of A Soil Vapor Extraction System 11-18
11-12 Well Orientation And Site Conditions 11-19
II-13 Typical Vertical Soil Vapor Extraction
Well Construction 11-21
11-14 Typical Horizontal Soil Vapor Extraction
Wen Construction 11-22
11-15 Performance Curves For Three Types Of Blowers 11-23
11-16 Monitoring And Control Equipment 11-25
11-17 System Monitoring Recommendations . 11-27
11-18 Relationship Between Concentration
Reduction And Mass Removal H-29
n-iv October 1994
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Chapter II
Soil Vapor Extraction
Overview
Soil vapor extraction (SVE), also known as soil venting or vacuum
extraction, is an in situ remedial technology that reduces concentrations
of volatile constituents in petroleum products adsorbed to soils in the
unsaturated (vadose) zone. In this technology, a vacuum is applied to the
soil matrix to create a negative pressure gradient that causes movement
of vapors toward extraction wells. Volatile constituents are readily
removed from the subsurface through the extraction wells. The extracted
vapors are then treated, as necessary, and discharged to the atmosphere
or reinjected to the subsurface (where permissible).
This technology has been proven effective in reducing concentrations
of volatile organic compounds (VOCs) and certain semi-volatile organic
compounds (SVOCs) found in petroleum products at underground
storage tank (UST) sites. SVE is generally more successful when applied
to the lighter (more volatile) petroleum products such as gasoline. Diesel
fuel, heating oils, and kerosene, which are less volatile than gasoline, are
not readily treated by SVE but may be suitable for removal by bioventing
(see Chapter III). SVE is generally not successful when applied to
lubricating oils, which are non-volatile, but these oils may be suitable for
removal by bioventing. A typical SVE system is shown in Exhibit II-1. A
summary of the advantages and disadvantages of SVE is shown in
Exhibit II-2.
This chapter will assist you in evaluating a corrective action plan
(CAP) which proposes SVE as a remedy for petroleum-contaminated soil.
The evaluation process, which is summarized in a flow diagram shown in
Exhibit II-3, will serve as a roadmap for the decisions you will make
during your evaluation. A checklist has also been provided at the end of
this chapter to be used as a tool to evaluate the completeness of the CAP
and to help focus attention on areas where additional information may
be needed. The evaluation process can be divided into the following
steps.
O Step 1: An initial screening of SVE effectiveness, which will allow
you to quickly gauge whether SVE is likely to be effective, moderately
effective, or ineffective.
October 1994 n-1
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i
-Ss CO
** III
is
£ g
5
n-2
October 1994
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Exhibit 11-2
Advantages And Disadvantages Of SVE
Advantages
Disadvantages
o Proven performance; readily available
equipment; easy installation.
o Minimal disturbance to site operations.
o Short treatment times: usually 6 months
to 2 years under optimal conditions.
o Cost competitive: $20-50/ton of
contaminated soil.
o Easily combined with other technologies
(e.g., air sparging, bioremediation, and
vacuum-enhanced dual-phase
extraction).
o Can be used under buildings and other
locations that cannot be excavated.
o Concentration reductions greater than
about 90% are difficult to achieve.
o Effectiveness less certain when applied
to sites with low-permeability soil or
stratified soils.
o May require costly treatment for
atmospheric discharge of extracted
vapors.
o Air emission permits generally required.
o Only treats unsaturated-zone soils; other
methods may also be needed to treat
saturated-zone soils and groundwater.
O Step 2: A detailed evaluation of SVE effectiveness, which provides
further screening criteria to confirm whether SVE is likely to be
effective. To complete the detailed evaluation, you will need to find
specific soil and constituent characteristics and properties, compare
them to ranges where SVE is effective, decide whether pilot studies
are necessary to determine effectiveness, and conclude whether SVE
is likely to work at a site.
O Step 3: An evaluation of the SVE system, design, which will allow
you to determine if the rationale for the design has been appropriately
defined based on pilot study data or other studies, whether the
necessary design components have been specified, and whether the
construction process flow designs are consistent with standard
practice.
O Step 4: An evaluation of the operation and monitoring plans,
which will allow you to determine whether start-up and long-term
system operation monitoring is of sufficient scope and frequency and
whether remedial progress monitoring plans are appropriate.
October 1994
H-3
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Initial Screening Of SVE Effectiveness
Although the theories that explain how SVE works are well-
understood, determining whether SVE will work at a given site is not
simple. Experience and judgement are needed to determine whether SVE
will work effectively. The key parameters that should be used to decide
whether SVE will be a viable remedy for a particular site are:
O Permeability of the petroleum-contaminated soils. Permeability of the
soil determines the rate at which soil vapors can be extracted.
O Volatility of the petroleum constituents. Volatility determines the rate
(and degree) at which petroleum constituents will vaporize from the
soil-adsorbed state to the soil vapor state.
In general, the type of soil (e.g., clay, silt, sand) will determine its
permeability. Fine-grained soils (e.g., clays and silts) have lower
permeability than coarse-grained soils (e.g., sands and gravels). The
uotatility of a petroleum product or its constituents is a measure of its
ability to vaporize. Because petroleum products are highly complex
mixtures of chemical constituents, the volatility of the product can be
roughly approximated by its boiling point range.
Exhibit E-4 is an initial screening tool that you can use to help assess
the potential effectiveness of SVE for a given site. This exhibit provides a
range of soil permeabilities for typical soil types as well as ranges of
volatility (based on boiling point range) for typical petroleum products.
Use this screening tool to make an initial assessment of the potential
effectiveness of SVE. To use this tool, you should scan the CAP to
determine the soil type present and the type of petroleum product
released at the site.
Information provided in the following section will allow a more
thorough effectiveness evaluation and will identify areas that could
require special design considerations.
n-4
October 1994
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Exhibit 11-3
SVE Evaluation Process Flow Chart
DETAILED EVALUATION OF
SVE EFFECTIVENESS
INITIAL SCREENING OF
SVE EFFECTIVENESS
Identify site characteristics
important to SVE
effectiveness
Identify product
constituent properties
important to
SVE effectiveness
Determine the types of
soils that occur within
fiie contaminated area
Intrinsic Permeability
Soil Structure
Depth to Groundwater
Moisture Content
Vapor Pressure
Boiling Range
Henry's Law Constant
intrinsic
permeability ^
> 10"8 cm2, and is depth
to groundwater
> 4 feet?
Are vapor
pressures of product
constituents
> 0.5mm Hg?
Is clay soil
targeted for
remediation?
Is
soil free
impermeable !ayers\ NO
or other conditions that
would disrupt
airflow?
constituent boiling
range < 260-300° C?
Determine which petroleum
products are targeted for
remediation by SVE
• Gasoline
• Kerosene
• Diesel Fuel
• Heating Oil
• Lubricating Oil
SVE is not likely
to be effective at
the site.
Consider other
technologies.
• Bioventing
• Landfarming
• Biomounding
• Thermal
Desorption
Does
moisture content
of soils in contaminated
area appear to
below?
Is
Henry's Law
Constant
> 100 atm?
Pilot studies are required
to demonstrate effectiveness.
Review pilot study results.
Are
lubricating
oils targeted for
remediation?
Do pilot
study results
demonstrate SVE
effectiveness?
SVE will not be
effective at the site.
Consider other
technologies.
• Bioventing
• Landfarming
• Biomounding
• Thermal
Desorption
SVE has the potential to
be effective at the site.
Proceed to next panel.
SVE is likely to be
effective at the site.
Proceed to evaluate
October 1994
n-s
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Exhibit II-3
SVE Evaluation Process Flow Chart
EVALUATION OF SVE
SYSTEM DESIGN
EVALUATION OF SVE SYSTEM
(OPERATION & MONITORING PLANS
Determine the design elements
• Radius of Influence
• Wellhead Vacuum
* Extraction Flowrate
* initial Vapor Concentrations
» End-Point Vapor Concentrations
* Remedial Cleanup time
• Soil Volume to be Treated
• Pore Volume Calculations
• Discharge Limitations
• Construction Limitations
Have the
"design basics been^
identified and are they
^within appropriate,
ranges?,
Review the 0 & M plan for
the proposed SVE system
for the following:
• Start-Up Operations Plan
• Long-Term Operations &
Monitoring Plan
• Remedial Progress
Monitoring Plan
NO
,YES
Review the conceptual
process flow design & identify
the system components
| • Extraction Well Orientation,
Placement and Construction
• Manifold Piping
I • Vapor Pretreatment Equipment
I • Extraction Btower
> Instrumentation & Controls
> Injection Wills & Other Optional
Components
I • Vapor Treatment Equipment
Has the
"conceptual design^
been provided and is
it adequate?
SVE system
design is
incomplete.
Request
additional
information.
NO
YES
The SVE system design
is complete and its
elements are within
normal ranges. Proceed
to O&M evaluation.
n-6
Are
start-up
& monitoring
described, and are their
scope & frequency
adequate?
Request
additional
information
on startup
procedures and
monitoring
Is a
long-term O&M
plan described; is it of
adequate scope & frequency;
does it include
discharge permit
monitoring?
Request
additional
information
on long-term
O&M.
Is a
remedial prog
monitoring plan estab-
lished; is it of adequate scope
& frequency; does it include
provisions for detect-
ing asymptotic
Request
additional
information
on remedial
progress
monitoring.
The SVE system is
likely to be effective.
The design and O&M
plans are complete.
October 1994
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Exhibit 11-4
Initial Screening For SVE Effectiveness
Permeability
^Tn'eW^ctivex^^iS.M^^^16..10 Minimal Effective
Intrinsic Permeability, k (cm2)
10-1« 1Q-14 10-12 10-10 i0-e i0-« 10-4 1O~z
I Cloy I
Glacial Till
I Silt, Loess
Silty Sand
Clean Sand
| Grovel
Product Volatility
Ei^lixI^^SJ^Ef^ltivlne^1"101 Effective
Boiling Point (*C)
Nonvolatile 300 250 200 100
I Lube Oils ~|
Heating Oils
Diesel
erosene
Gasoline
Detailed Evaluation Of SVE Effectiveness
Once you have completed the initial screening and determined that
SVE may have the potential to be effective for the soils and petroleum
product present, further scrutinize the CAP to confirm that SVE will be
effective.
Begin by reviewing the two major factors that determine the
effectiveness of SVE: (1) permeability of the soil and (2) constituent
volatility. The combined effect of these two factors results in the initial
contaminant mass extraction rate, which will decrease during SVE
operation as concentrations of volatile organics in the soil (and soil
vapor) are reduced.
Many site-specific parameters can be used to determine permeability
and volatility. These parameters are summarized in Ejdiibit II-5.
October 1994 n-7
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Exhibit 11-5
Key Parameters Used To Evaluate Permeability Of Soil And
Constituent Volatility
Permeability Of Soil Constituent Volatility
Intrinsic permeability Vapor pressure
Soil structure and stratification Product composition and boiling point
Depth to groundwater Henry's law constant
Moisture content
The remainder of this section describes each parameter, why it is
important to SVE, how it can be determined, and a range of values over
which SVE is effective.
Factors That Contribute To Permeability Of Soil
Intrinsic Permeability
Intrinsic permeability is a measure of the ability of soils to transmit
fluids and is the single most important factor in determining the
effectiveness of SVE. Intrinsic permeability ranges over 12 orders of
magnitude (from 10"16 to 10"3 cm2) for the wide variety of earth
materials, although a more limited range applies for common soil types
(10~13 to 10"5 cm2). Intrinsic permeability is best determined from field
tests, but can be estimated within one or two orders of magnitude from
soil boring logs and laboratory tests. Coarse-grained soils (e.g., sands)
have greater intrinsic permeability than fine-grained soils (e.g., clays or
silts). Note that the ability of a soil to transmit air, which is of prime
importance to SVE, is reduced by the presence of soil water, which can
block the soil pores and reduce air flow. This is especially important in
fine-grained soil, which tend to retain water.
Intrinsic permeability can be determined in the field by conducting
permeability tests or SVE pilot studies, or in the laboratory using soil
core samples from the site. Procedures for these tests are described by
EPA (199la). Use the values presented in Exhibit II-6 to determine if
intrinsic permeability is within the effectiveness range for SVE.
n-8 October 1994
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Exhibit 11-6
Intrinsic Permeability And SVE Effectiveness
Intrinsic Permeability (k) SVE Effectiveness
k.> 10~8 cm2 Generally effective.
10"8 >. k > 10"10 cm2 May be effective; needs further evaluation.
k < 10'10 cm2 Marginal effectiveness to ineffective.
At sites where the soils in the saturated zone are similar to those
within the unsaturated zone, hydraulic conductivity of the soils may be
used to estimate the permeability of the soils. Hydraulic conductivity is a
measure of the ability of soils to transmit water. Hydraulic conductivity
can be determined from aquifer tests, including slug tests and pumping
tests. You can convert hydraulic conductivity to intrinsic permeability
using the following equation:
k = K (u / pg)
where: k = intrinsic permeability (cm2)
K = hydraulic conductivity (cm/sec)
u = water viscosity (g/cm • sec)
p = water density (g/cm3)
g = acceleration due to gravity (cm/sec2)
At 20°C: u/pg = 1.02 • 10'5 cm/sec
To convert k from cm2 to darcy, multiply by 108
Soil Structure And Stratification
Soil structure and stratification are important to SVE effectiveness
because they can affect how and where soil vapors will flow within the
soil matrix under extraction conditions. Structural characteristics such
as microfracturing can result in higher permeabilities than expected for
certain soil components (e.g., clays). However, the increased flow
availability will be confined within the fractures but not in the
unfractured media. This preferential flow behavior can lead to ineffective
or significantly extended remedial times. Stratification of soils with
different permeabilities can increase the lateral flow of soil vapors in the
more permeable stratum while dramatically reducing the soil vapor flow
through the less permeable stratum.
October 1994 H-9
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You can determine the intergranular structure and stratification of
the soil by reviewing soil boring logs for wells or borings and by
examining geologic cross-sections. You should verify that soil types have
been identified, that visual observations of soil structure have been
documented, and that sampling intervals are of sufficient frequency to
define any soil stratification. Stratified soils may require special
consideration in design to ensure less-permeable stratum are addressed.
Depth To Groundwater
Fluctuations in the groundwater table should also be considered
when reviewing a CAP. Significant seasonal or daily (tidal or
precipitation-related) fluctuations may, at times, submerge some of the
contaminated soil or a portion of the extraction well screen, making it
unavailable for air flow. This is most important for horizontal extraction
wells, where the screen is parallel to the water table surface.
SVE is generally not appropriate for sites with a groundwater table
located less than 3 feet below the land surface. Special considerations
must be taken for sites with a groundwater table located less than 10
feet below the land surface because groundwater upwelling can occur
within SVE wells under vacuum pressures, potentially occluding well
screens and reducing or eliminating vacuum-induced soil vapor flow.
Use Exhibit II-7 to determine whether the water-table depth is of
potential concern for SVE effectiveness.
Moisture Content
High moisture content in soils can reduce soil permeability and
thereafter, the effectiveness of SVE by restricting the flow of air through
soil pores. Airflow is particularly important for soils within the capillary
fringe where, oftentimes, a significant portion of the constituents can
accumulate. Fine-grained soils create a thicker capillary fringe than
coarse-grained soils. The thickness of the capillary fringe can usually be
determined from soil boring logs (i.e., in the capillary fringe, soils are
usually described as moist or wet). The capillary fringe usually extends
from inches to several feet above the groundwater table elevation. SVE is
not generally effective in removing contaminants from the capillary
fringe. When combined with other technologies (e.g., pump-and-treat to
lower the water table or air sparging to strip contaminants from the
capillary fringe) the performance of SVE-based systems is considerably
increased.
n-lO October 1994
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Exhibit 11-7
Depth To Groundwater And SVE Effectiveness
Depth To Groundwater SVE Effectiveness
> 10 feet Effective
3 feet < depth < 10 feet Need special controls (e.g., horizontal wells
or groundwater pumping)
< 3 feet Not generally effective
Moist soils can also occur from stormwater infiltration in unpaved
areas without sufficient drainage. This moisture may be a persistent
problem for fine-grained soils with slow infiltration rates. SVE does
dehydrate moist soils to some extent, but the dehydration process may
hinder SVE performance and extend operational time.
Factors That Contribute To Constituent Volatility
Vapor Pressure
Vapor pressure is the most important constituent characteristic in
evaluating the applicability and potential effectiveness of an SVE system.
The vapor pressure of a constituent is a measure of its tendency to
evaporate. More precisely, it is the pressure that a vapor exerts when in
equilibrium with its pure liquid or solid form. Constituents with higher
vapor pressures are more easily extracted by SVE systems. Those with
vapor pressures higher than 0.5 mm Hg are generally considered
amenable for extraction by SVE.
As previously discussed, gasoline, diesel fuel, and kerosene are each
composed of over a hundred different chemical constituents. Each
constituent will be extracted at a different rate by an SVE system,
generally according to its vapor pressure. Exhibit II-8 lists vapor
pressures of selected petroleum constituents.
October 1994 H-ll
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Vapor Pressures
Constituent
Methyl t-butyl ether
Benzene
Toluene
Ethyiene dibromide
Ethylbenzene
Xylenes
Naphthalene
Tetraethyl lead
Exhibit 11-8
Of Common Petroleum Constituents
Vapor Pressure
(mm Hg at 20°C)
245
76
22
11
7
6
0.5
0.2
Product Composition And Boiling Point
The most commonly encountered petroleum products from UST
releases are gasoline, diesel fuel, kerosene, heating oils, and lubricating
oils. Because of their complex constituent composition, petroleum
products are often classified by their boiling point range. Because the
boiling point of a compound is a measure of its volatility, the
applicability of SVE to a petroleum product can be estimated from its
boiling point range. The boiling point ranges for common petroleum
products are shown in Exhibit II-9.
Exhibit 11-9
Petroleum Product Boiling Point Ranges
Product
Boiling Point Range
Gasoline
Kerosene
Diesel fuel
Heating oil
Lubricating oils
40 to 225
180 to 300
200 to 338
>275
Nonvolatile
In general, constituents in petroleum products with boiling points less
than 250° to 300°C are sufficiently volatile to be amenable to removal by
SVE. Therefore, SVE can remove nearly all gasoline constituents, a
portion of kerosene and diesel fuel constituents, and a lesser portion of
heating oil constituents. SVE cannot remove lubricating oils. Most
petroleum constituents are biodegradable, however, and might be
n-i2
October 1994
-------
amenable to removal by bioventing. (See Chapter III for information
about Bioventing.) Injection of heated air also can be used to enhance
the volatility of these products because vapor pressure generally
increases with temperature. However, energy requirements for volatility
enhancement are so large as to be economically prohibitive.
Henry's LaaH Constant
Another indicator of the volatility of a constituent is by noting its
Henry's law constant. Henry's law constant is the partitioning
coefficient that relates the concentration of a constituent dissolved in
water to its partial pressure in the vapor phase under equilibrium
conditions. In other words, it describes the relative tendency for a
dissolved constituent to partition between the vapor phase and the
dissolved phase. Therefore, the Henry's law constant is a measure of the
degree to which constituents that are dissolved in soil moisture (or
groundwater) will volatilize for removal by the SVE system. Henry's law
constants for several common constituents found in petroleum products
are shown in Exhibit 11-10. Constituents with Henry's law constants of
greater than 100 atmospheres are generally considered amenable to
removal by SVE.
Henry's Law Constant
Constituent
Tetraethyl lead
Ethylbenzene
Xylenes
Benzene
Toluene
Naphthalene
Ethylene dibromide
Methyl t-butyl ether
Exhibit 11-10
Of Common Petroleum Constituents
Henry's Law Constant (atm)
4700
359
266
230
217
72
34
27
Other Considerations
There are other site-specific aspects to consider when evaluating the
potential effectiveness of an SVE system. For example, it may be
anticipated that SVE would be only marginally effective at a site as the
result of low permeability of the soil or low vapor pressure of the
constituents. In this case, bioventing may be the best available
alternative for locations such as under a building or other inaccessible
area.
October 1994 H-13
-------
SVE may also be appropriate near a building foundation to prevent
vapor migration into the building. Here, the primary goal may be to
control vapor migration and not necessarily to remediate soil.
Pilot Scale Studies
At this stage, you will be in a position to decide if SVE is likely to be
highly effective, somewhat effective, or ineffective. If it appears that SVE
will be only marginally to moderately effective at a particular site, make
sure that SVE pilot studies have been completed at the site and that
they demonstrate SVE effectiveness. Pilot studies are an extremely
important part of the design phase. Data provided by pilot studies is
necessary to properly design the full-scale SVE system. Pilot studies also
provide information on the concentration of volatile organic compounds
(VOCs) that are likely to be extracted during the early stages of operation
of the SVE system.
While pilot studies are important and recommended for evaluating
SVE effectiveness and design parameters for any site, they are
particularly useful at sites where SVE is expected to be only marginally
to moderately effective. Pilot studies typically include short-term (1 to 30
days) extraction of soil vapors from a single extraction well, which may
be an existing monitoring well at the site. However, longer pilot studies
(up to 6 months) which utilize more than one extraction well may be
appropriate for larger sites. Different extraction rates and wellhead
vacuums are applied to the extraction wells to determine the optimal
operating conditions. The vacuum influence at increasing distances from
the vapor extraction well is measured using vapor probes or existing
wells to establish the pressure field induced in the subsurface by
operation of the vapor extraction system. The pressure field
measurements can be used to define the design radius of influence for
SVE. Vapor concentrations are also measured at two or more intervals
during the pilot study to estimate initial vapor concentrations of a
full-scale system. The vapor concentration, vapor extraction rate and
vacuum data are also used in the design process to select extraction and
treatment equipment.
In some instances, it may be appropriate to evaluate the potential of
SVE effectiveness using a screening model such as Hyperventilate (EPA,
1993). Hyperventilate can be used to identify required site date, decide if
SVE is appropriate at a site, evaluate air permeability tests, and estimate
the minimum number of wells needed. It is not intended to be a detailed
SVE predictive modeling or design tool.
n-14 October 1994
-------
Evaluation Of The SVE System Design
Once you have verified that SVE is applicable, you can scrutinize the
design of the system. A pilot study that provides data used to design the
full-scale SVE system is highly recommended. The CAP should include a
discussion of the rationale for the design and presentation of the
conceptual engineering design. Detailed engineering design documents
might also be included, depending on state requirements. Further detail
about information to look for in the discussion of the design is provided
below.
Rationale For The Design
Consider the following factors as you evaluate the design of the SVE
system in the CAP.
O Design Radius of Influence (ROI) is the most important parameter to
be considered in the design of an SVE system. The ROI is defined as
the greatest distance from an extraction well at which a sufficient
vacuum and vapor flow can be induced to adequately enhance
volatilization and extraction of the contaminants in the soil. As a rule-
of-thumb, the ROI.is often considered to be the distance from the
extraction well at which a vacuum of at least 0.1 inches of water is
observed.
The ROI depends on many factors including: lateral and vertical
permeability; depth to the groundwater table; the presence or absence
of a surface seal; the use of injection wells; and the extent of soil
heterogeneity. Generally, the design ROI can range from 5 feet (for fine
grained soils) to 100 feet (for coarse grained soils). For sites with
stratified geology, design ROI should be defined for each soil type. The
ROI is important for determining the appropriate number and spacing
of extraction wells. The ROI should be determined based on the
results of pilot study testing; however, at sites where pilot tests can
not be performed, the ROI can be estimated using air flow modelling
or other empirical methods.
O Wellhead. Vacuum is the vacuum pressure that is required at the top
of the extraction well to produce the desired vapor extraction flow rate
from the extraction well. Although wellhead vacuum is usually deter-
mined through pilot studies, it can be estimated and typically ranges
from 3 to 100 inches of water vacuum. Less permeable soils generally
require higher wellhead vacuum pressures to produce a reasonable
October 1994 H-15
-------
radius of influence. It should be noted, however, that high vacuum
pressures (e.g., greater than 100 inches of water) can cause upwelling
of the water table and occlusion of the extraction well screens.
O Vapor Extraction Flow Rate is the volumetric flow rate of soil vapor
that will be extracted from each vapor extraction well. Vapor
extraction flow rate, radius of influence, and wellhead vacuum are
interdependent (e.g., a change in the extraction rate will cause a
change in the wellhead vacuum and radius of influence). Vapor
extraction flow rate should be determined from pilot studies but may
be calculated using mathematical or physical models (EPA 1993). The
flow rate will contribute to the operational time requirements of the
SVE system. Typical extraction rates can range from 10 to 100 cubic
feet per minute (cfm) per well.
O Initial Constituent Vapor Concentrations can be measured during pilot
studies or estimated from soil gas samples or soil samples. They are
used to estimate constituent mass removal rate and SVE operational
time requirements and to determine whether treatment of extracted
vapors will be required prior to atmospheric discharge or reinjection.
The initial vapor concentration is typically orders of magnitude higher
than the sustained vapor extraction concentration and can be
expected to last only a few hours to a day before dropping off
significantly. Vapor treatment is especially important during this early
phase of remediation.
O Required Final Constituent Concentrations in soils or vapors are either
defined by state regulations as "remedial action levels," or determined
on a site-specific basis using fate and transport modeling and risk
assessment. They will determine what areas of the site require
treatment and when SVE operation can be terminated.
O Required Remedial Cleanup Time may also influence the design of the
system. The designer may reduce the spacing of the extraction wells
to increase the rate of remediation to meet cleanup deadlines or client
preferences, as required.
O Soil Volume To Be Treated is determined by state action levels or a
site-specific risk assessment using site characterization data for the
soils.
O Pore Volume Calculations are used along with extraction flow rate to
determine the pore volume exchange rate. The exchange rate is
calculated by dividing the soil pore space within the treatment zone
by the design vapor extraction rate. The pore space within the
treatment zone is calculated by multiplying the soil porosity by the
H-16 October 1994
-------
volume of soil to be treated. Some literature suggests that one pore
volume of soil vapor should be extracted at least daily for effective
remedial progress.
You can calculate the time required to exchange one pore volume of
soil vapor using the following equation:
E = (m3 vapor / m3 soil) • (m3 soil) = ^
(m3 vapor / hr)
where: E = pore volume exchange time (hr)
8 = soil porosity (m3 vapor/m3 soil)
V = volume of soil to be treated (m3 soil)
Q = total vapor extraction flowrate (m3 vapor/hr)
O Discharge Limitations And Monitoring Requirements are usually
established by state regulations but must be considered by designers
of an SVE system to ensure that monitoring ports are included in the
system hardware. Discharge limitations imposed by state air quality
regulations will determine whether offgas treatment is required.
O Site Construction Limitations such as building locations, utilities,
buried objects, residences, and the like must be identified and
considered in the design process.
Components Of An SVE System
Once the rationale for the design is defined, the actual design of the
SVE system can be developed. A typical SVE system design will include
the following components and information:
O Extraction wells
O Well orientation, placement, and construction details
O Manifold piping
O Vapor pretreatment design
O Blower selection
O Instrumentation and control design
O Optional SVE components
Injection wells
Surface seals
Groundwater depression pumps
Vapor treatment systems
Exhibit II- 1 1 is a schematic diagram of an SVE system.
October 1994 n'17
-------
Exhibit 11-11
Schematic Of A Soil Vapor Extraction System
Ambient
Air
Condensate
Separator
Btow Back Loop
Discharge to
Atmosphere
(Permit May
Be Required)
Slotted Vertical
Extraction Vent
Pipe (Typical)
Pressure Indicator
Sampling Port
Flow Control Valve
Flow Meter
The following subsections provide guidance for reviewing the system
configuration, standard system components, and additional system
components.
Extraction Wells
Well Orientation. An SVE system can use either vertical or horizontal
extraction wells. Orientation of the wells should be based on site-specific
needs and conditions. Exhibit 11-12 lists site conditions and the
corresponding appropriate well orientation.
Well Placement And Number Of Wells. Determine the number and location of
extraction wells by using several methods. In the first method, you divide
the area of the site requiring treatment by the area of influence for a
single well to obtain the total number of wells needed. Then, space the
wells evenly within the treatment area to provide area! coverage so that
the areas of influence cover the entire area of contamination.
Area of influence for a single well = rc • (ROI)2
Number of wells needed =
Treatment area (m2)
Area of influence for single extraction well (m^/well)
n-is
October 1994
-------
Exhibit 11-12
Well Orientation And Site Conditions
Well Orientation Site Conditions
Vertical extraction well o Shallow to deep contamination (5 to
100+feet).
o Depth to groundwater > 10 feet.
Horizontal extraction well o Shallow contamination (< 25 feet). More
effective than vertical wells at depths
< 10 feet. Construction difficult at depths
> 25 feet.
o Zone of contamination confined to a
specific stratigraphic unit.
In the second method, determine the total extraction flow rate needed to
exchange the soil pore volume within the treatment area in a reasonable
amount of time (8 to 24 hours). Determine the number of wells required
by dividing the total extraction flow rate needed by the flow rate
achievable with a single well.
_ e V / t
Number of wells needed =
where: E = soil porosity (m3 vapor / m3 soil)
V = volume of soil in treatment area (m3 soil)
q = vapor extraction rate from single extraction well
(m vapor/hr).
t = pore volume exchange time (hours)
In the example below, an 8-hour exchange time is used.
Number of wells needed =
m3 vapor
soil
(m3 soil)
8 hrs
m3 vapor
hr
Consider the following additional factors in determining well spacing.
O Use closer well spacing in areas of high contaminant concentrations
to increase mass removal rates.
October 1994 H-19
-------
O If a surface seal exists or is planned for the design, space the wells
slightly farther apart because air is drawn from a greater lateral
distance and not directly from the surface. However, be aware that
this increases the need for air injection wells.
O At sites with stratified soils, wells that are screened in strata with low
intrinsic permeabilities should be spaced more closely than wells that
are screened in strata with higher intrinsic permeabilities.
Well Construction. Vertical Well Construction. Vertical extraction wells are
similar in construction to groundwater monitoring wells and are
installed using the same techniques. Extraction wells are usually
constructed of polyvinyl chloride (FVC) casing and screening. Extraction
well diameters typically range from 2 to 12 inches, depending on flow
rates and depth; a 4-inch diameter is most common. In general, 4-inch-
diameter wells are favored over 2-inch-diameter wells because 4-inch-
diameter wells are capable of higher extraction flow rates and generate
less fiictional loss of vacuum pressure.
Exhibit II-13 depicts a typical vertical extraction well. Vertical
extraction wells are constructed by placing the casing and screen in the
center of a borehole. Filter pack material is placed in the annular space
between the casing/screen and the walls of the borehole. The filter pack
material extends 1 to 2 feet above the top of the well screen and is
followed by a 1- to 2-foot-thick bentonite seal. Cement-bentonite grout
seals the remaining space up to the surface. Filter pack material and
screen slot size must be consistent with the grain size of the surrounding
soils.
The location and length of the well screen in vertical extraction wells
can vary and should be based on the depth to groundwater, the
stratification of the soil, and the location and distribution of
contaminants. In general, the length of the screen has little effect on the
ROI of an extraction well. However, because the ROI is affected by the
intrinsic permeability of the soils in the screened interval (lower intrinsic
permeability will result in a smaller ROI, other parameters being equal),
the placement of the screen can affect the ROI.
O At a site with homogeneous soil conditions, ensure that the well is
screened throughout the contaminated zone. The well screen may be
placed as deep as the seasonal low water table. A deeper well helps to
ensure remediation of the greatest amount of soil during seasonal low
groundwater conditions.
O At a site with stratified soils or lithology, check to see that the
screened interval is within the zone of lower permeability because
preferred flow will occur in the zones of higher permeability.
n-2O October 1994
-------
Exhibit 11-13
Typical Vertical Soil Vapor Extraction Well Construction
To Blower
Manifold
Grade
'•f^^" •
a
—
£
.H
'a
v
Q
2
%
£
|
7
j
7
;_
»
1
•c
^
f
"8
5
C
1
y^%&:^
""^ ~S^/"^- /'
'^
'^
^_>
•,^
^
^\X 5
^oS1^
^y>\
*^^
^<
i
i
1
*:*
*-.
rtj
:
i
i
i
K
i:
-'
3
r-
•-:
.-
-»
J
V:
"5
:
Note:
Piping may be buried
in utility trenches.
i^^-^f
S^&~^- Cerhent/Bentonit«
^sjj? — Bentonite
'JZZ- Sched. 40 PVC J
S$>
f
•^ Bore Hole
^-- — Sand Pack
*•
Sched. 40 Slottw
^"^ Well Screen
\
sixyV
-Flat Bottomed, Sched. 40
PVC Threaded Plug
Horizontal Well Construction. Look for horizontal extraction wells or
trench systems in shallow groundwater conditions. Exhibit 11-14 shows a
typical shallow horizontal well construction detail. Horizontal extraction
wells are constructed by placing slotted (PVC) piping near the bottom of
an excavated trench. Gravel backfill surrounds the piping. A bentonite
seal or impermeable liner is added to prevent air leakage from the
surface. When horizontal wells are used, the screen must be high
enough above the groundwater table that normal groundwater table
fluctuations do not submerge the screen. Additionally, vacuum
pressures should be monitored such that they do not cause upwelling of
the groundwater table that could occlude the well screen(s).
October 1994
H-21
-------
Manifold Piping
Manifold piping connects the extraction wells to the extraction blower.
Piping can either be placed above or below grade depending on site
operations, ambient temperature, and local building codes. Below-grade
piping is most common and is installed in shallow utility trenches that
lead from the extraction wellhead vault(s) to a central equipment
location. The piping can either be manifolded in the equipment area or
connected to a common vacuum main that supplies the wells in series,
in which case flow control valves are sited at the wellhead. Piping to the
well locations should be sloped toward the well so that condensate or
entrained groundwater will flow back toward the well.
Exhibit 11-14
Typical Horizontal Soil Vapor Extraction Well Construction
To Blower
Note:
Piping may be buried
in utility trenches.
Fabric Liner
Bentonite
Backfilled Soil
Grade
SLp ip-ft
-------
Blower Selection
The type and size of blower selected should be based on both the
vacuum required to achieve design vacuum pressure at the extraction
wellheads (including upstream and downstream piping losses) and the
total flow rate. The flow rate requirement should be based on the sum of
the flow rates from the contributing vapor extraction wells. In
applications where explosions might occur, blowers must have
explosion-proof motors, starters, and electrical systems. Exhibit 11-15
depicts the performance curves for the three basic types of blowers that
can be used in an SVE system.
O Centrifugal blowers (such as squirrel-cage fans) should be used for
high-flow (up to 280 standard cubic feet per minute), low-vacuum
(less than 30 inches of water) applications.
Exhibit 11-15
Performance Curves For Three Types Of Blowers
£
3
O
c
E
_3
O
O
M
o
o
c
• • - Rotary Lobe Blower
Regenerative Blower
Centrifugal Blower
80 12O 160 200 240 280
Airflow - Standard Cubic Feet per Minute (SCFM)
Notes:
Centrifugal blower type shown Is a New York model 2004A at 350O rpm. Regenerative
blower type shown is a Rotron model DR707. Rotary lobe blower type shown is a M-D
Pneumatics model 3204 at 3000 rpm.
From "Guidance for Design, Installation and Operation of Soil Venting Systems."
Wisconsin Department of Natural Resources, Emergency and Remedial Response
Section, PUBL-SW185-93, July 1993.
October 1994
n-23
-------
O Regenerative and turbine blowers should be used when a higher (up to
80 inches of water) vacuum is needed.
O Rotary lobe and other positive displacement blowers should be used
when a very high (greater than 80 inches of water) vacuum and
moderate air flow are needed.
Monitoring And Controls
The parameters typically monitored in an SVE system include:
O Pressure (or vacuum)
O Air/vapor flow rate
O Contaminant mass removal rates
O Temperature of blower exhaust vapors
The equipment in an SVE system used to monitor these parameters
provides the information necessary to make appropriate system
adjustments and track remedial progress. The control equipment in an
SVE system allow the flow and vacuum pressure to be adjusted at each
extraction well of the system, as necessary. Control equipment typically
includes flow control valves. Exhibit 11-16 lists typical monitoring and
control equipment for an SVE system, where each of these pieces of
monitoring equipment should be placed, and the types of equipment that
are available.
Optional SVE Components
Additional SVE system components might also be used when certain
site conditions exist or pilot studies dictate they are necessary. These
components include:
O Injection and passive inlet wells
O Surface seals
O Groundwater depression pumps
O Vapor treatment systems
Injection and Passive Inlet Wells. Air injection and inlet wells are
designed to help tune air flow distribution and may enhance air flow
rates from the extraction wells by providing an active or passive air
source to the subsurface. These wells are often used at sites where a
deeper zone (i.e., > 25 feet) is targeted for SVE or where the targeted zone
for remediation is isolated from the atmosphere by low permeability
materials. They are used also to help prevent short-circuiting of air flow
from the atmosphere at sites with shallower target zones. Passive wells
have little effect unless they are placed close to the extraction well. In
addition, air injection is used to eliminate potential stagnation zones
(areas of no flow) that sometimes exist between extraction wells.
n-24 October 1994
-------
Exhibit 11-16
Monitoring And Control Equipment
Monitoring Equipment
Flow meter
Vacuum gauge
Vapor temperature sensor
Sampling port
Vapor sample collection
equipment (used through a
sampling port)
Control Equipment
Flow control valves
Location In System
o At each wellhead
o Manifold to blower
o Blower discharge
o At each well head or
manifold branch
o Before and after filters
upstream of blower
o Before and after vapor
treatment
o Manifold to blower
o Blower discharge (prior to
vapor treatment)
o At each well head or
manifold branch
o Manifold to blower
o Blower discharge
o At each well head or
manifold branch
o Manifold to blower
o Blower discharge
o At each well head or
manifold branch
o Dilution or bleed valve at
manifold to blower
Example Of Equipment
o Pilot tube
o In-line rotameter
o Orifice plate
o Venturi or flow tube
o Manometer
o Magnehelic gauge
o Vacuum gauge
o Bi-metal dial-type
thermometer
o Hose barb
o Septa fitting
o Tedlarbags
o Sorbent tubes
o Sorbent canisters
o Polypropylene tubing for
direct GC injection
o Ball valve
o Gate/globe valve
o Butterfly valve
Air injection wells are similar in construction to extraction wells but
can be designed with a longer screened interval in order to ensure
uniform air flow. Active injection wells force compressed air into soils.
Passive air inlet wells, or inlets, simply provide a pathway that helps
extraction wells draw ambient air to the subsurface. Air injection wells
should be placed to eliminate stagnation zones, if present, but should
not be placed such that the injected air will force contaminants to an
area where they will not be recovered (i.e., off-site).
October 1994
n-25
-------
Surface Seals. Surface seals might be included in an SVE system
design to prevent surface water infiltration that can reduce air flow rates,
reduce emissions of fugitive vapors, prevent vertical short-circuiting of
air flow, or increase the design ROI. These results are accomplished
because surface seals force fresh air to be drawn from a greater distance
from the extraction well. If a surface seal is used, the lower pressure
gradients result in decreased flow velocities. This condition may require
a higher vacuum to be applied to the extraction well.
Surface seals or caps should be selected to match the site conditions
and regular business activities at the site. Options include high density
polyethylene (HDPE) liners (similar to landfill liners), clay or bentonite
seals (with cover vegetation or other protection), or concrete or asphalt
paving. Existing covers (e.g., pavement or concrete slab) might not
provide sufficient air confinement if they are constructed with a porous
subgrade material.
Groundwater Depression Pumps. Groundwater depression pumping
might be necessary at a site with a shallow groundwater table.
Groundwater pumps can reduce the upwelling of water into the
extraction wells and lower the water table and allow a greater volume of
soil to be remediated. Because groundwater depression is affected by
pumping wells, these wells must be placed so that the surface of the
groundwater is depressed in all areas where SVE is occurring.
Groundwater pumping, however, can create two additional waste
streams requiring appropriate disposal:
O Groundwater contaminated with dissolved hydrocarbons; and
O Liquid hydrocarbons (i.e., free product, if present).
Vapor Treatment Systems. Look for vapor treatment systems in the SVE
design if pilot study data indicate that extracted vapors will contain VOC
concentrations in excess of state or local air emission limits. Available
vapor treatment options include granular activated carbon (GAC),
catalytic oxidation, and thermal oxidation.
GAC is a popular choice for vapor treatment because it is readily
available, simple to operate, and can be cost competitive. Catalytic
oxidation, however, is generally more economical than GAC when the
contaminant mass loading is high. However, catalytic oxidation is not
recommended when concentrations of chemical constituents are
expected to be sustained at levels greater than 20 percent of their lower
explosive limit (LEL). In these cases, a thermal oxidizer is typically
employed because the vapor concentration is high enough for the
H-26 October 1994
-------
constituents to burn. Biofilters, an emerging vapor-phase biological
treatment technique, can be used for vapors with less than 10 percent
LEL, appear to be cost effective, and may also be considered.
Evaluation Of Operation And Monitoring Plans
Make sure that a system operation and monitoring plan has been
developed for both the system start-up phase and for long-term
operations. Operations and monitoring are necessary to ensure that
system performance is optimized and contaminant mass removal is
tracked.
Start-Up Operations
The start-up phase should include 7 to 10 days of manifold valving
adjustments. These adjustments should optimize contaminant mass
removal by concentrating vacuum pressure on the extraction wells that
are producing vapors with higher contaminant concentrations, thereby
balancing flow and optimizing contaminant mass removal. Flow
measurements, vacuum readings, and vapor concentrations should be
recorded daily from each extraction vent, from the manifold, and from
the effluent stack.
Long-Term Operations
Long-term monitoring should consist of flow-balancing, flow and
pressure measurements, and vapor concentration readings.
Measurements should take place at biweekly to monthly intervals for the
duration of the system operational period.
Exhibit 11-17 provides a brief synopsis of system monitoring
recommendations.
Exhibit 11-17
System Monitoring Recommendations
Phase
Start-up (7-10 days)
Remedial (ongoing)
Monitoring Frequency
Daily
Biweekly to monthly
What To Monitor
o Flow
o Vacuum
o Vapor concentrations
o Flow
o Vacuum
o Vapor concentrations
Where To Monitor
o Extraction vents
o Manifold
o Effluent stack
o Extraction vents
o Manifold
o Effluent stack
October 1994
H-27
-------
Remedial Progress Monitoring
Monitoring the performance of the SVE system in reducing
contaminant concentrations in soils is necessary to determine if remedial
progress is proceeding at a reasonable pace.
The mass removed during long-term monitoring intervals can be
calculated using vapor concentration and flow rate measurements taken
at the manifold. The instantaneous and cumulative mass removal is then
plotted versus time. The contaminant mass removed during an operating
period can be calculated using the equation provided below. This
relationship can be used for each extraction well (and then totalled) or
for the system as a whole, depending on the monitoring data that is
available.
M = C • Q • t
where: M = cumulative mass removed (kg)
C = vapor concentration (kg/m3)
Q = extraction flow rate (lET/hr)
t = operational period (hr)
kjj m
mass removed (kg) = —S. •- • hr
m3 h1"
Remedial progress of SVE systems typically exhibits asymptotic
'behavior with respect to both vapor concentration reduction and
cumulative mass removal. (See Exhibit 11-18.) At this point, the
composition of the vapor should be determined and compared with soil
vapor samples. This comparison will enable confirmation that there has
been a shift in composition toward less volatile components. Soil vapor
samples may indicate the composition and extent of the residual
contamination. When asymptotic behavior begins to occur, the operator
should closely evaluate alternatives that increase mass removal rate
such as increasing flow to extraction wells with higher vapor
concentrations by terminating vapor extraction from extraction wells
with low vapor concentrations or pulsing. Pulsing involves the periodic
shutdown and startup operation of extraction wells to allow the
subsurface environment to come to equilibrium (shutdown) and then
begin extracting vapors again (startup). Other more aggressive steps to
curb asymptotic behavior cari include installation of additional injection
wells or extraction wells.
H-28 October 1994
-------
Exhibit 11-18
Relationship Between Concentration Reduction And Mass Removal
TJ
a: c
o>
w u
c
oo
oo
Cumulative
VOC Mass
Removal (Ibs)
; Asymptotic:;
•Behavior:
(Irreducible)
VOC Concentrations
in Extracted Soil
Vapor (ppm)
Operation Time-
If asymptotic behavior is persistent for periods greater than about six
months and the concentration rebound is sufficiently small following
periods of temporary system shutdown, termination of operations may
be appropriate if residual levels are at or below regulatory limits. If not,
operation of the system as a bioventing system with reduced vacuum
and air flow may be an effective remedial alternative.
October 1994
H-29
-------
References
Beckett, G.D. and D. Huntley. Characterization of Flow Parameters
Controlling Soil Vapor Extraction. Groundwater. Vol. 32, No. 2, pp.
239-247, 1994.
DiGiulio, D. Evaluation of Soil Venting Application. Ada, OK: U.S.
Environmental Protection Agency, Office of Research and
Development. EPA/540/S-92/004, 1992.
Nyer, E.K. Practical Techniques for Groundwater and Soil Remediation,
Boca Raton, FL: Lewis Publishers, CRC Press, Inc., 1993.
U.S. Environmental Protection Agency (EPA). Soil Vapor Extraction
Technology: Reference Handbook. Cincinnati, OH: Office of Research
and Development. EPA/540/2-91/003, 199 la.
U.S. Environmental Protection Agency (EPA). Guide for TreatabUihj
Studies Under CERCLA: Soil Vapor Extraction. Washington, DC: Office
of Emergency and Remedial Response. EPA/540/2-91/019A, 1991b.
U.S. Environmental Protection Agency (EPA). Decision-Support Software
for Soil Vapor Extraction Technology Application: Hyperventilate.
Cincinnati, OH: Office of Research and Development.
EPA/600/R-93/028, 1993.
Wisconsin Department of Natural Resources (DNR). Guidance for Design,
Installation and Operation of Soil Venting Systems. Madison, WI:
Emergency and Remedial Response Section. PUBL-SW185-93, 1993.
Johnson, P.C., Stanley, C.C., Kemblowski, M.W., Byers, D.L., and J.D.
ColtharL "A Practical Approach to the Design, Operation and
Monitoring of In Situ Soil-Venting Systems." Ground Water Monitoring
Review, Vol. 10, No. 2, pp. 159-178, 1990.
n-3O October 1994
-------
Checklist: Can SVE Be Used At This Site?
This checklist can help you to evaluate the completeness of the CAP
and to identify areas that require closer scrutiny. As you go through the
CAP, answer the fpllowing questions. If the answer to several questions
is no, you will want to request additional information to determine if SVE
will accomplish the cleanup goals at the site.
1. Factors That Contribute To Permeability Of Soil
Yes No
Q Q Is the intrinsic permeability greater than 10"9 cm2?
Q Q Is the depth to greundwater greater than 3 feet?1
Q Q Are site soils generally dry?
2. Factors That Contribute To Constituent Volatility
Yes No
Q Q Is the contaminant vapor pressure greater than 0.5 mm Hg?
Q Q If the contaminant vapor pressure is not greater than 0.5
mm Hg, is some type of enhancement (e.g., heated air
injection) proposed to increase volatility?
Q Q Are the boiling points of the contaminant constituents less
than 300°C?
Q Q Is the Henry's law constant for the contaminant greater
than 100 atm?
1 If no, this parameter alone may not negate the use of SVE. However, provisions for
use of a surface seal, construction of horizontal wells, or for lowering the water table
should be incorporated into the CAP.
October 1994 H-31
-------
3. Evaluation Of The SVE System Design
Yes No
Q Q Does the radius of influence (ROI) for the proposed
extraction wells faH in the range 5 to 100 feet?
Q Q Has the ROI been calculated for each soil type at the site?
Q Q Examine the extraction flow rate. Will these flow rates
achieve cleanup in the time allotted for remediation in the
CAP?
Q Q Is the type of well proposed (horizontal or vertical)
appropriate for the site conditions present?
Q Q Is the proposed well density appropriate, given the total area
to be cleaned up and the radius of influence of each well?
Q Q Do the proposed well screen intervals match soil conditions
at the site?
Q Q Is the blower selected appropriate for the desired vacuum
conditions?
4. Optional SVE Components
Yes No
Q Q Are air injection or passive inlet wells proposed?
Q Q Is the proposed air injection/inlet well design appropriate for
this site?
Q Q Are surface seals proposed?
Q Q Are the sealing materials proposed appropriate for this site?
Q Q Will groundwater depression be necessary?
Q Q If groundwater depression is necessary, are the pumping
wells correctly spaced?
Q Q Is a vapor treatment system required?
Q Q If a vapor treatment system is required, is the proposed
system appropriate for the contaminant concentration at the
site?
n-32 October 1994
-------
4. Operation And Monitoring Plans
Yes No
Q Q Does the CAP propose daily monitoring for the first 7 to 10
days of flow measurements, vacuum readings, and vapor
concentrations from each extraction vent, the manifold, and
the effluent stack?
Q Q Does the CAP propose biweekly to monthly monitoring of
flow measurements, vacuum readings, and vapor
concentrations from each extraction vent, the manifold, and
the effluent stack?
October 1994 n-33
-------
-------
Chapter III
Bioventing
-------
-------
Contents
Overview III-l
Initial Screening Of Bioventing Effectiveness III-7
Detailed Evaluation Of Bioventing Effectiveness Ill-7
Site Characteristics III-9
Intrinsic Permeability III-9
Soil Structure And Stratification III-l 1
Microbial Presence III-l 1
Soil Ph 111-13
Moisture Content 111-13
Soil Temperature 111-14
Nutrient Concentrations 111-14
Depth To Groundwater ; HI-15
Constituent Characteristics 111-15
Chemical Structure 111-15
Concentration And Toxicity 111-16
Vapor Pressure . . 111-18
Product Composition And Boiling Point Ill-18
Henry's Law Constant 111-19
Pilot Scale Studies HI-20
Evaluation Of The Bioventing System Design 111-22
Rationale For The Design 111-22
Components Of A Bioventing System 111-25
Extraction Wells 111-25
Air Injection Wells 111-31
Manifold Piping 111-31
Vapor Pretreatment 111-32
Blower Selection 111-32
Instrumentation and Controls 111-32
Optional Bioventing Components 111-33
Evaluation Of Operation And Monitoring Plans 111-36
Start-Up Operations 111-36
Long-Term Operations 111-37
Remedial Progress Monitoring 111-37
References , 111-40
Checklist: Can Bioventing Be Used At This Site? 111-41
October 1994 m-iii
-------
List Of Exhibits
Number Title Page
III-l Typical Bioventing System Using Vapor Extraction III-2
III-2 Bioventing Summary III-3
III-3 Bioventing Evaluation Process Flow Chart III-4
III-4 Initial Screening For Bioventing Effectiveness III-8
III-5 Key Parameters Used To Evaluate Site
Characteristics And Constituent Characteristics III-9
III-6 Oxygen Provided Per Day From A Single
Well By A Vent System 111-10
III-7 Intrinsic Permeability And Bioventing Effectiveness .... Ill-10
in-8 Heterotrophic Bacteria And Bioventing Effectiveness . . . Ill-12
III-9 Soil Ph And Bioventing Effectiveness Ill-13
III-10 Depth To Groundwater And Bioventing Effectiveness . . . Ill-15
IE-11 Chemical Structure And Biodegradability Ill-16
III-12 Constituent Concentration And Bioventing
Effectiveness 111-17
III-13 Cleanup Concentrations And Bioventing Effectiveness . . Ill-18
III-14 Vapor Pressures Of Common Petroleum Constituents . . HI-19
m-15 Petroleum Product Boiling Flanges 111-19
III-16 Henry's Law Constant Of Common
Petroleum Constituents 111-20
III-l 7 Schematic Of Bioventing System Using
Vapor Extraction 111-26
III-l8 Well Orientation And Site Conditions 111-27
III-l9 Typical Bioventing Vertical Well Construction 111-29
m-20 Typical Horizontal Well 111-30
in-21 Performance Curves For Three Types Of Blowers 111-33
IH-22 Monitoring Equipment 111-34
111-23 System Monitoring Recommendations 111-37
IH-24 VOC/CO2 Concentration Reduction And
Constituent Mass Removal And Degradation
Behavior For Bioventing Systems 111-39
m-iv October 1994
-------
Chapter HI
Bioventing
Overview
Bioventing is an in-situ remediation technology that uses indigenous
microorganisms to biodegrade organic constituents adsorbed to soils in
the unsaturated zone. Soils in the capillary fringe and the saturated zone
are not affected. In bioventing, the activity of the indigenous bacteria is
enhanced by inducing air (or oxygen) flow into the unsaturated zone
(using extraction or injection wells) and, if necessary, by adding
nutrients. A bioventing layout using extraction wells is shown in
Exhibit III-l; air flow would be reversed if injection wells were used.
When extraction wells are used for bioventing, the process is similar
to soil vapor extraction (SVE). However, while SVE removes constituents
primarily through volatilization, bioventing systems promote
biodegradation of constituents and minimize volatilization (generally by
using lower air flow rates than for SVE). In practice, some degree of
volatilization and biodegradation occurs when either SVE or bioventing is
used. (See Chapter II for a discussion of SVE.)
All aerobically biodegradable constituents can be treated by
bioventing. In particular, bioventing has proven to be very effective in
remediating releases of petroleum products including gasoline, jet fuels,
kerosene, and diesel fuel. Bioventing is most often used at sites with
mid-weight petroleum products (i.e., diesel fuel and jet fuel), because
lighter products (i.e., gasoline) tend to volatilize readily and can be
removed more rapidly using SVE. Heavier products (e.g., lubricating oils)
generally take longer to biodegrade than the lighter products. A
summary of the advantages and disadvantages of bioventing is shown in
Exhibit III-2.
This chapter will assist you in evaluating a corrective action plan
(CAP) which proposes bioventing as a remedy for petroleum-
contaminated soil. The evaluation process is summarized in a flow
diagram shown on Exhibit ni-3; this flow diagram serves as a roadmap
for the decisions you will make during your evaluation. A checklist has
also been provided at the end of this chapter for you to use as a tool to
both evaluate the completeness of the CAP and focus attention on areas
where additional information may be needed. The evaluation process can
be divided into the four steps described below.
October 1994 m-1
-------
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October 1994
-------
Exhibit 111-2
Bioventing Summary
Advantages
Disadvantages
o Uses readily available equipment; easy to
install.
o Creates minimal disturbance to site
operations. Can be used to address
inaccessible areas (e.g., under buildings).
o Requires short treatment times: usually 6
months to 2 years under optimal
conditions.
o Is cost competitive: $45-140/ton of
contaminated soil.
o Easily combinable with other technologies
(e.g., air sparging, groundwater
extraction).
o May not require costly offgas treatment.
o High constituent concentrations may
initially be toxic to microorganisms.
o Not applicable for certain site conditions
(e.g., low soil permeabilities, high clay
content, insufficient delineation of
subsurface conditions).
o Cannot always achieve very low cleanup
standards.
o Permits generally required for nutrient
injection wells (if used). (A few states
also require permits for air injection.)
O Step 1: An initial screening ofbioventing effectiveness, which will
allow you to quickly gauge whether bioventing is likely to be effective,
moderately effective, or ineffective.
O Step 2: A detailed evaluation ofbioventing effectiveness, which
provides further screening criteria to confirm whether bioventing is
likely to be effective. To complete the detailed evaluation, you will
need to identify specific soil properties and product constituent
characteristics in the CAP, compare them to ranges where bioventing
is effective, evaluate the results of pilot studies reported in the CAP,
and conclude whether bioventing is likely to be effective.
O Step 3: An evaluation of the bioventing system design, which will
allow you to determine if the rationale for the design has been
appropriately defined based on pilot study data or other studies,
whether the necessary design components have been specified, and
whether the construction process flow designs are consistent with
standard practice.
October 1994
m-3
-------
Exhibit 111-3
Bioventing Evaluation Process Flow Chart
INITIAL SCREENING
OF BIOVENTING
EFFECTIVENESS
Determine the types of soils
that occur within the
contaminated area
Bioventing is not
likely to be effective
at the site.
Consider other
technologies.
Is clay
soil targeted for
remediation?
• Landfarming
• Biopiles
• Thermal
Desorption
Bioventing has the
potential for
effectiveness at the site.
Proceed to next panel
m-4
October 1994
-------
Exhibit III-3
Bioventing Evaluation Process Flow Chart
DETAILED EVALUATION OF
BIOVENTING EFFECTIVENESS
Identify product constituent
characteristics important to
Bioventing effectiveness
Identify site characteristics
important to Bioventing
effectiveness
Are
constituents all sufficient!
biodegradable?
Is intrinsic
permeability
Bioventing is
generally not
effective.
Consider other
technologies.
Landfarmmg
Biopites
Thermal
Desorptkm
OR
Is
soil free o
impermeable layers
or other conditions that
would disrupt
irflow?
IsTPH
< 26,000 ppm and
eavy metals <
ppm?
Conduct special
pilot studies to
address the out of
range parameters
Are vapor
pressures of product
constituents
< 0.5mm Hg?
Is depth
to groundwater
> 3 feet?
background
heterotrophic bacteria
> 1000 CPU/gram
Offgas may be
contaminated.
Pilot study and
system design
should consider
vapor control
Is
constituent
boiling range
< 260-300° C?
Is soil
pH between
6 and 8?
Is
Henry's Law
Constant
<100atm?
Pilot studies are
required to demonstrate
effectiveness. Review
pilot study results.
Is moisture
content of soils in
contaminated area between
40% and 85% of
saturation?
Do pilot
study results
demonstrate bioventi
effectiveness?
Bioventing will
not be effective
at the site.
Consider other
technologies.
Is soil
temperature
between 10°C and
45°C?
Landfarmmg
Biopites
Thermal
Desorption
Bioventing is likely to
be effective at the site.
Proceed to evaluate
October 1994
m-5
-------
Exhibit HI-3
Bioventing Evaluation Process Flow Chart
EVALUATION OF
BIOVENTING
SYSTEM DESIGN
EVALUATION OF BIOVENTING
SYSTEM OPERATION &
MONITORING PLANS
Determine the design elements
• Radius of Influence
• Wellhead Vacuum
• Extraction Flowrate
• Initial Vapor Concentrations
• End-Point Vapor Concentrations
• Son Volume to be Treated
• Pore Volume Calculations
* Discharge Limitations
• Construction Limitations
• Nutrient Formulation
• Nutrient Delivery Rate
Have design
elements been identified;
and are they within
normal ranges?
c
t
Identify & review the conceptual
process flow design &
the system components
• Extraction and/or Injection
Well Orientation, Spacing and
Construction
• Nutrient Delivery System
• Manifold Piping
• Vapor Pretreatment Equipment
• Blower
• Instrumentation & Controls
* Injection Wetts
• Vapor Treatment Equipment
Bioventing
system
design is
incomplete.
Request
additional
information.
NO
The Bioventing system
design is complete and
its elements are within
normal ranges. Proceed
to O&M evaluation.
Review the O&M plan
for the following:
• Start-Up Operations Plan
• Long-Term Operations &
Monitoring Plan
• Remedial Progress
Monitoring Plan
*
"I
I
i
•'
\*
Are
start-up
operations & monitoring
described, and are their
scope & frequency
adequate?
Is a
long-term O&M
plan described; is it
of adequate scope & frequency;
does it include
discharge permit
monitoring?
The proposed
Bioventing
system
operations and
monitoring plan
is incomplete.
Request
additional
information.
Is a
remedial progress
monitoring plan estab-
lished; is it of adequate scope
& frequency; does it include
provisions for detect-
ing asymptotic
behavior?
The Bioventing system
is likely to be effective.
The design and O&M
plans are complete.
m-6
October 1994
-------
O Step 4: An evaluation of the operation and monitoring plans,
which will allow you to determine whether start-up and long-term
system operation monitoring is of sufficient scope and frequency and
whether remedial progress monitoring plans are appropriate.
Initial Screening Of Bioventing Effectiveness
This section defines the key factors that should be used to decide
whether bioventing has the potential to be effective at a particular site.
These factors are:
O The permeability of the petroleum-contaminated soils. This will
determine the rate at which oxygen can be supplied to the
hydrocarbon-degrading microorganisms found in the subsurface.
O The biodegradabtiity of the petroleum constituents. This will
determine both the rate at which and the degree to which the
constituents will be metabolized by microorganisms.
In general, the type of soil will determine its permeability. Fine-grained
soils (e.g., clays and silts) have lower permeabilities than coarse-grained
soils (e.g., sands and gravels). The biodegradabUity of a petroleum
product constituent is a measure of its ability to be metabolized by
hydrocarbon-degrading bacteria that produce carbon dioxide and water
as byproducts of microbial respiration. Petroleum products are generally
biodegradable regardless of their molecular weight, as long as indigenous
microorganisms have an adequate supply of oxygen and nutrients. For
heavier constituents (which are less volatile and less soluble than many
lighter components), biodegradation will exceed volatilization as the
primary removal mechanism, even though biodegradation is generally
slower for heavier constituents than for lighter constituents.
Exhibit III-4 provides a screening tool you can use to make an initial
assessment of the potential effectiveness of bioventing. To use this tool,
first determine the type of soil present and the type of petroleum product
released at the site. Information provided in the following section will
allow a more thorough evaluation of effectiveness and will identify areas
that could require special design considerations.
Detailed Evaluation Of Bioventing Effectiveness
Once you have completed the initial screening and determined that
bioventing may be effective for the soil and petroleum product present,
review the CAP further to reconfirm effectiveness.
October 1994 m-7
-------
y~
S5
lO-
Sr"
Not
All
fo
Exhibit III-4
Initial Screening For Bioventing Effectiveness
Permeability %v
x^ffx^^™.^™™-!-^- 'iJi^JQrqfe £0 Minimal ^&"&^£^£^~"^"'^:i:®^s%&^
y-M^i--;^^^^^^^^' Effectiveness '^^^^^^^•^-M^;^^^;^'
Intrinsic Permeability, k (cm2) /^
16 1O"14 10~12 10~10 10~8 10~6 10"4 10~2
I Clay
I Glacial Till I
| Silt, Loess |
| Silty Sand |
| Clean Sand
I Grovel I
Product Composition \^>v
.ess Effective? aSSso^i • :i^p 5J:BSffi?!S? MsrS-JiffSSSY6. BS^^
""""^•":""""""""" """"":":: v.v.v.v.vv • • :.:""". : •""•:.".'"."""". "::•:•::. '::.':::::::"•:::::::":::::::.'"•:. "::::::Sr
\::;;fr
Lube Oils I /^
I Fuel Oils )
I Diesel I
I Kerosene I
I Gasoline I
te: •
petroleum products listed are amenable
r the bioventing remediation alternative.
While the initial screen focused on soil permeability and constituent
biodegradability, the detailed evaluation should consider a broader range
of site and constituent characteristics, which are listed in E3diibit III-5.
The remainder of this section describes each of these parameters, why
each is important to bioventing, how they can be determined, and the
range of each parameter considered appropriate for bioventing.
m-8
October 1994
-------
Exhibit 111-5
Key Parameters Used To Evaluate Site Characteristics And
Constituent Characteristics
Site Characteristics Constituent Characteristics
Intrinsic permeability Chemical structure
Soil structure and stratification Concentration and toxicity
Microbial presence Vapor pressure
Soil pH Product composition and boiling point
Moisture content Henry's law constant
Soil temperature
Nutrient concentrations
Depth to groundwater
Site Characteristics
Intrinsic Permeability
Intrinsic permeability is a measure of the ability of soils to transmit
air and is the single most important factor in determining the
effectiveness of bioventing because it determines how much oxygen can
be delivered (via extraction or injection) to the subsurface bacteria.
Hydrocarbon-degrading bacteria use oxygen to metabolize organic
material to yield carbon dioxide and water, a process commonly referred
to as aerobic respiration. To degrade large amounts of petroleum
hydrocarbons, a substantial bacterial population is required which, in
turn, requires oxygen for both the metabolic process and the growth of
the bacterial mass itself. Approximately 3 to SVz pounds of oxygen are
needed to degrade one pound of petroleum product. Exhibit III-6 shows
the relationship of oxygen provided per day from a single vent well for
different induced flow rates.
Intrinsic permeability, which will determine the rate at which oxygen
can be supplied to the subsurface, varies over 13 orders of magnitude
(from 10~16 to 10~3 cm2) for the wide range of earth materials, although a
more limited range applies for most soil types (10~13 to 10~5 cm2).
Intrinsic permeability is best determined from field or laboratory tests,
but can be estimated within one or two orders of magnitude from soil
boring log data and laboratory tests. Procedures for these tests are
described in EPA (199la). Coarse-grained soils (e.g., sands) have higher
intrinsic permeability than fine-grained soils (e.g., clays, silts). Note that
the ability of a soil to transmit air, which is of prime importance to
bioventing, is reduced by the presence of soil water, which can block the
October 1994 m-9
-------
Oxygen
Air
SCFM
1
5
10
20
50
100
Exhibit
Provided Per Day From A
Flow Rate
m3/min
2.83 -10'2
1.42-10'1
2.83 -10'1
5.66 • 10'1
1.42-10°
2.83-10°
III-6
Single Well By A Vent
System
Oxygen Provided
Ib/day
23
117
233
467
1,170
2,330
kg/day
10
52
106
212
529
1,060
soil pores and reduce air flow. This is especially important in fine-
grained soils, which tend to retain water. Use the values presented in
Exhibit III-7 to determine if intrinsic permeability is within the
effectiveness range for bioventing.
Exhibit Hl-7
Intrinsic Permeability And Bioventing Effectiveness
Intrinsic Permeability (cm )
Bioventing Effectiveness
k<10'
r10
Effective.
May be effective; needs further evaluation.
Not effective.
At sites where the soils in the saturated zone are similar to those
within the unsaturated zone, hydraulic conductivity of the soils may be
used to estimate the permeability of the soils. Hydraulic conductivity is a
measure of the ability of soils to transmit water. Hydraulic conductivity
can be determined from aquifer tests, including slug tests and pumping
tests. You can convert hydraulic conductivity to intrinsic permeability
using the following equation:
where:
k = K (u / pg)
k = intrinsic permeability (cm2)
K = hydraulic conductivity (cm/sec)
p = water viscosity (g/cm • sec)
p = water density (g/cm3)
g = acceleration due to gravity (cm/sec2)
At 20°C: u/pg = 1.02 • 10'5 cm/sec
To convert k from cm2 to darcy, multiply by 108
m-io
October 1994
-------
Soil Structure And Stratification
Soil structure and stratification are important to bioventing because
they affect how and where soil vapors will flow within the soil matrix
when extracted or injected. Structural characteristics such as
microfracturing can result in higher permeabilities than expected for
certain soils (e.g., clays). Increased flow will occur in the fractured but
not in the unfractured media. Stratification of soils with different
permeabilities can dramatically increase the lateral flow of soil vapors in
more permeable strata while reducing the soil vapor flow through less
permeable strata. This preferential flow behavior can lead to ineffective
or extended remedial times for less-permeable strata or to the possible
spreading of contamination if injection wells are used.
You can determine soil intergranular structure and stratification by
reviewing soil boring logs for wells or borings and by examining geologic
cross-sections. Verify that soil types have been identified, that visual
observations of soil structure have been documented, and that boring
logs are of sufficient detail to define any soil stratification.
The types of soils and their structures will determine their
permeabilities. In general, fine-grained soils composed of clays or silts
offer resistance to air flow. However, if the soils are highly fractured, they
may have sufficient permeability to use bioventing. Stratified soils may
require special consideration in design to ensure that less-permeable
strata are adequately vented.
Fluctuations in the groundwater table should also be considered
when reviewing the CAP. Significant seasonal or daily (e.g., tidal or
precipitation-related) fluctuations may, at times, submerge some of the
contaminated soil or a portion of the well screen, making it unavailable
for air flow. These fluctuations are most important for horizontal wells,
in which screens are placed parallel with the water table surface and a
water table rise could occlude the entire length of screen.
Microbial Presence
Soil normally contains large numbers of diverse microorganisms
including bacteria, algae, fungi, protozoa, and actinomycetes. In well-
aerated soils, which are most appropriate for bioventing, these
organisms are generally aerobic. Of these organisms, the bacteria are the
most numerous and biochemically active group, particularly at low
oxygen levels. Bacteria require a carbon source for cell growth and an
energy source to sustain metabolic functions required for growth.
Nutrients, including nitrogen and phosphorus, are also required for cell
growth.
October 1994 m-11
-------
The metabolic process used by bacteria to produce energy requires a
terminal electron acceptor (TEA) to enzymatically oxidize the carbon
source to carbon dioxide.
Microbes are classified by the carbon and TEA sources they use to
carry out metabolic processes. Bacteria that use organic compounds
(such as petroleum constituents and other naturally occurring organics)
as their source of carbon are called heterotrophic; those that use
inorganic carbon compounds such as carbon dioxide are called
autotrophic. Bacteria that use oxygen as their TEA are called aerobic;
those that use a compound other than oxygen (e.g., nitrate or sulfate)
are called anaerobic; and those that can utilize both oxygen and other
compounds as TEAs are called facultative. For bioventing applications
directed at petroleum products, bacteria that are both aerobic (or
facultative} and heterotrophic are most important in the degradation
process.
To evaluate the presence and population of naturally occurring
bacteria that will contribute to degradation of petroleum constituents,
laboratory analysis of soil samples from the site should be completed.
These analyses, at a minimum, should include plate counts for total
heterotrophic bacteria. Although heterotrophic bacteria are normally
present in all soil environments, plate counts of less than 1000 colony-
forming units (CFU)/gram of soil could indicate the presence of toxic
concentrations of inorganic or organic compounds or depletion of oxygen
or other essential nutrients. However, concentrations as low as 100 CFU
per gram of soil can be increased by bioventing to acceptable levels. The
total population of heterotrophic bacterial species that are capable of
degrading the specific petroleum constituents present should also be
measured. These conditions are summarized in Exhibit III-8.
Exhibit ill-8
Heterotrophic Bacteria And Bioventing Effectiveness
Total Heterotrophic Bacteria
(prior to bioventing)
>1000 CFU/gram dry soil
<1000 CFU/gram dry soil
Bioventing Effectiveness
Generally effective.
May be effective; needs further evaluation to
determine if toxic conditions are present.
October 1994
-------
SoilpH
The optimum pH for bacterial growth is approximately 7; the
acceptable range for soil pH in bioventing is between 6 and 8. Soils with
pH values outside this range prior to bioventing will require pH
adjustments prior to and during bioventing operations. Exhibit III-9
summarizes the effect of soil pH on bioventing effectiveness. Review the
CAP to verify that soil pH measurements have been made. If the soil pH
is less than 6 or greater than 8, make sure that pH adjustments are
included in the bioventing design and operational plans.
Exhibit 111-9
Soil pH And Bioventing Effectiveness
Soil pH
(prior to bioventing) Bioventing Effectiveness
6 < pH < 8 Generally effective.
6 > pH > 8 Soils will require amendments to correct pH
to effective range.
Moisture Content
Bacteria require moist soil conditions for proper growth. Excessive soil
moisture, however, reduces the availability of oxygen, which is also
necessary for bacterial metabolic processes, by restricting the flow of air
through soil pores. The ideal range for soil moisture is between 40 and
85 percent of the water-holding capacity of the soil. Generally, soils
saturated with water prohibit air flow and oxygen delivery to bacteria,
while dry soils lack the moisture necessary for bacterial growth.
Airflow is particularly important for soils within the capillary fringe,
where a significant portion of the constituents often reside. Fine-grained
soils create a thicker capillary fringe than coarse-grained soils. The
thickness of the capillary fringe can usually be determined from soil
boring logs (i.e., in the capillary fringe, soils are usually described as
moist or wet). The capillary fringe usually extends from one to several
feet above the elevation of the groundwater table. Moisture content of
soils within the capillary fringe may be too high for effective bioventing.
Depression of the water table by groundwater pumping may be
necessary to biovent soils within the capillary fringe.
October 1994 m-13
-------
Stormwater infiltration can create excessively moist soils in areas that
do not have surface covers, such as asphalt or concrete. This may be a
persistent problem with fine-grained soils that have slow infiltration
rates. Bioventing promotes dehydration of moist soils through increased
air flow through the soil, but excessive dehydration hinders bioventing
performance and extends operation time.
Soil Temperature
Bacterial growth rate is a function of temperature. Soil microbial
activity has been shown to decrease significantly at temperatures below
10°C and essentially to cease at 5°C. Microbial activity of most bacteria
important to petroleum hydrocarbon biodegradation also diminishes at
temperatures greater than 45°C. Within the range of 10°C to 45°C, the
rate of microbial activity typically doubles for every 10°C rise in
temperature. In most areas of the U.S., subsurface soils have a fairly
constant temperature of about 13°C throughout the year. However,
subsurface soil temperatures in the extreme northern states may be
lower, reducing the rate of biodegradation.
Nutrient Concentrations
Bacteria require inorganic nutrients such as ammonium and
phosphate to support cell growth and sustain biodegradation processes.
Nutrients may be available in sufficient quantities in the site soils but,
more frequently, nutrients need to be added to soils to maintain
bacterial populations. However, excessive amounts of certain nutrients
(i.e., phosphate and sulfate) can repress metabolism.
A rough approximation of minimum nutrient requirements can be
based on the stoichiometry of the overall biodegradation process:
C-source + N-source + O2 + Minerals + Nutrients —>
Cell mass + CO2 + H2O + products
Different empirical formulas of bacterial cell mass have been proposed;
the most widely accepted are C5H7O2N and C60H87O32N12P. Using the
empirical formulas for cell biomass and other assumptions, 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.
Chemical analysis of soil samples from the site should be completed
to determine the concentrations of nitrogen (expressed as ammonia) and
phosphate that occur naturally in the soil. Using the stoichiometric
ratios, the need for nutrient addition can be determined by using an
m-14 October 1994
-------
average concentration of the constituents (carbon source) in the soils to
be treated.
Depth To Groundwater
Bioventing is not appropriate for sites with groundwater tables located
less than 3 feet below the land surface. Special considerations must be
taken for sites with a groundwater table located less than 10 feet below
the land surface because groundwater upwelling can occur within
bioventing wells under vacuum pressures, potentially occluding screens
and reducing or eliminating vacuum-induced soil vapor flow. This
potential problem is not encountered if injection wells are used instead
of extraction wells to induce air flow. Use Exhibit III-10 to determine
whether the water-table depth is of potential concern for use of
bioventing.
Exhibit 111-10
Depth To Groundwater And Bioventing Effectiveness
Depth To Groundwater Bioventing Effectiveness
> 10 feet Effective.
3 feet < depth < 10 feet Need special controls (i.e., horizontal wells or
groundwater pumping).
< 3 feet Not effective.
Constituent Characteristics
Chemical Structure
The chemical structures of the constituents present in the soils
proposed for treatment by bioventing are important for determining the
rate at which biodegradation will occur. Although nearly all constituents
in petroleum products typically found at UST sites are biodegradable,
the more complex the molecular structure of the constituent, the more
difficult and less rapid is biological treatment. 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 III-l 1 lists, in
order of decreasing rate of potential biodegradability, some common
constituents found at petroleum UST sites.
October 1994 m-15
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Exhibit 111-11
Chemical Structure And Biodegradability
Biodegradability
More degradable
•
|
•
1
•B
\
Less degradable
Example Constituents
n-butane, l-pentane,
n-octane
Nonane
Methyl butane,
dimethylpentenes,
methyloctanes
Benzene, toluene,
ethylbenzene, xylenes
Propylbenzenes
Decanes
Dodecanes
Tridecanes
Tetradecanes
Naphthalenes
Fluoranthenes
Pyrenes
Acenaphthenes
Products In Which
Constituent Is Typically
Found
o Gasoline
o Diesel fuel
o Gasoline
o Gasoline
o Diesel, kerosene
o Diesel
o Kerosene
o Heating fuels
o Lubricating oils
o Diesel
o Kerosene
o Heating oil
o Lubricating oils
Evaluation of the chemical structure of the constituents proposed for
reduction by bioventing at the site will allow you to determine which
constituents will be the most difficult to degrade. You should verify that
remedial time estimates, biotreatability studies, field-pilot studies (if
applicable), and bioventing operation and monitoring plans are based on
the constituents that are the most difficult to degrade (or "rate limiting")
in the biodegradation process.
Concentration And Toxicity
The presence of very high concentrations of petroleum organics or
heavy metals in site soils can be toxic or inhibit the growth and
reproduction of bacteria responsible for biodegradation. In addition, very
low concentrations of organic material will also result in diminished
levels of bacterial activity.
m-16
October 1994
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In general, concentrations of petroleum hydrocarbons in excess of
25,000 ppm, or heavy metals in excess of 2,500 ppm, in soils are
considered inhibitory and/or toxic to aerobic bacteria. Review the CAP to
verify that the average concentrations of petroleum hydrocarbons and
heavy metals in the soils to be treated are below these levels.
Exhibit III-12 provides the general criteria for constituent concentration
and bioventing effectiveness.
Exhibit 111-12
Constituent Concentration And Bioventing Effectiveness
Constituent Concentration Bioventing Effectiveness
Petroleum constituents < 25,000 ppm Effective.
and
Heavy metals < 2,500 ppm
Petroleum constituents > 25,000 ppm Ineffective; toxic or inhibitory conditions to
or bacterial growth exist. Long remediation
Heavy metals > 2,500 ppm times likely.
In addition to maximum concentrations, you should consider the
cleanup concentrations proposed for the treated soils. Below a certain
"threshold" constituent concentration, the bacteria cannot obtain
sufficient carbon (from degradation of the constituents) to maintain
adequate biological activity. The threshold level can be determined from
laboratory studies and should be below the level required for cleanup.
Although the threshold limit varies greatly depending on bacteria-specific
and constituent-specific features, constituent concentrations below
0.1 ppm are generally not achievable by biological treatment alone. In
addition, experience has shown that reductions in total petroleum
hydrocarbon concentrations (TPH) greater than 95 percent can be very
difficult to achieve because of the presence of "recalcitrant" or
nondegradable petroleum species that are included in the TPH analysis.
Identify the average starting concentrations and the cleanup
concentrations in the CAP for individual constituents and TPH. If a
cleanup level lower than 0.1 ppm is required for any individual
constituent or a reduction in TPH greater than 95 percent is required to
reach the cleanup level for TPH, either a pilot study should be required
to demonstrate the ability of bioventing to achieve these reductions at
the site or another technology should be considered. These conditions
are summarized in Exhibit III-13.
October 1994 m-17
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Exhibit 111-13
Cleanup Concentrations And Bioventing Effectiveness
Cleanup Requirement Bioventing Effectiveness
Constituent concentration > 0.1 ppm Effective.
and
TPH reduction < 95%
Constituent concentration < 0.1 ppm Potentially ineffective; pilot studies are
or required to demonstrate reductions.
TPH reduction > 95%
Vapor Pressure
Vapor pressure is important in evaluating the extent to which
constituents will be volatilized rather than biodegraded. The vapor
pressure of a constituent is a measure of its tendency to evaporate. More
precisely, it is the pressure that a vapor exerts when in equilibrium with
its pure liquid or solid form. Constituents with higher vapor pressures
are generally volatilized rather than undergoing biodegradation.
Constituents with vapor pressures higher than 0.5 mm Hg will likely be
volatilized by the induced air stream before they biodegrade.
Constituents with vapor pressures lower than 0.5 mm Hg will not
volatilize to a significant degree and can instead undergo in situ
biodegradation by bacteria.
As previously discussed, petroleum products contain many different
chemical constituents. Each constituent will be volatilized (rather than
biodegraded) to different degrees by a bioventing system, depending on
its vapor pressure. If concentrations of volatile constituents are
significant, treatment of extracted vapors may be needed. Exhibit III-14
lists vapor pressures of select petroleum constituents.
Product Composition And Boiling Point
Boiling point is another measure of constituent volatility. Because of
their complex constituent compositions, petroleum products are often
classified by their boiling point ranges (rather than vapor pressures). In
general, nearly all petroleum-derived organic compounds are capable of
biological degradation, although constituents of higher molecular
weights and higher boiling points require longer periods of time to be
m-18 October 1994
-------
Vapor Pressures
Constituent
Methyl t-butyl ether
Benzene
Toluene
Ethylene dibromide
Ethylbenzene
Xylenes
Naphthalene
Tetraethyi lead
Exhibit 111-14
Of Common Petroleum Constituents
Vapor Pressure
(mm Hg at 20°C)
245
76
22
11
7
6
0.5
0.2
degraded. Products with boiling points of less than about 250°C to
300°C will volatilize to some extent and can be removed by a
combination of volatilization and biodegradation in a bioventing system.
The boiling point ranges for common petroleum products are shown in
Exhibit III-15.
Exhibit 111-15
Petroleum Product Boiling Ranges
Boiling Range
Product (°C)
Gasoline 40 to 205
Kerosene 175 to 325
Diesel fuel 200 to 338
Heating oil > 275
Lubricating oils Nonvolatile
Henry's Law Constant
Another method of gauging the volatility of a constituent is by noting
its Henry's law constant. Henry's law constant is the partition
coefficient that relates the concentration of a constituent dissolved in
water to its partial pressure in the vapor under equilibrium conditions.
October 1994 m'19
-------
In other words, it describes the relative tendency for a dissolved
constituent to exist in the vapor phase. Henry's law constants for
several common constituents found in petroleum products are shown in
Exhibit m-16. Constituents with Henry's law constants of greater than
100 atmospheres are generally considered volatile and are more likely to
be volatilized rather than biodegraded.
Henry's Law Constant
Constituent
Tetraethyl lead
Ethylbenzene
Xylenes
Benzene
Toluene
Naphthalene
Ethylene dibromide
Methyt-butyl ether
Exhibit 111-16
Of Common Petroleum Constituents
Henry's Law Constant
(atm)
4,700
359
266
230
217
72
34
27
Pilot Scale Studies
After you have examined the data in the CAP to gauge the potential
effectiveness of bioventing, you will be in a position to decide if
bioventing is likely to be highly effective, somewhat effective, or
ineffective for site conditions. In general, remedial approaches that rely
on biological processes should be subject to field pilot studies to verify
and quantify the potential effectiveness of the approach and provide data
necessary to design the system. For bioventing, these studies may range
in scope and complexily from a simple soil column test or microbial
count to field respirometry tests and soil vapor extraction (or injection)
pilot studies. The scope of pilot testing or laboratory studies should be
commensurate with the size of the area to be remediated, the reduction
in constituent concentration required, and the results of the initial
effectiveness screening.
A list and description of commonly used laboratory and pilot-scale
studies is provided below.
O Soil Vapor Extraction and Injection Treatabttty Tests are generally
used to determine the radius of influence that an extraction well or
injection well can exert in the surrounding soils, the optimum vapor
m-2o
October 1994
-------
flow rate and pressure (or vacuum) that should be applied to the
wells, and the concentration of petroleum constituents in the induced
air stream. The test most often includes short-term vapor extraction
or air injection from a single well while measuring the pressure effect
in monitoring wells or probes spaced at increasing distances from the
extraction well or the injection well. The test can assist in determining
the spacing, number, and type of wells needed for the full-scale
system. It is usually not economically attractive to perform this test
for sites with areas smaller than 5,000 cubic yards of in situ
contaminated soil or for sites with soil permeabilities greater
than 10"8 cm2.
O Respirometry Studies are generally used to determine the oxygen
transport capacity of the site soils and to estimate the biodegradation
rates under field conditions. The test includes short-term injection of
an oxygen/inert gas mixture into a well that has been screened in the
contaminated soil horizon. Carbon dioxide, inert gas (typically
helium), and oxygen concentrations are measured in the injection well
and surrounding wells periodically for about 1 to 5 days. The
measurements are then compared to baseline concentrations of the
gases prior to injection. Increases in carbon dioxide and decreases in
oxygen concentrations are indications of biological metabolism of
constituents; the inert gas concentration provides the baseline for
these calculations. Temperature of the extracted vapor may also be
monitored to serve as an additional indicator of biological activities.
Field respirometry studies are usually only needed for sites with large
areas of contamination, perhaps greater than 100,000 cubic yards of
in situ soils requiring remediation; at sites where soil permeability is
less than 10"8 cm2; or when reductions of more than 80 percent of the
constituents that have vapor pressures less than 0.5 mm Hg are
required.
O Laboratory Microbial Screening tests are used to determine the
presence of a population of naturally-occurring bacteria that may be
capable of degrading petroleum product constituents. Samples of soils
from the site are analyzed in an offsite laboratory. Microbial plate
counts determine the number of colony forming units (CFU) of
heterotrophic bacteria and petroleum-degrading bacteria are present
per unit mass of dry soil. These tests are relatively inexpensive.
O Laboratory Biodegradation Studies can be used to estimate the rate of
oxygen delivery and to determine if the addition of inorganic nutrients
is necessary. However, laboratory studies cannot duplicate field
conditions, and field tests are more reliable. There are two kinds of
laboratory studies: slurry studies and column studies. Slurry studies,
which are more common and less costly, involve the preparation of
October 1994 m-21
-------
numerous "soil microcosms" consisting of small samples of site soils
mixed into a slurry with site groundwater. The microcosms are
divided into several groups which may include control groups that are
"poisoned" to destroy any bacteria, non-nutrified test groups that
have been provided oxygen but not nutrients, and nutrified test
groups which are supplied both oxygen and nutrients. Microcosms
from each group are analyzed periodically (usually weekly) for the test
period duration (usually 4 to 12 weeks) for bacterial population
counts and constituent concentrations. Results of slurry studies
should be considered as representing optimal conditions because
slurry microcosms do not consider the effects of limited oxygen
delivery or soil heterogeneity. Column studies are set up in a similar
way using columns of site soils and may provide more realistic
expectations of bioventing performance.
Evaluation Of The Bioventing System Design
Once you have completed the detailed evaluation of bioventing
effectiveness, you can evaluate the design of the system. The CAP should
include a discussion of the design basis for the system and the
conceptual design. Detailed engineering design documents might also be
included, depending on state requirements. Further detail about
information to look for in the discussion of the design is provided below.
Rationale For The Design
The rationale for the design includes the fundamental design
decisions and requirements that form the foundation for the system
design. For bioventing systems, the design should include the following
information:
O Design Radius of Influence (ROI) is an estimate of the maximum
distance from a vapor extraction well (or injection well) at which
sufficient air flow can be induced to sustain acceptable degradation
rates. Establishing the design ROI is not a trivial task because it
depends on many factors including intrinsic permeability of the soil,
soil chemistry, moisture content, and desired remediation time. The
ROI should usually be determined through field pilot studies but can
be estimated from air flow modeling or other empirical methods.
Generally, the design ROI can range from 5 feet (for fine-grained soils)
to 100 feet (for coarse-grained soils). For sites with stratified geology,
radii of influence should be defined for each soil type. The ROI is
important in determining the appropriate number and spacing of
extraction or injection wells.
m-22 October 1994
-------
O Wellhead Pressure is the pressure (or vacuum) that is required at the
top of the vent well to produce the desired induced air stream flow
rate from the well. Although wellhead pressure (or vacuum) is usually
determined through field pilot studies, it can be estimated and
typically ranges from 3 to 100 inches of water vacuum for extraction
and 10 to 50 psi for injection. Less permeable soils generally require
higher vacuum or pressure to produce a reasonable radius of influ-
ence. It should be noted, however, that high vacuum pressures can
cause upwelling of the water table and occlusion of the extraction well
screens. For air injection, high pressure may push the contaminated
vapor to previously uncontaminated soil and ground water.
O Induced Vapor Flow Rate is the volumetric flow rate of soil vapor that
will be induced by each extraction or injection well and establishes
the oxygen delivery rate to the in situ treatment area. The induced
vapor flow rate, radius of influence, and wellhead pressure are all
interdependent (i.e., a certain vapor flow rate requires a certain
wellhead pressure and radius of influence). The induced vapor flow
rate should be determined from pilot studies, but it may be calculated
using mathematical or physical models (EPA, 1993). The flow rate will
contribute to the operational time requirements of the bioventing
system. Typical induced flow rates can range from 5 to 100 CFM per
well.
O Initial Constituent Vapor Concentrations can be measured during pilot
studies or estimated from soil gas samples or soil samples. They are
used to estimate constituent mass extraction rate to determine
whether treatment of extracted vapors will be required prior to
atmospheric discharge or reinjection. Be advised that state
regulations may not allow reinjection.
O Required Final Constituent Concentrations in soils or vapors are either
defined by state regulations as "remedial action levels" or determined
on a site-specific basis using transport modeling and risk assessment.
They will determine what areas of the site require treatment and when
bioventing operations can be terminated.
O Required Remedial Cleanup Time may also influence the design of the
system. The designer may vary the well spacing to speed remediation
to meet cleanup deadlines, if required.
O Soil Volume To Be Treated is determined by state action levels or a
site-specific risk assessment using site characterization data for the
soils.
October 1994 m-23
-------
O Pore Volume Calculations are used along with extraction flow rate to
determine the pore volume exchange rate and, therefore, oxygen
delivery rate. The exchange rate is calculated by dividing the soil pore
space within the treatment zone by the design vapor extraction rate.
The pore space within the treatment zone is calculated by multiplying
the soil porosity by the volume of soil to be treated. Some literature
suggests that one pore volume of soil vapor should be extracted at
least weekly for effective remedial progress.
You can calculate the time required to exchange one pore volume of
soil vapor using the following equation:
9
where: E = pore volume exchange time (hr)
e = soil porosity (m3 vapor/m3 soil)
V = volume of soil to be treated (m3 soil)
Q = total vapor extraction flowrate (m3 vapor/hr)
_ (m3 vapor / m3 soil) • (m3 soil) _ ,
(m3 vapor / hr)
O Discharge Limitations And Monitoring Requirements are usually
established by state air quality regulations. Such requirements must
be considered by designers of a bioventing system to ensure that
monitoring ports are included in the system for sites where volatile
constituents will be extracted. Discharge limitations imposed by state
air quality regulations will determine whether offgas treatment is
required.
O Site Construction Limitations, such as buildings, utilities, buried
objects, and residences, must be identified and considered in the
design process.
O Nutrient Formulation and Delivery Rate, which can be established
through either field or laboratory pilot studies, determines if nutrients
are required.
m~24 October 1994
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Components Of A Bioventing System
Once the design basis is defined, the design of the bioventtng system
can be developed. A typical bioventing system design will include the
following components and information:
O Extraction well (or injection well) orientation, placement, and
construction details
O Piping design
O Vapor pretreatment design (if necessary)
O Vapor treatment system selection (if necessary)
O Blower specification
O Instrumentation and control design
O Monitoring locations
Nutrient additions are sometimes included in bioventing designs. If
nutrients are added, the design should specify the nutrient addition well
orientation, placement, and construction details. Note that state
regulations may either require permits for nutrient injection wells or
prohibit them entirely. Exhibit III-17 is a conceptual schematic diagram
for a bioventing system using vapor extraction.
The following subsections provide guidance for selecting the
appropriate system configuration, standard system components, and
additional system components to adequately address petroleum
contaminated soils at a particular UST site.
Extraction Wells
Well Orientation. A bioventing system can use either vertical or horizontal
extraction wells. Orientation of the wells should be based on site-specific
needs and conditions. Exhibit III-18 lists site conditions and the
corresponding appropriate well orientation.
October 1994 m-25
-------
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Exhibit 111-18
Well Orientation And Site Conditions
Well Orientation Site Conditions
Vertical extraction well o Shallow to deep contamination (5 to
100+feet).
o Depth to groundwater > 10 feet.
Horizontal extraction well o Shallow contamination (< 25 feet). More
effective than vertical wells at depths
< 10 feet. Construction difficult at depths
> 25 feet.
o Zone of contamination confined to a
specific stratigraphic unit.
Well Placement and Number of Wells. You can determine the number and
location of extraction wells by using several methods. In the first
method, divide the area of the site requiring treatment by the area
corresponding to the design ROI of a single well to obtain the total
number of wells needed. Then space the wells evenly within the
treatment area to provide area! coverage so that the areas of influence
cover the entire area of contamination.
Number of wells needed = Treatment area (m2)
Area for single extraction well (m2 / well)
In the second method, determine the total extraction flow rate needed to
exchange the soil pore volume within the treatment area in a reasonable
amount of time (3 to 7 days). Determine the number of wells required by
dividing the total extraction flow rate needed by the flow rate achievable
with a single well.
Number of wells needed =
eV/ te
October 1994 m-27
-------
where: E = soil porosity (m3 vapor / m3 soil)
V = volume of soil in treatment area (m3 soil)
q = vapor extraction rate from single extraction well
(m vapor/hr).
te= time for exchange of pore volumefs), fhrs)
In the example below, a 7-day exchange time is used.
Number of wells needed
m3 vapor
m3 soU
soU)
168 hrs
m3 vapor
hr
Consider the following additional factors in evaluating proposed well
spacing.
O In areas of high contaminant concentrations, closer well spacing is
desired to increase oxygen flow and accelerate contaminant
degradation rates.
O Wells may be spaced slightly farther apart if a surface seal is planned
for installation or if one already exists. A surface seal increases the
radius of influence by forcing air to be drawn from a greater distance
by preventing short-circuiting from land surface. However, passive
vent wells or air injection wells may be required to supplement the
flow of air in the subsurface.
O In stratified or structured soils, well spacings may be irregular. Wells
screened in zones of lower intrinsic permeability must be spaced
closer together than wells screened in zones of higher intrinsic
permeability.
Well Construction. Vertical Well Construction. Vertical extraction wells are
similar in construction to monitoring wells and are installed using the
same techniques. Extraction wells are usually constructed of polyvinyl
chloride (FVC) casing and screen. Extraction well diameters typically
range from 2 to 12 inches, depending on flow rates and depth; a 4-inch
diameter is most common.
Exhibit III-19 depicts a typical vertical extraction well. Vertical
extraction wells are constructed by placing the casing and screen in the
center of a borehole. Filter pack material is placed in the annular space
between the casing/screen and the walls of the borehole. The filter pack
material extends 1-2 feet above the top of the well screen and is
m-28 October 1994
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Exhibit 111-19
Typical Bioventing Vertical Well Construction
Grode
ite Seal
Bentonite
Sched. 40 PVC Solid Casing
Sched. 40 Slotted PVC
Well Screen
Rat Bottomed, Sched. 40
PVC Threaded Plug
followed by a 1-2 foot thick bentonite seal. Cement-bentonite grout seals
the remaining space up to the surface. Filter pack material and screen
slot size must be consistent with the grain size of the surrounding soils.
The location and length of the well screen in vertical extraction or
injection wells can vary and should be based on the depth to
groundwater, the stratification of the soil, and the location and
distribution of contaminants. In general, the length of the screen has
little effect on the ROI of an extraction or injection well. However,
because the ROI is affected by the intrinsic permeability of the soils in
the screened interval flower intrinsic permeability will result in a smaller
ROI, other parameters being equal), the placement of the screen can
affect the ROI.
October 1994
m-29
-------
O At a site with homogeneous soil conditions, ensure that the well is
screened throughout the contaminated zone. The well screen may be
placed as deep as the seasonal low water table. A deep well helps to
ensure remediation of the greatest amount of soil during seasonal low
groundwater conditions.
O At a site with stratified soils or lithology, the screened interval can be
placed at a depth corresponding to a zone of lower permeability. This
placement will help ensure that air passes through this zone rather
than merely flow through adjacent zones of higher permeability.
Horizontal Well Construction. Horizontal extraction wells or trench
systems are generally used in shallow groundwater conditions.
Exhibit 111-20 shows a typical shallow horizontal well construction detail.
Horizontal extraction wells are constructed by placing slotted PVC piping
near the bottom of an excavated trench. Gravel bedding surrounds the
piping. A bentonite seal or impermeable liner prevents air leakage from
the surface. When horizontal wells are used, the screen must be high
enough above the groundwater table so that normal groundwater table
fluctuations do not submerge the screen. Additionally, if vacuum
extraction is used, pressures should be monitored to ensure that
induced groundwater upwelling does not occlude the screen(s).
Exhibit 111-20
Typical Horizontal Well
To Blower
Note:
Piping may be buried
in utility trenches.
Fabric Liner
Bentonite
Backfilled Soil
Grade
- High
Groundwater
Level
PVC Threaded Cap
Slotted PVC Pipe
Pea Gravel
m-so
October 1994
-------
Air Injection Wells
Air injection wells are similar in construction to extraction wells, but
air injection wells can be designed with a longer screened interval in
order to ensure uniform air flow. Other design criteria for injection wells'
orientation, well placement, and well construction are the same as that
of extraction wells described above. Horizontal wells are also applicable
for air injection. Active injection wells force compressed air into soils.
Passive injection wells, or inlets, simply provide a pathway that helps
extraction wells draw air from the atmosphere into the subsurface. Air
injection wells should be placed to eliminate stagnation zones, but
should not force contaminants to an area where they will not be
recovered (i.e., off-site) or could cause adverse health or safety effects.
Air injection wells can be used alone or, more commonly, in
conjunction with extraction wells. The injection well/extraction well
combination is often used at sites that are covered with an impermeable
cap (e.g., pavement or buildings) because the cap restricts direct air flow
to the subsurface. They are used also to help prevent short-circuiting the
air flow which may be restricted by preferential pathways in the
subsurface. In addition, air injection can be used to eliminate potential
stagnation zones (areas of no flow), which sometimes exist between
extraction wells.
Air injection wells are seldom used by themselves primarily because
the contaminated offgas can not be collected. Without the ability to
collect the offgas, contaminated vapor may spread to previously
uncontaminated areas. Also the offgas can not be used to evaluate the
extent of subsurface biological activities. In most cases, air injections are
limited to removing low or non-volatile petroleum products.
Manifold Piping
Manifold piping connects to the extraction or injection blower. Piping
can either be placed above or below grade depending on site operations,
ambient temperature, and local building codes. Below-grade piping is the
more common and is installed in shallow utility trenches that lead from
the wellhead vault to a central equipment location. The piping can either
be manifolded in the equipment area or connected to a common pressure
(or vacuum) main that supplies the wells in series, in which case flow
control valves are sited at the wellhead. Piping to extraction well
locations should be sloped toward the well so that condensate or
entrained groundwater will flow back toward the well.
October 1994 m-31
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Vapor Pretreatment
Extracted vapor can contain condensate, entrained groundwater, and
particulates that can damage blower parts and inhibit the effectiveness
of downstream treatment systems. In order to minimize the potential for
damage, vapors are usually passed through a moisture separator and a
particulate filter prior to entering the blower. Check the CAP to verify
that both a moisture separator and a particulate filter have been
included in the design.
Blower Selection
The type and size of blower selected should be based on (1) the
vacuum or pressure required to achieve design pressure at the wellheads
(Including upstream and downstream piping losses) and (2) the total flow
rate. The flow rate requirement should be based on the sum of the flow
rates from the contributing extraction or injection wells. In applications
where explosions may occur, be sure the CAP specifies blowers with
explosion-proof motors, starters, and electrical systems. Exhibit IH-21
depicts the performance curves for the three basic types of blowers that
can be used in a bioventing system.
O Centrifugal blowers (such as squirrel-cage fans) should be used for
high-flow, low-pressure, or low-vacuum applications (less than 20
inches of water).
O Regenerative and turbine blowers should be used when a higher
pressure or vacuum (up to 80 inches of water) is needed.
O Rotary lobe and other positive displacement blowers should be used
when a very high pressure or vacuum (greater than 80 inches of
water) is needed. Rotary lobe blowers are not generally applicable to
bioventing systems.
Instrumentation and Controls
The parameters typically monitored in a bioventing system include:
O Pressure (or vacuum)
O Air/vapor flow rate
O Carbon dioxide and/or oxygen concentration in extracted vapor
O Contaminant mass extraction rates
O Temperature
O Nutrient delivery rate (if nutrients are added)
m-32 October 1994
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Exhibit 111-21
Performance Curves For Three Types Of Blowers
E
3
3
U
c
I
O
M
0)
o
160 -
140 -
120 -
100 -
80 -
60 -
40 -
20 -
• • • • • Rotary Lobe Blower
Regenerative Blower
Centrifugal Blower
•
1
80 120 160 200 240 ,280
Airflow - Standard Cubic Feet per Minute (SCFM)
Notes:
Centrifugal blower type shown is a New York model 2004A at 3500 rpm. Regenerative
blower type shown is a Rotron model DR707. Rotary lobe blower type shown is a M-D
Pneumatics model 3204 at 3000 rpm.
From "Guidance for Design, Installation and Operation of Soil Venting Systems."
Wisconsin Department of Natural Resources, Emergency and Remedial Response
Section, PUBL-SW185-93, July 1993.
The monitoring equipment in a bioventing system enables you to
observe the progress of remediation and to control each component of
the system. Exhibit 111-22 describes where each of these pieces of
monitoring equipment is typically placed and the types of equipment
that are available.
Optional Bioventing Components
Additional bioventing system components might be used when certain
site conditions exist or when pilot studies dictate they are necessary.
These components include:
O Nutrient delivery systems (if needed)
O Surface seals
O Groundwater depression pumps
O Vapor treatment systems.
October 1994
m-33
-------
Exhibit 111-22
Monitoring Equipment
Instrument
Flow meter
Vacuum/Pressure gauge
Sampling port
Flow control valves
Vapor temperature sensor
Vapor sample collection
equipment (used through a
sampling port)
Control Equipment
Flow control valves
Location In System
o At each well head
o Manifold to blower
o Blower discharge
o Nutrient manifold
o At each well head or
manifold branch
o Before and after filters
before blower
o Before and after vapor
treatment
o At each well head or
manifold branch
o Manifold to blower
o Blower discharge
o At each well head or
manifold branch
o Dilution or bleed valve at
manifold to blower
o Manifold to blower
o Blower discharge (prior to
vapor treatment)
o At each well head or
manifold branch
o Manifold to blower
o Blower discharge
o At each well head or
manifold branch
o Dilution or bleed valve at
manifold to blower
Example Of Equipment
o Pitot tube
o In-line rotameter
o Orifice plate
o Turbine wheel
o Venturi or flow tube
o Manometer
o Magnehelic gauge
o Vacuum gauge
o Hose barb
o Septa fitting
o Ball valve
o Gate valve
o Dilution/ambient air bleed
valve
o Bi-metal dial-type
thermometer
o Tedlar bags
o Sorbent tubes
o Sorbent canisters
o Polypropylene tubing for
direct GC injection
o Ball valve
o Gate/globe valve
o Butterfly valve
m-34
October 1994
-------
Each of these system components is discussed below.
Nutrient Delivery Systems. If the addition of nutrients is required to
support biological growth, a nutrient delivery system will be needed.
Nutrients are usually supplied to the subsurface through topical
application or by injection through horizontal trenches or wells. Topical
application is either by hand-spraying or through conventional irrigation
systems (e.g., sprinklers). Horizontal wells are similar in design to those
used for extraction, and typically consist of slotted or perforated FVC
pipe installed in shallow (< 2 feet) trenches laid in a gravel bed. Nutrient
solutions can be prepared from solid formulations used in agricultural
applications of sodium tripolyphosphate.and ammonium salts, and "
should be added monthly to quarterly. Nutrient delivery systems may
also be used to add solutions to adjust pH as required.
Surface Seals. Surface seals might be included in a bioventing system
design in order to prevent surface water infiltration that can reduce air
flow rates, to reduce fugitive emissions, to prevent short-circuiting of air
flow, or to increase the design ROI. These results are accomplished
because surface seals force fresh air to travel a greater distance from the
extraction or injection well. If a surface seal is used, the lower pressure
gradients result in decreased flow velocities. This condition may require
a higher vacuum or pressure to be applied to the extraction or injection
well.
Surface seals or caps should be selected to match the site conditions
and regular business activities at the site. Options include high density
polyethylene (HDPE) liners (similar to landfill liners), clay or bentonite
seals, or concrete or asphalt paving. Existing covers (e.g., pavement or
concrete slabs) might not be applicable if they are constructed with a
porous subgrade material.
Groundwater Pumps. Groundwater depression pumping might be
necessary at a site with a shallow groundwater table or to expose
contaminated soils in the capillary or saturated zone. Groundwater
pumps reduce the upwelling of water into the extraction wells or lower
the water table and allow a greater volume of soil to be remediated.
Because groundwater depression is affected by pumping wells, these
wells must be placed so that the surface of the groundwater is depressed
in all areas where bioventing is to occur. Groundwater pumping,
however, can create two additional waste streams requiring appropriate
disposal:
O Groundwater contaminated with dissolved hydrocarbons; and
O Liquid hydrocarbons (i.e., free product), if present.
October 1994 DI-35
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Vapor Treatment. Look for vapor treatment systems in the bioventing
design if pilot study data indicate that extracted vapors will contain VOC
concentrations in excess of established air quality limits. Commonly
available treatment options are granular activated carbon (GAC),
catalytic oxidation, or thermal oxidation for vapor treatment.
GAC is a popular choice for vapor treatment because it is readily
available, simple to operate, and can be cost effective. Catalytic
oxidation, however, is generally more economical than GAC when the
contaminant mass loading is high. However, catalytic oxidation is not
recommended when concentrations of chemical constituents are
expected to be sustained at levels greater than 20 percent of their lower
explosive limit (LEL). In these cases, a thermal oxidizer is typically
employed because the vapor concentration is high enough for the
constituents to bum. Biofilters, an emerging vapor-phase biological
treatment technique, can be used for vapors with less than 10 percent
LEL, appear to be cost effective, and may also be considered.
Evaluation Of Operation And Monitoring Plans
It is important to make sure that a system operation and monitoring
plan has been developed for both the system start-up phase and for
long-term operations. Operations and monitoring are necessary to
ensure that system performance is optimized and contaminant mass
extraction and degradation are tracked. Monitoring of remedial progress
for bioventing systems is more difficult than for SVE systems in that
mass removal cannot be directly measured in extracted vapors.
Typically, both VOC concentrations (extracted mass) and carbon dioxide
concentrations (a product of microbial respiration) must both be
monitored.
Systems Involving only injection wells will have an especially limited
capability for performance monitoring because it is not possible to collect
the ofifgas. The monitoring plan should include subsurface soil sampling
to track constituent reduction and biodegradation conditions. Also, to
ensure the injected air is not causing contamination of the atmosphere
or previously uncontaminated soil or ground water, samples from each
medium should be analyzed for potential constituents.
Start-Up Operations
The start-up phase should include 7 to 10 days of manifold valving
adjustments. These adjustments should balance flow to optimize carbon
dioxide production and oxygen uptake rate while, to the extent possible,
minimizing volatilization by concentrating pressure (or vacuum) on the
m-36 October 1994
-------
wells that are in areas of higher contaminant concentrations. To
accomplish this, flow measurements, pressure or vacuum readings,
carbon dioxide concentrations, oxygen concentrations, and VOC
concentrations should be recorded daily from each extraction well, from
the manifold, and from the effluent stack. Nutrient delivery (if needed)
shguld not be performed until after start-up operations are complete.
Long-Term Operations
Long-term monitoring should consist of flow-balancing, flow and
pressure measurements, carbon dioxide measurements, oxygen
measurements, and VOC concentration readings. Measurements should
take place at weekly or biweekly intervals for the duration of the system
operational period. Nutrient addition, if necessary, should occur on a
periodic basis rather than continuously. Some literature suggests that
nutrient solutions be injected in wells or trenches or applied to the
surface at monthly qr quarterly intervals. Exhibit 111-23 provides a brie/
synopsis of system monitoring recommendations.
Exhibit 111-23
System Monitoring Recommendations
Phase Frequency
Start-up At least daily
Remedial Weekly to bi-weekly
What To Monitor
o Flow
o Vacuum readings
0 VOCs
o Carbon dioxide
o Oxygen
o Flow
o Vacuum
o VOCs
o Carbon dioxide
o Oxygen
Where To Monitor
o Extraction vents
o Manifold
o Effluent stack
o Extraction vents
o Manifold
o Effluent stack
Remedial Progress Monitoring
Monitoring the performance of the bioventing system in reducing
contaminant concentrations in soils is necessary to determine if remedial
progress is proceeding at a reasonable pace. A variety of methods can be
used.
Since concentrations of petroleum constituents may be reduced due
to both volatilization and biodegradation, both processes should be
monitored in order to track the cumulative effect. The constituent mass
October 1994
m-37
-------
extraction component can be tracked and calculated using the VOC
concentrations measured in the extraction manifold multiplied by the
extraction flow rate. The constituent mass that is degraded is more
difficult to quantify but can be monitored qualitatively by observing
trends in carbon dioxide and oxygen concentrations in the extracted soil
vapors.
Remedial progress of bioventing systems typically exhibits asymptotic
behavior with respect to VOC, oxygen, and carbon dioxide
concentrations in extracted vapors as shown in Exhibit 111-24. When
asymptotic behavior begins to occur, the operator should closely
evaluate alternatives that may increase bioventing effectiveness (e.g.,
increasing extraction flow rate or nutrient addition frequency). Other,
more aggressive steps to curb asymptotic behavior can include adding
injection wells, additional extraction wells, or injecting concentrated
solutions of bacteria.
If asymptotic behavior is persistent for periods greater than about
6 months, modification of the system design and operations (e.g., pulsing
of injection or extraction air flow) may be appropriate. If asymptotic
behavior continues, termination of operations may be appropriate.
m-38 October 1994
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Exhibit 111-24
VOC/C02 Concentration Reduction And Constituent Mass Removal And
Degradation Behavior For Bioventing Systems
•o
II
I!
oc. c
-------
References
Norris, R.D., Hinchee, R.E., Brown, R.A., McCarty, P.L., Semprini, L.t
Wilson, J.T., Kampbell, D.H., Reinhard, M., Bower, E.J., Borden, R.C.,
Vogel, T.M., Thomas, J.M., and C.H. Ward. Handbook of
Bioremediation. Boca Raton, FL:CRC Press, 1994.
U.S. Environmental Protection Agency (EPA). Guide for Conducting
Treatabitity Studies Under CERCLA: Aerobic Biodegradation Remedy
Screening. Washington, DC: Office of Emergency and Remedial
Response. EPA/540/2-91/0ISA, 1991a.
U.S. Environmental Protection Agency (EPA). Soil Vapor Extraction
Technology: Reference Handbook. Cincinnati, OH: Office of Research
and Development. EPA/540/2-91/003, 1991b.
U.S. Environmental Protection Agency (EPA). Guide for Conducting
Treatability Studies Under CERCLA.: Soil Vapor Extraction. Washington,
DC: Office of Emergency and Remedial Response.
EPA/540/2-91/019A, 1991c.
U.S. Environmental Protection Agency (EPA). Decision-Support Software
for Soil Vapor Extraction Technology Application: Hyperventilate.
Cincinnati, OH: Office of Research and Development.
EPA/600/R-93/028, 1993.
Wisconsin Department of Natural Resources (DNR). Guidance for Design,
Installation and Operation of Soil Venting Systems. Madison, WI:
Emergency and Remedial Response Section. PUBI^SWl 85-93, 1993.
m-4O October 1994
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Checklist: Can Bioventing Be Used At This Site?
This checklist can help you evaluate the completeness of the CAP and
to identify areas that require closer scrutiny. As you go through the CAP,
answer the following questions. If the answer to several questions is no,
you should request additional information to determine if bioventing will
accomplish cleanup goals at the site.
1. Site Characteristics
Yes No
Q Q Is the soil intrinsic permeability greater than 10"10 cm2?
Q Q Is the soil free of impermeable layers or other conditions that
would disrupt air flow?
Q Q Is the total heterotrophic bacteria count > 1,000 CFU/gram
dry soil?
Q Q Is soil pH between 6 and 8?
Q Q Is the moisture content of soil in contaminated area between
40% to 85% of saturation?
Q Q Is soil temperature between 10°C and 45°C during the
proposed treatment season?
Q Q Is the carbon:nitrogen:phosphorus ratio between 100:10:5
and 100:1:0.5?
Q Q Is the depth to groundwater > 3 feet?1
2. Constituent Characteristics
Yes No
Q Q Are constituents all sufficiently biodegradable?
Q Q Is the concentration of Total Petroleum Hydrocarbon
< 25,000 ppm and heavy metals < 2,500 ppm?
Q Q If there are constituents with vapor pressures greater than
0.5 mm Hg, boiling ranges above 300°C, or Henry's law
constants greater than 100 atm/mole fraction, has the CAP
addressed the potential environmental impact of the
volatilized constituents?
1 This parameter alone may not negate the use of bioventing. However, provisions for
the construction of horizontal wells or trenches or for lowering the water table should be
incorporated into the CAP.
October 1994 m-41
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3. Evaluation Of The Bioventing System Design
Yes No
Q Q Will the induced air flow rates achieve cleanup in the time
allotted for remediation in the CAP?
Q Q Does the radius of influence (ROI) for the proposed
extraction or injection wells fall in the range of 5 to 100 feet?
Q Q Has the ROI been calculated for each soil type at the site?
Q Q Is the type of well proposed (horizontal or vertical)
appropriate for the site conditions present?
Q Q Is the proposed well density appropriate, given the total area
to be cleaned up and the radius of influence of each well?
Q Q Do the proposed well screen intervals match soil conditions
at the site?
Q Q Are air injection wells proposed?
Q Q Is the proposed air injection well design appropriate for this
site?
Q Q Is the selected blower appropriate for the desired vacuum
conditions?
4. Optional Bioventing Components
Yes No
Q Q If nutrient delivery systems will be needed, are designs for
those systems provided?
Q Q Are surface seals proposed?
Q Q Are the proposed sealing materials appropriate for this site?
Q Q Will groundwater depression be necessary?
Q Q If groundwater depression is necessary, are the pumping
wells correctly spaced?
Q Q Is a vapor treatment system required?
Q Q If a vapor treatment system is required, is the proposed
system appropriate for the contaminant concentration at the
site?
m-42
October 1994
-------
5. Operation And Monitoring Plans
Yes No
Q Q Is monitoring qf offgas vapors for VOC and carbon dioxide
concentration proposed?
Q Q Is subsurface soil sampling proposed for tracking constituent
reduction and biodegradation conditions?
Q Q Are manifold valving adjustments proposed for the start-up
phase?
Q Q Is nutrient addition (if necessary) proposed to be controlled
on a periodic rather than continuous basis?
October 1994 m-43
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-------
Chapter IV
Biopiles
-------
-------
Contents
Overview W-l
Evaluation Of Biopile Effectiveness IV-7
Soil Characteristics . IV-8
Microbial Population Density IV-8
Soil pH IV-9
Moisture Content IV-10
Soil Temperature IV-10
Nutrient Concentrations IV-11
Soil Texture IV-12
Constituent Characteristics IV-13
Volatility , IV-13
Chemical Structure IV-13
Concentration And Toxicity IV-14
Climatic Conditions IV-16
Ambient Temperature IV-16
Rainfall IV-16
Wind IV-17
Biotreatability Evaluation IV-17
Evaluation Of The Biopile Design IV-19
Evaluation Of Operation And Remedial
Progress Monitoring Plans FV-21
Operations Plan IV-22
Remedial Progress Monitoring Plan IV-22
References - IV-24
Checklist: Can Biopiles Be Used At This Site? IV-25
October 1994
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List Of Exhibits
Number Title Page
IV-1 Typical Biopile System IV-2
IV-2 Advantages And Disadvantages Of Biopiles IV-3
IV-3 Biopile Evaluation Process Flow Chart IV-4
IV-4 Parameters Used To Evaluate The Effectiveness
Of Biopile Systems IV-7
IV-5 Heterotrophic Bacteria And Biopile Effectiveness IV-9
IV-6 Soil pH And Biopile Effectiveness IV-10
IV-7 Soil Moisture And Biopile Effectiveness IV-10
IV-8 Soil Temperature And Biopile Effectiveness IV-11
IV-9 Chemical Structure And Biodegradability IV-14
IV-10 Constituent Concentration And
Biopile Effectiveness IV-15
IV-11 Cleanup Requirements And Biopile Effectiveness IV-16
IV-12 Physical And Chemical Parameters For
Biotreatabilily Studies IV-18
IV-13 Construction Design Of A Typical Biopile IV-20
IV-14 Typical Remedial Progress Monitoring
Plan For Biopiles IV-23
IV-iv
October 1994
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Chapter IV
Biopiles
Overview
Biopiles, also known as biocells, bioheaps, biomounds, and compost
piles, are used to reduce concentrations of petroleum constituents in
excavated soils through the use of biodegradation. This technology
involves heaping contaminated soils into piles (or "cells") and stimulating
aerobic microbial activity within the soils through the aeration and/or
addition of minerals, nutrients, and moisture. The enhanced microbial
activity results in degradation of adsorbed petroleum-product
constituents through microbial respiration. Biopiles are similar to
landfarms in that they are both above-ground, engineered systems that
use oxygen, generally from air, to stimulate the growth and reproduction
of aerobic bacteria which, in turn, degrade the petroleum constituents
adsorbed to soil. While landfarms are aerated by tilling or plowing,
biopiles are aerated most often by forcing air to move by injection or
extraction through slotted or perforated piping placed throughout the
pile. (Chapter V provides a detailed description of landfarming.) A typical
biopile cell is shown in Exhibit IV-1.
Biopiles, like landfarms, have been proven effective in reducing
concentrations of nearly all the constituents of petroleum products
typically found at underground storage tank (UST) sites. Lighter (more
volatile) petroleum products (e.g., gasoline) tend to be removed by
evaporation during aeration processes (i.e., air injection, air extraction,
or pile turning) and, to a lesser extent, degraded by microbial
respiration. Depending upon your state's regulations for air emissions of
volatile organic compounds (VOCs), you may need to control the VOC
emissions. Control involves capturing the vapors before they are emitted
to the atmosphere, passing them through an. appropriate treatment
process, and then venting them to the atmosphere. The mid-range
hydrocarbon products (e.g., diesel fuel, kerosene) contain lower
percentages of lighter (more volatile) constituents than does gasoline.
Biodegradation of these petroleum products is more significant than
evaporation. Heavier (non-volatile) petroleum products (e.g., heating oil,
lubricating oils) do not evaporate during biopile aeration; the dominant
mechanism that breaks down these petroleum products is
biodegradation. However, higher molecular weight petroleum
constituents such as those found in heating and lubricating oils, and, to
a lesser extent, in diesel fuel and kerosene, require a longer period of
time to degrade than do the constituents in gasoline. A summary of the
advantages and disadvantages of biopiles is shown in Exhibit IV-2.
October 1994 IV-1
-------
CO
o
x
UJ 75
October 1994
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Exhibit IV-2
Advantages And Disadvantages Of Biopiles
Advantages
o Relatively simple to design and
implement.
o Short treatment times: usually 6 months
to 2 years under optimal conditions.
o Cost competitive: $30-90/ton of
contaminated soil.
o Effective on organic constituents with
slow biodegradation rates.
o Requires less land area than landfarms.
o Can be designed to be a closed system;
vapor emissions can be controlled.
o Can be engineered to be potentially
effective for any combination of site
conditions and petroleum products.
Disadvantages
o Concentration reductions > 95% and
constituent concentrations < 0.1 ppm are
very difficult to achieve.
o May not be effective for high constituent
concentrations (> 50,000 ppm total
petroleum hydrocarbons).
o Presence of significant heavy metal
concentrations (> 2,500 ppm) may inhibit
microbial growth.
o Volatile constituents tend to evaporate
rather than biodegrade during treatment.
o Requires a large land area for treatment,
although less than landfarming.
o Vapor generation during aeration may
require treatment prior to discharge.
o May require bottom liner if leaching from
the biopile is a concern.
This chapter will assist you in evaluating a corrective action plan
(CAP) that proposes biopiles as a remedy for petroleum-contaminated
soil. The evaluation guidance is presented in the three steps described
below. The evaluation process, summarized in a flow diagram shown in
Exhibit IV-3, will serve as a roadmap for the decisions you will make
during your evaluation. A checklist has been provided at the end of this
chapter for you to use as a tool for evaluating the completeness of the
CAP and for focusing on areas where additional information may be
needed. Because a biopile system can be engineered to be potentially
effective for any combination of site conditions and petroleum products,
the evaluation process for this technology does not include initial
screening. The evaluation process can be divided into the following steps.
O Step 1: An evaluation of biopile effectiveness, in which you can
identify the soil, constituent, and climatic factors that contribute to
the effectiveness of biopiles and compare them to acceptable operating
ranges. To complete the evaluation, you will need to compare these
properties to ranges in which biopiles are effective.
October 1994
IV-3
-------
Exhibit IV-3
Biopile Evaluation Process Flow Chart
EVALUATION OF
BIOPILE EFFECTIVENESS
Identify soil characteristics important
to biopile effectiveness
Mlcrobtel Population Density Soli Temperature
SollpH Nutrient Concentrations
Moisture Content Soil Texture
Are
background
heterotrophic bacteria
> 1000 CFU/gram?
Blotreatablltty studies
should Include special
studies to evaluate
out-of-range parameters.
Is soil
pH between
6 and 8?
Are
soils free of
clays that could cause
clumping and poor
aeration?
Do
btotreatability
studies demonstrate
biopile
effectiveness?
Is moisture
content of soils in
contaminated area between
40% and 85% of field
capacity?
Biopile will not
be effective at
the site.
Consider other
technologies.
Biopile design and
operation should Include
considerations to adjust
out-oftange parameters.
Is soil
attire between
10° C and 45° C during
treatment?
• Thermal
Desorption
Do
biopile design
and operation account
for out-of-range
parameters?
Do nutrient
concentrations have
a C:N:P ratio between
100:10:1 and
100:1:0.6?
Continue with evaluation
of biopile design.
IV-4
October 1994
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Exhibit IV-3
Biopile Evaluation Process Flow Chart
October 1994
EVALUATION OF
BIOPILE EFFECTIVENESS
Identify constituent characteristics
important to biopile effectiveness
Volatility
Chemical Structure
Concentration and Toxlclty
Is
gasoline or
other highly volatile
proposed for
treatme
Blotreatablltty studies
should Include special
studies to evaluate
out-of-rango parameters.
Are
constituents all suffici
biodegradable?
IsTPH
< 60,000 ppm and
eavy metals < 2,600
ppm?
Do
biotreatability
studies demonstrate
biopile
effectiveness?
Identify climate conditions
important to biopile effectiveness
Ambient Temperature
Rainfall
Wind
Biopile will not
be effective at
the site.
Consider other
technologies.
Are ambient
temperatures between
10° C and 4S°C for at least
4 months a year?
BlopHe design and
operation should Include
considerations to adjust
out-of^ange parameters.
• Thermal
Desorption
Is annual
precipitation less than
30 inches?
and operation account
for out-of-range
Is the site
subject to only light or
infrequent winds?
The biopile system is likely
to be effective at the site.
Proceed to evaluate the design.
1V-5
-------
Exhibit IV-3
Biopile Evaluation Process Flow Chart
EVALUATION OF
BIOPILE DESIGN
EVALUATION OF
BIOPILE OPERATION
MONITORING PLANS
Determine the design elements
• Land Requirements
• Biopile Layout
• Biopile Construction
• Aeration Equipment
• Water Management
• Soil Erosion Control
* pH Adjustment
• Moisture Addition
• Nutrient Supply
• Site Security
• Air Emission Controls
Have the
design elements been
identified and are they
appropriate?
The Biopile
design is
incomplete.
Request
additional
information.
The Biopue design
is complete. Proceed
to OAM evaluation.
Review the 0 & M plans
for the proposed biopile
for the following:
Operations Plan
Remedial Progress
Monitoring Plan
Are
operations procedures
described, and are their
scope & frequency
adequate?
Request
additional
information on
operations
procedures.
Is a
monitoring
plan described; is it of
adequate scope & frequency;
does it include
discharge permit
monitoring?
Request
additional
information on
monitoring
plans.
The Biopile system is
likely to be effective.
The design and O&M
plans are complete.
IV-6
October 1994
-------
O Step 2: An evaluation of the biopile system, design will allow you
to determine if the rationale for the design has been appropriately
defined, whether the necessary design components have been
specified, and whether the construction designs are consistent with
standard practice.
O Step 3: An evaluation of the operation and monitoring plans,
which are critical to the effectiveness of biopiles, will allow you to
determine whether start-up and long-term system operation and
monitoring plans are of sufficient scope.
Evaluation Of Biopile Effectiveness
The effectiveness of a biopile system depends on many parameters
which are listed in Exhibit IV-4. The parameters are grouped into three
categories: soil characteristics, constituent characteristics, and climatic
conditions.
Exhibit IV-4
Parameters Used To Evaluate The Effectiveness Of Biopile Systems
Soil Characteristics Constituent Characteristics Climatic Conditions
Microbial population density Volatility Ambient temperature
Soil pH Chemical structure Rainfall
Moisture content Concentration and toxicity Wind
Soil temperature
Nutrient concentrations
Soil texture
The following paragraphs contain descriptions of each parameter that
include: why it is important; how it can be determined; and what its
appropriate range is. During your evaluation, remember that because a
biopile is an above-ground treatment technique, most parameters (except
climatic conditions) can be controlled during the design and operation of
the biopile. Therefore, during your evaluation, identify those parameters
that fall outside the effective ranges provided and verify that the system
design and proposed operating specifications compensate for any site
conditions that are less than optimal.
October 1994 1V-7
-------
Soil Characteristics
Microbial Population Density
Soil normally contains large numbers of diverse microorganisms
including bacteria, algae, fungi, protozoa, and actinomycetes. In well-
drained soils, which are most appropriate for biopiles, these organisms
are generally aerobic. Of these organisms, bacteria are the most
numerous and biochemically active group, particularly at low oxygen
levels. Bacteria require a carbon source for cell growth and an energy
source to sustain metabolic functions required for growth. Bacteria also
require nitrogen and phosphorus for cell growth. Although sufficient
types and quantities of microorganisms are usually present in the soil .
for landfarming, recent applications of ex-situ soil treatment include
blending the soil with cultured microorganisms or animal manure
(typically from chickens or cows). Incorporating manure serves to both
augment the microbial population and provide additional nutrients.
Recently, the use of a certain fungi for biodegradation of organic
contaminants has been proposed based on promising laboratory tests.
The metabolic process used by bacteria to produce energy requires a
terminal electron acceptor (TEA) to enzymatically oxidize the carbon
source to carbon dioxide. Microbes are classified by the carbon and TEA
sources they use to carry out metabolic processes. Bacteria that use
organic compounds (e.g., petroleum constituents and other naturally
occurring organics) as their source of carbon are heterotrophic; those
that use inorganic carbon compounds (e.g., carbon dioxide) are
ovtotrophic. Bacteria that use oxygen as their TEA are aerobic; those that
use a compound other than oxygen, (e.g., nitrate, sulfate), are anaerobic;
and those that can utilize both oxygen and other compounds as TEAs
are facultative. For applications directed at cleaning up petroleum
products, only bacteria that are both aerobic (or facultative) and
heterotrophic are important in the degradation process.
In order to evaluate the presence and population of naturally
occurring bacteria that will contribute to degradation of petroleum
constituents, conduct laboratory analyses of soil samples from the site.
These analyses, at a minimum, should include plate counts for total
heterotrophic bacteria. Plate count results are normally reported in
terms of colony-forming units (CFUs) per gram of soil. Microbial
population densities in typical soils range from 104 to 107 CFU/gram of
soil. For biopiles to be effective the minimum heterotrophic plate count
should be 103 CFU/gram or greater. Plate counts lower than 103 could
indicate the presence of toxic concentrations of organic or inorganic (e.g.,
metals) compounds. In this situation, biopiles may still be effective if the
soil is conditioned or amended to reduce the toxic concentrations and
increase the microbial population density. More elaborate laboratory
tests are sometimes conducted to identify the bacterial species present.
This may be desirable if there is uncertainty about whether
IV-8 October 1994
-------
microbes capable of degrading specific petroleum hydrocarbons occur
naturally in the soil. If insufficient numbers or types of microorganisms
are present, the population density may be increased by introducing
cultured microbes that are available from numerous different vendors.
See Exhibit IV-5 for the relationship between counts of total
heterotrophic bacteria and the effectiveness of biopiles.
Exhibit IV-5
Heterotrophic Bacteria And Biopile Effectiveness
Total Heterotrophic Bacteria
(prior to biopile operation) Biopile Effectiveness
> 1,000 CFU/gram dry soil Generally effective.
< 1,000 CFU/gram dry soil May be effective; needs further evaluation to
determine if toxic conditions are present.
The use of fungi (specifically the white rot fungus) is emerging as a
remedial technology that may be effective on many types of organic
contaminants. These fungi do not metabolize contaminants; degradation
occurs outside their cells. The fungi degrade lignin, which must be
supplied to them, usually in the form of sawdust or woodchips blended
with the soil. In the process of degrading lignin, the fungi excrete other
chemicals that degrade the organic contaminants. This process is called
co-metabolism. Although the technology has not as yet been subject to
extensive field testing, laboratory tests show it can degrade organic
chemicals to non-detectable levels.
SoilpH
To support bacterial growth, the soil pH should be within the 6 to 8
range, with a value of about 7 (neutral) being optimal. Soils with pH
values outside this range prior to biopile operation will require pH
adjustment during construction of the biopile and during operation of
the biopile. Soil pH within the biopile soils can be raised through the
addition of lime and lowered by adding elemental sulfur during
construction. Liquid solutions may also be injected into the biopile
during operations to adjust pH. However, mixing with soils during
construction results in more uniform distribution. Exhibit IV-6
summarizes the effect of soil pH on biopile effectiveness. Review the CAP
to verify that soil pH measurements have been made. If the soil pH is
less than 6 or greater than 8, make sure that pH adjustments, in the
form of soil amendments, are included in the construction plans for the
biopile and that the operations plan includes monitoring of pH.
October 1994 IV-9
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Exhibit IV-6
Soil pH And Biopile Effectiveness
Soil pH
(prior to biopile construction) Biopile Effectiveness
6 < PH < 8 Generally effective.
6 > pH > 8 Biopile soils will require amendments to
correct pH to effective range.
Moisture Content
Soil microorganisms require moist soil conditions for proper growth.
Excessive soil moisture, however, restricts the movement of air through
the subsurface thereby reducing the availability of oxygen which is
essential for aerobic bacterial metabolic processes. In general, soils
should be moist but not wet or dripping wet. The ideal range for soil
moisture is between 40 and 85 percent of the water-holding capacity
(field capacity) of the soil or about 12 percent to 30 percent by weight.
Periodically, moisture must be added to the biopile because soils become
dry as a result of evaporation, which is increased during aeration
operations. Excessive accumulation of moisture can occur within
biopiles in areas with high precipitation or poor drainage. These condi-
tions should be considered in the biopile design. For example, an imper-
meable cover can mitigate excess infiltration and potential erosion of the
biopile. Exhibit IV-7 shows the optimal range for soil moisture content.
Exhibit IV-7
Soil Moisture And Biopile Effectiveness
Soil Moisture Biopile Effectiveness
40% < field capacity < 85% Effective.
Field capacity < 40% Periodic moisture addition is needed to
maintain proper bacterial growth.
Reid capacity > 85% Biopile design should include special water
drainage considerations or impervious cover.
Soil Temperature
Bacterial growth rate is a function of temperature. Soil microbial
activity has been shown to significantly decrease at temperatures below
10°C and to essentially cease below 5°C. The microbial activity of most
bacteria important to petroleum hydrocarbon biodegradation also
diminishes at temperatures greater than 45°C. Within the range of 10°C
IV-10 October 1994
-------
to 45°C, the rate of microbial activity typically doubles for every 10°C
rise in temperature. Because soil temperature varies with ambient
temperature, there will be certain periods during the year when bacterial
growth and, therefore, constituent degradation will diminish. When
ambient temperatures return to the growth range, bacterial activity will
be gradually restored.
In colder parts of the United States, such as the Northeastern states,
optimum operating temperatures typically exist for periods of 7 to 9
months. In very cold climates, special precautions can be taken,
including enclosing the biopile within a greenhouse-type structure,
injecting heated air into the biopile, or introducing special bacteria
capable of activity at lower temperatures. In warm regions, optimum
temperatures for biopile effectiveness can last all year. Exhibit IV-8
shows how soil temperature affects biopile operation.
Exhibit IV-8
Soil Temperature And Biopile Effectiveness
Soil Temperature Biopile Effectiveness
10°C < soil temperature < 45°C Effective.
10°C > soil temperature > 45°C Not generally effective; microbial activity
diminished during seasonal temperature •
extremes but restored during periods within
the effective temperature range.
Temperature-controlled enclosures, heated
(or cooled) air injection, or special bacteria
required for areas with extreme
temperatures.
Nutrient Concentrations
Microorganisms require inorganic nutrients such as nitrogen and
phosphorus to support cell growth and sustain biodegradation
processes. Nutrients may be available in sufficient quantities in the site
soils but, more frequently, nutrients need to be added to the biopile soils
to maintain bacterial populations. However, excessive amounts of certain
nutrients (i.e., phosphate and sulfate) can repress microbial metabolism.
The typical carbon:nitrogen:phosphorus ratio necessary for
biodegradation falls in the range of 100:10:1 to 100:1:0.5, depending on
the specific constituents and microorganisms involved in the
biodegradation process.
October 1994 IV-11
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The naturally occurring available nitrogen and phosphorus content of
the soil should be determined by chemical analyses of samples collected
from the site. These types of analyses are routinely conducted in
agronomic laboratories that test soil fertility for farmers. These
concentrations can be compared to the nitrogen and phosphorus
requirements calculated from the stoichiometric ratios of the
biodegradation process. A conservative approximation of the amount of
nitrogen and phosphorus required for optimum degradation of petroleum
products can be calculated by assuming that the total mass of
hydrocarbon in the soil represents the mass of carbon available for
biodegradation. This simplifying assumption is valid because the carbon
content of the petroleum hydrocarbons commonly encountered at UST
sites is approximately 90 percent carbon by weight.
As an example, assume that at a LUST site the volume of
contaminated soil is 90,000 ft3, the average TPH concentration in the
contaminated soil is 1,000 mg/kg, and the soil bulk density is 50 kg/ft3
(1.75 g/cm3).
The mass of contaminated soil is equal to the product of volume and
bulk density:
soil mass = 90,000 ft3 x 50 kg = 4.5 x 106 kg
The mass of the contaminant (and carbon) is equal to the product of
the mass of contaminated soil and the average TPH concentration in the
contaminated soil:
contaminant mass =
4.5 x 106 kg x 1,000 J2I = 4.5 x 103 kg « 10,000 Ibs
kg
Using the C:N:P ratio of 100:10:1, the required mass of nitrogen
would be 1,000 Ibs, and the required mass of phosphorus would be
100 Ibs. After converting these masses into concentration units
(56 mg/kg for nitrogen and 5.6 mg/kg for phosphorus), they can be
compared with the results of the soil analyses to determine if nutrient
addition is necessary. If nitrogen addition is necessary, slow release
sources should be used. Nitrogen additions can lower soil pH, depending
on the amount and type of nitrogen added.
Soil Texture
Texture affects the permeability, moisture content, and bulk density
of the soil. To ensure that oxygen addition (by air extraction or injection),
nutrient distribution, and moisture content of the soils can be
maintained within effective ranges, you must consider the texture of the
soils. For example, soils that tend to clump together (such as clays) are
difficult to aerate and result in low oxygen concentrations. It is also
difficult to uniformly distribute nutrients throughout these soils. They
also retain water for extended periods following a precipitation event.
IV-12 October 1994
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You should identify whether clayey soils are proposed for the biopile
at the site. Soil amendments (e.g., gypsum) and bulking materials (e.g.,
sawdust, or straw) should be blended into the soil as the biopile is being
constructed to ensure that the biopile medium has a loose or divided
texture. Clumpy soil may require shredding or other means of
pretreatment during biopile construction to incorporate these
amendments.
Constituent Characteristics
Volatility
The volatility of contaminants proposed for treatment in biopiles is
important because volatile constituents tend to evaporate from the
biopile into the air during extraction or injection, rather than being
biodegraded by bacteria. Constituent vapors in air that is injected into
the biopile will dissipate into the atmosphere unless the biopile is
covered and collection piping is installed beneath the cover. If air is
added to the pile by applying a vacuum to the aeration piping, volatile
constituent vapors will pass into the extracted air stream which can be
treated, if necessary. In some cases (where allowed), it may be acceptable
to reinject the extracted vapors back into the soil pile for additional
degradation. It is important to optimize the aeration rate to the biopile.
Evaporation of volatile constituents can be reduced by minimizing the air
extraction or injection rate, which also reduces degradation rates by
reducing oxygen supply to bacteria.
Petroleum products generally encountered at UST sites range from
those with a significant volatile fraction, such as gasoline, to those that
are primarily nonvolatile, such as heating and lubricating oils. Petroleum
products generally contain more than one hundred different constituents
that possess a wide range of volatility. In general, gasoline, kerosene,
and diesel fuels contain constituents with sufficient volatility to
evaporate from a biopile. Depending upon state-specific regulations for
air emissions of volatile organic compounds (VOCs), control of VOC
emissions may be required. Control involves capturing vapors before
they are emitted to the atmosphere and then passing them through an
appropriate treatment process before being vented to the atmosphere.
Chemical Structure
The chemical structures of the contaminants present in the soils
proposed for treatment by biopiles are important in determining the rate
at which biodegradation will occur. Although nearly all constituents in
petroleum products typically found at UST sites are biodegradable, the
more complex the molecular structure of the constituent, the more
difficult and less rapid is biological treatment. Most low molecular-weight
(nine carbon atoms or less) aliphatic and monoaromatic constituents are
more easily biodegraded than higher molecular weight aliphatic or
October 1994 IV-13
-------
polyaromatic organic constituents. Exhibit IV-9 lists, in order of
decreasing rate of potential biodegradability, some common constituents
found at petroleum UST sites.
Exhibit IV-9
Chemical Structure And Biodegradability
Biodegradability
More degradable
•
I-
\
T
Less degradable
Example Constituents
n-butane, l-pentane,
n-octane
Nonane
Methyl butane,
dimethylpentenes,
methyloctanes
Benzene, toluene,
ethylbenzene, xylenes
Propylbenzenes
Decanes
Dodecanes
Tridecanes
Tetradecanes
Naphthalenes
Fluoranthenes
Pyrenes
Acenaphthenes
Products In Which
Constituent Is Typically
Found
o Gasoline
o Diesel fuel
o Gasoline
o Gasoline
o Diesel, kerosene
o Diesel
o Kerosene
o Heating fuels
o Lubricating oils
o Diesel
o Kerosene
o Heating oil
o Lubricating oils
Evaluation of the chemical structure of the constituents proposed for
reduction by biopiles at the site will allow you to determine which
constituents will be the most difficult to degrade. You should verify that
remedial time estimates, biotreatability studies, field-pilot studies (if
applicable), and biopile operation and monitoring plans are based on the
constituents that are most difficult to degrade (or "rate limiting") in the
biodegradation process.
Concentration And Toxicity
The presence of very high concentrations of petroleum organics or
heavy metals in site soils can be toxic or inhibit the growth and
reproduction of bacteria responsible for biodegradation in biopiles.
Conversely, very low concentrations of organic material will result in
diminished levels of microbial activity.
IV-14
October 1994
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In general, soil concentrations of total petroleum hydrocarbons (TPH)
in the range of 10,000 to 50,000 ppm, or heavy metals exceeding 2,500
ppm, are considered inhibitory and/or toxic to most microorganisms. If
TPH concentrations are greater than 10,000 ppm, or the concentration of
heavy metals is greater than 2,500 ppm, then the contaminated soil
should be thoroughly mixed with clean soil to dilute the contaminants so
that the average concentrations are below toxic levels. Exhibit IV-10
provides the general criteria for constituent concentration and biopile
effectiveness.
Exhibit IV-10
Constituent Concentration And Biopile Effectiveness
Constituent Concentration Biopile Effectiveness
Petroleum constituents £ 50,000 ppm Effective, however, if contaminant
and concentration is > 10,000 ppm, then soil
Heavy metals < 2,500 ppm should be blended with clean soil to reduce
the concentration of the contaminants.
Petroleum constituents > 50,000 ppm Ineffective; toxic or inhibitory conditions to
or bacterial growth exist. Dilution by blending
Heavy metals > 2,500 ppm necessary.
In addition to maximum concentrations, you should consider the
cleanup goals proposed for the biopile soils. Below a certain "threshold"
constituent concentration, the bacteria cannot obtain sufficient carbon
(from degradation of the constituents) to maintain adequate biological
activity. The threshold level can be determined from laboratory studies
and should be below the level required for cleanup. Although the
threshold limit varies greatly depending on bacteria-specific and
constituent-specific features, generally constituent concentrations below
0.1 ppm are not achievable by biological treatment alone. In addition,
experience has shown that reductions in TPH concentrations greater
than 95 percent can be very difficult to achieve because of the presence
of "recalcitrant" or nondegradable hydrocarbon species that are included
in the TPH analysis. If a cleanup level lower than 0.1 ppm is required for
any individual constituent or a reduction in TPH greater than 95 percent
is required to reach the cleanup level for TPH, either a pilot study is
required to demonstrate the ability of a biopile system to achieve these
reductions at the site or another technology should be considered.
Exhibit IV-11 shows the relationship between cleanup requirements and
biopile effectiveness.
October 1994 IV-15
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Exhibit IV-11
Cleanup Requirements And Biopile Effectiveness
Cleanup Requirement Biopile Effectiveness
Constituent concentration > 0.1 ppm Effective.
and
TPH reduction < 95%
Constituent concentration < 0.1 ppm Potentially ineffective; pilot studies are
or required to demonstrate contaminant
TPH reduction > 95% reductions.
Climatic Conditions
Ambient Temperature
The ambient temperature is important because it influences soil
temperature. As described previously, the temperature of the soils in the
biopile impacts bacterial activity and, consequently, biodegradation. The
optimal temperature range for biopiles is 10°C to 45°C. Special
considerations (e.g., heating, covering, or enclosing) in biopile design can
overcome the effects of colder climates and extend the length of the
bioremediation season.
Rainfall
Some biopile designs do not include covers, leaving the biopile
exposed to climatic factors including rainfall, snow, and wind, as well as
ambient temperatures. Rainwater that falls on the biopile area will
increase the moisture content of the soil and cause erosion. As
previously described, effective biopile operation requires a proper range
of moisture content. During and following a significant precipitation
event, the moisture content of the soils may be temporarily in excess of
that required for effective bacterial activity. On the other hand, during
periods of drought, moisture content may be below the effective range
and additional moisture may need to be added.
If the site is located in an area subject to annual rainfall of greater
than 30 inches during the biopile season, a rain shield (such as a cover,
tarp, plastic tunnel, or greenhouse structure) should be considered in
the design of the biopile. In addition, rainfall runon and runoff from the
biopile area should be controlled using berms at the perimeter of the
biopile. A leachate collection system at the bottom of the biopile and a
leachate treatment system may also be necessary to prevent
groundwater contamination from the biopile.
IV-16 October 1994
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Wind
Erosion of the biopile soils can occur during windy periods. Wind
erosion can be limited by applying moisture periodically to the surface of
the biopile or by enclosing or covering the biopile.
Biotreatability Evaluation
Biotreatability studies are especially desirable if toxicity is a concern
or natural soil conditions are not conducive to biological activity.
Biotreatability studies are usually performed in the laboratory and
should be planned so that, if successful, the proper parameters are
developed to design and implement the biopile system. If biotreatability
studies do not demonstrate effectiveness, field trials or pilot studies will
be needed prior to implementation, or another remedial approach should
be evaluated. If the soil, constituents, and climatic characteristics are
within the range of effectiveness for biopiles, review biotreatability
studies to confirm that biopiles have the potential for effectiveness and
to verify that the parameters needed to design the full-scale biopile
system have been obtained. Biotreatability studies should provide data
on contaminant biodegradability, ability of indigenous microorganisms to
degrade contaminants, optimal microbial growth conditions and
biodegradation rates, and sufficiency of natural nutrients and minerals.
There are two types of biotreatability studies generally used to
demonstrate biopile effectiveness: (1) Flask Studies and (2) Pan Studies.
Both types of studies begin with the characterization of the baseline
physical and chemical properties of the soils to be treated in the biopile.
Typical physical and chemical analyses performed on site soil samples
for biotreatability studies are listed on Exhibit IV-12. The specific
objectives of these analyses are to:
O Determine the types and concentrations of contaminants in the soils
that will be used in the biotreatability studies.
O Assess the initial concentrations of constituents present in the study
samples so that reductions in concentration can be evaluated.
O Determine if nutrients (nitrogen and phosphorus) are present in
sufficient concentrations to support enhanced levels of bacterial
activity.
O Evaluate parameters that may inhibit bacterial growth (e.g., toxic
concentrations of metals, pH values lower than 6 or higher than 8).
October 1994 IV-17
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Exhibit IV-12
Physical And Chemical Parameters For Biotreatability Studies
Parameter
Soil toxicity
Soil texture
Nutrients
Contaminant biodegradability
Measured Properties
Type and concentration of contaminant
and/or metals present, pH.
Grain size, clay content, moisture content,
porosity, permeability, bulk density.
Nitrate, phosphate, other anions and cations.
Total organic carbon concentration, volatility,
chemical structure.
After you have characterized the soil samples, perform bench studies
to evaluate biodegradation effectiveness. Flask (or bottle) studies which
are simple and inexpensive, are used to test for biodegradation in water
or soils using soil/water slurry microcosms. Flask studies may use a
single slurry microcosm that is sampled numerous times or may have a
series of slurry microcosms, each sampled once. Flask studies are less
desirable than pan studies for evaluation of biopile effectiveness and are
primarily used for evaluation of water-phase bioremedial technologies.
Pan studies use soils, without dilution in an aqueous slurry, placed in
steel or glass pans as microcosms that more closely resemble biopiles.
In either pan or flask studies, degradation is measured by tracking
constituent concentration reduction and changes in bacterial population
and other parameters over time. A typical treatment evaluation using
pan or flask studies may include the following types of studies.
O JVo Treatment Control Studies measure the rate at which the existing
bacteria can degrade constituents under oxygenated conditions
without the addition of supplemental nutrients.
O Nutrient Adjusted Studies determine the optimum adjusted C:N:P ratio
to achieve maximum degradation rates using microcosms prepared
with different concentrations of nutrients.
O Inoculated Studies are performed if bacterial plate counts indicate that
natural microbial activity is insufficient to promote sufficient
degradation. Microcosms are inoculated with bacteria known to
degrade the constituents at the site and are analyzed to determine if
degradation can be increased by inoculation.
IV-18
October 1994
-------
O Sterile Control Studies measure the degradation rate due to abiotic
processes (including volatilization) as a baseline comparison with the
other studies that examine biological processes. Microcosm soils are
sterilized to eliminate bacterial activity. Abiotic degradation rates are
then measured over time.
Review the CAP to determine that biotreatability studies have been
completed, biodegradation is demonstrated, nutrient application and
formulation have been evaluated and defined, and potential inhibitors or
toxic conditions have been identified.
Evaluation Of The Biopile Design
Once you have verified that biopiles have the potential to be effective,
you can evaluate the design of the biopile system. The CAP should
include a discussion of the rationale for the design and present the
conceptual engineering design. Detailed engineering design documents
might also be included, depending on state requirements. Further detail
about information to look for in the discussion of the design is provided
below.
O Land Requirements can be determined by dividing the amount of soil
to be treated by the height of the proposed biopile(s). The typical
height of biopiles varies between 3 and 10 feet. Additional land area
around the biopile(s) will be required for sloping the sides of the pile,
for containment berms, and for access. The length and width of
biopiles is generally not restricted unless aeration is to occur by
manually turning the soils. In general, biopiles which wiH be turned
should not exceed 6 to 8 feet in width.
O Biopile Layout is usually determined by the configuration of and
access to the land available for the biopile(s). The biopile system can
include single or multiple piles.
O Biopile Construction includes: site preparation (grubbing, clearing, and
grading); berms; liners and covers(if necessary); air injection,
extraction and/or collection piping arrangement; nutrient and
moisture injection piping arrangement; leachate collection and
treatment systems; soil pretreatment methods (e.g., shredding,
blending, amendments for fluffing, pH control); and enclosures and
appropriate vapor treatment facilities (where needed). The
construction design of a typical biopile is shown as Exhibit IV-13.
O Aeration Equipment usually includes blowers or fans which will be
attached to the aeration piping manifold unless aeration is to be
accomplished by manually turning the soil.
October 1994 IV-19
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Exhibit IV-13
Construction Design Of A Typical Biopile
Timber Frame
2:1 Sloped Sides
Contaminated Soil
4
I I
I I
I I
I I
I I
\L
\l \
I I I
I I I
I I I
I I I
I I I
I I I
I I I
:*
9
g
-Pipe Spacing Depends on
Soil Permeability (5-20 Feet)
^—Pipii
Piping Manifold for Air Injection/
Extraction or Nutrient Addition
(Only 1 Manifold Shown for Clarity)
PLAN VIEW
NOT TO SCALE
Cover (Optional)
2:1 Slope
Air Injection and/or
Extraction Piping
Timber Frame for Liner/
Cover Attachment and
Containment Berm
Contaminated Soil (3—10 Feet)
Soil Vapor
Monitoring Probes
I /—Air Inlet/Exhaust
m i—Nutrient and Moisture
rf / Addition (Drip Irrigation)
Leachate Treatment
(optional)-
FTl
-Leachate Runoff Trench
(Optional)
Sand Layer (3-6 inches, Sloped)
For Leachate Collection (Optional)
Leachate Collection
System Piping (optional)
CROSS SECTION
NOT TO SCALE
IV-2O
October 1994
-------
O Water Management systems for control of runon and runoff are neces-
sary to avoid saturation of the treatment area or washout of the soils
in the biopile area. Runon is usually controlled by earthen berms or
ditches that intercept and divert the flow of stormwater. Runoff can be
controlled by diversion within the bermed treatment area to a reten-
tion pond where the runoff can be stored, treated, or released under a
National Pollution Discharge Elimination System (NPDES) permit.
O Soil Erosion. Control from wind or water generally includes sloping the ,
sides of the pile, covering the pile, constructing water management
systems, and spraying to minimize dust.
O pH Adjustment, Moisture Addition, and. Nutrient Supply methods
usually include incorporation of solid fertilizers, lime and/or sulfur
into the soils while constructing the biopile, or injection of liquid
nutrients, water and acid/alkaline solutions preferably through a
dedicated piping system during operation of the biopile. The
composition of nutrients and acid or alkaline solutions/solids for pH
control is developed in biotreatability studies, and the frequency of
their application is modified during biopile operation as needed.
O Site Security may be necessary to keep trespassers out of the
treatment area. If the biopile is accessible to the public, a fence or
other means of security is recommended to deter public contact with
the contaminated material within the biopile area.
O Air Emission Controls (e.g., covers or structural enclosures) may be
required if volatile constituents are present in the biopile soils. For
compliance with air quality regulations, the volatile organic emissions
should be estimated based on initial concentrations of the petroleum
constituents present. Vapors in extracted or injected air should be
monitored during the initial phases of biopile operation for compliance
with appropriate permits or regulatory limits on atmospheric
discharges. If required, appropriate vapor treatment technology
should be specified, including operation and monitoring parameters.
Evaluation Of Operation And Remedial
Progress Monitoring Plans
It is important to make sure that system operation and monitoring
plans have been developed for the biopile operation. Regular monitoring
is necessary to ensure optimization of biodegradation rates, to track
constituent concentration reductions, and to monitor vapor emissions,
migration of constituents into soils beneath the biopile (if unlined), and
groundwater quality. If appropriate, ensure that monitoring to determine
compliance with stormwater discharge or air quality permits is also
proposed.
October 1994 IV-21
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Operations Plan
Make certain that the plan for operating the biopile system described
in the CAP includes the anticipated frequency of aeration, nutrient
addition, and moisture addition. The plan should be flexible and
modified based on the results of regular monitoring of the biopile soils.
The plan should also account for seasonal variations in ambient
temperature and rainfall. In general, aeration and moisture and nutrient
applications should be more frequent in the warmer, drier months. If the
biopile is covered with impervious sheeting (e.g., plastic or geofabric/
geotextile), the condition of the cover must be checked periodically to
ensure that it remains in place and that it is free of rips, tears, or other
holes. Provision should be made for replacement of the cover in the event
that its condition deteriorates to the point where it is no longer effective.
Remedial Progress Monitoring Plan
Make certain that the monitoring plan for the biopile system is
described in detail and include monitoring of biopile soils for constituent
reduction and biodegradation conditions (e.g., CO2, O2, CH4, H2S), air
monitoring for vapor emissions if volatile constituents are present, soil
and groundwater monitoring to detect potential migration of constituents
beyond the biopile area, and runoff water sampling (if applicable) for
discharge permits. Make sure that the number of samples collected,
sampling locations, and collection methods are in accordance with state
regulations. A monitoring plan for a typical biopile operation is shown in
Exhibit IV-14.
Soils within the biopile should be monitored at least quarterly during
treatment to determine pH, moisture content, bacterial population,
nutrient content, and constituent concentrations. For biopiles using air
extraction or for those using air injection and off-gas collection,
biodegradation conditions can be tracked by measuring oxygen and
carbon dioxide concentrations in the vapor extracted from the biopile.
These measurements should be taken weekly during the first 3 months
of operation. The results of these analyses, which may be done using
electronic instruments, field test kits, or in a field laboratory are critical
to the optimal operation of the biopile. The results should be used to
adjust air injection or extraction flow rates, nutrient application rates,
moisture addition frequency and quantity, and pH. Optimal ranges for
these parameters should be maintained to achieve maximum
degradation rates.
IV-22 October 1994
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o
I
A
Exhibit IV-14
Typical Remedial Progress Monitoring Plan For Biopiles
1
Medium To Be Monitored
Soil in the biopile
Air extracted or collected from
the biopile
Air
Runoff water
Soil beneath the biopile
Groundwater downgradient of
biopile
Purpose
Sampling Frequency
Determine constituent degradation
and biodegradation conditions.
Determine constituent degradation
and biodegradation conditions.
Monthly to quarterly during the
operation.
Weekly during the first 3 months then
monthly or quarterly.
Parameters To Be Analyzed
Bacterial population, constituent
concentrations, pH, ammonia,
phosphorus, moisture content, other
rate limiting conditions.
C02,02, CH4, H2S, VOCs.
Site personnel and population health Twice during the first two weeks of Volatile constituents, particulates.
hazards.
operation, quarterly thereafter or to
meet air quality requirements.
Soluble or suspended constituents. As required for NPDES permit.
Migration of constituents.
Migration of soluble constituents.
Quarterly or twice per biopile season.
Once per biopile season (annually).
As specified for NPDES permit; also
hazardous organics.
Hazardous constituents.
Hazardous, soluble constituents.
-------
References
Alexander, M., Biodegradation and Bioremediation. San Diego, CA:
Academic Press, 1994.
Fan, C.Y. and A.N. Tafuri. "Engineering Application of Biooxidation
Processes for Treating Petroleum-Contaminated Soil," in D.L. Wise
and D.J. Trantolo, eds. Remediation, of Hazardous Waste
Contaminated Soils. New York, NY: Marcel Dekker, Inc., pp. 373-401,
1994.
Flathman, P.E. and D.E. Jerger. Bioremediation Field Experience. Boca
Raton, FL: CRC Press, 1993.
Freeman, H.M. Standard Handbook of Hazardous Waste Treatment and
Disposal New York, NY: McGraw-Hill Book Company, 1989.
Grasso, D. Hazardous Waste Site Remediation, Source Control Boca
Raton, FL: CRC Press, 1993.
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. Handbook of
Bioremediation. Boca Raton, FL:CRC Press, 1994.
Norris, R.D., Hinchee, RE., 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. In-Situ Bioremediation of
Ground Water and Geological Material A Review of Technologies. Ada,
OK: U.S. Environmental Protection Agency, Office of Research and
Development. EPA/5R-93/124, 1993.
Pope, Daniel F., and J.E. Matthews. Environmental Regulations and
Technology: Bioremediation Using the Land Treatment Concept. Ada,
OK: U.S. Environmental Protection Agency, Environmental Research
Laboratory. EPA/600/R-93/164, 1993.
IV-24 October 1994
-------
Checklist: Can Biopiies Be Used At This Site?
This checklist can help you to evaluate to completeness of the CAP
and to identify areas that require closer scrutiny. As you go through the
CAP, answer the following questions. If the answer to several questions
is no and biotreatability studies demonstrate marginal to ineffective
results, request additional information to determine if biopiles will
accomplish cleanup goals at the site.
1. Soil Characteristics That Contribute To Biopile Effectiveness
Yes No
Q Q Is the total heterotrophic bacteria count > 1,000 CFU/gram
dry soil?
Q Q Is the soil pH between 6 and 8?
Q Q Is the soil moisture between 40% and 85%?
Q Q Is the soil temperature between 10°C and 45°C?
Q Q Is the carbon:nitrogen:phosphorous ratio between 100:10:1
' and 100:1:0.5?
Q Q Does the soil divide easily and tend not to clump together?
2. Constituent Characteristics That Contribute To Biopile
Effectiveness
Yes No
Q Q Are products to be treated primarily kerosene or heavier (i.e.,
not gasoline), or will air emissions be monitored and, if
necessary, controlled?
Q Q Are most of the constituents readily degradable?
Q Q Are total petroleum constituents < 50,000 ppm and total
heavy metals <. 2,500 ppm?
3. Climatic Conditions That Contribute To Biopile Effectiveness
Yes No
Q Q Is the rainfall less than 30 inches during the biopile season?
Q Q Are high winds unlikely?
October 1994 IV-25
-------
4. Biotreatability Evaluation
Yes No
Q Q Has a biotreatability study been conducted?
Q Q Was biodegradation demonstrated, nutrient application and
formulation defined, and potential inhibitors or toxic
conditions checked?
5. Evaluation Of Biopile Design
Yes No
Q Q Is sufficient land available considering the biopile depth and
additional space for berms and access?
Q Q Is runon and runoff controlled?
Q Q Are erosion control measures specified?
Q Q Are the frequency of application and composition of
nutrients and pH adjustment materials specified?
Q Q Is moisture addition needed?
Q Q Are other sub-optimal natural site conditions addressed in
the biopile design (e.g., low temperatures, poor soil texture,
and excessive rainfall)?
Q Q Is the site secured?
Q Q Are air emissions estimated and will air emissions
monitoring be conducted?
Q Q Are provisions included for air emissions controls, if needed?
6. Operation And Monitoring Plans
Yes No
Q Q Are frequencies of aeration, nutrient addition, and moisture
addition provided in the operation plan?
Q Q Is monitoring for constituent reduction and biodegradation
conditions proposed?
IV-26 October 1994
-------
6. Operation And Monitoring Plans (continued)
Yes No
Q Q Are air, soil, and surface runoff water sampling (if applicable)
proposed to ensure compliance with appropriate permits?
Q Q Are the proposed number of samples to be collected,
sampling locations, and collection methods in accordance
with state regulations?
Q Q Is quarterly (or more frequent) monitoring for soil pH,
moisture content, bacterial population, nutrient content, and
constituent concentrations proposed?
October 1994 IV-27
-------
-------
Chapter V
Landfarming
-------
-------
Contents
Overview • • V-l
Evaluation Of Landfarming Effectiveness V-7
Soil Characteristics - V-8
Microbial Population Density V-8
Soil pH V-9
Moisture Content V-10
Soil Temperature V-10
Nutrient Concentrations - - • V-ll
Soil Texture V-12
Constituent Characteristics V-13
Volatility V-13
Chemical Structure V-13
Concentration And Toxiciry V-14
Climatic Conditions V-15
Ambient Temperature V-15
Rainfall V-16
Wind V-16
Biotreatabiliry Evaluation V-17
Evaluation Of The Landfarm Design V-19
Evaluation Of Operation And Remedial Progress Monitoring Plans . V-21
Operations Plan V-21
Remedial Progress Monitoring Plan V-22
References - • • V-24
Checklist: Can Landfarming Be Used At This Site? V-25
October 1994
-------
List Of Exhibits
Number
V-l
V-2
V-3
V-4
V-5
V-6
V-7
V-8
V-9
V-10
V-ll
V-12
V-13
V-14
Title Page
Typical Landfarming Operation V-2
Advantages And Disadvantages Of Landfarming V-3
Landfarming Evaluation Process Flow Chart V-4
Parameters Used To Evaluate The
Effectiveness Of Landfarming V-7
Heterotrophic Bacteria And Landfarming
Effectiveness
V-9
Soil pH And Landfarming Effectiveness V-9
Soil Moisture And Landfarming Effectiveness V-10
Soil Temperature And Landfarming Effectiveness V-ll
Chemical Structure And Biodegradability V-14
Constituent Concentration And
Landfarming Effectiveness V-15
Cleanup Requirements And
Landfarming Effectiveness V-16
Physical And Chemical Parameters For
Biotreatabiliiy Studies V-18
Construction Design Of A Typical Landfarm V-20
Typical Remedial Progress Monitoring
Plan For Landfarming V-23
V-iv
October 1994
-------
Chapter V
Landfarming
Overview
Landfarming, also known as land treatment or land application, is an
above-ground remediation technology for soils that reduces
concentrations of petroleum constituents through biodegradation. This
technology usually involves spreading excavated contaminated soils in a
thin layer on the ground surface and stimulating aerobic microbial
activity within the soils through aeration and/or the addition of
minerals, nutrients, and moisture. The enhanced microbial activity
results in degradation of adsorbed petroleum product constituents
through microbial respiration. If contaminated soils are shallow (i.e., <_ 3
feet below ground surface), it may be possible to effectively stimulate
microbial activity without excavating the soils. If petroleum-
contaminated soil is deeper than 5 feet, the soils should be excavated
and reapplied on the ground surface. A typical landfarming operation is
shown in Exhibit V-1.
Landfarming has been proven effective in reducing concentrations of
nearly all the constituents of petroleum products typically found at
underground storage tank (UST) sites. Lighter (more volatile) petroleum
products (e.g., gasoline) tend to be removed by evaporation during
landfarm aeration processes (i.e., tilling or plowing) and, to a lesser
extent, degraded by microbial respiration. Depending upon your state's
regulations for air emissions of volatile organic compounds (VOCs), you
may need to control the VOC emissions. Control involves capturing the
vapors before they are emitted to the atmosphere, passing them through
an appropriate treatment process, and then venting them to the
atmosphere. The mid-range hydrocarbon products (e.g., diesel fuel,
kerosene) contain lower percentages of lighter (more volatile)
constituents than does gasoline. Biodegradation of these petroleum
products is more significant than evaporation. Heavier (non-volatile)
petroleum products (e.g., heating oil, lubricating oils) do not evaporate
during landfarm aeration; the dominant mechanism that breaks down
these petroleum products is biodegradation. However, higher molecular
weight petroleum constituents such as those found in heating and
lubricating oils, and, to a lesser extent, in diesel fuel and kerosene,
require a longer period of time to degrade than do the constituents in
gasoline. A summary of the advantages and disadvantages of
landfarming is shown in Exhibit V-2.
The policies and regulations of your state determine whether
landfarming is allowed as a treatment option. Before reading this
chapter, consider whether your state allows the use of this remedial
option.
October 1994 V-l
-------
Exhibit V-1
Typical Landfarming Operation
Porous Cup Lysimetars
Tilling for Soil Aeration
Leachate Collection
and Treatment
(Optional) •
Groundwater
Monitoring Wells
-------
Exhibit V-2
Advantages And Disadvantages Of Landfarming
Advantages
o Relatively simple to design and
implement.
o Short treatment times: usually 6 months
to 2 years under optimal conditions.
o Cost competitive: $30-60/ton of
contaminated soil.
o Effective on organic constituents with
slow biodegradation rates.
Disadvantages
o Concentration reductions > 95% and
constituent concentrations < 0.1 ppm are
very difficult to achieve.
o May not be effective for high constituent
concentrations (> 50,000 ppm total
petroleum hydrocarbons).
o Presence of significant heavy metal
concentrations (> 2,500 ppm) may inhibit
microbial growth'.
o Volatile constituents tend to evaporate
rather than biodegrade during treatment.
o Requires a large land area for treatment.
o Dust and vapor generation during
landfarm aeration may pose air quality
concerns.
o May require bottom liner if leaching from
the landfarm is a concern.
This chapter will assist you in evaluating a corrective action plan
(CAP) that proposes landfarming as a remedy for petroleum
contaminated soil. The evaluation guidance is presented in the three
steps described below. The evaluation process, which is summarized in a
flow diagram shown in Exhibit V-3, will serve as a roadmap for the
decisions you will make during your evaluation. A checklist has also
been provided at the end of this chapter to be used as a tool to evaluate
the completeness of the CAP and to help you focus on areas where
additional information may be needed. The evaluation process can be
divided into the following steps.
O Step 1: An evaluation of landfarming effectiveness, in which you
can identify the soil, constituent, and climatic factors that contribute
to the effectiveness of landfarming and compare them to acceptable
operating ranges. To complete the evaluation, you will need to
compare these properties to ranges where landfarming is effective.
October 1994
V-3
-------
Exhibit V-3
Landfarming Evaluation Process Flow Chart
EVALUATION OF
LANDFARMING EFFECTIVENESS
Identify soil characteristics important
to landfarming effectiveness
MicrobtiJ Population Density Sou Temperature
Soil pH Nutrient Concentrations
Moistim Content Soil Texture
Are
background
heterotrophic bacteria
> 1000 CPU/gram?
BlotreatabHity studies
should Include special
studies to evaluate
out-of-range parameters.
Is soil
pH between
6 and 8?
Are
soils free of
clays that could cause
clumping and poor
aeration?
Do
biotreatability
studies demonstrate
landfarming
effectiveness?
Is moisture
content of soils in
contaminated area between
40% and 85% of field
capacity?
Landfarming will
not be effective
at the site.
Consider other
technologies.
Landtarm design and
operation should include
considerations to adjust
out-offange parameters.
Is soil
temperature between
10° C and 46° C during
treatment?
Thermal
Desorption
landfarm design
Do nutrient
concentrations have
a C:N:P ratio between
100:10:1 and
100:1:0.5?
and operation account
for out-of-range
Continue with evaluation
of landfarming design.
V-4
October 1994
-------
Exhibit V-3
Landfarming Evaluation Process Flow Chart
EVALUATION OF
LANDFARMING EFFECTIVENESS
Identify constituent characteristics
| important to landfarming effectiveness
Volatility
Chemical Structure
Concentration and Toxictty
Is
gasoline or
other highly volatile p
proposed for
Are
constituents all suffici
biodegradable?
IsTPH
< 50,000 ppm and
metals < 2,500
ppm?
^ NO
NO
IBiotreatal
should Im
out-of-rang
,
Mlty studies
dude special
to evaluate
e parameters.
i
Do
biotreatability
studies demonstrate
landfarming
effectiveness?
Identify donate conditions important
to landfarming effectiveness
Ambient Temperature
Rainfall
Wind
Landfarming will
not be effective
at the site.
Consider other
technologies.
Are ambient
temperatures between
10° C and 46° C for at least
4 months a year?
Landfamt design and
operation should Include
considerations to adjust
out-offange parameters.
• Thermal
Desorption
Is annual
precipitation less than
30 inches?
Do
landfarming
design and operation
account for out-of-range
parameters?
Is the site
subject to only light or
infrequent winds?
The landfarming system is
likely to be effective at the site.
Proceed to evaluate the desig
October 1994
V-5
-------
Exhibit V-3
Landfarming Evaluation Process Flow CJiart
EVALUATION OF
LANDFARMING DESIGN
EVALUATION OF
i LANDFARMING OPERATION
& MONITORING PLANS
Determine the design elements
• Land Requirements
• Landfarm Layout
• Landfarm Construction
• Aeration Equipment
• Water Management
• Soil Erosion Control
• pH Adjustment
• Moisture Addition
• Nutrient Supply
• Site Security
• Air Emission Controls
Havethe
design elements been
identified and are they
appropriate?
The Landfarm
design is
ncornptete.
Request
additional
information.
The Landfarm design
is complete. Proceed
to O&M evaluation.
V-6
Review the 0 & M plans
for the proposed landfarm
for the following:
• Operations Plan
• Remedial Progress
Monitoring Plan
Are
operations procedures
described, and are their
scope & frequency
adequate?
Request
additional
information on
operations
procedures.
Is a
monitoring
plan described; is it of
adequate scope & frequency;
does it include
discharge permit
monitoring?
Request
additional
information on
monitoring
plans.
The Landfarm system
is likely to be effective.
The design and O&M
plans are complete.
October 1994
-------
O Step 2: An evaluation of the landfarming system design will allow
you to determine if the rationale for the design has been appropriately
defined, whether the necessary design components have been
specified, and whether the construction designs are consistent with
standard practice.
O Step 3: An evaluation of the operation and monitoring plans,
which are critical to the effectiveness of landfarming, will allow you to
determine whether start-up and long-term system operation and
monitoring plans are of sufficient scope and frequency.
Evaluation Of Landfarming Effectiveness
The effectiveness of landfarming depends on many parameters which
are listed in Exhibit V-4. The parameters are grouped into three
categories: soil characteristics, constituent characteristics, and climatic
conditions.
Exhibit V-4
Parameters Used To Evaluate The Effectiveness Of Landfarming
Soil Characteristics Constituent Characteristics Climatic Conditions
Microbial population density Volatility Ambient temperature
Soil pH Chemical structure Rainfall
Moisture content Concentration and toxicity Wind
Soil temperature
Nutrient concentrations
Texture
The following paragraphs contain descriptions of each parameter that
include: why it is important; how it can be determined; and what its
appropriate range is. During your evaluation, remember that because
landfarming is an above-ground treatment technique, most parameters
(except climatic conditions) can be controlled during the design and
operation of the landfarm. Therefore, during your evaluation, identify
those parameters that fall outside the effectiveness ranges provided and
verify that the system design and proposed operating specifications
compensate for any site conditions that are less than optimal.
October 1994 V-7
-------
Soil Characteristics
Microbial Population Density
Soil normally contains large numbers of diverse microorganisms
including bacteria, algae, fungi, protozoa, and actinomycetes. In well-
drained soils, which are most appropriate for landfarming, these
organisms are generally aerobic. Of these organisms, bacteria are the
most numerous and biochemically active group, particularly at low
oxygen levels. Bacteria require a carbon source for cell growth and an
energy source to sustain metabolic functions required for growth.
Bacteria also require nitrogen and phosphorus for cell growth. Although
sufficient types and quantities of microorganisms are usually present in
the soil, recent applications of ex-situ soil treatment include blending the
soil with cultured microorganisms or animal manure (typically from
chickens or cows). Incorporating manure serves to both augment the
microbial population and provide additional nutrients.
The metabolic process used by bacteria to produce energy requires a
terminal electron acceptor (TEA) to enzymatically oxidize the carbon
source to carbon dioxide. Microbes are classified by the carbon and TEA
sources they use to carry out metabolic processes. Bacteria that use
organic compounds (e.g., petroleum constituents and other naturally
occurring organics) as their source of carbon are heterotrophic; those
that use inorganic carbon compounds (e.g., carbon dioxide) are
oufotrpphic. Bacteria that use oxygen as their TEA are aerobic; those that
use a compound other than oxygen, (e.g., nitrate, sulfate), are anaerobic;
and those that can utilize both oxygen and other compounds as TEAs
are facultative. For landfarming applications directed at petroleum
products, only bacteria that are both aerobic (or facultative) and
heterotrophic are important in the degradation process.
In order to evaluate the presence and population of naturally
occurring bacteria that will contribute to degradation of petroleum
constituents, conduct laboratory analyses of soil samples from the site.
These analyses, at a minimum, should include plate counts for total
heterotrophic bacteria. Plate count results are normally reported in
terms of colony-forming units (CFUs) per gram of soil. Microbial
population densities in typical soils range from 104 to 107 CFU/gram of
soil. For landfarming to be effective, the minimum heterotrophic plate
count should be ICr CFU/gram or greater. Plate counts lower than 103
could indicate the presence of toxic concentrations of organic or
inorganic (e.g., metals) compounds. In this situation, landfarming may
still be effective if the soil is conditioned or amended to reduce the toxic
concentrations and increase the microbial population density. More
elaborate laboratory tests are sometimes conducted to identify the
bacterial species present. This may be desirable if there is uncertainty
about whether or not microbes capable of degrading specific petroleum
V-8 October 1994
-------
hydrocarbons occur naturally in the soil. If insufficient numbers or types
of microorganisms are present, the population density may be increased
by introducing cultured microbes that are available from vendors.
Exhibit V-5 shows the relationship between plate counts of total
heterotrophic bacteria and the effectiveness of landfarming.
Exhibit V-5
Heterotrophic Bacteria And Landfarming Effectiveness
Total Heterotrophic Bacteria
(prior to landfarming) Landfarming Effectiveness
> 1000 CFU/gram dry soil Generally effective.
< 1000 CFU/gram dry soil May be effective; needs further evaluation to
determine if toxic conditions are present.
SoilpH
To support bacterial growth, the soil pH should be within the 6 to 8
range, with a value of about 7 (neutral) being optimal. Soils with pH
values outside this range prior to landfarming will require pH
adjustment prior to and during landfarming operations. Soil pH within
the landfarm can be raised through the addition of lime and lowered by
adding elemental sulfur. Exhibit V-6 summarizes the effect of soil pH on
landfarming effectiveness. Review the CAP to verify that soil pH
measurements have been made. If the soil pH is less than 6 or greater
than 8, make sure that pH adjustments, in the form of soil amendments,
are included in the design and operational plans for the landfarm.
Exhibit V-6
Soil pH And Landfarming Effectiveness
SoilpH
(prior to landfarming) Landfarming Effectiveness
6 < pH < 8 Generally effective.
6 > pH > 8 Landfarm soils will require amendments to
correct pH to effective range.
October 1994 V-9
-------
Moisture Content
Soil microorganisms require moisture for proper growth. Excessive
soil moisture, however, restricts the movement of air through the
subsurface thereby reducing the availability of oxygen which is also
necessary for aerobic bacterial metabolic processes. In general, the soil
should be moist but not wet or dripping wet. The ideal range for soil
moisture is between 40 and 85 percent of the water-holding capacity
(field capacity) of the soil or about 12 percent to 30 percent by weight.
Periodically, moisture must be added in landfarming operations because
soils become dry as a result of evaporation, which is increased during
aeration operations (i.e., tilling and/or plowing). Excessive accumulation
of moisture can occur at landfarms in areas with high precipitation or
poor drainage. These conditions should be considered in the landfarm
design. For example, an impervious cover can mitigate excessive
infiltration and potential erosion of the landfarm. Exhibit V-7 shows the
optimal range for soil moisture content.
Exhibit V-7
Soil Moisture And Landfarming Effectiveness
Soil Moisture Landfarming Effectiveness
40% < field capacity < 85% Effective.
Field capacity < 40% Periodic moisture addition is needed to
maintain proper bacterial growth.
Field capacity > 85% Landfarm design should include special
water drainage considerations.
Soil Temperature
Bacterial growth rate is a function of temperature. Soil microbial
activity has been shown to decrease significantly at temperatures below
10°C and to essentially cease below 5°C. The microbial activity of most
bacteria important to petroleum hydrocarbon biodegradation also
diminishes at temperatures greater than 45°C. Within the range of 10°C
to 45°C, the rate of microbial activity typically doubles for every 10°C
rise in temperature. Because soil temperature varies with ambient
temperatures, there will be certain periods during the year when
bacterial growth and, therefore, constituent degradation, will diminish.
When ambient temperatures return to the growth range, bacterial
activity will be gradually restored. The period of the year when the
ambient temperature is within the range for microbial activity is
commonly called the "landfarming season."
V-1O October 1994
-------
In colder parts of the United States, such as the Northeastern states,
the length of the landfarming season is shorter, typically ranging from
only 7 to 9 months. In very cold climates, special precautions can be
taken, including enclosing the landfarm within a greenhouse-type
structure or introducing special bacteria (psychrophiles), which are
capable of activity at lower temperatures. In warm regions, the
landfarming season can last all year. Exhibit V-8 shows how soil
temperature affects landfarming operation.
Exhibit V-8
Soil Temperature And Landfarming Effectiveness
Soil Temperature Landfarming Effectiveness
10°C < soil temperature < 45°C Effective.
10°C > soil temperature > 45°C Not generally effective; microbial activity
diminished during seasonal temperature
extremes but restored during periods within
the effective temperature range.
Temperature-controlled enclosures or special
bacteria required for areas with extreme
temperatures.
Nutrient Concentrations
Microorganisms require inorganic nutrients such as nitrogen and
phosphorus to support cell growth and sustain biodegradation
processes. Nutrients may be available in sufficient quantities in the site
soils but, more frequently, nutrients need to be added to landfarm soils
to maintain bacterial populations. However, excessive amounts of certain
nutrients (i.e., phosphate and sulfate) can repress microbial metabolism.
The typical carbon:nitrogen:phosphorus ratio necessary for
biodegradation falls in the range of 100:10:1 to 100:1:0.5, depending
upon the specific constituents and microorganisms involved in the
biodegradation process.
The naturally occurring available nitrogen and phosphorus content of
the soil should be determined by chemical analyses of samples collected
from the site. These types of analyses are routinely conducted in
agronomic laboratories that test soil fertility for farmers. These
concentrations can be compared to the nitrogen and phosphorus
requirements calculated from the stoichiometric ratios of the
biodegradation process. A conservative approximation of the amount of
nitrogen and phosphorus required for optimum degradation of petroleum
products can be calculated by assuming that the total mass of
hydrocarbon in the soil represents the mass of carbon available for
biodegradation. This simplifying assumption is valid because the carbon
October 1994 V-ll
-------
content of the petroleum hydrocarbons commonly encountered at UST
sites is approximately 90 percent carbon by weight.
As an example, assume that at a LUST site the volume of
contaminated soil is 90,000 ft3, the average TPH concentration in the
contaminated soil is 1,000 mg/kg, and the soil bulk density is 50 kg/ft3
(1.75 g/cm3).
The mass of contaminated soil is equal to the product of volume and
bulk density:
soil mass = 90,000 ft3 x 50 kg = 4.5 x 106 kg
ft3
The mass of the contaminant (and carbon) is equal to the product of
the mass of contaminated soil and the average TPH concentration in the
contaminated soil:
contaminant mass =
4.5 x 106 kg x 1,000 .E! = 4.5 x 103 kg = 10,000 Ibs
kg
Using the C:N:P ratio of 100:10:1, the required mass of nitrogen
would be 1,000 Ibs, and the required mass of phosphorus would be
100 Ibs. After converting these masses into concentration units
(56 mg/kg for nitrogen and 5.6 mg/kg for phosphorus), they can be
compared with the results of the soil analyses to determine if nutrient
addition is necessary. 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.
Soil Texture
Texture affects the permeability, moisture content, and bulk density
of the soil. To ensure that oxygen addition (by tilling or plowing),
nutrient distribution, and moisture content of the soils can be
maintained within effective ranges, you must consider the texture of the
soils. For example, soils which tend to clump together (such as clays) are
difficult to aerate and result in low oxygen concentrations. It is also
difficult to uniformly distribute nutrients throughout these soils. They
also retain water for extended periods following a precipitation event.
You should identify whether clayey soils are proposed for landfarming
at the site. Soil amendments (e.g., gypsum) and bulking materials (e.g.,
sawdust, or straw) should be blended into the soil as the landfarm is
being constructed to ensure that the landfarming medium has a loose or
divided texture. Clumpy soil may require shredding or other means of
pretreatment during landfarm construction to incorporate these
amendments.
V-12 October 1994
-------
Constituent Characteristics
Volatility
The volatility of contaminants proposed for treatment by landfarming
is important because volatile constituents tend to evaporate from the
landfarm, particularly during tilling or plowing operations, rather than
being biodegraded by bacteria. Constituent vapors emitted from a
landfarm will dissipate into the atmosphere unless the landfarm is
enclosed within a surface structure such as a greenhouse or plastic
tunnel or covered with a plastic sheet.
Petroleum products generally encountered at UST sites range from
those with a significant volatile fraction, such as gasoline, to those that
are primarily nonvolatile, such as heating and lubricating oils. Petroleum
products generally contain more than one hundred different constituents
that possess a wide range of volatility. In general, gasoline, kerosene,
and diesel fuels contain constituents with sufficient volatility to
evaporate from a landfarm. Depending upon state-specific regulations for
air emissions of volatile organic compounds (VOCs), control of VOC
emissions may be required. Control involves capturing vapors before
they are emitted to the atmosphere and then passing them through an
appropriate treatment process before being vented to the atmosphere.
Chemical Structure
The chemical structures of the contaminants present in the soils
proposed for treatment by landfarming are important in determining the
rate at which biodegradation will occur. Although nearly all constituents
in petroleum products typically found at UST sites are biodegradable,
the more complex the molecular structure of the constituent, the more
difficult, and less rapid, is biological treatment. 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 V-9 lists, in order
of decreasing rate of potential biodegradability, some common
constituents found at petroleum UST sites.
Evaluation of the chemical structure of the constituents proposed for
reduction by landfarming at the site will allow you to determine which
constituents will be the most difficult to degrade. You should verify that
remedial time estimates, biotreatability studies, field-pilot studies (if
applicable), and landfarm operation and monitoring plans are based on
the constituents that are most difficult to degrade (or "rate limiting") in
the biodegradation process.
October 1994 V-13
-------
Exhibit V-9
Chemical Structure And Biodegradability
Biodegradabilhy
More degradable
•
\
Less degradable
Example Constituents
n-butane, n-pentane,
n-oclane
Nonane
Methyl butane,
dimethylpentenes,
methyloctanes
Benzene, toluene,
ethylbenzene, xylenes
Propylbenzenes
Decanes
Dodecanes
Tridecanes
Tetradecanes
Naphthalenes
Fluoranthenes
Pyrenes
Acenaphthenes
Products In Which
Constituent Is Typically
Found
o Gasoline
o Diesel fuel
o Gasoline
o Gasoline
o Diesel, kerosene
o Diesel
o Kerosene
o Heating fuels
o Lubricating oils
o Diesel
o Kerosene
o Heating oil
o Lubricating oils
Concentration And Toxicfty
The presence of very high concentrations of petroleum organics or
heavy metals in site soils can be toxic or inhibit the growth and
reproduction of bacteria responsible for biodegradation in landfarms. In
addition, very low concentrations of organic material will also result in
diminished levels of bacteria activity.
In general, soil concentrations of total petroleum hydrocarbons (TPH)
in the range of 10,000 to 50,000 ppm, or heavy metals exceeding
2,500 ppm, are considered inhibitory and/or toxic to most
microorganisms. If TPH concentrations are greater than 10,000 ppm, or
the concentration of heavy metals is greater than 2,500 ppm, then the
contaminated soil should be thoroughly mixed with clean soil to dilute
the contaminants so that the average concentrations are below toxic
levels. Exhibit V-10 provides the general criteria for constituent
concentration and landfarming effectiveness.
V-14
October 1994
-------
Exhibit V-10
Constituent Concentration And Landfarming Effectiveness
Constituent Concentration Landfarming Effectiveness
Petroleum constituents < 50,000 ppm Effective; however, if contaminant
and concentration is > 10,000 ppm, the soil may
Heavy metals < 2,500 ppm need to be blended with clean soil to reduce
the concentration of the contaminants.
Petroleum constituents > 50,000 ppm Ineffective; toxic or inhibitory conditions to
or bacterial growth exist. Dilution by blending
Heavy metals > 2,500 ppm necessary.
In addition to maximum concentrations, you should consider the
cleanup goals proposed for the landfarm soils. Below a certain
"threshold" constituent concentration, the bacteria cannot obtain
sufficient carbon (from degradation of the constituents) to maintain
adequate biological activity. The threshold level can be determined from
laboratory studies and should be below the level required for cleanup.
Although the threshold limit varies greatly depending on bacteria-specific
and constituent-specific features, generally constituent concentrations
below 0.1 ppm are not achievable by biological treatment alone. In
addition, experience has shown that reductions in TPH concentrations
greater than 95 percent can be very difficult to achieve because of the
presence of "recalcitrant" or nondegradable species that are included in
the TPH analysis. If a cleanup level lower than 0.1 ppm is required for
any individual constituent or a reduction in TPH greater than 95 percent
is required to reach the cleanup level for TPH, either a pilot study is
required to demonstrate the ability of landfarming to achieve these
reductions at the site or another technology should be considered.
Exhibit V-l 1 shows the relationship between cleanup requirements and
landfarming effectiveness.
Climatic Conditions
Typical landfarms are uncovered and, therefore, exposed to climatic
factors including rainfall, snow, and wind, as well as ambient
temperatures.
Ambient Temperature
The ambient temperature is important because it influences soil tem-
perature. As described previously, the temperature of the soils in the
landfarm impacts bacterial activity and, consequently, biodegradation.
The optimal temperature range for landfarming is 10°C to 45°C. Special
considerations (e.g., heating, covering, or enclosing) can overcome the
effects of colder climates and extend the length of the landfarming
season.
October 1994 V-15
-------
Exhibit V-11
Cleanup Requirements And Landfarming Effectiveness
Cleanup Requirement
Landfarming Effectiveness
Constituent concentration > 0.1 ppm
and
TPH reduction < 95%
Constituent concentration £0.1 ppm
or
TPH reduction > 95%
Effective.
Potentially ineffective; pilot studies are
required to demonstrate contaminant
reduction.
Rainfall
Rainwater that falls directly onto, or runs onto; the landfarm area will
increase the moisture content of the soil and cause erosion. As
previously described, effective landfarming requires a proper range of
moisture content. During and following a significant precipitation event,
the moisture content of the soils may be temporarily in excess of that
required for effective bacterial activity. On the other hand, during
periods of drought, moisture content may be below the effective range
and additional moisture may need to be added.
If the site is located in an area subject to annual rainfall of greater
than 30 inches during the landfarming season, a rain shield (such as a
tarp, plastic tunnel, or greenhouse structure) should be considered in
the design of the landfarm. In addition, rainfall runon and runoff from
the landfarm should be controlled using berms at the perimeter of the
landfarm. A leachate collection system at the bottom of the landfarm and
a leachate treatment system may also be necessary to prevent
groundwater contamination from the landfarm.
Wind
Erosion of landfarm soils can occur during windy periods and
particularly during tilling or plowing operations. Wind erosion can be
limited by plowing soils into windrows and applying moisture
periodically.
V-16
October 1994
-------
Biotreatability Evaluation
Biotreatability studies are especially desirable if toxicity is a concern
or natural soil conditions are not conducive to biological activity.
Biotreatability studies are usually performed in the laboratory and
should be planned so that, if successful, the proper parameters are
developed to design and implement the landfarming approach. If
biotreatability studies do not demonstrate effectiveness, field trials or
pilot studies will be needed prior to implementation, or another remedial
approach should be evaluated. If the soil, constituents, and climatic
characteristics are within the range of effectiveness for landfarming,
review biotreatability studies to confirm that landfarming has the
potential for effectiveness and to verify that the parameters needed to
design the full-scale landfarm have been obtained. Biotreatability studies
should provide data on contaminant biodegradability, ability of
indigenous microorganisms to degrade contaminants, optimal microbial
growth conditions and biodegradation rates, and sufficiency of natural
nutrients and minerals.
There are two types of biotreatability studies generally used to
demonstrate landfarming effectiveness: (1) Flask Studies and (2) Pan
Studies. Both types of studies begin with the characterization of the
baseline physical and chemical properties of the soils to be treated in the
landfarm. Typical physical and chemical analyses performed on site soil
samples for biotreatability studies are listed on Exhibit V-12. The
specific objectives of these analyses are to:
O Determine the types and concentrations of contaminants in the soils
that will be used in the biotreatability studies.
O Assess the initial concentrations of constituents present in the study
samples so that reductions in concentration can be evaluated.
O Determine if nutrients (nitrogen and phosphorus) are present in
sufficient concentrations to support enhanced levels of bacterial
activity.
O Evaluate parameters that may inhibit bacterial growth (e.g., toxic
concentrations of metals, pH values lower than 6 or higher than 8).
After the characterization of the soil samples is complete, perform
bench studies to evaluate biodegradation effectiveness. Flask (or bottle)
studies, which are simple and inexpensive, are used to test for
biodegradation in water or soils using soil/water slurry microcosms.
Flask studies may use a single slurry microcosm that is sampled
numerous times or may have a series of slurry microcosms, each
sampled once. Flask studies are less desirable than pan studies for
evaluation of landfarming effectiveness and are primarily used for
evaluation of water-phase bioremedial technologies. Pan studies use
soils, without dilution in an aqueous slurry, placed in steel or glass pans
as microcosms that more closely resemble landfarming.
October 1994 V-17
-------
Exhibit V-12
Physical And Chemical Parameters For Biotreatability Studies
Parameter
Soil toxic'rty
Soil texture
Nutrients
Contaminant biodegradability
Measured Properties
Type and concentration of contaminant
and/or metals present, pH.
Grain size, clay content, moisture content,
porosity, permeability, bulk density.
Nitrate, phosphate, other anions and cations.
Total organic carbon concentration, volatility,
chemical structure.
In either pan or flask studies, degradation is measured by tracking
constituent concentration reduction and changes in bacterial population
and other parameters over time. A typical treatment evaluation using
pan or flask studies may include the following types of studies.
O No Treatment Control Studies measure the rate at which the existing
bacteria can degrade constituents under oxygenated conditions
without the addition of supplemental nutrients.
O Nutrient Adjusted Studies determine the optimum adjusted C:N:P ratio
to achieve maximum degradation rates using microcosms prepared
with different concentrations of nutrients.
O Inoculated Studies are performed if bacterial plate counts indicate that
natural microbial activity is insufficient to promote sufficient
degradation. Microcosms are inoculated with bacteria known to
degrade the constituents at the site and are analyzed to determine if
degradation can be increased by inoculation.
O Sterile Control Studies measure the degradation rate due to abiotic
processes (including volatilization) as a baseline comparison with the
other studies that examine biological processes. Microcosm soils are
sterilized to eliminate bacterial activity. Abiotic degradation rates are
then measured over time.
Review the CAP to determine that biotreatability studies have been
completed, biodegradation is demonstrated, nutrient application and
formulation have been evaluated and defined, and no potential inhibitors
or toxic conditions have been identified.
V-18
October 1994
-------
Evaluation Of The Landfarm Design
Once you have verified that landfarming has the potential for
effectiveness, you can evaluate the design of the landfarm. The CAP
should include a discussion of the rationale for the design and present
the conceptual engineering design. Detailed engineering design
documents might also be included, depending on state requirements.
Further detail about information to look for in the discussion of the
design is provided below.
O Land Requirements can be determined by dividing the amount of soil
to be treated by the depth of the landfarm soils. The depth of
landfarms can vary between 12 inches and 18 inches depending on
the capabilities of the tilling equipment to be used. Very powerful
tillers can reach as much as 24 inches deep to aerate landfarm soils.
Additional land area around the landfarm will be required for
containment berms and for access.
O Landfarm Layout is usually determined by the configuration of and
access to the land available for the landfarm. The landfarm can
include single or multiple plots.
O Landfarm. Construction includes: site preparation (grubbing, clearing
and grading); berms; liners (if necessary); leachate collection and
treatment systems; soil pretreatment methods (e.g., shredding,
blending and amendments for fluffing, pH control); and enclosures
and appropriate vapor treatment facilities (where needed). The
construction design of a typical landfarm is shown as Exhibit V-13.
O Aeration Equipment usually includes typical agricultural equipment
such as roto-tillers. The most favorable method is to use a disking
device towed behind a tractor so that aerated soils are not tamped by
the tractor tires.
O Water Management systems for control of runon and runoff are
necessary to avoid saturation of the treatment area or washout of the
soils in the landfarm. Runon is usually controlled by earthen berms
or ditches that intercept and divert the flow of stormwater. Runoff can
be controlled by diversion within the bermed treatment area to a
retention pond where the runoff can be stored, treated, or released
under a National Pollution Discharge Elimination System (NPDES)
permit.
O Soil Erosion Control from wind or water generally includes terracing
the soils into windrows, constructing water management systems, and
spraying to minimize dust.
October 1994 V-19
-------
Exhibit V-13
Construction Design Of A Typical Landfarm
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PLAN VIEW
NOT TO SCALE
-Porous Cup Lysimeters
Mr Emission Sampling
-Soil Cores
Contaminated Soil
•Windrows
Impermeable Liner
(optional)
-Groundwater
Monitoring Well
CROSS SECTION
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011-7
V-2O
October 1&94
-------
O pH Adjustment and Nutrient Supply methods usually include periodic
application of solid fertilizers, lime and/or sulfur while disking to
blend soils with the solid amendments, or applying liquid nutrients
using a sprayer. The composition of nutrients and acid or alkaline
solutions/solids for pH control is developed in biotreatability studies
and the frequency of their application is modified during landfarm
operation as needed.
O Site Security may be necessary to keep trespassers out of the
treatment area. If the landfarm is accessible to the public, a fence or
other means of security is recommended to deter public contact with
the contaminated material within the landfarm.
O Air Emission Controls (e.g., covers or structural enclosures) may be
required if volatile constituents are present in the landfarm soils. For
compliance with air quality regulations, the volatile organic emissions
should be estimated based on initial concentrations of the petroleum
constituents present. Vapors above the landfarm should be monitored
during the initial phases of landfarm operation for compliance with
appropriate permits or regulatory limits on atmospheric discharges. If
required, appropriate vapor treatment technology should be specified,
including operation and monitoring parameters.
Evaluation Of Operation And Remedial
Progress Monitoring Plans
It is important to make sure that system operation and monitoring
plans have been developed for the landfarming operation. Regular
monitoring is necessary to ensure optimization of biodegradation rates,
to track constituent concentration reductions, and to monitor vapor
emissions, migration of constituents into soils beneath the landfarm (if
unlined), and groundwater quality. If appropriate, ensure that
monitoring to determine compliance with stormwater discharge or air
quality permits is also proposed.
Operations Plan
Make certain that the plan for operating the landfann described in the
CAP includes the anticipated frequency of aeration, nutrient addition,
and moisture addition. The plan should be flexible and modified based
on the results of regular monitoring of the landfarm soils. The plan
should also account for seasonal variations in ambient temperature and
rainfall. In general, aeration and moisture and nutrient applications
should be more frequent in the warmer, drier months. If the landfarm is
covered with impervious sheeting (e.g., plastic or geofabric/textile), the
condition of the cover must be checked periodically to ensure that it
remains in place and that it is free of rips, tears, or other holes.
October 1994 V-21
-------
Provision should be made for replacement of the cover in the event that
its condition deteriorates to the point where it is no longer effective.
Particularly in the more northern states, operations may be suspended
altogether during the winter months.
Remedial Progress Monitoring Plan
Make certain that the monitoring plan for the landfarm is described in
detail and includes monitoring of landfarm soils for constituent
reduction and biodegradation conditions (e.g., CO2, O2, CH4, H2S), air
monitoring for vapor emissions if volatile constituents are present, soil
and groundwater monitoring to detect potential migration of constituents
beyond the landfarm, and runoff water sampling (if applicable) for
discharge permits. Make sure that the number of samples collected,
sampling locations, and collection methods are in accordance with state
regulations. A monitoring plan for a typical landfarm operation is shown
in Exhibit V-14.
Soils within the landfarm should be monitored at least quarterly
during the landfarming season to determine pH, moisture content,
bacterial population, nutrient content, and constituent concentrations.
The results of these analyses, which may be done using electronic
instruments, field test kits, or in a field laboratory are critical to the
optimal operation of the landfarm. The results should be used to adjust
aeration frequency, nutrient application rates, moisture addition
frequency and quantity, and pH. Optimal ranges for these parameters
should be maintained to achieve maximum degradation rates.
V-22 October 1994
-------
Exhibit V-14
Typical Remedial Progress Monitoring Plan For Landfarming
Medium To Be Monitored
Soil in the landfarm
Air
Runoff water
Soil beneath the landfarm
Purpose
Sampling Frequency
Determine constituent degradation
and biodegradation conditions.
Monthly to quarterly during the
landfarming season.
Site personnel and population health During first two aerations, quarterly
hazards. thereafter or to meet air quality
requirements.
Soluble or suspended constituents. As required for NPDES permit.
Migration of constituents.
Quarterly or twice per landfarming
season.
Qroundwater downgradient of Migration of soluble constituents. Once per landfarming season
landfarm (annually).
Parameters To Be Analyzed
Bacterial population, constituent
concentrations, pH, ammonia,
phosphorus, moisture content, other
rate limiting conditions.
Volatile constituents, particulates.
As specified for NPDES permit; also
hazardous organics.
Hazardous constituents.
Hazardous, soluble constituents.
2
09
-------
References
Alexander, M. Biodegradation and Bioremediation. San Diego, CA;
Academic Press, 1994.
Flathman, P.E. and D.E. Jerger. Bioremediation Field Experience. Boca
Raton, FL: CRC Press, 1993.
Freeman, H.M. Standard Handbook of Hazardous Waste Treatment and
Disposal New York, NY: McGraw-Hill Book Company, 1989.
Grasso, D. Hazardous Waste Site Remediation, Source Control Boca
Raton, FL: CRC Press, 1993.
Norris, R.D., Hinchee, R.E., Brown, RJL, McCarty, P.L., Semprini, L.,
Wilson, J.T., KampbeU, D.H., Reinhard, M., Bower, E.J., Borden, RC.,
Vogel, T.M., Thomas, J.M., and C.H. Ward. Handbook of
Bioremediation. Boca Raton, FLrCRC Press, 1994.
Norris, R.D., Hinchee, R.E., Brown, RJL, McCarty, P.L., Semprini, L.,
Wilson, J.T., KampbeU, D.H., Reinhard, M., Bower, E.J., Borden, R.C.,
Vogel, T.M., Thomas, J.M., and C.H. Ward. In-Situ Bioremediation of
Ground Water and Geological Material: A Review of Technologies. Ada,
OK: U.S. Environmental Protection Agency, Office of Research and
Development. EPA/5R-93/124, 1993.
Pope, Daniel F., and J.E. Matthews. Environmental Regulations and
Technology: Bioremediation Using the Land Treatment Concept Ada,
OK: U.S. Environmental Protection Agency, Environmental Research
Laboratory. EPA/600/R-93/164, 1993.
V-24
October 1994
-------
Checklist: Can Landfarming Be Used At This Site?
This checklist can help you to evaluate the completeness of the CAP
and to identify areas that require closer scrutiny. As you go through the
CAP, answer the following questions. If the answer to several questions
is no and biotreatability studies demonstrate marginal to ineffective
results, request additional information to determine if landfarming will
accomplish cleanup goals at the site.
1. Soil Characteristics That Contribute To Landfarming
Effectiveness
Yes No
Q Q Is the total heterotrophic bacteria count > 1,000 CFU/gram
dry soil?
Q Q Is the soil pH between 6 and 8?
Q Q Is the soil moisture between 40% and 85%?
Q Q Is the soil temperature between 10°C and 45°C?
Q Q Is the carbon:nitrogen:phosphorous ratio between 100:10:1
and 100:1:0.5?
Q Q Does the soil divide easily and tend not to clump together?
2. Constituent Characteristics That Contribute To Landfarming
Effectiveness
Yes No
Q Q Are products to be treated primarily kerosene or heavier (i.e.,
not gasoline), or will air emissions be monitored and, if
necessary, controlled?
Q Q Are most of the constituents readily degradable?
Q Q Are total petroleum constituents <, 50,000 ppm and total
heavy metals <. 2,500 ppm?
3. Climatic Conditions That Contribute To Landfarming
Effectiveness
Yes No
Q Q Is the rainfall less than 30 inches during the landfarming
season?
Q Q Are high winds unlikely?
October 1994 v'25
-------
4. Biotreatability Evaluation
Yes No
Q Q Has a biotreatability study been conducted?
Q Q Were biodegradation demonstrated, nutrient application and
formulation defined, and potential inhibitors or toxic
conditions checked?
5. Evaluation Of Landfarm Design
Yes No
Q Q Is sufficient land available considering the landfarm depth
and additional space for berms and access?
Q Q Are runon and runoff controlled?
Q Q Are erosion control measures specified?
Q Q Are the frequency of application and composition of
nutrients and pH adjustment materials specified?
Q Q Is moisture addition needed?
Q Q Are other sub-optimal natural site conditions addressed in
the landfarm design?
Q Q Is the site secured?
Q Q Are air emissions estimated and will air emissions
monitoring be conducted?
Q Q Are provisions included for air emissions controls, if needed?
6. Operation And Monitoring Plans
Yes No
Q Q Is monitoring for stormwater discharge or air quality permits
(if applicable) proposed?
Q Q Does the operation plan include the anticipated frequency of
aeration, nutrient addition, and moisture addition?
Q Q Does the monitoring plan propose measuring constituent
reduction and biodegradation conditions in the landfarm
soils?
V-26
October 1994
-------
6. Operation And Monitoring Plans (continued)
Yes No
Q Q Are air, soil, and surface runoff water sampling (if applicable)
proposed to ensure compliance with appropriate permits?
Q Q Are the proposed numbers of samples to be collected,
sampling locations, and collected methods in accordance
with state regulations?
Q Q Is quarterly (or more frequent) monitoring for soil pH,
moisture content, bacterial population, nutrient content, and
constituent concentrations proposed?
October 1994 V-27
-------
-------
Chapter VI
Low-Temperature Thermal Desorption
-------
-------
Contents
Evaluation Of The Applicability Of LTTD VI-7
Soil Characteristics VI-9
Soil Plasticity VI-9
Particle Size Distribution VI-10
Moisture Content VI-10
Heat Capacity VI-12
Concentration Of Humic Material VI-12
Metals Concentration VI-12
Bulk Density VI-13
Constituent Characteristics VI-13
Constituent Concentrations VI-13
Boiling Point Range ? VI-15
Vapor Pressure VI-15
Octanol/Water Partition Coefficient (K^J VI-16
Aqueous Solubility VI-16
Thermal Stability VI-16
Dioxin Formation VI-16
Process Operating Conditions VI-16
Types of Low-Temperature Thermal Desorption Systems VI-17
OffGas Treatment VI-21
Treatment Temperature VI-22
Residence Time VI-22
Pilot Testing VI-22
Determination Of The Practicality Of Using LTTD VI-23
Vertical And Horizontal Extent Of Contamination VI-23
Site Layout VI-25
Adjacent Land Use VI-25
Other Considerations VI-25
Evaluation Of The Effectiveness Of LTTD VT-26
References VI-28
Checklist: Can LTTD Be Used At This Site? VI-29
October 1994 Vl-iii
-------
List Of Exhibits
Number Title Page
VI-1 Parallel Flow (Co-Current) Rotary Low-Temperature
Thermal Desorption System VI-2
VI-2 Advantages And Disadvantages Of LTTD VI-3
VI-3 Low-Temperature Thermal Desorption
Process Flow Chart VI-4
VI-4 Recommended Soil Treatment Temperatures For
Selected Petroleum Products VI-8
VI-5 Key Soil And Constituent Characteristics That
Influence Applicability Of LTTD VI-9
VI-6 Energy Demand Versus Soil Moisture Content VI-11
VI-7 Feed Soil Moisture Content Limits VI-11
VI-8 Feed Soil TPH Concentration Limits VI-14
VI-9 Petroleum Product Boiling Ranges VI-15
VT-10 Thermal Desorption System Schematic Design VI-18
VI-11 Thermal Desorption Size Versus Amount Of
Soil To Be Treated VI-24
VI-12 Monitoring Recommendations ; VI-26
Vl-iv October 1994
-------
Chapter VI
Low-Temperature Thermal Desorption
Low-Temperature Thermal Desorption (LTTD), also known as low-
temperature thermal volatilization, thermal stripping, and soil roasting,
is an ex-sita remedial technology that uses heat to physically separate
petroleum hydrocarbons from excavated soils. Thermal desorbers are
designed to heat soils to temperatures sufficient to cause constituents to
volatilize and desorb (physically separate) from the soil. Although they
are not designed to decompose organic constituents, thermal desorbers
can, depending upon the specific organics present and the temperature
of the desorber system, cause some of the constituents to completely or
partially decompose. The vaporized hydrocarbons are generally treated in
a secondary treatment unit (e.g.f an afterburner, catalytic oxidation
chamber, condenser, or carbon adsorption unit) prior to discharge to the
atmosphere. Afterburners and oxidizers destroy the organic constituents.
Condensers and carbon adsorption units trap organic compounds for
subsequent treatment or disposal.
Some pre- and postprocessing of soil is necessary when using LTTD.
Excavated soils are first screened to remove large (> 2 inches in
diameter) objects. These may be sized (e.g., crushed or shredded) and
then introduced back into the feed material. After leaving the desorber,
soils are cooled, re-moistened to control dust, and stabilized (if
necessary) to prepare them for disposal/reuse. Treated soil may be
redeposited onsite, used as cover in landfills, or incorporated into
asphalt.
Thermal desorption systems fall into two general classes — stationary
facilities and mobile units. Contaminated soils are excavated and
transported to stationary facilities; mobile units can be operated directly
onsite. Desorption units are available in a variety of process
configurations including rotary desorbers, asphalt plant aggregate
dryers, thermal screws, and conveyor furnaces.
LTTD has proven very effective in reducing concentrations of
petroleum products including gasoline, jet fuels, kerosene, diesel fuel,
heating oils, and lubricating oils. LTTD is applicable to constituents that
are volatile at temperatures as great as 1,200°F. Exhibit VI-1 provides an
illustration of a typical LTTD operation. The advantages and
disadvantages of LTTD are listed in Exhibit VI-2.
October 1994 VI-1
-------
Exhibit VM
Parallel Flow (Co-Current) Rotary Low-Temperature Thermal Desorptlon System
Vapor Treatment
Unit (Incinerator)
Contaminated
Waste Input
Fuel
Emergency Relief
Heat Exchanger
Particulate Collector
(Baghouse)
Discharge
Gas
Decontaminated
Solid Waste
Rotary Cooler
& Moisturizer
-------
Exhibit VI-2
Advantages And Disadvantages Of LTTD
Advantages
Disadvantages
o Readily available equipment for onsite or
offsite treatment.
o Very rapid treatment time; most
commercial systems capable of over
25 tons per hour throughput.
o Cost competitive for large volumes
(> 1,000 yd3) of soils: $30-70/ton of
contaminated soil, exclusive of excavation
and transportation costs.
o Can be used to mitigate "hot spot" source
areas with very high concentrations of
petroleum hydrocarbons.
o Easily combinable with other
technologies, such as air sparging or
groundwater extraction.
o Treated soil can be redeposited onsite or
used for landfill cover (if permitted by a
regulatory agency).
o Can consistently reduce TPH to below
10 ppm and BTEX below 100 ppb (and
sometimes lower).
o Requires excavation of soils; generally
limited to 25 feet below land surface.
o Onsite treatment will require significant
area (> 1/a acre) to locate LTTD unit and
store process soils.
o Offsite treatment will require costly
transportation of soils and possibly
manifesting.
o Soils excavated from below the
groundwater table require dewatering
prior to treatment because of high
moisture content.
This chapter will assist you in evaluating a corrective action plan
(CAP) which proposes LTTD as a remedy for petroleum-contaminated
soil. It is not intended to serve as a guide for designing, operating,
monitoring, or permitting thermal desorption systems. Further, LTTD
processes generate additional waste streams (e.g., gaseous and/or liquid)
that require treatment and typically come under the authority of
different regulatory agencies. Desorption units are permitted by these
other agencies and must comply with monitoring and treatment
requirements that are beyond the purview of most UST programs. The
evaluation process is summarized in a flow diagram shown on
Exhibit VI-3 and will serve as a roadmap for the decisions you will make
during your evaluation. A checklist has also been provided at the end of
this chapter to be used as a tool to evaluate the completeness of the CAP
October 1994
VI-3
-------
Exhibit VI-3
Low-Temperature Thermal Desorption Process Flow Chart
EVALUATION OF THE
APPLICABILITY OF LTTD
Identify soil characteristics that
determine applicability of LTTD
SonPfasGclty
ftwffcte Sizes
Molsturo Content
Detennine whether constituent
parameters are within normal operating
ranges for proposed LTTD system
Constituent Concentrations
Boiling Point
Vapor Pressure
OctanoVWater Partition Coefficient (K^)
Aqueous Solubility
Thermal Stability
Dloxin Formation
Humic Material
Metals Concentration
Is
contaminated
soil highly
plastic?
Pretreatment of soil
is probably required.
Pretreatment may
involve shredding,
crushing, blending,
amending, and/or
drying.
Are
contamination
concentration, boiling
point, and vapor pressure
within acceptable
ranges?
Does
contaminated
soil contain large
particles?
Is
moisture
content between
10% and 25%?
Are
contaminant
(Kow), solubility, and
thermal stability within
acceptable
ranges?
high concentration of
humic material?
Pilot-test or
bum" may be
necessary to
demonstrate that
LTTD is applicable
for this site.
Are
dioxin precursors'
present in the
soil?
high concentration
of metals?
Do results
of pilot test indicate
that LTTD is
applicable?
LTTD is not
applicable.
Consider other
remedial
technologies:
• Landfanring
• Biopiles
• Bioventing
SVE
LTTD is an applicable
remedial technology
for the soils and
contaminants at this site.
Proceed to evaluate
if LTTD is practical.
VI-4
October 1994
-------
Exhibit VI-3
Low-Temperature Thermal Desorption Process Flow Chart
EVALUATION OF THE
PRACTICALITY OF USING LTTD
Excavation of contaminated
soil is practical.
Determine if soil can be
treated on-site or if it must be
transported off-site.
Determine if excavation of
contaminated soil is practical
Is depth
of contamination
> 25 feet?
Is
sufficient
area (> 1/2 acre) a
volume of contaminated
soil (> 300 yd3)
available?
Excavation of
contaminated
soil is not
practical.
Consider other
technologies:
Is the
lateral extent of
contamination outside
site boundaries
• Bioventing
• SVE
• Air Sparging
Biosparging
Is
distance
to off-site facility
> 200 miles?
Is
contamination
beneath buildings
or close to
foundations?
Can
current site
use accommodate
on-site
treatment?
surround)
land use permit
on-site
Off-site treatment
is a potential
option.
On-site treatment
Is not
On-site
treatment is
a potential
option.
LTTD is an applicable remedial
technology for the soils and
contaminants at this site.
Proceed to evaluate
whether LTTD is practical.
October 1994
VI-S
-------
Exhibit VI-3
Low-Temperature Triermal Desorption Process Flow Chart
EVALUATION OF THE
LTTD EFFECTIVENESS
Evaluate whether
sampling/monitoring
plans are adequate to
demonstrate effectiveness
Will
an adequate
number of soil samples
be collected and
analyzed?
Sampling
plans are not
adequate.
Request that
sampling plan
be improved.
Will an
adequate number
of treated soil samples
be collected and
analyzed?
Has
proposed
LTTD unit successfully
treated similar
soils?
Pilot-test or test
burn results are
necessary to
demonstrate LTTD
effectiveness.
ultimate disposal
Request
additional
information.
LTTD is likely to be an
effective remedial
technology for this site and
the proposed corrective
action plan is complete.
VI-6
October 1994
-------
and to help focus attention on areas where additional information may
be needed. The evaluation process is divided into the following three
steps:
O Step 1: An evaluation of the applicability ofLTTD. Factors that
influence the applicability of thermal desorption include physical and
chemical properties of the soil and constituents present at the site,
and the process operating conditions of the desorption system. To
complete the evaluation, you will need to verify that these properties
are within the range of LTTD effectiveness. Pre- and post-treatment of
the soil should be also be considered. If factors are outside the
demonstrated range of LTTD effectiveness, then pilot studies (e.g. test
bums) may be appropriate to verify that LTTD will be effective.
O Step 2: An evaluation of the practicality of using LTTD.
Determination of the practicality of using thermal desorption depends
upon site-specific factors such as volume of contaminated soil,
horizontal and vertical extent of contamination, site area, site usage
and surrounding land use. In addition, desorption process parameters
(e.g., soil processing rate, mobile vs stationary unit) and target
residual levels should also be considered. Other considerations
include economic factors and disposition of treated soils.
O Step 3: An evaluation ofLTTD effectiveness. The effectiveness of
LTTD treatment systems may be evaluated by either (1) calculating
the percent reduction in constituent concentrations by comparing the
pre- and post-treatment levels in the soil or, (2) determining if
residual contaminant levels are at or below regulatory limits.
Monitoring plans should specify an adequate number of samples of
treated soil to be analyzed.
Evaluation Of The Applicability Of LTTD
This section defines the key parameters that should be used to
decide whether LTTD will be a viable remedy for a particular site. In
order to determine if LTTD is an applicable remedial alternative, factors
to be considered include the characteristics of the soil and constituents
present at the site, as well as the LTTD process operating conditions.
Thermal desorption is applicable to a wide range of organic constituents,
including most petroleum hydrocarbon fuels (Exhibit VI-4). Specific soil
and constituent characteristics that influence the applicability of LTTD
are summarized in Exhibit VI-5.
October 1994 , VI-7
-------
Exhibit Vl-4
Recommended Treatment Temperatures For Selected Petroleum Products
i
i
o
o>
c
o
1
2
0
Q.
E
0)
1-
05
E
o
i_
c
Autorr
"f""J\
3 2<
No. 1
Je
Jet Fuel-
No
Lube Oil
N<
4 Fuel Oil
No. 3 Fuel Oil
Mo. 2 Fuel Oil (Diesel)
Fuel Oil (K«
t Fuel-A (JF
Used Motoii Oil. Crude i
6 Fuel Oil
— — I
rosene) ]
-5)
^B (JP-4)
obile Gasoline
i
30 400 600 800 1.OOO 1.200
r . 1 1 1 1 J
£
Ufe
*^E
1_ 8)
.Q
_ O
L.
.C O
Thermal
Legend:
Screw - St«
rhermal Sere
Tempera
am Heated
f - Hot Oil
Asphalt Aggregate D
Rotary Dr
ture (°F)
Heated
fer
i
rer — Carbon Steel
Ce nveyor Fumqce j
Rotary Dryer - ^lloy
Yttjj^ui R«eommended Product Treatment Temperature Range
•iMHi Typical Thermal Desorber Soil Discharge Temperature
VI-8
October 1994
-------
Exhibit VI-5
Key Soil And Constituent Characteristics That Influence Applicability Of LTTD
Soil Characteristics Constituent Characteristics
Soil plasticity Contaminant concentrations
Particle size distribution Boiling point range
Moisture content Vapor pressure
Heat capacity Octanol/water partition coefficient
Concentration of humic material Aqueous solubility
Metals concentration Thermal stability
Bulk density Dioxin formation
The remainder of this section describes each of these parameters, why
each is important to LTTD, how each can be determined, and the range
of each parameter considered appropriate for LTTD.
Soil Characteristics
Essentially all soil types are amenable for treatment by LTTD systems.
However, different soils may require varying degrees and types of
pretreatment. For example, coarse-grained soils (e.g., gravel and cobbles)
may require crushing; fine-grained soils that are excessively cohesive
(e.g., clay) may require shredding.
Soil Plasticity
The plasticity of the soil is a measure of its ability to deform without
shearing and is to some extent a function of water content. Plastic soils
tend to stick to screens and other equipment, and agglomerate into large
clumps. In addition to slowing down the feed rate, plastic soils are
difficult to treat. Heating plastic soils requires higher temperatures
because of the low surface area to volume ratio and increased moisture
content. Also, because plastic soils tend to be very fine-grained, organic
compounds tend to be tightly sorbed. Thermal treatment of highly plastic
soils requires pretreatment, such as shredding or blending with more
friable soils or other amendments (e.g., gypsum).
Plasticity characteristics are formally measured using a set of
parameters known as Atterberg Limits. Atterberg Limits are defined as
the moisture contents which define a soil's liquid limit, plastic limit, and
sticky limit. The range of water content where the soil is in a plastic state
is defined as the plasticity index.
October 1994 VT-9
-------
The plasticity index is the difference between the soil's liquid and
plastic limits, and indicates the range of water content through which
the soil remains plastic. Thus, the greater the plastic index, the more
likely the soil will clump. In general, clumping is most likely for silt and
clay soils.
From a practical standpoint, formal determination of a soil's plasticity
index is unnecessary. One of the first stages in the LTTD treatment train
is screening to remove material larger than about 2 inches in diameter.
Desorption unit operators will take the steps necessary to ensure that
the soils will move freely through the treatment process, whether this
requires shredding, blending, or amending. If the soils are to be blended,
the characteristics of the blending stock should be determined to ensure
that no contaminants are present that could adversely affect treatment
of the soils excavated from the UST site.
Particle Size Distribution
Particle size distribution is important for proper selection of the type
of thermal desorber and pretreatment process to be used. Material larger
than 2 inches in diameter will need to be crushed or removed. Crushed
material is recycled back into the feed to be processed. Coarser-grained
soils tend to be free-flowing and do not agglomerate into clumps. They
typically do not retain excessive moisture, therefore, contaminants are
easily desorbed. Finer-grained soils tend to retain soil moisture and
agglomerate into clumps. When dry, they may yield large amounts of
particulates that may require recycling after being intercepted in the
baghouse. Other consequences of fine-grained soils are discussed under
Soil Plasticity and Moisture Content.
Moisture Content
The solids processing capacity of a thermal desorption system is
inversely proportional to the moisture content of the feed material. The
presence of moisture in the excavated soils to be treated in the LTTD
unit will determine the residence time required and heating
requirements for effective removal of contaminants. In order for
desorption of petroleum constituents to occur, most of the soil moisture
must be evaporated in the desorber. This process can require significant
additional thermal input to the desorber and excessive residence time for
the soil in the desorber (Exhibit VI-6). In general, soil moisture content
ranges from 5 to 35 percent. Exhibit VI-7 shows the applicability of
various LTTD system configurations for various soil moisture ranges. For
LTTD treatment, the optimal soil moisture range is from 10 to 25
percent. For moisture content above 10 percent by weight,
VI-1O October 1994
-------
Energy
Exhibit Vi-6
Demand Versus Soil Moisture Content
^
*
*O
_ 400-
X
£ 300-
? 200-
c
UJ
100-
f\ _
-I
° 5
I
—
I
1
1
1
n
I
•*• — Moisture
•^ — Ornanlc
*-* — Dry Soil
10 15 20 25 30 35
Soil Moisture Content (%)
Exhibit VI-7
Feed Soil Moisture Content Limits
(
1
'
As|
IHHH^H
Rotor
•••••
R
wm^-mm
•••^H
) 1
|
A
Legend:
^^^^^^ «
tWxWyVI '
ffffffff\ ,
Then
Therr
•halt Aggrec
•••^•H
r Dryer — <
••••
otary Dryer
~mmmam
Conveyor F
•••••
0 2
1 j
nai Screw
nol Screw -
ate Dryer
Carbon Stee
- Alloy
jrnace
- Steam H<
- Hot Oil H
0 30 4
1 |
sated
sated
0 5
1 1
0 6
1 1
0
1
verage Feed Soil Moisture Conten+(%)
slormal Operating Range (No Upper Limit
>n Moisture Content for Ttwrmal Screws)
potential Operating Range - Determine
f Moisture is Below Plastic Limit
October 1994
VI-11
-------
moisture is the major heat sink in the system. Moisture content also
influences plasticity which affects handling of the soil. Soils with
excessive moisture content (> 20 percent) must be dewatered. Typical
dewatering methods include air drying (if storage space is available to
spread the soils), mixing with drier soils, or mechanical dewatering. For
example, if 10 feet of soil will be excavated, including 1 foot in the
capillary fringe, and 9 feet of drier soil, the excavated soils when mixed
would likely be suitable for LTTD.
If soils located beneath the water table or those with moisture
contents greater than 20 to 35 percent are proposed for treatment by
LTTD, you should verify that dewatering is planned. If the soil is to be
mixed with drier soils there needs to be a sufficient volume of this
material available to produce a mixture with an acceptable moisture
level.
Heat Capacity
Heat capacity of soil partially determines the amount of heat that
must be transferred to raise the temperature of the soil sufficiently to
volatilize the organic contaminants. However, since the typical range in
heat capacity values of various soils is relatively small, variations are not
likely to have a major impact on application of thermal desorption
processes.
Concentration Of Humic Material
Humic material is composed of organic material formed by the decay
of vegetation. Humic material is found in high concentrations in peat
and other highly organic soils. The presence of humic material can cause
analytical interferences, yielding a false positive indication of the
presence of TPH or BTEX. Organic material in soil also enhances the
adsorption of certain organic compounds, making desorption more
difficult.
Metals Concentration
In the past, various lead compounds (e.g., tetraethyl lead) were
commonly used as fuel additives to boost the octane rating in gasoline.
Although the use of lead has been discontinued, sites of older spills may
have relatively high lead concentrations in the soil. The presence of
metals in soil can have two implications: (1) limitations on disposal of
the solid wastes generated by desorption, and (2) attention to air
pollution control regulations that limit the amount of metals that may be
released in stack emissions. At normal LTTD operating temperatures,
heavy metals are not likely to be significantly separated from soils.
VI-12 October 1994
-------
Bulk Density
Bulk density is required to estimate the mass of contaminated soil
from the volume of soil excavated. The typical in situ, [bank) bulk density
range is 80-120 lb/ft3. Ex situ (excavated) soil bulk density ranges from
75 to 90 percent of the in situ bulk density.
Constituent Characteristics
The concentrations and characteristics of constituents are the key
parameters to be evaluated during screening studies to evaluate the
potential use of thermal desorption processes. The thermal treatment
contractor will want to know the concentration of total petroleum
hydrocarbons (TPH) in the soil. A number of state and local regulatory
agencies require testing of the soils for other specific hazardous
characteristics. The following analyses may be required to be conducted
during screening studies:
O Benzene, toluene, ethylbenzene, xylenes (BTEX)
O Total organic halides (TOX)
O Toxicity Characteristic Leaching Procedure (TCLP) for volatiles,
semivolatiles, and metals
O Total metals
O Polychlormated biphenyls (PCBs)
O Ignitability
O Corrosivity
O Reactivity
Constituent Concentrations
Constituent concentrations have several impacts on the thermal
desorption process. The selection of the appropriate LTTD process
configuration is dependent to some extent on constituent concentrations
because they influence the soil treatment temperature and residence
time required to meet soil cleanup criteria. Each petroleum product
possesses a heating value that is a measure of the amount of thermal
energy that will be released when the product is burned. High
concentrations of petroleum products in soil can result in high soil
heating values. Heat released from soils can result in overheating and
damage to the desorber. Soils with heating values > 2,000 Btu/lb require
blending with cleaner soils to dilute the high concentration of
hydrocarbons. High hydrocarbon concentrations in the offgas may
exceed the thermal capacity of the afterburner and potentially result in
the release of untreated vapors into the atmosphere.
October 1994 VI-13
-------
Excessive constituent levels in soil could also potentially result in the
generation of vapors in the desorber at concentrations exceeding the
lower explosive limit (LEL). The LEL for most organics is generally 1-5
percent by volume. For safety reasons, the concentration of organic
compounds in the exhaust gas of a thermal desorption device operating
in an oxygen-rich environment should be limited to < 25 percent of the
lower explosive limit. For directly heated rotary dryers, the maximum
concentration of TPH in feed material that can be treated without
exceeding the lower explosive limits is generally in the range of 1-
3 percent. If the organic content exceeds 3 percent, the soil must be
blended with soil that has a lower organics content to avoid exceeding
the LEL. Systems that operate in an inert atmosphere (e.g., thermal
screws) do not have limitations on the concentration of organics that can
be processed. In an inert atmosphere, the concentration of oxygen is too
low (< 2 percent by volume) to support combustion. Exhibit VI-8 shows
feed soil TPH concentration limits for various LTTD system
configurations.
Exhibit VI-8
Feed Soil TPH Concentration Limits
Thermal Screw •- Steam H
-------
Boiling Point Range
Petroleum products are often classified by their boiling point ranges.
Because the boiling point of a compound is a measure of its volatility,
the applicability of LTTD at a site can be estimated from the boiling point
range of the petroleum product present. In general, most petroleum-
related organics are capable of removal by LTTD, but higher molecular
weight (and higher boiling point) constituents require longer residence
time in the desorber and higher desorber operating temperatures.
Heavier products tend to break down before volatilizing, or they may
form non-toxic wax-like compounds that do not volatilize. The boiling
point ranges for common petroleum products are shown in Exhibit VI-9.
Petroleum
Product
Gasoline
Kerosene
Diesel fuel
Heating oil
Lubricating oils
Exhibit VI-9
Product Boiling Ranges
Boiling Range
(°C)
40 to 225
180 to 300
200 to 338
>275
Nonvolatile
Boiling Range
(°F)
104 to 437
356 to 572
392 to 640
>527
Nonvolatile
Most desorbers operate at temperatures between 300°F-1,000°F.
Desorbers constructed of special alloys can operate at temperatures up
to 1,200°F. More volatile products (e.g., gasoline) can be desorbed at the
lower operating range, while semivolatile products (e.g., kerosene, diesel
fuel) generally require temperatures in excess of 700°F, and relatively
nonvolatile products (e.g., heating oil, lubricating oils) require even
higher temperatures.
Vapor Pressure
Vapor pressure is the force per unit area exerted by a vapor in an
equilibrium state with its pure solid, liquid, or solution at a given
temperature. Along with boiling point, vapor pressure is used to
measure a compound's volatility. Vapor pressure influences the rate of
thermal desorption and increases exponentially with an increase in
temperature. Therefore, modest increases in desorption temperature
result in large increases in the rate of desorption.
October 1994 VI-15
-------
Octanol/Water Partition Coefficient (Kow)
The octanol/water partition coefficient (K^) represents 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 K^, the more non-polar the
compound. Log E^, is generally used as a relative indicator of the
tendency of an organic compound to absorb to soil. Log Kow values are
generally inversely related to aqueous solubility and directly proportional
to molecular weight. Compounds with high Log Kow values tend to
remain sorbed to soil for a long period of time and are more difficult to
desorb than compounds with low values.
Aqueous Solubility
Aqueous solubility is a measure of the extent to which a compound
will dissolve in water. Solubility- is generally inversely related to
molecular weight the higher the molecular weight, the lower the
solubility. Compounds with higher molecular weight are also generally
more difficult to desorb from soil than are compounds with lower
molecular weight
Thermal Stability
Petroleum hydrocarbons are not expected to significantly
decompose/combust in LTTD units, provided that the offgas temperature
is below the temperature at which a compound will spontaneously
combust (the autoignition temperature). Autoignition temperature is,
therefore, an indicator of the thermal stability of petroleum
hydrocarbons, and the degree of thermal decomposition is related to the
maximum temperature exposure.
Dioxin Formation
Dioxins can be formed from the thermal destruction of PCBs and
other chlorinated compounds. The petroleum hydrocarbons typically
present at UST sites do not contain PCBs; therefore, formation of dioxins
is usually not of concern. Waste oils that contain chlorinated
hydrocarbons may, however, be potential precursors of dioxins. Soils
from waste oil spills should be analyzed for PCBs and other chlorinated
hydrocarbons.
Process Operating Conditions
Process operating conditions are dependent upon the type of thermal
desorption system and vary over a wide range. Each system
configuration has its own advantages and disadvantages, and each is
VI-16 October 1994
-------
applicable for treatment of specific ranges of constituents (Exhibit VI-10).
LTTD systems vary in the manner in which the soils are transported
through the desorber, the method used to heat the soils; the
temperature at which the desorber operates; the time required to treat
the soils; and the offgas treatment method used to control emissions.
Types Of Low-Temperature Thermal Desorption Systems
The term thermal desorber describes the primary treatment operation
that heats petroleum-contaminated materials and desorbs organic
materials into a purge gas. Mechanical design features and process
operating conditions vary considerably among the various types of LTTD
systems. Desorption units are available in the following configurations:
rotary dryer, asphalt plant aggregate dryer, thermal screw, and conveyor
furnace. Systems may either be stationary facilities or mobile units.
Contaminated soils are excavated and transported to stationary facilities,
while mobile units can be operated directly on the site of the
contaminated soil.
Although all LTTD systems use heat to separate (desorb) organic
contaminants from the soil matrix, each system has a different
configuration with its own set of advantages and disadvantages. The
decision to'use one system over another depends on the nature of the
contaminants as well as machine availability, system performance, and
economic considerations. System performance may be evaluated on the
basis of pilot tests (e.g., test burns) or examination of historical machine
performance records. Pilot tests to develop treatment conditions are
generally not necessary for petroleum-contaminated soils.
Mechanical design features and process operating conditions vary
among the different types of LTTD systems. The four systems mentioned
above are briefly described below, and the advantages and disadvantages
of each are listed.
Rotary Dryers. Rotary dryer systems use a cylindrical metal reactor
(drum) that is inclined slightly from the horizontal. A burner located at
one end provides heat to raise the temperature of the soil sufficiently to
desorb organic contaminants. The flow of soil may be either cocurrent
with or countercurrent to the direction of the purge gas flow. As the
drum rotates, soil is conveyed through the drum. Lifters raise the soil,
carrying it to near the top of the drum before allowing it to fall through
the heated purge gas. Mixing in a rotary dryer enhances heat transfer by
convection and allow soils to be rapidly heated. Rotary desorber units
are manufactured for a wide range of treatment capacities; these units
may be either stationary or mobile.
October 1994 VI-17
-------
CO
!
1
•Is
<-« to
s
§
CO
1
O
3
9
OS
•o
P
TJ
5
"o
o
VI-18
October 1994
-------
The maximum soil temperature that can be obtained in a rotary dryer
depends on the composition of the dryer shell. The soil discharge
temperature of carbon steel drums is typically 300°-600° F. Alloy drums
are available that can increase the soil discharge temperature to
1,200° F. Most rotary dryers that are used to treat petroleum
contaminated soil are made of carbon steel. After the treated soil exits
the rotary dryer, it enters a cooling conveyor where water is sprayed on ,
the soil for cooling and dust control. Water addition may be conducted in
either a screw conveyor or a pugmill.
Besides the direction of purge gas flow relative to soil feed direction,
there is one major difference in configuration between countercurrent
and cocurrent rotary dryers. The purge gas from a countercurrent rotary
dryer is typically only 350°F-500°F and does not require cooling before
entering the baghouse where fine particles are trapped. A disadvantage
is that these particles may not have been decontaminated and are
typically recycled to the dryer. Countercurrent dryers have several
advantages over cocurrent systems. They are more efficient in
transferring heat from purge gas to contaminated soil, and the volume
and temperature of exit gas are lower, allowing the gas to go directly to a
baghouse without needing to be cooled. The cooler exit gas temperature
and smaller volume eliminates the need for a cooling unit, which allows
downstream processing equipment to be smaller. Countercurrent
systems are effective on petroleum products with molecular weights
lower than No.2 fuel oil.
In cocurrent systems, the purge gas is 50°-100°F hotter than the soil
discharge temperature. The result is that the purge gas exit temperature
may range from 400°-1,000°F and cannot go directly to the baghouse.
Purge gas first enters an afterburner to decontaminate the fine particles,
then goes into a cooling unit prior to introduction into the baghouse.
Because of the higher temperature and volume of the purge gas, the
baghouse and all other downstream processing equipment must be
larger than in a countercurrent system. Cocurrent systems do have
several advantages over countercurrent systems. The afterburner is
located upstream of the baghouse ensuring that fine particles are
decontaminated. In addition, because the heated purge gas is
introduced at the same end of the drum as the feed soil, the soil is
heated faster, resulting in a longer residence time. Higher temperatures
and longer residence time mean that cocurrent systems can be used to
treat soils contaminated with heavier petroleum products. Cocurrent
systems are effective for light and heavy petroleum products including
No. 6 fuel oil, crude oil, motor oil, and lubricating oil.
October 1994 VI-19
-------
Asphalt Plant Aggregate Dryers. Hot-mix asphalt plants use aggregate
that has been processed in a dryer before it is mixed with liquid asphalt.
The use of petroleum contaminated soils for aggregate material is
widespread. Aggregate dryers may either be stationary or mobile. Soil
treatment capacities range from 25-150 tons per hour. The soil may be
incorporated into the asphalt as a recycling process or the treated soil
may be used for other purposes.
Asphalt rotary dryers are normally constructed of carbon steel and
have a soil discharge temperature of 300°-600°F. Typically, asphalt plant
aggregate dryers are identical to countercurrent rotary desorbers
described above and are effective on the same types of contaminants.
The primary difference is that an afterburner is not required for
incorporation of clean aggregate into the asphalt mix. In some areas,
asphalt plants that use petroleum contaminated soil for aggregate may
be required to be equipped with an afterburner.
Thermal Screws. A thermal screw desorber typically consists of a series
of 1-4 augers. The auger system conveys, mixes, and heats contaminated
soils to volatilize moisture and organic contaminants into a purge gas
stream. Augers can be arranged in series to increase the soil residence
time, or they can be configured in parallel to increase throughput
capacity. Most thermal screw systems circulate a hot heat-transfer oil
through the hollow flights of the auger and return the hot oil through the
shaft to the heat transfer fluid heating system. The heated oil is also
circulated through the jacketed trough in which each auger rotates.
Thermal screws can also be steam-heated. Systems heated with oil can
achieve soil temperatures of up to 500°F, and steam-heated systems can
heat soil to approximately 350°F. .
Most of the gas generated during heating of the heat-transfer oil does
not come into contact the waste material and can be discharged directly
to the atmosphere without emission controls. The remainder of the flue
gas maintains the thermal screw purge gas exit temperature above
300°F. This ensures that volatilized organics and moisture do not
condense. In addition, the recycled flue gas has a low oxygen content
(< 2 percent by volume) which minimizes oxidation of the organics and
reduces the explosion hazard. If pretreatment analytical data indicates a
high organic content (> 4 percent), use of a thermal screw is
recommended. After the treated soil exits the thermal screw, water is
sprayed on the soil for cooling and dust control. Thermal screws are
available with soil treatment capacities ranging from 3-15 tons per hour.
Vl-20 October 1994
-------
Since thermal screws are indirectly heated, the volume of purge gas
from the primary thermal treatment unit is less than one half of the
volume from a directly-heated system with an equivalent soil processing
capacity. Therefore, offgas treatment systems consist of relatively small
unit operations that are well suited to mobile applications. Indirect
heating also allows thermal screws to process materials with high
organic contents since the recycled flue gas is inert, thereby reducing the
explosion hazard.
Conveyor Furnace. A conveyor furnace uses a flexible metal belt to
convey soil through the primary heating chamber. A one-inch-deep layer
of soil is spread evenly over the belt. As the belt moves through the
system, soil agitators lift the belt and turn the soil to enhance heat
transfer and volatilization of organics. The conveyor furnace can heat
soils to temperatures from 300°-800°F. At the higher temperature range,
the conveyor furnace is more effective in treating some heavier petroleum
hydrocarbons than are oil or steam-heated thermal screws, asphalt plant
aggregate dryers, and carbon steel rotary dryers. After the treated soil
exits the conveyor furnace, it is sprayed with water for cooling and dust
control. As of February, 1993, only one conveyor furnace system was
currently in use for the remediation of petroleum contaminated soil. This
system is mobile and can treat 5-10 tons of soil per hour.
Off Gas Treatment
Treatment systems for LTTD system offgas are designed to address
three types of air pollutants: particulates, organic vapors, and carbon
monoxide. Particulates are controlled with both wet (e.g., venturi
scrubbers) and dry (e.g., cyclones, baghouses) unit operations. Rotary
dryers and asphalt aggregate dryers most commonly use dry gas
cleaning unit operations. Cyclones are used to capture large particulates
and reduce the particulate load to the baghouse. Baghouses are used as
the final particulate control device. Thermal screw systems typically use
a venturi scrubber as the primary particulate control.
The control of organic vapors is achieved by either destruction or
collection. Afterburners are used downstream of rotary dryers and
conveyor furnaces to destroy organic contaminants and oxidize carbon
monoxide. Conventional afterburners are designed so that exit gas
temperatures reach 1,400°-1,600°F. Organic destruction efficiency
typically ranges from 95 to > 99 percent.
October 1994 VI-21
-------
Condensers and activated carbon may also be used to treat the offgas
from thermal screw systems. Condensers may be either water-cooled or
electrically-cooled systems to decrease offgas temperatures to 100°-
140°F. The efficiency of condensers for removing organic compounds
ranges from 50 to > 95 percent. Noncondensible gases exiting the
condenser are normally treated by a vapor-phase activated carbon
treatment system. The efficiency of activated carbon adsorption systems
for removing organic contaminants ranges from 50-99 percent.
Condensate from the condenser is processed through a phase separator
where the non-aqueous phase organic component is separated and
disposed of or recycled. The remaining water is then processed through
activated carbon and used to rehumidify treated soil.
Treatment Temperature
Treatment temperature is a key parameter affecting the degree of
treatment of organic components. The required treatment temperature
depends upon the specific types of petroleum contamination in the soil.
Exhibit VI-4 illustrates the recommended treatment temperatures for
various petroleum products and the operating temperature ranges for
various LTTD systems. The actual temperature achieved by an LTTD
system is a function of the moisture content and heat capacity of the
soil, soil particle size, and the heat transfer and mixing characteristics of
the thermal desorber.
Residence Time
Residence time is a key parameter affecting the degree to which
decontamination is achievable. Residence time depends upon the design
and operation of the system, characteristics of the contaminants and the
soil, and the degree of treatment required.
Pilot Testing
The requirement for pilot testing of petroleum-contaminated soils, in
which a quantity of soil from the site is processed through the LTTD
system (a "test bum"), is specified by state and local regulations. The
results of preliminary testing of soil samples should identify the relevant
constituent properties, and examination of the machine's performance
records should Indicate how effective the system will be in treating the
soil. However, it should be noted that the proven effectiveness of a
particular system for a specific site or waste does not ensure that it will
be effective at all sites or that the treatment efficiencies achieved will be
acceptable at other sites. If a test burn is conducted, it is important to
VI-22
October 1994
-------
ensure that the soil tested is representative of average conditions and
that enough samples are analyzed before and after treatment to
confidently determine whether LTTD will be effective.
Determination Of The Practicality Of Using LTTD
This section identifies the factors that determine whether LTTD is a
practical remedial alternative. While many of these factors are dependent
upon site-specific characteristics (e.g., the location and volume of
contaminated soils, site layout), practicality is also determined by
regulatory, logistical, and economic considerations. The economics of
LTTD as a remedial option are highly site-specific. Economic factors
include site usage (because excavation and onsite soil treatment at a
retail site (e.g., gasoline station, convenience store) will most likely
prevent the business from operating for an extended period of time), the
cost of LTTD per unit volume of soil relative to other remedial options,
and the location of the nearest applicable LTTD system (because
transportation costs are a function of distance). Further discussion of
the economics of LTTD use is beyond the scope of this manual.
Vertical And Horizontal Extent Of Contamination
Because soils to be treated in an LTTD unit must be excavated, their
location must be suitable for removal by excavation techniques. Soils
that are located more than 25 feet below the land surface cannot be
removed by conventional equipment. In addition, soils that are located
beneath a building or near building foundations cannot be excavated
without removal of the building itself. In addition, as mentioned
previously, soils located beneath the groundwater table can be excavated
but generally cannot be treated in the LTTD unit unless dried,
dewatered, or blended with other soils to reduce moisture content.
You should identify the location of the proposed excavation and verify
that soils to be excavated are less than 25 feet below land surface, above
the water table, and not beneath or near buildings or other structures.
The vertical and horizontal extent of contamination determines the
volume of soil that must be treated. The cost of remediation and time
required for processing is directly proportional to the volume of contam-
inated soil to be treated. Volume also determines whether onsite treat-
ment is viable. A small mobile LTTD system with a throughput capacity
of 5 to 15 tons per hour may be able to stockpile materials and operate
in an area as small, as 14 acre. Exhibit VI-11 shows the relationship
between thermal desorber size and the amount of soil to be treated.
October 1994 VI-23
-------
O
E
o
"w
>.
to
Exhibit VI-11
Thermal Desorption Size Versus Amount Of Soil To Be Treated
Small Siz
System I
Me
;d Mobile
On-Site)
dium Sized
Stationai
Mobile Sys
Large 5
y Facility ((
|
i
!
em(On— Site)
iized Mobile
System (O
)ff-Site) 1
n-Site)
2,000 4,000 6,000 8,000 10,000 12,000
1 I I i I I
Amount of Soil to be Treated
System Type
System Characteristics
SmoH Medium Large Stationary
Mobile Mobile Mobile Facility
(On-Site) (On-Site) (On-Site) (Off-Site)
No. of Trailers
Primary Burner Capacity (MM Btu/Hr)
Secondary Burner Capacity (MM Btu/Hr)
Soil Processing Capacity (Tons/Hour)
1-2 3-6 7-10 NA
5-15 15-30 30-50 30-120
5-15° 15-30° 30-50 30-100b
5-15 15-30 25-100 30-300
Systems with Condensers do not Include Afterburners.
Some Fixed Base Asphalt Aggregate Dryers do not Include Afterburners.
VI-24
October 1994
-------
Site Layout
Site layout factors influence whether excavation of soils is possible at
all. If excavation is possible, consideration can be given to whether onsite
thermal treatment is a viable option. Site layout factors that must be
considered in evaluating onsite thermal desorption treatment include:
O Amount of space available for stockpiling treated and untreated
materials and operating process equipment,
O Space required for continuation of daily business, and
O Minimum distances required by fire and safety codes for operating
thermal desorption equipment in the vicinity of petroleum storage
facilities.
The amount of area available to stockpile soils and operate processing
equipment may dictate the maximum size of the treatment system that
can be operated at the site. In general, onsite treatment operations will
require a minimum of Vz acre. This has further economic implications
because the costs associated with LTTD are strongly affected by the
physical size and soil processing capacity of the thermal treatment
system.
Adjacent Land Use
When land adjacent to an UST site is being used for schools, parks,
health care facilities, high-value commercial development, or dense
residential development, problems may develop in obtaining permits for
the use of onsite thermal desorption. Air discharge restrictions may
require the use of expensive control measures that could make onsite
treatment economically infeasible. Thermal desorption units are most
economical when they are operated on a 24-hour-per-day schedule.
However, noise considerations may limit hours of operation in some
locations.
Other Considerations
Treatment goals are also important when considering the use of LTTD.
For soils contaminated with lighter petroleum hydrocarbons, residual
TPH levels can be reduced to 10 ppm or less. Some newer rotary units
can consistently achieve TPH levels of < 1 ppm and BTEX levels
< 100 ppb. System effectiveness can be evaluated based on the
treatment records for a specific machine.
October 1994 VI-25
-------
Treated soils are typically disposed of in a landfill, used as cover in
landfills, incorporated into asphalt, or returned to the site to backfill the
excavation. Final disposition of the soil depends upon the residual levels
of contaminants in the treated soil and economic factors such as
transportation and disposal costs, as well as costs for clean material to
backfill the excavation. It should be noted that treatment processes may
alter the physical properties of the material. A thorough geotechnical
evaluation of the treated material may be necessary to determine its
suitability for use in an engineering application (e.g., road bed, building
foundation support, grading and filling).
Evaluation Of The Effectiveness Of LTTD
For sites with petroleum contaminated soils, the primary concern is to
reduce the residual concentration of the organic constituents to or below
regulatory levels. This criterion applies to both the soil surrounding the
excavation and the soil that was excavated and thermally treated. An
appropriate number of soil samples should be collected from around the
walls and bottom of the excavation. These samples should then be
analyzed for the requisite parameters to ensure that all of the soil that
must be thermally treated has been excavated.
The effectiveness of an LTTD treatment system may be evaluated by
either (1) determining whether residual contaminant levels are at or
below regulatory limits or (2) calculating the percent reduction in soil
constituent concentrations by comparing pre- and post-treatment levels.
Monitoring plans should specify an adequate number of samples of
treated soil to be analyzed. A typical sample density is one sample per
100 cubic yards of treated soil. Exhibit VI-12 lists typical monitoring
locations and frequency for petroleum contaminated soils treated by
LTTD.
Exhibit VI-12
Monitoring Recommendations
Phase
Excavation
LTTD treatment
Frequency Where To Monitor
At proposed limit of o Excavation walls
excavation
o Excavation floor
Every 100 cu.yd. of feed o Feed soil
soil and treated soil o Treated soil
What To Monitor
o TPH, constituents of
concern
o TPH, constituents of
concern
VI-26
October 1994
-------
Operation of LTTD units requires various permits and demonstration
of compliance with permit requirements. Monitoring requirements for
LTTD systems are by their nature different from monitoring required at
an UST site. Monitoring of LTTD system waste streams (e.g.,
concentrations of particulates, volatiles, and carbon monoxide in stack
gas) are required by the agency(ies) issuing the permits for operation of
the facility. Compliance with limits specified by the permits is the
responsibility of the LTTD facility owner/operator. Other LTTD system
operating parameters (e.g., desorber temperature, soil feed rate,
afterburner temperature) are also the responsibility of the LTTD facility
owner/ operator.
October 1994 VI-27
-------
References
U.S. Environmental Protection Agency (EPA). Low-temperature Thermal
Treatment Technology: Applications Analysis Report Cincinnati, OH:
U.S. EPA, Office of Research and Development. EPA/540/AR-92/019,
1992.
Troxler, W.L., J.J. Cudahy, R.P. Zink, and S.I. Rosenthal. Thermal
Desorption Applications Manual for Treating Nonhazardous Petroleum
Contaminated Soils. Cincinnati, OH: U.S. EPA, Office of Research and
Development, 1994.
Anderson, W.C., ed. Innovative Site Remediation Technology: Thermal
Desorption, Volume 6. Washington, D.C.: U.Sr EPA, Office of Solid
Waste and Emergency Response. EPA 542-B-93-011, 1993.
VI-28 October 1994
-------
Checklist: Can LTTD Be Used At This Site?
This checklist can help you to evaluate the completeness of the CAP
and to identify areas that require closer evaluation. As you go through
the CAP, answer the following questions.
1. Evaluation Of LTTD Effectiveness
Yes No
Q Q Do soils have high plasticity?
Q Q Do soils contain large rocks or debris?
Q Q Is moisture content > 35%?
Q Q Is the TPH concentration > 2% by weight?
Q Q Are hydrocarbons highly volatile?
If the answer to any of the above questions is yes, then the soils
require pretreatment.
Q Q Do the soils have a high concentration of humic
material?
Q Q Do the soils have a high concentration of heavy
metals?
Q Q Are contaminant K^ relatively high?
Q Q Are dioxin precursors present in the soils?
If the answer to any of the above questions is yes, then a pilot test
or "test burn" should be conducted to demonstrate that LTTD is an
applicable remedial technology.
Q Q Do the results of the pilot test indicate that LTTD is
applicable?
October 1994 VI-29
-------
2. Evaluation Of The Practicality Of Using LTTD
Yes No
Q Q Is the depth of contaminated soil 25 feet or less below
land surface?
Q Q Is contaminated soil contained within site
boundaries?
Q Q Is there no contamination beneath buildings or near
building foundations?
If the answer to any of the above questions is no, then excavation of
the soil is not practical; therefore, LTTD is not practical. Consider
an in situ, remedial technology instead.
Q Q Is sufficient land area available for operation of
equipment and temporary storage (staging) of
contaminated soil and treated soil?
Q Q Is the distance to an off-site facility prohibitively far?
Q Q Will surrounding land use permit operation of an
onsite system in the neighborhood?
If the answer to any of the above questions is no, then excavated
soils must be transported to an off-site facility for treatment.
3. Evaluation Of The Effectiveness Of Using LTTD
Yes No
Q Q Will an adequate number of in situ soil samples be
collected and analyzed?
Q Q Will an adequate number of treated soil samples be
collected and analyzed?
VI-3O October 1994
-------
3. Evaluation Of The Effectiveness Of Using LTTD (continued)
Yes No
Q Q Has the proposed desorption unit successfully treated
similar soils with, similar contaminant concentration
levels?
Q Q Is the proposed ultimate disposal of the soil (e.g.,
return to excavation, transport to landfill for cover)
acceptable?
If the answer to any of the above questions is no, then additional
information is necessary to evaluate whether LTTD is likely to be an
effective remedial technology.
October 1994 VI-31
-------
-------
Chapter VII
Air Sparging
-------
-------
Contents
Overview VII-1
Initial Screening Of Air Sparging Effectiveness VII-6
Detailed Evaluation Of Air Sparging Effectiveness VII-8
Factors That Contribute To Constituent Vapor/Dissolved
Phase Partitioning VII-8
Henry's Law Constant VII-8
Product Composition And Boiling Point VII-9
Vapor Pressure VII-10
Constituent Concentrations VII-11
Solubility VII-11
Factors That Contribute To Permeability Of Soil VII-11
Intrinsic Permeability VII-11
Soil Structure And Stratification VII-14
Iron Concentration Dissolved In Groundwater VII-14
Field Pilot-Scale Studies VII-15
Evaluation Of The Air Sparging System Design VII-16
Rationale For The Design VII-16
What Are The Typical Components Of An
Air Sparging System? VII-19
Sparge And Extraction Wells VII-20
Manifold Piping . VII-24
Compressed Air Equipment VII-24
Monitoring And Controls VII-24
Evaluation Of Operation And Monitoring Plans VII-25
Startup Operations VII-26
Long-Term Operations VII-26
Remedial Progress Monitoring VII-26
References VII-29
Checklist: Can Air Sparging Be Used At This Site? VII-30
October 1994 VH-iii
-------
List Of Exhibits
Number Title Page
VIM Air Sparging System With SVE VII-2
VII-2 Advantages And Disadvantages Of Air Sparging VII-3
VII-3 Air Sparging Evaluation Process Flow Chart VII-4
VII-4 Initial Screening For Air Sparging Effectiveness VII-7
VII-5 Key Parameters Used To Evaluate Vapor/Dissolved
Phase Partitioning And Permeability Of Soil VII-8
V1I-6 Henry's Law Constant Of Common
Petroleum Constituents VII-9
VII-7 Petroleum Product Boiling Point Ranges VII-10
VII-8 Vapor Pressures Of Common
Petroleum Constituents VII-10
VII-9 Summary Of Air Sparging Applications
(Used With SVE) VII-12
VII-10 Solubility Of Common Petroleum Constituents VII-13
VII-11 Intrinsic Permeability And Air Sparging Effectiveness .. VII-13
VII-12 Dissolved Iron And Air Sparging Effectiveness VII-15
VII-13 Pilot Test Data Objectives VII-17
VII-14 Schematic Of Air Sparging System Used With SVE .... VII-19
VII-15 WeU Orientation And Site Conditions VII-20
VII-16 Air Sparging/Soil Vapor Extraction
WeU Configurations . . VII-21
VII-17 Combined Air Sparging/SVE System Layout VII-22
VII-18 Typical Vertical Air Sparging Well Construction VII-23
VII-19 Typical Horizontal Air Sparging Well Construction .... VII-23
VII-20 Monitoring And Control Equipment VII-25
VII-21 System Monitoring Recommendations VII-27
VII-22 Concentration Reduction And Mass Removal
Behavior For Both Air Sparging And SVE Systems . . VII-28
Vn-iv October 1994
-------
Chapter VII
Air Sparging
Overview
Air sparging (AS) is an in situ remedial technology that reduces
concentrations of volatile constituents in petroleum products that are
adsorbed to soils and dissolved in groundwater. This technology, which
is also known as "in situ air stripping" and "in situ volatilization,"
involves the injection of contaminant-free air into the subsurface
saturated zone, enabling a phase transfer of hydrocarbons from a
dissolved state to a vapor phase. The air is then vented through the
unsaturated zone. Air sparging is most often used together with soil
vapor extraction (SVE), but it can also be used with other remedial
technologies. When air sparging is combined with SVE, the SVE system
creates a negative pressure in the unsaturated zone through a series of
extraction wells to control the vapor plume migration. This combined
system is called AS/SVE. Chapter II provides a detailed discussion of
SVE.
The! existing literature contains case histories describing both the
success and failure of air sparging; however, since the technology is
relatively new, there are few cases with substantial documentation of
performance. When used appropriately, air sparging has been found to
be effective in reducing concentrations of volatile organic compounds
(VOCs) found in petroleum products at underground storage tank (UST)
sites. Air sparging is generally more applicable to the lighter gasoline
constituents (i.e., benzene, ethylbenzene, toluene, andxylene [BTEX]),
because they readily transfer from the dissolved to the gaseous phase.
Air sparging is less applicable to diesel fuel and kerosene. Appropriate
use of air sparging may require that it be combined with other remedial
methods (e.g., SVE or pump-and-treat). Exhibit VII-1 provides an
illustration of an air sparging system with SVE. Exhibit VII-2 provides a
summary of the advantages and disadvantages of air sparging.
This chapter will assist you in evaluating a corrective action plan
(CAP) that proposes air sparging as a remedy for petroleum-
contaminated soil. The evaluation guidance is presented in the four
steps described below. The evaluation process, which is summarized in a
flow diagram shown in Exhibit VII-3, serves as a roadmap for the
decisions you will make during your evaluation. A checklist has also
been provided at the end of the chapter for you to use as a tool both to
evaluate the completeness of the CAP and to focus on areas where
additional information may be needed.
October 1994 VH-1
-------
Exhibit VIM
Air Sparging System With SVE
Compressor
(Air Sparging)
Vapor
Treatment-
Atmosphertc
Discharge
Legend:
Vapor Phase
Adsorbed Phase
Dissolved Phase
-------
Exhibit VII-2
Advantages And Disadvantages Of Air Sparging
Advantages
o Readily available equipment; easy
installation.
o Implemented with minimal disturbance to
site operations.
o Short treatment times: usually less than 1
to 3 years under optimal conditions.
o At about $20-50/ton of saturated soil, air
sparging is less costly than aboveground
treatment systems.
o Requires no removal, treatment, storage,
or discharge considerations for
groundwater.
o Can enhance removal by SVE.
Disadvantages
o Cannot be used if free product exists
(i.e., any free product must be removed
prior to air sparging).
o Cannot be used for treatment of confined
aquifers.
o Stratified soils may cause air sparging to
be ineffective.
o Some interactions among complex
chemical, physical, and biological
processes are not well understood.
o Lack of field and laboratory data to
support design considerations.
o Potential for inducing migration of
constituents.
o Requires detailed pilot testing and
monitoring to ensure vapor control and
limit migration.
O Step 1: An initial screening of air sparging effectiveness allows
you to quickly gauge whether air sparging is likely to be effective,
moderately effective, or ineffective. You can use the initial screening
process as a yardstick to determine whether the technology has the
potential to be effective.
O Step 2: A detailed evaluation of air sparging effectiveness
provides further screening criteria to confirm whether air sparging is
likely to be effective. You will need to find specific soil and product
constituent characteristics and properties, compare them to ranges
where air sparging is effective, and evaluate pilot study plans.
O Step 3: An evaluation of the air sparging system design allows
you to determine if basic design information has been defined, if
necessary design components have been specified, if construction
process flow designs are consistent with standard practice, and if a
detailed field pilot scale test has been properly performed.
October 1994
vn-s
-------
Exhibit VII-3
Air Sparging Evaluation Process Flow Chart
W-4
DETAILED EVALUATION
OF AIR SPARGING
EFFECTIVENESS
INITIAL SCREENING
OF AIR SPARGING
EFFECTIVENESS
Identify product constituent
properties important to
air sparging effectiveness.
Identify site characteristics
important to
air sparging effectiveness.
Is floating
free product
present?
Remove free
product
Henry's Law Constant
Boiling Range
Vapor Pressure
Intrinsic Permeability
Soil Structure and Stratification
Dissolved Iron Content
basements, sewers.
or other subsurface
Is Henry's
Law constant
> 100 atm?
confined spaces
is intrinsic
permeability
> to-3 cm2?
Will
SVEbe
used to control
migration of
vapors
Is
soil free
NO ^Impermeable layers
or other conditions that
would disrupt
airflow?
Is
contaminated
groundwcter in a
confined
aquifer?
Is constituent
boiling range
< 260-300° C?
Determine which petroleum
products are targeted for
remediation by Air Sparging
• Kerosene
• Gasoline
• Diesel Fuel
• Heating Oil
• Lubricating Oil
Is the
dissolved iron
concentration at the
site < 10 mg/L?
Are
vapor pressures
of product
> 0.5 mm Hg?
Pilot studies are required to
demonstrate effectiveness.
Review pilot study results.
Are lubricating
oils targeted for
remediation
Have
pilot studies
been completed
do the results demonstrate
Air Sparging
effectiveness?
Determine the types of soils
that occur within the
contaminated area
Air Sparging will
not be
effective at the
site.
Consider other
technologies.
Air Sparging
is not likely to be
effective at the site.
Consider other
technologies.
Biosparging
Vacuum-enhanced
Pump and Treat
In-srtu
Groundwater
BJoremediation
son targeted for
• Rospargmg
Vacuum-enhanced
Pump and Treat
»In-situ
Groundwater
Bioremediation
Air Sparging is likely to
be effective at the site.
Proceed to evaluate
I Air Sparging
has the potential to be
effective at the site.
Proceed to next panel.
October 1994
-------
" Exhibit VI1-3
Air Sparging Evaluation Process Flow Chart
EVALUATION OF
AIR SPARGING
SYSTEM DESIGN
Determine the design elements
based on pilot study results
• Radius of Influence
• Sparging Air Flow Rate
• Sparging Air Pressure
• Required Final Dissolved Concentrations
• Required Cleanup Time
• Saturated Zone Volume to be Treated
• Pore Volume Calculations
• Discharge Limits
• Construction Limitations
| EVALUATION OF AIR SPARGING
SYSTEM OPERATION &
MONITORING PLANS
Review the O&M
plan for the proposed
Air Sparging system for
the following:
• Start-Up Operations Plan
• Long-Term Operations &
Monitoring Plan
• Remedial Progress
Monitoring Plan
Are
start-up
operations & monitoring
described, and are their
scope & frequency
adequate?
Have design
elements been identified
and are they within
appropriate
ranges?
Request
additional
information
on startup
procedures and
monitoring.
Review the conceptual
process flow design & identify
the system components
Is a
long-term O&M
plan described; is it
of adequate scope &
frequency?
Request
additional
information
on long-term
O&M.
• Sparging Well Orientation,
Placement, and Construction
• Manifold Piping
• Sparging Compressor
• Monitoring & Control
Equipment
Air Sparging
system
design is
incomplete.
Request
additional
information.
Is a
remedial progress
monitoring plan estab- \ K,o
lished; is it of adequate scope" INU
& frequency; does it include
provisions for detect-
ing asymptotic
behavior?
Hasthe
conceptual
design been provided
and is it
adequate?
Request
additional
information
on remedial
progress
monitoring.
The Air Sparging system
design is complete and its
elements are within
appropriate ranges.
Proceed to O&M
evaluation.
The Air Sparging
system is
likely to be effective.
The design and O&M
plans are complete.
October 1994
vn-s
-------
O Step 4: An evaluation of the operation and monitoring plans
allows you to determine whether start-up and long-term system
operation and monitoring is of sufficient scope and frequency and
whether remedial progress monitoring plans are appropriate.
Initial Screening Of Air Sparging Effectiveness
This section allows you to perform an initial screening of whether air
sparging will be effective at a site. First, you need to determine if site-
specific factors which prohibit the use of air sparging are present.
Second, you need to determine if the key parameters which contribute to
the effectiveness and design are within appropriate ranges for air
sparging.
Air sparging should not be used if the following site conditions exist:
O Free product is present. Air sparging can create groundwater
mounding which could potentially cause free product to migrate and
contamination to spread.
O Nearby basements, sewers, or other subsurface confined spaces are
present at the site. Potentially dangerous constituent concentrations
could accumulate in basements unless a vapor extraction system is
used to control vapor migration.
O Contaminated groundwater is located in a confined aquifer system. Air
sparging cannot be used to treat groundwater in a confined aquifer
because the injected air would be trapped by the saturated confining
layer and could not escape to the unsaturated zone.
The effectiveness of air sparging depends primarily on two factors:
O Vapor/dissolved phase partitioning of the constituents determines the
equilibrium distribution of a constituent between the dissolved phase
and the vapor phase. Vapor/dissolved phase partitioning is, therefore,
a significant factor in determining the rate at which dissolved
constituents can be transferred to the vapor phase.
O Permeability of the soil determines the rate at which air can be
injected into the saturated zone. It is the other significant factor in
determining the mass transfer rate of the constituents from the
dissolved phase to the vapor phase.
Effectiveness of air sparging can be gauged by determining these two
factors. In general, air sparging is more effective for constituents with
greater volatility and lower solubility and for soils with higher
permeability.
VTI-6 October 1994
-------
Exhibit VII-4 can be used as a screening tool to help you assess the
general effectiveness of air sparging for a given site. It provides boiling
point ranges for the petroleum products typically encountered at UST
sites as a rough gauge for vapor/dissolved phase partitioning. The higher
boiling point products contain more constituents of higher volatility (but
not necessarily lower solubility) which generally results in greater
partitioning to the vapor phase from the dissolved phase. Exhibit VII-4
also provides the range of intrinsic permeabilities for soil types typically
encountered at UST sites.
Exhibit VII-4
Initial Screening For Air Sparging Effectiveness
Permeability of Soil
Ineffective
Moderate to Minimal;
Effectiveness
Effective
Intrinsic Permeability, k (cm2)
-10-i« -i 0-i4 10-12 10-10 10-8 10-6 io~» 10-2
Cloy
Glacial Till
J
Sift, Loess
Silty Sond
Clean Sond |
Grovel |
Vapor/Dissolved Phase Partitioning
^Ineffective:
:Moderate to Minimal;
Effectiveness?
: Effective:;
Nonvolatile
Boiling Point (*C)
300 250 200
100
I Lube Oils I
Heating Oils
I
Diesel
Kerosene
Gasoline
October 1994
VH-7
-------
Detailed Evaluation Of Air Sparging Effectiveness
Once you have completed the initial screening and determined that air
sparging may have the potential to be effective for the soils and
petroleum product present, evaluate the CAP further to confirm that air
sparging will be effective.
Begin by reviewing the two major components that determine the
effectiveness of air sparging: (1) the vapor/dissolved phase partitioning of
the constituents and (2) the permeability of the soils. The combined
effect of these two components determines the rate at which the
constituent mass will be removed (i.e., the constituent mass removal
rate). This rate will decrease as air sparging operations proceed and
concentrations of dissolved constituents are reduced. They also
determine the placement and number of air sparge points required to
address the dissolved phase plume.
Many site-specific and constituent-specific parameters can be used to
determine vapor/dissolved partitioning and permeability. These
parameters are summarized in Exhibit VII-5. The remainder of this
section describes each parameter, why it is important to air sparging,
how it can be determined, and its range for effective air sparging.
Exhibit VII-5
Key Parameters Used To Evaluate Vapor/Dissolved Phase Partitioning And
Permeability Of Soil
Constituent Vapor/Dissolved
Phase Partitioning Permeability Of Soil
Henry's law constant Intrinsic permeability
Product composition and boiling point Soil structure and stratification
Vapor pressure Iron concentration dissolved in groundwater
Constituent concentration
Solubility
Factors That Contribute To Constituent Vapor/Dissolved
Phase Partitioning
Henry's Law Constant
The most important characteristic to evaluate vapor/dissolved phase
partitioning is the Henry's law constant, which quantifies the relative
tendency of a dissolved constituent to transfer to the vapor phase.
Henry's law states that, for ideal gases and solutions under equilibrium
conditions, the ratio of the partial pressure of a constituent in the vapor
Vn-8 October 1994
-------
phase to the concentration of the constituent in the dissolved phase is
constant. That is:
where:
Pa = partial pressure of constituent a in air (atm)
Ha = Henry's law constant (atm)
= Solution concentration of constituent (mole fraction)
Henry's law constants for several constituents commonly found in
petroleum products are shown in Exhibit VII-6. Constituents with
Henry's law constants greater than 100 atmospheres are generally
considered amenable to removal by air sparging.
Henry's Law Constant
Constituent
Tetraethyl lead
Ethylbenzene
Xylenes
Benzene
Toluene
Naphthalene
Ethylene dibromide
Methyl t-butyl ether
Exhibit VII-6
Of Common Petroleum Constituents
Henry's Law Constant At 20°C (atm)
4700
359
266
230
217
72
34
27
Product Composition And Boiling Point
Because petroleum products are often classified by their boiling point
range and because the boiling point of a compound is a measure of its
volatility, vapor/dissolved phase partitioning of the dissolved petroleum
product can be estimated from its boiling point range. However, because
vapor/dissolved phase partitioning is a function of both volatility and
solubility, boiling point range should be used only as a gauge to consider
effectiveness for the product in general.
The most commonly encountered petroleum products from UST
releases are gasoline, kerosene, diesel fuel, heating oils, and lubricating
oils. Petroleum products are a complex mixture often containing more
than 100 separate compounds. Each compound responds to air sparging
with differing levels of success based on its individual volatility. Shown
in Exhibit VII-7 are the boiling point ranges for common petroleum
products.
October 1994 VH-9
-------
Exhibit VII-7
Petroleum Product Boiling Point Ranges
Product
Boiling Point Range (°C)
Gasoline
Kerosene
Diesel fuel
Heating oil
Lubricating oils
40 to 225
180 to 300
200 to 338
>275
Nonvolatile
In general, constituents in petroleum products with boiling points less
than 250°C to 300°C are sufficiently volatile for removal from the
saturated zone by air sparging. Nearly all gasoline constituents and a
portion of kerosene and diesel fuel constituents can be removed from the
saturated zone by air sparging. Heating and lubricating oils cannot be
removed by air sparging. However, air sparging can promote biodegrada-
tion of semivolatile and nonvolatile constituents (see Chapter VIII:
Biosparging).
Vapor Pressure
Vapor pressure is another means by which the volatility of a
constituent can be determined and used as a gauge for vapor/dissolved
phase partitioning. The vapor pressure of a chemical is a measure of its
tendency to evaporate. More precisely, it is the pressure that a vapor
exerts when in equilibrium with its pure liquid or solid form.
Constituents with higher vapor pressures are generally transferred from
the dissolved phase to the vapor phase more easily. Those constituents.
with vapor pressures higher than 0.5 mm Hg are considered to be
amenable to air sparging. Exhibit VII-8 presents vapor pressures of some
common petroleum constituents.
Vapor Pressures
Constituent
Methyl t-butyl ether
Benzene
Toluene
Ethylene dibromide
Ethylbenzene
Xylenes
Naphthalene
Tetraethyl lead
Exhibit VII-8
Of Common Petroleum Constituents
Vapor Pressure
(mm Hg at 20°C)
245
76
22
11
7
6
0.5
0.2
vn-io
October 1994
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Constituent Concentrations
If it is determined that air sparging is a potentially viable technology
for the site, the initial and the target cleanup levels for the contaminants
in the groundwater must be evaluated. No apparent upper level of
contaminant concentration exists for air sparging to be effective;
however, if floating free product is present, air sparging is not suitable
because induced groundwater mounding can spread the contamination.
Thus, any free product must be removed prior to initiating air sparging.
The achievable cleanup level may vary greatly depending on the
contaminant type and soil concentrations. Exhibit VII-9 presents
examples of the effectiveness of air sparging (used with SVE). After
varying operational durations, each system reached a residual
contaminant level that could not be lowered {listed as the final
concentration).
Solubility
The aqueous solubility of a constituent is a measure of the maximum
weight of the constituent that can be dissolved in water. Solubility, like
volatility, is a component of the vapor/dissolved phase partitioning
behavior for a constituent. However, solubility is less important than
vapor pressure and Henry's law constant and should not be used as the
sole gauge for air sparging effectiveness. Thus, no threshold value can be
provided. Constituents with relatively high solubility, such as benzene,
can still exhibit sufficiently high vapor/dissolved phase partitioning for
air sparging when they possess high volatility (vapor pressure). When
considering a constituent for removal by air sparging, however, it is
important to consider that sparging creates turbulence in the subsurface
which will enhance dissolution of constituents adsorbed to saturated
zone soils. Constituents with relatively high solubilities and low Henry's
law constants, such as MTBE and ethylene dibromide, could be
mobilized through dissolution but not removed effectively by air
sparging. Exhibit VII-10 lists the solubilities of several constituents
typically found in petroleum products at UST sites.
Factors That Contribute To Permeability Of Soil
Intrinsic Permeability
Intrinsic permeability is a measure of the ability of soils to transmit
fluids and is the single most important characteristic of the soil in
determining the effectiveness of air sparging. Intrinsic permeability
varies over 13 orders of magnitude (from 10~16 to 10"3 cm2) for the wide
range of earth materials, although a more limited range applies to most
soil types (10'13 to 10"5 cm2). Although the intrinsic permeability of the
October 1994 VH-11
-------
I
>-l
to
Exhibit VII-9
Summary Of Air Sparging Applications (Used With SVE)
Site
A
B
E
F
G
Soil Type
Alluvial sands, silts, and
clay
Silly sand, interfering clay
layer
NR
Sand, silt
Fine-coarse sand, gravel
Depth to
Groundwater
(feet)
6.5-16
6.5
NR
8-13
15.5-16
Product
gasoline
gasoline
gasoline
gasoline
gasoline
Cleanup Time
(months)
2
5
10
24
2
Initial Groundwater
Concentrations (mg/L)
BTEX: 4-25
Benzene: 3-6
Benzene: > 30
BTEX: 6-24
BTEX: 21
Final Groundwater
Concentrations
(mg/L)
BTEX: 0.25-8
59% average reduction
Benzene: < 5
BTEX: 0.38-7.6
BTEX: < 1
O
I
BTEX = Benzene, Toluene, Ethylbenzene, and Xylene
NR = Not Reported
Source: Adapted from R.A. Brown et al., Treatment of a Solvent Contaminated She with Air Sparging/Soil Vapor Extraction.
-------
Exhibit VII-10
Solubility Of Common Petroleum Constituents
Solubility
Constituent (mg/L at 20°C)
Methyl t-butyl ether 48,000
Ethylene dibromide 4,310
Benzene 1,780
Toluene 515
Xylene 185
Ethylbenzene 152
Naphthalene 30
Tetraethyl lead 0.0025
saturated zone (for air sparging) and unsaturated zone (when SVE is
used) is best determined from field tests, it can be estimated from soil
boring logs and laboratory tests. Coarse-grained soils (e.g., sands) have
greater intrinsic permeability than fine-grained soils (e.g., clays and
silts). Use the values shown in Exhibit VII-11 to determine if intrinsic
permeability is within the range of effectiveness for air sparging.
Exhibit Vll-11
Intrinsic Permeability And Air Sparging Effectiveness
Intrinsic Permeability (k)(cnr) Air Sparging Effectiveness
k > 10'9 Generally effective
10"9 > k > 10"10 May be effective; needs further evaluation
k < 10"10 Marginal effectiveness to ineffective
Intrinsic permeability of saturated-zone soils is usually determined in
the field by aquifer pump tests that measure hydraulic conductivity. You
can convert hydraulic conductivity to intrinsic permeability using the
following equation:
k = K (u/pg)
where: k = intrinsic permeability (cm2)
K = hydraulic conductivity (cm/sec)
u = water viscosity (g/cm • sec)
p = water density (g/cm3)
g = acceleration due to gravity (cm/sec2)
October 1994 VH-13
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At 20°C: u/pg = 1.02 • 10"5 cm/sec
To convert k from cm2 to darcy, multiply by 10s
Intrinsic permeability of the unsaturated zone can be estimated from
the intrinsic permeability of the saturated zone if similar soil types are
present or can be determined in the field by conducting permeability
tests or SVE pilot studies. (See Chapter II: Soil Vapor Extraction.)
Soil Structure And Stratification
The types of soil present and their micro- and macro-structures
control the air sparging pressure and the distribution of air in the
saturated zone. For example, fine-grained soils require higher sparging
air pressures because gas pockets have a tendency to form in these
types of soils, thereby further reducing the minimal effectiveness of
sparging for treating them. Greater lateral dispersion of the air is likely
in fine-grained soils and can result in lateral displacement of the
groundwater and contaminants if groundwater control is not maintained.
Soil characteristics will also determine the preferred zones of vapor
flow in the vadose zone, thereby indicating the ease with which vapors
can be controlled and extracted using SVE (if used).
Stratified or highly variable heterogeneous soils typically create the
greatest barriers to air sparging. Both the injected air and the stripped
vapors will travel along the paths of least resistance (coarse-grained
zones) and could travel a great lateral distance from the injection point.
This phenomenon could result in the contaminant-laden sparge vapors
migrating outside the vapor extraction control area.
Information about soil type, structure, and stratification can be
determined from boring logs or geologic cross-section maps. You should
verify that soil types have been identified and visual observations of soil
structure have been documented.
Iron Concentration Dissolved In Groundwater
The presence of dissolved iron (Fe+2) in groundwater can reduce the
permeability of the saturated zone soils during air sparging operations.
When dissolved iron is exposed to oxygen, it is oxidized to insoluble iron
(Fe+3) oxide which can precipitate within the saturated zone and occlude
soil pore space, thereby reducing the region available for air (and
groundwater) flow, and reducing permeability. Precipitation of iron oxide
occurs predominantly in the saturated zone near air sparging well
screens where oxygen content (from injected air) is the highest. This
oxidation can render air sparging wells useless after even short periods
of operation and necessitate the installation of new wells in different
locations.
Vn-14 October 1994
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You should verify that laboratory measurements of total dissolved iron
have been completed for groundwater samples from the site. Use Exhibit
VII-12 to determine the range where dissolved iron is a concern for air
sparging effectiveness.
Exhibit Vil-12
Dissolved Iron And Air Sparging Effectiveness
Dissolved Iron Concentration (mg/L) Air Sparging Effectiveness
Fe+2 < 10 Air sparging effective
10 £ Fe+2 < 20 Air sparging wells require periodic testing
and may need periodic replacement
Fe+2 > 20 Air sparging not recommended
Field Pilot-Scale Studies
Field pilot studies are necessary to adequately design and evaluate
any air sparging system. However, pilot tests should not be conducted if
free product is known to exist at the groundwater table, if uncontrolled
vapors could migrate into confined spaces, sewers, or buildings, or if the
contaminated groundwater is in an unconfined aquifer. The air sparge
well used for pilot testing is generally located in an area of moderate
constituent concentrations. Testing the system in areas of extremely low
constituent concentrations may not provide sufficient data. In addition,
because sparging can induce migration of constituents, pilot tests are
generally not conducted in areas of extremely high constituent
concentrations. The air sparging pilot study should include an SVE pilot
study if SVE is to be included in the design of the air sparging system.
Pilot studies for air sparging often include SVE pilot testing to
determine if SVE can be used to effectively control the vapor plume. Pilot
studies, therefore, should include the installation of a single sparge
point, several vapor extraction points (if SVE is to be included in the
design), and soil gas monitoring points to evaluate vapor generation
rates and to define the vapor plume. Existing groundwater monitoring
wells (normally not fewer than three to five wells around the plume) that
have been screened above the saturated zone and through the dissolved
phase plume can be used to monitor both dissolved and vapor phase
migration, to monitor for changes in dissolved oxygen, and to measure
changes in the depth to the groundwater table surface. Additional vapor
probes should be used to further define the vapor plume and identify
any preferential migration pathways. These probes should be designed
and installed as discussed in Chapter II: Soil Vapor Extraction.
October 1994 VH-15
-------
If SVE is to be used in the air sparging system, the first portion of the
test should be conducted using vapor extraction only and evaluated as
described in Chapter II: Soil Vapor Extraction without the air sparging
system being operated. This portion of the pilot test will establish the
baseline vapor extraction levels, the extent of the non-sparged vapor
plume, the SVE well radius of influence, and the intrinsic permeability of
the unsaturated zone (discussed in Chapter II). The air sparging portion
of the test should be conducted with the sparging point operating at
variable sparge pressures (e.g., 5 pounds per square inch-gauge [psig],
10 psig) and different depths (e.g., 5 feet, 10 feet below the dissolved
phase plume). It is essential that vapor equilibrium be obtained prior to
changing the sparge rate or depth. When no change in vapor emission
rates from baseline occurs, the air sparging system may not be
controlling the sparge vapor plume, possibly due to soil heterogeneity.
Assess the potential for this problem by reviewing the site's soil lithology,
typically documented on soil boring logs. During this test, the hydraulic
gradient and VOC concentrations in soil vapors extracted from
monitoring wells must be monitored until equilibrium is reached.
The final portion of the pilot test is the concurrent operation of the
SVE pilot system and the air sparging system. This portion of the test
will determine the optimum SVE system (i.e., the number and orienta-
tion of wells) that will capture the sparged VOCs for various sparging
rates. In addition, this portion of the test requires monitoring of VOC
emissions, sparging pressure and flow rates, SVE vacuum and flow
rates, monitoring well vapor concentrations, and dissolved constituent
concentrations. Exhibit VII-13 presents a summary of the Pilot Test Data
Objectives.
Evaluation Of The Air Sparging System Design
Once you have verified that air sparging is applicable to your site, you
can evaluate the design of the system. The CAP should include a discus-
sion of the rationale for the system design and the results of the pilot
tests. Detailed engineering design documents might also be included,
depending on individual state requirements. Discussion of the SVE
portion of the design is included in Chapter II: Soil Vapor Extraction.
Rationale For The Design
The following factors should be considered as you evaluate the design
of the air sparging system in the CAP.
O Design ROI for air sparging wells. The ROI is the most important
parameter to be considered in the design of the air sparging system.
The ROI is defined as the greatest distance from a sparging well at
which sufficient sparge pressure and airflow can be induced to
enhance the mass transfer of contaminants from the dissolved phase
to the vapor phase. The ROI will help determine the number and
spacing of the sparging wells.
Vn-16 October 1994
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Exhibit VIM 3
Pilot Test Data Objectives
Data Requirement
Source
SVE Test Portion (if necessary)
SVE radius of influence (ROI)
Wellhead and monitoring point vacuum
Initial contaminant vapor concentrations
Initial hydraulic gradient
Air Sparging Test Portion
Air sparging ROI
Sparging rate
Sparging vapor concentrations
Hydraulic gradient influence
Dissolved oxygen and carbon dioxide
Combined Test (if necessary)
Sparging/SVE capture rates
Constituent vapor concentrations
Monitoring point pressure gauges
Well head pressure gauge
SVE exhaust flame ionization detector (FID)
readings (or other suitable detection device)
Water level tape at monitoring wells or
pressure transducers and data logger
Monitoring point pressure gauge
Compressor discharge flow gauge
Monitoring well and vapor point FID readings
(or other suitable detection device)
Water level tape at monitoring wells or
pressure transducers and data logger
Dissolved oxygen and carbon dioxide probes
at monitoring wells
Pressure/flow gauges
Blower discharge and monitoring points
The ROI should be determined based on the results of pilot tests. One
should be careful, however, when evaluating pilot test results because
the measurement of air flow, increased dissolved oxygen, or the
presence of air bubbles in a monitoring point can be falsely
interpreted as an air flow zone that is thoroughly permeated with
injected air. However, these observations may only represent localized
sparging around sparsely distributed air flow channels. The ROI
depends primarily on the hydraulic conductivity of the aquifer
material in which sparging takes place. Other factors that affect the
ROI include soil heterogeneities, and differences between lateral and
vertical permeability of the soils. Generally, the design ROI can range
from 5 feet for fine-grained soils to 100 feet for coarse-grained soils.
O Sparging Air Flow Rate. The sparging air flow rate required to provide
sufficient air flow to enhance mass transfer is site-specific and will be
determined via the pilot test Typical air flow rates range from 3 to 25
standard cubic feet per minute (scfm) per injection well. Pulsing of the
air flow (i.e., turning the system on and off at specified intervals) may
provide better distribution and mixing of the air in the contaminated
saturated zone, thereby allowing for greater contact with the dissolved
phase contaminants- The vapor extraction system should have a
October 1994
VH-17
-------
greater flow capacity and greater area of influence than the air
sparging system. The air sparging rate should vary between 20
percent and 80 percent of the soil vapor extraction flow rate.
O Sparging Air Pressure is the pressure at which air is injected into the
saturated zone. The saturated zone requires pressures greater than
the static water pressure (1 psi for every 2.3 ft of hydraulic head) and
the head necessary to overcome capillary forces of the water in the
soil pores near the injection point. A typical system will be operated at
approximately 10 to 15 psig. Excessive pressure may cause fracturing
of the soils and create permanent air channels that can significantly
reduce air sparging effectiveness.
O Initial Constituent Vapor Concentrations are measured during pilot -
studies. They are used to estimate constituent mass removal rates
and system operational time requirements and to determine whether
treatment of extracted vapors will be required prior to atmospheric
discharge or reinjection.
O Required. Final Dissolved Constituent Concentrations in the saturated
zone will determine which areas of the site require treatment and
when air sparging system operations can be terminated. These levels
are usually defined by state regulations as remedial action levels. In
some states, these levels are determined on a site-specific basis using
transport modeling and risk assessment.
O Required Remedial Cleanup Time may influence the design of the
system. The designer may vary the spacing of the sparging wells to
speed remediation to meet cleanup deadlines, if required.
O Saturated Zone Volume To Be Treated is determined by state action
levels or a site-specific risk assessment using site characterization
data for the groundwater.
O Pore Volume Calculations are used along with extraction flow rate to
determine the pore volume exchange rate. Some literature suggests
that at a minimum one pore volume of soil vapor should be extracted
daily for effective remedial progress.
O Discharge Limitations And Monitoring Requirements are usually
established by state regulations but must be considered by designers
of an air sparging system which uses SVE to ensure that monitoring
ports are included in the system hardware. Discharge limitations
imposed by state air quality regulations will determine whether offgas
treatment is required.
O Site Construction Limitations (e.g., building locations, utilities, buried
objects, residences) must be identified and considered in the design
process.
Vn-18 October 1994
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What Are The Typical Components Of An Air Sparging System?
Once the rationale for the design is defined, th§ design of the air
sparging system can be developed. A typical air sparging system design
may include the following components and information:
O Well orientation, placement, andx construction details.
O Manifold piping.
O Compressed air equipment.
O Monitoring and controls.
If an SVE system is used for vapor control, the following components
and information will also be needed:
O Vapor pretreatment design.
O Vapor treatment system selection.
O Blower specification.
Exhibit VII- 14 provides a schematic diagram of a typical air sparging
system used with SVE. Chapter II: Soil Vapor Extraction should be
consulted for information on the design of the SVE portion of the
remedial system (if necessary) including vapor pretreatment design,
vapor treatment system selection, and blower specification.
Exhibit VII-14
Schematic Of Air Sparging System Used With SVE
Ambient
Air _ , .
Cpndensati
Separator
Blow Back Loop
Discharge to
ospnere
Mav
Required)
Legend:
PI Pressure Indicator
SP Sampling Port
\&\ Flow Control Valve
1=3 Flow Meter
Slotted Vertical
Air Sparge Point
(Typical)
October 1994
VH-19
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Sparge And Extraction Wells
Well Orientation. An air sparging system can use either vertical or
horizontal sparge wells. Well orientation should be based on site-specific
needs and conditions. For example, horizontal systems should be
considered when evaluating sites that will require 10 or more sparge or
extraction points or if the affected area is under an operational facility.
Exhibit VH-15 lists site conditions and the corresponding appropriate
well orientation.
Exhibit VIMS
Well Orientation And Site Conditions
Well Orientation
Site Conditions
Vertical wells
Horizontal wells
o Deep contamination (> 25 feet)
o Depth to groundwater (> 10 feet)
o Fewer than 10 wells
o Shallow groundwater table (< 25 feet)
o Zone of contamination within a specific
stratigraphic unit
o System under an operational facility
Well Placement And Number of Wells. Exhibit VII-16, Air Sparging/Vapor
Extraction Well Configurations, shows various configurations that can be
used in laying out air sparging systems used in conjunction with SVE.
The essential goals in configuring the wells and monitoring points are (1)
to optimize the influence on the plume, thereby maximizing the removal
efficiency of the system and (2) to provide optimum monitoring and vapor
extraction points to ensure minimal migration of the vapor plume and no
undetected migration of either the dissolved phase or vapor phase
plumes. In shallow applications, in large plume areas, or in locations
under buildings or pavements, horizontal vapor extraction wells are very
cost effective and efficient for controlling vapor migration. Exhibit VII-17
is a typical layout of a system that surrounds and contains a plume and
includes air sparging and SVE wells.
vn-20
October 1994
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Exhibit VI1-16
Air Sparging/Soil Vapor Extraction Well Configurations
Extraction
Well
Sparging
Well
Extraction
Well
a) Spaced Configuration
b) Nesied Wells
Extraction
Well
iiiiiiimimiiiHiiiiiiMiiiiiiiiHirmr
"'"» "' iiiiiiiiiiiiniiiiii
Sparging
Well
Extraction
Well
Sparging
Well
iiiiiiiiiiiiiiiinnr
c) Horizontal Wells
d) Combined Horizontal/Vertical Wells
Source: "Advances in Air Sparging Design," The Hazardous Waste Consultant, VoL 11,
Issue 1, January/February 1993, p. 1-4.
The number and location of extraction wells can be determined by
using several methods as discussed in Chapter II: Soil Vapor Extraction.
However, the following general points should be considered:
O Closer well spacing is often appropriate in areas of high contaminant
concentrations in order to enhance air distribution and removal rates.
O If a surface seal exists or is planned for the design, the extraction
wells can be spaced slightly farther apart because air is drawn from a
greater distance and not directly from the surface.
O At sites with stratified soils, wells screened in strata with low
permeabilities might require closer well spacing than wells screened in
strata with higher permeabilities.
October 1994
VH-21
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Exhibit VII-17
Combined Air Sparging/SVE System Layout
Equipment
Compound
Legend:
& Air Sparging Well
H SVE Well
SVE (Vacuum) Manifold
Air Sparging (Compressed Air) Manifold
' Extent of Dissolved Petroleum Contamination
Well Construction. The air sparging (injection) wells are generally
constructed of 1 to 5 inch FVC or stainless steel pipe. The screened
interval is normally from 1 to 3 feet and is generally set from 5 to 15 feet
below the deepest extent of adsorbed contaminants. Setting the screen at
a deeper interval requires higher pressures on the system but generally
does not achieve higher sparge rates. Increased screened intervals do not
improve system efficiency because air tends to exit at the top portion of
the screen. Air sparging wells must be properly grouted to prevent short
circuiting of the air. Horizontal injection wells should be designed and
installed carefully to ensure that air exits from along the entire screen
length. Perforated pipe, rather than well screening, is sometimes
preferable. Exhibits VII-18 and VII-19 present typical vertical and
horizontal air sparging well constructions, respectively.
Injection wells should be fitted with check valves to prevent potential
line fouling caused by pressure in the saturated zone forcing water up
the point when the system is shut down. Each air sparging well should
also be equipped with a pressure gauge and flow regulator to enable
adjustments in sparging air distribution. Refer to Chapter II: Soil Vapor
Extraction for vapor extraction well details.
VTX-22
October 1994
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Exhibit VIMS
Typical Vertical Air Sparging Well Construction
Grade
/—Pressure Indicator
11—Flow Regulating Valve
//e—Check Valve
Manifold
From Air
Compressor
Sched. 40 PVC
Solid Casing
Cement/Bentonite Seal
Bentonite
Sand Pack
Slotted Sched. 40
PVC Well Screen
Flat Bottomed, Sched.
40 PVC Threaded Plug
Exhibit VII-19
Typical Horizontal Air Sparging Well Construction
From Air
Compressor
Note:
Piping may be buried
in utility trenches.
Fabric Liner
Bentonite
Backfilled Soil
II II I I I I II II I I I I I I I II II I I I I III II I I I I I I I II I II
5oV'T) n c%O Oa C* n C"iO o O,*5 a !O1-* '~~J j O£j O£? C ^O^mj vt
PVC Threaded Cap
Slotted PVC Pipe
Pea Gravel
October 1994
VH-23
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Manifold Piping
Manifold piping connects the sparging wells to the air compressor.
Rping can be placed either above or below grade depending on site
operations, ambient temperature, and local building codes. Below-grade
piping is more common and is installed in shallow utility trenches that
lead from the sparging wellhead vaultfs) to a central equipment location.
The piping can be either manifolded in the equipment area or connected
to a common compressor main that supplies the wells in series, in which
case flow control valves are located at the wellhead. Piping to the well
locations should be sloped toward the well so that condensate or
entrained groundwater will flow back toward the well.
The pressurized air distribution system can be made of metal pipe or
rubber-reinforced air hose. PVC pipe should not be connected directly to
the compressor because of the high temperatures of air leaving the
compressor which can diminish the integrity of the FVC. If pipe trenches
are used for the distribution system, they must be sealed to prevent
short circuiting of air flow.
Compressed Air Equipment
An oil-free compressor or a standard compressor equipped with
downstream coalescing and particulate filters should be used to ensure
that no contaminants are injected into the saturated zone. The
compressor should be rated for continuous duty at the maximum
expected flow rate and pressure to provide adequate flexibility during full
operations.
Monitoring And Controls
The parameters typically monitored in an air sparging system include:
O Pressure (or vacuum)
O Air/vapor flow rate
The equipment in an air sparging system used to monitor these
parameters provides the information necessary to make appropriate
system adjustments and track remedial progress. The control equipment
in an air sparging system allow the flow and sparge pressure to be
adjusted at each sparging well of the system, as necessary. Control
equipment typically includes flow control valves/regulators.
Exhibit VII-20 lists typical monitoring and control equipment for an air
sparging system, where each of these pieces of equipment should be
placed, and the types of equipment that are available.
VH-24
October 1994
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Exhibit Vll-20
Monitoring And Control Equipment
Monitoring Equipment
Flow meter
Pressure gauge
Vapor or air sparge
temperature sensor
Sampling port
Control Equipment
Flow control valves/
regulators
Location In System
o At each injection and
vapor extraction well
head
o Manifold to blower
o Stack discharge
o At each injection and
vapor extraction well
head or manifold branch
o Before blower (before
and after filters)
o Before and after vapor
treatment
o Manifold to blower
o Blower or compressor
discharge (prior to vapor
treatment)
o At each vapor extraction
well head or manifold
branch
o Manifold to blower
o Blower discharge
o At each vapor extraction
well head or manifold
branch
o Dilution or bleed valve at
manifold to blower
o At header to each sparge
point
Example Of Equipment
o Pitottube
o in-line rotameter
o Orifice plate
o Venturi or flow tube
o Manometer
o Magnehelic gauge
o Vacuum gauge
o Bi-metal dial-type
thermometer
o Thermocouple
o Hose barb
o Septa fitting
o Ball valve
o Gate valve
o Dilution/ambient air bleed
valve
o Gate valve
o Dilution/ambient air bleed
valve
Evaluation Of Operation And Monitoring Plans
The system operation and monitoring plan should include both
system startup and long-term operations. Operations and monitoring are
necessary to ensure optimal system performance and to track the rate of
contaminant mass removal.
October 1994
VH-25
-------
Startup Operations
The startup phase should begin with only the SVE portion of the
system (if used) as described in Chapter II. After the SVE system is
adjusted, the air sparging system should be started. Startup operations
should include 7 to 10 days of manifold valving adjustments to balance
injection rates and optimize mass flow rates. Injection and extraction
rates, pressures, depth to groundwater, hydraulic gradient, and VOC
levels should be recorded hourly during Initial startup until the flow is
stabilized. Injection rates should then be monitored daily. Vapor
concentration should also be monitored in any nearby utility lines,
basements, or other subsurface confined spaces. Other monitoring of the
system should be done in accordance with the SVE requirements from
Chapter II.
Long-Term Operations
Long-term monitoring should consist of contaminant level
measurements (in the groundwater, vapor wells, and blower exhaust),
flow-balancing (including flow and pressure measurements), and vapor
concentration readings. Measurements should take place at biweekly to
monthly intervals for the duration of the system operational period.
Samples collected during sparging operations may give readings that
show lower concentrations of dissolved contaminants than those found
in the surrounding aquifer. These readings could lead to the erroneous
conclusion that remediation is occurring throughout the aquifer.
Therefore, contaminant concentrations should be determined shortly
following system shutdown, when the subsurface environment has
reached equilibrium.
Exhibit VII-21 provides a brief synopsis of system monitoring
requirements.
Remedial Progress Monitoring
Monitoring the performance of the air sparging system in reducing
contaminant concentrations in the saturated zone is necessary to
determine if remedial progress is proceeding at a reasonable pace. A
variety of methods can be used. One method includes monitoring
contaminant levels in the groundwater and vapors in the monitoring
wells and blower exhaust, respectively. The vapor and contaminant
concentrations are then each plotted against time.
VH-26
October 1994
-------
Exhibit Vll-21
System Monitoring Recommendations
Phase
Startup (7-10 days)
Long-term
(ongoing)
Monitoring
Frequency
At least daily
.Biweekly to monthly
Quarterly to
annually
What To Monitor
o Sparge pressure
o Flow
o Vacuum readings (SVE)
o Vapor concentrations (SVE)
o Flow (SVE)
o Vacuum readings (SVE)
o Sparge pressure
o Vapor concentrations (SVE)
o Dissolved constituent
concentrations
o
o
o
o
o
o
o
0
0
Where To Monitor
Air sparging wellhead
Sparge and extraction
wells
Manifold
Effluent stack
Extraction vents
Manifold
Air sparging wellhead
Effluent stack
Groundwater
monitoring wells
Remedial progress of air sparging systems typically exhibits
asymptotic behavior with respect to both dissolved-phase and vapor-
phase concentration reduction (Exhibit VII-22). Systems that use SVE
can monitor progress through mass removal calculations. (See
Chapter II: Soil Vapor Extraction for calculations.) When asymptotic
behavior begins to occur, the operator should evaluate alternatives that
increase the mass transfer removal rate (e.g., pulsing, or turning off the
system for a period of time and then restarting it). Other more aggressive
steps to further reduce constituent concentrations can include
installation of additional air sparging or extraction wells.
If asymptotic behavior is persistent for periods greater than about 6
months and the concentration rebound is sufficiently small following
periods of temporary system shutdown, the appropriate regulatory
officials should be consulted; termination of operations may be
appropriate.
October 1994
VH-27
-------
Exhibit VII-22
Concentration Reduction And Mass Removal Behavior For Both
Air Sparging And SVE Systems
TJ
II
II
OL C
®
M U
2 C
13
Cumulative
VOC Mass
Removal (Ibs)
Asymptotic
•Behavior:
i(lrreducible)
VOC Concentrations
In Extracted Soft
Vapor (ppm)
Operation Time-
Vn-28
October 1994
-------
References
Brown, L.A. and R. Fraxedas, "Air sparging extending volatilization to
contaminated aquifers." Proceedings of the Symposium on Soil Venting,
April 29-May 1, 1991, Houston, Texas, pp. 249-269. U.S. EPA, Office
of Research and Development. EPA/600/R-92/174, 1992.
Johnson, R.L., P.C. Johnson, D.B. McWhorter, R.E. Hinchee, and I.
Goodman. "An overview of in situ air sparging." Ground Water
Monitoring Review. Vol. 13, No. 4, pp. 127-135, 1993.
Hinchee, R.E. Air Sparging for Site Remediation. Boca Raton, FL: Lewis
Publishers, 1994.
Marley, M., D.J. Hazenbronck, and M.T. Walsh. 'The application of in
situ air sparging as an innovative soils and groundwater remediation
technology." Ground Water Monitoring Review. Vol. 12, No. 2, pp. 137-
145, 1992.
Martin, L.M., R.J. Sarnelli, and M.T. Walsh. "Pilot-scale evaluation of
groundwater air sparging: site-specific advantages and limitations."
Proceedings ofR&D 92-Natipnal Research and Development Conference
on the Control of Hazardous Materials. Greenbelt, MD: Hazardous
Materials Control Research Institute, 1992.
U.S. Environmental Protection Agency. A Technology Assessment of Soil
Vapor Extraction and Air Sparging. Washington, D.C.: Office of
Research and Development. EPA/600/R-92/173, 1992.
October 1994 Vn-29
-------
Checklist: Can Air Sparging Be Used At This Site?
This checklist can help you to evaluate the completeness of the CAP
and to identify areas that require closer scrutiny. As you go through the
CAP, answer the following questions. If the answer to, several questions
is no, you will want to request additional information to determine if air
sparging will accomplish the cleanup goals at the site.
1. Factors That Contribute To The Vapor/Dissolved Phase
Partitioning Of The Constituents
Yes No
Q Q Is the Henry's law constant for the contaminant greater
than 100 atm?
Q Q Are the boiling points of the contaminant constituents less
than 300°C?
Q Q Is the contaminant vapor pressure greater than 0.5 mm Hg?
2. Factors That Contribute To Permeability Of Soil
Yes No
Q Q Is the intrinsic permeability greater than 10"9 cm2?
Q Q Is the soil free of impermeable layers or other conditions that
would disrupt air flow?
Q Q Is the dissolved iron concentration at the site < 10 mg/L?
3. Evaluation Of The Air Sparging System Design
Yes No
Q Q Does the radius of influence (ROI) for the proposed air
sparging wells fall in the range 5 to 100 feet?
Q Q Has the ROI been calculated for each soil type at the site?
Q Q Examine the sparging air flow rate. Will these flow rates
provide sufficient vapor/dissolved phase partitioning of
constituents to achieve cleanup in the time allotted for
remediation in the CAP?
VH-3O October 1994
-------
3. Evaluation Of The Air Sparging System Design (continued)
Yes No
Q Q Examine the sparging air pressure. Will the proposed
pressure be sufficient to overcome the hydraulic head and
capillary forces?
Q Q Is the number and placement of wells appropriate, given the
total area to be cleaned up and the radius of influence of
each well?
Q Q Do the proposed well screen intervals account for
contaminant plume location at the site?
Q Q Is the proposed well configuration appropriate for the site
conditions present?
Q Q Is the air compressor selected appropriate for the desired
sparge pressure?
4.Operation And Monitoring Plans
Yes No
Q Q Does the CAP propose starting up the SVE system prior to
starting the air sparging system?
Q Q Are manifold valving adjustments proposed during the first 7
to 10 days of operation?
Q Q Is monitoring for sparge pressure and flows, vacuum
readings (for SVE), groundwater depth, vapor concentrations,
dissolved oxygen levels, carbon dioxide levels, and pH
proposed for the first 7 to 10 days of operation?
Q Q Is weekly to biweekly monitoring of groundwater pH and
levels of contaminants, carbon dioxide, and dissolved oxygen
proposed following startup?
Q Q Is weekly to biweekly monitoring of the effluent stack for
levels of contaminants, oxygen, and carbon dioxide proposed
following startup?
October 1994 VH-31
-------
-------
Chapter VIII
Biosparging
-------
-------
Contents
Overview VIII-1
Initial Screening Of Biosparging Effectiveness VIII-7
Detailed Evaluation Of Biosparging Effectiveness . . . VIII-9
Site Characteristics That Affect Biosparging VIII-9
Intrinsic Permeability VIII-9
Soil Structure And Stratification VIII-11
Temperature Of The Groundwater VIII-11
pH Levels VIII-11
Microbial Population Density VIII-12
Nutrient Concentrations Vin-13
Iron Concentration Dissolved In Groundwater VIII-13
Constituent Characteristics That Affect Biosparging VIII-14
Chemical Structure . . . . VIII-14
Concentration And Toxicity VIII-15
Vapor Pressure VIII-17
Product Composition And Boiling Point VIII-17
Henry's Law Constant VIII-18
Laboratory Treatability And Field Pilot Scale Studies VIII-18
Evaluation Of The Biosparging System Design VIII-20
Rationale For The Design VIII-21
Components Of A Biosparging System VIII-23
Sparge And Extraction Wells VIII-24
Manifold Piping VIII-27
Compressed Air Equipment VIII-28
Monitoring And Controls VTII-29
Evaluation Of Operation And Monitoring Plans VIII-30
Startup Operations VIII-31
Long-Term Operations VIII-31
Remedial Progress Monitoring VIII-32
References VIII-34
Checklist: Can Biosparging Be Used At This Site? VIII-35
October 1994 Vm-iii
-------
List Of Exhibits
Number Title Page
VIII-1 Biosparging System (Used With
Soil Vapor Extraction) VIII-2
VIII-2 Advantages And Disadvantages Of Biosparging VIII-3
VIII-3 Biosparging Evaluation Process Flow Chart VIII-4
VIII-4 Initial Screening For Biosparging Effectiveness VIII-8
VIII-5 Key Parameters Used To Evaluate The
Suitability Of Biosparging VIII-9
VIII-6 Intrinsic Permeability And Biosparging
Effectiveness VTII-10
VTII-7 Heterotrophic Bacteria And Biosparging
Effectiveness VIII-13
VIII-8 Dissolved Iron And Biosparging Effectiveness VIII-14
VIII-9 Chemical Structure And Biodegradability VIII-15
VIII-10 Constituent Concentration And Biosparging
Effectiveness VIII-16
VIII-11 Cleanup Concentrations And Biosparging
Effectiveness VIII-16
VIII-12 Vapor Pressures Of Common Petroleum
Constituents VIII-17
VIII-13 Petroleum Product Boiling Ranges VIII-18
VIII-14 Henry's Law Constant Of Common Petroleum
Constituents VIII-19
VIII-15 Pilot Test Data Objectives VIII-21
VIII-16 Schematic Of Biosparging System Used
With Vapor Extraction VIII-24
VIII-17 Well Orientation And Site Conditions VIII-25
VIII-18 Biosparging/Vapor Extraction Well Configurations .... VIII-26
vm-iv October 1994
-------
List Of Exhibits (cont'd)
Number Title Page
VIII-19 Combined Biosparging/Vapor Extraction
System Layout VHI-27
VIII-20 Vertical Sparging Well Construction VIII-28
VIII-21 Horizontal Sparging WeU Construction Vffl-29
VIH-22 Monitoring And Control Equipment VIII-30
VIII-23 System Monitoring Recommendations VIII-32
VIII-24 Concentration Reduction And Mass Removal
Behavior For Biosparging Systems VIII-33
October 1994 vm-v
-------
-------
Chapter VIII
Biosparging
Overview
Biosparging is an in-situ remediation technology that uses indigenous
microorganisms to biodegrade organic constituents in the saturated
zone. In biosparging, air (or oxygen) and nutrients (if needed) are injected
into the saturated zone to increase the biological activity of the
indigenous microorganisms. Biosparging can be used to reduce
concentrations of petroleum constituents that are dissolved in
groundwater, adsorbed to soil below the water table, and within the
capillary fringe. Although constituents adsorbed to soils in the
unsaturated zone can also be treated by biosparging, bioventing is
typically more effective for this situation. (Chapter III provides a detailed
description of bioventing.)
The biosparging process is similar to air sparging. However, while air
sparging removes constituents primarily through volatilization,
biosparging promotes biodegradation of constituents rather than
volatilization (generally by using lower flow rates than are used in air
sparging). In practice, some degree of volatilization and biodegradation
occurs when either air sparging or biosparging is used. (Air sparging is
discussed in Chapter VII.)
When volatile constituents are present, biosparging is often combined
with soil vapor extraction or bioventing (collectively referred to as vapor
extraction in this chapter), and can also be used with other remedial
technologies. When biosparging is combined with vapor extraction, the
vapor extraction system creates a negative pressure in the vadose zone
through a series of extraction wells that control the vapor plume
migration. Chapters II and III provide detailed discussions of soil vapor
extraction and bioventing, respectively. Exhibit VIII-1 provides a
conceptual drawing of a biosparging system with vapor extraction.
The existing literature contains case histories describing both the
successes and failures of biosparging; however, because the technology
is relatively new, few cases provide substantial documentation of
performance. When used appropriately, biosparging is effective in
reducing petroleum products at underground storage tank (UST) sites.
Biosparging is most often used at sites with mid-weight petroleum
products (e.g., diesel fuel, jet fuel); lighter petroleum products (e.g.,
gasoline) tend to volatilize readily and to be removed more rapidly using
air sparging. Heavier products (e.g., lubricating oils) generally take
longer to biodegrade than the lighter products, but biosparging can still
be used at these sites. Exhibit VIII-2 provides a summary of the
advantages and disadvantages of biosparging.
October 1994 VHI-l
-------
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October 1994
-------
Exhibit VIII-2
Advantages And Disadvantages Of Biosparging
Advantages
Disadvantages
o Readily available equipment; easy to
install.
o Creates minimal disturbance to site
operations.
o Short treatment times, 6 months to 2
years under favorable conditions.
o Is cost competitive.
o Enhances the effectiveness of air
sparging for treating a wider range of
petroleum hydrocarbons.
o Requires no removal, treatment, storage,
or discharge of groundwater.
o Low air injection rates minimize potential
need for vapor capture and treatment.
o Can only be used in environments where
air sparging is suitable (e.g., uniform and
permeable soils, unconfined aquifer, no
free-phase hydrocarbons, no nearby
subsurface confined spaces).
o Some interactions among complex
chemical, physical, and biological
processes are not well understood.
o Lack of field and laboratory data to
support design considerations.
o Potential for inducing migration of
constituents.
This chapter will assist you in evaluating a corrective action plan
(CAP) that proposes biosparging as a remedy for petroleum-contaminated
groundwater and soil. The evaluation process is summarized in a flow
diagram shown in Exhibit VIII-3, which serves as a roadmap for the
decisions you will make during your evaluation. A checklist has also
been provided at the end of this chapter for you to use as a tool to both
evaluate the completeness of the CAP and to focus attention on areas
where additional information may be needed. The evaluation process can
be divided into the four steps described below.
O Step 1: An initial screening of biosparging effectiveness allows
you to quickly gauge whether biosparging is likely to be effective,
moderately effective, or ineffective.
O Step 2: A detailed evaluation of biosparging effectiveness
provides further screening criteria to confirm whether biosparging is
likely to be effective. You will need to identify site and constituent
characteristics, compare them to ranges where biosparging is
effective, and evaluate pilot study plans.
October 1994
vra-s
-------
Exhibit VIII-3
Biosparging Evaluation Process Flow Chart
INITIAL SCREENING
OF BIOSPARGING
EFFECTIVENESS
Is floating
free product
present?
Remove free
product
Are nearby
basements, sewers,
or other subsurface
confined spaces
present?
Will
SVEbe
used to control
migration of
vapors?
Is
contaminated
groundwaterin
a confined
aquifer?
Biosparging has
the potential to be
effective at the site.
Proceed to next panel.
Biosparging is
not likely to be
effective at the site.
Consider other
technologies.
• Vacuum-enhanced
Pump and Treat
• In-Situ
Groundwater
Bioremediation
vm-4
October 1994
-------
Exhibit VIII-3
Biosparging Evaluation Process Flow Chart
DETAILED EVALUATION
OF BIOSPARGING
EFFECTIVENESS
I
identify site characteristics important
to biosparging effectiveness.
Identify constituent characteristics
important to biosparging effectiveness.
Is soil free o
Me layers or
conditions that would
disrupt air flow?
NO /^Area!!targeted
constituents sufficiently
biodegradable?
Biosparging
will not be
effective at
the site.
Consider other
technologies.
IsTPH
< 50,080 ppm
and heavy metals
< 2,500 ppm?
Is intrinsic
permeability
>10'8cm2?
Vacuum-enhanced
Pump and Treat
In-Situ Groundwater
Bioremediation
Is
temperature or\ NO
groundwater between
10*Cand4S°C
Are vapor
pressures of
product constituents
< 0.5mm Hg?
IspH
of subsurface
environment between
6 and 8?
Offgas may be
contaminated.
Pilot study and
system design
should consider
vapor control.
constituent
boiling range
<250-300
Is heterotrophic
bacterial population density
> 1,000 CPU/gram?
Is Henry's
Law Constant
<100atm?
Is the
disserved iron
concentration at the
site<16mg/L
Pilot studies
are required to
demonstrate
effectiveness.
Review pilot
study results.
Do pilot
study results "X^ YES
demonstrate biosparging
effectiveness?
Biosparging is likely
to be effective at the
site.
Proceed to evaluate
the design.
October 1994
Biosparging will not
be effective at the site.
Consider other
technologies.
• Vacuum-enhanced
Pump and Treat
• In-Situ Groundwater
Bioremediation
vm-5
-------
Exhibit VIII-3
Biosparging Evaluation Process Flow Chart
EVALUATION OF
BIOSPARGING
SYSTEM DESIGN
EVALUATION OF BIOSPARGING
SYSTEM OPERATION &
MONITORING PLANS
Determine the design elements
based on pilot study results
• Bubble Radius
• Sparging Air Flow Rate
* Sparging Air Pressure
• Nutrient Formulation and Delivery Rate
»Initial Temperature, Concentrations of O2
and CO2, and Constituent Concentrations
* Required Final Dissolved Concentrations
* Required Cleanup Time
• Saturated Zone Volume to be Treated
* Discharge Limits and Monitoring
Requirements
• Site Construction Limitations
Review the O&M
plan for the proposed
biosparging system for
the following:
> Start-Up Operations Plan
> Long-Term Operations &
Monitoring Plan
> Remedial Progress
Monitoring Plan
Are
start-up
operations & monitoring
described, and are their
scope & frequency
adequate?
Request
additional
information
on startup
procedures and
monitoring.
Have design
elements been identified
and are they within
appropriate
ranges?
Is a
long-term O&M
plan described; is it
of adequate scope &
frequency?
Review the conceptual
process flow design & identify
the system components
Request
additional
information
onlong4erm
O&M.
• Sparging Well Orientation,
Placement, and Construction
* Manrfofd Piping
• Sparging Compressor
• Monitoring & Control Equipment
• Vapor Extraction System (Optional)
Biosparging
system
design is
incomplete.
Request
additional
information.
Is a
remedial progres
monitoring plan estab-
lished; is it of adequate scope
& frequency; does it include
provisions for detect-
ing asymptotic
behavior?
Request
additional
information
on remedial
design been provided
progress
monitoring.
The biosparging system
design is complete and its
eJements are within
appropriate ranges.
Proceed to O&M
evaluation.
The biosparging system
is likely to be effective.
The design and O&M
plans are complete.
October 1994
-------
O Step 3: An evaluation of the biosparging system design allows you
to determine whether basic design information has been defined,
whether necessary design components have been specified, whether
construction process flow designs are consistent with standard
practice, and if a detailed field pilot scale test has been properly
performed.
O Step 4: An evaluation of the operation and monitoring plans
allows you to determine whether start-up and long-term system
operation and monitoring is of sufficient scope and frequency and
whether remedial progress monitoring plans are appropriate.
initial Screening Of Biosparging Effectiveness
This section allows you to perform an initial screening of whether
biosparging will be effective at a site. First, you need to determine
whether or not any site-specific factors which could prohibit the use of
biosparging are present. Second, you need to determine if the key
parameters which contribute to the effectiveness and design are within
appropriate ranges for biosparging.
Biosparging should not be used if the following site conditions exist:
O Free product is present. Biosparging can create groundwater
mounding which could cause free product to migrate and
contamination to spread.
O Basements, sewers, or other subsurface coriflned spaces are located
near the site. Potentially dangerous constituent concentrations could
accumulate in basements and other subsurface confined spaces
unless a vapor extraction system is used to control vapor migration.
O Contaminated groundwater is located in a confined aquifer system.
Biosparging cannot be used to treat groundwater in a confined aquifer
because the air sparged into the aquifer would be trapped by the
saturated confining layer and could not escape to the unsaturated
zone.
The effectiveness of biosparging depends primarily on two factors:
O The permeability of the soil which determines the rate at which oxygen
can be supplied to the hydrocarbon-degrading microorganisms in the
subsurface.
O The biodegradability of the petroleum constituents which determines
both the rate at which and the degree to which the constituents will
be degraded by microorganisms.
October 1994 vm-7
-------
In general, the type of soil will determine its permeability. Fine-grained
soils (e.g., clays and silts) have lower permeabilities than coarse-grained
soils (e.g., sands and gravels). The biodegradabiUty of a petroleum
constituent is a measure of its ability to be metabolized by hydrocarbon-
degrading bacteria or other microorganisms. Petroleum constituents are
generally biodegradable, regardless of their molecular weight, as long as
indigenous microorganisms have an adequate supply of oxygen and
nutrients. For heavier constituents (which are generally less volatile and
less soluble than lighter constituents), biodegradation will exceed
volatilization as the primary removal mechanism, even though
biodegradation is generally slower for heavier constituents than for
lighter constituents.
Exhibit VIII-4 is an initial screening tool that you can use to help
assess the potential effectiveness of biosparging for a given site. To use
this tool, first determine the type of soil present and the type of
petroleum product released at the site. Information provided in the
following section will allow a more thorough evaluation of effectiveness
and will identify areas that could require special design considerations.
Exhibit VIII-4
Initial Screening For Biosparging Effectiveness
Permeability
Intrinsic Permeability, k (cm2)
10-'8 1O~14 10"12 10~10 10"* 10"8 ID"4
I ClayI
10-2
Glacial Till
J
I Silt. Loess 1
L
Silty Sand
I Clean Sand ~\
L
Grovel
Product Composition
I Fuel Oils I
Diesel
Kerosene
I Gasoline I
Note:
All petroleum products listed are amenable
for the biosparging remediation alternative.
vm-s
October 1994
-------
Detailed Evaluation Of Biosparging Effectiveness
Once you have completed the initial screening and determined that
biosparging may be effective for the soils and petroleum product present,
evaluate the CAP further to confirm that biosparging will be effective.
While the initial screen focused on soil permeability and constituent
biodegradabiliry, the detailed evaluation should consider a broader range
of site and constituent characteristics, which are listed in Exhibit VIII-5.
Exhibit Vili-5
Key Parameters Used To Evaluate The Suitability Of Biosparging
Site Characteristics Constituent Characteristics
Intrinsic permeability Chemical structure
Soil structure and stratification Concentration and toxicity
Temperature Vapor pressure
pH Product composition and boiling point
Microbial population density Henry's law constant
Nutrient concentrations
Dissolved iron concentration
The remainder of this section describes each parameter, why it is
important to biosparging, how it can be determined, and its range for
effective biosparging. If a vapor extraction system is considered for vapor
control requirements, additional factors such as depth to groundwater
and moisture content of the unsaturated zone should be examined to
determine if vapor extraction is suitable. See Chapter II: Soil Vapor
Extraction for the evaluation of the vapor extraction component, if used.
Site Characteristics That Affect Biosparging
Intrinsic Permeability
Intrinsic permeability is a measure of the ability of soil to transmit
fluids and is the single most important characteristic of the soil in
determining the effectiveness of biosparging because it controls how well
oxygen can be delivered to the subsurface microorganisms. Aerobic
hydrocarbon-degrading bacteria use oxygen to metabolize organic
material to yield carbon dioxide and water. To degrade large amounts of
a petroleum product, a substantial bacterial population is required
which, in turn, requires oxygen for both metabolic processes and an
increase in the overall bacterial population. Approximately 3 to 3Vz
pounds of oxygen are needed to degrade one pound of petroleum
product.
October 1994 vm-9
-------
Intrinsic permeability varies over 13 orders of magnitude (from 10"1
to 10"3 cm2) for the wide range of earth materials, although a more
limited range applies to most soil types (10~13 to 10"5 cm2). Intrinsic
permeability of the saturated zone for biosparging is best determined
from field tests, but it can also be estimated from soil boring logs and
laboratory tests. Procedures for these tests are described in EPA (199la).
Coarse-grained soils (e.g., sands) have greater intrinsic permeability than
fine-grained soils (e.g., clays and silts). Use the values shown in Exhibit
VIII-6 to determine if the intrinsic permeability of the soils at the site are
within the range of effectiveness for biosparging.
Exhibit VIII-6
Intrinsic Permeability And Biosparging Effectiveness
Intrinsic Permeability (k)(cm2) Biosparging Effectiveness
k > 10"9 Generally effective.
10"9 > k > 10"10 May be effective; needs further evaluation.
k < 10~10 Marginal effectiveness to ineffective.
Intrinsic permeability of saturated-zone soils is usually determined in
the field by aquifer pump tests that measure hydraulic conductivity. You
can convert hydraulic conductivity to intrinsic permeability using the
following equation:
k = K (u/pg)
where: k = intrinsic permeability (cm2)
K = hydraulic conductivity (cm/sec)
u = water viscosity (g/cm • sec)
p = water density (g/cm3)
g = acceleration due to gravity (cm/sec2)
At 20°C: u/pg = 1.02 • 10'5 cm/sec
Convert k from cm2 to darcy, multiply by 108.
Intrinsic permeability of the unsaturated zone can be estimated from
the intrinsic permeability of the saturated zone if similar soil types are
present Alternatively, it can be determined in the field by conducting
permeability tests or soil vapor extraction pilot studies. (See Chapter II:
Soil Vapor Extraction.)
VTJI-IO October 1994
-------
Soil Structure And Stratification
The types of soil present and their micro- and macro-structures will
control the biosparging pressure and distribution of oxygen and
nutrients in the saturated zone. For example, fine-grained soils require
higher sparging air pressures because air flow is restricted through
smaller pores, thereby reducing the efficiency of oxygen distribution. In
general, air injection rates used in biosparging are low enough that
vapor migration is not a major concern. However, this rate must be
assessed on a site-by-site basis.
Soil characteristics also determine the preferred zones of vapor flow in
the unsaturated zone, thereby indicating the ease with which vapors can
be controlled and extracted (if vapor extraction is used). Stratified or
highly variable heterogeneous soils typically create the greatest
impediments to biosparging. Both the injected air and the stripped
vapors will travel along the paths of least resistance (coarse-grained
zones) and could travel a great lateral distance from the injection point.
This phenomenon could result in enhanced migration of constituents.
Information about soil type, structure, and stratification can be
determined from boring logs or geologic cross-section maps. You should
verify that soil types have been identified and that visual observations of
soil structure have been documented.
Temperature Of The Groundwater
Bacterial growth rate is a function of temperature. Subsurface
microbial activity has been shown to decrease significantly at
temperatures below 10°C and essentially to cease below 5°C. Microbial
activity of most bacterial species important to petroleum hydrocarbon
biodegradation also diminishes at temperatures greater than 45°C.
Within the range of 10°C to 45°C, the rate of microbial activity typically
doubles for every 10°C rise in temperature. In most cases, because
biosparging is an in-situ technology, the bacteria are likely to experience
stable groundwater temperatures with only slight seasonal variations. In
most areas of the U.S., the average groundwater temperature is about
13°C, but groundwater temperatures may be somewhat lower or higher
in the extreme northern and southern states.
pH Levels
The optimum pH for bacterial growth is approximately 7; the
acceptable range for biosparging is between 6 and 8. If the groundwater
pH is outside of this range, it is possible to adjust the pH prior to and
during biosparging operations. However, pH adjustment is often not
cost-effective because natural buffering capacity of the groundwater
system generally necessitates continuous adjustment and monitoring
throughout the biosparging operation. In addition, efforts to adjust pH
October 1994 vm-11
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may lead to rapid changes in pH, which are also detrimental to bacterial
activity.
Microbial Population Density
Soil normally contains large numbers of diverse microorganisms
including bacteria, algae, fungi, protozoa, and actinomycetes. Of these
organisms, the bacteria are the most numerous and biochemically active
group, particularly at low oxygen levels. Bacteria require a carbon source
for cell growth and an energy source to sustain metabolic functions
required for growth. Nutrients, including nitrogen and phosphorus, are
also required for cell growth. The metabolic process used by bacteria to
produce energy requires a terminal electron acceptor (TEA) to
enzymatically oxidize the carbon source to carbon dioxide.
Microbes are classified by the carbon and TEA sources they use to
carry out metabolic processes. Bacteria that use organic compounds
(such as petroleum constituents and other naturally occurring organics)
as their source of carbon are heterotroptuc; those that Use inorganic
carbon compounds such as carbon dioxide are outotrophic. Bacteria that
use oxygen as their TEA are aerobic:, those that use a compound other
than oxygen (e.g., nitrate or sulfate) are anaerobic; and those that can
utilize both oxygen and other compounds as TEAs are/ocuttatiue. For
biosparging applications directed at petroleum products, bacteria that
are both aerobic (or facultative) and heterotrophic are most important in
the degradation process.
To evaluate the presence and population density of naturally
occurring bacteria that will contribute to degradation of petroleum
constituents, laboratory analysis of soil samples from the site (collected
from below the water table) should be conducted. These analyses, at a
minimum, should include plate counts for total heterotrophic bacteria.
Plate count results are normally reported in terms of colony-forming
units (CPUs) per gram of soil. Microbial population densities in typical
soils range from 104 to 107 CPU/gram of soil. For biosparging to be
effective, the minimum heterotrophic plate count should be 103
CFU/gram or greater. Plate counts lower than 103 could indicate the
presence of toxic concentrations of organic or inorganic (e.g., metals)
compounds. These conditions are summarized in Exhibit VIII-7.
Even when plate counts are lower than 103, biosparging may still be
effective if the soil is conditioned or amended to reduce the toxic concen-
trations and increase the microbial population density. More elaborate
laboratory tests are sometimes conducted to identify the bacterial
species present. Such tests may be desirable if you are uncertain
whether or not microbes capable of degrading specific petroleum
hydrocarbons occur naturally in the soil. If insufficient numbers or types
of microorganisms are present, .the population density may be increased
by introducing cultured microbes that are available from numerous
vendors. These conditions are summarized in Exhibit VIII-7.
Vm-12 October 1994
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Exhibit VIII-7
Heterotrophic Bacteria And Biosparging Effectiveness
Total Heterotrophic Bacteria
(prior to biosparging) Biosparging Effectiveness
> 1,000 CFU/gram dry soil Generally effective.
< 1,000 CFU/gram dry soil May be effective; needs further evaluation to
determine if toxic conditions are present.
Nutrient Concentrations
Bacteria require inorganic nutrients such as nitrogen and phosphate
to support cell growth and sustain biodegradation processes. Nutrients
may be available in sufficient quantities in the aquifer but, more
frequently, nutrients need to be added to maintain adequate bacterial
populations. However, excessive amounts of certain nutrients (i.e.,
phosphate and sulfate) can repress metabolism.
A rough approximation of minimum nutrient requirements can be
based on the stoichiometry of the overall biodegradation process:
C-source + N-source + O2 + Minerals + Nutrients —>
Cell mass + CO2 + H2O + other metabolic by-products
Different empirical formulas of bacterial cell mass have been proposed;
the most widely accepted are C5H7O2N and C60H87O32NI2P. Using the
empirical formulas for cell biomass and other assumptions, 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.
Chemical analyses of soil samples from the site (collected from below
the water table) should be completed to determine the available
concentrations of nitrogen (expressed as ammonia) and phosphate that
are naturally in the soil. These types of analyses are routinely conducted
in agronomic laboratories that test soil fertility for farmers. Using the
stoichiometric ratios, the need for nutrient addition can be determined
by using an average concentration of the constituents (carbon source) in
the soils to be treated. 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.
Iron Concentration Dissolved In Groundwater
The presence of dissolved ferrous iron (Fe+2) in groundwater can
reduce the permeability of the saturated zone soils during the sparging
operations. When dissolved iron is exposed to oxygen, it is oxidized to
ferric iron (Fe+3) oxide which, because it is less soluble than ferrous iron,
October 1994 VHI-13
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can precipitate within the saturated zone and occlude soil pore space.
On a large scale this could reduce the region available for air (and
groundwater) flow, thereby reducing permeability. Precipitation of iron
oxide occurs predominantly in the saturated zone near sparging well
screens where oxygen content (from injected air) is the highest. This
oxidation can render sparging wells useless after even short periods of
operation; installation of new wells in different locations would then be
required.
Verify that laboratory measurements of total dissolved iron have been
completed for groundwater samples from the site. Use Exhibit VIII-8 to
determine the range in which dissolved iron is a concern for biosparging
effectiveness.
Exhibit VIII-8
Dissolved iron And Biosparging Effectiveness
Dissolved Iron Concentration (mg/L) Biosparging Effectiveness
Fe*2 < 10 Biosparging effective.
10 < Fe*2 < 20 Sparging wells require periodic testing and
may need periodic replacement.
j.O
Fe > 20 Biosparging not recommended.
Constituent Characteristics That Affect Biosparging
Chemical Structure
The chemical structures of the constituents to be treated by
biosparging are important for determining the rate at which
biodegradation will occur. Although nearly all constituents in petroleum
products typically found at UST sites are biodegradable, the more
complex the molecular structure of the constituent, the more difficult
and less rapid is biological treatment. 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 VIII-9 lists, in order of
decreasing rate of potential biodegradability, some common constituents
found at petroleum UST sites.
Evaluation of the chemical structure of the constituents proposed for
reduction by biosparging at the site will allow you to determine which
constituents will be the most difficult to degrade. You should verify that
remedial time estimates, biotreatability studies, field-pilot studies (if
applicable), and biosparging operation and monitoring plans are based
on the constituents that are the most difficult to degrade (or "rate
limiting") in the biodegradation process.
VHE-14 October 1994
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Exhibit Vill-9
Chemical Structure And Biodegradability
Biodegradabilrty
More degradable
•
I
Less degradable
Example Constituents
n-butane, l-pentane,
n-octane
. Nonane
Methyl butane,
dimethylpentenes,
methyloctanes
Benzene, toluene,
ethylbenzene, xylenes
Propylbenzenes
Decanes
Dodecanes
Tridecanes
Tetradecanes
Naphthalenes
Fluoranthenes
Pyrenes
Acenaphthenes
Products In Which
Constituent Is Typically
Found
o Gasoline
o Diesel fuel
o Gasoline
o Gasoline
o Diesel, kerosene
o Diesel
o Kerosene
o Heating fuels
o Lubricating oils
o Diesel
o Kerosene
o Heating oil
o Lubricating oils
Concentration And Toxicfty
The presence of very high concentrations of petroleum organics or
heavy metals in site soils can be toxic or inhibit the growth and
reproduction of bacteria responsible for biodegradation. In addition, very-
low concentrations of organic material will also result in diminished
levels of bacterial activity.
In general, concentrations of petroleum hydrocarbons in excess of
50,000 ppm, or heavy metals in excess of 2,500 ppm, in soils are
considered inhibitory and/or toxic to aerobic bacteria. Review the CAP to
verify that the average concentrations of petroleum hydrocarbons and
heavy metals in the soils and groundwater to be treated are below these
levels. Exhibit VIII-10 provides the general criteria for constituent
concentration and biosparging effectiveness.
In addition to maximum concentrations, you should consider the
cleanup concentrations proposed for the treated soils. Below a certain
"threshold" constituent concentration, the bacteria cannot obtain
sufficient carbon (from degradation of the constituents) to maintain
adequate biological activity. The threshold level can be determined from
October 1994
vm-is
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Exhibit VIII-10
Constituent Concentration And Biosparging Effectiveness
Constituent Concentration
Biosparging Effectiveness
Petroleum constituents < 50,000 ppm
and
Heavy metals < 2,500 ppm
Petroleum constituents > 50,000 ppm
or
Heavy metals > 2,500 ppm
Effective.
Ineffective; toxic or inhibitory conditions to
bacterial growth exist. Long remediation
times likely.
laboratory studies and should be below the level required for cleanup.
Although the threshold limit varies greatly depending on bacteria-specific
and constituent-specific features, constituent concentrations below
0.1 ppm are generally not achievable by biological treatment alone. In
addition, experience has shown that reductions in total petroleum
hydrocarbon concentrations fTPH) greater than 95 percent can be very
difficult to achieve because of the presence of "recalcitrant" or
nondegradable petroleum hydrocarbons that are included in the TPH
analysis. Identify the average starting concentrations and the cleanup
concentrations in the CAP for individual constituents and TPH. If a
cleanup level lower than 0.1 ppm is required for any individual
constituent or a reduction in TPH greater than 95 percent is required to
reach the cleanup level for TPH, either a pilot study should be required
to demonstrate the ability of biosparging to achieve these reductions at
the site or another technology should be considered. These conditions
are summarized in Exhibit VIII-11.
Exhibit VIII-11
Cleanup Concentrations And Biosparging Effectiveness
Cleanup Requirement
Constituent concentration > 0.1 ppm
and
TPH reduction < 95%
Constituent concentration < 0.1 ppm
or
TPH reduction > 95%
Biosparging Effectiveness
Effective.
Potentially ineffective; pilot studies are
required to demonstrate reductions.
vm-i6
October 1994
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Vapor Pressure
Vapor pressure is important in evaluating the extent to which
constituents will be volatilized rather than biodegraded. The vapor
pressure of a constituent is a measure of its tendency to evaporate. More
precisely, it is the pressure that a vapor exerts when in equilibrium with
its pure liquid or solid form. Constituents with higher vapor pressures
are generally volatilized rather than biodegraded. In general, constituents
with vapor pressures higher than 0.5 mm Hg will likely be volatilized by
the induced air stream before they biodegrade. Constituents with vapor
pressures lower than 0.5 mm Hg will not volatilize to a significant degree
and can instead undergo in situ biodegradation by bacteria.
As previously discussed, petroleum products contain many different
chemical constituents. Each constituent will be volatilized (rather than
biodegraded) to different degrees by a biosparging system, depending on
its vapor pressure. If concentrations of volatile constituents are
significant, use of a vapor extraction system and treatment of extracted
vapors may be needed. Exhibit VIII-12 lists vapor pressures of select
petroleum constituents.
Vapor Pressures
Constituent
Methyl t-butyl ether
Benzene
Toluene
Ethylene dibromide
Ethylbenzene
Xylenes
Naphthalene
Tetraethyl lead
Exhibit VIII-12
Of Common Petroleum Constituents
Vapor Pressure
(mm Hg at 20°C)
245
76
22
11
7
6
0.5
0.2
Product Composition And Boiling Point
Boiling point is another measure of constituent volatility. Because of
their complex constituent compositions, petroleum products are often
classified by their boiling point ranges (rather than vapor pressures). In
general, nearly all petroleum-derived organic compounds are capable of
biological degradation, although constituents of higher molecular
weights and higher boiling points require longer periods of time to be
degraded. Products with boiling points of less than about 250°C to
300°C will volatilize to some extent and can be removed by a
October 1994 vm-17
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combination of volatilization and biodegradation in a biosparging system.
The boiling point ranges for common petroleum products are shown in
Exhibit VIII-13.
Exhibit VIII-13
Petroleum Product Boiling Ranges
Product
Boiling Range
Gasoline
Kerosene
Diesel fuel
Heating oil
Lubricating oils
40 to 225
180 to 300
200 to 338
>275
Nonvolatile
Henry's Law Constant
Another method of gauging the volatility of a constituent is by noting
its Henry's law constant, which quantifies the relative tendency of a
dissolved constituent to transfer to the vapor phase. Henry's law states
that, for ideal gases and solutions under equilibrium conditions, the
ratio of the partial pressure of a constituent in the vapor phase to the
concentration in the dissolved phase is constant. That is:
Pa = HaXa
where: Pa = partial pressure of constituent a in air
Ha = Henry's law constant (atm)
Xa = solution concentration of constituent a (mole fraction)
Henry's law constants for several common constituents found in
petroleum products are shown in Exhibit VHI-14. Constituents with
Henry's law constants of greater than 100 atmospheres are generally
considered volatile and, hence, more likely to be volatilized rather than
biodegraded.
Laboratory Treatability And Field Pilot Scale Studies
In general, remedial approaches that rely on biological processes
should be subjected to laboratory treatability tests and field pilot studies
to verify and quantify the potential effectiveness of the approach and
provide data necessary to design the system. However, field tests of
biosparging should never be conducted if free product is known to exist
at the water table, if uncontrolled vapors could migrate into nearby
confined spaces (e.g., sewers, basements) or if the contaminated
vm-i8
October 1994
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Henry's Law Constant
Constituent
Tetraethy! lead
Ethylbenzene
Xylenes
Benzene
Toluene
Naphthalene
Ethylene dibromide
Methyl t-butyl ether
Exhibit VII 1-1 4
Of Common Petroleum Constituents
Henry's Law Constant
(atm)
4,700
359
266
230
217
72 . •:
.. 34
27
ground-water is in a confined aquifer. The scope of laboratory studies or
pilot testing should be commensurate with the size of the area to be
treated, the reduction in constituent concentrations required, and'the
results of the initial effectiveness screening.
Some commonly used laboratory and pilot-scale studies are described
below.
O Laboratory Microbial Screening tests are used to determine the
presence of a population of naturally occurring bacteria that may be
capable of degrading petroleum product constituents. Samples of soils
from the aquifer are analyzed in an offsite laboratory. Microbial plate
counts determine the number of colony forming units (CPU) of
heterotrophic bacteria and petroleum-degrading bacteria present per
unit mass of dry soil. These tests are relatively inexpensive.
O Laboratory Biodegradation Studies can be used to estimate the rate of
oxygen delivery and to determine if the addition of inorganic nutrients
is necessary. However, laboratory studies cannot duplicate field
conditions, and field tests are more reliable. A common
biodegradation study for biosparging is the slurry study. Slurry
studies involve the preparation of numerous "soil microcosms"
consisting of small samples of site soils from the aquifer mixed into a
slurry with the site groundwater. The microcosms are divided into
several groups which may include control groups which are sterilized
to destroy any bacteria, non-nutrifled test groups which have been
provided oxygen but not nutrients, and nutrified test groups which
are supplied both oxygen and nutrients. Microcosms from each group
are analyzed periodically (usually weekly) during the test period
(usually 4 to 12 weeks) for bacterial population counts and
constituent concentrations. Results of slurry studies should be
October 1994
vra-i9
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considered as representing optimal conditions because sluny
microcosms do not consider the effects of limited oxygen delivery or
soil heterogeneity.
O Field Biosparging Treatabtiity Tests determine the effectiveness of
biosparging by characterizing the rate of biodegradation, the "bubble"
radius, and the potential for plume migration. Data collected from the
studies are used to specify design parameters such as the number
and density of the wells and the sparging rate. The study usually
includes sparging a single well while its effects are being measured in
monitoring wells or probes spaced at various distances. Ideally, three
or more monitoring wells surrounding the plume should be installed.
These monitoring wells should be screened above the saturated zone
and through the dissolved phase plume. They can be used to monitor
both dissolved and vapor phase migration, to monitor changes in
dissolved oxygen, and to measure changes in the depth to
groundwater.
If vapor extraction is to be included in the design, the pilot study
should be accomplished in two parts. The first portion of the test
should be conducted using vapor extraction only and evaluated as
described in Chapter II (Soil Vapor Extraction) without the
biosparging system being operated. This portion of the pilot test will
establish the baseline vapor extraction levels, the extent of the non-
sparged vapor plume, the extraction well radius of influence and
intrinsic permeability of the unsaturated zone (discussed in
Chapter II). The second portion of the study would involve the
installation of a sparge point with several vapor extraction points in
the vadose zone. Exhibit VIII-15 summarizes the parameters and data
that would be useful in a biosparging pilot study.
Evaluation Of The Biosparging System Design
Once you have verified that biosparging has the potential for
effectiveness at your site, you can evaluate the design of the system. The
CAP should include a discussion of the rationale for the system design
and the results of the pilot test(s). Detailed engineering design
documents might also be included, depending on individual state
requirements. Further detail about information to look for in the
discussion of the biosparging design is provided at the end of this
chapter. Discussion of the vapor extraction portion of the design is
included in Chapter II: Soil Vapor Extraction.
vm-2o
October 1994
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Exhibit VIII-15
Pilot Test Data Objectives
Data Requirement
Source
Vapor Extraction Test Portion (if
necessary)
Extraction well radius of influence (ROI)
Wellhead and monitoring point vacuum
Initial contaminant vapor and C02
concentrations
Initial hydraulic gradient
Biosparging Test Portion
Air sparging bubble radius
Sparging rate
Sparging vapor concentrations
C02 level in the exhaust vapors
Hydraulic gradient influence
Dissolved oxygen and carbon dioxide
Combined Test (if necessary)
Sparging/SVE capture rates
Contaminant vapor concentrations
Monitoring point pressure gauges
Well-head pressure gauge
Vapor extraction exhaust flame ionization
detector (FID) readings and C02 probe (or
other suitable detection device)
Water level tape at monitoring wells or
pressure transducers and data logger
Monitoring point pressure gauge
Compressor discharge flow gauge
Monitoring well and vapor point FID readings
(or other suitable detection device)
Carbon dioxide probe-- "
Water level tape at monitoring wells or
pressure transducers and data logger
Dissolved oxygen and carbon dioxide probes"
at monitoring wells
Pressure/flow gauges
Blower discharge and monitoring points
Rationale For The Design
The following factors should be considered as you evaluate the design
of the biosparging system in the CAP.
O Bubble radius for sparging wells. The bubble radius should be
considered in the design of the biosparging system. The bubble radius
is defined as the greatest distance from a sparging well at which
sufficient sparge pressure and airflow can be induced to enhance the
biodegradation of contaminants. The bubble radius will determine the
number and spacing of the sparging wells.
The bubble radius should be determined based on the results of pilot
tests. One should be careful, however, when evaluating pilot test
results. The measurement of air flow, increased dissolved oxygen, or
the presence of air bubbles in a monitoring point can be falsely
October 1994
vm-21
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interpreted as an air flow zone that is thoroughly permeated with
injected air when these observations actually represent localized
sparging around sparsely distributed air flow channels. The bubble
radius depends primarily on the hydraulic conductivity of the aquifer
material in which sparging takes place. Other factors that affect the
bubble radius include soil heterogeneities and differences between
lateral and vertical permeability of the soils. Generally, the design
bubble radius can range from 5 feet for fine-grained soils to 100 feet
for coarse-grained soils.
O Sparging Air Flow Rate. The sparging air flow rate required to provide
sufficient air flow to enhance biological activity is site specific and will
be determined via the pilot test. Typical air flow rates are much lower
than for air sparging, ranging from 3 to 25 standard cubic feet per
minute (scfm) per injection well. Pulsing of the air flow (i.e., turning
the system on and off at specified intervals) may provide better
distribution and mixing of the air in the contaminated saturated zone,
thereby allowing for greater contact with the dissolved phase
contaminants. If a vapor extraction system is used, it should have a
greater flow capacity and greater area of influence than the
biosparging system. Typically the SVE extraction rates range from
1.25 to 5 times greater than the biosparging rate.
O Sparging Air Pressure is the pressure at which air is injected below
the water table. Injection of air below the water table requires .
pressure greater than the static water pressure (1 psig for every 2.3 ft
of hydraulic head) and the head necessary to overcome capillary
forces of the water in the soil pores near the injection point. A typical
system will be operated at approximately 10 to 15 psig. Excessive
pressure may cause fracturing of the soils and create permanent air
channels that can significantly reduce biosparging effectiveness.
O Nutrient Formulation and Delivery Rate (if needed) will be based on the
results of the laboratory tests and pilot study results. 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 injection or prohibit them entirely.
O Initial Constituent Concentrations will be measured during pilot-scale
studies. They establish a baseline for estimating the constituent mass
removal rate and the system operation time requirements. In addition,
they will help to determine whether vapor treatment will be required.
O Initial Concentrations of Oxygen and CO2 in the saturated zone will be
measured during pilot studies. They are used to establish system
operating requirements, to provide baseline levels of subsurface
biological activity, and to allow measurement of the system's progress.
Vm-22 October 1994
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O Required Final Dissolved Constituent Concentrations in the saturated
zone are either defined by state regulations as "remedial action levels"
or determined on .a site-specific basis using transport models and risk
assessment calculations. They will determine which areas of the site
require treatment and when biosparging system operations can be
terminated.
O Required Remedial Cleanup Time may influence the design of the
system. The designer may vary the spacing of the sparging wells to
speed remediation to meet cleanup deadlines, if required.
O Saturated Zone Volume To Be Treated is determined by state action
levels or a site-specific risk assessment using site characterization
data for the groundwater.
O Discharge Limitations and Monitoring Requirements are usually
established by state regulations but must be considered by system
designers to ensure that monitoring ports are included in the system.
Discharge limitations imposed by state air quality regulations will
determine whether offgas treatment is required.
O Site Construction Limitations (e.g., building locations, utilities, buried
objects, residences) must be identified and considered in the design
process.
Components Of A Biosparging System
Once the design rationale is defined, the design of the biosparging
system can be developed. A typical biosparging system design includes
the following components and information:
O Sparging well orientation, placement, and construction details
O Manifold piping
O Compressed air equipment
O Monitoring and control equipment
A nutrient delivery system is sometimes included in biosparging
design. If nutrients are added, the design should specify the type of
nutrient addition and the construction details. Note that state
regulations may either require permits for nutrient injection wells or
prohibit them entirely.
If an SVE system is used for vapor control, the following components
and information will also be needed:
O Vapor pretreatment design
O Vapor treatment system selection
O Blower specification
October 1994 Vm-23
-------
Exhibit VIII-16 provides a schematic diagram of a typical biosparging
system used with vapor extraction. Chapter II: Soil Vapor Extraction,
should be consulted for information on the design of the vapor extraction
portion of the remedial system (if necessary), including vapor
pretreatment design, vapor treatment system selection, and blower
specification.
Exhibit VIII-16
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i
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Biosparging System Used With Vapor Extraction
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Sparge And Extraction Wells
Well Orientation. A biosparging system can use either vertical or
horizontal sparge wells. Well orientation should be based on site-specific
needs and conditions. For example, horizontal systems should be
considered when evaluating sites that will require 10 or more sparge or
extraction points, if the affected area is located under a surface
structure, or if the thickness of the saturated zone is less than 10 feet.
Exhibit VIII-17 lists site conditions and the corresponding appropriate
well orientation.
vm-24
October 1994
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Exhibit VIII-17
Well Orientation And Site Conditions
Well Orientation Site Conditions
Vertical wells o Deep contamination (> 25 feet)
o Depth to groundwater (> 10 feet)
o Fewer than 10 wells
o Thickness of saturated zone (> 10 feet)
Horizontal wells o Shallow groundwater table (< 25 feet)
o Zone of contamination within a specific
stratigraphic unit
o System under an operational facility
o Thickness of saturated zone (< 10 feet)
Well Placement And Number of Wells. Exhibit VIII-18, Biosparging/Vapor
Extraction Well Configurations, shows various configurations that can be
used in laying out biosparging systems used in conjunction with vapor
extraction. The essential goals in configuring the wells and monitoring
points are (1) to optimize the influence on the plume, thereby maximizing
the treatment efficiency of the system, and (2) to provide optimum moni-
toring and vapor extraction points to ensure minimal migration of the
vapor plume and no undetected migration of either the dissolved phase
or vapor phase plumes. In shallow applications, in large plume areas, or
in locations under buildings or pavements, horizontal vapor extraction
wells are very cost effective and efficient for controlling vapor migration.
Exhibit VIII-19 is a typical layout for a system that surrounds and
contains a plume and includes sparging wells and vapor extraction wells.
The number and location of extraction wells (if needed) can be
determined by using several methods as discussed in Chapter II: Soil
Vapor Extraction. However, the following general points should be
considered:
O Closer well spacing is often appropriate in areas of high contaminant
concentrations in order to enhance air distribution (and oxygen
delivery rate), thus increasing the rate of biodegradation.
O If a surface seal exists or is planned for the design, the extraction
wells can be spaced slightly farther apart. Surface seals force air to be
drawn from a greater distance rather than directly from the surface.
O At sites with stratified soils, wells screened in strata with low
permeabilities might require closer well spacing than wells screened in
strata with higher permeabilities.
October 1994 vm-25
-------
Exhibit VIII-18
Biosparging/Vapor Extraction Well Configurations
a) Spaced Configuration
b) Nested Wells
Extraction
Well
iiiinnmnmi miiiitiiiiiimr
linn
miinmniiiiiMiiHuiiMimi i i
Sparging
Well
Extraction
Well
IIIIIHIIUIIIMIIMI
Sparging
Well
III
c) Horizontal Wells
d) Combined Horizontal/Vertical Wells
Source: "Advances in Air Sparging Design," The Hazardous Waste Consultant, VoL 11,
Issue 1, January/February 1993, p. 1-4.
Well Construction. Sparging wells are generally constructed of 1- to 5-
inch PVC, galvanized steel, or stainless steel pipe. The screened interval
is normally 1-3 feet in length and is generally set 5-15 feet below the
deepest extent of adsorbed contaminants. Setting the screen at a deeper
interval requires higher pressures on the system, but generally does not
achieve higher sparge rates. Increased screen length will not improve
system efficiency because air tends to exit at the top portion of the
screen where hydraulic pressure head is lower. Sparge points must be
properly grouted to prevent short circuiting of the air. Horizontal
injection wells should be designed and installed carefully to ensure that
air exits from along the entire screen length. Perforated pipe, rather than
well screening, is sometimes preferred for horizontal wells. Exhibits VIII-
20 and VIII-21 present typical vertical and horizontal sparging well
constructions, respectively.
vm-26
October 1994
-------
Exhibit Vlll-19
Combined Biosparging/Vapor Extraction System Layout
Equipment
Compound
s-
eb
Legend:
A A?r Sparging Well
B SVE Well
—— SVE (Vacuum) Manifold
•Air Sparging (Compressed Air) Manifold
1 Extent of Dissolved Petroleum Contamination
Injection wells should be fitted with check valves to prevent potential
line fouling. Fouling occurs when pressure in the saturated zone forces
water up the sparge point while the system is shut down. Each sparging
well should also be equipped with a pressure gauge and flow regulator to
enable adjustments in sparging air distribution. Refer to Chapter II: Soil
Vapor Extraction for vapor extraction well details.
Manifold Piping
Manifold piping connects sparging wells to an air compressor. Piping
can be placed above or below grade depending on site operations,
ambient temperature, and local building codes. Below-grade piping is
more common and is installed in shallow utility trenches that lead from
the sparging wellhead vault(s) to a central equipment location. The
piping can either be manifolded in the equipment area or connected to a
common compressor main that supplies the wells in series; in this case,
flow control valves are located at the wellhead. Rping to the well
locations should be sloped toward the well so that condensate or
entrained groundwater will flow back toward the well.
October 1994
Vm-27
-------
Exhibit VIII-20
Vertical Sparging Well Construction
Grade
/—Pressure Indicator
It—Flow Regulating Valve
//T— Check Valve
Sched. 40 PVC
Solid Casing
Cement/Benton'rte Seal
Bentonite
Sand Pack
Slotted Sched. 40
PVC Well Screen
Flat Bottomed, Sched.
40 PVC Threaded Plug
The pressurized air distribution system can be made of metal pipe or
rubber-reinforced air hose. PVC pipe should not be connected directly to
the compressor because of the high temperatures of air leaving the
compressor which can diminish the integrity of the PVC. If pipe trenches
are used for the distribution system, they must be sealed to prevent
short circuiting of air flow.
Compressed Air Equipment
An. oil-free compressor or a standard compressor equipped with
downstream coalescing and particulate filters should be used to ensure
that no contaminants are injected into the saturated zone. The
compressor should be rated for continuous duly at the maximum
expected flow rate and pressure to provide adequate flexibility during full
operations.
vm-28
October 1994
-------
Exhibit VIII-21
Horizontal Sparging Well Construction
From Air
Compressor
Note:
Piping may be buried
in utility trenches.
Fabric Liner
Bentonite
Backfilled Soil
i ii 111111II i 111111 ii nil 11 ii 111 IN 111 ii n i!|
°& r> d» C?=C? o i? '^S'i V °9 OOS. ?>?t°£
PVC Threaded Cap
Slotted PVC Pipe
Pea Gravel
Monitoring And Controls
The parameters typically monitored in a sparging system include:
O Pressure
O Air/vapor flow rate
O Carbon dioxide and oxygen concentration in soil vapor and
groundwater
O Constituent concentrations in soil vapor and groundwater
O Nutrient delivery rate
The equipment in a sparging system used to monitor these parameters
provides the information necessary to make appropriate system
adjustments and track remedial progress. The control equipment in a
sparging system allows the flow and sparge pressure to be adjusted at
each sparging well of the system as necessary. Control equipment
typically includes flow control valves or regulators. Exhibit VIII-22 lists
typical monitoring and control equipment for a biosparging system, the
location for each of these pieces of equipment, and the types of
equipment that are available.
October 1994
vm-29
-------
Exhibit VIII-22
Monitoring And Control Equipment
Monitoring Equipment
Flow meter
Pressure gauge
Sampling port
Control Equipment
Flow control valves/
regulators
Location In System
o At each sparge and
vapor extraction well
head
o Manifold to blower
o Stack discharge
o Nutrient manifold
o At each sparge and
vapor extraction well
head or manifold branch
o Before blower (before
and after filters)
o Before and after vapor
treatment
o At each vapor extraction
well head or manifold
branch
o Manifold to blower
o Blower discharge
o At each vapor extraction
well head or manifold
branch
o Dilution or bleed valve at
manifold to blower
o At header to each sparge
point
Example Of Equipment
o Pilot tube
o In-line rotameter
o Orifice plate
o Venturi or flow tube
o Turbine wheel
o Manometer
o Magnehelic gauge
o Vacuum gauge
o Hose barb
o Septa fitting
o Ball valve
o Gate valve
o Dilution/ambient air bleed
valve
o Gate valve
o Dilution/ambient air bleed
valve
Evaluation Of Operation And Monitoring Plans __
The system operation and monitoring plan should include both
system startup and long-term operations. Operations and monitoring are
necessary to ensure optimal system performance and to track the rate of
contaminant mass removal/reduction.
vm-3o
October 1994
-------
Startup Operations
The startup phase should begin with only the SVE portion of the
system (if used) as described in Chapter II. After the SVE system is
adjusted, the air sparging system should be started. Generally, 7 to 10
days of manifold valving adjustments are required to adjust the air
sparging system. These adjustments should balance flow to optimize the
carbon dioxide production and oxygen uptake rate. Monitoring data
should include sparge pressure and flows, vacuum readings for SVE,
depth of groundwater, vapor concentrations, dissolved oxygen levels,
CO2 levels, and pH. During the initial start up, these parameters should
be monitored hourly once the flow is stabilized. Vapor concentration
should also be monitored in any nearby utility lines, basements, or other
subsurface confined spaces. Other monitoring of the system should be
done in accordance with the SVE requirements from Chapter II.
Long-Term Operations
To evaluate the performance of a biosparging system the following
parameters should be monitored weekly to biweekly after the startup
operation:
O Contaminant levels, carbon dioxide level, dissolved oxygen level, and
pH in the groundwater.
O Contaminant level, oxygen, and carbon dioxide in the effluent stack
and the manifold of the SVE system (if used).
O Pressures and flow rates in the sparging wells and, if SVE is used, in
the extraction wells.
It should be noted that the samples from the groundwater monitoring
wells that will be analyzed to track dissolved contaminant concentrations
should be collected after a short period of time following system
shutdown. Sampling at these times allows the subsurface environment
to reach equilibrium. Samples collected during sparging operations may
have lower concentrations of dissolved contaminants than does the
surrounding aquifer. This result could lead to the erroneous conclusion
that remediation is occurring throughout the aquifer because the
monitoring wells may serve as preferential flow paths for the injected air.
Exhibit VIII-23 provides a brief synopsis of system monitoring
requirements.
October 1994 vm-31
-------
Exhibit VIII-23
System Monitoring Recommendations
Phase
Monitoring
Frequency
What To Monitor
Where To Monitor
Startup (7-10
days)
At least daily
Remedial
(ongoing)
Weekly to bi-
weekly
Quarterly to
annually
o Sparge pressure
o Flow
o Air sparging wellheads
o Sparge and extraction wells
(if used)
o Manifold
o Extraction wells (if SVE is
used)
o Vacuum readings (if SVE is o Groundwater and soil vapor
used)
o D.O., C02, pH
o Depth to groundwater
o Vacuum readings
o Vapor concentrations
monitoring points
o Groundwater monitoring
wells
o Extraction wells (if SVE is
used)
o Effluent stack (if SVE is
used)
o Manifold (if SVE is used)
o Sparge pressure and flow o Air sparging wellheads
o D.O., C02, pH
o Dissolved constituent
concentrations
o Groundwater and soil vapor
monitoring points
o Groundwater monitoring
wells
Remedial Progress Monitoring
Monitoring the performance of the biosparging system in reducing
contaminant concentrations in the saturated zone is necessary to
determine if remedial progress is proceeding at a reasonable pace. A
variety of methods can be used. One method includes monitoring
contaminant levels in the groundwater in monitoring wells and, if vapor
extraction is used, vapors in the blower exhaust. The vapor and
contaminant concentrations are then each plotted against time.
vm-32
October 1994
-------
The plot can be used to show the impact of the biosparging operation.
As biosparging reaches the limit of its ability to biodegrade further, the
reduction of dissolved constituents reaches asymptotic conditions. This
effect is also reflected in the concentrations of oxygen, CO2, and VOC in
the vapors released from the system. A plot of this effect is demonstrated
in Exhibit VIII-24. When asymptotic behavior begins to occur, the
operator should evaluate alternatives that increase the mass transfer
removal rate (e.g., pulsing, or turning off the system for a period of time
and then restarting it). Other more aggressive steps to further reduce
constituent concentrations can include the installation of additional
sparging points or vapor extraction wells.
Exhibit VIII-24
Concentration Reduction And Mass Removal Behavior For Biosparging Systems
a: c
o
« o
c
oo
ft N
OO
oo
Cumulative VOC
and CO 2 Mass
Removal (Ibs.)
....^Asymptotic::,...,
^(Irreducible)^
VOC and C02
Concentrations
in Extracted Soil
Vapor (ppm)
Operational Time
If asymptotic behavior is persistent for periods greater than about six
months and the concentration rebound is sufficiently small following
periods of temporary system shutdown, the performance of the
biosparging system should be reviewed with regulatory agencies to
determine whether remedial goals have been reached. If further
contaminant reduction is desired, another remedial technology may need
to be considered.
October 1994
vm-33
-------
References
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. Handbook of
Btoremediation. Boca Raton, FL:CRC Press, 1994.
Norris, R.D., Hinchee, RE., 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. In-Situ Bioremediation of
Ground. Water and. Geological Material: A Review of Technologies. Ada,
OK: U.S. Environmental Protection Agency, Office of Research and
Development. EPA/5R-93/124, 1993.
Riser-Roberts, E. Btoremediation of Petroleum Contaminated Sites. NCEL,
Port Hueneme, CA: C. K. Smoley Publishers, CRC Press, 1992.
Flathman, P.E. and D.E. Jerger. Btoremediation Field Experience.
Environmental Research Laboratory, Ada, OK: Lewis Publishers, CRC
Press, Inc., 1994.
Weston, Inc., Roy F. Remedial Technologies for Leaking Underground
Storage Tanks. University of Massachusetts, Amherst, MA: Lewis
Publishers, 1988.
U.S. Environmental Protection Agency (EPA). A Technology Assessment
of Soil Vapor Extraction and. Air Sparging. Cincinnati, OH: Office of
Research and Development. EPA/600/R-92/173, 1992.
Vm-34 October 1994
-------
Checklist: Can Biosparging Be Used At This Site?
This checklist can help you to evaluate the completeness of the CAP
and to identify areas that require closer scrutiny. As you go through the
CAP, answer the following questions. If the answer to several questions
is no, you will want to request additional information to determine if
biosparging will accomplish the cleanup goals at the site.
1. Site Factors
Yes No
Q Q Is the aquifer clear of floating free product?
Q Q Is the soil intrinsic permeability greater than 10"9 cm2?
Q Q Is the soil free of impermeable layers or other conditions that
would disrupt air flow?
Q Q Is soil temperature between 10°C and 45°C during the
proposed treatment season?
Q Q Is the pH of groundwater between 6 and 8?
Q Q Is the total heterotrophic bacteria count > 1,000 CPU/gram
dry soil?
Q Q Is the carbon:nitrogen:phosphorus ratio between 100:10:1
and 100:1:0.5?
Q Q Is the dissolved iron concentration at the site < 10 mg/L?
Q Q Is vapor migration of constituents controlled?
2. Constituent Characteristics
Yes No
Q Q Are constituents all sufficiently biodegradable?
Q Q Is the concentration of Total Petroleum Hydrocarbon
< 50,000 ppm and heavy metals < 2,500 ppm?
Q Are the constituent vapor pressures less than 0.5 mm Hg?
Q Q Are the Henry's law constants for the constituents present
lower than 100 atm?
October 1994 vm-35
-------
3. Evaluation Of The Biospaxging System Design
Yes No
Q Q Examine the sparging air pressure. Will the proposed
pressure be sufficient to overcome the hydraulic head and
capillary forces?
Q Q Is the proposed well density appropriate, given the total area
to be cleaned up and the radius of influence of each well?
Q Q Do the proposed well screen intervals account for
contaminant plume location at the site?
Q Q Is the proposed well configuration appropriate for the site
conditions present?
Q Q Is the air compressor selected appropriate for the desired
sparge pressure?
Q Q If nutrient addition is needed, are nutrient formulation and
delivery rates appropriate for the site, based on laboratory or
field studies?
Q Q Have background concentrations of oxygen and CO2
(measured in pilot studies) been taken into account in
establishing operating requirements?
4. Operation And Monitoring Plans
Yes No
Q Q Are manifold valving adjustments proposed during the first 7
to 10 days of operation?
Q Q Are hourly recordings of injection and extraction rates,
pressures, depth to groundwater, hydraulic gradient, and
VOC levels proposed during the first 7 to 10 days of
operation?
Q Q Is daily monitoring of injection rates proposed during the
first 7 to 10 days of operation?
Q Q Are biweekly to monthly measurements of contaminant levels
in groundwater, vapor wells, and blower exhausts proposed?
Q Q Are biweekly to monthly measurements of vapor
concentration proposed?
VHI-36 October 1994
-------
Chapter IX
Natural Attenuation
-------
-------
Contents
Overview IX-1
Initial Screening Of Natural Attenuation Effectiveness IX-7
Constituent Concentrations IX-7
Nearby Receptors . IX-7
Detailed Evaluation Of Natural Attenuation Effectiveness IX-9
Natural Attenuation Mechanisms IX-9
Biological Processes «, IX-11
Physical Phenomena IX-11
Evaluation Of Site And Constituent Factors -. ... IX-12
Site Factors Affecting Constituent Migration IX-12
Soil Texture IX-13
Soil Structure IX-14
Adsorption Potential IX-14
Groundwater Flow Rate IX-15
Soil And Groundwater Aeration IX-15
Soil Moisture Content IX-16
Son pH IX-16
Microbial Community IX-16
Precipitation IX-17
Temperature IX-17
Soil Nutrient Concentration LX-17
Chemical Constituent Factors IX-17
Solubility IX-19
Vapor Pressure IX-20
Henry's Law Constant IX-21
Boiling Point IX-22
Organic Carbon Partition Coefficient (K^),
Adsorption Potential (K^) IX-22
Molecular Weight IX-23
Remedial Progress Monitoring EX-23
Indicators Of Natural Attenuation IX-23
Constituent Plume Characteristics IX-25
Dissolved Oxygen Indicators IX-25
Geochemical Indicators IX-26
Oxidation/Reduction Potential IX-26
Ongoing Monitoring IX-27
Soils IX-28
Groundwater IX-28
References IX-31
Checklist Can Natural Attenuation Be Used At This Site? IX-32
October 1994 JX-iii
-------
List Of Exhibits
Number
IX-1
EX-2
rx-s
EX-4
IX-5
IX-6
IX-7
IX-8
DC-9
IX-10
IX-11
IX-12
IX-13
IX-14
IX-15
IX-16
IX-17
Title
Page
A Typical Hydrocarbon Plume Undergoing Natural
Bioremediation; (a) Cross-section, (b) Plan View IX-3
Advantages And Disadvantages Of Natural Attenuation . IX-4
Natural Attenuation Evaluation Process Flow Chart .... IX-5
Factors Determining Groundwater Potability IX-8
Potential Natural Attenuation Mechanisms IX-10
Potential For Natural Attenuation: Site Factors IX-13
Relative Environmental Partitioning Of
BTEX Constituents IX-18
Potential For Natural Attenuation: Chemical
Constituent Factors IX-19
Solubilities Of BTEX Constituents IX-20
Vapor Pressures Of BTEX Constituents IX-21
Henry's Law Constant Of BTEX Constituents IX-22
Boiling Points Of BTEX Constituents IX-22
HOC Values For BTEX Constituents IX-23
Site Characterization Data Used To Evaluate
Effectiveness Of Natural Attenuation IX-24
Redox Potentials For Various Electron Acceptors IX-27
Ongoing Progress Monitoring IX-28
Recommended Groundwater Monitoring Well
Network For Demonstrating Natural Attenuation IX-30
EK-iv
October 1994
-------
Chapter IX
Natural Attenuation Of Petroleum Hydrocarbons
Overview
Natural attenuation, also known as passive bioremediation, intrinsic
bioremediation, or intrinsic remediation, is a passive remedial approach
that depends upon natural processes to degrade and dissipate petroleum
constituents in soil and groundwater. Some of the processes involved in
natural attenuation of petroleum products include aerobic and anaerobic
biodegradation, dispersion, volatilization, and adsorption. In general, for
petroleum hydrocarbons, biodegradation is the most important natural
attenuation mechanism; it is the only natural process that results in an
actual reduction of petroleum constituent mass.
This chapter describes chemical and environmental factors that
influence the rate of natural attenuation processes. Because of the
complex interrelationship among these controlling factors, using specific
numerical thresholds to determine whether natural attenuation will be
effective is frequently not possible. A detailed site investigation is
necessary to provide sufficient data on site conditions and hydrocarbon
constituents present to evaluate the potential effectiveness of natural
attenuation. In addition, site conditions will need to be monitored over
time to confirm whether or not contaminants are being naturally
degraded at reasonable rates to ensure protection of human health and
the environment. Site data should clearly indicate whether
concentrations of soil and groundwater contaminants are being
adequately reduced without active remediation treatment. If not, more
aggressive remedial alternatives should be considered.
-Petroleum hydrocarbon constituents are generally biodegradable,
regardless of their molecular weight, as long as indigenous
microorganisms have an adequate supply of nutrients and biological
activity is not inhibited by toxic substances. For heavier hydrocarbons,
which are less volatile and less soluble than many lighter components,
biodegradation will exceed volatilization as the primary removal
mechanism, even though degradation is generally slower for heavier
molecular weight constituents than for lighter ones.
The essential nutrients required for biodegradation are usually
naturally present in the subsurface. Aerobic biodegradation consumes
oxygen which, if not replenished, can limit the effectiveness of further
aerobic biodegradation. When the geologic materials at a site are
relatively porous and permeable, oxygen is naturally replenished through
the soil and groundwater. When, however, the permeability is high, the
possibility exists for greater downgradient migration of contaminants.
Conversely, when the geologic materials have low porosity and are
October 1994
-------
relatively impermeable, the potential for migration is reduced but so is
the rate of oxygen replenishment. In addition, less permeable materials
typically are finer grained and contain higher percentages of organic
carbon. Both of these features favor adsorption and retardation of
contaminant movement. In this case, contaminants may remain
relatively undegraded but in close proximity to the original source.
Anaerobic biodegradation is also a significant attenuation process.
Oxygen depletion in the subsurface is a characteristic of biodegradation
of petroleum hydrocarbons and is a consequence of the rate of metabolic
oxygen utilization exceeding the natural capacity for oxygen
replenishment. The core of a contaminant plume is typically under
anaerobic conditions and only the margins are aerobic, as illustrated in
Exhibit IX-1. Therefore, even though the rate of anaerobic biodegradation
is much slower than aerobic biodegradation (often by a factor of 10 to
several hundred), anaerobic processes may dominate the degradation of
hydrocarbon contaminants.
Exhibit IX-2 provides a summary of the advantages and
disadvantages of using natural attenuation as a remedial option for
petroleum-contaminated soils and groundwater. Under the appropriate
site conditions, natural attenuation can reduce the potential impact of
petroleum product release either by preventing constituents from being
transported to sensitive receptors or by reducing constituent
concentrations to less harmful levels. Natural attenuation may also be
an acceptable option for sites that have been subject to active
remediation and which now have substantially reduced concentrations of
contaminants. However, natural attenuation is not an appropriate option
at all sites. The rates of natural processes are typically slow;
contaminant levels may not be reduced to acceptable regulatory levels
for years. In addition, long-term monitoring is necessary to demonstrate
that contaminant concentrations are continually decreasing at a rate
sufficient to ensure that potential receptors are not adversely affected.
The policies and regulations of your state determine whether natural
attenuation will be allowed as a treatment option. Before beginning an
analysis of the potential effectiveness of natural attenuation, determine if
your state restricts the use of this remedial option. For example, natural
attenuation may not be allowed if groundwater is contaminated at levels
exceeding drinking water standards (i.e., Maximum Contaminant Levels
[MCLs]) or at concentrations that may pose risks to receptors or human
health. Natural attenuation is not generally an option at sites with free
product in the subsurface.
This chapter will assist you in evaluating a corrective action plan
(CAP) that proposes natural attenuation as a remedial option for
petroleum-contaminated soil and groundwater. The evaluation guidance
is presented in the four steps described below. The evaluation process,
which is summarized in a flow diagram shown in Exhibit IX-3, can serve
as a roadmap for the decisions you will make during your evaluation. A
IX-2 October 1994
-------
Exhibit IX-1
A Typical Hydrocarbon Plume Undergoing Natural Bioremediation;
(a) Cross-section, (b) Plan View
Anaerobic Core
Aerobic — Uncontaminated Groundwater
—•—
Legend:
Aerobic Margins
Anaerobic Core
Residual Phase
JL Water Table
(a) Cross Section
Oxygenated — Uncontaminated
Groundwater
Flow—^
Hydrocarbon
Aerobic
Margin
Flow
Oxygenated — Uncontaminated
Groundwater
Plan View
October 1994
EX-3
-------
Exhibit IX-2
Advantages And Disadvantages Of Natural Attenuation
Advantages
Disadvantages
o Lower costs than most active remedial
alternatives.
o Minimal disturbance to the site
operations.
o Potential use below buildings and other
areas that cannot be excavated.
o Not effective where constituent
concentrations are high (> 20,000 to
25,000 ppm TPH).
o Not suitable under certain site conditions
(e.g., impacted ground water supply,
presence of free product).
o Some migration of constituents may
occur; not suitable if receptors might be
affected.
o Long period of time required to remediate
heavier petroleum products.
o Longer period of time may be required to
mitigate contamination than for active
remedial measures.
o May not always achieve the desired
cleanup levels within a reasonable length
of time.
checklist has also been provided at the end of this chapter to be used as
a tool to evaluate the completeness of the CAP and to help focus
attention on areas where additional information may be needed.
O Step 1: Determine if state regulations permit natural
attenuation as a remedial option. If not, an alternative remedial
technology should be employed.
O Step 2: An initial screening of natural attenuation effectiveness
allows you to quickly gauge whether natural attenuation is likely to be
effective.
O Step 3: A detailed evaluation of natural attenuation
effectiveness provides further screening criteria to confirm whether
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 natural attenuation will
likely result in adequate reductions of constituent concentrations.
rx-4
October 1994
-------
Exhibit IX-3
Natural Attenuation Evaluation Process Flow Chart
INITIAL SCREENING OF
NATURAL ATTENUATION EFFECTIVENESS
Does
the state allow
natural attenuation
as a remedial
alternative?
AreTPH
concentrations
lower than 25,000
ppm in soil?
Natural attenuation
is not a remedial
option at the site.
Consider other
technologies.
Broventing
Landfarming
Biomounding
Thermal
Desorption
Is there
current or projected
groundwater use within
a 2 year travel time
from the site?
Are
there nearby
receptors that the
contamination
could affect?
Natural attenuation has
the potential to be
effective at the site.
Proceed to next panel.
October 1994
IX-5
-------
Exhibit IX-3
Natural Attenuation Evaluation Process Flow Chart
DETAILED EVALUATION OF
NATURAL ATTENUATION
EFFECTIVENESS
EVALUATION OF
REMEDIAL PROGRESS
MONITORING
Identify coil, groundwater,
and environmental
characteristics and product
constituent properties
important to natural
attenuation effectiveness
Sofl;
• Ic soil permeability > 10-8cm2?>
* Are soil oxygen levels > 2%?
• Does soil have a pH of 6-8?
• Does soil have a moisture content
of 40-86%?
» Does soil have a C:N:P ratio of
approximately 100:1:0.5 to
,.100:10:1?
.YES
NO
Ground-water:
Is contaminant travel
time to receptor at least
2 years?
Climate;
Is precipitation 10-60
inJyr.?
Is the average ambient
temperatures0-45°
Product Constituents:
• Are constituents at most
slightly soluble?
• Are constituents not highly
volatile?
• Are Koc and Kd values high
enough to adequately retard
migration?
» Are constituents sufficiently^
^biodegradable?
rYES
NO
You may consider
natural attenuation as
a remedial alternative.
Is monitoring
frequency annually for
soils and quarterly for.
ndwater?
Does
soil/groundwater
monitoring include analysis
for TPH, BTEX, and other
constituents of
concern?
Does
soil gas
analysis include
analysis for oxygen?
Does groundwater monitoring
include analysis for
oxygen. pH,
alkalinity?
The proposed
natural
attenuation
monitoring plan
is incomplete.
Request
additional
information.
Are an
adequate number
of samples being collected in
SOB and around the bounda
of contaminated
areas?
Are an
adequate number of
groundwater samples being
collected from appropriate
locations?
The remedial progress
monitoring plan is of
sufficient scope and
frequency and can be
considered complete.
re-6
October 1994
-------
O Step 4: An evaluation of monitoring plans allows you to determine
whether the proposed monitoring schedule will provide adequate data
to evaluate the effectiveness of natural attenuation.
Initial Screening Of Natural Attenuation Effectiveness
You should consider the following two critical factors early on when
evaluating the potential effectiveness of natural attenuation:
(1) Constituent concentrations—whether the petroleum
hydrocarbon constituent concentrations are low enough for
natural attenuation to be a viable alternative; and
(2) Nearby receptors—whether receptors located near the site
could be affected by the presence of petroleum constituents
during the remediation process.
Constituent Concentrations
If initial total petroleum hydrocarbon (TPH) concentrations are too
high, natural attenuation will not reduce concentrations to acceptable
levels within a reasonable time period (i.e., a few years). Natural
attenuation should not be used at sites where free product is present. In
general, natural attenuation is probably not effective at sites with soil
TPH concentrations greater than about 20,000 to 25,000 ppm. At
concentrations higher than this level it is probable that free phase
hydrocarbons exist in the subsurface. These limits are highly variable
and depend upon site-specific factors including the type and
concentration of contaminants, proximity and sensitivity of receptors,
and the hydrogeological conditions. These are typical values generally
considered acceptable for natural attenuation at sites with simple
geologic and hydrologic conditions, and minimal risk. For sites for which
conditions are highly complex or the risk to receptors is greater,
maximum acceptable TPH concentrations, will be lower.
Nearby Receptors
Because natural attenuation generally allows constituents to migrate
farther than active remedial measures, it is important to determine
whether individuals or sensitive environmental areas might be affected
by the release (e.g., through ingestion of contaminated soil and/or
groundwater, direct contact with contaminated groundwater at discharge
points (e.g., streams or marshes), direct contact with contaminated soil,
or inhalation of constituent vapors (especially in a basement or other
confined space).
October 1994 IX-7
-------
Look for information in the CAP regarding the location of potential
receptors as well as the quality of groundwater, depth to groundwater,
flow rate and direction of groundwater, discharge points for
groundwater, and use of groundwater in the vicinity of the site. If
potential receptors are located near the site, also look for modeling
and/or monitoring results that demonstrate that the constituents will
not reach the receptors. Determination of whether a receptor is in close
proximity to a site may be considered in terms of either contaminant
travel time from the source to the receptor or distance separating the
source 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 calculated from the average hydraulic gradient, hydraulic
conductivity, effective porosity, and distance between the source and the
receptor. Travel times of 2 years or more should allow for an evaluation
of the potential effectiveness of natural attenuation and provide
sufficient time to implement active remedial measures should natural
attenuation prove to be ineffective in protecting human health and the
environment. For example, if the average groundwater seepage velocity at
a site is 2 feet per day, it would require 2 years for a conservative
contaminant to travel 1,500 feet (approximately 1A mile). Therefore, as a
rule of thumb, downgradient receptors within 1A mile of the source
should be identified and probable travel times calculated. If travel times
to receptors within this radius are less than 2 years, the radius should
be extended to the distance that corresponds to a travel time of 2 years.
It should be noted that the presence of layers of high permeability soil or
rock, fractures or faults, or utility conduits could potentially accelerate
the migration of contaminants. If the groundwater is potable and future
land use is expected to be residential, potential future receptors should
also be considered. The most important parameters that determine water
potability are listed in Exhibit IX-4. If this information is not provided in
the CAP, you will need to request the missing data. If constituents are
expected to reach receptors, an active remedial technology should be
used instead of natural attenuation, unless the CAP demonstrates that
resulting exposures would not result in significant adverse
environmental or human health effects.
Exhibit IX-4
Factors Determining Groundwater Potability
Factor Potability Parameters
Total dissolved solids < 500 mg/L
Chemical concentrations < MCLs
Biological characteristics No harmful microbes (pathogens)
IX-8 October 1994
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Under some circumstances natural attenuation might be considered a
remedial option even when there is potential for groundwater
contamination. For instance, active remediation to protect a groundwater
resource may not be required if the affected groundwater is not currently
potable (e.g., because of high salinity or other chemical or biological
contamination) and it is not likely to be used as a potential source of
drinking water in the future.
In addition to reviewing the water use in the area, it is important to
determine whether there are receptors that could come into contact with
contaminated soil or groundwater. Because soils associated with LIST
contamination are generally below the surface of the ground, there will
usually be limited opportunity for receptors to come into contact with
contaminated soils. However, if the contaminated soils might be
excavated (e.g., for construction) before natural attenuation has
adequately reduced constituent concentrations, 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 implemented. If the potential for direct contact is not addressed in the
CAP, request additional information.
Exposure to petroleum constituent vapors may also be a concern at
some sites..Hazardous constituents can volatilize from free petroleum
product and from petroleum products adsorbed to soils or dissolved in
groundwater. 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 constituents can also be significant if
groundwater is used for bathing (even if it is not used for drinking). If
vapor migration and associated health and safety risks are not
addressed in the CAP, request additional information.
Detailed Evaluation Of Natural Attenuation Effectiveness
Once you have completed the initial screen and determined that
natural attenuation could potentially be effective at the site, review the
CAP further to confirm that natural attenuation will be effective. A
thorough understanding of natural attenuation processes, the site
conditions, and the constituents present will be necessary to make this
determination.
Natural Attenuation Mechanisms
In order to assess site conditions to determine whether natural
attenuation is an acceptable alternative to active treatment, it is
important to understand the mechanisms that degrade petroleum
products in soil and groundwater. Mechanisms may be classified as
either destructive (i.e., result in a net decrease in contaminant mass) or
October 1994 IX-9
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non-destructive (i.e., result in decrease in equilibrium concentrations
but no net decrease in mass). Destructive mechanisms are primarily
biological. The primary non-destructive mechanisms are abiotic, physical
phenomena. Chemical processes are important for many compounds
(including some gasoline additives such as ethylene dibromide [EDB]),
but relatively insignificant for the hydrocarbon fuels themselves. For this
reason chemical processes will not be considered in the following
discussion.
Also, although it is not likely that all environmental conditions will be
within optimal ranges under natural field conditions, natural attenuation
processes will still be occurring. The natural attenuation mechanisms
discussed in the following section are:
O Biological Processes—aerobic (requires oxygen), anaerobic (must occur
in the absence of oxygen), and hypoxic (can occur under conditions of
low oxygen content); and
O Physical Phenomena--volatilization, dispersion (mechanical mixing
and molecular diffusion), and sorption.
Both of these mechanisms and how they contribute to natural
attenuation effectiveness are described below and summarized in
Exhibit K-5.
Exhibit IX-5
Potential Natural Attenuation Mechanisms
Mechanism
Description
Potential For BTEX Attenuation
Biological
Aerobic
Anaerobic
Denitrffication
Sutfate reducing
Methanogenic
Fe reducing
Hypoxic
Physical
Volatilization
Dispersion
Sorption
Microbes utilize oxygen as an electron acceptor Most significant attenuation mechanism if
to convert contaminant to CO2, water, and sufficient oxygen is present. Soil air (02)
^ 2 percent. Groundwater D.O. >_ 1 to 2 mg/L.
biomass.
Alternative electron acceptors (e.g., N03",
S042", Fe3*, C02) are utilized by microbes to
degrade contaminants.
Secondary electron acceptor required at low
oxygen content for biodegradation of
contaminants.
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.
Rates are typically much slower than for
aerobic biodegradation: toluene is the only
component of BTEX that has been shown to
consistently degrade.
Has not been demonstrated in the field for
BTEX.
Normally minor contribution relative to
biodegradation. More significant for shallow or
highly fluctuating water table.
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.
Source: Adapted from McAisterand Chiang, 1994.
IX-1O
October 1994
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Biological Processes
Aerobic biodegradation of BTEX by naturally occurring
microorganisms is more rapid than anaerobic biodegradation, but both
are important. The rate of oxygen depletion due to microbial metabolism
typically exceeds the rate at which oxygen is naturally replenished to the
subsurface. This is especially true in the core region of the hydrocarbon
plume dissolved in groundwater. The result is that anaerobic processes
can become predominant. When oxygen is depleted, an alternative
electron acceptor (e.g., NO3", SO42', Fe3+) and a microorganism capable
of using the alternative electron acceptor must be available for
biodegradation to occur. Toluene is the only BTEX component that has
been shown to degrade under anaerobic conditions in the field.
Conditions where oxygen is partially depleted are referred to as hypoxic
(about 0.1 to 2 ppm oxygen). Biodegradation of BTEX under hypoxic
conditions may be possible, but it has not been demonstrated.
Anaerobic biodegradation is also a significant attenuation process.
Oxygen depletion in the subsurface is a characteristic of biodegradation
of petroleum hydrocarbons and is a consequence of the rate of metabolic
oxygen utilization exceeding the natural capacity for oxygen
replenishment. The core of a contaminant plume is typically under
anaerobic conditions and only the margins are aerobic (Exhibit IX-1).
Therefore, even though the rate of anaerobic biodegradation is much
slower than aerobic biodegradation (often by a factor of 10 to several
hundred), anaerobic processes may dominate the degradation of
hydrocarbon contaminants. Because a variety of models are available
and their appropriate use requires a high degree of technical expertise, a
more detailed discussion of modeling is beyond the scope of this manual.
Physical Phenomena
Physical processes such as volatilization, dispersion, and sorption
also contribute to natural attenuation. Volatilization removes
constituents 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
constituents, the expected mass loss due to volatilization is even lower.
Dispersion ("spreading out" of constituents through the soil profile or
groundwater unit) results in lower concentrations of constituents, 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
October 1994 IX-11
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process by which particles 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 constituents
reach a receptor). Although none of these three processes results in a
loss of mass, they can help to improve the rate at which natural
attenuation occurs.
Evaluation Of Site And Constituent Factors
At most sites, monitoring data collected during the site
characterization (prior to submission of the CAP) will not cover a
sufficient time span to demonstrate that natural attenuation is
occurring. In this case, fate and transport models can be used to predict
the effectiveness of natural attenuation. Even with historical data
showing reductions in a constituent plume, modeling may be useful in
evaluating the mechanism responsible for the observed concentration
reductions (e.g., are constituents simply being diluted?). Modeling is
especially appropriate for sites with nearby receptors.
Fate and transport models typically account for attenuation factors
such as biodegradation, adsorption/retardation, and dispersion/
dilution. Model calibration involves comparison of simulation results
with field measurements collected over time. Predictive simulations of
contaminant movement and degradation should only be made using a
properly calibrated model. After the model is calibrated, model
predictions may be used to evaluate potential risks associated with the
attenuated contaminants. Not every site will require sophisticated
modeling. Often there are not enough data available to construct a
representative model. In some cases, other techniques such as statistical
regression analysis or analytical solutions may be adequate to
demonstrate the effectiveness of natural attenuation.
Even without modeling results, an evaluation of site and chemical
constituent factors can help you determine whether natural attenuation
is likely to be effective. For example, calculation of contaminant travel
times using conservative assumptions can be made if the hydraulic
gradient, hydraulic conductivity, and effective porosity are known.
Site Factors Affecting Constituent Migration
The potential for natural attenuation to result in reduction of BTEX
concentrations can be determined by evaluation of the site factors listed
in Exhibit IX-6. Each of these site factors is discussed in detail below.
rx-12
October 1994
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Exhibit IX-6
Potential For Natural Attenuation: Site Factors
Factor
Potential For Natural Attenuation
Soil texture
Soil structure
Adsorption potential
Groundwater flow rate
Soil and groundwater
aeration
Soil moisture content
SoilpH
Microbial community
Precipitation
Temperature
Soil nutrient concentration
Coarse-grained soils provide the greatest drainage and aeration,
but may also promote contaminant migration.
Layered soils inhibit vertical migration and dispersion of
constituents, but may promote lateral spreading.
Higher organic carbon content and smaller grain size in soil
results in greater adsorption of chemicals and retards migration.
Greater groundwater flow rate will enhance constituent dispersion.
Greatest when soil 02 > 2%, and groundwater
D.O. > 1 to 2 mg/L
Greatest between 40 to 85 percent of field capacity.
Greatest between soil pH values of 6 to 8.
Greatest with soil/groundwater conditions that allow 02 flow and
in the absence of toxic levels of constituents.
Most favorable at 10 to 60 inches of rain per year.
Most active microbial activity occurs at ambient temperatures of
5° to 45°C. Activity typically doubles for every 10°C rise in
temperature.
Greatest when the C:N:P ratio is about 100:10:1.
Soil Texture
Soil texture refers to the size of mineral particles. It is a qualitative
measure of the soil permeability to both air and water. Fine-grained soils
(e.g., clays and silts), have lower permeabilities than coarse-grained soils
(e.g., sand and gravel). Thus, sandy soils (which have an intrinsic
permeability of about 1CT8 cm2 or greater) promote drainage and
aeration, which is favorable to both the dispersion and biodegradation of
constituents. However, high permeability also promotes faster and
farther downgradient migration of contaminants, which could adversely
impact potential receptors. Because of their high sorptive capacities
(owing to both small particle size and higher organic matter content),
clays and silts are associated with a slower migration (i.e., retardation) of
contaminants and less dilution than those of sands and gravels. But, at
the same time, the potential for downgradient migration is also reduced.
Thus, even though biodegradation may take longer, there may be little or
no risk to potential downgradient receptors.
October 1994
IX-13
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Also consider the amount of precipitation in the region, as
precipitation can increase leaching rates to groundwater. If the soil at
the site has a high permeability and there is moderate to heavy
precipitation in the area, aeration, dilution, and dispersion will be
enhanced, thereby reducing constituent concentrations. However, these
conditions also favor migration of constituents (especially lighter, more
soluble products) from soils to groundwater. Whether this migration is
acceptable depends on your state's policies and the presence of receptors
near the site.
Soil Structure
Soil structure refers to the arrangement of soil particles into groups.
Soil structure can enhance or inhibit constituent migration. Layered
soils tend to hinder the vertical migration of constituents, but may
promote lateral migration. Naturally occurring fissures, cracks, or
channels (or those created by roots or burrowing animals), however, can
facilitate the migration of constituents from soil to either the atmosphere
or groundwater. 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.
Adsorption Potential
Adsorption is the affinity of a chemical substance for particulate
surfaces and is an important factor that retards a constituent's
movement in the environment. Constituents that adsorb tightly may be
less subject to transport in the gaseous phase or in solution, whereas
constituents that are not tightly adsorbed can be transported through
soils, aquatic systems, and the atmosphere. With respect to the impact
on natural attenuation, the higher the adsorption potential, the greater
the retardation of contaminant migration. Increased adsorption can
increase the time required for constituents to reach receptors, allowing
greater time for biodegradation to occur.
Adsorption 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 to adsorb (retard) organic
compounds. The fraction of organic carbon (foc) in surficial soils typically
ranges from 1 to 3.5 percent. The organic matter content in subsurface
soils is typically an order of magnitude lower because most organic
residues are incorporated or deposited on the surface.
rx-14
October 1994
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Adsorption is also influenced by:
O Hydrophobicity of the compound—the more hydrophobic (i.e.,
insoluble in water), the greater its tendency to adsorb onto particulate
matter, while the more hydrophilic (i.e., soluble in water), the less of a
tendency it has to adsorb onto particulate matter; and
O Sorption to mineral surfaces—this may be more important than
sorption to organic carbon, if foc is low and soil particles have a large
surface area to volume ratio (e.g., small clay particles).
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.
Groundwater Flow Rate
Dispersion and migration of constituents increases with increasing
groundwater flow rate. True groundwater velocity is referred to as the
seepage velocity. Seepage velocity is equal to the product of the hydraulic
conductivity and the gradient divided by the effective porosity. For a
given hydraulic gradient, the higher the hydraulic conductivity the
higher the seepage velocity. High hydraulic conductivity (i.e., > 10"4
cm/sec) will contribute significantly to the dispersion of constituent
concentrations, while low hydraulic conductivity (i.e., < 10"7 cm/sec) will
generally result in concentrations remaining more or less localized. Of
course, a higher flow rate also increases migration of dissolved
constituents, which, at many sites, may not be desirable. Rapid
infiltration and groundwater flow can also promote reoxygenation in the
subsurface by transporting higher levels of dissolved oxygen.
Soil And Groundwater Aeration
Aerobic biodegradation is substantially faster than anaerobic
biodegradation. Aerobic biodegradation requires that soils are relatively
permeable (with an intrinsic permeability about 10"8 cm2 or greater) to
allow transfer of oxygen to subsurface soils where the microorganisms
are degrading the petroleum constituents. Soils with a low oxygen
content can hinder aerobic biodegradation. Oxygen levels greater than or
equal to 2 percent are optimal for aerobic biodegradation in the
unsaturated zone. Another indication of well-aerated soils is the
presence of chemicals in their oxidized state (e.g., ferric iron [Fe3+],
manganic manganese [Mn4"1"], nitrate [NO3~], and sulfate ISO42"]); the
presence of the reduced forms of these elements indicates restricted
drainage and poor aeration.
October 1994 IX-15
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If constituents are present in groundwater, it is important that the
groundwater have sufficient dissolved oxygen for aerobic biodegradation
to occur. Generally, aerobic biodegradation of BTEX constituents will
occur with dissolved oxygen concentrations greater than 1 to 2 mg/L.
Natural groundwater flow into the contaminated area may provide a
continuous supply of dissolved oxygen, which will continue to encourage
aerobic biodegradation.
Dissolved hydrocarbon plumes typically have three zones defined by
the concentration of dissolved oxygen: (1) anaerobic core, (2) aerobic
margins, and (3) hypoxic interface between the aerobic and anaerobic
zones (Exhibit IX-1). For many subsurface hydrocarbon releases,
anaerobic biodegradation predominates over aerobic biodegradation
because oxygen becomes depleted by microbial metabolism and remains
depleted.
Soil Moisture Content
Moisture is necessary for microbial growth. Microbes can only utilize
petroleum hydrocarbons when the hydrocarbons are in the dissolved
phase. In addition, water facilitates the movement of bacteria to other
parts of the soil, where these bacteria can continue to degrade petroleum
constituents. In the unsaturated zone, soil moisture content of between
40 and 85 percent of the total water-holding capacity (field capacity), or
about 12 to 30 percent by weight, is considered optimal for aerobic
microbial activity.
SoilpH
Ssoils that have a pH of 6 to 8 generally promote bacterial growth.
Soils with a pH significantly above or below these values generally result
in limited microbial activity.
Microbial Community
Because microbes capable of degrading petroleum products are
present in almost all subsurface environments. However, it may be
important in some situations to analyze soil samples with the intent of
confirming the presence of hydrocarbon degrading microorganisms. The
exercise of collecting soil samples and conducting laboratory microcosm
studies is generally not necessary. 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. Degradation rate constants determined
in the laboratory may not correspond to rates that occur under field
conditions. Therefore, it is best to determine these rate constants from in
situ, field measurements. Soil samples should be analyzed for toxic levels
IX-16 October 1994
-------
of chemicals (e.g., heavy metals, corrosive materials, and pesticides) that
would inhibit the effectiveness of the microbial community.
Precipitation
Moderate to heavy precipitation (i.e., 10 to 60 inches/year) is
favorable for maintaining soil moisture necessary to support microbial
populations. Precipitation also transports oxygen and nutrients as it
percolates downward through the subsurface soils, enhancing microbial
metabolic activity. As mentioned above, microbial activity is generally
greatest when the moisture content in the soils is 40 to 85 percent of the
water-holding capacity of the soils.
Temperature
Effective biodegradation can generally occur within a temperature
range of 5°C to 45°C; ideally, temperatures should be above 15°C for
optimum biological activity. Extreme temperatures (either hot or cold)
prohibit microbial growth. In most areas of the U.S., subsurface soils
generally have a fairly constant temperature of about 13°C throughout
the year. Surficial soil temperature at sites in colder states, especially
Alaska, are likely to be lower, reducing the rate of microbial activity. The
rate of microbial activity typically doubles for every 10°C rise in
temperature.
Soil Nutrient Concentration
In addition to requiring sufficient organic (carbon-rich) material (i.e.,
petroleum constituents) for consumption as a food source, adequate
levels of nitrogen and phosphorus also are necessary for bacterial
growth. For optimal microbial growth, the carbon to nitrogen to
phosphorus (C:N:P) ratio should be between 100:10:1 and 100:1:0.5.
Chemical Constituent Factors
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 more than 100 separate compounds. Individual constituents
will be attenuated to a varying degree, based on its chemical and
physical properties. This chapter focuses on the chemical properties of
the BTEX constituents, because regulatory clean-up levels are typically
established for BTEX. Tables in this section present the chemical
properties of the BTEX constituents and may be used to establish the
magnitude of dispersion or degradation of the individual constituents.
The chemical factors discussed below largely determine the
partitioning of constituents among the dissolved, gaseous, and adsorbed
October 1994 IX-17
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phases. Exhibit IX-7 shows the relative environmental partitioning of the
Exhibit IX-7
Relative Environmental Partitioning Of BTEX Constituents
Petroleum
Constituent
Benzene
Ethylbenzene
Toluene
o-Xylene
Adsorption On Soil
Particles
(%)
3
21
3
15
Volatilization
(%)
62
59
77
54
Soluble Portion In Groundwater
And Soil Moisture
(%)
35
20
20
31
Source: Adapted from McLearn et al. 1988, TTie IntemationalJoumal of Air Pollution Control and Waste Management.
BTEX constituents based on soil modeling (SESOIL) results. Each BTEX
constituent will migrate via multiple pathways depending on its chemical
and physical characteristics. Consequently, different chemicals prefer
different migration pathways. For example, benzene tends to partition
between the vapor phase, the adsorbed phase, and the dissolved phase.
As shown in Exhibit IX-7, the majority of the benzene mass will either
volatilize or dissolve in either soil moisture or groundwater. Only a
relatively small percentage will adsorb to soil particles. For example, if
the soil contains a higher percentage of organic carbon, a higher
percentage of benzene will potentially be adsorbed. In contrast to
benzene's behavior, ethylbenzene will more likely adsorb to soil particles
and would not be as soluble in water. Exhibit IX-7 is an illustrative
example and partitioning may not represent equilibrium conditions.
Also, partitioning depends upon many site-specific parameters and may
not be the same at other sites.
While reviewing the CAP, it should be noted that fate and transport
characteristics are much different for the heavier petroleum hydrocarbon
molecules, which do not tend to disperse as readily. The heavier constit-
uents should, therefore, be considered separately. Lighter hydrocarbon
constituents (e.g.f the BTEX constituents) tend to have greater mobility
(e.g., via volatilization and leaching), whereas heavier constituents have a
greater adsorption and lower volatilization potential. Thus, there is
greater potential for lighter constituents to migrate and disperse.
The lighter constituents of petroleum hydrocarbon fuels (e.g.,
benzene) tend to be more toxic than the heavier constituents. Although
natural attenuation is slower for heavier constituents than for lighter
constituents, it may still be an acceptable remedial approach for heavier
constituents, as long as there are no nearby receptors and an
indeterminate time frame is permitted for attaining cleanup levels. After
the free product (if any) has been recovered from the subsurface, natural
attenuation may be the most appropriate remedial approach for heavier
petroleum constituents because they are less soluble and less volatile
(hence less mobile) than lighter fuels and they contain only small
EX-18 October 1994
-------
fractions of toxic constituents (e.g.f benzene). In fact, after moderate
degradation or weathering, almost all of the lighter (more mobile and
more toxic) compounds have been stripped away, leaving the residue
enriched with the heavier, constituents that generally do not pose a
significant threat to distant receptors.
With the exception of lead, inorganic chemicals are not typically of
concern at sites with petroleum releases. Soils that are contaminated
with older gasoline products may contain relatively high concentrations
of lead, which can cause serious health and environmental effects. Many
organic lead compounds are volatile and toxic. Lead may also be leached
into the groundwater where it can be transported downgradient. The
presence of lead in site soils may require active remediation to eliminate
potential risk.
This section examines the most important factors that contribute to a
constituent's partitioning into the soil (adsorbed), groundwater
(dissolved), and air (gaseous) phases. The potential for natural
attenuation to be effective and for constituent concentration reduction to
occur as a result of chemical factors is shown in Exhibit DC-8. Each of
these factors is discussed below in more detail.
Exhibit IX-8
Potential For Natural Attenuation: Chemical Constituent Factors
Factor
Solubity
Vapor pressure
Henry's law
constant
Boling point
Molecular
weight
Description
Potential For Natural Attenuation
The extent to which a constituent wil
dissolve in another substance (e.g., water).
A measure of a constituent's tendency to
evaporate.
A measure of a constituent's tendency to
partition between the aqueous phase and
gaseous phase.
A measure of a constituent's tendency to
volatize.
The tendency of a constituent to adsorb
onto organic matter in the sot).
The mass of a chemical constituent.
The greater the constituent's solubility, the greater
the dispersion in groundwater and the greater the
migration in sol.
The higher the vapor pressure, the more Italy that
the constituent wil volatilize.
The higher the Henry's law constant, the greater the
tendency to votattize.
The lower the Doling point, the greater the tendency
forvolatlization.
The lower the K,,. and K,,, the less the adsoiption
potential.
In general, the lighter the constituent the more likely
thatitwilsolubiize.
Solubility
Solubility is the amount of a substance (e.g.. hydrocarbon) that will
dissolve in a given amount of another substance (e.g., water). Therefore,
a constituent's solubility provides insight to its fate and transport in the
aqueous phase. Constituents that are highly soluble have a tendency to
dissolve into the groundwater and are not likely to remain in the
October 1994
DC-19
-------
adsorbed phase. They are also less likely to volatilize from groundwater
and are more easily biodegraded. Conversely, chemicals that have low
water solubilities tend to remain in the adsorbed phase or are likely to
volatilize more readily, but they are less likely to biodegrade. In general,
lower molecular weight constituents tend to be more soluble and,
therefore, migrate and disperse much more readily in groundwater or
soil moisture than do heavier constituents.
In the field, aqueous concentrations rarely approach the solubility of a
substance because dissolved concentrations tend to be reduced through
processes such as biodegradation, dilution, and adsorption.
Nevertheless, the mobility of a constituent is largely determined by its
water solubility. Exhibit IX-9 lists the solubility of the BTEX
constituents. Note that these values are for pure components and
mixtures tend to result in lower aqueous concentrations for individual
constituents. The higher the solubility, the more likely it is that the
constituent 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)
are transported farther and faster than hydrocarbons. Often these
additives can be detected in distant wells long before hydrocarbons
arrive.
Exhibit IX-9
Solubilities of BTEX Constituents
Petroleum
Constituent
Ethylbenzene
o-Xylene
Toluene
Benzene
Typical Percentage in
Gasoline
2 to 8
5 to 20
2 to 10
1 to 4
Pure Compound
Solubility
' (mg/L)(20eC)
152
175
515
1,780
Solubility of Compound in
Typical Gasoline
(%)
4 to 8
10 to 20
30 to 80
30 to 60
As shown in the exhibit, benzene is relatively more soluble than the
other BTEX constituents and will therefore preferentially dissolve into
the aqueous phase. As a result, benzene is the most likely BTEX
constituent to be mobile and disperse in the aqueous phase.
Ethylbenzene has a much lower solubility, therefore its concentration in
the aqueous phase will be lower than the concentration of benzene.
Vapor Pressure
The vapor pressure of a constituent is a measure of its tendency to
evaporate. More precisely, it is the pressure that a vapor exerts when in
equilibrium with its pure liquid or solid form. Constituents with higher
IX-20 October 1994
-------
vapor pressures (i.e., those constituents 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, constituents with relatively low vapor pressures are
less likely to vaporize and become airborne. Volatilization from soil or
groundwater is highest for constituents with higher vapor pressures.
Exhibit IX-10 presents vapor pressures of the BTEX constituents. As
shown in the Exhibit, benzene is the BTEX constituent with the highest
vapor pressure, and is therefore the constituent most likely to volatilize
and disperse. Alternatively, xylenes will more readily adsorb to soil than
enter the vapor phase. If the constituents of concern have high vapor
pressures, they will likely volatilize and disperse readily, constituents
with low vapor pressures are more likely to remain in soil and
groundwater.
Exhibit IX-10
Vapor Pressures Of BTEX Constituents
Vapor Pressure
Constituent (mm Hg at 20°C)
Xylenes 6
Ethylbenzene 7
Toluene 22
Benzene 76
Henry's Law Constant
The Henry's law constant is a measure of a constituent's tendency to
preferentially partition between the aqueous phase and the gaseous
phase. The Henry's law constant provides a qualitative indication of the
importance of volatilization: for constituents with Henry's law constants
greater than about 100 atmospheres (atm), volatilization from the
aqueous phase tends to be rapid.
As shown by the Henry's law constant values in Exhibit IX-11, each of
the BTEX constituents has a value greater than 200, indicating that
each will volatilize from water fairly readily, resulting in constituent
migration from the aqueous phase. In the field, however, volatilization is
generally limited by mass transfer (i.e., from the groundwater, through
the capillary fringe and the unsaturated zone, and finally to the
atmosphere), decreasing the importance of volatilization as a natural
attenuation mechanism.
October 1994 EK-21
-------
Exhibit IX-11
Henry's Law Constant Of BTEX Constituents
Henry's Law Constant
Constituent (atm)
Ethylbenzene 359
Xylenes 266
Benzene 230
Toluene 217
Boiling Point
Petroleum products are often classified by their boiling point range
because the boiling point is a measure of volatility. As shown in
Exhibit IX-12, the boiling point for benzene is the lowest for the
constituents in BTEX, implying that benzene will tend to volatilize most
readily and be quite mobile in the soil vapor. Comparing the BTEX
constituents, dissipation via volatilization is most important for benzene
and least important for xylenes.
Boiling point ranges for heavier petroleum products are higher than
those of the BTEX constituents (e.g., 40 to 225°C for gasoline, 180 to
300°C for kerosene, and > 275°C for heating oil), indicating that these
constituents will volatilize less readily than the BTEX chemicals in
gasoline.
Exhibit IX-12
Boiling Points Of BTEX Constituents
Constituent Boiling Point (°C)
Benzene 80.1
Toluene 110.8
Ethylbenzene 136.2
Xylenes 144.4
Organic Carbon Partition Coefficient (Koc), Adsorption Potential (Kd)
The organic carbon partition coefficient (K^) is an approximation of
the propensity of a compound to adsorb to organic matter found in the
soil. The adsorption coefficient (B^) value is an expression of the
tendency of a constituent to remain adsorbed on soil and is the product
K-22 October 1994
-------
of KOC and the fraction organic carbon (foc) in the soil. 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 Kr>c and K^ values.
KQ,, values can range from 1 to 107. The K^s of BTEX constituents are
all low, indicating relatively weak adsorption potential, as shown in
Exhibit IX-13. None of the BTEX constituents 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 constituents will dissipate. Heavier
petroleum constituents tend to have greater K^ values and will thus
sorb more strongly to soils, retarding constituent migration.
Exhibit IX-13
Koc Values For BTEX Constituents
Constituent Koc Value (cm3/g)
Benzene 65
Toluene 120
Ethylbenzene 220
Xylenes 237
Molecular Weight
Aromatic constituents with lighter molecular weights (i.e., the more
soluble fractions, <_ C7) tend to dissipate and degrade more readily than
aromatic hydrocarbons with heavier molecular weights. Of the BTEX
constituents, benzene and toluene have the lightest molecular weights
(i.e., molecular weights of 78 g/mole for benzene and 92 g/mole for
toluene).
Remedial Progress Monitoring
Monitoring the progress of natural attenuation is necessary to confirm
whether petroleum constituents are being degraded or dissipated at
acceptable rates and that potential receptors are not likely to be
adversely affected.
Indicators Of Natural Attenuation
Site characterization data can provide numerous indicators to
demonstrate that natural attenuation is occurring (McAllister, 1994).
Some of the necessary data may be collected as part of a standard site
characterization, while other data would likely be collected specifically
for the purpose of evaluating natural attenuation effectiveness. Site
October 1994 IX-23
-------
assessment data useful in evaluating natural attenuation effectiveness
are listed in Exhibit IX-14. Note that sampling and analytical methods
Exhibit IX-14
Site Characterization Data Used To Evaluate Effectiveness Of
Natural Attenuation
Site Characterization Data
Application
Direction and gradient of groundwaterflow
Hydraulic conductivity
Definition of lithdogy
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 D.O. levels upgradient of the
source and plume
D.O. levels inside and outside the
contaminant plume
Alkalinity, hardness, pH, and soluble Fe
inside and outside the contaminant plume
Redox potential
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
groundwaterflow.
Estimate volatilization rates.
Evaluate potential source smearing, influence of
fluctuations on groundwater concentrations, and
variation in flow direction.
Compare expected extent without natural
attenuation to actual extent.
Estimate expected extent of plume migration.
Evaluate status of plume (i.e., steady state,
decreasing, migrating).
Determine if sufficient D.O. is present for aerobic
biodegradation (> 1 to 2 mg/L).
Identify inverse correlation indicative of aerobic
biodegradation.
Evaluate geochemical indicators of natural
attenuation.
Determine nature of biologically mediated
degradation of contaminants.
Identify areas of natural groundwater aeration.
Source: Adapted from McAllister and Chiang, 1994.
must be consistent and appropriate, and well and screen placement
must be appropriate to the site conditions, or the monitoring data might
not accurately reflect the rate at which natural attenuation is occurring.
A thorough evaluation of constituent mass balance can be used to
demonstrate the extent and rate of natural attenuation, but this
approach requires extensive monitoring data that completely define the
horizontal and vertical extent of the contaminant plume. This approach
has been used to investigate natural attenuation, but it is generally
IX-24
October 1994
-------
practical only for research. Several other indicators of natural
attenuation with less extensive data requirements are described below.
Constituent Plume Characteristics
In the absence of natural attenuation mechanisms, constituent
concentrations would remain relatively constant within the plume, and
then decrease rapidly at the edge of the plume. If natural attenuation is
occurring, constituent concentrations will decrease with distance from
the source along the flow path of the plume as a result of dispersion. If
other natural attenuation mechanisms are occurring, the rate at which
concentrations of constituents are reduced will be accelerated.
Monitoring of constituent concentrations in the groundwater over time
will give the best indication of whether natural attenuation is occurring.
If natural attenuation is occurring, the contaminant plume will migrate
more slowly than expected based on the average groundwater velocity.
Receding plumes typically occur when the source has been eliminated.
Natural attenuation may also be occurring in plumes that are expanding,
but at a slower than expected rate. For example, in sandy soils with
relatively low organic carbon content (about 0.1 percent), BTEX
constituents are expected to migrate at one-third to two-thirds of the
average groundwater speed velocity (McAllister, 1994). Higher organic
carbon content would further retard constituent migration. If
constituents are migrating more slowly than expected based on
groundwater flow rates and retardation factors, then other natural
attenuation mechanisms (primarily biodegradation) are likely reducing
constituent concentrations. For stable plumes, the rate at which
contaminants are being added to the system at the source is equal to the
rate of attenuation. A plume may be stable for a long period of time
before it begins to recede, and in some cases, if the source is not
eliminated, the plume may not recede.
Occurrence of biodegradation might also be deduced by comparison of
the relative migration of individual constituents. The relative migration
rates of BTEX constituents, based on the chemical properties, are
expected to be in the following order:
benzene > toluene, o-xylene > ethylbenzene, m-xylene, p-xylene.
If the actual migration rates do not follow this pattern, biodegradation
may be responsible.
Dissolved Oxygen Indicators
The rate of biodegradation will depend, in part, on the supply of
oxygen to the contaminated area. At levels of dissolved oxygen (D.O.)
below 1 to 2 mg/L in the groundwater, aerobic biodegradation rates are
very slow. If background D.O. levels (upgradient of the contaminant
October 1994 IX-25
-------
source) equal or exceed 1 to 2 mg/L, the flow of groundwater will supply
D.O. to the contaminated area, and aerobic degradation is possible.
Where aerobic biodegradation is occurring, an inverse relationship
between D.O. concentration and constituent concentrations can be
expected (i.e., D.O. levels increase as constituent levels decrease). Thus,
if D.O. is significantly below background within the plume, aerobic
biodegradation is probably occurring at the perimeter of the plume.
Geochemical Indicators
Certain geochemical characteristics can also serve as indicators that
natural attenuation, particularly aerobic biodegradation, is occurring.
Aerobic biodegradation of petroleum products produces carbon dioxide
and organic acids, both of which tend to cause a region of lower pH and
increased alkalinity within the constituent plume.
Anaerobic biodegradation may result in different geochemical
changes, such as increased pH. Under anaerobic conditions,
biodegradation of aromatic hydrocarbons typically causes reduction of
Fe3+ [insoluble] to Fe2+ [soluble], because iron is commonly used as an
electron acceptor under anaerobic conditions. Thus, soluble iron
concentrations in the groundwater tend to increase immediately
downgradient of a petroleum source as the D.O. is depleted, and
conditions change to become anaerobic (i.e., reduced). The concentration
of methane increases, another indication that anaerobic biodegradation
is occurring.
Oxidation/Reduction Potential
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. Redox
potential also can be used as an indicator of certain geochemical
activities (e.g., reduction of sulfate, nitrate, or iron). The redox potential
of groundwater generally ranges from 800 millivolts to about -400
millivolts (Exhibit IX-15). The lower the redox potential, the more
reducing and anaerobic the environment.
Measurement of redox potential of groundwater also allows for
approximate delineation of the extent of the contaminant plume. Redox
potential values taken from within the contaminant plume will be lower
than background (upgradient) redox values and values from outside the
plume. This is due in part to the anaerobic conditions that typically exist
within the core of the dissolved hydrocarbon plume.
IX-26 October 1994
-------
Exhibit IX-15
Redox Potentials For Various Electron Acceptors
u>
o>p
Ul G
-I
o 0
"cui
O O>
§c
0
O 0
oa:
Redox Potential (En°) in
Millivolts @ pH = 7 and T = 25° C
1000 —
Aerobic - -02 + 4H+ + 4e 2H20 (En°= 820)
- -2N03 + 12H+ + 10e- N2 + 6H20 (En° = +740)
Anaerobic
500'
- - Mn02(s) + HCOJ + 3H+ + 2e~ MnC03(s) + 2H20
"* (En° = +520)
0 —
-500«
-- FeOOH(s) + HC02
4
C02
9H
8H*
2H* + e~ — FeC03 + 2H20
(En° = -50)
8e~ - - HS + 4H20 (En°= -220)
8e- — — CH4 + 2H20 (En° « -240)
Source: Modified from Atoms e( a/., (1994).
Ongoing Monitoring
If the indicators described above suggest that natural attenuation is
occurring, it is important for ongoing monitoring to be conducted to
confirm effectiveness and estimate the rate. As with the active
remediation technologies also described in this manual, if natural
attenuation does not appear to be effective in remediating the
contamination at the site within a reasonable time frame, then an
alternative active remedial technology will be required. Alternatively, if
permissible under state regulations, a risk assessment (using fate and
transport modeling and/or exposure models) may be acceptable to
determine if an aggressive remedial alternative is necessary.
A table summarizing the constituents to monitor and the suggested
monitoring frequency is presented as Exhibit K-16, while more specific
details are discussed below.
October 1994
IX-27
-------
Exhibit IX-16
Ongoing Progress Monitoring
Monitoring
Medium Frequency
Soil Annually
Groundwater Quarterly for
the first year,
then annually
thereafter.
What To Monitor
BTEX; TPH; any other
constituents of
concern; 02, C02,
temperature, pH.
BTEX; TPH; any other
constituents of
concern; D.O., C02,
pH, alkalinity,
hardness, soluble Fe,
redox potential.
Where/Number Of
Samples To Monitor
Several representative
samples located
throughout the area of
contamination.
Minimum of 4 wells: 1
upgradient, 3
downgradient. Wells
along the plume
centerline and outside the
plume boundaries.
Soils
Soil samples should be collected from the unsaturated zone and
should be analyzed for the BTEX constituents, TPH, and any other
constituents of concern at the site. Sampling should take place annually
to demonstrate reductions in constituent concentrations.
At a minimum, samples should be collected in locations where the
contamination is known to be greatest (e.g., where the LUST was initially
located), to determine whether concentrations are being reduced. Any
other affected areas also should be sampled. Samples should also be
collected from the boundary of the contaminated soil area to evaluate
whether the extent of contamination in the soils is increasing or
decreasing. If the primary constituents of concern at the site are volatile
organic chemicals (VOCs), monitoring of soil gas may be substituted for
direct soil measurements at some locations.
In addition, analyses for O2, CO2, and methane may be conducted to
determine the microbial activity in the soils. As described above, reduced
O2 concentrations in the plume area (relative to background) and
elevated CO2 concentrations are a good indication that aerobic
biodegradation is occurring. These measurements are best collected from
permanent gas probes.
Groundwater
Groundwater monitoring should be designed to ensure that the
vertical and lateral extent of constituents in groundwater is evaluated. At
a minimum, the groundwater should be analyzed for VOCs and other
constituents of concern, TPH, dissolved oxygen (D.O.), pH, redox
potential, alkalinity, and hardness. Groundwater monitoring should be
IX-28
October 1994
-------
conducted quarterly during the first year. Sampling frequency can then
be reduced depending upon contaminant travel times and other site-
specific factors (e.g., distance to nearest receptor).
In order to demonstrate that natural attenuation is occurring, a
sufficient number of monitoring wells that are appropriately located are
necessary. A typical site characterization may involve installation of one
upgradient well and three downgradient wells. Appropriate monitoring of
natural attenuation requires additional wells; at least 1 close to the
original source and several along the centerline of the dissolved plume
and around the outer boundary of the plume as illustrated in
Exhibit IX-17. The number of wells required must be determined based
on site-specific considerations. If natural attenuation is occurring, .
concentrations in these wells should decrease with distance from the
source area. Optimally, some of these additional wells will be placed so
that they are outside the plume. Another type of well, a sentinel well, is
located between the leading downgradient edge of the dissolved plume
and a receptor (e.g., a drinking water supply well). Such wells provide
early warning to the well user should the plume continue to migrate.
Detection of contamination in sentinel wells or wells outside the plume
indicate that natural attenuation is not occurring at an acceptable rate
and more aggressive remedial alternatives should be considered.
\
If the potential exists for constituents in shallow gro'undwater to
discharge into surface water bodies such as streams or rivers, these
water bodies should be sampled for the same characteristics (e.g., VOCs,
TPH, D.O.) at the same frequency as groundwater. However, dilution and
biodegradation will potentially be of such magnitude that the presence of
the constituent, parameter, or characteristic of interest may be
completely masked.
October 1994 IX-29
-------
Exhibit IX-17
Recommended Groundwater Monitoring Well Network For Demonstrating
Natural Attenuation
•Contaminant
Plume
Boundary
4-
Upgradient
Well
MW-1
Groundwater
Flow—*,
MW-8
MW-6
Legend:
•$• Monitoring Well
Source: Adapted from McAllister and Chiang, 1994.
IX-3O
October 1994
-------
References
Harder, H., and T. Hopner. "Hydrocarbon Biodegradation in Sediments
and Soils: A Systematic Examination of Physical and Chemical
Conditions - Part 5 Moisture." Hydrocarbon Technology, pp. 329-333,
1991.
Hillel, D. "Movement and Retention of Organics In Soil: A Review and A
Critique of Modeling (Chapter 7)." in Kostecki, D.T. and E.J.
Calabrese, eds. Petroleum Contaminated Soils. Volume 1. Remediation
Techniques and Environmental Fate Risk Assessment Chelsea, MI:
Lewis Publishers, 1989.
Lyman, W.J., W.F. Reehl, and D.H. Rosenblatt Handbook of Chemical
Property Estimation Methods. Environmental Behavior of Organic
Compounds. New York: McGraw-Hill Book Company, 1982.
McAllister, P.M. and C.Y. Chiang. "A Practical Approach to Evaluating
Natural Attenuation of Contaminants in Groundwater." Groundwater
Monitoring Review, pp. 161-173, 1994.
McLeam, M.E., Miller, M.J., Kostecki, P.T., Calabrese, E.J., Preslo, L.M.,
Suyama, W., and W.A. Kucharski. "Remedial Options for Leaking
Underground Storage Tanks." The Journal of the Air Pollution Control
Association, pp. 428-435, 1988.
Norris, R.D., Hinchee, R.E., Brown, RA., McCarly, 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. Handbook of
Bioremediation. Boca Raton, FL: CRC Press, 1994.
Wiedemeier, T.H., J.T. Wilson, D.H. Kampbell, and R.N. Miller. "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, 1994.
October 1994 IX-31
-------
Checklist: Can Natural Attenuation Be Used At This Site?
This checklist can help you to evaluate the completeness of the CAP
and to identify areas that require closer scrutiny. As you go through the
CAP, answer the following questions. If the answer to several of the
questions below is no, you may want to request additional information to
determine if natural attenuation will accomplish the cleanup goals at the
site.
1. Initial Screening
Yes No
Q Q Are there no nearby human or sensitive ecological receptors
near the site that could be exposed to the petroleum
contamination in soil?
Q Q If potential receptors are present, are they located at a
distance that represents a minimum 2 year travel time?
Q Q Are maximum total constituent concentrations less than
20,000 to 25,000 ppm TPH?
Q Q Are there potential receptors who could be exposed to
contaminated groundwater, soil, or vapors?
2. Detailed Evaluation — Site Factors Affecting Constituent
Degradation
Yes No
Q Q Are the soils well aerated, allowing for transfer of oxygen to
subsurface soils?
Q Q Is the adsorption potential of the constituent/soil
combination high enough to adequately retard constituent
migration?
Q Q Is the seepage velocity low enough to prevent rapid migration
of constituents?
Q Q Is soil oxygen content>_2 percent and dissolved oxygen
content > 1 to 2 mg/L?
Q Q Is moisture available for transport of microorganisms (soil
moisture of 40 to 85 percent of field capacity)?
Q Q Is the pH of the soil between 6 and 8?
IX-32 October 1994
-------
2. Detailed Evaluation — Site Factors Affecting Constituent
Degradation (continued)
Yes No
Q Q Are concentrations of heavy metals and other toxic
compounds below levels that could inhibit microbial activity?
Q Q Is rainfall moderate to heavy (i.e., 10 to 60 inches/year)?
Q Q Is the climate moderate to warm (i.e., 5° to 45°C)?
Q Q Does the soil have a C:N:P ratio of about 100:1:0.5 to
100:10:1?
3. Detailed Evaluation — Chemical Constituent Factors Affecting
Migration For Those Constituents Requiring The Most Significant
Concentration Reduction
Yes No
Q Q Are the majority of the hydrocarbon constituents at most
slightly soluble in water?
Q Q Are the majority of the hydrocarbon constituents not highly
volatile (as measured by vapor pressure, Henry's law
constant, and boiling point)?
Q Q Are the K^,, and Kj values of constituents high enough to
adequately retard migration?
Q Q Are the constituents sufficiently biodegradable?
4. Remedial Monitoring
Yes No
Q Q Will soil samples be collected?
Q Q Will a minimum of 4 groundwater wells be sampled?
Q Q Is the groundwater monitoring frequency at least quarterly
during the first year?
Q Q Are the groundwater wells placed to detect the reductions of
constituent concentrations in the plume and potential
migration of constituents?
October 1994 DX-33
-------
-------
Chapter X
In-Situ Groundwater Bioremediation
-------
-------
Contents
Overview X-l
Initial Screening Of In-Situ Groundwater Bioremediation X-9
Detailed Evaluation Of In-Situ Groundwater Bioremediation
Effectiveness . X-l 1
Site Characteristics That Affect In-Situ Groundwater
Bioremediation X-12
Hydraulic Conductivity X-12
Soil Structure and Stratification X-13
Groundwater Mineral Content X-13
Groundwater pH X-14
Groundwater Temperature : X-15
Microbial Presence X-15
Terminal Electron Acceptors X-16
Nutrient Concentrations X-17
Constituent Characteristics That Affect In-Situ Groundwater
Bioremediation X-18
Chemical Structure X-18
Concentration And Toxicity X-20
Solubility X-21
Treatability Testing X-22
Bench-Scale Treatability Testing X-22
Pilot-Scale Treatability Testing X-23
Groundwater Modeling X-24
Evaluations Of In-Situ Groundwater Bioremediation System Design X-24
Rationale For The Design X-25
Components Of An In-Situ Groundwater Bioremediation System . . X-27
Well Placement X-29
Electron Acceptor and Nutrient Addition System X-30
System Controls and Alarms . . . . ; X-31
Evaluation Of Operation And Monitoring Plans X-31
Start-up Operation X-32
Normal Operation X-32
Remedial Progress Monitoring - X-32
References X-35
Checklist: Can In-Situ Groundwater Bioremediation Be Used At This
Site? X-36
May 1995 X-iii
-------
List Of Exhibits
Number
Title
Page
X-l Schematic Diagram Of Typical In-Situ Groundwater
Bioremediation System Using Infiltration Gallery X-2
X-2 Schematic Diagram Of Typical In-Situ Groundwater
Bioremediation System Using Injection Wells X-3
X-3 Advantages And Disadvantages Of In-Situ Groundwater
Bioremediation X-5
X-4 In-Situ Groundwater Bioremediation Evaluation Process
Flow Chart X-6
X-5 Initial Screening For In-Situ Groundwater Bioremediation
Effectiveness X-10
X-6 Key Parameters Used To Evaluate The Effectiveness Of In-
Situ Groundwater Bioremediation X-11
X-7 Dissolved Iron And In-Situ Bioremediation Effectiveness X-l4
X-8 Heterotrophic Bacteria And In-Situ Groundwater
Bioremediation Effectiveness X-16
X-9 Chemical Structure And Biodegradability X-l9
X-10 Constituent Concentration And In-Situ Groundwater
Bioremediation Effectiveness X-20
X-ll Cleanup Concentrations And In-Situ Groundwater
Bioremediation Effectiveness X-21
X-l 2 Schematic Diagram Of Typical In-Situ Groundwater
Bioremediation System Using Injection Wells X-28
X-l3 Idealized Layout Of Extraction, Injection, And Monitoring
Wells For In-Situ Groundwater Bioremediation X-30
X-14 System Monitoring Recommendations X-33
X-iv
May 1995
-------
Chapter X
In-Situ Groundwater Bioremediation
Overview
In-situ groundwater bioremediation is a technology that encourages
growth and reproduction of indigenous microorganisms to enhance
biodegradation of organic constituents in the saturated zone. In-situ
groundwater bioremediation can effectively degrade organic constituents
which are dissolved in groundwater and adsorbed onto the aquifer
matrix.
Bioremediation generally requires a mechanism for stimulating and
maintaining the activity of these microorganisms. This mechanism is
usually a delivery system for providing one or more of the following: An
electron acceptor (oxygen, nitrate); nutrients (nitrogen, phosphorus); and
an energy source (carbon). Generally, electron acceptors and nutrients
are the two most critical components of any delivery system.
In a typical in-situ bioremediation system, groundwater is extracted
using one or more wells and, if necessary, treated to remove residual
dissolved constituents. The treated groundwater is then mixed with an
electron acceptor and nutrients, and other constituents if required, and
re-injected upgradient of or within the contaminant source. Infiltration
galleries or injection wells may be used to re-inject treated water, as
illustrated in Exhibits X-l and X-2, respectively. In an ideal
configuration, a "closed-loop" system would be established. All water
extracted would be reinjected without treatment and all remediation
would occur in situ. This ideal system would continually recirculate the
water until cleanup levels had been achieved. If your state does not
allow re-injection of extracted groundwater, it may be feasible to mix the
electron acceptor and nutrients with fresh water instead. Extracted
water that is not re-injected must be discharged, typically to surface
water or to publicly owned treatment works (POTW).
In-situ bioremediation can be implemented in a number of treatment
modes, including: Aerobic (oxygen respiration); anoxic (nitrate
respiration); anaerobic (non-oxygen respiration); and co-metabolic (see
Abbreviations and Definitions). The aerobic mode has been proven most
effective in reducing contaminant levels of aliphatic (e.g., hexane) and
aromatic petroleum hydrocarbons (e.g., benzene, naphthalene) typically
present in gasoline and diesel fuel. In the aerobic treatment mode,
groundwater is oxygenated by one of three methods: Direct sparging of
air or oxygen through an injection well; saturation of water with air or
oxygen prior to re-injection; or addition of hydrogen peroxide directly
May 1995 X-l
-------
CD
CO
01
Exhibit X-1
Schematic Diagram Of Typical In-Situ Groundwater Bioremediation System Using Infiltration Gallery
Former
LUST Location-
Infiltration-
Gallery
Slotted
PVC Piping-
Water Supply
(If Necessary)
•Filter
—Nutrients
r—Aeration
-Water Treatment
(If Necessary)
Discharge
**r^
Legend:
Vapor Phase
Adsorbed Phase
Dissolved Phase
Monitoring
Wells
Groundwater
Flow
Direction
Groundwater
Extraction Well
-------
I
M
CO
CO
en
Exhibit X-2
Schematic Diagram Of Typical In-Situ Groundwater Bioremediation System Using Injection Wells
Former
LUST Location
Water Supply
(If Necessary)
Filter
-Water Treatment
(If Necessary)
Injection
Well
—Nutrients
—Aeration
Legend:
Vapor Phase
Adsorbed Phase
Dissolved Phase
Monitoring
Wells
Groundwater
Flow
Direction
Groundwater
Extraction Well
CO
-------
into an injection well or into reinjected water. Whichever method of
oxygenation is used, it is important to ensure that oxygen is being
distributed throughout the area of contamination. Anoxic, anaerobic,
and co-metabolic modes are sometimes used for remediation of other
compounds, such as chlorinated solvents, but are generally slower than
aerobic respiration in breaking down petroleum hydrocarbons.
In-situ groundwater bioremediation can be effective for the full range
of petroleum hydrocarbons. While there are some notable exceptions,
such as MTBE, the short-chain, low-molecular-weight, more water
soluble constituents are degraded more rapidly and to lower residual
levels than are long-chain, high-molecular-weight, less soluble
constituents. Recoverable free product should be removed from the
subsurface prior to operation of the in-situ groundwater bioremediation
system. This will mitigate the major source of contaminants as well as
reduce the potential for smearing or spreading high concentrations of
contaminants. A summary of the advantages and disadvantages of in-
situ bioremediation of the saturated zone is shown in Exhibit X-3.
In-situ bioremediation of groundwater can be combined with other
saturated zone remedial technologies (e.g., air sparging) and vadose zone
remedial operations (e.g., soil vapor extraction, bioventing).
This chapter will assist you in evaluating a corrective action plan
(CAP) that proposes in-situ groundwater bioremediation for a petroleum-
contaminated aquifer. The evaluation process, which is summarized in a
flow diagram shown in Exhibit X-4, will serve as a roadmap for the
decisions you will make during your evaluation. You can use the
checklist at the end of this chapter as a tool to evaluate the
completeness of the CAP and to help focus attention on areas where
additional information may be needed. The evaluation process can be
divided into the following steps:
O Step 1: An initial screening of in-situ groundwater
bioremediation effectiveness, which will allow you to quickly gauge
whether this technology is likely to be effective, moderately effective,
or ineffective.
O Step 2: A detailed evaluation of in-situ groundwater
bioremediation effectiveness, which provides further screening
criteria to confirm the effectiveness of this technology and develop
design standards and operating conditions. To complete the detailed
evaluation, you will need to identify specific soil and constituent
characteristics and properties, compare them to ranges where in-situ
groundwater bioremediation is potentially effective, decide whether
X-4 May 1995
-------
Exhibit X-3
Advantages And Disadvantages Of In-Situ Groundwater Bioremediation
Advantages
o Remediates contaminants that are
adsorbed onto or trapped within the
geologic materials of which the aquifer is
composed along with contaminants
dissolved in the groundwater.
o Application involves equipment that is
widely available and easy to install.
o Creates minimal disruption and/or
disturbance to on-going site activities.
o Time required for subsurface remediation
may be shorter than other approaches
(e.g., pump and treat).
o Is generally recognized as being less costly
than other remedial options (e.g., pump
and treat, excavation).
o Can be combined with other technologies
(e.g., bioventing, soil vapor extraction) to
enhance site remediation.
o In many cases, this technique does not
produce waste products that must be
disposed of.
Disadvantages
o Injection wells and/or infiltration galleries
may become plugged by microbial growth
or mineral precipitation.
o High concentrations (TPH > 50,000 ppm) of
low solubility constituents may be toxic
and/or not bioavailable.
o Difficult to implement in low-permeability
aquifers (<10~4 cm/sec).
o Re-injection wells or infiltration galleries
may require permits or may be prohibited.
Some states require permit for air injection.
o May require continuous monitoring and
maintenance.
o Remediation may only occur in more
permeable layer or channels within the
aquifer.
treatability studies are necessary to determine effectiveness, and
conclude whether this technology is likely to be effective at a site.
O Step 3: An evaluation of the in-situ groundwater bioremediation
system design, which will allow you to determine if the rationale for
the design has been appropriately defined based on treatability study
data, whether the necessary design components have been specified,
and whether the construction process flow designs are consistent with
standard practice.
O Step 4: An evaluation of the operation and monitoring plans,
which will allow you to determine whether plans for start-up and
long-term system operation monitoring are of sufficient scope and
frequency, and whether remedial progress monitoring plans are
appropriate.
May 1995
X-5
-------
Exhibit X-4
In-situ Groundwater Bioremediation Evaluation Process Flow Chart
INITIAL SCREENING OF IN-SITU
GROUNDWATER BIOREMEDIATION
EFFECTIVENESS
Determine the types
of contaminants that
occur in the
contaminated aquifer
Determine the types of soils
that occur within the
contaminated aquifer
In situ groundwater
bioremediation is not
likely to be effective
at the site.
Consider other
technologies.
Are
constituents
potentially
biodegradable?
Is clayey
medium targeted
for remediation?
• Dual-phase
extraction
• Natural
attenuation
In-situ groundwater
bioremediation has the
potential for effectiveness
at the site.
Proceed to next panel.
X-6
May 1995
-------
Exhibit X-4
In-situ Groundwater Bioremediation Evaluation Process Flow Chart
DETAILED EVALUATION OF IN-SITU
GROUNDWATER BIOREMEDIATION
EFFECTIVENESS
Identify constituent
characteristics important
to in-situ groundwater
bioremediation effectiveness
Identify site characteristics
important to in-situ
groundwater bioremediation
effectiveness
Are
constituents
all sufficiently
biodegradable
Is
hydraulic
conductivity
> 10"4 cm/sec
IsTPH
< 60,000 ppm,
solvents < 7,000 ppm,
and heavy metals
< 2,600 ppm?
Is
soil free of
ble layers orv^ NO
other conditions that would
disrupt groundwater
flow?
In-situ groundwater
bioremediation is
generally not effective
Consider other
technologies.
• Dual-phase
extraction
Are
desired
constituent concen-
trations > 0.1 ppm and is
desired hydrocarbon
reduction
<95%
Are
dissolved
iron concentrations
<10mg/L?
OR
Conduct special
treatability studies to
address the out of
range parameters.
Are
constituents
at least slightly
soluble in
water?
Is
groundwater
pH between
6 and 8?
Treatability studies are required
to demonstrate effectiveness.
Review treatability study results.
groundwater
temperature between
10°C and 45°C
Do
reatability study
results demonstrate
technology
effectiveness
In-situ groundwater
bioremediation will
not be effective
at the site.
Are
background
heterotrophic bacteria
> 1000 CFU/gram?
Consider other
technologies.
• Dual-phase
extraction
• Natural
attenuation
• Technology is likely to be
9 effective at the site.
•[Proceed to evaluate the design.
May 1995
X-7
-------
Exhibit X-4
In-situ Groundwater Bioremediation Evaluation Process Flow Chart
EVALUATION OF
IN-SITU GROUNDWATER
BIOREMEDIATION
SYSTEM DESIGN
L
Determine the design elements
Volume and area of aquifer to be treated
Initial concentrations of constituents of concern
Required final constituent concentrations
Estimates of electron acceptor and nutrient requirements
Layout of injection and extraction wells
Design area of Influence
Groundwater extraction and Injection flow rates
Site construction limitations
Electron acceptor system
Nutrient formulation and delivery system
Btoaugmentation
Extracted groundwater treatment and disposition
Rates of InJectlon/Infiltraton
Cleanup time
Freo product recovery system
Identify & review the conceptual
process flow design &
the system components
* Extraction and injection well(s)
lay-out and construction
* Filtration system
* Electron acceptor delivery system
* Nutrient delivery system
* Instrumentation
• Extracted groundwater treatment
and disposition
• System controls and alarms
In-situ
groundwater
bioremediation
system design
is incomplete.
Request
additional
information.
Has the
conceptual design
been provided and is
it complete?
Trie in-situ groundwater
bioremediation system design
Is complete and its elements
are within normal ranges.
Proceed to O&M evaluation.
X-8
EVALUATION OF IN-SSTU
GROUNDWATER BIOREMEDIATION
SYSTEM OPERATION &
MONITORING PLANS
Review the O&M plan
for the following:
• Start-Up Operations Plan
• Long-Term Operations &
Monitoring Plan
• Remedial Progress
Monitoring Plan
Are
start-up
operations & monitoring
procedures described, and are
their scope & frequency
adequate?
Request
additional
information
on start-up
procedure and
monitoring.
Is a
long-term O&M
plan described; is it
of adequate scope & frequency;
does it include
discharge permit
monitoring?
Request
additional
information
on long-term
operation and
monitoring.
Is a
remedial progress
monitoring plan estab-
lished; is it of adequate scope
& frequency; does it include
provisions for detect-
ing asymptotic
behavior?
Request
additional
information
on remedial
progress
monitoring.
The technology is likely
to be effective.
The design and O&M
plans are complete.
May 1995
-------
Initial Screening Of In-Situ Groundwater Bioremediation
This section allows you to quickly assess whether in-situ groundwater
bioremediation is likely to be effective at a site. The key parameters that
determine the effectiveness of this technology are:
O The hydraulic conductivity of the aquifer, which controls the
distribution of electron acceptors and nutrients in the subsurface;
O The biodegradability of the petroleum constituents, which determines
both the rate and degree to which constituents will be degraded by
microorganisms; and
O The location of petroleum contamination in the subsurface.
Contaminants must be dissolved in groundwater or adsorbed onto
more permeable sediments within the aquifer.
In general, the aquifer medium will determine hydraulic conductivity.
Fine-grained media (e.g., clays, silts) have lower intrinsic permeability
than coarse-grained media (e.g., sands, gravels).
Bioremediation is generally effective in permeable (e.g., sandy,
gravelly) aquifer media. However, depending on the extent of
contamination, bioremediation also can be effective in less permeable
silty or clayey media. In general, an aquifer medium of lower
permeability will require longer to clean up than a more permeable
medium.
The biodegradability of a petroleum constituent is a measure of its
ability to be metabolized (or co-metabolized) by hydrocarbon-degrading
bacteria or other microorganisms.
The chemical characteristics of the contaminants will dictate their
biodegradability. For example, heavy metals are not degraded by
bioremediation. The biodegradability of organic constituents depends on
their chemical structures and physical/chemical properties (e.g., water
solubility, water/octanol partition coefficient). Highly soluble organic
compounds with low molecular weights will tend to be more rapidly
degraded than slightly soluble compounds with high molecular weights.
The low water solubilities of the more complex compounds render them
less bioavailable to petroleum-degrading organisms. Consequently, the
larger, more complex chemical compounds may be slow to degrade or
may even be recalcitrant to biological degradation (e.g., asphaltenes in
No. 6 fuel oil).
The location, distribution, and disposition of petroleum contamination
in the subsurface can significantly influence the likelihood of success for
May 1995 X-9
-------
bioremediation. This technology generally works well for dissolved
contaminants and contamination adsorbed onto higher permeability
sediments (sands and gravels). However, if the majority of
contamination is (1) in the unsaturated zone; (2) trapped in lower
permeability sediments, or (3) outside the "flow path" for nutrients and
electron acceptors, this technology will have reduced impact or no
impact.
Exhibit X-5 is an initial screening tool that you can use to help assess
the potential effectiveness of in-situ groundwater bioremediation. To use
this tool, you must first determine the type of aquifer medium present
and the type of petroleum product released at the site. Information
provided in the following section will allow a more thorough evaluation of
Exhibit X-5
Initial Screening For In-Situ Groundwater Bioremediation Effectiveness
Hydraulic Conductivity
Effective
Hydraulic Conductivity, cm/sec
1Q-" 1Q-9 10~7 10-5 10-3 10"1 10
I Clay I
L
Glocial Till
J
| Silt, Loess I
Silty Sand
I Clean Sand I
L
Gravel
Product Composition
i-More Effective:;
I Lube Oils
I Fuel Oils
Diesel
Kerosene
Gasoline
Note:
All petroleum products listed are amenable to in-situ groundwater bioremediation.
X-10
May 1995
-------
effectiveness and will identify areas that could require special design
considerations.
Detailed Evaluation Of In-Situ Groundwater Bioremediation
Effectiveness
Once you have completed the initial screening and determined that
in-situ groundwater bioremediation may be effective for the aquifer
media and petroleum product present, evaluate the CAP further to
confirm that the technology will be effective.
While the initial screening focused on hydraulic conductivity and
constituent biodegradability, the detailed evaluation should consider a
broader range of site and constituent characteristics, which are listed in
Exhibit X-6.
Exhibit X-6
Key Parameters Used To Evaluate The Effectiveness Of In-Situ
Groundwater Bioremediation
Site Characteristics Constituent Characteristics
Hydraulic conductivity Chemical structure
Soil structure and stratification Concentration and toxicity
Groundwater mineral content Solubility
Groundwater pH
Groundwater temperature
Microbial presence i
Terminal electron acceptors
Nutrient concentrations
The remainder of this section describes each parameter, why it is
important to in-situ groundwater bioremediation, how it can be
determined, and a range of values over which in-situ groundwater
bioremediation is generally effective.
May 1995 X-ll
-------
Site Characteristics That Affect In-Situ Groundwater Bioremediation
Site characteristics that influence the potential effectiveness of in-situ
groundwater bioremediation are described below.
Hydraulic Conductivity
Hydraulic conductivity, which is a measure of water's ability to move
through the aquifer medium, is one of the important factors in
determining the potential effectiveness of in-situ groundwater
bioremediation. This characteristic controls the rate and the distribution
of electron acceptors and nutrients delivered to the bacteria in the
aquifer. Hydraulic conductivity can be determined from aquifer tests,
including slug tests and pumping tests. These tests must be designed
carefully to ensure that contaminants are not forced to spread further in
the aquifer and that a large volume of contaminated groundwater is not
generated which then requires expensive treatment or disposal. The
hydraulic conductivity of aquifer media varies over a wide range
depending on the constituent materials (e.g., sand, gravel, silt, clay). In
general, fine-grained soils composed of clays or silts offer resistance to
water flow. Soils that are highly fractured, however, may have sufficient
permeability to use in-situ bioremediation. For aquifers with hydraulic
conductivity greater than 10"4 cm/sec, in-situ groundwater
bioremediation is effective. For sites with lower hydraulic conductivities
(e.g., 10"4 to 10"6 cm/sec), the technology also could be effective, but it
must be carefully evaluated, designed, and controlled.
Intrinsic permeability, which is a measure of the ability of soils to
transmit fluids, is sometimes reported instead of hydraulic conductivity.
If intrinsic permeability is given, you calculate the hydraulic conductivity
from the following equation:
K_ k(pg)
H
where
K = hydraulic conductivity (cm/sec),
k = intrinsic permeability (cm2),
u = water viscosity (g/cm- sec),
p = water density (g/cm3),
g = acceleration of gravity (cm/sec2).
X-12 May 1995
-------
At 20°C: (pg/u) = 9.8x 104 (cm- sec)"1. To convert k from cm2 to darcy,
multiply by 108.
Soil Structure and Stratification
Soil structure and stratification are important to in-situ groundwater
bioremediation because they affect groundwater flowrates and patterns
when water is extracted or injected. Structural characteristics such as
microfracturing can result in higher permeabilities than expected for
certain soils (e.g., clays). In this case, however, flow will increase in the
fractured media but not in the unfractured media. The stratification of
soils with different permeabilities can dramatically increase the lateral
flow of groundwater in the more permeable strata while reducing the flow
through less permeable strata. This preferential flow behavior can lead
to reduced effectiveness and extended remedial times for less-permeable
strata.
The intergranular structure and stratification of aquifer media can be
determined by reviewing soil logs from wells or borings and by examining
geologic cross-sections. It will be necessary to verify that soil types have
been properly identified, that visual observations of soil structure have
been documented, and that boring logs are of sufficient detail to define
soil stratification. Stratified soils may require special design
consideration (e.g., special injection well(s)) to ensure that these less-
permeable strata are adequately handled.
Fluctuations in the groundwater table should also be determined.
Significant seasonal or daily (e.g., tidal, precipitation-related)
fluctuations will submerge some of the soil in the unsaturated zone,
which should be considered during design of the system.
Groundwater Mineral Content
Excessive calcium, magnesium, or iron in groundwater can react with
phosphate, which is typically supplied as a nutrient in the form of
tripolyphosphate, or with carbon dioxide, which is produced by
microorganisms as a by-product of aerobic respiration. The products of
these reactions can adversely affect the operation of an in-situ
bioremediation system. When calcium, magnesium, or iron reacts with
phosphate or carbon dioxide, crystalline precipitates or "scale" is formed.
Scale can constrict flow channels and can also damage equipment, such
as injection wells and sparge points. In addition, the precipitation of
calcium or magnesium phosphates ties up phosphorus compounds,
making them unavailable to microorganisms for use as nutrients. This
effect can be minimized by using tripolyphosphates in a mole ratio of
greater than 1:1 tripolyphosphates to total minerals (i.e., magnesium
May 1995 X-13
-------
and calcium). At these concentrations, the tripolyphosphate acts as a
sequestering agent to keep the magnesium and calcium in solution (i.e.,
prevent the metal ions from precipitating and forming scale).
When oxygen is introduced to the subsurface as a terminal electron
acceptor, it can react with dissolved iron (Fe+2) to form an insoluble iron
precipitate, ferric oxide. This precipitate can be deposited in aquifer flow
channels, reducing permeability. The effects of iron precipitation tend to
be most noticeable around injection wells, where oxygen concentration in
groundwater is highest and can render injection wells inoperable.
Exhibit X-7 provides a guide to assessing the potential impact of
dissolved iron in groundwater.
Exhibit X-7
Dissolved Iron And In-Situ Bioremediation Effectiveness
Dissolved Iron Concentration
(mg/L) Effectiveness
Fe*2 < 10 Probably effective
,t\
10 < Fe < 20 Injection wells require periodic testing and
may need periodic cleaning or replacement
Fe > 20 Not recommended
Other parameters that could be good indicators of potential
groundwater scaling are hardness, alkalinity, and pH. In particular, very-
hard water (i.e., >. 400-500 mg/L carbonate hardness) tends to promote
scaling. The potential adverse effects caused by excessive mineral
content (e.g., calcium, magnesium, iron, total carbonates) in the
groundwater warrants careful attention during site characterization
activities.
Groundwater pH
Extreme pH values (i.e., less than 5 or greater than 10) are generally
unfavorable for microbial activity. Typically, optimal microbial activity
occurs under neutral pH conditions (i.e., in the range of 6-8). The
optimal pH is site specific. For example, aggressive microbial activity
has been observed at lower pH conditions outside of this range (e.g., 4.5
to 5) in natural systems. Because indigenous microorganisms have
adapted to the natural conditions where they are found, pH adjustment,
even toward neutral, can inhibit microbial activity. If man-made
X-14 May 1995
-------
conditions (e.g., releases of petroleum) have altered the pH outside the
neutral range, pH adjustment may be needed. If the pH of the
groundwater is too low (too acid), lime or sodium hydroxide can be added
to increase the pH. If the pH is too high (too alkaline), then a suitable
acid (e.g., hydrochloric, muriatic) can be added to reduce the pH.
Changes to pH should be closely monitored because rapid changes of
more than 1 or 2 units can inhibit microbial activity and may require an
extended acclimation period before the microbes resume their activity.
Groundwater Temperature
Bacterial growth rate is a function of temperature. Subsurface
microbial activity has been shown to decrease significantly at
temperatures below 10°C and essentially to cease below 5°C. Microbial
activity of most bacterial species important to petroleum hydrocarbon
biodegradation also diminishes at temperatures greater than 45°C.
Within the range of 10°C to 45°C, the rate of microbial activity typically
doubles for every 10°C rise in temperature. In most cases, for in-situ
groundwater bioremediation, the bacteria living in an aquifer system are
likely to experience relatively stable temperatures with only slight
seasonal variations. In most areas of the U.S., the average groundwater
temperature is about 13°C, but groundwater temperatures may be
somewhat lower or higher in the extreme northern and southern states.
Microbial Presence
Soil normally contains large numbers of diverse microorganisms,
including bacteria, algae, fungi, protozoa, and actinomycetes. Of these
organisms, the bacteria are the most numerous and biochemically active
group, particularly at low oxygen levels, and they contribute significantly
to in-situ groundwater bioremediation.
At a contaminated site, the natural microbial population undergoes a
selection process. First, there is an acclimation period, during which
microbes adjust to their new environment and new source of food.
Second, those organisms that adapt most quickly tend to grow fastest
and can use up nutrients that other microbes would need. Third, as the
environmental conditions change and the nature of the food supply
changes, the microorganism populations change as well. Organisms
capable of withstanding the stress of their changing environment will
generally be those that will contribute to the bioremediation of the site.
To determine the presence and population density of naturally
occurring bacteria that will contribute to degradation of petroleum
constituents, laboratory analysis of soil samples from the site should be
completed. These analyses, at a minimum, should include plate counts
May 1995 X-15
-------
for total heterotrophic bacteria (i.e., bacteria that use organic
compounds as an energy source) and hydrocarbon-degrading bacteria.
Although heterotrophic bacteria are normally present in all soil
environments, plate counts of less than 1,000 colony-forming units
(CPU)/gram of soil could indicate depletion of oxygen or other essential
nutrients or the presence of toxic constituents. However, concentrations
as low as 100 CFU per gram of soil can be stimulated to acceptable
levels, assuming toxic conditions (e.g., exceptionally high concentrations
of heavy metals) are not present. These conditions are summarized in
Exhibit X-8.
Exhibit X-8
Heterotrophic Bacteria And In-Situ Groundwater Bioremediation Effectiveness
Total Heterotrophic Bacteria
> 1,000 CFU/gram dry soil
100 -1,000 CFU/gram dry soil
Effectiveness
<100
Generally effective.
May be effective; needs further evaluation to
determine whether toxic conditions are
present and/or whether population responds
to stimulation (e.g., increased supply of
electron acceptor and/or nutrients).
Not generally effective.
Some CAPs propose the addition of microorganisms (bioaugmentation)
into the aquifer environment when colony plate counts are low.
However, research has shown that most in-situ bioremediation projects
have been successfully completed without microbial augmentation.
Experience with microbial augmentation shows that it varies in
effectiveness. Except in coarse-grained, highly permeable material,
microbes tend not to move very far past the point of injection, therefore,
their effectiveness is limited in extent. In general, microbial
augmentation does not adversely affected bioremediation, but it could be
an unnecessary cost.
Terminal Electron Acceptors
Microorganisms require carbon as an energy source to sustain their
metabolic functions, which include growth and reproduction. The
metabolic process used by bacteria to produce energy requires a
terminal electron acceptor (TEA) to enzymatically oxidize the carbon
source (organic matter) to carbon dioxide.
X-16
May 1995
-------
Organic Matter + O2 + Biomass -» CO2 + H2O +AHf
where AHf is energy generated by the reaction to fuel other metabolic
processes including growth and reproduction. In this example, oxygen
serves as the TEA.
Microorganisms are classified by the carbon and TEA sources they
use to carry out metabolic processes. Bacteria that use organic
compounds as their source of carbon are called heterotrophs; those that
use inorganic carbon compounds such as carbon dioxide are called
autotrophs. Bacteria that use oxygen as their TEA are called aerobes;
those that use a compound other than oxygen (e.g., nitrate, sulfate) are
called anaerobes; and those that can utilize both oxygen and other
compounds as TEAs are called facultative. For in-situ groundwater
bioremediation applications directed at petroleum products, bacteria
that are both aerobic (or facultative) and heterotrophic are most
important in the degradation process.
Nutrient Concentrations
Microorganims require inorganic nutrients such as nitrogen and
phosphate to support cell growth and sustain biodegradation processes.
Nutrients may be available in sufficient quantities in the aquifer but,
more frequently, nutrients need to be added to maintain adequate
bacterial populations.
A rough approximation of maximum nutrient requirements can be
based on the stoichiometry of the overall biodegradation process:
C-source + N-source + O2 + Minerals + Nutrients —>
Cell mass + CO2 + H2O + other metabolic by-products
Different empirical formulas of bacterial cell mass have been proposed;
the most widely accepted are C5H7NO2 and C60H87O32N12P. Using the
empirical formulas for cell biomass and other assumptions, 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.
Chemical analyses of soil samples (collected from below the water
table) and groundwater samples should be completed to determine the
available concentrations of nitrogen (expressed as ammonia) and
phosphate. Soil analyses are routinely conducted in agronomic
laboratories that test soil fertility for farmers. These concentrations can
be compared to the nitrogen and phosphorus requirements calculated
May 1995 X-17
-------
from the stoichiometric ratios of the biodegradation process. Some
microbes can use nitrate as a nitrogen source. The drinking water
standard for nitrate is 40 mg/L and there may be regulatory prohibitions
against injecting nitrate into groundwater. If nitrogen addition is
necessary, slow release sources should be used and addition of these
materials should be monitored throughout the project to prevent
degradation of water quality. In addition, excessive nitrogen additions
can lower soil pH, depending on the amount and type of nitrogen added.
Because of water quality and soil chemistry considerations, in situ
groundwater bioremediation should be operated at near nutrient-limited
conditions.
Constituent Characteristics That Affect In-Situ Groundwater
Bioremediation
Chemical Structure
The chemical structures of the constituents to be treated by in-situ
groundwater bioremediation are important for determining the rate at
which biodegradation will occur. Although nearly all constituents in
petroleum products typically found at UST sites are biodegradable, the
more complex the molecular structure of the constituent, the more
difficult the product is to treat and the greater the time required for
treatment. 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. Straight chain, aliphatic (i.e., alkanes, alkenes, and
alkynes) hydrocarbon compounds are more readily degraded than their
branched isomers, and mono-aromatic compounds (e.g., benzene, ethyl
benzene, toluene, xylenes) are more rapidly degraded than the two-ring
compounds (e.g., naphthalene), which in turn are more rapidly degraded
than the larger multi-ringed compounds (i.e., polyaromatic hydrocarbons
or polynuclear aromatic hydrocarbons). The larger, more complex
chemical structures may be slow to degrade or be essentially resistant to
biological degradation (e.g., asphaltenes in No. 6 fuel oil). Exhibit X-9
lists, in order of decreasing rate of potential biodegradability, some
common constituents found at petroleum UST sites.
Petroleum hydrocarbon contamination is sometimes accompanied by
other organic contaminants, including both non-chlorinated solvents
(e.g., alcohols, ketones, esters, acids) and chlorinated compounds (e.g.,
trichloroethane, chlorinated phenols, polychlorinated biphenyls (PCBs)).
The non-chlorinated solvents tend to be readily biodegradable but can
exert toxic effects at high concentrations. Lightly chlorinated
compounds (e.g., chlorobenzene, dichlorobenzene, chlorinated phenols,
X-18 May 1995
-------
Exhibit X-9
Chemical Structure And Biodegradability
Biodegradability
More degradable
•
I
1
T
Less degradable
Resistant
Example Constituents
n-butane, l-pentane,
n-octane
Nonane
Methyl butane,
dimethylpentenes,
methyloctanes
Benzene, toluene,
ethylbenzene, xylenes
Propylbenzenes
Decanes
Dodecanes
Tridecanes
Tetradecanes
Naphthalenes
Fluoranthenes
Pyrenes
Acenaphthenes
Asphaltenes
MTBE
Products In Which
Constituent Is Typically
Found
o Gasoline
o Diesel fuel
o Gasoline
o Gasoline
o Diesel, kerosene
o Diesel
o Kerosene
o Heating fuels
o Lubricating oils
o Diesel
o Kerosene
o Heating oil
o Lubricating oils
o Fuel oil no. 6
o Gasoline
lightly chlorinated PCBs) are typically degradable under aerobic
conditions. The more highly chlorinated compounds tend to be more
resistant to aerobic degradation, but they can be degraded by
dechlorination under anaerobic conditions. Several common chlorinated
solvents (e.g., chlorinated ethanes, ethenes) can be degraded under
aerobic conditions if they exist in the presence of another contaminant
that can behave as a co-metabolite (e.g., methane, toluene, phenol).
Evaluation of the chemical structure of the constituents proposed for
reduction by in-situ groundwater bioremediation at the site will allow
you to determine which constituents will be the most difficult to degrade.
You should verify that remedial time estimates, treatability studies, and
operation and monitoring plans are based on the constituents that are
the most difficult to degrade in the biodegradation process.
May 1995
X-19
-------
Concentration And Toxicity
High concentrations of petroleum organics or heavy metals in site
soils can be toxic to or inhibit the growth and reproduction of bacteria
responsible for biodegradation. In addition, very low concentrations of
organic material will result in diminished levels of bacterial activity.
In general, concentrations of petroleum hydrocarbons (measured as
total petroleum hydrocarbons) in excess of 50,000 ppm, organic solvent
concentrations in excess of 7,000 ppm, or heavy metals in excess of
2,500 ppm in the groundwater or aquifer medium are considered
inhibitory and/or toxic to aerobic bacteria. Review the CAP to verify that
the average concentrations of petroleum hydrocarbons and heavy metals
in the soils and groundwater to be treated are below these levels.
Exhibit X-10 provides the general criteria for constituent concentration
and bioremediation effectiveness.
Exhibit X-10
Constituent Concentration And In-Situ Groundwater Bioremediation
Effectiveness
In-Situ Groundwater
Constituent Concentration Bioremediation Effectiveness
Petroleum constituents < 50,000 ppm, Effective.
Solvent constituents < 7,000 ppm,
and
Heavy metals < 2,500 ppm
Petroleum constituents > 50,000 ppm, Ineffective; toxic or inhibitory conditions to
Solvent constituents > 7,000 ppm, bacterial growth exist. Long remediation
or times likely.
Heavy metals > 2,500 ppm
In addition to maximum concentrations, you should consider the
cleanup concentrations proposed for the treated soils. Below a certain
"threshold" constituent concentration, the bacteria cannot obtain
sufficient carbon from degradation of the constituents to maintain
adequate biological activity. The threshold level determined from
treatability studies conducted in the laboratory is likely to be much
lower than what is achievable in the field under less than optimal
conditions. Although the threshold limit varies greatly depending on
bacteria-specific and constituent-specific features, constituent
concentrations below 0.1 ppm in the total aquifer matrix may be difficult
to achieve. However, concentrations in the groundwater for these
X-2O May 1995
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specific constituents may be below detection levels. Experience has
shown that reductions in petroleum hydrocarbon concentrations greater
than 95 percent can be very difficult to achieve because of the presence
of "resistant" or nondegradable petroleum constituents. Identify the
average starting concentrations and the desired cleanup concentrations
in the CAP. If a cleanup level lower than 0.1 ppm is required for any
individual constituent or a reduction in petroleum hydrocarbon
concentration of greater than 95 percent is required to reach the cleanup
level, either a treatability study should be required to demonstrate the
ability of bioremediation to achieve these reductions at the site, or
another technology should be considered. Another option is to combine
one or more technologies to achieve cleanup goals. These conditions are
summarized in Exhibit X-11.
Exhibit X-11
Cleanup Concentrations And In-Situ Groundwater Bioremediation Effectiveness
In-Situ Groundwater
Cleanup Requirement Bioremediation Effectiveness
Constituent concentration > 0.1 ppm Effective.
and
Petroleum hydrocarbon reduction < 95%
Constituent concentration < 0.1 ppm Potentially effective; pilot studies are
or required to demonstrate reductions.
Petroleum hydrocarbon reduction >. 95%
Solubility
Solubility is the amount of a substance (e.g., hydrocarbon) that will
dissolve in a given amount of another substance (e.g., water). Therefore,
a constituent's solubility provides insight to its fate and transport in the
aqueous phase. Constituents that are highly soluble have a tendency to
dissolve into the groundwater and are more available for biodegradation.
Conversely, chemicals that have low water solubilities tend to remain in
the adsorbed phase and will biodegrade more slowly. In general, lower
molecular weight constituents tend to be more soluble and biodegrade
more readily than do higher molecular weight or heavier constituents.
In the field, aqueous concentrations rarely approach the solubility of a
substance because dissolved concentrations tend to be reduced through
competitive dissolution of other constituents and degradation processes
such as biodegradation, dilution, and adsorption.
May 1995 X-21
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Treatability Testing
To implement an in-situ groundwater bioremediatlon system at a site
contaminated with compounds other than BTEX, treatability testing
should be performed to verify applicability of the proposed remedial
approach and to develop site specific design criteria and operating
conditions. Treatability testing is probably not necessary at typical
gasoline station sites. This is because the microbes capable of degrading
BTEX compounds are ubiquitous and the contaminants are known to be
degradable. This treatability testing can be accomplished in two phases
- bench-scale and pilot-scale testing. Bench-scale tests are conducted in
the laboratory (using apparatus sufficiently small to be placed on a
laboratory bench) to evaluate the feasibility of a process, while pilot-scale
tests simulate full-scale operations and are often conducted in the field.
The extent and scope of the treatability testing will depend greatly on the
volume of groundwater to be remediated.
For a relatively small volume of water with fairly well defined site and
constituent characteristics, bench-scale testing may suffice. However, for
large, more complex sites, it is recommended that both bench- and pilot-
scale treatability testing be performed.
Bench-Scale Treatability Testing
The purposes of a bench-scale treatability study are to determine the
following treatability characteristics:
O Presence of a responsive microbial population;
O Biodegradability of the groundwater contaminants;
O Degradation rate;
O Oxygen and nutrients required to sustain the biodegradation;
O Likely interactions between the introduced and generated compounds
and the aquifer media (plugging potential); and
O Achievable cleanup levels.
There are two basic types of bench-scale studies, the flask (slurry) study
and the column study. For flask studies, samples of aquifer material
and contaminated groundwater are analyzed to determine the presence
of organic, inorganic, and heavy metal compounds, and to estimate the
numbers of microbes present. A minimum of three treatment conditions
are generally tested - some combination of nutrients, a supply of
X-22 May 1995
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electron acceptors, and possibly an introduction of commercially
available microorganisms. Tests are conducted over a 4- to 12-week
period (most commonly 8 to 10 weeks) in both sterile and unamended
control conditions. During this time, analyses are periodically performed
to determine the rate of biodegradation. Results of flask studies should
be considered as representing optimal conditions because the flask
microcosm does not consider the effects of variables such as limited
oxygen and nutrient delivery or soil heterogeneity. At the completion of
the study, a preliminary treatment design is prepared that specifies the
anticipated rate of contaminant reduction (cleanup time) and the
quantities of oxygen and nutrients required.
Column studies employ the same approach as flask studies. Glass
columns are filled with aquifer material, and contaminated groundwater
is percolated through the columns; sterile and nutrient-amended
columns are also evaluated as controls. While the columns do not
accurately re-create actual in-situ conditions, they do provide an
indication of the likely effects of adsorption and precipitation within the
aquifer medium.
Pilot-Scale Treatability Testing
Pilot-scale treatability testing is a simulation of the full scale
operation. The objective of this type of treatability testing is to verify
treatability of constituents of concern under actual field conditions and
to generate data to design the full-scale system. At small, typical
gasoline stations, the pilot-scale system will be the same as the full-scale
system. This pilot testing could extend from a few weeks to several
months depending on the data generation requirements. Longer study
times are required to track contaminant reduction to project the time
required to attain clean-up goals.
A pilot testing program could also include the following:
O Pumping test to determine sustained groundwater extraction rate and
general aquifer response;
O Aquifer recharge response tests (tracer test);
O Microbial response to injection of electron acceptor and nutrients; and
O Long-term operability of the system (aquifer and/or injection well
fouling).
Information from these tests will be generated from measurements
collected from a network of monitoring wells. The results of these tests
will enable determination of (1) groundwater flow velocities and flow
May 1995 X-23
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paths in the vicinity of the injection well or infiltration gallery and
extraction well, (2) potential zones of anisotropy within the aquifer (i.e.,
areas where properties, such as hydraulic conductivity, vary depending
on the direction in which they are measured), (3) the distribution and
concentrations of electron acceptors and nutrients, and (4) site specific
remediation rates. Long-term operation of the pilot system also will
provide information on potential fouling/plugging of the aquifer matrix in
the vicinity of injection and extraction wells. Monitoring wells should be
sampled at a frequency which will allow statistical validation of data
generated.
Groundwater Modeling
For large, complex sites and even for some smaller sites, groundwater
modeling can be a valuable tool to develop a more accurate
conceptualization of the site and analyze the impacts of varying the
locations and pumping rates of injection and extraction wells. This can
be very important in determining whether the system can achieve and
maintain hydraulic containment of the contaminant plume. The
complexity and sophistication of the model used will depend on the site
characteristics and the amount of data available to develop the model.
The cost of groundwater modeling needs to be evaluated against the total
remediation costs of the site. The data generated in the site
characterization and pilot testing can be incorporated into a model that
provides projections and predictions of aquifer conditions with time.
Typical factors that can be determined by modeling include:
O Aquifer conditions, including flow rates and direction, water levels,
extraction/injection points, aquifer sensitivity;
O Numbers, locations, and configurations of injection, extraction, and
monitoring wells that will maximize system efficiency; and
O Fate and transport of contaminants, including concentration,
distribution, and degradation with time.
Evaluations Of In-Situ Groundwater Bioremediation System
Design
Once you have verified that in-situ groundwater bioremediation has
the potential to be effective, you can evaluate the design of the proposed
remedial system. The CAP should include a discussion of the rationale
for the design and present the conceptual engineering design. Detailed
engineering design documents might also be included, depending on
state requirements.
X-24 May 1995
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Rationale For The Design
The following design elements are presented in the order in which
design information might typically be collected.
O Volume and area of aquifer to be treated is generally determined by
site characterization combined with regulatory action levels or a site-
specific risk assessment.
O Initial concentration of constituents of concern can be measured during
initial site characterization and during treatability studies. These
concentrations will be used to predict likely toxic effects of the
contaminants on indigenous microorganisms and to estimate electron
acceptor and nutrient requirements, and the extent of treatment
required.
O Required Final Cqnsttiuent Concentrations are generally defined by
your state as remediation action levels or determined on a site-specific
basis using transport models and/or risk assessment calculations.
These limits will define the areal extent of the aquifer to be
remediated.
O Estimates of electron acceptor and nutrient requirements. As a rule of
thumb, 3 Ibs of oxygen are added per pound of hydrocarbon as an
electron acceptor. For nutrients, a maximum ratio of 100:10:1 for
C:N:P is typically used (assume 1 pound of hydrocarbon is equal to 1
pound of carbon). Often systems require substantially less, on the
order of 100:1:0.5, especially if plugging of injection wells/galleries is
a problem.
O Layout of injection and extraction wells. Probably the most critical '
factor is ensuring that the contaminant plume is hydraulically
controlled. This will prevent it from spreading and concentrate
bioremediation efforts on the contaminants. For large complex sites,
designing this layout can be facilitated by groundwater modeling.
Injection wells/infiltration galleries can be located upgradient of the
contaminant source, with extraction wells located downgradient of the
source. Alternatively, injection points can be located along the
centerline of the plume axis, with extraction wells located on the
edges of the plume. The latter arrangement can typically achieve
shorter remediation times, but at greater expense.
O Design Area of Influence. (AOI) is an estimate of the volume/area of
aquifer to which an adequate amount of electron acceptor and
nutrient can be supplied to sustain microbial activity. Establishing
the design AOI is not a trivial task because it depends on many
May 1995 X-25
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factors including intrinsic permeability of the soil, soil chemistry,
moisture content, and desired remediation time. Although the AOI
should usually be determined through field pilot studies, it can be
estimated from groundwater modeling or other empirical methods.
For sites with stratified geology, the area of influence should be
defined for each soil type. The AOI is important in determining the
appropriate number and spacing of extraction or injection wells or
infiltration galleries.
O Groundwater extraction and injection flow rates can vary from a few to
a few hundred gallons per minute, depending primarily on the
hydraulic conductivity of the aquifer. Although flow rates can be
estimated by groundwater modeling, they are best determined by pilot
studies. In general, only about 75 percent of extracted water can be
readily re-injected using either injection wells or infiltration galleries.
O Site Construction Limitations. Locations of buildings, utilities, buried
objects, etc. must be identified and considered in the design process.
O Electron Acceptor System. For aerobic processing, air, oxygen or
hydrogen peroxide can be used; for anaerobic processing, alternative
electron acceptors (e.g., nitrate, sulfate, or ferric iron) can be used.
The electron acceptors may be introduced using a direct air/oxygen
sparge system into the injection well (air sparging) or a water injection
system.
O Nutrient Formulation and Delivery System. Site characterization and
bench-scale treatability studies will determine if nutrients are
required. The nutrients selected should be compatible with aquifer
chemistry to minimize precipitation and flow-channel fouling.
O Bioaugmentation. Microorganisms can be added to the injected or
infiltrated water to increase microbial activity. However, as discussed
earlier, bioaugmentation is usually not necessary.
O Extracted Groundwater Treatment and Disposition. The above ground
treatment system for extracted groundwater should be of sufficient
size to process the volume of water extracted. Disposition of treated
groundwater will depend on specific state policies. Some states
discourage reinjection, although in most instances, re-injection makes
good technical sense without causing adverse impacts on the
receiving groundwater. Groundwater treatment systems could entail
biological, chemical, and/or physical treatment. The selection of the
appropriate extracted, groundwater treatment technology will depend
on the proposed duration of operation, size of treatment system, and
cost.
X-26 May 1995
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O Remedial Cleanup Time. Imposed remedial cleanup time could affect
the design of the remedial system. Ultimately, the duration of the
cleanup will depend on the rate of biological activity attainable, the
bio-availability of the contaminants of concern, and the locations and
spacings of the injection/extraction wells.
O Ratio of Injection/Infiltration to Extraction. The percentage of the
treated water that is reinjected or reinfiltrated should be based on
hydraulic control. Because dispersion and diffusion at the boundary
of the AOI is likely to allow some migration of contaminated
groundwater, less groundwater is generally injected or recharged to
the aquifer than is extracted. This provides for better hydraulic
, containment of the contamination.
O Free Product Recovery System. A system designed to recover free
product should be used to reduce "source" input effects to the
groundwater and generally optimize saturated zone remediation.
Components Of An In-Situ Groundwater Bioremediation
System
Once the design rationale is defined, the design of the in-situ
groundwater bioremediation system can be developed. Exhibit X-12 is a
schematic diagram of a typical in-situ groundwater bioremediation
system using injection wells. A typical in-situ groundwater
bioremediation system design includes the following components and
information:
O Extraction well(s) orientation, placement, and construction details;
O Injection well(s) or infiltration gallery(ies) orientation, placement, and
construction details;
O Filtration system to remove biomass and particulates that could
promote clogging of injection wells or galleries;
O Extracted groundwater treatment system (e.g., biological, chemical
oxidation, granular carbon adsorption) and methods for disposal or
re-use of treated groundwater (surface discharge, discharge to a
sewer, re-injection);
O Nutrient solution preparation system and storage;
O Microorganism addition system (if required);
O Electron acceptor system (e.g., air, oxygen, hydrogen peroxide);
May 1995 X-27
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Exhibit X-12
Schematic Diagram Of Typical In-Situ Groundwater Bioremediation System
Using Injection Wells
Oxygen
Addition
Water
Supply
Legend:
PI Pressure Indicator
SP Sampling Port
|<5) Row Control Valve
H Row Meter
O Monitoring well(s) orientation, placement, and construction details;
and
O System controls and alarms.
Extraction wells are generally necessary to achieve hydraulic control
over the plume to ensure that it does not spread contaminants into
areas where contamination does not exist or accelerate the movement
toward receptors. Placement of extraction wells is critical, especially in
systems that also use nutrient injection wells or infiltration galleries.
These additional sources of water can alter the natural groundwater flow
patterns which can cause the contaminant plume to move in an
unintended direction or rate. Without adequate hydraulic control, this
X-28
May 1995
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situation can lead to worsening of the original condition and complicate
the cleanup or extend it.
Nutrient injection systems may not be necessary at all, if the
groundwater contains adequate amounts of nutrients, such as nitrogen
and phosphorus.
The following sections provide more detailed descriptions of the
electron acceptor and nutrient addition systems and system control
alarms. For a detailed explanation of suggested well construction
guidelines, see Chapters VII and VIII, "Air Sparging" and "Biosparging."
In some cases, electron acceptor and nutrient supply systems are
combined rather than discrete systems (i.e., both the electron acceptor
and nutrients are added to the same stream for injection into the
aquifer).
Well Placement
Location of extraction wells, injection wells (or infiltration galleries),
and monitoring wells can vary substantially depending on site-specific
conditions. However, the essential goals in configuring these wells are
as follows:
O Extraction wells should be located such that hydraulic control is
achieved at the outer limits of the contaminant plume. In other
words, the cones of depression created by the pumping wells should
intersect so that hydraulic gradients throughout the plume are inward
in the direction of the pumping wells;
O Injection wells (and/or infiltration galleries) should be located to
provide distribution of the electron acceptor and nutrients throughout
the area targeted for remediation; the impacts on water table
gradients caused by injection well location and rate of liquid injection
should be considered carefully. Excessive mounding of the water table
could induce migration of contaminants in unintended directions, or
alter the effectiveness of the extraction well in achieving hydraulic
control; and
O Monitoring wells should be located outside the plume in each
direction and within the plume to track remedial progress and to
ensure that the extraction wells are achieving the desired hydraulic
control and preventing further migration.
One possible configuration of extraction, injection, and monitoring wells
is shown in Exhibit X-13.
May 1995 X-29
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Exhibit X-13
Idealized Layout Of Extraction, Injection, And Monitoring Wells
For In-Situ Groundwater Bioremediation
Vodose Zone
Contamination
• Groundwater
Contamination
.-B
Groundwater
Flow—*-
Legend:
5 Extraction Well
/\ Injection Well
4- Monitoring Well
The design area of influence of extraction and injection wells will
determine the number of wells needed. The area of influence of
neighboring extraction wells should overlap to achieve hydraulic control.
Electron Acceptor and Nutrient Addition System
For a given site, selection of an appropriate electron acceptor will
depend on the results of the treatability studies. The most widely used
electron acceptor in the remediation of petroleum hydrocarbons is
oxygen, which enhances the aerobic biological process. Oxygen can be
delivered by either a "carrier stream" of water which has been enriched
with atmospheric air or pure oxygen or by air or oxygen sparging. Air
sparging is covered in Chapter VII. Water saturated with atmospheric
air (20 percent oxygen) contains dissolved oxygen concentrations of 8-10
mg/1. Water saturated with pure oxygen can attain dissolved oxygen
concentrations of approximately 40 mg/1.
X-30
May 1995
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Higher dissolved oxygen concentrations in groundwater are attainable
with hydrogen peroxide. However, at levels greater than 500 to 1,000
mg/1, hydrogen peroxide behaves like a biocide; therefore, it should be
used with caution. Hydrogen peroxide degrades relatively rapidly and is
very difficult to disperse through the aquifer. Also, hydrogen peroxide is
very expensive, and its use may not be cost-effective.
A typical electron acceptor addition system would include:
O Oxygen Enriched Stream, including an air blower or pure
oxygen source and contacting chamber;
O Injection Well Sparging System, including an air blower or
pure oxygen source; or
O Hydrogen Peroxide System, including a hydrogen peroxide
supply, storage, and metering pump system.
A typical nutrient addition system could include the following
components:
O Reagent (e.g., hydrogen peroxide, ammonium salt, phosphate)
storage facilities
O Mixing tanks for reagent solutions (i.e., solutions of
ammonium or urea and phosphorus salt solutions)
O Meters to measure rate of Introduction of nutrient solutions
into carrier streams
O Control system for metering systems
System Controls and Alarms
In many cases, remediation sites are remote and have minimal
operation and maintenance staff. In these cases, equipment is fitted with
control devices to shut down the system in the event of failure or
unusual conditions (e.g., high water levels in injection wells because of
plugging). When these systems shut down, alarms are triggered. These
alarms can notify personnel on-site, or can be relayed to a remote
station from which control personnel can be summoned.
Evaluation Of Operation And Monitoring Plans
Monitoring operations of the in-situ groundwater bioremediation
system is necessary to ensure that equipment functions according to
May 1995 X-31
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specification, that nutrients and electron acceptors are being effectively
supplied and distributed, and that contaminant removal is proceeding
according to projections. A system operating and monitoring plan which
covers both start-up and normal operations must be developed.
Start-up Operation
Initial start-up should entail hydraulic balancing of rates of extraction
and injection of water. Depending on the system size and complexity,
this hydraulic balancing can take 1 to 3 days. Once the extraction and
injection flows are balanced and stabilized, addition of nutrients and the
electron acceptor should be initiated. After about two or three days, the
groundwater electron acceptor levels should be checked. In highly
contaminated areas, the electron acceptor concentration will be
depressed. Start-up adjustments are generally needed for the first 1 to 2
weeks of operation. Concentrations of the electron acceptor should be
measured daily; water levels across the site should be measured every
two to three days.
At the end of this start-up period, a set of samples (groundwater and
soils) should be collected for detailed analysis for constituents of
concern.
Normal Operation
The normal operation of the system should consist of weekly routine
checking of (a) the operation and maintenance of equipment (e.g.,
pumps, blowers, mixers and controllers); (b) groundwater levels;
(c) extraction and injection flow rates; (d) groundwater electron acceptor
concentrations; (e) nutrient levels (ammonium, phosphate, nitrate);
(f) pH; and (g) conductivity. System monitoring parameters can be
measured using field test kits. Nutrient addition can be an intermittent
operation and can be scheduled to coincide with routine operation
checks. Exhibit X-14 provides a brief synopsis of system monitoring
requirements.
Remedial Progress Monitoring
It is assumed that the objective of in-situ groundwater bioremediation
processing is remediation of the saturated zone. To monitor remedial
progress, samples of both groundwater and aquifer media (soil) should
be collected on a routine basis and analyzed for parameters of concern.
Groundwater samples should be collected and analyzed monthly to
quarterly. Soil samples should be collected prior to site closure to
demonstrate that cleanup objectives have been achieved.
X-32 May 1995
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Exhibit X-14
System Monitoring Recommendations
Phase
Monitoring Frequency What To Monitor
Where To Monitor
Start-up (1 -2 weeks) At least daily
o Extraction volume
o Injection volume
o Electron acceptor
concentration
o Extraction and
injection well heads
or manifolds
o Monitoring wells
Remedial (ongoing)
Every 2-3 days
Weekly
o Groundwater levels o Monitoring wells
Monthly to quarterly
o Groundwater levels
o Extraction and
injection flow rates
o Concentration of
electron acceptor,
ammonia,
phosphate, nitrate,
pH, and conductivity
o Constituent
concentrations in soil
and groundwater
o Monitoring wells
o Extraction and
injection wellheads
or manifolds
o Monitoring wells
o Extraction, injection,
and monitoring wells
o Soil borings
In developing the monitoring plan, it will be necessary to identity
potential monitoring points. Sampling points could be extraction,
injection and monitoring wells. In sampling at the injection wells, the
injection system should be shut down for approximately 24 hours to
allow ambient conditions to be re-established. This delay will help
ensure that the samples are representative of conditions in the aquifer
without the effects of dilution.
Groundwater sampling and analysis can be scheduled about once
every 1 to 3 months. Procedures for the collection of groundwater
samples should be described in a sampling and analysis plan provided in
the CAP. Analyses must be conducted according to prescribed or
approved procedures as required by state regulations. In addition, water
table maps can be useful in assessing whether injected water is flowing
toward extraction wells as predicted. These maps should be prepared on
a monthly to quarterly basis.
During remediation, contaminant levels decrease until they reach an
asymptotic level. Once asymptotic conditions are reached for several
successive sampling periods, continuing remediation activities generally
May 1995
X-33
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results in little further decrease of contaminant concentrations.
However, frequently when active remediation is ceased, levels of
dissolved contaminants abruptly increase. This increase is caused by
the diffusion into solution of contaminants that were previously
adsorbed onto the surfaces of individual grains that comprise the aquifer
media. When asymptotic behavior begins, the operator should evaluate
alternatives that will facilitate aquifer biodegradation. Alternatively, you
may need to re-evaluate the rates and concentrations of nutrients and
electron acceptors being injected, examine other remedial alternatives, or
consider changing from active to passive (natural attenuation)
remediation.
If asymptotic behavior is persistent for periods greater than about 6
months and the concentration rebound is sufficiently small following
periods of pulsing (i.e., varying the extraction rate or turning the system
off and on), the performance of the in-situ groundwater bioremediation
system should be reviewed to determine whether remedial goals have
been reached. If further contaminant reduction is necessary, another
remedial technology may need to be considered.
X-34 May 1995
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References
Brubaker, G.R. "In-situ Bioremediation of Groundwater." in D.E. Daniel,
ed., Geotechnical Practice for Waste DispQsaL London/New York:
Chapman & Hall, 1993.
Kinsella, J.V. and M.J.K. Nelson. "In-situ Bioremediation: Site
Characterization, System Design and Full-Scale Field Remediation of
Petroleum Hydrocarbon- and Trichloroethylene-Contaminated
Groundwater." in P.E. Flathman and D.E. Jerger, eds., Bioremediation
Field Experience. Boca Raton, FL: CRC Press, 1993.
Norris, R.D. "In-situ Bioremediation of Soils and Groundwater
Contaminated with Petroleum Hydrocarbons." in R.D. Norris, R.E.
Hinchee, R.A. Brown, P.L. McCarty, L. Semprini, J.T. Wilson, D.H.
Kampbell, M. Reinhard, E.J. Bower, R.C. Borden, Handbook of
Bioremediation. Boca Raton, FL: CRC Press, 1994.
Norris, R.D., R.E. Hinchee, R. Brown, P.L. McCarty, and L. Semprini. In-
situ Bioremediation of Groundwater and Geologic Material: A Review of
Technologies. Washington, DC: U.S. Environmental Protection
Agency, EPA/600/R-93/124, {NTIS: PB93-215564/XAB), July 1993.
Norris, R.D. and K.D. Dowd. "In-situ Bioremediation of Petroleum
Hydrocarbon-Contaminated Soil and Groundwater in a Low-
Permeability Aquifer." in P.E. Flathman and D.E. Jerger, eds.,
Bioremediation Field Experience. Boca Raton, FL: CRC Press, 1993.
Sims, J.L., J.M. Suflita, and H.H. Russell. In-situ Bioremediation of
Contaminated Groundwater. Washington, DC: U.S. Environmental
Protection Agency, EPA/540/S-92/003, (NTIS: PB92-224336/XAB),
February 1992.
Staps, S.J.J.M. International Evaluation ofln-Situ Biorestoration of
Contaminated Soil and Groundwater. Washington, DC: U.S.
Environmental Protection Agency, Office of Emergency and Remedial
Response. EPA 540/2-90/012, 1990.
Van der Heijde, P.K.M., and O.A. Elnawawy. Compilation of Groundwater
Models. Washington, DC: U.S. Environmental Protection Agency,
Office of Research and Development, EPA/600/R-93/118, (NTIS:
PB93-209401), May 1993.
May 1995 X-35
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Checklist: Can In-Situ Groundwater Bioremediation Be Used
At This Site?
This checklist can help you evaluate the completeness of the CAP and
identify areas that require closer scrutiny. As you go through the CAP,
answer the following questions. If the answer to several questions is no,
you should request additional information to determine if in-situ
groundwater bioremediation will accomplish cleanup goals at the site.
1. Site Characteristics
Yes No
Q Q Is the aquifer hydraulic conductivity greater than 10"4
cm/sec?
Q Q Have impermeable layers or other conditions that would
disrupt groundwater flow been considered in the design
of the remediation system?
Q Q Has the groundwater mineral content been quantified
and taken into consideration?
Q Q Are dissolved iron concentrations < 10 mg/1?
Q Q Is the groundwater pH between 6 and 8?
Q Q Is the groundwater temperature been 10°C and 45°C?
Q Q Is the total heterotrophic bacteria count > 1,000
CFU/gram in dry soil?
Q Q Is the carbon:nitrogen:phosphorus ratio between
100:10:1 and 100:1:0.5?
2. Constituent Characteristics
Yes No
Q Q Have all constituents of concern been identified?
Q Q Are constituents all sufficiently biodegradable?
Q Q Is the concentration of total petroleum hydrocarbon
< 50,000 ppm and heavy metals < 2,500 ppm?
Q Q Are organic solvent concentrations < 7,000 ppm?
X-36 May 1995
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Q Q Are desired constituent concentrations > 0.1 ppm and is
the desired hydrocarbon reduction < 95%?
Q Q Are the constituents present soluble in groundwater?
3. Evaluation Of The In-situ Groundwater Bioremediation System
Design
Yes No
Q Q Has treatability testing been performed?
Q Q Has groundwater modelling been used to calculate
aquifer conditions over time?
Q Q If not, has some othermethod been used to calculate
cleanup times?
Q Q Will the processing rates achieve cleanup in the time
allotted for remediation in the CAP?
Q Q Have remediation rates been established for the project?
Q Q Has the area of influence for the proposed extraction or
injection wells been determined?
Q Q Is the proposed well placement appropriate, given the
total area to be cleaned up and the area of influence of
each injection/extraction well system?
Q Q Has the amount of the contaminant to be remediated
been determined?
Q Q Has the quantity and type of electron acceptors required
for the remediation been determined?
Q Q If an electron acceptor system will be needed, is a design
for that system provided?
Q Q Will aboveground treatment of groundwater be required?
Q Q Has the quantity of nutrients required for remediation
been determined?
Q Q If nutrient delivery systems will be needed, are designs
for those systems provided?
Q Q Is bioaugmentation addressed, if needed, in the design?
May 1995 X-37
-------
Q Q Have groundwater extraction rates been determined?
Q Q Is a system control/alarm system included in the
design?
Q Q Is a free product recovery system needed?
4. Operating and Monitoring Plans
Yes No
Q Q Is hydraulic balancing proposed as the first activity in
start-up?
Q Q Is routine system operation and monitoring proposed?
Q Q Is subsurface soil and groundwater sampling proposed
for tracking constituent reduction and biodegradation
conditions?
Q Q Is a schedule for tracking constituent reduction
proposed?
Q Q Is nutrient addition (if necessary) proposed to be
controlled on a periodic rather than continuous basis?
X-38 May 1995
-------
Chapter XI
Dual-Phase Extraction
-------
-------
Contents
Overview ....... XI-1
Initial Screening Of DPE Effectiveness XI-11
Detailed Evaluation Of DPE Effectiveness XI-12
Site Characteristics XI-13
Intrinsic Permeability XI-13
Soil Structure And Stratification XI-15
Moisture Content In The Unsaturated Zone XI-15
Depth To Groundwater XI-16
Chemical Properties Xt-17
Effective Volatility XI-17
Chemical Sorptive Capacity XI-19
Pilot Scale Studies XI-19
Evaluation Of The DPE System Design XI-21
Rationale For The Design XI-21
Components Of A DPE System XI-24
Extraction Wells XI-25
Manifold Piping XI-29
Vapor Pretreatment XI-29
Vapor Treatment XI-30
Blower Selection XI-30
Instrumentation and Controls XI-31
Optional DPE Components XI-32
Land Surface Seals XI-33
Injection Wells XI-33
Evaluation Of Operation And Monitoring Plans XI-33
Start-Up Operations XI-34
Long-Term Operations XI-34
Remediation Progress Monitoring XI-34
References XI-37
Checklist: Can Dual-Phase Extraction Be Used At This Site? XI-38
May 1995 XI-ili
-------
List Of Exhibits
Number Title Page
XI-1 Typical Single-Pump DPE System XI-2
XI-2 Typical Multi-Pump DPE System XI-3
XI-3 Typical Single-Pump, DPE Extraction WeU XI-5
XI-4 Typical Multi-Pump, DPE Extraction Well XI-6
XI-5 Advantages and Disadvantages of Single-Pump DPE . XI-7
XI-6 Advantages and Disadvantages of Dual-Pump DPE . . XI-8
XI-7 DPE Evaluation Process Flow Chart XI-9
XI-8 Initial Screening for DPE Effectiveness XI-12
XI-9 Key Parameters Used To Evaluate Site Characteristics
And Chemical Properties XI-13
XI-10 Vapor Pressures Of Common Petroleum Constituents XI-18
XI-11 Petroleum Product Boiling Ranges XI-18
XI-12 Henry's Law Constant Of Common Petroleum
Constituents XI-19
XI-13 Schematic of Single Pump DPE System XI-26
XI-14 Schematic of Dual-Pump DPE System XI-27
XI-15 Performance Curves For Three Types Of Blowers . . . XI-31
XI-16 Monitoring And Control Equipment XE-32
XI-17 System Monitoring Recommendations XI-35
XI-18 Relationship Between Concentration Reduction And
Mass Removal XI-36
XI-iv May 1995
-------
Chapter X!
Dual-Phase Extraction
Overview
Dual-phase extraction, also known as multi-phase extraction,
vacuum-enhanced extraction, or bioslurping, is an in-situ technology
that uses pumps to remove various combinations of contaminated
groundwater, separate-phase petroleum product, and hydrocarbon vapor
from the subsurface. Extracted liquids and vapor are treated and
collected for disposal, or re-injected to the subsurface (where
permissible). Dual-phase extraction systems can be effective in removing
separate-phase product from the subsurface, thereby reducing
concentrations of petroleum hydrocarbons in both the saturated and
unsaturated zones of the subsurface. Dual-phase extraction systems are
typically designed to maximize extraction rates; however, the technology -
also stimulates biodegradation of petroleum constituents in the
unsaturated zone by increasing the supply of oxygen, in a manner
similar to bioventing (see Chapter III for more information on bioventing).
Although the general class of technologies presented in this chapter is
referred to as dual-phase extraction (DPE), significant variations in the
technology exist. DPE systems often apply relatively high vacuums to
the subsurface. Thus, the adjective "high-vacuum" is sometimes used to
describe DPE technologies, even though all DPE systems are not high-
vacuum systems. DPE technologies can be divided into two general
categories, depending on whether subsurface liquid(s) and soil vapor are
extracted together as a high-velocity dual-phase (liquid(s) and vapor)
stream using a single pump or whether the subsurface liquid(s) and soil
vapor are extracted separately using two or more pumps. Exhibits XI-1
and XE-2 display typical single- and dual-pump DPE systems,
respectively.
Single-pump systems rely on high-velocity airflow to lift suspended
liquid droplets upwards by factional drag through an extraction tube to
the land surface. Single-pump vacuum extraction systems can be used
to extract groundwater or combinations of separate-phase product and
groundwater. The somewhat more conventional dual-pump systems use
one pump to extract liquids from the well and a surface blower (the
second pump) to extract soil vapor. A third DPE configuration uses a
total of three pumps, including the surface blower together with one
pump to extract floating product and one to extract groundwater.
Because both double- and triple-pump DPE systems extract the well
liquids separately from the soil vapor and are similar in operation and
May 1995 XI-1
-------
Exhibit XI-1
Typical Single-Pump DPE System
Extraction Tube- =
Free-phase
Petroleum
Product
Extraction Well
Atmospheric
Discharge
Oil-Water-Gas Separator—Water Treatment
Oil Storage
• Appropriate
Vapor Treatment
/—Water Discharge
Legend:
Vapor Phase
Adsorbed Phase
Dissolved Phase
en
-------
I
H
CO
(0
en
Exhibit XI-2
Typical Multi-Pump DPE System
Oil-Water Separator
Water Treatment
System
Water
Discharge
Atmospheric
Discharge
Oil Storage Unit
Appropriate
Vapor Treatment -
Extraction
Tube
Free-phase
Petroleum
Product
Submersible
Pump
Legend:
Vapor Phase
Adsorbed Phase
Dissolved Phase
CO
-------
application, these systems will be discussed together under the heading
of "dual-pump DPE systems" in the remainder of this chapter. Exhibits
XI-3 and XI-4 are diagrams of typical single-pump and dual-pump DPE
extraction wells, respectively.
Vacuum groundwater extraction has been used for many decades as a
standard method for extracting groundwater to control seepage or effect
dewatering during construction and mining activities (Powers, 1981).
Single-pump DPE systems represent a recent adaptation of this long-
established technology to the task of subsurface remediation. Single-
pump DPE systems are generally better suited to low-permeability
conditions, and they are difficult to implement at sites where natural
fluctuations in groundwater levels are substantial. United States
patents exist on certain applications of single-pump DPE systems (Hess
et al., 1991; Hajali et al., 1992; Hess et al., 1993). Single-pump DPE
technology is sometimes referred to as bioslurping (U.S. Air Force, 1994).
Dual-pump DPE systems are simply a combination of traditional soil
vapor extraction (SVE) and groundwater (and/or floating product)
recovery systems. Dual-pump systems tend to be more flexible than
single-pump systems, making dual-pump systems easier to apply over a
wider range of site conditions (e.g., fluctuating water tables, wide
permeability ranges); however, equipment costs are higher.
The vacuum applied to the subsurface with DPE systems creates
vapor-phase pressure gradients toward the vacuum well. These vapor-
phase pressure gradients are also transmitted directly to the subsurface
liquids present, and those liquids existing in a continuous phase (e.g.,
water and "free" petroleum product) will flow toward the vacuum well in
response to the imposed gradients (the term "free" product is a
commonly used, though imprecise term because a greater fraction of
resident petroleum product may be recovered using vacuum-enhanced
DPE compared to the fraction of product recoverable using gravity
drainage alone). The higher the applied vacuum, the larger the hydraulic
gradients that can be achieved in both vapor and liquid phases, and
thus the greater the vapor and liquid recovery rates.
Dramatic enhancements in both water and petroleum product
recovery rates resulting from the large hydraulic gradients attainable
with DPE systems have been reported in the literature (Blake and Gates,
1986; Blake, et al., 1990; Bruce, et al., 1992). The depressed
groundwater table that results from these high recovery rates serves
both to hydraulically control groundwater migration and to increase the
efficiency of vapor extraction. The remedial effectiveness of DPE within
the zone of dewatering that commonly develops during DPE application
XI-4 May 1995
-------
Exhibit XI-3
Typical Single-Pump, DPE Extraction Well
Vacuum Extracted
Water And/Or
Floating Product
Vacuum Gauge
Air Inlet
Cement/Bentonite Seal
Sched. 40 PVC
Solid Casing
Slotted Sched. 40
PVC Well Screen
Bottomed, Sched. 40
PVC Threaded Plug
May 1995
XI-5
-------
Exhibit XI-4
Typical Multi-Pump, DPE Extraction Well
Extracted Water
And/Or
Floating Product
Extracted Air
To Blower '
Vacuum Gauge
Electrical Conduit for
Submersible Pump
Sealed Pass—through
For Power Cables
Submersible Pump
Liquids Extraction Tube
Cement/Bentonite Seal
Sched. 40 PVC
Solid Casing
Slotted Sched. 40
PVC Well Screen
Submersible Pump
^•^/^ ^V-S^piat Bottomed. Sched. 40
PVC Threaded Plug
XI-6
May 1995
-------
should be greater than that of air sparging due to the more uniform air
flow developed using DPE (Johnson, et al., 1992).
Because of the varied nature of DPE systems, the conceptual design
objectives for DPE can vary widely. DPE is often selected because it
enhances groundwater and/or product recovery rates, especially in
layered, fine-grained soils. The application of DPE also maximizes the
effectiveness of SVE by lowering the water table and therefore increasing
air-phase permeabilities in the vadose zone. Finally, DPE can enhance
biodegradation by substantially increasing the supply of oxygen to the
vadose zone. Exhibits XI-5 and XI-6 list the advantages and
disadvantages of single-pump and dual-pump DPE systems, respectively.
Exhibit XI-5
Advantages and Disadvantages of Single-Pump DPE
Advantages
o Proven performance in low-
permeability soils. Requires no
downhole pumps.
o Minimal disturbance to site
operations.
o Short treatment times (usually 6
months to 2 years under optimal
conditions).
o Substantially increases groundwater
extraction rates.
o Can be applied at sites with floating
product, and can be combined with
other technologies, such as air
sparging and bioremediation.
o Can be used under buildings and
other locations that cannot be
excavated.
o Can reduce the cost of groundwater
treatment through air stripping within
the vacuum extraction tube.
Disadvantages
o Expensive to implement at sites with
medium to high-permeability soils.
o Difficult to apply to sites where the
water table fluctuates.
o Treatment may be expensive for
extracted vapors and for oil-water
separation
o Can extract a large volume of
groundwater that may require
treatment.
o Requires specialized equipment with
sophisticated control capability.
o Requires complex monitoring and
control during operation.
This chapter will assist you in evaluating a corrective action plan
(CAP) that proposes DPE as a remedy for petroleum-contaminated soil
and groundwater. The evaluation process, which is summarized in the
flow diagram shown in Exhibit XI-7, will serve as a roadmap for the
May 1995
XI-7
-------
Exhibit XI-6
Advantages and Disadvantages of Multi-Pump DPE
Advantages
O Proven performance under a wide range
of conditions; readily available equipment.
o Minimal disturbance to site operations.
o Short treatment times (usually 6 months
to 2 years under optimal conditions).
O Substantially increases groundwater
extraction rates.
o Flexible applications to sites with water-
table fluctuations or widely ranging
permeabilities.
o Can be applied to sites with floating
product, and can be combined with other
technologies, such as air sparging and
bioremediation.
O Can be used under buildings and other
locations that cannot be excavated.
Disadvantages
o Effectiveness less certain when
applied to soils with very low
permeabilities or when applied with
lack of sufficient information of
subsurface conditions.
o May require costly treatment for
atmospheric discharge of extracted
vapors.
o May require costly oil/water
separation and groundwater
treatment.
o Requires complex monitoring and
control during operation.
decisions you will make during your evaluation. The evaluation can be
divided into the following steps.
O Step 1: An initial screening of DPE effectiveness, which will allow
you to quickly gauge whether DPE is likely to be effective, moderately
effective, or ineffective for a given site-specific application.
O Step 2: A detailed evaluation of DPE effectiveness, which provides
further screening criteria to confirm whether DPE is likely to be
effective at a given site. To complete the detailed evaluation, you will
need to identify key site characteristics and soil properties in the CAP,
compare them with conditions under which DPE is typically effective,
decide whether pilot studies are needed, and conclude whether DPE is
likely to be effective for the site-specific application.
O Step 3: An evaluation of the DPE system design, which will allow
you to determine whether the rationale for the design has been
appropriately defined based on pilot study data or other studies,
whether the necessary design components have been specified, and
XI-8
May 1995
-------
Exhibit XI-7
DPE Evaluation Process Flow Chart
INITIAL SCREENING OF
DPE EFFECTIVENESS
Determine the types of
soils that occur within
the contaminated area
Is
unstratifl
dense clay soil
targeted for
remediation?
DPE has the potential to
be effective at the site.
Proceed to next panel.
DPE is not likely
to be effective at
the site.
Consider ex-situ
technologies.
> Landfarming
' Biomounding
• Thermal
Desorption
DETAILED EVALUATION
OF DPE EFFECTIVENESS
Identify site characteristics
important to DPE effectiveness
Intrinsic Permeability
Soil Structure and Stratification
Moisture Content
Depth to Broundwator
Is
the intrinsic
permeability of
target soils and ground
water media
>10'12cm?
Is the
soil free of
impermeable layers
that would disrupt
air flow?
Is
soil water
content £85% of
saturation
Pilot studies are
required to demonstrate
effectiveness.
Review pilot study
results.
Is
depth to
groundwater
> 3 feet?
pilot
study results
demonstrate DPE
effectiveness
Are
the Henry's
Law Constants of
constituents
>100
Are
the chemical
sorptive capacities
of constituents
sufficiently
low?^
YES
DPE is likely to be
effective at the site.
Proceed to evaluate
the design.
May 1995
DPE will not be
effective at the site.
Consider other
technologies.
• Landfarming
• Biomounding
• Thermal
Desorption
M-9
-------
Exhibit XI-7
DPE Evaluation Process Flow Chart
[EVALUATION OF DPE
SYSTEM DESIGN
EVALUATION OF DPE SYSTEM
I OPERATION & MONITORING PLANS
Determine the design dements
• Design Radius of Influence
• Wellhead Vacuum
• Vapor Extraction Flow Rate
* Groundwater Extraction Rates
• Initial Constituent Vapor
Concentrations
* Required Final Constituent
Concentrations
* Required Final Cleanup Time
* Pore Volume Calculations
• Discharge Limitations and
Monitoring Requirements
• Site Construction Limitations
Have the
'design basics been"
Identified and are they
^within appropriate,,
jranges?,
,YES
Review the conceptual
process flow design & identify
the system components
• Extraction Well Orientation,
Placement and Construction
• Manifold Piping
• Vapor Pretreatment Equipment
• Blower Selection
* Instrumentation & Controls
* Surface Seals
Injection Wells
DPE system
design is
incomplete.
Request
additional
information.
Review the O & M plan for
the proposed DPE system
for the following:
• Start-Up Operations Plan
• Long-Term Operations &
Monitoring Plan
• Remedial Progress
Monitoring Plan
Has the
conceptual design
been provided and is
(t adequate?
DPE system
design is
incomplete.
Request
additional
information.
The DPE system design
Is complete and its
elements are within
normal ranges. Proceed
to O&M evaluation.
Are
start-up
8perations & monitoring
described, and are their
scope & frequency
adequate?
Request
additional
information
on startup
procedures and
monitoring
Is a
long-term O&M
plan described; is it of
adequate scope & frequency;
does it include
discharge permit
monitoring?
Request
additional
information
on Iong4erm
O&M.
Is a
remedial progres
monitoring plan estab-
lished; is it of adequate scope
& frequency; does it include
provisions for detect-
ing asymptotic
behavior
Request
additional
information
on remedial
progress
monitoring.
The DPE system is
likely to be effective.
The design and O&M
plans are complete.
XI-10
May 1995
-------
whether the construction process flow designs are consistent with
standard practice.
O Step 4: An evaluation of the operation and monitoring plans,
which will allow you to determine whether plans for start-up and
long-term monitoring of the system are of sufficient scope and
frequency and whether remedial progress monitoring plans are
appropriate.
Initial Screening Of DPE Effectiveness
Because of the differences in application of various types of DPE
systems and the complexity of DPE, deterrnining whether DPE will work
effectively at a given site is complex. This section discusses the key site
parameters that should be evaluated in deciding whether DPE will be a
viable remedy for a particular site. The key site parameters include:
O Permeability of the petroleum-contaminated soils and aquifer media.
Permeability affects the rates at which groundwater and soil vapors
can be extracted and controls the pore volume exchange rate.
O Volatility of the petroleum constituents. Volatility determines the rate
at and degree to which petroleum constituents will vaporize to the soil
vapor state.
In general, the type of soil (e.g., clay, silt, sand) will determine its
permeability. Fine-grained soils (e.g., clays and silts) have lower
permeability than coarse-grained soils (e.g., sands and gravels).
Permeability usually varies significantly with depth; for screening
purposes, consider the effects of the most permeable soil that is found
areally continuous through a significant portion of the chemically-
affected soil profile. Permeability affects remediation in both the vadose
and groundwater zones.
The volatility of a petroleum product or its constituents is a measure
of its ability to vaporize and can be measured in several ways. Because
petroleum products are highly complex mixtures of chemical constit-
uents, the volatility of the product mixture can be gauged most easily by
its boiling point. If the boiling point is low, the volatility of the product
will be high. Conversely, petroleum products with higher boiling points
are less volatile. If product volatility is low, DPE will be less effective in
removing petroleum constituents in the vapor phase from the
unsaturated zone.
Exhibit XI-8 is an initial screening tool that can be used to help
assess the potential effectiveness of DPE for a given site. It provides a
May 1995 XI-11
-------
Exhibit XI-8
Initial Screening for DPE Effectiveness
Permeability
im^ti&m Effective "^Sf i
vwH-SOTAiwH-HwvSCw: Effectives- •
: >>
,V.
".'.
YJ
•X v
•I *I
*I \
10
-16
Intrinsic Permeability, k (cm2)
1CT12 1CT10 1CT8 10~6 10~* TO"2
I Cloy I
L
Glacial Till
Silt, Loess |
Silty Sand
Clean Sand
Gravel
Product Composition
?More Effective;
I Lube Oils I
I Fuel Oils I
Diesel
Kerosene
Gasoline
Note:
All petroleum products listed are amenable for the DPE remediation alternative.
range of soil permeabilities for typical soil types as well as ranges in
composition for typical petroleum products. Use this screening tool to
make an initial assessment of the potential effectiveness of DPE. To use
this tool, you should scan the CAP to determine the soil type present and
the type of petroleum product released to assess the potential remedial
effectiveness of DPE at the site.
Information provided in the following section will allow a more
thorough effectiveness evaluation and will identify issues that could
require special design considerations.
Detailed Evaluation Of DPE Effectiveness
Once you have completed the initial screening and determined that
DPE is potentially effective for the soil permeability and petroleum
product composition present, you need to further evaluate the CAP to
XI-12
May 1995
-------
confirm that DPE will be effective. While the initial screen focused on
permeability and constituent volatility, the detailed evaluation should
consider a broader range of site and constituent characteristics, which
are listed in Exhibit XI-9.
The factors listed on Exhibit XI-9 largely control the initial
contaminant mass extraction rate, which will decrease during DPE
operation as concentrations of volatile organics in the soil (and soil
vapor) are reduced. However, based on the total contaminant mass
present in soils and a reasonable remediation time frame, acceptable
ranges for the site-specific factors can be determined.
Exhibit XI-9
Key Parameters Used To Evaluate Site Characteristics And Chemical Properties
Site Characteristics Chemical Properties
Intrinsic permeability Effective volatility
Soil structure and stratification Chemical sorptive capacity
Moisture content in the unsaturated zone
Depth to groundwater
The remainder of this section describes these parameters, why each is
important to DPE, how they can be determined, and the range of each
parameter considered appropriate for DPE. Keep in mind that the site-
specific factors that govern the effectiveness of DPE are generally the
same as those that govern the effectiveness of both SVE and bioventing.
Site Characteristics
Intrinsic Permeability
Intrinsic permeability is a measure of the ability of soil to transmit
fluids and is the most important factor in determining the effectiveness
of DPE because it controls the pore volume rates of groundwater and soil
vapor extraction. In addition, intrinsic permeability influences the
amount of oxygen supply that can be delivered to the unsaturated zone
bacteria and it controls the groundwater drawdown associated with given
extraction rates.
Intrinsic permeability varies over 13 orders of magnitude (from 1CT16
to 10~3 cm2) for the full range of earth materials, although a more limited
range applies for most soil types (1CT13 to 1CT5 cm2). Intrinsic
permeability is best determined from field or laboratory tests, but it can
May 1995 XI-13
-------
be estimated within one or two .orders of magnitude from soil boring log
data and laboratory tests. Coarse-grained soils (e.g., sands) have higher
intrinsic permeability than fine-grained soils (e.g., clays, silts). Note that
the ability of a soil to transmit air, which is of great importance in DPE,
is reduced by the presence of soil water, which can block the soil pores
and reduce air flow. The presence of soil water is especially important in
fine-grained soils, which tend to retain pore water.
The relatively high vacuum achievable with DPE systems is generally
effective in extracting liquids from relatively uniform soils with
permeabilities as low as 10"11 cm2. Single-pump DPE technology is best
suited to sites with intrinsic permeabilities ranging from 10"9 to 10"11
cm2, although it can be effective at sites with permeabilities as low as
10"12 cm2. Single-pump DPE systems are generally not economical at
sites with permeabilities greater than 10~9 cm2 because of the large air
flow required to maintain an adequate vacuum. There is no maximum
permeability limit for application of dual-pump DPE systems, provided
sufficient air confinement exists above the soils targeted for remediation
(see Depth to Groundwater, below). However, the added cost of vacuum
enhanced extraction is not warranted strictly to enhance groundwater
recovery rates in more permeable soils, and DPE should only be
considered for highly permeable soils in cases where soil and
groundwater remediation is required.
At most sites, intrinsic permeability varies significantly with depth,
and therefore the effectiveness of DPE systems depends on the soil
stratification. This relationship is further discussed in the "Soil
Structure and Stratification" section on page XI-15. Soils with very low
intrinsic permeabilities (i.e., < 10"11 cm2) can be dewatered if they are
interbedded with coarser-grained sediments. The coarser-grained
sediments are dewatered first, then the fine-grained sediments drain to
the dewatered layers, which are under high vacuum.
Hydraulic conductivity is a measure of the ability of soils to transmit
water. Hydraulic conductivity can be determined from aquifer tests,
including slug tests and pumping tests. You can convert hydraulic
conductivity to intrinsic permeability using the following equation:
k = K (u/pg)
where: k = intrinsic permeability (cm2)
K = hydraulic conductivity (cm/sec)
u = water dynamic viscosity (g/cm • sec)
p = water density (g/cm3)
XI-14 May 1995
-------
g = acceleration due to gravity (cm/sec2)
At 20°C: u/pg = 1.02 x 10"5 cm • sec
To convert k from cm2 to darcy, multiply by 108.
The effective air-phase permeability of the petroleum-contaminated
vadose-zone soils, along with the supply of air to the subsurface,
controls the air-flow rates achievable using DPE. The extracted air-flow
rate largely determines both the efficiency of vapor extraction and the
rates at which oxygen can be supplied to hydrocarbon-degrading
microorganisms in the subsurface. The effective air-phase permeability is
the product of the intrinsic soil permeability and the relative
permeability of the soil to the air phase in situ. The relative permeability
to air is greatest at low volumetric contents (or saturations) of soil-water
and petroleum product, and decreases as the liquid content increases
owing to the blockage of soil pores by the liquid(s).
Soil Structure And Stratification
Soil structure and stratification are important to DPE because they
affect how and where soil vapors will flow within the soil matrix during
extraction. Structural characteristics such as microfracturing and
secondary porosity features (e.g., root holes, mole holes, and worm holes)
can result in higher permeabilities than expected for certain soils (e.g.,
clays). Increased flow will occur in the fractured but not in the
unfractured media. Stratification of soils with different permeabilities
can dramatically increase the lateral flow of soil vapors in more
permeable strata while reducing the soil vapor flow through less
permeable strata. Consequently, a significant volume of contaminated
soil can remain untreated, and the remaining residual contamination
can act as a future source of groundwater contamination.
You can determine soil structure and stratification by reviewing soil
boring logs for wells or borings and by examining geologic cross-sections.
Verify that soil types have been identified, that visual observations of soil
structure have been documented, and that boring logs are of sufficient
detail to define soil stratification. Special design provisions may be
necessary for stratified soils to ensure that less-permeable strata are
adequately vented.
Moisture Content In The Unsaturated Zone
High moisture content in the unsaturated zone soils can reduce soil
permeability and, therefore, the effectiveness of DPE in removing
hydrocarbons from the unsaturated zone. Generally, with water
May 1995 XI-15
-------
saturation levels equal to or greater than 85 percent of field capacity, air
flow is blocked because the effective air permeability is essentially zero.
Airflow is particularly important for soils within the capillary fringe,
where a significant portion of petroleum constituents often accumulate.
Fine-grained soils create a thicker capillary fringe than coarse-grained
soils. The thickness of the capillary fringe can usually be determined
from soil boring logs (i.e., in the capillary fringe, soils are usually
described as moist or wet). By lowering the groundwater table, DPE can
effectively vent soils within the capillary fringe.
Depth To Groundwater
DPE is difficult to apply at sites where the water table is located less
than 3 feet below the land surface. This difficulty is due primarily to the
high potential for air-flow short circuiting due to large vertical air-flow
rates in the immediate vicinity of extraction wells within highly
permeable soils. Vertical short circuiting of air flow prevents more
uniform and lateral air flow through the affected soils. If a natural
barrier (e.g., shallow moist clay layer or sealed building slab) does not
exist to provide the necessary air confinement near the ground surface,
then an engineered surface seal must be installed to prevent the
undesirable air-flow short circuiting at sites with shallow groundwater.
Groundwater upwelling that can occur within SVE wells under
vacuum pressures generally does not pose a problem for DPE systems
because of the concurrent groundwater extraction that offsets potential
upwelling in the vicinity of DPE wells. Groundwater extraction with DPE
can be used to lower the water table and significantly expand the
thickness of unsaturated soil through which air can be circulated, thus
enhancing remedial effectiveness in shallow soils.
If water-table elevations fluctuate significantly at the site, special
design provisions must be made to accommodate them. Knowledge of
water table elevation fluctuations is especially critical if a single-pump
DPE system is in use because the inlet ends of the downhole extraction
tubes must be kept at or very near the liquid-gas interface in the wells to
maintain the entrainment of suspended liquid droplets within the
extracted air stream. As groundwater levels fluctuate, the liquid-gas
interface will move accordingly, making it difficult to keep the vacuum
extraction tubes in the optimal position for extraction of the air/droplet
stream.
XI-16 May 1995
-------
Chemical Properties
Effective Volatility
Effective volatility controls the rate at and degree to which
constituents will vaporize from the adsorbed and aqueous phases in the
unsaturated zone to the soil-vapor phase. The effective volatility of
petroleum constituents in the subsurface depends on whether mobile
free-phase product is present. If free-phase product is not present, the
effective volatility of petroleum constituents is characterized by their
Henry's law constants. In general, Henry's law constants increase as the
boiling points of the constituents decreases. When free petroleum
product exists in the subsurface, the product directly contacts soil
vapor, and the effective volatility is given by the constituent's saturation
vapor concentration times the mole fraction of the constituent in the
product mixture (Raoult's Law).
Vapor pressure of a constituent is the pressure that a vapor exerts
when in equilibrium with its pure liquid or solid form. This is an
approximate measure of its tendency to evaporate. Constituents with
higher vapor pressures (> 0.5 mm Hg) are generally volatilized efficiently
by the induced air stream of DPE systems. Constituents with vapor
pressures less than 0.5 mm Hg will not volatilize to a significant degree
and are primarily remediated by in-situ biodegradation by
microorganisms.
As previously discussed, petroleum products contain many different
chemical constituents. Depending on its vapor pressure, each
constituent will be volatilized to different degrees by a DPE system. If
concentrations of volatile constituents are significant, treatment of
extracted vapors may be needed. Exhibit XI-10 lists the vapor pressures
of common petroleum constituents.
Boiling point is another measure of constituent volatility. Because of
their complex constituent compositions, petroleum products are often
classified by their boiling point ranges rather than their vapor pressures.
Products with boiling points of less than about 250°C to 300°C are
sufficiently volatile to be amenable to physical removal from the
unsaturated zone by volatilization in a DPE system. Nearly all gasoline
constituents, a portion of kerosene and diesel fuel constituents, and a
lesser portion of heating oil constituents can be removed by
volatilization. Biodegradation will also contribute to removal of the these
constituents and will be a primary mechanism for removal of heavier,
less volatile constituents. If the petroleum product at the site comprises
predominantly low-volatility constituents, the DPE system should be
designed to maximize biodegradation, in a manner similar to bioventing
May 1995 XI-17
-------
Vapor Pressures
Constituent
Methyl t-butyl ether
Benzene
Toluene
Ethylene dibromide
Ethylbenzene
Xylenes
Naphthalene
Tetraethyl lead
Exhibit XI-10
Of Common Petroleum Constituents
Vapor Pressure
(mm Hg at 20°C)
245
76
22
11
7
6
0.5
0.2
(see Chapter III), or bioventing could be used in lieu of DPE. If, however,
the constituents are primarily volatile (e.g., gasoline), then higher air
flow, similar to that used in conventional SVE systems, would be
appropriate (see Chapter II). The boiling point ranges for common
petroleum products are shown in Exhibit XI-11.
Exhibit XI-11
Petroleum Product Boiling Ranges
Product
Boiling Range
Gasoline
Kerosene
Diesel fuel
Heating oil
Lubricating oils
40 to 205
180 to 300
200 to 338
>275
Nonvolatile
Henry's law constant is the partition coefficient that relates the
concentration of a constituent dissolved in water to its partial vapor
pressure under equilibrium conditions. In other words, it describes the
relative tendency for a dissolved constituent to exist in the vapor phase.
Henry's law constant is a measure of the degree to which constituents
that are dissolved in soil moisture or groundwater will volatilize for
physical removal by DPE. Henry's law constants for several common
constituents found in petroleum products are shown in Exhibit XI-12.
Constituents with Henry's law constants of greater than 100
atmospheres are considered sufficiently volatile to be physically removed
with extracted soil vapor.
XI-18
May 1995
-------
Henry's Law Constant
Constituent
Tetraethyl lead
Ethylbenzene
Xylenes
Benzene
Toluene
Naphthalene
Ethylene dibromide
Methyl t-butyl ether
Exhibit XI-12
Of Common Petroleum Constituents
Henry's Law Constant
(atm)
4,700
359
266
230
217
72
34
27
Chemical Sorptive Capacity
The chemical sorptive capacity determines the amount and degree of
adsorption of constituents onto the soils and aquifer media. The higher
the sorptive capacity of the soil, the more difficult the removal of
constituents from the subsurface. The sorptive capacity is described by
the soil-water partition coefficient, Kd, which is primarily a function of
the organic carbon-water partition coefficient, Koc (a chemical-specific
parameter) and the fractional content of soil organic carbon, foc (a soil-
specific parameter). For a given petroleum constituent, the sorptive
capacity (and thus the difficulty of remediation) tends to increase as the
soil becomes finer grained. The sorptive capacity affects the remedial
effectiveness in both the vadose and groundwater zones. Although
higher sorption decreases remedial effectiveness, it also reduces the risk
of hydrocarbon transport from affected soil to underlying groundwater or
to the atmosphere, thus decreasing the need for thorough remediation
(i.e., increasing the residual chemical concentrations) that may be safely
left behind in the targeted soils.
Pilot Scale Studies
After you have examined the data in the CAP to gauge the potential
effectiveness of DPE, you will be in a position to decide if DPE is likely to
be highly effective, somewhat effective, or ineffective given the site
conditions. If the site shows marginal to moderate potential for
effectiveness to DPE, you should evaluate the design closely and verify
that adequate DPE pilot studies have been completed at the site and that
the test results indicate DPE should be effective.
May 1995 XI-19
-------
While pilot studies are valuable to any DPE evaluation and design,
they are critical in cases where the screening-level assessment of the site
conditions indicates only moderate to marginal applicability to DPE.
Ideally, a small-scale pilot version of the actual DPE system intended for
use at the site should be tested. For small sites, where the volume of soil
requiring remediation is less than roughly 2000 cubic yards, it may not
be economically attractive to conduct thorough DPE pilot tests in the
field. Nevertheless, it is advisable to at least conduct a simple soil vapor
extraction (SVE) test to verify that soil vapor can be extracted at
achievable vacuum pressures. Also, aquifer testing is recommended to
gather information needed to design the groundwater extraction portion
of the DPE system.
For SVE testing, different extraction rates and wellhead vacuums are
applied to the well to determine optimal operating conditions. The
vacuum influence is measured at increasing distances from the
extraction well using vapor probes or existing wells to establish the
pressure field induced in the subsurface by the extraction system. The
pressure field measurements can be used to define the radius of vacuum
influence for the vadose-zone portion of the DPE system. Vapor
concentrations should also be measured two or more times during the
pilot testing to estimate the initial vapor concentrations that might be
expected of a full-scale system at the site. This information serves as the
basis for the vapor treatment system design. If an extended pilot test is
conducted, long-term changes in soil-vapor concentrations can be used
to assess how concentrations will vary over time, and to estimate the
time required for full remediation.
Mistakes in the SVE field test commonly lead to erroneous
conclusions regarding the potential effectiveness of DPE. Sometimes, the
applied vacuum is too great, and the water level within the well casing
rises rapidly to a level above the slotted portion of the well casing. When
the applied vacuum is too high, no air can be extracted from the
subsurface, leading to the erroneous conclusion that DPE cannot be
applied at the site. Similarly, DPE is erroneously thought to be infeasible
because the results from a simple pumping aquifer test (using
conventional pumps) indicate that insufficient production of
groundwater and/or petroleum product is obtained. In such cases, the
high vacuum achievable with DPE systems can greatly enhance
groundwater and/or product recovery, and DPE could still be a
potentially effective remedy for low permeability sites. This illustrates
the importance of actual DPE pilot testing.
To assess the groundwater flow parameters necessary to design the
groundwater extraction portion of the DPE system, aquifer testing
should be conducted. The use of DPE equipment for vacuum-assisted
XI-2O May 1995
-------
aquifer testing is desirable, because such testing yields information that
is directly relevant to the potential effectiveness of a full-scale DPE
system at the site. However, this approach may be prohibitively
expensive for smaller sites; in such cases, traditional aquifer testing
(using groundwater extraction alone) may be used. Aquifer pumping
tests are preferred over slug tests because slug tests only yield
information regarding the local transmissivity (hydraulic conductivity
times the thickness of the groundwater flow zone) in the immediate
vicinity of the tested well, whereas pumping tests yield information
regarding the transmissivity over a relatively wide area surrounding the
pumped well. When properly conducted and analyzed, aquifer tests will
yield reliable estimates of the relevant hydraulic parameters
(transmissivity and storage coefficients) of the tested groundwater flow
zone(s) that are targeted for remediation. These values should then be
used in an appropriate groundwater model to simulate the potential
groundwater extraction effectiveness under the applied vacuums
achievable with a DPE system. In this way, the feasibility of the
groundwater extraction portion of the DPE system can be properly
assessed.
If the success of the DPE application is particularly dependent on
biodegradation, relevant field and/or laboratory testing should be
conducted. Chapter III on Bioventing describes several types of pilot-
scale tests that can be performed to confirm the potential effectiveness of
biodegradation.
Evaluation Of The DPE System Design
Once you have verified that DPE is generally applicable to the site,
you can scrutinize the design of the system. A pilot study that provides
data used to design the full-scale DPE system is highly recommended.
The CAP should include a discussion of the rationale for the design and
a presentation of the conceptual engineering design. Detailed engineering
design documents might also be included, depending on state
requirements. Further detail about information to look for in the
discussion of the design is provided below.
Rationale For The Design
The primary basis for any subsurface remedial design is a definition
of the volume of the subsurface targeted for active remediation (volume
of attainment) and the cleanup levels or concentrations of constituents
that must be achieved within the volume of attainment to protect human
health and the environment. The cleanup levels may either be defined by
state regulated "remedial action levels" or be determined on a site-
May 1995 XI-21
-------
specific basis using transport modeling and risk assessment. Site
characterization data must be used to determine what volumes of site
soils and/or groundwater exceed the state action levels or site-specific,
health-based cleanup levels. The CAP should clearly describe how the
proposed DPE system is designed to meet the remedial action objectives.
In addition, information such as the following should be included:
O The Design Radius of Influence (ROI) is the maximum distance from a
vapor extraction well at which sufficient air flow can be induced to
sustain acceptable rates of remediation (as dictated by the desired or
required remediation time). The usefulness of the simple ROI concept
is limited to certain site conditions (e.g., a single extraction well
operating without air inlets or air-injection wells), and even when
applicable, the task of establishing a meaningful ROI is not trivial.
The ROI depends on many factors including the geometric
configuration of extraction and injection wells, intrinsic permeability
of the soil, soil moisture content, and desired remediation time. The
ROI is best determined through field pilot studies, but it can be
estimated from air flow modeling or other empirical methods.
Generally, the design ROI can range from 5 feet (for fine-grained soils)
to 100 feet (for coarse-grained soils) for a single well operating alone.
For sites with stratified geology, radii of influence should be defined
for each major soil type that occupies a significant portion of the
chemically affected soil profile.
For applications where the groundwater is shallow, ambient air is
supplied readily through the land surface to the soils requiring
treatment. In this case, the ROI can be used in a simple manner to
determine the appropriate number and spacing of extraction wells.
For applications in deeper treatment intervals, or treatment intervals
that are effectively isolated from surface air supply, air inlet wells are
required for effective remediation, and the simple ROI concept is not
directly applicable. In such cases, subsurface air-flow stimulation is
recommended to aid in properly designing a system of extraction and
injection wells (or passive air inlets) that provides reasonably uniform
air circulation throughout the targeted regions of the vadose zone.
O Wellhead Vacuum is the vacuum pressure that is required at the top
of the vent well to produce the desired radius of vacuum influence in
the soils. Required wellhead vacuums are usually determined with
the aid of field pilot studies, and they typically range from 3 to 100
inches of water vacuum. Less permeable soils generally require
higher wellhead vacuum pressures to produce reasonable influence
radii of influence. It should be noted, however, that high vacuums can
cause upwelling of the water table and occlusion of all or part of the
extraction well screens.
XI-22 May 1995
-------
O Vapor Extraction Flow Rate is the volumetric flow rate of soil vapor
that will be extracted from each extraction well. Vapor extraction rate,
radius of influence, and wellhead vacuum are all interdependent, (i.e.,
a change in the extraction rate will cause a change in wellhead
vacuum and radius of influence). Appropriate vapor extraction flow
rates are best determined from pilot studies, but they can be
estimated using mathematical models and estimated values of the air-
flow parameters such as effective air permeability and flow-zone
thickness. The flow rate will contribute to the operational time
requirements of the DPE system. Typical extraction rates range from 2
to 50 cubic feet per minute (cfm) per well.
O Groundwater Extraction Rates should, at a minimum, be sufficient to
capture groundwater that has constituent concentrations that exceed
applicable standards or that pose a threat to human health or the
environment. Higher groundwater extraction rates may also be
specified to produce greater water-table drawdowns and enhance the
effectiveness of vadose zone remediation. The design of the
groundwater extraction portion of the DPE system should be based on
the results of aquifer testing and groundwater flow modeling. This is
especially true when groundwater extraction is proposed from
multiple groundwater flow zones, or when the objectives of the
groundwater extraction include lowering the water table (dewatering).
O Initial Constituent Vapor Concentrations can be measured during pilot
studies or estimated from soil gas samples or soil samples. They are
used to estimate constituent mass removal rate and DPE operational
time requirements, and to determine whether treatment of extracted
vapors will be required prior to atmospheric discharge or reinjection (if
allowed).
The concentration of constituents in the extracted vapor is typically
much higher during system start up than during sustained, long-term
operations. The higher initial vapor concentrations usually last only a
few hours or days before dropping off significantly. Vapor treatment
requirements may be greater during this early phase of remediation,
compared to the long-term requirements.
O Required Final Constituent Concentrations in soils, or soil cleanup
levels, may be defined by state regulations as "remedial action levels,"
or they may be determined on a site-specific basis using fate and
transport modeling and risk assessment. The required soil cleanup
levels will determine what areas of the site require treatment and
when DPE operation can be terminated.
May 1995 XI-23
-------
O Required Remedial Cleanup Time may also influence the design of the
system. The designer may reduce the spacing of the extraction wells
to increase the rate of remediation to meet cleanup deadlines or client
preferences, as required.
O Soil Volume To Be Treated is determined by state action levels or a
site-specific risk assessment using site characterization data for the
soils.
O Pore Volume Calculations are used along with extraction flow rate to
determine the pore volume exchange rate. The exchange rate is
calculated by dividing the pore space within the treatment zone by the
design extraction rate (for vapor and groundwater separately). The
pore space within the treatment zone is calculated by multiplying the
soil porosity by the volume of soil to be treated. Some literature
suggests that one pore volume of soil vapor be extracted at least daily
for effective remedial progress if volatilization is intended to be the
primary removal mechanism.
You can calculate the time required to exchange one pore volume of soil
vapor using the following equation:
9
where: E = pore volume exchange time (hr)
s = soil porosity (m3 vapor / m3 soil)
V = volume of soil to be treated (m3 soil)
Q = total vapor extraction flow rate (m3 vapor / hr)
O Discharge Limitations And Monitoring Requirements are usually
established by state regulations, but they must be considered by
designers of a DPE system to ensure that monitoring ports are
included in the system hardware. Discharge limitations imposed by
state air quality regulations will determine the offgas treatment
requirements.
O Site Construction Limitations, such as building locations, utilities,
buried objects, and residences must be identified and considered in
the design process.
Components Of A DPE System
Once the design basis is defined, the design of the DPE system can be
developed. A typical DPE system design will include the following
components and information:
XI-24 May 1995
-------
O Extraction well orientation, placement, and construction details
O Manifold piping
O Vapor pretreatment (if necessary)
O Vapor treatment (if necessary)
O Blower selection
O Instrumentation and control design
O Optional DPE components
O Surface seals
O Injection wells
Exhibits XI-13 and XI-14 are schematic diagrams of single-pump and
dual-pump DPE systems, respectively.
The following subsections provide guidance for selecting the
appropriate system configuration, standard system components, and
additional system components to adequately address petroleum
contaminated soils at a particular UST site.
Extraction Wells
Well Orientation. DPE systems generally use vertical extraction wells,
although horizontal wells can be used for air injection and/or for
nutrient addition to enhance biodegradation, if needed.
Well Placement And Number Of Wells. This design element is .critical to the
effectiveness of a DPE system. For complex sites, numerical modeling
should be used to simulate subsurface vapor flow and groundwater flow.
For simpler, shallow groundwater sites, you can determine the number
and location of extraction wells by using several methods. In the first
method, divide the area of the site requiring treatment by the area of
influence for a single well to obtain the total number of wells needed.
Then, space the wells evenly within the treatment area to provide area!
coverage so that the areas of influence cover the entire area of
contamination.
Area of influence for a single well = n • (ROI)2
Number of wells needed = Treatment area (m2)
Area of influence for single extraction well (m2/well)
This approximation method will work reasonably well in cases where
ambient air is readily supplied to the extraction wells through the
affected soils from the land surface. When there is no significant airflow
from the land surface downward through the treatment zone,
May 1995 XI-25
-------
Exhibit XI-13
Schematic of Single Pump
Legen
PI
SP
Ambient Blow Back Loop
^Mr'I 4^ " Vacuum
** IO1 ggy \±^y •al'01
1 f_^ ^ n Blower
I""""" l M& ' tffl1 l v~* 1
uquca ftKJKJiOT? v^y f
Pump wntor °" Storage
Water Trangfer Tank
-, Pump -40f-
Oil/Water 1 „_
separator •£
/• D— L-JOf-* Wat
1 ( •" F Treatr
DPE System
it
1 C^\
i lO'sP f
-r ' ' IT T 1 1 *
zzz* •^'t i . i |
icer Row
L J Meter
Vapor
Treatment
(If Required)
T Fn i5i p. riisrh
su-'-td-*— £«K
discharge to
Atmosphere
'Permit May
ie Required)
Water
arge
t May
uired)
Water
Transfer
Pump
Combined Liquid te Gas Stream __ _
Row spe-
d; p n
Pressure Indicator Slotted Vertical /'Rxj
Sample Port Well Casing With 1 ' J~
m /. i i Vacuum. Extraction
Row Meter *x.: :,,
Optional Depending
on the Site Condons S|ottcd Vertica| ^
Inlet Vent (Typical) -
1 -f~
SPe- T SPe
LjK uJv! U^V L
rvjr r^y *3L r
1 •• 1
1 - • 1
< j h§
1
approximately half of the evenly spaced wells specified should be air
injection wells or passive air inlets.
It is important to note the potential for "short circuiting" of vertical air
flow downward from the land surface in the immediate vicinity of an
extraction well. Such short circuiting leads to ineffective remediation
because the resulting air-flow circulation pattern only affects a small
volume of soil surrounding the extraction well. Short circuiting may
occur at system startup, or it may begin to occur after a DPE system has
been in operation for some time. Short circuiting at initial system
startup usually results from placing screened intervals at shallow depths
in media with high effective vertical air permeabilities. This allows a
relatively large volume of air to enter near the extraction well, reducing
the well's effective radius of influence. The potential for this problem can
XI-26
May 1995
-------
Exhibit XI-14
Schematic of Multi-Pump DPE System
Ambient
Air
Blow Bock Loop
Discharge to
Atmosphere
(Permit May
Be Required)
I I
I I
I J
Vapor
Treatment
(If Required)
Legend:
PI Pressure Indicator
SP Sample Port
Flow Control Valve
Slotted Vertical
Well Casing
(Typical
Submersible
Groundwater Pump ^::
Or Total Fluids
Purnp (Typical)
Optional Depending
on the Site Conditions
Slotted
Vertical Air
Inlet Vent
(Typical)
Treated Water
Discharge
(Permit May
Be Required)
Transfer
Pump
usually be assessed by conducting field testing, as previously discussed,
and engineered surface seals may be used to overcome this type of
problem. Short circuiting can likewise be caused by improper sealing of
the well boring annulus during the well construction. In such cases, the
well must be carefully sealed or replaced with a well of more air-tight
design.
The potential for short circuiting after a period of sustained
operations can be difficult to predict based on the results of a short field
test. Short circuiting that develops after a period of system operation is
May 1995
XI-27
-------
usually caused by a significant increase in effective vertical air-flow
permeability due to the drainage of water and/or product from the soil
pores, which increases the air-filled porosity of the aquifer matrix and
hence the effective air permeability. Adequate monitoring systems are
therefore required to detect changes in the system vacuum and/or air-
flow rates over time.
In the second method used to estimate the number of wells needed,
determine the total extraction flow rate required to exchange the soil
pore volume within the treatment area in 1 to 7 days. Determine the
number of wells required by dividing the total extraction flow rate needed
by the flow rate achievable with a single well.
Number of wells needed = £ '
q
where: e = soil porosity (m3 vapor / m3 soil)
V = volume of soil in treatment area (m3 soil)
q = vapor extraction rate from single extraction well
(m vapor/hr)
t = pore volume exchange time (hours)
Similar calculations can be used for evaluating groundwater
extraction. Consider the following additional factors in determining well
spacing.
O Use closer well spacing in areas of high contaminant concentrations
to increase mass removal rates.
O If a surface seal exists or is planned for the design, space the wells
slightly farther apart because air is drawn from a greater lateral
distance and not directly from the surface. However, be aware that the
presence of a surface seal and the increase in extraction well spacing
increases the need for air injection wells.
O At sites with stratified soils, wells that are screened in strata with low
intrinsic permeabilities should be spaced more closely than wells that
are screened in strata with higher intrinsic permeabilities. Well
spacing may be irregular.
Well Construction. Typical single-pump and dual-pump DPE extraction
wells are shown in Exhibits XI-3 and XI-4, presented earlier. Extraction
wells are similar in construction to monitoring wells and are drilled
using the same techniques. Extraction wells are usually constructed of
polyvinyl chloride (PVC) casing and screening. Extraction well diameters
typically range from 2 to 8 inches, depending on flow rates and depth; a
4-inch diameter is most common.
XI-28 May 1995
-------
Extraction wells are constructed by placing the casing and screen in
the center of a borehole. Filter pack material is placed in the annular
space between the casing/screen and the walls of the borehole. The filter
pack material extends 1 to 2 feet above the top of the well screen; a 1 to
2 foot thick bentonite seal is placed above the filter pack.
Cement-bentonite grout seals the remaining space up to the land
surface. Filter pack material and screen slot size must be consistent with
the grain size of the surrounding soils.
The location and length of the well screen in vertical extraction wells
can vary and should be based on the depth to groundwater, the
stratification of the soil, and the location and distribution of
contaminants. The bottom of the screened interval must be sufficiently
deep to allow for the maximum anticipated groundwater drawdown.
O At a site .with homogeneous soil conditions, ensure that the well is
screened throughout the contaminated zone. A deeper well helps to
ensure remediation of the greatest amount of soil during seasonal low
groundwater conditions.
O At a site with stratified soils or lithology, check to see that an
adequate number of wells have been screened within the lower
permeability zones, as well as the higher permeability zones, because
these zones are generally more difficult to remediate.
Manifold Piping
Manifold piping connects the extraction wells to the surface blower.
Piping can either be placed above or below grade depending on site
operations, ambient temperature, and local building codes. Below-grade
piping is most common and is installed in shallow utility trenches that
lead from the extraction wellhead vault(s) to a central equipment
location. The piping can either be manifolded in the equipment area or
connected to a common vacuum main that supplies the wells in series,
in which case flow control valves are sited at the wellheads. Piping to the
well locations should be sloped toward the well-so that condensate or
groundwater that is entrained with the air flow stream will flow back
toward the well.
Vapor Pretreatment
Extracted vapor can contain condensate, entrained groundwater, and
particulates that can damage blower parts and inhibit the effectiveness
of downstream treatment systems. In order to minimize damage, vapors
are' usually passed through a moisture separator and a particulate filter
May 1995 XI-29
-------
prior to entering the blower. Check the CAP to verify that these elements
have been included in the design.
Vapor Treatment
Look for vapor treatment systems in the DPE design if pilot study
data indicate that extracted vapors will contain VOC concentrations in
excess of established air quality limits. Available options for vapor
treatment include granular activated carbon (GAC), catalytic oxidation,
or thermal oxidation.
GAC is a popular choice because of its simplicity and effectiveness.
Catalytic oxidation is often used, however, when the contaminant mass
loading rate is expected to be too high to make GAC economical, and
when concentrations are at or below 20 percent of their lower explosive
limit (LEL). A thermal oxidizer may be employed when concentrations of
chemical constituents are expected to be sustained at levels greater than
20 percent of their LELs.
Blower Selection
The type and size of blower selected should be based on (1) the
vacuum required to achieve design vacuum pressure at the vent
wellheads (including upstream and downstream piping losses) and
(2) the total flow rate required. The flow rate requirement should be
based on the sum of the flow rates from the contributing vapor
extraction wells. In applications where explosive concentrations of
hydrocarbon vapors can collect, be sure the CAP specifies blowers with
explosion-proof motors, starters, and electrical systems. Exhibit XI-15
depicts the performance curves for the three basic types of blowers that
can be used in a DPE system.
O Centrifugal blowers (such as squirrel-cage fans) should be used for
high-flow, low-vacuum applications. Centrifugal blowers are only
applicable for dual-pump DPE systems, because higher vacuums are
required for single-pump DPE systems.
O Regenerative and turbine blowers should be used when a moderately
high vacuum is needed.
O Rotary lobe and other positive displacement blowers should be used
when a very high vacuum is needed.
Liquid ring vacuum pumps are also commonly used for DPE applications
in very low permeability environments where high vacuums are required.
XI-3O May 1995
-------
Exhibit XI-15
Performance Curves For Three Types Of Blowers
•5 160 -
3
O
^ 140 -
| 120 -
3
0 100-
jjj 80 -
0
^ 60 -
O
M 40 -
JC
o 20 -
m
* m m m m m m Rotof/ Lobe
Blower
• ^— —" Regenerative Blower
• mmm mmm Centrifugal Blower
1
"*"**^ *
^^^^^.*
^^xs^
• ^^^^^
* ^^^JI111 ^JtTJl
"\." >W
1 1 1 1 1 1 1
40 80 120 160 200 240 280
Airflow - Standard Cubic Feet per Minute
(SCFM)
Notes: Centrifugal blower type shown is a New York model 2004A at 3500 rpm. Regenerative blower type
shown is a Rotron model DR707. Rotary lobe blower type shown is a M-D Pneumatics model 3204 at
3000 rpm.
From "Guidance for Design, Installation and Operation of Soil Venting Systems." Wisconsin Department
of Natural Resources, Emergency and Remedial Response Section, PUBL-SW185-93, July 1993.
Instrumentation and Controls
The parameters typically monitored in an DPE system include:
O Pressure (or vacuum)
O Air/vapor flow rate
O Groundwater extraction rates
O Carbon dioxide and/or oxygen concentrations in extracted air (to
monitor biodegradation)
O Contaminant mass removal rates
O Temperature
The equipment in a DPE system used to monitor these parameters
provides the information necessary to make appropriate system
adjustments and track remedial progress. The monitoring equipment in
a DPE system enables you to control each component of the system.
Exhibit XI-16 lists typical monitoring and control equipment for a DPE
May 1995
XI-31
-------
system, where each of these pieces of monitoring equipment should be
placed, and the types of equipment that are available.
Exhibit XI-16
Monitoring And Control Equipment
Monitoring Equipment
Flow meter
Vacuum gauge
Vapor temperature sensor
Sampling port
Vapor sample collection
equipment (used through a
sampling port)
Control Equipment
Flow control valves
Location In System
o At each wellhead
o Manifold to blower
o Blower discharge
o At each well head or
manifold branch
o Before and after filters
upstream of blower
o Before and after vapor
treatment
o Manifold to blower
o Blower discharge (prior
to vapor treatment)
o At each well head or
manifold branch
o Manifold to blower
o Blower discharge
o At each well head or
manifold branch
o Manifold to blower
o Blower discharge
o At each well head or
manifold branch
o Dilution or bleed valve
at manifold to blower
Example Of Equipment
o Pilot tube
o In-line rotameter
o Orifice plate
o Venturi or flow tube
o Manometer
o Magnehelic gauge
o Vacuum gauge
o Bi-metal dial-type
thermometer
o Hose barb
o Septa fitting
o Tedlarbags
o Sorbent tubes
o Sorbent canisters
o Polypropylene tubing
for direct GC
injection
o Ball valve
o Gate/globe valve
o Butterfly valve
Optional DPE Components
Additional DPE system components might be required when certain
site conditions exist or pilot studies dictate they are necessary. These
components include land surface seals and injection wells. Each of
these system components is discussed on the following pages.
XI-32
May 1995
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Land Surface Seals
Land surface seals might be included in an DPE system design in
order to prevent surface water infiltration that can reduce air flow rates,
reduce fugitive emissions, and increase the lateral extent of air flow to
increase the volume of soil being treated. These results are accomplished
because surface seals force fresh air to be drawn from a greater distance
from the extraction well. If a surface seal is used, lower pressure
gradients may exist and decreased flow velocities will result unless a
higher vacuum is applied to the extraction well, or additional air
injection wells are used.
Surface seals or caps should be selected to match the site conditions
and regular business activities at the site. Options include high density
polyethylene (HOPE) liners (similar to landfill liners), clay or bentonite
seals, or concrete or asphalt paving. Existing covers (e.g., pavement or
concrete slabs) might not be effective as an air-flow barrier if they are
constructed with a porous subgrade material.
Injection Wells
Air injection wells are used to enhance air flow rates from the
extraction wells by providing an active or passive air source to the
subsurface. These wells are often used at sites that are covered with an
impermeable cap (e.g., pavement or buildings) because the cap restricts
direct air flow to the subsurface. They may also be used to help reduce
short-circuiting of air flow in the subsurface. In addition, air injection is
used to eliminate potential stagnation zones (areas of no flow) which can
exist between extraction wells.
Air injection wells are similar in construction to extraction wells, and
they may be operated in either passive or active mode. Active injection
wells force compressed air into soils. Passive injection wells, or inlets, -
simply provide a pathway that helps extraction wells draw ambient air
into the subsurface. Air injection wells should be placed to eliminate
stagnation zones, but should not force contaminants toward areas from
which they cannot be recovered (i.e., away from the influence areas of
the systems' extraction wells).
Evaluation Of Operation And Monitoring Plans
Make sure that a system operation and monitoring plan has been
developed for both the system start-up phase and for long-term
operations. Operations and monitoring are necessary to ensure that
system performance is optimized and contaminant mass removal is
tracked. When significant biodegradation occurs in the subsurface, mass
May 1995 XI-33
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removal cannot be directly measured by simply monitoring extracted
liquid and vapors. Both constituent concentrations and carbon dioxide
concentrations (to measure microbial respiration) should be monitored in
the extracted vapor stream. Dissolved constituent concentrations in the
extracted groundwater and the quantity of petroleum product collected
must be monitored to assess constituent mass removal.
Start-Up Operations
The start-up phase of operations for single-pump DPE systems will
include about 7 to 10 days of adjustments in the vacuum pump settings
and the depth of the extraction tube inlet. Multi-pump systems will
require a similar period of adjusting the valves and pumps for the
separate groundwater and air extraction systems. The start-up phase
should also include manifold valving adjustments. These adjustments
should balance flow between the wells within the system. To optimize
DPE effectiveness, flow measurements, pressure or vacuum readings,
carbon dioxide concentrations, oxygen concentrations, and volatile
organic compound (VOC) concentrations should be recorded daily from
each extraction well, from the manifold, and from the effluent stack
during the start up adjustment period. These measurement can be used
to decide how to best operate the system. Nutrient delivery (if needed to
enhance biodegradation) should not be performed until after start-up
operations are complete.
Long-Term Operations
Long-term monitoring should consist of flow-balancing, flow and
pressure measurements, and vapor concentration readings.
Measurements are commonly made at weekly or biweekly intervals for
the duration of the system operational period.
Exhibit XI-17 provides a brief synopsis of system monitoring
requirements.
Remediation Progress Monitoring
Monitoring the performance of the DPE system in reducing
contaminant concentrations in soils is necessary to track the progress of
remediation. Since concentrations of petroleum constituents may be
reduced due to both volatilization and biodegradation, both processes
should be monitored in order to track their cumulative effect.
Constituent mass extraction can be tracked and calculated by
multiplying the vapor concentrations measured in the extraction
manifold by the extraction air flow rate and adding the rate of petroleum
XI-34 May 1995
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Phase
Start-up
(7-10 days)
Remedial
(ongoing)
System
Monitoring
Frequency
At least daily
Weekly to bi-
weekly
Exhibit XI-17
Monitoring Recommendations
What To Monitor
o Flow
o Vacuum readings
o Vapor concentrations
o Carbon dioxide
o Oxygen
o Flow-balancing
o Flow
o Vacuum
o Vapor concentrations
o Carbon dioxide
o Oxygen
Where To Monitor
o Extraction vents
o Manifold
o Effluent stack
o Extraction vents
o Manifold
o Effluent stack
product recovery and dissolved aqueous phase recovery. The constituent
mass that is biodegraded is more difficult to quantify but can be
estimated from monitoring data on carbon dioxide and oxygen
concentrations in the extracted air stream. The quantities of petroleum
hydrocarbons degraded can then be estimated stoichiometrically (see
Chapter III, Bioventing).
Remediation,progress of DPE systems typically exhibits asymptotic
behavior with respect to the rates of recovery of free product and
groundwater, and a reduction in vapor concentration and the overall rate
of mass removed. (See Exhibit XI-18.) When asymptotic behavior begins
to occur, the operator should evaluate alternatives that may increase
DPE effectiveness (e.g., altering the subsurface airflow patterns by
changing airflow rates). Other more aggressive steps to renew
remediation effectiveness can include installing additional injection wells
or extraction wells. If very low effluent concentrations persist, extraction
flow rates may be reduced significantly, or the system may be operated
in a pulsed mode (although pulsed operation is generally less efficient
than operating at a very low, sustained extraction rate). Pulsing involves
the periodic shutdown and startup of extraction wells to allow the
subsurface environment to come to equilibrium (shutdown) before
beginning to extract vapors again.
If asymptotic behavior persists for periods greater than about
6 months, the concentration rebound remains small following periods of
system shutdown, and residual contamination levels are at or below
regulatory limits, termination of operations may be appropriate. If not,
May 1995
XI-35
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Exhibit XI-18
Relationship Between Concentration Reduction And Mass Removal
•o
O C
I
-------
References
Blake, S.B., and N,M. Gates. 'Vacuum Enhanced Hydrocarbon
Recovery: A Case Study,1' in Proceedings o/NWWA/APi Conference 6ft
Petroleum Hydrocarbons and Organic Chemicals in Ground Water:
Prevention, Detection, and Restoration. November 12-14, Houston,
Texas, p. 709-721, 1986.
Blake, S., B. Hockman, and M. Martin. "Applications of Vacuum
Dewatering Techniques to Hydrocarbon Remediation," in Proceedings
oJNWWA/API Conference on Petroleum Hydrocarbons and Organic
Chemicals in Ground Water: Prevention, Detection, and Restoration.
October 31 - November 2, Houston, Texas, p. 211-226, 1990.
Bruce, L., B. Hockman, R. James-Deanes, J. King, and D. Laws.
"Vacuum Recovery Barrier Wall System Dewaters Contaminated
Aquifer: A Solution Based on Proper Evaluation of Hydrogeologic
Parameters," in Proceedings of the NGWA/API Conference on Petroleum
Hydrocarbons and Organic Chemicals in Ground Water: Prevention,
Detection, and Restoration. November 4-6, Houston, Texas, p. 303-
311, 1992.
Hajali, P.A., and W.F. Revely, III. Process and Apparatus for
Groundwater Extraction Using a High Vacuum Process. U.S. Patent
No. 5,172,764, December 22, 1992.
Hess, R.E., A.A. Hooper, S.R. Morrow, D.J. Walker, and E. Zimmerman.
Process for Two Phase Vacuum Extraction of Soil Contaminants. U.S.
Patent No. 5,050,676, September 24, 1991.
Hess, R.E., A.A. Hooper, S.R. Morrow, D.J. Walker, and E. Zimmerman.
Apparatus for Two Phase Vacuum Extraction of Soil Contaminants.
U.S. Patent No. 5,197,541, March 30, 1993.
Johnson, R.L., Bagby, W., Matthew, P., and Chien, C.T. "Experimental
Examination of Integrated Soil Vapor Extraction Techniques," in
Proceedings of Petroleum Hydrocarbons and Organic Chemicals in-
Ground Water: Prevention, Detection, and Restoration. National
Ground Water Association, November, 1992.
Powers, J.P. Construction Dewatering, A Guide to Theory and Practice.
Wiley Interscience, 1981.
U.S. Air Force. Technology Profile: Vacuum-Mediated LNAPL Free
Product Recovery/Bioremediation (Bioslurper). Air Force Center for
Environmental Excellence, March 1994.
May 1995 XI-37
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Checklist: Can Dual-Phase Extraction
Be Used At This Site?
This checklist can help you evaluate the completeness of the CAP and
to identify areas that require closer scrutiny. As you go through the CAP,
answer the following questions. If the answer to several questions is no,
you should request additional information to determine if DPE will
accomplish cleanup goals at the site:
1. Site Characteristics
Yes No
Q Q Are the soil and aquifer media intrinsic permeabilities
greater than 10'12 cm2?
Q Q Is the soil free of impermeable layers or other conditions
that would disrupt air flow?
Q Q Is the soil moisture in the unsaturated zone less than or
equal to 85 percent of saturation?
Q Q Is depth to groundwater at least three feet?
2. Constituent Characteristics
Yes No
Q Q Are constituent vapor pressures greater than 0.5 mm
Hg, boiling points less than 300°C, and Henry's law
constants greater than 100 atm?
Q Q Are the chemical sorptive capacities of the constituents
present sufficiently low?
3. Evaluation Of The DPE System Design
Yes No
Q Q Does the radius of influence (ROI) for the proposed
extraction wells fall within the range of 5 to 100 feet?
Q Q Has the ROI been calculated for each soil type at the
site?
XI-38 May 1995
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Q Q For more complex sites with multiple treatment depth
intervals and/or the need for multiple extraction wells,
was subsurface airflow modeling conducted to determine
well placement?
Q • Q Is wellhead vacuum determined from field pilot studies
and between 3t and 100 inches of water?
Q Q Is vapor extraction flow rate between 2 and 50 cfm per
well?
Q Q Are groundwater extraction rates sufficient to capture
groundwater with constituent concentrations above
cleanup goals?
Q Q Will initial constituent vapor concentrations be
monitored?
Q Q Are required final constituent concentrations specified?
Q Q Is a specified cleanup time required?
Q Q Is soil volume to be treated estimated?
Q Q Is the pore volume exchange rate calculated?
Q Q Are discharge limits specified?
Q Q Were site construction limitations considered?
Q Q Is the well density appropriate, given the total area to be
cleaned up and the radius of influence of each well?
Q Q Is manifold piping design addressed and do extraction
pipes slope toward the wells?
Q Q Is vapor pretreatment specified?
Q Q Is vapor treatment included, if warranted based on
treatability study?
Q Q Is the blower selected appropriate for the desired
vacuum conditions?
May 1995 XI-39
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Are appropriate instrumentation and controls specified,
including means to monitor pressure (or vacuum), air/
vapor flow rate, groundwater extraction rates, carbon
dioxide and/or oxygen concentrations in extracted air,
contaminant concentrations in extracted air, and
temperature.
4. Optional DPE Components
Yes No
Q Q Are land surface seals proposed?
Q Q Are air injection or passive inlet wells proposed and are
they appropriate to the site?
5. Operation And Monitoring Plans
Yes No
Q
Q
Q
Does the CAP propose daily monitoring for at least 1
week of flow measurements, constituent concentrations,
vacuum readings, and carbon dioxide and oxygen
concentrations?
Does the CAP propose weekly to biweekly ongoing
monitoring of these same parameters?
XI-4O
May 1995
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Chapter XII
Abbreviations and Definitions
-------
-------
Chapter XII
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
CFU . Colony Forming Units
DNAPL Dense Non-Aqueous Phase Liquid
DO Dissolved Oxygen
DPE Dual-Phase Extraction
FID Flame lonization Detector
GAG Granular Activated Carbon
GC Gas Chromatograph
HDPE High Density Polyethylene
Hg Mercury, elemental
LEL Lower Explosive Limit
LNAPL Light 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
May 1995
XII-1
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UST Underground Storage Tank
VOC Volatile Organic Compound
Definitions
abiotic: not biotic; not formed by biologic processes.
absoxption: 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: able to live, grow, or take place only when free oxygen is
present.
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
XII-2 May 1995
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saturated or unsaturated. Alkanes, alkenes, and alkynes are aliphatic
hydrocarbons.
alkalies: 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.
alkenes: the group of unsaturated hydrocarbons having the general
formula CnH2n and characterized by being highly chemically reactive.
Also referred to as oleflns.
alkynes: the group of unsaturated hydrocarbons with a triple Carbon-
Carbon bond having the general formula CnH2n_2.
ambient: surrounding.
anaerobic: able to live, grow, or take place where free oxygen is not
present.
analog: in chemistry, a structural derivative of a parent compound.
anisotropic: the condition in which hydraulic properties of an aquifer
are not equal when measured in all directions.
anoxic: total deprivation of oxygen.
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.
May 1995 XII-3
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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.
autoignition temperature: the temperature at which a substance will
spontaneously ignite. Autoignition temperature is an indicator of thermal
stability for petroleum hydrocarbons.
autotrophic: 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 toxiciry 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.
xn-4 May199S
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bioaugmentation: the introduction of cultured microorganisms into the
subsurface environment for the purpose of enhancing bioremediation of
organic contaminants. Generally the microorganisms are selected for
their ability to degrade the organic compounds present at the
remediation site. The culture can be either an isolated genus or a mix of
more than one genera. Nutrients are usually also blended with the
aqueous solution containing the microbes to serve as a carrier and
dispersant. The liquid is introduced into the subsurface under natural
conditions (gravity fed) or injected under pressure.
bioavailability: the availability of a compound for biodegradatlon,
influenced by the compound's location relative to microorganisms and its
ability to dissolve in water.
biocide: a substance capable of destroying (killing) living organisms.
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.
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.
May 1995 XII-5
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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.
cometabolism: the simultaneous metabolism of two compounds, in
which the degradation of the second compound (the secondary
substrate) depends on the presence of the first compound (the primary
substrate). For example, in the process of degrading methane, some
bacteria can degrade hazardous chlorinated solvents that they would
otherwise be unable to attack.
complexation: a reaction in which a metal ion and one or more anionic
ligands chemically bond. Complexes often prevent the precipitation of
metals.
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.
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.
XH-6 May 1995
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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.
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).
May 1995 XII-7
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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.
electron acceptor: a chemical entity that accepts electrons transferred
to it from another compound. It is an oxidizing agent that, by virtue of
its accepting electrons, is itself reduced in the process. See also terminal
electron acceptor and oxidation-reduction.
electron donor: a chemical entity that donates electrons to another
compound. It is a reducing agent that, by virtue of its donating
electrons, is itself oxidized in the process. See also electron acceptor and
oxidation-reduction.
empirical: relying upon or gained from experiment or observation.
entrained: particulates or vapor transported along with flowing gas or
liquid.
enzyme: a protein that a living organism uses in the process of
degrading a specific compound. The protein serves as a catalyst in the
compound's biochemical transformation.
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.
extraction well: a well employed to extract fluids (either water, gas, free
product, or a combination of these) from the subsurface. Extraction is
usually accomplished by either a pump located within the well or suction
created by a vacuum pump at the ground surface.
XH-8 May 1995
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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.
facultative: used to describe organisms that are able to grow in either
the presence or absence of a specific environmental factor (e.g., oxygen).
See also facultative anaerobe.
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.
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 grow 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.
May 1995 XII-9
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gradient: the rate of change in value of a physical or chemical parameter
per unit change in position. For example, hydraulic gradient is equal to
the difference in head measured at two points (usually wells) divided by
the distance separating the two points. The dimensions of head and
distance are both lengths, therefore the gradient is expressed as a
dimensionless ratio (L/L).
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.
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.
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.
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.
XH-1O May 1995
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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.
hydrogen peroxide: H2O2. Hydrogen peroxide is used to increase the
dissolved oxygen content of groundwater to stimulate aerobic
biodegradation of organic contaminants. Hydrogen peroxide is infinitely
soluble in water, but rapidly dissociates to form a molecule of water
(H2O) and one-half molecule of oxygen (O). Dissolved oxygen
concentrations of greater than 1,000 mg/L are possible using hydrogen
peroxide, but high levels of D.O. can be toxic to microorganisms.
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 situ: in its original place; unmoved; unexcavated; remaining in the
subsurface.
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.
indigenous: living or occurring naturally in a specific area or
environment; native.
infiltration gallery: an engineered structure that facilitates infiltration
of water into the subsurface. Infiltration galleries may consist of one or
more horizontal or vertical perforated pipes, a single gravel-filled trench
or a network of such trenches, or a combination of these.
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.
May 1995 XII-11
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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.
intergranular: 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 (LI/): 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
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.
metabolism: a term that encompasses all of the diverse reactions by
which a cell processes food material to obtain energy and the compounds
from which new cell components are made.
XII-12 May 1995
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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.
mineralization: the release of inorganic chemicals from organic matter
in the process of aerobic or anaerobic decay.
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 diffusion: process whereby molecules of various gases tend to
intermingle and eventually become uniformly dispersed.
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.
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 aerobes: organisms that require the presence of molecular
oxygen (O2) for their metabolism.
obligate anaerobes: organisms for which the presence of molecular
oxygen is toxic. These organisms derive the oxygen needed for cell
synthesis from chemical compounds.
occlude: to cause to become obstructed or closed and thus prevent
passage either into or from.
May 1995 XH-13
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octanol/water partition coefficient (Kow): 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
Kow 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.
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.
Xn-14 May 1995
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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.
pore volume: the volume of water (or air) that will completely fill all of
the void space in a given volume of porous matrix. Pore volume is
equivalent to the total porosity. The rate of decrease in the
concentration of contaminants in a given volume of contaminated porous
media is directly proportional to the number of pore volumes that can be
exchanged (circulated) through the same given volume of porous media.
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.
May 1995 XII-15
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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.
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.
reagent: a substance or solution used in a chemical reaction, especially
those used in laboratory work to detect, measure, or produce other
substances.
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 refractory 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.
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.
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
XII-16 May 1995
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contaminant can be injected through a septa closure and into a GC
column for separation analysis.
sequester: to undergo sequestration.
sequestration: the inhibition or stoppage of normal ion behavior by
combination with added materials, especially the prevention of metallic
ion precipitation from solution by formation of a coordination complex
with a phosphate.
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: the entry of ambient air into an extraction well
without first passing through the contaminated zone. Short circuiting
may occur through utility trenches, incoherent well or surface seals, or
layers of high permeability geologic materials.
short circuiting: as it applies to SVE and bioventing, the entry of
ambient air into the extraction well without first passing through the
contaminated 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.
May 1995 XII-17
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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.
stratification: layering or bedding of geologic materials (e.g., rock or
sediments).
stratum: a horizontal layer of geologic material of similar composition,
especially one of several parallel layers arranged one on top of another.
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.
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 desorber: describes the primary treatment unit that heats
petroleum-contaminated materials and desorbs the organic materials
into a purge gas or off-gas.
thermal desorption system: refers to a thermal desorber and associated
systems for handling materials and treated soils and treating offgases
and residuals.
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.
XH-18 May 1995
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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 downgiradient.
tripolyphosphates: Salts with P3O~510 anion. Most common is sodium
tripolyphosphate (Na5P3O10).
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.
unsaturated: the characteristic of a carbon atom in a hydrocarbon
molecule that shares a double bond with another carbon atom.
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.
upgradient: it the direction of increasing potentiometric (piezometric)
head.
vacuum draft tube: a narrow tube lowered into an extraction well
through which a strong vacuum is pulled via a suction pump at ground
surface. Fluids (gas, water, and/or free product) are drawn into the
draft tube and conveyed to the surface for treatment or disposal.
Depending upon the configuration of the extraction system, the inlet of
the draft tube may be either above or below the static level of the liquid
in the well.
May 1995 XII-19
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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 unconfined 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.
windrow: a low, elongated row of material left uncovered to dry.
Windrows are typically arranged in parallel.
XH-2O May 1995
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IO& ?50o 7
CORRECTION
Please substitute new page IX-19/IX-20 for the page with the same
number in the shrink-wrapped package.
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We would like your reactions to
"How To Evaluate Alternative Cleanup
Technologies For Underground
Storage Tank Sites: A Guide For
Corrective Action Plan Reviewers."
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U.S. Environmental Protection Agency
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